Disruption of potential alpha-helix in the G loop of the guinea pig 5-hydroxytryptamine2 receptor does not prevent receptor coupling to phosphoinositide hydrolysis

Long-term depression of nociceptive synapses by non-nociceptive afferent activity: role of endocannabinoids, Ca²+, and calcineurin

Activity in non-nociceptive afferents is known to produce long-lasting decreases in nociceptive signaling, often referred to as gate control, but the cellular mechanisms mediating this form of neuroplasticity are poorly understood. In the leech, activation of non-nociceptive touch (T) mechanosensory neurons induces a heterosynaptic depression of nociceptive (N) synapses that is endocannabinoid-dependent. This heterosynaptic, endocannabinoid-dependent long-term depression (ecLTD) is observed where the T- and N-cells converge on a common postsynaptic target, in this case the motor neuron that innervates the longitudinal muscles (L-cells) that contributes to a defensive withdrawal reflex. Depression in the nociceptive synapse required both presynaptic and postsynaptic increases in intracellular Ca²⁺. Activation of the Ca²⁺-sensitive protein phosphatase calcineurin was also required, but only in the presynaptic neuron. Heterosynaptic ecLTD was unaffected by antagonists for NMDA or metabotropic glutamate receptors, but was blocked by the 5-HT₂ receptor antagonist ritanserin. Depression was also blocked by the CB1 receptor antagonist rimonabant, but this is thought to represent an effect on a TRPV-like receptor. This heterosynaptic, endocannabinoid-dependent modulation of nociceptive synapses represents a novel mechanism for regulating how injury-inducing or painful stimuli are transmitted to the rest of the central nervous system.

Properties of cannabinoid-dependent long-term depression in the leech

Previously, a cannabinoid-dependent form of long-term depression (LTD) was discovered at the polysynaptic connection between the touch mechanosensory neuron and the S interneuron (Li and Burrell in J Comp Physiol A 195:831-841, 2009). In the present study, the physiological properties of this cannabinoid-dependent LTD were examined. Increases in intracellular calcium in the S interneuron are necessary for this form of LTD in this circuit. Calcium signals contributing to cannabinoid-dependent LTD are mediated by voltage-dependent calcium channel and release of calcium from intracellular stores. Inositol triphosphate receptors, but not ryanodine receptors, appear to mediate this store-released calcium signal. Cannabinoid-dependent LTD also requires activation of metabotropic serotonin receptors, possibly a serotonin type 2-like receptor. Finally, this form of LTD involves the stimulation of nitric oxide synthase and a decrease in cyclic adenosine monophosphate signaling, both of which appeared to be downstream of cannabinoid receptor activation. Based on these findings, the cellular signaling mechanisms of cannabinoid-dependent LTD in the leech are remarkably similar to vertebrate forms of cannabinoid-dependent synaptic plasticity.

5-HT2A receptor antagonist properties of cyamemazine in rat and guinea pig smooth muscle

5-HT(2A) receptor antagonism seems to explain the low incidence of extrapyramidal side effects with atypical neuroleptics. Whether the neuroleptic cyamemazine, which at low doses is also devoid of extrapyramidal side effects, possesses 5-HT(2A) receptor antagonist properties is unknown. Cyamemazine was tested for its ability to antagonize 5-HT(2A)-mediated responses in isolated rat aorta and guinea pig trachea and to displace [3H]ketanserin specifically bound to rat brain membranes. In isolated rat aorta, cyamemazine potently and competitively antagonized serotonin-dependent contractions (pA(2)=8.82+/-0.26, n=7; Schild's slope=1.02+/-0.29). In this test, cyamemazine was of similar potency as ketanserin (pA(2)=8.23). In isolated guinea pig trachea, cyamemazine reduced maximum contractile responses to serotonin with pIC(50)=7.92+/-0.35, (n=4), whereas ketanserin exhibited a pIC(50)=8.79. Finally, cyamemazine displaced [3H]ketanserin specifically bound to rat brain membranes with pK(i)=8.76+/-0.53 (n=3). In conclusion, cyamemazine behaves as a potent antagonist at 5-HT(2A) receptors, which compares well with the reference compound, ketanserin. Whether this 5-HT(2A) receptor antagonist action of cyamemazine can explain its low incidence of extrapyramidal side effects deserves further investigation.

Multiple 5-HT receptors in the guinea-pig superior cervical ganglion

1. We have studied the pharmacology of the depolarization by 5-hydroxytryptamine (5-HT) of the guinea-pig isolated superior cervical ganglion (SCG) using the grease-gap technique. We studied the effects of selective and non-selective antagonists on the responses to 5-HT and other 5-HT receptor agonists. 2. We have extended the pharmacology of the 5-HT3 receptor in this preparation by studying the effects of granisetron, BRL 46470 and mianserin on the concentration-response curve (CRC) to 2-methyl-5-HT. As with other 5-HT3 receptor antagonists, these compounds exhibited a lower affinity for guinea-pig 5-HT3 receptors than for rat 5-HT3 receptors. 3. We have confirmed that low concentrations of 5-HT (< or = 1 microM) mediate ketanserin-sensitive responses and higher concentrations of 5-HT also recruit 5-HT3 receptors. The responses to low concentrations of 5-HT were antagonized by low concentrations of ketanserin, spiperone, mianserin, DOI and LSD indicating probably mediation by 5-HT2A receptors. At high concentrations, the hallucinogen, DOI, but not LSD, evoked a ketanserin-sensitive depolarization. 4. Although mianserin could bind to the 5-HT2A receptors in this preparation, we could not demonstrate a down-regulation of depolarizations evoked by these receptors after a 10 day oral treatment with mianserin (10 mg kg-1, daily). 5. 5-Carboxamidotryptamine (5-CT) evoked a prolonged depolarization. Although high concentrations of 5-CT (> or = microM) appeared to activate 5-HT2A receptors, lower concentrations of 5-CT evoked a response with a distinct pharmacology. After studying the action of 20 selective and non-selective 5-HT receptor ligands we believe that this response may be mediated by a novel receptor; but its pharmacology is closest to that of receptors in the 5-HT2 receptor family. Like 5-CT, 5-HT (3-300 microM) could evoke an LSD-sensitive response in the presence of the 5-HT2 receptor antagonist, ketanserin and the 5-HT3 receptor antagonist, tropisetron (all 1 microM). 6. We conclude that 5-HT activates three pharmacologically distinct receptors to depolarize the guinea-pig SCG. Low concentrations of 5-HT appear to activate 5-HT2A receptors. Higher concentrations of 5-HT also activate 5-HT3 receptors and a possible novel 5-HT receptor. The novel receptor could be a species homologue of a 5-HT2 receptor or an, as yet, unclassified 5-HT receptor.

Release of acetylcholinesterase from guinea-pig substantia nigra: effects of tryptaminergic drugs and dorsal raphé nucleus stimulation

The guinea-pig substantia nigra receives a 5-hydroxytryptaminergic (5-HT ergic) projection from the dorsal raphé nucleus. In this study we have attempted to identify the 5-HT receptor subtype mediating release of acetylcholinesterase (AChE) from nigral neurones, measured by assay of perfusate obtained via chronically implanted push-pull cannulae. The effects of direct nigral application of 5-HT, 2-methyl-5-HT and 5-methoxytryptamine. Application of submicromolar concentrations of 5-HT, 2,5,-dimethoxy-4- iodoamphetamine and alpha-methyl-5-HT significantly enhanced release of AChE, whereas 5-carboxamidotryptamine, sumatriptan, 2-methyl-5-HT and 5-methoxytryptamine were ineffective at a similar concentration range. Electrical stimulation (50 Hz, 20-300 mu A) of the dorsal raphé nucleus evoked release of AChE from the substantia nigra, and induced a rotational behavioural effect for the duration of stimulation. Pretreatment with 5,7,-dihydroxytryptamine inhibited both DRN-evoked release of AChE and animal rotation. The 5-HT receptor antagonists ketanserin and ritanserin (10(-7)-10(-6)M, when applied to the substantia nigra, inhibited raphé-stimulated AChE release. Drugs which inhibited raphé-stimulated release of AChE had no effect on concomitant animal rotation, indicating that the behavioural events are mediated via distinct processes, unrelated to those mediating nigral AChE release. The data suggest that evoked release of AChE from the substantia nigra by stimulation of the dorsal raphé nucleus may be mediated in part via a 5-HT2 receptor type. The 5-HT1D agonists 5-carboxamidotryptamine (10(-6)M and sumatriptan (10(-5)M also inhibited raphé-evoked AChE release, suggesting a possible presynaptic autoinhibitory role for 5-HT1D receptors on raphé-nigral nerve terminals.

Autoradiographic comparison of [125I]LSD-labeled 5-HT2A receptor distribution in rat and guinea pig brain

Although the density and distribution of 5-HT2A (5-hydroxytryptamine-2A) receptors is well established for rat brain, the 5-HT2A receptor distribution and density in guinea pig brain has not been extensively studied. In the present in vitro study, we have utilized 125I-lysergic acid diethylamide ([125I]LSD) to quantify and compare 5-HT2A receptor density in coronal sections of rat and guinea pig brain. Spiperone (1 microM) and sulpiride (1 microM) were used to displace [125I]LSD binding from 5-HT2A and D2 binding sites, respectively. Ligand binding was quantified by computer-aided image analysis densitometry (MCID). Similar to the rat, areas of highest specific 5-HT2A receptor binding (fmol/mg protein) in guinea pig brain included the claustrum and Layer 4 of the cerebral cortex. Significant binding was also found in remaining neocortical layers, islands of Calleja, caudate putamen, olfactory bulb, nucleus accumbens, and choroid plexus. While the rat brain exhibited a high level of specific binding in the tenia tecta and mammillary nuclei, little binding was observed in these regions in the guinea pig. In both rat and guinea pig, low specific binding was found in amygdaloid, thalamic, or cerebellar areas. These studies indicate a general similarity between 5-HT2A binding site distribution and relative density in guinea pig and rat brain but point to a few brain regions where significant differences exist.

Relationship between 5-HT2A receptor mRNA density and contractility in trachea and aorta from guinea pig and rat

The present studies document marked differences in contractile responsiveness to serotonin in trachea and aorta between guinea pig and rat. For example, the guinea pig trachea and rat aorta markedly contract in response to serotonin via activation of 5-HT2A receptors. In contrast, the rat trachea and guinea pig aorta only modestly contract to serotonin. The availability of 5-HT2A receptor selective cDNA clones from brain of both guinea pig and rat permitted molecular probes to be designed and PCR amplification studies initiated to identify and quantify 5-HT2A receptor specific mRNA in these tissues. For trachea, 3-fold higher concentrations of 5-HT2A receptor specific mRNA were found in guinea pig relative to rat trachea. These data are consistent with the more profound contractile response to serotonin in guinea pig versus rat trachea and suggest that differences in tracheal contractility to serotonin correlate with the density of 5-HT2A receptor mRNA. In contrast, although rat aorta contracted more dramatically to serotonin than guinea pig aorta, rat aorta possessed a similar concentration of 5-HT2A receptor specific mRNA as compared to guinea pig aorta. Thus, for the aorta, differences in the concentration of 5-HT2A receptor mRNA are not sufficient to explain the observed differences in contractility between tissues from guinea pig and rat. These studies documenting 5-HT2A receptor mRNA in rat trachea and guinea pig aorta, two tissues that do not markedly contract in response to serotonin indicate that 5-HT2A receptor mRNA although present, has not resulted in a receptor capable of mediating a contractile response in these tissues.