Effects of Repeated Exposure to Ambient Cold on the Development of Inflammatory Pain in a Rat Model of Knee Arthritis

This study investigated the effects of ambient cold exposure on inflammatory pain development, synovial cytokine levels, and spinal cord glial cell activation. Male Sprague-Dawley rats (6 weeks old) were divided into Cold and RT groups. The Cold group was exposed to cold (4 ± 1 °C) for 6 h/day for 5 consecutive days, while the RT group remained at room temperature (22 ± 1 °C). On day 6, knee arthritis was induced via intra-articular carrageenan injection. Pain was assessed by weight-bearing forces (WBFs) of the affected limb. Synovial pro-inflammatory (IL-1 β, IL-6, and TNF-α) and anti-inflammatory (IL-10) cytokines were measured by ELISA, while spinal cord microglia and astrocytes activation were evaluated via immunohistochemistry. WBFs were maximally reduced 4 h post-carrageenan injection, gradually recovering afterward. Cold-exposed rats showed significantly decreased WBF on days 1 and 2 post-injection compared to the RT group. In the Cold group, synovial cytokines (IL-1β, IL-6, IL-10) were significantly elevated 4 h post-injection, with no change in TNF-α levels. Additionally, OX42-positive cells (microglia) significantly increased 1 h post-injection in the Cold group, while GFAP-positive cells (astrocyte) remained unchanged. Repeated ambient cold exposure enhances inflammatory pain development through the regulation of synovial cytokines and microglia activation in the spinal cord in carrageenan-induced knee arthritis.