Mrgprb4-Lineage Neurons Participate in Intervention of TENS on Chronic Pain and Anxiety-Like Symptoms in Inflammatory Pain Mice Model

Background :Mas-related G-protein-coupled receptor b4 (Mrgprb4)-lineage neurons in the peripheral nervous system, were a type of C fibers in the hairy skin. Our prior work demonstrated that these neurons respond to both noxious and innocuous mechanical and thermal stimuli. Ablating them eliminates the pleasant sensation elicited by gentle pressure on the mouse nape. However, their potential role in mitigating pain and pain-related negative emotions in response to somatic stimuli remains unclear. Methods:Animal experiments investigated the pivotal role of Mrgprb4-lineage neurons in mediating the analgesic and anxiolytic effects of transcutaneous electrical nerve stimulation (TENS) applied to the Zusanli (ST36) acupoint. In vivo calcium imaging of lumbar 4 dorsal root ganglia (DRG) neurons in Mrgprb4-GCaMP6s transgenic mice characterized neuronal encode of distinct TENS intensities. Mechanical pain thresholds and anxiety-like behaviors were assessed in a CFA-induced mouse model of comorbid chronic pain and anxiety. To simulate TENS, optogenetic stimulation was applied to the ST36 acupoint in Mrgprb4-ChR2 mice; intrathecal viral injection specifically ablated L3-L5 Mrgprb4-lineage neurons, and TENS effects were evaluated with their gain- or loss-of-function manipulation. Results: 0.5 mA TENS on ST36 ameliorated pain and anxiety-like behaviors in model mice and activated Mrgprb4-lineage neurons. Photostimulation on ST36 induced analgesic and anxiolytic effect in comorbidity of chronic pain and anxiety model of Mrgprb4-ChR2 mice. Ablating these neurons attenuated the therapeutic effects of 0.5 mA TENS in model mice. Conclusion:These genetic engineering-assisted findings may deepen our understanding of the analgesic and anxiolytic effect and mechanism of somatic stimulation and further improve the clinical efficacy.