RELEASE OF VASOPRESSIN WITHIN THE RAT PARAVENTRICULAR NUCLEUS IN RESPONSE TO EMOTIONAL-STRESS - A NOVEL MECHANISM OF REGULATING ADRENOCORTICOTROPIC HORMONE-SECRETION

The effects of emotional stressors on the release of arginine vasopres sin (AVP) and oxytocin (OXT) within the rat hypothalamus and the origi n and physiological significance of AVP released within the hypothalam ic paraventricular nucleus (PVN) were investigated. First, adult male Wistar rats with a microdialysis probe aimed at the PVN or the supraop tic nucleus were exposed to either a dominant male rat (social defeat) or a novel cage. Release of AVP within the PVN was significantly incr eased in response to social defeat but not to novelty. In contrast to an activation of the hypothalamic-pituitary-adrenal (HPA) system, neit her stressor stimulated the. hypothalamic-neurohypophysial system (unc hanged plasma AVP and OXT and unchanged release within the supraoptic nucleus [AVP] and the PVN [OXT]). Next, we demonstrated by simultaneou s microdialysis of the suprachiasmatic nucleus and the PVN that AVP me asured in PVN dialysates during social defeat was probably of intranuc lear origin. Finally, a mixture of a V1 AVP and the alpha-helical cort icotropin-releasing hormone (CRH) receptor antagonists administered vi a inverse microdialysis into the PVN caused a significant increase in the plasma adrenocorticotropic hormone (ACTH) concentration compared w ith vehicle-treated controls both under basal conditions and during so cial defeat, indicating inhibitory effects of intra-PVN-released AVP a nd/or CRH on HPA system activity. The antagonists failed to affect anx iety-related behavior of the animals as assessed with the elevated plu s-maze. Taken together, our results show for the first time that AVP i s released within the PVN in response to an emotional stressor. We hyp othesize that this intranuclear release provides a negative tonus on A CTH secretion.