Lack of involvement of endogenous mu-receptor opioids in the hypothermic effects of clonidine in normotensive and spontaneously hypertensive rats.

Abstract

The effects of successive injections of the alpha-adrenoceptor agonist clonidine (25, 50 and 100 ug/kg given at hourly intervals) on the body temperature of normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats, previously treated for 48 hr with slow release emulsions (subcutaneous) containing either morphine (morphine SR, 100 mg/kg), naloxone (naloxone SR, 80 mg/kg) or no drug (vehicle SR), were examined. The successive injections of clonidine produced dose-dependent falls in body temperature which were quantitatively similar in the vehicle-treated WKY and spontaneously hypertensive rats. The hypothermic effects of clonidine in the morphine-dependent WKY and spontaneously hypertensive rats, and in the naloxone-treated WKY and spontaneously hypertensive rats, were not different to those of the respective vehicle-treated controls. These results suggest that endogenous mu-receptor opioid peptides do not have a major involvement in the hypothermic actions of clonidine, in either normotensive or spontaneously hypertensive rats.