SLC6A4 and obesity due to melanocortin 4 receptor deficiency: Sex-specific differences in intestinal 5-HT physiology exist in rodents, with higher EC cell density and increased jejunal 5-HT availability reported in female mice, and a male-specific impairment of SERT-mediated 5-HT reuptake in the small intestine in diet-induced obesity [21,44].