The nociceptive innervation of the normal and osteoarthritic mouse knee

Objectives: To document the nociceptive innervation of the normal and osteoarthritic murine knee. Methods: Knees were collected from naïve male C57BL/6 Na_V1.8-tdTomato reporter mice aged 10, 26, and 52 weeks (n = 5/group). Destabilization of the medial meniscus (DMM) or sham surgeries (n = 5/group) were performed in the right knee of 10-week old male Na_V1.8-tdTomato mice, and knees were harvested 16 weeks later. Twenty 20-μm frozen sections from a 400-μm mid-joint region were collected for confocal microscopy. Integrated density of the tdTomato signal was calculated using Image J by two independent observers blinded to the groups. Consecutive sections were stained with hematoxylin & eosin. C57BL/6-Pirt-GCaMP3 mice (n = 5/group) and protein gene product 9.5 (PGP9.5) immunostaining of C57BL/6 wild type (WT) mice (n = 5/group) were used to confirm innervation patterns. Results: In naive 10-week old mice, nociceptive innervation was most dense in bone marrow cavities, lateral synovium and at the insertions of the cruciate ligaments. By age 26 weeks, unoperated knees showed a marked decline in nociceptors in the lateral synovium and cruciate ligament insertions. No further decline was observed by age 1 year. Sixteen weeks after DMM, the medial compartment of OA knees exhibited striking changes in Na_V1.8+ innervation, including increased innervation of the medial synovium and meniscus, and nociceptors in subchondral bone channels. All results were confirmed through quantification, also in Pirt-GCaMP3 and PGP9.5-immunostained WT mice. Conclusions: Nociceptive innervation of the mouse knee markedly declines by age 26 weeks, before onset of spontaneous OA. Late-stage surgically induced OA is associated with striking plasticity of joint afferents in the medial compartment of the knee.