Exposure of the rat tail to ultraviolet A light produces sustained hyperalgesia to noxious thermal and mechanical challenges.

We aimed to establish whether exposing the tails of rats to ultraviolet A (UVA) light generated sustained hyperalgesia to noxious thermal and mechanical challenges. The tails of 21 rats underwent eight 40s exposures of UVA light, with 260s between each exposure. As a control procedure, during UVA-light exposure the tails of 11 of those rats were shielded with aluminium foil. Thermal hyperalgesia was assessed by immersing the rat tail in 49 degrees C water (modified tail flick test). Mechanical hyperalgesia was assessed by applying a bar algometer onto the tail and timing the escape response. Exposure to direct UVA light produced hyperalgesia for 8 days to the noxious thermal challenge (P<0.05, two-way ANOVA, Tukey post hoc tests) and at least 16 days to the noxious mechanical challenge (P<0.05, two-way ANOVA, Tukey post hoc tests). They gained mass throughout the study at the same rate as the control rats. The control rats did not develop thermal nor mechanical hyperalgesia. The tails of a further 20 rats were exposed similarly, and tail tissue examined histologically. Both exposed and control rats developed mild chronic inflammation unrelated to the hyperalgesia.