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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 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]

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]

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]

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]

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]

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]

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]

Presents an index of terms used in the December 2002 issue of 'Ailment Pharmacology Therapeutics.' Alpha-antagonist; Adenosine triphosphate; Ampicillin; Amoxicillin; Antibiotic prophylaxis; Anticonvulsants; Antidepressants; Bisacodyl; Cyclo-oxygenase; Cytosolic hepatic function; Colonic motility; Complementary therapy.