Investigating the Neural Circuits of Itch

Abstract Chronic itch severely diminishes quality of life. Although several neuronal populations that are important for itch have been identified, we lack a population-level view of the neurons that detect itch stimuli in the dorsal root ganglia (DRG), integrate itch in the dorsal horn, and convey itch to the brain. Moreover, although kappa opioid receptor (KOR) agonists inhibit itch, we do not yet have a clear understanding of the neural basis of the inhibition of itch. We propose to bridge these critical gaps in knowledge with 2-photon (2P) Ca2+ imaging of neural populations that are engaged in response to itch and those that are inhibited by KOR agonists. A network view of how pruritic information is initially integrated—and where KOR agonists act to inhibit this circuitry—will provide key insight into the neural coding of itch, and may elucidate new avenues for treatment. Here, we propose to use 2P Ca2+ imaging in the ex vivo somatosensory preparation to define the neurons that are engaged by three types of disease-relevant itch: urticaria from mass cell degranulation, the AEW model of dry skin itch, and the SADBE model of contact dermatitis. In Aim 1 we will characterize the DRG neurons that mediate itch; in Aim 2 we will characterize the corresponding responses of spinal excitatory neurons and spinal projection neurons; and in Aims 3 we will analyze how this pattern of activity is altered in the presence of a KOR agonist, which inhibits itch. Overall, we will test the hypothesis that distinct types of itch engage common excitatory interneurons and common spinal output neurons, and that KOR agonists reduce activity in one or more of these populations, including spinal output neurons. These experiments will provide new insight into the neural circuits that mediate disease-relevant itch and how this circuitry can be inhibited by KOR agonists through a population view of the neurons that detect itch in the periphery, integrate itch in the dorsal horn, and convey itch to the brain.