Two types of receptors for 5-hydroxytryptamine on the cholinergic nerves of the guinea-pig myenteric plexus

Presynaptic modulation of cholinergic and non-cholinergic fast synaptic transmission in the myenteric plexus of guinea pig ileum

Abstract These studies investigated receptors modulating release of mediators of fast excitatory postsynaptic potentials (fEPSPs) in guinea pig ileum myenteric plexus using electrophysiological methods. Fast EPSPs inhibited by >95% by hexamethonium (100 micromol L(-1)) were cholinergic; mixed fEPSPs were inhibited <95% by hexamethonium. Non-cholinergic fEPSPs were studied in the presence of hexamethonium. The alpha2-adrenergic receptor agonist UK 14304 inhibited cholinergic (maximum inhibition = 76%, EC(50) = 18 nmol L(-1)), mixed (81%, 21 nmol L(-1)) and non-cholinergic (76%, 44 nmol L(-1)) fEPSPs equally. The 5-HT(1) receptor agonist 5-carboxamidotryptamine inhibited cholinergic, mixed and non-cholinergic fEPSPs equally. Renzapride, increased non-cholinergic (33%) less than mixed (97%, 13 micromol L(-1)) fEPSPs. Renzapride inhibited the purely cholinergic fEPSPs (-29%) but potentiated the cholinergic component of mixed fEPSPs (39%). Prucalopride potentiated all fEPSPs equally (30-33%). 5-HT (0.1 micromol L(-1)) induced potentiation of cholinergic (75%), mixed (97%) and non-cholinergic (84%) fEPSPs was not statistically different. The potentiating effects of renzapride and 5-HT on fEPSPs were inhibited by the 5-HT(4) receptor antagonist, SB 204070 (10 nmol L(-1)). Renzapride (0.3 micromol L(-1)) blocked 5-HT-induced increases in cholinergic fEPSPs. alpha2-Adrenergic and 5-HT(1) receptors mediate inhibition of transmitter release from cholinergic and mixed terminals. 5-HT and prucalopride, acting at 5-HT(4) receptors, facilitate all fEPSPs; renzapride facilitates the cholinergic and non-cholinergic components of mixed fEPSPs but not purely cholinergic fEPSPs. Cholinergic synapses may express few 5-HT(4) receptors or a renzapride-insensitive 5-HT(4) receptor isoform.

Muramyl dipeptide (MDP) and 5-HT receptors. Neuroimmunomodulatory effects of MDP are probably not mediated through 5-HT4 or 5-HT1A receptors

A possible interaction of immunomodulator muramyl dipeptide (MDP) with 5-HT4 and 5-HT1A receptors was investigated. The activation of 5-HT4 receptors releases acetylcholine from nerve terminals, thereby contracting the guinea-pig distal ileum. The whole ileum segments were therefore cut and placed into the bath. The preparations were contracted by 5-HT (10 nM-3.2 microM); these contractions were totally abolished in the presence of atropine (1 microM) and significantly attenuated in the presence of SDZ-205,557 (320 nM). The 5-HT evoked contractions remained unchanged in the presence of MDP (5, 50 or 500 nM). MDP (l0 nM-3.2 microM) could not directly contract the preparations. In further experiments, the possible interaction of MDP with 5-HT1A receptors was investigated. The activation of 5-HT1A receptors inhibits the release of acetylcholine from nerve terminals, thereby decreasing the height of electrically evoked neurogenic twitches of guinea-pig ileum. The whole ileum segments were cut, placed into the bath and stimulated electrically. Selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) decreased the height of twitches and this effect was significantly attenuated in the presence of 5-HT antagonist metergoline (1 microM). The effect of 8-OH-DPAT remained unchanged in the presence of MDP (5, 50 or 500 nM). MDP (10 nM-3.2 microM) did not exert any direct effect on the preparations. These results suggest that MDP interacts with neither 5-HT4 nor 5-HT1A receptors.

Effects of fluvoxamine on morphine-induced inhibition of gastrointestinal transit, antinociception and hyperlocomotion in mice

In the present study, the effects of fluvoxamine on the inhibition of gastrointestinal transit, antinociception and hyperlocomotion induced by morphine were investigated in mice. Treatment with morphine (10 mg/kg, subcutaneously (s.c.)) inhibited gastrointestinal transit and hyperlocomotion. These effects were attenuated by fluvoxamine in a dose-dependent manner. On the other hand, morphine (1 mg/kg, s.c.)-induced antinociception was significantly potentiated by fluvoxamine. These results suggest that fluvoxamine combined with morphine may be useful for the treatment of not only depression but also pain in cancer patients without constipation or activation of the mesolimbic dopaminergic system. However, further understanding and clinical studies will be required to confirm this possibility.

Review article: the therapeutic potential of 5-HT3 receptor antagonists in the treatment of irritable bowel syndrome

There is evidence from studies, in both animals and humans, that 5-HT3 receptor blockade has potential value in the treatment of irritable bowel syndrome, particularly in those patients with diarrhoea-predominant bowel habits. New findings suggest that 5-HT3 receptors exist on gut afferent neurones and that their activation by locally released 5-HT leads to visceral nociceptive stimulation, in addition to increased neuronally-mediated motor and secretory activity. If this concept is validated, it will provide a rationale for the use of 5-HT3 receptor antagonists in patients with increased gut motility, reduced fluid absorption and low nociceptive thresholds leading to abdominal pain. Alosetron is a highly selective, potent 5-HT3 receptor antagonist which is well absorbed with a long pharmacodynamic half-life. Its ability to provide long-lasting blockade of 5-HT3 receptors throughout the body make it an ideal candidate within its class to evaluate the clinical hypothesis that sustained and ubiquitous 5-HT3 receptor blockade is of value in the treatment of IBS.

The interaction of immunomodulatory muramyl dipeptide with peripheral 5-HT receptors: overview of the current state

Immunomodulator muramyl dipeptide (MDP) exerts also pronounced neuropharmacological activities which are probably mediated by an interaction with 5-HT receptors. Some of these effects are considered as undesirable by its clinical use. More precise information concerning MDP effects on 5-HT receptors with respect to their many subtypes could result from studies using isolated organs in vitro. Earlier conducted studies of this type provided data that are concisely overviewed and reinterpreted here from the view of current 5-HT receptor classification. Since new 5-HT receptor types have emerged recently, new studies are under way. The results might contribute to the development of novel immunomodulatory drugs devoid of adverse effects.

5-HT1A receptor-mediated inhibition of acetylcholine release from guinea pig myenteric plexus: potential mechanisms

The mechanisms through which presynaptic 5-HT1A receptors cause inhibition of acetylcholine release from the guinea pig myenteric plexus were investigated. The selective 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) and 5-hydroxytryptamine (5-HT) caused concentration-dependent inhibitions of the electrically evoked release of [3H]acetylcholine from myenteric plexus preparations that had been preincubated with [3H]choline. The inhibitory effects were not modified by the activator of adenylyl cyclase, forskolin (10 microM), the phosphodiesterase inhibitor, AH 21-132 (100 microM), or after pretreatment of the guinea pigs with pertussis toxin (60 micrograms/kg). In contrast, the protein kinase C activator 4 beta- phorbol-12,13-dibutyrate (0.1 microM) prevented the release-inhibiting effect of 8-OH-DPAT, whereas the inactive isomer 4 alpha-phorbol-12,13-dibutyrate (0.1 microM) was without effect. The results suggest that the presynaptic 5-HT1A receptor is not coupled to a pertussis toxin sensitive G protein or to adenylyl cyclase. However, protein kinase C seems to be involved in the mechanism of inhibition of acetylcholine release by presynaptic 5-HT1A receptors.

Serotonin modulates retinotectal and corticotectal convergence in the superior colliculus

A dense serotonin (5-HT)-containing projection to the superficial layers of the superior colliculus (SC) has been demonstrated in diverse mammalian species, but how 5-HT may affect visual signals within these laminae is largely unknown. This study undertook to investigate the distribution of 2 types of 5-HT receptors in the SC and to ascertain their physiological effects on transmission of visual signals to the SC from the retinotectal and corticotectual pathways. Autoradiography of tissue sections exposed to [3H]-8-OH-DPAT (8-hydroxy-dipropylaminotetraline) or to [125I]cyanopindolol plus isoproterenol showed that 5-HT1A and 5-HT1B receptors, respectively, were present in the superficial SC layers. In unilaterally enucleated animals, binding of ligand to 5-HT1B receptors was greatly reduced on the deafferented (contralateral) side, which is consistent with the possibility that these receptors are located on preterminal axons. Binding to 5-HT1A receptors was unaltered by enucleation. In recordings of superficial layer neurons from SC slices, application of 5-HT during blockade of 5-HT1A receptors with spiperone reduced the amplitude of EPSPs evoked by stimulation of the optic tract. The 5-HT concentration for a 50% reduction in EPSP amplitude was 6 microM. Under these conditions, there were no significant alterations in either membrane potential or input resistance concurrent with 5-HT mediated reduction in EPSPs. During extracellular in vivo recordings, 5-HT, applied by iontophoresis or micropressure or by endogenous release produced by electrical stimulation of the dorsal raphé nucleus, strongly suppressed visual activity in SC neurons. The effectiveness of 5-HT application was significantly stronger on responses evoked by electrical stimulation of the optic chiasm (an average response decrement of 92.2%) than on these evoked in the same neurons by stimulation of visual cortex (an average response reduction of 32.3%). These results support the following conclusions. The 5-HT1B receptors are located preferentially on optic axon terminals and exert presynaptic inhibition of retinotectal inputs. Secondly, 5-HT1A receptors probably have a postsynaptic localization and may affect activity of SC neurons irrespective of the source of input. The combined effect of 5-HT at both subtypes would bias SC visual activity toward information received from the corticotectal pathway.

Subclassification of presynaptic alpha 2-adrenoceptors: alpha 2D-autoreceptors and alpha 2D-adrenoceptors modulating release of acetylcholine in guinea-pig ileum

The study was designed to classify in terms of alpha 2A, alpha 2B, alpha 2C and alpha 2D the presynaptic alpha 2-autoreceptors, as well as the alpha 2-receptors modulating the release of acetylcholine, in the myenteric plexus-longitudinal muscle (MPLM) preparation of the guinea-pig ileum. A set of antagonists was chosen that was able to discriminate between the four subtypes. Small pieces of the MPLM preparation were preincubated with 3H-noradrenaline or 3H-choline and then superfused and stimulated electrically. The stimulation periods used (3H-noradrenaline: 3 trains of 20 pulses, 50 Hz, train interval 60 s; 3H-choline: single trains of 30 pulses, 0.2 Hz) did not lead to alpha 2-autoinhibition or inhibition of 3H-acetylcholine release by endogenous noradrenaline. The alpha 2-selective agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) reduced the evoked overflow of tritium in both 3H-noradrenaline and 3H-choline experiments. Most (3H-noradrenaline) or all (3H-choline) of the 10 antagonists shifted the concentration-inhibition curves of UK 14,304 to the right. pKd values of the antagonists were calculated from the shifts. pKd values from 3H-noradrenaline experiments correlated with pKd values from 3H-choline experiments (r = 0.981). It is concluded that alpha 2-autoreceptors and alpha 2-heteroreceptors modulating the release of acetylcholine in the MPLM preparation are of the same subtype. Comparison with antagonist affinities for prototypic native alpha 2 binding sites, binding sites in cells transfected with alpha 2 subtype genes, and previously classified presynaptic alpha 2-adrenoceptors--all taken from the literature--indicates that both are alpha 2D.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of 5-HT1A and 5-HT4 receptor agonists on slow synaptic potentials in enteric neurons

Intracellular electrophysiological methods were used to examine the effects of 5-hydroxytryptamine (5-HT), 5-carboxamidotryptamine (5-CT), 5-methoxytryptamine (5-MeOT), 4-amino-5-chloro-2-methoxy-N-(4-[1-azabicyclo[3,3,1]nonyl]) benzamide hydrochloride (renzapride), cis-4-amino-5-chloro-N[1-[3- (4-fluorophenoxy)propyl]-3-methoxy-4-piperidinyl[-2-methoxybenzamide monohydrate (cisapride) and endo-N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-2,3-dihydro-3- (1-methyl)ethyl-2-oxo-1 H-benzimidazole-1-carboxamidehydrochloride (BIMU 8) on noncholineric slow excitatory postsynaptic potentials (slow EPSPs) in myenteric afterhyperpolarization (AH) neurons of guinea pig ileum. 5-HT (0.01-1 microM) and 5-CT (0.001-0.1 microM) produced a concentration-dependent inhibition of slow EPSPs. The 5-HT1A receptor antagonist 1-(2-methoxyphenyl)-4-[4-(2-phthalimidobutyl]piperazine (NAN-190) produced rightward shifts in 5-HT and 5-CT concentration-response curves; facilitation of slow EPSPs was never observed. 5-MeOT caused a depolarization and inhibited spike afterhyperpolarizations in a concentration-dependent manner but this effect was not blocked by the 5-HT3/5-HT4 receptor antagonist, tropisetron (1 microM). Renzapride (0.01-0.3 microM), cisapride (0.01-1.0 microM) and BIMU 8 (0.01-1.0 microM) did not change the membrane potential of any neuron tested. Renzapride and BIMU 8 did not change the amplitude of slow EPSPs. In 13 of 19 neurons cisapride did not change the amplitude of slow EPSPs; in 6 neurons cisapride (1 microM) reversibly inhibited the slow EPSP. Responses to substance P which mimicked the slow EPSP were not affected by cisapride.(ABSTRACT TRUNCATED AT 250 WORDS)

Prejunctional M1 and postjunctional M3 muscarinic receptors in the circular muscle of the guinea-pig ileum

The effects of subtype-selective muscarinic receptor antagonists on electrically evoked release of acetylcholine and muscle contraction were compared in circular muscle preparations of the guinea-pig ileum. Incubation of the preparation with [3H]choline resulted in the formation of [3H]acetylcholine. Electrical stimulation caused the release of [3H]acetylcholine which was abolished by tetrodotoxin and omission of calcium from the medium. 5-Hydroxytryptamine (10 microM) and the nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium (300 microM) did not change acetylcholine release. The muscarinic antagonists pirenzepine (M1 selective), AF-DX 116 (M2 selective) and hexahydrosiladifenidol (M3 selective) caused concentration-dependent increases in the evoked release of acetylcholine, and inhibitions of the circular muscle contraction. The postjunctional affinity constants (pA2 values) obtained for hexahydrosiladifenidol (8.06), pirenzepine (6.95) and AF-DX 116 (6.60) identified the muscular receptor as an M3 subtype. Pirenzepine was more potent in facilitating the evoked release than hexahydrosiladifenidol and AF-DX 116. These findings suggest that the release of acetylcholine in the circular muscle is inhibited by M1 muscarinic autoreceptors whereas muscle contraction is mediated by M3 receptors.

On the nature of the 5-HT receptor subtype inhibiting acetylcholine release in the guinea-pig ileum

1. The nature of the 5-hydroxytryptamine (5-HT) receptor subtype controlling acetylcholine release and contraction induced by stimulation of the neurokinin NK3 receptor has been studied in the longitudinal muscle-myenteric plexus preparation from guinea-pig ileum. 2. In preparations preloaded with [3H]-choline, the selective NK3 agonist, senktide, produced a concentration-dependent increase in tritium overflow, an index of [3H]-acetylcholine release. Low concentrations of neurokinin B, also markedly increased tritium efflux. 3. The senktide-induced acetylcholine release was markedly increased by the same concentration of methysergide and mesulergine. The 5-HT2A/2C agonist DOI (1 microM) inhibited the tritium overflow while 8-OH-DPAT, sumatriptan and ketanserin (1 microM each) were without effect on the senktide-induced tritium efflux. 4. The contractile response to senktide in the guinea-pig ileum was attenuated by atropine, 0.1 microM. Methysergide, a 5-HT1/2 receptor antagonist, and mesulergine, a 5-HT2A/2B/2C receptor antagonist, (1 microM each), enhanced the contractile effect of the NK3 receptor agonist. 5. It is concluded that the acetylcholine release induced by a NK3 receptor agonist is inhibited by stimulation of a 5-HT receptor, possibly of the 5-HT2C or 5-HT2C subtype.

SCH 23390 equivalently, but YM-09151-2 differentially reduces the stimulant effects of methamphetamine, MK-801 and ketamine: assessment by discrete shuttle avoidance in mice

The inhibitory actions of the selective dopamine D1-and D2-antagonists SCH 23390 and YM-09151-2, respectively, on the mouse's discrete shuttle avoidance were almost equipotent at doses ranging from 0.01-0.1 mg/kg. SCH 23390 reduced the stimulant action of methamphetamine (0.5 mg/kg), MK-801 (0.1 mg/kg) and ketamine (10 mg/kg) with a similar potency. YM-09151-2 also antagonized the actions of these drugs, with the following order of effectiveness: ketamine > MK-801 > methamphetamine. The present results indicate that methamphetamine, MK-801 and ketamine have different characteristics of CNS stimulant action through dopamine D2-receptors.

Receptors implicated in the actions of serotonin on chicken ileum longitudinal smooth muscle

The presence of serotonin (5-HT) in the chicken gastrointestinal tract has been previously reported, but its motor effects have been poorly described. The aims of this work were: A) to define the effects of 5-HT on chicken longitudinal ileum; B) to explore the mechanisms by which such effects occur and C) to identify the subtype(s) of 5-HT-ergic receptors implicated. The motor responses to 5-HT were assayed in vitro using ileal strips taken from male White Leghorn chickens 7-9 week old. 5-HT elicited ileal contraction (EC50 9.6 x 10(-8) M), which was markedly decreased in the presence of tetrodotoxin (TTX). Repeated exposure of the tissue to supramaximal concentrations of 5-HT did not however lead to desensitization. Atropine (10(-6) M), ketanserin (10(-5) M), methysergide (10(-5) M) and methiothepine (10(-6) M) attenuated the response to 5-HT. Ketanserin was an effective inhibitor of the residual response to 5-HT obtained even in the presence of TTX. Several serotonergic agonists were assayed to further analyse the type of receptors involved in the response to 5-HT. 5-methoxytryptamine (5-MOT), a mixed 5-HT1, 5-HT2 and 5-HT4 agonist, reproduced all the effects of 5-HT. 8-OH-DPAT, a selective 5-HT1A agonist, trifluoromethylphenylpiperazine, a mixed 5-HT1B/C agonist, and m-chlorophenylbiguanide, a 5-HT3 agonist, did not induce any consistent contractile effects. Sumatriptan, a 5-HT1D agonist, exerted a slight agonistic effect which was blocked by methiothepine and decreased by TTX but not by atropine. Cisapride, a 5-HT4 partial agonist in mammals, decreased the effects of both 5-HT and 5-MOT. These results indicate that chicken ileum contains 5-HT1 receptors similar to the 5-HT1D mammalian subtype but not the 5-HT1A, 5-HT1B, 5-HT1C or 5-HT3 subtypes. 5-HT2 receptors are also present and would appear to be located on smooth muscle.

Gastrointestinal effects of 5-hydroxytryptamine and related drugs

This paper deals with the effects of 5-hydroxytryptamine (5-HT; serotonin) and related drugs on the gastrointestinal tract (GIT). The nomenclature and classification of 5-HT receptors, as well as their putative role in the GIT are updated in this review. Besides its effects on the cardiovascular system, which have been extensively described, several lines of evidence suggest a role for 5-HT in regulating gastrointestinal functions. 5-HT is present in the gastrointestinal tissues, and can elicit contraction or relaxation by activation of a wide variety of mechanisms and receptors. At least four main types of receptors (5-HT1, 5-HT2, 5-HT3 and 5-HT4) have been described and all the four types seem to influence the GIT. In this respect, the 5-HT2, and in some cases the 5-HT1 receptors, appear to be present on the gastrointestinal smooth muscle, while 5-HT3 and 5-HT4 are mainly neuronal.

SDZ 205-557, a selective, surmountable antagonist for 5-HT4 receptors in the isolated guinea pig ileum

A selective antagonist for the recently characterized 5-HT4 receptor is lacking. The only surmountable antagonist available, ICS 205-930, is a weak antagonist and is far more potent at 5-HT3-than at 5-HT4 receptors. In this paper, SDZ 205-557 (2-methoxy-4-amino-5-chloro-benzoic acid 2-(diethylamino) ethyl ester) is characterized as the first potent, selective and surmountable antagonist at 5-HT4 receptors in the isolated guinea pig ileum. SDZ 205-557 was investigated in the non-stimulated and in the field-stimulated guinea pig ileum longitudinal muscle preparation for its affinity for 5-HT4-, 5-HT3-, muscarine-, nicotine- and histamine H1 receptors. The affinity for 5-HT1-, 5-HT2-, alpha 1-, alpha 2- and opiate (mu) receptors was determined by binding assays. SDZ 205-557 was devoid of substantial affinity (pKD values below 5.6) for all receptors investigated except for 5-HT3- and 5-HT4 receptors. At these two receptors, SDZ 205-557 acted as an antagonist without measurable intrinsic activity. At the 5-HT4 receptors of the non-stimulated guinea pig ileum, responses to 5-HT and 5-methoxytryptamine were antagonized by SDZ 205-557 with identical pA2 values of 7.4. The effect of renzapride was also blocked with no significant change in the maximum response; Schild analysis, however, revealed that the interaction was not competitive with an "apparent" pA2 value of 7.6. A pA2 of 6.8 was obtained using zacopride as a contractile agent; this value differed significantly from 7.4, the value obtained for 5-HT and 5-methoxytryptamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of 5-HT4 receptor stimulation on basal and electrically evoked release of acetylcholine from guinea-pig myenteric plexus

The effects of 5-methoxytryptamine and 5-hydroxytryptamine (5-HT) on both basal and electrically evoked outflow of tritium were studied in guinea-pig myenteric plexus preparations preincubated with [3H]-choline. Basal outflow. 5-Methoxytryptamine caused a transient and calcium-dependent increase in basal outflow of [3H]acetylcholine that was abolished by tetrodotoxin. Ondansetron (1 mumol/l) did not affect the stimulatory response of 5-methoxytryptamine but ICS 205-930 (1 and 3 mumol/l) produced parallel rightward displacements of the concentration-response curve to 5-methoxytryptamine. The pKB value for ICS 205-930 was 6.6 suggesting an involvement of 5-HT4 receptors. 5-HT caused an increase in basal outflow of [3H]acetylcholine and a biphasic concentration-response curve was obtained. The maximal response of the first phase to 5-HT (release of 0.98% of tissue tritium) and the maximal response to 5-methoxytryptamine (0.94% of tissue tritium) were similar but 5-methoxytryptamine (-log EC50: 6.9) was less potent than 5-HT (-log EC50 of the high affinity component: 7.9). ICS 205-930 (0.01-1.0 mumol/l) acted as a competitive antagonist against the low affinity component of the 5-HT concentration-response curve with a pA2 value of 8.0. It is concluded that stimulation of both 5-HT4 receptors (by 5-methoxytryptamine and submicromolar concentrations of 5-HT) and 5-HT3 receptors (by micromolar concentrations of 5-HT) causes a release of acetylcholine which in turn leads to smooth muscle contraction. Electrically evoked outflow. This outflow of [3H]-acetylcholine was concentration-dependently inhibited by both 5-methoxytryptamine and 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of non-classical 5-HT receptor in serotonin and cisapride induced secretion in hen colon

1. In hen colon 5-HT induces a tetrodotoxin-resistant, bumetanide-sensitive, chloride secretion, positively coupled with adenylate cyclase activity. 2. The 5-HT receptor mediating this response seems non-classical since it cannot be blocked by 5-HT1-like, 5-HT2 or 5-HT3 antagonists. 3. Effects are presented of new putative 5-HT agonists and antagonists on short circuit current and cord conductance in the hen colon, using the Ussing chamber technique. 4. The substituted benzamides, cisapride and BRL 24924, induced a dose-dependent short circuit currents but both with less potency than 5-HT. 5. Cisapride mediated this dose-dependent bumetanide sensitive response mainly by release of acetylcholine, since atropine reduced cisapride response by 70%. 6. Neither BRL 24924, 5-HTP-DP, ketanserin, ICS 205-930, prazosin, yohimbine, atropine nor piroxicam, covering the 5-HT1P, 5-HT2P, 5-HT2, 5-HT3, 5-HT4, adrenergic and muscarinic receptor types and the prostaglandin synthesis, altered 5-HT induced increases in short circuit current and cord conductance. 7. Results suggest (a) cisapride mediates it's response mainly by releasing acetylcholine, which then stimulates muscarinic receptors to release 5-HT. (b) Involvement of a non-classical 5-HT receptor subtype in 5-HT induced chloride secretion in hen colon.

Sigma receptor ligand N,N'-di-(ortho-tolyl)guanidine inhibits release of acetylcholine in the guinea pig ileum

The inhibition of stimulated contractions of the guinea pig ileum longitudinal muscle/myenteric plexus preparation by sigma receptor ligands has been previously described. In this study, the stimulated release of [3H]acetylcholine from cholinergic nerve terminals in this same preparation was monitored in the presence and absence of sigma receptor ligands. N,N'-Di-(orthotolyl)guanidine (DTG) and other compounds selective for the sigma receptor inhibited stimulated [3H]acetylcholine release. These results suggest that their inhibition of stimulated contractions in this preparation was mediated by inhibition of acetylcholine release.

Cisplatin increases the release of 5-hydroxytryptamine (5-HT) from the isolated vascularly perfused small intestine of the guinea-pig: involvement of 5-HT3 receptors

Isolated segments of the guinea-pig small intestine were vascularly perfused and the release of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) into the portal venous effluent determined by high pressure liquid chromatography with electrochemical detection. Release of acetylcholine from isolated superfused intestinal segments was determined as outflow of [3H]radioactivity from preparations preincubated with [3H]choline. Cisplatin (3 microM) increased the outflow of 5-HT and 5-HIAA by about 90%. At 30 and 100 microM cisplatin decreased the outflow of 5-HT and its metabolite by 40%-50%. The stimulatory effect of cisplatin was consistently observed only when the bicarbonate-phosphate buffer of the Tyrode's solution was replaced by HEPES-buffer. The stimulatory effect of cisplatin was abolished in the absence of extracellular calcium or presence of tetrodotoxin (1 microM). The stimulatory effect of cisplatin was also prevented by hexamethonium (100 microM) or scopolamine (100 nM). The 5-HT3 receptor antagonists ondansetron and ICS 205-930 in concentrations as low as 1 pM also abolished the stimulatory effect of cisplatin. The 5-HT3 receptor antagonist MDL 72222 prevented the stimulatory effect of cisplatin only at a concentration of 1 microM. None of the 5-HT3 receptor antagonists alone significantly altered the outflow of 5-HT and 5-HIAA. Cisplatin (3 microM) enhanced the outflow of [3H]radioactivity from intestinal segments and caused longitudinal muscle contractions that were abolished by 100 nM scopolamine. In conclusion, cisplatin, at concentrations which occur during anti-cancer therapy in humans and induce emesis, increases the release of 5-HT from the enterochromaffin cells of the small intestine of the guinea-pig.(ABSTRACT TRUNCATED AT 250 WORDS)

The differential contribution of endogenous prostaglandins to the release of acetylcholine from the myenteric plexus of the guinea-pig ileum

1. Prostaglandin E (PGE) may be essential for maintaining the sensitivity of the myenteric plexus of guinea-pig ileum to nicotine. The contributions of prostaglandins to nervous activity evoked by different stimuli have now been investigated by measuring the amount of acetylcholine (ACh) released from the myenteric plexus of the guinea-pig ileum. 2. The amount of ACh released in response to dimethylphenylpiperazinium (DMPP) or substance P was depressed to about 40% of control by 2.8 microM indomethacin (Ind), whereas the release of ACh induced by 5-hydroxytryptamine (5-HT) was not affected. The inhibitory effects of Ind were overcome by 14.3 nM PGE2. 3. Mepacrine 5 microM, an inhibitor of phospholipase A2, depressed the release of ACh in response to DMPP and substance P to the same extent as Ind. These inhibitory effects of mepacrine were overcome by arachidonic acid (10 microM), but not by arachidonic acid plus Ind. The release of ACh evoked by 5-HT or electrical field stimulation (EFS) was also inhibited to about 60% of control by mepacrine but these inhibitions were overcome by arachidonic acid (10 microM) either in the absence or the presence of Ind. 4. The results suggest that endogenous prostaglandins and arachidonic acid contribute to the maintenance of the excitability of the myenteric plexus by DMPP and substance P. By contrast, the release of ACh induced by 5-HT and EFS may be regulated by arachidonic acid and not by prostaglandins.

Receptors mediating the contractile effect of serotonin (5-HT) in the isolated common bile duct of guinea-pig

The contractile effect of 5-HT in the isolated common bile ducts of guinea-pigs was studied. 5-HT, administered non-cumulatively, evoked contractions which were concentration-dependent. Responses due to the low concentrations of 5-HT were antagonised significantly by ketanserin (10(-7) M and 10(-6) M) and methysergide (10(-6)M and 10(-5) M) whereas those induced by the higher concentrations of 5-HT remained unchanged. Atropine (3 x 10(-8) and 10(-7) M) and ICS 205-930 (10(-7) M and 10(-6) in contrast inhibited the contractions elicited by high concentrations of 5-HT without altering significantly the responses due to the lower ones. The results led us to conclude that 5-HT evoked contractions at low concentrations are predominantly mediated by 5-HT2 receptors whereas those at high concentrations are dependent on acetylcholine release via the stimulation of 5-HT3 receptors.

An examination of the 5-HT3 receptor mediating contraction and evoked [3H]-acetylcholine release in the guinea-pig ileum

1. The relative contributions of two classes of 5-hydroxytryptamine (5-HT) receptor (5-HT2 and 5-HT3) to the contractile action of 5-HT, 2-methyl-5-hydroxytryptamine (2-methyl-5-HT) and alpha-methyl-5-hydroxytryptamine (alpha-methyl-5-HT) were studied in the guinea-pig ileum longitudinal muscle-myenteric plexus strip (LMMP) preparation. Contractility studies were combined with an analysis of the effects of the three agonists on [3H]-acetylcholine ([3H]-ACh) release from preparations preincubated with [3H]-choline. 2. In contracting the LMMP, 5-HT was approximately one order of magnitude more active than 2-methyl-5-HT and alpha-methyl-5-HT, with relative activities for 5-HT: 2-methyl-5-HT: alpha-methyl-5-HT of 1.00: 0.13: 0.10. 3. Ketanserin (1 microM) was without effect on the concentration-response curves for concentration to 5-HT. 2-methyl-5-HT or alpha-methyl-5-HT, whilst ondansetron (GR38032F: 1 microM) produced a parallel rightward displacement of the upper part of the concentration-response curves to 5-HT and alpha-methyl-5-HT and of the entire curve to 2-methyl-5-HT. 4. In increasing the spontaneous release of [3H]-ACh from the LMMP, 5-HT was again approximately one order of magnitude more active than 2-methyl-5-HT and alpha-methyl-5-HT with relative activities for 5-HT: 2-methyl-5-HT: alpha-methyl-5-HT of 1.00: 0.19: 0.11. 5. Ondansetron (1 microM) greatly attenuated the increase in spontaneous [3H]-ACh release evoked by all three agonists. pKB estimates of 7.62 + 0.12 and 7.64 + 0.09 were obtained for ondansetron antagonism of 5-HT and 2-methyl-5-HT-evoked increases respectively. 6. These data suggest that the contractile action of 5-HT, 2-methyl-5-HT and a-methyl-5-HT in the guinea-pig ileum can, under these conditions, be accounted for largely in terms of 5-HT3 receptor activation. Estimates for pKB obtained with ondansetron are in accordance with those previously obtained from contractility studies in this preparation and these findings are discussed in terms of the postulated existence of subtypes of 5-HT3 receptors.

Characterization of 5-HT3 and 'atypical' 5-HT receptors mediating guinea-pig ileal contractions in vitro

1. Neuronal 5-hydroxytryptamine (5-HT) receptors mediating contraction of guinea-pig ileal segments have been characterized in vitro by the use of methysergide to block 5-HT1-like and 5-HT2 receptors. Concentration-response curves to 5-HT were biphasic (first phase, defined as those responses occurring between 1 nM and 0.32 microM 5-HT, -log EC50 = 7.15 +/- 0.08; second phase, defined as these responses occurring between 0.32 microM and 32 microM 5-HT, -log EC50 = 5.32 +/- 0.03) but monophasic to 5-methoxytryptamine (-log EC50 = 7.0 +/- 0.08) and 2 methyl 5-HT (-log EC50 = 5.2 +/- 0.13). The maximal response of the first phase to 5-HT and the maximal response to 5-methoxytryptamine were 30 +/- 4% and 35 +/- 5% respectively of the maximum response to the second phase of the 5-HT concentration-effect curve (set at 100%). In contrast, the maximal response to 2-methyl-5-HT equalled that obtained with 5-HT (second phase). 2. The responses comprising the second phase of the concentration-effect curve to 5-HT were antagonized by 1 microM ICS 205-930, ondansetron, granisetron, quipazine, N-methyl-quipazine and (R,S)-zacopride and the following pKB values, with 5-HT as the agonist, were obtained at the 5-HT3 receptor: ICS 205-930 7.61 +/- 0.05, ondansetron 6.90 +/- 0.04, granisetron 7.90 +/- 0.04, (S)-zacopride 8.11 +/- 0.06, (R,S)-zacopride 7.64 +/- 0.11, and (R)-zacopride 7.27 +/- 0.06. 3. Under conditions of 5-HT1-like, 5-HT2 and 5-HT3 receptor blockade, the following rank order of agonism was observed: 5-HT > 5-methoxytryptamine = renzapride > (S)-zacopride > (R,S-zacopride > 5-carboxamidotryptamine > BRL 24682 > (R-zacopride > metoclopramide > 2-methyl-5-HT > sulpiride. 8-Dihydroxydiphenylaminotetralin (8-OHDPAT), GR 43175, N,N-dipropyl-5-carboxamidotryptamine, ondansetron, ICS 205-930, granisetron, quipazine and N-methyl-quipazine were inactive as agonists and antagonists. Relative to 5-HT, (R,S)-zacopride acted as a partial agonist (intrinsic activity, alpha = 0.80; -log EC50 = 6.3 + 0.12; -log KA = 6.1 + 0.03) as did (R)-zacopride (alpha = 0.4, -log EC,0 5.7 + 0.08, -log KA = 5.5 + 0.11). (S)-zacopride acted as a full agonist (-log EC,0 = 6.9 + 0.03). ICS 205-930 (3 microM) antagonized competitively responses to 5-HT, 5 methoxytryptamine, (RS)- and (S)- zacopride and 5-carboxamidotryptamine yielding -log KB estimates ranging from 6.1-6.5. 4. It is concluded that two different 5-HT receptors mediate excitatory neuronal responses in the guineapig ileum. 5-HT3 receptors mediate the second phase of the biphasic concentration-response curve, whereas a receptor with properties distinct from the 5-HT1-like, 5-HT2 and 5-HT3 subtypes mediates the initial phase of the concentration-response curve. This receptor, which exhibits a close similarity to the 5-HT4 subtype is: (1) stimulated by 5-methoxytryptamine but not 2-methyl-5-HT; (2) stimulated selectively by certain substituted benzamides; (3) recognizes the optical isomers of zacopride and (4) is blocked by relatively high concentrations ICS 205-930 (pKB = 6.0-6.5) but not ondansetron, granisetron, quipazine or N-methyl-quipazine.

5-Methoxytryptamine and 2-methyl-5-hydroxytryptamine-induced desensitization as a discriminative tool for the 5-HT3 and putative 5-HT4 receptors in guinea pig ileum

Agonist-induced desensitization has been utilized to discriminate and independently "isolate" the neuronal excitatory receptors to 5-hydroxytryptamine (5-HT) in the guinea pig ileum (5-HT3 and putative 5-HT4 receptors). Electrically stimulated longitudinal muscle myenteric plexus preparations, and non-stimulated segments of whole ileum were used. Exposure to 5-methoxytryptamine (10 mumol/l) inhibited completely responses to 5-HT at the putative 5-HT4 receptor without affecting 5-HT3-mediated responses. Conversely, exposure to 2-methyl-5-HT (10 mumol/l) inhibited completely responses to 5-HT at the 5-HT3 receptor without affecting putative 5-HT4-mediated responses. The inhibition with 5-methoxytryptamine and 2-methyl-5-HT, either alone or in combination, appeared selective as responses to KCl, DMPP, carbachol, histamine, and substance P were unaffected or only very slightly modified. Furthermore, the pA2 values for ICS 205-930 at the putative 5-HT4 (pA2 = 6.2 to 6.5) and 5-HT3 (pA2 = 7.6 to 8.1) receptors (estimated in the presence of 2-methyl-5-HT and 5-methoxytryptamine, respectively) were consistent with those estimated in the absence of desensitization. 5-Methoxytryptamine, but not 2-methyl-5-HT, suppressed completely but reversibly the concentration-effect curve to renzapride, suggesting that responses to this agent are mediated exclusively via agonism at the putative 5-HT4 receptor. It is concluded that 5-methoxytryptamine and 2-methyl-5-HT can be utilized as selective probes to discriminate the putative 5-HT4 receptor from the 5-HT3 receptor in guinea pig ileum. This finding is of importance as no selective antagonist exists for the putative 5-HT4 receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation by reverse phase HPLC of [3H]acetylcholine release evoked from the myenteric plexus of the rat

Myenteric plexus-longitudinal muscle strips isolated from the small intestine of rats were incubated with [3H]choline to measure the synthesis and the release of [3H]acetylcholine. To separate different radioactive compounds (acetylcholine, choline, phosphorylcholine) from both the tissue and the overflow a new method, the reverse phase HPLC, was used. The radiochromatogram following the injection of a [3H]choline-standard and a [14C]acetylcholine-standard onto the HPLC showed a clear separation of both isotopes with a recovery rate of roughly 100%. Incubation of the muscle strips with [3H]choline caused the synthesis of [3H]acetylcholine (30,000 dpm/preparation) that increased 2-fold, when the electrical field stimulation during labelling was increased from 0.2 Hz to 1 Hz. Electrical field stimulation (3 Hz, 2 min) caused an increase in tritium efflux that was abolished by the removal of extracellular calcium or by the addition of tetrodotoxin. Analysis by reverse phase HPLC of the overflow showed that the stimulated increase in tritium overflow was balanced by the enhanced release of [3H]acetylcholine. whereas the overflow of [3H]choline was not affected by the electrical field stimulation. Oxotremorine (1 mumol/l) suppressed the release of [3H]acetylcholine by 60%. Scopolamine (0.1 mumol/l) prevented this inhibition and, given alone, enhanced the release of [3H]acetylcholine by 43%. The release of [3H]acetylcholine evoked at 0.2, 2 or 20 Hz did not consistently decline at increasing frequencies. The present experiments show the synthesis and the calcium-dependent release of [3H]acetylcholine from the myenteric plexus-longitudinal muscle preparation of rats correspondingly to the same in-vitro preparation isolated from guinea-pigs. Muscarinic autoinhibition operates also in the small intestine of rats.(ABSTRACT TRUNCATED AT 250 WORDS)

R 50 595, a selective non-competitive antagonist of cisapride, BRL 24924 and 5-hydroxytryptamine on the guinea-pig ileum

5-Hydroxytryptamine and substituted benzamides such as cisapride and BRL 24924 enhance the twitch responses of the electrically stimulated longitudinal muscle-myenteric plexus preparation of the guinea-pig. The effects of these benzamides and 5-HT could possibly be mediated via similar receptor-effector systems. The aim of our study was therefore to determine whether R 50 595, an analogue of cisapride devoid of intrinsic activity, could specifically interfere with the effects of cisapride and BRL 24924 and if so, whether it would also affect the responses to serotonin. R 50 595 had no effect on the twitch responses of the electrically stimulated preparation up to a concentration of 3 X 10(-7) M. Cisapride and BRL 24924 both enhanced the contractile response to electrical stimulation by a maximum of 37 +/- 7% at 3 X 10(-7) M for cisapride and 36 +/- 6% for BRL 24924, also at 3 X 10(-7) M. R 50 595 (10(-7)(-3) X 10(-7) M) antagonized the effects of cisapride and BRL 24924 in a non-competitive way. 5-HT enhanced the contractile responses by a maximum of 24 +/- 3.2% at 3 X 10(-8) M. The effects of 5-HT were completely abolished at a concentration of 3 X 10(-7) M R 50 595. R 50 595 also antagonized the effects of 5-HT in a non-competitive way.(ABSTRACT TRUNCATED AT 250 WORDS)

Dual effects of 5-hydroxytryptamine on the release of gamma-aminobutyric acid from myenteric neurones of the guinea-pig ileum

The effects of 5-hydroxytryptamine (5-HT) on the release of gamma-aminobutyric acid (GABA) were examined in the longitudinal muscle-myenteric plexus (LM-MP) preparation of guinea-pig ileum. 5-HT increased the spontaneous release and inhibited the electrically-evoked release of [3H]-GABA. The 5-HT-evoked release was Ca2+-dependent and tetrodotoxin-sensitive, and was antagonized by (3 alpha-tropanyl)-1H-indole-3-carboxylic acid ester (ICS 205-930), but not by methysergide and ketanserin. The inhibitory effect of 5-HT was antagonized by methysergide, but not by ketanserin and ICS 205-930. 8-Hydroxy-2-(di-n-propylamino)tetralin mimicked the inhibitory effect of 5-HT. Thus, 5-HT may exert an excitatory effect on the enteric GABAergic neurone via the 5-HT3 receptor and an inhibitory effect via the 5-HT1A receptor.

Effects of alaproclate, potassium channel blockers, and lidocaine on the release of 3H-acetylcholine from the guinea-pig ileum myenteric plexus

The guinea-pig ileum longitudinal muscle-myenteric plexus preparation, preincubated with 3H-choline or 3H-noradrenaline, was mounted in an organ bath and superfused with Tyrode's solution. Alaproclate (2-(4-chlorophenyl)-1,1-dimethyl 2-aminopropanoate) (0.01-0.5 mmol/l) reduced (IC50 = 0.1 mmol/l) and at about 0.5 mmol/l completely blocked the electrically evoked 3H-acetylcholine secretion. The depressing effect of alaproclate (0.2 mmol/l) was not counteracted by atropine (0.01, 1 or 10 mumol/l), hexamethonium (0.1 mmol/l), phentolamine (1 mumol/l) yohimbine (1 mumol/l), haloperidol (1 mumol/l), 8-phenyltheophylline (10 mumol/l), cyproheptadine (1 mumol/l), metitepine (1 mumol/l), bicuculline (10 mumol/l), picrotoxinin (0.1 mmol/l), forskolin (25 mumol/l), 3-isobutyl-1-methylxanthine (5 mmol/l), nifedipine (1 mumol/l), verapamil (1 mumol/l), dilitiazem (1 mumol/l), high calcium (6 mmol/l), high potassium (10 or 15 mmol/l), tetraethylammonium (2 mmol/l), 4-aminopyridine (0.5 mmol/l), apamin (0.5 mumol/l), barium (0.5 mmol/l) or quinine (0.1 mmol/l). Among the potassium channel blockers tested only quinine (at 0.5 or 1 mmol/l), in the same manner as lidocaine, reduced the evoked secretion of 3H-acetylcholine. The results are in agreement with the hypothesis that the effect of alaproclate on the evoked 3H-acetylcholine secretion is not mediated by a neurotransmitter receptor, or a potassium channel sensitive to tetraethylammonium, 4-aminopyridine, apamin, or barium or quinine, but is due to a local anaesthetic effect. In contrast to the evoked secretion, the spontaneous release of 3H-acetylcholine was enhanced by high concentrations of alaproclate (0.4-1 mmol/l). The mechanism underlying the effect of alaproclate on the spontaneous release remains to be established. Alaproclate (0.25 or 0.5 mmol/l) also enhanced the spontaneous release and reduced the electrically evoked 3H-noradrenaline secretion.

Effects of cromakalim on acetylcholine release and smooth muscle contraction in guinea-pig small intestine

The effects of the potassium channel opener cromakalim on smooth muscle contraction and 3H-acetyl-choline release were studied simultaneously in guinea-pig longitudinal muscle myenteric plexus preparations which had been preincubated with 3H-choline. Cromakalim (10 mumol/l) inhibited more markedly the smooth muscle contractions caused by the release of endogenous acetylcholine (via electrical stimulation or via activation of nicotine- and 5-HT3-receptors) than contractions induced by pilocarpine. Cromakalim (10 mumol/l) did not affect the release of 3H-acetylcholine evoked by electrical stimulation or by stimulation of nicotine- and 5-HT3-receptors. In contrast, the release of 3H-acetylcholine caused by stimulation of M1-receptors was concentration-dependently reduced by cromakalim (1-100 mumol/l). The results suggest that the relaxant effect of cromakalim on smooth muscle contraction is not caused by a reduction of acetylcholine release from myenteric neurones. An opening of cromakalim-sensitive potassium channels may be involved in the inhibition of the M1-receptor mediated acetylcholine release.

Effect of 5-hydroxytryptamine on [3H]-acetylcholine release from guinea-pig striatal slices

1. The effect of 5-hydroxytryptamine (5-HT) on spontaneous and electrically-evoked tritium efflux was studied in guinea-pig caudate nucleus slices preloaded with [3H]-choline. 2. 5-HT, 10-300 mumol l-1, temporarily increased the spontaneous tritium efflux (as well as the endogenous acetylcholine (ACh) release) and, after 15 min perfusion, inhibited it. The facilitatory effect of 5-HT on spontaneous efflux was increased while the inhibitory effect did not occur in slices taken from dopamine-depleted guinea-pigs. 3. The increase in spontaneous tritium efflux by 5-HT was blocked by methiothepin, methysergide (pA2 8.7) and by the selective 5-HT2 antagonist, ritanserin (pA2 6.7). 4. The inhibition of spontaneous tritium efflux by 5-HT was prevented by methysergide and methiothepin but not by ritanserin and (-)-propranolol. 5. 5-HT, 100 mumol l-1, inhibited the electrically-evoked tritium efflux and this effect was unchanged in dopamine-depleted slices. 6. The inhibition of electrically-evoked tritium efflux by 5-HT was blocked by methiothepin and methysergide but not by (-)-propranolol or ritanserin. 7. These results suggest that 5-HT may exert a rapid and transient (excitatory) and a more prolonged (inhibitory) control over striatal cholinergic neurones.

Selective and functional 5-hydroxytryptamine3 receptor antagonism by BRL 43694 (granisetron)

The activity of BRL 43694 (granisetron) was investigated using established models of 5-HT3 receptor activity. In guinea-pig isolated ileum, BRL 43694 antagonised the contractions evoked by relatively high concentrations of 5-HT (pA2 = 8.1 +/- 0.2). However, except in high concentrations, BRL 43694 did not affect the contractions of similar preparations of ileum, evoked by electrical field stimulation (cholinergically mediated), the nicotinic agonist dimethylphenyl piperazinium (DMPP) or by cholecystokinin octapeptide. Similarly, BRL 43694 did not affect electrically evoked, cholinergically mediated contractions of rat or human isolated stomach. In other models of 5-HT3 receptor activity (rabbit isolated heart, Bezold-Jarisch reflex in anaesthetised rats), potent antagonism by BRL 43694 was demonstrated. In radioligand binding studies on rat brain membranes, BRL 43694 had little or no affinity for 5-HT1A, 5-HT1B, 5-HT2 or for many other binding sites. BRL 43694 may therefore be a potent and selective 5-HT3 receptor antagonist.

Presynaptic regulation of the electrically evoked release of endogenous dopamine from the isolated neurointermediate lobe or isolated neural lobe of the rat pituitary gland in vitro

Isolated neurointermediate lobes (NILs) or isolated neural lobes (NLs) of the rat pituitary gland were incubated in Krebs-HEPES solution which contained pargyline and the dopamine uptake inhibitor GBR 12921. The release of endogenous dopamine was determined by HPLC with electrochemical detection. Electrical stimulation of the pituitary stalk induced a frequency-dependent release of dopamine. The release of dopamine from the combined NIL evoked by stimulation at 15 Hz was increased by 130% in the presence of the dopamine D2 receptor antagonist, (-)-sulpiride; the (+)-enantiomer of sulpiride had virtually no effect. When the stimulation frequency was 3 Hz (-)-sulpiride caused an increase in dopamine release by 230%. A similar increase was observed in the presence of domperidone, another dopamine D2 receptor antagonist. The dopamine receptor agonists, apomorphine and quinpirole, had no significant effects on the evoked release of dopamine indicating that under the present incubation conditions endogenous dopamine may have been maximally activating the autoinhibition. However, in the presence of 1 mumol/l (-)-sulpiride, apomorphine as well as quinpirole reduced the evoked release of dopamine in a concentration-dependent manner. The dopamine D1 receptor selective antagonist, SCH 23390, had no effect on the evoked release of dopamine at a concentration of 1 mumol/l. Only at a concentration of 10 mumol/l did SCH 23390 cause a small increase in dopamine release; this effect was, however, abolished in the presence of 1 mumol/l (-)-sulpiride.(ABSTRACT TRUNCATED AT 250 WORDS)

MDL 72832: a potent and stereoselective ligand at central and peripheral 5-HT1A receptors

In receptor binding assays (+/-)MDL 72832, 8-[4-(1,4-benzodioxan-2-ylmethylamino)butyl]-8-azaspiro++ +[4,5] decane-7,9-dione, was a potent (pIC50 9.1), selective and stereospecific ligand for central 5-HT1A recognition sites. In functional tests, (+/-)MDL 72832 and its S(-) and R(+) enantiomers blocked stereoselectively the 8-OH-DPAT-induced neuronal inhibition of the transmurally stimulated guinea-pig ileum and the cardiovascular effects of 8-OH-DPAT in anaesthetized rats. In contrast, (+/-)MDL 72832 and its enantiomers were exclusively '8-OH-DPAT-like' in their ability to fully and stereoselectively generalize to the 8-OH-DPAT discriminative stimulus and, in reserpinised rats, to induce forepaw treading and flat body posture. These results characterize (+/-)MDL 72832 as a potent, stereoselective ligand with mixed agonist and antagonist properties at central and peripheral 5-HT1A receptors. The similar stereoselective requirements for the recognition site and functional effects provides compelling evidence that the 5-HT1A recognition site is indeed a functional receptor.

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.

Electrophysiological characterization of functionally distinct 5-hydroxytryptamine receptors on guinea-pig submucous plexus

Intracellular recordings were made from neurons of the guinea-pig submucous plexus and the actions of 5-hydroxytryptamine on the postsynaptic membrane and on evoked synaptic potentials were examined. 5-Hydroxytryptamine produced two types of direct postsynaptic responses: (1) A depolarization associated with a fall in input resistance was observed in all cells. Voltage-clamp and ion substitutions showed that this depolarization resulted primarily from an inward sodium current. This response could be as brief as 30 ms; it showed desensitization and was selectively abolished by 0.2-2 microM ICS 205-930. (2) A depolarization (or inward current) associated with a decreased conductance was observed in about 50% of neurons, usually after the first response was blocked by ICS 205-930. This response was due to a decreased potassium conductance; the minimum time course of this response was 8-10 s. It did not show desensitization and was not sensitive to blockade by currently available antagonists of 5-hydroxytryptamine, nicotinic and/or muscarinic receptors. Higher concentrations of 5-hydroxytryptamine were required to produce the sodium conductance increase than the potassium conductance decrease; 2-methyl-5-hydroxytryptamine was equally effective in producing these responses. 5-Hydroxytryptamine also caused a barrage of "spontaneous" nicotinic excitatory post-synaptic potentials which were sensitive to tetrodotoxin. This response desensitized, was blocked by ICS 205-930 and is presumed to reflect excitation of other cholinergic cell bodies in the plexus by the sodium conductance increase mechanism described. The evoked nicotinic excitatory postsynaptic potential and the adrenergic inhibitory postsynaptic potential were decreased by 5-hydroxytryptamine; a portion of this inhibition showed desensitization and was blocked by ICS 205-930 as well as by the muscarinic receptor antagonists, atropine and pirenzepine. The ICS 205-930-insensitive portion of this inhibition could not be attributed to activation of 5-hydroxytryptamine-1 or 5-hydroxytryptamine-2 receptors. Thus, the following conclusions are drawn: 5-hydroxytryptamine excites submucous plexus neurons by activating two distinct 5-hydroxytryptamine receptors. Activation of the 5-hydroxytryptamine-3 receptor (sensitive to ICS 205-930) produces a depolarization mediated by an increased sodium conductance. The same effect occurring in other cholinergic cell bodies initiates action potentials which are responsible for the 5-hydroxytryptamine-induced release of acetylcholine.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional correlates of serotonin 5-HT1 recognition sites

A short overview is given of the evidence supporting the existence of subtypes of 5-HT1 receptors. As 5-HT1 receptors were first described using radioligand binding studies, a brief description of 5-HT1A, 5-HT1B, 5-HT1C and 5-HT1D receptor binding is given. Then, recent data obtained in biochemical, electrophysiological, behavioural and other functional studies is compared to results obtained in radioligand binding studies. The paper shows that functional correlates do indeed exist for subtypes of 5-HT1 recognition sites; moreover, these are markedly different from the effects mediated by 5-HT2 (5-HT D) or 5-HT3 (5-HT M) receptors.

Evidence for a neurotransmitter function of acetylcholine in rabbit superior colliculus

Acetylcholinesterase staining and studies on the uptake of [3H]choline into the subsequent efflux of tritium from collicular slices were carried out in order to provide evidence for a neurotransmitter function of acetylcholine in rabbit superior colliculus. Acetylcholinesterase staining was dense and homogeneous in superficial layers whereas the staining was arranged in patches with slightly higher density caudally than rostrally in the intermediate layers. The accumulation of tritium in slices incubated with [3H]choline depended on time, temperature and concentration, and was inhibited by hemicholinium-3. Accumulation was slightly higher in caudal than in rostral slices. Electrical stimulation enhanced tritium outflow from slices preincubated with [3H]choline. Tetrodotoxin and a low calcium medium inhibited the evoked overflow whereas hemicholinium-3 caused an enhancement. Oxotremorine decreased the evoked overflow; atropine prevented this effect. The opioids [D-Ala2, MePhe4, Glycol5]enkephalin, [D-Ala2, D-Leu5]enkephalin and ethylketocyclazocine caused an inhibition. The effects of the latter two agonists were antagonized by naloxone. The GABAB-receptor-agonist (-)-baclofen decreased the evoked overflow at lower concentrations than GABA, whereas the GABAA-receptor-agonist muscimol was ineffective. Serotonin produced an inhibition which was prevented by metitepin, alpha- and beta-adrenoceptor as well as dopamine-receptor ligands caused no change. It is concluded that in the rabbit superior colliculus the pattern of acetylcholinesterase staining is comparable, but not identical to the distribution in other species. The accumulation of [3H]choline, as well as the tetrodotoxin-sensitive and calcium-dependent overflow of tritium upon electrical stimulation (reflecting presumably release of [3H]acetylcholine) indicate that acetylcholine has a neurotransmitter function in this tissue. The release of [3H]acetylcholine was modulated by various transmitter substances and related compounds. The pattern of modulation of release differed from the pattern in other cholinergically innervated tissues.

The action of dazopride to enhance gastric emptying and block emesis

The substituted benzamide derivatives, dazopride and metoclopramide, enhanced field stimulation-induced contractions of guinea-pig stomach strips and gastric emptying in the guinea-pig after peripheral, intracerebroventricular and intrahypothalamic injection. In the isolated vagal nerve preparation from the rabbit, both compounds were shown to be 5-hydroxytryptamine M-receptor antagonists. Dazopride and metoclopramide were equipotent in antagonising cisplatin-induced emesis in the ferret, whereas metoclopramide was approximately 200 times more potent than dazopride in antagonising the emesis caused by the dopamine agonist 2-di-n-propylamino-5,6-dihydroxytetralin in the marmoset. In behavioural tests which indicate dopamine receptor antagonism in the rat, metoclopramide induced catalepsy, antagonised amphetamine-induced stereotypy and the hyperactivity induced by the intrastriatal injection of dopamine, caused body asymmetry on unilateral injection into the striatum and also antagonised apomorphine-induced climbing and circling behaviour in the mouse. In contrast, dazopride had little or no action in these tests and failed to displace [3H]spiperone in radioligand binding assays. The use of dazopride provides evidence to dissociate a dopamine receptor blockade from an ability to facilitate gastric emptying and to antagonise cisplatin-emesis, and indicates that antagonism of 5-hydroxytryptamine M-receptors is the essential basis of action for dazopride and plays an important role in the actions of metoclopramide.

Activation of a myenteric 5-hydroxytryptamine-like receptor by metoclopramide

Cholinergically mediated contractions were evoked by electrical field stimulation (EFS) of guinea-pig distal ileum longitudinal muscle-myenteric plexus strips. Metoclopramide, 0.1-100 microM, dose-dependently increased the contractions, probably by increasing acetylcholine (ACh) release; contractions evoked by exogenous ACh in the presence of tetrodotoxin were not increased by metoclopramide. 5-Hydroxytryptamine (5-HT) 0.3-30 nM similarly increased the height of the EFS-evoked contractions, although the maximum increase was less than for metoclopramide; higher concentrations of 5-HT (3 and 30 microM) had no effects or caused inhibition. The compound, ICS 205-930, 0.001-1 microM, had no effect on the EFS-evoked contractions and caused inhibition at higher concentrations. Preincubation of the tissues with 5-HT, 0.3-30 nM, did not affect the increase in EFS-evoked contractions caused by metoclopramide, 1 or 100 microM, whereas 5-HT, 3 and 30 microM, prevented the response caused by metoclopramide 1 microM, but not 100 microM. ICS 205-930, 0.1 microM, had no effect on the increase in EFS-evoked contractions caused by metoclopramide 1 or 100 microM. The drug, at least in low concentrations, may therefore increase cholinergically mediated contractions of guinea-pig ileum by stimulating 5-HT-like receptors within the myenteric plexus, which differ from those antagonized by ICS 205-930.

Increased gut cholinergic activity and antagonism of 5-hydroxytryptamine M-receptors by BRL 24924: potential clinical importance of BRL 24924

The mechanisms by which BRL 24924 ([(+/-)- (endo)])-4-amino-5-chloro-2-methoxy-N-(1-azabicyclo-[3.3.1]-non- 4-yl) benzamide hydrochloride stimulates gut motility and the relationships between BRL 24924 and 5-hydroxytryptamine (5-HT) receptors have been studied. In guinea-pig isolated ileum, BRL 24924 (10(-14)-10(-6) M) increased electrically-evoked, cholinergically-mediated contractions, probably by increasing acetylcholine (ACh) release. This action of BRL 24924 was prevented by the presence of high concentrations of 5-HT, but not by hexamethonium, phentolamine and propranolol, methysergide or ICS 205-930. The mechanism by which BRL 24924 can increase gut ACh release is not certain, but most likely involves activation of an enteric 5-HT receptor which differs from those 5-HT M-receptors antagonized by ICS 205-930 or by higher concentrations of BRL 24924 in other test systems. BRL 24924 antagonized 5-HT-evoked, cholinergically-mediated contractions of guinea-pig isolated ileum (pA2 = 7.56 +/- 0.12). Similar and higher concentrations of BRL 24924 did not antagonize contractions evoked by nicotinic receptor stimulation. In rabbit isolated heart, BRL 24924 1-10 nM reduced the tachycardia evoked by 5-HT. In anaesthetized rats, BRL 24924 0.3-83 nmol kg-1 i.v. antagonized the Bezold-Jarisch reflex evoked by 5-HT; the ID50 for BRL 24924 was 10.2 +/- 3.0 nmol kg-1 (3.7 +/- 1.1 microgram kg-1). A direct action of BRL 24924 on nerve function was excluded. In rat cortex, BRL 24924 10(-6) M did not displace [3H]-5-HT or [3H]-ketanserin binding to 5-HT1 and 5-HT2 receptors. The actions of BRL 24924 are discussed in terms of its potential clinical use as a stimulant of gastric motility and as a 5-HT M-receptor antagonist.

Increase by 5-hydroxykynuramine of spontaneous acetylcholine release from myenteric neurons: mediated by serotonin M receptors

The effects of 5-hydroxykynuramine (5-OH-K) and of 3-(2-amino-5-hydroxyphenyl)-propaneamine (AHPP) on spontaneous and electrically evoked release of [3H]acetylcholine were studied in the guinea-pig myenteric plexus longitudinal muscle preparation preincubated with [3H]choline. 5-OH-K caused a concentration-dependent increase in spontaneous [3H]acetylcholine release (EC50 5.3 microM). This effect was diminished in the presence of a desensitizing concentration of 5-hydroxytryptamine (5-HT). AHPP (1-100 microM) did not affect the spontaneous outflow of [3H]acetylcholine. The electrically evoked release of [3H]acetylcholine was significantly reduced in the presence of 100 microM of either 5-OH-K or AHPP. Metitepine did not antagonize the inhibition, which suggests that 100 microM of either compound non-specifically depressed the evoked release. It is concluded that 5-OH-K is inactive at the release-inhibitory 5-HT1 receptor, but is a potent agonist at the M receptor of myenteric neurons.

Modification of electrical field stimulation-induced contractions in the guinea-pig ileum by metoclopramide and ICS 205-930 depends on the integrity of the mucosa

Contractions induced by electrical field stimulation of guinea-pig ileum longitudinal muscle strips were enhanced by metoclopramide and ICS 205-930 at concentrations similar to those required to antagonize at 5-hydroxytryptamine 'M' receptors. The enhancement of contraction was observed in intact ileum strips but was not recorded in the longitudinal muscle myenteric plexus preparation or from the ileum with the mucosal layer removed. It is concluded that an intact mucosal layer is required for metoclopramide and ICS 205-930 to enhance electrical field stimulation-induced contractions of the guinea-pig ileum.