A myriad of counter-stimuli, including algogens and cooling, could inhibit itch sensation; however, the underlying molecular and neural mechanisms remain poorly understood. Here, we show that the spinal neurons expressing gastrin releasing peptide receptor (GRPR) primarily comprise excitatory interneurons that receive direct and indirect inputs from C and Aδ fibers and form contacts with projection neurons expressing the neurokinin 1 receptor (NK1R). Optical or chemogenetic activation of GRPR neurons evokes itch behavior that is partly dependent on NK1R activation. Importantly, we show that noxious or cooling counter-stimuli inhibit the activity of GRPR neurons via GABAergic signaling. By contrast, capsaicin, which could evoke a mix of itch and pain sensations, could exert both excitatory and inhibitory effects on GRPR neurons. These data strengthen the role of GRPR neurons as a key circuit for itch transmission and illustrate a spinal mechanism whereby counter-stimuli inhibit itch by suppressing the function of GRPR neurons.HighlightsActivation of GRPR neurons evokes itch and is dependent upon NK1R activationGRPR neurons receive both direct and indirect inputs from C/Aδ fibersCounter-stimuli inhibit GRPR neurons via GABAergic signalingIncreased excitability of GRPR neurons in chronic itch condition

The rostral nucleus of the solitary tract (rNST) serves as the first central relay in the gustatory system. In addition to synaptic interactions, central processing is also influenced by the ion-channel composition of individual neurons. For example, voltage-gated K+ channels such as IA can modify the integrative properties of neurons. IA currents are prevalent in rNST projection cells but are also found to a lesser extent in GABAergic interneurons. However, characterization of the kinetic properties of IA, the molecular basis of these currents as well as the consequences of IA on spiking properties of identified rNST cells is lacking. Here we show that IA in rNST GABAergic (G+) and non-GABAergic (G-) neurons share a common molecular basis. In both cell types, there was a reduction in IA following treatment with the specific Kv4 channel blocker AmmTX3. However, the kinetics of activation and inactivation of IA in the two cell types were different with G+ neurons having significantly more negative half-max activation and inactivation values. Likewise, under current clamp, G- cells had significantly longer delays to spike initiation in response to a depolarizing stimulus preceded by a hyperpolarizing pre-pulse. Computational modeling and dynamic clamp suggest that differences in the activation half-max may account for the differences in delay. We further observed evidence for a window current under both voltage clamp and current clamp protocols. We speculate that the location of Kv4.3 channels on dendrites, together with a window current for IA at rest, serves to regulate excitatory afferent inputs.