Gabapentin depresses C-fiber-evoked field potentials in rat spinal dorsal horn only after induction of long-term potentiation

C-fiber-evoked field potentials in response to electrical stimulation of the sciatic nerve were recorded in the dorsal horn of the rat lumbar spinal cord, and their long-term potentiation (LTP) was induced by high-frequency stimulation applied on the sciatic nerve as a synaptic model of hypersensitivity underlying an increased efficacy of nociceptive transmission. We evaluated the effect of gabapentin on the basal C-fiber-evoked field potentials and their established LTP. Intravenously administered gabapentin (10 and 30 mg/kg, i.v.) reduced the LTP of C-fiber-evoked field potentials in a dose-dependent manner when applied 60 min after establishment of the LTP. However, gabapentin did not affect the basal C-fiber-evoked field potentials or induction of the LTP. Thus, gabapentin was effective only in sensitized conditions. By contrast, morphine HCl (1 and 3 or 10 mg/kg, i.v.) reduced both the basal responses and their established LTP. The combination of gabapentin and morphine at lower doses of each drug appeared to result in a stronger reduction on the established LTP than that of each drug alone, suggesting that combination therapy can generate better analgesia in the treatment of chronic pain.