The Monodelphis melanoma model: Initial report on large ultraviolet A exposures of suckling young

The objective of this study was to determine whether exposure of early suck ling young of the opossum Monodelphis domestica to ultraviolet A (UVA) radi ation (320-400 nm) can lead to the development of melanocytic lesions simil ar to those induced after exposure to ultraviolet B (WB) radiation (280-320 nm) to total doses as low as 380 J/m(2). A total of 576 sucklings received nine exposures of 0.6, 2.6 or 15.5 kJ/m(2) per dose (total doses similar t o 6, 23 and 140 kJ/m(2), respectively) from a Blak Ray lamp source with a n arrow range emission at 365 nm. A further 280 sucklings were exposed in the same way to doses of 2.6 kJ/m(2) per dose (total similar to 23 kJ/m(2)) br oad-band UVA with visible wavelengths from a Dermalight lamp. Frequency of litter loss following all of the UVA-exposure protocols was similar to that within the same stocks in the colony at large. Only one of the 856 UVA-exp osed individuals possessed a melanocytic lesion at the 5 month assessment p oint. No radiation-induced lesions of any type were evident on the skin of the other animals exposed as sucklings. The affected male was from a group of 70 individuals exposed to the highest total dose (140 kJ/m(2)) from the Blak Ray light source. The melanocytic hyperplasia was provisionally identi fied as a potential melanoma but it slowly regressed as the animal aged. We conclude that in the opossum suckling exposure system, the potency of UVA for melanoma induction is extremely low compared with that of UVB. Possible explanations, amenable to further investigations, are given for the low UV A sensitivity of the suckling model compared to the adult exposure model of Ley.