Impaired osmoregulatory responses in rats with area postrema lesions

Area postrema lesions (APX) in adult male rats produced a robust spontaneous intake of 0.5 M NaCl, as reported previously. The largest NaCl intakes (up to 108 ml/day) were observed when there was little incidental damage in the medial subnucleus of the nucleus of the solitary tract adjacent to the caudal and middle portions of the area postrema. Rats with discrete APX also drank substantial amounts of 0.5 M NaCl when access to saline was restricted to 7 h/day (up to 30 ml in 1 h, 48 ml in 7 h). Such large NaCl intakes stimulated considerable water ingestion and renal sodium excretion, but together these responses usually were insufficient for osmoregulation during the 7-h test period. After systemic administration of hypertonic NaCl solution, rats with APX excreted less Na⁺ in urine and secreted less vasopressin and oxytocin than control rats did. The prominent salt appetite, insufficient thirst and natriuresis in response to an ingested NaCl load, and blunted natriuresis and neurohypophysial hormone secretion in response to an injected NaCl load, all indicate that osmoregulatory responses are impaired in rats after APX.