Differential analysis of experimental hypermelanosis induced by UVB, PUVA, and allergic contact dermatitis using a brownish guinea pig model

In moderately colored guinea-pig skin, UVB, PUVA, and allergic contact dermatitis were shown to induce hyperpigmentation that resembled the pigmentary changes observed in mongoloid human skin. Using this model, we examined the effects of chemical agents, including tyrosinase inhibitors and sunscreen agents, on the color changes induced by UV irradiation. The daily exposure of brownish guinea-pig skin to UVB irradiation at a variety of energies for 3 successive days induced clearly visible black pigmentation on the irradiated rectangular areas of the flank within a few days of irradiation, the maximum being reached about 1 week after irradiation, i.e., similar to the changes that occur in pigmented human skin. Split epidermal sheets prepared from untreated pigmented guinea pigs exhibited 200-400 melanocytes/mm2; 1 week after UV irradiation, the applied areas show an increased number of strongly dopa-positive melanocytes with stout dendrites (800-1,000 cells/mm2). UVA irradiation following an intraperitoneal injection of 8-methoxypsoralen (8-MOP) also produced black pigmentation 1 week after irradiation, and this was paralleled by a marked increase in the number of dopa-positive melanocytes in dopa-reacted split epidermal sheets. Allergic contact dermatitis produced by the application of 1-phenylazo-2-naphthol induced hyperpigmentation after an interval of about 14 days in 10 of the 21 allergy-acquiring animals examined. This induced pigmentation was accompanied by an increase in the number of dopa-positive melanocytes as compared to the number seen in controls. In contrast, allergic contact dermatitis produced by the application of dinitrochlorobenzene failed to induce such a high ratio of postpigmentation, with only 3 of the 21 allergy-acquiring animals showing hyperpigmentation and 5 showing depigmentation; in the latter, there was a slight decrease in the number of dopa-positive melanocytes. To study the preventive effect of tyrosine inhibitors on UVB-induced pigmentation, daily topical applications of these compounds were performed after three daily UVB irradiations. Treatment with 10% hydroquinone for 10 days interrupted UVB-induced pigmentation and resulted in a marked reduction in the number of epidermal melanocytes as compared to the number found in UVB-irradiated, untreated control skin.