Mediation of Exoticin analgesia by a truncated 6-transmembrane of the
mu-opioid receptor
Abstract
Background and Purpose:Mu-opioids remain vastly important for pain
treatment, and would represent ideal analgesics if their analgesic
effects could be separated from their many side effects.Exoticin, an
extract of Murraya exotica, has been shown to have an analgesic effect
which can be blocked by naloxone.Here, we hypothesized that the
analgesic effect of Exoticin is mediated by 6-transmembrane (6TM) of the
mu-opioid receptor. Experimental Approach:We determines the analgesic
EC50 of Exoticin by using the hot-plate assay.The analgesia of Exoticin
was evaluated after intrathecal administration with mu, delta, and kappa
antagonists in C57BL/6 mice. Antinociceptive effects of Exoticin on
formalin or Complete Freund’s adjuvant (CFA) mediated allodynia and
heat-hyperalgesia were assessed in exon 11 mu-opioid knockout (E11 KO)
and exon 1/ exon 11 mu-opioid double knockout (E1/E11 KO) mice.
Opioid-related side effects, such as respiration depression,
constipation, locomotor activity and dependence were assessed.Finally,
to verify the effect of 6TM on Exoticin, the analgesic effect of
Exoticin mediated by 6TM of the mu-opioid receptor was determined by
over-expression of the adeno-associated virus (AAV) in E1/E11KO
mu-opioid mice. Key Results:Delta, kappa antagonists in WT mice had no
impact on Exoticin analgesia, while mice which received the mu
antagonist markedly lowered. Loss of the truncated 6TM variants in the
E11 KO mouse eliminated Exoticin analgesia. Restoring expression of a
6TM variant with an AAV-mMor-1G rescued Exoticin analgesia in E1/E11 KO
mouse that no expressed the 7-transmembrane (7TM) variants. Conclusion
and Implications:Exoticin analgesia depended upon the expression of exon
11-associated truncated 6TM variants of mu-opioid receptor.