Changes In Satellite Glial Cells Research Articles

Satellite Glial Cells: Morphology, functional heterogeneity, and role in pain.

Neurons in the somatic, sympathetic, and parasympathetic ganglia are surrounded by envelopes consisting of satellite glial cells (SGCs). Recently, it has become clear that SGCs are highly altered after nerve injury, which influences neuronal excitability and, consequently, the development and maintenance of pain in different animal models of chronic pain. However, the exact mechanism underlying chronic pain is not fully understood yet because it is assumed that SGCs in different ganglia share many common peculiarities, making the process complex. Here, we review recent data on morphological and functional heterogeneity and changes in SGCs in various pain conditions and their role in response to injury. More research is required to decipher the role of SGCs in diseases, such as chronic pain, neuropathology, and neurodegenerative diseases.

Exercise reduces pain behavior and pathological changes in dorsal root ganglia induced by systemic inflammation in mice

Emerging research indicates that physical activity can ameliorate chronic pain, but the underlying mechanisms are still largely obscure. In particular, little is known on the mechanisms behind exercise-induced analgesia in the setting of inflammatory pain. In our previous studies on systemic inflammation in mice using lipopolysaccharide (LPS) administration, we characterized satellite glial cells (SGCs) and neurons in dorsal root ganglia (DRG). We found that a week post-LPS injection, the sensitivity to mechanical stimulation was lowered, SGCs were activated and coupling among SGCs increased 3 to 4.5-fold. In the present work, we examined the effects of exercise (free wheel running) on tactile sensitivity and on pathological changes in mouse DRG in the LPS model. We found that exercise prevented tactile hypersensitivity, and also reversed the cellular changes in the DRG induced by LPS that were listed above. We propose that the analgesic effect of exercise is at least partly mediated by reversing the pathological changes in SGCs.

Profiling the molecular signature of satellite glial cells at the single cell level reveals high similarities between rodents and humans.

Peripheral sensory neurons located in dorsal root ganglia relay sensory information from the peripheral tissue to the brain. Satellite glial cells (SGCs) are unique glial cells that form an envelope completely surrounding each sensory neuron soma. This organization allows for close bidirectional communication between the neuron and its surrounding glial coat. Morphological and molecular changes in SGC have been observed in multiple pathological conditions such as inflammation, chemotherapy-induced neuropathy, viral infection, and nerve injuries. There is evidence that changes in SGC contribute to chronic pain by augmenting the neuronal activity in various rodent pain models. Satellite glial cells also play a critical role in axon regeneration. Whether findings made in rodent model systems are relevant to human physiology have not been investigated. Here, we present a detailed characterization of the transcriptional profile of SGC in mice, rats, and humans at the single cell level. Our findings suggest that key features of SGC in rodent models are conserved in humans. Our study provides the potential to leverage rodent SGC properties and identify potential targets in humans for the treatment of nerve injuries and alleviation of painful conditions.

Emerging importance of satellite glia in nervous system function and dysfunction.

Satellite glial cells (SGCs) closely envelop cell bodies of neurons in sensory, sympathetic and parasympathetic ganglia. This unique organization is not found elsewhere in the nervous system. SGCs in sensory ganglia are activated by numerous types of nerve injury and inflammation. The activation includes upregulation of glial fibrillary acidic protein, stronger gap junction-mediated SGC–SGC and neuron–SGC coupling, increased sensitivity to ATP, downregulation of Kir4.1 potassium channels and increased cytokine synthesis and release. There is evidence that these changes in SGCs contribute to chronic pain by augmenting neuronal activity and that these changes are consistent in various rodent pain models and likely also in human pain. Therefore, understanding these changes and the resulting abnormal interactions of SGCs with sensory neurons could provide a mechanistic approach that might be exploited therapeutically in alleviation and prevention of pain. We describe how SGCs are altered in rodent models of four common types of pain: systemic inflammation (sickness behaviour), post-surgical pain, diabetic neuropathic pain and post-herpetic pain.

NOCICEPTIVE STIMULUS IN THE NEONATAL PERIOD AFFECTS SATELLITE GLIAL CELLS MORPHOLOGY IN ADULT LIFE, WITH DIFFERENCES BETWEEN MALE AND FEMALE RATS

Satellite glial cells (SGCs) are found in the dorsal root ganglia (DRG) surrounding completely and individually sensory neurons, forming a thin sheath cells with regulatory function of the neuronal microenvironment. During the nociceptive stimulus the SGCs are activated and express glial fibrillary acidic protein (GFAP). Sustained nociceptive stimulus may cause morphological changes in SGCs influencing the mechanisms of pain chronification that may differ between genders. Understanding the plasticity in peripheral pain pathways considering gender differences is crucial, particularly when noxious stimuli are applied to neonates, when the nervous system is still under rapid and intense remodeling. We investigated the density (number/area) of neurons in the DRG that were enveloped by GFAP immunoreactive SGCs. Male (N=6 per group) and female (N=6 per group) Wistar rats, 15 and 180 days old after neonatal nociceptive stimulation [1] were used. Right and left L5 DRG were removed, frozen and 8 μm thick criossections were incubated in rabbit polyclonal anti‐GFAP antibody (1:2.000; 24hr). Immunostaining protocol was used as described [2]. The DRG area was measured with the aid of computer software. Neurons enveloped by a ring of GFAP‐labeled SGCs were counted, their density (number/area) was calculated and data were compared between right and left sides, 15 and 180 days of age and genders. Differences were considered significant if p<0.05. There was an increased density of neurons enveloped by a ring of GFAP‐labeled SGCs on both genders, 15 days after the nociceptive stimulus, compared to controls, with females presenting a larger density of these neurons compared to males at this age, on the stimulated side. At age of 180 days, the difference between pain and control groups persisted only in females. The comparison between genders showed that males present a smaller density of neurons enveloped by a ring of GFAP‐labeled SGCs compared to females on both ages with statistical significance only at 15 days old groups. There is an extensive literature clearly pointing to the fact that men and women are different in their responses to pain. Women present increased sensitivity to pain and differ in pain intervention responsivity. Despite that sociocultural and psychological factors certainly play a role in these gender differences, evidence from animals studies reveal important, qualitative and quantitative differences at low levels of the neuroaxis. Our results add extra evidence that multiple biological factors and mechanisms underlie gender differences in sensitivity and responses to pain.Support or Funding InformationFinancial support: FAPESP, CNPq, CAPES and FAEPAThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Role of satellite glial cells in gastrointestinal pain.

Gastrointestinal (GI) pain is a common clinical problem, for which effective therapy is quite limited. Sensations from the GI tract, including pain, are mediated largely by neurons in the dorsal root ganglia (DRG), and to a smaller extent by vagal afferents emerging from neurons in the nodose/jugular ganglia. Neurons in rodent DRG become hyperexcitable in models of GI pain (e.g., gastric or colonic inflammation), and can serve as a source for chronic pain. Glial cells are another element in the pain signaling pathways, and there is evidence that spinal glial cells (microglia and astrocytes) undergo activation (gliosis) in various pain models and contribute to pain. Recently it was found that satellite glial cells (SGCs), the main type of glial cells in sensory ganglia, might also contribute to chronic pain in rodent models. Most of that work focused on somatic pain, but in several studies GI pain was also investigated, and these are discussed in the present review. We have shown that colonic inflammation induced by dinitrobenzene sulfonic acid (DNBS) in mice leads to the activation of SGCs in DRG and increases gap junction-mediated coupling among these cells. This coupling appears to contribute to the hyperexcitability of DRG neurons that innervate the colon. Blocking gap junctions (GJ) in vitro reduced neuronal hyperexcitability induced by inflammation, suggesting that glial GJ participate in SGC-neuron interactions. Moreover, blocking GJ by carbenoxolone and other agents reduces pain behavior. Similar changes in SGCs were also found in the mouse nodose ganglia (NG), which provide sensory innervation to most of the GI tract. Following systemic inflammation, SGCs in these ganglia were activated, and displayed augmented coupling and greater sensitivity to the pain mediator ATP. The contribution of these changes to visceral pain remains to be determined. These results indicate that although visceral pain is unique, it shares basic mechanisms with somatic pain, suggesting that therapeutic approaches to both pain types may be similar. Future research in this field should include additional types of GI injury and also other types of visceral pain.

Chronic Stress-Induced Changes in Pro-Inflammatory Cytokines and Spinal Glia Markers in the Rat: A Time Course Study

Background/Aims: Spinal glia activation has been proposed as one mechanism underlying visceral hyperalgesia in a rodent model of chronic stress. In order to assess the possible role of changes in circulating cytokines and in blood-spinal cord barrier (BSCB) permeability in spinal glia activation, we studied the time course of peripheral and spinal pro-inflammatory cytokines and of spinal and satellite glia markers in response to repeated water avoidance (WA) stress. Methods: Spinal cords and dorsal root ganglion cells (DRGs) were collected from control rats, rats exposed to 1-hour WA, or 1-hour WA daily for 5 days or 1-hour WA daily for 10 days. Results: We demonstrated a time-dependent change in circulating IL-1β and spinal IL-1β, IL-6 and TNF-α in stressed animals compared with controls. We found altered expression of the astrocyte markers GFAP and Connexin 43 in spinal and DRG samples at different time points. Finally, WA was associated with increased BSCB permeability. Conclusions: These findings confirm the concept that both peripheral and spinal immune markers are altered after chronic WA and suggest a possible link between stress-induced increase of peripheral pro-inflammatory cytokines, changes in satellite glial cells, increase in BSCB permeability and increase in spinal pro-inflammatory mediators suggesting glia activation.

Peripheral inflammation upregulates P2X receptor expression in satellite glial cells of mouse trigeminal ganglia: A calcium imaging study

Satellite glial cells (SGCs) in sensory ganglia are altered structurally and biochemically as a result of nerve injury. Whereas there is ample evidence that P2 purinergic receptors in central glial cells are altered after injury, there is very little information on similar changes in SGCs, although it is well established that SGCs are endowed with P2 receptors. Using calcium imaging, we characterized changes in P2 receptors in SGCs from mouse trigeminal ganglia in short-term cultures. Seven days after the induction of submandibular inflammation with complete Freund’s adjuvant, there was a marked increase in the sensitivity of SGCs to ATP, with the threshold of activation decreasing from 5 μM to 10 nM. A similar observation was made in the intact trigeminal ganglion after infra-orbital nerve axotomy. Using pharmacological tools, we investigated the receptor mechanisms underlying these changes in cultured SGCs. We found that in control tissues response to ATP was mediated by P2Y (metabotropic) receptors, whereas after inflammation the response was mediated predominantly by P2X (ionotropic) receptors. As the contribution of P2X1,3,6 receptors was excluded, and the sensitivity to a P2X7 agonist did not change after inflammation, it appears that after inflammation the responses to ATP are largely due to P2X2 and/or 5 receptors, with a possible contribution of P2X4 receptors. We conclude that inflammation induced a large increase in the sensitivity of SGCs to ATP, which involved a switch from P2Y to P2X receptors. We propose that the over 100-fold augmented sensitivity of SGCs to ATP after injury may contribute to chronic pain states.

Phenotypic changes in satellite glial cells in cultured trigeminal ganglia

Satellite glial cells (SGCs) are specialized cells that form a tight sheath around neurons in sensory ganglia. In recent years, there is increasing interest in SGCs and they have been studied in both intact ganglia and in tissue culture. Here we studied phenotypic changes in SGCs in cultured trigeminal ganglia from adult mice, containing both neurons and SGCs, using phase optics, immunohistochemistry and time-lapse photography. Cultures were followed for up to 14 days. After isolation virtually every sensory neuron is ensheathed by SGCs, as in the intact ganglia. After one day in culture, SGCs begin to migrate away from their parent neurons, but in most cases the neurons still retain an intact glial cover. At later times in culture, there is a massive migration of SGCs away from the neurons and they undergo clear morphological changes, and at 7 days they become spindle-shaped. At one day in culture SGCs express the glial marker glutamine synthetase, and also the purinergic receptor P2X7. From day 2 in culture the glutamine synthetase expression is greatly diminished, whereas that of P2X7 is largely unchanged. We conclude that SGCs retain most of their characteristics for about 24h after culturing, but undergo major phenotypic changes at later times.

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.

Augmentation in gap junction-mediated cell coupling in dorsal root ganglia following sciatic nerve neuritis in the mouse

Recent findings highlight the participation of central glial cells in chronic pain, but less is known of a comparable role for satellite glial cells (SGCs), in dorsal root ganglia (DRG). Our previous work showed that sciatic nerve axotomy augmented SGC coupling by gap junctions. The aim of the present research was to find out whether similar changes occur in a mouse inflammation model. Sciatic nerve neuritis was induced by complete Freund's adjuvant (CFA), and isolated ganglia were examined 1 week later. Cell coupling was monitored by intracellular injection of the fluorescent dye Lucifer Yellow. Changes in gap junctions were assessed quantitatively by electron microscopy. Withdrawal threshold in the foot on the side of the inflamed nerve decreased from an average of 3.9 g in control to 0.94 g using Von Frey hairs (P<0.05). In CFA-treated animals dye coupling incidence between SGCs belonging to different glial envelopes increased from 6.9% in controls to 22.5% (P<0.05). Whereas in controls there was no coupling between neurons or between neurons and SGCs, after CFA application the incidence of neuron-neuron and neuron-SGC coupling was 8%. Electron microscopy showed formation of bridges between SGC sheaths surrounding different neurons, which were completely absent in controls. The mean number of gap junctions/100 μm2 of surface of the section occupied by SGCs increased from 0.215 in controls to 0.709 (P<0.01) in CFA-treated mice. The size of individual gap junctions remained the same. This is the first evidence for ultrastructural changes in SGCs following inflammation. The results support the idea that SGCs are sensitive to a variety of peripheral nerve injuries. We propose that the observed changes may alter signal transmission in DRG and thus may contribute to chronic pain.

Gap junctions in dorsal root ganglia: Possible contribution to visceral pain

Peripheral injuries can lead to sensitization of neurons in dorsal root ganglia (DRGs), which can contribute to chronic pain. The neurons are sensitized by a combination of physiological and biochemical changes, whose full details are still obscure. Another cellular element in DRGs are satellite glial cells (SGCs), which surround the neurons, but little is known about their role in nociception. We investigated the contribution of SGCs to neuronal sensitization in isolated S1 DRGs from a mouse model of colonic inflammation induced by local application of dinitrosulfonate benzoate (DNBS). Retrograde labeling was used to identify DRG neurons projecting to the colon. Cell-to-cell coupling was determined by intracellular dye injection, and the electrical properties of the neurons were studied with intracellular electrodes. Pain behavior was assessed with von-Frey hairs. The dye injections showed that 10–12 days after DNBS application there was a 6.6-fold increase in gap junction–mediated coupling between SGCs surrounding adjacent neurons, and this occurred preferentially (another 2-fold increase) near neurons that project to the colon. Neuron–neuron coupling incidence increased from 0.7% to 12.1% by colonic inflammation. Inflammation led to an augmented neuronal excitability, and to a reduced pain threshold. Gap junction blockers abolished the inflammation-induced changes in SGCs and neurons, and significantly reversed the pain behavior. We propose that inflammation induces augmented cell coupling in DRGs that contributes to neuronal hyperexcitability, which in turn leads to visceral pain. Gap junction blockers may have potential as analgesic drugs.