Clonidine (hydrochloride)

Clonidine hydrochloride is an agonist of α2-adrenoceptor and potent antihypertensive agent. In Vitro: Clonidine (0.01, 0.1 or 1 μM) significantly induces CGRP (α and β) mRNA expression in a dose-dependent manner in endothelial cells. Clonidine treatment (1 μM) for 24 h significantly increases the NO level in endothelial cells. NO pathway modulates CGRP production induced by clonidine^[2]. In Vivo: Clonidine (50 μg/kg, i.p.) induces a significant decrease in body temperature of rat lasting 3 hr, with the maximum at 1 hr after administration. An intracerebroventricular pretreatment of rats with neutral doses of phentolamine 15 min before clonidine considerably antagonizes the clonidine-induced hypothermia^[1]. Clonidine (0.003-0.05 mg/kg, i.p.) potently suppresses dopamine efflux in the prefrontal cortex induced by PCP. Pretreatment with the alpha-2A receptor antagonist (BRL-44408) prevents clonidine from suppressing PCP-induced dopamine overflow in the prefrontal cortex^[3]. In DMSO-pretreated SO rats, clonidine (0.6 μg i.c.) has no effect on blood pressure. However, after central adenosine A1R blockade (DPCPX) in SO rats, clonidine significantly (P < 0.05, one-way ANOVA) reduces blood pressure. In contrast, in DMSO-pretreated ABD rats, clonidine (0.6 μg i.c.) causes significant reduction in blood pressure; importantly, central A1R blockade (DPCPX pretreatment) does not influence (P > 0.05, one-way ANOVA) clonidine-evoked reduction in blood pressure in ABD rats. In DPCPX-pretreated SO rats and along with the appearance of the hypotensive response, clonidine causes a significant (P < 0.05) increase in the RVLM pERK1/2 level compared with basal or clonidine treatment in DMSO-pretreated SO rats. In vehicle (DMSO)-pretreated ABD rats, clonidine significantly (P < 0.05) enhances RVLM pERK1/2, and this response is not affected by DPCPX pretreatment^[4].