The differential response of acute swim stress on c-Fos expression on the ipsilateral and contralateral sides in the rat spinal cord after formalin- induced pain

This study was conducted to determine the effects of acute swim stress on the pain behavior response and c-Fos expression in the rat spinal cord. Male Sprague-Dawley rats weighing 250-300g were divided into four groups consisting of rats subjected to acute swim stress (S), formalin injection (F), acute swim stress with formalin injection (S+F) and control rat (C) (n=6 per groups). Rats were acutely stressed by swimming for 3 minutes in water at 21 ̊C. Ten minutes after cessation of acute swim stress, formalin was injected subcutaneously into the plantar surface of the left hind paw of the rats and pain behavior score was recorded for 60 minutes. Two hours after formalin injection, the rats were sacrificed and the L4/L5 segments of the spinal cords removed for immunohistochemistry to detect Fos-like immunoreactivity (FLI). Pain behavior score was significantly lower in the S+F group compared to the F group (P < 0.01) during phase 2. FLI was found significantly higher in groups injected with formalin (F and S+F) on both ipsilateral (injected) and contralateral (notinjected) sides (P < 0.01) when compared to the non-injected groups (S and C). However, S+F group showed significantly reduced FLI in laminae I/II (P < 0.05) and laminae V/VI (P < 0.01) on the ipsilateral side but significantly increased in laminae I/II (P < 0.01) and laminae V/VI P < 0.01) on the contralateral side compared to the F group. In conclusion, acute swim stress not only modulates the pain behavior response but also can modulate the FLI expression differentially on ipsilateral and contralateral sides in the rat spinal cord after formalin injection possibly due to neuroplastic changes in neural pain pathways. J Physiol Biomed Sci. 2012; 25(1): 13-18