In vitro studies with ICI 169,369, a chemically novel 5-HT antagonist

Functional serotonin and histamine receptor subtypes in porcine ciliary artery in comparison with middle cerebral artery

Functional serotonin (5-HT) and histamine receptor subtypes were investigated in porcine middle cerebral and ciliary arteries. An H(1) antagonist, mepyramine, antagonized histamine-induced responses with pK(B) values of 8.91-9.10. In the presence of 1 muM mepyramine, however, histamine caused dilation through H(2) receptors in the middle cerebral but not in the ciliary artery. A 5-HT(2A) antagonist, ketanserin, antagonized 5-HT-induced responses, causing rightward shifts in the concentration-response curves with pK(B) values of 8.52-8.71. A 5-HT(1B) antagonist, SB224289, produced rightward shifts of the concentration-response curves to sumatriptan with pK(B) values (6.66) only in the middle cerebral artery. In contrast, a 5-HT(1D) antagonist, BRL15572, had no effect in either artery. An RT-PCR study demonstrated the gene expression of the mRNAs of all three receptors (5HT(1B), 5HT(1D) and 5HT(2A)) in both arteries. These results suggest that histamine-induced contraction is mediated only through functional H(1) receptor in these arteries. Interestingly, there are functional 5-HT(2A) and 5-HT(1B) receptor subtypes in the middle cerebral artery, whereas the only functional receptor is 5-HT(2A) in the ciliary artery. The difference may be important for treatment with 5-HT(1B/1D) agonists (e.g. for migraine) without ocular side effect.

The role of 5-HT on the cardiovascular and renal systems and the clinical potential of 5-HT modulation

The main peripheral sources of 5-hydroxytryptamine (5-HT) are as a neurotransmitter and local hormone in the gastrointestinal tract, and stored in circulating platelets and pulmonary neuroepithelial bodies. 5-HT has been shown to have many possible physiological and pathophysiological roles on the cardiovascular and renal systems. Thus, 5-HT may contribute to valvular heart disease, coronary artery disease, pulmonary hypertension, pulmonary embolism, pre-eclampsia, peripheral vascular disease and diabetic nephropathy. Consequently, modulators of the 5-HT system have diverse clinical potential. For instance, selective 5-HT subtype 3 receptor (5-HT(3)) antagonists may have potential in the treatment of the pain associated with myocardial infarction. MCI-9042 (sarpogrelate) or other 5-HT(2A) antagonists may have clinical potential for the treatment of vasospastic angina, ischaemic heart disease, reperfusion injury and hindlimb ischaemia. Several modulators of 5-HT (5-HT transporter inhibitors, 5-HT(1B) and (2B) antagonists) may have potential alone or in combination in the treatment of pulmonary hypertension. In hypertension, agonists at the 5-HT(7) and antagonists at the 5-HT(2B) may reduce blood pressure, and in diabetes, sarpogrelate may protect against nephropathy.

Smooth muscle 5-HT2A receptors mediating contraction of porcine isolated proximal stomach strips

1. The aim of this study was to characterize the 5-HT receptors involved in the 5-HT-induced contraction of longitudinal muscle (LM) strips of porcine proximal stomach. This was done in a classical organ bath set-up for isotonic measurement. 2. The concentration-contraction curve to 5-HT was not modified by 5-HT(3) and 5-HT(4) receptor antagonism. Methysergide, ketanserin and mesulergine antagonized the curve to 5-HT. Concomitantly, increasing concentrations of ketanserin and mesulergine progressively revealed a biphasic nature of the 5-HT curve. Ketanserin antagonized the low-affinity receptor while it did not modify the high-affinity receptor. 3. Tetrodotoxin did not influence the concentration-contraction curve to 5-HT neither in the absence nor presence of ketanserin, indicating that nerves are not involved. 4. Ketanserin competitively antagonized the monophasic concentration-response curve to alpha-Methyl-5-HT, yielding a Schild slope that was not significantly different from unity. After constraining the Schild slope to unity, a pK(B) estimate of 8.23+/-0.90 was obtained. This affinity estimate of ketanserin closely approximates previously reported affinities at 5-HT(2A) receptors. 5. In the presence of ketanserin (0.1 microM; exposing the high-affinity receptor), a wide range of 5-HT receptor antagonists covering all 5-HT receptors known, was tested. Only methysergide and ritanserin inhibited the response to 5-HT, thus expressing affinity for the high-affinity receptor. This did not reveal the identity of the receptor involved. 6 It can be concluded that 5-HT induces pig proximal stomach (LM) contraction via 5-HT(2A) receptors located on smooth muscle. A ketanserin-insensitive phase of contractions could not be characterized between the actually known classes of 5-HT receptors with the pharmacological tools that were used.

Pharmacological characterization of the 5-HT receptors mediating contraction and relaxation of canine isolated proximal stomach smooth muscle

1. We aimed to characterize 5-HT receptors mediating contraction and relaxation to 5-HT in dog proximal stomach longitudinal muscle (LM) strips. 2. Of the tryptamine analogues tested, 5-HT was the most potent contractile agent at basal length, while 5-CT was the most potent relaxant of PGF(2alpha)-induced contraction. Neither the contractions to 5-HT, nor the relaxations to 5-CT were influenced by tetrodotoxin, illustrating that action potential propagation is not involved. 3. The 5-HT-induced contraction was antagonized by mesulergine (0.03 to 0.3 microM) and ketanserin (2 - 20 nM), but the antagonism was not of a simple competitive nature, indicating multiple receptor involvement. Ketanserin (3 to 30 nM) and mesulergine (30 nM) competitively antagonized the alpha-Me-5-HT-induced contraction (pK(B): 8.83+/-0.09 and pA(2): 8.25+/-0.06 respectively). These affinity values are in line with literature affinities of ketanserin and mesulergine at 5-HT(2A) receptors in various bioassays. 4. The 5-CT-induced inhibition of PGF(2alpha)-induced contraction was competitively antagonized by mesulergine (pK(B) estimate: 8.52+/-0.12) and by the selective 5-HT(7) receptor antagonist SB-269970 (pK(B) estimate: 9.36+/-0.14). Both pK(B) estimates are in line with literature affinities of these compounds for 5-HT(7) receptors. Mesulergine (30 nM) and SB-269970 (10 nM) shifted the relaxant curve to 5-HT parallel to the right in the presence of ketanserin (0.3 microM) (pA(2) estimates of 8.08+/-0.10 and 8.75+/-0.14 respectively), indicative of 5-HT(7) receptor involvement. 5. It is concluded that 5-HT induces dog proximal stomach (LM) contraction via smooth muscle 5-HT(2A) receptors and relaxation via smooth muscle 5-HT(7) receptors.

Effects of ritanserin on haloperidol-induced dopamine (D2) receptor up-regulation in the rat

Rats were treated with vehicle, ritanserin (5 mg/kg, i.p.), haloperidol (1 mg/kg, i.p.) or both ritanserin and haloperidol for 19 days to determine whether chronic administration of the serotonin (5HT2) antagonist ritanserin affects D2 receptor up-regulation produced by haloperidol. Brain sections were prepared for D2 and 5HT2 receptor autoradiography with [3H]spiperone and [3H]ketanserin, respectively. Ritanserin significantly reduced 5HT2 receptors to 80% of vehicle in the sulcal area of the frontal cortex but had no effect on D2 receptors. Haloperidol significantly increased striatal and n. accumbens D2 receptors with no effect on 5HT2 receptors. In ritanserin/haloperidol-treated rats, D2 receptors were significantly increased along with significant decreases in 5HT2 receptors of the frontal cortex. These results suggest 5HT2 receptor antagonism by ritanserin does not significantly affect D2 receptor up-regulation produced by haloperidol.

YM-14673, a thyrotropin-releasing hormone analogue, injected into the nucleus accumbens and the striatum produces repetitive jaw movements in rats

Bilateral injections of the thyrotropin-releasing hormone (TRH) analogue, N alpha-[((S)-4-oxo-2-azetidinyl)-carbonyl]-L-histidyl-L-prolinamide dihydrate (YM-14673, 0.1 microgram and 1 microgram/0.2 microliters), into the nucleus accumbens, the dorsal and ventrolateral striatum produced repetitive jaw movements in a dose-dependent manner. The effects were greatest in the nucleus accumbens and smallest in the ventrolateral striatum. Pattern of the movements differed from that produced by injections of a mixture of SKF 38393 (5 micrograms) and quinpirole (10 micrograms); frequent tongue protrusions were evident in rats treated with the mixture but those were not seen in YM-14673-treated rats. TRH (1 microgram, 10 micrograms and 30 micrograms/0.2 microliters) did not evoke jaw movements from any of the sites. The non-selective dopamine receptor antagonist, cis-(Z)-flupentixol (10 micrograms), significantly reduced the response to administration of YM-14673 (1 microgram) into the nucleus accumbens or dorsal striatum, while the 5-hydroxytryptamine (5-HT)2A receptor antagonist, 2-(2-dimethylaminoethylthio)-3-phenylquinoline hydrochloride (ICI 169,369, 0.2 micrograms), did not affect the response to YM-14673 (1 microgram). Given intrathecally (0.5 microgram/5 microliters), both YM-14673 and TRH produced wet-dog shakes. Although the mechanisms giving rise to the display of jaw movements after intrastriatal injections of YM-14673 remain unknown, stimulation of the dopamine D1/D2 receptors may at least partly contribute to these effects. Anyhow, these mechanisms differ from that underlying the ability of YM-14673 and TRH to elicit wet-dog shakes, a mechanism that is known to involve serotonergic processes.

Clozapine injected into the nucleus accumbens potentiates apomorphine-induced jaw movements

The effects of clozapine injected into the nucleus accumbens on apomorphine-induced jaw movements were studied. Jaw movements induced by apomorphine (0.5 mg/kg i.v.) were potentiated by clozapine (10 micrograms/0.2 microliters) injected into the nucleus accumbens 10 min before apomorphine. Enhancement of the apomorphine-induced jaw movements was also found with the muscarinic acetylcholine receptor antagonist, methylscopolamine (2.5 micrograms), whereas the acetylcholine receptor agonist, carbachol (2.5 micrograms), inhibited the effects of apomorphine. Injection of a smaller dose of carbachol (0.1 microgram) alone into the nucleus accumbens 10 min before failed to alter the effects of apomorphine but prevented the potentiation induced by clozapine. Both the 5-hydroxytryptamine(5-HT)2A receptor antagonist, 2-(2-dimethylaminoethylthio)-3-phenylquinoline hydrochloride (ICI 169,369, 0.1 and 0.2 microgram), and the alpha 1-adrenoceptor antagonist, prazosin (0.05 and 0.2 microgram), failed to affect the effects of apomorphine(0.5 mg/kg i.v.). In contrast, clozapine (1, 5 and 10 micrograms), ICI 169,369 (0.1 and 0.2 microgram) or prazosin (0.05 and 0.2 microgram) given into the ventral striatum inhibited the effects of apomorphine (0.5 mg/kg i.v.). It is suggested that the clozapine-induced potentiation in the nucleus accumbens might be due to its antimuscarinic properties.

Further characterization of 5-hydroxytryptamine receptors (putative 5-HT2B) in rat stomach fundus longitudinal muscle

1. The present study was undertaken to isolate and characterize pharmacologically homogeneous populations of 5-hydroxytryptamine (5-HT) receptors from a possible mixed receptor population mediating concentration of the longitudinal muscle of rat stomach fundus. Our aim was to extend the pharmacological characterization of the 5-HT2B receptor which is reported to be expressed in this preparation. 2. To minimize spontaneous activity and any influence of circular muscle on the contractile response, narrow (1-1.5 x 20 mm) segments of mucosa-denuded longitudinal muscle were used. Under these conditions, blockade of monoamine oxidase with pargyline (100 microM for 15 min) caused a leftward displacement of concentration-effect curves for both 5-methoxytryptamine (5-MeO-T) and tryptamine. Neither pargyline nor a number of uptake inhibitors affected responses to 5-HT. 3. In pargyline pretreated preparations, the order of potency of a number of tryptamine analogues was as follows: 5-MeO-T > or = alpha-Me-5-HT > or = 5-HT > 5-carboxamidotryptamine (5-CT) > tryptamine > 2-Me-5-HT. In addition several ligands known to act as agonists at either 5-HT2A or 5-HT2C receptors including 1-m-chlorophenylpiperazine (m-CPP), Ru 24969, MK 212 and SCH 23390 were also agonists in rat fundus whilst sumatriptan, renzapride and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) were very weak or inactive. With the exception of 2-Me-5-HT and m-CPP, most agonists produced monophasic concentration-effect curves consistent with an interaction at a single site. High concentrations of 2-Me-5-HT evoked relaxations which were blocked by phentolamine (1 MicroM) suggesting an interaction with alpha-adrenoceptors. m-CPP often evoked biphasic concentration-effect curves with a second contractile phase which was insensitive to yohimbine at concentrations higher than required for antagonism of responses to 5-HT.4. LY 53857, methiothepin, methysergide, ritanserin and ICI 170809 were potent but non-surmountable antagonists of 5-HT in rat fundus. In contrast, several ligands behaved as surmountable antagonists with the following order of potency: rauwolscine >yohimbine = mesulergine > mianserin = SB 204070 >WY 26703 > SB 200646> pirenpirone> renzapride. DAU 6285, granisetron, spiperone, ketanserin,phentolamine and GR 127935 did not affect responses to 5-HT at concentrations up to 1 pM. The agonist and concentration independent profile of antagonism supported a single site interaction for both agonists and antagonists.5. We conclude that despite small differences concerning the enantiomeric selectivity and affinity of rauwolscine and yohimbine, the close pharmacological identity of 5-HT receptors in rat stomach fundus and the recently cloned 5-HT2B receptor is maintained. SB 200646, which demonstrates some selectivity for 5-HT receptors in rat stomach fundus, should provide a useful ligand for confirmation of this view and allow discrimination of 5-HT2B function both in vitro and in vivo.

Evidence that 5-HT2c receptor antagonists are anxiolytic in the rat Geller-Seifter model of anxiety

Four non-selective 5-HT2C/5-HT2A receptor antagonists, mianserin (2-8 mg/kg), 1-naphthyl piperazine (1-NP) (0.5-1 mg/kg), ICI 169,369 (20 mg/kg) and LY 53857 (5 mg/kg), increased punished responding for a food reward in the rat Geller-Seifter test 30 min after subcutaneous (SC) administration. This property was shared by the benzodiazepine anxiolytic chlordiazepoxide (5 mg/kg SC). However, the selective 5-HT2A receptor antagonists ketanserin (0.2-1 mg/kg SC) and altanserin (0.5, 1 mg/kg SC) had little effect. The 5-HT1A, 5-HT1B and beta-adrenergic receptor antagonists pindolol and cyanopindolol (6 mg/kg SC) did not affect punished responding either, nor did the 5-HT1D receptor partial agonist and alpha 2 adrenergic receptor antagonist yohimbine (2.5 mg/kg SC) or the histamine H1 receptor antagonist mepyramine (1 mg/kg SC). Unpunished responding was also modestly increased after some doses of the 5-HT2C/5-HT2A receptor antagonists. However, this effect was inconsistent and was also seen after chlordiazepoxide. Furthermore, it was not associated with the increase in punished responding observed in rats orally treated with mianserin (10, 20 mg/kg), 1-NP (10, 20 mg/kg) or ICI 169,369 (50 mg/kg). The action of the 5-HT2C/5-HT2A receptor antagonists tested is therefore consistent with anxiolysis. The results also strongly suggest that this effect is mediated by blockade of the 5-HT2C receptor, although the possibility of 5-HT2B receptor mediation is discussed.

Complex of rapid movements and vocalization induced by RX336-M in mice: possible relevance to Tourette's syndrome

RX336-M (7,8-dihydro-5',6'-dimethylcyclohex-5'-eno-1',2',8',14 codeinone) induced a complex of simultaneous rapid movements of body parts accompanied by a single squeak-vocalization, which occurred at irregular intervals, when injected i.p. in mice. The complex included whole body jerks and head-shakes and its frequency was dose dependent between 0.1 and 20 mg/kg. Frequency after a 5 mg/kg dose was reduced by haloperidol (0.5-10 mg/kg, i.p.), ritanserin (0.1-10 mg/kg, i.p.) and ICI 169,369 (2.0 mg/kg, i.p.), suggesting the possible involvement of 5-HT(2A) and/or dopamine receptors. The possible relevance of the behavioural effects of RX336-M to the tics and vocalization of Tourette's syndrome is discussed.

No effect of the 5HT2-antagonist ICI 169,369 on systolic, ergotamine-induced blood pressure changes in man

The 5HT-antagonistic effect of drugs in man is difficult to study because of side effects to 5HT. Ergotamine, however contracts human arteries, probably by acting on 5HT receptors. This effect can be antagonized in vitro by the 5HT antagonist ICI 169,369. After an initial ergotamine challenge to select responders to ergotamine, 10 selected volunteers were given in a double blind study an oral dose of either placebo, 30 mg or 120 mg of ICI 169,369. After 2 hr an intravenous dose of 0.5 mg ergotamine tartrate was given. From 6 to 8 hr after the administration, ergotamine caused a mean decrease in the toe-arm systolic gradient, measured by strain-gauge plethysmography of 32 mmHg (P < 0.001), which was not influenced by either 30 mg or 120 mg ICI 169,369 (P < 0.4). The most likely explanation for our inability to detect any effect of ICI 169,369 on blood pressure changes, induced by ergotamine may be high binding (99%) of ICI 169,369 to plasma proteins. There was, however, indication of a per se vasodilatory effect of ICI 169,369 since the dose of 120 mg increased the toe-arm systolic gradient by 4.7 mmHg.

ICI 169,369, A 5-HT2/5-HT1C antagonist, that can evoke endothelium-dependent relaxation in rabbit aorta

1. The direct effects of ICI 169,369 on vascular reactivity were investigated in rabbit aortic rings with and without endothelium. 2. ICI 169,369 evoked an endothelium-dependent relaxation in aortic rings precontracted with PGF2 alpha. No direct effects on vascular reactivity were found in endothelial denuded rings. 3. The relaxations induced by ICI 169,369 were inhibited by haemoglobin, an agent known to interfere with the responses to endothelium-derived nitric oxide (EDRF) but not by the cyclo-oxygenase inhibitor, indomethacin. Inhibition of the ICI 169,369-induced relaxation by the L-arginine analogue, NG-monomethyl L-arginine (L-NMMA) confirmed that the relaxations evoked by ICI 169,369 were mediated by the endothelial L-arginine: nitric oxide pathway. 4. Studies with competitive receptor antagonists showed that in the rabbit aorta, ICI 169,369 evoked relaxations which were not elicited by the activation of any known 5-HT1, 5-HT2, 5-HT3, muscarinic, histamine, adenosine receptor or adrenoceptors. 5. The diacylglycerol kinase inhibitor, R 59022 also failed to affect these relaxations. It is concluded that ICI 169,369 has a post-receptor action, possibly by directly affecting intracellular calcium levels in the endothelial cells.

Effects of chronic treatment with two selective 5-HT2 antagonists on sleep in the rat

The effect of chronic administration of 2(2-dimethylaminoethylthio)-3-phenylquinoline (ICI-169,369) and 2(2-dimethylamino-2-methylpropylthio)-3-phenylquinoline (ICI-170,809), two selective 5-HT2 antagonists, on sleep was studied in rats. As previously shown, the acute effect of ICI-170,809 was to increase latency to rapid eye movement sleep (REMS), decrease the number of REM periods (REMPs), suppress the cumulative amount of REMS over 12 h, and increase the duration of REMPs in the first 6 h, while having no effect on non-REM sleep (NREMS). Administration of ICI-169,369 had similar effects except no change was seen in the duration of REMPs and cumulative REMS was suppressed for 24 h. When given 2 x daily for 5 days, tolerance to the REMS suppressant effects developed in both drugs. After discontinuation of treatment, a REMS rebound occurred after ICI-170,809, but not ICI-169,369. No significant effect on NREMS was seen after administration of ICI-170,809, whereas ICI-169,369 lowered 24-h cumulative NREMS on the fifth day of administration.

Autoradiographic studies of RP 62203, a potent 5-HT2 receptor antagonist. In vitro and ex vivo selectivity profile

In this study, quantitative autoradiography was used to determine the selectivity of RP 62203, a novel naphtosultam derivative, for 5-HT2 receptors in vitro and ex vivo, using [125I]7-amino-8-iodo-ketanserin ([125I]AMIK) and [3H]mesulergine as radioligands. The density of [125I]AMIK or [3H]mesulergine binding sites was determined by quantitative image analysis. In in vitro experiments, RP 62203 displaced [125I]AMIK from 5-HT2 receptors with an IC50 of 0.21 nM in rat frontal cortex. Its affinity for 5-HT1C receptors was 100-fold lower (IC50 25 nM versus [3H]mesulergine in rat choroid plexus). RP 62203 showed moderate affinity for alpha 1-adrenoceptors in the rat thalamus (IC50 14 nM) and for histamine H1 receptors in the guinea-pig cerebellum (IC50 13 nM). The tetrabenazine sites were not affected by RP 62203 at a concentration of 30 nM. In ex vivo experiments, RP 62203 was about 4 times more potent than ritanserin in displacing [125I]AMIK from 5-HT2 receptors (ED50 0.58 mg/kg p.o.). A dose of 10 mg/kg of RP 62203 did not displace [3H]mesulergine from 5-HT1C receptors or [125I]AMIK from alpha 1-adrenoceptors and tetrabenazine sites in the rat brain and from histamine H1 receptors in the guinea-pig brain. These results demonstrate that RP 62203 specifically recognizes 5-HT2 receptors in rodent brain.

Functional activity of 5-HT2 receptors in the modulation of the sleep/wakefulness states

In the light of recent pharmacological investigations using agonists and antagonists that have potent actions on 5-hydroxytryptamine-2 (5-HT2) receptors, the possible functional role of 5-HT2 receptors in the modulation of the sleep/wakefulness states was examined. Data obtained from animals and from clinical studies suggest that serotonin may exert an inhibitory control on deep slow-wave sleep (SWS) through 5-HT2 receptors. In further investigations, the existence of a diurnal variation in the functional activity of 5-HT2 receptors, that depends on the day/night cycle and/or the melatonin rhythm, was revealed. Questions remain with regard to the physiological significance, of the 5-HT2 receptor-mediated deep SWS regulation, the anatomical site(s) of 5-HT2 receptors involved in this regulation and the mechanism underlying diurnal fluctuations in the functional activity of 5-HT2 receptors.

Contribution of antiplatelet activity to the effects of 5-HT2 receptor antagonists on reperfusion-induced arrhythmias in anaesthetized rats

The effects of certain 5-HT receptor antagonists were examined on ischaemia-induced and reperfusion-induced arrhythmias, on ex vivo platelet aggregation and on isolated cardiac muscle. Methiothepin (1 mg kg-1) reduced the total number of ischaemia-induced ventricular premature beats whereas ICI 170,809 (1 mg kg-1) reduced reperfusion-induced mortality to 10% compared with 70% in controls. ICI 169,369 did not significantly alter either ischaemia- or reperfusion-induced arrhythmias. High concentrations of both ICI 169,369 and ICI 170,809 caused reductions in the maximum driving frequency of isolated cardiac muscle but methiothepin had no significant effect. Administration of ketanserin, ritanserin, methiothepin or ICI 170,809, but not ICI 169,369, abolished the ability of 5-HT to enhance platelet aggregation. The results of these experiments suggest that the ability of 5-HT2 receptor antagonists to reduce reperfusion-induced arrhythmias may be related to their antiplatelet activity.

Molecular structural basis of ligand selectivity for 5-HT2 versus 5-HT1C cortical receptors

A molecular structural criterion of ligand selectivity for the 5-HT2 versus 5-HT1C receptor was hypothesized on the basis of radioligand binding data. Despite the large number of compounds which have been tested at both receptors, analysis of published data led to the identification of only five agents which are greater than 10-fold selective for the 5-HT2 versus the 5-HT1C receptor. Comparison of the two-dimensional structures revealed that, although these five compounds represent three distinct structural classes, they share a common structural feature located in the region hypothesized to be involved in receptor binding: a carbonyl or carboxyl oxygen interposed spatially between an aromatic ring and nitrogen atom. This structural feature was used to predict the relative selectivity of compounds that had not previously been analyzed at both the 5-HT2 and 5-HT1C receptors. All six drugs tested which contain the identified reactive carbonyl or carboxyl group were found to be selective for the 5-HT2 versus the 5-HT1C receptor with selectivity ratios ranging from 26 to 380. By contrast, three agents which are structurally similar but do not contain the reactive carbonyl or carboxyl group displayed equally high affinity for both receptor binding sites. Since the physiological roles of the 5-HT2 and 5-HT1C receptor are markedly different, it would be of potential clinical and scientific value to utilize this molecular structural feature to further identify chemical compounds which would selectively interact with only one of the two receptors.

The effects of the specific serotonin antagonist ICI 169,369 on the pituitary hormone response to insulin-induced hypoglycaemia in humans

OBJECTIVE

To examine the role of serotonin in pituitary hormone release by studying the effect of a specific 5HT2 receptor antagonist, ICI 169,369, on the ACTH, prolactin, growth hormone and AVP response to insulin-induced hypoglycaemia in healthy humans.

DESIGN

A double-blind, within-subject trial using a crossover design to compare the effect of placebo with two doses of ICI 169,369 on pituitary hormone responses to insulin-induced hypoglycaemia.

PATIENTS

Ten healthy subjects were studied in the low-dose (30 mg x 2) limb and 11 healthy volunteers in the high-dose (80 mg x 2) limb.

MEASUREMENTS

Plasma concentrations of prolactin, growth hormone, ACTH, cortisol and AVP, and blood glucose.

RESULTS

In the low-dose study, pretreatment with 30 mg ICI 169,369, 10 and 2 hours before the study, had no effect on the fall in blood glucose or the rise in plasma ACTH, prolactin, growth hormone, AVP or plasma cortisol following insulin injection, when compared with placebo. In the high-dose study the effect of a higher dose (80 mg) of ICI 169,369 on the pituitary hormone response to hypoglycaemia was compared with that of placebo. Although the fall in blood glucose was similar following drug (4.3 +/- 0.1 to 1.5 +/- 0.5 mmol/l, mean +/- SEM, P less than 0.001) and placebo (4.3 +/- 0.1 to 1.4 +/- 0.4 mmol/l, P less than 0.001), the rise in plasma AVP was lower (P less than 0.05) following pretreatment with drug (0.5 +/- 0.2 to 2.1 +/- 0.6 pmol/l, P less than 0.05) than with placebo (0.7 +/- 0.2 to 3.4 +/- 0.9 pmol/l, P less than 0.01).

CONCLUSIONS

The ACTH, prolactin, growth hormone and cortisol responses were unaffected by ICI 169,369. The data are compatible with an inhibitory effect of the serotonin antagonist ICI 169,369 on the AVP, but not the ACTH, prolactin or growth hormone response to insulin-induced hypoglycaemia in humans.

The effects of a selective 5-HT2 receptor antagonist (ICI 170,809) on platelet aggregation and pupillary responses in healthy volunteers

1. ICI 170,809 (2-(2-dimethylamino-2-methylpropylthio)-3-phenylquinoline hydrochloride) is a potent 5-hydroxytryptamine (5-HT) type 2 postsynaptic receptor antagonist. 2. Effects of ICI 170,809 as single oral doses (3, 7, 15 and 30 mg) or placebo were studied on the duration of antagonism for the ex vivo platelet aggregatory response to 5-HT and to the pupillary light constrictor response in eight healthy male volunteers. 3. Pupillary dark adapted responses to a 0.5 s light stimulus were measured using a portable infrared pupillometer, for up to 24 h after dosing. 4. The in vitro platelet 5-HT aggregation response was reduced by ICI 170,809, with depression of the dose-response curve to 5-HT at all concentrations of 5-HT and with no evidence for a parallel shift. 5. The ex vivo platelet 5-HT response demonstrated a dose related significant (P less than 0.02) decrease in aggregation reaching a maximum at 2 h after dosing with the effect persisting for at least 8 h after dosing with the 7 and 15 mg doses. 6. Resting pupil diameter (RPD), and light induced pupillary responses in the dark adapted pupil, showed a significant (P less than 0.01) dose related reduction with significant (P less than 0.05) effects still present with the 15 and 30 mg doses at 8 h after dosing. 7. We conclude that, changes in both ex vivo platelet aggregation to 5-HT and dark adapted pupil size, are significantly correlated (P less than 0.0001) with log plasma concentrations (ng ml-1) of ICI 170,809, enabling the assessment of 5-HT2-receptor antagonism in man.

Centrally active 5-HT receptor agonists and antagonists

Eleven subtypes of central 5-HT receptor have so far been postulated, four of which have been cloned (5-HT1A, 5-HT1C, 5-HT1D and 5-HT2) and a fifth (the 5-HT3 receptor) purified. The present review discusses the agonists and antagonists which act at these subtypes with respect to their degree of selectivity and in vivo potency. Selective agonists exist for the 5-HT1A, 5-HT1B and 5-HT3 receptors and selective antagonists for the 5-HT2 and 5-HT3 receptors.

Endocrine and receptor pharmacology of serotonergic anxiolytics, antipsychotics and antidepressants

Several classes of drugs that modify serotonin (5-HT) neurotransmission are either currently used, or are being evaluated for their potential use in the treatment of anxiety, schizophrenia, and depression. 5-HT1A agonists are considered potential anxiolytics, while some atypical antipsychotics are potent 5-HT2 antagonists (and also have modest dopamine D2 affinity). Furthermore, there is a diverse group of serotonergic drugs that may be effective antidepressants. Secretion of ACTH, corticosterone/cortisol, prolactin, renin, oxytocin and vasopressin are stimulated by activation of different 5-HT receptor subtypes, while other neurotransmitter receptors also influence the secretion of these hormones. We compared the receptor binding profiles of 5-HT anxiolytics, antipsychotics and antidepressants with their endocrine effects. These comparisons could aid in understanding both the therapeutic and side effects of these drugs.

5-HT1C receptor antagonists have anxiolytic-like actions in the rat social interaction model

The effects of a range of 5-HT receptor antagonists were examined in an animal model of anxiety--the social interaction test. Six antagonists with high affinity for 5-HT1C receptors; mianserin, (+) mianserin, 1-naphthyl piperazine, ICI 169 369, pizotifen and LY 53857 all increased the time spent in active social interaction by pairs of weight-matched rats under high light unfamiliar conditions. As locomotion was only increased by 1-NP and then only at high doses, the effect of the drugs is consistent with anxiolysis. These properties were shared by the benzodiazepine anxiolytic chlordiazepoxide but not by the specific 5-HT2 antagonists ketanserin and altanserin, nor by the 5-HT1A and 5-HT1B antagonists cyanopindolol and pindolol. Similarly, neither the adrenergic alpha 2 antagonist idazoxan, the alpha 2 antagonist and putative 5-HT1D partial agonist yohimbine nor the H1 antagonist mepyramine had any significant effect. Since (+)mianserin, LY 53857 and ICI 169 369 at least have low affinity for 5-HT3 receptors these receptors are also unlikely to be involved. The results therefore imply that the observed anxiolytic effects of the drugs are likely to be mediated by 5-HT1C receptor blockade.

Pharmacological characterization of RP 62203, a novel 5-hydroxytryptamine 5-HT2 receptor antagonist

1. RP 62203 (2-[3-(4-(4-fluorophenyl)-piperazinyl)propyl]naphto[1,8- ca]isothiazole-1,1-dioxide) is a novel naphtosultam derivative which shows very high affinity for 5-HT2 receptors in the rat cerebral cortex (Ki = 50.0 pM). 2. RP 62203 is relatively selective for this sub-type of 5-hydroxytryptamine (5-HT) receptor, having lower affinity for the 5-HT1A receptor and very low affinity for the 5-HT, receptor. RP 62203 displayed low to moderate affinity for alpha 1-adrenoceptors, dopamine D2 receptors and histamine H1 receptors. 3. In vivo binding experiments demonstrated that oral administration of low doses of RP 62203 led to a long-lasting (greater than 6 h) occupation of cortical 5-HT2 receptors (ID50 = 0.39 mgkg-1). 4. In cortical slices from the neonatal rat, RP 62203 potently inhibited inositol phosphate formation evoked by 5-HT, with an IC50 of 7.76 nM. 5. The activity of neurones in the raphé and their responses to microiontophoretically applied 5-HT were studied with extracellular recording electrodes in the anaesthetized rat. RP 62203 potently and dose-dependently blocked excitations evoked by 5-HT when administered at doses of 0.5-4.0 mg kg-1, i.p. In contrast, neither 5-HT-evoked depressions nor glutamate-evoked excitations of raphé neuronal firing were blocked by RP 62203 at doses as high as 8.0 mg kg-1, i.p. 6. Head twitches induced by 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) could be abolished by low doses of RP 62203 in mice (ED50 = 0.44 mg kg-1, p.o.) and in rats (ED50 = 1.54 p.o.). Similar results were obtained with mescaline and 5-hydroxytryptophan (5-HTP). 7. The potency of RP 62203 was compared with that of three other 5-HT2 receptor antagonists, ritanserin, ICI 169,369 and ICI 170,809. In all models, RP 62203 showed similar activity to ritanserin, whilst either ICI 169,369 or ICI 170,809 was several fold less active. 8. It is concluded that RP 62203 is a potent and selective antagonist at 5-HT2 receptors in the rodent central nervous system.

Differential effects of novel ligands for 5-HT receptor subtypes on nonopioid defensive analgesia in male mice

The effects of a number of 5-HT receptor ligands were examined on nonopioid defensive analgesia in male DBA/2 mice. MDL 73005EF (0.05-1.0 mg/kg), a selective 5-HT1A receptor agonist, potently and dose-dependently inhibited the analgesic consequences of social defeat. CGS 12066B (0.5-10.0 mg/kg) and MK-212 (0.3-10.0 mg/kg), selective agonists for 5-HT1B and 5-HT1C sites, respectively, failed to influence this particular form of adaptive pain inhibition. Two 5-HT2/1C receptor antagonists, ritanserin (0.05-10.0 mg/kg) and ICI 169.369 (0.3-10.0 mg/kg), were also devoid of specific effects upon defensive analgesia. Both ritanserin and ICI 169,369 were found to have intrinsic analgetic efficacy and to induce behavioural changes indicative of increased defensiveness. These data, together with previous findings, confirm the specific involvement of 5-HT1A receptor mechanisms in the analgesic consequences of social defeat in male mice. Results are discussed in relation to the role of anxiety in adaptive pain inhibition.

The effects of a 5-HT2 receptor antagonist (ICI 169,369) on changes in waking EEG, pupillary responses and state of arousal in human volunteers

1. ICI 169,369 (2-(2-dimethylamino ethylthio)-3-phenyl quinoline) is a potent selective competitive antagonist of the 5-HT2 receptor in animal models. Effects of ICI 169,369 as single oral doses (80 and 120 mg) separated by 1 week, on the power spectrum of waking EEG, dark adapted pupil responses and sedation score, were studied in a double-blind, placebo controlled, randomised cross over within subject comparison, in six healthy male volunteers. 2. Pupillary responses were measured using a portable infrared pupillometer following 15 min dark adaptation, assessing resting vertical pupil diameter (RPD), light constricted diameter (MPD) and recovered final diameter (FPD) at the end of a 3 s measurement cycle. 3. Both doses of ICI 169,369 produced a mean 36% (range 10-54%) decrease in log 10 power of the waking EEG alpha activity with eyes closed (P less than 0.02), and mean 38% (range 2-86%) increase in theta activity at 2 h compared with placebo. 4. Both 80 and 120 mg doses of ICI 169,369 reduced RPD by approximately 30% from a predose value of 6.25 mm (+/- 0.87; 95% CI) and from placebo values 6.41 mm (+/- 1.06) and 7.48 mm (+/- 1.49) at 3 and 5 h after dosing. MPD was reduced by 50% with the 120 mg dose at 5 h after dosing (placebo 5.2 mm; ICI 169,369 2.7 mm; P less than 0.05). FPD was significantly reduced (P less than 0.01) by both doses at 3 h after dosing.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of (+/-)-DOI on neuromuscular transmission: a microelectrode study

DOI (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) significantly depressed end plate current (EPC) amplitude. It decreased quantum content, increased the extent of neurally evoked EPC rundown during the train, produced a nonlinear current-voltage relationship, shortened time constant of decay, and depressed iontophoretically evoked EPC. The depressant response of DOI on EPC amplitude was antagonized by 5-HT1-like receptor antagonists, but was resistant to 5-HT2 and 5-HT3 receptor antagonists. This suggests that inhibitory 5-HT receptors roughly correspond to 5-HT1-like receptors.

Mechanisms of action of atypical antipsychotic drugs. Implications for novel therapeutic strategies for schizophrenia

The mechanisms which contribute to the actions of atypical antipsychotic drugs, such as clozapine and the putative atypical agents remoxipride and raclopride, are reviewed. Examination of available preclinical and clinical data leads to two hypotheses concerning the mode of action of atypical antipsychotic drugs. The first hypothesis is that antagonism of the dopamine D2 receptor is both necessary and sufficient for the atypical profile, but that interaction with subtypes of the D2 receptor differentiates typical from atypical antipsychotic drugs. The second hypothesis has been previously advanced, and suggests that a relatively high ratio of serotonin 5-HT2:dopamine D2 receptor antagonism may subserve the atypical profile. It seems likely that the atypical antipsychotic drug profile may be achieved in more than one way.

Methylenedioxymethamphetamine induces spontaneous tail-flicks in the rat via 5-HT1A receptors

In rats lightly restrained in horizontal cylinders, (+/-)-3,4-methylenedioxymethamphetamine (MDMA) dose dependently (0.16-10.0 mg/kg, s.c.) elicited spontaneous tail-flicks; that is, tail-flicks in the absence of extraneous stimulation. In contrast, amphetamine over a similar dose-range was inactive. Selective inhibitors of 5-hydroxytryptamine (5-HT) uptake and carrier-mediated 5-HT release, paroxetine and citalopram, did not induce spontaneous tail-flicks themselves and blocked those induced by MDMA. In distinction, maprotiline and bupropion, selective inhibitors of noradrenaline and dopamine uptake, respectively, failed to modify the action of MDMA. Spontaneous tail-flicks elicited by MDMA were unaffected by the selective 5-HT3 receptor antagonists, ICS 205,930 and GR 38032F. They were attenuated by the mixed 5-HT1/5-HT2 receptor antagonist, methiotepin, the mixed 5-HT1A/5-HT1B receptor antagonist, (-)-alprenolol and the mixed 5-HT1A/5-HT2 receptor antagonist, spiperone, but not by the selective 5-HT1C/5-HT2 receptor antagonists, ritanserin, ICI 169,369 and ketanserin. The novel 5-HT1A receptor antagonists, BMY 7378 and NAN-190, each abolished MDMA-evoked spontaneous tail-flicks. Selective D1, D2, alpha 1, alpha 2, beta 1 and beta 2 antagonists had little influence upon induction of spontaneous tail-flicks by MDMA. These data indicate that MDMA evokes spontaneous tail-flicks in the rat via a release of 5-HT which acts at 5-HT1A receptors. Thus, 5-HT1A receptors appear to be involved in the acute functional actions of MDMA.

5-hydroxytryptamine antagonistic effects of ICI 169,369, ICI 170,809 and methysergide in human temporal and cerebral arteries

ICI 169,369 and ICI 170,809 are two chemically novel 5-HT antagonists that have high affinity for the 5-HT2 binding site in rat cortex (Ki 1.79 x 10(-8)M and 6.6 x 10(-10)M, respectively). In human temporal artery preparations ICI 169,369 was shown to cause a progressive rightward shift of the 5-HT-response curve over the range 10(-7)-10(-5)M, while ICI 170,809 in these concentrations shifted the curve to the same degree (no dose dependency). In human cerebral vessels no effect was observed until a high concentration (10(-5)M) was used for either compounds. The mixed 5-HT1/5-HT2 antagonist, methysergide, induced a non parallel rightward shift of the 5-HT-induced concentration-effect curve with a depression of the maximum achievable response in both the temporal and cerebral artery. The mode of effect of ICI 169,369 and ICI 170,809 to block the 5-HT-induced contractions in human temporal vessels resembles that of the pure 5-HT2 antagonist ketanserin, thus suggesting that the two ICI compounds are mainly 5-HT2 antagonists. In high concentrations both ICI 169,369 and ICI 170,809 have vasorelaxant properties, explaining the reduction in maximum 5-HT-induced contraction seen at high antagonist concentrations.

Evidence suggesting that the 5-HT2 antagonist ICI 169,369 activates vagal afferents and in addition has a central hypotensive action in anaesthetized rats

1. The cardiovascular effects of the 5-HT2 antagonist ICI 169,369 have been investigated in pentobarbitone anaesthetized rats and in pithed rats whose blood pressure was supported by a vasopressin infusion. 2. In anaesthetized rats, cumulative doses (0.1-3.0 mg kg-1) of ICI 169,369 caused dose-related large transient falls in heart rate and blood pressure followed by a slow dose-related decline in both parameters. 3. Pretreatment with atropine methonitrate (1 mg kg-1) alone or in combination with atenolol (1 mg kg-1) or bi-vagotomy blocked the transient changes in blood pressure and heart rate caused by ICI 169,369. The long-term fall in heart rate was also attenuated by either atropine or atenolol; however, the combination of atenolol and atropine was more effective. None of the above pretreatments affected the long-term fall in blood pressure caused by ICI 169,369. 4. The cardiovascular effects of ICI 169,369 were unaffected by pretreatment with MDL 72222 (1 mg kg-1) and failed to show cross-tachyphylaxis with phenylbiguanide. 5. In pithed rats whose blood pressure was maintained with vasopressin, ICI 169,369 failed to cause any of the above transient and long-term effects on blood pressure and heart rate. 6. The study indicates that ICI 169,369 is capable of stimulating cardiopulmonary afferents through a non 5-HT3 receptor mechanism and has a central hypotensive action.

Agonist action at 5-HT1C receptors facilitates 5-HT1A receptor-mediated spontaneous tail-flicks in the rat

In rats lightly restrained in plastic cylinders, subcutaneous administration of the selective, high efficacy 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), induced spontaneous tail-flicks, that is, tail-flicks in the absence of extraneous stimulation. The putative 5-HT1B receptor agonist, CGS 12066B, the mixed 5-HT1B/1C receptor agonists, 1-((3-(trifluoromethyl)phenyl]piperazine (TFMPP) and 1-(3-chlorophenyl)piperazine (mCPP), the 5-HT1C/2 receptor agonist, [+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the 5-HT1B/1C/2 receptor agonist, quipazine, did not, in contrast, elicit tail-flicks when applied alone. However, TFMPP, mCPP, DOI and quipazine, but not CGS 12066B, each potentiated the action of 8-OH-DPAT. Further, in the presence of TFMPP, mCPP and DOI, the dose-response curve for the induction of tail-flicks by 8-OH-DPAT was both steeper and shifted to the left. Tail-flicks induced by another high efficacy 5-HT1A receptor agonist, lisuride, were also enhanced by TFMPP, mCPP and DOI. The 5-HT1A receptor partial agonists, buspirone and (+/-)-flesinoxan, evoked tail-flicks only in the presence of TFMPP, mCPP or DOI. The mixed 5-HT1C/2 receptor antagonists, ritanserin and ICI 169,369, did not modify the action of 8-OH-DPAT alone but abolished the potentiation of 8-OH-DPAT-induced tail-flicks by DOI and TFMPP. Further, the selective 5-HT1A receptor antagonist, BMY 7378, blocked tail-flicks induced by both 8-OH-DPAT alone and 8-OH-DPAT plus DOI or TFMPP. A common property of those drugs potentiating 8-OH-DPAT-induced tail-flicks is an agonist action at 5-HT1C receptors and the data indicate that it is this mechanism which underlies the facilitation of tail-flicks.

Studies on the role of 5-HT receptors in satiation and the effect of d-fenfluramine in the runway test

d-Fenfluranine has previously been shown to reduce food-rewarded runway behaviour in the rat, an effect thought to be mediated through activation of central 5-HT pathways. We now examined in more detail the mechanism by which d-fenfluramine reduces runway performance and food intake in the 15-trial runway test. The non-selective 5-HT receptor antagonist, metergoline (1.0 mg/kg), significantly antagonised the effect of d-fenfluramine (2.5 mg/kg) in the runway test. In contrast, neither ritanserin, the potent 5-HT2 and 5-HT1C receptor antagonist (0.5 mg/kg), nor the peripheral 5-HT receptor antagonist, xylamidine (3.0 mg/kg), attenuated the effect of d-fenfluramine in this test situation. Metergoline, but not ritanserin or xylamidine significantly increased runway performance when administered alone. These data indicate that d-fenfluramine reduces runway performance and food intake through activation of 5-HT1 receptors. In addition, blockade of 5-HT1 receptors can attenuate the development of satiation normally observed under control conditions in the runway test.

Pharmacological studies in vivo with ICI 169,369, a chemically novel 5-HT2/5-HT1C receptor antagonist

ICI 169,369 (2-(2-dimethylaminoethylthio-3-phenylquinoline hydrochloride) has been tested in vivo for its potency and selectivity as an antagonist at 5-HT2 and 5-HT1C receptors. It caused a 50% inhibition of 5-HTP-induced head twitches in mice and fenfluramine-induced hyperthermia in the rat at approximately 1 mg/kg following parenteral administration. Results showed that ICI 169,369 had good oral bioavailability, since in the fenfluramine test the oral and s.c. ID50 values were similar. ICI 169,369 was a selective antagonist of 5-HT-induced bronchoconstriction in the guinea-pig and 5-HT-induced pressor effects in the anaesthetised dog. In a series of other tests in vivo the compound was shown to be devoid of significant activity at alpha 1- and alpha 2-adrenoceptors, dopamine (D2), muscarinic (M1) and histamine (H1) receptors at 30-100 times its ID50 values used in the 5-HT tests. Thus, ICI 169,369 is a selective, orally active 5-HT2/5-HT1C antagonist that should prove useful in the analysis of the role of 5-HT in physiological and pathological states.

Drugs affecting serotonin neurons

Advances in serotonin pharmacology, the development of drugs that intervene at specific sites to modify serotonergic function, have accompanied advances in the understanding of physiologic roles of serotonin present in neurons and elsewhere and of serotonin receptors that are widely distributed in brain and many peripheral tissues. The pharmacologic advances have sometimes been stimulated by developments in serotonin physiology, such as the recognition of multiple serotonin receptor subtypes, and in other cases have been a major factor in providing new insights into physiologic roles of serotonin. Drugs that modify serotonin function have a variety of therapeutic applications currently and many more potential therapeutic uses to be explored in the future. Having drugs that act with high specificity or selectivity on particular enzymes in serotonin biosynthesis, on particular serotonin receptors, or at other sites such as uptake carriers for serotonin not only offers the hope of improved clinical therapy in diseases caused by abnormal serotonergic function or in which alteration of serotonergic function can alleviate symptoms, but also provides valuable pharmacologic tools for learning more about serotonin physiology and probing the functional status of serotonergic systems. The next few years promise to yield important new serotonergic drugs.

Ergometrine and 5-hydroxytryptamine binding sites in rat brain and myometrium

Functional studies suggest that ergometrine is a partial agonist involving 5-hydroxytryptamine (5-HT) receptors in rat uterus. Ergometrine displaced [3H]5-HT from specific binding sites in rat brain, but did not displace [3H]5-HT at functionally important concentrations in rat myometrium. These binding studies indicate that the agonist and antagonist actions of ergometrine in rat uterus arise from its initial interaction with binding sites other than those for 5-HT.

Suppressant effects of selective 5-HT2 antagonists on rapid eye movement sleep in rats

The effects of the novel, highly selective serotonin-2 (5-HT2) antagonists, ICI 169,369 and ICI 170,809, on 24 h EEG sleep-wake activity were studied in the rat. Both compounds caused a dose-related increase in the latency to rapid eye movement sleep (REMS) and significantly suppressed cumulative REMS time up to 12 h postinjection. In contrast, neither drug disrupted slow-wave sleep continuity in as much as the latency to non-REMS (NREMS) and cumulative NREMS time were unchanged. However, at the highest dose tested (20 mg/kg) ICI 170,809 did produce a significant increase in total NREMS time during the second half of the sleep-awake cycle. These results demonstrate effects of selective 5-HT2 antagonists on sleep in rats which appear to be specific for REMS behavior, suggesting that the priming influence of serotonin on REMS may involve 5-HT2 receptor subtypes. The relationship between the REMS suppressant actions of these compounds and their consideration as therapeutic agents in depression is discussed.

Does a selective 5-hydroxytryptamine antagonist (ICI 169, 369) lower blood pressure in hypertensive patients?

1. The effect of single doses (10, 30 and 50 mg) of a selective 5-HT2 receptor antagonist, ICI 169, 369, on blood pressure, heart rate and the electrocardiogram was studied using a double-blind, placebo-controlled, within subject design in hypertensive patients. 2. ICI 169, 369 did not reduce blood pressure or increase QT interval as has been reported with ketanserin. This suggests that it is the other properties of ketanserin which are responsible for its antihypertensive effect. 3. Plasma concentrations of AUC for ICI 169, 369 were low. This is consistent with low bioavailability due to extensive first pass metabolism. 4. ICI 169, 369 was well tolerated and none of the symptoms reported by the patients was thought to be drug related.

Ergometrine--a partial agonist at 5-HT receptors in the uterus isolated from the oestrogen-primed rat

The mechanism of action of ergometrine has been studied in isolated uterus from the oestrogen-primed non-pregnant rat. Ergometrine (30 nM-1 microM) induced spasm and was antagonised selectively by methysergide and ICI 169,369, a proposed competitive antagonist at 5-hydroxytryptamine (5-HT2) receptors. The pA2 values of ICI 169,369 against 5-HT and ergometrine were not significantly different. Ergometrine (0.1-10 microM) was also a selective antagonist of 5-HT with no effect against acetylcholine or potassium chloride. It is suggested that ergometrine is a partial agonist involving 5-HT receptors in rat uterus.

ICI 169,369 selectively blocks 5-hydroxytryptamine2 receptors and lowers portal pressure in portal hypertensive rats

The contractile effects of 5-hydroxytryptamine on isolated portal veins and superior mesenteric veins of portal hypertensive rats (portal vein constricted) were antagonized competitively by ICI 169,369. The equilibrium dissociation constant of 1-3 nM for ICI 169,369, estimated in the veins, agrees with affinity estimates for arterial 5-hydroxytryptamine2 receptors. The receptors of portal veins of sham-operated rats had the same affinity for ICI 169,369 as the receptors of portal hypertensive rats. The systemic administration of ICI 169,369 to portal hypertensive rats decreased portal pressure from 13.0 +/- 0.4 to 11.3 +/- 0.5 mmHg (p less than 0.01) but did not affect arterial pressure. ICI 169,369 induced nonsignificant changes in both portal venous inflow and portocollateral resistance, as estimated by the radioactive microsphere technique. It is estimated that the combined changes in portal flow and resistance could explain the decrease in portal pressure. The results are consistent with an involvement of 5-hydroxytryptamine, acting through 5-hydroxytryptamine2 receptors, in prehepatic portal hypertension.

Examination of the effects of some 5-HT2 receptor antagonists on central sympathetic outflow and blood pressure in anaesthetised cats

Thoracic preganglionic sympathetic nerve activity, blood pressure, heart rate and femoral arterial conductance were recorded in anaesthetised, paralysed cats. Cumulative dose response curves were constructed for the 5-HT2 antagonists cinanserin, ritanserin, cyproheptadine, methiothepin, metergoline and ICI 169,369. These antagonists showed differing effects on the above parameters. Methiothepin and cyproheptadine caused hypotension and sympathoinhibition at low and high doses, whilst metergoline and ritanserin caused these effects at high doses, above 1 mg kg-1. Cinanserin and ICI 169,369 did cause large transient changes in heart rate, blood pressure and sympathetic outflow. However, all the antagonists except ICI 169,369 and methiothepin caused an increase in femoral arterial conductance which was not associated with hypotension. It was therefore concluded that using the above antagonists it was difficult to attribute changes in blood pressure and central sympathetic outflow to blockade of 5-HT2 receptors. However, it is suggested that 5-HT2 receptors may be involved in the control of skeletal muscle and/or skin vascular beds.