Spinal microglial β‐endorphin signaling mediates IL‐10 and exenatide‐induced inhibition of synaptic plasticity in neuropathic pain

Aim This study aimed to investigate the regulation of pain hypersensitivity induced by the spinal synaptic transmission mechanisms underlying interleukin (IL)‐10 and glucagon‐like peptide 1 receptor (GLP‐1R) agonist exenatide‐induced pain anti‐hypersensitivity in neuropathic rats through spinal nerve ligations. Methods Neuropathic pain model was established by spinal nerve ligation of L5/L6 and verified by electrophysiological recording and immunofluorescence staining. Microglial expression of β‐endorphin through autocrine IL‐10‐ and exenatide‐induced inhibition of glutamatergic transmission were performed by behavioral tests coupled with whole‐cell recording of miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs) through application of endogenous and exogenous IL‐10 and β‐endorphin. Results Intrathecal injections of IL‐10, exenatide, and the μ‐opioid receptor (MOR) agonists β‐endorphin and DAMGO inhibited thermal hyperalgesia and mechanical allodynia in neuropathic rats. Whole‐cell recordings of bath application of exenatide, IL‐10, and β‐endorphin showed similarly suppressed enhanced frequency and amplitude of the mEPSCs in the spinal dorsal horn neurons of laminae II, but did not reduce the frequency and amplitude of mIPSCs in neuropathic rats. The inhibitory effects of IL‐10 and exenatide on pain hypersensitive behaviors and spinal synaptic plasticity were totally blocked by pretreatment of IL‐10 antibody, β‐endorphin antiserum, and MOR antagonist CTAP. In addition, the microglial metabolic inhibitor minocycline blocked the inhibitory effects of IL‐10 and exenatide but not β‐endorphin on spinal synaptic plasticity. Conclusion This suggests that spinal microglial expression of β‐endorphin mediates IL‐10‐ and exenatide‐induced inhibition of glutamatergic transmission and pain hypersensitivity via presynaptic and postsynaptic MORs in spinal dorsal horn.
Schematic diagram shows the role of microglial expression of β‐endorphin in IL‐10‐ and specific GLP‐1 receptor agonist exenatide‐induced inhibition of spinal excitatory synaptic transmission and pain hypersensitivity in neuropathic pain. Following activation of GLP‐1 receptors, IL‐10 is released and then activates IL‐10 receptors via a microglial autocrine mechanism. Afterward, the β‐endorphin is released to microglial neuronal synapses and activates neuronal presynaptic and postsynaptic μ‐opioid receptors (MORs) to inhibit the enhanced glutamatergic transmission, leading to pain anti‐hypersensitivity.