Animal experiment:

On the day of the experiment, the flow rate is increased to 2 μL/min approximately 2 h before beginning the collection of baseline samples. Dialysates are collected every 20 min; after 4 baseline samples are collected, animals are pretreated with an intra-peritoneal (i.p.) injection of either 0.9% saline (the vehicle), clonidine (0.0033, 0.01 or 0.05 mg/kg) or guanfacine (0.05 or 0.5 mg/kg), before receiving an injection of PCP (2.5 mg/kg, i.p.) 20 min later. In a separate study, BRL (1.0 mg/kg) is administered 20 min prior to clonidine. In addition, for some control experiments, the animals only receive one injection of saline, clonidine (0.01 or 0.05 mg/kg), guanfacine (0.5 mg/kg) or BRL (1.0 mg/kg).

Clonidine hydrochloride is an agonist of α2-adrenoceptor and potent antihypertensive agent.

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].

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) 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].

References:
[1]. Bugajski J, et al. The involvement of central alpha-adrenergic and histamine H2-receptors in the hypothermia induced by clonidine in the rat. Neuropharmacology. 1980 Jan;19(1):9-15.
[2]. Nassar N, et al. Brainstem adenosine A1 receptor signaling masks phosphorylated extracellular signal-regulated kinase 1/2-dependent hypotensive action of clonidine in conscious normotensive rats. J Pharmacol Exp Ther. 2009 Jan;328(1):83-9.
[3]. Zhang YM, et al. Clonidine induces calcitonin gene-related peptide expression via nitric oxide pathway in endothelial cells. Peptides. 2009 Sep;30(9):1746-52.
[4]. Jentsch JD, et al. Clonidine and guanfacine attenuate phencyclidine-induced dopamine overflow in rat prefrontal cortex: mediating influence of the alpha-2A adrenoceptor subtype. Brain Res. 2008 Dec 30;1246:41-6.