Behavioural effects of LTP-inducing sciatic nerve stimulation in the rat

A short-lasting tetanic sciatic nerve stimulation that previously has been shown to be nociceptive only during the stimulation, induces long-term potentiation (LTP) of nociceptive evoked responses in wide dynamic range neurons in the dorsal horn of rats. The LTP may contribute to the process of central sensitization. We have shown that the tetanic conditioning stimulation with muscular contractions induces LTP of both Aβ- and C-fibre evoked responses. However, the same stimulation during muscular paralysis induces LTP only of C-fibre evoked responses. In the present study, we investigated the effects of this conditioning stimulation with or without muscular paralysis in behavioural tests in rats. Conditioning stimulation with muscular contractions caused a significant reduction of weight borne on the stimulated side, suggesting muscular soreness and peripheral sensitization. Conditioning stimulation during neuromuscular paralysis, which only has given LTP of C-fibre evoked responses in intact animals, caused no change in the weight borne on the stimulated side, suggesting less or even absence of allodynia. However, in these animals the response temperature in the hot plate test was increased both on the stimulated and on the contralateral side compared to sham-operated rats. In view of our recent results indicating that a descending inhibition reduces the expression of LTP in dorsal horn cells, and the suggestion by others that long-term descending inhibition may override a segmental facilitation, it is suggested that an increased long-lasting endogenous nociceptive inhibition is induced after LTP-inducing stimulation. This is an interesting parallel to stimulation-induced analgesia in humans. C 1999 European Federation of Chapters of the International Association for the Study of Pain