On the role of brain 5-HT7 receptor in the mechanism of hypothermia: comparison with hypothermia mediated via 5-HT1A and 5-HT3 receptor

Intracerebroventricular administration of selective agonist of serotonin 5-HT₇ receptor LP44 (4-[2-(methylthio)phenyl]-N-(1,2,3,4-tetrahydro-1-naphthalenyl)-1-pyperasinehexanamide hydrochloride; 10.3, 20.5 or 41.0 nmol) produced considerable hypothermic response in CBA/Lac mice. LP44-induced (20.5 nmol) hypothermia was significantly attenuated by the selective 5-HT₇ receptor antagonist SB 269970 (16.1 fmol, i.c.v.) pretreatment. At the same time, intraperitoneal administration of LP44 in a wide range of doses 1.0, 2.0 or 10.0 mg/kg (2.0, 4.0, 20.0 μmol/kg) did not cause considerable hypothermic response. These findings indicate the implication of central, rather than peripheral 5-HT₇ receptors in the regulation of hypothermia. The comparison of LP44-induced (20.5 nmol) hypothermic reaction in eight inbred mouse strains (DBA/2J, CBA/Lac, C57BL/6, BALB/c, ICR, AKR/J, C3H and Asn) was performed and a significant effect of genotype was found.In the same eight mouse strains, functional activity of 5-HT_1A and 5-HT₃ receptors was studied. The comparison of hypothermic responses produced by 5-HT₇ receptor agonist LP44 (20.5 nmol, i.c.v.) and 5-HT_1A receptor agonist 8-OH-DPAT 1.0 mg/kg, i.p. (3.0 μmol/kg), 5-HT₃ receptor agonist m-CPBG (40.0 nmol, i.c.v.) did not reveal considerable interstrain correlations between 5-HT₇ and 5-HT_1A or 5-HT₃ receptor-induced hypothermia. The selective 5-HT₇ receptor antagonist SB 269970 (16.1 fmol, i.c.v.) failed to attenuate the hypothermic effect of 8-OH-DPAT 1.0 mg/kg, i.p. (3.0 μmol/kg) and m-CPBG (40.0 nmol, i.c.v.) indicating that the brain 5-HT₇ receptor is not involved in the hypothermic effects of 8-OH-DPAT or m-CPBG. The obtained results suggest that the central 5-HT₇ receptor plays an essential role in the mediation of thermoregulation independent of 5-HT_1A and 5-HT₃ receptors.     

5-Hydroxytryptamine is emetogenic in the house musk shrew,Suncus murinus

Summary The emetic effects of 5-hydroxytryptamine (5-HT) and 5-HT₃ receptor agonists were investigated in the house musk shrew, Suncus murinus. 5-Hydroxytryptamine (5-HT; i.p., i.v., s.c.) and 2-methyl-5-HT (2-Me-5HT; i.p.) but not 5-hydroxyindoleacetic acid (i.p.) or 5-ethoxytryptamine (i.p.) induced emesis with very short latency. Tropisetron (ICS 205-930, a 5-HT₃ receptor antagonist, s.c.) blocked the emesis induced by 5-HT (10 mg/kg, i.p.) and 2-Me-5-HT (5 mg/kg, i.p.) with respective ID₅₀ values of 7.8 and 70.9 g/kg. Pindolol (5-HT₁ receptor antagonist) and ketanserin (5-HT₂ receptor antagonist) were about 100 times less potent than tropisetron. The emesis induced by 5-HT was prevented by surgical vagotomy but not by pretreatment with a combination of atropine (0.1 mg/kg, s.c.) and hexamethonium (10 mg/kg, s.c.). These results clearly indicate that 5-HT is emetogenic probably through a stimulation of peripheral 5-HT₃ receptors. Send offprint requests to N. Matsuki at the above address     

5-HT1A receptor antagonists increase the activity of serotonergic cells in the dorsal raphe nucleus in rats treated acutely or chronically with citalopram

In this study we have examined the acute effects of systemic administration of the selective serotonin reuptake inhibitor (SSRI), citalopram, in combination with either of the two selective 5-HT_1A receptor antagonists, (S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin [(S)-UH-301] or (+)-N-tertbutyl 3-(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenylpropionamide dihydrochloride [(+)-WAY100135], on the activity of single 5-HT neurons in the dorsal raphe nucleus (DRN) of anesthetized rats using extracellular recording techniques. Acute administration of citalopram (0.3 mg/kg i.v.) significantly decreased the firing rate of DRN-5-HT cells most likely as a result of indirect stimulation of inhibitory somatodendritic 5-HT_1A autoreceptors located on 5-HT cells in the DRN. This effect of citalopram was completely reversed by (S)-UH-301 (0.5 mg/kg i.v.) and partly by (+)WAY100135 (0.5 mg/kg i.v.). Furthermore, the inhibitory effect of citalopram on the activity of 5-HT neurons was significantly attenuated by pretreatment with (S)-UH-301 (0.25 mg/kg i.v.) or (+)-WAY100135 (0.25 mg/kg i.v.).We have also studied the effects of (S)-UH-301 (0.03–0.50 mg/kg i.v.) on the firing rate of single DRN5-HT cells in rats chronically treated with citalopram (20 mg/kg/day i.p. × 14 days). Administration of (S)UH-301 significantly and dose-dependently increased the activity of 5-HT cells in citalopram-treated rats, but did not affect these neurons in saline-treated (1 m1/kg/day i.p. × 14 days), control rats. Our results thus suggest that 5-HT_1A receptor antagonists can augment both the acute and chronic effects of citalopram on central serotonergic neurotransmission. Since the antidepressant effect of SSRIs is critically linked to the availability of 5-HT, these findings support the notion that 5-HT_1A receptor antagonists may not only shorten the latency of onset of SSRIs in the treatment of depression, but also increase their efficacy.     

Gastroprokinetic properties of the benzimidazolone derivative BIMU 1, an agonist at 5-hydroxytryptamine4 and antagonist at 5-hydroxytryptamine3 receptors

We have investigated the in vivo motor stimulating and gastroprokinetic properties of the azabicycloalkyl benzimidazolone derivative BIMU 1 (3-ethyl-2,3dihydro-N-(8-methyl-8-azabicyclo [3.2.1] oct-3-yl)-2-oxo 1H-benzimidazole-1-carboxamide hydrochloride) and its binding profile at 5-hydroxytryptamine₃ and 5-hydroxytryptamine₄ receptors, in an attempt to assess the serotonergic mechanism underlying its prokinetic action.BIMU 1 dose-dependently (0.01–0.3 mg/kg iv.) increased the motility of a denervated pouch of canine stomach. This excitatory action was sensitive to muscarinic blockade. A similar stimulatory effect was exerted by the benzamidic prokinetic agent cisapride (0.03–0.3 mg/kg i.v.) but not by the 5-HT₃ receptor antagonist ondansetron (up to 1 mg/kg i.v ). The significance for propulsive efficacy of the motor stimulating activity of BIMU 1 was evaluated in a model of gastric emptying of liquids in the conscious dog. The emptying rate of a non-caloric liquid meal instilled through a gastric fistula was accelerated by both BIMU 1 (0.01–1 mg/kg i.v. and 0.1–3 mg/kg p.o.) and cisapride (0.03–1 mg/kgiv.and0.3–10 mg/kgp.o.).Ondansetron (1 mg/kg i.v.) did not show any effect. The activity of the 5-HT₄ receptor antagonist DAU 6285 was evaluated in the gastric emptying model per se and in interaction experiments on the accelerating action of BIMU 1 (0.3 mg/kg L v.). At 1 mg/kg iv., DAU 6285 was ineffective on its own and failed to antagonize BIMU 1-induced prokinetic action;at the dose of 3 mg/kg i.v., it depressed the gastric emptying rate per se by 15% and totally abolished the accelerating effect of BIMU 1.In the binding assay, BIMU 1 exhibited an appreciable affinity for 5-HT3 receptors in NG 108-15 cells (K_D: 0.8 nmol/l) and for 5-HT₄ receptors in pig striatum (K_D: 26.5 nmol/l). Compared to BIMU 1, cisapride bound with a similar affinity to 5-HT₄ (K_D: 35.2 mnol/l) and a much lower affinity to 5-HT₃ receptors (K_D: 155 nmol/l). By contrast, ondansetron was highly selective for 5-HT₃ sites (K_D: 4.7 nmol/l), being ineffective in the assay for 5-HT₄ receptors (K_D > 10000).Our results show that BIMU 1, like cisapride and unlike ondansetron, is an effective stimulant of gastric motility and propulsion. The action of BIMU 1 appears to depend on 5-HT₄ receptor stimulation and to involve the activation of cholinergic nerve pathways. Correspondence to: C. A. Rizzi at the above address     

Antagonism by SB 204070 of 5-HT-evoked Contractions in the Dog Stomach: an In-vivo Model of 5-HT4 Receptor Function

The ability of 5-hydroxytryptamine (5-HT) to evoke contractile activity in the gastric Heidenhain pouch was measured in conscious dogs using a method in which 5-HT₄ receptor-antagonist activity can be measured in-vivo. At doses of 5-HT which evoked short-lived measurable responses (5 or 10 μg kg^?1, i.v.), it was found that this activity was greatly reduced by atropine (100 μg kg^?1, i.v.), but was unaffected by methysergide, methiothepin, ketanserin (each at 100 μg kg^?1, i.v.) or granisetron (10 or 100 μg kg^?1, i.v.). At best SDZ 205–557 2-diethylaminoethyl-[2-methoxy-4-amino-5-chloro] benzoate; 100 μg kg^?1, i.v.) reduced the action of 5-HT in 4/5 animals and increased it in the other but its effects were variable in magnitude and not consistently maintained. However, the more potent and selective 5-HT₄-receptor antagonist SB 204070 (1-butyl-4-piperidinylmethyl 8-amino-7-chloro-1,4-benzodioxan-5-carboxylate hydrochloride) dose-dependently antagonized the 5-HT-evoked contractions in all dogs tested. This action was reversible, but long-lasting with an effective half-life of 18·0 h when administered at 1 μg kg^?1. The estimated ID50 value was 0·55 μg kg^?1.     

Inhibition of the 5-HT-induced cardiogenic hypertensive chemoreflex by the selective 5-HT3 receptor antagonist ICS 205-930

Summary We investigated the effect of ICS 205-930 [(3-tropanyl)-1H-indole-3-carboxylic acid ester], a selective antagonist at 5-HT₃ receptors, on the cardiogenic hypertensive chemoreflex in the anaesthetized dog. The reflex was elicited by injection of 5-HT (12.5–1600 g) into the left cardiac ventricle and consisted of a dose-dependent systemic hypertension associated with a decrease in heart rate. ICS 205-930 (10, 30, and 100 g/kg i.v.) caused a displacement to the right of both the dose-response curves of 5-HT-induced blood pressure increase and heart rate reduction. Its blocking effects upon the action of 5-HT could be surmounted by increasing the dose of the agonist. The selective 5-HT₂ receptor antagonist, ketanserin (0.1 mg/kg i.v.) and the combined 5-HT₁ and 5-HT₂ receptor antagonist, methiothepin (0.1 mg/kg i.v.) had no influence on the hypertensive reflex. When the reflex was elicited by the ganglionic stimulant, 1,1-dimethyl-4-phenyl-piperazinium (DMPP; 100–1600 g), ICS 205-930 had no blocking effect. The results suggest that the 5-HT-induced cardiogenic hypertensive chemoreflex is mediated by 5-HT₃ receptors. Send offprint requests to H. Berthold at the above address     

Analysis of the heart rate effects of 5-hydroxytryptamine in the cat; mediation of tachycardia by 5-HT1-like receptors

Summary The effects of 5-hydroxytryptamine (5-HT) on heart rate in anaesthetized cats were analysed both in intact animals and after spinal section plus vagotomy.The intact cat responded to 5-HT (3, 10 and 30 g·kg^–1, i.v.) with a brief, but intense, bradycardia and a longerlasting hypotension. Administration of MDL 72222, a selective antagonist of M-type 5-HT receptors, blocked bradycardia elicited by 5-HT without affecting that caused by stimulation of the vagus nerve.In spinal cats the same doses of 5-HT increased heart rate and blood pressure. These effects remained essentially unchanged after bilateral adrenalectomy, guanethidine, propranolol and burimamide, suggesting that 5-HT acted directly on the myocardium and blood vessels. The tachycardic responses to 5-HT in spinal cats were little affected by 0.5 mg·kg^–1 doses of MDL 72222 or of the 5-HT₂ receptor antagonists, ketanserin, ritanserin or cyproheptadine. In contrast, the non-selective 5-HT receptor antagonist, methysergide, which binds to both 5-HT₁ and 5-HT₂ recognition sites in rat brain membranes, potently antagonized the 5-HT-induced tachycardia in doses of 0.05 to 0.5 mg·kg^–1. However pizotifen and mianserin, two other 5-HT₂ antagonists which show poor affinity for 5-HT₁ recognition sites, were also effective against the tachycardic response to 5-HT in doses of 0.5–4.5 mg·kg^–1. The pressor responses to 5-HT in the spinal cat were markedly inhibited by all six 5-HT₂ antagonists at a dose of 0.5 mg·kg^–1.5-Carboxamido-tryptamine, which has a high and selective affinity for 5-HT₁ recognition sites, elicited marked tachycardia in doses of 0.1–10 g/kg^–1 in spinal cats treated with saline. These responses were not affected in animals treated with 4.5 mg·kg^–1 of ketanserin, which was able to shift the dose-response curve for 5-HT to the right. On the other hand, methysergide (0.5 mg·kg^–1) displaced the dose-response curves for both 5-carboxamidotryptamine and 5-HT to a similar extent.Unlike on the dog saphenous vein, methysergide showed no agonist effects on heart rate in the spinal cat.On the basis of the above results, we conclude that: (i) the reflexogenic bradycardic response elicited by 5-HT overshadows its direct tachycardic response on heart rate in the intact cat; (ii) M-type 5-HT receptors mediate the bradycardic response; (iii) the pressor response to 5-HT in the spinal cat involves 5-HT₂ receptors; (iv) tachycardia by 5-HT in the spinal cat is mediated mainly by 5-HT₁-like receptors, but an additional, though less important, non-5-HT₁ mechanism may also be involved; (v) the cardiac 5-HT₁ receptors are similar, but perhaps not identical, to those delineated in the dog saphenous vein or rat brain membrane preparations; and (vi) the tachycardic responses to 5-HT and, in particular the more selective, 5-carboxamidotryptamine may be conveniently utilized to characterize new chemical compounds designed for potential 5-HT₁ receptor antagonist activity.     

Simultaneous blockade of 5-HT<Subscript>1A/B</Subscript> receptors and 5-HT transporters results in acute increases in extracellular 5-HT in both rats and guinea pigs: in vivo characterization of the novel 5-HT<Subscript>1A/B</Subscript> receptor antagonist/5-HT transport inhibitor SB-649915-B

Rationale The delay in onset and treatment resistance of subpopulations of depressed patients to conventional serotonin reuptake inhibitors has lead to new drug development strategies to produce agents with improved antidepressant efficacy. Objectives We report the in vivo characterization of the novel 5-HT_1A/1B autoreceptor antagonist/5-HT transporter inhibitor (6-[(1-{2-[(2-methyl-5-quinolinyl)oxy]ethyl}-4-piperidinyl)methyl]-2H-1,4-benzoxazin-3(4H)-one), SB-649915-B. Materials and methods Ex vivo binding was used to ascertain 5-HT_1A receptor and serotonin transporter occupancy. 8-OH-DPAT-induced hyperlocomotion and SKF-99101-induced elevation of seizure threshold were used as markers of central blockade of 5-HT_1A and 5-HT_1B receptors, respectively. In vivo electrophysiology in the rat dorsal raphe and microdialysis in freely moving guinea pigs and rats were used to evaluate the functional outcome of SB-649915-B. Results SB-649915-B (1–10 mg/kg p.o.) produced a dose-related inhibition of 5-HT_1A receptor radioligand binding and inhibited ex vivo [³H]5-HT uptake in both guinea pig and rat cortex. SB-649915-B (0.1–10 mg/kg p.o.) reversed both 8-OH-DPAT-induced hyperlocomotor activity and SKF-99101-induced elevation of seizure threshold in the rat, demonstrating in vivo blockade of both 5-HT_1A and 5-HT_1B receptors, respectively. SB-649915-B (0.1–3 mg/kg i.v.) produced no change in raphe 5-HT neuronal cell firing per se but attenuated the inhibitory effect of 8-OH-DPAT. Acute administration of SB-649915-B resulted in increases (approximately two- to threefold) in extracellular 5-HT in the cortex of rats and the dentate gyrus and cortex of guinea pigs. Conclusions Based on these data, one may speculate that the 5-HT autoreceptor antagonist/5-HT transport inhibitor SB-649915-B will have therapeutic efficacy in the treatment of affective disorders with the potential for a faster onset of action compared to current selective serotonin reuptake inhibitors.     

Inhibitory effects of the psychoactive drug modafinil on γ-aminobutyric acid outflow from the cerebral cortex of the awake freely moving guinea-pig

Summary The effects of modafmil on acetylcholine and GABA outflow from the cerebral cortex of awake freely moving guinea pigs provided with an epidural cup were studied. In the dose range of 3–30 mg/kg s. c. modafmil produced a dose dependent significant inhibition of GABA outflow without influencing cortical acetylcholine release. Methysergide (2 mg/kg, i.p.) and ketanserin (0.5 mg/kg, i. p.) but not prazosin (0.14 mg/kg, i. p.) counteracted the inhibitory action of modafinil on cortical GABA outflow. Modafinil both acutely and chronically in the same dose range increased striatal 5-HIAA levels and 5-HT utilization in the rat (acute) and mouse (chronic). The action on cortical GABA release may be dependent on activity at 5-HT₂ receptors, since the action of modafmil in this respect is blocked by the non-selective 5-HT antagonist methysergide and the 5-HT₂ antagonist ketanserin. The involvement of 5-HT mechanisms in the inhibitory action of modafmil on cortical GABA release is also suggested by the findings that 5-HT metabolism may become increased by modafmil at least in the striatum. The reduction of cortical GABA outflow via 5-HTZ receptors by modafmil is probably related to some of its actions on the central nervous system including behavioural effects. Send offprint requests to K. Fuxe at the above address     

Antiemetic effects of YM060, a potent and selective serotonin (5HT)3-receptor antagonist, in ferrets and dogs.

YM060, (R)-5-[(1-methyl-3-indolyl)carbonyl]-4,5,6,7-tetrahydro-1H-benzimidazole hydrochloride, is a new serotonin (5HT)3-receptor antagonist. We examined the effects of YM060 on chemotherapeutic agent-, apomorphine- and copper sulfate-induced emesis. Intravenous YM060 potently prevented cisplatin (10 mg/kg, i.v.)-induced emesis with ED50 values of 0.06 (0.05-0.07) micrograms/kg, i.v. in ferrets. Based on the ED50 values, YM060 was 300, 20 and 100 times more potent than ondansetron, granisetron and the S-isomer of YM060, respectively. The relative potencies of these drugs described above were similar to those in the previously reported 5HT3-receptor antagonism. YM060 given orally also potently inhibited cisplatin (10 mg/kg, i.p.)- and cyclophosphamide (200 mg/kg, i.p.)-induced emesis in ferrets with ED50 values of 0.1 (0.09-0.11) and 0.02 (0.16-0.27) micrograms/kg, p.o., respectively. All tested 5HT3-receptor antagonists including YM060 failed to prevent apomorphine (0.1 mg/kg, s.c.)-induced emesis in dogs and copper sulfate (1%, 10 ml, p.o.)-induced emesis in ferrets. Our data indicate that YM060 is a highly potent inhibitor of chemotherapeutic agent-induced emesis and that the antiemetic effect of YM060 may be depend on 5HT3-receptor antagonism.     

Diltiazem potentiates pentobarbital-induced hypnosis via 5-HT1A and 5-HT2A/2C receptors: role for dorsal raphe nucleus

It has been reported that the sedative component of pentobarbital is mediated by GABA receptors in an endogenous sleep pathway and the ventrolateral preoptic area (VLPO)-tuberomammillary nucleus (TMN) or VLPO-dorsal raphe nucleus (DRN) neural circuit is important in the sedative response to pentobarbital. Our previous findings indicated that the VLPO-TMN neuronal circuit may play crucial part in the augmentative effect of diltiazem on pentobarbital sleep and the serotonergic system may be involved. This study was designed to investigate the role of DRN and the serotonergic receptors 5-HT_1A and 5-HT_2A/2C in the augmentative effect of diltiazem on pentobarbital-induced hypnosis in rats. The results showed that diltiazem (5 mg/kg, i.g.) significantly reversed pentobarbital-induced (35 mg/kg, i.p.) reduction of c-Fos expression in 5-HT neurons of DRNV (at − 7.5 mm Bregma), DRND, DRNVL and MRN (at − 8.0 mm Bregma). However it did not influence this reducing effect of pentobarbital on non-5-HT neurons either in DRN or in MRN. Moreover, the effect of diltiazem (1 or 2 mg/kg, i.g.) on pentobarbital-induced (35 mg/kg, i.p.) hypnosis was significantly inhibited by 5-HT_1A agonist 8-OH-DPAT (0.5 mg/kg, i.p.) and 5-HT_2A/2C agonist DOI (0.5 mg/kg, i.p.), and potentiated by 5-HT_1A antagonist p-MPPI (2 mg/kg, i.p.) and 5-HT_2A/2C antagonist ritanserin (2 mg/kg, i.p.), respectively. From these results, it should be presumed that the augmentative effect of diltiazem on pentobarbital-induced sleep may be related to 5-HT_1A and 5-HT_2A/2C receptors, and DRN may be involved. In addition, it also suggested that the DRN may play a multi-modulating role in sleep-wake regulation rather than being recognized simply as arousal nuclei.     

Serotonergic modulation of extrapyramidal motor disorders in mice and rats: Role of striatal 5-HT3 and 5-HT6 receptors

Previous studies showed that 5-HT_1A and 5-HT₂ receptors play an important role in controlling the extrapyramidal motor disorders. However, the functions of other 5-HT receptor subtypes remain elusive. To elucidate the role of 5-HT receptors, specifically of 5-HT₃～5-HT₇ subtypes, in modifying antipsychotic- induced extrapyramidal side effects (EPS), we studied the effects of the 5-HT stimulant 5-hydroxytryptophan (5-HTP) and various 5-HT receptor antagonists on haloperidol (HAL)-induced bradykinesia and catalepsy in mice and rats. Pretreatment of mice with 5-HTP (25–100 mg/kg, i.p.) dose-dependently enhanced HAL (0.3 mg/kg, i.p.)-induced bradykinesia and catalepsy. The potentiation of HAL-induced EPS by 5-HTP (50 mg/kg, i.p.) was significantly inhibited by ritanserin (5-HT₂ antagonist, 0.3-3 mg/kg, i.p.), ondansetron (5-HT₃ antagonist, 0.1–1 mg/kg, i.p.), or SB-258585 (5-HT₆ antagonist, 1–10 mg/kg, i.p.) in a dose-dependent manner. However, neither WAY-100135 (5-HT_1A antagonist, 1–10 mg/kg, i.p.), GR-125487 (5-HT₄ antagonist, 1–10 mg/kg, i.p.), SB-699551 (5-HT_5A antagonist, 1–10 mg/kg, i.p.) nor SB-269970 (5-HT₇ antagonist, 1–10 mg/kg, i.p.) reduced the 5-HTP and HAL-induced bradykinesia or catalepsy. In addition, both ondansetron (0.1–1 mg/kg, i.p.) and SB-258585 (3 and 10 mg/kg, i.p.) also alleviated bradykinesia and catalepsy induced by HAL (0.5 mg/kg, i.p.) alone in mice. Furthermore, bilateral microinjection of ondansetron (5 μg (13.7 nmol) per side) or SB-258585 (5 μg (8.92 nmol) per side) into the dorsolateral striatum (dlST) attenuated haloperidol-induced catalepsy in rats. These results suggest that serotonergic stimulation augments extrapyramidal motor disorders by activating the striatal 5-HT₃ and 5-HT₆ receptors and the antagonism of these receptors effectively alleviates antipsychotic-induced EPS.     

Involvement of 5-hydroxytryptamine and bradykinin in the hyperalgesia induced in rats by collagenase from clostridium histolyticum

The involvement of bradykinin, 5-hydroxytryptamine, substance P and prostanoids in the hyperalgesia elicited by collagenase in rat paw was investigated. Collagenase (100μg) induced a slight hyperalgesia in kininogen deficient rats in comparison with the behavioural response obtained in normal rats. Lisinopril (10^–5M), and angiotensin-converting enzyme inhibitor, increased the duration of the hyperalgesia elicited in normal rats. Ondansetron (0.5 to 5μmol/kg), a 5-HT₃ antagonist, suppressed the hyperalgesia as did methysergide (1.1 to 11μmol/kg), a mixed 5-HT₁ and 5-HT₂ receptor antagonist. However, the hyperalgesia was not modified by RP67580 (1.8 to 18μmol/kg), a NK₁ receptor antagonist, and was only slightly delayed by indomethacin (2mg/kg), a cyclo-oxygenase inhibitor. The oedema-promoting effect of 5-HT (6nmol) was inhibited by methysergide but not by ondansetron. The swelling induced by collagenase in rat paw was reduced by methysergide but not by ondansetron. We conclude that the behavioural reponse induced by collagenase depends on an interactions between bradykinin and 5-HT. Prostanoids play a minor role in the beginning of the reaction whereas substance P is not significantly involved in this hyperalgesia. Received: 23 September 1996 / Accepted: 20 January 1997     

Functional behavioral homology between rat 5-HT1B and guinea pig 5-HT1D receptors in the modulation of prepulse inhibition of startle

The serotonin (5-HT) 1B receptor in rats and mice appears to be homologous to the 5-HT_1D receptor found in other mammals, such as guinea pigs and humans. The present series of experiments explored the functional similarity between the rat 5-HT_1B receptor and the guinea pig 5-HT_1D receptor on two behavioral measures known to be influenced by 5-HT_1B receptor manipulations in rats: prepulse inhibition of the startle response (PPI) and locomotor activity. Because the 5-HT_1B agonist RU 24969 disrupts PPI and stimulates locomotor behavior in rats, it was predicted that the 5-HT_1D agonist, SDZ 219–964, would demonstrate a similar behavioral profile in guinea pigs. In support of this hypothesis, SDZ 219–964 was found to disrupt PPI dose-dependently (1.0 and 2.0 mg/kg) without significantly affecting startle amplitude and to increase locomotor activity (0.5–2.0 mg/kg) in guinea pigs. In guinea pigs, RU 24969 failed to affect PPI, although it did increase locomotor activity, indicating that RU 24969 may have activity at the 5-HT_1D receptor. As expected, RU 24969 in rats disrupted PPI (2.5 and 5.0 mg/kg) and significantly increased locomotor activity (1.25–5.0 mg/kg). In rats, however, SDZ 219–964 had generalized, stimulatory effects on startle reactivity, without independent effects on PPI or locomotor activity. The spatial patterns of locomotion exhibited by guinea pigs treated with SDZ 219–964 versus those of rats treated with RU 24969 demonstrate important qualitative differences in structure, indicating that the neural substrates subserving these effects may be different. It is concluded that a functional similarity exists between 5-HT_1D and 5-HT_1B receptors with regard to the modulation of sensorimotor inhibition and, to a lesser extent, locomotor activity.     

Inhibition of noradrenaline release from the sympathetic nerves of the human saphenous vein via presynaptic 5-HT receptors similar to the 5-HT1D subtype

Summary The human saphenous vein preincubated with [³H]noradrenaline was used to determine the pharmacological properties of the release-inhibiting presynaptic serotonin (5-HT) receptor on the sympathetic nerves. The overflow of tritium evoked by transmural electrical stimulation (2 Hz) was concentration-dependently inhibited by drugs known to stimulate 5-HT receptors in the following rank order: oxymetazoline 5-HT 5-carboxamidotryptamine = 5-methoxytryptamine = sumatriptan > tryptamine > N,N(CH₃)₂-5-HT = yohimbine = 8-hydroxy-2-(di-n-propylamino)-tetraline. The potencies of these agonists in inhibiting overflow were significantly correlated with their affinities for 5-HT_1B and 5-HT_1D binding sites, but not with those for 5-HT_1A or 5-HT_1C binding sites. 5-Aminotryptamine, methysergide, ipsapirone, cyanopindolol, SDZ 21009 and metergoline dit not produce a significant inhibition. Metitepine and methysergide antagonized the inhibitory effect of 5-HT, whereas spiroxatrine, propranolol, ketanserin and ICS 205-930 did not.These data exclude the idea that the inhibitory presynaptic 5-HT receptor on the sympathetic nerves belongs to the 5-HT₂ and 5-HT₃ receptor class; the pattern of agonist potencies suggests that the receptor is very similar to the 5-HT_1D receptor subtype. Send offprint requests to M. Gothert at the above address     

Actions of non-peptide ergot alkaloids at 5-HT1-like and 5-HT2 receptors mediating vascular smooth muscle contraction

Summary This report describes the actions of the non-peptide ergot alkaloids methysergide, methylergometrine and ergometrine at two types of 5-HT receptor mediating vascular contraction; the well established 5-HT₂ receptor in rabbit aorta and a non-5-HT₂ receptor in rabbit saphenous vein which resembles the 5-HT₁-like receptor in dog saphenous vein.In the rabbit aorta ergometrine (1 mol/l) and methylergometrine (0.3 mol/l), but not methysergide, produced small contractions (14% and 7% respectively of the maximal response to 5-HT). This contraction was not related to activation of 5-HT₂ receptors since it was resistant to blockade by ketanserin (0.3 mol/l). When examined as antagonists of 5-HT-induced contractions of rabbit aorta, each ergot displayed nanomolar affinity at the 5-HT₂ receptor but only methysergide behaved as a simple competitive antagonist (pK_B = 8.25). Methylergometrine and ergometrine produced surmountable blockade which was accompanied by a non-parallel displacement of the 5-HT concentration-effect curves. The selective 5-HT₁-like receptor agonist GR43175 ( 30 mol/l) was devoid of affinity at the 5-HT₂ receptor in rabbit aorta.In the rabbit saphenous vein each of the ergots produced concentration-dependent contractions which resulted in overtly biphasic concentration-effect curves. Only the first phase of contraction mimicked the effects of 5-HT and GR43175 since contractions were not blocked by MDL 72222 (1 mol/l), but were surmountably antagonised by methiothepin (10 nmol/1), ketanserin (0.3 mol/l) and spiperone (0.3 mol/l). These results are expected for interactions at the 5-HT₁-like receptor in this preparation (Martin and MacLennan 1990). The mechanism(s) underlying the second phase of contraction with the ergots remains to be established. Receptor inactivation studies using the alkylating agent benextramine tetrahydrochloride enabled each agonists' affinity and efficacy at the 5-HT₁-like receptor to be estimated. Affinity estimates (pK_A) decreased in the order: methylergo metrine (7.79), ergometrine (7.75), 5-HT (7.19), methysergide (6.76), GR43175 (6.20), whereas efficacies () decreased in the order: 5-HT (3.28), methylergometrine (2.24), GR43175 (2.14), ergometrine (1.94), methysergide (0.99). Of particular interest, methysergide was significantly lower in affinity and efficacy than its primary demethylated metabolite methylergometrine. Evidently, at the 5-HT₁-like receptor mediating vascular contraction the ergots ergometrine and methylergometrine are both higher in affinity than, and comparable in efficacy to, the natural receptor agonist 5-HT. This contrasts with their actions at the 5-HT₂ receptor in rabbit aorta where they demonstrated a higher affinity but much lower intrinsic efficacy than 5-HT. These results favour the view that vascular contraction induced by these ergots is more likely to be mediated by 5-HT₁-like, rather than 5-HT₂ receptors. These results are discussed in relation to the therapeutic applications of these ergots, particularly in obstetrics and in migraine, and to their utility as diagnostic agents in patients with Prinzmetal's variant form of angina.Send offprint requests to S. J. MacLennan at the above address     

The pharmacology of the hypothermic response in mice to 8-hydroxy-2-(DI-n-propylamino)tetralin (8-OH-DPAT) : A model of presynaptic 5-HT1 function

In the mouse, injection (subcutaneously) of the putative 5-HT₁ agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), produced a dose-related hypothermia (ED₅₀:0.36mg/kg). A maximum response was elicited by intracerebroventricular (i.c.v.) injection of 8-OH-DPAT (3 μg) and almost abolished by lesion of 5-HT-containing terminals in the brain with 5,7-dihydroxytryptamine (5,7-DHT; i.c.v.) or long-term treatment with p-chlorophenylalanine. The response was unaltered by a range of neurotransmitter antagonists: prazosin (α₁-adrenoceptor), idazoxan (α₂-adrenoceptor), metoprolol (β₁-adrenoceptor), erythro-dl-1-(7-methylindan-4-yloxy)-3-isopropylamino-butan-2-ol (β₂-adrenoceptor), (?)propranolol or (±)pindolol (β-adrenoceptor), flupenthixol (dopamine) or Ro 15–1788 (benzodiazepine binding site). Classical 5-HT antagonists (methysergide, metergoline, cinanserin and methiothepin) were either without effect or facilitated the response and the 5-HT₂ antagonist, ritanserin was also without effect. In contrast, quipazine and haloperidol produced a dose-related antagonism of the response.Since the response was almost abolished by a lesion induced by 5,7-DHT and was antagonised by quipazine, which is known to antagonise presynaptic 5-HT function in vitro, it is suggested that the hypothermie response is due to 8-OH-DPAT acting as an agonist at a presynaptic 5-HT receptor, which also appears to be sensitive to butyrophenones (the antagonism elicited by haloperidol but not by flupenthixol). The hypothermie response of mice to 8-OH-DPAT, therefore, may provide a simple and convenient in vivo model in which to measure the function of the presynaptic 5-HT receptor.     

AN INVESTIGATION INTO THE MECHANISMS OF THE CARDIOVASCULAR EFFECTS OF 5-HYDROXYTRYPTAMINE IN CONSCIOUS NORMOTENSIVE AND DOCA-SALT HYPERTENSIVE RATS

• 1 The actions of intravenously administered 5-hydroxytryptamine (5-HT) have been analysed in conscious DOCA-salt hypertensive rats using selective 5-HT receptor agonists and antagonists to determine the receptor mechanisms involved and to compare them with those in conscious normotensive rats.
• 2 In both normotensive and hypertensive rats 5-HT, 3 and 10 μg i.v., produced a complex triphasic effect on blood pressure consisting of an initial short lasting depressor response, which was followed by a pressor response and then, finally, a hypotensive phase. Marked decreases in heart rate were observed immediately after dosing, which were followed by small increases in rate.
• 3 The selective 5-HT₃-receptor agonist, 2-methyl 5-HT, 3–30 μg i.v., produced immediate and marked dose-related decreases in blood pressure and heart rate in both normotensive and DOCA-salt hypertensive rats. The 5-HT₃-receptor antagonist, MDL 72222, 0.03 and 0.1 mg/kg i.v., antagonised these effects in both normotensive and DOCA-salt hypertensive rats. Treatment with MDL 72222, 0.3 mg/kg i.v., abolished the initial depressor response and bradycardia produced by 5-HT.
• 4 The 5-HT₂ receptor agonist, α-methyl 5-HT, 3–30 μg i.v., produced dose-related increases in blood pressure which were significantly greater in magnitude in DOCA-salt hypertensive than normotensive rats. Bradycardia was observed consistently at 30 μg only. The 5-HT₂ receptor antagonist, ketanserin, 0.03-0.3 mg/kg i.v., caused a dose-dependent antagonism of the pressor responses produced by α-methyl 5-HT, but had no effect on the increases in blood pressure produced by angiotensin. Ketanserin also antagonised the pressor responses produced by 5-HT in rats pretreated with MDL 72222.
• 5 5-Carboxamidotryptamine (5-CT), the selective ‘5-HT₁-like’ receptor agonist, at doses of 0.1–3 μg i.v. produced dose-related decreases in blood pressure which were more pronounced in the DOCA-salt hypertensive rats than in normotensive rats. These depressor responses were dose-dependently antagonised by methiothepin, 0.3 and 1 mg/kg, i.v. which did not antagonise the depressor responses produced by isoprenaline 0.1 μg/kg i.v. In rats pretreated with MDL 72222 and ketanserin, 5-HT produced dose-related depressor responses which were also antagonised by methiothepin 1 mg/kg i.v. and presumably mediated by 5-HT₁-like receptors.
• 6 In this study, selective 5-HT receptor agonists and antagonists have been used to mimic and block, respectively, the various phases of the 5-HT response. It is concluded that in the conscious rat, the complex cardiovascular effects of 5-HT involve stimulation of at least three different 5-HT receptors (for nomenclature see Bradley, Engel, Feniuk, Fozard, Humphrey, Middlemiss, Mylecharane, Richardson & Saxena, 1986). The initial depressor response and bradycardia involves activation of 5-HT₃-receptors, the secondary vasopressor effect, which is significandy greater in DOCA-salt than normotensive rats results from stimulation of 5-HT₂ receptors and the late vasodepressor response is due to vasodilatation via ‘5-HT₁-like’ receptors.

     

Effects of the 5-HT1 receptor agonists DP-5-CT,CGS 12066B,and RU 24969 on plasma adrenaline and glucose levels in the rat

Summary Recent results have indicated that the 5-HT_1A receptor subtype mediates the adrenaline-releasing and hyperglycemic effects of 8-hydroxy-2-(di-n-propylamino)tetralin in the rat. The aim of this study was to analyse, by means of the peripherally acting 5-HT_1A receptor agonist, N,N-dipropyl-5-carboxamidotryptamine (DP-5-CT), whether these 5-HT_1A receptors are peripherally or centrally localised. In view of the appreciable affinity of DP-5-CT for the 5-HT_1D receptor subtype, the effects of the mixed 5-HT_1B/5-HT_1D receptor agonist 7-trifluoromethyl-4-(4-methyl-1-piperazinyl)-pyrrolo(1,2-a)quinoxaline (CGS 1206613), and the mixed 5-HT_1A5-HT_1B/5-HT_1D receptor agonist 5-methoxy-3(1,2,3,6-tetrahydropyridine-4-yl)1H-indole (RU 24969) were also investigated.Administration of DP-5-CT (0.3 and 1 mg/kg i. v.) increased plasma glucose levels dose-dependently, whereas only the 1 mg/kg dose of DP-5-CT elicited a rise in plasma adrenaline levels. In contrast, CGS 1206613(1.5 and 4.5 mg/kg 1. v.) did not affect either plasma adrenaline or plasma glucose levels. Administration of RU 24969 (0.5–4.5 mg/kg i. v.) increased dose-dependently both plasma adrenaline and glucose levels. The data suggest that central 5-HT_1A receptors, but neither 5-HT_1B nor 5-HT_1D receptors, regulate plasma adrenaline and glucose levels. Send offprint requests to F. Chaouloff at the above address