Anatomical And Functional Interrogation of 5HT3R+ Sensory Neurons in Knee Arthritis

The aim of the current study was to map the distribution, phenotype and function of 5HT3AR positive sensory neurons in the normal and inflamed mouse knee joint. We collected lumbar dorsal root ganglia (DRG) from 5HT3R-Flpo:Ai65F reporter mice to characterize the neurochemical and molecular phenotype of labeled neurons using immunohistochemistry and multiplex fluorescent in situ hybridization. We confirmed a high degree of co-expression between Tomato mRNA with Htr3a mRNA (94.5 +/- 3.6%) and observed between 10-12% Tomato-immunoreactive (IR) neurons in lumbar DRG. Phenotypically, Tomato-IR was highly expressed in myelinated trkA+ and CGRP+ neurons but minimally in IB4+ non-peptidergic C fibers. A significant percentage of Tomato-IR neurons colocalized with putative markers of A delta high threshold mechanoreceptors and silent nociceptors. In cleared thick tissue sections (80 micron), we observed Tomato-IR neurites throughout the knee joint including aligning ligaments, adjacent to menisci and within the synovium and fat pads. Repeat intra-articular injection of nerve growth factor (NGF) induced mechanical knee hyperalgesia and increased the density of 5HT3R+ neurites in the ipsilateral knee. Viral vector targeted chemogenetic inhibition of 5HT3AR+ neurons reduced knee hyperalgesia in NGF injected mice. Based on these results, 5HT3AR+ sensory neurons may be major contributors to painful symptoms associated with knee osteoarthritis (OA) and therapeutic interventions that target these neurons may be beneficial. In ongoing studies, we are using in vivo intracellular recordings and calcium imaging in the DRG to characterize the physiological profile of 5HT3AR afferents in mice under normal conditions and following postsurgical knee OA. R21 AR078366.