Systemic angiotensin II and volume expansion release norepinephrine in the preoptic recess

The periventricular tissue surrounding the anteroventral portion of the third cerebral ventricle (AV3V) of the preoptic recess is critical for several responses evoked by central and peripheral angiotensin II (ang II), and the natriuresis produced by extracellular fluid volume expansion. In addition, several other studies have suggested that the pressor and natriuretic responses evoked by these stimuli are regulated by activation of noradrenergic systems in AV3V tissue. The purpose of the present experiments was to directly determine if i.v. infusion of ang II and volume expansion increase norepinephrine release in the AV3V area using in vivo microdialysis. Male, Sprague-Dawley rats were implanted with microdialysis probes in the AV3V and were tested conscious and unrestrained. Dialysate was collected before, during, and following i.v. administration of ang II or phenylephrine which raised blood pressure 40-50 mmHg, or volume expansion with isotonic saline. Dialysate norepinephrine concentrations from these rats were compared to dialysate norepinephrine content from animals which received no treatment. Both i.v. infusion of ang II and isotonic volume expansion significantly increased dialysate norepinephrine concentration, while infusion of phenylephrine did not alter norepinephrine release in AV3V tissue. These data demonstrate that peripheral ang II and volume expansion selectively increase norepinephrine release in the AV3V region, and are consistent with the conclusion that activation of noradrenergic systems in the AV3V region contribute to cardiovascular and drinking responses evoked by peripheral ang II and to natriuresis following volume expansion.