Effects of a [Met]-enkephalin analogue ( [D-Ala2,N-Me-Phe4,Met-(O)5-ol]-enkephalin) on canine pituitary function

In adult healthy beagle dogs, plasma concentrations of ACTH, cortisol, alpha-MSH, GH, prolactin and arginine vasopressin (AVP) were measured after i.v. administration of [D-Ala2,N-Me-Phe4,Met-(O)5-ol]-enkephalin (DAMME) at doses of 0.1, 0.5, 1, 5 and 10 micrograms/kg body weight. Significant dose-dependent increases occurred for ACTH, cortisol and GH at dose rates of 0.5, 1, 5 and 10 micrograms/kg body weight. Increments in plasma concentrations of prolactin were significant only at 5 and 10 micrograms DAMME/kg, and there was no significant effect on plasma concentrations of alpha-MSH and AVP. Prior i.v. administration of the opiate antagonist naloxone (0.1 mg/kg) attenuated the DAMME (10 micrograms/kg)-stimulated release of ACTH and cortisol. The results demonstrate that the [Met]-enkephalin analogue DAMME stimulates the release of ACTH, cortisol, GH and prolactin in dogs, and that this stimulation is, at least in part, mediated by mu-opioid receptors. The observations for ACTH and cortisol are different from those in man, where DAMME lowers their basal concentrations.