Involvement of nitric oxide in UVB-induced pigmentation in guinea pig skin

Ultraviolet (UV) B irradiation evokes erythema and delayed pigmentation in skin, where a variety of toxic and modulating events are known to be involved. Nitric oxide (NO) is generated from L-arginine by NO synthases (NOS). Production of NO is enhanced in response to UVB-stimulation and has an important role in the development of erythema. NO has recently been demonstrated as a melanogen which stimulates melanocytes in vitro, however, no known in vivo data has been reported to support this finding. In this study, we investigated the contribution of NO with UV-induced pigmentation in an animal model using an NOS inhibitor. UVB-induced erythema in guinea pig skin was reduced when an NOS inhibitor, L-NAME (N-nitro-L-arginine methylester hydrochloride), was topically applied to the skin daily, beginning 3 days before UVB-irradiation. Delayed pigmentation and an increased number of DOPA-positive melanocytes in the skin were markedly suppressed by sequential daily treatment with L-NAME. Furthermore, melanin content 13 days after UVB-irradiation was significantly lower in skin treated with L-NAME than in the controls. In contrast, D-NAME (N-nitro-D-arginine methylester hydrochloride), an ineffective isomer of L-NAME, demonstrated no effect on these UV-induced skin responses. These results suggest that NO production may contribute to the regulation of UVB-induced pigmentation.