Effects of CRF, AVP and opioid peptides on pituitary-adrenal responses in sheep.

Intravenous administration of equimolar doses of CRF (30 micrograms) and AVP (6 micrograms) to mature female sheep resulted in elevated plasma concentrations of ACTH and cortisol. Simultaneous administration of equimolar amounts of CRF and AVP resulted in a greater ACTH response compared with the sum of the responses to CRF or AVP given independently. Intravenous bolus administration of the endogenous opioid, Met-enkephalin (2.5 mg), and its potent and long-acting analogue, [D-Ala2,N-Phe4,Met(O)ol5]-enkephalin [FK33-824 (250 micrograms)], did not alter ACTH or cortisol secretion. Furthermore, naloxone, an opioid receptor antagonist given alone or concurrently with Met-enkephalin or FK33-824, was without effect. Pituitary-adrenal responses to CRF were unaltered by simultaneous administration of Met-enkephalin, FK33-824 or naloxone. These results suggest that in the sheep, opioid involvement in the tonic regulation of pituitary-adrenal function is absent. However, CRF and AVP may act alone or in synergy to control the release of biologically active ACTH from the sheep pituitary gland.