Feeding-related immunoreactive insulin changes in the PVN-VMH revealed by microdialysis

The presence of insulin in the brain and its anorectic effect when centrally infused are well-established today. The question of physiological and dynamic changes in brain insulin in relation to meals is still unanswered and addressed here. Immunoreactive insulin (IRI) was measured using a sensitized RIA in 30-min microdialysates from VMH and PVN nuclei during and after a scheduled meal in male Wistar rats. We indeed observed elevations in hypothalamic IRI during the first 30 min of 1-h meals with a progressive return towards premeal levels in spite of a robust satiety. When the rats were accustomed to the scheduled meals, an anticipatory rise in IRI was found in the hypothalamus, but not in the plasma, during the 30 min preceding the due time of the meal whether the meal was presented or not. This anticipatory rise was proportional to the number of repeated scheduled meals. These results first suggest that hypothalamic IRI changes reflect in some instances those in the plasma although there are exceptions that cannot be accounted for by a simple plasma-brain tissue delivery. Besides, hypothalamic IRI can hardly be proposed as a satiety signal. The present data suggest a role in satiation rather than in satiety or, perhaps, in the inhibition of the behavioral response of feeding that can include the anticipatory rise.