Effects of chronic central infusion of oxytocin on the response of the magnocellular neuroendocrine system to osmotic stimulation.

The effect of continuous intracerebroventricular (i.c.v.) infusion of oxytocin (OT) on the release of OT and vasopressin (VP) following osmotic stimulation was studied in ovariectomized rats treated peripherally with gonadal steroids to simulate late gestation/lactation. Artificial cerebrospinal fluid (CSF) with or without OT (2 ng/microg) was infused (0.5 microl/h) i.c.v. continuously for 7 days along with sequential peripheral administration of progesterone (2 mg/kg i.m.) for 4 days, then 17-beta-estradiol (200 microg/kg i.m.) for 2 days. Following 7 days of OT infusion, isotonic (0.15 mol/l NaCl) or hypertonic (1.5 mol/l NaCl) saline was injected (15 ml/kg s.c.); the animals were decapitated 1 h later. Animals infused centrally with OT had higher basal levels of OT in plasma (p < 0.01 vs. CSF). While osmotic stimulation increased plasma levels of both OT and VP (0.15 mol/l NaCl < 1.5 mol/l NaCl; p < 0.01), only circulating VP was enhanced further (p < 0.01) in animals infused with OT compared with those receiving CSF. These changes in hormone levels could not be explained by differences in neural lobe stores of OT or VP or by alterations in daily water intake during the infusion period. Thus, chronic i.c.v. infusion of OT stimulates basal release of OT and increases the response of the VP system to osmotic stimulation.