Drinking after osmotic challenge depends on circadian phase in rats with free-running rhythms.

Rhythmic phase-dependent modulation of the amount of water intake of rats occurs after several types of dipsogenic treatments. Previous studies conducted on rats in light-dark cycles confound direct effects of the light-dark cycle with the effects of circadian rhythmicity entrained to the light-dark cycle. The present study isolates the effect of circadian phase on the amount of drinking induced by injection of hypertonic saline. Free-running rhythms were induced in rats, and then water intake was measured after injections of both hypertonic and isotonic saline at different phases of the rats' circadian cycle. After injections of hypertonic saline, rats drank more water in their active phase than in their inactive phase. No difference in the amount of intake was found in rats that received isotonic saline at different phases of their circadian rhythms. The results indicate a circadian modulation of the drinking induced by cellular dehydration.