Transcriptional profile of spinal dynorphin-lineage interneurons in the developing mouse

Early life stress modulates neonatal somatosensation and the transcriptional profile of immature sensory neurons

ABSTRACT

Early life stress (ELS) is associated with an increased risk of experiencing chronic pain during adulthood, but surprisingly little is known about the short-term influence of ELS on nociceptive processing in the immature nervous system and the concomitant effects on somatosensation in the neonate. Here, we investigate how ELS modulates pain in neonatal mice and the transcriptional and electrophysiological signatures of immature dorsal root ganglia (DRG). Shortly after the administration of a neonatal limiting bedding (NLB) paradigm from postnatal days (P)2 to P9, both male and female pups exhibited robust hypersensitivity in response to tactile, pressure, and noxious cold stimuli compared with a control group housed under standard conditions, with no change in their sensitivity to noxious heat. Bulk RNA-seq analysis of L3-L5 DRGs at P9 revealed significant alterations in the transcription of pain- and itch-related genes following ELS, highlighted by a marked downregulation in Sst , Nppb , Chrna6 , Trpa1 , and Il31ra . Nonetheless, ex vivo whole-cell patch-clamp recordings from putative A- and C-fiber sensory neurons in the neonatal DRG found no significant changes in their intrinsic membrane excitability following NLB. Overall, these findings suggest that ELS triggers hyperalgesia in neonates across multiple pain modalities that is accompanied by transcriptional plasticity within developing sensory neurons. A better understanding of the mechanisms governing the interactions between chronic stress and pain during the neonatal period could inform the future development of novel interventional strategies to relieve pain in infants and children who have experienced trauma.

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).

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.

Spatial transcriptomics and single-nucleus RNA sequencing reveal a transcriptomic atlas of adult human spinal cord

Despite the recognized importance of the spinal cord in sensory processing, motor behaviors, and neural diseases, the underlying organization of neuronal clusters and their spatial location remain elusive. Recently, several studies have attempted to define the neuronal types and functional heterogeneity in the spinal cord using single-cell or single-nucleus RNA sequencing in animal models or developing humans. However, molecular evidence of cellular heterogeneity in the adult human spinal cord is limited. Here, we classified spinal cord neurons into 21 subclusters and determined their distribution from nine human donors using single-nucleus RNA sequencing and spatial transcriptomics. Moreover, we compared the human findings with previously published single-nucleus data of the adult mouse spinal cord, which revealed an overall similarity in the neuronal composition of the spinal cord between the two species while simultaneously highlighting some degree of heterogeneity. Additionally, we examined the sex differences in the spinal neuronal subclusters. Several genes, such as SCN10A and HCN1, showed sex differences in motor neurons. Finally, we classified human dorsal root ganglia (DRG) neurons using spatial transcriptomics and explored the putative interactions between DRG and spinal cord neuronal subclusters. In summary, these results illustrate the complexity and diversity of spinal neurons in humans and provide an important resource for future research to explore the molecular mechanisms underlying spinal cord physiology and diseases.

Deciphering Precise Gene Transcriptional Expression Using gwINTACT in Tomato

Functional gene transcription mainly occurs in the nucleus and has a significant role in plant physiology. The isolation of nuclei tagged in specific cell type (INTACT) technique provides an efficient and stable nucleus purification method to investigate the dynamic changes of nuclear gene transcriptional expression. However, the application of traditional INTACT in plants is still limited to seedlings or root cells because of severe chloroplast pollution. In this study, we proposed a newly designed and simplified INTACT based on mas-enhanced GFP (eGFP)-SlWIP2 (gwINTACT) for nuclear purification in tomato (Solanum lycopersicum) leaves, flowers, and fruits for the first time. The yield of the nucleus purified using gwINTACT from transgenic tomato leaves was doubled compared with using a traditional INTACT procedure, accompanied by more than 95% removal of chloroplasts. Relative gene expression of ethylene-related genes with ethylene treatment was reevaluated in gwINTACT leaves to reveal more different results from the traditional gene expression assay based on total RNA. Therefore, establishing the gwINTACT system in this study facilitates the precise deciphering of the transcriptional status in various tomato tissues, which lays the foundation for the further experimental study of nucleus-related molecular regulation on fruit ripening, such as ChIP-seq and ATAC-seq.

Circuit Mechanisms of Itch in the Brain

Itch is an unpleasant somatic sensation with the desire to scratch, and it consists of sensory, affective, and motivational components. Acute itch serves as a critical protective mechanism because an itch-evoked scratching response will help to remove harmful substances invading the skin. Recently, exciting progress has been made in deciphering the mechanisms of itch at both the peripheral nervous system and the CNS levels. Key neuronal subtypes and circuits have been revealed for ascending transmission and the descending modulation of itch. In this review, we mainly summarize the current understanding of the central circuit mechanisms of itch in the brain.

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.

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.

Endogenous opiates and behavior: 2019

This paper is the forty-second consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2019 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Postnatal maturation of spinal dynorphin circuits and their role in somatosensation

Inhibitory interneurons in the adult spinal dorsal horn (DH) can be neurochemically classified into subpopulations that regulate distinct somatosensory modalities. Although inhibitory networks in the rodent DH undergo dramatic remodeling over the first weeks of life, little is known about the maturation of identified classes of GABAergic interneurons, or whether their role in somatosensation shifts during development. We investigated age-dependent changes in the connectivity and function of prodynorphin (DYN)-lineage neurons in the mouse DH that suppress mechanosensation and itch during adulthood. In vitro patch clamp recordings revealed a developmental increase in primary afferent drive to DYN interneurons and a transition from exclusive C-fiber monosynaptic input to mixed A-fiber and C-fiber innervation. Although most adult DYN interneurons exhibited tonic firing as expected from their inhibitory phenotype, neonatal and adolescent DYN cells were predominantly classified as phasic or single-spiking. Importantly, we also found that most of the inhibitory presynaptic terminals contacting lamina I spinoparabrachial projection neurons (PNs) originate from DYN neurons. Furthermore, inhibitory synaptic input from DYN interneurons onto PNs was weaker during the neonatal period, likely reflecting a lower number of GABAergic terminals and a reduced probability of GABA release compared to adults. Finally, spinal DYN interneurons attenuated mechanical sensitivity throughout development, but this population dampened acute nonhistaminergic itch only during adulthood. Collectively, these findings suggest that the spinal "gates" controlling sensory transmission to the brain may emerge in a modality-selective manner during early life due to the postnatal tuning of inhibitory synaptic circuits within the DH.

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.

Intrinsic braking role of descending locus coeruleus noradrenergic neurons in acute and chronic itch in mice

Itch is defined as an unpleasant sensation that provokes a desire to scratch. Our understanding of neuronal circuits for itch information transmission and processing in the spinal dorsal horn (SDH) has progressively advanced following the identification of SDH neuron subsets that are crucial for scratching behavior in models of itch. However, little is known about the control of acute and chronic itch by descending signals from the brain to the SDH. In this study, using genetic approaches that enable cell-type and circuit-specific functional manipulation, we reveal an intrinsic potential of locus coeruleus (LC)-noradrenergic (NAergic) neurons that project to the SDH to control acute and chronic itch. Activation and silencing of SDH-projecting LC-NAergic neurons reduced and enhanced scratching behavior, respectively, in models of histamine-dependent and -independent acute itch. Furthermore, in a model of chronic itch associated with contact dermatitis, repetitive scratching behavior was suppressed by the activation of the descending LC-NAergic pathway and by knocking out NA transporters specific to descending LC-NAergic neurons using a CRISPR-Cas9 system. Moreover, patch-clamp recording using spinal slices showed that noradrenaline facilitated inhibitory synaptic inputs onto gastrin-releasing peptide receptor-expressing SDH neurons, a neuronal subset known to be essential for itch transmission. Our findings suggest that descending LC-NAergic signaling intrinsically controls acute and chronic itch and provide potential therapeutic strategies for the treatment of acute and chronic itch.

Neonatal Injury Evokes Persistent Deficits in Dynorphin Inhibitory Circuits within the Adult Mouse Superficial Dorsal Horn

Neonatal tissue damage induces long-term deficits in inhibitory synaptic transmission within the spinal superficial dorsal horn (SDH) that include a reduction in primary afferent-evoked, feedforward inhibition onto adult projection neurons. However, the subpopulations of mature GABAergic interneurons which are compromised by early-life injury have yet to be identified. The present research illuminates the persistent effects of neonatal surgical injury on the function of inhibitory SDH interneurons derived from the prodynorphin (DYN) lineage, a population that synapses directly onto lamina I spinoparabrachial neurons and is known to suppress mechanical pain and itch in adults. The results demonstrate that hindpaw incision at postnatal day 3 (P3) significantly decreased the strength of primary afferent-evoked glutamatergic drive onto DYN neurons within the adult mouse SDH while increasing the appearance of afferent-evoked inhibition onto the same population. Neonatal injury also dampened the intrinsic membrane excitability of mature DYN neurons, and reduced their action potential discharge in response to sensory input, compared with naive littermate controls. Furthermore, P3 incision decreased the efficacy of inhibitory DYN synapses onto adult spinoparabrachial neurons, which reflected a prolonged reduction in the probability of GABA release. Collectively, the data suggest that early-life tissue damage may persistently constrain the ability of spinal DYN interneurons to limit ascending nociceptive transmission to the adult brain. This is predicted to contribute to the loss of feedforward inhibition onto mature projection neurons, and the "priming" of nociceptive circuits in the developing spinal cord, following injuries during the neonatal period.SIGNIFICANCE STATEMENT Neonatal injury has lasting effects on pain processing in the adult CNS, including a reduction in feedforward inhibition onto ascending projection neurons in the spinal dorsal horn. While it is clear that spinal GABAergic interneurons are comprised of multiple subpopulations that play distinct roles in somatosensation, the identity of those interneurons which are compromised by tissue damage during early life remains unknown. Here we document persistent deficits in spinal inhibitory circuits involving dynorphin-lineage interneurons previously implicated in gating mechanical pain and itch. Notably, neonatal injury reduced the strength of dynorphin-lineage inhibitory synapses onto mature lamina I spinoparabrachial neurons, a major output of the spinal nociceptive network, which could contribute to the priming of pain pathways by early tissue damage.

A preliminary study on DRGs and spinal cord of a galanin receptor 2-EGFP transgenic mouse

The neuropeptide galanin functions via three G-protein coupled receptors, Gal_1-3-R. Both Gal₁-R and ₂-R are involved in pain signaling at the spinal level. Here a Gal₂-R-EGFP transgenic (TG) mouse was generated and studied in pain tests and by characterizing Gal₂-R expression in both sensory ganglia and spinal cord. After peripheral spared nerve injury, mechanical allodynia developed and was ipsilaterally similar between wild type (WT) and TG mice. A Gal₂-R-EGFP-positive signal was primarily observed in small and medium-sized dorsal root ganglion (DRG) neurons and in spinal interneurons and processes. No significant difference in size distribution of DRG neuronal profiles was found between TG and WT mice. Both percentage and fluorescence intensity of Gal₂-R-EGFP-positive neuronal profiles were overall significantly upregulated in ipsilateral DRGs as compared to contralateral DRGs. There was an ipsilateral reduction in substance P-positive and calcitonin gene-related peptide (CGRP)-positive neuronal profiles, and this reduction was more pronounced in TG as compared to WT mice. Moreover, Gal₂-R-EGFP partly co-localized with three pain-related neuropeptides, CGRP, neuropeptide Y and galanin, both in intact and injured DRGs, and with galanin also in local neurons in the superficial dorsal horn. Taken together, the present results provide novel information on the localization and phenotype of DRG and spinal neurons expressing the second galanin receptor, Gal₂-R, and on phenotypic changes following peripheral nerve injury. Gal2-R may also be involved in autoreceptor signaling.