Mechanisms underlying antinociception exerted by endomorphin in the spinal dorsal horn

Abstract

BackgroundEndomorphin (EM) is a new opioid peptide and considered as the endogenous ligand for the mu‐opioid receptor (MOR) with high affinity. EM has similar effect in pain inhibition as morphine, but with much less side effects. In the dorsal root ganglion (DRG) and superficial layers (laminae I and II) of the spinal dorsal horn, many substance P (SP), EM and MOR immunpositive neurons and dense fibers and terminals are found, respectively. Based on these phenomena, we proposed a hypothesis that EM binding to its autoreceptor MOR on the pre‐synaptic buttons will inhibit further release of the noxious neurotransmitters, such as SP.AimsTo prove whether the hypothesis is true or not, the antinociceptive effects of EM through pre‐synaptic mechanism in the superficial layers of the spinal dorsal horn were investigated.MethodsBehavioral observation, whole‐cell patch clamp recording, immunofluorescent histochemistry and post‐embedding immunohistochemical staining methods were used.Results(1) EM pre‐treatment through intrathecal administration could dose‐dependent inhibit the primary mechanical allodynia induced by subcutaneous injection of complete Freund's adjuvant (CFA). This effect might be reversed by antagonists of neurokinin 1 receptor, and attenuated by SP or co‐administration of SP and EM, respectively. (2) EM remarkably reduced the frequency instead of the amplitude of miniature EPSCs in lamina II. This effect was antagonized by MOR antagonist beta‐FNA. EM could also inhibit the EPSCs induced by stimulating of the attached dorsal root. The average amplitude of the residual EPSCs increased during high concentration of SP being perfused. (3) SP, EM and MOR were co‐localized in the DRG neurons. SP and EM were observed within the same large dense‐coated granular synaptic vesicles in the pre‐synaptic buttons, whereas MOR was localized on the pre‐synaptic membrane of the pre‐synaptic buttons in which SP and EM were found to co‐exist in the same vesicles.ConclusionsSP and EM are co‐localized in the pre‐synaptic button, of which MOR can be found on the pre‐synaptic membrane. EM inhibits excitatory synaptic transmission and decreases the release of SP from primary afferent terminals in the superficial layers of the spinal dorsal horn through activating MOR on primary afferent terminals to exert their analgesic effects.