Role of adrenergic activity in pressor responses to chronic melanocortin receptor activation

Acute studies have shown that MC3/4-R stimulation increases sympathetic activity, but the role of adrenergic activation in mediating the cardiovascular and renal responses to chronic melanocortin 3- and 4-receptor (MC3/4-R) activation is unknown. The present study tested whether chronic MC3/4-R activation raises blood pressure and whether these changes are attenuated by alpha1+beta-adrenergic blockade. Rats were instrumented with an intracerebroventricular (ICV) cannula and arterial and venous catheters for measurements of mean arterial pressure (MAP) and heart rate (HR) 24 hours per day, and intravenous infusions. After control measurements, rats were intravenously infused with either saline vehicle (n=7) or alpha1+ beta-adrenergic antagonists (n=6, terazosin+propranolol, 10 mg/kg per day each) for 21 days. Five days after starting the vehicle or adrenergic blockade, the MC3/4-R agonist, MTII (10 ng/h), was infused ICV for 11 days followed by a 5-day recovery period. Another group of rats was infused with the adrenergic antagonists for 21 days but received the saline vehicle ICV for 11 days (n=7). MC3/4-R activation decreased food intake from 21+/-1 to 8+/-2 g/d by day 3 of MC3/4-R activation, and increased MAP and HR by an average of 8+/-2 mm Hg and 9+/-5 bpm, respectively. Adrenergic blockade did not alter the MC3/4-R-mediated decrease in food intake but abolished the increases in MAP and HR (1+/-2 mm Hg and -12+/-5 bpm, respectively, compared with control). ICV vehicle infusion during adrenergic blockade did not alter food intake or MAP. Glomerular filtration rate was unchanged in both the vehicle-infused and adrenergic blocked rats during MC3/4-R activation. These results indicate that the chronic actions of MC3/4-R activation on MAP and HR are mediated by adrenergic activation.