Effects of Chronic Forced Swim Stress with Different Session on the Interphase and Termination of Phase 2 in the Formalin Test

Stress have bidirectional effects on pain threshold and behaviours. Whereas acute stress often results in analgesia، chronic stress can trigger hyperalgesia/allodynia. The formalin test as an inflammation model، consist of three phases and the effects of repeated forced swim stress with different session numbers on these phases have not been investigated. Therefore، in this study the effects of chronic forced swim stress with different intensities evaluated in formalin test were performed in adult male Wistar rats. In this study، the formalin test (50 μL، 2%) was used to evaluate the effects of repeated swim stress with different duration and sessions on nociceptive responses. Animals were initially submitted to 6 minutes in day with different sessions (3، 5 and 10 days) of forced swim stress and after 24 hours of the last session، animals were submitted to formalin injection in hind paw to evaluate nociceptive behaviours. Exposing animals to 3 days for 6 minutes had an increasing effect on formalin-induced pain behavior only in the final stage of phase 2. This data showed that increase session number of stress have same effect on nociceptive behaviours in termination of phase 2. Moreover increasing exposure to forced swim stress for 5 and 10 sessions effected nociceptive behaviours in interphase. These findings suggest that increase sessions of chronic forced swim stress (3،5 and 10 days) effect on the nociceptive behaviours and significantly increase interphase of the Formalin Test. which could be related to modulating mechanism during this phase