Long-term changes in sciatic-evoked A-fiber dorsal horn field potentials accompany loose ligation of the sciatic nerve in rats

The goal of the present study was to examine whether loose ligation of the sciatic nerve was associated with long-term changes in neuronal excitabilit y in the spinal dorsal horn in urethane-anesthetized rats. The sciatic nerv e was stimulated with 0.1 ms long pulses at 1 stimulus/5 min, and the evoke d dorsal horn field potentials remained stable in the absence of tetanic st imulation. In one set of control and ligated animals, high-frequency tetani c stimulation was applied to the nerve at 50 Hz (one 400 ms train of twenty 0.1 ms pulses), and the field potentials were recorded again (1 stimulus/5 min) for up to 4 h post-tetanus. In control animals, this protocol produce d significant increases in field potential amplitudes at 15, 30 and 60 min post-tetanus. Interestingly, after this time the evoked field potentials be gan to decrease, and attained less than 50% of their pre-tetanic values at 240 min post-tetanus, In contrast, in ligated rats the pattern of post-teta nic potentiation was significantly different as the increases in amplitude persisted, and at 240 min past-tetanus the field potentials were almost twi ce their baseline values. In another set of control and ligated animals, low-frequency tetanic stimul ation was applied at 5 Hz (one 400 ms train of two 0.1 ms pulses). Again a differential pattern of post-tetanic responses between control and ligated rats was seen. In control animals, a significant decrease in amplitude was evident within 30 min, and the depression became progressively more pronoun ced as the field potentials attained about a quarter of their baseline valu es at 180 min, and remained at these low levels at 240 min post-tetanus. On the other hand, in ligated animals, the depression was not significant, an d at 240 min post-tetanus the field potentials were still at about 80% of t heir baseline values. These data demonstrate that long-term changes in spinal dorsal horn neurona l excitability accompany sciatic ligation to perhaps contribute to the deve lopment of neuropathic pain. These changes may result from a lessening of n ormally strong inhibitory processes in the spinal dorsal horn to generate c onditions which favor post-tetanic potentiation over depression of dorsal h orn neuronal responses. (C) 2000 International Association for the Study of Pain. Published by Elsevier Science B.V.