Impact of a galanin antagonist on exogenous galanin and natural patterns of fat ingestion.

The peptide galanin (GAL) has a potent stimulatory effect on fat ingestion after administration into the hypothalamic paraventricular nucleus (PVN). This study examined a newly synthesized GAL antagonist, M40, in two separate experiments involving: (1) PVN injections of M40 alone in freely feeding animals, to investigate the importance of endogenous GAL receptor activity in determining natural patterns of fat ingestion, and (2) PVN injections of M40 in combination with exogenous GAL, to determine whether endogenous GAL receptors mediate this peptide-induced response. The results demonstrate that PVN injection of M40 by itself dose-dependently (2-108 pmol) reduces spontaneous ingestion of the fat diet. This phenomenon is robust, behaviorally specific and opposite to that induced by GAL itself. Moreover, the stimulatory effect of PVN-injected GAL on fat ingestion can be blocked by prior PVN administration of M40 at relatively low doses (2-6 pmol), indicating that M40 is a potent antagonist of GAL receptors in the hypothalamus. Together, these results provide the first evidence for the existence of endogenous GAL receptors in mediating the action of exogenous GAL in the hypothalamus. They also constitute a crucial step in demonstrating a physiological function of these PVN GAL receptors in controlling natural patterns of fat ingestion.