Subfornical organ-median preoptic connections and drinking and pressor responses to angiotensin II.

The subfornical organ (SFO) and the periventricular tissue of the anteroventral third ventricle (AV3V) have been shown to be important for the central action of circulating angiotensin. Recent anatomical findings have elucidated neural connections between the SFO and several structures within the AV3V region. The present study examined the function of fibers between the SFO and the median preoptic nucleus. Horizontal knife cuts rostral to the anterior commissure severed precommissural fibers between the SFO and the median preoptic nucleus. Cuts immediately dorsal to the anterior commissure interrupted both pre- and postcommissural connections. Rats with cuts of both sets of fibers evidenced a virtual absence of drinking responses following subcutaneous injections of angiotensin, while rats with selective cuts of precommissural fibers manifested a partial, but significant, reduction in responding. The two groups of animals showed similar attenuations in drinking responses following subcutaneous injections of hypertonic saline. Rats with cuts of both pre- and postcommissural fibers evidenced an attenuation of drinking responses elicited by centrally administered angiotensin. Pressor responses following intravenous and intraventricular angiotensin injections were not reduced specifically by the experimental knife cuts. These findings are consistent with a model that postulates that angiotensin receptors, and perhaps osmoreceptors, in the SFO send excitatory neural information to the median preoptic nucleus for the mobilization of thirst.