Differences in serum leptin concentration (a), body weight (b) and leptin concentration/body weight (c) between old (20 months) male and female Hcrt KO and WT mice. Each bar represents the mean value + SEM. a = Significant difference between female WT (+/+) and female KO (−/−) mice, b = significant difference between female and male KO mice, c = significant difference between female and male WT mice, at the level of p < 0.05*, p < 0.01**.