Degeneration is a hallmark of painful joint disease and is mediated by many proteases that degrade joint tissues, including collagenases. We hypothesized that purified bacterial collagenase would initiate nociceptive cascades in the joint by degrading the capsular ligament’s matrix and activating innervating pain fibers. Intra-articular collagenase in the rat facet joint was investigated for its effects on behavioral sensitivity, joint degeneration, and nociceptive pathways in the peripheral and central nervous systems. In parallel, a co-culture collagen gel model of the ligament was used to evaluate effects of collagenase on microscale changes to the collagen fibers and embedded neurons. Collagenase induced sensitivity within one day, lasting for 3 weeks (p < 0.001) but did not alter ligament structure, cartilage health, or chondrocyte homeostasis. Yet, nociceptive mediators were increased in the periphery (substance P, pERK, and MMP-1; p ≤ 0.039) and spinal cord (substance P and MMP-1; p ≤ 0.041). The collagen loss (p = 0.008) induced by exposing co-cultures to collagenase was accompanied by altered neuronal activity (p = 0.002) and elevated neuronal MMP-1 (p < 0.001), suggesting microscale collagen degradation mediates sensitivity in vivo. The induction of sustained sensitivity and nociception without joint damage may explain the clinical disconnect in which symptomatic joint pain patients present without radiographic evidence of joint destruction.

Mechanical overloading of the temporomandibular joint (TMJ) and biochemical changes, like inflammation and hypoxia, contribute to cartilage degeneration and pain associated with osteoarthritis (OA). Yet, how overloading contributes to early dysregulation of chondrocytes is not understood, limiting the development of diagnostics and treatments for TMJ OA. Hypoxia‐inducible factors (HIF)‐1α/2α in chondrocytes were evaluated at Days 8 and 15 in a rat TMJ pain model induced by jaw loading (1 h/day for 7 days) using immunohistochemistry and compared between cases that induce persistent (3.5 N), acute (2 N), or no (0 N) sensitivity. Hypoxia was measured on Day 8 by immunolabeling of the tracer EF5 and 18F‐EF5 PET imaging. To assess the role of tumor necrosis factor (TNF) in painful TMJ loading, intra‐articular etanercept was given before loading. Orofacial sensitivity was evaluated during and after loading. Facial grimace, TNF‐α, HIF‐2α, and hypoxia levels in the TMJ were measured after loading. HIF‐2α was elevated (P = .03) after 3.5 N loading at Day 8, but HIF‐1α was unchanged. EF5 uptake increased on Day 8 in the 3.5 N group (P < .048) by tissue assay and 18F‐EF5 PET. At Day 8, both HIF‐2α (P = .01) and EF5 uptake (P = .005) were correlated with loading magnitude. Etanercept attenuated sensitivity (P < .01) and the facial grimace on Day 7 (P = .01). It also reduced (P < .01) HIF‐2α and EF5 uptake on Day 8; but TNF‐α levels were not different from controls at that time. Findings suggest that TMJ loading that induces persistent sensitivity upregulates the catabolic factor HIF‐2α and reduces oxygen levels in the cartilage, which may be TNF‐driven.

Neuropathic injury is accompanied by chronic inflammation contributing to the onset and maintenance of pain after an initial insult. In addition to their roles in promoting immune cell activation, inflammatory mediators like secretory phospholipase A2 (sPLA2) modulate nociceptive and excitatory neuronal signaling during the initiation of pain through hydrolytic activity. Despite having a known role in glial activation and cytokine release, it is unknown if sPLA2 contributes to the maintenance of painful neuropathy and spinal hyperexcitability later after neural injury. Using a well-established model of painful nerve root compression, this study investigated if inhibiting spinal sPLA2 7 days after painful injury modulates the behavioral sensitivity and/or spinal dorsal horn excitability that is typically evident. The effects of sPLA2 inhibition on altered spinal glutamatergic signaling was also probed by measuring spinal intracellular glutamate levels and spinal glutamate transporter (GLAST and GLT1) and receptor (mGluR5, GluR1, and NR1) expression. Spinal sPLA2 inhibition at day 7 abolishes behavioral sensitivity, reduces both evoked and spontaneous neuronal firing in the spinal cord, and restores the distribution of neuronal phenotypes to those of control conditions. Inhibiting spinal sPLA2 also increases intracellular glutamate concentrations and restores spinal expression of GLAST, GLT1, mGluR5, and GluR1 to uninjured expression with no effect on NR1. These findings establish a role for spinal sPLA2 in maintaining pain and central sensitization after neural injury and suggest this may be via exacerbating glutamate excitotoxicity in the spinal cord.

Orofacial pain is among the most common chronic pain conditions and can result from temporomandibular disorders (TMDs) of the temporomandibular joint (TMJ). Matrix metalloproteinases (MMPs) drive degeneration of TMJ tissues and likely mediate pain in TMJ disorders given their role in nociception. However, few studies have assessed MMPs in the TMJ innervated tissues nor in the context of pain. This study defined the extent of MMP‐1, MMP‐9, and MMP‐2 in TMJ tissues from patients undergoing total joint replacement (TJR) or arthroplasty discectomy for painful TMJ disorders. Protein expression was probed by Western blot in TMJ disc and capsular ligaments taken during TJR (n = 6) or discectomy (n = 3) for osteoarthritis or internal derangement in an IRB‐approved study. Pro‐ and active MMP‐1, active MMP‐9, and pro‐ and active MMP‐2 are detectable. MMP‐1 and MMP‐9 correlate positively to each other (Kendall's τ = 0.63; p = 0.01), strengthening the hypothesis that they are mechanistically related in regulatory cascades. Active MMP‐1 and active MMP‐9 correlate positively with self‐reported pain scores (τ ≥ 0.51; p ≤ 0.04), suggesting their involvement in peripheral nociception. Overall, neither MMPs nor pain correlate with the functional vertical opening of the jaw. MMP‐1 varies with the observed stage of degeneration during surgery (p = 0.04). Neither overall MMPs nor pain correlate with the overall magnetic resonance imaging scores, corroborating the longstanding, but confounding, clinical observation that pain and radiological evidence of joint damage are not always related. Clinical significance: These findings suggest that MMPs mediate pain in innervated soft tissues and may be targets for diagnosing disease stage and treatments in painful TMJ disorders.