Satellite glial cells in sensory ganglia: from form to function

ATP in neuron-glia bidirectional signalling

ATP accomplishes important roles in brain, where it functions as neurotransmitter or co-transmitter, being stored and released either as single mediator or together with other neuromodulators. In the last years, the purinergic system has emerged as the most relevant mechanism for intercellular signalling in the nervous system, affecting communication between many types of neurons and all types of glia. In this review, we will focus on recently reported data which describe the role of ATP in bidirectional signalling between neurons and different populations of glial cells, in both peripheral and central system.

Satellite glial cells in sympathetic and parasympathetic ganglia: in search of function

Glial cells are established as essential for many functions of the central nervous system, and this seems to hold also for glial cells in the peripheral nervous system. The main type of glial cells in most types of peripheral ganglia - sensory, sympathetic, and parasympathetic - is satellite glial cells (SGCs). These cells usually form envelopes around single neurons, which create a distinct functional unit consisting of a neuron and its attending SGCs. This review presents the knowledge on the morphology of SGCs in sympathetic and parasympathetic ganglia, and the (limited) available information on their physiology and pharmacology. It appears that SGCs carry receptors for ATP and can thus respond to the release of this neurotransmitter by the neurons. There is evidence that SGCs have an uptake mechanism for GABA, and possibly other neurotransmitters, which enables them to control the neuronal microenvironment. Damage to post- or preganglionic nerve fibers influences both the ganglionic neurons and the SGCs. One major consequence of postganglionic nerve section is the detachment of preganglionic nerve terminals, resulting in decline of synaptic transmission. It appears that, at least in sympathetic ganglia, SGCs participate in the detachment process, and possibly in the subsequent recovery of the synaptic connections. Unlike sensory neurons, neurons in autonomic ganglia receive synaptic inputs, and SGCs are in very close contact with synaptic boutons. This places the SGCs in a position to influence synaptic transmission and information processing in autonomic ganglia, but this topic requires much further work.

Peripheral inflammation augments gap junction-mediated coupling among satellite glial cells in mouse sympathetic ganglia

Intercellular coupling by gap junctions is one of the main features of glial cells, but very little is known about this aspect of satellite glial cells (SGCs) in sympathetic ganglia. We used the dye coupling method to address this question in both a prevertebral ganglion (superior mesenteric) and a paravertebral ganglion (superior cervical) of mice. We found that in control ganglia, the incidence of dye coupling among SGCs that form the envelope around a given neuron was 10–20%, and coupling between SGCs around different envelopes was rare (1.5–3%). The dye injections also provided novel information on the structure of SGCs. Following peripheral inflammation, both types of coupling were increased, but most striking was the augmentation of coupling between SGCs forming envelopes around different neurons, which rose by 8–14.6-fold. This effect appeared to be non-systemic, and was blocked by the gap junction blocker carbenoxolone. These changes in SGCs may affect signal transmission and processing in sympathetic ganglia.

Calcitonin gene-related peptide differentially regulates gene and protein expression in trigeminal glia cells: findings from array analysis

Calcitonin gene-related peptide (CGRP) is a multifunctional neuropeptide implicated in inflammatory diseases involving trigeminal ganglion nerve activation. Within trigeminal ganglia, satellite glia and Schwann cells are found in close association with neuronal cell bodies and fibers, respectively, and are known to express functional CGRP receptors. The goal of this study was to use array analysis to provide a more comprehensive understanding of CGRP regulation of inflammatory proteins and genes in trigeminal glia. Primary trigeminal ganglia cultures enriched for glia were treated with 500 nM CGRP for 8 or 24h. CGRP caused a >3-fold increase in the level of 19 cytokines 8h after CGRP treatment and the levels of each of these cytokines remained significantly elevated over basal unstimulated levels at 24h. While mRNA levels of many genes involved in mitogen-activated protein (MAP) kinase signaling were increased 8h after CGRP treatment, the number of responsive genes was greatly increased at 24h. Specifically, CGRP was shown to temporally regulate expression of multiple MAP kinases as well as numerous MAP kinase-responsive genes including transcription factors, scaffold/anchoring proteins, and cell cycle proteins. Thus, our data provide evidence of an emerging role of CGRP as an important modulator of trigeminal ganglion glia by stimulating cytokine release as well as inducing expression of a diverse array of proteins involved in MAP kinase signaling.

Spatio-temporal changes of SDF1 and its CXCR4 receptor in the dorsal root ganglia following unilateral sciatic nerve injury as a model of neuropathic pain

There is a growing evidence that chemokines and their receptors play a role in inducing and maintaining neuropathic pain. In the present study, unilateral chronic constriction injury (CCI) of rat sciatic nerve under aseptic conditions was used to investigate changes for stromal derived factor-1 (SDF1) and its CXCR4 receptor in lumbal (L4-L5) and cervical (C7-C8) dorsal root ganglia (DRG) from both sides of naïve, CCI-operated and sham-operated rats. All CCI-operated rats displayed mechanical allodynia and thermal hyperalgesia in hind paws ipsilateral to CCI, but forepaws exhibited only temporal changes of sensitivity not correlated with alterations in SDF1 and CXCR4 proteins. Naïve DRG displayed immunofluorescence for SDF1 (SDF1-IF) in the satellite glial cells (SGC) and CXCR4-IF in the neuronal bodies with highest intensity in small- and medium-sized neurons. Immunofluorescence staining and Western blot analysis confirmed that unilateral CCI induced bilateral alterations of SDF1 and CXCR4 proteins in both L4-L5 and C7-C8 DRG. Only lumbal DRG were invaded by ED-1+ macrophages exhibiting SDF1-IF while elevation of CXCR4-IF was found in DRG neurons and SGC but not in ED-1+ macrophages. No attenuation of mechanical allodynia, but reversed thermal hyperalgesia, in ipsi- and contralateral hind paws was found in CCI-operated rats after i.p. administration of CXCR4 antagonist (AMD3100). These results indicate that SDF1/CXCR4 changes are not limited to DRG associated with injured nerve but that they also spread to DRG non-associated with such nerve. Functional involvement of these alterations in DRG non-associated with injured nerve in neuropathic pain remains to be elucidated.

Alterations in the vascular architecture of the dorsal root ganglia in a rat neuropathic pain model

An alteration in the structural arrangement of blood vessels identified by RECA immunohistochemistry was studied in a rat L4 dorsal root ganglia (L4-DRG) neuropathic pain model. We compared a three-dimensional (3-D) reconstruction of the vascular architecture surrounding bodies of the primary sensory neurons in the L4-DRG of naïve rats with that of rats that had surgically undergone unilateral sciatic nerve ligature. Rhodamine-conjugated dextran (Fluoro-Ruby) was used for retrograde labelling of neurons, the axons of which had been injured by nerve ligature. In contrast to DRG from naïve rats and contralateral DRG from operated rats, an increased proportion of RECA+ vascular area and the appearance of nest-like arrangements of blood vessels around neuronal bodies with injured axons were observed in L4-DRG ipsilateral to the sciatic nerve ligature. Fractal analysis confirmed a higher degree of vascular branching, irregularity, and tortuosity in L4-DRG related with sciatic nerve injury. The results suggest that nerve injury induces changes in vascular architecture in associated DRG.

Inwardly rectifying potassium channel Kir4.1 is responsible for the native inward potassium conductance of satellite glial cells in sensory ganglia

Satellite glial cells (SGCs) surround primary afferent neurons in sensory ganglia, and increasing evidence has implicated the K(+) channels of SGCs in affecting or regulating sensory ganglion excitability. The inwardly rectifying K(+) (Kir) channel Kir4.1 is highly expressed in several types of glial cells in the central nervous system (CNS) where it has been implicated in extracellular K(+) concentration buffering. Upon neuronal activity, the extracellular K(+) concentration increases, and if not corrected, causes neuronal depolarization and uncontrolled changes in neuronal excitability. Recently, it has been demonstrated that knockdown of Kir4.1 expression in trigeminal ganglia leads to neuronal hyperexcitability in this ganglia and heightened nociception. Thus, we investigated the contribution of Kir4.1 to the membrane K(+) conductance of SGCs in neonatal and adult mouse trigeminal and dorsal root ganglia. Whole cell patch clamp recordings were performed in conjunction with immunocytochemistry and quantitative transcript analysis in various mouse lines. We found that in wild-type mice, the inward K(+) conductance of SGCs is blocked almost completely with extracellular barium, cesium and desipramine, consistent with a conductance mediated by Kir channels. We then utilized mouse lines in which genetic ablation led to partial or complete loss of Kir4.1 expression to assess the role of this channel subunit in SGCs. The inward K(+) currents of SGCs in Kir4.1+/- mice were decreased by about half while these currents were almost completely absent in Kir4.1-/- mice. These findings in combination with previous reports support the notion that Kir4.1 is the principal Kir channel type in SGCs. Therefore Kir4.1 emerges as a key regulator of SGC function and possibly neuronal excitability in sensory ganglia.

Focal Inflammation Causes Carbenoxolone-Sensitive Tactile Hypersensitivity in Mice

A focal and transitory inflammation induced by injection of complete Freund's adjuvant (CFA) in the submandibular skin of mice elicits pain behavior that persists for several weeks after the initial inflammation has resolved. Chronic pain, assessed as tactile hypersensitivity to stimulation with von Frey filaments, was evident from 1-7 weeks following CFA injection, although inflammation at the injection site was resolved by 3-4 weeks. In contrast, there were no changes in tactile sensitivity in the paw (un-injected site for comparison), no alterations in open field behavior and no differences in a functional observation battery evident in CFA-treated mice compared to controls (saline-injected) or to baseline (before CFA injection). Neither strain (Balb/c vs. C57BL/6) nor sex differences in baseline tactile threshold were significant in the submandibular skin. CFA-induced tactile hypersensitivity was also not a function of strain or sex. A single intraperitoneal injection of the gap junction blocker carbenoxolone (CBX) restored normal tactile thresholds in CFA-treated mice when administered at the peak of inflammation (1 week), after significant resolution of inflammation (3 weeks) or after total resolution of inflammation (4 and 5 weeks) without altering the tactile threshold of control subjects, tactile threshold in the paw or open field behavior. Thus, in this novel model of post-inflammatory pain, transitory inflammation induced persistent sex- and strain-independent behavioral hypersensitivity that was reversed by the gap junction blocker CBX, suggesting neuronal and/or glial plasticity as a major component of the chronic pain.

The role of glial cells in influencing neurite extension by dorsal root ganglion cells

When pretreated with pertussis toxin (PTX), the neurites of adult rat dorsal root ganglion (DRG) cells in mixed cell cultures retract over a period of 2 h following the initial stimulus of removal from the cell culture incubator for brief periods of observation. The purpose of this investigation was to determine whether this PTX-dependent response was specific to any one of the three subpopulations of DRG neurons. However, no neurite retraction response was observed in neuron-enriched populations of cells, or in cultures enriched in isolectin B4 (IB4)-positive neurons or in IB4-negative neurons. But, the addition of non-neuronal cells, and/or medium conditioned by non-neuronal cells, was sufficient to restore the PTX-dependent neurite retraction response, but only in large diameter IB4-negative neurons. In conclusion, we have identified a regulatory response, mediated by Gi/o-proteins, which prevents retraction of neurites in large diameter IB4-negative cells of adult rat DRG. The non-neuronal cells of adult rat DRG constitutively release factor/s that can stimulate neurite retraction of a subset of isolated DRG neurons, but this property of non-neuronal cells is only observed when the Gi/o-proteins of large diameter IB4-negative cells are inhibited.

Differential regulation of immune responses and macrophage/neuron interactions in the dorsal root ganglion in young and adult rats following nerve injury

BACKGROUND

Neuropathic pain is an apparently spontaneous experience triggered by abnormal physiology of the peripheral or central nervous system, which evolves with time. Neuropathic pain arising from peripheral nerve injury is characterized by a combination of spontaneous pain, hyperalgesia and allodynia. There is no evidence of this type of pain in human infants or rat pups; brachial plexus avulsion, which causes intense neuropathic pain in adults, is not painful when the injury is sustained at birth. Since infants are capable of nociception from before birth and display both acute and chronic inflammatory pain behaviour from an early neonatal age, it appears that the mechanisms underlying neuropathic pain are differentially regulated over a prolonged postnatal period.

RESULTS

We have performed a microarray analysis of the rat L4/L5 dorsal root ganglia (DRG), 7 days post spared nerve injury, a model of neuropathic pain. Genes that are regulated in adult rats displaying neuropathic behaviour were compared to those regulated in young rats (10 days old) that did not show the same neuropathic behaviour. The results show a set of genes, differentially regulated in the adult DRG, that are principally involved in immune system modulation. A functional consequence of this different immune response to injury is that resident macrophages cluster around the large A sensory neuron bodies in the adult DRG seven days post injury, whereas the macrophages in young DRG remain scattered evenly throughout the ganglion, as in controls.

CONCLUSIONS

The results show, for the first time, a major difference in the neuroimmune response to nerve injury in the dorsal root ganglion of young and adult rats. Differential analysis reveals a new set of immune related genes in the ganglia, that are differentially regulated in adult neuropathic pain, and that are consistent with the selective activation of macrophages around adult, but not young large A sensory neurons post injury. These differences may contribute to the reduced incidence of neuropathic pain in infants.

Altered functional properties of satellite glial cells in compressed spinal ganglia

The cell bodies of sensory neurons in the dorsal root ganglion (DRG) are enveloped by satellite glial cells (SGCs). In an animal model of intervertebral foraminal stenosis and low-back pain, a chronic compression of the DRG (CCD) increases the excitability of neuronal cell bodies in the compressed ganglion. The morphological and electrophysiological properties of SGCs were investigated in both CCD and uninjured, control lumbar DRGs. SGCs responded within 12 h of the onset of CCD as indicated by an increased expression of glial fibrillary acidic protein (GFAP) in the compressed DRG but to lesser extent in neighboring or contralateral DRGs. Within 1 week, coupling through gap junctions between SGCs was significantly enhanced in the compressed ganglion. Under whole-cell patch clamp recordings, inward and outward potassium currents, but not sodium currents, were detected in individual SGCs. SGCs enveloping differently sized neurons had similar electrophysiological properties. SGCs in the compressed vs. control DRG exhibited significantly reduced inwardly rectifying potassium currents (Kir), increased input resistances and positively shifted resting membrane potentials. The reduction in Kir was greater for nociceptive medium-sized neurons compared to non-nociceptive neurons. Kir currents of SGCs around spontaneously active neurons were significantly reduced 1 day after compression but recovered by 7 days. These data demonstrate rapid alterations in glial membrane currents and GFAP expression in close temporal association with the development of neuronal hyperexcitability in the CCD model of neuropathic pain. However, these alterations are not fully sustained and suggest other mechanisms for the maintenance of the hyperexcitable state.

Glial precursors clear sensory neuron corpses during development via Jedi-1, an engulfment receptor

During the development of peripheral ganglia, 50% of the neurons that are generated undergo apoptosis. How the massive numbers of corpses are removed is unknown. We found that satellite glial cell precursors are the primary phagocytic cells for apoptotic corpse removal in developing mouse dorsal root ganglia (DRG). Confocal and electron microscopic analysis revealed that glial precursors, rather than macrophages, were responsible for clearing most of the dead DRG neurons. Moreover, we identified Jedi-1, an engulfment receptor, and MEGF10, a purported engulfment receptor, as homologs of the invertebrate engulfment receptors Draper and CED-1 expressed in the glial precursor cells. Expression of Jedi-1 or MEGF10 in fibroblasts facilitated binding to dead neurons, and knocking down either protein in glial cells or overexpressing truncated forms lacking the intracellular domain inhibited engulfment of apoptotic neurons. Together, these results suggest a cellular and molecular mechanism by which neuronal corpses are culled during DRG development.

Bidirectional calcium signaling between satellite glial cells and neurons in cultured mouse trigeminal ganglia

Astrocytes communicate with neurons, endothelial and other glial cells through transmission of intercellular calcium signals. Satellite glial cells (SGCs) in sensory ganglia share several properties with astrocytes, but whether this type of communication occurs between SGCs and sensory neurons has not been explored. In the present work we used cultured neurons and SGCs from mouse trigeminal ganglia to address this question. Focal electrical or mechanical stimulation of single neurons in trigeminal ganglion cultures increased intracellular calcium concentration in these cells and triggered calcium elevations in adjacent glial cells. Similar to neurons, SGCs responded to mechanical stimulation with increase in cytosolic calcium that spread to the adjacent neuron and neighboring glial cells. Calcium signaling from SGCs to neurons and among SGCs was diminished in the presence of the broad-spectrum P2 receptor antagonist suramin (50 muM) or in the presence of the gap junction blocker carbenoxolone (100 muM), whereas signaling from neurons to SGCs was reduced by suramin, but not by carbenoxolone. Following induction of submandibular inflammation by Complete Freund's Adjuvant injection, the amplitude of signaling among SGCs and from SGCs to neuron was increased, whereas the amplitude from neuron to SGCs was reduced. These results indicate for the first time the presence of bidirectional calcium signaling between neurons and SGCs in sensory ganglia cultures, which is mediated by the activation of purinergic P2 receptors, and to some extent by gap junctions. Furthermore, the results indicate that not only sensory neurons, but also SGCs release ATP. This form of intercellular calcium signaling likely plays key roles in the modulation of neuronal activity within sensory ganglia in normal and pathological states.

Regulation of boundary cap neural crest stem cell differentiation after transplantation

Success of cell replacement therapies for neurological disorders will depend largely on the optimization of strategies to enhance viability and control the developmental fate of stem cells after transplantation. Once transplanted, stem/progenitor cells display a tendency to maintain an undifferentiated phenotype or differentiate into inappropriate cell types. Gain and loss of function experiments have revealed key transcription factors which drive differentiation of immature stem/progenitor cells toward more mature stages and eventually to full differentiation. An attractive course of action to promote survival and direct the differentiation of transplanted stem cells to a specific cell type would therefore be to force expression of regulatory differentiation molecules in already transplanted stem cells, using inducible gene expression systems which can be controlled from the outside. Here, we explore this hypothesis by employing a tetracycline gene regulating system (Tet-On) to drive the differentiation of boundary cap neural crest stem cells (bNCSCs) toward a sensory neuron fate after transplantation. We induced the expression of the key transcription factor Runx1 in Sox10-expressing bNCSCs. Forced expression of Runx1 strongly increased transplant survival in the enriched neurotrophic environment of the dorsal root ganglion cavity, and was sufficient to guide differentiation of bNCSCs toward a nonpeptidergic nociceptive sensory neuron phenotype both in vitro and in vivo after transplantation. These findings suggest that exogenous activation of transcription factors expression after transplantation in stem/progenitor cell grafts can be a constructive approach to control their survival as well as their differentiation to the desired type of cell and that the Tet-system is a useful tool to achieve this.

Activation of satellite glial cells in rat trigeminal ganglion after upper molar extraction

The neurons in the trigeminal ganglion (TG) are surrounded by satellite glial cells (SGCs), which passively support the function of the neurons, but little is known about the interactions between SGCs and TG neurons after peripheral nerve injury. To examine the effect of nerve injury on SGCs, we investigated the activation of SGCs after neuronal damage due to the extraction of the upper molars in rats. Three, 7, and 10 days after extraction, animals were fixed and the TG was removed. Cryosections of the ganglia were immunostained with antibodies against glial fibrillary acidic protein (GFAP), a marker of activated SGCs, and ATF3, a marker of damaged neurons. After tooth extraction, the number of ATF3-immunoreactive (IR) neurons enclosed by GFAP-IR SGCs had increased in a time-dependent manner in the maxillary nerve region of the TG. Although ATF3-IR neurons were not detected in the mandibular nerve region, the number of GFAP-IR SGCs increased in both the maxillary and mandibular nerve regions. Our results suggest that peripheral nerve injury affects the activation of TG neurons and the SGCs around the injured neurons. Moreover, our data suggest the existence of a neuronal interaction between maxillary and mandibular neurons via SGC activation.

Modes and regulation of glial migration in vertebrates and invertebrates

Neurons and glial cells show mutual interdependence in many developmental and functional aspects of their biology. To establish their intricate relationships with neurons, glial cells must migrate over what are often long distances. In the CNS glial cells generally migrate as single cells, whereas PNS glial cells tend to migrate as cohorts of cells. How are their journeys initiated and directed, and what stops the migratory phase once glial cells are aligned with their neuronal counterparts? A deeper understanding of glial migration and the underlying neuron-glia interactions may contribute to the development of therapeutics for demyelinating diseases or glial tumours.

Bilateral changes in IL-6 protein, but not in its receptor gp130, in rat dorsal root ganglia following sciatic nerve ligature

Local intracellular signaling cascades following peripheral nerve injury lead to robust axon regeneration and neuropathic pain induction. Cytokines are classic injury-induced mediators. We used sciatic nerve ligature (ScNL) to investigate temporal changes in IL-6 and its receptor gp130 in both ipsilateral and contralateral lumbal (L4-L5) dorsal root ganglia (DRG). Rats were operated aseptically on unilateral ScNL and allowed to survive for 1, 3, 7, and 14 days. Immunohistochemistry and Western blot analysis were used to determine levels of IL-6 and gp130 in DRG. A distinct increase in immunostaining for IL-6 was found in the neuronal cell bodies of sections through both ipsilateral and contralateral DRG at 1 and 3 days after operation. After 7 and 14 days, the DRG sections displayed only a moderate elevation in immunostaining when compared with sections of naïve DRG. The levels of IL-6 protein increased in both ipsilateral and contralateral lumbal DRG following peripheral nerve injury. The elevation of IL-6 protein was significant in both ipsilateral and contralateral DRG 1, 3, 7, and 14 days after operation. On the other hand, the levels of gp130 receptor did not change significantly. The data provide evidence for changes in IL-6 levels not only in the DRG associated with the damaged nerve but also in those unassociated with nerve injury during the experimental neuropathic pain model.

Possible role of glial cells in the onset and progression of Lyme neuroborreliosis

BACKGROUND

Lyme neuroborreliosis (LNB) may present as meningitis, cranial neuropathy, acute radiculoneuropathy or, rarely, as encephalomyelitis. We hypothesized that glia, upon exposure to Borrelia burgdorferi, the Lyme disease agent, produce inflammatory mediators that promote the acute cellular infiltration of early LNB. This inflammatory context could potentiate glial and neuronal apoptosis.

METHODS

We inoculated live B. burgdorferi into the cisterna magna of rhesus macaques and examined the inflammatory changes induced in the central nervous system (CNS), and dorsal root nerves and ganglia (DRG).

RESULTS

ELISA of the cerebrospinal fluid (CSF) showed elevated IL-6, IL-8, CCL2, and CXCL13 as early as one week post-inoculation, accompanied by primarily lymphocytic and monocytic pleocytosis. In contrast, onset of the acquired immune response, evidenced by anti-B. burgdorferi C6 serum antibodies, was first detectable after 3 weeks post-inoculation. CSF cell pellets and CNS tissues were culture-positive for B. burgdorferi. Histopathology revealed signs of acute LNB: severe multifocal leptomeningitis, radiculitis, and DRG inflammatory lesions. Immunofluorescence staining and confocal microscopy detected B. burgdorferi antigen in the CNS and DRG. IL-6 was observed in astrocytes and neurons in the spinal cord, and in neurons in the DRG of infected animals. CCL2 and CXCL13 were found in microglia as well as in endothelial cells, macrophages and T cells. Importantly, the DRG of infected animals showed significant satellite cell and neuronal apoptosis.

CONCLUSION

Our results support the notion that innate responses of glia to B. burgdorferi initiate/mediate the inflammation seen in acute LNB, and show that neuronal apoptosis occurs in this context.

Differential expression of connexins in trigeminal ganglion neurons and satellite glial cells in response to chronic or acute joint inflammation

Trigeminal nerve activation in response to inflammatory stimuli has been shown to increase neuron-glia communication via gap junctions in trigeminal ganglion. The goal of this study was to identify changes in the expression of gap junction proteins, connexins (Cxs), in trigeminal ganglia in response to acute or chronic joint inflammation. Although mRNA for Cxs 26, 36, 40 and 43 was detected under basal conditions, protein expression of only Cxs 26, 36 and 40 increased following capsaicin or complete Freund's adjuvant (CFA) injection into the temporomandibular joint (TMJ). While Cx26 plaque formation between neurons and satellite glia was transiently increased following capsaicin injections, Cx26 plaque formation between neurons and satellite glia was sustained in response to CFA. Interestingly, levels of Cx36 and Cx40 were only elevated in neurons following capsaicin or CFA injections, but the temporal response was similar to that observed for Cx26. In contrast, Cx43 expression was not increased in neurons or satellite glial cells in response to CFA or capsaicin. Thus, trigeminal ganglion neurons and satellite glia can differentially regulate Cx expression in response to the type and duration of inflammatory stimuli, which likely facilitates increased neuron-glia communication during acute and chronic inflammation and pain in the TMJ.

Proinflammatory-activated trigeminal satellite cells promote neuronal sensitization: relevance for migraine pathology

Background

Migraine is a complex, chronic, painful, neurovascular disorder characterized by episodic activation of the trigeminal system. Increased levels of calcitonin gene-related peptide (CGRP) are found at different levels during migraine attacks. Interestingly, CGRP is also released within the trigeminal ganglia suggesting possible local effects on satellite cells, a specialized type of glia that ensheaths trigeminal neurons. CGRP was shown to enhance satellite-cell production of interleukin 1β (IL-1β), while trigeminal neurons express an activity-dependent production of nitric oxide (NO). Thus, in the present study we tested the hypothesis that IL-1β and NO induce trigeminal satellite cell activation, and that once activated these cells can influence neuronal responses.

Results

Primary cultures of rat trigeminal satellite cells isolated from neuronal cultures were characterized in vitro. Cyclooxygenase (COX) expression and activity were taken as a marker of glial pro-inflammatory activation. Most of the experiments were carried out to characterize satellite cell responses to the two different pro-inflammatory stimuli. Subsequently, medium harvested from activated satellite cells was used to test possible modulatory effects of glial factors on trigeminal neuronal activity. IL-1β and the NO donor diethylenetriamine/nitric oxide (DETA/NO) elevated PGE2 release by satellite cells. The stimulatory effect of IL-1β was mediated mainly by upregulation of the inducible form of COX enzyme (COX2), while NO increased the constitutive COX activity. Regardless of the activator used, it is relevant that short exposures of trigeminal satellite cells to both activators induced modifications within the cells which led to significant PGE2 production after removal of the pro-inflammatory stimuli. This effect allowed us to harvest medium from activated satellite cells (so-called 'conditioned medium') that did not contain any stimulus, and thus test the effects of glial factors on neuronal activation. Conditioned medium from satellite cells activated by either IL-1β or NO augmented the evoked release of CGRP by trigeminal neurons.

Conclusion

These findings indicate that satellite cells contribute to migraine-related neurochemical events and are induced to do so by autocrine/paracrine stimuli (such as IL-1β and NO). The responsiveness of IL-1β to CGRP creates the potential for a positive feedback loop and, thus, a plurality of targets for therapeutic intervention in migraine.

Adenovector GAD65 gene delivery into the rat trigeminal ganglion produces orofacial analgesia

Background

Our goal is to use gene therapy to alleviate pain by targeting glial cells. In an animal model of facial pain we tested the effect of transfecting the glutamic acid decarboxylase (GAD) gene into satellite glial cells (SGCs) of the trigeminal ganglion by using a serotype 5 adenovector with high tropisms for glial cells. We postulated that GABA produced from the expression of GAD would reduce pain behavior by acting on GABA receptors on neurons within the ganglion.

Results

Injection of adenoviral vectors (AdGAD65) directly into the trigeminal ganglion leads to sustained expression of the GAD65 isoform over the 4 weeks observation period. Immunohistochemical analysis showed that adenovirus-mediated GAD65 expression and GABA synthesis were mainly in SGCs. GABAA and GABAB receptors were both seen in sensory neurons, yet only GABAA receptors decorated the neuronal surface. GABA receptors were not found on SGCs. Six days after injection of AdGAD65 into the trigeminal ganglion, there was a statistically significant decrease of pain behavior in the orofacial formalin test, a model of inflammatory pain. Rats injected with control virus (AdGFP or AdLacZ) had no reduction in their pain behavior. AdGAD65-dependent analgesia was blocked by bicuculline, a selective GABAA receptor antagonist, but not by CGP46381, a selective GABAB receptor antagonist.

Conclusion

Transfection of glial cells in the trigeminal ganglion with the GAD gene blocks pain behavior by acting on GABAA receptors on neuronal perikarya.

Neuron-interacting satellite glial cells in human trigeminal ganglia have an APC phenotype

Satellite glial cells (SGC) in sensory ganglia tightly envelop the neuronal cell body to form discrete anatomical units. This type of glial cell is considered neuroectoderm-derived and provides physical support to neuron somata. There are scattered hints in the literature suggesting that SGC have an immune-related function within sensory ganglia. In this study, we addressed the hypothesis that SGC are tissue-resident APC. The immune phenotype and function of a large series (n = 40) of human trigeminal ganglia (TG) were assessed by detailed flow cytometry, in situ analyses, and functional in vitro assays. Human TG-resident SGC (TG-SGC) uniformly expressed the common leukocyte marker CD45, albeit at lower levels compared with infiltrating T cells, and the macrophage markers CD14, CD68, and CD11b. In addition, TG-SGC expressed the myeloid dendritic cell (DC) marker CD11c, the T cell costimulatory molecules CD40, CD54, CD80, and CD86 and MHC class II. However, the mature DC marker CD83 was absent on TG-SGC. Functionally, TG-SGC phagocytosed fluorescent bacteria, but were unable to induce an allogeneic MLR. Finally, TG-infiltrating T cells expressed the T cell inhibitory molecules CD94/NKG2A and PD-1, and the interacting TG-SGC expressed the cognate ligands HLA-E and PD-L1, respectively. In conclusion, the data demonstrate that human TG-SGC have a unique leukocyte phenotype, with features of both macrophages and immature myeloid DC, indicating that they have a role as TG-resident APC with potential T cell modulatory properties.

Post herpetic neuralgia, schwann cell activation and vitamin D

While the underlying pathophysiology of herpes zoster infection has been well characterised, many of the mechanisms relating to the subsequent development of post herpetic neuralgia (PHN) remain uncertain. The dorsal horn atrophy and reduction in skin innervation seen in PHN patients does not adequately explain many clinical features or the efficacy of a number of topical treatments. In the central nervous system the glia, their receptors and their secreted signalling factors are now known to have a major influence on neural function. In the peripheral nervous system, schwann cell activation in response to infection and trauma releases a number of neuroexcitatory substances. Activation of the nervi nervorum in the peripheral nervous system also leads to the release of calcitonin gene related peptide, substance P and nitric oxide. Schwann cell and/or nervi nervorum activation could be an additional mechanism of pain generation in PHN. Such a paradigm shift would mean that drugs useful in the treatment of glial cell activation such as naloxone, naltrexone, minocycline, pentoxifyllline, propentofylline, AV411 (ibudilast) and interleukin 10 could be useful in PHN. These drugs could be used systemically or even topically. High dose topical vitamin D would appear to offer particular promise because vitamin D has the ability to both reduce glial inflammation and reduce nitric oxide production.

Profiling of dynamically changed gene expression in dorsal root ganglia post peripheral nerve injury and a critical role of injury-induced glial fibrillary acidic protein in maintenance of pain behaviors [corrected]

To explore cellular changes in sensory neurons after nerve injury and to identify potential target genes contributing to different stages of neuropathic pain development, we used Affymetrix oligo arrays to profile gene expression patterns in L5/6 dorsal root ganglia (DRG) from the neuropathic pain model of left L5/6 spinal nerve ligation at different stages of neuropathic pain development. Our data indicated that nerve injury induced changes in expression of genes with similar biological functions in a temporal specific manner that correlates with particular stages of neuropathic pain development, indicating dynamic neuroplasticity in the DRG in response to peripheral nerve injury and during neuropathic pain development. Data from post-array validation indicated that there was a temporal correlation between injury-induced expression of the glial fibrillary acidic protein (GFAP), a marker for activated astrocytes, and neuropathic pain development. Spinal nerve ligation injury in GFAP knockout mice resulted in neuropathic pain states with similar onset, but a shortened duration compared with that in age, and gender-matched wild-type littermates. Intrathecal GFAP antisense oligonucleotide treatment in injured rats with neuropathic pain states reversed injury-induced behavioral hypersensitivity and GFAP upregulation in DRG and spinal cord. Together, these findings indicate that injury-induced GFAP upregulation not only serves as a marker for astrocyte activation, but it may also play a critical, but yet identified, role in the maintenance of neuropathic pain states.

CGRP stimulation of iNOS and NO release from trigeminal ganglion glial cells involves mitogen-activated protein kinase pathways

Clinical and basic science data support an integral role of calcitonin gene-related peptide (CGRP) in the pathophysiology of temporomandibular joint disorders. Recently, we have shown that CGRP can stimulate the synthesis and release of nitric oxide (NO) from trigeminal ganglion glial cells. The goal of this study was to determine the role of mitogen-activated protein kinase (MAPK) signaling pathways in CGRP regulation of iNOS expression and NO release from cultured trigeminal ganglion glial cells from Sprague-Dawley rats. CGRP treatment for 2 h significantly increased activity of the MAPK reporter genes, Elk, ATF-2, and CHOP. In addition, CGRP increased nuclear staining for the active forms of the MAPKs: extracellular signal-regulated kinase, c-Jun amino-terminal kinase, and p38. This stimulatory event was not observed in cultures pre-treated with the CGRP receptor antagonist peptide CGRP(8-37). Similarly, pre-treatment with selective MAPK inhibitors repressed increases in reporter gene activity as well as CGRP-induced increases in iNOS expression and NO release mediated by MAPKs. In addition, over-expression of MAPK kinase 1 (MEK1), MEK3, MEK6, and MEK kinase significantly increased iNOS expression and NO production in glial cells. Results from our study provide evidence that CGRP binding to its receptor can stimulate iNOS gene expression via activation of MAPK pathways in trigeminal ganglion glial cells.

Tonabersat inhibits trigeminal ganglion neuronal-satellite glial cell signaling

BACKGROUND

Sensitization and activation of trigeminal neurons are implicated in the underlying pathology of migraine, acute sinusitis, and allergic rhinitis. Cell bodies of trigeminal neurons that provide sensory innervation of the dura and nasal mucosa reside in the trigeminal ganglion in association with satellite glial cells where they communicate via gap junctions. Gap junctions, channels formed by connexins, modulate the excitability state of both neurons and glia under pathological conditions. Tonabersat, a compound being tested as an antimigraine drug, is thought to block gap junction activity.

OBJECTIVE

To investigate the cellular events within trigeminal ganglia that may account for the significant comorbidity of migraine and rhinosinusitis and determine the effect of tonabersat on neuron-satellite glia communication.

METHODS

Sprague Dawley rats injected with True Blue were used to localize neuronal cell bodies in the ganglion and study neuron-glia signaling via gap junctions in the trigeminal ganglion. Dye coupling studies were conducted under basal conditions and in response to tumor necrosis factor-alpha injection into the whisker pad and/or capsaicin injection into the eyebrow. Changes in connexin 26 and active p38 levels were determined by immunohistochemistry. In addition, the effect of tonabersat prior to chemical stimulation on gap junction activity and expression of connexins and active p38 was investigated.

RESULTS

Injection of tumor necrosis factor-alpha, a cytokine implicated in the pathology of acute sinusitis and allergic rhinitis, into the V2 region was shown to lower the amount of capsaicin required to stimulate neurons located in the V1 region of the ganglion. While injection of tumor necrosis factor-alpha into the whisker pad or capsaicin injection into the eyebrow alone did not cause increased dye movement, the combination of both stimuli greatly increased neuron-satellite glia communication via gap junctions in both V1 and V2 regions. The change in gap junction activity was accompanied by increased expression of connexin 26 and active p38 levels in both neurons and satellite glia in V1 and V2 regions. Pretreatment with tonabersat inhibited gap junction communication between neurons and satellite glia and blocked the increase in connexin 26 and active p38 levels in response to injection of both tumor necrosis factor-alpha (V2) and capsaicin (V1).

CONCLUSIONS

We propose that increased levels of tumor necrosis factor-alpha, as reported during acute sinusitis and allergic rhinitis, reduces the amount of capsaicin necessary to stimulate V1 neurons that leads to cellular changes in both V1 and V2 regions. The cellular events observed in this study may help to explain, in part, the significant comorbidity reported with migraine and rhinosinusitis. In addition, we have provided evidence to suggest that tonabersat can prevent increased neuron-satellite glia signaling and, thus, may be useful in the treatment of migraine, acute sinusitis, and allergic rhinitis.

Early blockade of injured primary sensory afferents reduces glial cell activation in two rat neuropathic pain models

Satellite glial cells in the dorsal root ganglion (DRG), like the better-studied glia cells in the spinal cord, react to peripheral nerve injury or inflammation by activation, proliferation, and release of messengers that contribute importantly to pathological pain. It is not known how information about nerve injury or peripheral inflammation is conveyed to the satellite glial cells. Abnormal spontaneous activity of sensory neurons, observed in the very early phase of many pain models, is one plausible mechanism by which injured sensory neurons could activate neighboring satellite glial cells. We tested effects of locally inhibiting sensory neuron activity with sodium channel blockers on satellite glial cell activation in a rat spinal nerve ligation (SNL) model. SNL caused extensive satellite glial cell activation (as defined by glial fibrillary acidic protein [GFAP] immunoreactivity) which peaked on day 1 and was still observed on day 10. Perfusion of the axotomized DRG with the Na channel blocker tetrodotoxin (TTX) significantly reduced this activation at all time points. Similar findings were made with a more distal injury (spared nerve injury model), using a different sodium channel blocker (bupivacaine depot) at the injury site. Local DRG perfusion with TTX also reduced levels of nerve growth factor (NGF) in the SNL model on day 3 (when activated glia are an important source of NGF), without affecting the initial drop of NGF on day 1 (which has been attributed to loss of transport from target tissues). Local perfusion in the SNL model also significantly reduced microglia activation (OX-42 immunoreactivity) on day 3 and astrocyte activation (GFAP immunoreactivity) on day 10 in the corresponding dorsal spinal cord. The results indicate that early spontaneous activity in injured sensory neurons may play important roles in glia activation and pathological pain.

The versatility of the vagus

The gut is one of several organs contributing to the peripheral signalling network that controls food intake. Afferent neurons of the vagus nerve provide an important pathway for gut signals that act by triggering ascending pathways from the brain stem to hypothalamus. Recent work indicates the existence of mechanisms operating at the level of vagal afferent neurons to modulate the effect of gastrointestinal satiety signals. Thus, the well known satiety hormone cholecystokinin (CCK) not only stimulates the discharge of these neurons but also controls their expression of both G-protein coupled receptors and peptide neurotransmitters known to influence food intake. When plasma CCK concentrations are low e.g. in fasting, the expression by vagal afferent neurons of cannabinoid (CB)-1 and melanin concentrating hormone (MCH)-1 receptors is increased. Release of CCK by feeding leads to a rapid down-regulation of expression of both receptors and to increased expression of Y2 receptors. In fasting, there is also increased expression in these neurons of the appetite-stimulating neuropeptide transmitter MCH, and depressed expression of the satiety-peptide cocaine and amphetamine regulated transcript (CARTp); endogenous CCK decreases MCH expression and stimulates CART expression. The gastric orexigenic hormone ghrelin blocks these actions of CCK at least in part by excluding phosphoCREB from the nucleus. The data suggest that CCK acts as a gatekeeper to determine the capacity of other neuroendocrine signals to act via vagal afferent neurons to influence food intake.

Cholecystokinin regulates expression of Y2 receptors in vagal afferent neurons serving the stomach

The intestinal hormones CCK and PYY3-36 inhibit gastric emptying and food intake via vagal afferent neurons. Here we report that CCK regulates the expression of Y2R, at which PYY3-36 acts. In nodose ganglia from rats fasted up to 48 h, there was a fivefold decrease of Y2R mRNA compared with rats fed ad libitum; Y2R mRNA in fasted rats was increased by administration of CCK, and by refeeding through a mechanism sensitive to the CCK1R antagonist lorglumide. Antibodies to Y2R revealed expression in both neurons and satellite cells; most of the former (89 +/- 4%) also expressed CCK1R. With fasting there was loss of Y2R immunoreactivity in CCK1R-expressing neurons many of which projected to the stomach, but not in satellite cells or neurons projecting to the ileum or proximal colon. Expression of a Y2R promoter-luciferase reporter (Y2R-luc) in cultured vagal afferent neurons was increased in response to CCK by 12.3 +/- 0.1-fold and by phorbol ester (16.2 +/- 0.4-fold); the response to both was abolished by the protein kinase C inhibitor Ro-32,0432. PYY3-36 stimulated CREB phosphorylation in rat nodose neurons after priming with CCK; in wild-type mice PYY3-36 increased Fos labeling in brainstem neurons but in mice null for CCK1R this response was abolished. Thus Y2R is expressed by functionally distinct subsets of nodose ganglion neurons projecting to the stomach and ileum/colon; in the former expression is dependent on stimulation by CCK, and there is evidence that PYY3-36 effects on vagal afferent neurons are CCK dependent.

Altered ATP release and metabolism in dorsal root ganglia of neuropathic rats

Background

Adenosine 5'-triphosphate (ATP) has a ubiquitous role in metabolism and a major role in pain responses after tissue injury. We investigated the changes in basal and KCl-evoked ATP release from rat dorsal root ganglia (DRG) after peripheral neuropathy induction by unilateral sciatic nerve entrapment (SNE).

Results

After SNE, rats develop long-lasting decreases in ipsilateral hindpaw withdrawal thresholds to mechanical and thermal stimulation. At 15–21 days after neuropathy induction, excised ipsilateral L4-L5 DRG display significantly elevated basal extracellular ATP levels compared to contralateral or control (naive) DRG. However, KCl-evoked ATP release is no longer observed in ipsilateral DRG. We hypothesized that the differential SNE effects on basal and evoked ATP release could result from the conversion of extracellular ATP to adenosine with subsequent activation of adenosine A1 receptors (A1Rs) on DRG neurons. Adding the selective A1R agonist, 2-chloro-N⁶-cyclopentyladenosine (100 nM) significantly decreased basal and evoked ATP release in DRG from naïve rats, indicating functional A1R activation. In DRG ipsilateral to SNE, adding a selective A1R antagonist, 8-cyclopentyl-1,3-dipropylxanthine (30 nM), further increased basal ATP levels and relieved the blockade of KCl-evoked ATP release suggesting that increased A1R activation attenuates evoked ATP release in neurons ipsilateral to SNE. To determine if altered ATP release was a consequence of altered DRG metabolism we compared O₂ consumption between control and neuropathic DRG. DRG ipsilateral to SNE consumed O₂ at a higher rate than control or contralateral DRG.

Conclusion

These data suggest that peripheral nerve entrapment increases DRG metabolism and ATP release, which in turn is modulated by increased A1R activation.

Nitric oxide-proton stimulation of trigeminal ganglion neurons increases mitogen-activated protein kinase and phosphatase expression in neurons and satellite glial cells

Elevated nitric oxide (NO) and proton levels in synovial fluid are implicated in joint pathology. However, signaling pathways stimulated by these molecules that mediate inflammation and pain in the temporomandibular joint (TMJ) have not been investigated. The goal of this study was to determine the effect of NO-proton stimulation of rat trigeminal neurons on the in vivo expression of mitogen-activated protein kinases (MAPKs) and phosphatases (MKPs) in trigeminal ganglion neurons and satellite glial cells. Low levels of the active MAPKs extracellular signal-regulated kinase (ERK), Jun amino-terminal kinase (JNK), and p38 were localized in the cytosol of neurons and satellite glial cells in unstimulated animals. However, increased levels of active ERK and p38, but not JNK, were detected in the cytosol and nucleus of V3 neurons and satellite glial cells 15 min and 2 h following bilateral TMJ injections of an NO donor diluted in pH 5.5 medium. While ERK levels returned to near basal levels 24 h after stimulation, p38 levels remained significantly elevated. In contrast to MKP-2 and MKP-3 levels that were barely detectable in neurons or satellite glial cells, MKP-1 staining was readily observed in satellite glial cells in ganglia from unstimulated animals. However, neuronal and satellite glial cell staining for MKP-1, MKP-2, and MKP-3 was significantly increased in response to NO-protons. Increased active ERK and p38 levels as well as elevated MKP levels were also detected in neurons and satellite glial cells located in V2 and V1 regions of the ganglion. Our data provide evidence that NO-proton stimulation of V3 neurons results in temporal and spatial changes in expression of active ERK and p38 and MKPs in all regions of the ganglion. We propose that in trigeminal ganglia these cellular events, which are involved in peripheral sensitization as well as control of inflammatory and nociceptive responses, may play a role in TMJ pathology.

Contribution of activated interleukin receptors in trigeminal ganglion neurons to hyperalgesia via satellite glial interleukin-1beta paracrine mechanism

The present study investigated whether under in vivo conditions, inflammation alters the excitability of nociceptive Adelta-trigeminal ganglion (TRG) neurons innervating the facial skin via a cytokine paracrine mechanism. We used extracellular electrophysiological recording with multibarrel-electrodes in this study, and complete Freund's adjuvant (CFA) was injected into the rat facial skin. The threshold for escape from mechanical stimulation applied to the whisker pad area in inflamed rats (2 days after CFA injection) was significantly lower than that in control rats. A total of 45 Adelta-nociceptive-TRG neurons responding to electrical stimulation of the whisker pad were recorded in pentobarbital-anesthetized rats. The number of Adelta-TRG neurons with spontaneous firings and their firing rate in inflamed rats were significantly larger than those in control rats. The firing rates of the Adelta-TRG neuronal spontaneous activity were current-dependently decreased by local iontophoretic application of an interleukin I receptor type I antagonist (IL-1ra) in inflamed rats, but not in controls, and current-dependently increased by iontophoretic application of interleukin 1beta (IL-1beta) in both control and inflamed rats. IL-1ra also inhibited Adelta-TRG neuron activity evoked by mechanical stimulation in the inflamed rats. The mechanical threshold of nociceptive-TRG neurons in inflamed rats was significantly lower than that in control rats, but was not significantly different between control and inflamed rats after application of an IL-1ra. These results suggested that inflammation modulates the excitability of nociceptive Adelta-TRG neurons innervating the facial skin via IL-1beta paracrine action within trigeminal ganglia. Such an IL-1beta release could be important in determining trigeminal inflammatory hyperalgesia.

Mutations in the nervous system--specific HSN2 exon of WNK1 cause hereditary sensory neuropathy type II

Hereditary sensory and autonomic neuropathy type II (HSANII) is an early-onset autosomal recessive disorder characterized by loss of perception to pain, touch, and heat due to a loss of peripheral sensory nerves. Mutations in hereditary sensory neuropathy type II (HSN2), a single-exon ORF originally identified in affected families in Quebec and Newfoundland, Canada, were found to cause HSANII. We report here that HSN2 is a nervous system-specific exon of the with-no-lysine(K)-1 (WNK1) gene. WNK1 mutations have previously been reported to cause pseudohypoaldosteronism type II but have not been studied in the nervous system. Given the high degree of conservation of WNK1 between mice and humans, we characterized the structure and expression patterns of this isoform in mice. Immunodetections indicated that this Wnk1/Hsn2 isoform was expressed in sensory components of the peripheral nervous system and CNS associated with relaying sensory and nociceptive signals, including satellite cells, Schwann cells, and sensory neurons. We also demonstrate that the novel protein product of Wnk1/Hsn2 was more abundant in sensory neurons than motor neurons. The characteristics of WNK1/HSN2 point to a possible role for this gene in the peripheral sensory perception deficits characterizing HSANII.

Evidence for a role of connexin 43 in trigeminal pain using RNA interference in vivo

The importance of glial cells in the generation and maintenance of neuropathic pain is becoming widely accepted. We examined the role of glial-specific gap junctions in nociception in the rat trigeminal ganglion in nerve-injured and -uninjured states. The connexin 43 (Cx43) gap-junction subunit was found to be confined to the satellite glial cells (SGCs) that tightly envelop primary sensory neurons in the trigeminal ganglion and we therefore used Cx43 RNA interference (RNAi) to alter gap-junction function in SGCs. Using behavioral evaluation, together with immunocytochemical and Western blot monitoring, we show that Cx43 increased in the trigeminal ganglion in rats with a chronic constriction injury (CCI) of the infraorbital nerve. Reducing Cx43 expression using RNAi in CCI rats reduced painlike behavior, whereas in non-CCI rats, reducing Cx43 expression increased painlike behavior. The degree of painlike behavior in CCI rats and intact, Cx43-silenced rats was similar. Our results support previous suggestions that increases in glial gap junctions after nerve injury increases nociceptive behavior but paradoxically the reduction of gap junctions in normal ganglia also increases nociceptive behavior, possibly a reflection of the multiple functions performed by glia.

Modulation of chloride homeostasis by inflammatory mediators in dorsal root ganglion neurons

Background

Chloride currents in peripheral nociceptive neurons have been implicated in the generation of afferent nociceptive signals, as Cl^- accumulation in sensory endings establishes the driving force for depolarizing, and even excitatory, Cl^- currents. The intracellular Cl^- concentration can, however, vary considerably between individual DRG neurons. This raises the question, whether the contribution of Cl^- currents to signal generation differs between individual afferent neurons, and whether the specific Cl^- levels in these neurons are subject to modulation. Based on the hypothesis that modulation of the peripheral Cl^- homeostasis is involved in the generation of inflammatory hyperalgesia, we examined the effects of inflammatory mediators on intracellular Cl^- concentrations and on the expression levels of Cl^- transporters in rat DRG neurons.

Results

We developed an in vitro assay for testing how inflammatory mediators influence Cl^- concentration and the expression of Cl^- transporters. Intact DRGs were treated with 100 ng/ml NGF, 1.8 μM ATP, 0.9 μM bradykinin, and 1.4 μM PGE₂ for 1–3 hours. Two-photon fluorescence lifetime imaging with the Cl^--sensitive dye MQAE revealed an increase of the intracellular Cl^- concentration within 2 hours of treatment. This effect coincided with enhanced phosphorylation of the Na⁺-K⁺-2Cl^- cotransporter NKCC1, suggesting that an increased activity of that transporter caused the early rise of intracellular Cl^- levels. Immunohistochemistry of NKCC1 and KCC2, the main neuronal Cl^- importer and exporter, respectively, exposed an inverse regulation by the inflammatory mediators. While the NKCC1 immunosignal increased, that of KCC2 declined after 3 hours of treatment. In contrast, the mRNA levels of the two transporters did not change markedly during this time. These data demonstrate a fundamental transition in Cl^- homeostasis toward a state of augmented Cl^- accumulation, which is induced by a 1–3 hour treatment with inflammatory mediators.

Conclusion

Our findings indicate that inflammatory mediators impact on Cl^- homeostasis in DRG neurons. Inflammatory mediators raise intracellular Cl^- levels and, hence, the driving force for depolarizing Cl^- efflux. These findings corroborate current concepts for the role of Cl^- regulation in the generation of inflammatory hyperalgesia and allodynia. As the intracellular Cl^- concentration rises in DRG neurons, afferent signals can be boosted by excitatory Cl^- currents in the presynaptic terminals. Moreover, excitatory Cl^- currents in peripheral sensory endings may also contribute to the generation or modulation of afferent signals, especially in inflamed tissue.

Activation of interleukin-1beta receptor suppresses the voltage-gated potassium currents in the small-diameter trigeminal ganglion neurons following peripheral inflammation

The glial cytokine, interleukin-1beta (IL-1beta), potentiates the excitability of nociceptive trigeminal ganglion (TRG) neurons via membrane depolarization following peripheral inflammation. Perforated patch-clamp technique was used this study to show that the mechanism underlying the excitability of small-diameter TRG neurons following inflammation is due to IL-1beta. Inflammation was induced by injection of complete Freund's adjuvant (CFA) into the whisker pad. The TRG neurons innervating the site of inflammation were identified by fluorogold (FG) labeling. The threshold for escape from mechanical stimulation applied to the orofacial area in inflamed rats was significantly lower than observed for control rats. IL-1beta at 1nM suppressed total voltage-gated K(+) currents in most TRG neurons (70%) under voltage-clamp conditions in control and inflamed rats. IL-1beta significantly decreased the total, transient (I(A)) and sustained (I(K)) currents in FG-labeled small-diameter TRG neurons in both groups. The IL-1beta-induced suppression of TRG neuron excitability was abolished by co-administration of ILra, an IL-1beta receptor blocker. The magnitude of inhibition of I(A) and I(K) currents by IL-1beta was significantly greater in inflamed rats than in controls. IL-1beta inhibited I(A) to a significantly greater extent than I(K). These results suggest that the inhibitory effect of I(A) and I(K) currents by IL-1beta in small-diameter TRG neurons potentiates neuronal excitability thereby contributing to trigeminal inflammatory hyperalgesia. These findings provide evidence for the development of voltage-gated K(+) channel openers and IL-1beta antagonists as therapeutic agents for the treatment of trigeminal inflammatory hyperalgesia.

Chicken protocadherin-1 functions to localize neural crest cells to the dorsal root ganglia during PNS formation

In vertebrate embryos, neural crest cells emerge from the dorsal neural tube and migrate along well defined pathways to form a wide diversity of tissues, including the majority of the peripheral nervous system (PNS). Members of the cadherin family of cell adhesion molecules play key roles during the initiation of migration, mediating the delamination of cells from the neural tube. However, a role for cadherins in the sorting and re-aggregation of the neural crest to form the PNS has not been established. We report the requirement for a protocadherin, chicken protocadherin-1 (Pcdh1), in neural crest cell sorting during the formation of the dorsal root ganglia (DRG). In embryos, cPcdh1 is highly expressed in the developing DRG, where it co-localizes with the undifferentiated and mitotically active cells along the perimeter. Pcdh1 can promote cell adhesion in vivo and disrupting Pcdh1 function in embryos results in fewer neural crest cells localizing to the DRG, with a concomitant increase in cells that migrate to the sympathetic ganglia. Furthermore, those cells that still localize to the DRG, when Pcdh1 is inhibited, are no longer found at the perimeter, but are instead dispersed throughout the DRG and are now more likely to differentiate along the sensory neuron pathway. These results demonstrate that Pcdh1-mediated cell adhesion plays an important role as neural crest cells coalesce to form the DRG, where it serves to sort cells to the mitotically active perimeter.

Contralateral neuropathic pain and neuropathology in dorsal root ganglion and spinal cord following hemilateral nerve injury in rats

STUDY DESIGN

The contralateral pain-related behavioral and immunohistochemical changes after hemilateral spinal nerve injury in rats were investigated.

OBJECTIVES

We evaluated the longitudinal changes in contralateral mechanical allodynia, expression of tumor necrosis factor (TNF)-alpha and glial fibrillary acidic protein (GFAP)-positive satellite cells in the contralateral dorsal root ganglion (DRG), and expression of astrocytes and microglia in the contralateral spinal dorsal horn after hemilateral spinal nerve injury in rats.

SUMMARY OF BACKGROUND DATA

In previous studies, hemilateral nerve injury has sometimes induced contralateral neuropathic pain. TNF-alpha expression and glial cell reactions in the DRG and spinal cord play an important role in the neuropathic pain state, and TNF-alpha is released from glial cells in the nervous system.

METHODS

Adult male Sprague-Dawley rats were used. The spinal L5 nerve distal to the DRG was crushed once for 3 seconds. At days 2, 7, 14, and 21 after surgery, mechanical allodynia was determined in bilateral hind paws by the von Frey test. Expression of TNF-alpha and GFAP in bilateral L5 DRGs and expression of GFAP and ionized calcium-binding adaptor molecule-1 (Iba-1) in bilateral L5 spinal dorsal horns were studied using immunohistochemistry and immunoblotting.

RESULTS

Mechanical withdrawal threshold of the ipsilateral hind paw was significantly decreased for 21 days. Conversely, mechanical withdrawal threshold of the contralateral hind paw was significantly decreased from 5 to 10 g for 7 days, and was <5 g at days 14 and 21. TNF-alpha expression and GFAP-positive satellite cells in the contralateral DRG significantly increased from day 7 to day 21. In the contralateral spinal dorsal horn, GFAP-positive astrocytes significantly increased for 21 days, but Iba-1 was not significant.

CONCLUSION

These results suggest that contralateral mechanical allodynia induced by hemilateral spinal nerve injury is associated with upregulation of satellite cells and TNF-alpha in the contralateral DRG. In addition, our results suggest that spinal astrocytes play an important role in these contralateral changes.

Novel mRNA isoforms of the sodium channels Na(v)1.2, Na(v)1.3 and Na(v)1.7 encode predicted two-domain, truncated proteins

The expression of voltage-gated sodium channels is regulated at multiple levels, and in this study we addressed the potential for alternative splicing of the Na(v)1.2, Na(v)1.3, Na(v)1.6 and Na(v)1.7 mRNAs. We isolated novel mRNA isoforms of Na(v)1.2 and Na(v)1.3 from adult mouse and rat dorsal root ganglia (DRG), Na(v)1.3 and Na(v)1.7 from adult mouse brain, and Na(v)1.7 from neonatal rat brain. These alternatively spliced isoforms introduce an additional exon (Na(v)1.2 exon 17A and topologically equivalent Na(v)1.7 exon 16A) or exon pair (Na(v)1.3 exons 17A and 17B) that contain an in-frame stop codon and result in predicted two-domain, truncated proteins. The mouse and rat orthologous exon sequences are highly conserved (94-100% identities), as are the paralogous Na(v)1.2 and Na(v)1.3 exons (93% identity in mouse) to which the Na(v)1.7 exon has only 60% identity. Previously, Na(v)1.3 mRNA has been shown to be upregulated in rat DRG following peripheral nerve injury, unlike the downregulation of all other sodium channel transcripts. Here we show that the expression of Na(v)1.3 mRNA containing exons 17A and 17B is unchanged in mouse following peripheral nerve injury (axotomy), whereas total Na(v)1.3 mRNA expression is upregulated by 33% (P=0.003), suggesting differential regulation of the alternatively spliced transcripts. The alternatively spliced rodent exon sequences are highly conserved in both the human and chicken genomes, with 77-89% and 72-76% identities to mouse, respectively. The widespread conservation of these sequences strongly suggests an additional level of regulation in the expression of these channels, that is also tissue-specific.