Long-term administration of MC4 receptor antagonist HS014 causes hyperphagia and obesity in rats

In this study we investigated the long term effects of a potent and selective melanocortin 4 (MC4) receptor antagonist (HS014) on food intake, body weight, body composition and blood glucose levels in adult rats. HS014 was injected i.c.v. either by twice-daily injections (2 x 1 nmol) for 6 days or administered by continuous infusion with osmotic minipumps (0.16 nmol/h) for 2 weeks. The results show a considerable increase in food intake and body weight after both of the treatments without any signs of tachyphylaxis. After 2 weeks of treatment with osmotic pumps, the HS014-treated rats (average weight 425g) had 20% higher body weight than the controls rats (average 360 g). When i.c.v. injections were terminated, the body weight of the twice-daily HS014-treated rats returned to the levels of control group, whereas the rats treated with continuous infusion of HS014 remained hyperphagic and still gained weight. Blood glucose levels in the rats treated with HS014 infusion were significantly increased. Analysis of body composition in HS014-infused rats indicated that body weight was increased due to fat deposits. These data show for the first time that chronic administration of exogenous MC4 receptor antagonist causes hyperphagia and severe obesity in rats. These data also indicate that the melanocortic control of food intake is very robust and suggest that changes induced by such treatment overcome negative feedback signals.