Melanocortin 1 Receptor - Function - in Mammals

In Mammals

The MC1R protein lies within the cell membrane, and is signalled by melanocyte-stimulating hormone (MSH) released by the pituitary gland. When activated by one of the variants of MSH, typically α-MSH, MC1R initiates a complex signaling cascade that leads to the production of the brown or black pigment eumelanin. In contrast, the receptor can also be antagonized by agouti signalling peptide (ASIP), which reverts the cell back to producing the yellow or red phaeomelanin.

The pulsatile nature of ASIP signalling through MC1R produces the characteristic yellow and black agouti banding pattern observed on most mammalian hair. In some species ASIP signaling is not of a pulsative nature, but is limited to certain regions. This is especially conspicuous in horses, where a bay horse has black legs, mane and tail, but a reddish body. A notable exception to this is human hair, which is neither banded nor particoloured, and thus is thought to be regulated by α-MSH signaling through MC1R exclusively.

In the United States, approximately 25 percent of the population are carrying the mutated Melanocortin 1 Receptor that causes red hair. The chance of two people having a child with red hair is 1-3 in every hundred (approximately 1 in 64).

In mutant yellow-orange mice and human redheads, both with non-functional MC1R, show that both genotypes display reduced sensitivity to noxious stimuli and increased analgesic responsiveness to morphine-metabolite analgetics. These observations suggests a role for mammalian MC1R outside the pigment cell, though the exact mechanism through which the protein can modulate pain sensation is not known. People with freckles and no red hair have an 85 percent chance of carrying the defective MC1R gene that is connected to red hair. People with no freckles and no red hair have an 18 percent chance of carrying the defective MC1R gene linked to red hair.