Proposals for the classification and nomenclature of functional receptors for 5-hydroxytryptamine

The pharmacological characterisation of pilocarpine-induced purposeless chewing behaviour in the rat

Purposeless chewing in rats was induced by the acute administration of the cholinergic agonist pilocarpine or by physostigmine. Pilocarpine-induced chewing was antagonised by the centrally acting anticholinergic drugs scopolamine, benzhexol and secoverine, but not by the peripherally acting anticholinergic drug methylscopolamine. Both benzhexol and secoverine caused dose-dependent inhibition of pilocarpine-induced chewing. The D-2 antagonist sulpiride and the D-1 antagonist SCH 23390 did not inhibit pilocarpine-induced chewing. The non-selective neuroleptics pimozide, trifluoperazine and thioridazine also were inactive. In contrast, clozapine caused a dose-related inhibition of pilocarpine-induced chewing. The alpha-1 antagonist prazosin, the alpha-2 antagonist idazoxan, the beta-antagonists propranolol and metoprolol and the H-1 antagonist mepyramine did not reduce pilocarpine-induced chewing. Purposeless chewing behaviour induced by pilocarpine was reduced in a dose-related manner by the administration of the 5-HT antagonists methiothepin and mianserin, but not by spiperone or ketanserin. These data confirm that pilocarpine-induced chewing behaviour in the rat is a model of central cholinergic activity, but suggest that a serotonergic component may be involved in the mediation of this behaviour.

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)

Behavioural evidence for simultaneous dual changes of 5-HT receptor subtypes: mode of antidepressant action?

Effects of the classic antidepressant imipramine and of an imipramine-like potential antidepressant dihydroergosine were studied in mice, rats and guinea pigs using behavioural models associated with the activation of 5-HT2 and 5-HT1 receptors respectively. Both drugs given in a single dose inhibited the 5-HT2 mediated behaviour for 24 and 48 h respectively and simultaneously stimulated 5-HT1 mediated behaviour for 6 days. Blockade of 5-HT2 receptors could have reduced their inhibitory influence on 5-HT1 receptors. We propose that the interplay between the two receptor subtypes controls the serotoninergic transmission. This idea throws a new light on the mode of action of antidepressants.

The 5-HT1A receptor probe [3H]8-OH-DPAT labels the 5-HT transporter in human platelets

The present study characterizes a serotonin (5-HT) binding site on human platelet membranes, using [3H]8-OH-DPAT as the radioligand. [3H]8-OH-DPAT binds specifically and saturably to a site on human platelet membranes with an average KD of 43 nM and Bmax of 1078 fmol/mg protein. Determinations of IC50 values for various serotonergic characterizing agents in platelets for displacement of [3H]8-OH-DPAT were performed. For example, 8-OH-DPAT 5HT1A had an IC50 of 117 nM; TFMPP 5HT1B (2.3 microM0 and PAPP 1A + 5HT2 (9 microM); ipsapirone 5HT1A (21.1 microM) and buspirone 5HT1A (greater than 100 microM); ketanserin 5HT2 (greater than 100 microM); 5-HT uptake inhibitors: paroxetine (13 nM); chlorimipramine (73 nM) and fluoxetine (653 nM). The pharmacological inhibitory profile of the platelet 8-OH-DPAT site is not consistent with profiles reported for brain. 8-OH-DPAT does not inhibit [3H]imipramine binding, however, it does inhibit [3H]5-HT uptake in human platelets near 5-HT's Km value (IC50 = 2-4 microM). These results suggest that the human platelet site labeled by [3H]8-OH-DPAT is pharmacologically different from the neuronal site and probably is a component of the 5-HT transporter.

Serotonin induced vasodilatation in the human forearm is antagonized by the selective 5-HT3 receptor antagonist ICS 205-930

The role of 5-HT3 receptors in the biphasic vasodilator response to serotonin (5-hydroxytryptamine; 5-HT) was investigated in the forearm of 7 young healthy volunteers (aged 22-32 years). Single dose infusions of 5-HT (1 ng/kg/min) and of acetylcholine (ACh, 500 ng/kg/min) were administered into the brachial artery. Subsequently combined infusions of 5-HT together with the selective 5-HT3 receptor antagonist ICS 205-930 (350 and 700 ng/kg/min), and ACh together with ICS 205-930 (700 ng/kg/min) were given. After a pause of at least 1 hour the single infusions of 5-HT and ACh were repeated. Subsequently, 5-HT and ACh were infused together with atropine (100 ng/kg/min). Forearm blood flow (FBF) was measured by R-wave triggered venous occlusion plethysmography. Heart rate (HR) and i.a. blood pressure (BP) were recorded semi-continuously. None of the drugs in the doses used did induce systemic hemodynamic effects. After an initial rapid transient increase in FBF of 316 +/- 55%, 5-HT elicited a persistent increase in FBF of 90 +/- 22% (mean +/- SEM, p less than 0.05 for both). ACh induced a monophasic vasodilatation of 475 +/- 123% (p less than 0.05). Both the initial transient and the persistent dilatator response to 5-HT were attenuated by ICS 205-930 350 ng/kg/min (p = 0.057, n = 5) and 700 ng/kg/min (p less than 0.05, n = 7). The highest dose of ICS 205-930 did not significantly influence the dilatator response to ACh. Atropine abolished the ACh induced vasodilatation (p less than 0.05), but did not influence the biphasic dilatator response to 5-HT. Thus the 5-HT induced biphasic vasodilatation was antagonized by ICS 205-930, indicating that this response was mediated by 5-HT3 receptor activation. The fact that atropine did not influence the vascular response to 5-HT suggests that 5-HT did not induce vascular relaxation indirectly by the release of ACh from cholinergic nerve endings.

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

1. 5-Hydroxytryptamine (5-HT) was applied by pressure ejection to coeliac ganglion cells of the guinea-pig maintained in vitro and responses measured intracellularly. 2. Cells responded in one of three ways to 5-HT: by (a) a fast, transient depolarization (43%), (b) a fast transient followed by a slow depolarization (biphasic response, 30%) or (c) a slow sustained depolarization (25%). 3. Fast depolarizations (response (a) above] were graded according to the duration of the ejection pulse. Maximal responses had a mean amplitude of 12 +/- 0.8 mV, a duration of 6.4 +/- 1.0 s, a latency of 0.4 +/- 0.1 s, were associated with a fall in membrane input resistance, increased in amplitude by hyperpolarization and probably mediated by an increased conductance to Na and K. The estimated reversal potential was -22.8 +/- 2.4 mV (n = 14). The maximal fast response seen in biphasically-responding cells (b) appeared similar to fast response (a). 4. Fast depolarizations (a) showed marked tachyphylaxis and were abolished by superfusion of the ganglion with 5-HT (100 microM). They were reduced in amplitude by tubocurarine (10-100 microM, pIC50 4.4), MDL 72222 (1-5 microM, pIC50 5.8), quipazine (1 microM reduced responses by 65 +/- 15%, n = 3), ICS 205-930 (1 microM reduced responses by 64 +/- 14%, n = 7) and metoclopramide (10 microM reduced responses by about 45%), but were unafected by methysergide (up to 1 microM) or hexamethonium (up to 1 mM). 5. Slow depolarizations (c) varied in amplitude with the duration of the ejection pulse. Maximal responses had a mean amplitude of 6.4 +/- 0.7 mV, a duration of 62 +/- 6 s, a latency of 3.5 +/- 0.8 s and were reduced in amplitude by methysergide (0.1-1 microM, pIC50 6.5) but not by MDL 72222 (1 microM). The maximal slow component in biphasically-responding cells (b) was similar in amplitude and duration to slow response (c), was partially blocked by methysergide (1-5 microM) in 4 of 6 cells and was enhanced by tubocurarine (50 microM) which reduced the fast component. 6. Slow depolarizations (b,c) were associated with either a small reduction or no change in membrane input resistance depending on the cell studied. Hyperpolarization had variable effects on slow depolarization amplitude. 7. It was concluded that the fast, phasic depolarization is mediated by an ionic mechanism and by receptors both of which are distinct from those involved in the slow depolarization. The receptor mediating the fast depolarization is a 5-HT3 receptor while that mediating the slow depolarization has yet to be identified.

Relationship of increased food intake and plasma ACTH levels to 5-HT1A receptor activation in rats

Various putative agonists of the 5-HT1A receptor subtype induce feeding in rats, probably by activating raphé somatodendritic 5-HT autoreceptors. These drugs also produce a marked increase in plasma concentrations of corticotropin (ACTH). In the present experiment we attempted to localize the site of action of 5-HT1A agonists on the secretion of ACTH and examined the relationship between 5-HT1A agonist-induced feeding and ACTH secretion. Rats were injected with either the high affinity 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) (0.016-1.0 mg/kg, s.c.) or the novel anxiolytics buspirone, gepirone or ipsapirone (2.0-16.0 mg/kg, s.c.), and either had their food intake measured 2 hr post injection or were sacrificed 30-40 min post injection for measurement of plasma ACTH. Plasma ACTH also was measured in rats pretreated with the serotonin synthesis inhibitor, para-chlorophenylalanine (PCPA) for three days (150 mg/kg, i.p. per day) and subsequently injected with 8-OH-DPAT (0.3 mg/kg, s.c.). As previously reported, the 5-HT1A agonists increased both food agonists increased both food intake and plasma ACTH concentrations. After 8-OH-DPAT, ipsapirone and gepirone the amount of food consumed was positively correlated with the concentration of plasma ACTH. No such correlation was evident following buspirone. PCPA pretreatment resulted in near total depletion of brain 5-HT content but had no effect on the ACTH rise induced by 8-OH-DPAT. Therefore, in contrast to the presynaptic site previously proposed for 5-HT1A agonist-induced feeding, the present results suggest a agonist-induced feeding, the present results suggest a postsynaptic location for the 5-HT1A receptor mediating ACTH release.

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.

Potential use of drugs modulating 5HT activity in the treatment of anxiety

1. It has been long suggested that central 5HT-mediated systems may be involved in modulation of anxiety and in the anxiolytic effect of benzodiazepines. However, recent evidence has questioned this hypothesis, particularly with respect to the mode of action of benzodiazepines. 2. Development of 5HT agonists and antagonists selective for different 5HT receptor sub-types (5HT1A, 5HT1B, 5HT2, 5HT3) has opened a new avenue for investigation of the potential role of 5HT in anxiety. 3. Buspirone is clinically active in the treatment of anxiety and it, and other anxiolytic candidates, gepirone and isapirone, may act as agonists (or perhaps partial agonists) on 5HT1A receptors. 4. The prototype 5HT1A agonist 8OH-DPAT may also have potential anxiolytic effects. 5HT1A agonists may act to suppress the activity of 5HT neurones as a major part of their action. 5. Although there is some supporting evidence, there is no clear indication of anxiolytic activity with agonists with some selectivity for 5HT1B sites (RU24969, mCPP, TMPP). 6. A selective 5HT2 antagonist, ritanserin, has anxiolytic effects in clinical studies but, like the 5HT1A agonists, does not show a similar profile to benzodiazepines in models of anxiety. 7. This raises the question of clinical predictivity of the various models used. 8. A recently developed 5HT3 antagonist, GR38032F, has been claimed to possess potential anxiolytic activity but its mode of action in this respect requires further elucidation.

The serotonin 5-HT2 receptor system, but not the alpha 1-adrenergic receptor system, is involved in the estrogen-induced afternoon prolactin surge in the rat

Ketanserin (Ket), a new serotonergic (5-HT2) antagonist, has recently been shown to block the estrogen-induced afternoon PRL surge (Endocrinology 120: 2070-2077, 1987). It is not certain, however, whether the effect of Ket was due to its serotonergic or adrenergic receptor antagonistic property. Another 5-HT2 receptor antagonist, LY53857, which possesses no alpha 1-adrenergic receptor affinity, as well as an alpha 1-adrenergic receptor antagonist, prazosin, were used in this study to further clarify the mechanism of 5-HT in the control of PRL secretion. Adult female Sprague-Dawley rats ovariectomized for 2-3 weeks and given a single injection of polyestradiol phosphate were studied 6 days later. Ket, LY53857 and prazosin were examined singly or in combination and animals were injected twice on the sampling day at 1200 and 1300h, respectively. The dosages were as follows: Ket and LY53857, 3 mg/kg BW, ip and 2 mg/kg BW, sc; prazosin, 1 mg/kg BW, ip and 0.7 mg/kg BW, sc. Blood samples were drawn from indwelling intraatrial catheters throughout the afternoon PRL surge.

Understanding anorexia in the elderly: formulating biopsychological research strategies

Anorexia in the elderly may arise from a disregulated system rather than from the operation of an isolated pathological mechanism. The properties of a disregulated system can be diagnosed through a functional analysis. Anorexia could arise through a weak read-out of metabolic signals of need rather than from hyperactive satiety agents. Serotonin systems may integrate regulatory actions.

Serotonin-induced pulmonary responses in the perfused guinea pig lung: evidence for 5HT2 receptor-mediated pulmonary vascular and airway smooth muscle constriction

The isolated perfused guinea pig lung was used to investigate pharmacologic characteristics of serotonin receptors in pulmonary vascular and airway smooth muscle. Serotonin caused a marked dose-related increase in pulmonary arterial pressure and peak intratracheal pressure when injected into the pulmonary artery. In contrast to serotonin, 5-carboxamidotryptamine, a 5HT1A and 5HT1B receptor agonist, and 2-methylserotonin, a 5HT3 receptor agonist produced only weak vascular and airway responses. Furthermore, vasoconstriction and bronchoconstriction elicited by serotonin were antagonized by the potent 5HT2 receptor antagonists, LY53857, ketanserin, and ritanserin. Antagonist specificity for 5HT2 receptors was demonstrated in the perfused guinea pig lung since similar responses induced by histamine were not blocked. High concentrations of serotonin were tachyphylactic on vascular but not airway constriction. Tachyphylaxis of vascular responses was not observed to an equipotent concentration of leukotriene D4 (LTD4). Thus, both the pulmonary vascular and airway constriction to serotonin were predominantly due to 5HT2 receptor activation. Furthermore, tachyphylaxis of vascular responses to serotonin might prove useful to differentiate 5HT receptor systems in pulmonary blood vessels from those in airways.

A 5-HT1-like receptor mediates a sympathetic ganglionic hyperpolarization

5-HT induced a hyperpolarization of the rat superior cervical ganglion in vitro which was resistant to both MDL 72222 (10 microM), a 5-HT3 antagonist, and ketanserin (1 microM), a 5-HT2 antagonist. The 5-HT1-selective ligands 5-carboxamidotryptamine and 8-OH-DPAT also hyperpolarized the ganglion. The 5-HT-induced hyperpolarization was potently antagonised by spiperone. These results suggest that 5-HT hyperpolarizes the rat superior cervical ganglion via a 5-HT1-like receptor which resembles the central 5-HT1A binding site.

The effects of intraperitoneal administration of antagonists and development of morphine tolerance on the antinociception induced by stimulating the anterior pretectal nucleus of the rat

1 The effects of intraperitoneal administration of antagonists to morphine, 5-hydroxytryptamine (5-HT), noradrenaline and dopamine have been studied on the antinociceptive effects of electrical stimulation of the anterior pretectal nucleus (APtN) of the rat. 2 A 15 s period of 35 microA sine wave stimulation of APtN significantly increased the latency of the tail flick reflex to noxious heat for periods up to 1 h. 3 Naloxone (0.25-1.0 mg kg-1) attenuated the effects of APtN stimulation in a dose-dependent manner. In rats made tolerant to morphine by daily administration of morphine, the antinociceptive effects of APtN stimulation were significantly reduced. 4 The 5-HT receptor antagonists methysergide (5 mg kg-1) and ketanserin (1 mg kg-1), the dopamine receptor antagonist haloperidol (1 mg kg-1) and the beta-adrenoceptor antagonist propranolol (1 mg kg-1) had little effect on the antinociceptive effects of stimulating the APtN. 5 alpha-Adrenoceptor antagonists caused a dose-dependent antagonism of the response. The order of potency was; idazoxan greater than prazosin greater than phenoxybenzamine, the respective ED50 for each drug being 0.08: 0.45: 1.5 mg kg-1. 6 It is concluded that antagonism at opioid receptors and alpha-adrenoceptors but not beta-adrenoceptors, dopamine or 5-HT receptors reduces the antinociceptive effects of APtN stimulation. This differs from the reported effects of these antagonists on the antinociception caused by stimulating other sites in the brain.

The modulation of excitatory amino acid responses by serotonin in the cat neocortex in vitro

1. The electrophysiological actions of excitatory amino acids and serotonin were investigated in slices from cat neocortex in vitro. Intracellular recordings were obtained from neurons (mainly in layer V) and the drugs applied extracellularly to the same neurons by microiontophoresis. 2. Serotonin, and to some extent noradrenaline, facilitated the excitatory actions of N-methyl-D-aspartate (NMDA), glutamate, and quisqualate but caused no changes in the passive neuronal membrane properties when presented alone. Serotonin had no effect on evoked excitatory postsynaptic potentials (EPSPs) or spike afterhyperpolarizations. 3. The facilitatory effect of serotonin on the responses to NMDA was observed with both somatic and dendritic applications. It persisted during Mg2+ depletion and in the presence of tetrodotoxin and tetraethylammonium. The effect was attenuated by the serotonin antagonist cinanserin but not by methysergide. A possible underlying receptor modulation is discussed.

Antiemetic properties of the 5HT3-receptor antagonist, GR38032F

GR38032F is a highly selective 5HT3-receptor antagonist which inhibits vomiting induced by cisplatin, cyclophosphamide or X-radiation in the ferret. Since cisplatin selectively increased the levels of 5HT and 5HIAA in the intestinal mucosa, a possible site of the antiemetic action of GR38032F may be on 5HT3-receptors on vagal afferents in the small intestine. The potent antiemetic action of GR38032F should be of clinical value in reducing the nausea and vomiting associated with radiotherapy or chemotherapy of cancer.

Pharmacological analysis of male rat sexual behavior

Pharmacological influences on male rat sexual behavior are reviewed in an attempt to identify neurotransmitters and their respective receptor types that regulate various factors comprising the behavioral pattern. Evidence is presented that: (1) serotonergic influence is generally inhibitory to sexual behavior, although two receptor subtypes may lower ejaculation threshold; (2) dopaminergic agonists facilitate several aspects of copulatory behavior and ex copula genital responses; (3) noradrenergic activity appears to increase sexual arousal; (4) cholinergic agonists facilitate ejaculation, or in some cases, delay or prevent initiation of copulation; (5) GABA agonists inhibit sexual responses both in and ex copula; (6) opiate agonists appear to inhibit copulation and penile reflexes, although antagonists have mixed effects; (7) ACTH and MSH peptides promote copulatory behavior and genital responses; (8) oxytocin facilitates ex copula penile responses, but may contribute to postejaculatory refractoriness; and (9) long-term exposure to prolactin inhibits sexual behavior and penile responses. Although some progress has been made in identifying neurotransmitter-receptor effects on behavioral components, copulatory behavior is complex and no drug has been found to affect only a single component. Furthermore, drug specificity is only relative.

Biochemical and behavioural studies on indole-pyruvic acid: a keto-analogue of tryptophan

The effects of acute or repeated administration of indole-pyruvic acid (IPA), a keto-analogue of tryptophan (TRP), were studied in various brain areas of rats by measuring the changes of 5-hydroxytryptamine (5-HT) and of norepinephrine (NE) content and metabolism. The analgesic and sedative properties of the molecule were evaluated by measuring the tail-flick latency, the spontaneous activity and the potentiation of the barbiturate-induced sleep. Acute or repeated administrations of IPA (20 or 50 mg/kg) increased the utilization of 5-HT in the cortex, hippocampus, diencephalon and brain-stem of rats fed a standard laboratory diet. IPA, however, did not substitute TRP in rats fed a TRP-free diet. The administration of this keto-analogue resulted also in a decreased content of 3-methoxy-4-hydroxy-phenylglycol (MHPG) in the cortex and in the brain-stem, thus suggesting a decreased utilization of NE in these areas. Furthermore, IPA administration decreased the rats' spontaneous activity, increased the duration of barbiturate-induced sleep and increased the tail-flick time, thus indicating that it has sedative and analgesic properties.

Neuronal involvement in type 1 hypersensitivity reactions in gut epithelia

1. Using a number of 5-hydroxytryptamine (5-HT)-antagonists we have compared their activity against the chloride-secretory response in guinea-pig ileal and colonic epithelia when challenged with 5-HT or antigen. Guinea-pigs sensitized to beta-lactoglobulin (beta LG) were used throughout; these were obtained by providing animals with cows' milk for drinking. 2. Of methysergide, ketanserin, cyproheptadine and ICS 205-930, only the latter inhibited both the response to 5-HT and to antigen challenge. Methysergide caused a minor, significant effect on 5-HT but not on beta LG responses. Ketanserin had no effect on the responses to 5-HT, but both ketanserin and cyproheptadine inhibited the challenge to beta LG. 3. The data are considered in relation to the current views of receptor subtypes for 5-HT. Some of the reported inhibitors may be non-specific, while we consider there is evidence to support the view that 5-HT3-receptors (neuronal receptors) are involved both in the responses to 5-HT and to antigen challenge. 4. Tetrodotoxin mimicked the effect of ICS 205-930 on both the response to 5-HT and to antigen challenge in sensitized tissues, confirming a neuronal involvement for both types of stimuli.

A thermodynamic study of 5-[3H]hydroxytryptamine binding to human cortex membranes

Kinetic and equilibrium measurements of [3H]-serotonin (5-hydroxytryptamine) binding to human frontal cortex membranes have been made between 4 and 30 degrees C. The effects of spiperone and ascorbate on binding have also been determined. Under the conditions used, binding was saturable and reversible. Affinity constants derived from kinetic and equilibrium data were comparable. Serotonin binding to several sites had substantial enthalpic as well as entropic components.

Characterization of presynaptic 5-HT receptors on adrenergic nerves supplying the bovine ovarian follicle

1. The effects of 5-hydroxytryptamine (5-HT) on contraction and release of [3H]-noradrenaline were investigated in vitro in bovine ovarian follicle strips. Using available selective agonists and antagonists, an effort was made to characterize the type of receptor mediating the inhibitory effect of 5-HT on neurogenic contraction and release of [3H]-noradrenaline by electrical field stimulation. 2. 5-Hydroxytryptamine inhibited the neurogenic contraction and release of [3H]-noradrenaline evoked by electrical field stimulation in a concentration-dependent manner. Like 5-HT, 5-carboxamidotryptamine (5-CT) and methysergide reduced the transmitter release as well as the neurogenic contraction, whereas 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) failed to inhibit both responses in concentrations up to 0.1 microM. 3. The 5-HT (1 microM)-induced inhibition of contractile responses was more evident during stimulation at low frequencies (4 and 8 Hz) than during high frequency electrical stimulation (16 and 32 Hz). 4. Methiothepin (1 microM) and methysergide (10 microM) significantly antagonized the inhibitory effect of 5-HT on the electrically evoked release of tritium, whereas cyanopindolol, MDL 72222 and ketanserin (all 0.1 microM) were without effect. In addition, ketanserin, MDL 72222, cimetidine, pyrilamine, atropine, propranolol and indomethacin were without effect on the 5-HT-induced inhibition of the neurogenic contraction. 5. It is suggested that 5-HT inhibits the electrically evoked transmitter release from adrenergic nerves in the bovine ovarian follicle wall via prejunctional 5-HT1-like receptors. This was based on the findings that 5-CT was a potent agonist, methiothepin an antagonist and the lack of effect of MDL 72222, cyanopindolol and ketanserin.

Relative selectivity of some conformationally constrained tryptamine analogs at 5-HT1, 5-HT1A and 5-HT2 recognition sites

In an attempt to define pharmacophoric differences between 5-HT1, 5-HT1A and 5-HT2 recognition sites, a number of rigid analogs were studied and compared to analogous free chain tryptamines. Racemic partial ergolines RU 27849 and RU 28306 showed reduced potency at all 5-HT1 sites, but were at least equipotent to analogous tryptamines at the 5-HT2 site. A nonergoline-like constrained analog of tryptamine was similar in potency to RU 27849 at all 5-HT1 sites, but showed a 4-fold enhancement in potency over RU 27849 and tryptamine at the 5-HT2 site. At all 3 sites, 3-(tetrahydropyridyl) indoles (unless substituted at the indole 2-position) were the most potent rigid analogs studied and represent the most promising class for the development of selective compounds.

Two distinct effects of 5-hydroxytryptamine on single cortical neurons

The ability of the indoleamine serotonin (5-hydroxytryptamine; 5-HT) to alter membrane characteristics of neocortical neurons was analyzed using intracellular recording techniques. The present study demonstrates that 5-HT primarily depolarized 68% of cortical neurons probably by decreasing a resting K+ conductance, an effect blocked by the antagonists ritanserin and cinanserin and apparently mediated by 5-HT2 receptors. A hyperpolarization associated with an increased conductance state and insensitive to 5-HT2 antagonists was observed in 26% of the neurons and could be mimicked by the selective 5-HT1A agonist (+/-)-8-hydroxy-2-(di-N-propyl-amino)tetralin (8-OH-DPAT). Therefore cortical pyramidal neurons contain at least two distinct functional 5-HT receptors whose activation produces opposing effects on membrane potential and conductance.

Senktide, a selective neurokinin B-like agonist, elicits serotonin-mediated behaviour following intracisternal administration in the mouse

Behavioural responses to tachykinins were observed following intracisternal administration in mice. The synthetic NK-3 agonists senktide and L-363,851 caused behaviour typically associated with serotonergic stimulation, including head twitches, reciprocal forepaw treading and hindlimb splaying. Neurokinin B produced some features of the serotonin (5-HT) syndrome, while substance P, neurokinin A and eledoisin failed to elicit any such behaviours. Senktide-induced head twitches were prevented by pretreatment with the 5-HT2 antagonists ketanserin and ritanserin, while forepaw treading was attenuated by the 5-HT1 antagonists (-)-pindolol and methysergide. These data suggest that NK-3 agonists interact with central 5-HT mechanisms.

Methiothepin enhances the potassium-evoked release of [3H]-noradrenaline in rat pineal gland

The 5-hydroxytryptamine (5-HT) autoreceptor antagonist methiothepin increased in a concentration-dependent manner the K+-evoked release of [3H]-noradrenaline in pineal glands from normal and parachlorophenylalanine (PCPA)-treated rats. However, 5-HT and the 5-HT receptor agonists, LSD and 5-methoxytryptamine, were inactive at modulating the K+-evoked release of [3H]-noradrenaline in pineal glands from normal and PCPA-treated rats. When tested on the uptake of [3H]-noradrenaline in the pineal gland, methiothepin was found to be a potent inhibitor (IC50 = 10.6 nmol/l). Exposure to methiothepin failed to increase the K+-evoked release of [3H]-noradrenaline when tested in the presence of cocaine. While the K+-evoked release of [3H]-noradrenaline was shown to be modulated through inhibitory presynaptic alpha 2-adrenoceptors in pineal glands from normal and PCPA-treated rats, no evidence was obtained for a presynaptic modulation through 5-HT receptors of [3H]-noradrenaline release. The facilitation by methiothepin of the K+-evoked release of [3H]-noradrenaline in rat pineal gland appears to be due to the inhibition of noradrenaline uptake by this compound.

Comparative analysis of two types of 5-hydroxytryptamine receptor mediating vasorelaxation: differential classification using tryptamines

Two receptors mediating relaxant responses to 5-hydroxytryptamine (5-HT) were studied comparatively in rings of rabbit jugular vein contracted with U-46619 (10 nmol/l). At low concentrations of 5-HT (0.001-0.1 mumol/l) vascular relaxation was mediated indirectly by the endothelial 5-HT receptor previously described by Leff et al. (1987). In preparations denuded of endothelium, higher concentrations of the amine (0.03-30 mumol/l) caused relaxation responses directly, presumably via a receptor located on the smooth muscle cells. Similarity between the receptors was evident in that both were susceptible to antagonism by methysergide, but resistant to blockade by ketanserin and MDL 72222. In these respects, the receptors qualified for a '5-HT1-like' classification. Consistent with this, 5-carboxamidotryptamine demonstrated a higher agonist potency than 5-HT at the receptor mediating relaxation directly. However, in endothelium-intact jugular vein rings this potency order was reversed, showing that the endothelial 5-HT receptor did not satisfy completely the criteria for a '5-HT1-like' designation. When the activities of a single set of tryptamines were compared in endothelium-intact and -denuded jugular vein rings, different affinity and relative efficacy estimates were obtained, confirming that two distinct 5-HT receptors mediate relaxation responses in this tissue. The most striking difference between these two receptor types was demonstrated by alpha-methyl-5-HT since it expressed a high affinity comparable to 5-HT at the endothelial receptor, but was inactive at the receptor in endothelium-denuded preparations at concentrations up to 30 mumol/l.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological characterization of 5-hydroxytryptamine-induced hyperpolarization of the rat superior cervical ganglion

1 A study has been made of the pharmacology of 5-hydroxytryptamine (5-HT)-induced hyperpolarization responses recorded extracellularly from the rat isolated superior cervical ganglion (SCG). 2 Hyperpolarization responses induced by 5-HT (1 X 10(-8)-1 X 10(-4) M) in the presence of MDL 72222 (1 X 10(-5) M) were not antagonized by phentolamine (1 X 10(-6) M), prazosin (1 X 10(-7)-3 X 10(-7) M), haloperidol (1 X 10(-6) M) or ketanserin (1 X 10(-7)-1 X 10(-6) M). However, the latter two compounds both potentiated and increased the persistence of the hyperpolarization induced by moderate to high concentrations of 5-HT. Spiperone (1 X 10(-7) M) caused similar effects. All further experiments were performed in the presence of ketanserin (1 X 10(-6) M) as well as MDL 72222. 3 8-Hydroxy-2(di-n-propylamino)-tetralin (8-OH-DPAT; 1 X 10(-7)-1 X 10(-4) M) and ipsapirone (3 X 10(-5)-3 X 10(-4) M) behaved as weak hyperpolarizing agonists on the SCG. However, at concentrations below those required to produce hyperpolarization, both compounds acted as unsurmountable antagonists of 5-HT-induced hyperpolarization. 4 5-Carboxamidotryptamine (5-CT; 1 X 10(-9)-1 X 10(-5) M) mimicked the hyperpolarizing activity of 5-HT on the SCG. The EC50 for 5-CT was approximately 9 fold lower than that for 5-HT. 5 Spiperone (1 X 10(-7) - 1 X 10(-5) M) behaved as a reversible competitive antagonist of hyperpolarization responses induced by 5-HT with a pKB value of 7.40 +/- 0.09. Spiperone (1 X 10(-7)-1 X 10(-6) M) also caused concentration-dependent rightward displacement of the 5-CT concentration-hyperpolarization response curve. In this case, the pKB was 7.80 +/- 0.05. 6 (+/-)-Cyanopindolol (3 X 10(-7)-3 X 10(-6) M) caused non-parallel rightward displacements of the 5-HT concentration-response curve. Against 5-CT, (+/-)-cyanopindolol (3 X 10(-7)-3 X 10(-6) M) caused a concentration-independent rightward displacement of the concentration-response curve, accompanied by a large increase in the maximum response. 5-CT-induced hyperpolarization recorded in the presence of (+/-)-cyanopindolol (3 X 10(-7) M) was not significantly antagonized by methiothepin (1 X 10(-6) M) or methysergide (1 X 10(-6) M). 7. It is concluded that 5-HT-induced hyperpolarization of the rat SCG is mediated via a 5-HT1-like receptor which resembles the 5-HT1A binding site. However, a lack of selective drugs precludes more definitive characterization of this receptor.

Inhibition of noradrenaline release via presynaptic 5-HT1B receptors of the rat vena cava

In the rat inferior vena cava preincubated with 3H-noradrenaline, the effects of nine serotonin (5-HT) receptor agonists and of eight antagonists (including two beta-adrenoceptor blocking agents) on the electrically evoked 3H overflow were determined. 1. 5-HT, 5-carboxamido-tryptamine, 5-methoxy-3(1,2,3,6-tetrahydropyridine-4-yl)-1H-indole (RU 24969), 5-methoxytryptamine, N,N-dimethyl-5HT, tryptamine and 5-aminotryptamine inhibited the evoked 3H overflow. The potencies of these agonists in inhibiting overflow were significantly correlated with their affinities for 5-HT1B binding sites, but not with their affinities for 5-HT1A, 5-HT1C or 5-HT2 binding sites. 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, and ipsapirone, a partial agonist at these receptors, did not inhibit overflow. 2. Cyanopindolol facilitated the evoked 3H overflow, an effect which was abolished by propranolol. The maximum inhibition of overflow obtainable with 5-HT was diminished by cyanopindolol. 3. The concentration-response curve for 5-HT was shifted to the right by metitepine, metergoline, quipazine, 6-chloro-2-(1-piperazinyl)pyrazine (MK 212) and propranolol which, given alone, did not affect 3H overflow. The apparent pA2 values of these antagonists tended to be correlated with their affinities for 5-HT1B (but not 5-HT1A, 5-HT1C or 5-HT2) binding sites. Ketanserin, a 5-HT2 receptor antagonist, and spiperone, which blocks 5-HT2 and 5-HT1A but not 5-HT1B or 5-HT1C receptors, failed to antagonize the effect of 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple mechanisms of serotonergic signal transduction

In this article we review serotonergic signal transduction mechanisms in the central and peripheral nervous systems and in a variety of target organs. The various classes of pharmacologically defined serotonergic receptors are coupled to three major effector systems: (1) adenylate cyclase; (2) phospholipase C mediated phosphoinositide (PI) hydrolysis and (3) ion channels (K+ and Ca++). Long term occupancy of serotonergic receptors also appears to induce alterations in mRNA and protein synthesis. For all three types of signal transduction there is evidence accumulating which suggests the involvement of guanine nucleotide regulatory proteins. Recent findings suggest that the distinct types of pharmacologically defined serotonergic receptors (5HT1A, 5HT1B, 5HT1c, 5HT2) may be coupled to one or more signal transduction systems. Thus, 5HT1 receptors may both activate and inhibit adenylate cyclase and increase K+-ion conductance in the hippocampus. 5HT2 receptors which activate PI hydrolysis in the brain, both open voltage-gated calcium channels and activate PI metabolism in certain smooth muscle preparations. Thus, each class of serotonergic receptor may be linked to one or more distinct biochemical transduction mechanisms. The possibility is raised that selective agonists and antagonists might be developed which have specific effects on a particular receptor-linked effector system.

Effect of 5-HT2-selective agonists on cat platelet aggregation

The effects of the 5-HT2-selective agonists 1-(4-bromo-2,5-dimethoxyphenyl)-2-aminopropane (DOB) and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on cat platelet aggregation were investigated and compared with those produced by serotonin (5-HT) and a positional isomer of DOB (i.e., isoDOB). Serotonin, DOB, and DOI enhanced the aggregation of platelets induced by a suboptimal concentration of ADP. This effect was completely inhibited by pre-incubation of the platelet suspension with the 5-HT2-selective antagonist ketanserin. IsoDOB, an isomer of DOB with a very low affinity for central 5-HT2 binding sites, was inactive in the platelet aggregation assay. The present results are consistent with the proposed role of 5-HT2 receptors in serotonin-induced platelet aggregation.

Comparison of effects of some 5-HT1 agonists on blood pressure and heart rate of normotensive anaesthetized rats

The present experiments served to compare the effects of the 3 5-HT1 agonists, 8-OH-DPAT, 5-MeODMT and TFMPP on the blood pressure and heart rate of normotensive anaesthetized rats. All the agonists induced, after i.v. injection, a decrease in blood pressure and heart rate. The hypotensive effects of 5-MeODMT and TFMPP were preceded by an increase, suppressed by both ketanserin and methysergide. The decrease in blood pressure induced by 5-MeODMT and 8-OH-DPAT was not antagonized by ketanserin, cocaine (and methysergide for 8-OH-DPAT) but was antagonized by methysergide (for 5-MeODMT) and spiroxatrine (for both). Bradycardia was not susceptible to ketanserin and cocaine (for 5-MeODMT) or to ketanserin and methysergide (for 8-OH-DPAT) but to methysergide and spiroxatrine (for 5-MeODMT) and cocaine and spiroxatrine (for 8-OH-DPAT). These results suggested that the hypotension and bradycardia induced by 5-MeODMT and 8-OH-DPAT are due to the stimulation of '5-HT1-like' receptors and probably to the 5-HT1A subtype; the 5-MeODMT-induced hypertension being ascribed to the stimulation of 5-HT2 receptors.

M-chlorophenylpiperazine increases blood pressure and heart rate in pithed and conscious rats

In pithed rats, m-chlorophenylpiperazine (m-CPP) produced marked, dose-dependent (ED50 = 0.18 mumol) increases in mean arterial blood pressure which peaked within 1 minute and were sustained over 15 minutes. Two serotonin antagonists, metergoline and ritanserin, completely blocked the pressor responses to 2.5 mg/kg m-CPP in pithed adrenal demedullated rats, while alpha-adrenergic blockade by prazosin plus yohimbine was without effect, suggesting that the doubling in blood pressure produced by m-CPP was mediated via serotonin receptors within blood vessels. Somewhat smaller increases in blood pressure over baseline values were observed after m-CPP administration to conscious, freely moving rats. A small but statistically significant increase in heart rate peaked 5 minutes after m-CPP and also was blocked by metergoline but was only minimally affected by ritanserin or the prazosin-yohimbine combination. These results with m-CPP support other evidence for two or more separable effects of serotonergic agonists on the peripheral cardiovascular system.

Effect of serotonin and serotonin analogues on passive membrane properties of lateral septal neurons in vitro

In this study we have tested the effect of serotonin (5-HT) and serotonin-analogues on passive membrane properties of septal neurons in a rat brain slice. Superfusion with 5-HT hyperpolarized the membrane potential and decreased the membrane resistance of lateral septal neurons in a concentration dependent manner. The lowest effective concentration was 1 microM while maximal effects were observed with 30 microM. Hyperpolarizing responses to 5-HT persisted in the presence of haloperidol, phentolamine or ritanserin. The 5-HTIa agonist 8-hydroxy-2-[n-dipropylamino]tetralin (8OHDPAT) hyperpolarized septal neurons like 5-HT though the agonist was active at a lower concentration than 5-HT and induced longer-lasting effects. Two other 5-HT1a analogues, TVX Q 7821 and buspirone, could also mimick the effect of 5-HT. The 5-HT1b agonist 1-(m-trifluoromethylphenyl)piperazine (TFMPP) did not appreciably affect the membrane potential or resistance of septal neurons. The present results are consistent with data from binding studies and suggest that the effect of 5-HT on lateral septal neurons is at least partly mediated through 5-HT1a-like receptors.

Identification of the 5-HT1A receptor binding subunit in rat brain membranes using the photoaffinity probe [3H]8-methoxy-2-[N-n-propyl, N-3-(2-nitro-4-azidophenyl)aminopropyl]aminotetralin

The synthesis of a tritiated derivative of the 5-HT1A photoaffinity probe 8-methoxy-2-[N-n-propyl, N-3-(2-nitro-4-azidophenyl)aminopropyl]aminotetralin ([3H]8-methoxy-3'-NAP-amino-PAT) allowed the use of this probe for attempting the irreversible labeling of specific binding sites in rat brain membranes. Sodium dodecyl-sulfate-polyacrylamide gel electrophoresis of proteins solubilized from hippocampal microsomal membranes that had been incubated with 20 nM [3H]8-methoxy-3'-NAP-amino-PAT under UV light revealed a marked incorporation of 3H label into a 63-kilodalton protein termed PI. As expected of a possible correspondence between PI and 5-HT1A receptor binding sites, 3H labeling by the photoaffinity probe could be prevented by selective 5-HT1A ligands such as 8-hydroxy-2-(di-n-propylamino)tetralin, ipsapirone, buspirone, and gepirone and by N-ethylmaleimide, but not by the 5-HT2 antagonist ketanserin, noradrenaline- and dopamine-related drugs, monoamine oxidase inhibitors, and chlorimipramine. Furthermore, the regional and subcellular distributions of PI were identical to those of specific 5-HT1A binding sites. These results indicated that the binding subunit of the 5-HT1A receptor is a 63-kilodalton protein with a functionally important sulfhydryl group(s).

Evidence that 5-hydroxytryptamine3 receptors mediate cytotoxic drug and radiation-evoked emesis

The involvement of 5-hydroxytryptamine (5-HT) 5-HT3 receptors in the mechanisms of severe emesis evoked by cytotoxic drugs or by total body irradiation have been studied in ferrets. Anti-emetic compounds tested were domperidone (a dopamine antagonist), metoclopramide (a gastric motility stimulant and dopamine antagonist at conventional doses, a 5-HT3 receptor antagonist at higher doses) and BRL 24924 (a potent gastric motility stimulant and a 5-HT3 receptor antagonist). Domperidone or metoclopramide prevented apomorphine-evoked emesis, whereas BRL 24924 did not. Similar doses of domperidone did not prevent emesis evoked by cis-platin or by total body irradiation, whereas metoclopramide or BRL 24924 greatly reduced or prevented these types of emesis. Metoclopramide and BRL 24924 also prevented emesis evoked by a combination of doxorubicin and cyclophosphamide. These results are discussed in terms of a fundamental role for 5-HT3 receptors in the mechanisms mediating severely emetogenic cancer treatment therapies.

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.