Diurnal variations in the feeding responses to norepinephrine, neuropeptide Y and galanin in the PVN.

The feeding responses elicited by injection of norepinephrine (NE), neuropeptide Y (NPY) and galanin (GAL) into the paraventricular nucleus (PVN) were studied at two different times of the dark (active) cycle in male Sprague-Dawley rats maintained ad lib on pure nutrient diets. The feeding response elicited by NE in the PVN, characterized by a potent and selective stimulatory effect on ingestion of the carbohydrate diet, was significantly stronger during the early dark period (+11.7 kcal over vehicle baseline) relative to the late dark period (+7.6 kcal). A similar pattern of effects was observed with NPY in the PVN, which also selectively potentiated carbohydrate ingestion. The effects of GAL were different from those observed with NE and NPY. Whereas the total amount of food consumed after PVN GAL injection was similar in the early and late dark periods, the macronutrient selection patterns exhibited at these two times were different. During the early dark period, PVN GAL had a small stimulatory effect on carbohydrate, in addition to a strong enhancement of fat intake; in the late dark period, in contrast, GAL stimulated intake only of the fat diet. These findings may reflect differential functions of these hypothalamic neurotransmitters in controlling nutrient ingestion at different periods of the circadian cycle.