The effect of intrathecal gabapentin on pain behavior and hemodynamics on the formalin test in the rat

In this study, we examined the effect of intrathecal (IT) gabapentin, administered before and after the injection of formalin into the rat hindpaw, on pain behavior and hemodynamics. Formalin evoked a biphasic flinching behavior and hypertension. II gabapentin administered 10 min before formalin produced a dose-dependent reduction of the Phase 2, but not Phase 1, flinching and cardiovascular response. In contrast, IT gabapentin administered 9 min after formalin had no effect on either phase of flinching. II D-serine (100 mu g) administered 10 min before IT gabapentin reversed the Phase 2 effect of gabapentin. IT gabapentin did not affect the thermal escape latency or the baseline cardiovascular measures even at the largest dose (300 mu g). These results indicate that the spinal effect of gabapentin reduces the somatosympathetic reflex and somatosensory response to tissue injury without an accompanying effect on acute nociception or resting sympathetic outflow. Implications: After tissue injury, there is an enhanced pain behavior and cardiovascular response, representing a facilitated state of spinal processing. Spinally delivered gabapentin had no evident effect on resting heart rate or blood pressure, but it attenuated the enhanced pain behavior and cardiovascular response otherwise produced by injury.