Phenobarbital effects on weight gain and circadian cycling of food intake and body temperature

Rats fed a diet supplemented with phenobarbital at a concentration of 0.25% gained less weight than rats fed the unsupplemented diet. The reduced weight gain in the phenobarbital-treated rats accompanied the induction of marked hepatomegaly. Circadian cycling of food consumption in control rats followed a biphasic pattern, with the first feeding episode occurring during the middle of the 12-h dark phase and the second occurring as the dark phase ended. In rats on phenobarbital, eating activity was confined to the first feeding episode, with the level of intake during this interval increasing to compensate for the absence of significant subsequent feeding, so that the daily levels of food consumption were similar in both groups. Measurements of circadian cycling of deep body temperature showed that ingestion of 0.25% dietary phenobarbital approximately doubled the amplitude of the temperature cycle and advanced the time at which peak temperature was attained by aproximately 2 h. It is suggested that the lower weight gain in rats chronically exposed to 0.25% dietary phenobarbital results primarily from alterations in hepatic metabolism, but phenobarbital-mediated changes in the circadian cycling of food intake and deep-body temperature may also contribute to the growth reduction.