Role of melanocortin signaling in the regulation of the hypothalamic–pituitary–thyroid (HPT) axis

The melanocortin signaling system is orchestrated by two, independent groups of neurons in the hypothalamic arcuate nucleus with opposing functions that synthesize either alpha-melanocyte stimulating hormone (α-MSH) or agouti-related protein (AGRP). These neurons exert regulatory control over hypophysiotropic TRH neurons in the hypothalamic paraventricular nucleus (PVN) at least in part through direct, overlapping, monosynaptic projections to the PVN. Alpha-MSH has an activating effect on hypophysiotropic TRH neurons via the phosphorylation of CREB, and when administered exogenously, can completely reverse fasting-induced suppression of TRH mRNA in the PVN. AGRP has a potent inhibitory effect on the hypothalamic–pituitary–thyroid axis in normally fed animals, mediated through actions at melanocortin 4 receptors. Inhibition of the HPT axis by fasting may be explained by inhibition of melanocortin signaling as a result of a reduction in α-MSH and increase in AGRP. Neuropeptide Y may also modulate the effects of the melanocortin signaling system during fasting by potentiating the inhibitory actions of AGRP on hypophysiotropic TRH neurons to prevent the phosphorylation of CREB and through direct inhibitory effects on α-MSH-producing neurons in the arcuate nucleus.