Different Analgesic Effects of Intrathecal Endomorphin-2 on Thermal Hyperalgesia and Evoked Inflammatory Pain in Ovariectomized Rats

Hormone replacement remains one of the common therapies for menopause-related pain but is associated with risk of orofacial or back pain. Spinal endomorphin-2 (EM-2) is involved in varied pain and its release is steroid-dependent, but whether increasing spinal EM-2 can inhibit thermal hyperalgesia and inflammatory pain in ovariectomized (OVX) female rats, an animal model mimicking menopause, is not clear, nor is the potential involvement of spinal mu-opioid receptor (MOR). In the current study, we revealed that the temporal decrease of spinal EM-2 is accompanied with OVX-induced thermal hyperalgesia that was dose-dependently attenuated by intrathecal (IT) delivery of EM-2. The subcutaneous injection of formalin-induced inflammatory pain in OVX rats was exacerbated and IT delivery of EM-2 dose-dependently inhibited the inflammatory pain. However, the ED50 for IT delivery of EM-2 on thermal hyperalgesia is smaller than that on inflammatory pain in OVX rats, suggesting different contributions of the EM-2 system to these 2 pain modalities in OVX rats. IT pretreatment with MOR antagonist, beta-funaltrexamine (beta-FNA), attenuated IT EM-2 analgesia on both thermal hyperalgesia and inflammatory pain in OVX rats. Furthermore, IT delivery of EM-2 did not affect the animals' locomotion or anxiety status. Our findings suggested that IT EM-2 might be a safer analgesia strategy than hormone replacement therapy in reducing risk of orofacial or back pain. However, a long-lasting form of EM-2 with less tolerance is needed to induce sustained analgesia.