Paraventricular and supraoptic nuclei of the hypothalamus are not equally important for oxytocin release during stress.

The relative importance of the paraventricular (PVN) and the supraoptic nuclei (SON) for the secretion of oxytocin was evaluated by comparison of stress-induced oxytocin release under normal conditions, in the absence of vasopressin and/or corticoliberin (CRF). We introduced an incomplete anterolateral cut (iALC) around the mediobasal hypothalamus designed to leave intact the SON-neurohypophysial connections but to inflict damage to the nerve fibers from the PVN. The studies were performed in conscious cannulated rats using immobilization as the stress stimulus. Stress-induced oxytocin release was found in heterozygous Brattleboro rats as well as in homozygous animals lacking vasopressin, yet in the latter it was less pronounced and in both cases it was prevented by iALC. In Wistar rats, stress-induced oxytocin release was markedly reduced after iALC and absent after PVN lesion. Both hypothalamic interventions failed to influence basal oxytocin levels and resulted in a similar reduction of ACTH release. It is concluded that a functional diversity exists between the hypothalamic magnocellular nuclei. At least in relation to immobilization stress, the PVN is essential for stress-induced oxytocin release and it is evident that the SON without the PVN cannot preserve oxytocin secretion during stress.