The effect of leptin is enhanced by macroinjection into the ventromedial hypothalamus

To determine whether changes in food intake produced by leptin involve targeting the hormone to distinct central nervous system regions, guide cannulas were positioned stereotaxically into three brain regions-the ventromedial hypothalamus (VMH) (bilaterally, n = 6), the dorsal raphe nucleus (n = 3), and the lateral ventricle (n = 3)-of nonobese male rats (400-500 g). Daily food intake and body weight changes were measured during twice-daily injections of saline (0.1 mu l) followed by recombinant human leptin (0.05 mu g) for 3 days via the brain cannulas, VMH-injected rats also were followed during a postleptin saline recovery interval, This small dose of leptin did not change food intake or body weight from that during the preceding saline injection period in ventricle-injected or dorsal raphe-injected rats, In sharp contrast, VMH-injected rats ate much less food (56 +/- 8% basal) and lost 9 +/- 3 g/day or 5% of their body weight during 3 days of leptin administration. VMH-injected animals fully recovered from leptin-induced effects within 3 days, We conclude that small doses of leptin that do not effect eating behavior when delivered to the ventricle or the dorsal raphe (another brain region believed to regulate feeding), suppress food intake when injected into the VMH, These data suggest that the VMH or a brain region in close proximity to it is a key target for the biological actions of leptin.