Met-enkephalin and the enkephalin analogue FK-33824 centrally inhibit adrenocorticotrophic hormone secretion in sheep

Abstract

1. The effects of central administration of met-enkephalin, leu-enkephalin, the enkephalin analogue FK-33824 and the opiate antagonist naloxone on plasma concentration of adrenocorticotrophic hormone (ACTH) were examined in conscious sheep. 2. Intracerebroventricular infusion of met-enkephalin and FK-33824 significantly decreased the basal plasma concentration of ACTH. 3. Intracerebroventricular infusion of FK-33824 inhibited the haemorrhage-induced increase in plasma concentration of ACTH. 4. Intracerebroventricular infusion of naloxone attenuated the central inhibition of plasma concentration of ACTH induced by FK-33824, but intravenous infusion of naloxone had no effect on the reduction in plasma concentration of ACTH induced by FK-33824. 5. These studies suggest that in sheep met-enkephalin may play a central inhibitory role in the control of ACTH secretion.