LEP and obesity due to melanocortin 4 receptor deficiency: However, studies with larger size samples reported a negative correlation between plasma leptin levels and major depression, hence supporting the idea that decreased leptin signaling may be a shared biological alteration in both obesity and depression (Kraus et al., 2001; Atmaca et al., 2002; Westling et al., 2004; Jow et al., 2006; Lawson et al., 2012).