THE EFFECTS OF CENTRALLY ADMINISTERED PORCINE RELAXIN ON DRINKING BEHAVIOR IN MALE AND FEMALE RATS

Of the reproductive hormones it has been suggested that relaxin may pl ay an important role in the increased sodium appetite of pregnancy. IC V injection of porcine relaxin caused water-replete male and female Wi star rats with access to water and 0.9% or 2.7% NaCl to drink on avera ge about 3 to 8 ml of water within 1 h of injection. By 24 h the cumul ative intake of water was no different from the control intake. The am ounts of water drunk were similar after doses of 50, 100, 250 and 500 ng of relaxin. A dose of 5 ng was ineffective. Male rats generally dra nk more water than female rats after ICV injection of angiotensin or r elaxin. Male SH rats which drink more water than male WKY rats in resp onse to ICV angiotensin also drank more after ICV relaxin. Intakes of 0.9% or 2.7% NaCl were unaffected for up to 24 h after injection of re laxin, whereas angiotensin-injected rats showed a significant increase in 0.9% NaCl 1 h after injection though this difference was no longer evident in the 24 h cumulative intake, Relaxin did not cause any incr ease in NaCl intake in SH rats. Insulin, which is similar in structure and molecular weight to relaxin, was without effect on drinking when doses comparable to dipsogerically effective doses of relaxin were inj ected ICV. In male Wistar rats treated with DOCA for 5-15 days, relaxi n retained its weak stimulatory action on water intake but did not aff ect NaCI intake despite the increased baseline NaCI intake during DOCA . These results indicate that relaxin is a dipsogen in the rat but tha t it seems to have little short-term effect on sodium appetite.