Abstract
The analgesic effect of clonidine in spontaneously hypertensive rats (SHR) and in normotensive Sprague-Dawley (SD) rats was assessed by using the formalin pain test. The analgesic response of SD rats to low doses (15–60 μg/kg i.p.) but not to a high dose (150 μg/kg i.p.) of clonidine was inhibited by naloxone, 2 mg/kg i.p., and a similar interaction was noted in SHR. In both rat strains, the analgesic response to low i.p. doses of clonidine was also inhibited by injection of 5 μg of naloxone or 7 μg of β-funaltrexamine, a μ-receptor antagonist, into the lateral cerebral ventricle. I.c.v. injection of 5 μg of ICI 174864, a δ-receptor antagonist, potentiated or did not influence clonidine analgesia in SD rats and SHR, respectively. It is concluded that the analgesic response to clonidine involves activation of central μ-opioid receptors in both SHR and SD rats, possibly by an endogenous opioid released by clonidine.
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