Discharge patterns of nociceptive primary afferent fibres in the rat coccygeal nerve after UVA-light exposure

We have shown that administration of the local anaesthetic bupivacaine into the base of rats' tails prevents the development of hyperalgesia induced by UVA-light to a noxious thermal challenge, but not to noxious mechanical challenges. The aim of this study was to determine whether exposure of rats' tails to UVA-light altered the responses of Aδ- and C-fibres innervating the tail to noxious mechanical and noxious thermal challenges. We dissected the right ventral coccygeal nerve in Sprague-Dawley rats that had their tails exposed to, or shielded from, UVA-light 24-h earlier. Single afferent Aδ- and C-fibres receptive fields were located and the response of a fibre to noxious blunt (3.9 N bar algometer) and punctate (rat toothed forceps) mechanical challenges, as well as a noxious thermal (49 °C) challenge was measured. In rats exposed to UVA-light, the peak firing rates and areas under the curve of post-challenge histograms of Aδ- and C-fibres evoked neural activity were increased when noxious blunt and punctate challenges were applied to the rats' tails. However, the peak firing rate and areas under the curve of post-challenge histograms of C-fibre evoked neural activity, which were responsive to noxious thermal challenges, were not increased after UVA-light exposure. We have shown that the Aδ- and C-fibres that encode for noxious punctate and blunt challenges were sensitized after UVA-light exposure, but C-fibres responsible for encoding noxious thermal challenges were not sensitized by UVA-light exposure.