Capsaicin-induced activation of fine afferent fibres from rat skin in vitro.

A preparation of the hindpaw-skin together with the saphenous nerve from the adult rat was maintained in vitro. This was used to characterize the properties of sensory receptors with slowly conducting nerve fibres (C- and A delta) and to evaluate the effects of capsaicin and the capsaicin antagonist, capsazepine. Mechano-heat sensitive C-fibres were the most sensitive to capsaicin (threshold < 0.3 microM) applied to the receptive field. Other types of C-fibres were less sensitive (mechano-cold sensitive fibres threshold 1 microM) or insensitive (high- and low-threshold mechano-sensitive fibres). Mechano-heat and mechano-cold sensitive A delta-receptors were also activated by capsaicin but high- and low-threshold mechano-sensitive A delta-fibres were insensitive to capsaicin (maximum concentration 3 microM). The capsaicin-induced activation of mechano-heat sensitive C-fibres was concentration dependent with an EC50 = 350 nM. Responses to capsaicin, administered at submaximal concentrations were highly reproducible when administrations were separated by 30 min. Administrations at greater frequency reduced responsiveness to capsaicin. This was accompanied by a slowing of conduction velocity or production of a conduction blockade which was reversible after a few minutes. The activation of mechano-heat sensitive C-fibres by capsaicin could be prevented by capsazepine, indicating the involvement of specific capsaicin receptor-sites. These data show that fine afferents in the rat hindpaw-skin retain receptive properties when maintained in vitro. These fibres exhibit differential sensitivity to capsaicin; mechano-heat sensitive C-fibres being the most sensitive. The activation of this class of fibre was mediated via a specific capsaicin-receptor.