Thyroid stimulating and pigmentary effects of synthetic peptides related to alpha-MSH and ACTH

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 (alpha-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 alpha-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 alpha-MSH-producing neurons in the arcuate nucleus.

Relationship of melanocyte-stimulating hormone to photoreceptor damage

Three separate experiments involving intact, sham-operated and hypophysectomized albino rats, examined the relationship of alpha-MSH to the severity of photically-induced retinal damage. alpha-MSH was administered at 1430 hours and 0900 hours, when endogenous MSH levels are different, and continuously by osmotic minipumps. alpha-MSH was ineffective in enhancing or reducing photoreceptor damage under these experimental conditions. A highly significant reduction in outer nuclear layer and total retinal thicknesses occurred in intact and hypophysectomized rats exposed to continuous illumination as compared to those in cyclic photoperiod. Hypophysectomy provided a significant degree of protection to the retina from photic damage.