Postnatal maturation of spinal dynorphin circuits and their role in somatosensation

An electrophysiologist's guide to dorsal horn excitability and pain

The dorsal horn of the spinal cord represents the first site in the central nervous system (CNS) where nociceptive signals are integrated. As a result, there has been a rapid growth in the number of studies investigating the ionic mechanisms regulating the excitability of dorsal horn neurons under normal and pathological conditions. We believe that it is time to look back and to critically examine what picture emerges from this wealth of studies. What are the actual types of neurons described in the literature based on electrophysiological criteria? Are these electrophysiologically-defined subpopulations strongly linked to specific morphological, functional, or molecular traits? Are these electrophysiological properties stable, or can they change during development or in response to peripheral injury? Here we provide an in-depth overview of both early and recent publications that explore the factors influencing dorsal horn neuronal excitability (including intrinsic membrane properties and synaptic transmission), how these factors vary across distinct subtypes of dorsal horn neurons, and how such factors are altered by peripheral nerve or tissue damage. The meta-research presented below leads to the conclusion that the dorsal horn is comprised of highly heterogeneous subpopulations in which the observed electrophysiological properties of a given neuron often fail to easily predict other properties such as biochemical phenotype or morphology. This highlights the need for future studies which can more fully interrogate the properties of dorsal horn neurons in a multi-modal manner.

Molecular Anatomy of Synaptic and Extrasynaptic Neurotransmission Between Nociceptive Primary Afferents and Spinal Dorsal Horn Neurons

Sensory signals generated by peripheral nociceptors are transmitted by peptidergic and nonpeptidergic nociceptive primary afferents to the superficial spinal dorsal horn, where their central axon terminals establish synaptic contacts with secondary sensory spinal neurons. In the case of suprathreshold activation, the axon terminals release glutamate into the synaptic cleft and stimulate postsynaptic spinal neurons by activating glutamate receptors located on the postsynaptic membrane. When overexcitation is evoked by peripheral inflammation, neuropathy or pruritogens, peptidergic nociceptive axon terminals may corelease various neuropeptides, neurotrophins and endomorphin, together with glutamate. However, in contrast to glutamate, neuropeptides, neurotrophins and endomorphin are released extrasynaptically. They diffuse from the site of release and modulate the function of spinal neurons via volume transmission, activating specific extrasynaptic receptors. Thus, the released neuropeptides, neurotrophins and endomorphin may evoke excitation, disinhibition or inhibition in various spinal neuronal populations, and together with glutamate, induce overall overexcitation, called central sensitization. In addition, the synaptic and extrasynaptic release of neurotransmitters is subjected to strong retrograde control mediated by various retrogradely acting transmitters, messengers, and their presynaptic receptors. Moreover, the composition of this complex chemical apparatus is heavily dependent on the actual patterns of nociceptive primary afferent activation in the periphery. This review provides an overview of the complexity of this signaling apparatus, how nociceptive primary afferents can activate secondary sensory spinal neurons via synaptic and volume transmission in the superficial spinal dorsal horn, and how these events can be controlled by presynaptic mechanisms.

An atlas of neuropathic pain-associated molecular pathological characteristics in the mouse spinal cord

Peripheral nerve injury (PNI)-induced neuropathic pain (NP) is a severe disease with high prevalence in clinics. Gene reprogramming and tissue remodeling in the dorsal root ganglia (DRG) and spinal cord (SC) drive the development and maintenance of neuropathic pain (NP). However, our understanding of the NP-associated spatial molecular processing landscape of SC and the non-synaptic interactions between DRG neurons and SC cells remains limited. We here integrate spatial transcriptomics (ST) with single-nucleus RNA-sequencing (snRNA-seq) and bulk RNA-sequencing (bulk RNA-seq) to characterize regional pathological heterogeneity of the SC under NP conditions. First, the SC of NP mice manifests unique spatial atlases of genes, cell populations, cell-cell cross-talks, signaling pathways, and transcriptional regulatory networks compared to sham mice. We further report that injured DRG sensory neurons and the corresponding ventral horn of the SC show similar expression patterns after PNI. In addition, for the first time, we systematically exhibit "cross-talk omics" between the DRG neurons and SC dorsal horn neurons and glial cells, indicating an altered communication profile under NP conditions. Together, our findings decode the spatial and cellular heterogeneity of molecular pathological mechanisms underlying NP, providing a foundation for designing therapeutic targets for this disorder.

Lateral lamina V projection neuron axon collaterals connect sensory processing across the dorsal horn of the mouse spinal cord

Spinal projection neurons (PNs) are defined by long axons that travel from their origin in the spinal cord to the brain where they relay sensory information from the body. The existence and function of a substantial axon collateral network, also arising from PNs and remaining within the spinal cord, is less well appreciated. Here we use a retrograde viral transduction strategy to characterise a novel subpopulation of deep dorsal horn spinoparabrachial neurons. Brainbow assisted analysis confirmed that virally labelled PN cell bodies formed a discrete cell column in the lateral part of Lamina V (LVlat) and the adjoining white matter. These PNs exhibited large dendritic territories biased to regions lateral and ventral to the cell body column and extending considerable rostrocaudal distances. Optogenetic activation of LVLat PNs confirmed this population mediates widespread signalling within spinal cord circuits, including activation in the superficial dorsal horn. This signalling was also demonstrated with patch clamp recordings during LVLat PN photostimulation, with a range of direct and indirect connections identified and evidence of a postsynaptic population of inhibitory interneurons. Together, these findings confirm a substantial role for PNs in local spinal sensory processing, as well as relay of sensory signals to the brain.

Influence of Early-Life Stress on the Excitability of Dynorphin Neurons in the Adult Mouse Dorsal Horn

While early-life adversity has been associated with a higher risk of developing chronic pain in adulthood, the cellular and molecular mechanisms by which chronic stress during the neonatal period can persistently sensitize developing nociceptive circuits remain poorly understood. Here, we investigate the effects of early-life stress (ELS) on synaptic integration and intrinsic excitability in dynorphin-lineage (DYN) interneurons within the adult mouse superficial dorsal horn (SDH), which are important for inhibiting mechanical pain and itch. The administration of neonatal limited bedding between postnatal days (P)2 and P9 evoked sex-dependent effects on spontaneous glutamatergic signaling, as female SDH neurons exhibited a higher amplitude of miniature excitatory postsynaptic currents (mEPSCs) after ELS, while mEPSC frequency was reduced in DYN neurons of the male SDH. Furthermore, ELS decreased the frequency of miniature inhibitory postsynaptic currents selectively in female DYN neurons. As a result, ELS increased the balance of spontaneous excitation versus inhibition (E:I ratio) in mature DYN neurons of the female, but not male, SDH network. Nonetheless, ELS weakened the total primary afferent-evoked glutamatergic drive onto adult DYN neurons selectively in females, without modifying afferent-evoked inhibitory signaling onto the DYN population. Finally, ELS failed to significantly change the intrinsic membrane excitability of mature DYN neurons in either males or females. Collectively, these data suggest that ELS exerts a long-term influence on the properties of synaptic transmission onto DYN neurons within the adult SDH, which includes a reduction in the overall strength of sensory input onto this important subset of inhibitory interneurons. PERSPECTIVE: This study suggests that chronic stress during the neonatal period influences synaptic function within adult spinal nociceptive circuits in a sex-dependent manner. These findings yield new insight into the potential mechanisms by which early-life adversity might shape the maturation of pain pathways in the central nervous system (CNS).

Astrocyte D1/D5 Dopamine Receptors Govern Non-Hebbian Long-Term Potentiation at Sensory Synapses onto Lamina I Spinoparabrachial Neurons

Recent work demonstrated that activation of spinal D1 and D5 dopamine receptors (D1/D5Rs) facilitates non-Hebbian long-term potentiation (LTP) at primary afferent synapses onto spinal projection neurons. However, the cellular localization of the D1/D5Rs driving non-Hebbian LTP in spinal nociceptive circuits remains unknown, and it is also unclear whether D1/D5R signaling must occur concurrently with sensory input in order to promote non-Hebbian LTP at these synapses. Here we investigate these issues using cell-type-selective knockdown of D1Rs or D5Rs from lamina I spinoparabrachial neurons, dorsal root ganglion (DRG) neurons, or astrocytes in adult mice of either sex using Cre recombinase-based genetic strategies. The LTP evoked by low-frequency stimulation of primary afferents in the presence of the selective D1/D5R agonist SKF82958 persisted following the knockdown of D1R or D5R in spinoparabrachial neurons, suggesting that postsynaptic D1/D5R signaling was dispensable for non-Hebbian plasticity at sensory synapses onto these key output neurons of the superficial dorsal horn (SDH). Similarly, the knockdown of D1Rs or D5Rs in DRG neurons failed to influence SKF82958-enabled LTP in lamina I projection neurons. In contrast, SKF82958-induced LTP was suppressed by the knockdown of D1R or D5R in spinal astrocytes. Furthermore, the data indicate that the activation of D1R/D5Rs in spinal astrocytes can either retroactively or proactively drive non-Hebbian LTP in spinoparabrachial neurons. Collectively, these results suggest that dopaminergic signaling in astrocytes can strongly promote activity-dependent LTP in the SDH, which is predicted to significantly enhance the amplification of ascending nociceptive transmission from the spinal cord to the brain.

Epigenetic Regulation and Molecular Mechanisms of Burn Injury-Induced Nociception in the Spinal Cord of Mice

Epigenetic mechanisms, including histone post-translational modifications (PTMs), play a critical role in regulating pain perception and the pathophysiology of burn injury. However, the epigenetic regulation and molecular mechanisms underlying burn injury-induced pain remain insufficiently explored. Spinal dynorphinergic (Pdyn) neurons contribute to heat hyperalgesia induced by severe scalding-type burn injury through p-S10H3-dependent signaling. Beyond p-S10H3, burn injury may impact various other histone H3 PTMs. Double immunofluorescent staining and histone H3 protein analyses demonstrated significant hypermethylation at H3K4me1 and H3K4me3 sites and hyperphosphorylation at S10H3 within the spinal cord. By analyzing Pdyn neurons in the spinal dorsal horn, we found evidence of chromatin activation with a significant elevation in p-S10H3 immunoreactivity. We used RNA-seq analysis to compare the effects of burn injury and formalin-induced inflammatory pain on spinal cord transcriptomic profiles. We identified 98 DEGs for burn injury and 86 DEGs for formalin-induced inflammatory pain. A limited number of shared differentially expressed genes (DEGs) suggest distinct central pain processing mechanisms between burn injury and formalin models. KEGG pathway analysis supported this divergence, with burn injury activating Wnt signaling. This study enhances our understanding of burn injury mechanisms and uncovers converging and diverging pathways in pain models with different origins.

Evaluating the Influence of Acute and Chronic Orofacial Pains on the Overall Comprehensive Quality of Life

Background Orofacial discomfort refers to various disorders that affect the mouth, jaws, and face. These conditions may substantially influence a person's quality of life (QoL). This kind of pain may be categorised into two primary classifications: acute and chronic. Acute orofacial pain (OFP) usually occurs suddenly and lasts for a short period. It is commonly caused by specific factors such as dental treatments, traumas, or infections. Hence, this study aimed to assess the influence of acute and chronic orofacial discomfort on the overall QoL. Methodology This research used a convenience sample to gather data from the five groups. It was conducted as a cross-sectional study. Four categories of OFP syndromes were utilised: temporomandibular disorders (TMDs), atypical dental pain (ADP), trigeminal neuralgia (TN), and persistent dentoalveolar pain disorder (PDAP). In addition, a control group consisting of individuals who did not experience any discomfort was also included in the study. Participants received a standardised explanation of the questionnaires utilised, and in most instances, they completed them at the clinic under the supervision of one of the investigators. Results The correlations between each version of the Oral Health Impact Profile (OHIP) were statistically significant (p < 0.001), with correlation values ranging from 0.92 to 0.97. When comparing the occurrence of OHIP items across the four pain types, we identified 18 variations that exceeded the 35% prevalence threshold we established for identifying important items that frequently occur out of the 315 comparisons. The question "Have you had a painful aching in your mouth?" showed a low frequency in patients with TN, TMD, and ADP but a significant prevalence in patients with PDAP (90%). This difference in prevalence across pain types was the biggest seen. The question that seemed to distinguish between the pain categories most effectively was "Have you experienced a toothache?" This question had a high occurrence in PDAP (65%) and ADP (60%), perhaps allowing for differentiating these two diseases from TN and TMD. The highest prevalence differences, over 30%, were most often seen when PDAP was included as one of the disorders being compared. ADP had the fewest variances, over 30%, occurring just seven times. Conclusion Both acute and chronic OFPs have a major negative influence on QoL, but they affect it differently and to differing extents. Injury-related acute pain obviously poses rapid and severe restrictions on physical function and causes temporary psychological distress and temporary social exclusion. On the other hand, chronic pain for the patient always implies being locked in a physical world that does not allow him or her to bypass physical limitations, psychological disorders remain constant, and isolation from other people persists for life.

Development and characterization of a Gucy2d-cre mouse to selectively manipulate a subset of inhibitory spinal dorsal horn interneurons

Recent transcriptomic studies identified Gucy2d (encoding guanylate cyclase D) as a highly enriched gene within inhibitory dynorphin interneurons in the mouse spinal dorsal horn. To facilitate investigations into the role of the Gucy2d+ population in somatosensation, Gucy2d-cre transgenic mice were created to permit chemogenetic or optogenetic manipulation of this subset of spinal neurons. Gucy2d-cre mice created via CRISPR/Cas9 genomic knock-in were bred to mice expressing a cre-dependent reporter (either tdTomato or Sun1.GFP fusion protein), and the resulting offspring were characterized. Surprisingly, a much wider population of spinal neurons was labeled by cre-dependent reporter expression than previous mRNA-based studies would suggest. Although the cre-dependent reporter expression faithfully labeled ~75% of cells expressing Gucy2d mRNA in the adult dorsal horn, it also labeled a substantial number of additional inhibitory neurons in which no Gucy2d or Pdyn mRNA was detected. Moreover, cre-dependent reporter was also expressed in various regions of the brain, including the spinal trigeminal nucleus, cerebellum, thalamus, somatosensory cortex, and anterior cingulate cortex. Injection of AAV-CAG-FLEX-tdTomato viral vector into adult Gucy2d-cre mice produced a similar pattern of cre-dependent reporter expression in the spinal cord and brain, which excludes the possibility that the unexpected reporter-labeling of cells in the deep dorsal horn and brain was due to transient Gucy2d expression during early stages of development. Collectively, these results suggest that Gucy2d is expressed in a wider population of cells than previously thought, albeit at levels low enough to avoid detection with commonly used mRNA-based assays. Therefore, it is unlikely that these Gucy2d-cre mice will permit selective manipulation of inhibitory signaling mediated by spinal dynorphin interneurons, but this novel cre driver line may nevertheless be useful to target a broader population of inhibitory spinal dorsal horn neurons.

Early-life adversity increases morphine tolerance and persistent inflammatory hypersensitivity through upregulation of δ opioid receptors in mice

ABSTRACT

Exposure to severely stressful events during childhood is associated with poor health outcomes in later life, including chronic pain and substance use disorder. However, the mediators and mechanisms are unclear. We investigated the impact of a well-characterized mouse model of early-life adversity, fragmented maternal care (FC) between postnatal day 2 and 9, on nociception, inflammatory hypersensitivity, and responses to morphine. Male and female mice exposed to FC exhibited prolonged basal thermal withdrawal latencies and decreased mechanical sensitivity. In addition, morphine had reduced potency in mice exposed to FC and their development of tolerance to morphine was accelerated. Quantitative PCR analysis in several brain regions and the spinal cords of juvenile and adult mice revealed an impact of FC on the expression of genes encoding opioid peptide precursors and their receptors. These changes included enhanced abundance of δ opioid receptor transcript in the spinal cord. Acute inflammatory hypersensitivity (induced by hind paw administration of complete Freund's adjuvant) was unaffected by exposure to FC. However, after an initial recovery of mechanical hypersensitivity, there was a reappearance in mice exposed to FC by day 15, which was not seen in control mice. Changes in nociception, morphine responses, and hypersensitivity associated with FC were apparent in males and females but were absent from mice lacking δ receptors or β-arrestin2. These findings suggest that exposure to early-life adversity in mice enhances δ receptor expression leading to decreased basal sensitivity to noxious stimuli coupled with accelerated morphine tolerance and enhanced vulnerability to persistent inflammatory hypersensitivity.

Early-Life Iron Deficiency Persistently Alters Nociception in Developing Mice

Clinical association studies have identified early-life iron deficiency (ID) as a risk factor for the development of chronic pain. While preclinical studies have shown that early-life ID persistently alters neuronal function in the central nervous system, a causal relationship between early-life ID and chronic pain has yet to be established. We sought to address this gap in knowledge by characterizing pain sensitivity in developing male and female C57Bl/6 mice that were exposed to dietary ID during early life. Dietary iron was reduced by ∼90% in dams between gestational day 14 and postnatal day (P)10, with dams fed an ingredient-matched, iron-sufficient diet serving as controls. While cutaneous mechanical and thermal withdrawal thresholds were not altered during the acute ID state at P10 and P21, ID mice were more sensitive to mechanical pressure at P21 independent of sex. During adulthood, when signs of ID had resolved, mechanical and thermal thresholds were similar between early-life ID and control groups, although male and female ID mice displayed increased thermal tolerance at an aversive (45 °C) temperature. Interestingly, while adult ID mice showed decreased formalin-induced nocifensive behaviors, they showed exacerbated mechanical hypersensitivity and increased paw guarding in response to hindpaw incision in both sexes. Collectively, these results suggest that early-life ID elicits persistent changes in nociceptive processing and appears capable of priming developing pain pathways. PERSPECTIVE: This study provides novel evidence that early-life ID evokes sex-independent effects on nociception in developing mice, including an exacerbation of postsurgical pain during adulthood. These findings represent a critical first step towards the long-term goal of improving health outcomes for pain patients with a prior history of ID.

Neuropeptide Y-expressing dorsal horn inhibitory interneurons gate spinal pain and itch signalling

Somatosensory information is processed by a complex network of interneurons in the spinal dorsal horn. It has been reported that inhibitory interneurons that express neuropeptide Y (NPY), either permanently or during development, suppress mechanical itch, with no effect on pain. Here, we investigate the role of interneurons that continue to express NPY (NPY-INs) in the adult mouse spinal cord. We find that chemogenetic activation of NPY-INs reduces behaviours associated with acute pain and pruritogen-evoked itch, whereas silencing them causes exaggerated itch responses that depend on cells expressing the gastrin-releasing peptide receptor. As predicted by our previous studies, silencing of another population of inhibitory interneurons (those expressing dynorphin) also increases itch, but to a lesser extent. Importantly, NPY-IN activation also reduces behavioural signs of inflammatory and neuropathic pain. These results demonstrate that NPY-INs gate pain and itch transmission at the spinal level, and therefore represent a potential treatment target for pathological pain and itch.

Calretinin-expressing islet cells are a source of pre- and post-synaptic inhibition of non-peptidergic nociceptor input to the mouse spinal cord

Unmyelinated non-peptidergic nociceptors (NP afferents) arborise in lamina II of the spinal cord and receive GABAergic axoaxonic synapses, which mediate presynaptic inhibition. However, until now the source of this axoaxonic synaptic input was not known. Here we provide evidence that it originates from a population of inhibitory calretinin-expressing interneurons (iCRs), which correspond to lamina II islet cells. The NP afferents can be assigned to 3 functionally distinct classes (NP1-3). NP1 afferents have been implicated in pathological pain states, while NP2 and NP3 afferents also function as pruritoceptors. Our findings suggest that all 3 of these afferent types innervate iCRs and receive axoaxonic synapses from them, providing feedback inhibition of NP input. The iCRs also form axodendritic synapses, and their targets include cells that are themselves innervated by the NP afferents, thus allowing for feedforward inhibition. The iCRs are therefore ideally placed to control the input from non-peptidergic nociceptors and pruritoceptors to other dorsal horn neurons, and thus represent a potential therapeutic target for the treatment of chronic pain and itch.

Calretinin-expressing islet cells: a source of pre- and post-synaptic inhibition of non-peptidergic nociceptor input to the mouse spinal cord

Unmyelinated non-peptidergic nociceptors (NP afferents) arborise in lamina II of the spinal cord and receive GABAergic axoaxonic synapses, which mediate presynaptic inhibition. However, until now the source of this axoaxonic synaptic input was not known. Here we provide evidence that it originates from a population of inhibitory calretinin-expressing interneurons (iCRs), which correspond to lamina II islet cells. The NP afferents can be assigned to 3 functionally distinct classes (NP1-3). NP1 afferents have been implicated in pathological pain states, while NP2 and NP3 afferents also function as pruritoceptors. Our findings suggest that all 3 of these afferent types innervate iCRs and receive axoaxonic synapses from them, providing feedback inhibition of NP input. The iCRs also form axodendritic synapses, and their targets include cells that are themselves innervated by the NP afferents, thus allowing for feedforward inhibition. The iCRs are therefore ideally placed to control the input from non-peptidergic nociceptors and pruritoceptors to other dorsal horn neurons, and thus represent a potential therapeutic target for the treatment of chronic pain and itch.

Intrinsic and synaptic properties of adult mouse spinoperiaqueductal gray neurons and the influence of neonatal tissue damage

ABSTRACT

The periaqueductal gray (PAG) represents a key target of projection neurons residing in the spinal dorsal horn. In comparison to lamina I spinoparabrachial neurons, little is known about the intrinsic and synaptic properties governing the firing of spino-PAG neurons, or whether such activity is modulated by neonatal injury. In this study, this issue was addressed using ex vivo whole-cell patch clamp recordings from lamina I spino-PAG neurons in adult male and female FVB mice after hindpaw incision at postnatal day (P)3. Spino-PAG neurons were classified as high output, medium output, or low output based on their action potential discharge after dorsal root stimulation. The high-output subgroup exhibited prevalent spontaneous burst firing and displayed initial burst or tonic patterns of intrinsic firing, whereas low-output neurons showed little spontaneous activity. Interestingly, the level of dorsal root-evoked firing significantly correlated with the resting potential and membrane resistance but not with the strength of primary afferent-mediated glutamatergic drive. Neonatal incision failed to alter the pattern of monosynaptic sensory input, with most spino-PAG neurons receiving direct connections from low-threshold C-fibers. Furthermore, primary afferent-evoked glutamatergic input and action potential discharge in adult spino-PAG neurons were unaltered by neonatal surgical injury. Finally, Hebbian long-term potentiation at sensory synapses, which significantly increased afferent-evoked firing, was similar between P3-incised and naive littermates. Collectively, these data suggest that the functional response of lamina I spino-PAG neurons to sensory input is largely governed by their intrinsic membrane properties and appears resistant to the persistent influence of neonatal tissue damage.

A cellular taxonomy of the adult human spinal cord

The mammalian spinal cord functions as a community of cell types for sensory processing, autonomic control, and movement. While animal models have advanced our understanding of spinal cellular diversity, characterizing human biology directly is important to uncover specialized features of basic function and human pathology. Here, we present a cellular taxonomy of the adult human spinal cord using single-nucleus RNA sequencing with spatial transcriptomics and antibody validation. We identified 29 glial clusters and 35 neuronal clusters, organized principally by anatomical location. To demonstrate the relevance of this resource to human disease, we analyzed spinal motoneurons, which degenerate in amyotrophic lateral sclerosis (ALS) and other diseases. We found that compared with other spinal neurons, human motoneurons are defined by genes related to cell size, cytoskeletal structure, and ALS, suggesting a specialized molecular repertoire underlying their selective vulnerability. We include a web resource to facilitate further investigations into human spinal cord biology.

CRISPR/Cas9-Based Mutagenesis of Histone H3.1 in Spinal Dynorphinergic Neurons Attenuates Thermal Sensitivity in Mice

Burn injury is a trauma resulting in tissue degradation and severe pain, which is processed first by neuronal circuits in the spinal dorsal horn. We have recently shown that in mice, excitatory dynorphinergic (Pdyn) neurons play a pivotal role in the response to burn-injury-associated tissue damage via histone H3.1 phosphorylation-dependent signaling. As Pdyn neurons were mostly associated with mechanical allodynia, their involvement in thermonociception had to be further elucidated. Using a custom-made AAV9_mutH3.1 virus combined with the CRISPR/cas9 system, here we provide evidence that blocking histone H3.1 phosphorylation at position serine 10 (S10) in spinal Pdyn neurons significantly increases the thermal nociceptive threshold in mice. In contrast, neither mechanosensation nor acute chemonociception was affected by the transgenic manipulation of histone H3.1. These results suggest that blocking rapid epigenetic tagging of S10H3 in spinal Pdyn neurons alters acute thermosensation and thus explains the involvement of Pdyn cells in the immediate response to burn-injury-associated tissue damage.

A harmonized atlas of mouse spinal cord cell types and their spatial organization

Single-cell RNA sequencing data can unveil the molecular diversity of cell types. Cell type atlases of the mouse spinal cord have been published in recent years but have not been integrated together. Here, we generate an atlas of spinal cell types based on single-cell transcriptomic data, unifying the available datasets into a common reference framework. We report a hierarchical structure of postnatal cell type relationships, with location providing the highest level of organization, then neurotransmitter status, family, and finally, dozens of refined populations. We validate a combinatorial marker code for each neuronal cell type and map their spatial distributions in the adult spinal cord. We also show complex lineage relationships among postnatal cell types. Additionally, we develop an open-source cell type classifier, SeqSeek, to facilitate the standardization of cell type identification. This work provides an integrated view of spinal cell types, their gene expression signatures, and their molecular organization.

Postnatal development of inner lamina II interneurons of the rat medullary dorsal horn

ABSTRACT

Pain processing in young mammals is immature. Despite the central role of the medullary dorsal horn (MDH) in processing orofacial sensory information, the maturation of the neurons within the MDH has been largely overlooked. Combining in vitro electrophysiological recordings and 3D morphological analysis over the first postnatal month in rats, we investigated the age-dependent development of the neurons within the inner lamina II (IIi) of the MDH. We show the lamina IIi neuronal population transition into a more hyperpolarized state, with modification of the action potential waveform, and a shift from single spiking, at early postnatal ages, to tonic firing and initial bursting at later stages. These physiological changes are associated with a strong structural remodelling of the neuronal morphology with most of the modifications occurring after the third postnatal week. Among the lamina IIi neuronal population, the subpopulation of interneurons expressing the γ isoform of the protein kinase C (PKCγ+) are key elements for the circuits underlying facial mechanical allodynia. How do they develop from the rest of the lamina IIi constitute an important question that remained to be addressed. Here, we show that PKCγ+ interneurons display electrophysiological changes over time comparable with the PKCγ- population. However, they show a distinctive increase of the soma volume and primary branches length, as opposed to the PKCγ- population. Together, our data demonstrate a novel pattern of late postnatal maturation of lamina IIi interneurons, with a spotlight on PKCγ+ interneurons, that may be relevant for the development of orofacial sensitivity.

Gucy2d selectively marks inhibitory dynorphin neurons in the spinal dorsal horn but is dispensable for pain and itch sensitivity

Introduction

Inhibitory neurons in the spinal dorsal horn can be classified based on expression of neurochemical marker genes. However, these marker genes are often expressed throughout the central nervous system, which poses challenges for manipulating genetically identified spinal neurons without undesired off-target effects.

Objectives

We investigated whether Gucy2d, previously identified as a highly selective marker of dynorphin-lineage neurons in the dorsal horn, is expressed in other locations within the adult mouse spinal cord, dorsal root ganglia (DRG), or brain. In addition, we sought to molecularly characterize Gucy2d-expressing dorsal horn neurons and investigate whether the disruption of Gucy2d gene expression affects sensitivity to itch or pain.

Methods

In situ hybridization experiments assessed Gucy2d mRNA expression in the adult mouse spinal cord, DRG, and brain, and its colocalization with Pax2, Bhlhb5, and Pde2a in dorsal horn neurons. Knockout mice lacking Gucy2d expression were compared with littermate controls to assess sensitivity to chloroquine-induced itch and dry skin-mediated chronic itch, as well as heat, cold, or mechanical stimuli.

Results

Gucy2d is selectively expressed in dynorphin-lineage neurons in lamina I-III of the adult mouse spinal cord but not in the brain or DRG. Spinal Gucy2d-expressing neurons are inhibitory neurons that also express the transcription factor Bhlhb5 and the cGMP-dependent phosphodiesterase Pde2a. Gucy2d knockout mice did not exhibit altered responses to itch or pain.

Conclusions

The selective expression of Gucy2d within a subpopulation of inhibitory dorsal horn neurons may yield a means to selectively manipulate inhibitory signaling at the level of the spinal cord without effects on the brain.

Spinal Inhibitory Interneurons: Gatekeepers of Sensorimotor Pathways

The ability to sense and move within an environment are complex functions necessary for the survival of nearly all species. The spinal cord is both the initial entry site for peripheral information and the final output site for motor response, placing spinal circuits as paramount in mediating sensory responses and coordinating movement. This is partly accomplished through the activation of complex spinal microcircuits that gate afferent signals to filter extraneous stimuli from various sensory modalities and determine which signals are transmitted to higher order structures in the CNS and to spinal motor pathways. A mechanistic understanding of how inhibitory interneurons are organized and employed within the spinal cord will provide potential access points for therapeutics targeting inhibitory deficits underlying various pathologies including sensory and movement disorders. Recent studies using transgenic manipulations, neurochemical profiling, and single-cell transcriptomics have identified distinct populations of inhibitory interneurons which express an array of genetic and/or neurochemical markers that constitute functional microcircuits. In this review, we provide an overview of identified neural components that make up inhibitory microcircuits within the dorsal and ventral spinal cord and highlight the importance of inhibitory control of sensorimotor pathways at the spinal level.

The burden of early life stress on the nociceptive system development and pain responses

For a long time, the capacity of the newborn infant to feel pain was denied. Today it is clear that the nociceptive system, even if still immature, is functional enough in the newborn infant to elicit pain responses. Unfortunately, pain is often present in the neonatal period, in particular in the case of premature infants which are subjected to a high number of painful procedures during care. These are accompanied by a variety of environmental stressors, which could impact the maturation of the nociceptive system. Therefore, the question of the long-term consequences of early life stress is a critical question. Early stressful experience, both painful and non-painful, can imprint the nociceptive system and induce long-term alteration in brain function and nociceptive behavior, often leading to an increase sensitivity and higher susceptibility to chronic pain. Different animal models have been developed to understand the mechanisms underlying the long-term effects of different early life stressful procedures, including pain and maternal separation. This review will focus on the clinical and preclinical data about early life stress and its consequence on the nociceptive system.

Single-nucleus characterization of adult mouse spinal dynorphin-lineage cells and identification of persistent transcriptional effects of neonatal hindpaw incision

Neonatal tissue damage can have long-lasting effects on nociceptive processing in the central nervous system, which may reflect persistent injury-evoked alterations to the normal balance between synaptic inhibition and excitation in the spinal dorsal horn. Spinal dynorphin-lineage (pDyn) neurons are part of an inhibitory circuit which limits the flow of nociceptive input to the brain and is disrupted by neonatal tissue damage. To identify the potential molecular underpinnings of this disruption, an unbiased single-nucleus RNAseq analysis of adult mouse spinal pDyn cells characterized this population in depth and then identified changes in gene expression evoked by neonatal hindpaw incision. The analysis revealed 11 transcriptionally distinct subpopulations (ie, clusters) of dynorphin-lineage cells, including both inhibitory and excitatory neurons. Investigation of injury-evoked differential gene expression identified 15 genes that were significantly upregulated or downregulated in adult pDyn neurons from neonatally incised mice compared with naive littermate controls, with both cluster-specific and pan-neuronal transcriptional changes observed. Several of the identified genes, such as Oxr1 and Fth1 (encoding ferritin), were related to the cellular stress response. However, the relatively low number of injury-evoked differentially expressed genes also suggests that posttranscriptional regulation within pDyn neurons may play a key role in the priming of developing nociceptive circuits by early-life injury. Overall, the findings reveal novel insights into the molecular heterogeneity of a key population of dorsal horn interneurons that has previously been implicated in the suppression of mechanical pain and itch.

Spinal GABAergic neurons are under feed-forward inhibitory control driven by A and C fibers in Gad2 td-Tomato mice

BACKGROUND

Spinal GABAergic neurons act as a critical modulator in sensory transmission like pain or itch. The monosynaptic or polysynaptic primary afferent inputs onto GABAergic neurons, along with other interneurons or projection neurons make up the direct and feed-forward inhibitory neural circuits. Previous research indicates that spinal GABAergic neurons mainly receive excitatory inputs from Aδ and C fibers. However, whether they are controlled by other inhibitory sending signals is not well understood.

METHODS

We applied a transgenic mouse line in which neurons co-expressed the GABA-synthesizing enzyme Gad65 and the enhanced red fluorescence (td-Tomato) to characterize the features of morphology and electrophysiology of GABAergic neurons. Patch-clamp whole cell recordings were used to record the evoked postsynaptic potentials of fluorescent neurons in spinal slices in response to dorsal root stimulation.

RESULTS

We demonstrated that GABAergic neurons not only received excitatory drive from peripheral Aβ, Aδ and C fibers, but also received inhibitory inputs driven by Aδ and C fibers. The evoked inhibitory postsynaptic potentials (eIPSPs) mediated by C fibers were mainly Glycinergic (66.7%) as well as GABAergic mixed with Glycinergic (33.3%), whereas the inhibition mediated by Aδ fibers was predominately both GABA and Glycine-dominant (57.1%), and the rest of which was purely Glycine-dominant (42.9%).

CONCLUSION

These results indicated that spinal GABAergic inhibitory neurons are under feedforward inhibitory control driven by primary C and Aδ fibers, suggesting that this feed-forward inhibitory pathway may play an important role in balancing the excitability of GABAergic neurons in spinal dorsal horn.