Project 4: Genetic dissection of pain and itch pathways The long-range goal of this project is to understand how pain and itch information is processed in mammals. A great achievement in modern neurobiology has been the discovery of sensory receptors and ion channels critical for the detection of noxious painful stimuli. However, itch-related fibers can also respond to painful stimuli, as a result of expressing "pain" channels and receptors such as TRPV1. As a result, the cellular identifies of itch-specific versus pain-specific fibers are still poorly characterized. In this project, we will use genetic strategies to silence or ablate specific subsets of DRG neurons, followed by determining the effects on pain and itch. In preliminary studies, we have generated the Vglut2 conditional knockout (CKO) mice, in which excitatory glutamatergic synaptic transmission from a specific population of nociceptors expressing the TTX-resistant sodium channel Nav1 .8 is eliminated, by selectively removing from these neurons VGLUT2, a transporter that packages glutamate into synaptic vesicles. These animals show deficits in specific types of pain and more strikingly, show greatly enhanced itching. Meanwhile, we are using an intersectional genetic strategy to create DRG-Tac1-GFP-Toxin mice, in which DRG neurons expressing the tachykinin 1 gene Tad will be either silenced or ablated, without affecting Tad* neurons in the central nervous system. Additionally, we will genetically mark Nav1.8* and Tad* neurons with the expression of red or green fluorescent proteins (RFP or GFP). By using these mice, we will first perform in vitro calcium imaging and electrophysiological recording to determine the proportions of Nav1 .8*, Navl.8", Tad*, and Tad" neurons that respond to agonists of individual TRP or P2X channels, as well as to itch-inducing compounds. We will then use the behavioral core to measure acute and chronic pain and itch behaviors in Vglut2 CKO mice, DRG-Tad-GFP-Toxin mice, and their control littermates. These studies will allow us to determine the contributions of Navl.8*, Navl.8", Tad*, and Tad" neurons to pain or itch behaviors. We will also determine if local application of the charged sodium channel blocker QX-314 or calcium channel blockers can be used to treat chronic neurogenic itch developed in Vglut2 CKO mice. The genetic silencing/ablation strategies will complement pharmacological silencing strategies used in other three projects, and will collectively help to characterize the identities of pain-related and itch related sensory neurons.
Pain and itch management remains a major medical problem in a variety of human diseases. Chronic pain or itch, moreover, is associated with worse disease outcome and depression. In the fullness of time, the work may allow us to characterize specific sensory neurons dedicated to the processing of pain or itch, thereby providing new therapeutic targets for pain or itch management.
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