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

5-HT receptor regulation of neurotransmitter release

Serotoninergic neurons in the central nervous system impinge on many other neurons and modulate their neurotransmitter release. This review focuses on 1) the function of presynaptic 5-hydroxytryptamine (5-HT) heteroreceptors on axon terminals of central cholinergic, dopaminergic, noradrenergic, or GABAergic neurons and 2) the role of GABAergic interneurons expressing 5-HT heteroreceptors in the regulation of acetylcholine, dopamine, or noradrenaline release. In vitro studies on slices or synaptosomes and in vivo microdialysis experiments have shown that 5-HT(1A), 5-HT(1B), 5-HT(2A), 5-HT(2C), 5-HT(3), and/or 5-HT(4) heteroreceptors mediate this modulation. 5-HT(1B) receptors on neocortical cholinergic, striatal dopaminergic, or hippocampal GABAergic axon terminals are examples for release-inhibiting 5-HT heteroreceptors; 5-HT(3) receptors on hippocampal GABAergic or 5-HT(4) receptors on hippocampal cholinergic axon terminals are examples for release-facilitating 5-HT heteroreceptors. GABA released from GABAergic interneurons upon activation of facilitatory 5-HT receptors, e.g., 5-HT(2A) or 5-HT(3) receptors, mediates inhibition of the release of other neurotransmitters such as prefrontal neocortical dopamine or neocortical acetylcholine release, respectively. Conversely, attenuated GABA release in response to activation of inhibitory 5-HT heteroreceptors, e.g., 5-HT(1A) or 5-HT(1B) receptors on GABAergic interneurons is involved in paradoxical facilitation of hippocampal acetylcholine and striatal dopamine release, respectively. Such 5-HT heteroreceptors are considered potential targets for appropriate 5-HT receptor ligands which, by enhancing the release of a relevant neurotransmitter, can compensate for its hypothesized deficiency in distinct brain areas. Examples for such deficiencies are the impaired release of hippocampal or neocortical acetylcholine, striatal dopamine, and hippocampal or neocortical noradrenaline in disorders such as Alzheimer's disease, Parkinson's disease, and major depression, respectively.

Effect of 5-hydroxytryptamine on neurogenic vasoconstriction in the isolated, autoperfused hindquarters of the rat

1. In the present study, we analysed the effect of different doses of 5-hydroxytryptamine (5-HT; intravenous infusions of 0.001-40 microg/kg per min) in the autoperfused hindquarters of the rat subjected to electrical stimulation (frequencies of 0.5-20 Hz) of the lumbar chains, investigating the relationship between the adrenergic and serotonergic systems in this vascular bed. 2. Because we observed that 5-HT inhibited the increases in perfusion pressure induced by electrical stimulation of the lumbar chains, we used different agonists and antagonists to analyse the mechanism of action of 5-HT. 3. The effect of 5-HT was inhibited by methiothepin (a non-specific 5-HT receptor antagonist), but not by ritanserin (a selective 5-HT2 receptor antagonist). The effects of 5-HT were mimicked by 5-carboxamidotryptamine (a 5-HT1 receptor agonist) and L-694 247 (a selective 5-HT1D receptor agonist), but not by 8-hydroxy-2-dipropylaminotetralin (a 5-HT1A receptor agonist), CGS-12066B (a 5-HT1B receptor agonist), alpha-methyl-5-HT (a 5-HT2 receptor agonist), 1-(3-chlorophenyl) piperazine (a 5-HT2C receptor agonist) or 1-phenylbiguanide (a 5-HT3 receptor agonist). The selective 5-HT1D/1B receptor antagonist BRL 15572 inhibited the effect of the agonist L-694 247. 4. Our data suggest that 5-HT inhibits the increases in perfusion pressure induced by the electrical stimulation of the lumbar chains, acting on presynaptic 5-HT1D receptors and decreasing the release of noradrenaline from the sympathetic nerves in the hindquarter vascular bed of the rat.

Modifications by sumatriptan and acetylcholine of nitric oxide-mediated neurogenic dilatation in dog cerebral arteries

Canine cerebral arterial strips denuded of endothelium responded to nicotine and transmural electrical stimulation with relaxations, which were abolished by NG-nitro-L-arginine and methylene blue. Magnitudes of relaxation did not differ in the arteries contracted with prostaglandin F2alpha and sumatriptan, an effective therapeutic of migraine. Sumatriptan concentration-dependently contracted the arteries responding to 2 Hz stimulation with persistent relaxations, and the concentration of this 5-HT1B/1D/1F receptor agonist to overcome the relaxation averaged 1.06 x 10(-7) M. Acetylcholine inhibited the response to nerve stimulation due possibly to its action on prejunctional nitroxidergic nerves; the inhibition did not differ in the arteries contracted with prostaglandin F2alpha and K+. It appears that sumatriptan does not interfere with the release of nitric oxide from nerves but counteracts the neurogenic relaxation by functional antagonistic action on smooth muscle. Prejunctional inhibition by muscarinic receptor activation is unlikely associated with opening of neuronal K+ channels.

Characterization of mechanisms involved in presynaptic inhibition of sympathetic pressor effects induced by some 5-HT1 receptor antagonists

1. In a previous study, we showed that the presynaptic inhibitory action of 5-hydroxytryptamine receptor agonists on sympathetic pressor effects obtained in the pithed rats were mainly mediated by activation of 5-HT1A and 5-HT1D receptor subtypes. At the time, we observed that some 5-HT1 receptors antagonists - WAY 100,635 and NAN-190 (both 5-HT1A receptor antagonists), methiothepin (a 5-HT1,2,5,6,7 receptor antagonist) and spiperone (a 5-HT1,2 receptor antagonist) - reduced per se the pressor effects obtained by electrical stimulation. The aim of the present work was to investigate the mechanism participating in this inhibitory effect. 2. The inhibition induced by WAY 100,635 (1000 microg kg-1, i.v.) was blocked after i.v. treatment with idazoxan, an alpha2-adrenoceptor antagonist (300 and 1000 microg kg-1) and was not modified after i.v. treatment with propranolol, a beta-adrenoceptor antagonist (1000 microg kg-1) and sulpiride, a D2 receptor antagonist (1000 microg kg-1). The inhibition induced by spiperone (500 microg kg-1 i.v.) was significantly blocked by sulpiride (1000 microg kg-1) and was not modified by idazoxan or propranolol. 3. Sulpiride (1000 microg kg-1) partially blocked the inhibition induced by methiothepin (50 microg kg-1 i.v.). Only pretreatment with idazoxan (300 microg kg-1) modified the inhibition induced by NAN-190 (100 microg kg-1 i.v.), such inhibition increasing after intravenous administration of idazoxan. 4. All the antagonists used in our experiments failed to inhibit the pressor responses elicited by i.v. noradrenaline administration. 5. The above results suggest that the inhibitory effects of these 5-HT1 receptor antagonists are presynaptic in nature, but not related to the blockade of 5-HT1 receptors subtypes. The simultaneous activation or inhibition of other receptor systems could explain the inhibition produced by each 5-HT1 receptor antagonist studied.

5-Hydroxytryptamine inhibits the tachycardia induced by selective preganglionic sympathetic stimulation in pithed rats

It has been shown in several species that serotonin (5-hydroxytryptamine; 5-HT) is able to inhibit the responses produced by sympathetic stimulation in a wide variety of blood vessels and other organs, including the heart. However, in pithed rats, the analysis of potential sympatho-inhibitory actions of 5-HT is hampered by the fact that 5-HT (given as i.v. bolus injections) produces tachycardia per se. Moreover, most studies have investigated 5-HT-induced sympatho-inhibition at only one frequency of stimulation. Thus, the present study set out to find the experimental conditions to overcome these problems. In this regard, we analyzed the potential ability of 5-HT, administered as i.v. continuous infusions, to inhibit the tachycardia caused by stimulation of the preganglionic (C7-T1) sympathetic outflow in pithed rats. Sympathetic cardiostimulation (0.01-3 Hz) resulted in frequency-dependent increases in heart rate; these responses were potentiated after desipramine (50 microg/kg, i.v.). During continuous infusions of 5-HT (3.1-10 microg/kg.min, i.v.), but not saline, the sympathetically-induced tachycardia was dose-dependently inhibited in both control and desipramine-pretreated rats. This inhibitory effect of 5-HT was significantly more pronounced at lower frequencies of stimulation. In contrast, the above infusions of 5-HT did not inhibit the tachycardia induced by i.v. bolus injections of noradrenaline in both control and desipramine-pretreated rats. Taken together, the above findings confirm that 5-HT induces inhibition of the sympathetic chronotropic outflow in the rat by acting at receptors located prejunctionally, without evoking tachycardia, over a wide range of stimulation frequencies.

Characterization of prejunctional 5-HT1 receptors that mediate the inhibition of pressor effects elicited by sympathetic stimulation in the pithed rat

1. A study was made of the effects of 5-carboxamidotryptamine (5-CT) on pressor responses induced in vivo by electrical stimulation of the sympathetic outflow from the spinal cord of pithed rats. All animals had been pretreated with atropine. Sympathetic stimulation (0.1, 0.5, 1 and 5 Hz) resulted in frequency-dependent increases in blood pressure. Intravenous infusion of 5-CT at doses of 0.01, 0.1 and 1 mg kg(-1) min(-1) reduced the pressor effects obtained by electrical stimulation. The inhibitory effect of 5-CT was significantly more pronounced at lower frequencies of stimulation. In the present study we characterized the pharmacological profile of the receptors mediating the above inhibitory effect of 5-CT. 2. The inhibition induced by 0.01 microg kg(-1) min(-1) of 5-CT on sympathetically-induced pressor responses was partially blocked after i.v. treatment with methiothepin (10 microg kg(-1)), WAY-100,635 (100 microg kg(-1)) or GR127935T (250 microg kg(-1)), but was not affected by cyanopindolol (100 microg kg(-1)). 3. The selective 5-HT1A receptor agonist 8-OH-DPAT and the selective 5-HT(1B/1D) receptor agonists sumatriptan and L-694,247 inhibited the pressor response, whereas the 5-HT1B receptor agonists CGS-12066B and CP-93,129 and the 5-HT2C receptor agonist m-CPP did not modify the pressor sympathetic responses. 4. The selective 5-HT1A receptor antagonist WAY-100,635 (100 microg kg(-1)) blocked the inhibition induced by 8-OH-DPAT and the selective 5-HT(1B/1D) receptor antagonist GR127935T (250 microg kg(-1)) abolished the inhibition induced either by L-694,247 or sumatriptan. 5. None of the 5-HT receptor agonists used in our experiments modified the pressor responses induced by exogenous noradrenaline (NA). 6. These results suggest that the presynaptic inhibitory action of 5-CT on the electrically-induced pressor response is mediated by both r-5-HT1D and 5-HT1A receptors.

Characterization of prejunctional 5-HT receptors mediating inhibition of sympathetic vasopressor responses in the pithed rat

1. It has recently been shown that continuous infusions of 5-hydroxytryptamine (5-HT) are able to inhibit, in a dose-dependent manner, the pressor responses induced by preganglionic (T7-T9) sympathetic stimulation in pithed rats pretreated with desipramine (50 micrograms kg-1, i.v.). This inhibitory effect, besides being significantly more pronounced at lower frequencies of stimulation (0.03-I Hz) and devoid of tachyphylaxis, is reversible after interrupting the infusions of 5-HT (up to 5.6 micrograms kg-1 min-1). In the present study we have characterized the pharmacological profile of the receptors mediating the above inhibitory effect of 5-HT. 2. The inhibition induced by 5.6 micrograms kg-1 min-1 of 5-HT on sympathetically-induced pressor responses was not blocked after i.v. treatment with physiological saline (1 ml kg-1), ritanserin (0.1 mg kg-1), MDL 72222 (0.15 mg kg-1) or tropisetron (3 mg kg-1), which did not modify the sympathetically-induced pressor responses per se, but was significantly antagonized by the 5-HT1-like and 5-HT2 receptor antagonist, methysergide (0.3 mg kg-1), which also produced a slight attenuation of the pressor responses to 0.03 and 0.1 Hz per se. 3. Unexpectedly and contrasting with methysergide, the 5-HT1-like and 5-HT2 receptor antagonists, methiothepin (0.01, 0.03 and 0.1 mg kg-1) and metergoline (1 and 3 mg kg-1), apparently failed to block the above 5-HT-induced inhibition. Nevertheless, it is noteworthy that these antagonists also blocked the electrically-induced pressor responses per se, presumably by blockade of vascular alpha 1-adrenoceptors and, indeed, this property might have masked their potential antagonism at the inhibitory 5-HT1-like receptors. 4. Consistent with the above findings, 5-carboxamidotryptamine (5-CT, a potent 5-HT1-like receptor agonist), metergoline and methysergide mimicked the inhibitory action of 5-HT with the following rank order of agonist potency: 5CT > > 5-HT > metergoline > or = methysergide. 5. Taken together, the above results suggest that the inhibitory action of 5-HT on the electrically-induced pressor responses is primarily mediated by an action on inhibitory prejunctional 5-HT1-like receptors leading to a decrease in the sympathetic nerve discharge. Interestingly, 5-HT-induced excitatory mechanisms could be made manifest once the inhibitory action of 5-HT had been antagonized.

5-hydroxytryptamine inhibits pressor responses to preganglionic sympathetic nerve stimulation in pithed rats

5-Hydroxytryptamine (5-HT) inhibits contractile responses to adrenergic nerve stimulation in several blood vessels and organs. We have now investigated the potential ability of 5-HT to inhibit the pressor responses caused by preganglionic sympathetic stimulation (T7-T9) in pithed rats. Sympathetic stimulation (0.03, 0.1, 0.3, 1 and 3 Hz) resulted in frequency-dependent increases in diastolic blood pressure; these effects were augmented after i.v. treatment with desipramine (50 micrograms/kg). During continuous infusions of 5-HT (1.8, 3.1, 5.6 and 10 micrograms/kg.min, i.v.), but not of saline, the pressor responses were dose-dependently inhibited in both control and desipramine-pretreated rats; this inhibitory effect of 5-HT was significantly more pronounced at lower frequencies of stimulation. In contrast, the above infusions of 5-HT did not inhibit the pressor responses induced by i.v. bolus injections of exogenous norepinephrine (up to 3 micrograms/kg). Taken together, the above findings suggest an operative 5-HT-induced prejunctional inhibition of sympathetic nerve activity on the systemic vasculature in vivo.

5-Carboxamidotryptamine attenuates the development of deoxycorticosterone acetate-salt hypertension in rats

The effect of chronic i.v. infusion of the 5-HT1 receptor agonist, 5-carboxamidotryptamine (5-CT), was evaluated during the development of deoxycorticosterone acetate-salt (DOCA-salt) hypertension in rats over 4 weeks. Vehicle-treated (n = 10) Sprague-Dawley rats given DOCA (100 mg/kg, s.c.) and 1% saline as drinking fluid developed hypertension with systolic blood pressure reaching 194.6 +/- 8.99 mm Hg at 27 days. In DOCA-salt rats treated with 5-CT infusions (15.0 micrograms/kg per day, n = 10) for 4 weeks via osmotic minimpumps, systolic blood pressure was significantly lower by 41.7 mm Hg at day 27 when compared to vehicle-treated DOCA-salt rats. Systolic blood pressure values on day 27 in 5-CT-treated DOCA-salt rats were however greater than those in vehicle-treated control rats which were not given DOCA. Systolic blood pressure in 5-CT-treated DOCA-salt rats was significantly lower by day 7 compared to vehicle-treated DOCA-salt rats and remained lowered for the rest of the observation period. Heart rate was significantly greater in 5-CT-treated DOCA-salt rats on day 7 when compared to vehicle-treated DOCA-salt rats. Baroreflex sensitivity on day 28 was significantly greater in 5-CT-treated DOCA-salt rats as compared to vehicle-treated DOCA-salt rats. On day 28, hypotensive responses to hexamethonium (20 mg/kg) in 5-CT-treated DOCA-salt rats were markedly reduced compared to those in vehicle-treated DOCA-salt rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Histamine and serotonin released from the rat perfused heart by compound 48/80 or by allergen challenge influence noradrenaline or acetylcholine exocytotic release

Terminal nerve fibres of the autonomic nervous system closely approach mast cells in peripheral organs, and mutual influences between release of neurotransmitters or mast cell mediators may cause neuro-immunological interactions. We have studied the influence of mast cell degranulation on the release of endogenous noradrenaline and newly incorporated acetylcholine (such as 14C-choline/acetylcholine overflow) evoked by stimulation of extrinsic postganglionic sympathetic or preganglionic vagal nerves in the rat Langendorff heart perfused with Tyrode solution. Compound 48/80 perfused in normal hearts, or ovalbumin infused into hearts from rats sensitized to ovalbumin, enhanced the overflow of endogenous histamine and serotonin. Both stimuli increased the release of mediators to a similar extent and with fast kinetics. Maximum average concentrations in the perfusate of histamine were about 800 nmol/l, and of serotonin 40 nmol/l, in a sample collected within 4 min after mast cell degranulation. Stimulation of autonomic nerves did not affect basal histamine or serotonin overflow. Whereas basal overflows were unaffected, the stimulation-evoked releases of both noradrenaline and acetylcholine, were facilitated when compound 48/80 was perfused before and during nerve stimulation. The facilitation of noradrenaline overflow was more pronounced (by 60%) when compound 48/80-induced mediator overflow started 4 min before nerve stimulation as compared to 30 s (15%), and was reduced by cocaine (by 50%), and, in the presence of cocaine, abolished by cimetidine (but was unaffected by mepyramine and thioperamide) and NG-nitro-(L)-(-)-arginine. In the presence of cimetidine and cocaine, when the facilitatory components were abolished, the evoked noradrenaline overflow observed 30 s after the start of infusion of compound 48/80 was inhibited, and the inhibition was partly reduced by methiotepin and ketanserin. Ovalbumin infusion in hearts from sensitized animals caused an inhibition of evoked noradrenaline overflow sensitive to methiotepin and also partly to ketanserin, and no facilitation was observed. The facilitation (> 100%) of evoked overflow of acetylcholine observed at 4 min after the start of perfusion with compound 48/80 was partly reduced by thioperamide (but not mepyramine or cimetidine) and to a comparable extent either by tropisetron (3 mumol/l) alone or by tropisetron plus methiotepin. In conclusion, degranulation of immunological cells is followed by histamine and serotonin release in the rat heart and may affect the release of autonomic neurotransmitters in rather unusual ways, by i) an uptake1-dependent and ii) an H2-mediated facilitation which probably involves nitric oxide as a permissive mediator, and iii) a serotonergic inhibition, of noradrenaline release, and iv) an H3- and serotonergic facilitation of acetylcholine release.

The 5-HT1-like receptor mediating the increase in canine external carotid blood flow: close resemblance to the 5-HT1D subtype

1. It has recently been shown that the increase in external carotid blood flow induced by 5-hydroxy-tryptamine (5-HT) in the anaesthetized dog, being mimicked by 5-carboxamidotryptamine (5-CT), inhibited by methiothepin, vagosympathectomy and sympatho-inhibitory drugs, and resistant to blockade by ritanserin and MDL 72222, is mediated by stimulation of prejunctional 5-HT1-like receptors leading to an inhibitory action on carotid sympathetic nerves; these 5-HT1-like receptors are unrelated to either the 5-HT1A, 5-HT1B or 5-HT1C (now 5-HT2C) receptor subtypes. Inasmuch as 5-CT, 5-methoxytryptamine, sumatriptan and metergoline display high affinity, amongst other 5-HT binding sites, for the 5-HT1D subtype, in the present study we have used these drugs in an attempt to determine whether the above inhibitory prejunctional 5-HT1-like receptors correlate with the 5-HT1D subtype. 2. One-minute intracarotid (i.c.) infusions of 5-HT (0.3, 1, 3 and 10 micrograms), 5-CT (0.01, 0.03, 0.1 and 0.3 micrograms), 5-methoxytryptamine (1, 3, 10 and 30 micrograms) and sumatriptan (1, 3, 10, 30 and 100 micrograms) resulted in dose-dependent increases in external carotid blood flow (without changes in mean arterial blood pressure or heart rate) with the following rank order of agonist potency: 5-CT >> 5-HT > 5-methoxytryptamine > or = sumatriptan. Interestingly, sumatriptan-induced vasodilatation was followed by a more pronounced vasoconstriction. 3. The external carotid vasodilator effects of 5-HT, 5-CT, 5-methoxytryptamine and sumatriptan were dose-dependently and specifically antagonized by metergoline (10, 30 and/or 100 micrograms kg-1, i.v.). In addition, 5-methoxytryptamine- and sumatriptan-induced vasodilator effects were, respectively, markedly inhibited or abolished after vagosympathectomy, as previously shown for 5-CT and 5-HT.4. Sumatriptan showed tachyphylaxis in its vasodilator component and antagonized 5-HT-induced external carotid vasodilatation in a specific manner, suggesting that a common site of action may be involved.5. Taken together, the above results support our contention that 5-HT, 5-CT, 5-methoxytryptamine and sumatriptan produce external carotid vasodilatation in the dog by an action that might primarily involve a prejunctional inhibition on carotid sympathetic nerves; a secondary component of this vasodilator response may be postsynaptic (endothelium-dependent and/or even directly on the vasculature).Based on the rank order of agonist potency, inhibition by vagosympathectomy and blockade by metergoline, we suggest that the inhibitory prejunctional 5-HT1-like receptors mediating external carotid vasodilatation in the dog closely resemble the 5-HTID receptor subtype. The pharmacological profile of these receptors is similar (sympathetic nerves of the rat kidney and human saphenous vein, as well as porcine coronary endothelium) to other putative 5-HTID receptors mediating vascular responses.

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.

Antihypertensive effect of chronic i.v. administration of 5-carboxamidotryptamine in spontaneously hypertensive rats

The effects of chronic i.v. administration of the serotonin 5-HT1 receptor agonist, 5-carboxamidotryptamine (5-CT), on blood pressure (BP), heart rate (HR) and baroreflex sensitivity were studied in Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Acute i.v. injection of increasing doses of 5-CT resulted in a dose-dependent reduction in mean arterial pressure (MAP) in SHR and WKY with concomitant tachycardia. In chronic experiments, 5-CT (15.0 micrograms/kg per day) or vehicle (24.0 microliters/day) was infused i.v. for 7 days, using osmotic minipumps. Systolic blood pressure (SBP) and HR were monitored daily before and during infusions. In SHR (n = 8) and WKY rats (n = 9) receiving 5-CT, a significant reduction in SBP was observed during the infusion period. HR was slightly increased in WKY rats on days 1 and 2. There were no HR changes in the SHR group. The fall in SBP was significantly larger in the SHR than in the WKY rats. Baroreflex sensitivity on day 7 was significantly greater in 5-CT-treated SHR than in control rats. There was no change in baroreflex sensitivity in WKY rats. Administration of a single dose of 5-CT (0.5 microgram/kg i.v.) on day 7 of infusion resulted in attenuated responses in WKY rats while SHR responded as their respective controls. Our data suggest that chronic administration of 5-CT results in a sustained antihypertensive effect. This is associated with an improved BRS in the SHR either as a consequence of a resetting of the baroreflex due to sustained lowering of BP or a direct action of 5-CT on baroreflex sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of 6-hydroxydopamine on pre- and post-junctional 5-HT1-like receptor-mediated responses in dog saphenous vein

To investigate whether 5-HT1-like receptor-mediated inhibition of adenosine 3':5'-cyclic monophosphate (cyclic AMP) accumulation occurs in nerves or smooth muscle of saphenous vein, infusions of 6-hydroxydopamine (6-OHDA) were administered to dogs with the aim of inducing sympathetic nerve damage. The effects of 6-OHDA on other 5-HT1-like receptor-mediated responses at the pre- and post-junctional level were investigated for comparison by studying 5-hydroxytryptamine (5-HT)-induced inhibition of 3H-noradrenaline release and contraction of smooth muscle respectively. Disruption of nerve function by 6-OHDA was revealed by the lack of catecholaminergic fluorescence and neurogenic contractile responses in saphenous veins from dogs treated with 6-OHDA. In addition, severe impairment of neuronal uptake mechanisms were apparent since basal efflux of 3H-noradrenaline, electrically-evoked release of 3H-noradrenaline and remaining 3H-noradrenaline content were considerably reduced. Some 3H-noradrenaline was taken up and released in 6-OHDA-treated tissues which is consistent with the existence of nerve varicosities resistant to the present dosing regime of 6-OHDA, an observation substantiated by electron microscopy studies showing inconsistent lesions of nerve terminals. 6-OHDA pre-treatment potentiated the smooth muscle contractile responses mediated by 5-HT1-like receptors as well as potentiating 5-HT-evoked inhibition of prostaglandin E2-stimulated cyclic AMP accumulation. It did not, however, affect 5-HT-induced inhibition of 3H-noradrenaline release. The present results suggest that inhibition of cyclic AMP accumulation by 5-HT occurs predominantly in smooth muscle.

Evidence for a noradrenergic innervation to alpha 1A-adrenoceptors in rat kidney

1. Experiments were undertaken to characterize the alpha 1-adrenoceptor subtype mediating vasoconstrictor responses to periarterial noradrenergic nerve stimulation (PNS) in the isolated perfused kidney of the rat. 2. Vasoconstrictor responses to nerve stimulation were inhibited by prazosin (10 nM), 5-methyl-urapidil (30 nM), and nitrendipine (1 microM) but were resistant to inhibition by chloroethylclonidine (100 microM). 3. 5-Methyl-urapidil (30 nM), chloroethylclonidine (100 microM) and nitrendipine (1 microM) did not inhibit the neuronal release of tritium from nerves loaded with [3H]-noradrenaline. 4. The results suggest that renovascular alpha 1A-adrenoceptors receive a noradrenergic innervation and that the innervated receptors are coupled to dihydropyridine-sensitive calcium channels.

Slow off-rate of prazosin accounts for deviations from competitive antagonism in the isolated perfused kidney of rat

1. The kinetics of the interaction of prazosin with noradrenaline, administered by bolus injection and infusion, was examined in the isolated perfused rat kidney. 2. Prazosin (1-30 nM) competitively antagonized vasoconstrictor responses to infusion of noradrenaline (pA2 = 9.51) whereas antagonism toward bolus injections of noradrenaline deviated from competition. 3. In the presence of prazosin, responses to bolus injection of noradrenaline became biphasic whereas the kinetics of the responses to infusions of noradrenaline were unchanged. 4. In contrast, phentolamine (30-300 nM) competitively antagonized responses to bolus injection of noradrenaline without altering response kinetics and in coperfusion studies with prazosin, prevented prazosin from inducing biphasic responses to bolus injection of noradrenaline. 5. It is concluded that prazosin acts as a pseudoirreversible antagonist toward vasoconstrictor responses to bolus injections noradrenaline (non-steady state conditions) whereas steady state conditions and competitive kinetics are observed with infused noradrenaline.

An endothelial 5-HT receptor that mediates relaxation in guinea-pig isolated jugular vein resembles the 5-HT1D subtype

1. Endothelium-dependent and -independent, concentration-related, relaxations to 5-hydroxytryptamine (5-HT) are described in a preparation of guinea-pig isolated jugular vein. 2. An endothelial 5-HT receptor was studied in the presence of mesulergine (at 10.0 microM, a concentration sufficient to antagonize 5-HT2 receptor-mediated contractions and endothelium-independent relaxations to 5-HT). Relaxations mediated by the endothelial 5-HT receptor were resistant to antagonism by mesulergine. 3. Several 5-HT receptor agonists activated the endothelial receptor with the following rank order of potency: 5-carboxamidotryptamine (5-CT) greater than 5-HT greater than methysergide greater than or equal to alpha-methyl-5-HT greater than sumatriptan greater than 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) greater than 2-methyl-5-HT. 4. Relaxations to 5-HT were not blocked by (+/-)-pindolol (1.0 microM), (-)-propranolol (1.0 microM), spiperone (1.0 microM), ondansetron (1.0 microM) or ICS 205-930 (10.0 microM). 5. Both 5-HT and sumatriptan evoked endothelium-dependent relaxations which were sensitive to antagonism (pA2 and apparent pA2 values respectively) by methiothepin (8.1 and 8.6), metergoline (7.4 and 7.5), PAPP (8.2 and 8.2), yohimbine (7.1 and 6.8), rauwolscine (6.8 and 6.7), but not by corynanthine (10.0 microM). 6. These observations are consistent with a 5-HT1D receptor-mediated effect, and provide further support for the concept that differences exist between endothelial 5-HT receptors in different tissues and species.

Analysis of the 5-HT receptors mediating contractions in the rabbit isolated renal artery

1. Using a number of agonist and antagonist compounds, we have attempted to characterize the responses and receptors involved in the effects of 5-hydroxytryptamine (5-HT) in the rabbit isolated renal artery. 2. In vessel segments precontracted with the thromboxane-mimetic agent, U46619 (100 nM), neither 5-HT (10(-8) to 10(-4) M) nor 5-carboxamidotryptamine (5-CT; 10(-8) to 3 x 10(-4) M) caused relaxations like those observed with methacholine. Both 5-HT and 5-CT further increased the tone of the vessels, with pD2 values of 7.1 and 7.9, respectively. 3. In the absence of U46619, both 5-HT (10(-7) to 3 x 10(-3) M) and 5-CT (10(-7) to 10(-3) M) contracted the rabbit renal artery, but with reduced potencies. The contractions to 5-HT were reproducible and the rank order of potency (pD2) of the agonists was: alpha-methyl-5-HT (5.7), sumatriptan (5.3), 5-HT (5.1), 8-hydroxy-2(di-n-propylamino)tetralin (5.0), 5-CT (4.7) and 5-methoxytryptamine (4.3). 1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane, flesinoxan and RU 24969 elicited either only small contractions or none at all. 4. The contractile effect of 5-HT was unaffected by MDL 72222 (10(-6) M) and metergoline (10(-8) and 10(-7) M), was weakly antagonized by ketanserin and phentolamine (pKB: 6.6 and 6.8, respectively), but was effectively antagonized by methiothepin (pKB: 8.6). Responses to 5-CT and sumatriptan were affected by ketanserin, phentolamine and methiothepin similarly to 5-HT-induced responses. 5. Ketanserin was ineffective against noradrenaline-induced contractions, which were antagonized by phentolamine with a pKB of 7.3. The pKB values of phentolamine against 5-HT, 5-CT or sumatriptan were about half a log unit lower than against noradrenaline.6. In vascular preparations treated with cocaine (3 x 10- I M), the potency (pKB) of phentolamine as an antagonist of the responses to noradrenaline (7.6) and 5-HT (6.7) did not differ significantly from the values in untreated preparations. However, the difference between the pKB values of phentolamine against the two agonists was now about one log unit.7. Pretreatment of the vascular strips with 6-hydroxydopamine (1.5 x 10- 3M) did not significantly affect responses to 5-HT or 5-CT, but almost eliminated those to tyramine. Cocaine (3 x 10- 5M) slightly potentiated noradrenaline-induced contractions, but did not significantly affect those induced by 5-HT.8. These data suggest that: (i) 5-HT receptors mediating vasodilatation are not present in the rabbit renal artery smooth muscle or endothelium; (ii) the contractile effect of 5-HT does not involve the release of noradrenaline from sympathetic nerve stores; (iii) the 5-HT receptor in the rabbit renal artery is not of the 5-HT2, 5-HT3 or 5-HT4 type. The pharmacological properties of this receptor most closely resemble those described for the heterogeneous 5-HT1-like category.

The effects of ageing on prejunctional 5-hydroxytryptamine receptors in the rat vas deferens

The prejunctional inhibitory effects of a series of 5-HT1 receptor agonists were examined against the isometric contraction of epididymal portions of rat vas deferens evoked by single stimulus pulses in the presence of nifedipine (10 mumol/l). The 5-HT1A ligand flesinoxan produced inhibition of contractions which was not inhibited by cyanopindolol or yohimbine. However, the prejunctional inhibitory concentration response curve for the 5-HT1 agonist 5-carboxamidotryptamine (5-CT) was biphasic in tissues from 1.5 month old animals but monophasic in tissues from 24 months animals. Cyanopindolol (1 mumol/l) antagonised the inhibitory effects of 5-CT in tissues from 1.5 and 3 month animals but not in tissues from 8 or 24 months animals. Inhibitory actions of 5-CT were not prevented by pretreating animals with pertussis toxin (6 micrograms/kg i.v.), a dose which abolished the negative inotropic response to acetylcholine in rat left atria. It is concluded that the nerve terminals of vas deferens from 1.5 month old animals contain both 5-HT1B and other as yet unclassified 5-HT1 receptors, but that this 5-HT1B-mediated response is lost in maturation and ageing.

Hemodynamic and renin responses to (+-)-DOI, a selective 5-HT2 receptor agonist, in conscious rats

The effect of the selective 5-HT2 agonist (+-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI) on arterial pressure (AP), heart rate (HR), renal blood flow (RBF) and plasma renin activity (PRA) was determined in conscious rats. DOI increased AP and PRA, but decreased HR and RBF. All responses to DOI were abolished by central (LY 53857) or peripheral (xylamidine) 5-HT2 antagonists. Prazosin did not alter the AP or HR response to DOI. Chlorisondamine abolished the bradycardia but slightly increased the hypertension produced by DOI, while enalapril attenuated the pressor response. No further reduction was produced by the combination of enalapril and prazosin. Propranolol attenuated but did not eliminate the renin response, and blocked the bradycardia elicited by DOI. The data suggest that DOI activates 5-HT2 receptors located on vascular smooth muscle and/or the circumventricular organs of the brain to: (1) increase AP and reflexly decrease HR, (2) decrease RBF and (3) increase PRA. The hypertension is mediated by angiotensin II and direct vascular effects whereas the increase in PRA is mediated by an interaction of increased sympathetic nerve activity and decreased renal perfusion pressure.

Renal hemodynamic responses to 5-hydroxytryptamine (5-HT): involvement of the 5-HT receptor subtypes in the canine kidney

The study was designed to define the serotonin (5-HT) receptor subtypes in the canine kidney. An intrarenal infusion of 5-HT at a dose of 5 micrograms/min in anesthetized dogs resulted in a biphasic response of renal blood flow which decreased transiently then increased above the control level during prolonged infusion. The decrease of renal blood flow was abolished by infusion of methysergide but not by ketanserin, and the subsequent increase was abolished by infusion of either ketanserin or methysergide. Terazosin, an alpha1-adrenoceptor antagonist, did not modify the renal action of 5-HT. These findings suggest that the renal blood flow response induced by 5-HT did not depend on an indirect effect via the sympathetic nervous system, the initial vasoconstriction was mediated via a 5-HT1-like receptor, and that the latter vasodilatation was mediated via a 5-HT2 receptor. The infusion of 5-HT also increased urine flow and urinary excretion of sodium. These increases were reversed by pretreatment with ketanserin and abolished by methysergide. We propose that 5-HT may exert its antidiuretic action via a 5-HT1-like receptor in the tubules but that the renal hemodynamic changes induced by 5-HT may overcome its antidiuretic action. The present results suggest the existence of a 5-HT1-like and 5-HT2 receptor in the renal vasculature and a 5-HT1-like receptor in the renal tubules.

The role of the central serotonergic system in pilocarpine-induced seizures: receptor mechanisms

Modification of central serotonergic transmission resulted in alterations of pilocarpine convulsive activity in male Wistar rats. Seizure activity was increased after pizotifen injection and the latency period to onset of convulsions was shortened in animals pretreated with mianserine and quipazine. Stimulation of 5-HT1A receptors with 8-hydroxy-di-N,N-propylaminotetralin (8-OH-DPAT) and blockade of 5-HT1B receptors with cyanopindolol resulted in seizure protection. Intracerebroventricular injections of 5,6-dihydroxytryptamine (5,6-DHT) did not change the protective effect of cyanopindolol. Other agents specifically affecting serotonergic receptors, the agonists 1-(3-chlorophenyl)piperazine (mCPP) and 5-methoxytryptamine (5-MT) and the antagonists spiperone, metergoline, methysergide, cyproheptadine and metoclopramide, did not influence pilocarpine-induced seizures. In conclusion, the present study suggests that the inhibition of pilocarpine-induced seizures may be mediated by stimulation of 5-HT1A and by blockade of 5-HT1B receptors, located probably on the cholinergic terminals.

High affinity serotonin binding sites in human brain: a comparison of cerebral cortex and basal ganglia

The high-affinity binding of 3H-serotonin and 3H-DPAT was studied in membrane preparations and tissue sections of cerebral cortex and basal ganglia of human brain. In tissue sections, 3H-serotonin bound to sites present at high density in the cerebral cortex, hippocampus and basal ganglia. 3H-DPAT bound predominantly to the outer layers of the cerebral cortex and the hippocampus, no significant binding was observed in the basal ganglia. In cortical membranes 3H-serotonin bound to a heterogeneous population of sites, one of which was similar to a binding site labelled by 3H-DPAT (the putative 5HT-1A receptor). 3H-serotonin binding to membranes prepared from the putamen displayed low affinity for DPAT, mesulergine and spiperone, RU24969 was considerably less potent than serotonin itself. This 3H-serotonin binding site does not have the properties of those sites described in rodent brain (5HT-1A, 5HT-1B, and 5HT-1C), and may represent a novel serotoninergic binding site.

[Modulation of renal transmitter release by presynaptic receptors]

Renal sympathetic nerve varicosities possess a variety of receptors which when activated by appropriate agonists can modulate noradrenaline release at the local level of the kidney. Thus, activation of prejunctional alpha 1- and alpha 2-adrenoceptors, prostaglandin (PG), dopamine, adenosine and serotonin receptors inhibits, whereas activation of prejunctional beta 2-adrenoceptors and angiotensin (A) II receptors enhances renal noradrenaline release. Moreover, neuronally released noradrenaline itself activates prejunctional inhibitory alpha 1- and alpha 2-adrenoceptors forming a "negative feedback loop" of its own release (autoinhibition). PGE2 and adenosine locally formed in the kidney by renal nerve stimulation inhibits noradrenaline release through activation of their specific prejunctional receptor system (transjunctional inhibition).

Phentolamine blocks presynaptic serotonin autoreceptors in rabbit and rat brain cortex

Possible antagonist effects of phentolamine at presynaptic serotonin autoreceptors were studied in slices of the occipito-parietal cortices of the rabbit and the rat. The slices were preincubated with 3H-serotonin and then superfused and stimulated electrically with single pulses or pulse trains. Nitroquipazine 1 mumol/l, a compound that inhibits the high affinity neuronal uptake of serotonin, was present in the superfusion medium in all one pulse-experiments as well as in experiments in which the effect of unlabelled serotonin was examined. In rabbit cortical slices, unlabelled serotonin reduced the single pulse-evoked overflow of tritium. Its concentration-response curve was not changed by the selective alpha 2-adrenoceptor antagonist idazoxan 1 mumol/l but was shifted to the right by phentolamine 1 and 10 mumol/l. Phentolamine 10 mumol/l also shifted to the right the concentration-inhibition curve of the selective 5-HT1-receptor agonist 5-carboxamidotryptamine. When the slices were stimulated by trains of 30 pulses at 3 Hz, phentolamine 1 and 10 mumol/l but not 0.1 mumol/l increased the evoked overflow of tritium, the maximal increase amounting to 178%; its effect was enhanced in the presence of nitroquipazine 1 mumol/l plus idazoxan 10 mumol/l (a drug combination that, when given alone, slightly increased the evoked overflow of tritium). The serotonin receptor antagonist metitepin at concentrations of 0.01-1 mumol/l also increased the overflow of tritium elicited by 30 pulses/3 Hz, the maximal increase amounting to 280%; its effect was potentiated in the presence of nitroquipazine 1 mumol/l plus idazoxan 10 mumol/l but was abolished or almost abolished in the presence of nitroquipazine 1 mumol/l plus phentolamine 10 mumol/l (a drug combination that, given alone, greatly increased the evoked overflow of tritium). When slices were stimulated by trains of 360 pulses at 3 Hz, there was no apparent antagonism of phentolamine 10 mumol/l against the inhibitory effect of unlabelled serotonin. In rat brain cortex slices, unlabelled serotonin reduced the overflow of tritium elicited by 4 pulses delivered at 100 Hz. Again, phentolamine 10 mumol/l shifted the concentration-response curve to the right. It is concluded that phentolamine blocks presynaptic serotonin autoreceptors in rabbit and rat brain cortex with pA2 values of 6.44 and 5.95, respectively. Previous failures to detect the antagonistic effect against exogenous agonists were probably due to stimulation conditions that led to marked endogenous autoinhibition of serotonin release.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Cardiovascular effects from stimulation of 5-hydroxytryptamine receptors

The cardiovascular effects of 5-hydroxytryptamine (5-HT), consisting of bradycardia or tachycardia, hypotension or hypertension, and vasodilatation or vasoconstriction, are mediated by three main types of receptors called 5-HT1-like, 5-HT2, and 5-HT3. In intact animals 5-HT elicits a short-lasting bradycardia, accompanied by hypotension, via stimulation of 5-HT3 receptors located on sensory vagal nerve endings in the heart (Bezold-Jarisch reflex). The nature of 5-HT receptors mediating tachycardiac responses is species-dependent. Myocardial 5-HT1-like and 5-HT2 receptors subserve tachycardia in the cat and rat, respectively. Tachycardia in the dog and rabbit is due to a release of catecholamines effected via the 5-HT2 receptors on the adrenal medulla and the 5-HT3 receptors on postganglionic cardiac sympathetic nerve fibres, respectively. The receptors mediating tachycardia in the pig are unique as they do not resemble any of the three 5-HT receptors characterized so far. The blood pressure response to 5-HT is usually triphasic: initial short-lasting hypotension due to reflex bradycardia (via 5-HT3 receptors), a middle pressor phase (via 5-HT2 receptors), and a longer-lasting hypotension (via 5-HT1-like receptors). Vascular contraction by 5-HT is generally mediated by 5-HT2 receptors (located primarily on the large conducting vessels), though in some instances (e.g., dog saphenous vein, dog and human basilar artery, and porcine arteriovenous anastomoses) the contractile response is (also) mediated via 5-HT1-like receptors. Venous dilatation and arteriolar dilatation (leading to increased capillary ['nutrient'] blood flow) occur via 5-HT1-like receptors located mainly on the vascular smooth muscles but also on the endothelium; the smooth muscle and endothelial 5-HT1-like receptors seem to be heterogeneous. In addition, 5-HT can elicit vasodilatation and hypotension as a result of decreased sympathetic nervous tone by acting within the central nervous system and by inhibiting noradrenaline release by a presynaptic action. Both these effects also involve 5-HT1-like receptors that do not appear to be identical. Last, knowledge of the cardiovascular effects of 5-HT and the nature of the receptors involved should be helpful in developing 5-HT-related compounds that may be useful in the treatment of hypertension, migraine, and peripheral vascular diseases.

Different receptor subtypes mediate the dual presynaptic effects of 5-hydroxytryptamine on peripheral sympathetic neurones

1. The aim of the present work was to characterize the presynaptic 5-HT receptors that mediate either the facilitation of the responses to nerve stimulation in the nictitating membrane of the cat or the inhibition of the responses to nerve stimulation in the guinea-pig atria. 2. In the nictitating membrane of the cat, the shift to the left in the frequency-response curves produced by 5-HT (0.1 microM) was prevented by the 5-HT3 receptor antagonists, metoclopramide (1 microM) and MDL 72222 (0.01 microM). 3. The facilitatory effect of 5-HT is also prevented by the 5-HT2 receptor antagonist, 0.01 microM ketanserin. Nevertheless, this drug reduced by itself the responses to both nerve stimulation and exogenous NA in the nictitating membrane. 4. In the guinea-pig isolated atria, the inhibitory effect of 5-HT on the chronotropic responses to cardioaccelerans nerve stimulation was mimicked by the mixed 5-HT1A + 5-HT1B + 5-HT1D receptor agonist 5-carboxamidotryptamine (5-CT 0.1 and 1 microM). The 5-HT1A receptor agonist 8-OH-DPAT (0.1 and 1 microM) did not modify the responses of the atria to the nerve stimulation. 5. The 5-HT2 receptor antagonists, ketanserin (0.01 and 0.1 microM) and cyproheptadine (1 microM), did not prevent the inhibitory effect of 5-HT in the guinea-pig atria. 6. The present results suggest that the facilitatory effects of 5-HT in the nictitating membrane of the cat are linked to the activation of 5-HT3 receptors whereas the inhibitory effects observed in the guinea-pig atria are mediated by 5-HT1-like receptors.

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)

A pharmacological analysis of the 5-HT receptor mediating inhibition of 5-HT release in the guinea-pig frontal cortex

The release of [3H]5-HT from guinea-pig frontal cortex slices was elicited by continuous exposure to Krebs solution containing elevated K+ ions (30 mM) and 10 microM fluvoxamine. K+-stimulated release was inhibited by 5-carboxamidotryptamine (pIC25 8.1), 5-HT (7.4), RU 24969 (6.5) and GR 43175 (6.4). 8-OH-DPAT was without effect on K+-evoked release of [3H]5-HT at concentrations up to 1 microM. The inhibitory effects of 5-HT were antagonised by metitepine (pA2 8.2), metergoline (7.0), methysergide (6.5), cyanopindolol (6.5), yohimbine (6.5) and mesulergine (6.2) but not by the 5-HT3 antagonist, ICS 205-930 (1 microM). The results are discussed in the context of the known pharmacology of 5-HT receptor subtypes. It is concluded that the 5-HT receptor modulating 5-HT release in the guinea-pig frontal cortex does not correspond to any of the 5-HT1-subtype recognition sites or to 5-HT2 or 5-HT3 receptors.

Quipazine reduces food intake in the rat by activation of 5-HT2-receptors

1. To determine which subtype(s) of 5-hydroxytryptamine (5-HT) receptor are involved in the anorectic action of quipazine, the ability of selective antagonists at 5-HT2- and 5-HT3-receptors, and an antagonist at 5-HT1-like receptors, to block this response were investigated in non-deprived rats, trained to eat a palatable diet. 2. Quipazine (0.5-8 mg kg-1, i.p.) produced a dose-related reduction in the intake of palatable diet. 3. The anorectic effect of 4 mg kg-1 quipazine was antagonized by the nonselective 5-HT-receptor antagonist methysergide (5 mg kg-1, i.p.) and by the selective 5-HT2-receptor antagonists ketanserin (1 mg kg-1 and 2.5 mg kg-1, i.p.) and ritanserin (0.5 mg kg-1 and 1 mg kg-1, i.p.). The selective 5-HT3-receptor antagonist GR38032F (1 mg kg-1, i.p.) and (-)-pindolol (4 mg kg-1, i.p.), which blocks some of the effects mediated at 5-HT1-like receptors, did not block the reduction in food intake produced by this dose of quipazine. 4. None of the 5-HT-receptor antagonists had any effect on food intake when they were administered alone, suggesting that endogenous 5-HT is not involved in the tonic control of food intake under the conditions of these experiments. 5. It is concluded that the anorectic action of quipazine is mediated, at least in part, by activation of 5-HT2-receptors.

Antagonistic properties of RU 24969, a preferential 5-HT1 receptor agonist, at presynaptic alpha 2-adrenoceptors of central and peripheral neurones

The effect of RU 24969 (5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)-1 H-indole) on the electrically evoked 3H overflow was studied in superfused rat brain cortex slices preincubated with 3H-noradrenaline or 3H-serotonin and in superfused segments of the rat vena cava preincubated with 3H-noradrenaline. In cortex slices preincubated with 3H-noradrenaline, RU 24969 facilitated the electrically (3 Hz) evoked 3H overflow. This effect was abolished by phentolamine but was not affected by desipramine or the 5-HT3 receptor antagonist ICS 205-930. The concentration-response curve of noradrenaline for its inhibitory effect on the evoked overflow (determined in the presence of desipramine) was shifted to the right by RU 24969 32 and 100 mumol/l. In this respect, RU 24969 was about 500 times less potent than phentolamine. In cortex slices preincubated with 3H-serotonin, the inhibitory effect of 3.2 mumol/l RU 24969 on the electrically evoked 3H overflow was increased by phentolamine. In segments of the vena cava, RU 24969 inhibited the electrically (0.66 Hz) evoked 3H overflow. The concentration-response curve of RU 24969 was U-shaped, since at concentrations higher than 0.1 mumol/l the extent of inhibition decreased with increasing concentrations of RU 24969. In the presence of phentolamine, the concentration-dependent attenuation of the RU 24969-induced inhibition of overflow was no longer detectable. The present results suggest that RU 24969 is a weak antagonist at presynaptic alpha 2-adrenoceptors (by more than 2.5 log units less potent than as an agonist at presynaptic 5-HT1B auto- and heteroreceptors).

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.

Intrathecal 5-hydroxytryptamine and electrical stimulation of the nucleus raphe magnus in rats both reduce the antinociceptive potency of intrathecally administered noradrenaline

The antinociceptive potency of noradrenaline (NA), as assessed by suppression of the spinal nocifensive tail flick and paw withdrawal reflexes was examined. The tail flick latency (TFL) was increased to the cut-off value for a period of approximately 120 min following the intrathecal microinjection of a standard 15 nmol dose of NA. A similar intrathecal dose of NA produced an increase in threshold to elicit the paw withdrawal reflex for a period of approximately 20 min. When preceded by a standard intrathecal microinjection of 260 nmol 5-hydroxytryptamine, the antinociceptive potency of NA was significantly reduced as reflected by both the tail flick and paw withdrawal tests. In addition, electrical stimulation of the posterior raphe complex immediately before and during the NA-induced increase in TFL, significantly reduced the antinociceptive potency of NA. It is concluded that spinal tryptaminergic activity can reduced the duration of the antinociceptive action of intrathecally applied NA.

Evidence for excitatory 5-HT2-receptors on rat brainstem neurones

1. The technique of microiontophoresis was used to investigate the identity of the receptor mediating the excitatory effects of 5-hydroxytryptamine (5-HT) upon neurones in the midline of the medullary brainstem of the rat in vivo. 2. The 5-HT1-like receptor agonists 5-carboxamidotryptamine (5-CT) and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) failed to excite the majority of neurones excited by 5-HT. The mobilities of 5-CT and 8-OH-DPAT when tested in vitro were found not to differ significantly from that of 5-HT, suggesting that the lack of effect of these agonists was not due to a lower rate of release from the microelectrodes. 3. The excitatory responses to 5-HT were attenuated by the 5-HT 2-receptor antagonists ketanserin and methysergide when applied microiontophoretically or administered intravenously (0.3 and 1 mg kg-1 respectively). Excitatory responses to glutamate and noradrenaline were not reduced. 4. The 5-HT3-receptor antagonist MDL 72222 failed to attenuate selectively the excitatory response to 5-HT when applied either by microiontophoresis or administered intravenously (1 mg kg-1). 5. Microiontophoretic application of the alpha 1-adrenoceptor antagonist prazosin did not attenuate excitatory responses to either 5-HT or noradrenaline. Intravenously administered prazosin (0.8 mg kg-1) also failed to attenuate excitatory responses to 5-HT, but did block excitatory responses to noradrenaline. 6. These results suggest that 5-HT2-receptors, but not 5-HT1-like receptors, 5-HT3-receptors or alpha 1-adrenoceptors, are involved in the excitatory response of midline medullary neurones to 5-HT.

5-Hydroxytryptamine stimulates two distinct adenylate cyclase activities in rat brain: high-affinity activation is related to a 5-HT1 subtype different from 5-HT1A, 5-HT1B, and 5-HT1C

5-HT binding sites of the 5-HT1 type are heterogeneous and appear to comprise several subtypes (5-HT1A, 5-HT1B and 5-HT1C); their physiological role is as yet unclear. The stimulation of adenylate cyclase induced by 5-HT has been investigated in membrane fractions prepared from rat brain cortex. Enzymatic activity was determined by measuring cAMP production with an HPLC technique. It was shown that 5-HT stimulates adenylate cyclase activity with 2 activation constants (Kact): one shows a high apparent affinity (Kact = 0.8 nM) and the other a lower apparent affinity (Kact = 0.30 microM). The latter activity, induced by micromolar concentrations of 5-HT, was inhibited by spiperone at concentrations that block 5-HT1A binding. 5-Methoxytryptamine, bufotenin, and LSD also had a stimulatory biphasic effect on adenylate cyclase activity, whereas trifluoromethylphenylpiperazine, 5-carboxyamidotryptamine, 8-hydroxy-(2-di-n-propylamino)tetralin, RU 24969 had a monophasic effect. Enzyme activation by drugs acting in the micromolar range was inhibited by spiperone (1 microM), suggesting a link between this activation and 5-HT1A sites. On the other hand, the high-affinity activation of the enzyme induced by 5-HT, 5-methoxytryptamine, bufotenin, LSD, and the activation induced by TFMPP were not inhibited by spiperone (1 microM), by propranolol (3 microM), or by mesulergine (0.1 microM), which selectively block 5-HT1A, 5-HT1B, and 5-HT1C sites. Inhibition was produced by dihydroergotamine, methysergide, cinanserin, and mianserin, but not by naloxone, phenoxybenzamine, and phentolamine. Therefore, these activations seem related to 5-HT1 receptors but not to 5-HT1A, 5-HT1B, or 5-HT1C sites. Accordingly, binding of [3H]5-HT to 5-HT1-like sites was examined in the presence of spiperone (1 microM) and propranolol (3 microM); in these conditions, a high-affinity site (KD = 3.4 nM) was indeed revealed. The relative potencies of a series of drugs that stimulate or inhibit the activation of the adenylate cyclase with a high affinity and their ability to inhibit this binding of [3H]5-HT showed a positive correlation, strongly suggesting a direct relation between this recognition site for 5-HT and the production of a second messenger (cAMP). Moreover, this potential receptor is shown to be heterogeneously distributed within the brain, and was localized postsynaptically at serotonergic synapses.

Detection and characterization of the serotonin 5-HT 1D receptor in rat and human brain

In the presence of 1 microM ( +/- )-pindolol [to block 5-hydroxytryptamine (5-HT, serotonin) 5-HT 1A and 5-HT 1B receptors] and 100 nM mesulergine (to block 5-HT 1C receptors), 2.0 nM [3H]5-HT binding to rat cortical homogenates is specific, saturable, and reversible. Scatchard analysis of [3H]5-HT binding, in the presence of 1 microM ( +/- )-pindolol and 100 nM mesulergine, produced a KD of 3.2 nM and Bmax of 43 fmol/mg protein. Distribution studies show this site to be present in most rat brain regions. This site is also detectable in human caudate. The pharmacological profile of this site is distinct from the previously identified 5-HT receptor subtypes. Compounds with high affinity for 5-HT 1A (8-hydroxydipropylaminotetralin), 5-HT 1B (trifluoromethylphenylpiperazine), 5-HT 1C (mesulergine), 5-HT 2 (4-bromo-2,5-dimethoxyphenylisopropylamine), and 5-HT3 (ICS 205-930) receptors have low affinity for this site. These data suggest the presence of an additional, previously unidentified, 5-HT binding site in rat and human brain tissue. This putative novel 5-HT receptor has a similar pharmacology to the "5-HT 1D" site detected in bovine brain by Heuring and Peroutka.

Identification of a 5-HT1 recognition site in human brain membranes different from 5-HT1A, 5-HT1B and 5-HT1C sites

In human caudate and cortex membranes, [3H]serotonin ([3H]5-HT) labels 5-HT1A and 5-HT1C recognition sites which show nanomolar affinity for 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)-tetralin) and mesulergine respectively, whereas no 5-HT1B binding could be identified. However, the majority of the sites labelled by [3H]5-HT (greater than or equal to 60% in cortex, 90% in caudate) are different from 5-HT1A, 5-HT1B and 5-HT1C sites. Competition experiments were performed in human caudate membranes incubated with [3H]5-HT in the presence of 100 nM 8-OH-DPAT and 100 nM mesulergine. Under those conditions, [3H]5-HT labelled an apparently homogeneous population of 5-HT1-like sites which display nanomolar affinity for tryptamines (5-carboxamido-tryptamine, (5-CT) greater than 5-HT greater than or equal to 5-methoxytryptamine (5-MeOT) greater than tryptamine) and some ergolines (metergoline greater than methysergide). In contrast, these sites showed low affinity for drugs with high affinity and/or selectivity for 5-HT1A (8-OH-DPAT, buspirone), 5-HT1B (21-009, RU 24969), 5-HT1C (mesulergine, mianserin) and 5-HT2 sites (ketanserin, cinanserin). The pharmacological profile of these sites is different from that of 5-HT1A, 5-HT1B, 5-HT1C, 5-HT2 and 5-HT3 sites but is consistent with the pharmacology of a 5-HT1-like receptor. It is very similar to that of the 5-HT1D site recently described in bovine brain by Heuring and Peroutka.

Functional correlates of serotonin 5-HT1 recognition sites

A short overview is given of the evidence supporting the existence of subtypes of 5-HT1 receptors. As 5-HT1 receptors were first described using radioligand binding studies, a brief description of 5-HT1A, 5-HT1B, 5-HT1C and 5-HT1D receptor binding is given. Then, recent data obtained in biochemical, electrophysiological, behavioural and other functional studies is compared to results obtained in radioligand binding studies. The paper shows that functional correlates do indeed exist for subtypes of 5-HT1 recognition sites; moreover, these are markedly different from the effects mediated by 5-HT2 (5-HT D) or 5-HT3 (5-HT M) receptors.

A 5-HT1-like receptor mediates a sympathetic ganglionic hyperpolarization

5-HT induced a hyperpolarization of the rat superior cervical ganglion in vitro which was resistant to both MDL 72222 (10 microM), a 5-HT3 antagonist, and ketanserin (1 microM), a 5-HT2 antagonist. The 5-HT1-selective ligands 5-carboxamidotryptamine and 8-OH-DPAT also hyperpolarized the ganglion. The 5-HT-induced hyperpolarization was potently antagonised by spiperone. These results suggest that 5-HT hyperpolarizes the rat superior cervical ganglion via a 5-HT1-like receptor which resembles the central 5-HT1A binding site.

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.