Pain‐related behavior and the characteristics of dorsal‐root ganglia in a rat model of hip osteoarthritis induced by mono‐iodoacetate

The principal aim of this study was to clarify the time course of pain‐related behavior and pain‐related sensory innervation in a rat model of hip osteoarthritis (OA) induced by intra‐articular injection of mono‐iodoacetate (MIA). Using 6‐week‐old male Sprague Dawley rats, 25 μl of sterile saline of 1% Fluoro‐Gold solution (FG) (control group; n = 30) and 25 μl of sterile saline of 1% FG with 2 mg of MIA (MIA group; n = 30) was injected into the right hip joints. Gait function was evaluated using a CatWalk system after 7, 14, 28, 42, and 56 days (n = 5, respectively). Neurons in the dorsal root ganglion (DRG) between L1 and L5 were immunostained for calcitonin gene‐related peptide (CGRP) and activating transcription factor‐3 (ATF3). Gait analysis revealed the mean six parameters of hind paws at all time points were significantly lower in the MIA group (p = 0.05). The number of CGRP‐immunoreactive (‐IR) DRG neurons was significantly increased on days 7, 14, 28, and 42 peaking at 14 days in the MIA group. By contrast, expression of ATF3‐IR in FG‐labeled DRG neurons was significantly increased on days 42 and 57. The FG‐labeled DRG neurons were distributed between L1 and L5, mainly at the L4 level. Pain‐related behavior indicated by gait disturbance was observed in a MIA model of hip OA in rat. Early elevation of CGRP expression and late expression of ATF‐3 were demonstrated in DRG neurons, possibly reflecting inflammatory pain and neuropathic pain in hip OA.