Effects of galanin-like peptide on food intake and the hypothalamo-pituitary-thyroid axis

Galanin-like peptide (GALP) is a novel hypothalamic peptide synthesised in neurons in the arcuate nucleus which project to the paraventricular nucleus (PVN). GALP has recently been identified as an orexigenic peptide. In this study we aimed to further characterise the hypothalamic action of this peptide in energy homeostasis. Firstly, we investigated the orexigenic effect of GALP in the PVN and compared its effects with galanin and galanin 2-29. Secondly, we examined the effect of PVN administration of GALP and galanin on circulating thyroid-stimulating hormone (TSH). PVN administration of GALP significantly increased the food intake of satiated rats 1 h after administration at doses of 0.3, 1 and 3 nmol. In comparison with paraventricular administration of galanin, GALP was a more potent orexigen, whereas galanin 2-29, the relatively selective GAL R2 agonist, had no effect on food intake. Both GALP and galanin administration (1 nmol) into the PVN significantly decreased the level of circulating TSH. To investigate the mechanism of these effects, we examined the effect of GALP and galanin application on neuropeptide release from hypothalamic explants in vitro. GALP peptide (100 nM) stimulated the release of the orexigenic peptide neuropeptide Y from hypothalamic explants and decreased the release of the anorectic peptide cocaine-and-amphetamine-regulated transcript, whereas galanin (100 nM) peptide had no significant effect on the release of either peptide. Both GALP (100 nM) and galanin (100 nM) inhibited the release thyrotrophin-releasing hormone. These data suggest that in the PVN, GALP may play a role in energy homeostasis by stimulating food intake and suppressing TSH release.