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Testosterone and UVL‐B stimulation of epidermal melanocytes in rat scrotal skin

The effects of UVL‐B and/or testosterone replacement therapy are compared in normal and castrated rats in order to determine whether testosterone is required for UVL‐B (280‐315 nm) stimulation of melanogenesis in the testosterone‐dependent epidermal melanocyte system of the scrotal skin of black Long Evans rats. Testosterone is not a prerequisite for UVL‐B stimulation of melanocytes as in both castrates and normal animals the melanocytes respond to UVL‐B by increases in size, length and number of dendrites (dendriticness), and tyrosinase activity (intensity of Dopa reaction). Addition of testosterone to castrates does enhance the effects of UVL‐B. However, UVL‐B with or without testosterone cannot maintain normal melanogenesis in rats irradiated immediately after castration nor can it restore normal melanogenesis following long term castration. Both the amount of UVL energy/exposure and the number of exposures are important variables in stimulation of the epidermal melanocytes. Administration of a dose of UVL‐B to castrates in a single exposure is ineffective, while the same overall dose spread over several exposures increases the size and dendriticness of melanocytes. Testosterone and UVL‐B act synergistically in affecting melanogenesis although neither singly nor in combination are they able to fully restore normal melanogenesis.