Suppression by Naloxone of Rise in Plasma Growth Hormone and Prolactin Induced by Suckling

Abstract

The effects of 3 different doses of the specific opiate antagonist, naloxone (0.2, 2.0 and 5.0 mg/kg body wt), on release of growth hormone (GH) and prolactin (PRL) induced by suckling were determined in postpartum lactating rats. Blood was collected from mother rats via an indwelling atrial cannula 8 h after separation from their pups. Suckling for 30 min induced a 3-fold increase in plasma GH and more than a 10-fold increase in plasma PRL. Injection of naloxone into mother rats just prior to suckling by their pups produced significant inhibition of both GH and PRL release. Doses of 0.2 and 5.0 mg naloxone/kg body wt almost completely inhibited GH release, whereas an intermediate dose (2.0 mg/kg body wt) was partially effective. Inhibition of PRL release by naloxone was dose related, and the highest dose (5.0 mg/kg body wt) decreased plasma PRL values by 60-80%. Apparently endogenous opioid peptides are involved in release of GH and PRL induced by the suckling stimulus in the rat.