ABSTRACT
The present investigation was designed to study, whether endogenous antinociceptive system is effective on mirror-image pain induced by peripheral inflammation. After Complete Freund's adjuvant (CFA) was subcutaneously injected into one hindpaw, besides heat hyperalgesia and mechanical allodynia from 1 h to 72 h at the injured site, contralateral mechanical allodynia was also induced at 1 h and significantly lasted for 24 h after injection, which was called mirror-image pain. To explore the effects of endogenous antinociceptive system on mirror-image pain, endomorphin (EM) 2 was intrathecally administered at doses of 0.2 μg, 2 μg, 20 μg and EM1 was given at the maximum dose of 20 μg by the same way, respectively, 10 min prior to CFA injection. The present results showed that three doses of EM2could reverse the decreased contralateral mechanical threshold from 48.03 ± 9.07 mN ( pre-treatment with vehicle) to 200.49 ± 53.68mN, 247.63 ± 49.43 mN and 250.57 ± 55.34 mN ( pre-treatments with EM2 ), respectively, but not in a significantly dose-dependent manner. On the other hand, intrathecal pre-treatment with EM1, the contralateral mechanical threshold was 51.24 ± 12.59 mN after CFA injection, which was similar to that pre-treatment with vehicle. It indicates that spinal EM1 did not have remarkable effect on mirror-image pain behavior. The present results provide evidence for that spinal EM2, but not EM1, mainly originated from the endogenous antinociceptive system might play an inhibitory role in mirror-image mechanical allodynia induced by peripheral tissue inflammation.