RESUMO
OBJECTIVE: Hypothalamic neuropeptide CART (cocaine-amphetamine-regulated transcript) is a leptin-dependent endogenous satiety factor in the rat, and single central injections of recombinant CART(42-89) lowers food intake in rats and mice. To assess the potential role of CART as a long-term regulator of food intake, we investigated the effects of continuous infusion of recombinant CART(42-89) on food consumption and body weight. RESEARCH METHODS AND PROCEDURES: Two doses of CART(42-89) were tested: 12 or 4.8 microg/d. Adult male, both lean (+/?) and Zucker (fa/fa) obese, rats were equipped with intracerebroventricular cannulae in the right lateral ventricle. The cannulae were connected to subcutaneously placed osmotic mini-pumps. Pumps were filled with either CART(42-89) or vehicle (50 mM phosphate-buffered saline, pH 7.4). The pumps delivered a continuous infusion of CART(42-89) or vehicle, and food intake and body weight were followed for 10 days (12 microg/d) or 7 days (4.8 microg/d). Animals given the low dose had the pump removed on Day 7, and from half of the group, trunk blood was collected after decapitation, whereas the other half of the group had their mini-pumps removed and were followed for another 7 days before being decapitated. RESULTS: Animals receiving the high doses displayed overt motor disturbances, whereas the low dose was devoid of such behavioral side effects. Both doses significantly lowered food intake with maximal effect on days 3 to 5 of the infusion period. The high dose of CART decreased body weight of normal animals to 85% of initial weight at days 3 to 5, whereas the weight of Zucker (fa/fa) obese rats dropped to 95% of the initial weight. In animals receiving 4.8 microg/d, moderate effects on body weight were seen between days 4 and 6 of the treatment period, but soon after termination of the treatment animals regained lost weight. To assess the biological activity of the contents of the osmotic mini-pumps, the pumps were removed from the subcutaneous implantation site, and 5 microL of their contents were injected intracerebroventricularly to naive animals kept on a restricted feeding schedule. The content of pumps from animals receiving 4.8 microg/d of CART(42-89) potently inhibited food intake, confirming full biological activity despite being kept for 7 days at body temperature. DISCUSSION: Due to obvious effects on motor behavior, it is impossible with certainty to conclude that the observed effects on feeding and body weight are primary interference with satiety centers or secondary to effects on locomotor pathways. Also, the present experiments suggest that hypothalamic appetite-regulating neurons are subject to pharmacological desensitization upon prolonged exposure to CART peptide. The underlying mechanism of such desensitization is as yet unknown.