Differential effects of naloxone on basal and stress-induced release of ACTH and prolactin in the male rat.

The effect of i.v. injection of various doses of naloxone (NAL) on plasma adrenocorticotropin (ACTH) and prolactin (Prl) in conscious animals bearing an indwelling intrajugular catheter was assessed. The effects were evaluated in animals which were left undisturbed and in others subjected to either restraint or ether stress. The results revealed that the dose of 3 mg/kg of NAL significantly reduced basal Prl levels, whereas a dose of 6 mg/kg of NAL was required to block completely either ether or restraint stress-induced release of Prl. The behavior of ACTH contrasted with that of Prl. There was no effect whatsoever of the 3 mg/kg dose of NAL on either resting or stress-induced ACTH levels, whereas a 6 mg/kg or 12 mg/kg dose of NAL elevated resting ACTH levels and only partially attenuated the further elevation induced by stress in these animals. The results clearly indicate a NAL sensitive step in the control of resting and stress-induced Prl release but indicate that the control of resting and stress-induced release of ACTH is different in that the predominantly millimicron receptor blocker, NAL, can elevate ACTH at high doses and can only partially block the response to stress. In contrast to Prl where opioid peptide control is solely stimulatory, this control of ACTH secretion appears to have both stimulatory and inhibitory features.