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

Abstract

We have shown that administration of the local anaesthetic bupivacaine into the base of rats’ tails prevents the development of hyperalgesia induced by UV A-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 UV A-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, UV A-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 UV A-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 UV A-light exposure. We have shown that the Aδ- and C-fibres that encode for noxious punctate and blunt challenges were sensitized after UV A-light exposure, but C-fibres responsible for encoding noxious thermal challenges were not sensitized by UV A-light exposure.