Abstract
The effects of various alpha adrenoceptor agonists and antagonists injected into the spinal subarachnoid space on blood pressure and heart rate were investigated in pentobarbital-anesthetized male rats. A dose-dependent decrease in blood pressure and heart rate was induced by intrathecal injections of clonidine (0.3-3 micrograms), at the T6-T7 level. Guanabenz (3-30 micrograms) and an azepine derivative B-HT 920 (1-3 micrograms) also reduced blood pressure and heart rate. In contrast, an imidazolidine derivative St 587 (10 micrograms) was ineffective. The clonidine-induced hypotensive effect was antagonized by yohimbine but not by prazosin. Yohimbine (3-10 micrograms) alone caused an increase in blood pressure and heart rate while only a weak hypotenion occurred with prazosin (10 micrograms). Pretreatment with intrathecal 6-hydroxydopamine (50 micrograms X 2) did not impair the hypotensive action of clonidine (1 microgram) whereas the hypertensive effect of yohimbine was reduced markedly by this treatment. When injected i.v. or intrathecally at the C1-C2 level, clonidine (1 microgram) produced only a slight decrease in blood pressure. It is concluded that, in rat spinal cord, alpha-2 adrenoceptors located postsynaptically are involved in blood pressure control. Endogenous catecholamines, especially norepinephrine in the spinal cord seem to activate tonically the alpha-2 adrenoceptors.
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