Effect of chronic hyponatremia on central and peripheral oxytocin and vasopressin secretion in rats

Previous studies have shown that many treatments that stimulate the peripheral secretion of oxytocin (OT) and vasopressin (AVP) from the pituitary simultaneously increase the levels of these peptides in the cerebrospinal fluid (CSF). Since osmotically and nonosmotically stimulated pituitary secretion of OT and AVP is markedly blunted in hyponatremic rats, the present studies evaluated whether central OT and AVP secretion into the CSF is similarly inhibited during sustained hyponatremia. Adult male rats with indwelling cisterna magna cannulae were rendered hyponatremic (plasma [Na+] < 110 mmol/l) by s.c. infusion of desmopressin (dDAVP; 10 ng/h) in combination with ingestion of a liquid diet for 3 days, then subjected to osmotic (i.v. or i.p. injection of 2 M NaCl; HS) or nonosmotic (6 mmol/kg of 0.15 M LiCl i.p.) stimulation. In normonatremic rats both i.v. and i.p. HS caused marked increases in plasma OT and AVP levels 30 min after treatment. Significant elevations of OT, but not AVP, were also present in CSF. Despite similar increases in plasma Na+ concentrations, plasma OT responses in the hyponatremic rats were absent after HS i.v. and were significantly blunted after HS i.p., but neither group had increased plasma AVP. In parallel with the plasma results, CSF OT responses were absent in hyponatremic rats given HS i.v. and significantly blunted in hyponatremic rats given HS i.p., but neither group had increased CSF AVP. Nonosmotic stimulation with isotonic LiCl increased OT levels both in plasma and CSF in normonatremic rats 20 min after treatment.(ABSTRACT TRUNCATED AT 250 WORDS)