Serotonin and beyond-a tribute to Manfred Göthert (1939-2019).

Manfred Göthert, who had served Naunyn-Schmiedeberg's Arch Pharmacol as Managing Editor from 1998 to 2005, deceased in June 2019. His scientific oeuvre encompasses more than 20 types of presynaptic receptors, mostly on serotoninergic and noradrenergic neurones. He was the first to identify presynaptic receptors for somatostatin and ACTH and described many presynaptic receptors, known from animal preparations, also in human tissue. In particular, he elucidated the pharmacology of presynaptic 5-HT receptors. A second field of interest included ligand-gated and voltage-dependent channels. The negative allosteric effect of anesthetics at peripheral nACh receptors is relevant for the peripheral clinical effects of these drugs and modified the Meyer-Overton hypothesis. The negative allosteric effect of ethanol at NMDA receptors in human brain tissue occurred at concentrations found in the range of clinical ethanol intoxication. Moreover, the inhibitory effect of gabapentinoids on P/Q Ca2+ channels and the subsequent decrease in AMPA-induced noradrenaline release may contribute to their clinical effect. Another ligand-gated ion channel, the 5-HT3 receptor, attracted the interest of Manfred Göthert from the whole animal via isolated preparations down to the cellular level. He contributed to that molecular study in which 5-HT3 receptor subtypes were disclosed. Finally, he found altered pharmacological properties of 5-HT receptor variants like the Arg219Leu 5-HT1A receptor (which was also shown to be associated with major depression) and the Phe124Cys 5-HT1B receptor (which may be related to sumatriptan-induced vasospasm). Manfred Göthert was a brilliant scientist and his papers have a major impact on today's pharmacology.

Potential involvement of a propranolol-insensitive atypical beta-adrenoceptor the vasodilator effect of cyanopindolol in the human pulmonary artery.

The aim of our study was to examine whether non beta(1)-/beta(2)-adrenoceptors participate in the relaxation of the human pulmonary artery. For this purpose the vasodilatory effect of the non-conventional partial beta-adrenoceptor agonist cyanopindolol was examined. Cyanopindolol (1-300 microM), studied in the presence of the beta(1)-/beta(2)-adrenoceptor antagonist propranolol, relaxed the human pulmonary artery preconstricted with serotonin 1 microM in a concentration-dependent manner (maximally by about 80%). This effect was diminished by bupranolol 10 microM (an antagonist of beta(1)-beta(3)-adrenoceptors and the low affinity state of the beta(1)-adrenoceptor) and CGP 20712 10 microM (known to antagonize the low-affinity state of the beta(1)-adrenoceptor at high concentrations). In further experiments, the effect of beta-adrenoceptor ligands on the serotonin-induced vasoconstriction was examined. The concentration-response curve for serotonin was not affected by cyanopindolol 30 microM, bupranolol 10 microM and CGP 20712 10 microM but shifted to the right by cyanopindolol 100 and 300 microM; the serotonin 5-HT(2A) receptor antagonist ketanserin 0.3 microM abolished the maximum contraction elicited by serotonin. In conclusion, the present study reveals that the vasodilatory effect of cyanopindolol in the human pulmonary artery consists of two components, i.e. activation of a propranolol-insensitive atypical beta-adrenoceptor and antagonism against 5-HT(2A) receptors.

Bi-directional CB1 receptor-mediated cardiovascular effects of cannabinoids in anaesthetized rats: role of the paraventricular nucleus.

The activation of cannabinoid CB1 receptors decreases and increases blood pressure (BP) in anaesthetized and conscious rats, respectively. The aim of our study was to check the possible involvement of CB1 receptors in the paraventricular nucleus of the hypothalamus (PVN) in the cardiovascular effects of cannabinoids in rats. Methanandamide (metabolically stable analogue of the endocannabinoid anandamide) and the synthetic cannabinoid receptor agonist CP55940 were microinjected into the PVN of urethane-anaesthetized rats twice (S1 and S2, 20 min apart). Receptor antagonists were administered intravenously (i.v.) 5 min before S1. Methanandamide and CP55940 decreased blood pressure by 15 - 20%. The CB1 receptor antagonist AM251 reversed the depressor effect into a pressor response of 20 - 30%. The pressor effect of CP55940 observed in the presence of AM251 i.v. was reduced by AM251 given additionally into the PVN but not by the i.v. injection of the CB2 antagonist SR144528 or the vanilloid TRPV1 antagonist ruthenium red. In the presence of the peripherally restricted CB1 receptor antagonist AM6545, CP55940 given into the PVN increased BP by 40%. AM6545 reversed the decrease in BP induced by CP55940 i.v. into a marked increase. Bilateral chemical lesion of the PVN by kainic acid abolished all cardiovascular effects of CP55940 i.v. In conclusion, the cannabinoid CP55940 administered to the PVN of urethane-anaesthetized rats can induce depressor and pressor effects. The direction of the response probably depends on the sympathetic tone. The centrally induced hypertensive response of CP55940 can, in addition, be masked by peripheral CB1 receptors.

Mediation of the positive chronotropic effect of CGP 12177 and cyanopindolol in the pithed rat by atypical beta-adrenoceptors, different from beta 3-adrenoceptors.

1. The influence of beta 1-, beta 2-, and beta 3-adrenoceptor agonists and of CGP 12177 and cyanopindolol on heart rate and diastolic blood pressure was studied in the pithed rat. 2. The beta 1-adrenoceptor agonist, prenalterol, increased heart rate and the beta 2-adrenoceptor agonist, fenoterol, caused a fall in blood pressure. The effect of prenalterol was antagonized by the beta 1-adrenoceptor antagonist, CGP 20712 0.1 mumol kg-1 and the action of fenoterol was attenuated by the beta 2-adrenoceptor antagonist, ICI 118551 0.1 mumol kg-1. Both effects were markedly diminished by the non-selective beta-adrenoceptor antagonist, bupranolol 0.1 mumol kg-1. 3. The non-selective beta-adrenoceptor agonist, isoprenaline, three beta 3-agonists as well as CGP 12177 and cyanopindolol elicited a positive chronotropic effect, exhibiting the following pED delta 60 values (negative log values of the doses increasing heart rate by 60 beats min-1): isoprenaline 10.4, CGP 12177 8.3, cyanopindolol 7.2, BRL 37344 6.9, ZD 2079 5.2 and CL 316243 < 5. 4. CGP 20712 0.1 mumol kg-1, given together with ICI 118551 0.1 mumol kg-1, markedly attenuated the positive chronotropic effect of isoprenaline, BRL 37344, ZD 2079 and CL 316243 without affecting the increase in heart rate produced by CGP 12177 and cyanopindolol. 5. The positive chronotropic effect of CGP 12177 and cyanopindolol was attenuated by CGP 20712, 1 and 10 mumol kg-1 and bupranolol, 10 mumol kg-1 but was not affected by ICI 118551, 10 mumol kg-1. The effect of CGP 12177 was also not changed by BRL 37344 1 mumol kg-1, ZD 2079 10 mumol kg-1, CL 316243 10 mumol kg-1, the alpha 1-adrenoceptor antagonist, prazosin 1 mumol kg-1 and the 5-hydroxytryptamine 5-HT2A receptor antagonist, ketanserin 3 mumol kg-1. 6. CGP 12177 0.002 mumol kg-1 and cyanopindolol 0.003 mumol kg-1 shifted to the right the dose-response curve of prenalterol for its positive chronotropic effect. The -log values of the doses causing a twofold shift to the right were 9.6 and 9.5, respectively. 7. Isoprenaline 0.00001-0.001 mumol kg-1, BRL 37344 0.01-1 mumol kg-1 and CGP 12177 0.1 mumol kg-1 caused a fall in diastolic blood pressure which was markedly attenuated by combined administration of CGP 20712 and ICI 118551, 0.1 mumol kg-1 each. 8. CGP 12177 0.01 and 0.1 mumol kg-1 and cyanopindolol 1 mumol kg-1 elicited an increase in diastolic blood pressure. CGP 20712, ICI 118551, bupranolol and, in the case of CGP 12177, also BRL 37344, ZD 2079, CL 316243, prazosin and ketanserin did not influence this effect. 9. In conclusion, the positive chronotropic effect of CGP 12177 and cyanopindolol is not mediated via beta 1-, beta 2-, beta 3-, alpha 1-adrenoceptors or 5-HT2A receptors. This effect may involve atypical beta-adrenoceptors, similar or identical to those described by Kaumann (1989) in isolated heart preparations.

Further evidence for differences between cardiac atypical beta-adrenoceptors and brown adipose tissue beta3-adrenoceptors in the pithed rat.

1. We have previously shown (Malinowska & Schlicker, 1996) that the atypical beta-adrenoceptor involved in the positive chronotropic effect of the so-called non-conventional partial beta-adrenoceptor agonists CGP 12177 and cyanopindolol in the pithed rat possesses properties markedly different from those observed for beta3-adrenoceptors in the literature. In the present study, we have directly compared the pharmacological properties of the atypical cardiostimulant beta-adrenoceptor and of the beta3-adrenoceptor mediating the thermogenic response in the brown adipose tissue in pithed and vagotomized rats. 2. Heart rate was dose-dependently increased by CGP 12177 and cyanopindolol by maximally 150 and 100 beats min(-1), yielding pED50 values of 8.0 and 7.3, respectively (pED50, -log10 of the dose in mol kg(-1) body weight i.v. causing the half-maximum effect), but not affected by the selective beta3-adrenoceptor agonist CL 316243 (pED50 > 6.0). 3. CGP 12177, cyanopindolol and CL 316243 increased temperature in the brown adipose tissue by maximally 1 degree C (pED50 values 7.4, 6.3 and 8.6, respectively). 4. The beta1-adrenoceptor antagonist CGP 20712 10 micromol kg(-1), attenuated the cardiostimulatory effect of CGP 12177 and, at a still higher dose (30 micromol kg(-1)), also antagonized its thermogenic effect. The -log10 values of the doses causing a two fold shift of the dose-response curves (DRCs) of CGP 12177 to the right were 6.1 and 5.2, respectively, and were much lower than the corresponding value for the antagonism of CGP 20712 against the beta1-adrenoceptor-mediated positive chronotropic effect which was 8.6. 5. The cardiostimulant and the thermogenic effect of CGP 12177 were not affected by the beta2-adrenoceptor antagonist ICI 118551 10 micromol kg(-1). 6. The beta3-adrenoceptor antagonist SR 59230A (which, by itself, caused a beta1-adrenoceptor-mediated increase in heart rate and, for this reason, was studied after administration of a low dose of CGP 20712) attenuated the cardiostimulant and the thermogenic effect of CGP 12177 to a similar extent. The -log10 values of the doses causing two fold rightward shifts of the DRCs of CGP 12177 were 5.9 and 5.7, respectively. 7. The non-selective beta-adrenoceptor antagonist bupranolol diminished the cardiostimulant and thermogenic response to a very similar extent. The -log10 values causing two fold rightward shifts of the DRCs of CGP 12177 were 5.6 and 5.7, respectively, and were much lower than the corresponding values for the antagonism of bupranolol against the beta1-adrenoceptor-mediated positive chronotropic effect and the beta2-adrenoceptor-mediated decrease in diastolic blood pressure which were 7.6 and 8.3, respectively. 8. The rank order of agonistic potencies for the cardiostimulant effect (CGP 12177 > cyanopindolol > CL 316243) differs from that for the thermogenic response in the brown adipose tissue (CL 316243 > CGP 12177 > cyanopindolol); furthermore, there is a difference with respect to the rank orders of antagonistic potencies for cardiostimulation (CGP 20712 > or = SR 59230A > or = bupranolol > ICI 118551) and thermogenesis (SR 59230A = bupranolol > CGP 20712 > ICI 118551). 9. In conclusion, our study provides further evidence that the atypical cardiostimulant beta-adrenoceptors (causing an increase in heart rate) and beta3-adrenoceptors are pharmacologically different.

Inhibition of noradrenaline release in the rat vena cava via prostanoid receptors of the EP3-subtype.

1. In segments of the rat vena cava preincubated with [3H]-noradrenaline and superfused with physiological salt solution (containing desipramine and corticosterone), we studied the effects of prostaglandins of the D, E and F series, of a prostacyclin analogue and a thromboxane-mimetic and of subtype-selective prostaglandin E-receptor (EP-receptor) ligands on the electrically (0.66 Hz)-evoked tritium overflow. 2. The electrically-evoked tritium overflow was inhibited by prostaglandin E2 (maximum inhibition by about 80%; pIC40 7.49). The effect of prostaglandin E2 was not affected by rauwolscine, which, by itself, increased the evoked overflow; the alpha 2-adrenoceptor antagonist was added to the superfusion medium in all subsequent experiments. Indomethacin failed to affect either the evoked tritium overflow or its inhibition by prostaglandin E2. 3. The inhibitory effect of prostaglandin E2 on the electrically-evoked tritium overflow was not altered by the EP1-receptor antagonist. AH 6809 (6-isopropoxy-9-oxoxanthene-2-carboxylic acid) at a concentration at least 30 fold higher than its pA2 value at EP1-receptors. The following compounds mimicked the effect of prostaglandin E2 showing the following rank order of potencies: misoprostol (EP2-/EP3-receptor agonist) congruent to sulprostone (EP1-/EP3-receptor agonist) congruent to prostaglandin E1 = prostaglandin E2 >> iloprost (EP1-/IP-receptor agonist) = prostaglandin F2 alpha. The evoked overflow was not affected by high concentrations of prostaglandin D2 or the thromboxane-mimetic U46619 (9,11-dideoxy-11 alpha, 9 alpha-epoxy-methano-prostaglandin F2 alpha). 4. The present results suggest that the postganglionic sympathetic nerve fibres innervating the rat vena cava are endowed with presynaptic EP3-receptors.They are not tonically activated by endogenously formed products of cyclo-oxygenase and do not interact with the presynaptic M2-adrenoceptors.

Noradrenaline release in the rat vena cava is inhibited by gamma-aminobutyric acid via GABAB receptors but not affected by histamine.

1. Segments of the rat vena cava preincubated with [3H]-noradrenaline were superfused with [3H]-noradrenaline-free solution containing desipramine and corticosterone and the effects of gamma-aminobutyric acid (GABA) receptor ligands and of histamine on tritium overflow evoked by transmural electrical stimulation were studied. 2. GABA inhibited, and histamine failed to affect, the electrically (0.66 Hz) evoked tritium overflow both in the absence and presence of rauwolscine (which was present in the superfusion medium in the subsequent experiments). The effect of GABA was less pronounced at a stimulation frequency of 2 Hz. 3. The inhibitory effect of GABA (pIC35 5.83) on the electrically (0.66 Hz) evoked overflow was mimicked by the GABAB receptor agonist, R-(-)-baclofen (6.07) and less potently by S-(+)-baclofen (3.30) and the GABAA receptor agonist, muscimol (3.70). The concentration-response curve of GABA was shifted to the right by the GABAB receptor antagonist, CGP 35348 (P-(3-aminopropyl)-P- diethoxymethyl-phosphinic acid; apparent pA2 4.76), but not affected by the GABAA receptor antagonist, (-)-bicuculline methiodide 100 mumol l-1. Given alone, (-)-bicuculline methiodide slightly increased, and CGP 35348 did not affect, the evoked overflow. 4. The effect of GABA in veins from rats treated for 14 days with RS-baclofen (10 mg kg-1, i.p. once daily) did not differ from that in veins from rats which received the vehicle instead. The effect of GABA also did not differ in veins from rats treated once either with RS-baclofen or with its vehicle. 5. The results suggest that the postganglionic sympathetic nerve fibres in the rat vena cava are endowed with presynaptic GABAB receptors, but not with receptors for histamine. (-)-Bicuculline methiodide increases noradrenaline release by a mechanism not related to GABAA, GABAB, alpha2-receptors or noradrenaline uptake.

Acetyl-RYYRIK-NH(2) is a highly efficacious OP(4) receptor agonist in the cardiovascular system of anesthetized rats.

Nociceptin, the endogenous ligand of the OP(4) or ORL(1) (opioid receptor-like(1)) receptor, decreases blood pressure and heart rate in anesthetized rats. Since the OP(4) receptor antagonist [Phe(1)Psi(CH(2)-NH)Gly(2)]-nociceptin(1-13)NH(2) possesses an agonistic effect in this model, we examined whether other purported OP(4) receptor antagonists, acetyl-RYYRIK-NH(2) and naloxone benzoylhydrazone, antagonize the depressant effects of nociceptin. Acetyl-RYYRIK-NH(2), like nociceptin and [Phe(1)Psi(CH(2)-NH)Gly(2)]-nociceptin(1-13)NH(2) and unlike naloxone benzoylhydrazone, decreased diastolic blood pressure and heart rate (rank order of potencies: nociceptin approximately equal to acetyl-RYYRIK-NH(2) >> [Phe(1)Psi(CH(2)-NH)Gly(2)]-nociceptin(1-13)NH(2)). The depressant effects were insensitive to the OP(1-3) receptor antagonist naloxone but diminished by naloxone benzoylhydrazone. In conclusion, the hypotensive and bradycardic effects of nociceptin in the anesthetized rat are mediated via OP(4) receptors, at which acetyl-RYYRIK-NH(2) is a highly potent and efficacious agonist.

H3 receptor-mediated inhibition of the neurogenic vasopressor response in pithed rats.

In pithed rats, the H3 agonist R-(-)-alpha-methylhistamine (R alpha MeHA) inhibited the electrically induced increase in blood pressure without affecting the vasopressor response to exogenous noradrenaline. The effect of R alpha MeHA was not affected by the H1 and H2 antagonists dimetindene and ranitidine, but attenuated by the H3 antagonist thioperamide. At higher doses, R alpha MeHA itself increased basal blood pressure; this effect was not affected by the H1, H2 and H3 antagonists. In conclusion, the neurogenic vasopressor response can be modulated via H3 receptors, probably located presynaptically on postganglionic sympathetic nerve fibres.

EP3 receptor-mediated inhibition of the neurogenic vasopressor response in pithed rats.

In pithed rats, we studied the effects of prostaglandin E2 and of subtype-selective prostaglandin E receptor (EP receptor) ligands on the rise in blood pressure induced by electrical stimulation of the preganglionic sympathetic nerves. Prostaglandin E2, the EP1/EP3 receptor agonist sulprostone and the EP2/EP3 receptor agonist misoprostol inhibited the electrically induced increase in diastolic blood pressure (rank order of potencies sulprostone > or = misoprostol > or = prostaglandin E2); the rise in blood pressure induced by exogenously added noradrenaline was not affected by these compounds. The inhibitory effect of sulprostone on the electrically induced vasopressor response was not significantly changed by indomethacin. Iloprost (an agonist at EP1 and prostacyclin receptors (IP receptors)) failed to affect the electrically evoked increase in blood pressure. The present study suggests that prostaglandin E2 inhibits the release of catecholamines in pithed rats via prostanoid receptors of the EP3 subtype, probably located presynaptically on the postganglionic sympathetic nerve fibres.

Facilitation by substance P and inhibition by (+)-tubocurarine of the 5-HT3 receptor-mediated Bezold-Jarisch reflex in rats.

The influence of substance P 3 (microgram/kg) and (+)-tubocurarine (850 micrograms/kg) on the Bezold-Jarisch reflex in urethane-anaesthetized rats was studied. The Bezold-Jarisch reflex was induced by the 5-HT3 receptor agonist phenylbiguanide (0.3, 1, 3 and 10 micrograms/kg i.v.) and by capsaicin (10 micrograms/kg i.v.). The 5-HT3 receptor antagonist ondansetron (10 micrograms/kg) abolished the phenylbiguanide- but not the capsaicin-stimulated bradycardia, indicating that phenylbiguanide and capsaicin act via different trigger mechanisms (5-HT3 receptor-dependent and -independent, respectively). Substance P significantly potentiated the phenylbiguanide- but not the capsaicin-induced decrease in heart rate. Also, when the phenylbiguanide-induced response was amplified by substance P, it was abolished by ondansetron. (+)-Tubocurarine inhibited the phenylbiguanide-induced bradycardia, but did not affect the capsaicin-stimulated decrease in heart rate. Our results demonstrate that substance P potentiates but (+)-tubocurarine inhibits the 5-HT3 receptor-mediated Bezold-Jarisch reflex. Both effects are probably due to direct influences of the drugs on the 5-HT3 receptors on sensory vagal nerves in the heart.

Ifenprodil influences changes in mouse behaviour related to acute and chronic ethanol administration.

The aim of the present study was to examine the influence of ifenprodil (a non-competitive NMDA receptor antagonist which also blocks 5-HT3 receptors and alpha1-adrenoceptors) on the effects of ethanol in the mouse in vivo and to elucidate the role of various receptors in these actions. The ethanol (4 g/kg i.p.)-induced sleeping time was shortened by ifenprodil 1 mg/kg but was not affected by ifenprodil 0.3 mg/kg, the 5-HT3 receptor antagonist ondansetron 0.03 mg/kg and the non-competitive NMDA receptor antagonist MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cycloheptan-5,10-imine maleate) 0.01 mg/kg. Ifenprodil 10 mg/kg mimicked the alpha1-adrenoceptor antagonist prazosin 1 mg/kg in that it prolonged the hypnotic response to ethanol (no additive effect when both drugs were given in combination); this is compatible with an involvement of alpha1-adrenoceptors in this effect of ifenprodil. Chronic exposure to ethanol (7%) induced physical dependence. The severity of ethanol withdrawal was suppressed by ifenprodil 1 and 10 mg/kg. In conclusion, ifenprodil influences ethanol-related changes in mouse behaviour and may prove to be useful in the treatment of alcoholism.

Inhibitory effect of ethanol on the 5-hydroxytryptamine-induced Bezold-Jarisch reflex--involvement of peripheral 5-HT3 receptors.

The influence of ethanol (0.5, 1.0 and 2.0 g/kg i.p.) on the Bezold-Jarisch reflex in urethane-anaesthetized rats was studied. 5-Hydroxytryptamine (serotonin; 5-HT; 1, 3, 10 and 30 micrograms/kg i.v.) and capsaicin (1, 3 and 10 micrograms/kg i.v.) reflexly decreased heart rate in a dose-dependent manner. The 5-HT3 receptor antagonist ondansetron 10 micrograms/kg i.v. abolished the 5-HT- but not the capsaicin-stimulated bradycardia, indicating that 5-HT and capsaicin acted via different trigger mechanisms (5-HT3 receptor-dependent and -independent, respectively). Ethanol at 1.0 and 2.0 g/kg i.p. inhibited in a dose-dependent manner (by 20-45%) the 5-HT- but not the capsaicin-stimulated decrease in heart rate. Our results demonstrate that the inhibitory effect of ethanol on the 5-HT3 receptor-mediated Bezold-Jarisch reflex may be related to the direct effect of ethanol on 5-HT3 receptors on sensory vagal nerves in the heart.

Cardiovascular effects of ethanol in anaesthetized, conscious and pithed rats.

The influence of ethanol (2.0 g/kg p.o.) on heart rate and blood pressure in anaesthetized, conscious and pithed rats was studied. In anaesthetized rats we observed an increase in heart rate and a marked and very sudden decrease in blood pressure. In pithed rats blood pressure decreased more slowly than in anaesthetized and conscious animals and there were no changes in heart rate. These results show that the central nervous system may be responsible for the increase in heart rate and early phase of hypotension caused by acute ethanol administration. The slower decrease (up to 1 h) in blood pressure may be caused by central and different indirect peripheral mechanisms.

Identification of endothelial H1, vascular H2 and cardiac presynaptic H3 receptors in the pithed rat.

In pithed and vagotomized rats the effects of the H3 receptor agonist R-(-)-alpha-methylhistamine, the H1 receptor agonist 2-(2-thiazolyl)ethylamine and the H2 receptor agonist dimaprit on basal diastolic blood pressure, basal heart rate and the electrically induced rise in heart rate were examined. Basal diastolic blood pressure was not altered by low, but increased by high doses of R-(-)-alpha-methylhistamine; the latter effect was not affected by selective H1, H2 or H3 receptor antagonists and by prazosin, but was attenuated by rauwolscine. Rauwolscine also unmasked a vasodepressor response to R-(-)-alpha-methylhistamine not affected by the H3 receptor antagonist thioperamide, but counteracted by the H1 receptor antagonist dimetindene or the H2 receptor antagonist ranitidine. The vasodepressor responses to 2-(2-thiazolyl)ethylamine and dimaprit were antagonized by dimetindene and ranitidine, respectively. The vasodepressor response to 2-(2-thiazolyl)ethylamine was not altered by indomethacin, but reduced by an inhibitor of endothelial nitric oxide synthase, N omega-nitro-L-arginine methyl ester (which, by itself, markedly increased blood pressure). Both drug tools did not alter the effect of dimaprit. Basal heart rate was not affected by 2-(2-thiazolyl)ethylamine (examined after administration of propranolol), dimaprit and R-(-)-alpha-methylhistamine. The electrically induced increase in heart rate (studied in animals which had received rauwolscine) was decreased by R-(-)-alpha-methylhistamine but not affected by 2-(2-thiazolyl)ethylamine and dimaprit. The effect of R-(-)-alpha-methylhistamine was abolished by thioperamide. R-(-)-alpha-methylhistamine did not influence the increase in heart rate produced by isoprenaline.(ABSTRACT TRUNCATED AT 250 WORDS)

Anandamide and methanandamide induce both vanilloid VR1- and cannabinoid CB1 receptor-mediated changes in heart rate and blood pressure in anaesthetized rats.

In anaesthetized rats activation of vanilloid receptors on sensory vagal nerves elicits rapid bradycardia and hypotension (Bezold-Jarisch reflex). Recent in vitro experiments revealed that the endogenous cannabinoid ligand anandamide acts as an agonist at the vanilloid VRI receptors. The present study was aimed at examining whether vanilloid VR1 receptors are involved in the cardiovascular effects of anandamide in the anaesthetized rat. Intravenous injection of anandamide, its stable analogue methanandamide and the vanilloid receptor agonist capsaicin produced a dose-dependent immediate and short-lasting decrease in heart rate and blood pressure with the following rank order of potencies: capsaicin > methanandamide > anandamide. This bradycardia was dose-dependently diminished by the selective vanilloid receptor antagonist capsazepine (0.3-3 micromol/kg) and the nonselective inhibitor of these receptors, ruthenium red (1-10 micromol/kg). Both antagonists reduced or tended to reduce the hypotension stimulated by the agonists. Following this bradycardia and hypotension (presumably evoked by the Bezold-Jarisch reflex; phase I), capsaicin, anandamide and methanandamide led to a brief vasopressor effect (phase II). Subsequently both anandamides, but not capsaicin, induced a more prolonged decrease in blood pressure (phase III). Capsazepine and ruthenium red (at doses up to 3 tmol/kg and 10 micromol/kg, respectively) failed to affect these changes in blood pressure. The cannabinoid CB1 receptor antagonist SR 141716 at 3 micromol/kg abolished the prolonged decrease in blood pressure (phase III) induced by anandamide and methanandamide, but had no effect on the reflex bradycardia and hypotension (phase I) and on the subsequent vasopressor effect (phase II) evoked by capsaicin, anandamide and methanandamide. In conclusion, the endogenous cannabinoid receptor agonist anandamide and its stable analogue methanandamide induce reflex bradycardia and hypotension (phase I) by activating the vanilloid VRI receptor. Whereas the mechanism underlying the brief vasopressor effect (phase II) is unknown, the prolonged hypotension (phase III) results from stimulation of the cannabinoid CB1 receptor.

Nociceptin inhibits the neurogenic vasopressor response in the pithed rat via prejunctional ORL1 receptors.

Nociceptin (or orphanin FQ), the endogenous ligand for the opioid receptor-like 1 (ORL1) receptor, decreases blood pressure in the conscious and anesthetized rat. This study examined whether prejunctional inhibitory ORL1 receptors located on the postganglionic sympathetic neurons innervating the resistance vessels are detectable in pithed rats. In pithed and vagotomized rats electrical stimulation of the preganglionic sympathetic nerve fibers, injection of nicotine (2 micromol/kg) or noradrenaline (1 nmol/kg) increased blood pressure by about 30 mmHg. The electrically induced vasopressor response was decreased dose-dependently by nociceptin (0.001-1 micromol/kg; decrease by about 60% at 1 micromol/kg). Nociceptin 0.1 micromol/kg reduced the nicotine-induced vasopressor response by about 40% but at doses up to 1 micromol/kg failed to affect the increase in blood pressure evoked by noradrenaline. The inhibitory action of nociceptin on the electrically and nicotine-induced increase in blood pressure was attenuated by the ORL1 receptor antagonists naloxone benzoylhydrazone (5 micromol/kg) and/or [Phe1psi(CH2-NH)Gly2]-nociceptin(1-13)NH2 (1 micromol/kg) but was not affected by naloxone 10 micromol/kg. In conclusion, the present data suggest that the postganglionic sympathetic nerve fibers innervating the resistance vessels of the rat are endowed with prejunctional ORL1 receptors, activation of which causes inhibition of noradrenaline release.

Cannabinoid CB1 receptor-mediated inhibition of the neurogenic vasopressor response in the pithed rat.

The effects of two cannabinoid receptor agonists, R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4- benzoxazin-yl]-(1-naphthalenyl)-methanone (WIN 55,212-2) and (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydr oxypropyl)-cyclohexanol (CP-55,940), were studied on (i) the vasopressor response elicited in pithed rats by electrical stimulation of the sympathetic outflow and (ii) the release of 3H-noradrenaline and the vasoconstriction elicited in isolated rat tail arteries by transmural electrical stimulation. In pithed rats, the electrical (1 Hz for 10 s) stimulation of the preganglionic sympathetic nerve fibres increased diastolic blood pressure by about 30 mmHg. This neurogenic vasopressor response (which under the conditions of our study was almost exclusively due to the release of catecholamines) was decreased by WIN 55-212,2 and CP-55,940 in a dose-dependent manner (inhibition by WIN 55,212-2 and CP-55,940, 0.1 micromol/kg each, about 25-30%). The inhibition was identical in adrenalectomized rats and in animals with intact adrenals. The inhibitory action of WIN 55,212-2 and CP-55,940 was abolished by a dose of 0.03 micromol/kg of the CB1 receptor antagonist N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazo le-carboxamide (SR 141716), which, by itself, had no effect. WIN 55,212-2, CP-55,940 and SR 141716 failed to affect the vasopressor response to exogenous noradrenaline (1 nmol/kg), which also increased diastolic blood pressure by about 30 mmHg. In isolated rat tail arteries, the electrically (0.4 Hz) evoked tritium overflow and vasoconstriction were not modified by WIN 55,212-2 and CP-55,940 (1 micromol/l each). In conclusion, the neurogenic vasopressor response in the pithed rat can be modulated via cannabinoid CB1 receptors probably located presynaptically on the postganglionic sympathetic nerve fibres innervating resistance vessels.

Inhibitory H3 receptors on sympathetic nerves of the pithed rat: activation by endogenous histamine and operation in spontaneously hypertensive rats.

Our previous results demonstrate the occurrence of presynaptic inhibitory histamine H3 receptors on sympathetic neurons innervating resistance vessels of the pithed rat. The present study, in which new H3 receptor ligands with increased potency and selectivity (imetit, clobenpropit) were used, was designed to further explore the role of H3 receptors in the regulation of the rat cardiovascular system. In particular we were interested whether these receptors may be activated by endogenous histamine and whether they are detectable in an experimental model of hypertension. All experiments were performed on pithed and vagotomized rats treated with rauwolscine 1 mumol/kg. In normotensive Wistar rats the electrical (1 Hz, 1 ms, 50 V for 20 s) stimulation of the preganglionic sympathetic nerve fibres increased diastolic blood pressure by about 35 mmHg. Two H3 receptor agonists, R-(-)-alpha-methylhistamine and imetit, inhibited the electrically induced increase in diastolic blood pressure in a dose-dependent manner. The maximal effect (about 25%) was obtained for R-(-)-alpha-methylhistamine at about 10 mumol/kg and for imetit at about 1 mumol/kg. Two H3 receptor antagonists, thioperamide 1 mumol/kg and clobenpropit 0.1 mumol/kg, attenuated the inhibitory effect of imetit. The neurogenic vasopressor response was increased by about 15% by thioperamide 1 mumol/kg and clobenpropit 0.1 mumol/kg and decreased by 25% by the histamine methyltransferase inhibitor metoprine 37 mumol/kg. R-(-)-alpha-Methylhistamine, imetit, thioperamide, clobenpropit and metoprine did not affect the vasopressor response to exogenously added noradrenaline 0.01 mumol/kg (which increased diastolic blood pressure by about 40 mmHg). Metoprine had only a very low affinity for H3 binding sites (labelled by 3H-N alpha-methylhistamine; pKi 4.46). In pithed Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats, electrical (1 Hz, 1 ms, 50 V for 10 s) stimulation increased diastolic blood pressure by 28 and 37 mmHg, respectively. Imetit inhibited the neurogenic vasopressor response to about the same extent in WKY and SHR rats (maximal effect of about 30%). The inhibitory influence of imetit was diminished by thioperamide 1 mumol/kg to about the same degree in rats of either strain. The present study confirms the occurrence of presynaptic H3 receptors on sympathetic nerve fibres involved in the inhibition of the neurogenic vasopressor response. Moreover, it demonstrates that these H3 receptors are probably activated by endogenous histamine and appear to be operative in hypertension.

Mutual interaction of histamine H3-receptors and alpha 2-adrenoceptors on noradrenergic terminals in mouse and rat brain cortex.

Brain cortex slices were preincubated with 3H-noradrenaline and superfused with physiological salt solution containing desipramine. We studied the inhibition of the electrically evoked tritium overflow caused by histamine in the presence of alpha-adrenoceptor ligands (mouse and rat brain cortex), and the inhibition caused by talipexole (the former B-HT 920) in the presence of H3-receptor ligands (mouse brain cortex). In mouse brain cortex slices, the inhibitory effect of histamine on the tritium overflow evoked by 36 pulses, 0.3 Hz was not changed by the alpha 1-adrenoceptor antagonist prazosin, but increased by the alpha 2-adrenoceptor antagonist rauwolscine. When the current strength or the duration of electrical pulses was reduced to compensate for the increase in evoked tritium overflow produced by rauwolscine, the latter still enhanced the effect of histamine. The histamine-induced inhibition of tritium overflow evoked by 360 pulses, 3 Hz was not affected by the alpha 1-adrenoceptor agonist phenylephrine but attenuated by the alpha 2-adrenoceptor agonist talipexole. Finally, the inhibition by histamine of the tritium overflow evoked by 3 pulses, 100 Hz was attenuated by talipexole but not affected by rauwolscine. Conversely, the inhibitory effect of talipexole on tritium overflow elicited by 360 pulses, 3 Hz was slightly attenuated by the H3-receptor agonist R-(-)-alpha-methylhistamine but not affected by the H3-receptor antagonist thioperamide. In rat brain cortex slices, histamine only tended to inhibit tritium overflow evoked by 360 pulses, 3 Hz, both in the absence of alpha-adrenoceptor antagonists and in the presence of prazosin. However, histamine markedly inhibited the evoked overflow in the presence of rauwolscine.(ABSTRACT TRUNCATED AT 250 WORDS)