An Experimental Study on Spinal Cord μ-Opioid and α2-Adrenergic Receptors mRNA Expression Following Stress-Induced Hyperalgesia in Male Rats

Background: intense stress can change pain perception and induce hyperalgesia; a phenomenon called stress-induced hyperalgesia (sih). however, the neurobiological mechanism of this effect remains unclear. the present study aimed to investigate the effect of the spinal cord μ-opioid receptors (mor) and α2-adrenergic receptors (α2-ar) on pain sensation in rats with sih. methods: eighteen sprague-dawley male rats, weighing 200-250 g, were randomly divided into two groups (n=9 per group), namely the control and stress group. the stress group was evoked by random 1-hour daily foot-shock stress (0.8 ma for 10 seconds, 1 minute apart) for 3 weeks using a communication box. the tail-flick and formalin tests were performed in both groups on day 22. the real-time rt-pcr technique was used to observe mor and α2-ar mrna levels at the l4-l5 lumbar spinal cord. statistical analysis was performed using the graphpad prism 5 software (san diego, ca, usa). student’s t test was applied for comparisons between the groups. p<0.05 was considered statistically significant. results: there was a significant (p=0.0014) decrease in tailflick latency in the stress group compared to the control group. nociceptive behavioral responses to formalin-induced pain in the stress group were significantly increased in the acute (p=0.007) and chronic (p=0.001) phases of the formalin test compared to the control group. a significant reduction was also observed in mor mrna level of the stress group compared to the control group (p=0.003). there was no significant difference in α2-ar mrna level between the stress and control group. conclusion: the results indicate that chronic stress can affect nociception and lead to hyperalgesia. the data suggest that decreased expression of spinal cord mor causes hyperalgesia.