Trpv1 mediates spontaneous firing and heat sensitization of cutaneous primary afferents after plantar incision

TrpV1, the receptor for capsaicin, contributes to nociception in animals but appears to be much more important for signaling increased behavioral sensitivity in the injured state. The current study examined the relationship between the marked reduction in heat hyperalgesia after incision in TrpV1 knockout (KO) mice and the activity of the nociceptors in these same mice. Also, the role of TrpV1 in spontaneous activity (SA) of afferents after incision was examined. Standard teased-fiber techniques were used to record from glabrous skin afferents from incised and control TrpV1 KO and C57Bl6 mice. The loss of TrpV1 had minimal effect on the responses of mechano-heat-sensitive C-fiber afferents in the normal and incised states. However, a different group of heat sensitive afferents, termed unclassified afferents, was sensitized to heat by incision and had markedly reduced sensitization in the TrpV1 KO mice. These unclassified afferents also developed SA after incision, and generally had a lower threshold temperature compared to unclassified afferents without SA. The rate of SA was inversely correlated to the threshold temperature for heat; afferents that exhibited a higher rate of SA had a lower heat threshold. The proportion of unclassified afferents with SA was also reduced in incised TrpV1 KO mice compared to incised C57Bl6 mice. We conclude that a distinct class of afferents outside the mechano-heat-sensitive afferent population likely contributes to heat hypersensitivity after plantar incision. KO of TrpV1 influences SA in these unclassified afferents in incised skin. SA in these afferents is perhaps a manifestation of heat sensitization.