Acute nerve injury induces long-term potentiation of C-fiber evoked field potentials in spinal dorsal horn of intact rat.

Nerve injury produces a long lasting neuropathic pain, manifested as allodynia, a decrease in pain threshold and hyperalgesia, an increase in response to noxious stimuli. The mechanism underlying the lasting abnormal pain is not well understood. Our previous works have shown that electrical tetanic stimulation of the sciatic nerve induces long-term potentiation (LTP) of C-fiber evoked field potentials in the spinal dorsal horn, which is considered as a synaptic model of pathological pain. In the present study we tested if nerve injury, which is proved to produce neuropathic pain, induced the spinal LTP in intact rats. C-fiber evoked field potentials in spinal dorsal horn produced by electrical stimulation (10-20 V, 0.5 ms, 1/min) of the sciatic nerve were recorded. For induction of LTP of C-fiber evoked field potentials, three types of noxious stimuli were applied. (1) Electrical tetanic stimulation (40 V, 0.5 ms pulses at 100 Hz for 1 s repeated four times at 10 s intervals). (2) Transection of the sciatic nerve at 4-5 mm distal to the stimulation electrode. (3) Crushing the sciatic nerve with a forceps four times at 4-5 mm distal to stimulation electrode (from distal to proximal with 1 mm spacing at 10 s intervals), which simulated electrical tetanic stimulation. Acute nerve injury was made by either transection of the sciatic nerve at the distal to the stimulating electrode or crushing the sciatic nerve. We found that nerve injury by cutting or crushing the sciatic nerve produced LTP of C-fiber evoked field potentials lasting until the end of the experiments (3-9 h), and that pretreatment of the sciatic nerve with lidocaine 10 min prior to the nerve transectoin completely blocked LTP induced by nerve transection. The nerve transection-induced LTP was blocked by NMDA receptor antagonist AP5. LTP produced by nerve transection could not be further potentiated by electrical tetanic stimulation, while LTP induced by single electrical tetanic stimulation could be further potentiated by transection of the sciatic nerve. However, when LTP was saturated by several times of electrical tetanic stimulation, nerve transection did not affect the spinal LTP. We conclude that acute nerve injury induces LTP of C-fiber evoked field potentials in intact animals and that nerve transection is more powerful than electrical tetanic stimulation for induction of the spinal LTP. The results further support the notion that LTP of C-fiber evoked field potentials may underlie neuropathic pain.