Functional characterization of ovine dorsal root ganglion neurons reveal peripheral sensitization after osteochondral defect

Knee joint trauma can cause an osteochondral defect (OD), a risk factor for osteoarthritis and cause of debilitating pain in patients. Rodent OD models are less translatable due to their smaller joint size and open growth plate. This study proposes sheep as a translationally relevant model to understand the neuronal basis of OD pain. Unilateral 6 mm deep OD was induced in adult female sheep. 2-6 weeks post-operation, lumbar dorsal root ganglia (DRG) neurons were collected from the contralateral and OD side of operated sheep. Functional assessment of neuronal excitability and activity of the pain-related ion channels TRPV1 and P2X3 was carried out using electrophysiology and Ca2+-imaging. Immunohistochemistry was utilized to verify expression of pain-related proteins. We observed that an increased proportion of OD DRG neurons (sheep, N = 3; Ctrl neurons, n =15, OD neurons, n = 16) showed spontaneous electrical excitability (Ctrl: 20.33 ± 4.5%; OD: 50 ± 10%; p = 0.009, unpaired t-test) and an increased proportion fired a greater number of spikes above baseline in response to application of a TRPV1 agonist (capsaicin) application (Ctrl: 40%; OD: 75%; p = 0.04, chi-sq test). Capsaicin also produced Ca2+ influx in an increased proportion of OD DRG neurons isolated (Ctrl: 25%; OD: 44%; p = 0.001, chi-sq test). Neither protein expression, nor functionality of the P2X3 ion channel were altered in OD neurons. Overall, we provide evidence of increased excitability of DRG neurons (an important neural correlate of pain) and TRPV1 function in an OD sheep model. Our data show that functional assessment of sheep DRG neurons can provide important insights into the neural basis of OD pain and thus potentially prevent its progression into arthritic pain. Significance statement Pain is the primary symptom of osteoarthritis (OA) and often the main reason for patients seeking medical care. Understanding pain mechanisms in OA can boost the development of disease specific pain relief. While small animals such as mouse and rat have been widely used in OA pain studies, the genetic and anatomical differences between rodents and humans can hinder clinical translation. Here, we studied pain in an early osteochondral defect (OD) model in sheep, a commonly used large animal model in OA research. We found increased excitability and TRPV1 function in dorsal root ganglia (DRG) neurons innervating the site of OD. This study thus demonstrates the utility of using a large animal, such as sheep, for studying mechanisms of joint pain.