Expressions of spinal microglia activation, BDNF, and DREAM proteins correlated with formalin-induced nociceptive responses in painful and painless diabetic neuropathy rats

Role of microglia in diabetic neuropathic pain

Approximately one-third of the patients with diabetes worldwide suffer from neuropathic pain, mainly categorized by spontaneous and stimulus-induced pain. Microglia are a class of immune effector cells residing in the central nervous system and play a pivotal role in diabetic neuropathic pain (DNP). Microglia specifically respond to hyperglycemia along with inflammatory cytokines and adenosine triphosphate produced during hyperglycemic damage to nerve fibers. Because of the presence of multiple receptors on the microglial surface, microglia are dynamically and highly responsive to their immediate environment. Following peripheral sensitization caused by hyperglycemia, microglia are affected by the cascade of inflammatory factors and other substances and respond accordingly, resulting in a change in their functional state for DNP pathogenesis. Inhibition of receptors such as P2X reporters, reducing cytokine expression levels in the microglial reactivity mechanisms, and inhibiting their intracellular signaling pathways can effectively alleviate DNP. A variety of drugs attenuate DNP by inhibiting the aforementioned processes induced by microglial reactivity. In this review, we summarize the pathological mechanisms by which microglia promote and maintain DNP, the drugs and therapeutic techniques available, and the latest advances in this field.

miRNA-130a-3p targets sphingosine-1-phosphate receptor 1 to activate the microglial and astrocytes and to promote neural injury under the high glucose condition

As a common complication of diabetes, diabetic pain neuropathy (DPN) is caused by neuron intrinsic and extrinsic factors. Neuron intrinsic factors include neuronal apoptosis and oxidative stress, while extrinsic factors are associated with glial activation. The present study was performed to reveal the functions of miR-130a-3p in apoptosis and oxidative stress of the high glucose (HG)-stimulated primary neurons as well as in the activation of microglial and astrocytes. Primary neurons, microglial, and astrocytes were isolated from newborn mice. Apoptosis was assessed by flow cytometry analysis and western blotting. Reactive oxygen species and glutathione levels were assessed to determine the oxidative stress. Markers of glial cells were detected by immunofluorescence staining. The results revealed that miR-130a-3p deficiency alleviated apoptosis and oxidative stress of HG-stimulated neurons as well as suppressed microglial and astrocyte activation. Moreover, sphingosine-1-phosphate receptor 1 (S1PR1) was found as a target downstream of miR-130a-3p. S1PR1 knockdown partially rescued the inhibitory effects of silenced miR-130a-3p on neuronal injury and glial activation. In conclusion, miR-130a-3p targets S1PR1 to activate the microglial and astrocytes and to promote apoptosis and oxidative stress of the HG-stimulated primary neurons. These findings may provide a novel insight into DPN treatment.

Inflammatory-associated apoptotic markers: are they the culprit to rheumatoid arthritis pain?

BACKGROUND

Rheumatoid arthritis (RA) is a prolonged inflammatory disease resulting from autoimmune reactions that leads to local and systemic bone erosion, joint defects and functional impairment. Although the inflammation is subsided through the prescription of anti-inflammatory therapeutics, the patients persistently complained of sleepless nights due to flare pain. This indicates the possible contribution of other pathways besides inflammation in leading to RA pain. This review aims to uncover the roles and involvement of several inflammatory-associated apoptotic markers in facilitating pain transmission and processing during the pathogenesis of RA.

MATERIALS AND METHODS

This narrative review focused on the reports from the previous literature based on the search string of "apoptotic marker AND inflammation AND 'chronic pain' OR 'neuropathic pain' and apoptosis AND 'rheumatoid arthritis' OR arthritis from the databases including Science Direct and Scopus, considering the exclusion criteria of the published abstracts, proceedings or articles on other neuropathic pain types such as painful bowel syndrom, insterstitial cystitis, fibrosis and so on.

RESULTS

Several studies in the literature demonstrate a close association between imbalanced apoptotic regulations and an increased number of synovial fibroblasts and inflammatory cells in RA. Cell death or specific cell survival has been linked with increased central hypersensitivity in various types of chronic and neuropathic pain.

CONCLUSION

The RA-related flare pain is possibly contributed by the abnormal regulation of apoptosis through several inflammatory-related pathways, and further studies need to modulate these pathways for the putative anti-nociceptive benefits.

A Novel Progress: Glial Cells and Inflammatory Pain

Inflammatory pain is the perception of noxious stimuli that occurs during inflammation or an immune response. Glial cells are widespread in the central and peripheral nervous systems, supporting and guiding the migration of neurons, participating in the immune response, forming the myelin sheath and blood-brain barrier, and maintaining the concentration of potassium ions outside nerve cells. Recent studies have shown that glial cells have a significant connection with the production and development of inflammatory pain. This article reviews the relationship, mechanisms, therapeutic targets between five types of glial cells and inflammatory pain, and the medicine composition that can effectively inhibit inflammatory pain. It expands the study on the mechanism of glial cells regulating pain and provides new ideas for the therapy of inflammatory pain.

Tannins in the Treatment of Diabetic Neuropathic Pain: Research Progress and Future Challenges

Diabetes mellitus and its consequences continue to put a significant demand on medical resources across the world. Diabetic neuropathic pain (DNP) is a frequent diabetes mellitus chronic microvascular outcome. Allodynia, hyperalgesia, and aberrant or lack of nerve fibre sensation are all symptoms of DNP. These clinical characteristics will lead to worse quality of life, sleep disruption, depression, and increased mortality. Although the availability of numerous medications that alleviate the symptoms of DNP, the lack of long-term efficacy and unfavourable side effects highlight the urgent need for novel treatment strategies. This review paper systematically analysed the preclinical research on the treatment of DNP using plant phytochemicals that contain only tannins. A total of 10 original articles involved in in-vivo and in-vitro experiments addressing the promising benefits of phytochemical tannins on DNP were examined between 2008 and 2021. The information given implies that these phytochemicals may have relevant pharmacological effects on DNP symptoms through their antihyperalgesic, anti-inflammatory, and antioxidant properties; however, because of the limited sample size and limitations of the studies conducted so far, we were unable to make definitive conclusions. Before tannins may be employed as therapeutic agents for DNP, more study is needed to establish the specific molecular mechanism for all of these activities along the pain pathway and examine the side effects of tannins in the treatment of DNP.

Painful diabetic peripheral neuropathy: Role of oxidative stress and central sensitisation

AIMS

Diabetic peripheral neuropathy (DPN) occurs in about half of people with diabetes, of whom a quarter may develop chronic pain. Pain may remain for years yet be difficult to treat because the underlying mechanisms remain unclear. There is consensus that processing excessive glucose leads to oxidative stress, interfering with normal metabolism. In this narrative review, we argue that oxidative stress may also contribute to pain.

METHODS

We reviewed literature in PubMed published between January 2005 and August 2021.

RESULTS AND CONCLUSIONS

In diabetes, hyperglycaemia and associated production of reactive species can directly increase pain signalling and activate sensory neurons; or the effects can be indirect, mediated by mitochondrial damage and enhanced inflammation. Furthermore, pain processing in the central nervous system is compromised in painful DPN. This is implicated in central sensitisation and dysfunctional pain modulation. However, central pain modulatory function is understudied in diabetes. Future research is required to clarify whether central sensitisation and/or disturbances in central pain modulation contribute to painful DPN. Positive results would facilitate early detection and future treatment.

Lower Formalin-Induced Pain Responses in Painless Diabetic Neuropathy Rat Correlate with the Reduced Spinal Cord NR2B Subunit of N-Methyl-D-Aspartate Receptor Activation

Diabetic neuropathy (DN) is a late complication of diabetic mellitus and may rise into painful and painless variants. Limited studies have looked at nociceptive mechanisms of painless DN variant. The study aimed to determine phosphorylation and total NR2B subunit of N-methyl-D-aspartate receptor in the spinal cord of painless DN rat during early phase following formalin injection. Thirty-six Sprague-Dawley male rats were randomly assigned into three groups: control, painful, and painless DN (n = 12). The rats were developed into the early phase of DN for 2 weeks following diabetic induction. Two weeks later, the rats were injected with 5% formalin solution and flinching and licking responses were recorded for 60 min. The rats were sacrificed 3 days later, and the spinal cord enlargement region was collected. Immunohistochemistry and Western blot procedures were conducted to determine the phosphorylated and total NR2B subunit expressions. The results showed reduced flinching and licking responses in painless DN rats compared to control and painful DN groups, followed by a significant reduction in phosphorylated and total NR2B expression at both ipsilateral and contralateral regions of the spinal cord. In conclusion, reduced pain behavior responses in painless DN rats following formalin injection is possibly contributed by the reduced expression of phosphorylated and total NR2B subunit in the spinal cord.