title: Different analgesic effects of intrathecal endomorphin-2 on thermal hyperalgesia and evoked inflammatory pain in ovariectomized rats.
abstract: 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 (β-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.
authors: Department of Anatomy, Histology and Embryology, K.K. Leung Brain Research Centre, The Fourth Military Medical University, P. R. China; Department of Physical Education, Xi'an University of Architecture and Technology, PR China; Department of Neurosurgery.
authors: Xiao-Hui
authors: XH
authors: Zhao
authors: Ya-Qun
authors: YQ
authors: Zhao
authors: Chao
authors: C
authors: Zhu
authors: Lei
authors: L
authors: Chen
authors: Wei
authors: W
authors: Hu
authors: Ting
authors: T
authors: Zhang
authors: Yu-Lin
authors: YL
authors: Dong
authors: Sheng-Xi
authors: SX
authors: Wu
authors: Alan David
authors: AD
authors: Kaye
authors: Wen
authors: W
authors: Wang
authors: Yun-Qing
authors: YQ
authors: Li
mesh_terms: D000701:Analgesics, Opioid / Q000008:administration & dosage*; D000818:Animals; D004361:Drug Tolerance / Q000502:physiology; D005260:Female; D006358:Hot Temperature / Q000009:adverse effects*; D006930:Hyperalgesia / Q000188:drug therapy* / Q000473:pathology; D007249:Inflammation / Q000188:drug therapy; D009842:Oligopeptides / Q000008:administration & dosage*; D010052:Ovariectomy / Q000009:adverse effects* / Q000639:trends; D010146:Pain / Q000188:drug therapy / Q000473:pathology; D059408:Pain Management / Q000379:methods; D010147:Pain Measurement / Q000187:drug effects / Q000379:methods*; D051381:Rats; D017207:Rats, Sprague-Dawley; D016896:Treatment Outcome
chemical_list: D000701:Analgesics, Opioid; D009842:Oligopeptides; C105294:endomorphin 2