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1 A microsomal fraction was prepared from human umbilical arteries by differential centrifugation. The preparation was capable of an oxalate-stimulated Ca2+ uptake at a mean rate of 0.74 nmol Ca2+ mg-1 protein min-1 which could be inhibited by a Ca2+ ionophore, A 23 187, and by Tween 80. 2 The rate of Ca2+ uptake in the fraction obtained by density gradient fractionation paralleled 5'-nucleotidase activity suggesting that vesicles of predominantly sarcolemmal origin were responsible for the microsomal Ca2+ uptake. 3 Cyclic adenosine 3',5'-monophosphate-dependent protein kinase enhanced membrane phosphorylation but did not affect Ca2+ uptake. Preincubation with alkaline phosphatase reduced membrane phosphorylation to a greater extent than Ca2+ uptake. These data are not in favour of a close correlation between Ca2+ uptake and phosphorylation. 4 None of 15 vasodilator drugs (bencyclane, carbocromen, diazoxide, dilazep, hydralazine, indapamide, isosorbide dinitrate, methyl-isobutyl-xanthine, minoxidil, naftidrofuryl, nitroglycerine, prenylamine, sodium nitroprusside, tetracaine, and verapamil) had any effect on Ca2+ uptake at 10(-5) M. This suggests that vasodilator drugs do not act by a direct influence on the Ca2+ pumps of vascular smooth muscle cells.

Rat brain tissues preincubated with [3H]serotonin ([3H]5-HT) or [3H]norepinephrine ([3H]NE) were superfused in the presence of an inhibitor of serotonin or NE uptake, respectively. The electrically (3 Hz) evoked [3H]5-HT release from slices of the medulla oblongata [containing the nucleus tractus solitarii (NTS)] from Wistar rats was inhibited by 5-HT and NE, and these effects were counteracted by metitepine and phentolamine, respectively. The evoked [3H]5-HT release in slices from the cortex, medulla oblongata, and hypothalamus of 5-7-, 9-11-, and 19-22-week-old spontaneously hypertensive rats (SHR) did not differ from that in slices from age-matched Wistar-Kyoto rats (WKY). Nor was there any difference between strains for: the inhibitory effects of 5-HT and NE and the facilitatory effect of metitepine on the evoked [3H]5-HT release; the shift to the right of the concentration-response curves of 5-HT and NE by metitepine and phentolamine, respectively; the potassium (12 mM)-evoked [3H]5-HT release from cortex synaptosomes and its inhibition by 5-HT; the electrically evoked [3H]NE release in cortex slices, its inhibition by NE, and the shift to the right of the concentration-response curve of NE by phentolamine. The results provide evidence that 5-HT release in the rat brain NTS can be inhibited by 5-HT receptors and alpha-adrenoceptors. 5-HT release and its modulation by presynaptic 5-HT1 receptors and alpha 2-adrenoceptors as well as NE release and its modulation by presynaptic alpha 2-adrenoceptors do not differ between SHR and WKY rats. It may be of therapeutic relevance that according to these results the effects of 5-HT1 receptor agonists activating presynaptic 5-HT autoreceptors are not attenuated in this type of hypertension. It has been suggested that such agonists can be developed as a new class of antihypertensive drugs.

Rat brain frontal cortex or striatal slices preincubated with 3H-serotonin were superfused with physiological salt solution and tritium overflow was evoked by electrical field stimulation at a frequency of 3 Hz. 1. GABA 1 mmol/1 inhibited the evoked overflow from frontal cortex slices by about 30%. The inhibitory effect was abolished when the frequency of electrical stimulation was 10 instead of 3 Hz. Progabide and R(-)-baclofen were more potent, whereas muscimol was less potent than GABA itself in inhibiting the evoked overflow; the IC15 values of progabide, R(-)-baclofen, GABA and muscimol were 8.3, 15, 170 and greater than 320 mumol/1, respectively. S(+)-baclofen behaved as a partial agonist with a maximum inhibitory effect by about 15%. Nipecotic acid and aminooxyacetic acid were ineffective. The same held true for bicuculline, picrotoxin and diazepam. 2. Bicuculline, picrotoxin, diazepam, phentolamine and the serotonin receptor antagonist metitepin did not influence the inhibitory effect of GABA. By contrast, S(+)-baclofen attenuated the effects of GABA and R(-)-baclofen. 3. The evoked overflow from striatal slices was inhibited by GABA and progabide (IC15 values: 480 and 13 mumol/1, respectively). Nipecotic acid was ineffective. The results suggest that exogenous GABA inhibits serotonin release in the rat brain via GABAB receptors which may be assumed to be located presynaptically.

Strips of human saphenous veins were incubated with [3H]noradrenaline and subsequently superfused with physiological salt solution containing cocaine, corticosterone and propranolol. The electrically (6 Hz) evoked overflow of tritium (78% of which was accounted for by unmetabolized [3H]noradrenaline) was abolished by tetrodotoxin or omission of Ca2+ from the superfusion fluid. Unlabelled noradrenaline, alpha-methylnoradrenaline, B-HT 920 and clonidine inhibited the evoked overflow (maximum effect of clonidine lower than that of the other compounds) whereas methoxamine was ineffective. The alpha 2-adrenoceptor antagonists, BDF 6143 and rauwolscine, facilitated the evoked overflow but no effect was obtained with prazosin. Rauwolscine produced a shift to the right of the concentration-response curve of B-HT 920 for its inhibitory effect on evoked outflow and BDF 6143 caused a shift to the right of the corresponding curve of clonidine. It is concluded that the stimulation-evoked release of noradrenaline from the sympathetic nerve fibres of the human saphenous vein is modulated via presynaptic alpha 2-adrenoceptors.

The effects of histamine and related drugs on the evoked tritium overflow from superfused rat brain cortex slices preincubated with 3H-noradrenaline were determined. Tritium overflow was stimulated electrically (3 Hz; slices superfused with normal physiological salt solution) or by introduction of CaCl2 1.3 mmol/l (slices superfused with Ca2(+)-free medium containing K+ 20 mmol/l). Histamine slightly decreased the electrically evoked 3H overflow in slices superfused in the presence of desipramine. The degree of inhibition obtained with histamine was doubled when both desipramine and phentolamine were present in the superfusion medium (pIC15 6.46). Under the latter condition, the evoked overflow was inhibited by the H3 receptor agonist R-(-)-alpha-methylhistamine and its S-(+) enantiomer (pIC15 7.36 and 5.09, respectively), but was not affected by the H2 receptor agonist dimaprit and the H1 receptor agonist 2-thiazolylethylamine (both at up to 32 mumols/l). The concentration-response curve of histamine was shifted to the right by the H3 receptor antagonists thioperamide, impromidine and burimamide (apparent pA2 8.37, 6.86 and 7.05, respectively), by the H2 receptor antagonist ranitidine (apparent pA2 4.27) and was not affected by the H1 receptor antagonist dimetindene (32 mumols/l). The inhibitory effect of R-(-)-alpha-methylhistamine on the evoked overflow was also counteracted by thioperamide. Given alone, none of the five histamine receptor antagonists affected the evoked overflow. In the absence of desipramine plus phentolamine, impromidine and burimamide facilitated the electrically evoked 3H overflow whereas thioperamide had no effect. The facilitatory effects of impromidine and burimamide were abolished by phentolamine, but not affected by desipramine.(ABSTRACT TRUNCATED AT 250 WORDS)

Rats were pithed, vagotomized and adrenalectomized and the effect of procaterol on the pressor response to electrical stimulation of the thoracolumbar preganglionic sympathetic outflow from the spinal cord or to exogenous noradrenaline was studied in the absence and presence of beta-adrenoceptor antagonists and drugs interfering with the renin-angiotensin system. 1. Basal diastolic blood pressure was decreased by captopril, ramiprilate (angiotensin converting enzyme inhibitors), saralasin (an angiotensin II receptor antagonist), pepstatin A (a protease inhibitor with renin antagonistic properties) and by functional nephrectomy (ligation of both renal hili), but was not affected by procaterol (a beta 2-adrenoceptor agonist), nebivolol (a beta 1-adrenoceptor antagonist) and ICI 118,551 (erythro-dl-1-(7-methylindan-4-yloxy)-3-isopropylaminobut an-2-ol; a beta 2-adrenoceptor antagonist). 2. The vasopressor response induced by electrical stimulation of the preganglionic sympathetic nerve fibres was increased by procaterol, whereas the increase in blood pressure evoked by exogenous noradrenaline was not affected. The pressor response to both electrical stimulation and exogenous noradrenaline was decreased by captopril, ramiprilate, saralasin and nephrectomy but was not affected by nebivolol and ICI 118,551. 3. The facilitatory effect of procaterol on the neurogenic, electrically induced pressor response, which was also obtained when basal blood pressure was decreased by nephrectomy and increased by Lys8-vasopressin, was abolished by ICI 118,551 but not affected by nebivolol. Under none of these experimental conditions did procaterol alter the vasopressor response to exogenous noradrenaline. 4. The facilitatory effect of procaterol on the neurogenic, electrically induced rise in blood pressure was abolished by captopril, ramiprilate, saralasin and pepstatin A.(ABSTRACT TRUNCATED AT 250 WORDS)

Rat brain cortex slices preincubated with 3H-noradrenaline were superfused with physiological salt solution, and the effect of vinpocetine on the electrically (3 Hz) evoked 3H overflow was studied. Vinpocetine at 10–100 μmol/liter facilitated the evoked overflow; at 32 and 100 μmol/liter, basal efflux was slightly increased. The facilitatory effect of vinpocetine on the evoked overflow was not altered by desipramine, rolipram (an inhibitor of cAMP phosphodiesterase), or ICS 205-930 ([3α-tropanyl]-1H-indole-3-carboxylic acid ester; an antagonist at 5-HT3 (M) receptors); it was strongly attenuated by phentolamine. In turn, vinpocetine moderately decreased the facilitatory effect of phentolamine on the evoked overflow. The concentrationresponse curve of noradrenaline (obtained in the presence of desipramine) for its inhibitory effect on the evoked overflow was not shifted to the right by vinpocetine at 32 and 100 μmol/liter. The present results show that vinpocetine facilitates noradrenaline release. This effect may be related to the blockade of presynaptic α2-adrenoceptors (although evidence is not unequivocal), but does not appear to involve inhibition of noradrenaline uptake or cAMP phosphodiesterase nor activation of 5-HT3 (M) receptors. The facilitatory effect of vinpocetine on noradrenaline release occurs at concentrations higher than those obtained under treatment with this drug.

1. In rat brain cortex slices preincubated with [3H]5-HT, the potencies of 17 5-HT receptor agonists to inhibit the electrically evoked3H overflow and the affinities of 13 antagonists (including several β-adrenoceptor blocking agents) to antagonize competitively the inhibitory effect of unlabelled 5-HT on evoked3H overflow were determined. 2. The affinities of the compounds for 5-HT1B and 5-HT2 binding sites in rat brain cortex membranes (labelled by [125I]cyanopindolol = [125I]-CYP in the presence of 30 μmol/l isoprenaline and [3H]ketanserin, respectively), for 5-HT1A binding sites in pig and rat brain cortex membranes (labelled by [3H]8-hydroxy-2-(di-n-propylamino)tetralin = [3H]8-OH-DPAT) and for 5-HT1C binding sites in pig choroid plexus membranes (labelled by [3H]mesulergine) were also determined. The affinities of the drugs for the various 5-HT recognition sites ranged over 4–5 log units (the functional experiments revealed the same range of differences between the drugs). 3. There were no significant correlations between the affinities of the drugs at 5-HT1C and 5-HT2 binding sites and their potencies or affinities, determined for the 5-HT autoreceptors. In contrast, significant correlations were found between the potencies or affinities of the drugs for the autoreceptors and their affinities at 5-HT1A or 5-HT1B binding sites; the best correlations were obtained with the 5-HT1B binding site. 4. Some of the drugs investigated were not included in the correlation since their agonistic or antagonistic effects on the autoreceptors were weak and pEC30 or apparent pA2 values could not be determined ( 7.2). 5. In conclusion, the evidence indicates that the presynaptic 5-HT autoreceptor belongs to the 5-HT1B receptor subtype.

1. In pithed rats, the blood pressure effects of ATP, alpha,beta-methylene ATP (mATP), alpha-adrenoreceptor agonists and of electrical stimulation of the thoracolumbar sympathetic outflow were studied in the absence and presence of mATP, suramin and adrenoreceptor antagonists. 2. ATP elicited an initial rise in mean blood pressure followed by a decrease and a second increase. mATP produced a short-lived increase in blood pressure, whereas equieffective doses of noradrenaline, methoxamine and B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-(5,4-d)-azepine) produced a more prolonged, biphasic pressor response. 3. In the presence of high doses of prazosin, rauwolscine plus propranolol, the initial vasopressor and the vasodepressor effect to ATP were not affected, whereas the delayed vasopressor response to ATP, the vasopressor response to electrical stimulation and even more so that to noradrenaline were suppressed. 4. Suramin, which by itself produced a short-lived decrease, followed by a persistent increase in blood pressure, decreased the pressor responses to ATP (initial phase), to mATP and to electrical stimulation without affecting the fall and second rise in blood pressure elicited by ATP and the pressor response to noradrenaline. 5. Desensitization of P2x receptors by a low dose of mATP abolished the initial vasopressor response to ATP but failed to affect the subsequent blood pressure effects of ATP as well as the pressor responses to noradrenaline and electrical stimulation. A high dose of mATP, in addition, decreased the vasopressor responses to noradrenaline, methoxamine, B-HT 920 and electrical stimulation; the delayed effects of ATP on blood pressure were not changed. 6. The electrically induced increase in blood pressure subsequent to administration of high doses of prazosin, rauwolscine plus propranolol was diminished by suramin and by the low and high dose of mATP. 7. The present study suggests that under certain circumstances ATP, which, added exogenously, has a triphasic effect on mean blood pressure, contributes to the electrically induced vasopressor response by activation of P2x receptors.

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).

We have shown previously that the cannabinoid receptor agonist CP55940 microinjected into the paraventricular nucleus of the hypothalamus (PVN) of urethane-anaesthetized rats induces depressor and pressor cardiovascular effects in the absence and presence of the CB1 antagonist AM251, respectively. The aim of our study was to examine whether the hypotension and/or hypertension induced by CP55940 given into the PVN results from its influence on glutamatergic and GABAergic neurotransmission. CP55940 was microinjected into the PVN of urethane-anaesthetized rats twice (S1 and S2, 20 min apart). Antagonists of the following receptors, NMDA (MK801), β2-adrenergic (ICI118551), thromboxane A2–TP (SQ29548), angiotensin II–AT1 (losartan) or GABAA (bicuculline), or the NO synthase inhibitor L-NAME were administered intravenously 5 min before S2 alone or together with AM251. The CP55940-induced hypotension was reversed into a pressor response by AM251, bicuculline and L-NAME, but not by the other antagonists. The CP55940-induced pressor effect examined in the presence of AM251 was completely reversed by losartan, reduced by about 50–60 % by MK801, ICI118551 and SQ29548, prevented by bilateral adrenalectomy but not modified by bicuculline and L-NAME. Parallel, but smaller, changes in heart rate accompanied the changes in blood pressure. The bi-directional CB1 receptor-mediated cardiovascular effects of cannabinoids microinjected into the PVN of anaesthetized rats depend on stimulatory glutamatergic and inhibitory GABAergic inputs to the sympathetic tone; the glutamatergic input is related to AT1, TP and β2-adrenergic receptors and catecholamine release from the adrenal medulla whereas the GABAergic input is reinforced by NO.

Slices of cerebral cortex from young adult and aged rats, preincubated with [3H]serotonin or [3H]noradrenaline were superfused and the electrically (3 Hz) evoked overflow of tritium and its modulation by the respective agonists and antagonists were studied. The electrically evoked overflow in slices preincubated with [3H]serotonin from 2.5-year-old rats was smaller (by about 35%) than in slices from 3-month-old animals. No difference was found for the inhibition by serotonin and the facilitation by metitepine of the evoked overflow and for the metitepine-induced shift of the concentration-response curve for serotonin to the right. There was no difference between slices preincubated with [3H]noradrenaline from 3 months, 1, 2 and 2.5-year-old rats, with respect to the electrically evoked overflow, its inhibition and facilitation by noradrenaline and phentolamine, respectively, and the phentolamine-induced shift of the concentration-response curve for noradrenaline to the right. The present results suggest that the release of serotonin is reduced in the cerebral cortex of aged rats whereas the release of noradrenaline and the autoreceptor-mediated modulation of the release of either monoamine is not altered.

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

Superfused rat brain cortex slices, hypothalamic slices and cortex synaptosomes preincubated with 3H-serotonin or 3H-noradrenaline were used to study the effects of eicosanoids on tritium overflow evoked either electrically (3 Hz; slices) or by potassium 12 mmol/l (synaptosomes). 1. The electrically evoked 3H overflow from cortex slices preincubated with 3H-serotonin was inhibited by prostaglandins E1 and E2 and by the prostacyclin analogue iloprost. No effect was seen with prostaglandin F2 alpha, prostaglandin D2, CG 4203 (another prostacyclin analogue), U 46619 (a thromboxane A2 analogue) and leukotriene C4. The same held true for indomethacin and the prostaglandin receptor antagonists SC 19220 and N-0164. The inhibitory effect of prostaglandin E2 was slightly more pronounced in the presence of indomethacin than in its absence, but was not affected by SC 19220, N-0164 or forskolin plus AH 21-132 (an inhibitor of cAMP phosphodiesterase). Yohimbine and the serotonin receptor antagonist metitepin failed to influence the inhibitory effect of prostaglandin E1. 2. The potassium-evoked 3H overflow from cortex synaptosomes preincubated with 3H-serotonin was inhibited by prostaglandin E2. 3. Prostaglandin E2 also inhibited the electrically evoked 3H overflow from hypothalamic slices preincubated with 3H-serotonin. 4. The electrically evoked 3H overflow from cortex slices preincubated with 3H-noradrenaline was inhibited by prostaglandin E2, but was not affected by SC 19220, which, in turn, did also not alter the effect of prostaglandin E2. The present results are compatible with the view that presynaptic SC 19220-insensitive prostaglandin E receptors may be involved in the inhibitory effect of prostaglandins E1 and E2 on serotonin (and noradrenaline) release.

Rat brain cortex slices preincubated with 3H-serotonin or 3H-noradrenaline were superfused with physiological salt solution, and the effect of quipazine on the 3H overflow evoked by electrical field stimulation was examined. 1. The electrically evoked tritium overflow from brain slices preloaded with 3H-serotonin was increased by quipazine in a concentration-dependent manner. The concentration-response curves of unlabelled serotonin and noradrenaline for their inhibitory effects on impulse-evoked 3H overflow were shifted to the right by quipazine. 2. In brain slices preincubated with tritiated noradrenaline, quipazine caused an increase in electrically stimulated 3H overflow which was less marked than the effect on brain slices preloaded with 3H-serotonin. The concentration-response curve of unlabelled noradrenaline for its decreasing effect on stimulation-evoked 3H overflow was shifted to the right by quipazine. We conclude that quipazine blocks presynaptic inhibitory serotonin receptors and alpha-adrenoceptors on monoaminergic neurones of the rat brain cortex.

Stimulation-evoked tritium overflow was examined in superfused rat brain cortex slices (stimulus: electrical impulses; 3 Hz) and synaptosomes (stimulus: potassium 12 mmol/l) preincubated with 3H-5-HT. 1. In slices and synaptosomes, the evoked 3H overflow was facilitated by forskolin and 8-Br-cAMP, but was not affected by AH 21-132 (an inhibitor of cAMP phosphodiesterase; cis-6-(p-acetamidophenyl)-1, 2, 3, 4, 4 a,10b-hexahydro-8, 9-dimethoxy-2-methylbenzo [c] [1,6]-naphthyridine). In the presence of AH 21-132, the facilitatory effect of forskolin on evoked overflow was increased. 2. In slices, AH 21-132 or combined administration of forskolin plus AH 21-132 did not change the percentage of basal or evoked 3H overflow represented by unmetabolized 3H-serotonin (about 30% and 60%, respectively). 3. In slices, cocaine or 6-nitroquipazine, an inhibitor of serotonin uptake, did not influence the increase in evoked overflow produced by forskolin plus AH 21-132. Forskolin plus AH 21-132 did not alter the inhibitory effect of serotonin (examined in the presence of 6-nitroquipazine) and the facilitatory effect of metitepin (a serotonin receptor antagonist) on evoked 3H overflow, but considerably decreased the inhibitory effect of clonidine or B-HT 920 (2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo-[5,4-d]-azepine). The present results suggest that the serotoninergic nerve terminals in the rat brain cortex are endowed with an adenylate cyclase, which is negatively coupled to the presynaptic α2-adrenoceptors, but is not linked to the presynaptic autoreceptors.

In the rat inferior vena cava preincubated with 3H-noradrenaline, the effects of nine serotonin (5-HT) receptor agonists and of eight antagonists (including two β-adrenoceptor blocking agents) on the electrically evoked 3H overflow were determined. 1. 5-HT, 5-carboxamidotryptamine, 5-methoxy-3(1,2,3,6-tetrahydropyridine-4-yl)-1H-indole (RU 24969), 5-methoxytryptamine, N,Ndimethyl-5-HT, tryptamine and 5-aminotryptamine inhibited the evoked 3H overflow. The potencies of these agonists in inhibiting overflow were significantly correlated with their affinities for 5-HT1B binding sites, but not with their affinities for 5-HT1A, 5-HT1C or 5-HT2 binding sites. 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5HT1A receptor agonist, and ipsapirone, a partial agonist at these receptors, did not inhibit overflow. 2. Cyanopindolol facilitated the evoked 3H overflow, an effect which was abolished by propranolol. The maximum inhibition of overflow obtainable with 5-HT was diminished by cyanopindolol. 3. The concentration-response curve for 5-HT was shifted to the right by metitepine, metergoline, quipazine, 6-chloro-2-(1-piperazinyl)pyrazine (MK 212) and propranolol which, given alone, did not affect 3H overflow. The apparent pA2 values of these antagonists tended to be correlated with their affinities for 5-HT1B (but not 5-HT1A, 5-HT1c or 5-HT2) binding sites. Ketanserin, a 5-HT2 receptor antagonist, and spiperone, which blocks 5-HT2 and 5-HT1A but not 5-HT1B or 5-HT1C receptors, failed to antagonize the effect of 5-HT. These results suggest that the inhibitory presynaptic 5-HT receptors on the sympathetic nerve terminals of the rat vena cava appear to belong to the 5-HT1B subtype. Cyanopindolol may act as a partial agonist at these receptors, as it does at the facilitatory prosynaptic β-adrenoceptors.

The effects of serotonin receptor agonists and antagonists on the electrically (3 Hz) evoked 3H overflow were determined on pig brain cortex slices preincubated with 3H-serotonin and superfused with physiological salt solution containing indalpine (an inhibitor of serotonin uptake) plus phentolamine. The potencies of the serotonin receptor agonists and antagonists were compared with their affinities for 5-HT1A, 5-HT1B, 5-HT1c, and 5-HT1D binding sites in pig or rat tissue membranes; in addition, the potencies of the agonists were compared to their potencies in inhibiting adenylate cyclase activity in membranes of calf substantia nigra. In the superfusion experiments on pig brain cortex slices the following rank orders of potencies were obtained: agonists, serotonin > 5-methoxytryptamine = 5-carboxamidotryptamine >R U 24969 (5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole) > SDZ 21009 (4(3-terbutylamino- 2-hydroxypropoxy)indol- 2-carbonic-acid-isopropylester) ≥ yohimbine ≥ cyanopindolol > 8-OHDPAT (8-hydroxy-2-(di-n-propylamino)tetralin) ≥ CGS 12066 B (7-trifluoromethyl-4(4-methyl-l-piperazinyl)-pyrrolo[1,2-a]quinoxaline); ipsapirone and urapidil were ineffective; antagonists (antagonism determined against 5methoxytryptamine as an agonist), metitepine > metergoline > mianserin. Propranolol, spiperone or mesulergine did not produce a shift of the concentration-response curve for 5-methoxytryptamine. The potencies of the serotonin receptor agonists in pig brain cortex slices were significantly correlated with their affinities for 5-HT1c and 5-HT1D binding sites in membranes of the pig choroid plexus and caudate nucleus, respectively, but not with their affinities for 5-HT1A and 5-HT1B sites in membranes of the cerebral cortex of pig and rat, respectively. The agonist potencies in decreasing 3H overflow were also significantly correlated with their potencies in inhibiting adenylate cylase activity in calf substantia nigra (i.e., a 5-HT1D receptor-mediated effect). In conclusion, the pig brain cortical 5-HT autoreceptor probably belongs to the 5-HT1D subtype. The involvement of 5-HT1c recognition sites was excluded by the low potency of mianserin as an antagonist and, in particular, by the ineffectiveness of the 5-HT1c receptor antagonist mesulergine.

Pig brain cortex synaptosomes and slices preincubated with 3H-5-hydroxytryptamine (3H-5-HT) were superfused with physiological salt solution containing citalopram (an inhibitor of 5-HT uptake), and the effects of indolethylamines and 5-HT receptor antagonists on the potassium- or electrically evoked 3H overflow were determined. The potassium (25 mmol/l)-evoked tritium overflow from cortex synaptosomes was inhibited by 5-HT; the inhibitory effect of 5-HT was counteracted by metitepine, which, by itself, did not affect the evoked overflow. 5-Methoxytryptamine (examined in the absence of citalopram) also produced an inhibition of the evoked overflow. In cortex slices, the electrically (3 Hz) evoked overflow was inhibited by 5-HT and 5-carboxamidotryptamine. The inhibitory effect of 5-HT was antagonized by metitepine, which, given alone, increased the evoked overflow, but was not attenuated by ketanserin and ICS 205-930 ([3 alpha-tropanyl]-1H-indole-3-carboxylic acid ester), which, by themselves, did not influence the evoked overflow. The present results suggest that the serotoninergic nerve fibres of the pig brain cortex are endowed with presynaptic 5-HT1 receptors, which can be activated by endogenous and exogenous 5-HT.

Rat brain cortex slices preincubated with 3H-serotonin were superfused with physiological salt solution (containing citalopram, an inhibitor of serotonin uptake) and the effect of histamine on the electrically (3 Hz) evoked 3H overflow was studied. Histamine decreased the evoked overflow in a concentration-dependent manner. The inhibitory effect of histamine was antagonized by impromidine and burimamide, but was not affected by pheniramine, ranitidine, metitepine and phentolamine. Given alone, impromidine facilitated the evoked overflow, whereas burimamide, pheniramine and ranitidine had no effect. The results suggest that histamine inhibits serotonin release in the rat brain cortex via histamine H3 receptors, which may be located presynaptically.

The development of beta-adrenoreceptor density and beta 1- and beta 2-adrenoreceptor distribution has been investigated in heart, kidney and lung of spontaneously hypertensive (SHR) and normotensive (WKY) rats by (-)125I-iodocyanopinolol binding at the age of 5-6, 9-10 and 19-21 weeks. At all ages beta-adrenoreceptor density was similar in hearts of both strains, while it was increased in kidney and lung of SHR compared to WKY. The beta 2-adrenoreceptor density was higher in all three tissues of SHR at all ages investigated. On the other hand, beta 1-adrenoreceptor density was decreased in heart, unchanged in lung and increased in kidney of SHR compared to WKY. Destruction of presynaptic nerve terminals by treatment of WKY with 6-hydroxydopamine produced a 24% loss of cardiac beta 2-adrenoreceptors, whereas the beta 1-adrenoreceptor density remained unchanged, suggesting that at least part of the cardiac beta 2-adrenoreceptor population is localized prejunctionally. It is suggested that beta 2-adrenoreceptors are involved in the development or maintenance of high blood pressure in SHR, possibly by facilitating noradrenaline release.

Rat brain cortex slices preincubated with 3H-5-hydroxytryptamine (3H-5-HT) were superfused with physiological salt solution containing paroxetine, an inhibitor of 5-hydroxytryptamine (5-HT) uptake. The effects of various indolethylamines on the electrically evoked tritium overflow (containing 66.3% unmetabolized 3H-5-HT) were investigated (the percentage of unmetabolized 3H-5-HT was not altered by the indolethylamines or metitepin). 6,7-Dihydroxytryptamine (6,7-DHT) did not affect the stimulation-evoked tritium overflow, whereas the latter was inhibited by the other tryptamine derivatives investigated; when the compounds were compared to each other on the basis of their inhibitory potencies the following rank order was obtained: unlabelled 5-HT greater than 5-methoxytryptamine greater than 4-HT greater than 6-HT greater than 5,6-DHT greater than tryptamine greater than 7-HT greater than 5,7-DHT. The inhibitory effects of these compounds were antagonized by metitepin. It is concluded that the indolethylamines inhibit the stimulation-evoked 3H-5-HT release by activating the presynaptic 5-HT autoreceptors on the 5-HT neurones of the rat brain cortex. Similarities may exist between these receptors and the postsynaptic 5-HT1 binding sites of this brain area.

Background The endocannabinoid anandamide is implicated in the pathogenesis of hypotension in haemorrhagic, endotoxic, and cardiogenic shock. It has been demonstrated in animal, but not in human, vessels that the vasodilatory effects of anandamide and abnormal cannabidiol are partially mediated by an as yet unidentified endothelial cannabinoid receptor. Our study was performed to examine the influence of abnormal cannabidiol on the human pulmonary artery. Methods Isolated human pulmonary arteries were obtained from patients without clinical evidence of pulmonary hypertension during resection of lung carcinoma. Vasodilatory effects of abnormal cannabidiol were examined on endothelium-intact vessels preconstricted with serotonin or potassium chloride. Results Anandamide and abnormal cannabidiol relaxed serotonin-preconstricted vessels concentration-dependently. The effect of abnormal cannabidiol was reduced by endothelium denudation, pertussis toxin and two antagonists of the novel endothelial receptor, cannabidiol and O-1918, but not by the nitric oxide synthase inhibitor L-NAME given together with the cyclooxygenase inhibitor indomethacin. It was also diminished by blockade of calcium-activated potassium channels by the nonselective blocker tetraethylammonium or by combination of selective blockers of small (apamin) and intermediate and large (charybdotoxin) conductance calcium-activated potassium channels. The potency of abnormal cannabidiol to relax vessels was lower in potassium chloride than in serotonin-preconstriced preparations. Conclusions Abnormal cannabidiol relaxes human pulmonary arteries in an endothelium-independent and endothelium-dependent manner. The latter component is probably mediated via the putative endothelial cannabinoid receptor, activation of which may release endothelium-derived hyperpolarizing factor, which in turn acts via calcium-activated potassium channels. Abnormal cannabidiol is behaviourally inactive; it may have a therapeutic implication in vascular diseases, especially in the treatment of pulmonary hypertension.