Light-dark rhythms and drinking behavior in the rat

Male Wistar rats were divided into two groups maintained either on a normal lighting schedule with the 12-hr light period from 7 am to 7 pm, or on a reversed lighting schedule. The intake of water in response to ip injections of hypertonic saline or renin, given at 9 am, was found to be approximately equal for both groups, while drinking of a palatable 0.01 M saccharin solution was significantly greater for the group in the dark. The results suggest that the sensitivity of the primary thirst mechanisms does not vary during a 24-hr day, whereas those controlling secondary drinking are more responsive during the dark period. The relation of these observations to the circadian pattern of water intake in the rat is discussed.