Counter-stimuli Inhibit GRPR Neurons via GABAergic Signaling in the Spinal Cord

Abstract

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

Presynaptic inhibition of cutaneous afferents prevents self-generated itch

“…To test the role of Ptf1a EX1 neurons in mechanosensory information gating, we generated a condition of increased itch sensation by activating GRPR neurons using the same intersectional approach (Figure S7K-L). CNO-mediated activation of GRPR neurons induces 20-fold scratching increase compared to vehicle-treated animals ( Figure 7C), as recently reported using a different approach (Bardoni et al, 2018). Simultaneous activation of Ptf1a EX1 and GRPR neurons in the same animal ( Figure 7B), resulted in a 70% reduction in scratching compared to GRPR-activated animals ( Figure 7C).…”

“…Considering that 90% of body surface in mammals is covered by hairy skin (Djouhri et al, 2016), it is tempting to speculate that mechanosensory input to the circuit is used to maintain GRPR neurons at a high subthreshold level of excitation. Some evidence for this mechanism comes from the observation that GRPR specific activation ( Figure 7C and Bardoni et al, 2018) evokes not only scratching behavior but also intense grooming activity ( Figure 7D). It remains to be tested, albeit difficult to accomplish, the behavioral response elicited by GRPR activation in the absence of cutaneous mechanosensory information.…”

Spinal Inhibitory Ptf1a-Derived Neurons Prevent Self-Generated Itch

“…To assess the function of Ptf1a-Cre neurons in controlling GRPR-mediated itch, we used a chemogenetic intersectional approach in which GRPR-Cre and/or Ptf1a-Cre neurons were stimulated (Sciolino et al, 2016) ( Figure 6A; Figures S5A and S5B). Clozapine-N-oxide (CNO)-mediated activation of GRPR-Cre neurons induced a 20-fold increase in the number of scratching bouts compared with vehicle-or CNO-treated control animals ( Figure 6B), as also recently reported using a different approach (Bardoni et al, 2018). Chemogenetic activation of inhibitory Ptf1a-Cre neurons did not elicit changes in somatosensory or motor behavior or in a conditioned place aversion assay ( Figures S5C-S5K).…”

“…Considering that 90% of the body surface of mammals is covered by hairy skin (Djouhri, 2016), it is tempting to speculate that mechanosensory input to the circuit is used to maintain GRPR neurons at a high subthreshold level of excitation. Some evidence for this mechanism comes from the observation that GRPR neuronal activation ( Figure 6B) (Bardoni et al, 2018) evokes not only scratching behavior but also intense grooming activity ( Figure 6C), which could be the outcome of lower-level activation of GRPR neurons. The delayed-scratching phenotype reaches peak intensity, which suggests that other itch-signaling neurons, distinct from GRPR ones, become stimulated upon Ptf1a-Cre ablation.…”

Central circuit mechanisms of itch

“…1a) 22,23 . This is evidenced by data showing that ablation of spinal GRPR + neurons in mice almost completely abolishes the scratching behavior induced by both histamine-dependent and histamine-independent pruritogens 23 , while optogenetic or pharmacogenetic activation of GRPR + neurons directly evokes itch-like scratching or biting behaviors 24 . Consistently, the number of spinal GRPR + neurons are substantially decreased in mice lacking TR4, which show deficits in scratch responses 21 .…”