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Kinetic on and off rate constants for many receptor ligands are difficult to determine with regular radioligand binding technique since only few of the ligands are available in labeled form. Here we developed a new and simple radioligand binding method for determining the kinetic off-rate constant for unlabeled ligands, using whole cells expressing alpha(2A)- and alpha(2C)-adrenoceptors. The new method involves pre-incubation with unlabeled ligand, centrifugation of microtiter plates in order to adhere the cells to the bottom surface, and then upside-down centrifugation of the plates for few seconds to wash away the non-bound fraction of the pre-incubated ligand. The final on-reaction assay for the radioligand is then started by quick addition of a relatively fast-associating radioligand to the cells. The curve obtained is defined by a fairly simple mathematical formula that reflects the simultaneous dissociation of pre-incubated ligand and association of the radioligand. The method proved to produce highly reproducible results in determining the k(off) constants for various unlabeled ligands. The results show that the alpha(2C)-selectivity of MK912 depends mainly on a very slow off-rate at the alpha(2C)-adrenoceptor subtype. Regarding the markedly alpha(2C)- over alpha(2A)-selective compound spiroxatrine, its much faster on-rate at alpha(2C)- than alpha(2A)-adrenoceptors explains much of its exceptional alpha(2C)-selectivity. Several new techniques for determining the kinetic component of ligand-receptor interactions at molecular level are currently developing. As a reference, based on standard radioligand binding techniques, the present study describes a simple and robust experimental and mathematical procedure for determining k(off) constants of unlabeled drugs.

We investigated the effects of serotonin (5-hydroxytryptamine; 5-HT) on nitric oxide (NO) synthesis in vascular smooth muscle cells. We measured the production of nitrite, a stable metabolite of NO, and the expression of inducible NO synthase protein in cultured rat vascular smooth muscle cells. Incubation of the cultures with interleukin-1beta (10 ng/ml) caused a significant increase in nitrite production. 5-HT inhibited nitrite production by interleukin-1beta -stimulated vascular smooth muscle cells in a concentration-dependent manner (10(-8)-10(-5) M). 5-HT-induced inhibition of nitrite production was accompanied by decreased inducible NO synthase protein accumulation in vascular smooth muscle cells. Addition of the 5-HT2 receptor antagonist ketanserin, but not the 5-HT1A receptor antagonist spiroxatrine, inhibited the effect of 5-HT. On the other hand, the 5-HT2 receptor agonist alpha-methyl-5-HT, but not the 5-HT1A receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin, decreased interleukin-1beta-induced nitrite production by vascular smooth muscle cells. 5-HT significantly increased protein kinase C activity in vascular smooth muscle cells, and the protein kinase C inhibitor calphostin C dose-dependently abolished the effect of 5-HT on nitrite production. After protein kinase C activity was functionally depleted by treatment of cells with phorbol 12-myristate 13-acetate for 24 h, the effect of 5-HT was abolished. These results indicate that 5-HT acts on 5-HT2 receptors and inhibits NO synthesis in interleukin-1beta-stimulated vascular smooth muscle cells at least partially through a protein kinase C-dependent pathway.

The effects of acute and repeated treatment with 1-(2-pyrimidinyl)piperazine (1-PP), a metabolite of the 5-HT 1A receptor ligand azapirones, were investigated on hippocampal excitatory synaptic transmission. Recordings of the electrically evoked field population excitatory post-synaptic potentials (e.p.s.p.s) were carried out in the stratum radiatum of the CA1 region of the dorsal hippocampus of alert rats. Acute i.p. administration of 1-PP transiently reduced the e.p.s.p. amplitude in a dose-dependent (0.25–1 mg/kg) manner. This effect was blocked by the 5-HT 1A receptor antagonists spiroxatrine (1 mg/kg) and MDL 73005EF (8-[2-(2,3-dihydro-1,4-benzodioxin-2-yl methylaminoethyl]-8-azaspirol[4,5]decane-7,9-dione methyl sulphonate, 2 mg/kg). Intraphippocampal administration of 1-PP (5 μg) evoked a transient reduction of the e.p.s.p. amplitude which was similar to that obtained with 5-HT (10 μg). 1-PP (0.25 mg/kg per day) administered for 9 days produced a gradual reduction in the daily pre-injection baseline e.p.s.p. amplitude coupled with a decrease in the acute response to the drug. The chronic baseline reduction was transiently reversed by spiroxatrine and full recovery to pretreatment levels was observed 4 days after the last 1-PP dose. These findings indicate that the previously reported reduction in the e.p.s.p. produced by the azapirone group of 5-HT 1A receptor ligands may be mediated in part by their metabolite 1-PP through activation of 5-HT 1A receptors.

The effects of acute and repeated treatment with the 5-HT1A receptor ligand gepirone on hippocampal excitatory synaptic transmission were investigated. Recordings of the electrically evoked field population excitatory postsynaptic potentials (e.p.s.p.s) were made in the stratum radiatum of the CA1 region of the dorsal hippocampus of alert male Wistar rats. Acute injection of gepirone reduced the e.p.s.p. amplitude in a transient dose-dependent (0.5-10 mg/kg, i.p.) manner. This effect was blocked by the 5-HT1A receptor antagonist MDL 73005EF (8-[2-(2,3-dihydro-1,4-benzodioxin-2-yl methylaminoethys]-8- azaspiro[4,5]decane-7,9-dione methyl sulphonate, 2 mg/kg, i.p.). Gepirone (1 mg/kg per day, i.p.) administered for 7 days produced a gradual reduction in the daily pre-injection baseline e.p.s.p. amplitude coupled with a concomitant reduction of the acute response to the drug. The chronic baseline reduction was transiently reversed by the 5-HT1A receptor antagonist spiroxatrine and complete recovery to pretreatment levels was observed 48 h after the last gepirone dose. The data indicate that with repeated administration, a prolongation and enhancement of the 5-HT1A receptor-mediated reduction in the e.p.s.p. by gepirone occurs. This delayed effect may contribute to the slow onset of therapeutic action of gepirone.

The long-lasting descending inhibition of lumbar segmental reflexes in the neonatal rat spinal cord has been investigated in vitro by recording from lumbar ventral roots on stimulation of a single lumbar dorsal root. Descending inhibition was elicited by a single stimulus to the latero-ventral thoracic cord. A number of strategies were used to clarify the role of 5-hydroxytryptamine (5-HT) in inhibiting the monosynaptic reflex, the ipsilateral polysynaptic response and the contralateral fast response evoked on the opposite side of the lumbar cord. The 5-HT uptake inhibitor, citalopram (10 nM), potentiated both short-interval (0.5–2 s) inhibition and long-interval (5–100 s) inhibition of the monosynaptic reflex, and also inhibition of the polysynaptic response 10–100 s after the thoracic stimulus. Inhibition of the monosynaptic reflex was blocked by ketanserin (1 μM), spiperone (1 μM) and methiothepin (1 μM), but not by spiroxatrine (0.1 μM) or sulpiride (1 μM). Sumatriptan (20 nM) and methysergide (10 nM) enhanced inhibition of the monosynaptic reflex 0.2–1 s after the thoracic stimulus. It was concluded that 5-HT acting through 5-HT2A/2C receptors is the transmitter responsible for monosynaptic reflex inhibition, at intervals of 0.5–100 s, but a stronger stimulus to the thoracic cord may elicit a non-serotonergic component at intervals of 0.1–2 s. There was no unequivocal evidence that endogenous 5-HT activates 5-HT1 receptors to produce inhibition. 5-HT acting through 5-HT2A/2C receptors is also responsible for part of the inhibition of the polysynaptic response, but the transmitter(s) responsible for the ketanserin-resistant component of inhibition and for inhibition of the contralateral fast response has still to be identified.

Administration of 2 and 5 but not 0.4 microgram/microliter 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) into the CA1 region of the dorsal hippocampus 10 min before the acquisition trial in a passive avoidance task significantly reduced retention latencies 24 h later. The effect of 5 micrograms 8-OH-DPAT on retention latencies was completely antagonized by 1 microgram/microliter spiroxatrine, a 5-HT1A receptor antagonist, infused into the dorsal hippocampus 5 min before 8-OH-DPAT. Administered 5 min after the acquisition trial, 5 micrograms/microliters 8-OH-DPAT had no effect on retention latencies 24 h later. Administration of 5 micrograms/microliters 8-OH-DPAT into the dorsal hippocampus did not significantly change the thresholds for responses to the same electrical stimuli used in the passive avoidance task and had no antidepressant-like effect in the forced swimming test. The dose of 5 micrograms/microliters 8-OH-DPAT administered into the dorsal hippocampus caused anxiolytic-like effects assessed by stress-induced deficit in open field locomotor activity. The results suggest that stimulation of 5-HT1A receptors in the dorsal hippocampus impairs rats' performance in a passive avoidance task by interfering with memory processes or by attenuating the emotional impact of the shock through an anxiolytic action.

The roles of the alpha(2)-adrenoceptor subtypes in the regulation of cervical resistance have previously not been investigated. The aim of the present study was to identify these receptors in the late-pregnant cervix and determine their functions in vitro in the rat. The expressions of the alpha(2)-adrenoceptor subtypes were determined by means of RT-PCR and Western blotting techniques. The changes in cervical resistance due to subtype-selective antagonists were investigated in stretching tests. The cyclic AMP immunoassay technique was used to detect the level of cyclic AMP following stimulation of the alpha(2)-adrenoceptors with or without pertussis toxin. On pregnancy days 18, 20, 21 and 22, the RT-PCR and Western blotting studies revealed the expressions of all three alpha(2)-adrenoceptor subtype mRNAs and proteins. On days 18 and 20, noradrenaline increased and decreased the resistance, respectively. Its effect was blocked by each of the antagonists used, except ARC 239 on both days. On day 21, noradrenaline again increased the resistance, this effect being maintained only in the presence of spiroxatrine. Noradrenaline was ineffective on day 22. These results were supported by the changes in cyclic AMP levels. Pertussis toxin pretreatment eliminated the changes in the cyclic AMP level on days 18 and 21. We presume that the alpha(2A)- and alpha(2C)-adrenoceptors play predominant roles in the regulation of cervical resistance on days 18-21. Depending on the day of pregnancy, stimulation of these alpha(2)-adrenoceptors could even result in opposite effects. This fluctuation can be explained by the changes in the G(i)/G(s)-coupling of the alpha(2A)- and alpha(2C)-adrenoceptors.

The effect of buspirone on corticotropin-releasing factor (CRF) and stress-stimulated cecal motility and its antagonism by 5-HT1A (spiroxatrine) and sigma (BMY 14802) antagonists were evaluated by electromyography in rats equipped with chronically implanted electrodes on the cecum and a small catheter into the right lateral ventricle of the brain. Exposure to mental stress, consisting of a fear-conditioned response, increased during 30 min the frequency of cecal spike bursts significantly (P less than 0.01). The frequency of cecal spike bursts was also increased following intracerebroventricular injection of CRF (500 ng/kg). Buspirone (1 mg/kg s.c.) abolished the stimulatory effects of mental stress and CRF on cecal motility. Whereas spiroxatrine (0.5 mg/kg s.c.) blocked the effect of buspirone on the colonic hypermotility induced by i.c.v. injection of CRF, BMY 14802 at a similar dose (0.5 mg/kg s.c.) was unable to block the action of buspirone. It is concluded that s.c. administration of buspirone suppresses the stress-induced cecal motor response through 5-HT1A receptors, probably by inhibiting the central or peripheral pathways involved in CRF mediation of these effects.

Selective activation of the 5-HT1A receptor induces lower lip retraction (LLR) in rats. 8-Hydroxy-dipropylamino tetralin (8-OH-DPAT)-induced LLR could not be antagonised by the 5-HT antagonists methysergide, metergoline or mesulergine. In fact, some 5-HT antagonists induced LLR. However, 8-OH-DPAT-induced LLR could be antagonised by pindolol, spiperone, spiroxatrine and NAN-190, but not by the beta 1-adrenoceptor antagonist metoprolol, the beta 2-adrenoceptor antagonist butoxamine or the dopamine antagonist haloperidol. This antagonism was competitive as the dose-response curve of 8-OH-DPAT was shifted to the right. Pindolol, spiperone, spiroxatrine and NAN-190 all have a high affinity for the 5-HT1A receptor. This indicates that blockade of 8-OH-DPAT-induced LLR is only possible by selective blockade of 5-HT1A receptors. A possible mechanism of action is discussed. The increased defecation induced by 8-OH-DPAT could be antagonised by pindolol and NAN-190. The effect of spiroxatrine and haloperidol on the 8-OH-DPAT-induced increase in defecation was bimodal: an increase after a low and a decrease after a high dose of 8-OH-DPAT. Metoprolol and butoxamine had no effect on the 8-OH-DPAT-induced increase in defecation, thereby excluding an influence of beta-adrenoceptors.

Naftopidil, a novel antihypertensive compound, possesses 5-HT1A agonistic properties in addition to being an alpha 1-adrenoceptor antagonist. The IC50 values for alpha 1-adrenoceptors (235 nmol/l) and for 5-HT1A receptors (108 nmol/l) lie in the same concentration range. The reduction in blood pressure of anesthetized cats by 8-OH-DPAT and urapidil was completely abolished by spiroxatrine, a 5-HT1A antagonist. However, the decreases in blood pressure induced by naftopidil were only partly antagonized by spiroxatrine.

We examined the effects of (R)- and (S)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT) on the monosynaptic spinal reflex in rats. In intact rats, (R)-8-OH-DPAT (10 microg/kg, i.v.) enhanced the amplitude of the monosynaptic reflex, whereas at 100 microg/kg, it reduced the amplitude. (S)-8-OH-DPAT enhanced the monosynaptic reflex dose-dependently. In spinalized rats, (R)-8-OH-DPAT produced dose-dependent inhibition, but the (S)-enantiomer did not affect the monosynaptic reflex. Pretreatment with spiroxatrine or 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]-piperazine (NAN-190) inhibited (R)-8-OH-DPAT-induced monosynaptic reflex enhancement in intact rats, as did 5-hydroxytryptamine (5-HT) depletion. Ketanserin reduced the effect of (R)-8-OH-DPAT. These pretreatment regimens had no effect on the monosynaptic reflex depression produced by the (R)-enantiomer in intact and spinalized rats. Pretreatment with prazosin inhibited (S)-8-OH-DPAT-induced monosynaptic reflex enhancement in intact rats, as did noradrenaline and 5-HT depletion. These results suggest that supraspinal 5-HT1A receptors and the descending serotonergic system are involved in the stimulatory effect of (R)-8-OH-DPAT on the monosynaptic reflex, while both the descending serotonergic and noradrenergic systems, the latter acting via alpha1-adrenoceptors, are involved in the effect of the (S)-enantiomer on this reflex.

The aim of the current investigation was to characterize the 5-HT receptors that mediate neurogenic relaxation of the guinea-pig proximal colon. After blockade of 5-HT 2A , 5-HT 3 and 5-HT 4 receptor-mediated contractions, 5-hydroxytryptamine (5-HT) induced relaxations yielding a biphasic concentration-response curve. Other tryptamines were also agonists with the following rank order of potency: 5-HT > 5-carboxamidotryptamine = 5-methoxytryptamine ≥ α-methyl-5-HT (partial agonist) > tryptamine (partial agonist). 5-Hydroxytryptophan, 2-methyl-5-HT and N -methyltryptamine were virtually inactive as agonists. The curve to 5-HT was not affected by pargyline, citalopram, phentolamine, or by the 5-HT 4 receptor antagonists 2-methoxy-4-amino-5-chloro-benzoic acid 2-(diethylamino)ethyl ester (SDZ 205–557) and (1-butyl-4-piperidinylmethyl)-8-amino-7-chloro-1,4-benzodioxan-5-carboxylate (SB 204070). 8-Hydroxy-2-(di- n -propylamino)-tetralin (8-OH-DPAT), 5-methoxy-3[1,2,3,6-tetrahydroxypyridin-4-yl]-1 H -indole (RU 24969), 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101), 1-(3-chlorophenyl)piperazine (mCPP), 1-( m -trifluoromethylphenyl)-piperazine (TFMPP), flesinoxan, sumatriptan and 6-chloro-2-(piperazinyl)-pyrazine (MK212) were inactive as 5-HT receptor agonists. The first phase of the curve to 5-HT was inhibited by: metergoline (pA 2 = 8.8 ± 0.3, against 5-methoxytryptamine 9.3 ± 0.3), methysergide (non-surmountable), methiothepin (non-surmountable), spiroxatrine (non-surmountable), MK212 (non-surmountable), mesulergine (7.8 ± 0.3), mCPP (7.1 ± 0.1), mianserin (7.0 ± 0.4), ritanserin (8.9 ± 0.2), rauwolscine (7.0 ± 0.2), yohimbine (6.2 ± 0.2), 1-(1-naphthyl)-piperazine (7.7 ± 0.2) and RU 24969 (6.4 ± 0.1), but not by 1-(2-methoxyphenyl)-4-[4-(2-phthalimidobtyl]-piperazine (NAN-190), spiperone, sumatriptan, 8-OH-DPAT and flesinoxan. It is suggested that the 5-HT receptor under study could be considered an unknown 5-HT 2 -like receptor.

Feeding a vitamin B6-deficient diet to rats causes a moderate hypertension. The blood pressure responses to 5-HT1A receptor agonists were studied in conscious vitamin B6-deficient hypertensive rats. They were all effective in lowering blood pressure with the following rank order of potency: 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) > flesinoxan > 5-methylurapidil > urapidil. The putative 5-HT1A receptor antagonist spiroxatrine by itself, did not have any effect on the blood pressure at the doses used (0.01-1 mumol/kg). However, dose dependently, it antagonized the hypotensive effect of flesinoxan and urapidil. The alpha 1-adrenoreceptor antagonist prazosin, on prior treatment, did not change the hypotensive effect of either flesinoxan or urapidil. The alpha 2-adrenoreceptor agonist clonidine dose dependently (0.01-0.1 mumol/kg) reduced blood pressure. This effect of clonidine was unaffected by spiroxatrine, but was dose dependently antagonized by the alpha 2-adrenoreceptor antagonist yohimbine. Binding studies with [3H]8-OH-DPAT indicated that the affinity and Bmax of 5-HT1A receptors was increased in vitamin B6-deficient hypertensive rats. The results suggest that decreased synthesis of 5-HT in brain regions and the consequent alterations in 5-HT receptors in the vitamin B6-deficient rats may be the underlying cause of the hypertension seen in these animals.

Selective 5-HT1A receptor agonists such as 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), flesinoxan, 5-methylurapidil and others, increased locomotor behaviour in rats following s.c. injection. Unfortunately, all available 5-HT1A receptor antagonists are non-selective, a fact which severely hampers their use. The inhibitory effects of (+)- and (-)-pindolol (1-10 mg/kg, -45 min s.c.) on locomotion induced by 8-OH-DPAT were investigated in rats pretreated with the beta-adrenoceptor antagonists, ICI 118,551 and betaxolol (both 1 mg/kg, -45 min s.c.). ICI 118,551 and betaxolol significantly enhanced the response to 8-OH-DPAT. Co-administration of (-)-pindolol (2-10 mg/kg) dose dependently antagonised the hyperlocomotion whereas (+)-pindolol was weakly effective at 10 mg/kg. The partial agonists ipsapirone (1-10 mg/kg, -45 min s.c.) and buspirone (0.1-1 mg/kg, -45 min) antagonised the effects of 8-OH-DPAT. This inhibition may, however, be due to alpha 1-adrenoceptor blockade and DA receptor antagonism, respectively. Finally, although spiroxatrine (0.1 and 1 mg/kg, -45 min s.c.) inhibited the response to 8-OH-DPAT, this inhibition may also be non-specific since spiroxatrine strongly reduced spontaneous locomotor activity. In conclusion, whilst much of our data is consistent with 8-OH-DPAT-induced locomotion being mediated by 5-HT1A receptors, only the data obtained with the pindolol enantiomers provide direct evidence for this. The results also suggest that, under normal circumstances, the 5-HT1A receptor antagonist effect of pindolol may be masked by the facilitating effect of beta-adrenoceptor blockade.

The cardiovascular response to flesinoxan and 8-OH-DPAT (8-hydroxy-2-(di-N-propylamino)tetralin), 5-HT 1A receptor agonists, has been investigated in anaesthetized Wistar rats and spontaneously hypertensive rats (SHR) and in conscious SHR. Flesinoxan and 8-OH-DPAT potently lowered blood pressure and heart rate in these models. In conscious SHR, atropine reversed the bradycardia induced by flesinoxan partially and that induced by 8-OH-DPAT completely. In pithed rats with vasopressin-raised blood pressure, neither flesinoxan nor 8-OH-DPAT lowered blood pressure or heart rate. Intracisternal administration of either flesinoxanor 8-OH-DPAT was less efficacious than intravenous administration. The cardiovascular responses to flesinoxan and 8-OH-DPAT in the anaesthetized Wistar were inhibited by the putative 5-HT 1A antagonists methiothepin, buspirone, spiroxatrine and 8-MeO-C1EPAT (8-methoxy-2-(N-2-chloroethyl-N-n-propylamino)tetralin). 8-MeO-C1EPAT appeared to be the most suitable antagonist in this model. The 5-HT 1C , 5-HT 2 antagonist ritanserin or the 5-HT 3 antagonist GR 38032F had no effect on the responses to flesinoxan or 8-OH-DPAT. In conscious SHR however, 8-MeO-C1EPAT did not antagonize these cardiovascular responses. This study confirms the involvement of central 5-HT 1A receptors in the cardiovascular effects of flesinoxan and 8-OH-DPAT.

A comparison was made of the doses of neuroleptic and related agents required to inhibit the peri-oral biting movements induced in the guinea pig by s.c. administered apomorphine and the s.c. and bilateral intrastriatal injection of 2-(N,N-dipropyl)amino-5,6-dihydroxytetralin. The bilateral intracerebral injection of 2-(N,N-dipropyl)amino-5,6-dihydroxytetralin into areas surrounding the caudate-putamen and into the area preoptica and thalamus elicited similar peri-oral movements but these had a longer latency of onset or shorter duration (anterior, dorsal and lateral cortex), or were associated with a hyperactivity (area preoptica and thalamus). Typical neuroleptics such as haloperidol and fluphenazine were found to be less active as antagonists of the peri-oral movements induced by the aminotetralin compound than oxiperomide, spiroxatrine and tiapride. These latter agents were approximately 16, 8 and > 4 times respectively more effective as antagonists of the aminotetralin biting than that induced by apomorphine. Bilateral intrastriatal injections of sodium valproate, γ-aminobutyric acid and serotonin failed to modify peripherally induced apomorphine stereotypy, though these treatments inhibited the peri-oral movements induced by intrastriatal 2-(N,N-dipropyl)amino-5,6-dihydroxytetralin. It is suggested that the mechanisms involved with apomorphine stereotypy differ from those activated by peripherally and intrastriatally administered 2-(N,N-dipropyl)amino-5,6-dihydroxytetralin, and that the preferential detection of oxiperomide and tiapride (clinically effective antidyskinetic drugs), may indicate the usefulness of 2-(N,N-dipropyl)amino-5,6-dihydroxytetralin as a potential model for predicting antidyskinetic activity.

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

A comparison was made of the doses of neuroleptic and related agents required to inhibit the stereotyped biting/gnawing/licking response induced in the guinea pig by systemically administered apomorphine and d-amphetamine or by dopamine administered bilaterally into the striatum. Haloperidol. lenperone, fluphenazine (0.5-8 mg/kg i.p.) and fluspirilene (0.125-8 mg/kg i.p.) each inhibited the stereotyped behaviour induced by apomorphine and amphetamine but doses up to 16 mg/kg i.p. failed to modify the dyskinesias induced by intrastriatal dopamine. Pimozide similarly abolished the stereotypies (0.125-8 mg/kg i.p.) but a larger dose (16 mg/kg i.p.) also abolished the dopamine-induced dyskinesias. However, oxiperomide and spiroxatrine were both shown to possess marked anti-dyskinetic (1-2 mg/kg i.p.) as well as anti-stereotypic (0.25-5 mg/kg i.p.) properties. Thioridazine, clothiapine, clozapine, sulpiride and metoclopramide were generally inactive against the stereotypies and dyskinesias although 1.25-5 mg/kg s.c. morphine effectively abolished stereotyped behaviour and the dopamine-induced dyskinesias were inhibited in a small proportion of animals. It is suggested that the dopamine mechanisms involved with stereotypy induction differ from those activated by intrastriatally administered dopamine to induce abnormal oro-facial movements, and that, since only the effects of intrastriatal dopamine showed the same relative degree of resistance to neuroleptic inhibition as clinical dyskinesias, that this may be more applicable to the clinical situation than the stereotypy model.

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.

DOI (1–100 μg/kg i.v.) induced an increase in mean blood pressure in the anaesthetized rat. Similarly, in the pithed rat, DOI (1–100 μg/kg i.v.) induced a dose-dependent increase in mean blood pressure, as did 5-HT. However, in contrast to 5-HT, DOI did not change the heart rate in either intact or pithed rats. In the pithed rat, the dose-pressor response curves to both 5-HT and DOI were unaffected by MDL 72222 (5-HT 3 receptor antagonist), spiroxatrine or (±)-pindolol) (5-HT 1A receptor antagonists), idazoxan ( α 2 -adrenoceptor blocking agent) and AR-C 239 ( α 1 -adrenoceptor blocking agent). Only the selective 5-HT 2 receptor antagonist, LY 53857, significantly and dose dependently shifted to the right the dose-response curves to both 5-HT and DOI. These results indicated that DOI possesses 5-HT 2 agonistic properties and that the pressor response induced by DOI in the pithed rat is mediated via 5-HT 2 receptors.

The effects of acute and long-term treatment with buspirone on synaptic transmission in the hippocampus were compared in alert rats with chronic indwelling electrodes and cannula. Buspirone produced a transient dose-dependent reduction in the amplitude of the excitory postsynaptic potential (EPSP) when acutely injected either systemically (0.3–3.0 mg/kg i.p.) or directly into the hippocampus (0.1–1.0 μg i.h.). Whereas acute application of 0.5 mg/kg i.p. produced a 20% reduction which reversed within 2 h, during long-term treatment with this relatively low dose there was a gradual reduction of the baseline EPSP amplitude which reached a maximum (40%) between days 7–14 and which did not reverse completely until 72 h after the last injection. Intrahippocampal injection of either buspirone or 5-hydroxytryptamine did not have any additional effect during the period of baseline reduction. The 5-HT 1A receptor antagonist spiroxatrine (1 mg/kg i.p.) produced a transient reversal of the effect of chronic buspirone. It is concluded that the chronic inhibitory effect of buspirone is probably an extension of its acute action on 5-HT 1A receptors in the hippocampus.

5-HT induced an increase in blood pressure in the pithed rat which was antagonized by LY 53857 a selective 5-HT 2 receptor antagonist. It was not antagonized by spiroxatrine, MDL 72222, idazoxan or AR-C 239, respectively 5-HT 1 -like and 5-HT 3 receptor antagonists, α 2 - and α 1 -adrenocepto antagonists. 5-MeODMT also induced an increase in blood pressure which was antagonized by LY 53857 but not by the other 5-HT receptor antagonists and α-adrenoceptor antagonists used, suggesting a 5-HT 2 component in the pressor effect of 5-MeODMT. The maximal effect of 5-MeODMT was less marked than that of 5-HT. 8-OH-DPAT, RU 24969 and TFMPP were far less effective than 5-HT and 5-MeODMT to increase blood pressure. In contrast, 5-CT induced a vasodepressor effect. It is therefore suggested that the vasoconstriction induced by 5-HT and 5-MeODMT in pithed rats could be due mainly to the selective stimulation of postjunctional 5-HT 2 receptors because selective α 2 - and α 2 -adrenoceptor antagonists were ineffective against the vasoconstrictor effects of 5-HT and 5-MeODMT. The relative lack of effect of 8-OH-DPAT, RU 24969 and TFMPP to increase blood pressure suggested that postjunctional 5-HT 1 -like receptors play only a minor role — if any — 5-HT induced vasoconstriction in the pithed rat.

beta-Endorphin administered intrathecally (i.t.) in rats produced a dose-dependent elevation in tail-flick latency. Naltrexone administered i.t. as a pretreatment reversed the spinal antinociceptive action of beta-endorphin, suggesting that the opioid interacts directly with spinal opiate receptors. Spinal administration of the alpha 1-adrenoceptor antagonist WB-4101 failed to alter the analgesic effects of the opioid, whereas the alpha 2-adrenoceptor antagonist yohimbine completely blocked beta-endorphin-induced elevations in tail-flick latency. Thus, there is an apparent specificity for the alpha 2-adrenoceptor to mediate the spinal action of beta-endorphin. The 5-HT1 and 5-HT3 receptor antagonists (spiroxatrine and ICS 205-930, respectively) also reversed the analgesic effects of the opioid, while the 5-HT2 receptor antagonist ritanserin only partially blocked beta-endorphin-induced elevations in tail-flick latency. The present results suggest that beta-endorphin produces analgesia at the spinal level via an opiate receptor-mediated interaction with spinal monoaminergic nerve terminals.