Dorsal root ganglion neurons and tyrosine hydroxylase--an intriguing association with implications for sensation and pain

Regenerative failure of sympathetic axons contributes to deficits in functional recovery after nerve injury

Renewed scientific interest in sympathetic modulation of muscle and neuromuscular junctions has spurred a flurry of new discoveries with major implications for motor diseases. However, the role sympathetic axons play in the persistent dysfunction that occurs after nerve injuries remains to be explored. Peripheral nerve injuries are common and lead to motor, sensory, and autonomic deficits that result in lifelong disabilities. Given the importance of sympathetic signaling in muscle metabolic health and maintaining bodily homeostasis, it is imperative to understand the regenerative capacity of sympathetic axons after injury. Therefore, we tested sympathetic axon regeneration and functional reinnervation of skin and muscle, both acute and long-term, using a battery of anatomical, pharmacological, chemogenetic, cell culture, analytical chemistry, and electrophysiological techniques. We employed several established growth-enhancing interventions, including electrical stimulation and conditioning lesion, as well as an innovative tool called bioluminescent optogenetics. Our results indicate that sympathetic regeneration is not enhanced by any of these treatments and may even be detrimental to sympathetic regeneration. Despite the complete return of motor reinnervation after sciatic nerve injury, gastrocnemius muscle atrophy and deficits in muscle cellular energy charge, as measured by relative ATP, ADP, and AMP concentrations, persisted long after injury, even with electrical stimulation. We suggest that these long-term deficits in muscle energy charge and atrophy are related to the deficiency in sympathetic axon regeneration. New studies are needed to better understand the mechanisms underlying sympathetic regeneration to develop therapeutics that can enhance the regeneration of all axon types.

Catecholaminergic dysfunction drives postural and locomotor deficits in a mouse model of spinal muscular atrophy

Development and maintenance of posture is essential behavior for overground mammalian locomotion. Dopamine and noradrenaline strongly influence locomotion, and their dysregulation initiates the development of motor impairments linked to neurodegenerative disease. However, the precise cellular and circuit mechanisms are not well defined. Here, we investigated the role of catecholaminergic neuromodulation in a mouse model of spinal muscular atrophy (SMA). SMA is characterized by severe motor dysfunction and postural deficits. We identify progressive loss of catecholaminergic synapses from spinal neurons that occur via non-cell autonomous mechanisms. Importantly, the selective restoration of survival motor neuron (SMN) in either catecholaminergic or serotonergic neurons is sufficient to correct impairments in locomotion. However, only combined SMN restoration in both catecholaminergic and serotonergic neurons or pharmacological treatment with l-dopa improve the severe postural deficits. These findings uncover the synaptic and cellular mechanisms responsible for the postural and motor symptoms in SMA and identify catecholaminergic neuromodulation as a potential therapeutic target.

Conditioning electrical stimulation fails to enhance sympathetic axon regeneration

Peripheral nerve injuries are common, and there is a critical need for the development of novel treatments to complement surgical repair. Conditioning electrical stimulation (CES) is a novel variation of the well-studied perioperative electrical stimulation treatment paradigm. CES is a clinically attractive alternative because of its ability to be performed at the bedside prior to a scheduled nerve repair surgery. Although 60 minutes of CES has been shown to enhance motor and sensory axon regeneration, the effects of CES on sympathetic regeneration are unknown. We investigated how two clinically relevant CES paradigms (10 minutes and 60 minutes) impact sympathetic axon regeneration and distal target reinnervation. Our results indicate that the growth of sympathetic axons is inhibited by CES at acute time points, and at a longer survival time point post-injury, there is no difference between sham CES and the CES groups. We conclude sympathetic axons may retain some regenerative ability, but no enhancement is exhibited after CES, which may be accounted for by the inability of the electrical stimulation paradigm to recruit the small-caliber sympathetic axons into activity. Furthermore, 10-minute CES did not enhance motor and sensory regeneration with a direct repair, and neither 60-minute nor 10-minute CES enhanced motor and sensory regeneration through a graft. Further studies will be needed to optimize electrical stimulation parameters to enhance the regeneration of all neuron types.

CD34 distribution in C-fiber low threshold mechanoreceptors in the mouse dorsal root ganglion and spinal cord

CD34 is a well-known cell marker of hematopoietic stem/ progenitor cells, endothelial cells, and fibrocytes. In the peripheral nervous system, a certain type of primary sensory neuron C-fiber low threshold mechanoreceptors (C-LTMRs) are reported to express CD34 mRNA. Here, we investigated the distribution of CD34 protein among putative C-LTMRs (pC-LTMR) using pC-LTMR markers such as VGLUT3 and TH in the dorsal root ganglion (DRG) and spinal cord. CD34 was frequently observed in DRG neurons double-positive for VGLUT3 and TH and single-positive for VGLUT3 in C8 and L4 levels, however, in C4 and L1 levels most of CD34-positive DRG neurons were demonstrated to be double-positive for VGLUT3 and TH. As for the termination, CD34-positive DRG neurons terminated in the ventral part of inner lamina II (lamina IIiv). At C4 and L1 levels of the dorsal horn, CD34 was observed in the entire region of lamina IIiv, however, in C8 and L4 levels of the dorsal horn CD34 was not detected in the medial part of lamina IIiv, which receives neural inputs from DRG neurons that innervate palm or sole skin. These results indicate that CD34 is expressed in pC-LTMRs and suggest that CD34 may play a role in providing C-LTMRs with a specific sensation by maintaining neural circuits.

Serum Spexin Level Is Negatively Associated With Peripheral Neuropathy and Sensory Pain in Type 2 Diabetes

Background: Spexin is a novel peptide hormone and has shown antinociceptive effects in experimental mice. This study is aimed at evaluating the association of serum spexin level with diabetic peripheral neuropathy (DPN) and related pain in a Chinese population. Methods: We enrolled 167 type 2 diabetes mellitus (T2DM) including 56 patients without DPN (non-DPN), 67 painless DPN, and 44 painful DPN. Serum spexin was measured using ELISA. Logistic regression models were performed to analyze the independent effects of spexin on prevalence of DPN and painful DPN. In streptozotocin (STZ)-induced diabetic mice, mechanical pain threshold was measured using electronic von Frey aesthesiometer. Human peripheral blood mononuclear cells (PBMCs) were isolated and further stimulated with lipopolysaccharide without or with spexin. The gene expression was assayed by qPCR. Results: Compared with non-DPN, serum spexin level decreased in painless DPN and further decreased in painful DPN. The odds of DPN was associated with low spexin level in T2DM, which was similar by age, sex, BMI, and diabetes duration, but attenuated in smokers. The odds of having pain was associated with decreased spexin level in DPN, which was similar by age, sex, smoking status, and diabetes duration, but attenuated in normal weight. Furthermore, we observed that mechanical pain threshold increased in spexin-treated diabetic mice. We also found that lipopolysaccharide treatment increased the mRNA level of TNF-α, IL-6, and MCP-1 in human PBMCs, while spexin treatment prevented this increase. Conclusions: These results suggested that spexin might serve as a protective factor for diabetes against neuropathology and pain-related pathogenesis.

Chronic Cannabigerol as an Effective Therapeutic for Cisplatin-Induced Neuropathic Pain

Cannabigerol (CBG), derived from the cannabis plant, acts as an acute analgesic in a model of cisplatin-induced peripheral neuropathy (CIPN) in mice. There are no curative, long-lasting treatments for CIPN available to humans. We investigated the ability of chronic CBG to alleviate mechanical hypersensitivity due to CIPN in mice by measuring responses to 7 and 14 days of daily CBG. We found that CBG treatment (i.p.) for 7 and 14 consecutive days significantly reduced mechanical hypersensitivity in male and female mice with CIPN and reduced pain sensitivity up to 60-70% of baseline levels (p < 0.001 for all), 24 h after the last injection. Additionally, we found that daily treatment with CBG did not evoke tolerance and did not incur significant weight change or adverse events. The efficacy of CBG was independent of the estrous cycle phase. Therefore, chronic CBG administration can provide at least 24 h of antinociceptive effect in mice. These findings support the study of CBG as a long-lasting neuropathic pain therapy, which acts without tolerance in both males and females.

Specific regulation of mechanical nociception by Gβ5 involves GABA-B receptors

Mechanical, thermal, and chemical pain sensation is conveyed by primary nociceptors, a subset of sensory afferent neurons. The intracellular regulation of the primary nociceptive signal is an area of active study. We report here the discovery of a Gβ5-dependent regulatory pathway within mechanical nociceptors that restrains antinociceptive input from metabotropic GABA-B receptors. In mice with conditional knockout (cKO) of the gene that encodes Gβ5 (Gnb5) targeted to peripheral sensory neurons, we demonstrate the impairment of mechanical, thermal, and chemical nociception. We further report the specific loss of mechanical nociception in Rgs7-Cre+/- Gnb5fl/fl mice but not in Rgs9-Cre+/- Gnb5fl/fl mice, suggesting that Gβ5 might specifically regulate mechanical pain in regulator of G protein signaling 7-positive (Rgs7+) cells. Additionally, Gβ5-dependent and Rgs7-associated mechanical nociception is dependent upon GABA-B receptor signaling since both were abolished by treatment with a GABA-B receptor antagonist and since cKO of Gβ5 from sensory cells or from Rgs7+ cells potentiated the analgesic effects of GABA-B agonists. Following activation by the G protein-coupled receptor Mrgprd agonist β-alanine, enhanced sensitivity to inhibition by baclofen was observed in primary cultures of Rgs7+ sensory neurons harvested from Rgs7-Cre+/- Gnb5fl/fl mice. Taken together, these results suggest that the targeted inhibition of Gβ5 function in Rgs7+ sensory neurons might provide specific relief for mechanical allodynia, including that contributing to chronic neuropathic pain, without reliance on exogenous opioids.

Locus coeruleus inhibition of vibrissal responses in the trigeminal subnucleus caudalis are reduced in a diabetic mouse model

Diabetic neuropathy is the loss of sensory function beginning distally in the lower extremities, which is also characterized by pain and substantial morbidity. Furthermore, the locus coeruleus (LC) nucleus has been proposed to play an important role in descending pain control through the activation of α2-noradrenergic (NA) receptors in the spinal dorsal horn. We studied, on control and diabetic mice, the effect of electrical stimulation of the LC nucleus on the tactile responses in the caudalis division of the spinal trigeminal nucleus (Sp5C), which is involved in the relay of orofacial nociceptive information. Diabetes was induced in young adult C57BL/6J mice with one intraperitoneal injection of streptozotocin (50 mg/kg) daily for 5 days. The diabetic animals showed pain in the orofacial area because they had a decrease in the withdrawal threshold to the mechanical stimulation in the vibrissal pad. LC electrical stimulation induced the inhibition of vibrissal responses in the Sp5C neurons when applied at 50 and 100 ms before vibrissal stimulation in the control mice; however, the inhibition was reduced in the diabetic mice. These effects may be due to a reduction in the tyrosine hydroxylase positive (TH+) fibers in the Sp5C, as was observed in diabetic mice. LC-evoked inhibition was decreased by an intraperitoneal injection of the antagonist of the α2-NA receptors, yohimbine, indicating that it was due to the activation of α2-NA receptors. The decrease in the LC-evoked inhibition in the diabetic mice was partially recovered when clonidine, a non-selective α2-agonist, was injected intraperitoneally. These findings suggest that in diabetes, there is a reduction in the NA inputs from the LC in the Sp5C that may favor the development of chronic pain.

Axonal mapping of the motor cranial nerves

Basic behaviors, such as swallowing, speech, and emotional expressions are the result of a highly coordinated interplay between multiple muscles of the head. Control mechanisms of such highly tuned movements remain poorly understood. Here, we investigated the neural components responsible for motor control of the facial, masticatory, and tongue muscles in humans using specific molecular markers (ChAT, MBP, NF, TH). Our findings showed that a higher number of motor axonal population is responsible for facial expressions and tongue movements, compared to muscles in the upper extremity. Sensory axons appear to be responsible for neural feedback from cutaneous mechanoreceptors to control the movement of facial muscles and the tongue. The newly discovered sympathetic axonal population in the facial nerve is hypothesized to be responsible for involuntary control of the muscle tone. These findings shed light on the pivotal role of high efferent input and rich somatosensory feedback in neuromuscular control of finely adjusted cranial systems.

Somatosensory innervation of adipose tissues

The increasing prevalence of obesity and type 2 diabetes has led to a greater interest in adipose tissue physiology. Adipose tissue is now understood as an organ with endocrine and thermogenic capacities in addition to its role in fat storage. It plays a critical role in systemic metabolism and energy regulation, and its activity is tightly regulated by the nervous system. Fat is now recognized to receive sympathetic innervation, which transmits information from the brain, as well as sensory innervation, which sends information into the brain. The role of sympathetic innervation in adipose tissue has been extensively studied. However, the extent and the functional significance of sensory innervation have long been unclear. Recent studies have started to reveal that sensory neurons robustly innervate adipose tissue and play an important role in regulating fat activity. This brief review will discuss both historical evidence and recent advances, as well as important remaining questions about the sensory innervation of adipose tissue.

Sensory ataxia and cardiac hypertrophy caused by neurovascular oxidative stress in chemogenetic transgenic mouse lines

Oxidative stress is associated with cardiovascular and neurodegenerative diseases. Here we report studies of neurovascular oxidative stress in chemogenetic transgenic mouse lines expressing yeast D-amino acid oxidase (DAAO) in neurons and vascular endothelium. When these transgenic mice are fed D-amino acids, DAAO generates hydrogen peroxide in target tissues. DAAO-TGCdh5 transgenic mice express DAAO under control of the putatively endothelial-specific Cdh5 promoter. When we provide these mice with D-alanine, they rapidly develop sensory ataxia caused by oxidative stress and mitochondrial dysfunction in neurons within dorsal root ganglia and nodose ganglia innervating the heart. DAAO-TGCdh5 mice also develop cardiac hypertrophy after chronic chemogenetic oxidative stress. This combination of ataxia, mitochondrial dysfunction, and cardiac hypertrophy is similar to findings in patients with Friedreich's ataxia. Our observations indicate that neurovascular oxidative stress is sufficient to cause sensory ataxia and cardiac hypertrophy. Studies of DAAO-TGCdh5 mice could provide mechanistic insights into Friedreich's ataxia.

The metabolic recovery of marathon runners: an untargeted 1H-NMR metabolomics perspective

Introduction: Extreme endurance events may result in numerous adverse metabolic, immunologic, and physiological perturbations that may diminish athletic performance and adversely affect the overall health status of an athlete, especially in the absence of sufficient recovery. A comprehensive understanding of the post-marathon recovering metabolome, may aid in the identification of new biomarkers associated with marathon-induced stress, recovery, and adaptation, which can facilitate the development of improved training and recovery programs and personalized monitoring of athletic health/recovery/performance. Nevertheless, an untargeted, multi-disciplinary elucidation of the complex underlying biochemical mechanisms involved in recovery after such an endurance event is yet to be demonstrated. Methods: This investigation employed an untargeted proton nuclear magnetic resonance metabolomics approach to characterize the post-marathon recovering metabolome by systematically comparing the pre-, immediately post, 24, and 48 h post-marathon serum metabolite profiles of 15 athletes. Results and Discussion: A total of 26 metabolites were identified to fluctuate significantly among post-marathon and recovery time points and were mainly attributed to the recovery of adenosine triphosphate, redox balance and glycogen stores, amino acid oxidation, changes to gut microbiota, and energy drink consumption during the post-marathon recovery phase. Additionally, metabolites associated with delayed-onset muscle soreness were observed; however, the mechanisms underlying this commonly reported phenomenon remain to be elucidated. Although complete metabolic recovery of the energy-producing pathways and fuel substrate stores was attained within the 48 h recovery period, several metabolites remained perturbed throughout the 48 h recovery period and/or fluctuated again following their initial recovery to pre-marathon-related levels.

Catecholaminergic axon innervation and morphology in flat-mounts of atria and ventricles of mice

Sympathetic efferent axons regulate cardiac functions. However, the topographical distribution and morphology of cardiac sympathetic efferent axons remain insufficiently characterized due to the technical challenges involved in immunohistochemical labeling of the thick walls of the whole heart. In this study, flat-mounts of the left and right atria and ventricles of FVB mice were immunolabeled for tyrosine hydroxylase (TH), a marker of sympathetic nerves. Atrial and ventricular flat-mounts were scanned using a confocal microscope to construct montages. We found (1) In the atria: A few large TH-immunoreactive (IR) axon bundles entered both atria, branched into small bundles and then single axons that eventually formed very dense terminal networks in the epicardium, myocardium and inlet regions of great vessels to the atria. Varicose TH-IR axons formed close contact with cardiomyocytes, vessels, and adipocytes. Multiple intrinsic cardiac ganglia (ICG) were identified in the epicardium of both atria, and a subpopulation of the neurons in the ICG were TH-IR. Most TH-IR axons in bundles traveled through ICG before forming dense varicose terminal networks in cardiomyocytes. We did not observe varicose TH-IR terminals encircling ICG neurons. (2) In the left and right ventricles and interventricular septum: TH-IR axons formed dense terminal networks in the epicardium, myocardium, and vasculature. Collectively, TH labeling is achievable in flat-mounts of thick cardiac walls, enabling detailed mapping of catecholaminergic axons and terminal structures in the whole heart at single-cell/axon/varicosity scale. This approach provides a foundation for future quantification of the topographical organization of the cardiac sympathetic innervation in different pathological conditions.

Schwann cells contribute to demyelinating diabetic neuropathy and nerve terminal structures in white adipose tissue

Peripheral neuropathy, which can include axonal degeneration and/or demyelination, impacts adipose tissues with obesity, diabetes, and aging. However, the presence of demyelinating neuropathy had not yet been explored in adipose. Both demyelinating neuropathies and axonopathies implicate Schwann cells (SCs), a glial support cell that myelinates axons and contributes to nerve regeneration after injury. We performed a comprehensive assessment of SCs and myelination patterns of subcutaneous white adipose tissue (scWAT) nerves, and changes across altered energy balance states. We found that mouse scWAT contains both myelinated and unmyelinated nerves and is populated by SCs, including SCs that were associated with synaptic vesicle-containing nerve terminals. BTBR ob/ob mice, a model of diabetic peripheral neuropathy, exhibited small fiber demyelinating neuropathy and alterations in SC marker gene expression in adipose that were similar to obese human adipose. These data indicate that adipose SCs regulate the plasticity of tissue nerves and become dysregulated in diabetes.

Cisplatin Neurotoxicity Targets Specific Subpopulations and K+ Channels in Tyrosine-Hydroxylase Positive Dorsal Root Ganglia Neurons

Among the features of cisplatin chemotherapy-induced peripheral neuropathy are chronic pain and innocuous mechanical hypersensitivity. The complete etiology of the latter remains unknown. Here, we show that cisplatin targets a heterogeneous population of tyrosine hydroxylase-positive (TH+) primary afferent dorsal root ganglion neurons (DRGNs) in mice, determined using single-cell transcriptome and electrophysiological analyses. TH+ DRGNs regulate innocuous mechanical sensation through C-low threshold mechanoreceptors. A differential assessment of wild-type and vitamin E deficient TH+ DRGNs revealed heterogeneity and specific functional phenotypes. The TH+ DRGNs comprise; fast-adapting eliciting one action potential (AP; 1-AP), moderately-adapting (≥2-APs), in responses to square-pulse current injection, and spontaneously active (SA). Cisplatin increased the input resistance and AP frequency but reduced the temporal coding feature of 1-AP and ≥2-APs neurons. By contrast, cisplatin has no measurable effect on the SA neurons. Vitamin E reduced the cisplatin-mediated increased excitability but did not improve the TH+ neuron temporal coding properties. Cisplatin mediates its effect by targeting outward K+ current, likely carried through K2P18.1 (Kcnk18), discovered through the differential transcriptome studies and heterologous expression. Studies show a potential new cellular target for chemotherapy-induced peripheral neuropathy and implicate the possible neuroprotective effects of vitamin E in cisplatin chemotherapy.

Referred Somatic Hyperalgesia Mediates Cardiac Regulation by the Activation of Sympathetic Nerves in a Rat Model of Myocardial Ischemia

Myocardial ischemia (MI) causes somatic referred pain and sympathetic hyperactivity, and the role of sensory inputs from referred areas in cardiac function and sympathetic hyperactivity remain unclear. Here, in a rat model, we showed that MI not only led to referred mechanical hypersensitivity on the forelimbs and upper back, but also elicited sympathetic sprouting in the skin of the referred area and C8-T6 dorsal root ganglia, and increased cardiac sympathetic tone, indicating sympathetic-sensory coupling. Moreover, intensifying referred hyperalgesic inputs with noxious mechanical, thermal, and electro-stimulation (ES) of the forearm augmented sympathetic hyperactivity and regulated cardiac function, whereas deafferentation of the left brachial plexus diminished sympathoexcitation. Intradermal injection of the α₂ adrenoceptor (α₂AR) antagonist yohimbine and agonist dexmedetomidine in the forearm attenuated the cardiac adjustment by ES. Overall, these findings suggest that sensory inputs from the referred pain area contribute to cardiac functional adjustment via peripheral α₂AR-mediated sympathetic-sensory coupling.

Sympathetic axonal sprouting induces changes in macrophage populations and protects against pancreatic cancer

Neuronal nerve processes in the tumor microenvironment were highlighted recently. However, the origin of intra-tumoral nerves remains poorly known, in part because of technical difficulties in tracing nerve fibers via conventional histological preparations. Here, we employ three-dimensional (3D) imaging of cleared tissues for a comprehensive analysis of sympathetic innervation in a murine model of pancreatic ductal adenocarcinoma (PDAC). Our results support two independent, but coexisting, mechanisms: passive engulfment of pre-existing sympathetic nerves within tumors plus an active, localized sprouting of axon terminals into non-neoplastic lesions and tumor periphery. Ablation of the innervating sympathetic nerves increases tumor growth and spread. This effect is explained by the observation that sympathectomy increases intratumoral CD163+ macrophage numbers, which contribute to the worse outcome. Altogether, our findings provide insights into the mechanisms by which the sympathetic nervous system exerts cancer-protective properties in a mouse model of PDAC.

Cutaneous innervation in impaired diabetic wound healing

Type 2 diabetes is associated with several potential comorbidities, among them impaired wound healing, chronic ulcerations, and the requirement for lower extremity amputation. Disease-associated abnormal cellular responses, infection, immunological and microvascular dysfunction, and peripheral neuropathy are implicated in the pathogenesis of the wound healing impairment and the diabetic foot ulcer. The skin houses a dense network of sensory nerve afferents and nerve-derived modulators, which communicate with epidermal keratinocytes and dermal fibroblasts bidirectionally to effect normal wound healing after trauma. However, the mechanisms through which cutaneous innervation modulates wound healing are poorly understood, especially in humans. Better understanding of these mechanisms may provide the basis for targeted treatments for chronic diabetic wounds. This review provides an overview of wound healing pathophysiology with a focus on neural involvement in normal and diabetic wound healing, as well as future therapeutic perspectives to address the unmet needs of diabetic patients with chronic wounds.

Expression Pattern of 5-HT (Serotonin) Receptors during Normal Development of the Human Spinal Cord and Ganglia and in Fetus with Cervical Spina Bifida

The expression of 5-HT (serotonin) receptors (sr) was analyzed in the spinal cord and ganglia of 15 human conceptuses (5–10-weeks), and in the 9-week fetus with spina bifida. We used immunohistochemical method to detect sr-positive, apoptotic (caspase-3) and proliferating (Ki-67) cells, double immunofluorescence for co-localization with protein gene peptide (pgp) 9.5 and GFAP, as well as semiquantification and statistical measurements. Following the neurulation process, moderate (sr1 and sr2) and mild (sr3) expression characterized neuroblasts in the spinal cord and ganglia. During further development, sr1 expression gradually increased in the motoneurons, autonomic and sensory neurons, while sr2 and sr3 increased strongly in floor and roof plates. In the ganglia, sr3 expression increased during limited developmental period, while sr1 and sr2 increased throughout the investigated period. Co-expression of sr/pgp 9.5 characterized developing neurons, while sr/GFAP co-localized in the roof plate. In the spinal cord and ganglia of malformed fetus, weaker sr1 and sr2 and stronger sr3 expression accompanied morphological abnormalities. Anomalous roof plate morphology showed an excess of apoptotic and proliferating cells and increased sr3 expression. Our results indicate a human-species specific sr expression pattern, and the importance of sr1 in neuronal differentiation, and sr2 and sr3 in the control of the roof plate morphogenesis in normal and disturbed development.

Comparative Analysis of Dorsal Root, Nodose and Sympathetic Ganglia for the Development of New Analgesics

Interoceptive and exteroceptive signals, and the corresponding coordinated control of internal organs and sensory functions, including pain, are received and orchestrated by multiple neurons within the peripheral, central and autonomic nervous systems. A central aim of the present report is to obtain a molecularly informed basis for analgesic drug development aimed at peripheral rather than central targets. We compare three key peripheral ganglia: nodose, sympathetic (superior cervical), and dorsal root ganglia in the rat, and focus on their molecular composition using next-gen RNA-Seq, as well as their neuroanatomy using immunocytochemistry and in situ hybridization. We obtained quantitative and anatomical assessments of transmitters, receptors, enzymes and signaling pathways mediating ganglion-specific functions. Distinct ganglionic patterns of expression were observed spanning ion channels, neurotransmitters, neuropeptides, G-protein coupled receptors (GPCRs), transporters, and biosynthetic enzymes. The relationship between ganglionic transcript levels and the corresponding protein was examined using immunohistochemistry for select, highly expressed, ganglion-specific genes. Transcriptomic analyses of spinal dorsal horn and intermediolateral cell column (IML), which form the termination of primary afferent neurons and the origin of preganglionic innervation to the SCG, respectively, disclosed pre- and post-ganglionic molecular-level circuits. These multimodal investigations provide insight into autonomic regulation, nodose transcripts related to pain and satiety, and DRG-spinal cord and IML-SCG communication. Multiple neurobiological and pharmacological contexts can be addressed, such as discriminating drug targets and predicting potential side effects, in analgesic drug development efforts directed at the peripheral nervous system.

Expression of huntingtin-associated protein 1 in adult mouse dorsal root ganglia and its neurochemical characterization in reference to sensory neuron subpopulations

•

This study is the first to examine HAP1-expression in dorsal root ganglia (DRG).

•

HAP1 is highly co-expressed with the markers of nociceptive/proprioceptive neurons.

•

HAP1 is completely lacking in the touch-sensitive DRG neurons.

•

HAP1 may play an important role in modulating nociceptive/proprioceptive functions.

•

It will be of great interest to clarify the pathophysiological role of HAP1 in DRG.

Huntingtin-associated protein 1 (HAP1) is a polyglutamine (polyQ) length-dependent interactor with causal agents in several neurodegenerative diseases and has been regarded as a protective factor against neurodegeneration. In normal rodent brain and spinal cord, HAP1 is abundantly expressed in the areas that are spared from neurodegeneration while those areas with little HAP1 are frequent targets of neurodegeneration. We have recently showed that HAP1 is highly expressed in the spinal dorsal horn and may participate in modification/protection of certain sensory functions. Neurons in the dorsal root ganglia (DRG) transmits sensory stimuli from periphery to spinal cord/brain stem. Nevertheless, to date HAP1 expression in DRG remains unreported. In this study, the expression of HAP1 in cervical, thoracic, lumbar and sacral DRG in adult male mice and its relationships with different chemical markers for sensory neurons were examined using Western blot and immunohistochemistry. HAP1-immunoreactivity was detected in the cytoplasm of DRG neurons, and the percentage of HAP1-immunoreactive (ir) DRG neurons was ranged between 28–31 %. HAP1-immunoreactivity was comparatively more in the small cells (47–58 %) and medium cells (40–44 %) than that in the large cells (9–11 %). Double-immunostaining for HAP1 and markers for nociceptive or mechanoreceptive neurons showed that about 70–80 % of CGRP-, SP-, CB-, NOS-, TRPV1-, CR- and PV-ir neurons expressed HAP1. In contrast, HAP1 was completely lacking in TH-ir neurons. Our current study is the first to clarify that HAP1 is highly expressed in nociceptive/proprioceptive neurons but absent in light-touch-sensitive TH neurons, suggesting the potential importance of HAP1 in pain transduction and proprioception.

The neuropathic phenotype of the K/BxN transgenic mouse with spontaneous arthritis: pain, nerve sprouting and joint remodeling

The adult K/BxN transgenic mouse develops spontaneous autoimmune arthritis with joint remodeling and profound bone loss. We report that both males and females display a severe sustained tactile allodynia which is reduced by gabapentin but not the potent cyclooxygenase inhibitor ketorolac. In dorsal horn, males and females show increased GFAP⁺ astrocytic cells; however, only males demonstrate an increase in Iba1⁺ microglia. In dorsal root ganglia (DRG), there is an increase in CGRP⁺, TH⁺, and Iba1⁺ (macrophage) labeling, but no increase in ATF3⁺ cells. At the ankle there is increased CGRP⁺, TH⁺, and GAP-43⁺ fiber synovial innervation. Thus, based on the changes in dorsal horn, DRG and peripheral innervation, we suggest that the adult K/BxN transgenic arthritic mice display a neuropathic phenotype, an assertion consistent with the analgesic pharmacology seen in this animal. These results indicate the relevance of this model to our understanding of the nociceptive processing which underlies the chronic pain state that evolves secondary to persistent joint inflammation.

Transgene expression within the spinal cord of hTH-eGFP rats

The enzyme tyrosine hydroxylase (TH) is broadly expressed in catecholaminergic neurons. In the spinal cord, four components contain TH, including A11 diencephalospinal dopaminergic (DA-ergic) pathways, intraspinal DA-related neurons, supraspinal noradrenergic projections, and afferents of TH-expressing sensory neurons. A human TH-enhanced green fluorescent protein (hTH-eGFP) transgenic rat was recently developed to tag TH⁺ profiles in the nervous system for visualization. Using immunostaining, we found that only A11 pathways express GFP whereas the other 3 components do not in the spinal cord. Thus, this may suggest a genetic difference among these TH⁺ elements even though they produce the same protein.

Infrapatellar Fat Pad/Synovium Complex in Early-Stage Knee Osteoarthritis: Potential New Target and Source of Therapeutic Mesenchymal Stem/Stromal Cells

The infrapatellar fat pad (IFP) has until recently been viewed as a densely vascular and innervated intracapsular/extrasynovial tissue with biomechanical roles in the anterior compartment of the knee. Over the last decade, secondary to the proposition that the IFP and synovium function as a single unit, its recognized tight molecular crosstalk with emerging roles in the pathophysiology of joint disease, and the characterization of immune-related resident cells with varying phenotypes (e.g., pro and anti-inflammatory macrophages), this structural complex has gained increasing attention as a potential therapeutic target in patients with various knee pathologies including osteoarthritis (KOA). Furthermore, the description of the presence of mesenchymal stem/stromal cells (MSC) as perivascular cells within the IFP (IFP-MSC), exhibiting immunomodulatory, anti-fibrotic and neutralizing activities over key local mediators, has promoted the IFP as an alternative source of MSC for cell-based therapy protocols. These complementary concepts have supported the growing notion of immune and inflammatory events participating in the pathogenesis of KOA, with the IFP/synovium complex engaging not only in amplifying local pathological responses, but also as a reservoir of potential therapeutic cell-based products. Consequently, the aim of this review is to outline the latest discoveries related with the IFP/synovium complex as both an active participant during KOA initiation and progression thus emerging as a potential target, and a source of therapeutic IFP-MSCs. Finally, we discuss how these notions may help the design of novel treatments for KOA through modulation of local cellular and molecular cascades that ultimately lead to joint destruction.

Characterization of a cell bridge variant connecting the nodose and superior cervical ganglia in the mouse: Prevalence, anatomical features, and practical implications

While autonomic ganglia have been extensively studied in rats instead of mice, there is renewed interest in the anatomy of the mouse autonomic nervous system. This study examined the prevalence and anatomical features of a cell bridge linking two autonomic ganglia of the neck, namely, the nodose ganglion (NG) and the superior cervical ganglion (SCG) in a cohort of C57BL/6J mice. We identified a cell bridge between the NG and the cranial pole of the SCG. This cell bridge was tubular shaped with an average length and width of 700 and 240 μm, respectively. The cell bridge was frequently unilateral and significantly more prevalent in the ganglionic masses from males (38%) than females (21%). On each of its extremities, it contained a mixed of vagal afferents and postganglionic sympathetic neurons. The two populations of neurons abruptly replaced each other in the middle of the cell bridge. We examined the mRNA expression for selected autonomic markers in samples of the NG with or without cell bridge. Our results indicated that the cell bridge was enriched in both markers of postganglionic sympathetic and vagal afferents neurons. Lastly, using FluoroGold microinjection into the NG, we found that the existence of a cell bridge may occasionally lead to the inadvertent contamination of the SCG. In summary, this study describes the anatomy of a cell bridge variant consisting of the fusion of the mouse NG and SCG. The practical implications of our observations are discussed with respect to studies of the mouse vagal afferents, an area of research of increasing popularity.

Decreased neural expression of the noradrenaline transporter in the papillary dermis after partial sciatic nerve lesion

After peripheral nerve injury, regeneration or collateral sprouting of noradrenergic nerve fibres in the papillary dermis of the injured limb may contribute to sympathetically-maintained pain. The aim of this study was to determine whether noradrenergic nerve fibre regeneration after partial sciatic nerve ligation (PSL) in Wistar rats was accompanied by parallel shifts in expression of the noradrenaline transporter (NAT). Four or 28 days after PSL surgery, immunohistochemistry was used to examine NAT expression in plantar hind paw skin in relation to pan-neuronal markers (class III beta-tubulin and protein gene product 9.5), peptidergic afferents containing calcitonin gene-related peptide (CGRP), nonpeptidergic afferents labelled by isolectin B₄ (IB₄), and tyrosine hydroxylase (TH), a marker for cutaneous noradrenergic nerve fibres. Most dermal nerve fibre populations decreased shortly after PSL. However, four weeks after PSL, an increase in staining intensity of CGRP and novel expression of TH were observed in the papillary dermis on the injured side. In contrast, neural expression of NAT was reduced in this region. Loss of NAT might have implications for sympathetically-maintained pain, as failure to rapidly clear noradrenaline could exacerbate aberrant sympathetic-sensory signalling between closely apposed noradrenergic and peptidergic nerve fibres.

Electroacupuncture relieved visceral and referred hindpaw hypersensitivity in colitis rats by inhibiting tyrosine hydroxylase expression in the sixth lumbar dorsal root ganglia

Irritable bowel syndrome patients frequently complain of pain in body regions somatotopically distinct from the gut, suggesting the involvement of an exaggerated signaling process in both visceral and somatic sensory pathways. Increasing evidence has shown that sprouting of tyrosine hydroxylase immunoreactive (TH-IR) fibers toward sensory neurons in dorsal root ganglia maintains and exacerbates the neuropathic and inflammatory pain, as well as colonic inflammation. The aim of the present study was to determine whether electroacupuncture could alleviate the visceral and secondary somatic hyperalgesia in colitis rats by suppressing the TH-IR expression in related dorsal root ganglia. After trinitrobenzene sulfonic acid irritation, rats developed inflammatory tissue damage in the distal colon, which was accompanied by visceral hypersensitivity and secondary hind paw hyperalgesia, as indicated by enhanced visceromotor response to colorectal distension and decreased mechanical and thermal withdrawal latency of the hind paw. Additionally, excessive TH-IR fibers sprouted toward calcitonin gene-related peptide immunoreactive sensory neurons, and TH-IR neurons also increased in the sixth lumbar dorsal root ganglia of colitis rats. Both electroacupuncture and guanethidine attenuated visceral and referred hind paw hyperalgesia by inhibiting the over-expression of TH-IR neurons and fibers in the sixth lumbar dorsal root ganglia. Moreover local inflammatory damage in the distal colon was restored after 7 days of electroacupuncture intervention. These results suggest that electroacupuncture relieved visceral and referred hind paw hypersensitivity in colitis rats by inhibiting TH expression in the sixth lumbar dorsal root ganglia.

An integrated cell isolation and purification method for rat dorsal root ganglion neurons

Objective

Neurobiology studies are increasingly focused on the dorsal root ganglion (DRG), which plays an important role in neuropathic pain. Existing DRG neuron primary culture methods have considerable limitations, including challenging cell isolation and poor cell yield, which cause difficulty in signaling pathway studies. The present study aimed to establish an integrated primary culture method for DRG neurons.

Methods

DRGs were obtained from fetal rats by microdissection, and then dissociated with trypsin. The dissociated neurons were treated with 5-fluorouracil to promote growth of neurons from the isolated cells. Then, reverse transcription polymerase chain reaction and immunofluorescence assays were used to identify and purify DRG neurons.

Results

Isolated DRGs were successfully dissociated and showed robust growth as individual DRG neurons in neurobasal medium. Both mRNA and protein assays confirmed that DRG neurons expressed neurofilament-200 and neuron-specific enolase.

Conclusions

Highly purified, stable DRG neurons could be easily harvested and grown for extended periods by using this integrated cell isolation and purification method, which may help to elucidate the mechanisms underlying neuropathic pain.

A disease mutation reveals a role for NaV1.9 in acute itch

Itch (pruritis) and pain represent two distinct sensory modalities; yet both have evolved to alert us to potentially harmful external stimuli. Compared with pain, our understanding of itch is still nascent. Here, we report a new clinical case of debilitating itch and altered pain perception resulting from the heterozygous de novo p.L811P gain-of-function mutation in NaV1.9, a voltage-gated sodium (NaV) channel subtype that relays sensory information from the periphery to the spine. To investigate the role of NaV1.9 in itch, we developed a mouse line in which the channel is N-terminally tagged with a fluorescent protein, thereby enabling the reliable identification and biophysical characterization of NaV1.9-expressing neurons. We also assessed NaV1.9 involvement in itch by using a newly created NaV1.9-/- and NaV1.9L799P/WT mouse model. We found that NaV1.9 is expressed in a subset of nonmyelinated, nonpeptidergic small-diameter dorsal root ganglia (DRGs). In WT DRGs, but not those of NaV1.9-/- mice, pruritogens altered action potential parameters and NaV channel gating properties. Additionally, NaV1.9-/- mice exhibited a strong reduction in acute scratching behavior in response to pruritogens, whereas NaV1.9L799P/WT mice displayed increased spontaneous scratching. Altogether, our data suggest an important contribution of NaV1.9 to itch signaling.

A Dual Noradrenergic Mechanism for the Relief of Neuropathic Allodynia by the Antidepressant Drugs Duloxetine and Amitriptyline

In addition to treating depression, antidepressant drugs are also a first-line treatment for neuropathic pain, which is pain secondary to lesion or pathology of the nervous system. Despite the widespread use of these drugs, the mechanism underlying their therapeutic action in this pain context remains partly elusive. The present study combined data collected in male and female mice from a model of neuropathic pain and data from the clinical setting to understand how antidepressant drugs act. We show two distinct mechanisms by which the selective inhibitor of serotonin and noradrenaline reuptake duloxetine and the tricyclic antidepressant amitriptyline relieve neuropathic allodynia. One of these mechanisms is acute, central, and requires descending noradrenergic inhibitory controls and α_2A adrenoceptors, as well as the mu and delta opioid receptors. The second mechanism is delayed, peripheral, and requires noradrenaline from peripheral sympathetic endings and β₂ adrenoceptors, as well as the delta opioid receptors. We then conducted a transcriptomic analysis in dorsal root ganglia, which suggested that the peripheral component of duloxetine action involves the inhibition of neuroimmune mechanisms accompanying nerve injury, including the downregulation of the TNF-α-NF-κB signaling pathway. Accordingly, immunotherapies against either TNF-α or Toll-like receptor 2 (TLR2) provided allodynia relief. We also compared duloxetine plasma levels in the animal model and in patients and we observed that patients' drug concentrations were compatible with those measured in animals under chronic treatment involving the peripheral mechanism. Our study highlights a peripheral neuroimmune component of antidepressant drugs that is relevant to their delayed therapeutic action against neuropathic pain.SIGNIFICANCE STATEMENT In addition to treating depression, antidepressant drugs are also a first-line treatment for neuropathic pain, which is pain secondary to lesion or pathology of the nervous system. However, the mechanism by which antidepressant drugs can relieve neuropathic pain remained in part elusive. Indeed, preclinical studies led to contradictions concerning the anatomical and molecular substrates of this action. In the present work, we overcame these apparent contradictions by highlighting the existence of two independent mechanisms. One is rapid and centrally mediated by descending controls from the brain to the spinal cord and the other is delayed, peripheral, and relies on the anti-neuroimmune action of chronic antidepressant treatment.

Connexin36 Expression in Primary Afferent Neurons in Relation to the Axon Reflex and Modality Coding of Somatic Sensation

Electrical coupling mediated by connexin36-containing gap junctions that form electrical synapses is known to be prevalent in the central nervous system, but such coupling was long ago reported also to occur between cutaneous sensory fibers. Here, we provide evidence supporting the capability of primary afferent fibers to engage in electrical coupling. In transgenic mice with enhanced green fluorescent protein (eGFP) serving as a reporter for connexin36 expression, immunofluorescence labeling of eGFP was found in subpopulations of neurons in lumbar dorsal root and trigeminal sensory ganglia, and in fibers within peripheral nerves and tissues. Immunolabeling of connexin36 was robust in the sciatic nerve, weaker in sensory ganglia than in peripheral nerve, and absent in these tissues from Cx36 null mice. Connexin36 mRNA was detected in ganglia from wild-type mice, but not in those from Cx36 null mice. Labeling of eGFP was localized within a subpopulation of ganglion cells containing substance P and calcitonin gene-releasing peptide, and in peripheral fibers containing these peptides. Expression of eGFP was also found in various proportions of sensory ganglion neurons containing transient receptor potential (TRP) channels, including TRPV1 and TRPM8. Ganglion cells labeled for isolectin B4 and tyrosine hydroxylase displayed very little co-localization with eGFP. Our results suggest that previously observed electrical coupling between peripheral sensory fibers occurs via electrical synapses formed by Cx36-containing gap junctions, and that some degree of selectivity in the extent of electrical coupling may occur between fibers belonging to subpopulations of sensory neurons identified according to their sensory modality responsiveness.

Opposing expression gradients of calcitonin-related polypeptide alpha (Calca/Cgrpα) and tyrosine hydroxylase (Th) in type II afferent neurons of the mouse cochlea

Type II spiral ganglion neurons (SGNs) are small caliber, unmyelinated afferents that extend dendritic arbors hundreds of microns along the cochlear spiral, contacting many outer hair cells (OHCs). Despite these many contacts, type II afferents are insensitive to sound and only weakly depolarized by glutamate release from OHCs. Recent studies suggest that type II afferents may be cochlear nociceptors, and can be excited by ATP released during tissue damage, by analogy to somatic pain-sensing C-fibers. The present work compares the expression patterns among cochlear type II afferents of two genes found in C-fibers: calcitonin-related polypeptide alpha (Calca/Cgrpα), specific to pain-sensing C-fibers, and tyrosine hydroxylase (Th), specific to low-threshold mechanoreceptive C-fibers, which was shown previously to be a selective biomarker of type II versus type I cochlear afferents (Vyas et al., ). Whole-mount cochlear preparations from 3-week- to 2-month-old CGRPα-EGFP (GENSAT) mice showed expression of Cgrpα in a subset of SGNs with type II-like peripheral dendrites extending beneath OHCs. Double labeling with other molecular markers confirmed that the labeled SGNs were neither type I SGNs nor olivocochlear efferents. Cgrpα starts to express in type II SGNs before hearing onset, but the expression level declines in the adult. The expression patterns of Cgrpα and Th formed opposing gradients, with Th being preferentially expressed in apical and Cgrpα in basal type II afferent neurons, indicating heterogeneity among type II afferent neurons. The expression of Th and Cgrpα was not mutually exclusive and co-expression could be observed, most abundantly in the middle cochlear turn.