A Functional Melanocortin System May Be Required for Chronic CNS-Mediated Antidiabetic and Cardiovascular Actions of Leptin

OBJECTIVE: We recently showed that leptin has powerful central nervous system (CNS)-mediated antidiabetic and cardiovascular actions. This study tested whether the CNS melanocortin system mediates these actions of leptin in diabetic rats. RESEARCH DESIGN AND METHODS: A cannula was placed in the lateral ventricle of Sprague-Dawley rats for intracerebroventricular infusions, and arterial and venous catheters were implanted to measure mean arterial pressure (MAP) and heart rate 24 h/day and for intravenous infusions. After recovery from surgery for 8 days, rats were injected with streptozotocin (STZ), and 5 days later, either saline or the melanocortin 3 and 4 receptor (MC3/4R) antagonist SHU-9119 (1 nmol/h) was infused intracerebroventricularly for 17 days. Seven days after starting the antagonist, leptin (0.62 μg/h) was added to the intracerebroventricular infusion for 10 days. Another group of diabetic rats was infused with the MC3/4R agonist MTII (10 ng/h i.c.v.) for 12 days, followed by 7 days at 50 ng/h. RESULTS: Induction of diabetes caused hyperphagia, hyperglycemia, and decreases in heart rate (−76 bpm) and MAP (−7 mmHg). Leptin restored appetite, blood glucose, heart rate, and MAP back to pre-diabetic values in vehicle-treated rats, whereas it had no effect in SHU-9119–treated rats. MTII infusions transiently reduced blood glucose and raised heart rate and MAP, which returned to diabetic values 5–7 days after starting the infusion. CONCLUSIONS: Although a functional melanocortin system is necessary for the CNS-mediated antidiabetic and cardiovascular actions of leptin, chronic MC3/4R activation is apparently not sufficient to mimic these actions of leptin that may involve interactions of multiple pathways.