Melanocortin System

MC1R: Skin Pigmentation and Immune Modulation

MC1R is expressed on melanocytes in the skin and hair follicles, and on immune cells including macrophages, dendritic cells, and neutrophils. In melanocytes, MC1R activation by alpha-MSH drives the Gs-cAMP-PKA pathway, activating MITF, which transcriptionally upregulates tyrosinase, TYRP1, and DCT -- the enzymes that convert tyrosine to melanin. UV-induced DNA damage in keratinocytes triggers local POMC processing to alpha-MSH, which acts in a paracrine fashion on adjacent melanocytes, explaining the UV-tanning response. The hair follicle determines coat colour in rodents through the relative activation of MC1R by alpha-MSH (produces eumelanin, black/brown pigment) versus competition by agouti (shifts to phaeomelanin, yellow/red pigment). On immune cells, MC1R activation suppresses NF-kB-driven pro-inflammatory cytokine production, a pathway contributing to the anti-inflammatory properties observed with Alpha-MSH.

MC2R: Adrenocortical Steroidogenesis

MC2R (the ACTH receptor) is expressed almost exclusively in the adrenal cortex. It is the receptor for ACTH (adrenocorticotropic hormone), not for shorter melanocortin peptides like alpha-MSH, which do not have high affinity for MC2R. ACTH binding drives cAMP-PKA signalling in adrenal cortex cells, activating the steroidogenic acute regulatory protein (StAR), which enables cholesterol transport into mitochondria, the rate-limiting step of cortisol biosynthesis. MC2R is therefore not a primary target of the synthetic research melanocortin peptides (PT-141, MT-II, Alpha-MSH) at typical research doses.

MC3R: Energy Homeostasis and Peripheral Inflammation

MC3R is expressed in the hypothalamus (particularly the arcuate nucleus), the limbic system, the brainstem, and in peripheral immune cells. Its hypothalamic role involves energy homeostasis: MC3R-deficient mice develop obesity with increased fat mass and reduced lean mass, indicating that MC3R normally contributes to appetite suppression and energy partitioning. MC3R is also expressed on NPY/AgRP neurons themselves, where its activation produces autoinhibitory feedback, dampening the AgRP-driven hunger signal. Synthetic melanocortin agonists with MC3R activity (including Melanotan II and Alpha-MSH) may contribute to their appetite-suppressing effects through this receptor, distinct from MC4R-mediated mechanisms.

MC4R: Sexual Arousal, Appetite Suppression, and Energy Expenditure

MC4R is expressed broadly in the central nervous system, particularly in the hypothalamic paraventricular nucleus (PVN), dorsomedial nucleus, and brainstem nuclei. It is the receptor that mediates appetite suppression and increased energy expenditure in response to alpha-MSH and is the reason that MC4R loss-of-function mutations cause the most common single-gene form of obesity in humans. Separately, MC4R activation in the medial preoptic area and limbic circuits triggers downstream oxytocin and dopaminergic signalling, generating the central arousal and desire response that is the basis of PT-141's (Bremelanotide's) FDA-approved mechanism. This central mechanism is mechanistically distinct from PDE5 inhibitors, which act peripherally on vascular smooth muscle. MC4R agonism produces appetite suppression and the sexual arousal effect through the same receptor, which explains why higher doses of melanocortin agonists produce dose-dependent anorexia alongside sexual arousal.

MC5R: Exocrine Gland Regulation

MC5R is expressed predominantly in exocrine glands (lacrimal, salivary, preputial, Harderian, sebaceous) and in skeletal muscle and immune cells. Its physiological role is less well characterised than MC1R-MC4R; MC5R-deficient mice exhibit reduced exocrine secretions and altered pheromone production. MC5R is not a primary target for the research compounds currently profiled on WikiPeptide; its activation at pharmacological doses of non-selective agonists like Melanotan II may contribute to some peripheral effects but is not the basis for their primary research applications.