To further elucidate the role of the preoptic-hypothalamic region in fluid and electrolyte balance we studied the effect of surgical preoptic-hypothalamic disconnection using either a large (preoptic-hypothalamic disconnection) or a small (medial preoptic-hypothalamic disconnection) microknife. Both the large and small cuts seemed to transect the posterior projection originating in the periventricular tissue surrounding the anteroventral third ventricle (AV3V) and extending to supraoptic nucleus, but the supraoptic-neurohypophysial pathway was severed only by the large cut. Seven-day metabolic studies showed a disruption in hydromineral balance only in large cut rats; they had increased water intake and urine volume on day 1, a near-recovery of function on days 2 and 3, and polydipsia and polyuria on days 4 to 7. There was no difference between small cut rats and sham-operated rats in metabolic measurements. The large cut rats also had sustained hypernatremia and hyperosmolality, which was enhanced after water restriction for 48 h but was not accompanied by an increase in plasma arginine vasopressin. Our data therefore suggest that the efferent fibers running caudally from the AV3V are not involved in mediation of the hydromineral regulation of the AV3V.
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