Angiotensin acts at the subfornical organ to increase plasma oxytocin concentrations in the rat

We have examined the effects of systemic angiotensin II (AII) on plasma oxytocin (OXY) concentrations in freely moving male Sprague-Dawley rats. We have also examined the role of the subfornical organ (SFO) as a CNS site at which circulating AII acts to influence secretion of this neurohypophysial peptide. OXY concentrations were measured by radioimmunoassay in plasma samples obtained by drawing blood samples through indwelling atrial catheters. In SFO intact animals ( n = 8) AII infusion (1.0 μg/kg/min) resulted in increases in plasma OXY concentrations from baseline values of 6.8 ± 2.5 pg/ml to postinfusion concentrations of 44.9 ± 11.9 pg/ml. In a second series of experiments electrolytic lesions were placed in the region of the SFO prior to testing the effects of AII infusion on OXY concentrations. Two further experimental groups were thus established according to the histologically verified location of lesions in either the rostral or caudal SFO. In the caudal SFO lesioned group AII infusion resulted in increases in plasma OXY concentrations from control values of 6.9 ± 1.4 pg/ml to postinfusion levels of 45.1 ± 9.8 pg/ml. These changes were not significantly different from the SFO intact group. In contrast rostral SFO lesions resulted in significantly elevated basal concentrations of OXY (17.4 ± 3.4 pg/ml, n = 6) while postinfusion concentrations were found to be 22.8 ± 4.9 pg/ml indicating that AII infusion was without effect following such lesions. These data are in accordance with the hypothesis that circulating AII acts at the SFO to influence SFO efferents which in turn activate OXY secreting neurons in the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei. These neuroendocrine cells then release this peptide into the systemic circulation from the posterior pituitary.