Interactions between Autophagy, Proinflammatory Cytokines, and Apoptosis in Neuropathic Pain: Granulocyte Colony Stimulating Factor as a Multipotent Therapy in Rats with Chronic Constriction Injury

Neuroprotective effects of granulocyte colony-stimulating factor against tramadol-induced cerebellar neurotoxicity

BACKGROUND

Tramadol (TRM) is a centrally acting synthetic opioid and serotonin/norepinephrine reuptake inhibitor. Despite being a potent painkiller, long-term use can induce permanent neurotoxicity. Granulocyte colony-stimulating factor (G-CSF) is a cytokine that helps to mobilize stem cells and facilitate their integration over injured neurons.

AIM

This work aims to study the histopathological, biochemical, and molecular alterations in the cerebellar cortex induced by TRM in comparison to the postulated protective effect of G-CSF versus TRM withdrawal.

METHODS

32 adult male albino rats were equally divided into four groups: control, TRM, TRM+G-CSF-treated, and TRM withdrawal groups. The TRM group received a daily dose of 80 mg/kg body weight orally via gastric tube for 12 weeks. The TRM+G-CSF-treated group received subcutaneous injections of 100 μg/kg body weight of G-CSF for seven consecutive days, then TRM from the 8th day. The TRM withdrawal group received TRM for 12 weeks; then, the rats were left without TRM administration for a further 12 weeks. The structural, biochemical, and molecular changes of the cerebellum were measured.

RESULTS

The study revealed that TRM not only induced cerebellar atrophy but also triggered microgliosis, neuroinflammation, and apoptotic indicators, all while suppressing autophagy. However, G-CSF and TRM withdrawal reversed these alterations with superiority to G-CSF.

CONCLUSION

The current investigation shows that G-CSF may improve behavioral, neurochemical, immunohistochemical, and molecular metrics in the rat cerebellum after tramadol-induced injury. G-CSF exhibits a superior protective effect compared to tramadol withdrawal. This is achieved through its antioxidant, anti-apoptotic, and autophagic enhancement properties, as well as its ability to reduce cerebellar gliosis.

High-voltage pulsed radiofrequency improves ultrastructure of DRG and enhances spinal microglial autophagy to ameliorate neuropathic pain induced by SNI

Neuropathic pain (NeP) is intractable for which many therapies are ineffective. High-voltage pulsed radiofrequency (HVPRF) on dorsal root ganglion (DRG) is considered an effective treatment for NeP. The aim of this study is to explore the therapeutic voltage for the optimal efficacy of PRF and the underlying mechanisms. The radiofrequency electrode was placed close to the L5 DRG of rats with spared nerve injury (SNI) and emitted current by the corresponding voltage in different groups. Four different voltages (45 V, 65 V, 85 V, and 100 V) of PRF on DRG significantly alleviated the SNI-induced NeP, reduced the levels of activating transcription factor 3 (ATF3) in DRG, improved the ultrastructure of DRG, and promoted autophagy in spinal microglia to varying degrees and partially reversed the increased expression of TNF-α and the reduced expression of IL-10 in spinal cord dorsal horn (SCDH). The beneficial effect of 85V-PRF was superior to those of other three PRF treatments. The underlying mechanisms may be related to repairing the DRG damage and improving the DRG ultrastructure while regulating spinal microglial autophagy and thereby alleviating neuroinflammation.

Pulsed radiofrequency on DRG inhibits hippocampal neuroinflammation by regulating spinal GRK2/p38 expression and enhances spinal autophagy to reduce pain and depression in male rats with spared nerve injury

Evidence indicates that microglial G protein-coupled receptor kinase 2 (GRK2) is a key regulator of the transition from acute to chronic pain mediated by microglial products via the p38 mitogen-activated protein kinase (MAPK) pathway in the spinal cord dorsal horn (SCDH). Increasing studies have shown that autophagic dysfunction in the SCDH and neuroinflammation in the hippocampus underlie NeP. However, whether GRK2/p38MAPK and autophagic flux in the SCDH and hippocampal neuroinflammation are involved in NeP and depression comorbidity has not been determined. Here, we explored the effects of high-voltage pulsed radiofrequency (PRF) (85 V-PRF; HV-PRF) to the dorsal root ganglion (DRG) on pain phenotypes in Wistar male rats with spared nerve injury (SNI) and the underlying mechanisms. The exacerbation of pain phenotypes was markedly relieved by PRF-DRG. The SNI-induced reduction in GRK2 expression, elevation of p-p38 MAPK levels in the SCDH, and increase in IL-1β and TNF-α levels in the hippocampus were reversed by PRF, which was accompanied by an increase in autophagic flux in spinal microglia. The beneficial effect of 85 V-PRF was superior to that of 45 V-PRF. In addition, the improvements elicited by 85 V-PRF were reversed by intrathecal injection of GRK2 antisense oligonucleotide, and these changes were accompanied by GRK2 downregulation and p-p38 upregulation in the SCDH, increased pro-inflammatory factor levels in the hippocampus, and excessive autophagy in spinal microglia. In conclusion, our data indicate that the application of HV-PRF to the DRG could serve as an excellent therapeutic technique for regulating neuroimmunity and neuroinflammation to relieve pain phenotypes.

Interplay between exosomes and autophagy machinery in pain management: State of the art

Despite recent progress regarding inexpensive medical approaches, many individuals suffer from moderate to severe pain globally. The discovery and advent of exosomes, as biological nano-sized vesicles, has revolutionized current knowledge about underlying mechanisms associated with several pathological conditions. Indeed, these particles are touted as biological bio-shuttles with the potential to carry specific signaling biomolecules to cells in proximity and remote sites, maintaining cell-to-cell communication in a paracrine manner. A piece of evidence points to an intricate relationship between exosome biogenesis and autophagy signaling pathways at different molecular levels. A close collaboration of autophagic response with exosome release can affect the body's hemostasis and physiology of different cell types. This review is a preliminary attempt to highlight the possible interface of autophagy flux and exosome biogenesis on pain management with a special focus on neuropathic pain. It is thought that this review article will help us to understand the interplay of autophagic response and exosome biogenesis in the management of pain under pathological conditions. The application of therapies targeting autophagy pathway and exosome abscission can be an alternative strategy in the regulation of pain.

Neuroimmune Mechanisms Underlying Neuropathic Pain: The Potential Role of TNF-α-Necroptosis Pathway

The neuroimmune mechanism underlying neuropathic pain has been extensively studied. Tumor necrosis factor-alpha (TNF-α), a key pro-inflammatory cytokine that drives cytokine storm and stimulates a cascade of other cytokines in pain-related pathways, induces and modulates neuropathic pain by facilitating peripheral (primary afferents) and central (spinal cord) sensitization. Functionally, TNF-α controls the balance between cell survival and death by inducing an inflammatory response and two programmed cell death mechanisms (apoptosis and necroptosis). Necroptosis, a novel form of programmed cell death, is receiving increasing attraction and may trigger neuroinflammation to promote neuropathic pain. Chronic pain is often accompanied by adverse pain-associated emotional reactions and cognitive disorders. Overproduction of TNF-α in supraspinal structures such as the anterior cingulate cortex (ACC) and hippocampus plays an important role in pain-associated emotional disorders and memory deficits and also participates in the modulation of pain transduction. At present, studies reporting on the role of the TNF-α–necroptosis pathway in pain-related disorders are lacking. This review indicates the important research prospects of this pathway in pain modulation based on its role in anxiety, depression and memory deficits associated with other neurodegenerative diseases. In addition, we have summarized studies related to the underlying mechanisms of neuropathic pain mediated by TNF-α and discussed the role of the TNF-α–necroptosis pathway in detail, which may represent an avenue for future therapeutic intervention.

Novel Therapies for the Treatment of Neuropathic Pain: Potential and Pitfalls

Neuropathic pain affects more than one million people across the globe. The quality of life of people suffering from neuropathic pain has been considerably declining due to the unavailability of appropriate therapeutics. Currently, available treatment options can only treat patients symptomatically, but they are associated with severe adverse side effects and the development of tolerance over prolonged use. In the past decade, researchers were able to gain a better understanding of the mechanisms involved in neuropathic pain; thus, continuous efforts are evident, aiming to develop novel interventions with better efficacy instead of symptomatic treatment. The current review discusses the latest interventional strategies used in the treatment and management of neuropathic pain. This review also provides insights into the present scenario of pain research, particularly various interventional techniques such as spinal cord stimulation, steroid injection, neural blockade, transcranial/epidural stimulation, deep brain stimulation, percutaneous electrical nerve stimulation, neuroablative procedures, opto/chemogenetics, gene therapy, etc. In a nutshell, most of the above techniques are at preclinical stage and facing difficulty in translation to clinical studies due to the non-availability of appropriate methodologies. Therefore, continuing research on these interventional strategies may help in the development of promising novel therapies that can improve the quality of life of patients suffering from neuropathic pain.

The Role of Autophagy and Apoptosis in Neuropathic Pain Formation

Neuropathic pain indicates pain caused by damage to the somatosensory system and is difficult to manage and treat. A new treatment strategy urgently needs to be developed. Both autophagy and apoptosis are critical adaptive mechanisms when neurons encounter stress or damage. Recent studies have shown that, after nerve damage, both autophagic and apoptotic activities in the injured nerve, dorsal root ganglia, and spinal dorsal horn change over time. Many studies have shown that upregulated autophagic activities may help myelin clearance, promote nerve regeneration, and attenuate pain behavior. On the other hand, there is no direct evidence that the inhibition of apoptotic activities in the injured neurons can attenuate pain behavior. Most studies have only shown that agents can simultaneously attenuate pain behavior and inhibit apoptotic activities in the injured dorsal root ganglia. Autophagy and apoptosis can crosstalk with each other through various proteins and proinflammatory cytokine expressions. Proinflammatory cytokines can promote both autophagic/apoptotic activities and neuropathic pain formation, whereas autophagy can inhibit proinflammatory cytokine activities and further attenuate pain behaviors. Thus, agents that can enhance autophagic activities but suppress apoptotic activities on the injured nerve and dorsal root ganglia can treat neuropathic pain. Here, we summarized the evolving changes in apoptotic and autophagic activities in the injured nerve, dorsal root ganglia, spinal cord, and brain after nerve damage. This review may help in further understanding the treatment strategy for neuropathic pain during nerve injury by modulating apoptotic/autophagic activities and proinflammatory cytokines in the nervous system.

Potamogeton perfoliatus L. Extract Attenuates Neuroinflammation and Neuropathic Pain in Sciatic Nerve Chronic Constriction Injury-Induced Peripheral Neuropathy in Rats

Sciatic nerve injury is often associated with neuropathic pain and neuroinflammation in the central and peripheral nervous systems. In our previous work, Potamogeton perfoliatus L. displayed anti-inflammatory, antipyretic and analgesic properties, predominantly via the inhibition of COX-2 enzyme and attenuation of oxidative stress. Herein, we extended our investigations to study the effects of the plant's extract on pain-related behaviors, oxidative stress, apoptosis markers, GFAP, CD68 and neuro-inflammation in sciatic nerve chronic constriction injury (CCI) rat model. The levels of the pro-inflammatory marker proteins in sciatic nerve and brainstem were measured with ELISA 14 days after CCI induction. Pretreatment with the extract significantly attenuated mechanical and cold allodynia and heat hyperalgesia with better potential than the reference drug, pregabalin. In addition, CCI lead to the overexpression of prostaglandin E2 (PGE2), inducible nitric oxide synthase (iNOS), tumor necrosis alpha (TNFα), nuclear factor κB (NF-κB), cyclooxygenase-2 (COX-2), 5-lipoxygenase (5-LOX), and NADPH oxidase-1 (NOX-1) and decreased the catalase level in sciatic nerve and brainstem. The observed neuro-inflammatory changes were accompanied with glial cells activation (increased GFAP and CD68 positive cells), apoptosis (increased Bax) and structural changes in both brainstem and sciatic nerve. The studied extract attenuated the CCI-induced neuro-inflammatory changes, oxidative stress, and apoptosis while it induced the expression of Bcl-2 and catalase in a dose dependent manner. It also decreased the brainstem expression of CD68 and GFAP indicating a possible neuroprotection effect. Taking together, P. perfoliatus may be considered as a novel therapy for neuropathic pain patients after performing the required clinical trials.