Spinal Neuropeptide Y1 Receptor-Expressing Neurons Form an Essential Excitatory Pathway for Mechanical Itch

Abstract

Graphical Abstract Highlights d Excitatory NPYR1 Cre (Y1 Cre ) neurons are required for mechanical itch transmission d Spinal Y1 Cre neurons receive LTMR input and mediate light punctate touch d NPY::Cre interneurons inhibit Y1-expressing neurons in the dorsal horn d NPY signaling via dorsal horn Y1-expressing neurons gates mechanical itch In Brief Acton et al. identify the excitatory neurons in the dorsal spinal cord that drive mechanical itch. These cells mediate responses to light punctate touch and are inhibited by neuropeptide Y (NPY)::Cre interneurons. Light touch sensitivity and mechanical itch responses are gated by NPY signaling mediated by Y1-expressing neurons in the dorsal horn. Mechanical itch Disrupted inhibitory gating Normal inhibitory gating Y1 + Low-threshold mechanoreceptors NPY + Normal touch discrimination NPY Y1 + NPY + Mechanical itch NPY Low-threshold mechanoreceptors SUMMARYAcute itch can be generated by either chemical or mechanical stimuli, which activate separate pathways in the periphery and spinal cord. While substantial progress has been made in mapping the transmission pathway for chemical itch, the central pathway for mechanical itch remains obscure. Using complementary genetic and pharmacological manipulations, we show that excitatory neurons marked by the expression of the neuropeptide Y1 receptor (Y1 Cre neurons) form an essential pathway in the dorsal spinal cord for the transmission of mechanical but not chemical itch. Ablating or silencing the Y1 Cre neurons abrogates mechanical itch, while chemogenetic activation induces scratching. Moreover, using Y1 conditional knockout mice, we demonstrate that endogenous neuropeptide Y (NPY) acts via dorsalhorn Y1-expressing neurons to suppress light punctate touch and mechanical itch stimuli. NPY-Y1 signaling thus regulates the transmission of innocuous tactile information by establishing biologically relevant thresholds for touch discrimination and mechanical itch reflexes.

Presynaptic inhibition of cutaneous afferents prevents self-generated itch

“…The positive correlation of animal activity with scratching frequency and that of hair stimulation with increased dorsal horn neural activity suggest that selfgenerated activation of hair mechanosensory inputs during locomotion triggers an exacerbated scratch response in the absence of Ptf1a EX1 neurons inhibition. Lack of an acute mechanical itch phenotype as the one previously described for NPY-ablated animals (Bourane et al, 2015;Acton et al, 2019;Pan et al, 2019) agrees with our Ptf1a EX1 targeting data, which reveals unaffected expression of NPY in the dorsal horns of Ptf1a-ablated animals. These results point to diverse subsets of presynaptic inhibitory neurons specialized in the gating of different mechanosensory information submodalities.…”

“…Our discovery that DREADD-mediated activation of spinal GRPR neurons leads to increased scratching and grooming together with a delayed appearance of scratching phenotype in the simultaneous Ptf1a EX1 and GRPR neurons ablation paradigm, begs the question about the connection between itch and innocuous tactile information processing. Previous studies reported no changes in response to acute mechanical itch stimuli presented to GRPR-ablated mice (Bourane et al, 2015;Pan et al, 2019;Acton et al, 2019). However, such type of stimuli presentation, in which skin is pre-conditioned by removing hair before the experiment, would reduce hair mechanosensory input to the GRPR circuit, probably masking any outcome in response to hair activation.…”

“…Ablation was induced by intraperitoneal administration of diphtheria toxin at 50ng/g of body weight (D0564, Sigma-Aldrich) in saline solution. Injection was performed twice with a 72 hours interval (Duan et al, 2014;Bourane et al, 2015;Acton et al, 2019;Pan et al, 2019). GRPR antagonist RC-3095 (Tocris) was dissolved in 0.1% DMSO in saline to 2µg/µl and injected intraperitoneally at 10µg/g of body weight 30 minutes before behavioral evaluation.…”

“…Recently, it was postulated that only chemical but not mechanical itch signals are conveyed through the GRPR pathway. Indeed, it has been shown very recently that dorsal horn neurons expressing the neuropeptide Y1 receptor (NPY1R, Acton et al, 2019) and/or urocortin 3 (UBC3, Pan et al, 2019) transmit exclusively mechanical itch information. However, the nature of the stimulus inducing the spontaneous scratching in these mutant/ablated mice, that leads to developing self-inflicted skin wounds, has remained elusive.…”

The Challenge of Basic Itch Research

“…This suggests that Merkel cells connected to slowly adapting type I (SAI) afferents excite NPY-expressing interneurons to inhibit spinal itch transmission. It was very recently reported that activation of neurons expressing the NPY-1 receptor promotes mechanical itch (68). Mechanical itch was not affected following ablation of spinal neurons expressing the NK-1 receptor, implying that mechanical itch is transmitted via a pathway independent of that for chemical itch (Fig.…”