Chronic pain affects millions of patients worldwide, costing over 600 billion dollars annually for the United States alone. Neuropathic pain is a form of chronic pain that arises as a direct consequence of a lesion or diseases affecting the somatosensory system and is very difficult to treat as the underlying mechanisms of this disease are poorly understood. A stronger excitatory tone in the delicate balance between excitatory and inhibitory interneurons in the dorsal horn of the spinal cord has been strongly implicated in the mechanical and thermal hypersensitivities seen in patients that suffer from neuropathic pain. Exactly how nerve injury disrupts this balance to generate a net pronociceptive tone, however, remains unclear. Promising preliminary data within our laboratory has implicated one specific excitatory interneuron population, those expressing the neuropeptide Y (NPY) Y1 receptor. First, selective lesioning of Y1 receptor-expressing interneurons (Y1Rs) reduces behavioral signs of neuropathic pain. Second, intrathecal administration of a Y1 agonist dose- dependently reduces mechanical and cold hypersensitivity after nerve injury. Third, chemogenetic activation of Y1Rs induces both mechanical and thermal hypersensitivities. These observations provide the premise for my central hypothesis that nerve injury increases the excitability of Y1R interneurons in response to peripheral input, ultimately leading to neuropathic pain. The overarching goals of the following three specific aims are to increase our understanding of how nerve injury increases the excitability of Y1Rs and provide rationale for targeting Y1Rs as a novel approach to treat neuropathic pain.
Specific Aim 1 will test the hypothesis that nerve injury will favor the anatomical expression of excitatory, as compared to inhibitory, synaptic contacts onto Y1Rs. I will utilize high-throughput synapse quantification tools to quantify both putative-excitatory and putative-inhibitory synapses onto Y1Rs after spared nerve injury (SNI). I predict that nerve injury will increase the number of excitatory inputs and/or decrease the number of inhibitory inputs onto Y1Rs.
Specific Aim 2 will test the hypothesis that nerve injury increases presynaptic excitatory drive and/or decreases inhibitory drive onto Y1Rs in dorsal horn. Npy1rGFP mice will undergo SNI or sham surgery followed 14 days later by whole-cell voltage-clamp recordings in spinal cord slices. I will measure both excitatory and inhibitory miniature postsynaptic currents of Y1Rs in sham and SNI slices in order to assess physiological changes in pre- or postsynaptic activity to Y1Rs after injury.
Specific Aim 3 will test the hypothesis that spinal Y1Rs are necessary for neuropathic pain and sufficient to produce pain hypersensitivity. I will use innovative optogenetic spinal LED implants to optogenetically inhibit Y1Rs in nave and nerve injured mice and excite Y1Rs in nave mice in order to understand the behavioral correlates of Y1R activation and inhibition in both healthy and neuropathic mice.
Neuropathic pain is a form of chronic pain caused by a lesion or disease of the somatosensory system that affects millions of individuals worldwide. Imbalances between excitatory and inhibitory signaling within the dorsal horn of the spinal cord have been heavily implicated in the development, maintenance, and progression of neuropathic pain. This study uses next-generation synapse quantification, electrophysiology, and cutting edge in vivo optogenetics to determine whether neuropathic injury increases the excitability of a distinct population of dorsal horn interneurons (those that contain the type 1 receptor for neuropeptide Y), ultimately leading to neuropathic pain.
