Spinal neuronal plasticity is evident within 1 day after a painful cervical facet joint injury

Excessive stretch of the cervical facet capsular ligament induces persistent pain and spinal plasticity at later time points. Yet, it is not known when such spinal modifications are initiated following this painful injury. This study investigates the development of hyperalgesia and neuronal hyperexcitability in the spinal cord after a facet joint injury. Behavioral sensitivity was measured in a model of painful C6/C7 facet joint injury in the rat, and neuronal hyperexcitability in the spinal cord was evaluated at 6 h and 1 day after injury or a sham procedure, in separate groups. Extracellular recordings of C6/C7 dorsal horn neuronal activity (229 neurons) were used to quantify spontaneous and evoked firing. Rats exhibited no change in sensitivity to mechanical stimulation of the forepaw at 6 h, but did exhibit increased sensitivity at 1 day after injury (p = 0.012). At 6 h, both spontaneous neuronal activity and firing evoked by light brushing, pinch, and von Frey filaments (1.4-26 g) applied at the forepaw were not different between sham and injury. At 1 day, spontaneous firing was noted in a greater number of neurons after injury than sham (p < 0.04). Evoked firing was also increased 1 day after injury compared to normal and sham (p < 0.03). Dorsal horn hyperexcitability and increased spontaneous firing developed between 6 and 24 h after painful facet injury, suggesting that the development of hyperalgesia parallels dorsal horn hyperexcitability following mechanical facet joint injury, and these spinal mechanisms are initiated as early as 1 day after injury.