Table 2.

Melanocortin receptors (MCRs), their agonists, function, signaling system, and site of expression.

MCRs type	Distribution	Function	Agonists	Signaling system	References
MC1R		Pigmentation, regulation of skin physiology, anti-inflammation, pain	α-MSH, β-MSH, γ-MSH and its analogues (MT I, MT II)	cAMP-PKA pathway, Ca++ signaling	[1, 3, 4, 29–32]
MC2R	Adrenal cortex, murine adipocytes, skin, melanoma cells	Adrenal steroids secretion	ACTH	cAMP-PKA pathway	[1, 3, 4]
MC3R	CNS, stomach, kidneys, heart, gut, thymus, placenta	Feeding, energy homeostasis and anti-inflammation	ACTH and γ-MSH	cAMP-PKA, MAPK and IP/Ca++ signaling	[1, 3, 29, 33]
MC4R	CNS	Anti-inflammatory, control of feeding and sexual behaviors, energy homeostasis	α-MSH, ACTH	cAMP-PKA and MAPK	[1, 3, 31, 33]
MC5R	CNS and many peripheral tissues, exocrine glands, spleen, skin, lung, sexual organs, adipose tissues.	Exocrine secretion, lipolysis, regulation of body temperature	ACTH(1–24), α-MSH, MT I & MT II	cAMP-PKA and Jak/STAT phosphorylation pathway	[1, 3, 29, 33]

Mitogen activated protein kinase: (MAPK), Protein kinase A: (PKA), ionositol phosphate: (IP), and cyclic AMP: (cAMP).