A pre-junctional action of 5-hydroxytryptamine and methysergide on noradrenergic nerves in dog isolated saphenous vein

History of Methysergide in Migraine

Harold Wolff's theory of vasodilation in migraine is well-known. Less known is his search for a perivascular factor that would damage local tissues and increase pain sensitivity during migraine attacks. Serotonin was found to be among the candidate agents to be included. In the same period, serotonin was isolated (1948) and, because of its actions, an anti-serotonin drug was needed. Methysergide was synthesized from lysergic acid (LSD) by adding a methyl group and a butanolamid group. This resulted in a compound with selectivity and high potency as a serotonin (5-HT) inhibitor. Based on the possible involvement of serotonin in migraine attacks, it was introduced in 1959 by Sicuteri as a preventive drug for migraine. The clinical effect was often excellent, but 5 years later it was found to cause retroperitoneal fibrosis after chronic intake. Consequently, the use of the drug in migraine declined considerably, but it was still used as a 5-HT antagonist in experimental studies. In 1974 Saxena showed that methysergide had a selective vasoconstrictor effect in the carotid bed and in 1984 he found an atypical receptor. This finding provided an incentive for the development of sumatriptan. Bredberg et al. showed that methysergide is probably a prodrug for its active metabolite methylergometrine. Whereas methysergide is ‘a clean drug’, methylergometrine is ‘a relatively dirty drug’ with additional dopaminergic activity. The mechanism for the preventive effect of methysergide (methylergometrine) in migraine remains elusive. We describe the rise, fall and subsequent use as a third-choice drug of the first effective migraine prophylactic, methysergide.

Serotonin receptors: subtypes, functional responses and therapeutic relevance

Recent, rapid progress in the molecular biology of serotonin (5-HT) receptors requires conceptual re-thinking with respect to receptor classification. Thus, based on operational criteria (agonist and antagonist rank order), as well as transduction mechanisms involved and the structure of the receptor protein, the Nomenclature Committee of the Serotonin Club has proposed the following classification and nomenclature: the main receptor types 5-HT1 to 5-HT4, recombinant receptors (e.g. 5-ht5 to 5-ht7) and 'orphan' receptors. The aim of the present review is to discuss the events leading to this classification, the criteria for and functional responses mediated by various 5-HT receptors, as well as the therapeutic possibilities with 5-HT ligands.

Inhibition of serotonin-induced increase in canine external carotid blood flow by drugs that decrease the sympathetic outflow

1. The present study was designed to analyse the effect of the centrally-acting sympatho-inhibitory drugs, prazosin and ketanserin, on the increase in external carotid blood flow (external CBF) produced by 5-hydroxytryptamine (5-HT) in pentobarbital-anaesthetized dogs. 2. Intracarotid (i.c.) infusions of 5-HT (10 micrograms min-1 during 1 min) produced an increase in external CBF without changes in mean arterial blood pressure or heart rate. This response to 5-HT was dose-dependently blocked by intravenous (i.v.) administration of prazosin (1, 3, 10, 30 and 100 micrograms kg-1) or ketanserin (10, 30, 100 and 300 micrograms kg-1). 3. Furthermore, 5-HT-induced increase in external CBF was inhibited by either the ganglionic blocking agent, mecamylamine (0.03, 0.1, 0.3, 1, 3 and 10 mg kg-1), the mixed 5-HT1-like and 5-HT2 receptor antagonist, methiothepin (3, 10 and 30 micrograms kg-1) or the 5-HT1A ligand, spiroxatrine (10, 30, 100 and 300 micrograms kg-1). In contrast, the selective 5-HT2 and 5-HT1C receptor antagonist, ritanserin (30 and 100 micrograms kg-1, i.v.), was unable to block the above response to 5-HT. 4. It is concluded that the inhibition of 5-HT-induced increase in external CBF by prazosin, ketanserin, mecamylamine and spiroxatrine is due to a reduction in the sympathetic tone and not to a blockade of 5-HT receptors.

Vascular receptors for 5-hydroxytryptamine: distribution, function and classification

The complex actions of 5-hydroxytryptamine (5-HT) on blood vessels result from interactions with a multiplicity of specific 5-HT receptors. Dramatic advances recently have been made in defining the receptor subtypes involved in terms of their pharmacology and biochemistry, as well as their molecular biology, provoking an evolution of the criteria used for classifying and naming them. This article reviews the distribution and function of different 5-HT receptor types present on vascular smooth muscle, endothelium and perivascular neurones, and considers ways in which they can be positively defined and differentiated using traditional pharmacological approaches. The characteristics of each receptor type are also considered in terms of current biochemical and molecular perspectives on 5-HT receptor classification.

Effect of serotonin receptor blockade on endocrine and cardiovascular responses to head-up tilt in humans

Effects of blockade of serotonin (5-HT) receptors on the integrated cardiovascular and endocrine adaptations during head-up tilt were investigated in normal men. In control experiments 50 degrees head-up tilt increased heart rate (HR), total peripheral resistance (TPR), plasma renin activity (PRA) and sympathetic activity (plasma noradrenaline; NA). A moderate increase in pituitary-adrenal hormones (plasma ACTH, beta-END and cortisol) was observed. After a mean tilt time of 30 +/- 5 min (n = 20) presyncopal symptoms associated with decreases in HR, TPR and arterial pressure occurred. At this time pituitary hormones, cortisol, adrenomedullary (plasma adrenaline; A) as well as vagal activity (plasma pancreatic polypeptide) were markedly increased, whereas sympathetic activity (plasma NA) decreased. The 5-HT1+2 receptor antagonist methysergide did not significantly interfere with cardiovascular variables but attenuated the response of NA, prolactin (PRL), beta-endorphin (beta-END) and PRA (P < 0.02). The 5-HT2-receptor antagonist ketanserin reduced the tolerated tilt time (10 +/- 4 vs. 32 +/- 2 min; P < 0.0003, n = 7) but had no significant effects on hormonal variables. The 5-HT3-receptor antagonist ondansetron abolished the adrenomedullary response to hypotension without affecting cardiovascular tolerance or the activity of the pituitary-adrenal axis. The results suggest that serotonergic mechanisms may be involved in the integrated cardiovascular and endocrine responses to central blood volume depletion in humans.

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 that the reversible MAO-A inhibitor moclobemide increases prolactin secretion by a serotonergic mechanism in healthy male volunteers

The serotonin receptor antagonist methysergide was used to investigate the mechanism mediating stimulation of prolactin release after single doses of the reversible MAO-A inhibitor moclobemide. Eight healthy male volunteers participated in a placebo-controlled cross-over study, where pretreatment with methysergide almost totally prevented the moclobemide-induced increase in plasma prolactin levels. MAO-A inhibition, as evidenced by up to 80% decreases in the plasma concentration of 3,4-dihydroxyphenylglycol, a deaminated metabolite of norepinephrine, was similar after both pretreatments. This result suggests that moclobemide stimulates prolactin release through activation of serotonergic receptors, and provides evidence that the drug is capable of augmenting central serotonergic neurotransmission in humans.

Partial agonistic activity of GR43175 at the inhibitory prejunctional 5-HT1-like receptor in rat kidney

1. The pharmacological effect of the selective 5-HT receptor agonist GR43175 has been studied in the isolated perfused rat kidney with particular reference to the inhibitory prejunctional 5-HT1-like receptor. 2. GR43175 (1 x 10(-8) to 1 x 10(-5) M) inhibited the electrically evoked release of tritium from renal noradrenergic nerves but did not affect the basal (non-stimulated) release of tritium or perfusion pressure. 3. The inhibitory prejunctional action of GR43175, as well as that of 5-HT, was blocked by methiothepin, suggesting that both agonists act at the inhibitory 5-HT1-like site. 4. GR43175, relative to 5-HT, acted as a weak partial agonist at the inhibitory prejunctional 5-HT1-like receptor and antagonized 5-HT-induced inhibition of tritium release, suggesting again that a common site of action is involved. 5. The results support the conclusion that the 5-HT1-like receptor activated by GR43175 in the rat kidney is very similar to that identified in dog saphenous vein and certain other tissues.

Inhibition of noradrenaline release in the pig coronary artery via a novel serotonin receptor

In pig coronary artery preincubated with [3H]noradrenaline, the effects of serotonin (5-HT) receptor agonists and antagonists on the electrically evoked (0.66 Hz) tritium overflow were determined. Tritium overflow was inhibited by 5-HT, 5-aminotryptamine, N,N-dimethyl-5-hydroxytryptamine, 5-hydroxytryptamine, 5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole (RU 24969) and tryptamine. The maximum inhibition obtainable with 5-HT was by about 35%, its pIC20 value was 7.85. 8-Hydroxy-di(n-propylamino)tetralin, urapidil, ipsapirone, 5-carboxamidotryptamine, 4-hydroxytryptamine, 5-methoxytryptamine and alpha-methyl-5-hydroxytryptamine did not decrease 3H overflow. The inhibitory effect of 5-HT was not antagonized by ketanserin, mesulergine, metitepine, propranolol, (3 alpha-tropanyl)-1H-indole-3-carboxylic acid ester (ICS 205-930) and yohimbine. Additionally, it was not altered by indomethacin. We conclude from the present data that the sympathetic nerves of the pig coronary artery are endowed with inhibitory presynaptic 5-HT receptors which do not belong to the 5-HT1, 5-HT2 or 5-HT3 receptor type but seem to represent a so far unknown receptor class.

5-HT(1)-like receptor agonists and the pathophysiology of migraine

Recently, AG-25086 and GR-43175 have been reported to be highly effective in aborting acute migraine attacks. These compounds seem selectively to stimulate a sub-population of 5-HT(1)-like receptors to inhibit noradrenaline release from certain sympathetic neurovascular terminals, to contract dog saphenous vein and dog, monkey and human basilar arteries, and to decrease the arteriovenous anastomotic component of carotid blood flow in the cat. GR-43175 neither has any antinociceptive effect nor crosses the blood-brain barrier. Pramod Saxena and Michel Ferrari review the clinical effectiveness of these 5-HT(1)-like receptor agonists and their selective pharmacology, both of which strongly suggest that excessive dilatation in the extracerebral cranial (scalp and/or dural) vasculature is an integral part of the pathophysiology of migraine.

The selective carotid arterial vasoconstrictor action of GR43175 in anaesthetized dogs

1. GR43175 is a highly selective agonist at 5-HT1-like receptors in the dog saphenous vein. This study describes the haemodynamic effects of GR43175 in barbitone-anaesthetized dogs. 2. GR43175 (1-1000 micrograms kg-1, i.v.) produced dose-dependent decreases in carotid arterial blood flow with little or no change in arterial blood pressure. The decrease in blood flow was associated with an increase in carotid arterial vascular resistance. In preliminary studies, the dose of GR43175 producing 50% of the maximum carotid vasoconstrictor response was 39 +/- 8 micrograms kg-1, i.v. 3. In comparative regional haemodynamic studies, GR43175 (1-1000 micrograms kg-1, i.v.) had little effect on total peripheral resistance or resistance in the mesenteric, vertebral and coronary arterial vascular beds. Low doses of GR43175 decreased, whilst high doses (100 micrograms kg-1, i.v. and above) increased femoral arterial vascular resistance. GR43175 (1-1000 micrograms kg-1, i.v.) had no effect on respiratory inflation pressure. In doses of 100 micrograms kg-1 i.v. and above, GR43175 caused small decreases in heart rate. 4. The carotid arterial vasoconstrictor action of GR43175 was resistant to antagonism by the 5-HT2 receptor, 5-HT3 receptor and alpha-adrenoceptor blocking drugs, ketanserin, MDL72222 and phentolamine respectively, but could be antagonized by the non-selective 5-HT1-like receptor blocking drug methiothepin. Methiothepin had no effect on the carotid vasoconstrictor action of the thromboxane A2 mimetic, U46619. 5. The results demonstrate that GR43175 produces a selective vasoconstriction in the carotid arterial circulation of anaesthetized dogs via activation of 5-HT1-like receptors, which appear similar to those mediating contraction of the dog isolated saphenous vein.

The actions of 5-hydroxytryptamine receptor agonists and antagonists at pre- and postjunctional level on the canine saphenous vein

The prejunctional and postjunctional 5-HT receptors of the canine saphenous vein were studied. The release of 3H-noradrenaline (3H-NA) from incubated saphenous vein strips was inhibited by 5-hydroxytryptamine (5-HT) in a concentration-dependent way (5-HT concentrations: 0.01, 0.1 and 1.0 mumol.l-1), but not by the selective 5-HT1A agonist 8-hydroxy-dipropylaminotetralin (8-OH-DPAT; 1 and 10 mumol.l-1). The inhibitory effect of 5-HT was antagonized by metitepine and methysergide, but not by yohimbine, (-)-pindolol or ketanserin. In strips preincubated with 5-HT (1.2 mumol.l-1), the fractional release of 3H-NA was slightly reduced (paired experiments). 5-HT and 8-OH-DPAT caused concentration-dependent contractions of the saphenous smooth muscle. A parallel shift of the concentration-response curve for 8-OH-DPAT to the right was caused by metitepine and yohimbine, but not by ketanserin. The contractions caused by 5-HT were antagonized by metitepine and yohimbine (parallel displacement of the curves to the right), as well as by ketanserin and methysergide (with a depression of the upper part of the curve). Blockade of alpha-adrenoceptors (due to prazosin plus a low concentration of yohimbine) also resulted in a weak antagonistic effect. Ketanserin and metitepine displaced the noradrenaline concentration-response curve to the right. We conclude that the saphenous vein of the dog is endowed with prejunctional receptors of the 5-HT1 type which can not be classified as belonging either to the 1A or 1B subtype; and that at the postjunctional level 5-HT1 (possibly of the 1D subtype) and 5-HT2 receptors are present.(ABSTRACT TRUNCATED AT 250 WORDS)

GR43175, a selective agonist for the 5-HT1-like receptor in dog isolated saphenous vein

1. We describe the actions of a novel and selective 5-HT1-like receptor agonist, GR43175, in a range of isolated tissue preparations containing different 5-hydroxytryptamine (5-HT) receptor types. 2. GR43175 was a potent agonist at 5-HT1-like receptors mediating contraction of the dog isolated saphenous vein and also at those inhibiting neuronally mediated contractions in the same preparations. For both actions, GR43175 was approximately four times weaker than 5-HT. 3. GR43175 was devoid of agonist properties at 5-HT1-like receptors mediating relaxation of the cat isolated saphenous vein. 4. GR43175 was devoid of agonist properties at 5-HT2 receptors mediating contraction of the rabbit isolated aorta, pig coronary artery, greyhound coronary artery and beagle femoral artery. 5. GR43175 was devoid of agonist properties at 5-HT3 receptors mediating depolarization of the rat isolated vagus nerve. 6. The contractile response to GR43175 in the dog isolated saphenous vein was selectively antagonized by methiothepin but was resistant to antagonism by the 5-HT2 receptor blocking drug ketanserin and the 5-HT3 receptor blocking drug MDL 72222. Methiothepin antagonized the contractile action of 5-HT and GR43175 to an equal extent suggesting that both agonists act at the same receptor. 7. The results demonstrate that GR43175 is a highly selective agonist for the 5-HT1-like receptors found in the dog saphenous vein. The absence of an action of GR43175 at 5-HT1-like receptors mediating relaxation of the cat isolated saphenous vein provides further evidence that 5-HT1-like receptors are heterogeneous.

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

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

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

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

[Pilot studies with a serotonin agonist (AH 25086 B). Efficacy and tolerance in acute migraine attacks.]

AH 25086 B is a selective agonist of the newly determined 5-HT(1) receptors, which are sited mainly in the intracranial section of the carotid artery. According to experimental studies, the effect of AH 25086 B is decidedly more highly selective than that of ergotamine; the blood flow through the arteriovenous anastomoses of the internal carotid artery is clearly reduced, while the blood flow through the capillaries supplying the brain is increased. With AH 25086 B administered in an infusion rapid abolition of migraine attacks already in progress proved possible. The first 12 patients worldwide to receive this preparation were treated, some on several occasions, for a total of 21 migraine attacks: 7 received one infusion, 3 received two, 1 three, and 1 patient received five infusions. There were some side-effects with some infusions: nausea (18), vomiting (9), and photophobia (19). On average it took 31 min (range 10-60 min) for the headache to be relieved, regardless of the duration of migraine symptoms before the start of treatment. This was not a controlled study, but the results (14 very good, 6 good or satisfactory, in 21 attacks treated) were better than could have been expected by chance. Tolerance of the preparation was good, all side-effects being transitory and mild; with dosages up to 1.6 mug kg(-1) min(-1) no changes were seen in heart rate or blood pressure.

Ketanserin potentiates the prejunctional inhibitory effect of 5-hydroxytryptamine on rat vas deferens

5-Hydroxytryptamine (5-HT) slightly inhibited the twitch contractions of rat vas deferens caused by single pulse field stimulation at 0.1 Hz. The inhibitory effect of 5-HT was much less in the epididymal portion than in the prostatic portion of the vas deferens. Ketanserin potentiated the prejunctional inhibitory effect of 5-HT and attenuated its stimulatory effect. This potentiation was observable only in the epididymal portion, of the vas deferens. Cyproheptadine and mianserin, but not methysergide, had essentially similar potentiating effects to those of ketanserin. These results suggest that the 5-HT receptor that mediates prejunctional inhibition is not of the 5-HT2 type, and that ketanserin acts by suppressing the 5-HT-induced stimulatory effect, which is possibly mediated by a postjunctional 5-HT2 receptor, thus unmasking the inhibitory effect of 5-HT.

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

As a result of controversy in the literature regarding the classification and nomenclature of functional receptors for 5-hydroxytryptamine (5-HT), a framework for classification is proposed. The formulation of these proposals has only been made possible by the recent advent of new drug tools. It is considered that there are three main types of 5-HT receptor, two of which have been well characterised pharmacologically, using selective antagonists, and which it is proposed to name 5-HT2 and 5-HT3. These two groups broadly encompass the "D" and "M" receptors, respectively, which Gaddum identified in the guinea-pig ileum (Gaddum and Picarelli, 1957). The 5-HT2 receptor, which mediates a variety of actions of 5-HT, has been definitively shown to correlate with the 5-HT2 binding site in the brain. No binding studies in brain tissue have yet been published with radiolabelled ligands specific for 5-HT3 receptors. A number of other actions of 5-HT appear to be mediated via receptors distinct from 5-HT2 or 5-HT3 receptors. Since selective antagonists are not yet available, these receptors cannot be definitively characterised, although in many cases they do have some similarities with 5-HT1 binding sites, which are a heterogeneous entity. Criteria are proposed for tentatively classifying these receptors as "5-HT1-like" (Table 1). Definitive characterisation of these receptors will await the identification of specific antagonists. This classification of 5-HT receptors into three main groups (Table 1) is based largely, but not exclusively, on data from studies in isolated peripheral tissues where definitive classification is possible. However, it is believed that this working classification will be relevant to functional responses to 5-HT in the central nervous system.

Inhibitory presynaptic 5-hydroxytryptamine (5-HT) receptors on the sympathetic nerves of the human saphenous vein

Superfused strips of the human saphenous vein preincubated with 3H-noradrenaline were used to investigate the influences of serotonin (5-HT) receptor agonists and antagonists on the electrically evoked tritium overflow. 5-HT and the preferential 5-HT1A receptor agonist 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin] concentration-dependently inhibited the evoked 3H overflow. The evoked 3H overflow was not affected by 0.1 or 1 mumol/l TVX Q 7821 (2-(4-[4-(2-pyrimidinyl)-1-piperazinyl]-butyl)-1,2-benzoisothiazol -3(2H)one-1,1-dioxide), which selectively binds to 5-HT1A sites; TVX Q 7821 10 mumol/l produced an increase in overflow. The inhibitory effect of 5-HT on the impulse-evoked 3H overflow was abolished by the nonselective 5-HT receptor antagonist metitepin, but was not attenuated by propranolol. Metitepin also abolished the inhibitory effect of 8-OH-DPAT on evoked 3H overflow, whereas the 5-HT2 receptor antagonist ketanserin was inactive in this respect. There was also no antagonism of the effect of 8-OH-DPAT by the alpha 2-adrenoceptor antagonist rauwolscine or the dopamine receptor antagonist flupenthixol. These results suggest that both 5-HT and 8-OH-DPAT inhibit noradrenaline release by activating inhibitory 5-HT receptors on the sympathetic nerves of the human saphenous vein. These receptors possess similarities to 5-HT1 recognition sites, but a further subclassification is not yet possible on the basis of the available data.

Receptor-mediated effects of serotonin and 5-methoxytryptamine on noradrenaline release in the rat vena cava and in the heart of the pithed rat

In segments of the rat inferior vena cava preincubated with 3H-noradrenaline, it was examined whether presynaptic serotonin (5-HT) receptors exist on the postganglionic sympathetic nerves of the circulatory system; for this purpose the effects of 5-HT receptor agonists and antagonists on the electrically evoked 3H overflow were studied. Furthermore, vagotomized pithed rats (treated with atropine and captopril) were used to investigate the effects of these drugs on heart rate and on the tachycardia induced by electrical stimulation of the preganglionic sympathetic nerves (C7--T1) via the pithing rod; these experiments were carried out to provide evidence that the presynaptic 5-HT receptors are operative in vivo. 5-HT and 5-methoxytryptamine (5-OCH3-T) concentration-dependently inhibited the electrically evoked 3H overflow from the vena cava. The inhibitory effect was more pronounced at 0.66 Hz than at 2 Hz. 8-Hydroxy-2-(di-n-propylamino)tetralin did not alter the evoked overflow. The inhibitory effect of 5-HT or 5-OCH3-T was antagonized by metitepin but not affected by ketanserin or rauwolscine. In pithed rats 5-HT and 5-OCH3-T by themselves dose-dependently increased heart rate. The positive chronotropic effect of 5-HT 10 mumol/kg, which was not affected by ketanserin, was considerably decreased by desipramine, indicating that 5-HT at least at this high dose acts predominantly by a tyramine-like indirect sympathomimetic effect.(ABSTRACT TRUNCATED AT 250 WORDS)

An inhibitory prejunctional 5-HT1-like receptor in the isolated perfused rat kidney. Apparent distinction from the 5-HT1A, 5-HT1B and 5-HT1C subtypes

The present study has identified a receptor for 5-hydroxytryptamine (5-HT) which functions to inhibit the stimulus-induced release of [3H] noradrenaline following sympathetic periarterial nerve stimulation to the isolated perfused rat kidney. In addition to 5-HT (IC30 = 4.5 X 10(-8) mol/l), both 5-carboxamidotryptamine (IC30 = 8 X 10(-9) mol/l) and 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl) indole (RU-24969, IC30 = 2.5 X 10(-7) mol/l) acted as agonists whereas 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) was inactive. The inhibitory effect of 5-HT on the electrically-evoked release of tritium was antagonized in a concentration-dependent manner by methiothepin (IC50 = 4 X 10(-9) mol/l), metergoline (IC50 = 4 X 10(-8) mol/l) and methysergide (IC50 = 1.3 X 10(-7) mol/l) but not by cyproheptadine, ketanserin, mesulergine, (-)-propranolol, (+/-)-pindolol, (+/-)-cyanopindolol, metoclopramide or phentolamine. It is concluded that the receptor to 5-HT conforms to general criteria defining 5-HT1-like receptors but at the present time the receptor site cannot be fitted to the designated 5-HT1A, 5-HT1B or 5-HT1C subtypes.

Mechanisms involved in the vasodilator action of 5-hydroxytryptamine in the dog femoral arterial circulation in vivo

The possibility that the vasodilator effect of 5-hydroxytryptamine (5-HT) in vivo involves a presynaptic inhibitory effect on sympathetic nerve activity was investigated in the femoral arterial circulation of pentobarbitone-anaesthetized dogs. The vasodilator effect of intraarterial (i.a.) 5-HT was abolished following ganglion blockade with mecamylamine, and remained inhibited after restoration of femoral vascular tone with i.a. ornipressin. These procedures had no effect on the vasodilator response to i.a. acetylcholine. The vasoconstrictor response to i.a. noradrenaline was not inhibited by mecamylamine. These findings suggest that a presynaptic inhibition of sympathetic neurotransmission is responsible for 5-HT-induced vasodilatation in vivo. Pizotifen (0.1-0.4 mg/kg i.v.) inhibited the 5-HT dilator response, but the doses required were too high to be commensurate with an action at 'D' type 5-HT receptors. Ketanserin (0.1-0.4 mg/kg i.v.) specifically inhibited the dilator response to 5-HT; higher doses (1-4 mg/kg i.v.) also inhibited noradrenaline-induced vasoconstriction. Ketanserin, at all doses used, reduced systemic blood pressure and femoral vascular resistance. The effects of the lower doses of ketanserin (0.1-0.4 mg/kg) cannot be due to peripheral alpha-adrenoceptor antagonism; blockade of the dilator effect of 5-HT may simply be due to inhibition of sympathetic nerve activity by ketanserin itself.

The effects of ketanserin, methysergide and LY 53857 on sympathetic nerve activity

Preganglionic sympathetic nerve activity, blood pressure, heart rate and femoral arterial conductance were recorded in anaesthetised, paralysed cats. Three 5-HT2 antagonists, ketanserin, methysergide and LY 53857, were infused separately over 1 h periods. Ketanserin caused a fall in, methysergide a rise followed by a fall and LY 53857 only a rise in preganglionic sympathetic nerve activity. The sympathoexcitation caused by both LY 53857 and methysergide was not associated with any changes in blood pressure and heart rate. The sympathoinhibition caused by ketanserin and methysergide was accompanied by a fall in blood pressure and heart rate. Both ketanserin and LY 53857 caused increases in femoral arterial conductance, while methysergide caused a transient decrease. These differences are explained on the basis that ketanserin possesses alpha 1-adrenoceptor antagonist and methysergide 5-HT receptor agonist properties. It is suggested that 5-HT2 sympathoinhibitory receptors are involved in central cardiovascular control.

Excitatory and inhibitory effects of 5-hydroxytryptamine in mesenteric arteries of spontaneously hypertensive rats

Some of the presynaptic and postsynaptic excitatory and inhibitory actions of 5-hydroxytryptamine (5-HT) in the rat mesenteric arteries were reexamined with particular reference to the genetic hypertensive rat model. Mesenteric arteries were perfused and the perfusion pressure monitored in the presence or absence of 5-HT, norepinephrine or periarterial sympathetic nerve stimulation. The vasoconstrictor response to 5-HT was resistant to prazosin but effectively inhibited by cyproheptadine and ketanserin. The vasoconstrictor responses to norepinephrine and nerve stimulation were markedly potentiated by 5-HT, and this potentiation was blocked by ketanserin at concentrations which inhibited the 5-HT-induced vasoconstriction, while much higher concentrations were required for cyproheptadine. No significant difference was found in these regards between mesenteric arteries from the spontaneously hypertensive rats (SHR) and those from the normotensive Kyoto Wistar rats (WKY). 5-HT significantly increased the nerve stimulation-evoked 3H overflow in [3H]norepinephrine-treated mesenteric arteries of SHR, but reduced the 3H overflow in the WKY preparations. These results suggest that ketanserin-sensitive 5-HT2 receptors are involved in the potentiating effect of 5-HT in the rat mesenteric arteries, and that an increase in transmitter release by 5-HT may contribute to its potentiation of nerve stimulation-induced vasoconstriction in SHR.

Presynaptic alpha-adrenoceptors: do exogenous and neuronally released noradrenaline act at different sites?

The effects of dopamine receptor and alpha-adrenoceptor agonists and antagonists on the stimulation-evoked overflow of radioactivity from strips of dog saphenous vein previously loaded with [3H]-noradrenaline have been examined alone and in combination. In the presence of neuronal and extraneuronal catecholamine uptake inhibitors, noradrenaline (0.1-1 X 10(-6)M) and dopamine (0.01-1 X 10(-6)M) both inhibited the stimulation-evoked overflow of radioactivity. Sulpiride (1 X 10(-6)M) was without effect and prazosin (1 X 10(-7)M) had little effect on stimulation-evoked overflow but yohimbine enhanced it approximately 2 fold; the effect of yohimbine was similar at concentrations of 1 X 10(-7) and 1 X 10(-6)M. Sulpiride abolished the inhibitory effect of dopamine on stimulation-evoked overflow, but was without effect against noradrenaline. When allowance was made for the effects of yohimbine, alone, on overflow, yohimbine (1 X 10(-7)M) had no effect against dopamine and minimal effects against noradrenaline. A similar result was obtained when the concentration of yohimbine was increased to 1 X 10(-6)M. Prazosin did not antagonize the effect of noradrenaline. In the absence of the uptake inhibitors, clonidine (0.01-1 X 10(-5)M) inhibited stimulation-evoked overflow of radioactivity. Yohimbine (1 X 10(-6)M) was without effect on its own and antagonized the effects of clonidine at a concentration of 0.1 X 10(-5)M, but not at 0.01 or 1.0 X 10(-5)M. These findings suggest that dopamine inhibits overflow by stimulating presynaptic dopamine receptors on the terminals of the noradrenergic nerves supplying the dog saphenous vein. The interaction between yohimbine and noradrenaline is discussed in terms of the current concepts of control of transmitter release mediated via presynaptic alpha 2-adrenoceptors.

Dual presynaptic effects of 5-hydroxytryptamine on peripheral noradrenergic synapses

The aim of the present experiments was to study the effects of 5-hydroxytryptamine (5-HT) on the responses to postganglionic stimulation of two models of the peripheral sympathetic nervous system: the isolated nictitating membrane of the cat and the guinea-pig isolated atria. In the nictitating membrane of the cat, 5-HT (0.1 microM) shifted to the left the frequency-response curve to nerve stimulation. This potentiating effect was prevented by 5-HT receptor antagonists (0.1 microM methysergide, 0.1 microM pizotifen and 0.1 microM morphine) and also by the beta-adrenoreceptor blocker propranolol (0.1 microM). The alpha 2-adrenoreceptor antagonist yohimbine (0.1 microM) had no effect on the 5-HT-induced potentiation. In the guinea-pig isolated atria the responses to cardioaccelerans nerve stimulation were diminished by 5-HT (0.1 to 1.0 microM). The shift to the right in the frequency-response curve induced by 5-HT (1.0 microM) was additive to the antagonism caused in the atria by propranolol (0.1 microM). The inhibitory effect of 5-HT on the pacemaker responses to nerve stimulation was prevented by the 5-HT receptor antagonists methysergide (1.0 microM) and pizotifen (1.0 microM) and also by the alpha-adrenoreceptor antagonist phentolamine (0.1 microM). The selective alpha 2-adrenoreceptor agonist clonidine (0.01 microM) reduced to the same extent as 5-HT (1.0 microM) the responses to the guinea-pig atria to nerve stimulation. The inhibitory effect of clonidine was prevented by the alpha-adrenoreceptor blocker phentolamine (0.1 microM) but not by the 5-HT receptor blocker pizotifen (1.0 microM). With the exception of propranolol, which in the atria shifted to the right the concentration-response curve to exogenous noradrenaline (NA), neither 5-HT nor the different antagonists employed modified the sensitivity to NA in the tissues studied. The present observations show that 5-HT can produce a dual effect on the sympathetic neurotransmission. It is proposed that a modification in the overflow of NA in response to nerve stimulation is caused by 5-HT and results from the interaction of 5-HT with specific receptors located on the sympathetic fibres. These presynaptic 5-HT receptors behave as excitatory (cat nictitating membrane) or inhibitory (guinea-pig atria) depending on the tissue studied.

Evidence that serotonin receptors mediate the cutaneous vasoconstriction produced by 5-hydroxytryptamine in canine forelimbs

The effects of local intra-arterial infusions of serotonin (5 or 25 micrograms base/min) or norepinephrine (1 or 5 micrograms base/min) on cutaneous (skin) and skeletal muscle vasculatures were investigated in canine forelimbs perfused at constant flow in dogs anesthetized with pentobarbital. Norepinephrine produced dose-related constriction of the skin and skeletal muscle vasculatures. In the cutaneous vascular circuit, norepinephrine produced large artery, small vessel, and large vein constriction. The increase in cutaneous vascular resistance was primarily due to an increase in small vessel resistance. Serotonin did not increase skeletal muscle vascular resistance but produced marked cutaneous vasoconstriction subsequent to large artery and large vein constriction. The small vessels, if anything, tended to dilate. The skin and skeletal muscle vascular responses to serotonin and norepinephrine were similar in innervated and acutely denervated forelimbs. Phentolamine pretreatment completely blocked all vascular actions of norepinephrine, and largely inhibited the cutaneous vasoconstriction produced by the infusion of the low dose of serotonin. However, the cutaneous large artery and large vein constriction produced by the infusion of the high dose of serotonin was not affected by phentolamine pretreatment. Cyproheptadine pretreatment blocked or largely inhibited the cutaneous vasoconstriction produced by serotonin only in doses which also inhibited norepinephrine and vasopressin cutaneous vasoconstriction. Pretreatment with methysergide blocked or largely inhibited the cutaneous large artery and large vein constriction produced by infusions of serotonin. Norepinephrine and vasopressin produced significant vasoconstriction in the presence of methysergide. These data suggest that the cutaneous large artery and large vein constriction produced by serotonin is not due to the activation of postjunctional alpha-adrenergic receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for common pharmacological properties of [3H]5-hydroxytryptamine binding sites, presynaptic 5-hydroxytryptamine autoreceptors in CNS and inhibitory presynaptic 5-hydroxytryptamine receptors on sympathetic nerves

The affinities of 16 5-hydroxytryptamine (5-HT) receptor agonists (indole derivatives) and 7 5-HT receptor antagonists for [3H]5-hydroxytryptamine [( 3H]5-HT) binding sites in rat cerebral cortex membranes were determined. In addition, the potencies of the agonists for inhibiting the electrically induced tritium overflow from rat brain cortex slices preincubated with [3H]5-HT and from canine saphenous veins preincubated with [3H]noradrenaline were measured. Furthermore, the potencies of the indole derivatives for inducing contractile responses of canine saphenous veins were recorded. In addition, the interaction of the antagonists with unlabelled 5-HT at the 5-HT autoreceptor was studied in rat brain cortex slices. There was a good correlation between the binding affinities of the indole derivatives for the [3H]5-HT sites of rat brain cortex membranes and their potencies for inhibiting the evoked tritium overflow from both rat brain cortex slices and strips of canine saphenous vein. Comparison of the inhibition constants derived from the overflow experiments in both tissues again revealed a high correlation coefficient while there was only weak correlation between the binding affinities in rat brain cortex and the contractile potencies of the drugs in canine saphenous vein strips. When 5-HT receptor antagonists were investigated, metitepin and metergoline showed moderate affinities for the 5-HT autoreceptors in rat brain cortex slices, whereas quipazine had only weak affinity, and ketanserin, metoclopramide, cinanserin and cyproheptadine exhibited no antagonistic property. In binding experiments, the competition curves of most 5-HT receptor antagonists were biphasic, suggesting that the [3H]5-HT binding sites are heterogeneous.(ABSTRACT TRUNCATED AT 250 WORDS)