Naloxone affects the release of cortisol, but not of vasopressin or oxytocin, in dehydrated sheep.

Ovariectomized ewes (N = 7) were dehydrated for 24 h and then given iv injections of saline vehicle or 8 or 64 mg naloxone hydrochloride in saline. Blood samples were taken by jugular venepuncture before and after dehydration and at intervals during the 90 min period directly following naloxone treatment. Plasma concentrations of AVP, OT and cortisol were measured by radioimmunoassay. Plasma AVP levels and osmolality increased with dehydration, OT concentrations showed no consistent change, and cortisol levels were unaffected. After administration of naloxone, AVP and OT concentrations did not alter. The sampling procedure increased plasma cortisol levels and the duration of this response was prolonged by the 64 mg dose of naloxone.