LEP and obesity due to melanocortin 4 receptor deficiency: As expected, obesity in fa/fa rats resulting from impaired leptin receptor signaling was manifested by hyperleptinemia; thus, a significant age and fa/fa genotype interaction exists (F(1,20) = 12.36; p < 0.01), and a subsequent Bonferroni post-hoc test revealed an increase in the plasma leptin levels in young fa/fa rats compared with young controls (p < 0.01); this increase was more pronounced in old fa/fa rats compared with old controls (p < 0.001) (Table 1).