Effects of leptin and cholecystokinin in rats with a null mutation of the leptin receptor Lepr(fak)

The Koletsky ('corpulent',) obese rat is homozygous for an autosomal recessive mutation of the leptin receptor (Lepr) that results in hyperphagia, obesity, and hyperlipidemia. Unlike the Lepr mutation that characterizes the fatty Zucker rat (Lepr(fa)) the Koletsky mutation (Lepr(fak)) is null. Because the Lepr(fak) mutation is null, exogenous leptin should have no effect on body weight or food intake in fa(k)/fa(k) rats. We confirmed that prediction: murine leptin, administered into the third ventricle for 5 consecutive days, did not affect daily food intake or body weight in fa(k)/fa(k) rats but produced dose-related inhibitions of food intake and body weight in +/+ and +/fa(k) rats. Although fa(k)/fa(k) rats did not respond to leptin, their response to CCK-8 (4 μg/kg ip) injected before 30- min test meals of 10% sucrose was not different from that of +/+ or +/fa(k) rats. These results demonstrate that the fa(k)/fa(k) rat is a good model in which to analyze the controls of food intake, energy expenditure, and energy storage in the absence of leptin effects.