Comprehensive Phenotyping of Specific Populations of Spinal Neurons Processing Cutaneous Information Before and After Injury

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ABSTRACT Chronic pain is a debilitating condition for which there is a pressing need for safe, effective treatments. These patients experience enhanced pain sensations and often experience pain when innocuous stimuli are presented. However, the neural basis for this increased sensitivity is poorly understood. Here, we propose to investigate the neural circuit basis for central hyperexcitability that may contribute to persistent pain. We will be combining novel physiological, and pharmacological approaches to address our goals. Using existing transgenic mouse lines, we can express the calcium indicator GCamp6s in all excitatory spinal neurons. We can then use our novel ex vivo skin-spinal cord preparation and 2-photon calcium imaging to examine activity in the spinal dorsal horn activity to cutaneous stimulation in naïve mice and those following injury. While this approach will allow us to image the responses of many (~200) neurons simultaneously, it does not allow us to identify different types of neurons, thereby severely hampering the degree to which the data can be interpreted. Now, we have developed a novel approach to circumvent this limitation through post hoc pharmacological identification of cell types. The underlying concept is that most neurons express one or more Gq-coupled G-protein coupled receptors (GPCRs) whose activation results in the release of Ca2+ from internal stores. In the presence of tetrodotoxin (TTX) to silence neuronal activity, the only neurons that show a Ca2+ transient in response to a given agonist are those that express its receptor. This approach, which we have termed CICADA (Cell-type Identification by Ca2+-coupled Activity through Drug Activation), allows us to unambiguously define cell types based on their responses to a series of GPCR agonists. Now we are uniquely poised to address specific questions about the function of spinal circuitry and how the functional properties of these circuits are altered following injury. In the first Aim we will extend and validate this analysis with the goal of developing a complete repertoire dorsal horn subtypes that can be identified in population imaging studies. While neurons respond to different types of sensory stimuli (e.g., heat, cold, and mechanical), how this coding is manifest across neuronal populations is unclear. In the second Aim we will catalog functional response properties across populations of CICADA-defined cell subtypes. In the third Aim we will examine the effects of capsaicin induced acute central sensitization on these subpopulations. The chronification of pain is thought to be associated with long-term changes in central network activity that perpetuate hyperalgesic states. In the fourth Aim we will identify the CICADA-defined cell types that show altered activity in the context of chronic pain using the spared nerve injury (SNI) as a model. The studies we are proposing here will begin to identify specific spinal circuitry involved in central sensitization and investigate how these specific microcircuits are altered in conditions of acute and chronic injury. This knowledge may elucidate new therapeutic targets for the treatment of pain, which is the long-term goal of research of our program.