Estrogen increases hair pigmentation in female recessive yellow mice.

Murine recessive yellow (Mc1re) is a loss-of-function mutation in the receptor for a-melanocyte-stimulating hormone, melanocortin receptor 1 (Mc1r), and results in a yellow coat by inducing pheomelanin synthesis in hair follicular melanocytes. We previously showed that eumelanin and pheomelanin content in dorsal hair in female Mc1re/Mc1re mice 5 weeks after birth was greater than that in male mice. To better understand the sex difference in hair pigmentation, estrogen, progesterone, and androgen were injected subcutaneously to newborn mice, and the effects of these sex hormones on hair pigmentation at 8.5 days were investigated. Although these sex hormones failed to affect the ratio of pheomelanic to eumelanic hair, they increased total hair pigmentation. Chemical analyses showed that total melanin content was increased by a low dose of estrogen in female mice. Moreover, estrogen increased pheomelanin content in female hair, whereas the hormone greatly decreased both eumelanin and pheomelanin content in male hair. High doses of progesterone, in contrast, increased total melanin content in both female and male hair. Moreover, a high dose of androgen increased total melanin content exclusively in male hair. These results suggest that estrogen is the main factor in determining the higher content of eumelanin and pheomelanin in female hair of Mc1re/Mc1re mice