Fluid intake of rats in constant light and during feeding restricted to the light or dark portion of the illumination cycle

Rats restricted to eating and drinking during the light phase of a 12:12 light-dark (L-D) cycle (L-drinkers) consumed much smaller volumes of saccharin and NaCl than did rats whose feeding and drinking was restricted to the dark phase (D-drinkers). The patterns of fluid intake within the respective 12-hr feeding periods were markedly different for L- and D-drinkers and the food/fluid ratios were significantly lower for D- than for L-drinkers. It was concluded that illumination affects fluid intake independently of changes in food intake and that light limits the rat's capacity for fluid ingestion. L- and D-drinkers did not differ in their preference for saccharin over water in two bottle preference tests; this suggests that light does not interfere with the rat's ability to detect and determine the quality of the taste stimulus. Differences in ingestive behavior between L- and D-drinkers may in part reflect constraints imposed by changes in the sleep-wakefulness cycle. Intake of a wide concentration range of saccharin solutions was decreased in ad lib fed rats housed in constant light (L-L); differences in fluid intake between the L-L and L-D groups varied directly with the hedonic value of the saccharin solutions. Only 44% of rats housed in constant light as compared to 100% of rats maintained in a L-D cycle decreased their intake of a saccharin solution whose ingestion was followed by injection of lithium chloride. This finding is relevant to studies of illness-induced aversions and was related to circadian rhythms in drug susceptibility and their alteration in constant light. The necessity for controlling and specifying environmental illumination in studies of ingestive behavior was emphasized and various methodological considerations related to illumination cycles were discussed.