Melatonin Inhibits Central Sensitization Via MT2 Receptor In The Treatment Of Radicular Pain

BackgroundLumbar disc herniation(LDH)is a common clinical disease in the majority of young adults.Patients can suffer a series of symptoms such as waist pain,leg radiation pain,lower limb paralysis and limited activity.Radicular pain(RP)caused by LDH is a type of pathological pain that affects patients for a long time.Although many scholars have done a lot of research on this disease,the specific pathogenesis of LDH has not been fully understood.The mechanisms of pathologic pain include peripheral sensitization and central sensitization.Peripheral sensitization refers to the change of expression and function of ion channels in primary afferent neurons,which leads to the enhancement of neuronal excitability and the increase of pain signal generation.Central sensitization refers to that the primary afferent fibers receiving nociceptive stimulation transmit pain signals to the dorsal horn neurons of the spinal cord,leading to long-term potentiation(LTP)caused by the enhancement of signal exchange efficiency between synapses and the expansion of pain signals.The pathogenesis of LDH is generally believed to be closely related to peripheral sensitization.In 2017,our team confirmed and reported that central sensitization is an important cause of RP,which provides a new direction for the treatment of RP.Melatonin(MLT)is one of the fat-soluble neuroendocrine hormones,mainly derived from the secretion of the pineal gland,and plays an important regulatory role in multiple systems of the body through receptors MT1 and MT2.Some studies have shown that MLT can inhibit the induction of LTP in hippocampal CA1 region of normal mice,but other studies have found that MLT can improve the hippocampal LTP damage caused by Alzheimer’s disease and epilepsy,and improve the spatial learning ability of tall mice,that is,MLT can promote the induction and maintenance of hippocampal LTP.These studies suggest that MLT may have completely opposite effects on hippocampal LTP in physiological and pathological states.What is the role of MLT in LDH-induced spinal cord LTP? There have been no reports yet.The study found that MLT levels in normal mice were higher at night,when they were insensitive to painful stimuli.MLT has an analgesic effect in pathological pain caused by mechanical nerve injury and inflammation.But whether MLT can relieve radicular pain? How is its specific mechanism? It’s worth exploring further.ObjectivesTo confirmed that MLT can treat RP caused by LDH by inhibiting pain transmission,and to explore the specific mechanism from MLT related receptors.Method and ResultsIn this experiment,we used autologous NP(Nucleus pulposus)transplantation to construct rat LDH model,and injected MLT intraperitoneally.Pain behavior test showed that MLT increased mechanical paw withdrawal threshold(PWT)and thermal withdraw latency(PWL)of NP rats,which indicated that MLT could alleviate mechanical and thermal pain sensitivity of NP rats.Electrophysiological experiments showed that MLT significantly reduced the amplitude of C fiber evoked potential in spinal dorsal horn of NP rats,which indicated that MLT could inhibit central sensitization The expression of N-methyl-d-aspartate receptor subunit 2A(NR2A)and N-methyl-d-aspartate receptor subunit 2B(NR2B)protein was detected by Western blot.And the protein expressions of calcitonin gene related peptide(CGRP),Isolectin b4(Ib4)and NF200 were detected by immunofluorescence.It is found that MLT can inhibit central sensitization by inhibiting the expression of NR2 B,CGRP and Ib4.These results indicate that MLT can alleviate pain by inhibiting the central sensitization of pain transmission.The expression of MT1 and MT2 protein was detected by Westernb lot.the results showed that the expression of MT2 in NP group was significantly higher than that in sham operation group,but there was no significant difference in MT1.Immunofluorescence double staining showed that MT2 was co-located with neurons and microglia.Further intrathecally injected MLT receptor tools:8-methoxy-2-propionamidoteralin(8MP,MT2 receptor agonist),luzindole(MT1,MT2 receptor antagonist),4-phenyl-2-propionamidoteralin(4PP,MT2 receptor antagonist).It was found that 8MP could improve PWT and PWL of model rats,reducing the amplitude of C fiber evoked potential and decreasing the expressions of NR2 B,CGRP and Ib4 while luzindole and 4PP can reverse the effect of 8MP,which indicates that MLT may play the role of anti-central sensitization and pain relief mainly through MT2.ConclusionMelatonin can alleviate the development of radicular pain by inhibiting central sensitization of pain transmission,the mechanism may be related to the down-regulation of glutamate receptor NR2 B and the decrease of the release of important C fiber terminal transmitters CGRP and Ib4,and MLT may mainly inhibit central sensitization and relieve pain through MT2 receptor.Therapeutic programs targeting melatonin and its MT2 receptor may provide a new research direction for clinical treatment of radicular pain.