Brain insulin response to feeding in the rat is both macronutrient and area specific

Using microdialysis, we showed recently that hypothalamic immunoreactive insulin (IRI) levels increased after a meal of chow and decreased in response to a fat meal. In the present study, we have compared extracellular hypothalamic and extrahypothalamic basal IRI levels and investigated the effect of meals composed exclusively of either carbohydrates (85% starch, 15% sucrose) or casein on both plasma and medial hypothalamic (PVN-VMH) insulin. The response of IRI to a carbohydrate meal was also investigated in the cerebellum. Basal hypothalamic IRI was twofold higher in the hypothalamus as compared to the cerebellum (33 +/- 4 and 15 +/- 2 pg/mL, respectively). Hypothalamic IRI increased twofold in response to the carbohydrate meal (72 +/- 15 pg/mL) but remained unchanged during the casein meal. No IRI change was found in the cerebellum after a meal of carbohydrates (16 +/- 2 pg/mL). Insulinemia was increased by both the carbohydrate and the casein meal. However, the protein-induced increase was less pronounced (maximum + 359% compared to 1650% for carbohydrates). The present data show a dual specificity of brain insulin response to feeding; in addition to the macronutrient specific variations, a regional specificity was also observed. Taken together with previous observations, the present data are in favor of an involvement of PVN-VMH insulin in the control of feeding and macronutrient-specific appetites.