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ABSTRACT

Tyrosinase mRNA, synthesis and activity were measured in the skin during the first 2 weeks of life of C₃H-HeA^vy mice. Tyrosinase mRNA levels were found to peak on days 3–4 and were followed by increases in tyrosinase synthesis and activity which peaked on days 6–7 and 7–8 respectively. These changes in tyrosinase expression were presumably associated with the growth of the first coat of hair that in neonatal C₃H-HeA^vy mice is yellow in colour as a result of the increased proportion of phaeomelanin. By the time hair growth had ceased there was no expression of tyrosinase at both mRNA and protein levels. Daily administration of α-melanocyte stimulating hormone (α-MSH) enhanced the expression of tyrosinase mRNA transcripts, tyrosinase synthesis and activity. The increase in tyrosinase activity paralleled the change in the amount of tyrosinase, suggesting that the primary action of α-MSH is to stimulate new synthesis of the enzyme. This induction of tyrosinase was associated with the growth of hair that was darker in colour than that of the controls and contained an increased proportion of eumelanin. This increase in eumelanin reflected a decrease in phaeomelanin content. It was concluded that, through its actions on the enzyme tyrosinase, α-MSH is able to switch the synthesis of phaeomelanin to that of eumelanin in hair follicular melanocytes.

Journal of Endocrinology (1993) 137, 189–195

ABSTRACT

Skin tyrosinase levels and the eumelanin and phaeomelanin contents of the hair were measured in pubertal and adult C₃H–HeA*^vy mice that grow dark and golden yellow hair respectively. Hair growth was initiated by plucking and the skin tyrosinase levels, which increased during the growth of new hair and peaked at around 9 days after plucking, were higher during the growth of dark hair in the pubertal mice than during the growth of yellow hair in the adult mice. Although there was only a twofold difference in the phaeomelanin contents of these two types of hair, the dark hair of the pubertal mice contained over 20 times more eumelanin than the golden-yellow hair of the adult mice. These results suggest that the changes in coat colour in C₃H–HeA*^vy mice are due mainly to changes in eumelanin synthesis by the hair follicular melanocytes and that the production of this pigment requires higher levels of the enzyme tyrosinase than does the production of phaeomelanin. These changes did not appear to be related to plasma α-MSH levels. Nevertheless, administration of α-MSH increased skin tyrosinase activity in the pubertal mice that were growing dark hair and produced a twofold increase in the eumelanin content of the hair. However, it had no such effects in adult mice and also failed to affect the phaeomelanin content of the hair in both groups of mice. In contrast to α-MSH, bromocriptine decreased skin tyrosinase levels and the eumelanin content and increased the phaeomelanin content of the hair in pubertal mice. These effects of bromocriptine were unrelated to plasma immunoreactive α-MSH levels and were not restored when α-MSH was administered together with the dopamine agonist.

Although the present results support the idea that α-MSH increases coat darkening in the C₃H–HeA*^vy mouse through its actions on tyrosinase activity and eumelanin synthesis, it seems that these actions are more dependent on changes at the melanocyte level than changes in circulating α-MSH. The present results further suggest that dopaminergic mechanisms may also play a direct regulatory role in the control of coat colour in this mouse.

J. Endocr. (1986) 109, 15–21