Protein kinase C gamma interneurons in the rat medullary dorsal horn: distribution and synaptic inputs to these neurons, and subcellular localization of the enzyme

Spinal Glycine Receptor Alpha 3 Cells Communicate Sensations of Chemical Itch in Hairy Skin

Glycinergic neurons regulate nociceptive and pruriceptive signaling in the spinal cord, but the identity and role of the glycine-regulated neurons are not fully known. Herein, we have characterized spinal glycine receptor alpha 3 (Glra3) subunit-expressing neurons in Glra3-Cre female and male mice. Glra3-Cre(+) neurons express Glra3, are located mainly in laminae III-VI, and respond to glycine. Chemogenetic activation of spinal Glra3-Cre(+) neurons induced biting/licking, stomping, and guarding behaviors, indicative of both a nociceptive and pruriceptive role for this population. Chemogenetic inhibition did not affect mechanical or thermal responses but reduced behaviors evoked by compound 48/80 and chloroquine, revealing a pruriceptive role for these neurons. Spinal cells activated by compound 48/80 or chloroquine express Glra3, further supporting the phenotype. Retrograde tracing revealed that spinal Glra3-Cre(+) neurons receive input from afferents associated with pain and itch, and dorsal root stimulation validated the monosynaptic input. In conclusion, these results show that spinal Glra3(+) neurons contribute to acute communication of compound 48/80- and chloroquine-induced itch in hairy skin.

Synaptic Targets of Glycinergic Neurons in Laminae I-III of the Spinal Dorsal Horn

A great deal of evidence supports the inevitable importance of spinal glycinergic inhibition in the development of chronic pain conditions. However, it remains unclear how glycinergic neurons contribute to the formation of spinal neural circuits underlying pain-related information processing. Thus, we intended to explore the synaptic targets of spinal glycinergic neurons in the pain processing region (laminae I-III) of the spinal dorsal horn by combining transgenic technology with immunocytochemistry and in situ hybridization accompanied by light and electron microscopy. First, our results suggest that, in addition to neurons in laminae I-III, glycinergic neurons with cell bodies in lamina IV may contribute substantially to spinal pain processing. On the one hand, we show that glycine transporter 2 immunostained glycinergic axon terminals target almost all types of excitatory and inhibitory interneurons identified by their neuronal markers in laminae I-III. Thus, glycinergic postsynaptic inhibition, including glycinergic inhibition of inhibitory interneurons, must be a common functional mechanism of spinal pain processing. On the other hand, our results demonstrate that glycine transporter 2 containing axon terminals target only specific subsets of axon terminals in laminae I-III, including nonpeptidergic nociceptive C fibers binding IB4 and nonnociceptive myelinated A fibers immunoreactive for type 1 vesicular glutamate transporter, indicating that glycinergic presynaptic inhibition may be important for targeting functionally specific subpopulations of primary afferent inputs.

GABAA and Glycine Receptor-Mediated Inhibitory Synaptic Transmission onto Adult Rat Lamina IIi PKCγ-Interneurons: Pharmacological but not Anatomical Specialization

Mechanical allodynia (pain to normally innocuous tactile stimuli) is a widespread symptom of inflammatory and neuropathic pain. Spinal or medullary dorsal horn (SDH or MDH) circuits mediating tactile sensation and pain need to interact in order to evoke mechanical allodynia. PKCγ-expressing (PKCγ⁺) interneurons and inhibitory controls within SDH/MDH inner lamina II (II_i) are pivotal in connecting touch and pain circuits. However, the relative contribution of GABA and glycine to PKCγ⁺ interneuron inhibition remains unknown. We characterized inhibitory inputs onto PKCγ⁺ interneurons by combining electrophysiology to record spontaneous and miniature IPSCs (sIPSCs, mIPSCs) and immunohistochemical detection of GABA_ARα2 and GlyRα1 subunits in adult rat MDH. While GlyR-only- and GABA_AR-only-mediated mIPSCs/sIPSCs are predominantly recorded from PKCγ⁺ interneurons, immunohistochemistry reveals that ~80% of their inhibitory synapses possess both GABA_ARα2 and GlyRα1. Moreover, nearly all inhibitory boutons at gephyrin-expressing synapses on these cells contain glutamate decarboxylase and are therefore GABAergic, with around half possessing the neuronal glycine transporter (GlyT2) and therefore being glycinergic. Thus, while GABA and glycine are presumably co-released and GABA_ARs and GlyRs are present at most inhibitory synapses on PKCγ⁺ interneurons, these interneurons exhibit almost exclusively GABA_AR-only and GlyR-only quantal postsynaptic inhibitory currents, suggesting a pharmacological specialization of their inhibitory synapses.

Glycine Receptors in Spinal Nociceptive Control-An Update

Diminished inhibitory control of spinal nociception is one of the major culprits of chronic pain states. Restoring proper synaptic inhibition is a well-established rational therapeutic approach explored by several pharmaceutical companies. A particular challenge arises from the need for site-specific intervention to avoid deleterious side effects such as sedation, addiction, or impaired motor control, which would arise from wide-range facilitation of inhibition. Specific targeting of glycinergic inhibition, which dominates in the spinal cord and parts of the hindbrain, may help reduce these side effects. Selective targeting of the α3 subtype of glycine receptors (GlyRs), which is highly enriched in the superficial layers of the spinal dorsal horn, a key site of nociceptive processing, may help to further narrow down pharmacological intervention on the nociceptive system and increase tolerability. This review provides an update on the physiological properties and functions of α3 subtype GlyRs and on the present state of related drug discovery programs.

Mechanical Allodynia Circuitry in the Dorsal Horn Is Defined by the Nature of the Injury

The spinal dorsal horn is a major site for the induction and maintenance of mechanical allodynia, but the circuitry that underlies this clinically important form of pain remains unclear. The studies presented here provide strong evidence that the neural circuits conveying mechanical allodynia in the dorsal horn differ by the nature of the injury. Calretinin (CR) neurons in lamina II inner convey mechanical allodynia induced by inflammatory injuries, while protein kinase C gamma (PKCγ) neurons at the lamina II/III border convey mechanical allodynia induced by neuropathic injuries. Cholecystokinin (CCK) neurons located deeper within the dorsal horn (laminae III-IV) are important for both types of injuries. Interestingly, the Maf⁺ subset of CCK neurons is composed of transient vesicular glutamate transporter 3 (tVGLUT3) neurons, which convey primarily dynamic allodynia. Identification of an etiology-based circuitry for mechanical allodynia in the dorsal horn has important implications for the mechanistic and clinical understanding of this condition.

Recent advances in our understanding of the organization of dorsal horn neuron populations and their contribution to cutaneous mechanical allodynia

The dorsal horns of the spinal cord and the trigeminal nuclei in the brainstem contain neuron populations that are critical to process sensory information. Neurons in these areas are highly heterogeneous in their morphology, molecular phenotype and intrinsic properties, making it difficult to identify functionally distinct cell populations, and to determine how these are engaged in pathophysiological conditions. There is a growing consensus concerning the classification of neuron populations, based on transcriptomic and transductomic analyses of the dorsal horn. These approaches have led to the discovery of several molecularly defined cell types that have been implicated in cutaneous mechanical allodynia, a highly prevalent and difficult-to-treat symptom of chronic pain, in which touch becomes painful. The main objective of this review is to provide a contemporary view of dorsal horn neuronal populations, and describe recent advances in our understanding of on how they participate in cutaneous mechanical allodynia.

PKCγ interneurons, a gateway to pathological pain in the dorsal horn

Chronic pain is a frequent and disabling condition that is significantly maintained by central sensitization, which results in pathological amplification of responses to noxious and innocuous stimuli. As such, mechanical allodynia, or pain in response to a tactile stimulus that does not normally provoke pain, is a cardinal feature of chronic pain. Recent evidence suggests that the dorsal horn excitatory interneurons that express the γ isoform of protein kinase C (PKCγ) play a critical role in the mechanism of mechanical allodynia during chronic pain. Here, we review this evidence as well as the main aspects of the development, anatomy, electrophysiology, inputs, outputs, and pathophysiology of dorsal horn PKCγ neurons. Primary afferent high-threshold neurons transmit the nociceptive message to the dorsal horn of the spinal cord and trigeminal system where it activates second-order nociceptive neurons relaying the information to the brain. In physiological conditions, low-threshold mechanoreceptor inputs activate inhibitory interneurons in the dorsal horn, which may control activation of second-order nociceptive neurons. During chronic pain, low-threshold mechanoreceptor inputs now activate PKCγ neurons that forward the message to second-order nociceptive neurons, turning thus tactile inputs into pain. Several mechanisms may contribute to opening this gate, including disinhibition, activation of local astrocytes, release of diffusible factors such as reactive oxygen species, and alteration of the descending serotoninergic control on PKCγ neurons through 5-HT_2A serotonin receptors. Dorsal horn PKCγ neurons, therefore, appear as a relevant therapeutic target to alleviate mechanical allodynia during chronic pain.

Cav3.2 T-type calcium channels shape electrical firing in mouse Lamina II neurons

The T-type calcium channel, Cav3.2, is necessary for acute pain perception, as well as mechanical and cold allodynia in mice. Being found throughout sensory pathways, from excitatory primary afferent neurons up to pain matrix structures, it is a promising target for analgesics. In our study, Cav3.2 was detected in ~60% of the lamina II (LII) neurons of the spinal cord, a site for integration of sensory processing. It was co-expressed with Tlx3 and Pax2, markers of excitatory and inhibitory interneurons, as well as nNOS, calretinin, calbindin, PKCγ and not parvalbumin. Non-selective T-type channel blockers slowed the inhibitory but not the excitatory transmission in LII neurons. Furthermore, T-type channel blockers modified the intrinsic properties of LII neurons, abolishing low-threshold activated currents, rebound depolarizations, and blunting excitability. The recording of Cav3.2-positive LII neurons, after intraspinal injection of AAV-DJ-Cav3.2-mcherry, showed that their intrinsic properties resembled those of the global population. However, Cav3.2 ablation in the dorsal horn of Cav3.2^GFP-Flox KI mice after intraspinal injection of AAV-DJ-Cav3.2-Cre-IRES-mcherry, had drastic effects. Indeed, it (1) blunted the likelihood of transient firing patterns; (2) blunted the likelihood and the amplitude of rebound depolarizations, (3) eliminated action potential pairing, and (4) remodeled the kinetics of the action potentials. In contrast, the properties of Cav3.2-positive neurons were only marginally modified in Cav3.1 knockout mice. Overall, in addition to their previously established roles in the superficial spinal cord and in primary afferent neurons, Cav3.2 channel appear to be necessary for specific, significant and multiple controls of LII neuron excitability.

Synaptic Organization of VGLUT3 Expressing Low-Threshold Mechanosensitive C Fiber Terminals in the Rodent Spinal Cord

Low-threshold mechanosensitive C fibers (C-LTMRs) that express the vesicular glutamate transporter VGLUT3 are thought to signal affective touch, and may also play a role in mechanical allodynia. However, the nature of the central termination of C-LTMRs in the dorsal horn remains largely unexplored. Here, we used light and electron microscopy in combination with VGLUT3 immunolabeling as a marker of C-LTMR terminations to investigate this issue. VGLUT3⁺ C-LTMRs formed central terminals of Type II glomeruli in the inner part of lamina II of the dorsal horn, often establishing multiple asymmetric synapses with postsynaptic dendrites but also participating in synaptic configurations with presynaptic axons and dendrites. Unexpectedly, essentially all VGLUT3⁺ C-LTMR terminals showed substantial VGLUT1 expression in the rat, whereas such terminals in mice lacked VGLUT1. Most VGLUT3⁺ C-LTMR terminals exhibited weak-to-moderate VGLUT2 expression. Further, C-LTMR terminals formed numerous synapses with excitatory protein kinase Cγ (PKCγ) interneurons and inhibitory parvalbumin neurons, whereas synapses with calretinin neurons were scarce. C-LTMR terminals rarely if ever established synapses with neurokinin 1 receptor (NK1R)-possessing dendrites traversing lamina II. Thus, VGLUT3⁺ C-LTMR terminals appear to largely correspond to neurofilament-lacking central terminals of Type II glomeruli in inner lamina II and can thus be identified at the ultrastructural level by morphological criteria. The participation of C-LTMR terminals in Type II glomeruli involving diverse populations of interneuron indicates highly complex modes of integration of C-LTMR mediated signaling in the dorsal horn. Furthermore, differences in VGLUT1 expression indicate distinct species differences in synaptic physiology of C-LTMR terminals.

5-HT2A Receptor-Induced Morphological Reorganization of PKCγ-Expressing Interneurons Gates Inflammatory Mechanical Allodynia in Rat

Mechanical allodynia, a widespread pain symptom that still lacks effective therapy, is associated with the activation of a dorsally directed polysynaptic circuit within the spinal dorsal horn (SDH) or medullary dorsal horn (MDH), whereby tactile inputs into deep SDH/MDH can gain access to superficial SDH/MDH, eliciting pain. Inner lamina II (II_i) interneurons expressing the γ isoform of protein kinase C (PKCγ⁺) are key elements for allodynia circuits, but how they operate is still unclear. Combining behavioral, ex vivo electrophysiological, and morphological approaches in an adult rat model of facial inflammatory pain (complete Freund's adjuvant, CFA), we show that the mechanical allodynia observed 1 h after CFA injection is associated with the following (1) sensitization (using ERK1/2 phosphorylation as a marker) and (2) reduced dendritic arborizations and enhanced spine density in exclusively PKCγ⁺ interneurons, but (3) depolarized resting membrane potential (RMP) in all lamina II_i PKCγ⁺/PKCγ^- interneurons. Blocking MDH 5HT_2A receptors (5-HT_2AR) prevents facial mechanical allodynia and associated changes in the morphology of PKCγ⁺ interneurons, but not depolarized RMP in lamina II_i interneurons. Finally, activation of MDH 5-HT_2AR in naive animals is enough to reproduce the behavioral allodynia and morphological changes in PKCγ⁺ interneurons, but not the electrophysiological changes in lamina II_i interneurons, induced by facial inflammation. This suggests that inflammation-induced mechanical allodynia involves strong morphological reorganization of PKCγ⁺ interneurons via 5-HT_2AR activation that contributes to open the gate for transmission of innocuous mechanical inputs to superficial SDH/MDH pain circuitry. Preventing 5-HT_2AR-induced structural plasticity in PKCγ⁺ interneurons might represent new avenues for the specific treatment of inflammation-induced mechanical hypersensitivity.SIGNIFICANCE STATEMENT Inflammatory or neuropathic pain syndromes are characterized by pain hypersensitivity such as mechanical allodynia (pain induced by innocuous mechanical stimuli). It is generally assumed that mechanisms underlying mechanical allodynia, because they are rapid, must operate at only the level of functional reorganization of spinal or medullary dorsal horn (MDH) circuits. We discovered that facial inflammation-induced mechanical allodynia is associated with rapid and strong structural remodeling of specifically interneurons expressing the γ isoform of protein kinase C (PKCγ) within MDH inner lamina II. Moreover, we elucidated a 5-HT_2A receptor to PKCγ/ERK1/2 pathway leading to the behavioral allodynia and correlated morphological changes in PKCγ interneurons. Therefore, descending 5-HT sensitize PKCγ interneurons, a putative "gate" in allodynia circuits, via 5-HT_2A receptor-induced structural reorganization.

Inhibition Mediated by Glycinergic and GABAergic Receptors on Excitatory Neurons in Mouse Superficial Dorsal Horn Is Location-Specific but Modified by Inflammation

The superficial dorsal horn is the synaptic termination site for many peripheral sensory fibers of the somatosensory system. A wide range of sensory modalities are represented by these fibers, including pain, itch, and temperature. Because the involvement of local inhibition in the dorsal horn, specifically that mediated by the inhibitory amino acids GABA and glycine, is so important in signal processing, we investigated regional inhibitory control of excitatory interneurons under control conditions and peripheral inflammation-induced mechanical allodynia. We found that excitatory interneurons and projection neurons in lamina I and IIo are dominantly inhibited by GABA while those in lamina IIi and III are dominantly inhibited by glycine. This was true of identified neuronal subpopulations: neurokinin 1 receptor-expressing (NK1R+) neurons in lamina I were GABA-dominant while protein kinase C gamma-expressing (PKCγ+) neurons at the lamina IIi-III border were glycine-dominant. We found this pattern of synaptic inhibition to be consistent with the distribution of GABAergic and glycinergic neurons identified by immunohistochemistry. Following complete Freund's adjuvant injection into mouse hindpaw, the frequency of spontaneous excitatory synaptic activity increased and inhibitory synaptic activity decreased. Surprisingly, these changes were accompanied by an increase in GABA dominance in lamina IIi. Because this shift in inhibitory dominance was not accompanied by a change in the number of inhibitory synapses or the overall postsynaptic expression of glycine receptor α1 subunits, we propose that the dominance shift is due to glycine receptor modulation and the depressed function of glycine receptors is partially compensated by GABAergic inhibition.SIGNIFICANCE STATEMENT Pain associated with inflammation is a sensation we would all like to minimize. Persistent inflammation leads to cellular and molecular changes in the spinal cord dorsal horn, including diminished inhibition, which may be responsible for enhance excitability. Investigating inhibition in the dorsal horn following peripheral inflammation is essential for development of improved ways to control the associated pain. In this study, we have elucidated regional differences in inhibition of excitatory interneurons in mouse dorsal horn. We have also discovered that the dominating inhibitory neurotransmission within specific regions of dorsal horn switches following peripheral inflammation and the accompanying hypersensitivity to thermal and mechanical stimuli. Our novel findings contribute to a more complete understanding of inflammatory pain.

Neuronal networks and nociceptive processing in the dorsal horn of the spinal cord

The dorsal horn (DH) of the spinal cord receives a variety of sensory information arising from the inner and outer environment, as well as modulatory inputs from supraspinal centers. This information is integrated by the DH before being forwarded to brain areas where it may lead to pain perception. Spinal integration of this information relies on the interplay between different DH neurons forming complex and plastic neuronal networks. Elements of these networks are therefore potential targets for new analgesics and pain-relieving strategies. The present review aims at providing an overview of the current knowledge on these networks, with a special emphasis on those involving interlaminar communication in both physiological and pathological conditions.

A novel relay nucleus between the inferior colliculus and the optic tectum in the chicken (Gallus gallus)

Processing multimodal sensory information is vital for behaving animals in many contexts. The barn owl, an auditory specialist, is a classic model for studying multisensory integration. In the barn owl, spatial auditory information is conveyed to the optic tectum (TeO) by a direct projection from the external nucleus of the inferior colliculus (ICX). In contrast, evidence of an integration of visual and auditory information in auditory generalist avian species is completely lacking. In particular, it is not known whether in auditory generalist species the ICX projects to the TeO at all. Here we use various retrograde and anterograde tracing techniques both in vivo and in vitro, intracellular fillings of neurons in vitro, and whole-cell patch recordings to characterize the connectivity between ICX and TeO in the chicken. We found that there is a direct projection from ICX to the TeO in the chicken, although this is small and only to the deeper layers (layers 13-15) of the TeO. However, we found a relay area interposed among the IC, the TeO, and the isthmic complex that receives strong synaptic input from the ICX and projects broadly upon the intermediate and deep layers of the TeO. This area is an external portion of the formatio reticularis lateralis (FRLx). In addition to the projection to the TeO, cells in FRLx send, via collaterals, descending projections through tectopontine-tectoreticular pathways. This newly described connection from the inferior colliculus to the TeO provides a solid basis for visual-auditory integration in an auditory generalist bird. J. Comp. Neurol. 525:513-534, 2017. © 2016 Wiley Periodicals, Inc.

Lamina specific postnatal development of PKCγ interneurons within the rat medullary dorsal horn

Protein kinase C gamma (PKCγ) interneurons, located in the superficial spinal (SDH) and medullary dorsal horns (MDH), have been shown to play a critical role in cutaneous mechanical hypersensitivity. However, a thorough characterization of their development in the MDH is lacking. Here, it is shown that the number of PKCγ-ir interneurons changes from postnatal day 3 (P3) to P60 (adult) and such developmental changes differ according to laminae. PKCγ-ir interneurons are already present at P3-5 in laminae I, IIo, and III. In lamina III, they then decrease from P11-P15 to P60. Interestingly, PKCγ-ir interneurons appear only at P6 in lamina IIi, and they conversely increase to reach adult levels at P11-15. Analysis of neurogenesis using bromodeoxyuridine (BrdU) does not detect any PKCγ-BrdU double-labeling in lamina IIi. Quantification of the neuronal marker, NeuN, reveals a sharp neuronal decline (∼50%) within all superficial MDH laminae during early development (P3-15), suggesting that developmental changes in PKCγ-ir interneurons are independent from those of other neurons. Finally, neonatal capsaicin treatment, which produces a permanent loss of most unmyelinated afferent fibers, has no effect on the development of PKCγ-ir interneurons. Together, the results show that: (i) the expression of PKCγ-ir interneurons in MDH is developmentally regulated with a critical period at P11-P15, (ii) PKCγ-ir interneurons are developmentally heterogeneous, (iii) lamina IIi PKCγ-ir interneurons appear less vulnerable to cell death, and (iv) postnatal maturation of PKCγ-ir interneurons is due to neither neurogenesis, nor neuronal migration, and is independent of C-fiber development. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 102-119, 2017.

Subpopulations of PKCγ interneurons within the medullary dorsal horn revealed by electrophysiologic and morphologic approach

Mechanical allodynia, a cardinal symptom of persistent pain, is associated with the unmasking of usually blocked local circuits within the superficial spinal or medullary dorsal horn (MDH) through which low-threshold mechanical inputs can gain access to the lamina I nociceptive output neurons. Specific interneurons located within inner lamina II (IIi) and expressing the gamma isoform of protein kinase C (PKCγ⁺) have been shown to be key elements for such circuits. However, their morphologic and electrophysiologic features are still unknown. Using whole-cell patch-clamp recordings and immunohistochemical techniques in slices of adult rat MDH, we characterized such lamina IIi PKCγ⁺ interneurons and compared them with neighboring PKCγ⁻ interneurons. Our results reveal that PKCγ⁺ interneurons display very specific activity and response properties. Compared with PKCγ⁻ interneurons, they exhibit a smaller membrane input resistance and rheobase, leading to a lower threshold for action potentials. Consistently, more than half of PKCγ⁺ interneurons respond with tonic firing to step current. They also receive a weaker excitatory synaptic drive. Most PKCγ⁺ interneurons express Ih currents. The neurites of PKCγ⁺ interneurons arborize extensively within lamina IIi, can spread dorsally into lamina IIo, but never reach lamina I. In addition, at least 2 morphologically and functionally different subpopulations of PKCγ⁺ interneurons can be identified: central and radial PKCγ⁺ interneurons. The former exhibit a lower membrane input resistance, rheobase and, thus, action potential threshold, and less PKCγ⁺ immunoreactivity than the latter. These 2 subpopulations might thus differently contribute to the gating of dorsally directed circuits within the MDH underlying mechanical allodynia.

Protein Kinase C γ Interneurons Mediate C-fiber–induced Orofacial Secondary Static Mechanical Allodynia, but Not C-fiber–induced Nociceptive Behavior

Background

Tissue injury enhances pain sensitivity both at the site of tissue damage and in surrounding uninjured skin (secondary hyperalgesia). Secondary hyperalgesia encompasses several pain symptoms including pain to innocuous punctate stimuli or static mechanical allodynia. How injury-induced barrage from C-fiber nociceptors produces secondary static mechanical allodynia has not been elucidated.

Methods

Combining behavioral, immunohistochemical, and Western blot analysis, the authors investigated the cell and molecular mechanisms underlying the secondary static mechanical allodynia in the rat medullary dorsal horn (MDH) using the capsaicin model (n = 4 to 5 per group).

Results

Intradermal injection of capsaicin (25 μg) into the vibrissa pad produces a spontaneous pain and a secondary static mechanical allodynia. This allodynia is associated with the activation of a neuronal network encompassing lamina I–outer lamina III, including interneurons expressing the γ isoform of protein kinase C (PKCγ) within inner lamina II (IIi) of MDH. PKCγ is concomitantly phosphorylated (+351.4 ± 79.2%, mean ± SD; P = 0.0003). Mechanical allodynia and innocuous punctate stimulus–evoked laminae I to III neuronal activation can be replicated after intracisternally applied γ-aminobutyric acid receptor type A (GABAA) antagonist (bicuculline: 0.05 μg) or reactive oxygen species (ROS) donor (tert-butyl hydroperoxide: 50 to 250 ng). Conversely, intracisternal PKCγ antagonist, GABAA receptor agonist, or ROS scavenger prevent capsaicin-induced static mechanical allodynia and neuronal activation.

Conclusions

Sensitization of lamina IIi PKCγ interneurons is required for the manifestation of secondary static mechanical allodynia but not for spontaneous pain. Such sensitization is driven by ROS and GABAAergic disinhibition. ROS released during intense C-fiber nociceptor activation might produce a GABAAergic disinhibition of PKCγ interneurons. Innocuous punctate inputs carried by Aδ low-threshold mechanoreceptors onto PKCγ interneurons can then gain access to the pain transmission circuitry of superficial MDH, producing pain.

Activation of medullary dorsal horn γ isoform of protein kinase C interneurons is essential to the development of both static and dynamic facial mechanical allodynia

The γ isoform of protein kinase C (PKCγ), which is concentrated in a specific class of interneurons within inner lamina II (IIi ) of the spinal dorsal horn and medullary dorsal horn (MDH), is known to be involved in the development of mechanical allodynia, a widespread and intractable symptom of inflammatory or neuropathic pain. However, although genetic and pharmacological impairment of PKCγ were shown to prevent mechanical allodynia in animal models of pain, after nerve injury or reduced inhibition, the functional consequences of PKCγ activation alone on mechanical sensitivity are still unknown. Using behavioural and anatomical approaches in the rat MDH, we tested whether PKCγ activation in naive animals is sufficient for the establishment of mechanical allodynia. Intracisternal injection of the phorbol ester, 12,13-dibutyrate concomitantly induced static as well as dynamic facial mechanical allodynia. Monitoring neuronal activity within the MDH with phospho-extracellular signal-regulated kinases 1 and 2 immunoreactivity revealed that activation of both lamina I-outer lamina II and IIi -outer lamina III neurons, including lamina IIi PKCγ-expressing interneurons, was associated with the manifestation of mechanical allodynia. Phorbol ester, 12,13-dibutyrate-induced mechanical allodynia and associated neuronal activations were all prevented by inhibiting selectively segmental PKCγ with KIG31-1. Our findings suggest that PKCγ activation, without any other experimental manipulation, is sufficient for the development of static and dynamic mechanical allodynia. Lamina IIi PKCγ interneurons have been shown to be directly activated by low-threshold mechanical inputs carried by myelinated afferents. Thus, the level of PKCγ activation within PKCγ interneurons might gate the transmission of innocuous mechanical inputs to lamina I, nociceptive output neurons, thus turning touch into pain.

Dorsal Horn Circuits for Persistent Mechanical Pain

Persistent mechanical hypersensitivity that occurs in the setting of injury or disease remains a major clinical problem largely because the underlying neural circuitry is still not known. Here we report the functional identification of key components of the elusive dorsal horn circuit for mechanical allodynia. We show that the transient expression of VGLUT3 by a discrete population of neurons in the deep dorsal horn is required for mechanical pain and that activation of the cells in the adult conveys mechanical hypersensitivity. The cells, which receive direct low threshold input, point to a novel location for circuit initiation. Subsequent analysis of c-Fos reveals the circuit extends dorsally to nociceptive lamina I projection neurons, and includes lamina II calretinin neurons, which we show also convey mechanical allodynia. Lastly, using inflammatory and neuropathic pain models, we show that multiple microcircuits in the dorsal horn encode this form of pain.

Dorsal Horn Parvalbumin Neurons Are Gate-Keepers of Touch-Evoked Pain after Nerve Injury

Neuropathic pain is a chronic debilitating disease that results from nerve damage, persists long after the injury has subsided, and is characterized by spontaneous pain and mechanical hypersensitivity. Although loss of inhibitory tone in the dorsal horn of the spinal cord is a major contributor to neuropathic pain, the molecular and cellular mechanisms underlying this disinhibition are unclear. Here, we combined pharmacogenetic activation and selective ablation approaches in mice to define the contribution of spinal cord parvalbumin (PV)-expressing inhibitory interneurons in naive and neuropathic pain conditions. Ablating PV neurons in naive mice produce neuropathic pain-like mechanical allodynia via disinhibition of PKCγ excitatory interneurons. Conversely, activating PV neurons in nerve-injured mice alleviates mechanical hypersensitivity. These findings indicate that PV interneurons are modality-specific filters that gate mechanical but not thermal inputs to the dorsal horn and that increasing PV inter-neuron activity can ameliorate the mechanical hypersensitivity that develops following nerve injury.

Plasticity of inhibition in the spinal cord

Inhibitory interneurons, which use GABA and/or glycine as their principal transmitter, have numerous roles in regulating the transmission of sensory information through the spinal dorsal horn. These roles are likely to be performed by different populations of interneurons, each with specific locations in the synaptic circuitry of the region. Peripheral nerve injury frequently leads to neuropathic pain, and it is thought that loss of function of inhibitory interneurons in the dorsal horn contributes to this condition. Several mechanisms have been proposed for this disinhibition, including death of inhibitory interneurons, decreased transmitter release, diminished activity of these cells and reduced effectiveness of GABA and glycine as inhibitory transmitters. However, despite numerous studies on this important topic, it is still not clear which (if any) of these mechanisms contributes to neuropathic pain after nerve injury.