SummaryThe deep dorsal horn is a poorly characterized spinal cord region implicated in processing low-threshold mechanoreceptor (LTMR) information. We report an array of mouse genetic tools for defining neuronal components and functions of the dorsal horn LTMR-recipient zone (LTMR-RZ), a role for LTMR-RZ processing in tactile perception, and the basic logic of LTMR-RZ organization. We found an unexpectedly high degree of neuronal diversity in the LTMR-RZ: seven excitatory and four inhibitory subtypes of interneurons exhibiting unique morphological, physiological, and synaptic properties. Remarkably, LTMRs form synapses on between four and 11 LTMR-RZ interneuron subtypes, while each LTMR-RZ interneuron subtype samples inputs from at least one to three LTMR classes, as well as spinal cord interneurons and corticospinal neurons. Thus, the LTMR-RZ is a somatosensory processing region endowed with a neuronal complexity that rivals the retina and functions to pattern the activity of ascending touch pathways that underlie tactile perception.

Stimuli that elicit itch are detected by sensory neurons that innervate the skin. This information is processed by the spinal cord; however, the way in which this occurs is still poorly understood. Here we investigated the neuronal pathways for itch neurotransmission, in particular the contribution of the neuropeptide somatostatin. We find that in the periphery, somatostatin is exclusively expressed in Nppb neurons, and we demonstrate that Nppb/somatostatin-cells function as pruriceptors. Employing chemogenetics, pharmacology and cell-specific ablation methods, we demonstrate that somatostatin potentiates itch by inhibiting inhibitory dynorphin neurons, which results in disinhibition of GRPR neurons. Furthermore, elimination of somatostatin from primary afferents and/or from spinal interneurons demonstrates differential involvement of the peptide released from these sources in itch and pain. Our results define the neural circuit underlying somatostatin-induced itch, and characterize a contrasting anti-nociceptive role for the peptide.

The balance between excitatory and inhibitory synapses is crucial for normal brain function. Wnt proteins stimulate synapse formation by increasing synaptic assembly. However, it is unclear whether Wnt signaling differentially regulates the formation of excitatory and inhibitory synapses. Here, we demonstrate that Wnt7a preferentially stimulates excitatory synapse formation and function. In hippocampal neurons, Wnt7a increases the number of excitatory synapses, whereas inhibitory synapses are unaffected. Wnt7a or postsynaptic expression of Dishevelled-1 (Dvl1), a core Wnt signaling component, increases the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs), but not miniature inhibitory postsynaptic currents (mIPSCs). Wnt7a increases the density and maturity of dendritic spines, whereas Wnt7a-Dvl1-deficient mice exhibit defects in spine morphogenesis and mossy fiber-CA3 synaptic transmission in the hippocampus. Using a postsynaptic reporter for Ca 2+ /Calmodulin-dependent protein kinase II (CaMKII) activity, we demonstrate that Wnt7a rapidly activates CaMKII in spines. Importantly, CaMKII inhibition abolishes the effects of Wnt7a on spine growth and excitatory synaptic strength. These data indicate that Wnt7a signaling is critical to regulate spine growth and synaptic strength through the local activation of CaMKII at dendritic spines. Therefore, aberrant Wnt7a signaling may contribute to neurological disorders in which excitatory signaling is disrupted.Wnt7a | Dvl1 mutant | plasticity T he formation of functional neuronal circuits requires the assembly of different types of synapses with great specificity. The development of an appropriate balance of glutamatergic excitatory and GABAergic inhibitory synapses (E/I ratio) is essential for proper circuit function (1, 2) because an imbalance in the E/I ratio can result in neurological disorders (3-5). Some synaptogenic factors regulate both excitatory and inhibitory synapses (6, 7), whereas other synaptic organizers are more specific (8-10). However, the precise mechanisms by which synaptic organizing signals regulate excitatory and inhibitory synapses remain poorly understood.In the central nervous system, most excitatory inputs are located on dendritic spines, postsynaptic protrusions that function as domains where synaptic activity is regulated in a compartmentalized manner (11-13). Several intracellular molecules have been implicated in the formation and maturation of dendritic spines (14-16), but the mechanisms by which extracellular factors signal through intracellular regulators to promote spine development and maturation remain poorly characterized.Wnt secreted proteins are synaptic organizers that stimulate the formation of central and peripheral synapses (17-19) by promoting presynaptic assembly (20) and the clustering of postsynaptic components (19,(21)(22)(23)(24). In cultured neurons, Wnt5a regulates postsynaptic development of both GABAergic and glutamatergic synapses (24,25). However, it is unclear whether other Wnts play a...

Key points• Perception of normal bodily sensations relies on the precise regulation of sensory information entering the dorsal horn of the spinal cord.• Inhibitory, axoaxonic, synapses provide a mechanism for this regulation, but the source of these important inhibitory connections remains to be elucidated.• This study shows that a subpopulation of spinal interneurons that expresses parvalbumin and have specific morphological, connectivity and functional characteristics are a likely source of the inhibitory inputs that selectivity regulate non-noxious tactile input in the spinal cord.• Our findings suggest that a loss of normal function in parvalbumin positive dorsal horn neurons may result in the development of tactile allodynia, where non-painful stimuli gain the capacity to evoke the sensation of pain.Abstract Axo-axonic synapses on the central terminals of primary afferent fibres modulate sensory input and are the anatomical correlate of presynaptic inhibition. Although several classes of primary afferents are under such inhibitory control, the origin of these presynaptic inputs in the dorsal horn is unknown. Here, we characterize the neurochemical, anatomical and electrophysiological properties of parvalbumin (PV)-expressing cells in wild-type and transgenic mice where enhanced green fluorescent protein (eGFP) is expressed under the PV promoter. We show that most PV cells have either islet or central cell-like morphology, receive inputs from myelinated primary afferent fibres and are concentrated in laminae II inner and III. We also show that inhibitory PV terminals in lamina II inner selectively target the central terminals of myelinated afferents (∼80% of 935 PVeGFP boutons) and form axo-axonic synapses (∼75% of 71 synapses from PV boutons). Targeted whole-cell patch-clamp recordings from PVeGFP positive cells in laminae II and III showed action potential discharge was restricted to the tonic firing and initial bursting patterns (67% and 33% respectively; n = 18), and virtually all express I h subthreshold voltage-gated currents (94%; n = 18). These neurons show higher rheobase current than non-eGFP cells but respond with high frequency action potential discharge upon activation. presynaptic input on to myelinated primary afferents. Consequently PV cells are ideally placed to play an important role in the development of central sensitization and tactile allodynia.

Key pointsr The superficial spinal dorsal horn contains a heterogeneous population of neurons that process sensory inputs.r Information on the properties of excitatory interneurons in this region is limited. As calretinin is a protein thought to be restricted to an excitatory population in this region, the aim of this study was to characterize calretinin-expressing neurons.r Most calretinin cells (85%) exhibited large A-type potassium currents and delayed firing action potential discharge, and received strong excitatory synaptic input, whereas the remainder exhibited hyperpolarization-activated cation currents and low threshold T-type calcium currents, and tonic-or initial bursting firing patterns, and received weak excitatory synaptic input. These respective features are consistent with properties of excitatory and inhibitory interneuron populations in this region of the spinal cord.r Our findings have resolved a previously unidentified population of inhibitory interneurons. Furthermore, the contrasting excitability patterns of excitatory and inhibitory calretinin-expressing neurons suggest that they play distinct roles in spinal sensory processing circuits.Abstract Neurons in the superficial dorsal horn (SDH) of the spinal cord play an important role in nociceptive, thermal, itch and light touch sensations. Excitatory interneurons comprise ß65% of all SDH neurons but surprisingly few studies have investigated their role in spinal sensory processing. Here we use a transgenic mouse to study putative excitatory SDH neurons that express the calcium binding protein calretinin (CR). Our immunocytochemical, morphological and electrophysiological analysis identified two distinct populations of CR-expressing neurons, which we termed 'Typical' and 'Atypical' . Typical CR-expressing neurons comprised ß85% of the population and exhibited characteristic excitatory interneuron properties including delayed firing discharge, large rapid A-type potassium currents, and central, radial or vertical cell morphologies. Atypical neurons exhibited properties consistent with inhibitory interneurons, including tonic firing or initial bursting discharge, I h currents, and islet cell morphology. Although both

Summary Chronic pain presents a major unmet clinical problem. The development of more effective treatments is hindered by our limited understanding of the neuronal circuits underlying sensory perception. Here, we show that parvalbumin (PV)-expressing dorsal horn interneurons modulate the passage of sensory information conveyed by low-threshold mechanoreceptors (LTMRs) directly via presynaptic inhibition and also gate the polysynaptic relay of LTMR input to pain circuits by inhibiting lamina II excitatory interneurons whose axons project into lamina I. We show changes in the functional properties of these PV interneurons following peripheral nerve injury and that silencing these cells unmasks a circuit that allows innocuous touch inputs to activate pain circuits by increasing network activity in laminae I–IV. Such changes are likely to result in the development of tactile allodynia and could be targeted for more effective treatment of mechanical pain.

HighlightsNeurochemistry of lamina I–II inhibitory neurons in mouse is similar to that in rat.Five neurochemical classes account for all lamina I–II inhibitory neurons in mouse.Excitatory dynorphin cells are largely restricted to glabrous skin territory.

Expression of the substance P precursor preprotachykinin A defines a distinct population of superficial dorsal horn excitatory neurons, many of which respond to noxious or pruritic stimuli.