Endocrine responses to acute stress in castrated rams: no increase in oxytocin but evidence for an inverse relationship between cortisol and vasopressin.

Plasma concentrations of cortisol, oxytocin, vasopressin and prolactin were measured by radioimmunoassay in eight castrated rams subjected to handling (control), restraint or isolation. Determinations were also made of hematocrit and plasma osmolality. Blood was collected by jugular venipuncture 60 and less than 1 min before, and 5, 15, 30, 60 and 120 min after the start of treatment. Plasma cortisol was raised above pre-treatment levels during isolation and restraint but only transiently after handling. In the first 30 min of treatment, cortisol concentrations were higher following isolation than after restraint or handling and, in the final 60 min, levels were higher during both isolation and restraint than after handling. Oxytocin concentrations showed little change but vasopressin levels were reduced 15, 30 and 60 min after the start of isolation and were negatively correlated with those of cortisol. Plasma prolactin concentrations decreased during handling and restraint but remained unaltered after isolation. Osmolality and hematocrit decreased in all three treatment conditions. These findings indicate that stress does not alter oxytocin levels in sheep, but changes in vasopressin secretion and water balance may occur if cortisol release is greatly enhanced.