EVIDENCE FOR A SPECIFIC ROLE OF VASOPRESSIN IN SUSTAINING PITUITARY-ADRENOCORTICAL STRESS RESPONSE IN THE RAT

In the adult male Wistar rat a 2-fold 2-min restraint stress exposure, repeated 15 min apart, activated the adrenocortical secretion more than a single one would have. However, in rats with a pharmacological block of the endogenous CRF release, exogenous CRH (0.03-mu-g/kg iv), administered 15 min after a first similar dose, was unable to stimulate pituitary-adrenocortical activity above the level attained with the first peptide injection. On the contrary, in the same conditions arginine vasopressin (AVP) (0.3-mu-g/kg iv) administered 15 min after CRH, was able to further stimulate pituitary-adrenocortical activity. Using the same experimental procedure, oxytocin (0.3-mu-g/kg iv) was found to be totally inactive. The physiological import of these findings was investigated in the Brattleboro rat, genetically lacking in endogenous AVP, in which, unlike the control Long-Evans strain, the 2-fold stress exposure did not cause an increase in plasma corticosterone concentration greater than that of a single exposure. These results suggest that endogenous AVP is essential in sustaining adrenocortical activation in circumstances in which pituitary refractoriness towards CRH stimulation intervenes.