Studies using Wistar–Kyoto (WKY) rats, a genetic animal model of depression, have uncovered reduced prepro-orexin mRNA expression, a lower OX-A level in various brain areas, and an 18% reduction in orexin neuron numbers, along with a 15% reduction in soma size, compared with control Wistar rats [17,18]. Here, HCRT is linked to major depressive disorder.