Impaired osmoregulatory responses in rats with area postrema lesions

Area postrema lesions (APX) in adult male rats produced a robust spontaneou s intake of 0.5 M NaCl, as reported previously. The largest NaCl intakes (u p to 108 ml/day) were observed when there was little incidental damage in t he medial subnucleus of the nucleus of the solitary tract adjacent to the c audal 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 restrict ed to 7 h/day (up to 30 ml in 1 h, 48 ml in 7 h). Such large NaCl intakes s timulated considerable water ingestion and renal sodium excretion, but toge ther these responses usually were insufficient for osmoregulation during th e 7-h test period. After systemic administration of hypertonic NaCl solutio n, rats with APX excreted less Na+ in urine and secreted less vasopressin a nd oxytocin than control rats did. The prominent salt appetite, insufficien t thirst and natriuresis in response to an ingested NaCl load, and blunted natriuresis and neurohypophysial hormone secretion in response to an inject ed NaCl load, all indicate that osmoregulatory responses are impaired in ra ts after APX.