Intrinsic Homeostatic Plasticity in Mouse and Human Sensory Neurons.

In response to changes in activity induced by environmental cues, neurons in the central nervous system undergo homeostatic plasticity to sustain overall network function during abrupt changes in synaptic strengths. Homeostatic plasticity involves changes in synaptic scaling and regulation of intrinsic excitability. Increases in spontaneous firing and excitability of sensory neurons are evident in some forms of chronic pain in animal models and human patients. However, whether mechanisms of homeostatic plasticity are engaged in sensory neurons under normal conditions or altered after chronic pain is unknown. Here, we showed that sustained depolarization induced by 30mM KCl induces a compensatory decrease in the excitability in mouse and human sensory neurons. Moreover, voltage-gated sodium currents are robustly reduced in mouse sensory neurons contributing to the overall decrease in neuronal excitability. Decreased efficacy of these homeostatic mechanisms could potentially contribute to the development of the pathophysiology of chronic pain.

Harmonized cross-species cell atlases of trigeminal and dorsal root ganglia.

Peripheral sensory neurons in the dorsal root ganglion (DRG) and trigeminal ganglion (TG) are specialized to detect and transduce diverse environmental stimuli including touch, temperature, and pain to the central nervous system. Recent advances in single-cell RNA-sequencing (scRNA-seq) have provided new insights into the diversity of sensory ganglia cell types in rodents, non-human primates, and humans, but it remains difficult to compare transcriptomically defined cell types across studies and species. Here, we built cross-species harmonized atlases of DRG and TG cell types that describe 18 neuronal and 11 non-neuronal cell types across 6 species and 19 studies. We then demonstrate the utility of this harmonized reference atlas by using it to annotate newly profiled DRG nuclei/cells from both human and the highly regenerative axolotl. We observe that the transcriptomic profiles of sensory neuron subtypes are broadly similar across vertebrates, but the expression of functionally important neuropeptides and channels can vary notably. The new resources and data presented here can guide future studies in comparative transcriptomics, simplify cell type nomenclature differences across studies, and help prioritize targets for future pain therapy development.

The impact of tumor immunogenicity on cancer pain phenotype using syngeneic oral cancer mouse models.

Head and neck squamous cell carcinoma (HNSCC) patients report severe function-induced pain at the site of the primary tumor. The current hypothesis is that oral cancer pain is initiated and maintained in the cancer microenvironment due to secretion of algogenic mediators from tumor cells and surrounding immune cells that sensitize the primary sensory neurons innervating the tumor. Immunogenicity, which is the ability to induce an adaptive immune response, has been widely studied using cancer cell transplantation experiments. However, oral cancer pain studies have primarily used xenograft transplant models in which human-derived tumor cells are inoculated in an athymic mouse lacking an adaptive immune response; the role of inflammation in oral cancer-induced nociception is still unknown. Using syngeneic oral cancer mouse models, we investigated the impact of tumor cell immunogenicity and growth on orofacial nociceptive behavior and oral cancer-induced sensory neuron plasticity. We found that an aggressive, weakly immunogenic mouse oral cancer cell line, MOC2, induced rapid orofacial nociceptive behavior in both male and female C57Bl/6 mice. Additionally, MOC2 tumor growth invoked a substantial injury response in the trigeminal ganglia as defined by a significant upregulation of injury response marker ATF3 in tongue-innervating trigeminal neurons. In contrast, using a highly immunogenic mouse oral cancer cell line, MOC1, we found a much slower onset of orofacial nociceptive behavior in female C57Bl/6 mice only as well as sex-specific differences in the tumor-associated immune landscape and gene regulation in tongue innervating sensory neurons. Together, these data suggest that cancer-induced nociceptive behavior and sensory neuron plasticity can greatly depend on the immunogenic phenotype of the cancer cell line and the associated immune response.

Sensory Neurotransmitter Calcitonin Gene-Related Peptide Modulates Tumor Growth and Lymphocyte Infiltration in Oral Squamous Cell Carcinoma.

Head and neck squamous cell carcinoma are highly innervated by peripheral sensory neurons. Local neurotransmitter release (e.g., calcitonin gene-related peptide (CGRP)) from sensory neurons innervating cancer is linked to tumorigenesis. CGRP-immunoreactive nerve presence comprised 9.53±1.9% of total nerve area across 11 HNSCC patients. A syngeneic tongue tumor transplant mouse model of oral cancer and a global Calca knockout mouse (CGRPKO ) are used to investigate the impact of CGRP signaling on tumor growth and the associated immune response in vivo. In tumor-bearing CGRPKO mice, there is a significant reduction in tumor size over time compared to wildtype mice using two different mouse oral cancer cell lines. Furthermore, tumor tissue from CGRPKO mice had a significant increase in tumor-infiltrating CD4+ T cells, cytotoxic CD8+ T cells, and NK1.1+ NK cells compared to wildtype. Fluorescent-activated cell sorting and real-time qPCR are used to confirm that CD4+ T cells are isolated from tumor-bearing wildtype mice containing a high expression of Ramp1 compared to sham mice. These data suggest that sensory neurotransmitter CGRP may modulate oral cancer progression via tumor immunosurveillance. Understanding the relationship between sensory neurons and cancer will aid in repurposing clinically available nervous system drugs for the treatment of cancer.

Pharmacological target-focused transcriptomic analysis of native vs cultured human and mouse dorsal root ganglia.

Dorsal root ganglion (DRG) neurons detect sensory inputs and are crucial for pain processing. They are often studied in vitro as dissociated cell cultures with the assumption that this reasonably represents in vivo conditions. However, to the best of our knowledge, no study has directly compared genome-wide transcriptomes of DRG tissue in vivo versus in vitro or between laboratories and culturing protocols. Comparing RNA sequencing-based transcriptomes of native to cultured (4 days in vitro) human or mouse DRG, we found that the overall expression levels of many ion channels and G-protein-coupled receptors specifically expressed in neurons are markedly lower although still expressed in culture. This suggests that most pharmacological targets expressed in vivo are present under the condition of dissociated cell culture, but with changes in expression levels. The reduced relative expression for neuronal genes in human DRG cultures is likely accounted for by increased expression of genes in fibroblast-like and other proliferating cells, consistent with their mitotic status in these cultures. We found that the expression of a subset of genes typically expressed in neurons increased in human and mouse DRG cultures relative to the intact ganglion, including genes associated with nerve injury or inflammation in preclinical models such as BDNF, MMP9, GAL, and ATF3. We also found a striking upregulation of a number of inflammation-associated genes in DRG cultures, although many were different between mouse and human. Our findings suggest an injury-like phenotype in DRG cultures that has important implications for the use of this model system for pain drug discovery.

Battery-free, fully implantable optofluidic cuff system for wireless optogenetic and pharmacological neuromodulation of peripheral nerves.

Studies of the peripheral nervous system rely on controlled manipulation of neuronal function with pharmacologic and/or optogenetic techniques. Traditional hardware for these purposes can cause notable damage to fragile nerve tissues, create irritation at the biotic/abiotic interface, and alter the natural behaviors of animals. Here, we present a wireless, battery-free device that integrates a microscale inorganic light-emitting diode and an ultralow-power microfluidic system with an electrochemical pumping mechanism in a soft platform that can be mounted onto target peripheral nerves for programmed delivery of light and/or pharmacological agents in freely moving animals. Biocompliant designs lead to minimal effects on overall nerve health and function, even with chronic use in vivo. The small size and light weight construction allow for deployment as fully implantable devices in mice. These features create opportunities for studies of the peripheral nervous system outside of the scope of those possible with existing technologies.

A wireless closed-loop system for optogenetic peripheral neuromodulation.

The fast-growing field of bioelectronic medicine aims to develop engineered systems that can relieve clinical conditions by stimulating the peripheral nervous system1-5. This type of technology relies largely on electrical stimulation to provide neuromodulation of organ function or pain. One example is sacral nerve stimulation to treat overactive bladder, urinary incontinence and interstitial cystitis (also known as bladder pain syndrome)4,6,7. Conventional, continuous stimulation protocols, however, can cause discomfort and pain, particularly when treating symptoms that can be intermittent (for example, sudden urinary urgency)8. Direct physical coupling of electrodes to the nerve can lead to injury and inflammation9-11. Furthermore, typical therapeutic stimulators target large nerve bundles that innervate multiple structures, resulting in a lack of organ specificity. Here we introduce a miniaturized bio-optoelectronic implant that avoids these limitations by using (1) an optical stimulation interface that exploits microscale inorganic light-emitting diodes to activate opsins; (2) a soft, high-precision biophysical sensor system that allows continuous measurements of organ function; and (3) a control module and data analytics approach that enables coordinated, closed-loop operation of the system to eliminate pathological behaviours as they occur in real-time. In the example reported here, a soft strain gauge yields real-time information on bladder function in a rat model. Data algorithms identify pathological behaviour, and automated, closed-loop optogenetic neuromodulation of bladder sensory afferents normalizes bladder function. This all-optical scheme for neuromodulation offers chronic stability and the potential to stimulate specific cell types.

Angiotensin II Triggers Peripheral Macrophage-to-Sensory Neuron Redox Crosstalk to Elicit Pain.

Injury, inflammation, and nerve damage initiate a wide variety of cellular and molecular processes that culminate in hyperexcitation of sensory nerves, which underlies chronic inflammatory and neuropathic pain. Using behavioral readouts of pain hypersensitivity induced by angiotensin II (Ang II) injection into mouse hindpaws, our study shows that activation of the type 2 Ang II receptor (AT2R) and the cell-damage-sensing ion channel TRPA1 are required for peripheral mechanical pain sensitization induced by Ang II in male and female mice. However, we show that AT2R is not expressed in mouse and human dorsal root ganglia (DRG) sensory neurons. Instead, expression/activation of AT2R on peripheral/skin macrophages (MΦs) constitutes a critical trigger of mouse and human DRG sensory neuron excitation. Ang II-induced peripheral mechanical pain hypersensitivity can be attenuated by chemogenetic depletion of peripheral MΦs. Furthermore, AT2R activation in MΦs triggers production of reactive oxygen/nitrogen species, which trans-activate TRPA1 on mouse and human DRG sensory neurons via cysteine modification of the channel. Our study thus identifies a translatable immune cell-to-sensory neuron signaling crosstalk underlying peripheral nociceptor sensitization. This form of cell-to-cell signaling represents a critical peripheral mechanism for chronic pain and thus identifies multiple druggable analgesic targets.SIGNIFICANCE STATEMENT Pain is a widespread health problem that is undermanaged by currently available analgesics. Findings from a recent clinical trial on a type II angiotensin II receptor (AT2R) antagonist showed effective analgesia for neuropathic pain. AT2R antagonists have been shown to reduce neuropathy-, inflammation- and bone cancer-associated pain in rodents. We report that activation of AT2R in macrophages (MΦs) that infiltrate the site of injury, but not in sensory neurons, triggers an intercellular redox communication with sensory neurons via activation of the cell damage/pain-sensing ion channel TRPA1. This MΦ-to-sensory neuron crosstalk results in peripheral pain sensitization. Our findings provide an evidence-based mechanism underlying the analgesic action of AT2R antagonists, which could accelerate the development of efficacious non-opioid analgesic drugs for multiple pain conditions.

Metabotropic Glutamate Receptor 2/3 (mGluR2/3) Activation Suppresses TRPV1 Sensitization in Mouse, But Not Human, Sensory Neurons.

The use of human tissue to validate putative analgesic targets identified in rodents is a promising strategy for improving the historically poor translational record of preclinical pain research. We recently demonstrated that in mouse and human sensory neurons, agonists for metabotropic glutamate receptors 2 and 3 (mGluR2/3) reduce membrane hyperexcitability produced by the inflammatory mediator prostaglandin E2 (PGE2). Previous rodent studies indicate that mGluR2/3 can also reduce peripheral sensitization by suppressing inflammation-induced sensitization of TRPV1. Whether this observation similarly translates to human sensory neurons has not yet been tested. We found that activation of mGluR2/3 with the agonist APDC suppressed PGE2-induced sensitization of TRPV1 in mouse, but not human, sensory neurons. We also evaluated sensory neuron expression of the gene transcripts for mGluR2 (Grm2), mGluR3 (Grm3), and TRPV1 (Trpv1). The majority of Trpv1+ mouse and human sensory neurons expressed Grm2 and/or Grm3, and in both mice and humans, Grm2 was expressed in a greater percentage of sensory neurons than Grm3. Although we demonstrated a functional difference in the modulation of TRPV1 sensitization by mGluR2/3 activation between mouse and human, there were no species differences in the gene transcript colocalization of mGluR2 or mGluR3 with TRPV1 that might explain this functional difference. Taken together with our previous work, these results suggest that mGluR2/3 activation suppresses only some aspects of human sensory neuron sensitization caused by PGE2. These differences have implications for potential healthy human voluntary studies or clinical trials evaluating the analgesic efficacy of mGluR2/3 agonists or positive allosteric modulators.

Sex-, stress-, and sympathetic post-ganglionic-dependent changes in identity and proportions of immune cells in the dura.

Aim of investigation Due to compelling evidence in support of links between sex, stress, sympathetic post-ganglionic innervation, dural immune cells, and migraine, our aim was to characterize the impacts of these factors on the type and proportion of immune cells in the dura. Methods Dural immune cells were obtained from naïve or stressed adult male and female Sprague Dawley rats for flow cytometry. Rats with surgical denervation of sympathetic post-ganglionic neurons of the dura were also studied. Results Immune cells comprise ∼17% of all cells in the dura. These included: macrophages/granulocytes ("Macs"; 63.2% of immune cells), dendritic cells (0.88%), T-cells (4.51%), natural killer T-cells (0.51%), natural killer cells (3.08%), and B-cells (20.0%). There were significantly more Macs and fewer B- and natural killer T-cells in the dura of females compared with males. Macs and dendritic cells were significantly increased by stress in males, but not females. In contrast, T-cells were significantly increased in females with a 24-hour delay following stress. Lastly, Macs, dendritic cells, and T-cells were significantly higher in sympathectomized-naïve males, but not females. Conclusions It may not only be possible, but necessary to use different strategies for the most effective treatment of migraine in men and women.

Sex-, Stress-, and Sympathetic Post-Ganglionic Neuron-Dependent Changes in the Expression of Pro- and Anti-Inflammatory Mediators in Rat Dural Immune Cells.

BACKGROUND: Migraine attacks are associated with sterile inflammation of the dura. Immune cells are a primary source of inflammatory mediators, and we therefore sought to further explore the link between dural immune cells and migraine. OBJECTIVE: Based on the observations that migraine is more common in women than in men, stress is the most common trigger for a migraine attack, and sympathetic post-ganglionic innervation of the dura enables local control of dural immune cells, we hypothesized that stress shifts the balance of inflammatory mediator expression in dural immune cells toward those that trigger a migraine attack, where these changes are larger in females and dependent, at least in part, on sympathetic post-ganglionic innervation of the dura. Our objective was to test this hypothesis. METHODS: Dura were obtained from naïve or stressed, intact or surgically sympathectomized, adult male and female rats. Dura were assessed immediately or 24 hours after termination of 4 continuous days of unpredictable, mild stressors. Following enzymatic digestion of each dura, myeloid and lymphoid-derived dural immune cells were isolated by fluorescence-activated cell sorting for semi-quantitative polymerase chain reaction analysis. RESULTS: In myeloid-derived dural immune cells, there was an increase in pro-inflammatory mediator mRNA following stress, particularly in females, which remained elevated with a 24-hour delay after stress. There was a stress-induced decrease in anti-inflammatory mediator mRNA immediately after stress in females, but not males. The stress-induced changes were attenuated in sympathectomized females. In lymphoid-derived dural immune cells, there was a persistent increase in pro-inflammatory mediator mRNA following stress, particularly in females. A stress-induced increase in anti-inflammatory mediator mRNA was also observed in both males and females, and was further attenuated in sympathectomized females. CONCLUSIONS: Consistent with our hypothesis, there is a stress-induced shift in the balance of pro- and anti-inflammatory mediator expression in dural immune cells that is more pronounced in females, and is dependent, at least in part, on sympathetic post-ganglionic innervation in females. This shift in the balance of inflammatory mediator expression may not only play an important role in triggering migraine attacks, but also suggests it may be possible, if not necessary, to employ different strategies to most effectively treat migraine in men and women.

Distribution of artemin and GFRalpha3 labeled nerve fibers in the dura mater of rat: artemin and GFRalpha3 in the dura.

OBJECTIVE: We examined the distribution of artemin and its receptor, glial cell line-derived neurotrophic factor family receptor alpha3 (GFRalpha3), in the dura mater of rats. BACKGROUND: Artemin, a member of the glial cell line-derived neurotrophic factor family, is a vasculature-derived growth factor shown to regulate migration of sympathetic neuroblasts and targeting of sympathetic innervation. The artemin receptor, GFRalpha3, is present in both sympathetic efferents and a subpopulation of nociceptive afferents. Recent evidence has shown that artemin may contribute to inflammatory hyperalgesia. The extent to which artemin is present in the dural vasculature and its relationship to GFRalpha3 containing fibers have yet to be investigated. METHODS: We used retrograde labeling, double and triple labeling with immunohistochemistry on the dura mater and trigeminal ganglia of female Sprague-Dawley rats. RESULTS: Artemin-like immunoreactivity (-LI) was detected in the smooth muscle of dural vasculature. GFRalpha3-LI was present in nerve fibers that closely associated with tyrosine hydroxylase or calcitonin gene-related peptide (CGRP). CGRP-LI and transient receptor potential ion channel 1 (TRPV1)-LI were present in all GFRalpha3-positive dural afferents, which constituted 22% of the total population of dural afferents. CONCLUSIONS: These anatomical results support the hypothesis that artemin contributes to dural afferent activity, and possibly migraine pain, through modulation of both primary afferent and sympathetic systems.