Opiate receptors may mediate the suppressive but not the excitatory action of ACTH on motor activity in rats

Abstract

Subcutaneous injections of adrenocorticotropin (ACTH) or of the opiate antagonist naltrexone produced a dose-dependent biphasic effect on motor activity in rats. High doses of ACTH (100 or 200 μg/kg) and of naltrexone (2.0 mg/kg) depressed, whereas smaller doses of ACTH (50 μg/kg) and of naltrexone (0.125 and 0.25 mg/kg) stimulated motor activity in the open field test. Furthermore, naltrexone at a dose level that had no effect on motor activity blocked the suppressive effect of the high doses of ACTH but had no effect on the stimulating effect of the intermediate dose of ACTH. Finally, chronic naltrexone administration resulted in enhanced sensitivity to the suppressive but not to the stimulating effect of ACTH on motor activity. It is argued that opiate receptors may play a selective role in the effect of ACTH on motor activity. Such receptors may mediate the suppressive effect of high doses of ACTH whereas other, naltrexone insensitive receptor systems may mediate the stimulating effect of ACTH on activity functions.