In vitro inflammatory knee pain: Of Mice and Men

The ongoing pain associated with arthritis reduces the quality of human life. Injection of complete Freund’s adjuvant (CFA) into the knee of mice induced inflammation and a concomitant decrease in their natural digging behaviour within 24-hours. After inflammation, retrogradely labelled knee-innervating dorsal root ganglion neurons showed a decreased action potential threshold and increased transient receptor potential vanilloid-1 (TRPV1) expression (validated with immunohistochemistry). Subsequently, systemic administration of a TRPV1 antagonist normalized mouse digging behaviour within 30-minutes, likely due to inhibition of pain signalling. To test the translational potential of these results, we recorded from mouse sensory neurons incubated overnight with synovial fluid from humans with osteoarthritis experiencing chronic pain (OA-SF). We find that OA-SF directly sensitizes mouse knee neurons by decreasing the action potential threshold and increasing the resting membrane potential implicating voltage-gated sodium (Na_v) and potassium (K_v) channels. Using Ca²⁺-imaging we also find that OA-SF increases TRPV1 and TRPM8 activity compared to synovial fluid from healthy individuals, highlighting the importance of TRP channels in OA pain. Taken together, our data suggest that inflammatory mediators in OA-SF excite sensory neurons and hence drive knee pain. This in vitro, translational model can be incorporated into clinical trials to unbiasedly assess the efficacy of drugs on sensory neurons and hence identify drivers of OA pain.