Painful Cervical Facet Joint Injury Is Accompanied by Changes in the Number of Excitatory and Inhibitory Synapses in the Superficial Dorsal Horn That Differentially Relate to Local Tissue Injury Severity

Immunohistochemistry labeled pre- and postsynaptic structural markers to quantify excitatory and inhibitory synapses in the spinal superficial dorsal horn at 14 days after painful facet joint injury in the rat. The objective of this study was to investigate the relationship between pain and synapse density in the spinal cord after facet injury. Neck pain is a major contributor to disability and often becomes chronic. The cervical facet joints are susceptible to loading-induced painful injury, initiating spinal central sensitization responses. Although excitatory synapse plasticity has been reported in the superficial dorsal horn early after painful facet injury, whether excitatory and/or inhibitory synapse density is altered at a time when pain is maintained is unknown. Rats underwent either a painful C6/C7 facet joint distraction or sham surgery. Mechanical hyperalgesia was measured and immunohistochemistry techniques for synapse quantification were used to quantify excitatory and inhibitory synapse densities in the superficial dorsal horn at day 14. Logarithmic correlation analyses evaluated whether the severity of facet injury correlated with either behavioral or synaptic outcomes. Facet joint injury induces pain that is sustained until day 14 (P