Light mechanical stimulation of the hairy skin can induce a form of itch known as mechanical itch. This itch sensation is normally suppressed by inputs from mechanoreceptors, however, in many forms of chronic itch, including alloknesis, this gating mechanism is lost. Here we demonstrate that a population of spinal inhibitory interneurons (INs) that are defined by the expression of neuropeptide Y::Cre (NPY::Cre) act to gate mechanical itch. Mice in which dorsal NPY::Cre-derived neurons are selectively ablated or silenced develop mechanical itch without an increase in sensitivity to chemical itch or pain. This chronic itch state is histamine-independent and is transmitted independently of the GRP-GRPR signaling pathway. Our studies thereby reveal a dedicated spinal cord inhibitory pathway that gates the transmission of mechanical itch

Sensory circuits in the dorsal spinal cord integrate and transmit multiple cutaneous sensory modalities including the sense of light touch. Here we identify a population of excitatory interneurons (INs) in the dorsal horn that are important for transmitting innocuous light touch sensation. These neurons express the ROR alpha (RORα) nuclear orphan receptor and are selectively innervated by cutaneous low threshold mechanoreceptors (LTMs). Targeted removal of RORα INs in the dorsal spinal cord leads a marked reduction in behavioral responsiveness to light touch without affecting responses to noxious and itch stimuli. RORα IN-deficient mice also display a selective deficit in corrective foot movements. This phenotype, together with our demonstration that the RORα INs are innervated by corticospinal and vestibulospinal projection neurons, argues that the RORα INs direct corrective reflex movements by integrating touch information with descending motor commands from the cortex and cerebellum.

Animals depend on sensory feedback from mechanosensory afferents for the dynamic control of movement. This sensory feedback needs to be selectively modulated in a task- and context-dependent manner. Here, we show that inhibitory interneurons (INs) expressing the RORβ orphan nuclear receptor gate sensory feedback to the spinal motor system during walking and are required for the production of a fluid locomotor rhythm. Genetic manipulations that abrogate inhibitory RORβ IN function result in an ataxic gait characterized by exaggerated flexion movements and marked alterations to the step cycle. Inactivation of RORβ in inhibitory neurons leads to reduced presynaptic inhibition and changes to sensory-evoked reflexes, arguing that the RORβ inhibitory INs function to suppress the sensory transmission pathways that activate flexor motor reflexes and interfere with the ongoing locomotor program. VIDEO 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.

Inhibitory neurons in the spinal cord perform dedicated roles in processing somatosensory information and shaping motor behaviors that range from simple protective reflexes to more complex motor tasks such as locomotion, reaching and grasping. Recent efforts examining inhibition in the spinal cord have been directed toward determining how inhibitory cell types are specified and incorporated into the sensorimotor circuitry, identifying and characterizing molecularly-defined cohorts of inhibitory neurons and interrogating the functional contribution these cells make to sensory processing and motor behaviors. Rapid progress is being made on all these fronts, driven in large part by molecular genetic and optogenetic approaches that are being creatively combined with neuroanatomical, electrophysiological and behavioral techniques.

Glutamate is the neurotransmitter released from hair cells. Its clearance from the synaptic cleft can shape neurotransmission and prevent excitotoxicity. This may be particularly important in the inner ear and in other sensory organs where there is a continually high rate of neurotransmitter release. In the case of most cochlear and type II vestibular hair cells, clearance involves the diffusion of glutamate to supporting cells, where it is taken up by EAAT1 (GLAST), a glutamate transporter. A similar mechanism cannot work in vestibular type I hair cells as the presence of calyx endings separates supporting cells from hair-cell synapses. Because of this arrangement, it has been conjectured that a glutamate transporter must be present in the type I hair cell, the calyx ending, or both. Using whole-cell patch-clamp recordings, we demonstrate that a glutamate-activated anion current, attributable to a high-affinity glutamate transporter and blocked by DL-TBOA, is expressed in type I, but not in type II hair cells. Molecular investigations reveal that EAAT4 and EAAT5, two glutamate transporters that could underlie the anion current, are expressed in both type I and type II hair cells and in calyx endings. EAAT4 has been thought to be expressed almost exclusively in the cerebellum and EAAT5 in the retina. Our results show that these two transporters have a wider distribution in mice. This is the first demonstration of the presence of transporters in hair cells and provides one of the few examples of EAATs in presynaptic elements.