There is emerging evidence that the morphology and function of SGCs change under the influence of harmful factors, such as nerve injury or inflammation [46, 47, 48, 49]. The response of SGCs to peripheral nerve injury is rapid and takes place within 4 hours. This reaction peaks at 1 week and begins to alleviate after 3 weeks [48, 49, 50].

Activated SGCs synthesize and release pro-inflammatory cytokines (i.e., interleukin (IL)-1$\beta$, tumor necrosis factor (TNF)-$\alpha$) and contribute to pain reaction through the activation of several signaling pathways, i.e., mitogen-activated protein kinase (MAPK) pathways [51].

The family of MAPK includes extracellular signal-regulated kinase 1 and 2 (ERK1 and ERK2), p38, and c-Jun N-terminal kinases (JNK). MAPK signaling pathway maintains an important role in intracellular signaling in neurons during persistent pain. In several studies, it was evidenced that nerve fiber injury results in augmented phosphorylation (activation) of p38 in SGCs of DRG [52, 53], while the peripheral inflammation of temporomandibular joint results in increased activation of this pathway in SGCs of TG [54]. Noteworthy, axonal injury leads to the phosphorylation of ERK in SGCs in DRG [55], whereas the inflammation of the temporomandibular joint promotes activation of ERK in SGCs of TG [54]. MAPK pathways are inactivated by phosphatases (MAPK phosphatase 1, 2, 3, MKP). It was assessed that the expression of MKP1, MKP2 and MKP3 rapidly increases in SCGs of TG during inflammation [54].

Following nerve injury, SGCs proliferation increases, and these cells display increased expression of GFAP [56] and enhanced production of cytokines (IL-1$\beta$, IL-6, TNF-$\alpha$) [57, 58]. Furthermore, SGCs can release glial mediators (i.e., basic fibroblast growth factor 4, (bFGF4) or matrix metalloproteinase-2, (MMP-2)) which may activate other glial cells via paracrine/autocrine mechanisms [59].

Finally, besides previously mentioned changes, adenosine triphosphate (ATP) sensitivity increases and purinergic signaling is altered.

Nutraceuticals with antioxidant capacities have been evaluated in several studies. Indeed, some diseases exert damaging effects on SGCs due to free radicals and nutraceuticals seem to exert beneficial effects in these conditions.

One such antioxidant compound is N-acetylcysteine (NAC), a safe and commonly used drug in the clinics, also present in some fruits and vegetables [71]. The main antioxidant effect is due to the ability of NAC to act as a reduced glutathione (GSH) precursor; GSH is a well-known direct antioxidant and a substrate of several antioxidant enzymes. Moreover, in some conditions where a significant depletion of endogenous cysteine (Cys) and GSH occurs, NAC can act as a direct antioxidant for some oxidant species such as NO${}_2$. Thus, NAC would act as a Cys donor. The antioxidant activity of NAC could also be due to its effect in breaking thiolated proteins, thus releasing free thiols and reduced proteins. Besides being involved in these antioxidant mechanisms, the disulphide breaking activity of NAC also explains its well-known mucolytic activity which is due to its effect in reducing heavily cross-linked mucus glycoproteins [72]. All these properties justify its already extended clinical use to counteract acetaminophen-induced hepatotoxicity and to clear airways [73] and the search for new potential uses, like treatment of peripheral pain, which involves SGCs.

An investigation on postoperative hyperalgesia induced by remifentanil in Sprague-Dawley rats focused on the usefulness of NAC in modulating the action of MMP-9 metalloproteases, which were up-regulated and activated by remifentanil in the DRGs [62]. Matrix metalloproteases are a group of calcium-dependent enzymes responsible for the cleavage of extracellular matrix proteins and cytokines [74]. The excessive proteolytic activity of MMP-9 damages DRGs and may increase neuronal transmission and produce neuropathic pain by stimulating the activation of microglia [75]. MMP-9 is mainly expressed in neurons that co-express mu opioid receptors (MOR) [76] and causes maturation of pro-inflammatory cytokines through the cleavage of IL-1$\beta$. Acting as a Cys donor, NAC prevented the cysteine residue present in MMP-9 from being oxidized, which is vital for its activation. Thus, it was demonstrated that NAC inhibits the cleavage of IL-1$\beta$ in DRGs and suppresses glial activation and neuronal excitability induced by remifentanil, alleviating hyperalgesia [62].

In Sprague-Dawley rats, Hart et al. [63] investigated the effects of systemic NAC upon cell death within DRG, which characterizes many peripheral neuropathies, particularly those of traumatic etiology. NAC was applied after peripheral nerve injury, and its neuroprotective potential was analyzed. Specifically, the effect of treatment upon cell survival was quantified using terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL), a method for detecting apoptotic deoxyribonucleic acid (DNA) fragmentation, widely used to identify and quantify apoptotic cells [77]. TUNEL positive cells were more numerous in axotomized ganglia, and NAC application provoked a reduction in the number of TUNEL positive glial cells. Additionally, mitochondrial architecture, usually damaged in this kind of neuropathies, was preserved. In vivo treatment with NAC might therefore increase glial glutathione production, potentially protecting glia not only directly but indirectly, through neuroprotection secondary to increased neuronal glutathione levels.

Curcumin, a spice obtained from the rhizome of turmeric, is another compound with antioxidant action, well-known to act as a free radical scavenger [78, 79]. Pathological processes such as sciatic nerve crush lead to poor function of the affected limb and induce histological changes including neuronal loss, as already mentioned. Norafshan et al. [64] studied this process in Sprague-Dawley rats and found a reduction in the number of SGCs in the lumbar 5 (L5) DRG. The damaging insult produces a period of ischemia with the release of chemical mediators, an increase in vascular permeability and a deficiency in the blood-nerve barrier [80], with edema, inflammation and metabolic disability in the cells [81, 82]. All these events favor the production of toxic oxygen metabolites such as superoxide anions, hydrogen peroxide and hydroxyl radicals due to the infiltration of polymorphonuclear leukocytes into the lesioned areas. These free radicals and cytokines released by neutrophils are responsible for cell damage. SGCs damage (measured histologically in the DRG and the associated signs of painful neuropathy (measured in vivo) were less intense with curcumin treatment, probably due to the reduction of toxic oxygen-derived metabolites, although this mechanism was not directly evaluated.

Curcumin may also act through purinergic receptors. Specifically, this mechanism of action has been shown for curcumin in the field of diabetes mellitus and the neuropathic pain that it entails. Jia et al. [65] used the classical type 1 diabetes mellitus model, induced by streptozotocin (STZ) injection, in adult Sprague-Dawley rats, where they tested the effect of curcumin-loaded nanoparticles, since the low in vivo metabolic stability of this compound and its low bioavailability make it necessary to find more efficient forms of administration [83]. The G protein-coupled type 2 (P2) purinergic receptor P2Y12 is expressed in SGCs and plays an important role in transmitting pain signals. These P2Y12 receptors are increased in SGCs of animals with diabetes that suffer thermal hyperalgesia, associated with increased production of the pro-inflammatory cytokine IL-1$\beta$ and an increased expression of Cx43 (connexin 43) in DRG [65]. As also previously mentioned, gap junctions between SGCs play an important role in neuronal excitability and contribute to the induction and maintenance of pain. Thus, the upregulation of P2Y12 in SGCs and of IL-1$\beta$ and Cx43 in the DRG indicates activation of SGCs. Treatment with nanoparticulate curcumin, probably through the prevention of P2Y12 receptor up-regulation, inhibited the activation of SGCs with a consequent decrease in thermal and mechanical hyperalgesia [65].

Inhibition of other types of purinergic receptors 2 (P2X4 and P2X7) appears to be the mechanism of action by which other nutraceuticals such as quercetin, resveratrol, and osthole exert their effects on SGCs.

Quercetin is a flavonoid widely distributed in fruits and vegetables such as onions, blueberries, or apples with antioxidant, anti-inflammatory and antinociceptive properties [84]. It is also a supplement for the alternative treatment of allergies, asthma, arthritis, hypertension, or neurodegenerative disorders [85, 86, 87, 88]. Using a rat model that developed neuropathic pain as a consequence of spared nerve injury, it was found that the administration of quercetin before surgery attenuated mechanical allodynia and inhibited GFAP expression (measured by Western blot) and labeling (measured immunohistochemically) in L5 DRG [66]. After the peripheral nerve damage, SGCs changed their number and function, revealing an increase in gap junctions that enhanced intercellular coupling of the SGCs and decreased their membrane resistance, causing pain. In addition, SGCs upregulated the production of pro-inflammatory cytokines such as TNF-$\alpha$. The application of quercetin in this model caused the inhibition of SGCs, as indicated by the suppression of GFAP labeling, thus contributing to pain reduction [66]. Interestingly, in the same report, the authors demonstrated a similar, dose-dependent effect of quercetin on GFAP levels, elevated by IL-6 in a C6 rat glioma cultured cell line used as a cellular astrocyte model [66]. These results suggest that quercetin may have a broad glioprotective action.

Using STZ-induced diabetic Sprague-Dawley rats, the mechanism of action of quercetin was studied in depth [67]. Thus, combining real-time reverse-transcription polymerase chain reaction (RT-PCR) for P2X4 receptor, Western blot for P2X4 and p38 MAPK, immunofluorescence for P2X4 and GFAP and molecular docking techniques (computer simulation tools used to predict the binding mode of a ligand in the active site of a protein), quercetin appeared to act through the inhibition of P2X4 receptors located on SGCs. Upregulation of these nociceptive ion channels contributes to developing and maintaining pain-related symptoms of diabetes mellitus induced by STZ. These studies showed the ability of quercetin to bind the P2X4 receptor, effectively decreasing its upregulation in DRG SGCs, and consequently inhibiting the activation of p38 MAPK mediated by this receptor, which led to thermal and mechanical hyperalgesia alleviation in diabetic rats.

The P2X4 receptor also appears to be behind the beneficial effects of osthole, a component extracted from the seeds of the Cnidium monnieri (L) cusson plant with anti-inflammatory and antioxidant properties in the diabetic neuropathy that was induced in male Sprague-Dawley rats after STZ injection [68]. Again, different techniques were used, including RT-PCR for P2X4 detection, Western blot analysis of GFAP, P2X4, p38 MAPK, p-P38 MAPK and brain-derived neurotrophic factor (BDNF), and immunofluorescence labeling of P2X4 receptor and GFAP. Osthole decreased the upregulation of this receptor and the activation of SGCs in the DRGs that occurs during diabetes mellitus, followed by downregulation of BDNF, IL-1$\beta$ and p38 MAPK. In this way, thermal and mechanical hyperalgesia would be inhibited. To confirm the involvement of P2X4 receptors in the effect of quercetin, the authors performed whole-cell patch-clamp recordings of HEK293 cells transfected with the pEGFP-hP2X4 plasmid. ATP activated HEK293 cells expressing P2X4 receptors, and quercetin significantly inhibited ATP-activated current in these cells, strongly suggesting that quercetin may relieve pain behaviors in diabetic rats by inhibiting P2X4 signaling [68].

As already mentioned, resveratrol is a naturally occurring polyphenolic compound found in peanuts, blackberries, grapes, and red wine with antinociceptive and anti-inflammatory effects [89]. Two studies using Sprague-Dawley rats have analyzed its mechanism of action in two different animal models: a chronic pain model caused by administration of gp120, derived from the human immunodeficiency virus (HIV) [69]; and a neuropathic pain model caused by chronic constriction injury [70]. In both cases, resveratrol seemed to act through the inhibition of P2X7 purinergic receptors located on SGCs of DRG, which are related to the induction and maintenance of neuropathic and inflammatory pain. In both animal models, treatment with resveratrol reduced pain transmission, suppressing the expression of the P2X7 receptor and preventing an elevation of the pain threshold. The mechanism by which it relieves pain includes the inhibition of P2X7 and GFAP in SCGs and the consequent reduction in the release of cytokines such as TNF-$\alpha$ and IL-6 that would block the receptor’s downstream signals ERK1/2 and p38 MARK, leading to pain attenuation. ATP is a signaling molecule that can activate P2X7 receptors on SCGs, mediating pain, and other cells, like macrophages, mediating inflammation [90]. As in the previous studies with osthole, HEK293 cells were transfected with the P2X7 plasmid and the ATP-activated currents in these cells were recorded by whole-cell patch clamping. It was shown that resveratrol significantly inhibited those ATP-activated currents. These data further suggest that resveratrol relieves pain in these models by acting on the P2X7.