Neural Circuit Mechanisms of Peripheral Nerve Stimulation for Trigeminal Neuropathic Pain - PROJECT SUMMARY/ABSTRACT Trigeminal neuropathic pain (TNP) is characterized by constant unilateral facial pain caused by a lesion or disease affecting the trigeminal branch of the peripheral nervous system. This type of pain is non-triggerable, unremitting, and varies in intensity. Clinically, TNP poses a significant therapeutic challenge due to a lack of understanding of its pathophysiological mechanisms. Opioids are commonly used to treat this condition, often combined with other medications and interventional pain procedures. However, long- term opioid use leads to tolerance and dependence, necessitating escalating doses to maintain pain relief. Peripheral nerve stimulation (PNS) has emerged as a promising treatment for neuropathic pain, including TNP, without the concerns of tolerance and dependence associated with opioids. Despite its clinical use, the underlying analgesic mechanisms of PNS remain unclear. Evidence suggests that the thalamic reticular nucleus (TRN) plays a crucial role in sensory processing and pain regulation. Animal studies have shown that PNS increases TRN firing in rats and that activating the ventral TRN reduces pain sensitivity in mice. Clinical evidence indicates altered neuronal discharge in the TRN of patients with neuropathic pain. These findings highlight the TRN's importance in neuropathic pain regulation. Our preliminary data showed that PNS effectively alleviated hyperalgesia and suppressed ventral posteromedial nucleus (VPM) hyperactivity induced by infraorbital nerve- chronic constriction injury (IoN-CCI) in mice. c-Fos mapping and intravital imaging revealed that PNS activates TRN GABAergic neurons. Chemogenetic inhibition by designer receptors exclusively activated by designer drugs (DREADDs) of these PNS-activated TRN interneurons abolished the PNS effect of analgesia and suppression of VPM hyperactivity in IoN-CCI mice, while activation of the PNS-activated neurons produced similar analgesic effects to PNS, attenuating IoN-CCI-induced hyperalgesia and VPM hyperactivity. Immunofluorescence staining indicated that the activated TRN cells were predominantly parvalbumin (PV)-expressing neurons (PVNs), and neural tracing in PV-Cre mice showed that these TRN-PVNs primarily project to VPM glutamatergic neurons. Based on these preliminary findings, we hypothesize that the TRN-PVNs activation and the inhibitory projection from TRN to VPM excitatory neurons are crucial to the analgesic effects of PNS. Our proposed aims are: 1) To examine whether the TRN-PVNs is involved in the analgesic effect of PNS in IoN-CCI mice. 2) To investigate if the inhibitory projection of TRN-PVNs to VPM excitatory neurons contributes to the PNS effect on dampening VPM hyperactivity induce by IoN-CCI. We plan to use the Cre-loxP system for optogenetic manipulation of TRN- PVNs in PV-Cre mice to determine the role of the PVNs in the PNS analgesic effect for TNP. Multidisciplinary approaches including DeepLabCut machine learning, multichannel two-photon imaging and optogenetics will be applied. This proposed work will provide a novel conceptual framework for understanding the neural circuit mechanisms of PNS and generate proof-of-concept data for new circuit-based manipulations for TNP treatment.
