Abstract Persistent itch is an aversive condition that results in a severely diminished quality of life. Our approach to address this important health issue is to gain a better understanding of the underlying neural circuits and pathways. Specifically, we propose to investigate the mechanisms through which kappa opioid receptor (KOR) signaling modulates itch. The long-term goal of our research program is aimed at the development of novel interventions for itch that are both safe and effective. We previously found that mice lacking the transcription factor Bhlhb5 show elevated itch, and that this effect is caused by the loss of a specific population spinal inhibitory interneurons that express dynorphin. However, the neurons that respond to dynorphin ? those that express KOR ? remained unknown, and the neural circuits through which KOR agonists inhibit itch are unclear. To address these gaps, our lab developed novel tools (KOR-Cre allele) and approaches (ex vivo preparation) to study this circuitry. We also discovered that in models of pathological itch there is abnormal bursting behavior in lamina I spinal neurons. Here, we propose to elucidate the circuitry through which KOR agonists inhibit itch, combining anatomical (AIM 1), behavioral (AIM 2), physiological (AIM 3), and translational (AIM 4) approaches. Our overall hypothesis is that KOR signaling acts on both primary afferents and spinal neurons to inhibit acute pruritoception, and that KOR agonists will continue to be effective in the presence of persistent itch. Our proposal is innovative because it combines these state-of-the-art tools and approaches to elucidate the neural circuits through which KOR agonists inhibit pruritoception. The combination of conceptual advances and therapeutic insight into the neural circuitry through which KOR agonists inhibit itch makes this proposal highly significant to human health.