Receptor mechanisms for 5-hydroxytryptamine in rabbit arteries

1 Previous investigations into the vascular actions of biogenic amines implicated in migraine have shown that the contractile effects of both 5-hydroxytryptamine (5-HT) and noradrenaline (NA) in the rabbit ear artery are mediated by a direct sympathomimetic action at alpha-adrenoceptors, while in the rabbit aorta, 5-HT and NA act on pharmacologically distinct receptors. The purpose of the present investigation was to determine whether the absence of 5-HT receptors in rabbit ear arteries is characteristic of distributing arteries in general, or is confined to particular regional circulations.2 Agonist-antagonist interactions were studied in various rabbit vascular preparations (common carotid, external carotid and femoral arterial strips, and perfused ear arteries) by determining pA(2) values for pizotifen and phentolamine against 5-HT- and NA-induced contractile responses.3 In common carotid and femoral arteries, pizotifen was a potent competitive antagonist of 5-HT, but weak against NA. The converse applied to phentolamine. In external carotid and ear arteries, pizotifen was a weak competitive antagonist of both 5-HT and NA, whereas phentolamine was a potent competitive antagonist of both. Cocaine did not influence pA(2) values against NA.4 5-HT and NA were of similar potency in common carotid and femoral arteries, but 5-HT was much less potent than NA in external carotid and ear arteries.5 The results indicate that rabbit common carotid and femoral arteries contain both D-type 5-HT receptors and alpha-adrenoceptors, as does the aorta. However, external carotid arteries, like ear arteries, do not contain specific 5-HT receptors. The action of 5-HT in the external carotid artery is mediated by alpha-adrenoceptors; this is a direct sympathomimetic action since it was not inhibited by cocaine or reserpine-pretreatment.6 The absence of 5-HT receptors in the rabbit extracranial circulation may limit the usefulness of this species as a model for research relating to migraine.     

EFFECTS OF HISTAMINE H1 AND H2-RECEPTOR ANTAGONISTS IN THE CRANIAL CIRCULATION OF THE MONKEY

1. The effects of the histamine H₂-receptor antagonists, metiamide and cimetidine, and the Hi-receptor antagonist, mepyramine, on cranial vasodilator and systemic vasodepressor responses to histamine have been investigated in anaesthetized monkeys. 2. In monkeys treated with successively increasing intravenous doses of 0.25, 1 and 5 mg/kg of metiamide, histamine-induced decreases in external and internal carotid vascular resistances were blocked, but systemic vasodepressor responses were not affected. 3. In monkeys treated similarly with 0.25, 1 and 5 mg/kg of cimetidine, external carotid vascular responses to histamine were blocked, but internal carotid vascular and systemic vasodepressor responses to histamine were not affected. 4. In monkeys treated similarly with 0.5, 2 and 10 mg/kg of mepyramine, histamine-induced systemic vasodepressor responses were blocked, but responses in the external and internal carotid circulations were unaffected. 5. In those monkeys treated with mepyramine after initial H₂-receptor antagonist treatment, blockade of internal, but not external, carotid vascular responses to histamine appeared to be increased. 6. The histamine antagonists did not affect cranial vascular responses to noradrenaline, serotonin, prostaglandin E₁ or bradykinin. Metiamide and cimetidine had no appreciable direct effects, but mepyramine produced short-lasting changes (vasodilatation followed by vasoconstriction). 7. These results indicate that H₂-receptors are involved in the external and internal carotid vascular effects of histamine. Hi-Receptors do not appear to contribute to the external carotid vasodilator response to histamine, but their possible involvement in the internal carotid response cannot be excluded.     

Receptors for 5-hydroxytryptamine and noradrenaline in rabbit isolated ear artery and aorta.

5-Hydroxytryptamine (5-HT) is thought to be implicated in the vascular disturbances of the external carotid artery bed associated with migraine. As part of a study of the pharmacology of some 5-HT antagonists used in the treatment of migraine we have examined the interactions of these drugs with 5-HT and noradrenaline in rabbit isolated ear artery and aortic strip. The results provide new information on the distribution of 5-HT-receptors in these preparations. In the aorta, 5-HT and noradrenaline were of similar potency in producing contractions. Methysergide produced very small contractions and was about 1000 times less potent than the other two agonists. In the ear artery noradrenaline produced monophasic vasoconstrictor responses, whereas 5-HT and methysergide produced prolonged biphasic responses. 5-HT was about 700 times less potent and methysergide about 4500 times less potent than noradrenaline. Methysergide was a better agonist in the ear artery than in the aorta. Biphasic responses to 5-HT and methysergide were also obtained in ear arteries from reserpine-treated rabbits indicating that neither agonist was acting by releasing endogenous noradrenaline. Pizotifen, cyproheptadine and phentolamine had no agonistic actions in either the aorta or ear artery. In the aorta methysergide, pizotifen and cyproheptadine were potent antagonists of 5-HT and much weaker antagonists of noradrenaline. Phentolamine possessed the opposite profile of selectivity. These results show that there are distinct receptors for 5-HT and noradrenaline in rabbit aorta. In the ear artery the pA2 values for each of the four antagonists were virtually identical against 5-HT and noradrenaline and similar to those obtained on alpha-adrenoceptors in the aorta. We conclude that 5-HT and noradrenaline act directly at alpha-receptors to produce vasoconstriction in the ear artery and that this preparation does not contain specific 5-HT receptors. This insight into the distribution of 5-HT receptors and alpha-receptors allows interpretation of the various actions of methysergide. In the aorta, methysergide was a potent antagonist at 5-HT-receptors and a weak partial agonist at alpha-receptors. In the ear artery, methysergide was a partial agonist at alpha-receptors; it was only a weak antagonist of 5-HT because this preparation does not contain specific 5-HT-receptors. The cross-reactivity demonstrated throughout these experiments indicates that 5-HT-receptors and alpha-receptors, although distinct entities, have features in common. These results are discussed in relation to the mode of action of methysergide, pizotifen and cyproheptadine in the treatment of migraine.     

Influence of pizotifen and ergotamine on the venoconstrictor effect of 5-hydroxytryptamine and noradrenaline in man

The influence of locally infused pizotifen (80 ng) and ergotamine (16 ng and 4 ng) on the compliance of superficial hand veins in man, and their interactions with the venoconstrictor effects of noradrenaline and 5-hydroxytryptamine (5-HT), were investigated in a placebo-controlled study in healthy volunteers. Pizotifen alone reduced venous compliance and produced a parallel displacement to the right of the 5-HT dose-response curve suggestive of competitive antagonism. The venoconstrictor effect of noradrenaline was not influenced by pizotifen. This confirms the selective antagonism of 5-HT by pizotifen and supports the existence of specific 5-HT receptors on human veins. After infusion of 16 ng ergotamine, which by itself reduced venous compliance, the venoconstrictor effects of the lower doses of 5-HT and of all doses of noradrenaline were larger but still never exceeded the arithmetic sum of the separate effects of noradrenaline or 5-HT and ergotamine. A lower dose of ergotamine (4 ng) induced only a small venoconstriction and did not influence the constrictor effect of noradrenaline. Therefore, in contrast to previous observations, no potentiation of the venoconstrictor effect of noradrenaline by ergotamine was observed under the present experimental conditions. The additive effect of noradrenaline and ergotamine may well explain its therapeutic action in the treatment of migraine.     

Comparative effects of four migraine prophylactic drugs on an isolated extracranial artery.

Clonidine and methysergide constrict the rabbit auricular artery by activating smooth muscle alpha-adrenoceptors. Clonidine inhibits and methysergide enhances responses to stimulation of the sympathetic nerves. Both drugs sensitise the artery to a variety of vasoconstrictor stimuli, although not to potassium chloride. This weak generalised sensitisation may depend on clonidine and methysergide themselves being vasoconstrictor since sensitisation after clonidine did not occur when vasoconstriction was abolished with phentolamine. The marked potentiation of 5-HT and tryptamine observed during clonidine perfusion may reflect a property of the tryptamines since it was observed during perfusion with histamine but not when histamine vasoconstriction was abolished with mepyramine. Cyproheptadine and pizotifen neither constrict the artery nor sensitise it to vasoconstrictor agents. They inhibit responses to nerve stimulation, alpha-adrenoceptor agonists, potassium chloride and particularly histamine. Sensitisation of blood vessels reinforced by direct vasoconstriction may contribute to the mechanism of action of clonidine and methysergide in migraine. Conversely, alterations in vascular function may be less important to the antimigraine actions of either cyproheptadine or pizotifen.     

Pizotifen, an antimigraine drug with venoconstrictor activity in vivo

Determination of the diameter of saphenous veins in conscious dogs revealed a considerable agonist activity of pizotifen not only after local but also after systemic i.v. and oral administration. By contrast, stimulation of vascular smooth muscle in vitro occurred only at very high concentrations (greater than 1 mumol/l). Treatment of dogs with 30 micrograms/kg ketanserin i.v. produced a two-fold shift to the right of the dose-response curve of locally infused 5-hydroxytryptamine (5-HT) but increased only slightly the ED50 of locally infused pizotifen. Furthermore, pizotifen proved to be a highly selective 5-HT receptor blocking agent both in vivo (on saphenous veins) and in vitro (on saphenous veins and basilar arteries) from dogs and humans. By contrast, antagonism of responses to both noradrenaline and CaCl2 occurred only at about 100 times higher pizotifen concentrations than required for blockade of 5-HT effects. It is suggested that the clinical effectiveness of pizotifen in migraine prophylaxis derives at least in part from an agonist activity the mechanism of which remains to be elucidated.     

Effects of low doses of clonidine in the monkey cranial circulation

Migrane therapy using low doses of clonidine has been based on the proposal that clonidine directly inhibits vascular smooth muscle reactivity. In anaesthetized monkeys in which internal and external carotid vascular resistances were measured, the only significant effects of clonidine administered acutely (0.5 and 2 μg · kg^?1 i.v.) or chronically (2 μg · kg^?1 i.m. daily for 7 days) on cranial vascular responses to the constrictions noradrenaline and 5-hydroxytryptamine, and the dilators histamine, prostaglandin E₁ and bradykinin, were small potentiations of some of the responses. Acute clonidine initially increased blood pressure and constricted the cranial vasculature, then induced hypotension without involvement of the cranial circulation. It also decrease the external carotid vasoconstrictor response to low frequency cervical sympathetic nerve stimulation. The low chronic dose of clonidine had no hypotensive effect. The pressor response to common carotid occlusion was inhibited by both acute and chronic clonidine. These experiments thus provide no evidence that clonidine inhibits vascular reactivity at doses equivalent to those used in migraine.     

Evidence for stimulation of 5-HT receptors in canine saphenous arteries by ergotamine

Summary Changes in tension of spiral strips from dog saphenous arteries were monitored isometrically. Dose-response curves for noradrenaline, 5-HT and ergotamine were established without and after a 30 min incubation with phentolamine or pizotifen. Phentolamine was about 15 times more potent in antagonizing responses to noradrenaline (pA₂ value=7.4) than those to 5-HT (pA₂ value=6.2) but it was nearly equipotent in antagonizing responses to ergotamine (pA₂ value=6.5) and those to 5-HT. Pizotifen was about 500 times less potent in antagonizing noradrenaline effects than 5-HT but again nearly equipotent when tested against ergotamine and 5-HT.It is suggested that in canine saphenous arteries the stimulant activity of ergotamine is mediated mainly through 5-HT receptors.     

Regional differences in the responsiveness of isolated arteries from cattle, dog and man

Changes in tension of spiral strips from basilar, external carotid, temporal and muscle arteries from cattle, dog and man were monitored isometrically. Dose-response curves for 5-HT, noradrenaline and ergotamine indicated a variation in responsiveness between different arteries of the same species as well as between the same arteries of different species. When tested against 5-HT ergotamine consistently acted as non-competitive dualist but it antagonized responses to to noradrenaline only in peripheral arteries from dog and man. In all arteries from the carotid vasculature and in bovine muscle arteries the vasoconstrictor effects of ergotamine and noradrenaline were independent additive. It is suggested that the therapeutic value of ergotamine in the treatment of migraine headache is due to its selective vasoconstrictor activity in the external carotid vasculature mediated through independent additive vasoconstrictor effects of ergotamine and noradrenaline.     

Actions of serotonin antagonists on dog coronary artery

Serotonin released from platelets may initiate coronary vasospasm in patients with variant angina. If this hypothesis is correct, serotonin antagonists without constrictor activity may be useful in this form of angina. We have investigated drugs classified as serotonin antagonists on dog circumflex coronary artery ring segments in vitro. Ergotamine, dihydroergotamine, bromocriptine, lisuride, ergometrine, ketanserin, trazodone, cyproheptadine and pizotifen caused non-competitive antagonism of serotonin concentration-response curves. In addition, ketanserin, trazodone, bromocriptine and pizotifen inhibited noradrenaline responses in concentrations similar to those required for serotonin antagonism. All drugs with the exception of ketanserin, cyproheptadine and pizotifen showed some degree of intrinsic constrictor activity. Methysergide antagonized responses to serotonin competitively but also constricted the coronary artery. The lack of a silent competitive serotonin antagonist precludes a definite characterization of coronary serotonin receptors at this time. However, the profile of activity observed for the antagonist drugs in the coronary artery differs from that seen in other vascular tissues. Of the drugs tested, ketanserin may be the most useful in variant angina since it is a potent 5HT antagonist, lacks agonist activity and has alpha-adrenoceptor blocking activity.     

Pharmacological profile of the receptors that mediate external carotid vasoconstriction by 5-HT in vagosympathectomized dogs.

1. 5-Hydroxytryptamine (5-HT) can produce vasodilatation or vasoconstriction of the canine external carotid bed depending upon the degree of carotid sympathetic tone. Hence, external carotid vasodilatation to 5-HT in dogs with intact sympathetic tone is primarily mediated by prejunctional 5-HT1-like receptors similar to the 5-HT1D subtype, which inhibit the carotid sympathetic outflow. The present investigation is devoted to the pharmacological analysis of the receptors mediating external carotid vasoconstriction by 5-HT in vagosympathectomized dogs. 2. Intracarotid (i.c.) infusions for 1 min of 5-HT (0.3, 1, 3, 10, 30 and 100 micrograms) resulted in dose-dependent decreases in both external carotid blood flow and the corresponding conductance; both mean arterial blood pressure and heart rate remained unchanged during the infusions of 5-HT. These responses to 5-HT were resistant to blockade by antagonists at 5-HT2 (ritanserin) and 5-HT3/5-HT4 (tropisetron) receptors, but were partly blocked by the 5-HT1-like and 5-HT2 receptor antagonist, methiothepin (0.3 mg kg-1); higher doses of methiothepin (1 and 3 mg kg-1) caused little, if any, further blockade. These methiothepin (3 mg kg-1)-resistant responses to 5-HT were not significantly antagonized by MDL 72222 (0.3 mg kg-1) or tropisetron (3 mg kg-1). 3. The external carotid vasoconstrictor effects of 5-HT were mimicked by the selective 5-HT1-like receptor agonist, sumatriptan (3, 10, 30 and 100 micrograms during 1 min, i.c.), which produced dose-dependent decreases in external carotid blood flow and the corresponding conductance; these effects of sumatriptan were dose-dependently antagonized by methiothepin (0.3, 1 and 3 mg kg-1), but not by 5-HT1D-like receptor blocking doses of metergoline (0.1 mg kg-1). 4. The above vasoconstrictor effects of 5-HT remained unaltered after administration of phentolamine, propranolol, atropine, hexamethonium, brompheniramine, cimetidine and haloperidol, thus excluding the involvement of alpha- and beta-adrenoceptors, muscarinic, nicotinic, histamine and dopamine receptors. Likewise, inhibition of either 5-HT-uptake (with fluoxetine) or cyclo-oxygenase (with indomethacin), depletion of biogenic amines (with reserpine) or blockade of calcium channels (with verapamil) did not modify the effects of 5-HT. 5. Taken together, the above results support our contention that the external carotid vasoconstrictor responses to 5-HT in vagosympathectomized dogs are mainly mediated by activation of sumatriptan-sensitive 5-HT1-like receptors. It must be emphasized, notwithstanding, that other mechanisms of 5-HT, including an interaction with a novel 5-HT receptor (sub)type and/or an indirect action that may lead to the release of a known (or even unknown) neurotransmitter substance cannot be categorically excluded.     

Migraine: pathophysiology, pharmacology, treatment and future trends

Migraine treatment has evolved into the scientific arena, but it seems still controversial whether migraine is primarily a vascular or a neurological dysfunction. Irrespective of this controversy, the levels of serotonin (5-hydroxytryptamine; 5-HT), a vasoconstrictor and a central neurotransmitter, seem to decrease during migraine (with associated carotid vasodilatation) whereas an i.v. infusion of 5-HT can abort migraine. In fact, 5-HT as well as ergotamine, dihydroergotamine and other antimigraine agents invariably produce vasoconstriction in the external carotid circulation. The last decade has witnessed the advent of sumatriptan and second generation triptans (e.g. zolmitriptan, rizatriptan, naratriptan), which belong to a new class of drugs, the 5-HT1B/1D/1F receptor agonists. Compared to sumatriptan, the second-generation triptans have a higher oral bioavailability and longer plasma half-life. In line with the vascular and neurogenic theories of migraine, all triptans produce selective carotid vasoconstriction (via 5-HT1B receptors) and presynaptic inhibition of the trigeminovascular inflammatory responses implicated in migraine (via 5-HT1D/5-ht1F receptors). Moreover, selective agonists at 5-HT1D (PNU-142633) and 5-ht1F (LY344864) receptors inhibit the trigeminovascular system without producing vasoconstriction. Nevertheless, PNU-142633 proved to be ineffective in the acute treatment of migraine, whilst LY344864 did show some efficacy when used in doses which interact with 5-HT1B receptors. Finally, although the triptans are effective antimigraine agents producing selective cranial vasoconstriction, efforts are being made to develop other effective antimigraine alternatives acting via the direct blockade of vasodilator mechanisms (e.g. antagonists at CGRP receptors, antagonists at 5-HT7 receptors, inhibitors of nitric oxide biosynthesis, etc). These alternatives will hopefully lead to fewer side effects.     

Ability of ketanserin to block different receptors in human placental vessels

5-Hydroxytryptamine (5-HT), noradrenaline (NA) and histamine induced concentration-dependent contractions in segments of human chorionic arteries and veins, whereas clonidine, an alpha 2-adrenoceptor agonist, had no effect. 5-HT and histamine induced strong contractions, while NA elicited weak contractions in some segments. The maximal response was similar for 5-HT and histamine. The order of potency (EC50 values) was: 5-HT greater than or equal to NA greater than or equal to histamine. These agonists induced tachyphylaxis, and single concentrations caused transient contractions. Contractions elicited by 5-HT were antagonized by ketanserin, a 5-HT2-receptor antagonist, which also antagonized the responses to NA and histamine, but at greater concentrations than those needed for 5-HT responses. Contractions elicited by histamine were reduced by diphenydramine. Low concentrations of 5-HT amplified contractions caused by NA and histamine. The results indicate that: (i) 5-HT is the most potent constrictor agent tested in these vessels; (ii) its effects are mediated by 5-HT2-receptors; and (iii) ketanserin at therapeutic plasma concentrations (10(-7) M) seems to block mainly 5-HT2-receptors, and alpha 1-adrenergic- and H1-receptors to a small extent only.     

Atypical tryptamine receptors in sheep pulmonary vein.

Both the pulmonary artery and vein of the sheep contracted dose-dependently to histamine, carbamoylcholine, prostaglandin F2a, noradrenaline and bradykinin and relaxed in the presence of isoprenaline or prostaglandin E1. 2 The effect of 5-hydroxytryptamine (5-HT) on the artery was consistently to produce dose-dependent contractions without tachyphylaxis. The effect on the vein was biphasic. 5HT 5 X 10(-10) to 5 X 10(-8) M relaxed the partially constricted vein. 5-HT 10(-7) to 10(-6) m caused brief venoconstriction followed by relaxation. 5-HT greater than 10(-6) M caused dose-related contraction of the vein. 3 Methysergide effectively blocked the contractile response of the artery to 5-HT, but only weakly inhibited the contractions of the vein (dose-ratio less than 20). 4 Each of ten antagonists tested failed to inhibit the 5-HT-induced relaxation of the vein. Sheep pulmonary vein possesses tryptamine receptors which mediate relaxation and which are not of the classicl M- or D-type. These receptors appear not to be involved directly or indirectly with responses to acetylcholine, catecholamines, histamine or prostaglandins.     

5—羟色胺受体介导迷走交感神经切断犬颈外血管收缩

One specific example reflecting the complexity of cardiovascular responses induced by serotonin (5-hydroxytryptamine; 5-HT) and the progress achieved in the pharmacological characterization of the receptors involved can be illustrated by the effects of 5-HT on the canine external carotid artery bed. Within this framework, it has been shown that the external carotid vasoconstrictor response to 5-HT in the dog is mediated by '5-HT1-like' receptors, which being blocked by the 5-HT1B/1D receptor antagonist GR127935, resemble 5-HT1B/1D (previously called 5-HT1D beta/1D alpha) receptors. It was proposed that these receptors could belong to the 5-HT1B, rather than the 5-HT1D, subtype on the basis of their resistance to blockade by a high dose of ritanserin (a potential 5-HT1D receptor antagonist) and the presence of mRNA for 5-HT1B(5-HT1D beta) receptors, but not for 5-HT1D(5-HT1D alpha) receptors, in vascular smooth muscle. With the advent of subtype-selective antagonists it was subsequently shown that external carotid vasoconstriction to 5-HT and sumatriptan is dose-dependently antagonized by the selective 5-HT1B receptor antagonist SB224289 (2,3,6,7-tetrahydro-1'-methyl-5-[2'-methyl-4' (5-methyl-1,2,4-oxadiazol-3-yl) biphenyl-4-carbonyl] furo [2,3-f] indole-3-spiro-4'-piperidine hydrochloride), whereas the selective 5-HT1D receptor antagonist BRL15572 (1-(3-chlorophenyl)-4-[3,3-diphenyl (2-(S,R) hydroxypropanyl) piperazine] hydrochloride) was ineffective. These findings represent the first in vivo evidence showing that vascular constriction induced by 5-HT and sumatriptan is mediated primarily via 5-HT1B, but not 5-HT1D receptors. The pharmacological profile of these receptors could be similar (isolated human temporal artery and porcine carotid arteriovenous anastomoses) to other putative 5-HT1B receptors mediating vasoconstrictor responses. In view of the putative pathophysiologic role of external carotid (and extracerebral) vasodilation in migraine, the constriction of these blood vessels by sumatriptan via 5-HT1B receptors may be, at least partly, responsible for its therapeutic efficacy in migraine.     

Effects of mianserin, desipramine and maprotiline on blood pressure responses evoked by acetylcholine, histamine and 5-hydroxytryptamine in rats.

1 In rats anaesthetized with pentobarbitone, intravenous administration of desipramine (0.1 mg/kg), maprotiline (0.5 mg/kg) or mianserin (0.3-3.0 mg/kg) did not modify the blood pressure lowering effects of acetylcholine (0.25-1.0 micrograms/kg, i.v.) which were significantly reduced by atropine (3.0 micrograms/kg, i.v.). 2 Maprotiline and mianserin, like promethazine (0.1 mg/kg, i.v.), inhibited the vasodepressor responses evoked by histamine (2.5-10.0 micrograms/kg,i.v.). however, desipramine was inactive against histamine. 3 In pithed rats, the pressor effects of intravenous 5-hydroxytryptamine (5-HT: 5.0-20.0 micrograms/kg) were antagonized by mianserin (0.01-0.3 mg/kg, i.v.) and cyproheptadine (0.01 mg/kg) but were unaffected by maprotiline and desipramine. 4 In syrosingopine pretreated rats given mianserin 0.1 mg/kg, intravenously, 5-HT (20.0 micrograms/kg, i.v.) produced a significant fall in blood pressure which could be reduced by a large dose of mianserin (10.0 mg/kg, i.v.). 5 In conclusion, desipramine, maptrotiline and mianserin, in doses previously found to inhibit noradrenaline neuronal reuptake in the rat cardiovascular system, lack muscarinic receptor antagonist properties. Whilst maprotiline and mianserin blocked vascular histamine receptors, only mianserin (10.0 mg/kg, i.v.). 5 In conclusion, desipramine, maptrotiline and mianserin, in doses previously found to inhibit noradrenaline neuronal reuptake in the rat cardiovascular system, lack muscarinic receptor antagonist properties. Whilst maprotiline and mianserin blocked vascular histamine receptors, only mianserin, like cyproheptadine, was a potent antagonist of the 5-HT receptors that mediate increases in blood pressure in rats. Finally, the vasodepressor effects of 5-HT in syrosingopine pretreated rats given a small dose of mianserin were antagonized by a large dose of mianserin, suggesting that 5-HT may activate two distinct types of receptors in the rat.     

Comparison of the neuronal responses to 5-hydroxytryptamine, noradrenaline and phenylephrine in the cerebral cortex: effects of haloperidol and methysergide

The technique of microelectrophoresis was used in order to compare the effect of 5-hydroxytryptamine (5-HT) with those of noradrenaline and phenylephrine on single neurones in the somatosensory cerebral cortex of the rat. Both 5-HT and noradrenaline evoked excitatory, depressant and biphasic (depression followed by excitation) responses; phenylephrine evoked only excitatory responses. There was no statistically significant association between the agonistic effects of 5-HT and noradrenaline, and of 5-HT and phenylephrine compared on the same cells; 5-HT depressed many cells that were excited by noradrenaline or phenylephrine. 5-Hydroxytryptamine appeared to be less potent than either noradrenaline or phenylephrine in evoking excitatory responses. The excitatory response to 5-HT had a longer latency and a shorter recovery time than did excitatory responses to either noradrenaline or phenylephrine; this difference in time-course presumably reflects the lower excitatory potency of 5-HT. The neuroleptic haloperidol discriminated between excitatory responses to 5-HT and phenylephrine: the response to phenylephrine was preferentially antagonized; probably reflecting the greater affinity of haloperidol for the α-adrenoceptor than for the “excitatory 5-HT receptor”. Methysergide, a 5-HT receptor blocking agent, discriminated between excitatory responses to 5-HT and phenylephrine, and also to 5-HT and noradrenaline: responses to 5-HT were antagonized, while responses to the other two remained unaffected. The antagonism of the excitatory response to 5-HT was often accompanied by the “unmasking” of a latent depressant response to 5-HT. Depressant responses to 5-HT were resistant to antagonism by methysergide: these responses were potentiated rather than antagonized by the drug. The results show that 5-HT can stimulate pharmacologically distinct “inhibitory” and “excitatory” receptors on cortical neurones; it is suggested that the “inhibitory” receptor may correspond to the 5-HT₁ site, and the “excitatory” receptor to the 5-HT₂ site identified in receptor binding studies. Furthermore, the excitatory 5-HT receptor seems to be different from the excitatory α-adrenoceptor: the excitatory 5-HT receptor is susceptible to antagonism by methysergide, and is relatively resistant to antagonism by haloperidol, whereas the converse applies to the neuronal α-adrenoceptor.     

Dual effect of dihydroergotamine at vascular 5-hydroxytryptamine receptors in pithed rats

Dihydroergotamine (DHE), administered intravenously to pithed normotensive rats, increased the mean arterial blood pressure dose-dependently. This pressor effect was noncompetitively inhibited by the 5-hydroxytryptamine (5-HT) receptor antagonists pizotifen and cyproheptadine (0.1 mg/kg each). The alpha 1-adrenoceptor blocker prazosin (1 mg/kg) had no influence on the DHE effect whereas the alpha 2-adrenoceptor blocker yohimbine, at the same dose, proved to be a noncompetitive inhibitor of this pressor effect. The results indicate that both the 5-HT receptors and alpha 2-adrenoceptors are involved in the DHE-induced vasoconstriction. On the other hand, in pithed normotensive rats, DHE proved to be a potent inhibitor of the pressor response to 5-HT but it did not inhibit that to noradrenaline. The results obtained characterize DHE as a noncompetitive dualist at vascular 5-HT receptors.     

Alpha-adrenoceptors, 5-hydroxytryptamine receptors and the action of dihydroergotamine in human venous preparations obtained during saphenectomy procedures for varicose veins

Summary Changes in tension were monitored isometrically on spiral strips from human saphenous veins obtained during surgical removal of varicose veins. Concentration-response curves for noradrenaline and 5-hydroxytryptamine (5-HT) were established by cumulative administrations, curves for dihydroergotamine were constructed from the mean responses to single concentrations. The use of the antagonists prazosin, yohimbine and pizotifen provided evidence for the existence of both postjunctional ₁- and ₂-adrenoceptors and for the existence of 5-HT receptors. The venoconstrictor effects of dihydroergotamine were unchanged by prazosin. Yohimbine antagonized both dihydroergotamine and 5-HT at about 60 times higher concentrations than required against noradrenaline whereas pizotifen inhibited responses to both dihydroergotamine and 5-HT at about 100 times lower concentrations than those to noradrenaline.These new results are in contrast to conclusions drawn from animal studies and do not support the suggestion that in man the venoconstrictor activity of dihydroergotamine is mediated through stimulation of -adrenoceptors. The present results strongly suggest that in human saphenous veins the constrictor activity of dihydroergotamine is mediated at least in part through stimulation of 5-HT receptors.     

Responses of cortical neurones to stimulation of the nucleus raphé medianus: a pharmacological analysis of the role of indoleamines

Single shock stimulation of the nucleus raphé medianus evoked complex responses from the large majority of neurones tested. These responses consisted of a short-latency (mean 21 msec) inhibition of firing followed by a longer latency (mean 160 msec) increase in firing rate. Occasionally, cells were encountered which exhibited pure inhibitory or pure excitatory responses. Prior treatment with the tryptophan hydroxylase inhibitor rho-chlorophenylalanine reduced the excitatory effects of raphé stimulation but greatly increased the inhibitory effects. Pretreatment with 5,7-dihydroxytryptamine (5,7-DHT), on the other hand, reduced both inhibitory and excitatory effects of raphé stimulation. Methysergide was found to be an effective antagonist of excitatory responses to iontophoretically applied 5-hydroxytryptamine (5-HT) but less effective against depressant responses to either 5-HT or tryptamine. In contrast metergoline consistently antagonized 5-HT and tryptamine-induced depressions but not 5-HT elicited excitations. When tested against stimulation evoked responses of cortical neurones methysergide antagonized the excitatory effects of raphé stimulation but had much less effect on the inhibitory responses, while the reverse held true for metergoline. The present results may be compatible with a mediation of the excitatory effects of stimulation by 5-HT and the inhibitory effects by tryptamine.