Blockade of endogenous GRF at dark onset selectively suppresses protein intake.

This study examined whether endogenous central GRF activity contributes to the increase in macronutrient intake shown by rats at dark onset. Animals were habituated to two diets: carbohydrate-fat and protein-fat. Antiserum raised against GRF (aGRF; 1% and 10% solutions) was microinjected into the suprachiasmatic nucleus/medial preoptic area (SCN/MPOA) at dark onset, and macronutrient intake was determined at 1, 2, and 4 h postinjection. aGRF blocked the increase in protein intake normally seen at dark onset, but had no effect on carbohydrate intake. These findings suggest that endogenous GRF activity in the SCN/MPOA region of the brain contributes to the circadian and nutritional organization of food intake.