Atrial natriuretic peptide mediates oxytocin secretion induced by osmotic stimulus

Atrial natriuretic peptide (ANP), first discovered in the heart, has been also detected in various brain regions involved in the control of cardiovascular function and water and sodium balance. The anteroventral region of the third ventricle (AV3V) and the subfornical organ (SFO) have ANP-immunoreactive projections towards the paraventricular (PVN) and supraoptic (SON) nuclei of the hypothalamus. Extracellular fluid (ECF) hyperosmolality stimulates the secretion of oxytocin (OT) which induces ANP release by the atrium. On the other hand, passive immunoneutralization of ANP reduces OT secretion in response to ECF hypertonicity. Previous studies have shown the co-localization of ANP and OT in PVN and SON neurons and in the periventricular region, as well as the presence of ANPergic and oxytocinergic neurons in the median eminence. The aim of the present study was to investigate the OT and ANP content in the SON and PVN of the hypothalamus and in the posterior pituitary (PP) after an osmotic stimulus that induces OT secretion. The results showed that intracerebroventricular microinjection of normal rabbit serum (NRS) or of ANP antiserum followed or not by an intraperitoneal injection of isotonic saline did not alter OT secretion or OT content in the PVN, SON, and PP; passive ANP immunoneutralization reduced the basal content of ANP in the PVN, SON, and PP of animals in a situation of isotonicity; the ANP antiserum inhibited the increase of OT secretion and content of OT and ANP in the PVN, SON and PP induced by the osmotic stimulus. Thus, the increase in plasma OT and oxytocinergic neurons of the hypothalamus-posterior pituitary system in response to hypertonicity depends on the action of endogenous ANP, i.e., ECF hypertonicity must activate ANPergic neurons which directly or indirectly stimulate OT release. (C) 2002 Elsevier Science Inc. All rights reserved.