Different increase in C-fibre evoked responses after nociceptive conditioning stimulation in sham-operated and neuropathic rats

Mechanisms of neuropathic pain are not clear. Recently we showed long-term potentiation (LTP) of wide dynamic range (WDR) neurones after electrical co nditioning stimulation of the sciatic nerve in normal rats. In this study w e investigated the effects of the same conditioning on both the evoked resp onses of WDR cells and on vital signs in neuropathic rats. Nerve injury was produced by tight ligation of the L5 and L6 spinal nerves and a control gr oup received sham surgery. The electrical conditioning gave a significantly smaller LTP in neuropathic rats as compared to controls. Despite giving a smaller LTP in neuropathic rats, the rise in mean arterial blood pressure a nd heart rate induced by the conditioning was exactly the same as in the co ntrol group. The threshold for activating C-fibres was significantly lower and the baseline tended to be higher in neuropathic rats. Thus, it is sugge sted that the nerve injury itself is likely to induce an LTP-like state in the neurones studied. Further studies using the LTP-inducing model can prov ide new information on the mechanisms of neuropathic pain. (C) 2000 Elsevie r Science Ireland Ltd. All rights reserved.