Neuronal hyperexcitability in the dorsal horn after painful facet joint injury

Abstract

Excessive cervical facet capsular ligament stretch has been implicated as a cause of whiplash-associated disorders following rear-end impacts, but the pathophysiological mechanisms that produce chronic pain in these cases remain unclear. Using a rat model of C6-C7 cervical facet joint capsule stretch that produces sustained mechanical hyperalgesia, the presence of neuronal hyperexcitability was characterized 7 days after joint loading. Extracellular recordings of spinal dorsal horn neuronal activity between C6 and C8 (117 neurons) were obtained from anesthetized rats, with both painful and non-painful behavioral outcomes established by the magnitude of capsule stretch. The frequency of neuronal firing during noxious pinch (p<0.0182) and von Frey filaments applications (4-26g) to the forepaw was increased (p<0.0156) in the painful group compared to the non-painful and sham groups. In addition, the incidence and frequency of spontaneous and after discharge firing were greater in the painful group (p<0.0307) relative to sham. The proportion of cells in the deep laminae that responded as wide dynamic range neurons also was increased in the painful group relative to non-painful or sham groups (p<0.0348). These findings suggest that excessive facet capsule stretch, while not producing visible tearing, can produce functional plasticity of dorsal horn neuronal activity. The increase in neuronal firing across a range of stimulus magnitudes observed at day 7 post-injury provides the first direct evidence of neuronal modulation in the spinal cord following facet joint loading, and suggests that facet-mediated chronic pain following whiplash injury is driven, at least in part, by central sensitization.

Figure 1. Forepaw stimulation protocol

(a) Once a neuron was identified, the forepaw location that evoked maximal firing was defined. This location was most frequently found at the distal end of digits 4 and 5. (b) Light brush, (c) noxious pinch, and (d–g) von Frey (vF) filament stimulation were applied to that forepaw location. Neurons were classified as wide dynamic range (WDR) or low threshold mechanoreceptive (LTM) based on their response measured between 3–8 seconds during noxious pinch, as in (c). Spontaneous firing was characterized during the 1-second prior to von Frey stimulation in (d–g). Evoked firing from each von Frey application in (d–g) was defined by the response during each stimulation and immediately after. Afterdischarge following each von Frey application in (d–g) was defined as the difference between firing immediately after stimulation (a) and spontaneous firing (s).

Figure 2. Representative extracellular recordings during the application of a 26 g von Frey filament to the forepaw 7 days after facet joint loading

For each neuron, a series of 5 stimulations was applied with each filament magnitude. Extracellular (EC) data were spike-sorted to isolate the response of single units. Superimposed traces of all single unit activity counted in the histograms are provided. (a) Firing was evoked in sham rats during the initial application of the filament to the forepaw and upon its removal. (b) Rats that underwent a targeted 0.2 mm vertebral displacement also responded to forepaw stimulation predominantly during the initial application of the filament and upon its removal. (c) Neuronal firing was more frequent throughout the entire application of the filament and after removal for rats that underwent a vertebral displacement capable of producing a painful behavioral response.

Figure 3. Neuronal hyperexcitability in the dorsal horn are produced at day 7 following facet joint loading that produces mechanical hyperalgesia

(a) The withdrawal threshold was significantly lower (*p=0.004) at day 7 in the painful group compared to the corresponding baseline values. (b) Neuronal excitability was significantly greater (*p<0.038) in the painful group compared to sham for brush, pinch, and 26 g von Frey stimuli; the painful group was also significantly greater (p<0.0182) than the non-painful group for pinch and von Frey stimulation.

Figure 4. Evoked neuronal firing increases with increasing von Frey stimulus magnitude

The number of evoked spikes in the painful group was significantly greater than the non-painful and sham groups at 4, 10, and 26 g (*p<0.0156).

Figure 5. The average number of spikes evoked during von Frey filament stimulation of the forepaw increases with respect to the filament strength and depends on the order of application

A significant interaction effect was found between the von Frey magnitude and the ordinal rank of the stimulus application (p<0.0001); at greater filament magnitudes, firing was greater in response to the first application of the filament relative to the subsequent applications at that magnitude.

Figure 6. Afterdischarge rates increase after painful facet injury in response to von Frey filament stimulation of the forepaw

Afterdischarge was measured as the increase in firing rate above spontaneous baseline firing rates. The afterdischarge rate was significantly higher after 26 g filament stimulation in the painful group compared to non-painful and sham groups (*p<0.0006).