Cell type-specific calcium imaging of central sensitization in mouse dorsal horn

Differential modification of ascending spinal outputs in acute and chronic pain states

Pain hypersensitivity arises from the induction of plasticity in peripheral and spinal somatosensory neurons, which modifies nociceptive input to the brain, altering pain perception. We applied longitudinal calcium imaging of spinal dorsal projection neurons to determine whether and how the representation of somatosensory stimuli in the anterolateral tract, the principal pathway transmitting nociceptive signals to the brain, changes between distinct pain states. In healthy mice, we identified stable outputs selective for cooling or warming and a neuronal ensemble activated by noxious thermal and mechanical stimuli. Induction of acute peripheral sensitization by topical capsaicin transiently re-tuned nociceptive output neurons to encode low-intensity stimuli. In contrast, peripheral nerve injury resulted in a persistent suppression of innocuous spinal outputs coupled with persistent activation of a normally silent population of high-threshold neurons. These results demonstrate differential modulation of spinal outputs to the brain during nociceptive and neuropathic pain states.

Spike rate inference from mouse spinal cord calcium imaging data

Calcium imaging is a key method to record the spiking activity of identified and genetically targeted neurons. However, the observed calcium signals are only an indirect readout of the underlying electrophysiological events (single spikes or bursts of spikes) and require dedicated algorithms to recover the spike rate. These algorithms for spike inference can be optimized using ground truth data from combined electrical and optical recordings, but it is not clear how such optimized algorithms perform on cell types and brain regions for which ground truth does not exist. Here, we use a state-of-the-art algorithm based on supervised deep learning (CASCADE) and a non-supervised algorithm based on non-negative deconvolution (OASIS) to test spike rate inference in spinal cord neurons. To enable these tests, we recorded specific ground truth from glutamatergic and GABAergic somatosensory neurons in the superficial dorsal horn of spinal cord in mice of both sexes. We find that CASCADE and OASIS algorithms that were designed for cortical excitatory neurons generalize well to both spinal cord cell types. However, CASCADE models re-trained on our ground truth further improved the performance, resulting in a more accurate inference of spiking activity from spinal cord neurons. We openly provide re-trained models that can be applied to spinal cord data with variable noise levels and frame rates. Together, our ground-truth recordings and analyses provide a solid foundation for the interpretation of calcium imaging data from spinal cord dorsal horn and showcase how spike rate inference can generalize between different regions of the nervous system.Significance Statement Calcium imaging is a powerful method for measuring the activity of genetically identified neurons. However, accurate interpretation of calcium transients depends on having a detailed understanding of how neuronal activity correlates with fluorescence. Such calibration recordings have been performed for cerebral cortex but not yet for most other CNS regions and neuron types. Here, we obtained ground truth data in spinal cord by conducting simultaneous calcium and electrophysiology recordings in excitatory and inhibitory neurons. We systematically investigated the transferability of cortical algorithms to spinal neuron subpopulations and generated inference algorithms optimized to excitatory and inhibitory neurons. Our study provides a foundation for the rigorous interpretation of calcium imaging data from spinal cord.

Spike rate inference from mouse spinal cord calcium imaging data

Calcium imaging is a key method to record the spiking activity of identified and genetically targeted neurons. However, the observed calcium signals are only an indirect readout of the underlying electrophysiological events (single spikes or bursts of spikes) and require dedicated algorithms to recover the spike rate. These algorithms for spike inference can be optimized using ground truth data from combined electrical and optical recordings, but it is not clear how such optimized algorithms perform on cell types and brain regions for which ground truth does not exist. Here, we use a state-of-the-art algorithm based on supervised deep learning (CASCADE) and a non-supervised algorithm based on non-negative deconvolution (OASIS) to test spike rate inference in spinal cord neurons. To enable these tests, we recorded specific ground truth from glutamatergic and GABAergic somatosensory neurons in the superficial dorsal horn of spinal cord in mice of both sexes. We find that CASCADE and OASIS algorithms that were designed for cortical excitatory neurons generalize well to both spinal cord cell types. However, CASCADE models re-trained on our ground truth further improved the performance, resulting in a more accurate inference of spiking activity from spinal cord neurons. We openly provide re-trained models that can be applied to spinal cord data of variable noise levels and frame rates. Together, our ground-truth recordings and analyses provide a solid foundation for the interpretation of calcium imaging data from spinal cord dorsal horn and showcase how spike rate inference can generalize between different regions of the nervous system.

Long-term optical imaging of the spinal cord in awake behaving mice

Advances in optical imaging and fluorescent biosensors enable study of the spatiotemporal and long-term neural dynamics in the brain of awake animals. However, methodological difficulties and fibrosis limit similar advances in the spinal cord. Here, to overcome these obstacles, we combined in vivo application of fluoropolymer membranes that inhibit fibrosis, a redesigned implantable spinal imaging chamber and improved motion correction methods that together permit imaging of the spinal cord in awake behaving mice, for months to over a year. We demonstrated a robust ability to monitor axons, identified a spinal cord somatotopic map, performed months-long imaging in freely moving mice, conducted Ca2+ imaging of neural dynamics in behaving mice responding to pain-provoking stimuli and observed persistent microglial changes after nerve injury. The ability to couple in vivo imaging and behavior at the spinal cord level will drive insights not previously possible at a key location for somatosensory transmission to the brain.

TRPA1 Agonist-Responsive Afferents Contribute to Central Sensitization by Suppressing Spinal GABAergic Interneurons Through Somatostatin 2A Receptors

Altered nociception, a key feature of nociplastic pain, often involves central sensitization. We previously found that central sensitization underlying a nociplastic pain state in female mice depends on the ongoing activity of TRPA1 agonist-responsive afferents. Here, we investigated how the activity of these afferents induces and maintains central sensitization at the spinal level. We hypothesized that, in the superficial dorsal horn where somatostatin (SST) is expressed in excitatory interneurons and the SST2A receptor (SST2A-R) in GABAergic inhibitory interneurons (GABAn), TRPA1 agonist-responsive afferents stimulate SST-expressing excitatory interneurons (SSTn), leading to GABAn suppression through SST2A-R and resulting in altered nociception. We tested this hypothesis using ex vivo Ca2+ imaging of dorsal root-attached spinal cord slices expressing GCaMP6f in either SSTn or GABAn and in vivo assessment of mechanical hypersensitivity. The dorsal root was chemically (with allyl isothiocyanate [AITC]) and electrically stimulated to activate TRPA1-expressing nociceptors and all afferents, respectively. The stimulation of dorsal root with AITC excited SSTn. During activation of AITC-responsive afferents, a subset of SSTn showed potentiated responses to both low- and high-threshold afferent inputs, whereas a subset of GABAn showed suppressed responses to those afferents in an SST2A-R-dependent manner. Intrathecally administered SST2A-R antagonist inhibited the development of mechanical hypersensitivity by intraplantar AITC injection and alleviated persistent mechanical hypersensitivity in the murine model of nociplastic pain. These results suggest that the activity of TRPA1 agonist-responsive afferents induces and maintains central sensitization by activating dorsal horn SSTn and suppressing GABAn via SST2A-R, resulting in altered nociception that manifests as mechanical hypersensitivity. PERSPECTIVE: This article presents experimental evidence that TRPA1 agonist-responsive afferents induce and maintain central sensitization at the spinal level by activating SST-expressing excitatory interneurons and suppressing GABAergic inhibitory interneurons via SST2A-R. Spinal SST2A-R may represent a promising target for treating mechanical pain hypersensitivity due to central sensitization by TRPA1 agonist-responsive afferents.

Kappa opioids inhibit spinal output neurons to suppress itch

Itch is a protective sensation that drives scratching. Although specific cell types have been proposed to underlie itch, the neural basis for itch remains unclear. Here, we used two-photon Ca2+ imaging of the dorsal horn to visualize neuronal populations that are activated by itch-inducing agents. We identify a convergent population of spinal interneurons recruited by diverse itch-causing stimuli that represents a subset of neurons that express the gastrin-releasing peptide receptor (GRPR). Moreover, we find that itch is conveyed to the brain via GRPR-expressing spinal output neurons that target the lateral parabrachial nuclei. We then show that the kappa opioid receptor agonist nalfurafine relieves itch by selectively inhibiting GRPR spinoparabrachial neurons. These experiments provide a population-level view of the spinal neurons that respond to pruritic stimuli, pinpoint the output neurons that convey itch to the brain, and identify the cellular target of kappa opioid receptor agonists for the inhibition of itch.

Parabrachial neurons promote nociplastic pain

The parabrachial nucleus (PBN) in the dorsal pons responds to bodily threats and transmits alarm signals to the forebrain. Parabrachial neuron activity is enhanced during chronic pain, and inactivation of PBN neurons in mice prevents the establishment of neuropathic, chronic pain symptoms. Chemogenetic or optogenetic activation of all glutamatergic neurons in the PBN, or just the subpopulation that expresses the Calca gene, is sufficient to establish pain phenotypes, including long-lasting tactile allodynia, that scale with the extent of stimulation, thereby promoting nociplastic pain, defined as diffuse pain without tissue inflammation or nerve injury. This review focuses on the role(s) of molecularly defined PBN neurons and the downstream nodes in the brain that contribute to establishing nociplastic pain.

Neural circuit-selective, multiplexed pharmacological targeting of prefrontal cortex-projecting locus coeruleus neurons drives antinociception

Selective manipulation of neural circuits using optogenetics and chemogenetics holds great translational potential but requires genetic access to neurons. Here, we demonstrate a general framework for identifying genetic tool-independent, pharmacological strategies for neural circuit-selective modulation. We developed an economically accessible calcium imaging-based approach for large-scale pharmacological scans of endogenous receptor-mediated neural activity. As a testbed for this approach, we used the mouse locus coeruleus due to the combination of its widespread, modular efferent neural circuitry and its wide variety of endogenously expressed GPCRs. Using machine learning-based action potential deconvolution and retrograde tracing, we identified an agonist cocktail that selectively inhibits medial prefrontal cortex-projecting locus coeruleus neurons. In vivo, this cocktail produces synergistic antinociception, consistent with selective pharmacological blunting of this neural circuit. This framework has broad utility for selective targeting of other neural circuits under different physiological and pathological states, facilitating non-genetic translational applications arising from cell type-selective discoveries.

Vitamin C Drives Reentrant Actin Phase Transition: Biphasic Exocytosis Regulation Revealed by Single-Vesicle Electrochemistry

Unveiling molecular mechanisms that dominate protein phase dynamics has been a pressing need for deciphering the intricate intracellular modulation machinery. While ions and biomacromolecules have been widely recognized for modulating protein phase separations, effects of small molecules that essentially constitute the cytosolic chemical atmosphere on the protein phase behaviors are rarely understood. Herein, we report that vitamin C (VC), a key small molecule for maintaining a reductive intracellular atmosphere, drives reentrant phase transitions of myosin II/F-actin (actomyosin) cytoskeletons. The actomyosin bundle condensates dissemble in the low-VC regime and assemble in the high-VC regime in vitro or inside neuronal cells, through a concurrent myosin II protein aggregation-dissociation process with monotonic VC concentration increase. Based on this finding, we employ in situ single-cell and single-vesicle electrochemistry to demonstrate the quantitative modulation of catecholamine transmitter vesicle exocytosis by intracellular VC atmosphere, i.e., exocytotic release amount increases in the low-VC regime and decreases in the high-VC regime. Furthermore, we show how VC regulates cytomembrane-vesicle fusion pore dynamics through counteractive or synergistic effects of actomyosin phase transitions and the intracellular free calcium level on membrane tensions. Our work uncovers the small molecule-based reversive protein phase regulatory mechanism, paving a new way to chemical neuromodulation and therapeutic repertoire expansion.

Selective modification of ascending spinal outputs in acute and neuropathic pain states

Pain hypersensitivity arises from the plasticity of peripheral and spinal somatosensory neurons, which modifies nociceptive input to the brain and alters pain perception. We utilized chronic calcium imaging of spinal dorsal horn neurons to determine how the representation of somatosensory stimuli in the anterolateral tract, the principal pathway transmitting nociceptive signals to the brain, changes between distinct pain states. In healthy conditions, we identify stable, narrowly tuned outputs selective for cooling or warming, and a neuronal ensemble activated by intense/noxious thermal and mechanical stimuli. Induction of an acute peripheral sensitization with capsaicin selectively and transiently retunes nociceptive output neurons to encode low-intensity stimuli. In contrast, peripheral nerve injury-induced neuropathic pain results in a persistent suppression of innocuous spinal outputs coupled with activation of a normally silent population of high-threshold neurons. These results demonstrate the differential modulation of specific spinal outputs to the brain during nociceptive and neuropathic pain states.

Novel aspects of signal processing in lamina I

The most superficial layer of the spinal dorsal horn, lamina I, is a key element of the nociceptive processing system. It contains different types of projection neurons (PNs) and local-circuit neurons (LCNs) whose functional roles in the signal processing are poorly understood. This article reviews recent progress in elucidating novel anatomical features and physiological properties of lamina I PNs and LCNs revealed by whole-cell recordings in ex vivo spinal cord. This article is part of the Special Issue on "Ukrainian Neuroscience".

A mouse DRG genetic toolkit reveals morphological and physiological diversity of somatosensory neuron subtypes

Dorsal root ganglia (DRG) somatosensory neurons detect mechanical, thermal, and chemical stimuli acting on the body. Achieving a holistic view of how different DRG neuron subtypes relay neural signals from the periphery to the CNS has been challenging with existing tools. Here, we develop and curate a mouse genetic toolkit that allows for interrogating the properties and functions of distinct cutaneous targeting DRG neuron subtypes. These tools have enabled a broad morphological analysis, which revealed distinct cutaneous axon arborization areas and branching patterns of the transcriptionally distinct DRG neuron subtypes. Moreover, in vivo physiological analysis revealed that each subtype has a distinct threshold and range of responses to mechanical and/or thermal stimuli. These findings support a model in which morphologically and physiologically distinct cutaneous DRG sensory neuron subtypes tile mechanical and thermal stimulus space to collectively encode a wide range of natural stimuli.

Nerve injury disrupts temporal processing in the spinal cord dorsal horn through alterations in PV+ interneurons

How mechanical allodynia following nerve injury is encoded in patterns of neural activity in the spinal cord dorsal horn (DH) remains incompletely understood. We address this in mice using the spared nerve injury model of neuropathic pain and in vivo electrophysiological recordings. Surprisingly, despite dramatic behavioral over-reactivity to mechanical stimuli following nerve injury, an overall increase in sensitivity or reactivity of DH neurons is not observed. We do, however, observe a marked decrease in correlated neural firing patterns, including the synchrony of mechanical stimulus-evoked firing, across the DH. Alterations in DH temporal firing patterns are recapitulated by silencing DH parvalbumin+ (PV+) interneurons, previously implicated in mechanical allodynia, as are allodynic pain-like behaviors. These findings reveal decorrelated DH network activity, driven by alterations in PV+ interneurons, as a prominent feature of neuropathic pain and suggest restoration of proper temporal activity as a potential therapeutic strategy to treat chronic neuropathic pain.

In-Vivo Calcium Imaging of Sensory Neurons in the Rat Trigeminal Ganglion

Genetically encoded calcium indicators (GECIs) enable imaging techniques to monitor changes in intracellular calcium in targeted cell populations. Their large signal-to-noise ratio makes GECIs a powerful tool for detecting stimulus-evoked activity in sensory neurons. GECIs facilitate population-level analysis of stimulus encoding with the number of neurons that can be studied simultaneously. This population encoding is most appropriately done in vivo. Dorsal root ganglia (DRG), which house the soma of sensory neurons innervating somatic and visceral structures below the neck, are used most extensively for in vivo imaging because these structures are accessed relatively easily. More recently, this technique was used in mice to study sensory neurons in the trigeminal ganglion (TG) that innervate oral and craniofacial structures. There are many reasons to study TG in addition to DRG, including the long list of pain syndromes specific to oral and craniofacial structures that appear to reflect changes in sensory neuron activity, such as trigeminal neuralgia. Mice are used most extensively in the study of DRG and TG neurons because of the availability of genetic tools. However, with differences in size, ease of handling, and potentially important species differences, there are reasons to study rat rather than mouse TG neurons. Thus, we developed an approach for imaging rat TG neurons in vivo. We injected neonatal pups (p2) intraperitoneally with an AAV encoding GCaMP6s, resulting in >90% infection of both TG and DRG neurons. TG was visualized in the adult following craniotomy and decortication, and changes in GCaMP6s fluorescence were monitored in TG neurons following stimulation of mandibular and maxillary regions of the face. We confirmed that increases in fluorescence were stimulus-evoked with peripheral nerve block. While this approach has many potential uses, we are using it to characterize the subpopulation(s) of TG neurons changed following peripheral nerve injury.

Understanding of Spinal Wide Dynamic Range Neurons and Their Modulation on Pathological Pain

The spinal dorsal horn (SDH) transmits sensory information from the periphery to the brain. Wide dynamic range (WDR) neurons within this relay site play a critical role in modulating and integrating peripheral sensory inputs, as well as the process of central sensitization during pathological pain. This group of spinal multi-receptive neurons has attracted considerable attention in pain research due to their capabilities for encoding the location and intensity of nociception. Meanwhile, transmission, processing, and modulation of incoming afferent information in WDR neurons also establish the underlying basis for investigating the integration of acupuncture and pain signals. This review aims to provide a comprehensive examination of the distinctive features of WDR neurons and their involvement in pain. Specifically, we will examine the regulation of diverse supraspinal nuclei on these neurons and analyze their potential in elucidating the mechanisms of acupuncture analgesia.

Identification of a convergent spinal neuron population that encodes itch

Itch is a protective sensation that drives scratching. Although specific cell types have been proposed to underlie itch, the neural circuit basis for itch remains unclear. Here, we used two-photon Ca2+ imaging of the dorsal horn to visualize the neuronal populations that are activated by itch-inducing agents. We identify a convergent population of spinal neurons that is defined by the expression of GRPR. Moreover, we discover that itch is conveyed to the brain via GRPR-expressing spinal output neurons that target the lateral parabrachial nucleus. Further, we show that nalfurafine, a clinically effective kappa opioid receptor agonist, relieves itch by inhibiting GRPR spinoparabrachial neurons. Finally, we demonstrate that a subset of GRPR spinal neurons show persistent, cell-intrinsic Ca2+ oscillations. These experiments provide the first population-level view of the spinal neurons that respond to pruritic stimuli, pinpoint the output neurons that convey itch to the brain, and identify the cellular target of kappa opioid receptor agonists for the inhibition of itch.

Long-term optical imaging of the spinal cord in awake, behaving animals

Advances in optical imaging approaches and fluorescent biosensors have enabled an understanding of the spatiotemporal and long-term neural dynamics in the brain of awake animals. However, methodological difficulties and the persistence of post-laminectomy fibrosis have greatly limited similar advances in the spinal cord. To overcome these technical obstacles, we combined in vivo application of fluoropolymer membranes that inhibit fibrosis; a redesigned, cost-effective implantable spinal imaging chamber; and improved motion correction methods that together permit imaging of the spinal cord in awake, behaving mice, for months to over a year. We also demonstrate a robust ability to monitor axons, identify a spinal cord somatotopic map, conduct Ca2+ imaging of neural dynamics in behaving animals responding to pain-provoking stimuli, and observe persistent microglial changes after nerve injury. The ability to couple neural activity and behavior at the spinal cord level will drive insights not previously possible at a key location for somatosensory transmission to the brain.

Contralateral Afferent Input to Lumbar Lamina I Neurons as a Neural Substrate for Mirror-Image Pain

Mirror-image pain arises from pathologic alterations in the nociceptive processing network that controls functional lateralization of the primary afferent input. Although a number of clinical syndromes related to dysfunction of the lumbar afferent system are associated with the mirror-image pain, its morphophysiological substrate and mechanism of induction remain poorly understood. Therefore, we used ex vivo spinal cord preparation of young rats of both sexes to study organization and processing of the contralateral afferent input to the neurons in the major spinal nociceptive projection area Lamina I. We show that decussating primary afferent branches reach contralateral Lamina I, where 27% of neurons, including projection neurons, receive monosynaptic and/or polysynaptic excitatory drive from the contralateral Aδ-fibers and C-fibers. All these neurons also received ipsilateral input, implying their involvement in the bilateral information processing. Our data further show that the contralateral Aδ-fiber and C-fiber input is under diverse forms of inhibitory control. Attenuation of the afferent-driven presynaptic inhibition and/or disinhibition of the dorsal horn network increased the contralateral excitatory drive to Lamina I neurons and its ability to evoke action potentials. Furthermore, the contralateral Aβδ-fibers presynaptically control ipsilateral C-fiber input to Lamina I neurons. Thus, these results show that some lumbar Lamina I neurons are wired to the contralateral afferent system whose input, under normal conditions, is subject to inhibitory control. A pathologic disinhibition of the decussating pathways can open a gate controlling contralateral information flow to the nociceptive projection neurons and, thus, contribute to induction of hypersensitivity and mirror-image pain.SIGNIFICANCE STATEMENT We show that contralateral Aδ-afferents and C-afferents supply lumbar Lamina I neurons. The contralateral input is under diverse forms of inhibitory control and itself controls the ipsilateral input. Disinhibition of decussating pathways increases nociceptive drive to Lamina I neurons and may cause induction of contralateral hypersensitivity and mirror-image pain.

A DRG genetic toolkit reveals molecular, morphological, and functional diversity of somatosensory neuron subtypes

Mechanical and thermal stimuli acting on the skin are detected by morphologically and physiologically distinct sensory neurons of the dorsal root ganglia (DRG). Achieving a holistic view of how this diverse neuronal population relays sensory information from the skin to the central nervous system (CNS) has been challenging with existing tools. Here, we used transcriptomic datasets of the mouse DRG to guide development and curation of a genetic toolkit to interrogate transcriptionally defined DRG neuron subtypes. Morphological analysis revealed unique cutaneous axon arborization areas and branching patterns of each subtype. Physiological analysis showed that subtypes exhibit distinct thresholds and ranges of responses to mechanical and/or thermal stimuli. The somatosensory neuron toolbox thus enables comprehensive phenotyping of most principal sensory neuron subtypes. Moreover, our findings support a population coding scheme in which the activation thresholds of morphologically and physiologically distinct cutaneous DRG neuron subtypes tile multiple dimensions of stimulus space.