Melanocortin receptor agonist transiently increases oxygen consumption in rats.

Acute injections of melanocortin (MC) agonist and antagonist are highly effective in reducing or increasing food take, respectively. Much less is known about how injection of MC receptor active substances affects metabolism, in particular during long term administration. Here we investigated the effect of 8 days continuous i.c.v. infusion of either MC receptor agonist MTII or the selective MC4 receptor antagonist HS024 on oxygen consumption, food intake and body weight in rats. We observed significant increase in oxygen consumption 2 days after the start of the MTII infusion. However, this increase had disappeared by day 4 of the study. No difference was observed in the oxygen consumption after injection of HS024. MTII substantially decreased the food intake during the first days, but then the feeding recovered and the body weight stabilised at a new level. The immediate effect of the MC receptor agonist on both food intake and metabolism was thus transient, even though the weight loss was maintained. The HS024 treated rats were hyperphagic throughout the test period, continuously gaining weight, resulting in increased fat pads and high leptin levels. This is the first study that describes long term effects of MC receptor agonist and antagonist on metabolism and energy balance.