A comparison of fast and slow depolarizations evoked by 5-HT in guinea-pig coeliac ganglion cells in vitro

Spontaneous Slow Fluctuation of EEG Alpha Rhythm Reflects Activity in Deep-Brain Structures: A Simultaneous EEG-fMRI Study

The emergence of the occipital alpha rhythm on brain electroencephalogram (EEG) is associated with brain activity in the cerebral neocortex and deep brain structures. To further understand the mechanisms of alpha rhythm power fluctuation, we performed simultaneous EEGs and functional magnetic resonance imaging recordings in human subjects during a resting state and explored the dynamic relationship between alpha power fluctuation and blood oxygenation level-dependent (BOLD) signals of the brain. Based on the frequency characteristics of the alpha power time series (APTS) during 20-minute EEG recordings, we divided the APTS into two components: fast fluctuation (0.04–0.167 Hz) and slow fluctuation (0–0.04 Hz). Analysis of the correlation between the MRI signal and each component revealed that the slow fluctuation component of alpha power was positively correlated with BOLD signal changes in the brain stem and the medial part of the thalamus and anterior cingulate cortex, while the fast fluctuation component was correlated with the lateral part of the thalamus and the anterior cingulate cortex, but not the brain stem. In summary, these data suggest that different subcortical structures contribute to slow and fast modulations of alpha spectra on brain EEG.

5-Hydroxytryptamine depolarizes neurons of cat pancreatic ganglia

Pancreatic ganglia contain 5-hydroxytryptamine (5-HT)-immunoreactive axons, some of which are extensions of myenteric neurons located in the pyloric antrum and proximal duodenum. The present study investigated the effect of 5-HT on the membrane potential of cat pancreatic ganglion neurons by means of intracellular recordings in vitro. Pressure application of 5-HT evoked a fast depolarization in 29 of 147 neurons and a slow depolarization in 89 of 147 neurons. A biphasic response was observed in 10 of 108 neurons. The 5-HT-induced slow depolarizing response was not altered in a low Ca2+ (0.1 mM), high Mg2+ (15 mM) solution nor by hexamethonium (10(-4) M) or atropine (10(-6) M). The fast depolarizing response was associated with a decrease of membrane input resistance (-17.2%). The slow depolarizing response was associated with either a decrease (-19.6%) in 24, an increase (+25.0%) in 20, or without a detectable change of membrane input resistance in 10 out of 54 neurons tested. Conditioning hyperpolarization increased the amplitude of both fast and slow depolarizing responses. A low Na+ (68.5 mM) solution and a high K+ (23.5 mM) solution significantly reduced the amplitude of the slow depolarizing response. A low Cl- (9.6 mM) solution had no significant effect on the slow depolarization. The 5-HT3 receptor antagonist MDL 72222 (Bemesetron) blocked the 5-HT-evoked fast depolarizing response. BRL 24924 (Renzapride) and 5 HT-DP, antagonists for the putative 5-HT1P receptor, blocked the slow depolarizing response. The 5-HT3 receptor agonist 2-methyl-5-HT evoked a fast depolarizing response and MCPP, an agonist for the putative 5-HT1P receptor, evoked a slow depolarizing response. Spiperone (a 5-HT1A receptor antagonist) and mianserin (a 5-HT2 receptor antagonist) had no effect on either depolarizing response to 5-HT. The results show that pancreatic ganglion neurons responded to 5-HT with fast and slow depolarizing responses. The data suggest that these responses were mediated by the 5-HT3 receptor and the putative 5-HT1P receptor, respectively.

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.

Evidence that the 5-HT3 receptors of the rat, mouse and guinea-pig superior cervical ganglion may be different

1. Using grease-gap recordings from the isolated superior cervical ganglion of mouse, rat and guinea-pig, we have compared the depolarization evoked by 5-hydroxytryptamine (5-HT) with that evoked by the selective 5-HT3 receptor agonist 2-methyl-5-HT (2-Me-5-HT). 2. The maximum depolarization induced by 2-Me-5-HT was smaller than that induced by 5-HT in all three species, and particularly in the guinea-pig. 3. The 5-HT2 receptor antagonist ketanserin (1 microM) caused a clear rightward shift of the dose-response curve to 5-HT on the guinea-pig ganglion, but not on the mouse or rat ganglion. Spiperone (0.03 microM) had a quantitatively similar action to ketanserin (0.1 microM) on the 5-HT dose-response curve of the guinea-pig ganglion. Ketanserin had no significant effect on the dose-response curve to 2-Me-5-HT on any of these ganglia. 4. Using 2-Me-5-HT as the agonist, we determined the pA2 values for two 5-HT3 receptor antagonists. The potency of ICS 205-930 varied by approximately 100 fold between the species and that of (+)-tubocurarine varied by over 1000 fold. The differences in the pA2 values of these compounds varied independently among the species. 5. We conclude that 5-HT3 receptors are present on the superior cervical ganglion from the rat, mouse and guinea-pig, but these receptors may be pharmacologically distinct from each other. In addition, the depolarization of the guinea-pig superior cervical ganglion by low concentrations of 5-HT is largely mediated by ketanserin-sensitive receptors.

Monoamines modulate the electrically-evoked efflux of 3H-choline from slices of guinea pig nucleus basalis magnocellularis

The influence exerted by monoamines on acetylcholine release was studied in electrically stimulated slices of guinea pig nucleus basalis magnocellularis (nbM) prelabelled with 3H-choline (3H-Ch). Noradrenaline, 30 microM, and clonidine, 1 microM, reduced the evoked 3H-Ch efflux by about 50%, but phenylephrine, 100 microM, did not; idazoxan, 0.1 microM, but not prazosin, 1 microM, antagonized these effects, pointing to the involvement of alpha 2 receptors. Apomorphine, 1 or 30 microM, reduced 3H-Ch efflux from nbM slices as well. The effect was shared by quinpirole, 1 or 10 microM, but not by 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine (SKF 38393), 10 microM, and was antagonized by sulpiride, 1 microM, but not by R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin++ +-7-ol (SCH 23390), 1 microM, suggesting the involvement of the D2 receptor subtype. 5-hydroxytryptamine (5-HT) 0.3-30 microM, and alpha-methyl-5-HT, 10 microM, significantly increased 3H-Ch efflux from nbM slices; the 5-HT2 antagonist ritanserin, 1 microM, prevented this response. 2-methyl-5-HT, 1-30 microM, inhibited the evoked 3H-Ch efflux and its effect was prevented by the 5-HT3 antagonist 1 alpha H,3 alpha,5 alpha H-tropan-3-yl-3,5-dichlorobenzoate (MDL 72222), 1 microM. These findings indicate that i) catecholamines inhibit nbM neurons through alpha 2 and D2 receptors and that ii) a complex serotonergic modulation of cholinergic function exists in the nbM, involving the activation of various receptor subtypes, which can mediate opposite responses.

Effects of vasoactive intestinal polypeptide on antigen-induced bronchoconstriction and thromboxane release in guinea-pig lung

1. Exogenous vasoactive intestinal polypeptide (VIP) infused into the pulmonary artery of isolated and ventilated lungs of guinea-pigs decreased, in a dose-dependent fashion (1.0-10.0 nmol), airway resistance and thromboxane B2 (TXB2, the stable hydrolysis product of TXA2) release in the perfusion medium. Prostacyclin (PGI2) synthesis, as reflected by the release of its stable hydrolysis product 6-oxo-PGF1 alpha, was unaffected. Pretreatment with the 5-lipoxygenase inhibitor BWA4c (3.5 x 10(-5) M) did not modify the bronchodilatory effect of VIP or its inhibitory action on TXB2 release. 2. Basal release of immunoreactive VIP from perfused lungs decreased from an initial value of 0.96 +/- 0.10 ng min-1 (mean +/- s.e.mean) in the first 2 min to an average of 0.58 +/- 0.10 ng min-1 in the following 15-20 min. 3. Antigen challenge with ovalbumin (0.1%) in sensitized lungs caused an anaphylactic reaction in 45% of tested lungs, concomitant with a 5 fold increase in both VIP and TXB2 release. Tetrodotoxin pretreatment (10(-6) M) reduced basal VIP release by > 80% and abolished the VIP increase observed during anaphylaxis, without modifying TXB2 release or the bronchoconstrictor response. 4. Indomethacin (10(-6) M) inhibited TXB2 synthesis and release by > 90%, delayed the bronchoconstrictor response and blunted the increased VIP release during lung anaphylaxis, without influencing basal VIP release. 5. The 5-lipoxygenase inhibitor BWA4c (3.5 x 10(-5) M) blunted the increase of TXB2 and VIP release from guinea-pig lung and attenuated the bronchoconstrictor response following ovalbumin challenge. 6. The administration of exogenous VIP as a continuous infusion (10-8 M) attenuated the bronchoconstriction and the release of cyclo-oxygenase metabolites following antigen challenge.7. Acetylcholine (10-6-l0-5 M) infused into the pulmonary artery induced a dose-dependent bronchoconstriction not associated with enhanced VIP or TXB2 release.8. The TXA2 mimetic U-46619 (0.1-1.0 nmol) caused dose-dependent increases in airway resistance,concomitant with an up to 10 fold increase in VIP release. VIP inhibited arachidonate-induced in vitro aggregation of washed rabbit platelets in a dose-dependent manner over a dose range 10-8 10-6 M.Despite the antiaggregatory effect of VIP, TXB2 and PGE2 synthesis was reduced only to a minor extent,and there was no redirection of arachidonate metabolism from TXA2 to PGE2, indicating that VIP does not act as a TX synthase inhibitor in vitro.9. We conclude that VIP may play a role in regulating bronchial smooth muscle reactivity in lung anaphylaxis by inhibiting the synthesis and release of TXA2, a potent vasoactive and bronchoconstrictor agent. TXA2, on the other hand, strongly enhances neuronal VIP release.

Blockade of 5-HT3 receptor-mediated currents in dissociated frog sensory neurones by benzoxazine derivative, Y-25130

1. The effect of Y-25130, ((+-)-N-(1-azabicyclo[2.2.2]oct-3-yl)-6-chloro-4-methyl-3-oxo-3,4-dih ydr o- 2H-1,4-benzoxazine-8-carboxamide hydrochloride), a high affinity 5-hydroxytryptamine3 (5-HT3) receptor ligand, was examined on the 5-HT-induced response in dissociated frog dorsal root ganglion (DRG) neurones by use of the extremely rapid concentration-jump ('concentration-clamp') and the conventional whole-cell patch-clamp techniques. 2. 5-HT induced a rapid transient inward current associated with an increase in membrane conductance at a holding potential of -70 mV. The current amplitude increased sigmoidally as 5-HT concentration increased. The half-maximum value (Ka) and the Hill coefficient estimated from the concentration-response curve were 1.7 x 10(-5) M and 1.7, respectively. 3. The current-voltage (I-V) relationship of 5-HT-induced current (I5-HT) showed inward rectification at potentials more positive than -40 mV. The reversal potential (E5-HT) was -11 mV. The E5-HT value was unaffected by total replacement of intracellular K+ by Cs+, indicating that the 5-HT-gated channels might be large cation channels. 4. Both the activation and inactivation phases of I5-HT were single exponentials. The time constants of activation and inactivation (tau a and tau i) decreased with increasing 5-HT concentration. 5. The 5-HT response was mimicked by a selective 5-HT3 receptor agonist, 2-methyl-5-HT, but the maximum response induced was approximately 25% that of 5-HT. The 5-HT response was reversibly antagonized by the 5-HT3 receptor antagonists, ICS 205-930, metoclopramide and Y-25130, but not by a 5-HTIA receptor antagonist, spiperone, and a 5-HT2 receptor antagonist, ketanserin. The half-inhibition concentrations (IC50) were 4.9 x 10-10 M for Y-25130, 4.8 x 10-10 M for ICS 205-930 and 8.6 x 10-9 M for metoclopramide.6. Y-25130 (5 x 10-10 M) caused a rightward shift of the concentration-response curve for 5-HT while decreasing the maximum response.7. The results suggest that Y-25130 is a potent antagonist of the 5-HT3 receptor-channel complex.

Serotonin is a directly-acting hyperalgesic agent in the rat

In this study, we have investigated serotonin hyperalgesia employing the mechanical paw withdrawal nociceptive threshold test in the rat. Intradermally injected serotonin was found to produce a dose-dependent hyperalgesia that was not attenuated by procedures which eliminate the known indirect mechanisms of hyperalgesia such as sympathectomy, polymorphonuclear leukocyte depletion or cyclooxygenase inhibition. In addition, the latency to onset of serotonin hyperalgesia is extremely short, with maximal hyperalgesia observed in less than 1 min, a similar temporal onset to direct-acting hyperalgesic agents such as prostaglandin E2. The results suggest, therefore, that the hyperalgesic effects of serotonin in our animal model are exerted by direct action on primary afferent neurons. Only the intradermal injection of selective serotonin (5-hydroxytryptamine; 5-HT) agonists for the 1A receptor subset (5-HT1A), (+/-)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthaline hydrobromide and N,N-dipropyl-5-carboxamido-tryptamine maleate, produced dose-dependent hyperalgesia. No hyperalgesia was seen after 5-HT1B, CGS-12066B maleate and m-trifluoromethylphenyl-piperazine hydrochloride; 5-HT2+IC, alpha methyl 5HT and (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl; or 5-HT3, 2-methyl-5-hydroxytryptamine maleate and phenylbiguanide, agonists. Similarly, only the 5-HT1A antagonists, spiroxatrine and spiperone, attenuated the hyperalgesia induced by intradermally injected serotonin. 5-HT2+IC antagonists, mesulergine and ketanserin, and 5-HT3 antagonists, quipazine and 3-tropanyl-indole-3-carboxylate, did not significantly attenuate 5-HT hyperalgesia. We conclude that serotonin produces hyperalgesia by a direct action on the primary afferent neuron via the 5-HT1A subset of serotonin receptors.

Response to 5-hydroxytryptamine on neurons of guinea pig celiac and inferior mesenteric ganglia in primary culture

The electrophysiological effects of serotonin, a putative neurotransmitter in prevertebral sympathetic ganglia, were evaluated in cultured celiac and inferior mesenteric ganglia (IMG) neurons. Intracellular microelectrode recordings were performed in neurons that were maintained in culture an average of 26 days. Seventy-eight of 85 neurons responded when serotonin (10 microM) was applied by pressure ejection from a micropipette to the surface of the isolated cells. The majority of the neurons (n = 48) generated fast depolarizations, although slow depolarizations (n = 17), bipolar responses (n = 5), hyperpolarizations (n = 7), and a biphasic response (n = 1), were also seen. Hyperpolarizing responses were evoked in celiac neurons only. All responses were inhibited by the 5-HT3 antagonist MDL 72,222 (5 microM). Fast responses were not inhibited by tetrodotoxin (n = 3). These results demonstrate that serotonin evokes a variety of membrane potential changes in cultured prevertebral sympathetic neurons by activating 5-HT3 receptors.

The effects of 5-HT on articular sensory receptors in normal and arthritic rats

1. The effects of intra arterial (i.a.) injections of 5-hydroxytryptamine (5-HT, 1-100 micrograms) on the discharge of (a) identified articular high threshold mechanoreceptors and (b) unidentified chemosensitive receptors in the ankle joint have been studied electrophysiologically in anaesthetized normal and arthritic rats. Recordings were made from a fine branch of the medial plantar nerve. 2. 5-HT increased the mechanical responsiveness of high threshold nociceptive mechanoreceptors with C and A delta fibre afferents in both normal and adjuvant-arthritic rats. Receptors in arthritic joints were more sensitive to 5-HT than were those from normal joints. 3. 5-HT produced a complex response from both types of articular receptors following i.a. injection. Two separate components were identified: (a) a fast transient burst of activity was obtained within 10 s of this injection in 66% of units from normal animals and 45% from arthritics, followed by (b) a delayed slow longer-lasting excitation seen in 62% of the units examined from normals and 77% of units from arthritic rats. 4. Increased mechanoreceptor responsiveness produced by 5-HT was reduced or abolished by the 5-HT3 receptor antagonists studied (MDL 72222, ICS 205-930, or GR 38032F, in single doses of 100 micrograms kg-1, i.a.). 5. Fast excitation showed marked tachyphylaxis and was antagonized by MDL 72222, ICS 205-930 or GR 38032F. It was unaffected by ketanserin (100 micrograms kg-1, i.a.). Delayed excitation was reduced or abolished by ketanserin but was unaffected by the 5-HT3-receptor antagonists. 6. Administration of MDL 72222, ICS 205-930 or GR 38032F caused short lasting (< 5 min) reductions in background activity from both types of unit recorded in arthritic rats, as well as in normal rats in which activity had increased following administration of 5-HT. Ketanserin caused similar reductions in background activity in chemosensitive units, but had no effect on mechanoreceptors. 7. At least two types of receptor are involved in the actions of 5-HT on articular sensory receptors with fine afferent fibres. Increased mechano-responsiveness involves a 5-HT3-receptor as does fast excitation. Delayed excitation probably involves a 5-HT2-receptor. Endogenous 5-HT appears not to play a crucial role in sensitization of high threshold mechanoreceptors in this model of chronic inflammation and arthritis, although its local release may potentiate the actions of other inflammatory mediators on sensory receptors in the ankle joint.

Electrophysiological properties of frog spinal dorsal horn neurons and their responses to serotonin: an intracellular study in the isolated hemisected spinal cord

Frog dorsal horn neurons and their responses to serotonin (5-HT) were studied in intracellular recordings from isolated hemisected spinal cords. Electrophysiological properties were comparable to those of mammals. Bath application of 5-HT (10-50 microM) increased the excitability and caused membrane depolarizations in 7/14 cells, reduced or abolished activity and caused hyperpolarizations in 4 neurons, exerted a biphasic effect in two cells, and produced no detectable change in one neuron. The multiple effects of 5-HT may be mediated through different 5-HT receptor subtypes.

Effects of 5-HT3 receptor antagonists on 5-HT and nicotinic depolarizations in guinea-pig submucosal neurones

1. Intracellular recordings were made from neurones of the guinea-pig submucosal plexus. The effects of several 5-hydroxytryptamine3 (5-HT3) receptor antagonists on depolarizations produced by ionophoretic application of 5-HT and acetylcholine, as well as on fast excitatory postsynaptic potentials (fast e.p.s.ps) produced by nerve stimulation were examined. 2. ICS 205-930, GR 38032F, MDL 72222, cocaine and curare all inhibited the fast e.p.s.p. as well as the depolarizations in response to 5-HT and acetylcholine (ACh) ionophoresis in a dose-dependent fashion. 3. IC50 values for ICS 205-930, GR 38032F, MDL 72222, cocaine and curare in inhibiting the 5-HT mediated depolarizations were 12 nM, 100 nM, 3 microM, 3 microM and 20 microM, respectively. 4. IC50 values for ICS 205-930, GR 38032F, MDL 72222, cocaine and curare in inhibiting the nicotinic depolarizations were 4 microM, 12 microM, 11 microM, 6 microM and 17 microM, respectively. Similar IC50 values were obtained for inhibition of the fast e.p.s.ps by these antagonists. 5. The nicotinic receptor blocker, hexamethonium, inhibited the nicotinic depolarization and the fast e.p.s.p. with IC50 values of 10 microM. Hexamethonium (10 microM-5 mM) did not alter the depolarization induced by 5-HT. 6. These results demonstrate that the pharmacological profile of 5-HT3 receptors present on submucosal neurones is identical to that of 5-HT3 receptors on myenteric neurones and, thus, provide evidence that the enteric neuronal 5-HT3 receptor forms a receptor subtype distinct from that characterized in other parts of the autonomic nervous system.

5-Hydroxytryptamine evokes three distinct responses on the rat superior cervical ganglion in vitro

In addition to the 5-HT3-mediated fast depolarisation, 5-hydroxytryptamine (5-HT) evoked two additional responses on the rat superior cervical ganglion: a hyperpolarisation and a slow depolarisation. These responses appeared to be direct actions on 5-HT receptors since they were present in a low calcium medium containing tetrodotoxin and were not abolished by a variety of non-serotonin antagonists. The hyperpolarisation was not antagonised by 5-HT3 or 5-HT2 antagonists. The 5-HT1 ligands 5-carboxamidotryptamine (5-CT) and 8-OH-DPAT also evoked a hyperpolarisation. The hyperpolarisation was antagonised by six 5-HT1A antagonists including WB-4101 and spiroxatrine. It was therefore concluded to be mediated by a 5-HT1A receptor. The slow depolarisation was only evoked by 5-HT. The receptor involved in this response, however, could not be determined. We conclude that in addition to 5-HT3 receptors the rat superior cervical ganglion possesses 5-HT1A receptors and another uncharacterised 5-HT receptor.

Calcitonin gene-related peptide evokes distinct types of excitatory response in guinea pig coeliac ganglion cells

Pressure application of calcitonin gene-related peptide (CGRP) evoked in a population of guinea pig coeliac neurons 3 types of response: a fast, a slow and a biphasic depolarization. The responses were not appreciably affected in low Ca/high Mg or tetrodotoxin-containing Krebs solution. The fast depolarization was associated with a fall in membrane resistance; it was made larger on hyperpolarization and the estimated reversal potential was -24 mV. The fast response was reversibly blocked in a Na-free medium as well as by relatively high concentrations of d-tubocurarine (50-100 microM) but not by hexamethonium. The slow, CGRP-induced depolarization resistant to nicotinic and muscarinic antagonists, was associated with either a small increase or decrease of input resistance. Membrane hyperpolarization increased the slow response in the majority of coeliac neurons, with an estimated reversal potential of -44 mV. The biphasic depolarization displayed electrophysiological and pharmacological characteristics resembling the fast and slow responses. These results raise the possibility that CGRP acting via two distinct types of receptor elicits, respectively, a fast, Na-dependent excitatory response and a slow response, the mechanism of which remains to be established.

The depolarizing action of 5-hydroxytryptamine on rabbit isolated preganglionic cervical sympathetic nerves

This study describes a depolarizing action of 5-hydroxytryptamine (5-HT) on rabbit isolated preganglionic cervical sympathetic nerves using an extracellular recording technique. From cumulative concentration-response curves for 5-HT (1 mumol/l-1 mmol/l), the mean maximal depolarization was shown to be 277 +/- 32 microV and EC50 was 9.4 mumol/l (6.5-13.6 mumol/l, geometric mean, 95% confidence limits, n = 42). The responses to 5-HT displayed marked tachyphylaxis. When cumulative concentration-response curves to 5-HT and 2-methyl-5-HT were determined in the same preparations (n = 4), the mean maximal response to 5-HT was 519 +/- 167 microV, EC50 32.2 mumol/l (8.8-118 mumol/l) and the mean maximal response to 2-methyl-5-HT was 317 +/- 63 microV, EC50 35.1 mumol/l (12.9-95.5 mumol/l, geometric means, 95% confidence limits). The action of selective 5-HT antagonists was tested on repeated cumulative concentration-response curves to 5-HT. Neither methiothepin (0.1-1 mumol/l, n = 3) nor ketanserin (0.1-1 mumol/l, n = 3) had an action on 5-HT responses. The selective 5-HT3 antagonists MDL 72222, ICS 205-930 and SDZ 206-830 were all potent antagonists of the 5-HT depolarizations. The action of these antagonists was quantified by determining the apparent pA2 from the dose ratios and a Schild plot.(ABSTRACT TRUNCATED AT 250 WORDS)

Drug receptors on single enteric neurons

There are many substances contained within enteric nerves which excite or inhibit other nerves when these substances are applied to single neurons. The actions of these substances and of drugs which mimic these actions is to open or close membrane ion channels. The effects on membrane potential are dependent on the nature of the ions which pass through the channel and whether the channel is opened or closed. In the enteric nervous system, drugs can act at one of three broad classes of receptors: [1] those which are part of an ion channel complex and which open either cation channels or chloride channels, both of which result in membrane depolarization [2] those which open potassium channels resulting in hyperpolarization or [3] those which close potassium channels resulting in depolarization. Receptors which open potassium channels are coupled to the channel via a G-protein while receptors which close potassium channels are coupled to the channel, in some cases, via a cyclic AMP-dependent system while in other cases another second messenger system is involved.

Fast and slow depolarizing responses of guinea-pig coeliac ganglion cells to 5-hydroxytryptamine

Responses to 5-hydroxytryptamine (5-HT) applied by pressure ejection to guinea-pig coeliac ganglion cells were recorded intracellularly in vitro. Three types of responses were observed: a fast depolarization (43% of cells), a slow depolarization (25%) and a biphasic response comprising an initial fast followed by a slow depolarization (30%). Fast depolarizations had a mean duration of 6.4 s and were associated with a decrease in membrane input resistance. On hyperpolarizing the membrane, responses increased in amplitude; the mean reversal potential was -23 mV. Fast responses were nearly eliminated in a Na-free medium but were unaffected by low Ca/high Mg solution or by low Cl medium. Fast responses were depressed by MDL 72222, quipazine, metoclopramide, and by relatively high concentrations of D-tubocurarine, but unaffected by methysergide. Slow responses had a mean duration of 62 s and were associated either with no change or a slight fall in membrane resistance. hyperpolarizing the membrane, slow responses were increased in most but not all cells tested. Slow depolarizations were depressed by methysergide but unaffected by agents that suppressed the fast responses. The biphasic response displayed electrophysiological and pharmacological characteristics of both the fast and slow depolarizations. It is concluded that guinea-pig coeliac ganglion cells display either a rapid depolarization, a slow depolarization or a biphasic response to 5-HT, that the fast depolarization appears to be mediated by a conductance increase to Na and K, while a different mechanism appears to underlie the slow depolarization, and that the two responses are mediated by different kinds of 5-HT receptor.