GHRL and major depressive disorder: In addition to ghrelin's role in glucose and energy homeostasis, research over the last 15 years has revealed a surprising variety of additional physiological functions of ghrelin in areas as distinct as learning and memory [188–190], psychological stress, mood and anxiety [191,192], depression [26,193,194], thymopoiesis [195], sleep/wake rhythm [7–9,196,197], and aging [198,199].