Effect of prazosin on spontaneous sympathetic-cholinergic activity

Inhibition of serotonin-induced increase in canine external carotid blood flow by drugs that decrease the sympathetic outflow

1. The present study was designed to analyse the effect of the centrally-acting sympatho-inhibitory drugs, prazosin and ketanserin, on the increase in external carotid blood flow (external CBF) produced by 5-hydroxytryptamine (5-HT) in pentobarbital-anaesthetized dogs. 2. Intracarotid (i.c.) infusions of 5-HT (10 micrograms min-1 during 1 min) produced an increase in external CBF without changes in mean arterial blood pressure or heart rate. This response to 5-HT was dose-dependently blocked by intravenous (i.v.) administration of prazosin (1, 3, 10, 30 and 100 micrograms kg-1) or ketanserin (10, 30, 100 and 300 micrograms kg-1). 3. Furthermore, 5-HT-induced increase in external CBF was inhibited by either the ganglionic blocking agent, mecamylamine (0.03, 0.1, 0.3, 1, 3 and 10 mg kg-1), the mixed 5-HT1-like and 5-HT2 receptor antagonist, methiothepin (3, 10 and 30 micrograms kg-1) or the 5-HT1A ligand, spiroxatrine (10, 30, 100 and 300 micrograms kg-1). In contrast, the selective 5-HT2 and 5-HT1C receptor antagonist, ritanserin (30 and 100 micrograms kg-1, i.v.), was unable to block the above response to 5-HT. 4. It is concluded that the inhibition of 5-HT-induced increase in external CBF by prazosin, ketanserin, mecamylamine and spiroxatrine is due to a reduction in the sympathetic tone and not to a blockade of 5-HT receptors.

Comparison of peripheral and central nervous system sympatholytic actions of prazosin using the cat nictitating membrane

Prazosin is a highly selective alpha 1-adrenoceptor antagonist that decreases blood pressure by actions on both the peripheral and central (CNS) divisions of the nervous system. The present investigation was undertaken in an attempt to characterize the relative contribution of these two sympatholytic sites of action. Submaximal contractions of the nictitating membranes were evoked by electrical stimulation of the preganglionic cervical sympathetic nerve trunk and by stimulation of the posterior hypothalamus in anesthetized cats. In initial control experiments, phenoxybenzamine (0.1-3.0 mg/kg i.v.) produced an equivalent depression of evoked nictitating membrane responses from both peripheral and CNS sites of activation which suggests only a peripheral blocking action as well as functional equivalence of the intensity of CNS and peripheral nerve stimulation. In contrast, prazosin (3-300 micrograms/kg i.v.) caused a differential dose-related depression of the evoked responses with ED50s of 81.5 micrograms/kg (peripheral stimulation) and 12.5 micrograms/kg (CNS stimulation) respectively; P less than 0.05. Pretreatment with rauwolscine (500 micrograms/kg i.v.) totally prevented the differential CNS sympatho-inhibition produced by prazosin. These results indicate that, although both CNS and peripheral sites of drug action are manifest, the ED50 for prazosin-induced CNS sympatho-inhibition is approximately 6-fold less than that required for direct alpha 1-adrenoceptor blockade at the end organ. In addition, prazosin produces CNS sympatho-inhibition indirectly by means of an alpha 2-adrenoceptor mechanism.

The noradrenaline uptake inhibitor, (+)-oxaprotiline, but not the inactive enantiomer, (-)-oxaprotiline, inhibits sympathetic nerve activity in the rabbit: involvement of adrenoceptors

The effect of the enantiomers of oxaprotiline on the sympathetic nervous system was studied in anesthetized rabbits. (+)-Oxaprotile 0.2, 0.6 and 1.8 mg kg-1 dose dependently reduced prostganglionic renal sympathetic nerve activity and the clearance of [3H]noradrenaline from the plasma but increased the plasma noradrenaline concentration. The spillover of noradrenaline into the blood was not changed significantly, nor were blood pressure and heart rate, except transiently. The same doses of (-)-oxaprotiline had no effect on any of the parameters measured. (+)-Oxoprotiline also reduced lumbar preganglionic sympathetic nerve activity. The effect of (+)-oxaprotiline on renal sympathetic nerve activity was only minimally antagonized by yohimbine given alone. (+)-Oxaprotiline also inhibited renal sympathetic nerve activity after treatment with prazosin and propranolol. Subsequent administration of yohimbine restored sympathetic nerve activity to the value seen before the administration of (+)-oxaprotiline. The results show that inhibition of the re-uptake of released noradrenaline is the basis of the sympathoinhibition produced by (+)-oxaprotiline. Noradrenaline probably acts on alpha 2-adrenoceptors in the central nervous system to produce sympathoinhibition.

Response of the rat myometrium to phenylephrine in early pregnancy and the effects of 6-hydroxydopamine

1. The contractile responses of the longitudinal and circular muscle layers of the rat uterus to the alpha 1-adrenoceptor agonist phenylephrine were measured on days 3-6 of gestation. There was a progressive increase in sensitivity to phenylephrine in both muscle layers between days 3 and 6 of gestation. Overall, this amounted to a 13 and 9 fold increase in sensitivity in longitudinal and circular muscles, respectively. In longitudinal muscle the slope of the Hill plot was 2 on day 3 of pregnancy and was decreased to 1 thereafter. 2. The sympathetic nerve terminals innervating the smooth muscle of the uterus were destroyed by administration of 6-hydroxydopamine (2 x 50 mg kg-1) 4-7 days before testing with phenylephrine. Following this treatment there was a significant increase in sensitivity to phenylephrine on day 3 in both muscle layers. After day 4, the longitudinal muscle was less sensitive to phenylephrine. 3. In the longitudinal muscle there was a progressive increase in the contractile response to maximal concentrations of phenylephrine and to high potassium (100 mM) between days 3 and 6 of pregnancy. In the circular muscle the responsiveness to both phenylephrine and potassium remained unchanged between days 3 and 6 of gestation. 6-Hydroxydopamine had no effect on the maximal responses to phenylephrine or high potassium in either muscle layer. 4. In conclusion, denervation supersensitivity of uterine smooth muscle following injection of 6-hydroxydopamine is observed only on day 3 of pregnancy and appears to be replaced by subsensitivity by day 6. The decrease in the slope of the Hill plot in longitudinal muscle after day 3 may be explained by changes in events between activation of alpha 1-adrenoceptors and contraction.

Mechanism of ketanserin-induced sympatho-inhibition

Experiments were undertaken to determine if sympatho-inhibition produced by ketanserin is due to antagonism of central nervous system alpha 1-adrenoceptors rather than central 5-HT2 receptors and if (like prazosin) it produces sympatho-inhibition indirectly via a central (presynaptic) alpha 2-adrenoceptor mechanism. Administration of ketanserin (0.03-3.0 mg/kg i.v.) caused a dose-related depression of sympathetic-cholinergic electrodermal responses evoked by electrical stimulation of the hypothalamus in pentobarbital anesthetized cats. No effect of ketanserin was observed on electrodermal responses evoked by preganglionic sympathetic nerve stimulation nor did the more specific 5-HT2 receptor antagonist, cinanserin, produce a central sympatholytic effect at dosages up to 3 mg/kg i.v. Pretreatment with alpha 2-adrenoceptor blockers yohimbine, idazoxan, or rauwolscine significantly antagonized ketanserin-induced sympatho-inhibition. Depletion of central nervous system (CNS) monoamines totally prevented ketanserin-induced sympatho-inhibition although clonidine (30 micrograms/kg i.v.) continued to be effective. These results suggest that ketanserin acts in the CNS to reduce sympathetic reactivity by blocking alpha 1-adrenoceptors and not 5-HT2 receptors. In this regard, ketanserin appears to act in a manner similar to other alpha 1-adrenoceptor antagonists (e.g. prazosin and indoramin) by an apparent presynaptic facilitation of alpha 2-adrenoceptor mediated tonic inhibition descending from the lower brainstem.