The dipsogenic effects of rat relaxin: The effect of photoperiod and the 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 (i.c.v.) cannula. Rat relaxin (2.5, 5, 10, 25, 50, or 100 ng/2 microl in 0.9% saline) was injected into the right 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 > or = 5 ng, but the sensitivity and magnitude of the response varied 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 the 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 significantly with either the dose or time of day. A second set of experiments was done to examine the effects of neutralizing the central actions of relaxin on drinking behavior in pregnancy. Pregnant rats were injected daily, through a chronically implanted i.c.v. 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. There was a significant decrease in water consumed at night, but no effect on drinking during the day in relaxin-neutralized rats. These animals also showed a decrease in weight gain during pregnancy compared with controls and gave birth to lighter-weight litters. These data provide 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 drinking behavior during the second half of pregnancy.