Is the hypothermic effect of alpha-methyldopa mediated by opioid peptides?

At an ambient temperature of 21 degrees C alpha-methyldopa (25-200 mg kg-1) induces a dose-dependent decrease in body temperature in rats. A relationship between adrenergic and opioid neuronal systems has been reported. Therefore, in this study the hypothesis that alpha-methyldopa produces hypothermia through release of endogenous opioid peptides has been investigated using the opiate antagonist naltrexone. The hypothermic effect of alpha-methyldopa is potentiated by naltrexone pointing to antagonism of an hyperthermic acting opioid system. However, at an ambient temperature of 6 degrees C, pretreatment with naltrexone did not significantly alter the hypothermic effect of alpha-methyldopa. Although the hypothesis proved not to be correct, it is concluded that depending on ambient temperature opioid peptides are involved in the determination of the ultimate effect of alpha-methyldopa on body temperature in rats.