Studies of naloxone-induced secretion of beta-endorphin immunoreactivity in dogs

The stimulating effect of naloxone on plasma beta-endorphin immunoreactivity (beta EI) was examined in dogs. Intravenous naloxone at 5, 1, or 0.1 mg/kg caused a significant increase in beta EI while doses of 0.01 or 0.001 mg/kg had no effect. The peak plasma beta EI levels occurred at 25 mins after naloxone. The neurotransmitter and antagonists metergoline, atropine, diphenhydramine and phentolamine all failed to significantly alter basal beta EI secretion; further, they all failed to prevent the increase in beta EI resulting from naloxone administration. Dexamethasone prevented the naloxone-induced rise in beta EI. Our results suggest naloxone's effect on beta EI is not mediated through several neurotransmitter systems known to affect ACTH secretion. Additionally, beta EI secreted in response to naloxone appears to originate mainly from the anterior lobe of the pituitary.