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

Abstract

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 6h 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 6h, but did exhibit increased sensitivity at 1 day after injury (p=0.012). At 6h, both spontaneous neuronal activity and firing evoked by light brushing, pinch, and von Frey filaments (1.4-26g) 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 24h 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.

Copyright © 2013 Elsevier Ireland Ltd. All rights reserved.

Figures

Figure 1

Paw withdrawal threshold (PWT) at baseline (BL) prior to surgery, and at either 6 hours or 1 day. The PWT at BL for each group was not different from normal. At 6 hours, PWT was not different from baseline in either group. PWT was significantly reduced (*p=0.012) from baseline at 1 day after injury but was unchanged at that time point in sham.

Figure 2

Spontaneous firing in the spinal dorsal horn. Example recorded voltages from neurons with (a) no baseline firing and (b) baseline firing greater than 1 spike/sec. (c) The number of quiescent neurons (presented as percent of total neurons) was unchanged at 6 hours relative to sham at that time point, but at 1 day was significantly lower (*p=0.038) after injury compared to sham. (d) The number of active neurons exhibited a similar trend with a significantly greater number of neurons (*p=0.006) at 1 day after injury. Percentages of cells in the normal group are shown (dotted line) for (c) and (d).

Figure 3

Evoked firing in the spinal dorsal horn. (a) Normal rats and those at 1 day after sham exhibited significantly lower evoked responses to light brush and noxious pinch than those at 1 day after injury (*p<0.014), while only normal rats were significantly lower than those at 6 hours after sham (#p<0.0008) and 6 hours after injury (^p<0.004). (b) In response to von Frey filament stimuli, normal rats and those at 1 day after sham exhibited significantly less evoked firing than those at 1 day after injury (*p<0.03), 6 hours after sham (#p<0.0006), and 6 hours after injury (^p<0.017).