THE DIPSOGENIC EFFECTS OF RAT RELAXIN - THE EFFECT OF PHOTOPERIOD ANDTHE POTENTIAL ROLE OF RELAXIN ON DRINKING IN PREGNANCY

Experiments were done to examine whether rat relaxin is dipsogenic and whether such dipsogenic effects of rat relaxin are related to time of injection during the light-dark cycle. Female rats were fitted with a chronic intra-cerebro-ventricular (icv) cannula. Rat relaxin (2.5, 5, 10, 25, 50, or 100 ng/2 mu l in 0.9% saline) was injected into the ri ght lateral ventricle at either morning (0800-1000 h), afternoon (1400 -1600 h), or night (2200-2400 h), and water consumption was measured. Relaxin caused a dose-dependent dipsogenesis at doses greater than or equal to 5 ng, but the sensitivity and magnitude of the response varie d with the photoperiod. Water consumption was smallest (3.5 +/- 0.7 ml at 50 ng) and least sensitive (minimal effective dose at 25 ng) in th e afternoon and maximal(17.7 +/- 2.3 ml at 50 ng) and most; sensitive (minimal effective dose 5 ng) at night. The latency from injection to drinking was 55.8 +/- 10.4 sec (mean +/- SEM) and did not vary signifi cantly with either the dose or time of day. A second set of experiment s was done to examine the effects of neutralizing the central actions of relaxin on drinking behavior in pregnancy. Pregnant rats were injec ted daily, through a chronically implanted icy cannula, with either a specific monoclonal antibody raised against rat relaxin from day 12 to day 22 of gestation or with saline as a control. Drinking and eating behavior and weight gain were monitored every 12 h during pregnancy. T here was a significant decrease in water consumed at night, but no eff ect on drinking during; the day in relaxin-neutralized rats. These ani mals also showed a decrease in weight gain during pregnancy compared w ith controls and gave birth to Lighter-weight litters. These data prov ide evidence that the dipsogenic response to exogenous rat relaxin in female rats varies with time of injection during the light-dark cycle and suggest that relaxin in the brain may have a role in nighttime dri nking behavior during the second half of pregnancy.