Inflammation unmasks gabapentin's effect on Aδ-fiber evoked excitatory postsynaptic currents in substantia gelatinosa neurons of rat spinal cord

Objective. To study the analgesic mechanism of gabapentin, an anticonvulsant, during antinociceptive clinical treatment. Methods. Whole-cell voltage-clamp recordings were taken from adult rat spinal cord slices to investigate the effect of gabapentin on primary afferent Aδ-fiber evoked excitatory postsynaptic currents (EPSCs) to substantia gelatinosa (SG) neurons in normal and inflamed (established by plantar injection of carrageenan) rats. Results. Gabapentin (5-20 μmol/L for 5 min) depressed dorsal root Aδ fiber evoked polysynaptic, but not monosynaptic EPSCs to SG experiencing inflammation by about 25% (n=10, P < 0.01). However, gabapentin did not depress the evoked polysynaptic or monosynaptic EPSCs in normal rats. Gabapentin failed to block a glutamate receptor subtype, N-methyl-D-aspartate (NMDA), -induced slow excitatory currents on SG neurons. Conclusions. Inflammation, at least in part, unmasks the gabapentin depression on nociception transmission in the dorsal horn, and this depression is not due to the blockade of postsynaptic NMDA receptor.