Insulin and glucose-induced changes in feeding and medial hypothalamic monoamines revealed by microdialysis in rats

Microdialysis from the ventromedian (VMH) and paraventricular (PVN) regions and simultaneous infusion of insulin alone (orexigenic) or with glucose (anorexigenic) was performed in the free-feeding rat. Intravenous insulin infusion (1 IU in 1 ml over 1 h) resulted in the expected glucoprivic feeding and a decrease in dopamine (DA) and serotonin (5-hydroxytryptamine [5-HT]) with an increase in their respective metabolites, dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindolacetic acid (5-HIAA). These data are quite different from those observed in spontaneous ad lib feeding: increase in DA, 5-HT and 5-HIAA and decrease in DOPAC. These last changes were not superimposed on insulin effects when insulin-induced meals occurred. When food was not available, insulin infusion induced the same changes except an increase in DA levels which could probably be ascribed to stress. When insulin was infused together with a sufficient amount of glucose to prevent hypoglycemia, we observed the usual anorexigenic effect. Although this treatment had an effect on induced-feeding opposite to that following insulin alone, the changes in hypothalamic monoamines were similar. The only consistent overall change is finally the rise in 5-HIAA found in both spontaneous and induced feeding conditions. These data suggest that monoaminergic changes in the VMH and PVN are not directly implicated in the control of feeding but they rather reflect metabolic events that accompany this behavior.