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The medial prefrontal cortex has been shown to participate in pain processing and to undergo structural and functional changes in chronic visceral pain syndromes. Its infralimbic region (IL) is directly involved in central autonomic regulation and provides extensive projections to pain-related brain centers, being most capable of modulating visceral nociception. However, the precise neuronal and molecular mechanisms underlying IL-exerted descending control of visceral pain and their probable alterations in pathology remain unknown. Considering the high levels of serotonin 5-HT3 receptors in the IL and their important role in visceral pain regulation, in this study performed in anesthetized adult male Wistar rats, we aimed to determine whether 5-HT3 receptor-dependent mechanisms are involved in the IL influence on visceral pain-responsive neurons of the caudal ventrolateral medulla (CVLM) and whether intestinal inflammation affects the process. Our results showed that electrical stimulation of the IL caused a facilitation of both excitatory and inhibitory CVLM neuron responses to noxious colorectal distension (CRD). Both effects were 5-HT3-dependent, being decreased following intracerebroventricular injection of a 5-HT3-antagonist, granisetron (20 µg in 10 µL saline). In trinitrobenzenesulfonic acid-induced colitis, the IL stimulating influence on CRD-excited CVLM neurons intensified, while the cortex-driven facilitation of CRD-inhibited cells reduced; both appeared resistant to the 5-HT3 receptor blockade. These data provide an insight to the specific mechanisms underlying cortical modulation of acute and gut pathology-associated visceral nociception, promoting the development of differentiated strategies for their treatment. [ABSTRACT FROM AUTHOR]

Background: There is an urgent clinical need to provide a theoretical basis for silver needle thermal therapy to Myofacial pain syndrome (MPS).Objective: This study was conducted to explore the effect of silver needle thermal therapy on myofascial pain syndrome in rats.Methods: MPS rat models were duplicated, and the rats were subsequently divided into model and treatment groups. A normal control group was synchronously set up. No treatment was given to the model group, whereas silver needle thermal therapy was administered to the treatment group. The thermal and mechanical pain threshold, the morphological structure as well as the expression of 5-HT3 receptors in the spinal cord were observed.Results: Rats from the treatment group presented with a significantly higher pain threshold compared to the untreated model group.The myofascial arrangement of the affected part of the model group was disordered, and some muscle fibers were atrophied and deformed. Meanwhile, the myofascial arrangement of the treatment group became more regular than that of the model group. The expression levels of 5-HT3 receptor in the spinal cord of the untreated model group were significantly increased, while being markedly decreased in the treatment group.Conclusions: Silver needle thermal therapy can augment the pain threshold of rats with MPS, repair the damaged myofascial membrane in the rats, and further reduce the expression of 5-HT3 receptors in the spinal cord of the MPS rats. [ABSTRACT FROM AUTHOR]

Chronic psychological stress develops and exacerbates irritable bowel syndrome (IBS). 5-hydroxytryptamine (5-HT) via activation of intestinal 5-HT 3 receptors involves impairment of intestinal functions. This study aimed to investigate the effects of ondansetron, a 5-HT 3 receptor antagonist, on locomotor activity, anxiety-related behaviors, and colonic functions in repeated water avoidance stress. Food intake and fecal pellet output (FPO) of sham stress (SS), water avoidance stress (WS), and water avoidance stress with oral administration of ondansetron (1 mg/kg BW) (WA) groups were monitored along the water avoidance stress protocol for 10 consecutive days. On day 11, locomotor activity and anxiety-related behaviors were determined using an open field test. Contractile properties of colonic tissues in response to KCl and a cumulative dose of carbachol (CCh) were determined using in vitro organ bath technique. FPO was significantly increased in the WS group after 7 days of water avoidance stress, which was reversed in WA group. WS group decreased unsupported rearing behavior compared to WS group, which was not altered in the WA group. The colon of the WS group had a higher tonic contraction in response to CCh than the SS and WA groups, which was reversed with ondansetron pre-incubation. Oral administration of ondansetron prevented increased FPO but did not affect anxiety-related behavior in repeated stress model. Colonic hypercontractility in the stressed mice was related to increased responses to cholinergic-induced contractions, which involved 5-HT 3 receptors. Our findings suggest the modulatory roles of 5-HT 3 receptors to mediate stress-induced colonic dysfunction. • Ondansetron administration could not prevent a decrease in unsupported rearing behavior in repeated water avoidance stress-induced mice • Mice that received oral administration of ondansetron reversed the increased fecal pellet output after exposure to repeated water avoidance stress. • 5-HT 3 receptors modulate augmented tonic contraction in response to cholinergic stimulation in water avoidance stress-induced colonic hypercontractility. [ABSTRACT FROM AUTHOR]

Cohabitation with a partner undergoing chronic pain induces pain hypersensitivity. Among a lot of other neurochemical pathways, the serotonin (5-HT) role, specifically the 5-HT3 receptor (5-HT3R), in the amygdala has never been evaluated in this model. Here we studied the effects of the amygdala's chemical inhibition, its neuronal activation pattern, and 5-HT, 5-HIAA, and 5-HT turnover within the amygdala. Furthermore, the systemic and intra-amygdala 5-HT3R activation and blockade in mice that cohabited with a conspecific subjected to chronic constriction injury were investigated. Male Swiss mice were housed in partners for 28 days. The dyads were divided into two groups on the 14th day: cagemate nerve constriction (CNC) and cagemate sham (CS). On the 24th day, cagemates underwent a stereotaxic surgery (when necessary) and, on the 28th day, they were evaluated on the writhing test. The amygdala inactivation promotes pain-hypersensitivity behaviors in groups and dyads; cohabitation with a partner with chronic pain did not change FosB-labeled cells in the amygdala's nucleus and increases 5-HT turnover in cagemates. Systemic and intra-amygdala 5-HT3R activation attenuated and enhanced the number of writhes, respectively. In contrast, 5-HT3R blockade reduced hypersensitivity pain response. Results suggest the involvement of amygdala serotonergic signaling via 5-HT3R in empathy-like behavior. [ABSTRACT FROM AUTHOR]

Rationale: 2-Amino-6-chloro-3,4-dihydroquinazoline (e.g., A6CDQ) represents a novel putative antidepressant originally thought to act through a 5-HT3 serotonin receptor antagonist mechanism. Here, we investigated this further by examining a positional isomer of A6CDQ (i.e., A7CDQ). Materials and methods: 5-HT3 receptor and transporter activity (uptake-1 and uptake-2) were investigated using a variety of in vitro assays and the in vivo mouse tail suspension test (TST). Results: Although A7CDQ binds at 5-HT3 receptors with low affinity (Ki = 1975 nM) compared to A6CDQ (Ki = 80 nM), it retained 5-HT3 receptor antagonist action (IC50 = 5.77 and 0.26 μM, respectively). In the mouse TST A7CDQ produced antidepressant-like actions (ED50 = 0.09 mg/kg) comparable to that of A6CDQ. In addition, A6CDQ was found to be a 5-HT releasing agent (Km = 2.8 μM) at hSERT and a reuptake inhibitor (IC50 = 1.8 μM) at hNET, whereas A7CDQ was a weak reuptake inhibitor (Km = 43.6 μM) at SERT but a releasing agent (EC50 = 3.3 μM) at hNET. Moreover, A6CDQ and A7CDQ were potent inhibitors of uptake-2 (e.g.; OCT3 IC50 = 3.9 and 5.9 μM, respectively). Conclusions: A simple shift of a substituent in a common quinazoline scaffold from one position to another (i.e., a chloro group from the 6- to the 7-position) resulted in a common action in the TST but via a somewhat different mechanism. A6CDQ and A7CDQ might represent the first members of a new class of potential antidepressants with a unique multi-modal mechanism of action. [ABSTRACT FROM AUTHOR]

Abstract This review analyses the knowledge about the incidence of transient apnoeic spells, induced by substances which activate vagal chemically sensitive afferents. It considers the specificity and expression of appropriate receptors, and relevant research on pontomedullary circuits contributing to a cessation of respiration. Insight is gained into an excitatory drive of 5-HT 1A serotonin receptors in overcoming opioid-induced respiratory inhibition. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

• The activation of 5-HT3 receptors in medullary areas increases the intravesical pressure. • 5-HT3 receptors are involved in the phasic control of urinary bladder. • 5-HT3 receptors in the medulla oblongata are involved in the pathways of the tonic control of the cardiovascular system. The control of micturition depends on reflex mechanisms, however, it undergoes modulation from cortex, pons and medullary areas. This study investigated if the activation of 5-HT3 receptors in the medulla influences the urinary bladder (UB) regulation in rats. Isoflurane female Wistar rats were submitted to catheterization of the femoral artery and vein for mean arterial pressure (MAP) and heart rate (HR) recordings and injection of drugs, respectively. The UB was cannulated for intravesical pressure (IP) measurement. The Doppler flow probe was placed around the left renal artery for renal conductance (RC) recordings. Phenylbiguanide (PB) and granisetron (GN) were injected into the 4th brain ventricle in rats with guide cannulas implanted 5 days prior to the experiments; or PB and GN were randomly injected intravenously or applied topically (in situ) on the UB. PB injection into 4th V significantly increased IP (68.67 ± 11.70%) and decreased MAP (−29 ± 6 mmHg) compared to saline (0.34 ± 0.64% and −2 ± 2 mmHg), with no changes in the HR and RC. GN injection into the 4th V did not significantly change the IP and RC compared to saline, nevertheless, significantly increased MAP (25 ± 4 mmHg) and heart rate (36 ± 9 bpm) compared to saline. Intravenous PB and GN only produced cardiovascular effects, whilst PB but not GN in situ on the UB evoked increase in IP (111.60 ± 30.36%). Therefore, the activation of 5HT-3 receptors in medullary areas increases the intravesical pressure and these receptors are involved in the phasic control of UB. In contrast, 5-HT3 receptors in the medulla oblongata are involved in the pathways of the tonic control of the cardiovascular system. The activation of 5-HT3 receptors in the bladder cause increase in intravesical pressure and this regulation seem to be under phasic control as the blockade of such receptors elicits no changes in baseline intravesical pressure. [ABSTRACT FROM AUTHOR]

The caudal ventrolateral reticular formation of the medulla oblongata is the first layer of visceral nociceptive processing. In experiments on rats, neuronal responses in this zone to nociceptive stimulation of the large intestine were examined and the effects of selective blockade of 5-HT3 receptors on these responses were assessed. By the character of responses to nociceptive colorectal stimulation (CRS), the recorded medullary neurons were divided into three groups-excited, inhibited and indifferent. Intravenous injection of 5-HT3 antagonist granisetron (1 and 2 mg/kg) as well as local application of this agent on the surface of the medulla oblongata (1.25 and 2.5 nmole) suppressed the background and evoked firing of CRS-excited reticular neurons in a dose-dependent manner but did not exert as pronounced influence on the cells inhibited by visceral nociceptive stimulation. Spike activity in the group of CRS-indifferent neurons under similar conditions was 5-HT3-independent. The results obtained provide evidence that 5-HT3 receptors mediate the facilitating effect of serotonin on supraspinal transmission of the abdominal nociceptive stimulus which, at least in part, is realized via selective activation of visceral medullary nociceptive neurons. A shutdown of this mechanism may underlie the analgesic effect of 5-HT3 antagonists in abdominal pain syndromes. [ABSTRACT FROM AUTHOR]

Serotonin fibers densely innervate the cortical sheath to regulate neuronal excitability, but its role in shaping network dynamics remains undetermined. We show that serotonin provides an excitatory tone to cortical neurons in the form of spontaneous synaptic noise through 5-HT3 receptors, which is persistent and can be augmented using fluoxetine, a selective serotonin re-uptake inhibitor. Augmented serotonin signaling also increases cortical network activity by enhancing synaptic excitation through activation of 5-HT2 receptors. This in turn facilitates the emergence of epileptiform network oscillations (10-16 Hz) known as fast runs. A computational model of cortical dynamics demonstrates that these two combined mechanisms, increased background synaptic noise and enhanced synaptic excitation, are sufficient to replicate the emergence fast runs and their statistics. Consistent with these findings, we show that blocking 5-HT2 receptors in vivo significantly raises the threshold for convulsant-induced seizures. [ABSTRACT FROM AUTHOR]

The three most common Cys-loop receptors expressed by myenteric neurons are nACh, 5-HT3 and GABAA. To investigate the function of these proteins researchers have used channel inhibitors such as hexamethonium (antagonist of nACh receptors), ondansetron (antagonist of 5-HT3 receptors), picrotoxin and bicuculline (both antagonists of GABAA receptors). The aim of this study was to investigate the specificity of these inhibitors on Cys-loop receptors of primary cultured neurons obtained from the guinea-pig small intestine. The whole-cell configuration of the patch clamp techniques was used to record membrane currents induced by ACh (IACh), 5-HT (I5-HT) and GABA (IGABA) in the absence and the presence of various concentrations of hexamethonium, ondansetron, picrotoxin or bicuculline., The three Cys-loop receptors present in enteric neurons are expressed independently and they do not cross-desensitized. Hexamethonium inhibited IACh without affecting I5-HT and IGABA. Ondansetron inhibited I5-HT and also IACh but did not affect IGABA. Picrotoxin and bicuculline inhibited I5-HT, IACh and IGABA with different potency, being the lowest potency on 5-HT3 receptors. All these inhibitory effects were concentration dependent and reversible., Our observations showed that except for hexamethonium, all other inhibitors used here show different degrees of selectivity, which has to be considered when these antagonists are used in experimental studies aimed to investigate the functions of these receptors. In particular, in tissues expressing nACh receptors because these are the targets of all other inhibitors used here. The low potency of picrotoxin and bicuculline to inhibit 5-HT3 receptors suggests that these receptors are heteromeric proteins. [ABSTRACT FROM AUTHOR]

Background and Objective: Although 5-fluorouracil (5-FU) is a widely used as chemotherapy agent, severe mucositis develops in approximately 80 % of patients. 5-FU-induced small intestinal mucositis can cause nausea and vomiting. The current study was designed to investigate peripheral alterations due to the 5-FU-induced mucositis of neuronal and non-neuronal 5-HT 3 and NK 1 receptor expression by immunohistochemical analysis. Methods: 5-FU was administered by i.p. injection to C57BL/6 mice. After 4 days, segments of the jejunum were removed. The specimens were analyzed by immunohistochemistry, real-time PCR, and enzyme immunoassay. Results: The numbers of 5-HT 3 receptor immunopositive cells and nerve fibers in mucosa were increased by 5-FU treatment. The 5-HT 3 receptor immunopositive cell bodies were found only in jejunal submucosa and myenteric plexus in the 5-FU-treated mice. The numbers of NK 1 receptor cells in mucosa and immunopositive expression of NK 1 receptors in deep muscular plexus were dramatically increased in 5-FU-treated mice. Real-time PCR demonstrated that 5-FU treatment significantly increased mRNA levels of 5-HT 3A, 5-HT 3B, and NK 1 receptors. The amounts of 5-HT and substance P increased after 5-FU treatment. The 5-HT 3 or NK 1 receptor immunopositive cells colocalized with both 5-HT and substance P. Furthermore, 5-HT 3 and NK 1 receptors colocalized with CD11b. Conclusions: The 5-HT 3 and NK 1 immunopositive macrophages and mucosal mast cells in lamina propria release 5-HT and substance P, which in turn activate their corresponding receptors on mucosal cells in autocrine and paracrine manners. It is assumed to result in the release of 5-HT and substance P in mucosa. [ABSTRACT FROM AUTHOR]

Abstract: 2-Amino-6-chloro-3,4-dihydroquinazoline HCl (A6CDQ, 4) binds at 5-HT3 serotonin receptors and displays antidepressant-like action in the mouse tail suspension test (TST). Empirically, 4 was demonstrated to be a 5-HT3 receptor antagonist (two-electrode voltage clamp recordings using frog oocytes; IC50 =0.26μM), and one that should readily penetrate the blood–brain barrier (log P =1.86). 5-HT3 receptor antagonists represent a potential approach to the development of new antidepressants, and 4 is an example of a structurally novel 5-HT3 receptor antagonist that is active in a preclinical antidepressant model (i.e., the mouse TST). [Copyright &y& Elsevier]

Abstract: It has been described that endogenous ATP via activation of P2X3 and P2X2/3 receptors contributes to inflammatory nociception in different models, including the formalin injected in subcutaneous tissue of the rat's hind paw. In this study, we have evaluated whether TRPA1, 5-HT3 and 5-HT1A receptors, whose activation is essential to formalin-induced inflammatory nociception, are involved in the nociception induced by activation of P2X3 receptors on subcutaneous tissue of the rat's hind paw. We have also evaluated whether the activation of P2X3 receptors increases the susceptibility of primary afferent neurons to formalin action modulated by activation of TRPA1, 5-HT3 or 5-HT1A receptors. Nociceptive response intensity was measured by observing the rat's behavior and considering the number of times the animal reflexively raised its hind paw (flinches) in 60min. Local subcutaneous administration of the selective TRPA1, 5-HT3 or 5-HT1A receptor antagonists HC 030031, tropisetron and WAY 100,135, respectively, prevented the nociceptive responses induced by the administration in the same site of the non-selective P2X3 receptor agonist αβmeATP. Administration of the selective P2X3 and P2X2/3 receptor antagonist A-317491 or pretreatment with oligonucleotides antisense against P2X3 receptor prevented the formalin-induced behavioral nociceptive responses during the first and second phases. Also, the co-administration of a subthreshold dose of αβmeATP with a subthreshold dose of formalin induced nociceptive behavior, which was prevented by local administration of tropisetron, HC 030031 or WAY 100, 135. These findings have demonstrated that the activation of P2X3 receptors induces inflammatory nociception modulated by TRPA1, 5-HT3 and 5-HT1A receptors. Also, they suggest that inflammatory nociception is modulated by the release of endogenous ATP and P2X3 receptor activation, which in turn, increases primary afferent nociceptor susceptibility to the action of inflammatory mediators via interaction with TRPA1, 5-HT3 and 5-HT1A receptors in the peripheral tissue. [Copyright &y& Elsevier]

Segmentation is an important process in nutrient mixing and absorption; however, the mechanisms underlying this motility pattern are poorly understood. Segmentation can be induced by luminal perfusion of fatty acid in guinea pig small intestine in vitro and mimicked by the serotonin (5-HT) reuptake inhibitor fluoxetine (300 nM) and by cholecystokinin (CCK). Serotonergic and CCK-related mechanisms underlying nutrient-induced segmentation were investigated using selective 5-HT and CCK receptor antagonists on isolated segments of small intestine luminally perfused with 1 mM decanoic acid. Motility patterns were analyzed using video imaging and spatiotemporal maps. Segmenting activity mediated by decanoic acid was depressed following luminal application of the 5-HT receptor antagonists granisetron (5-HT3, 1 μM) and SB-207266 (5-HT4, 10 nM) and the CCK receptor antagonists devazepide (CCK-1, 300 nM) and L-365260 (CCK-2, 300 nM), but these antagonists did not further depress segmentation when combined. The P2 receptor antagonist pyridoxal phosphate-6-azophenyl-2=,4=-disulfonate (10M) had no effect on activity. Serosal application of 5-HT antagonists had little effect on segmentation in the duodenum but reduced activity in the jejunum when granisetron and SB-207266 were applied together. These results reveal that 5-HT3 and 5-HT4 receptors, as well as CCK-1 and CCK-2 receptors, are critical in regulating decanoic acid-induced segmentation. Computational simulation indicated that these data are consistent with decanoic acid activating two pathways in the mucosa that converge within the enteric neural circuitry, while contraction-induced release of 5-HT from the mucosa provides feedback into the neural circuit to set the time course of the overall contractile activity. [ABSTRACT FROM AUTHOR]

Background The purinergic component of enteric inhibitory neurotransmission is important for normal motility in the gastrointestinal (GI) tract. Controversies exist about the purine(s) responsible for inhibitory responses in GI muscles: ATP has been assumed to be the purinergic neurotransmitter released from enteric inhibitory motor neurons; however, recent studies demonstrate that β-nicotinamide adenine dinucleotide (β-NAD+) and ADP-ribose mimic the inhibitory neurotransmitter better than ATP in primate and murine colons. The study was designed to clarify the sources of purines in colons of Cynomolgus monkeys and C57BL/6 mice. Methods High-performance liquid chromatography with fluorescence detection was used to analyze purines released by stimulation of nicotinic acetylcholine receptors (nAChR) and serotonergic 5-HT3 receptors (5-HT3R), known to be present on cell bodies and dendrites of neurons within the myenteric plexus. Key Results Nicotinic acetylcholine receptor or 5-HT3R agonists increased overflow of ATP and β-NAD+ from tunica muscularis of monkey and murine colon. The agonists did not release purines from circular muscles of monkey colon lacking myenteric ganglia. Agonist-evoked overflow of β-NAD+, but not ATP, was inhibited by tetrodotoxin (0.5 μmol L−1) or ω-conotoxin GVIA (50 nmol L−1), suggesting that β-NAD+ release requires nerve action potentials and junctional mechanisms known to be critical for neurotransmission. ATP was likely released from nerve cell bodies in myenteric ganglia and not from nerve terminals of motor neurons. Conclusions & Inferences These results support the conclusion that ATP is not a motor neurotransmitter in the colon and are consistent with the hypothesis that β-NAD+, or its metabolites, serve as the purinergic inhibitory neurotransmitter. [ABSTRACT FROM AUTHOR]

BACKGROUND AND PURPOSE Disturbed cortical gamma band oscillations (30-80 Hz) have been observed in schizophrenia: positive symptoms of the disease correlate with an increase in gamma oscillation power, whereas negative symptoms are associated with a decrease. EXPERIMENTAL APPROACH Here we investigated the effects of first and second generation antipsychotics (FGAs and SGAs, respectively) on gamma oscillations. The FGAs haloperidol, flupenthixol, chlorpromazine, chlorprothixene and the SGAs clozapine, risperidone, ziprasidone, amisulpride were applied on gamma oscillations induced by acetylcholine and physostigmine in the CA3 region of rat hippocampal slices. KEY RESULTS Antipsychotics inhibited the power of gamma oscillations and increased the bandwidth of the gamma band. Haloperidol and clozapine had the highest inhibitory effects. To determine which receptor is responsible for the alterations in gamma oscillations, the effects of the antipsychotics were plotted against their p Ki values for 19 receptors and analysed for correlation. Our results indicated that 5-HT3 receptors have an enhancing effect on gamma oscillations whereas dopamine D3 receptors inhibit them. To test this prediction, m-chlorophenylbiguanide, PD 128907 and CP 809101, selective agonists at 5-HT3, D3 and 5-HT2C receptors were applied and revealed that 5-HT3 receptors indeed enhanced the gamma power whereas D3 receptors reduced it. As predicted, 5-HT2C receptors had no effects on gamma oscillations. CONCLUSION AND IMPLICATIONS Our data suggest that antipsychotics alter hippocampal gamma oscillations by interacting with 5-HT3 and dopamine D3 receptors. Moreover, a correlation of receptor affinities with the biological effects can be used to predict targets for the pharmacological effects of multi-target drugs. [ABSTRACT FROM AUTHOR]

Abstract: Emerging evidence from therapeutic trials in humans and animal models suggests that in the treatment of depression, antidepressants play a role by targeting the glutamatergic system. Fluvoxamine is one of the widely used SSRIs which has been considered to target monoamine neurotransmitter reuptake mechanisms. However, whether fluvoxamine has an effect on the glutamate release is still unclear. The present experiment studied the effect of fluvoxamine on presynaptic glutamate release in prelimbic cortex, both in control C57BL/6 mice and chronic restraint stress C57BL/6 mice, and further investigated the mechanism underlying this effect by using patch clamp, on-line fluorimetry, pharmacological approaches combined with other techniques. The results showed that fluvoxamine increased the glutamate release in the depression model mice but it had no effect on the glutamate release in the control mice. The mechanism underlying these effects in depression model mice was that, fluvoxamine firstly activated presynaptic 5-HT3 receptors, which transiently increased the Ca2+ concentration. The increase of Ca2+ concentration via 5-HT3 receptors caused the activation of sigma-1 receptors, which were activated by fluvoxamine. The activation of sigma-1 receptors increased the intrasynaptosomal Ca2+ concentration significantly through the outflow of endoplasmic reticulum calcium and finally activated PKC. These results suggested that fluvoxamine may have a selective effect and different mechanism based on the condition of animal. [Copyright &y& Elsevier]

Abstract: The early postnatal period represents a critical time window for brain development. Transient Cajal–Retzius cells in layer I of the cortex play an important role in cortical lamination by modulating neuronal migration and maturation. Recent data have demonstrated that the 5-HT3 receptor antagonist and alpha7 nicotinic receptor partial agonist tropisetron, acting via 5-HT3 receptors expressed on Cajal–Retzius cells, can disturb the formation of cortical columns at perinatal stages. This process is thought to be involved in several neuropsychiatric disorders. Here we investigated the possible long-term behavioral effects of exposure to tropisetron at early postnatal stages in mice. We found that the administration of 1mg/kg, intraperitoneal (i.p.) tropisetron from postnatal days 2–12 (P2–P12) did not induce significant cognitive, schizophrenia-like or emotional alterations in tropisetron-treated animals as compared to controls, when tested in multiple behavioral assays. These results may be of relevance regarding the possible protracted deleterious neuropsychiatric effects of tropisetron during early life. [Copyright &y& Elsevier]

Abstract: Ovariectomized rats, hormonally primed with 10μg estradiol benzoate and 500μg progesterone are resistant to the lordosis-inhibiting effects of a 5min restraint experience. However, modulation of the serotonergic (5-HT) system alters this resistance to stress. In the following experiment, ovariectomized Fischer inbred rats were hormonally primed with 10μg estradiol benzoate and 500μg progesterone. The effect of 5min restraint on sexual behavior was examined after bilateral hypothalamic infusion or intraperitoneal (ip) treatment with the 5-HT3 receptor antagonist, 3-tropanylindole-3-carboxylate hydrochloride (tropisetron). Infusion with 50 or 100ng tropisetron inhibited lordosis behavior. When rats were infused with 10 or 25ng tropisetron, rats showed normal lordosis behavior. However, when infusion with 10 or 25ng tropisetron was combined with 5min restraint, lordosis behavior was inhibited. These findings are consistent with prior work that has implicated hypothalamic serotonin in control of lordosis behavior and in the effect of mild restraint on the behavior. In contrast to the effects of the intracranial infusion, intraperitoneal injection with 1.0 or 2.0mg/kg tropisetron did not amplify the effects of restraint. [Copyright &y& Elsevier]

Serotonin (5-HT)3 receptors are the only ligand-gated ion channel of the 5-HT receptors family. They are present both in the peripheral and central nervous system and are localized in several areas involved in mood regulation (e.g., hippocampus or prefrontal cortex). Moreover, they are involved in regulation of neurotransmitter systems implicated in the pathophysiology of major depression (e.g., dopamine or GABA). Clinical and preclinical studies have suggested that 5-HT3 receptors may be a relevant target in the treatment of affective disorders. 5-HT3 receptor agonists seem to counteract the effects of antidepressants in non-clinical models, whereas 5-HT3 receptor antagonists, such as ondansetron, present antidepressant-like activities. In addition, several antidepressants, such as mirtazapine, also target 5-HT3receptors. In this review, we will report major advances in the research of 5-HT3 receptor's roles in neuropsychiatric disorders, with special emphasis on mood and anxiety disorders. [ABSTRACT FROM AUTHOR]

Abstract: Previous studies showed that 5-HT1A and 5-HT2 receptors play an important role in controlling the extrapyramidal motor disorders. However, the functions of other 5-HT receptor subtypes remain elusive. To elucidate the role of 5-HT receptors, specifically of 5-HT3∼5-HT7 subtypes, in modifying antipsychotic- induced extrapyramidal side effects (EPS), we studied the effects of the 5-HT stimulant 5-hydroxytryptophan (5-HTP) and various 5-HT receptor antagonists on haloperidol (HAL)-induced bradykinesia and catalepsy in mice and rats. Pretreatment of mice with 5-HTP (25–100mg/kg, i.p.) dose-dependently enhanced HAL (0.3mg/kg, i.p.)-induced bradykinesia and catalepsy. The potentiation of HAL-induced EPS by 5-HTP (50mg/kg, i.p.) was significantly inhibited by ritanserin (5-HT2 antagonist, 0.3-3mg/kg, i.p.), ondansetron (5-HT3 antagonist, 0.1–1mg/kg, i.p.), or SB-258585 (5-HT6 antagonist, 1–10mg/kg, i.p.) in a dose-dependent manner. However, neither WAY-100135 (5-HT1A antagonist, 1–10mg/kg, i.p.), GR-125487 (5-HT4 antagonist, 1–10mg/kg, i.p.), SB-699551 (5-HT5A antagonist, 1–10mg/kg, i.p.) nor SB-269970 (5-HT7 antagonist, 1–10mg/kg, i.p.) reduced the 5-HTP and HAL-induced bradykinesia or catalepsy. In addition, both ondansetron (0.1–1mg/kg, i.p.) and SB-258585 (3 and 10mg/kg, i.p.) also alleviated bradykinesia and catalepsy induced by HAL (0.5mg/kg, i.p.) alone in mice. Furthermore, bilateral microinjection of ondansetron (5μg (13.7nmol) per side) or SB-258585 (5μg (8.92nmol) per side) into the dorsolateral striatum (dlST) attenuated haloperidol-induced catalepsy in rats. These results suggest that serotonergic stimulation augments extrapyramidal motor disorders by activating the striatal 5-HT3 and 5-HT6 receptors and the antagonism of these receptors effectively alleviates antipsychotic-induced EPS. [Copyright &y& Elsevier]

Aim: Stimulation of pulmonary C fibre receptors by phenylbiguanide (PBG, 5-HT3 agonist) produces hypotension, bradycardia and tachypnoea or apnoea. However, tachypnoeic or apnoeic responses are not consistent. Therefore, this study was undertaken to delineate the actions of PBG on respiration and compared with those evoked by capsaicin (TRPV1 agonist). Methods: Blood pressure, respiratory excursions and ECG were recorded in urethane anaesthetized adult rats. The effect of PBG or capsaicin was evaluated before and after ondansetron (5-HT3 antagonist), capsazepine (TRPV1 antagonist) or bilateral vagotomy. In addition, their effect on vagal afferent activity was also evaluated. Results: Bolus injection of PBG produced concentration-dependent (0.1–100 μg kg−1) hypotensive and bradycardiac responses, while there was tachypnoea at lower concentrations (0.1–3 μg kg−1) and apnoea at higher concentrations (10–100 μg kg−1). After vagotomy or after exposure to ondansetron both tachypnoeic and apnoeic responses were abolished along with cardiovascular responses. However, capsazepine (3 mg kg−1) did not block the PBG-induced reflex responses. Capsaicin (0.1–10 μg kg−1), on the other hand, produced a concentration-dependent apnoea, hypotension and bradycardia but tachypnoea was not observed. Ondansetron failed to block the capsaicin-induced reflex response while bilateral vagotomy abolished bradycardiac and hypotensive responses and attenuated the apnoeic response. In another series, vagal afferent activity and cardio-respiratory changes evoked by PBG were blocked by ondansetron. However, capsaicin failed to activate the PBG-sensitive vagal afferents even though cardio-respiratory alterations were observed. Conclusions: The present observations indicate that PBG produced tachypnoea at a lower concentration and apnoea at a higher concentration involving vagal afferents which are different from those excited by capsaicin. [ABSTRACT FROM AUTHOR]

Abstract: In the present study, we examined changes in the firing rate and firing pattern of putative slow-spiking (SS) and fast-spiking (FS) interneurons in medial prefrontal cortex (mPFC) and the effect of 5-hydroxytryptamine-3 (5-HT3) receptor agonist SR 57227A on the neuronal firing in rats with 6-hydroxydopamine lesions of the substantia nigra pars compacta (SNc) by using extracellular recording. The lesion of the SNc in rats decreased the firing rate of FS interneurons and the firing pattern of both SS and FS interneurons changed towards a more burst-firing. Systemic administration of SR 57227A (40–640 μg/kg, i.v.) increased the firing rate of SS interneurons, and decreased FS interneurons in sham-operated and the lesioned rats, respectively. The doses producing excitation or inhibition in the lesioned rats were higher than sham-operated rats. The local application of SR 57227A (0.01 μg) in mPFC excited SS interneurons, and inhibited FS interneurons in sham-operated rats, while having no effects on firing rate in the lesioned rats. Systemic administration of GABAA receptor antagonist bicuculline (2 mg/kg, i.v.) excited FS interneurons in sham-operated rats, whereas bicuculline did not change the activity of FS interneurons in the lesioned rats. Our findings indicate that the putative SS and FS interneurons activity is modulated through activation of 5-HT3 receptor by direct or indirect action, and the lesion of the SNc leads to changes in firing activity of the SS and FS interneurons and decreased response of these interneurons to SR 57227A, suggesting dysfunction and/or down-regulation of 5-HT3 receptor on interneurons in the 6-hydroxydopamine-lesioned rats. [ABSTRACT FROM AUTHOR]

Abstract: To determine the role of central serotonin 5-HT3 receptors in respiratory motor control, respiratory motor bursts were recorded from hypoglossal (XII) nerve rootlets on isolated adult turtle brainstems during bath-application of 5-HT3 receptor agonists and antagonists. mCPBG and PBG (5-HT3 receptor agonists) acutely increased XII burst frequency and regularity, and decreased bursts/episode. Tropisetron and MDL72222 (5-HT3 antagonists) increased bursts/episode, suggesting endogenous 5-HT3 receptor activation modulates burst timing in vitro. Tropisetron blocked all mCPBG effects, and the PBG-induced reduction in bursts/episode. Tropisetron application following mCPBG application did not reverse the long-lasting (2h) mCPBG-induced decrease in bursts/episode. We conclude that endogenous 5-HT3 receptor activation regulates respiratory frequency, regularity, and episodicity in turtles and may induce a form of respiratory plasticity with the long-lasting changes in respiratory regularity. [Copyright &y& Elsevier]

Converging lines of evidence demonstrate a vagally mediated antinociceptive pathway in animals undergoing acute visceral insults, the contribution of this system to visceral pain following chronic noxious stimuli is unknown. 5-HT3 receptor (5-HT3Rs) on spinal afferents are crucially involved in nociceptive processing, the role of 5-HT3Rs on vagal afferents is unclear. The aim of the present study was to determine the contribution of vagal afferents to visceral nociception in rats undergoing chronic luminal allergen stimulation and whether it involves vagal 5-HT3Rs. Sensitized rats received chicken egg albumin (EA, 1 mg mL−1) in drinking water for 2 weeks (day 1–14). Visceromotor response (VMR) to colorectal distension [colorectal distension (CRD), 60 mmHg] and the levels of mRNA encoding 5-HT3R (including 3A and 3B subunits) in the nodose ganglia (NG) were evaluated on day 2, 4, 8 and 15. Chronic EA challenge induced gradually increased visceral nociception, with a peak on day 15. Subdiaphragmatic vagotomy or functional deafferentation with capsaicin abolished this time-dependent manner, inducing hyperalgesia from day 2, lasting to day 15. Intraluminal infusion of a 5-HT3R antagonist (granisetron), whether alone or infused after local mucosa anaesthetic with 1% lidocaine, mimicked the effects of vagotomy. The mRNA levels for 5-HT3B or 5-HT3A subunit in the NG showed an opposite time-course to that of visceral pain, which increased from day 2, then decreased gradually to levels lower than those of controls. Our results demonstrate a time-dependent vagal afferent modulation of chronic allergen-sensitized visceral hyperalgesia, which may involve a 5-HT3R pathway. [ABSTRACT FROM AUTHOR]

Abstract: Heteroaromatic carboxylic esters of (nor)tropine were synthesized. Tropine esters displaced [3H]strychnine binding to glycine receptors of rat spinal cord with low Hill slopes. Two-site displacement resulted in nanomolar IC50,1 and micromolar IC50,2 values, and IC50,2/IC50,1 ratios up to 615 depending on the heteroaromatic rings and N-methyl substitution. Nortropeines displayed high affinity and low heterogeneity. IC50,1 and IC50,2 values of tropeines did not correlate suggesting different binding modes/sites. Glycine potentiated only the nanomolar displacement reflecting positive allosteric interactions and potentiation of ionophore function. Affinities of three (nor)tropeines were different for glycine receptors but identical for 5-HT3 receptors. [Copyright &y& Elsevier]

This study aimed to examine the distribution of 5-HT receptors in the human colon. 5-HT induces desensitization of the circular muscle and as this is facilitated by G-protein coupled receptor kinases (GRKs) and other proteins, we also examined their distribution. Human sigmoid colon samples were dissected into three separate layers (mucosa, taeniae coli and intertaenial strips) and RNA was amplified by RT-PCR. The 5-HT2B receptor and all 5-HT7 receptor splice variants were expressed in all tissues. 5-HT4 a,b,c and n splice variants were also expressed in all tissues and 5-HT4d, 5-HT4g and 5-HT4i were only detected in some samples. The 5-HT2A receptor was seen predominantly in the intertaenial strips of the colon. Only one transcript of the serotonin transporter (SERT) was detected in the muscle layers. Variation was seen in GRK expression with GRK2 and 3 predominantly expressed in the mucosa, while GRK5 and 6 were found more commonly in the taeniae coli. PDZ (named after postsynaptic density protein, Drosophila disc large tumour suppressor and tight junction protein ZO-1) domain containing proteins, which may be involved in 5-HT receptor trafficking, were also detected throughout the sigmoid colon. The 5-HT3A subunit was expressed in all tissues, whereas the 5-HT3E subunit was mainly found in the mucosa layer while the 5-HT3B subunit was more common in the muscle layers. Receptor interacting chaperone (RIC-3), which is involved in transporting 5-HT3 receptor subunits, is expressed less in mucosa compared to muscle layers. In conclusion, these results show that there is variation in distribution of 5-HT receptors and interacting proteins within the sigmoid colon that may contribute to colonic function. [ABSTRACT FROM AUTHOR]

Abstract: Involvement of vanilloid and 5-HT3 receptors in the cardiorespiratory reflexes evoked by intra-arterial (i.a.) injection of Mesobuthus tamulus (BT) venom was examined. In anaesthetised rats, blood pressure, respiratory excursions and ECG were recorded for 60min after the injection of venom in the absence or presence of antagonists. Injection of BT venom (1mg/kg, i.a.) produced alterations in respiratory frequency (RF), blood pressure (BP) and heart rate (HR). The changes in RF were manifested as immediate increase (40%) followed by a decrease (40%) and subsequent sustained increase (60%). In case of BP, the increase began around 40s, peaked at 5min (50%) and remained above the initial level subsequently. The bradycardiac response began around 5min which peaked (50% of the initial) around 25min and remained at that level. Thus, exhibiting immediate-tachypnoeic, intermediate-hypertensive and delayed-bradycardiac responses. Pretreatment with lignocaine, blocked the respiratory responses and attenuated the pressor responses evoked by venom. Pretreatment with capsazepine, vanilloid receptor 1 (VR1) antagonist, antagonized all the three parameters of cardiorespiratory responses evoked by venom. Whereas, ondansetron (5-HT3 antagonist) attenuated the pressor and bradycardiac responses significantly but not the respiratory responses. These observations indicate that the cardiorespiratory changes induced by intra-arterial injection of venom are carried by afferents in addition to somatic nerves, involving mainly VR1 receptors and partially by 5-HT3 receptors. [Copyright &y& Elsevier]

The roles of serotonin (5-HT) and/or noradrenaline in acute pruriceptive processing have been demonstrated using antidepressants, such as milnacipran, a serotonin and noradrenaline reuptake inhibitor, and mirtazapine, a noradrenergic and specific serotonergic antidepressant; however, the involvement of 5-HT in acute pruriceptive processing has not yet been elucidated in detail. Scratching events induced by chloroquine (CQ) were attenuated by the administration of milnacipran or mirtazapine, and these effects were reversed by a treatment with ondansetron, a 5-HT 3 antagonist, or SB26970, a 5-HT 7 antagonist. CQ-induced scratching events were also ameliorated by the intrathecal administration of 5-HT, SR572227A and RS56812 (5-HT 3 agonists), and LP211 and LP44 (5-HT 7 agonists), indicating the modulation of CQ-induced scratching events by 5-HT and noradrenaline. By contrast, histamine-induced scratching events were not markedly affected by the administration of 5-HT and 5-HT 7 agonists, whereas 5-HT 3 agonists exerted attenuating effects. Similarly, they were not clearly reversed by the administration of the 5-HT 7 antagonist, unlike a 5-HT 3 antagonist. Therefore, 5-HT is involved in the attenuating effects of milnacipran and mirtazapine on CQ- and histamine-induced scratching events, and 5-HT 3 and 5-HT 7 receptors play different roles in pruriceptive processing induced by histamine or CQ. • Milnacipran and mirtazapine inhibit histamine-/chloroquine (CQ)-induced scratching. • An antagonist of 5-HT 3 receptors reverses this inhibition. • A 5-HT 7 antagonist reverses the inhibition of CQ-induced scratching. • 5-HT and agonists of 5-HT 3 and 5-HT 7 receptors inhibit CQ-induced scratching. • 5-HT 3 agonists, but not 5-HT or 5-HT 7 agonists, inhibit histamine-induced scratching. [ABSTRACT FROM AUTHOR]

Endogenous ghrelin causes interdigestive contractions of the stomach in rats. In contrast, previous studies showed that 5-HT3 and 5-HT4 receptors were involved in regulating intestinal interdigestive contractions. We studied the possible role of endogenous ghrelin and 5-HT regulating interdigestive gastrointestinal (GI) contractions in rats. Four strain gauge transducers were implanted on the antrum, duodenum, and proximal and distal jejunum. After an overnight fast, GI contractions were recorded in freely moving conscious rats and ghrelin receptor antagonists [(D-lys3)GHRP6; 1 µmol/kg], 5-HT3 antagonists (Ondansetron; 0.5 mg/kg) and 5-HT4 antagonists (GR 125,487; 1 mg/kg) were administered (bolus iv). To evaluate the relationship between the luminal concentrations of 5-HT and phase III-like contractions of the duodenum, duodenal juice was collected via the intraduodenal catheter. 5-HT content of the duodenal juice was measured by HPLC. (D-lys3)GHRP6 significantly attenuated the occurrence and amplitude of phase Ill-like contractions of the antrum, but not the duodenum and jejunum. 5-HT4 antagonists significantly reduced spontaneous phase Ill-like contractions of the jejunum, without affecting those of the antrum and duodenum. In contrast, 5-HT3 antagonists did not affect phase Ill-like contractions in GI tract. Luminal concentration of 5-HT at the phase Ill-like contraction (36.0 ± 13.3 ng/ml, n = 9) was significantly higher than that at the phase I-like contractions of the duodenum (4.9 ± 1.6 ng/ml, n = 9, P < 0.05). It is suggested that released ghrelin from the gastric mucosa mediates gastric phase Ill-like contractions, whereas 5-HT released from enterochromaffin cells of the duodenal mucosa mediates intestinal phase Ill-like contractions via 5-HT4 receptors. [ABSTRACT FROM AUTHOR]

Abstract: The notion of functional interactions between the α7 nicotinic acetylcholine (α7 nACh) and the cannabinoid systems is emerging from recent in vitro and in vivo studies. Both the α7 nACh receptor and the cannabinoid receptor 1 (CB1) are highly expressed in the hippocampus. To begin addressing possible anatomical interactions between the α7 nACh and the cannabinoid systems in the rat hippocampus, we investigated the distribution of neurons expressing α7 nACh mRNA in relation to those containing CB1 mRNA. By in situ hybridization we found that the α7 nACh mRNA is diffusely expressed in principal neurons and is highly expressed in a subset of interneurons. We observed that the pattern of distribution of hippocampal interneurons co-expressing transcripts encoding α7 nACh and glutamate decarboxylase (GAD; synthesizing enzyme of GABA) closely resembles the one displayed by interneurons expressing CB1 mRNA. By double in situ hybridization we established that the majority of hippocampal interneurons expressing α7 nACh mRNA have high levels of CB1 mRNA. As CB1 interneurons contain cholecystokinin (CCK), we investigated the degree of cellular co-expression of α7 nACh mRNA and CCK, and found that the cellular co-existence of α7 nACh and CCK varies within the different layers of the hippocampus. In summary, we established that most of the hippocampal α7 nACh expressing interneurons are endowed with CB1 mRNA. We found that these α7 nACh/CB1 interneurons are the major subpopulation of hippocampal interneurons expressing CB1 mRNA. The α7 nACh expressing interneurons represent half of the detected population of CCK containing neurons in the hippocampus. Since it is well established that the vast majority of hippocampal interneurons expressing CB1 mRNA have 5-HT type 3 (5-HT3) receptors, we conclude that these hippocampal α7 nACh/5HT3/CB1/CCK interneurons correspond to those previously postulated to relay inputs from diverse cortical and subcortical regions about emotional, motivational, and physiological states. [Copyright &y& Elsevier]

Serotoninergic modulation of GABAergic mIPSCs was investigated in immature (postnatal 12–16-days old) rat CA3 pyramidal neurons using a conventional whole-cell patch clamp technique. Serotonin or 5-hydroxytryptamine (5-HT) (10 μmol/L) transiently and explosively increased mIPSC frequency with a small increase in the current amplitude. However, 5-HT did not affect the GABA-induced postsynaptic currents, indicating that 5-HT acts presynaptically to facilitate the probability of spontaneous GABA release. The 5-HT action on GABAergic mIPSC frequency was completely blocked by 100 nmol/L MDL72222, a selective 5-HT3 receptor antagonist, and mimicked by mCPBG, a selective 5-HT3 receptor agonist. The 5-HT action on GABAergic mIPSC frequency was completely occluded either in the presence of 200 μmol/L Cd2+ or in the Na+-free external solution, suggesting that the 5-HT3 receptor-mediated facilitation of mIPSC frequency requires a Ca2+influx passing through voltage-dependent Ca2+channels from the extracellular space, and that presynaptic 5-HT3 receptors are less permeable to Ca2+. The 5-HT action on mIPSC frequency in the absence or presence of extracellular Na+ gradually increased with postnatal development. Such a developmental change in the 5-HT3 receptor-mediated facilitation of GABAergic transmission would play important roles in the regulation of excitability as well as development in CA3 pyramidal neurons. [ABSTRACT FROM AUTHOR]

The contractile activity of the novel 5-HT3 receptor agonist MKC-733 was determined in intestinal tissues of rats, guinea pigs and mice. The potential influence of non-5-HT3 receptors was removed by the inclusion of methysergide and GR125487. MKC-733 had a lower efficacy than 5-HT in the rat jejunum, ileum and distal colon; however, it had similar efficacy and potency to 5-HT in the rat proximal colon. The activity profile of MKC-733 was different in the guinea pig intestine where it exhibited greater potency and efficacy than 5-HT in all regions. MKC-733 showed little to no response in the regions of the mouse intestine. Responses to MKC-733 in the rat and guinea pig tissues were inhibited by ondansetron, confirming its action on 5-HT3 receptors. These in vitro studies indicate that MKC-733 displays both regional and species specificities. Copyright © 2008 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Abstract: Despite its beneficial effect in IBS patients, the mechanism of action of the 5-HT3 receptor (5-HT3R) antagonist alosetron is still incompletely understood. We aimed to characterize the effect and site(s) of action in a model of stress-induced sensitization of visceral nociception in rats. Adult male Wistar rats were equipped for recording of visceromotor response (VMR) to phasic colorectal distension (CRD; 10–60mmHg). VMR to CRD was recorded 24h after an acute session of water avoidance (WA) stress (post-WA). Baseline and post-WA responses were measured in rats exposed to WA or sham-WA, treated with alosetron at 0.3mg/kg subcutaneously (s.c.) 25nmol intrathecally (i.t.) or vehicle before post-WA CRD. Some rats were treated with capsaicin/vehicle on the cervical vagus nerve and received alosetron (0.3mg/kg, s.c.) 15min before post-WA CRD. WA stress led to visceral hyperalgesia 24h later. Alosetron (0.3mg/kg, s.c.), failed to inhibit WA-induced exacerbation of VMR to CRD. Stress-induced visceral hyperalgesia was abolished when alosetron was injected intrathecally (P <0.05) in intact rats or subcutaneously (0.3mg/kg) in capsaicin-pretreated animals (P <0.05). Capsaicin-pretreatment did not affect the exacerbating effect of stress on visceral sensitivity. Alosetron had no inhibitory effect on normal visceral pain responses when administered subcutaneously or intrathecally. We demonstrated that 5-HT3Rs on central terminals of spinal afferents are engaged in the facilitatory effect of stress on visceral sensory information processing. In addition, we showed that stress-induced sensitization of visceral nociception is independent of 5-HT3R activation on vagal afferents. [Copyright &y& Elsevier]

The motility of the gut depends on the chemicals contained in the lumen, but the stimuli that modify motility and their relationship to enteric neural pathways are unclear. This study examined local inhibitory reflexes activated by various chemical stimulants applied to the mucosa to characterize effective physiological stimuli and the pathways they excite. Segments of the jejunum were dissected to allow access to the circular muscle on one-half of the preparation while leaving the mucosa intact on the circumferentially adjacent half. Chemicals were transiently applied to the mucosa, and responses were recorded intracellularly in nearby circular muscle cells. The amino acids L-phenylalanine, L-alanine, or L-tryptophan (all 1 mM) evoked inhibitory junction potentials (IJPs; latency 150–300 ms, amplitude 3–8 mV, each n > 6) that were blocked by TTX and partially blocked by antagonists of P2X receptors and/or a combination of antagonists at 5-HT3 and 5-HT4 receptors. The putative mediators 5-HT (10 μM), ATP (1 mM), and CCK-8 (1–10 μM) elicited IJPs mediated via 5-HT3, P2X, and CCK-B receptors, respectively. Responses were only partially reduced by the effective antagonists. IJPs evoked by electrically stimulating the mucosa were unaffected by antagonists that reduced chemically evoked responses. Both chemically and electrically evoked IJPs were resistant to nicotinic, NK1, NK3, α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, N-methyl-D-aspartate, or CGRP receptor blockade. We conclude that mucosal stimulation by amino acids activates local neural pathways whose pharmacology depends on the nature of the stimulus. Transmitters involved at some synapses in these pathways remain to be identified. [ABSTRACT FROM AUTHOR]

Abstract: The effect of the 5-HT3 receptor partial agonist MD-354 (meta-chlorophenylguanidine) was examined on the discriminative stimulus produced by (+)amphetamine. Using male Sprague–Dawley rats trained to discriminate 1.0 mg/kg (i.p.) of (+)amphetamine from saline vehicle (VI 15-s schedule of reinforcement) in a two-lever operant procedure for appetitive reward, tests of stimulus generalization (substitution) and antagonism showed that MD-354 neither substituted for, nor antagonized, the amphetamine stimulus at the doses evaluated. Administration of (+)amphetamine doses in combination with a fixed (i.e., 1.0 mg/kg) dose of MD-354 shifted the (+)amphetamine dose–response curve to the left such that, following 0.3 mg/kg of (+)amphetamine, stimulus generalization occurred. Furthermore, MD-354 doses of 0.1, 0.3 and 1.0 mg/kg, but not doses of 0.01, 0.5, 1.5 or 3.0 mg/kg (i.p.), administered in combination with the ED50 dose (0.33 mg/kg) of (+)amphetamine resulted in stimulus generalization (i.e., >80% drug-appropriate responding). It is concluded that even though MD-354 lacks amphetamine-like central stimulant actions of its own it can modulate the discriminative stimulus effects of (+)amphetamine in rats. [Copyright &y& Elsevier]

Abstract: Several studies demonstrated the involvement of 5-hydroxytryptamine (5-HT) and its different receptor subtypes in the modulation of neurotransmission of cardiovascular reflexes in the nucleus tractus solitarii (NTS). Moreover, anatomic evidence suggests that nucleus raphe obscurus (ROb) is a source of 5-HT-containing terminals within the NTS. In the present study we investigated the possible changes in the cardiovascular responses to peripheral chemoreceptor activation by potassium cyanide (KCN, i.v.) following ROb stimulation with l-glutamate (10 nmol/50 nL) and also whether 5-HT3 receptors in the caudal commissural NTS are involved in this neuromodulation. The results showed that stimulation of the ROb with l-glutamate in awake rats (n =15) produced a significant reduction in the bradycardic response 30 s after the microinjection (−182±19 vs −236±10 bpm; Wilcoxon test) but no changes in the pressor response to peripheral chemoreceptor activation (43±4 vs 51±3 mmHg; two-way ANOVA) in relation to the control. Microinjection of 5--HT3 receptors antagonist granisetron (500 pmol/50 nL), but not the vehicle, into the caudal commissural NTS bilaterally prevented the reduction of chemoreflex bradycardia in response to microinjection of l-glutamate into ROb. These data indicate that 5-HT-containing projections from ROb to the NTS play an inhibitory neuromodulatory role in the chemoreflex evoked bradycardia by releasing 5-HT and activating 5-HT3 receptors in the caudal NTS. [Copyright &y& Elsevier]

Abstract: Serotonin receptors are potential targets for treating functional bowel disorders. This study investigated the functional roles and expression of the 5-HT4 and the 5-HT7 receptor, which coexist in human colon circular smooth muscle. 5-HT3 receptor expression was also investigated. Part of the relaxant response to 5-HT was due to activation of 5-HT4 receptors as the apparent pK B value of the selective 5-HT4 antagonist, GR 113808, was 9.36. 5-HT4 mRNA levels were low in five tissues and undetectable in four others, but all responded to 5-HT with an EC50 value of 102.54±19.32 nM. The contribution of 5-HT7 receptors to the response was not readily demonstrated using the selective 5-HT7 antagonist, SB-269970, as its apparent pK B value of 7.19 (5-HT4 block with 1 μM GR 113808) was lower than the value obtained using the 5-HT7 guinea pig ileum assay (8.62). Nevertheless, the 5-HT7 receptor was expressed more consistently than the 5-HT4, but at similar levels. The 5-HT3Ashort and 5-HT3B subunits were co-expressed at similar levels, but the 5-HT3Along subunit was detected in only five of the nine samples tested. The findings show that 5-HT4-induced relaxation occurs at low to undetectable levels of tissue mRNA, as measured by qPCR. Although 5-HT7 receptor mRNA is detected at low, but consistent levels, the functional activity of this receptor is not readily identified given the currently available drugs. [Copyright &y& Elsevier]

MD-354 ( meta-chlorophenylguanidine) has been identified as a member of a novel class of 5-HT3 serotonin receptor agonists. MD-354 is a 5-HT3 receptor partial agonist that has been shown to behave as an agonist in some assays, and as an antagonist in others. MD-354 also binds at α-adrenoceptors (ARs) and displays an affinity for α2B-ARs comparable to its affinity for 5-HT3 receptors. Although devoid of antinociceptive actions following systemic administration alone, MD-354 markedly enhances the antinociceptive actions of clonidine in the mouse tail-flick assay without potentiating the sedative side effects of clonidine. Although studies with MD-354 are still in progress, some pharmacological findings are described here. MD-354-related agents may represent drug adjuvants for the relief of severe pain. [ABSTRACT FROM AUTHOR]

Abstract: Occipital artery (OA) injections of 5-HT elicit pronounced reductions in heart rate and mean arterial blood pressure (MAP) in urethane-anesthetized rats by activation of vagal afferent cell bodies in the ipsilateral nodose ganglion. In contrast, internal carotid artery (ICA) and i.v. injections elicit similar cardiovascular responses by activation of peripheral vagal afferent terminals. The aim of this study was to examine the roles of 5-HT3 and 5-HT2 receptors in the 5-HT-induced activation of vagal afferent cell bodies and peripheral afferent terminals in urethane-anesthetized rats. OA, ICA and i.v. injections of 5-HT elicited dose-dependent reductions in heart rate and MAP that were virtually abolished after i.v. administration of the 5-HT3 receptor antagonists, MDL 7222 or ICS 205-930. The responses elicited by the OA injections of 5-HT were markedly diminished after i.v. injection of the 5-HT2 receptor antagonists, xylamidine or ketanserin, whereas the responses elicited by i.v. or ICA injections of 5-HT were not affected. The present findings suggest that (1) 5-HT3 and 5-HT2 receptor antagonists gain ready access to nodose ganglion cells upon i.v. administration, and (2) functional 5-HT3 and 5-HT2 receptors exist on the cell bodies of vagal afferent neurons mediating the cardiovascular responses elicited by OA injections of 5-HT. These findings also support a wealth of evidence that 5-HT3 receptors exist on the peripheral terminals of vagal afferents, and although they do not discount the possibility that 5-HT2 receptors exist on peripheral vagal afferent terminals, it appears that activation of these receptors does not have pronounced effects on 5-HT3 receptor activity on terminals that mediate the hemodynamic responses to 5-HT. [Copyright &y& Elsevier]

Abstract: Prior studies have demonstrated inefficacy among dopamine receptor antagonists for treating cocaine dependence. An alternative approach would be to investigate the ability of indirect inhibitors of cortico-mesolimbic dopamine release, such as the 5-HT3 receptor antagonist ondansetron, to reduce cocaine''s reinforcing effects. We hypothesized that ondansetron might be more efficacious than placebo at reducing cocaine intake and promoting abstinence in cocaine-dependent individuals. In a pilot randomized, double-blind, 10-week controlled trial, 63 treatment-seeking, cocaine-dependent men and women received ondansetron (0.25mg, 1.0mg, or 4.0mg twice daily) or placebo. Up to three times per week, participants were assessed on several measures of cocaine use, including urine benzoylecgonine. Cognitive behavioral therapy was administered weekly. Ondansetron was well tolerated, causing no serious adverse events. The ondansetron 4.0mg group had the lowest dropout rate among all treatment groups and a greater rate of improvement in percentage of participants with a cocaine-free week compared with the placebo group (p =0.02), whereas the ondansetron 1.0mg group had a lower rate of improvement in percentage of weekly mean non-use days than did placebo recipients (p =0.04). These results suggest the possibility of a non-linear dose–response function, with evidence supporting efficacy for the 4.0mg group. [Copyright &y& Elsevier]

This study provides a comprehensive evaluation of 5-HT3 receptor functional distribution in both the rat and mouse intestinal tract.5-HT3A−S receptor splice variant mRNA was expressed throughout the intestine of the rat and mouse; the 5-HT3A−L variant being more common in the rat.5-HT, m-CPB, 1-PBG and 2-methyl-5-hydroxytryptamine (2m5-HT) induced contraction in the jejunum, ileum, proximal colon and distal colon of the rat (pEC50 range: 2m5-HT, 5.86±0.40 to m-CPB, 7.47±0.27) and mouse (pEC50 range: 1-PBG, 5.34±0.06 to m-CPB, 6.49±0.14) in the presence of nontarget 5-HT receptor antagonists, methysergide (1 μM) and GR125487 (0.1 μM). The rank orders of potency in the four regions of the rat and mouse intestine were concordant with the accepted order and the responses to 5-HT were inhibited by ondansetron (0.1 μM).5-HT3-induced contractions to 5-HT were reduced by tetrodotoxin (1 μM). Pargyline (10 μM) and fluoxetine (1 μM) potentiated responses in the rat jejunum. Atropine (0.1 μM) potentiated 5-HT3-induced responses in the rat jejunum (Emax 49–65%), but attenuated responses in most other regions of the rat and mouse (e.g. mouse ileum: Emax 57–26%). In the rat jejunum, L-NAME (100 μM) mimicked the effect of atropine, hexamethonium (100 μM) suppressed 5-HT3-induced responses, but tachykinin receptor antagonists were without effect.It is concluded that functional 5-HT3 receptors are present in nerves along the length of the rat and mouse intestinal tract. The mouse proximal colon was found to discriminate 5-HT3 receptor agonist profiles better than any other region in the rat or mouse. The rat jejunum shows evidence of 5-HT uptake and inactivation processes as well as inhibitory nitrergic and nontachykinin excitatory pathways associated with the 5-HT3-induced response.British Journal of Pharmacology (2006) 148, 1012–1021. doi:10.1038/sj.bjp.0706802; published online 12 June 2006 [ABSTRACT FROM AUTHOR]

Abstract: Functional studies have provided evidence that 5-HT3 ion-channel receptors (5-HT3Rs) on vagal cardiopulmonary afferents mediating the Bezold–Jarisch reflex (BJR) rapidly desensitize upon repeated exposure to selective 5-HT3R agonists. G-protein-coupled 5-HT2 receptors (5-HT2Rs) also exist on vagal afferents, although activation of these receptors does not elicit the BJR. However, there is in vivo evidence that 5-HT2Rs may regulate the activity of 5-HT3Rs. The aim of this study was to determine whether co-activation of 5-HT2Rs prevents desensitization of 5-HT3Rs mediating the BJR in conscious rats. The principal findings were that (1) tachyphylaxis rapidly developed to the BJR-mediated hemodynamic responses elicited by successive injections of 5-HT3R agonists and (2) co-injection of the selective 5-HT2R agonist, α-methyl-5-HT, prevented tachyphylaxis to the BJR-mediated hemodynamic responses elicited by the 5-HT3R agonists. Additional studies provided evidence that (1) tachyphylaxis to the 5-HT3R agonists was not due to impairment of the central or efferent processing of the BJR, and (2) the pressor responses elicited by α-methyl-5-HT were not responsible for preventing tachyphylaxis to the BJR reflex responses elicited by 5-HT3R agonists. These results suggest that the loss of response to 5-HT3R agonists is due to desensitization of 5-HT3Rs on vagal afferents mediating the BJR and that co-activation of 5-HT2Rs prevents the desensitization of these 5-HT3Rs. [Copyright &y& Elsevier]

Rationale and objectives: We previously found that systemic injections of the 5-HT uptake blocker fluoxetine attenuate intermittent footshock stress-induced reinstatement of alcohol seeking in rats, while inhibition of 5-HT neurons in the median raphe induces reinstatement of alcohol seeking. In this study, we further explored the role of 5-HT in footshock stress-induced reinstatement of alcohol seeking by determining the effects of the 5-HT releaser and reuptake blocker dexfenfluramine, and the 5- HT receptor antagonists ondansetron and tropisetron, which decrease alcohol self-administration and anxiety-like responses in rats, on this reinstatement. Methods: Different groups of male Wistar rats were trained to self-administer alcohol (12% v/v) for 28-31 days (1 h/day, 0.19 ml per alcohol delivery) and then their lever responding for alcohol was extinguished over 9-10 days. Subsequently, the effect of systemic injections of vehicle or dexfenfluramine (0.25 or 0.5 mg/kg, i.p), ondansetron (0.001, 0.01, or 0.1 mg/kg, i.p), or tropisetron (0.001, 0.01, and 0.1 mg/kg, i.p) on reinstatement induced by 10 min of intermittent footshock (0.8 mA) was determined. Results: Systemic injections of dexfenfluramine, ondansetron or tropisetron attenuated footshock-induced reinstatement of alcohol seeking. Injections of dexfenfluramine, ondansetron, or tropisetron had no effect on extinguished lever responding in the absence of footshock. Conclusions: The present results provide additional support for the hypothesis that brain 5-HT systems are involved in stress-induced reinstatement of alcohol seeking. The neuronal mechanisms that potentially mediate the unexpected observation that both stimulation of 5-HT release and blockade of 5-HT3 receptors attenuate footshock-induced reinstatement are discussed. [ABSTRACT FROM AUTHOR]

Rationale: Temporal differentiation refers to animals' ability to regulate their behaviour during an ongoing interval. Striatal dopaminergic mechanisms are purported to be involved in temporal differentiation, and recent evidence also implicates 5-hydroxytryptaminergic (5-HTergic) mechanisms, possibly mediated by 5-HT2A receptors. There is evidence that 5-HT3 receptors contribute to the regulation of dopamine release in the basal ganglia; however, it is not known whether 5-HT3 receptor stimulation can influence temporal differentiation. Objective: We examined the effects of a selective 5-HT3 receptor agonist m-CPBG, a mixed 5-HT2A/3 receptor agonist quipazine, and selective 5-HT3 and 5-HT2A receptor antagonists (MDL- 72222 and ketanserin, respectively) on temporal differentiation in a free-operant psychophysical procedure. Methods: Twenty-four rats were trained to respond on two levers (A and B) under a free-operant psychophysical schedule, in which sucrose reinforcement (0.6 M, 50 µl) was provided intermittently for responding on A during the first half and on B during the second half of 50-s trials. Logistic psychometric functions were fitted to the relative response rate data [percent responding on B (%B) vs time from trial onset (t)], and quantitative indices of timing performance [T50 (value of t corresponding to %B=50), Weber fraction, and mean time of switching from A to B, S50] were derived. Results: Quipazine (0.5, 1, and 2 mg kg-1) altered timing performance, dose-dependently reducing T50 and S50; m-CPBG (2.5, 5, and 10 mg kg-1) had no significant effect. The effect of quipazine was antagonized by ketanserin (2 mg kg-1), but not by MDL-72222 (1 mg kg-1). Conclusions: The present results provide no evidence for the involvement of 5- HT3 receptors in temporal differentiation and indicate that the effect of quipazine on performance was mediated by 5-HT2A receptor stimulation. The results are consistent with previous evidence for the involvement of 5-HT2A receptors in interval timing behaviour. [ABSTRACT FROM AUTHOR]

Hamsters repeatedly exposed to cocaine during adolescence display high levels of offensive aggression compared to saline-treated littermates. The escalated offensive phenotype observed in adolescent cocaine-treated animals is modulated by serotonin (5-HT) signaling and can be suppressed by inhibiting 5-HT type 3 receptors, suggesting that these receptors might play an important role in the aggression-stimulating effects of adolescent cocaine exposure. The current study examined this hypothesis and extended earlier studies investigating the relationship between 5HT3 receptor neural signaling and the offensive response patterns of aggressive, adolescent cocaine-treated animals compared to non-aggressive, saline-treated littermates. Adolescent cocaine-treated hamsters and saline-treated littermates were tested for offensive aggression after the administration of either the 5-HT3 antagonist 3-tropanylindole-3-carboxylate methiodide (tropisetron) or the 5-HT3 agonist 1-(m-chlorophenyl)-biguanide hydrochloride (mCPBG). Tropisetron significantly reduced the high levels of offensive responding observed in adolescent cocaine-treated animals, whereas treatment with the 5-HT3 receptor agonist mCPBG failed to affect the escalated offensive response. Conversely, tropisetron failed to affect very low, baseline levels of aggressive responding seen in adolescent saline-treated animals, while 5-HT3 receptor activation via mCPBG triggered highly escalated levels of offensive aggression in these animals. Together, these data support a stimulatory role for 5-HT3 neural signaling in offensive aggression. [Copyright &y& Elsevier]

We have studied the effects of concomitant blockade of 5-HT3 and GABAA receptors on acetylcholine (ACh) release in the frontal cortex of rats with a selective cholinergic lesion. Lesions were performed by microinjection of the cholinergic toxin 192 IgG-saporin into the nucleus basalis magnocellularis. Single treatment with either the 5-HT3 receptor antagonist ondansetron, 0.1 μg/kg, or the GABAA receptor benzodiazepine site antagonist flumazenil, 10 mg/kg, did not affect ACh release. However, the combined ondansetron+flumazenil administration significantly increased ACh release to a similar extent as a depolarising stimulus with K+, 100 mM, at both 7 and 30 days post-lesion. Cortical perfusion with the combined ondansetron+flumazenil treatment also increased [3H]ACh efflux “in vitro” 30 days after lesion, suggesting that local events within the frontal cortex may participate in the interaction of ondansetron with GABAergic neurons, modulating ACh release in situations of cholinergic hypoactivity. No differences in the expression of 5-HT3 and GABAA receptors in the frontal cortex were found after the cholinergic lesion. These results suggest that a combined ondansetron+flumazenil treatment would contribute to restoring a diminished cholinergic function and may provide a basis for using this treatment in the therapy of cognitive disorders associated with degeneration of the cholinergic system. [Copyright &y& Elsevier]

Cytotoxic drug-induced nausea and vomiting are the side effects most feared by cancer patients. Emesis is an instinctive defense reaction caused by the somatoautonomic nerve reflex, which is integrated in the medulla oblongata. Emesis caused by cytotoxic drugs such as cisplatin is associated with an increase in the concentration of 5-hydroxytryptamine (5-HT) in the intestine and the brainstem. It is proposed that cytotoxic drugs evoke 5-HT release from the enterochromaffin (EC) cells in the intestinal mucosa and that the released 5-HT stimulates the 5-HT receptors on the adjacent vagal afferent nerves. The depolarization of the vagal afferent nerves stimulates the vomiting center in the brainstem and eventually induces a vomiting reflex. 5-HT released from EC cells seems to mediate the cisplatin-induced emesis sensitive to 5-HT3 receptor antagonists. The release of 5-HT from the EC cells, however, is regulated by polymodal mechanisms on autoreceptors or heteroreceptors. The precise role of 5-HT on the occurrence of vomiting has not been fully elucidated. The present review aims to describe the role of 5-HT in anticancer drug-induced emesis from the viewpoint of 5-HT release and afferent vagus nerve activity. Various methods for predicting emesis are also evaluated. [Copyright &y& Elsevier]

The identification of a second 5-HT3 (5-HT3B) subunit provides an explanation for 5-HT3 receptor heterogeneity. We investigated whether introduction of recombinant 5-HT3B subunits would alter the functional properties of mouse neuroblastoma 5-HT3 receptors. RT-PCR analysis revealed that NB41A3 cells contain mRNAs encoding 5-HT3A and 5-HT3B subunits. 5-HT increased intracellular Ca2+ concentration ([Ca2+]i) and caused the concentration-dependent activation of inward currents recorded at –60 mV. Both actions of 5-HT were antagonized by ondansetron. The 5-HT concentration–response relationship of NB41A3 cells was indistinguishable from that of the related NG108-15 cell line. The selective 5-HT3-receptor agonist mCPBG also elevated [Ca2+]i and activated inward currents. 2-M-5HT was less efficacious than 5-HT as an activator of 5-HT3 receptors in NB41A3 cells and did not significantly increase [Ca2+]i. The 5-HT induced increase in [Ca2+]i did not involve caffeine- or thapsigargin-sensitive intracellular Ca2+ stores. The introduction of the 5-HT3B subunit by transient transfection of NB41A3 cells caused 5-HT to become less potent as an activator of 5-HT3 receptors and altered the kinetics of 5-HT activated currents so that they resembled currents mediated by 5-HT3AB receptors. The 5-HT3B subunit also abolished the 5-HT induced [Ca2+]i increase seen in untransfected NB41A3 cells. These data are consistent with the hypothesis that NB41A3 cells predominantly express homomeric 5-HT3A receptors that become heteromeric 5-HT3AB receptors upon introduction of the recombinant 5-HT3B subunit. [Copyright &y& Elsevier]

Serotonin-3 (5-HT3) receptor antagonists have been shown to attenuate drug-induced increases in mesolimbic dopamine (DA), locomotor activation, and drug self-administration. In the present study, we tested whether the selective 5-HT3 antagonist Y-25130 would attenuate cocaine-induced lowering of intracranial self-stimulation (ICSS) reward thresholds. Rats (n=6) were surgically prepared with bipolar stimulation electrodes and trained to self-administer electrical stimulation delivered to the medial forebrain bundle-lateral hypothalamus (MFB-LH). A discrete-trial, rate-free threshold determination procedure was used to detect pharmacologically induced changes from baseline reward thresholds. Four doses of Y-25130 (0.0, 0.03, 0.3, and 3.0 mg/kg ip) were given alone and in combination with cocaine (4.0 mg/kg ip). Y-25130 did not significantly alter reward thresholds or response latencies when given alone as compared to baseline measures. While there were no significant effects at lower doses, the middle and highest doses of Y-25130 (0.3 and 3.0 mg/kg) did attenuate the threshold-lowering effect of cocaine. These findings suggest that the rewarding effects of cocaine are mediated through 5-HT3 receptor activity. [Copyright &y& Elsevier]