LEP and obesity due to melanocortin 4 receptor deficiency: Subsequent studies from several laboratories have shown pharmacokinetic impairment in the blood-to-brain transport in several models of obesity including obesity of maturity, diet-induced obesity, the obesity prone rat, and the Koletsky rat; the latter two being genetic models in which leptin receptors in brain are deficient or absent.11–16 The obesity prone rat of Levin are born with defects in CNS leptin receptors but normal transport of leptin across the BBB.11 As they develop obesity, they acquire impairments in leptin transport.