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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]

The serotonin-gated ion channel (5-HT3R) mediates excitatory neuronal communication in the gut and the brain. It is the target for setrons, a class of competitive antagonists widely used as antiemetics, and is involved in several neurological diseases. Cryo-electron microscopy (cryo-EM) of the 5-HT3R in complex with serotonin or setrons revealed that the protein has access to a wide conformational landscape. However, assigning known high-resolution structures to actual states contributing to the physiological response remains a challenge. In the present study, we used voltage-clamp fluorometry (VCF) to measure simultaneously, for 5-HT3R expressed at a cell membrane, conformational changes by fluorescence and channel opening by electrophysiology. Four positions identified by mutational screening report motions around and outside the serotonin-binding site through incorporation of cysteine-tethered rhodamine dyes with or without a nearby quenching tryptophan. VCF recordings show that the 5-HT3R has access to four families of conformations endowed with distinct fluorescence signatures: ‘resting-like’ without ligand, ‘inhibited-like’ with setrons, ‘pre-active-like’ with partial agonists, and ‘active-like’ (open channel) with partial and strong agonists. Data are remarkably consistent with cryo-EM structures, the fluorescence partners matching respectively apo, setron-bound, 5-HT bound-closed, and 5-HT-bound-open conformations. Data show that strong agonists promote a concerted motion of all fluorescently labeled sensors during activation, while partial agonists, especially when loss-of-function mutations are engineered, stabilize both active and pre-active conformations. In conclusion, VCF, though the monitoring of electrophysiologically silent conformational changes, illuminates allosteric mechanisms contributing to signal transduction and their differential regulation by important classes of physiological and clinical effectors.

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]

The serotonin-gated ion channel (5-HT 3 R) mediates excitatory neuronal communication in the gut and the brain. It is the target for setrons, a class of competitive antagonists widely used as antiemetics, and is involved in several neurological diseases. Cryo-electron microscopy (cryo-EM) of the 5-HT 3 R in complex with serotonin or setrons revealed that the protein has access to a wide conformational landscape. However, assigning known high-resolution structures to actual states contributing to the physiological response remains a challenge. In the present study, we used voltage-clamp fluorometry (VCF) to measure simultaneously, for 5-HT 3 R expressed at a cell membrane, conformational changes by fluorescence and channel opening by electrophysiology. Four positions identified by mutational screening report motions around and outside the serotonin-binding site through incorporation of cysteine-tethered rhodamine dyes with or without a nearby quenching tryptophan. VCF recordings show that the 5-HT 3 R has access to four families of conformations endowed with distinct fluorescence signatures: 'resting-like' without ligand, 'inhibited-like' with setrons, 'pre-active-like' with partial agonists, and 'active-like' (open channel) with partial and strong agonists. Data are remarkably consistent with cryo-EM structures, the fluorescence partners matching respectively apo, setron-bound, 5-HT bound-closed, and 5-HT-bound-open conformations. Data show that strong agonists promote a concerted motion of all fluorescently labeled sensors during activation, while partial agonists, especially when loss-of-function mutations are engineered, stabilize both active and pre-active conformations. In conclusion, VCF, though the monitoring of electrophysiologically silent conformational changes, illuminates allosteric mechanisms contributing to signal transduction and their differential regulation by important classes of physiological and clinical effectors., Competing Interests: LP, SS, KM, PC No competing interests declared, (© 2023, Peverini et al.)

Pentameric ligand-gated ion channels (pLGICs) mediate fast synaptic transmission and are crucial drug targets. Their gating mechanism is triggered by ligand binding in the extracellular domain that culminates in the opening of a hydrophobic gate in the transmembrane domain. This domain is made of four α-helices (M1 to M4). Recently the outer lipid-facing helix (M4) has been shown to be key to receptor function, however its role in channel opening is still poorly understood. It could act through its neighboring helices (M1/M3), or via the M4 tip interacting with the pivotal Cys-loop in the extracellular domain. Mutation of a single M4 tyrosine (Y441) to alanine renders one pLGIC—the 5-HT3A receptor—unable to function despite robust ligand binding. Using Y441A as a proxy for M4 function, we here predict likely paths of Y441 action using molecular dynamics, and test these predictions with functional assays of mutant receptors in HEK cells and Xenopus oocytes using fluorescent membrane potential sensitive dye and two-electrode voltage clamp respectively. We show that Y441 does not act via the M4 tip or Cys-loop, but instead connects radially through M1 to a residue near the ion channel hydrophobic gate on the pore-lining helix M2. This demonstrates the active role of the M4 helix in channel opening.

Low birthweight (LBW) is associated with metabolic complications, such as glucose and lipid metabolism disturbances in early life. The objective of this study was to assess: (1) the effect of dietary tryptophan (Trp) on glucose and fat metabolism in an LBW piglet model, and (2) the role peripheral 5-hydroxytryptamine type 3 (5HT3) receptors in regulating the feeding behavior in LBW piglets fed with Trp-supplemented diets. Seven-day-old piglets were assigned to 4 treatments: normal birthweight-0%Trp (NBW-T0), LBW-0%Trp (LBW-T0), LBW-0.4%Trp (LBW-T0.4), and LBW-0.8%Trp (LBW-T0.8) for 3 weeks. Compared to LBW-T0, the blood glucose was decreased in LBW-T0.8 at 60 min following the meal test, and the triglycerides were lower in LBW-T0.4 and LBW-T0.8. Relative to LBW-T0, LBW-T0.8 had a lower transcript and protein abundance of hepatic glucose transporter-2, a higher mRNA abundance of glucokinase, and a lower transcript of phosphoenolpyruvate carboxykinase. LBW-T0.4 tended to have a lower protein abundance of sodium-glucose co-transporter 1 in the jejunum. In comparison with LBW-T0, LBW-T0.4 and LBW-T0.8 had a lower transcript of hepatic acetyl-CoA carboxylase, and LBW-T0.4 had a higher transcript of 3-hydroxyacyl-CoA dehydrogenase. Blocking 5-HT3 receptors with ondansetron reduced the feed intake in all groups, with a transient effect on LBW-T0, but more persistent effect on LBW-T0.8 and NBW-T0. In conclusion, Trp supplementation reduced the hepatic lipogenesis and gluconeogenesis, but increased the glycolysis in LBW piglets. Peripheral serotonin is likely involved in the regulation of feeding behavior, particularly in LBW piglets fed diets supplemented with a higher dose of Trp.

Cannabidiol (CBD) is a nonpsychotropic constituent of Cannabis sativa L. It is suggested to be useful in hypertension. Under in vitro conditions, it activates vanilloid TRPV1 and inhibits serotonin 5-HT3 receptors, i.e., receptors involved in the Bezold-Jarisch reflex stimulation. The aim of our study was to compare the cardiovascular effects of CBD in spontaneously hypertensive (SHR) and normotensive Wistar Kyoto (WKY) rats. Experiments were performed on conscious, urethane-anesthetized, and pithed rats. In pithed SHR and WKY, CBD increased heart rate (HR) and systolic blood pressure (SBP) and decreased diastolic BP (DBP) in a manner insensitive to adrenalectomy. Propranolol strongly impaired the CBD-induced increases in HR and SBP without affecting the decreases in DBP. Desipramine also reduced the CBD-induced effects on HR and SBP and further increased its effects on DBP. In anesthetized rats, bolus i.v. injection of single doses of CBD induced short-lasting decreases in HR, SBP, and DBP, stronger in SHR than in WKY and prevented by bilateral vagotomy. The CBD-induced fall in HR but not in BP was diminished by the TRPV1 receptor antagonist capsazepine and almost completely abolished if CBD was re-injected after previous administration. CBD reduced the Bezold-Jarisch reflex elicited by the 5-HT3 receptor agonist phenylbiguanide but not that evoked by the TRPV1 agonist capsaicin. In conscious rats, CBD did not affect cardiovascular parameters. In isolated left atria, CBD decreased contractile force. Conclusions: Cannabidiol (1) induces the Bezold-Jarisch reflex likely via TRPV1 receptors (which undergo tachyphylaxis) more markedly in SHR than in WKY; (2) inhibits the Bezold-Jarisch reflex induced by activation of 5-HT3 but not TRPV1 receptors; (3) has peripheral sympathomimetic, (4) vasodilatory, and (5) negative inotropic effects. The above properties of CBD should be taken under consideration when CBD is used for therapeutic purposes.

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]

Cannabidiol (CBD) is a nonpsychotropic constituent of Cannabis sativa L. It is suggested to be useful in hypertension. Under in vitro conditions, it activates vanilloid TRPV1 and inhibits serotonin 5-HT3 receptors, i.e., receptors involved in the Bezold-Jarisch reflex stimulation. The aim of our study was to compare the cardiovascular effects of CBD in spontaneously hypertensive (SHR) and normotensive Wistar Kyoto (WKY) rats. Experiments were performed on conscious, urethane-anesthetized, and pithed rats. In pithed SHR and WKY, CBD increased heart rate (HR) and systolic blood pressure (SBP) and decreased diastolic BP (DBP) in a manner insensitive to adrenalectomy. Propranolol strongly impaired the CBD-induced increases in HR and SBP without affecting the decreases in DBP. Desipramine also reduced the CBD-induced effects on HR and SBP and further increased its effects on DBP. In anesthetized rats, bolus i.v. injection of single doses of CBD induced short-lasting decreases in HR, SBP, and DBP, stronger in SHR than in WKY and prevented by bilateral vagotomy. The CBD-induced fall in HR but not in BP was diminished by the TRPV1 receptor antagonist capsazepine and almost completely abolished if CBD was re-injected after previous administration. CBD reduced the Bezold-Jarisch reflex elicited by the 5-HT3 receptor agonist phenylbiguanide but not that evoked by the TRPV1 agonist capsaicin. In conscious rats, CBD did not affect cardiovascular parameters. In isolated left atria, CBD decreased contractile force. Conclusions: Cannabidiol (1) induces the Bezold-Jarisch reflex likely via TRPV1 receptors (which undergo tachyphylaxis) more markedly in SHR than in WKY; (2) inhibits the Bezold-Jarisch reflex induced by activation of 5-HT3 but not TRPV1 receptors; (3) has peripheral sympathomimetic, (4) vasodilatory, and (5) negative inotropic effects. The above properties of CBD should be taken under consideration when CBD is used for therapeutic purposes. [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]

Brain-derived neurotrophic factor (BDNF) is a key regulator of neuronal plasticity and cognitive functions. BDNF val66met polymorphism, a human single-nucleotide polymorphism (SNP) in the pro-domain of BDNF gene, is associated with deficits in activity-dependent BDNF secretion and hippocampus-dependent memory. However, the underlying mechanism remains unclear. Here we show that in the BDNFMet/Met mouse line mimicking the human SNP, BDNF expression in the hippocampus was decreased. There was a reduction in the total number of cells in hippocampal CA1 region, while hippocampal expression of mRNAs for NR2a, 2b, GluR1, 2 and GABAARβ3 subunits were up-regulated. Although basal glutamatergic neurotransmission was unaltered, hippocampal long-term depression (LTD) induced by low-frequency stimulation was impaired, which was partially rescued by exogenous application of BDNF. Interestingly, 5-HT3a receptors were down-regulated in the hippocampus of BDNFMet/Met mice, whereas 5-HT2c receptors were up-regulated. Moreover, impaired LTD in BDNFMet/Met mice was reversed by 5-HT3aR agonist. Thus, these observations indicate that BDNF val66met polymorphism changes hippocampal synaptic plasticity via down-regulation of 5-HT3a receptors, which may underlie cognition dysfunction of Met allele carriers.

Introduction: Empathy is a fundamental prosocial behavior. It has been defined as perception, awareness, and understanding of others' emotional states, including painful processes. Mice living in pairs with conspecific chronic suffering from constriction injury exhibit pain hypersensitivity mediated by the amygdaloid complex. Nevertheless, the underlying mechanisms in the amygdala responsible for this response remain to be determined. This study investigated if the anxiolytic benzodiazepine midazolam (MDZ) and cannabidiol (CBD), a phytocannabinoid with multiple molecular targets, would attenuate this behavioral change. We also investigated if serotonergic and γ-aminobutyric acid (GABA)ergic mechanisms in the amygdala are involved in this effect. Materials and Methods: Male Swiss mice were housed in pairs for 28 days. The pairs were divided into two groups on the 14th day: cagemate nerve constriction and cagemate sham. On the 24th day, cagemates underwent a stereotaxic surgery and, on the 28th day, were evaluated on the writhing test. Results: The results showed that living with chronic pain leads to hypernociception in the cagemate and increases the expression of 5-HT 3 receptor (5-HT 3 R) and glutamic acid decarboxylase 67 within the amygdala. MDZ (3.0 and 30 nmol) and CBD (30 and 60 nmol) attenuated the hypernociceptive behavior. The 5-HT 3 R antagonist ondansetron (0.3 nmol) prevented the antinociceptive effects of MDZ and CBD. Conclusion: These findings indicate that 5-HT 3 R and GABAergic mechanisms within the amygdala are involved in the pain hypersensitivity induced by the empathy for pain model. They also suggest that MDZ and CBD could be a new potential therapy to alleviate emotional pain disorders.

• 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]

The roles of 5-HT3 and 5-HT4 receptors in the modulation of intestinal propulsion by luminal application of 5-HT and augmentation of endogenous 5-HT effects were studied in segments of guinea-pig ileum in vitro. Persistent propulsive contractions evoked by saline distension were examined using a modified Trendelenburg method. When 5-HT (30 nM), fluoxetine (selective serotonin reuptake inhibitor; 1 nM), 2-methyl-5-HT (5-HT3 receptor agonist; 1 mM) or RS 67506 (5-HT4 receptor agonist, 1 µM) was infused into the lumen, the pressure needed to initiate persistent propulsive activity fell significantly. A specific 5-HT4 receptor antagonist, SB 207266 (10 nM in lumen), abolished the effects of 5-HT, fluoxetine, and RS 67506, but not those of 2-methyl-5-HT. Granisetron (5-HT3 receptor antagonist; 1 µM in lumen) abolished the effect of 5-HT, fluoxetine, RS 67506 and 2-methyl-5-HT. The NK3 receptor antagonist SR 142801 (100 nM in lumen) blocked the effects of 5-HT, fluoxetine and 2-methyl-5-HT. SB 207266, granisetron and SR 142801 had no effect by themselves. Higher concentrations of fluoxetine (100 nM and 300 nM) and RS 67506 (3 µM and 10 µM) had no effect on the distension threshold for propulsive contractions. These results indicate that luminal application of exogenous 5-HT, or increased release of endogenous mucosal 5-HT above basal levels, acts to lower the threshold for propulsive contractions in the guinea-pig ileum via activation of 5-HT3 and 5-HT4 receptors and the release of tachykinins. The results further indicate that basal release of 5-HT is insufficient to alter the threshold for propulsive motor activity.

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 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]

Low birthweight (LBW) is associated with metabolic complications, such as glucose and lipid metabolism disturbances in early life. The objective of this study was to assess: (1) the effect of dietary tryptophan (Trp) on glucose and fat metabolism in an LBW piglet model, and (2) the role peripheral 5-hydroxytryptamine type 3 (5HT3) receptors in regulating the feeding behavior in LBW piglets fed with Trp-supplemented diets. Seven-day-old piglets were assigned to 4 treatments: normal birthweight-0%Trp (NBW-T0), LBW-0%Trp (LBW-T0), LBW-0.4%Trp (LBW-T0.4), and LBW-0.8%Trp (LBW-T0.8) for 3 weeks. Compared to LBW-T0, the blood glucose was decreased in LBW-T0.8 at 60 min following the meal test, and the triglycerides were lower in LBW-T0.4 and LBW-T0.8. Relative to LBW-T0, LBW-T0.8 had a lower transcript and protein abundance of hepatic glucose transporter-2, a higher mRNA abundance of glucokinase, and a lower transcript of phosphoenolpyruvate carboxykinase. LBW-T0.4 tended to have a lower protein abundance of sodium-glucose co-transporter 1 in the jejunum. In comparison with LBW-T0, LBW-T0.4 and LBW-T0.8 had a lower transcript of hepatic acetyl-CoA carboxylase, and LBW-T0.4 had a higher transcript of 3-hydroxyacyl-CoA dehydrogenase. Blocking 5-HT3 receptors with ondansetron reduced the feed intake in all groups, with a transient effect on LBW-T0, but more persistent effect on LBW-T0.8 and NBW-T0. In conclusion, Trp supplementation reduced the hepatic lipogenesis and gluconeogenesis, but increased the glycolysis in LBW piglets. Peripheral serotonin is likely involved in the regulation of feeding behavior, particularly in LBW piglets fed diets supplemented with a higher dose of Trp. [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-HT 3 receptor (5-HT 3 R), 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-HT 3 R 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 14 th day: cagemate nerve constriction (CNC) and cagemate sham (CS). On the 24 th day, cagemates underwent a stereotaxic surgery (when necessary) and, on the 28 th 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-HT 3 R activation attenuated and enhanced the number of writhes, respectively. In contrast, 5-HT 3 R blockade reduced hypersensitivity pain response. Results suggest the involvement of amygdala serotonergic signaling via 5-HT 3 R in empathy-like behavior.

Major depressive disorder (MDD) is a chronic and disabling psychiatric disorder characterized by a wide range of signs/symptoms, including cognitive dysfunction. Vortioxetine (VOR) is a multimodal antidepressant drug with pro-cognitive actions in animal models and MDD patients. The VOR-mediated blockade of 5-HT 3 -R in a subpopulation of GABA interneurons enhances pyramidal neuron activity in rat medial prefrontal cortex, an effect possibly underlying its pro-cognitive action. Brain oscillations are involved in regulation of cognitive function. We therefore examined VOR effects on oscillatory activity in four brain areas of freely-moving rats (prelimbic cortex, PrL; nucleus accumbens, NAc; dorsal hippocampus, dHPC; paraventricular thalamic nucleus, PVA), in standard and in serotonin-depleted rats showing recognition memory deficits. 4-chloro- dl -phenylalanine (p CPA) markedly reduced low frequency oscillations (LFO, mainly 1 Hz oscillations) and enhanced theta oscillations in PrL and NAc. It also reduced gamma and high frequency oscillations (HFO) in PVA. Subchronic VOR and escitalopram (ESC) treatments had little effect on oscillatory activity in standard rats. However, VOR –but not ESC- prevented recognition memory deficits in 5-HT-depleted rats, and normalized LFO and theta powers in PrL and NAc. In parallel, VOR –but not ESC- prevented the deficit in PrL-dHPC gamma coherence, but not the decrease in gamma and HFO powers in PVA. Overall, this supports a prominent role of serotonergic neurotransmission on brain oscillatory activity, particularly in cortico-striatal pathways linked to short-term recognition memory. Further, VOR prevented p CPA-induced cognitive deficits by normalizing oscillatory activity at lower frequencies in the PrL-NAc pathway, also normalizing the PrL-dHPC coherence at gamma frequencies. • 5-HT controls oscillatory activity in brain pathways linked to recognition memory. • Subchronic vortioxetine restores brain rhythms in 5-HT depleted rats. • Subchronic vortioxetine prevents recognition memory deficits in 5-HT depleted rats. • Acute vortioxetine enhances oscillatory activity only in 5-HT depleted rats. • Pro-cognitive actions of vortioxetine may involve changes in brain oscillations. [ABSTRACT FROM AUTHOR]

Pentameric ligand-gated ion channels (pLGICs) mediate fast synaptic transmission and are crucial drug targets. Their gating mechanism is triggered by ligand binding in the extracellular domain that culminates in the opening of a hydrophobic gate in the transmembrane domain. This domain is made of four α-helices (M1 to M4). Recently the outer lipid-facing helix (M4) has been shown to be key to receptor function, however its role in channel opening is still poorly understood. It could act through its neighboring helices (M1/M3), or via the M4 tip interacting with the pivotal Cys-loop in the extracellular domain. Mutation of a single M4 tyrosine (Y441) to alanine renders one pLGIC-the 5-HT 3A receptor-unable to function despite robust ligand binding. Using Y441A as a proxy for M4 function, we here predict likely paths of Y441 action using molecular dynamics, and test these predictions with functional assays of mutant receptors in HEK cells and Xenopus oocytes using fluorescent membrane potential sensitive dye and two-electrode voltage clamp respectively. We show that Y441 does not act via the M4 tip or Cys-loop, but instead connects radially through M1 to a residue near the ion channel hydrophobic gate on the pore-lining helix M2. This demonstrates the active role of the M4 helix in channel opening., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Crnjar, Mesoy, Lummis and Molteni.)

Certain 2-amino-3,4-dihydroquinazolines bind at 5-HT 3 serotonin receptors and act as antagonists (e.g. 6-chloro) whereas others bind with little to no affinity and lack functional activity (e.g. 8-chloro). The purpose of this investigation was to gain insight as to why this might be the case. X-Ray crystallographic studies revealed that the N–C–N distances in the examined analogs are nearly identical (1.31–1.34 Å), suggesting that differences in N–C–N delocalization does not account for differences in affinity/action. Homology modeling hydrophatic interactions (HINT) analysis revealed that the 6-chloro analog formed a greater number, and more favorable, interactions with the receptor, whereas the 8-chloro analog formed fewer, and unfavorable, interactions. The affinity and activity of the 6-chloro quinazoline relative to its 8-chloro counterpart are unrelated to the N–C–N delocalization pattern but might be related to specific (favorable and unfavorable) interactions of quinazoline substituents with certain receptor features as determined by HINT analysis. Image 1 • Certain quinazoline analogs bind at 5-HT 3 serotonin receptors; certain other do not. • X-Ray structures were solved to determine if delocalization was involved in their binding. • Modeling suggests that 6-Cl-2-amino-3,4-dihydroquinazoline binds in a positive fashion vs its 8-Cl positional isomer. [ABSTRACT FROM AUTHOR]

Background: Depression or Major depressive disorder (MDD) is a prolonged condition of sadness. MDD is the most common mental disorder that affects more than 264 million people worldwide. According to the monoamine hypothesis, serotonin (5-hydroxy tryptamine, 5-HT), dopamine (DA) and norepinephrine (NE) are the major neurotransmitters (NTs) involved in depression., Methods: The methodology adopted for writing this review article is essentially based on the secondary literature search through a systematic literature review. This review mainly focussed on the role of 5-HT 3 receptor antagonists (5-HT 3 RA) in depression and comorbid disorders like anxiety., Results: Out of three major NTs mentioned above, serotonin has a predominant role in the pathophysiology of depression. The serotonin type-3 receptors (5-HT 3 R) are well renowned to be expressed in the central nervous system (CNS) in regions which have significance in the vomiting reflex, perception of pain, the reward system, cognition, depression and anxiety control. 5-HT 3 R are the receptors of serotonergic family that belong to ligand-gated ion channel. 5-HT 3 RA inhibit the binding of serotonin to postsynaptic 5-HT 3 R and increases its availability to other receptors like 5- HT 1A , 1B and 1D as well as 5-HT 2 receptors and produces anti-depressant-like effect. 5-HT 3 RA also have an important role in mood and stress disorders. Some of the studies have shown the effectiveness of these agents in stress disorder., Conclusion: The present article focussed on the role of 5-HT 3 R and their antagonists in the treatment of depression and anxiety. Further studies are warranted to prove their efficacy with respect to other standard anti-depressants., (Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.net.)

Background: Since long back, it has been a matter of discussion regarding the role of peripheral blood vessels in the regulation of cardiorespiratory (CVR) system., Objective: The role of 5-HT3 and TRPV1 receptors present on perivascular nerves in elicitation of CVR reflexes was examined after intra-arterial instillation of bradykinin in urethane anesthetized rats., Materials and Methods: Femoral artery was cannulated retrogradely and was utilized for the instillation of saline/agonist/antagonist and recording of blood pressure (BP), using a double ported 24G cannula. BP, respiration and ECG were recorded for 30 min after bradykinin (1 μM) in the absence or presence of antagonists., Results: Instillation of bradykinin produced immediate hypotensive (40%), bradycardiac (17%), tachypnoeic (45%) and hyperventilatory (96%) responses of shorter latencies (5-8 s) favoring the neural mechanisms in producing the responses. In lignocaine (2%) pretreated animals, bradykinin- induced hypotensive (10%), bradycardiac (1.7%), tachypnoeic (13%) and hyperventilatory (13%) responses attenuated significantly. Pretreatment with ondansetron (100 μg/kg), 5-HT3-antagonist attenuated the hypotensive (10%), bradycardiac (1.7%), tachypnoeic (11%) and hyperventilatory (11%) responses significantly. Pretreatment with capsazepine (1 mg/kg), transient receptor potential vanilloid 1- antagonist blocked the hypotensive (5%), bradycardiac (1.2%), tachypnoeic (6%) and hyperventilatory (6%) responses significantly., Conclusion: In conclusion, presence of a nociceptive agent in the local segment of an artery evokes vasosensory reflex responses modulating CVR parameters involving TRPV1 and 5-HT3 receptors present on the perivascular sensory nerve terminals in anesthetized rats., (Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.net.)

Cannabidiol (CBD) is a nonpsychotropic constituent of Cannabis sativa L. It is suggested to be useful in hypertension. Under in vitro conditions, it activates vanilloid TRPV1 and inhibits serotonin 5-HT 3 receptors, i.e., receptors involved in the Bezold-Jarisch reflex stimulation. The aim of our study was to compare the cardiovascular effects of CBD in spontaneously hypertensive (SHR) and normotensive Wistar Kyoto (WKY) rats. Experiments were performed on conscious, urethane-anesthetized, and pithed rats. In pithed SHR and WKY, CBD increased heart rate (HR) and systolic blood pressure (SBP) and decreased diastolic BP (DBP) in a manner insensitive to adrenalectomy. Propranolol strongly impaired the CBD-induced increases in HR and SBP without affecting the decreases in DBP. Desipramine also reduced the CBD-induced effects on HR and SBP and further increased its effects on DBP. In anesthetized rats, bolus i.v. injection of single doses of CBD induced short-lasting decreases in HR, SBP, and DBP, stronger in SHR than in WKY and prevented by bilateral vagotomy. The CBD-induced fall in HR but not in BP was diminished by the TRPV1 receptor antagonist capsazepine and almost completely abolished if CBD was re-injected after previous administration. CBD reduced the Bezold-Jarisch reflex elicited by the 5-HT 3 receptor agonist phenylbiguanide but not that evoked by the TRPV1 agonist capsaicin. In conscious rats, CBD did not affect cardiovascular parameters. In isolated left atria, CBD decreased contractile force. Conclusions: Cannabidiol (1) induces the Bezold-Jarisch reflex likely via TRPV1 receptors (which undergo tachyphylaxis) more markedly in SHR than in WKY; (2) inhibits the Bezold-Jarisch reflex induced by activation of 5-HT 3 but not TRPV1 receptors; (3) has peripheral sympathomimetic, (4) vasodilatory, and (5) negative inotropic effects. The above properties of CBD should be taken under consideration when CBD is used for therapeutic purposes.

[This corrects the article on p. 306 in vol. 11, PMID: 29075179.].

Brain-derived neurotrophic factor (BDNF) is a key regulator of neuronal plasticity and cognitive functions. BDNF val66met polymorphism, a human single-nucleotide polymorphism (SNP) in the pro-domain of BDNF gene, is associated with deficits in activity-dependent BDNF secretion and hippocampus-dependent memory. However, the underlying mechanism remains unclear. Here we show that in the BDNF Met/Met mouse line mimicking the human SNP, BDNF expression in the hippocampus was decreased. There was a reduction in the total number of cells in hippocampal CA1 region, while hippocampal expression of mRNAs for NR2a, 2b, GluR1, 2 and GABA A Rβ3 subunits were up-regulated. Although basal glutamatergic neurotransmission was unaltered, hippocampal long-term depression (LTD) induced by low-frequency stimulation was impaired, which was partially rescued by exogenous application of BDNF. Interestingly, 5-HT3a receptors were down-regulated in the hippocampus of BDNF Met/Met mice, whereas 5-HT2c receptors were up-regulated. Moreover, impaired LTD in BDNF Met/Met mice was reversed by 5-HT3aR agonist. Thus, these observations indicate that BDNF val66met polymorphism changes hippocampal synaptic plasticity via down-regulation of 5-HT3a receptors, which may underlie cognition dysfunction of Met allele carriers.

The roles of 5-HT3 and 5-HT4 receptors in the modulation of intestinal propulsion by luminal application of 5-HT and augmentation of endogenous 5-HT effects were studied in segments of guinea-pig ileum in vitro. Persistent propulsive contractions evoked by saline distension were examined using a modified Trendelenburg method. When 5-HT (30 nM), fluoxetine (selective serotonin reuptake inhibitor; 1 nM), 2-methyl-5-HT (5-HT3 receptor agonist; 1 mM), or RS 67506 (5-HT4 receptor agonist, 1 μM) was infused into the lumen, the pressure needed to initiate persistent propulsive activity fell significantly. A specific 5-HT4 receptor antagonist, SB 207266 (10 nM in lumen), abolished the effects of 5-HT, fluoxetine, and RS 67506, but not those of 2-methyl-5-HT. Granisetron (5-HT3 receptor antagonist; 1 μM in lumen) abolished the effect of 5-HT, fluoxetine, RS 67506, and 2-methyl-5-HT. The NK3 receptor antagonist SR 142801 (100 nM in lumen) blocked the effects of 5-HT, fluoxetine, and 2-methyl-5-HT. SB 207266, granisetron, and SR 142801 had no effect by themselves. Higher concentrations of fluoxetine (100 and 300 nM) and RS 67506 (3 and 10 μM) had no effect on the distension threshold for propulsive contractions. These results indicate that luminal application of exogenous 5-HT, or increased release of endogenous mucosal 5-HT above basal levels, acts to lower the threshold for propulsive contractions in the guinea-pig ileum via activation of 5-HT3 and 5-HT4 receptors and the release of tachykinins. The results further indicate that basal release of 5-HT is insufficient to alter the threshold for propulsive motor activity.