Effects of Acute and Chronic Immobilization Stress on Formalin Test on the Male Rat

Acute and chronic stress induces hormonal and neuronal changes which affect both pain threshold and nociceptive behaviors. But the effect of acute and chronic immobilization stress on formalin induced nociceptive behaviors are unknown. Therefore، this study evaluated the effects of acute and chronic immobilization stress formalin test on the male rat.

In this study، the formalin test (50 μL، 2%) was used to evaluate the effects of acute restraint stress on nociceptive responses. Animals (42) were initially submitted to one session of acute restraint stress (15، 30، 60 min) or chronic restraint stress (10 and 20 days، each day 30 min) and immediately submitted to the formalin injection in hind paw to evaluate nociceptive behaviors.

Acute 60 minutes exposure to restraint stress did not reduce the nociceptive behaviors by chemical stimulation (formalin 2%) of the rats، while 15 minutes exposure to restraint stress reduced formalin induced nociceptive behaviors in phase 2، and 30 minutes exposure to restraint stress reduced formalin induced nociceptive behaviors in phase 1، interphase and phase 2. Chronic restraint stress for 10 and 20 days (each day 30 min) did not increase the stress induced analgesia. These findings suggest that acute exposure to restraint for 30 minutes produce greater decrease in nociceptive behaviors than 15 and 60 minutes.

Acute restraint stress can produce short-term and long-term SIA (Stress Induced Analgesia) for tonic pain. The short-term SIA is reflected as a decreasing in nociceptive behaviors during phase 1، whereas the long-term SIA is reflected as a decrease in nociceptive behaviors during phase 2.