Changes in respiratory quotient elicited in rats by a conditioned stimulus predicting food

The present study examined whether changes in energy expenditure and energy substrate utilization occur in rats exposed to a conditioned stimulus that signals food. In a differential conditioning procedure, rats were given conditioning sessions where one of two cues (either a flashing light or buzzer) predicted a carbohydrate-rich meal (CS+) while the other cue predicted no food (CS-). In two subsequent test sessions, indirect calorimetry was used to measure respiratory quotient, energy expenditure, and locomotor activity before, during, and after a 15-min CS+ or CS- presentation. The CS+ was found to significantly increase respiratory quotient, indicating a shift in the energy substrate being utilized toward carbohydrate. The CS+ also increased energy expenditure and locomotor activity, but these effects were more variable across rats. It is concluded that respiratory quotient may rise in anticipation of a carbohydrate-rich meal. Possible mechanisms underlying this effect are discussed.