Influence of ethanol and serotonin on rat platelet aggregation

We demonstrated that ethanol (1.0, 2.0 and 4.0 g/kg p.o.) significantly decreased blood platelet aggregation in a dose-dependent manner. The chronic administration of ethanol (6 g/kg daily for 4 weeks) also altered the sensitivity of rat platelets to ADP (4 mumol/l). We found that the acute and chronic administration of alcohol significantly increased the amplifying effect of 5-hydroxytryptamine (5-HT; 10(-6) mol/l) on ADP-induced aggregation. In all groups of rats, ketanserin (10(-5) mol/l) completely inhibited the amplification of aggregation induced by serotonin. In conclusion, the present results show that ethanol did not only produce inhibition of ADP-induced platelet aggregation but also affected the potentiating action of 5-HT on this process.     

Influence of captopril on the vasopressor effect of serotonin in pithed rats

In pithed rats intravenous administration of serotonin (5.0, 10.0, 20.0, 30.0 and 50.0 micrograms/kg) produced a dose-dependent increase in blood pressure. The pressor response to serotonin was significantly reduced by captopril (1.0, 5.0 and 50.0 mg/kg) in a dose-dependent manner. Captopril did not attenuate the response of the isolated tail artery to serotonin (5-HT) and did not amplify the dilator effect of serotonin in pithed rats pretreated with ketanserin (1.0 mg/kg). These data suggest that the action of captopril does not depend on its influence on serotoninergic receptors in blood vessels.     

Effect of ketanserin on blood pressure in rats

Ketanserin, a selective serotonergic (5-HT2) antagonist, also has affinity for alpha 1-adrenoceptors. It is not clear whether the hypotensive mechanism of ketanserin is due to its antagonistic action to 5-HT2 receptor or to its affinity for alpha 1 adrenoceptors. The hypotensive mechanism of ketanserin was studied in both stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar Kyoto rats (WKY). Anesthetized rats were used (alpha-chloralose + urethane, i.p.). Up to 3 ml of blood was drawn from each rat for analysis. Plasma norepinephrine (NE) was determined by radioenzymatic assay. Plasma serotonin (5-HT) was determined by HPLC-ECD. Adrenal nerve discharges were counted by a digital pulse counter. Ketanserin (0.5 mg/kg, 5.0 mg/kg, i.v.) produced a dose-dependent reduction of mean arterial pressure (MAP) in both SHRSP and WKY. MAP of SHRSP decreased significantly as compared with WKY. Both plasma NE and 5-HT showed a tendency to increase during ketanserin administration (5.0 mg/kg, i.v.). Ketanserin significantly antagonized the BP response induced by exogenously injected 5-HT (30 micrograms/kg) and NE (10 micrograms/kg). Adrenal nerve activity was reduced in parallel with the decrease in BP and HR. These findings suggest that ketanserin produced a decrease in BP via both peripheral and central action in rats.     

STUDIES ON THE MECHANISM OF ACTION OF THE HYPOTENSIVE EFFECT OF KETANSERIN

Ketanserin, 0.01-3.0 mg/kg i.v., was found to cause a dose-related fall in blood pressure in anaesthetized rats. However, these decreases were only significant after 1 and 3 mg/kg doses. Subsequent administration of prazosin, 0.2 mg/kg i.v., had no significant effect on blood pressure. In the presence of ketanserin 3 mg/kg, plus prazosin 0.2 mg/kg, adenosine caused a significant dose-dependent fall in blood pressure in anaesthetized rats. Methysergide, 1 mg/kg i.v., significantly attenuated the blood pressure responses to serotonin in anaesthetized rats whereas ketanserin (1 and 3 mg/kg) had no overall effect on serotonin responses. Ketanserin, 1 and 3 mg/kg i.v., was found to antagonize the pressor effects of phenylephrine and reverse the pressor effects of adrenaline in anaesthetized rats. Responses to angiotensin II were not significantly affected by ketanserin. These results suggest that ketanserin lowers blood pressure in anaesthetized rats by blockade of alpha-adrenoceptors.     

Effect of ketanserin on adrenal sympathetic nerve activity in rats

The present study was undertaken to clarify the site for the sympathoinhibitory action of ketanserin in anesthetized rats. Intravenous (i.v.) administration of ketanserin (0.5 and 5 mg/kg) produced a decrease in preganglionic adrenal sympathetic nerve activity (ANA) that is accompanied with hypotension and bradycardia. Intracerebroventricular (i.c.v.) administration of ketanserin (200 micrograms/rat) also decreased ANA, blood pressure (BP) and heart rate (HR). Intrathecal (i.t.) administration of ketanserin (200 micrograms/rat), on the other hand, affected neither ANA, BP nor HR. These results indicate that the site of the sympathoinhibitory action of ketanserin is the supraspinal structures, and not at the spinal cord level. In addition, the decrease in ANA after i.v. administration of ketanserin (0.5 and 5 mg/kg) was attenuated significantly with pretreatment of 5,7-dihydroxytryptamine (200 micrograms/rat, i.c.v.) or 6-hydroxydopamine (200 micrograms/rat, i.c.v.). These findings suggest that the adrenal sympathoinhibitory action of ketanserin may be centrally mediated via both serotonergic and noradrenergic pathways in rats.     

SELECTIVE STIMULATION OF VASCULAR POSTJUNCTIONAL α1-ADRENORECEPTORS BY (-)-AMIDEPHRINE IN RATS AND CATS

• 1 The vasopressor and chronotropic responses of (-)-amidephrine and the receptor types involved were studied in pithed rats of different strains and in pithed cats.
• 2 The increase in diastolic pressure of pithed rats after i.v. administration of (-)-amidephrine was not influenced by pretreatment with propranolol (1 mg/kg, i.v.), reserpine (2 times 5mg/kg in 48 h i.p.) or yohimbine (1 mg/kg, i.v.), but was strongly antagonized by prazosin (0.1 mg/kg, i.v.). In pithed cats, the pressor responses were antagonized by prazosin (1 mg/kg, i.v.) but much less so by yohimbine (1 mg/kg, i.v.).
• 3 (-)-Amidephrine elicited minor positive chronotropic responses in pithed rats and pithed cats. This tachycardia was not influenced by propranolol (1 mg/kg, i.v.) but was abolished by prazosin (0.1 – 1.0 mg/kg).
• 4 The results show that (-)-amidephrine acts as a selective agonist at vascular postjunctional α-adrenoreceptors in pithed rats and pithed cats. The positive chronotropic effects are attributable to stimulation of α-adrenoreceptors in the heart.

     

Inhibitory effect of N(G)-nitro-L-arginine methyl ester on the anti-adrenergic response elicited by ayanin in the pithed rat

In this study we evaluated the anti-adrenergic response elicited by ayanin, a flavonoid compound isolated from Croton schiedeanus Schlecht, in the pithed rat, and the inhibitory effect of NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME), and its acute toxicity profile in mice. In pithed rats ayanin (5 - 50 mg/kg i. v.) caused a dose-dependent decrease in the pressor and chronotropic responses induced by intravenous noradrenaline administration (0.25 microg/kg). This anti-adrenergic response was completely abolished by prior treatment with L-NAME (10 mg/kg i.v ) and the inhibitory effect of L-NAME was reversed after intravenous administration of L-arginine (100 mg/kg, i. v.). No lethal or major toxic effects were observed in mice receiving i. p. administration of ayanin up to a dose of 500 mg/kg. Our findings confirm that ayanin exerts protective cardiovascular effects against the increase in blood pressure and heart rate mainly through a mechanism that depends on the NO/cyclic guanosine monophosphate (cGMP) pathway without acute toxic effects. These results suggest that extracts of Croton schiedeanus, the native south American plant from which ayanin was isolated, might be beneficial in cardiovascular disease.     

Rapid desensitization of central beta-adrenoceptors in rat after subacute treatment with imipramine and calcium entry blockers.

The subcutaneous (s.c.) administration of isoprenaline to rats produced a dose-dependent increase in water drinking which was effectively antagonized by propranolol. This dipsogenic response was significantly inhibited after the intraperitoneal (i.p.) administration of imipramine (15 mg/kg/day), together with either of the following calcium entry blockers, for four days: diltiazem (15 mg/kg/day), verapamil (10 mg/kg/day), nifedipine (10 mg/kg/day) or nicardipine (15 mg/kg/day). Simultaneous injection of the inhibitor of the synthesis of serotonin, p-chlorophenylalanine (200 mg/kg/day, i.p.), did not affect this attenuation of the isoprenaline-induced response. Similarly, the selective 5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) or the 5-HT2 receptor antagonist, ketanserin, had no significant effect on the attenuation of isoprenaline-induced drinking behaviour. The inhibition of isoprenaline-induced drinking, was, however, effectively attenuated after treatment of the animals with 6-hydroxydopamine (2.5 micrograms) or clonidine (30 micrograms), injected intracerebroventricularly (i.c.v.). These results indicate that the calcium entry blockers accelerate the desensitization of central beta-adrenoceptors possibly by an action on central adrenoceptors of the rat.     

Participation of alpha-melanocyte stimulating hormone in ethanol-induced anxiolysis and withdrawal anxiety in rats

Although recent reports underscore a close association between the ethanol consumption and the central melanocortin (MC) system in rats, neurobehavioral component of this association has not been explored. In this study, we investigated the role of alpha-melanocyte stimulating hormone (alpha-MSH) in ethanol (1.5-2 g/kg, i.p.) induced anxiolysis and anxiety-like behavior following withdrawal from prolonged ethanol (9% v/v ethanol, 15 days) consumption, using elevated plus maze (EPM) test in rats. While alpha-MSH (1-5 microg/rat, i.c.v.) showed dose-dependent anxiogenic-like effect, the MC4 receptor antagonist HS014 (1-10 nM/rat, i.c.v.) or antiserum against alpha-MSH (1:500-1:50 dilution, 5 microl/rat, i.c.v.) failed to produce any effect in the EPM test. The anxiolytic-like effect of ethanol was suppressed by central administration of alpha-MSH (0.5 microg/rat, i.c.v.). On the other hand, pretreatment with either HS014 (5 nM/rat, i.c.v.) or antiserum against alpha-MSH (1:100 dilution, 5 microl/rat, i.c.v.) enhanced anxiolytic action of ethanol. Moreover, ethanol withdrawal anxiety was markedly blocked by HS014 (1-10 nM/rat, i.c.v.). These results suggest that alpha-MSH may be implicated in ethanol-induced anxiolysis and withdrawal anxiety. These findings also suggest MC4 receptors as possible therapeutic target for development of drugs to address the ethanol withdrawal-related conditions.     

Hypotensive actions of ephedradines, macrocyclic spermine alkaloids of ephedra roots*.

Ephedradines A, B, C and D (3 mg/kg, i.v.) elicited hypotensive effects in Wistar rats. Administration of ephedradine B (EB) (0.1-3 mg/kg, i.v.) to Wistar rats and to spontaneously hypertensive rats reduced blood pressure in a dose-dependent manner. EB (1 mg/kg, i.v.) slightly potentiated the pressor action of norepinephrine (1 microg/kg, i.v.) and significantly reduced that of 1,1-dimethyl-4-phenylpiperazinium (50 microg/kg, i.v.). The hypotensive activity of EB (1 mg/kg, i.v.) was inhibited by pretreatment with atropine (5 mg/kg, i.v.) or diphenhydramine (5 mg/kg, i.v.). In the hypogastric nerve-vas deferens of guinea pig, application of EB (3 x 10 (-7)-10 (-5) g/ml)to the ganglion inhibited the contraction of the vas deferens induced by electrical preganglionic nerve stimulation and by acetylcholine (10 (-4)-10 (-3) g/ml) applied to the ganglion. It is thus concluded that the hypotensive activity of ephedradine B is exerted mainly by ganglion block.     

The central sympatho-inhibitory effect of 5,6-dihydroxy-2-dimethylaminotetralin (M7) is mediated by α2-adrenoceptors

M7 (5,6-dihydroxy-2-dimethylaminotetralin) produces in anesthetized rats a hypotensive response previously attributed to peripheral dopaminergic mechanisms. We re-examined the effects of this drug on arterial blood pressure, heart rate and sympathetic nerve activity in anesthetized rats and dogs. M7 (1–100 μg/kg i.v.) produced in the rats transient dose-dependent pressor effects, with bradycardia and sympatho-inhibition, followed by long-lasting dose-dependent hypotension, bradycardia and sympatho-inhibition. The sympatho-inhibitory and hypotensive effects were comparable in baroreceptor-denervated rats and were reversed by idazoxan (0.1 mg/kg i.v.). The sympatho-inhibitory response induced by M7 (1–100 μg/kg) was prevented by treatment with the specific α₂-adrenoceptor antagonist, 2-methoxy-idazoxan (0.03 mg/kg i.v.). This central effect of M7 was not altered by treatment with the α₁-adrenoceptor antagonist, prazosin (0.1 mg/kg i.v.) and was reduced by treatment with the α₂-adrenoceptor antagonists, yohimbine (1 mg/kg i.v.) or idazoxan (0.3 mg/kg i.v.), and the dopaminergic antagonists, haloperidol (0.5 mg/kg i.v.) or sulpiride (3 mg/kg i.v.). Bilateral microinjections of M7 (0.3–3 nmol) into the rostroventral medulla in the rat produced dose-dependent hypotension, bradycardia and sympathetic nerve inhibition which were reversed and prevented by bilateral microinjection of 2-methoxy-idazoxan (1 nmol) into the same sites. Microinjections of 2-methoxy-idazoxan into the rostroventral medulla also inhibited the central effects of M7 at 0.03 mg/kg i.v. In anesthetized dogs, M7 administered into the cisterna magna (1–10 μg/kg) reduced arterial blood pressure, heart rate and sympathetic nerve activity; these effects were reversed by administration of 2-methoxy-idazoxan (0.03 mg/kg i.v.). In conclusion, M7, a rigid catecholamine, produces a potent central sympatho-inhibitory and hypotensive effect by activation of α₂-adrenoceptors.     

Intrinsic pressor activity of midaglizole, an alpha-2 adrenoceptor antagonist, in pithed rats

Midaglizole (3 and 30 mg/kg, i.v.) increased blood pressure in pithed rats. The pressor response was not inhibited by intravenous prazosin (0.3 mg/kg), yohimbine (1 mg/kg), ketanserin (1 mg/kg) or diphenhydramine (5 mg/kg). Diltiazem (1 mg/kg) antagonized the hypertension. Idazoxan (10 mg/kg) also increased blood pressure, and the pressor response was inhibited by prazosin, but not by yohimbine. These results suggest that the vascular effect of midaglizole is due to a mechanism different from that of idazoxan.     

ATTENUATION OF PRESSOR RESPONSES TO SYMPATHETIC STIMULI IN THE RAT BY ENALAPRIL

Intravenous administration to pithed Wistar rats of the angiotensin converting enzyme inhibitor enalapril (0.1-1.0 mg/kg) lowered the diastolic blood pressure and reduced pressor responses occurring during electrical stimulation (1-30 Hz) of the spinal sympathetic outflow. These doses of enalapril given intravenously also attenuated pressor responses to intravenous injection of the muscarinic ganglion stimulant McNeil-A-343 (50, 100, 150 micrograms/kg) and noradrenaline (0.1-5.0 micrograms/kg). Enalapril (1.0 mg/kg, i.v.) reduced pressor responses to the nicotinic ganglion stimulant 1,1-dimethyl-4-phenyl-piperazinium (300 micrograms/kg, i.v.). These results confirmed that the actions of enalapril resemble those of captopril in the pithed rat, by causing reductions in both blood pressure and pressor responses to sympathetic stimuli.     

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.     

Cardiovascular effects of acetaldehyde in pithed rats.

The influence of acetaldehyde (30 mg/kg i.v.) on blood pressure and heart rate in anesthetized and pithed rats was studied. In anesthetized rats we observed a small increase, and a subsequent considerable decrease in mean blood pressure. The latter did not occur in pithed rats. Stimulation of the circulatory system in anesthetized and in pithed rats was completely abolished by administration of phentolamine or reserpine. Both in anesthetized and in pithed rats acetaldehyde caused an increase in heart rate which was inhibited in animals treated earlier with propranolol or reserpine. Our results demonstrated that besides its peripheral action, acetaldehyde exerts central effects which cause severe hypotension.     

Ketanserin stabilizes blood pressure in conscious spontaneously hypertensive rats

1. It has been demonstrated that blood pressure variability (BPV) is increased in hypertension and related to organ damage. It will be important to lower BPV in the treatment of hypertension. The present study was designed to investigate the effect of ketanserin, a 5-HT2A receptor antagonist with a weak alpha1-adrenoceptor blocking effect, on BPV in conscious spontaneously hypertensive rats (SHR). 2. It was found that ketanserin decreased blood pressure (BP) and BPV in SHR when administered intravenously (3 mg/kg, i.v.). Ketanserin decreased BPV, but not the BP level, when administered intracerebroventricularly (50 microg/rat, i.c.v.). 3. Prazosin, an alpha1-adrenoceptor antagonist, lowered BP but did not affect BPV when given either i.v. (0.5 mg/kg) or i.c.v. (30 microg/rat). Ritanserin (0.625 mg/kg, i.v.; 40 microg/rat, i.c.v.), a 5-HT2A receptor antagonist, decreased BPV only when administered i.c.v. and did not modify the BP level. 4. Ketanserin enhanced arterial baroreflex function in SHR when given either i.v. or i.c.v. 5. The stabilizing effect of ketanserin on BP was persistent when administered intragastrically. This administration route is similar to oral administration clinically. 6. It is concluded that ketanserin is an antihypertensive agent with an effect of reducing BPV. This effect is mainly mediated by central 5-HT2A receptors and is probably attributable to the restoration of arterial baroreflex function.     

Effects of azelastine hydrochloride, a new antiallergic drug, on the gastrointestinal tract

Effects of 4-(p-chlorobenzyl)-2-(hexahydro-1-methyl-1H-azepin-4-yl)-(2H)-phthalazinone hydrochloride (azelastine hydrochloride), a new antiallergic drug, on the gastrointestinal tract were experimentally studied in comparison with diphenhydramine and clemastine. In the isolated guinea pig ileum, the dose-response curve for histamine was shifted to the right by 10(-8) mol/l of diphenhydramine, clemastine or azelastine to the same degree. Clemastine and azelastine reduced the maximum response in the dose-response curve, while diphenhydramine caused a parallel shift. In addition to the potent antihistamine action, azelastine at higher dose antagonized serotonin action and clemastine markedly inhibited the contractile responses of isolated intestinal preparations to acetylcholine and bradykinin. Azelastine and clemastine (50 mg/kg intraduodenally) reduced gastric secretion in pylorus ligated rats. Azelastine (5 mg/kg i.v.), clemastine (1 mg/kg i.v.) and diphenhydramine (1 mg/lg i.v.) depressed gastrointestinal motility in conscious rats. Biliary and pancreatic secretions of anesthetized rats were not affected by 1 mg/kg i.v. of azelastine or clemastine.     

Cardiovascular effects of KRN2391, nitroglycerin and cromakalim in dihydroergotamine-treated pithed rats

1. The effects of KRN2391 on the cardiovascular system were compared with those of nitroglycerin and cromakalim in pithed rats treated with dihydroergotamine (DHE) in order to examine the effects of these drugs on venous blood vessels.2. DHE (100 μg/kg, i.v.) produced increases in mean blood pressure (MBP), cardiac output (CO) and central venous pressure (CVP) without changes in total peripheral vascular resistance (TPR) and heart rate (HR) based on venoconstriction. The DHE-treated pithed rats, nitroglycerin (30 μg/kg, i.v.) decreased CO and CVP whereas cromakalim (30 μg/kg, i.v.) produced a slight increase in CO followed by a decrease and did not affect CVP. KRN2391 (30 μg/kg, i.v.) produced a decrease in CVP without affecting CO. Decreases in MBP and TPR were induced by all drugs.3. These results suggest that nitroglycerin acts predominantly as a venodilator and KRN2391 and cromakalim showed a venodilating action in addition to an arterial dilating action in DHE treated pithed rats. However, the venodilating action of KRN2391 in this condition is more potent than that of cromakalim.     

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

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

Effect of terodiline hydrochloride on the function of urinary bladder in rats

The effect of terodiline on the function of urinary bladder was investigated in anesthetized rats. When saline was infused continuously into the urinary bladder of rats, terodiline (1-10 mg/kg, i.v.) prolonged the time to micturition in a dose-dependent manner. When enough saline was infused into the urinary bladder to induce the voiding contraction in urethra-ligated rats, terodiline (1-10 mg/kg, i.v.) and verapamil (1 mg/kg, i.v.) abolished the contraction, of which amplitude and frequency were partially inhibited by atropine (1 mg/kg, i.v.). Efferent discharge from the pelvic nerve on the micturition reflex was inhibited by terodiline (3 mg/kg, iv.v.). Both of the bladder contractions evoked by the electrical stimulation of the peripheral or central cut end of the pelvic nerve were dose-dependently inhibited by terodiline (1-10 mg/kg, i.v.). At 3 mg/kg or more, terodiline significantly inhibited the contraction, and the effects were long lasting. The effect of atropine (1 mg/kg, i.v.) was similar to that of terodiline (3 mg/kg, i.v.). Increase in frequency of urination and decrease in total urinary volume per micturition after the bilateral transection of the hypogastric nerve were improved after on oral administration of terodiline (1-10 mg/kg).