Neurite growth of trigeminal ganglion neurons in vitro with near-infrared light irradiation

Chemokine receptor CXCR2 in primary sensory neurons of trigeminal ganglion mediates orofacial itch

The CXCR2 chemokine receptor is known to have a significant impact on the initiation and control of inflammatory processes. However, its specific involvement in the sensation of itch is not yet fully understood. In this study, we aimed to elucidate the function of CXCR2 in the trigeminal ganglion (TG) by utilizing orofacial itch models induced by incision, chloroquine (CQ), and histamine. Our results revealed a significant up-regulation of CXCR2 mRNA and protein expressions in the primary sensory neurons of TG in response to itch stimuli. The CXCR2 inhibitor SB225002 resulted in notable decrease in CXCR2 protein expression and reduction in scratch behaviors. Distal infraorbital nerve (DION) microinjection of a specific shRNA virus inhibited CXCR2 expression in TG neurons and reversed itch behaviors. Additionally, the administration of the PI3K inhibitor LY294002 resulted in a decrease in the expressions of p-Akt, Akt, and CXCR2 in TG neurons, thereby mitigating pruritic behaviors. Collectively, we report that CXCR2 in the primary sensory neurons of trigeminal ganglion contributes to orofacial itch through the PI3K/Akt signaling pathway. These observations highlight the potential of molecules involved in the regulation of CXCR2 as viable therapeutic targets for the treatment of itch.

Culture of Primary Neurons from Dissociated and Cryopreserved Mouse Trigeminal Ganglion

Corneal nerves originate from the ophthalmic branch of the trigeminal nerve, which enters the cornea at the limbus radially from all directions toward the central cornea. The cell bodies of the sensory neurons of trigeminal nerve are located in the trigeminal ganglion (TG), while the axons are extended into the three divisions, including ophthalmic branch that supplies corneal nerves. Study of primary neuronal cultures established from the TG fibers can therefore provide a knowledge basis for corneal nerve biology and potentially be developed as an in vitro platform for drug testing. However, setting up primary neuron cultures from animal TG has been dubious with inconsistency among laboratories due to a lack of efficient isolation protocol, resulting in low yield and heterogenous cultures. In this study, we used a combined enzymatic digestion with collagenase and TrypLE to dissociate mouse TG while preserving nerve cell viability. A subsequent discontinuous Percoll density gradient followed by mitotic inhibitor treatment effectively diminished the contamination of non-neuronal cells. Using this method, we reproducibly generated high yield and homogenous primary TG neuron cultures. Similar efficiency of nerve cell isolation and culture was further obtained for TG tissue cryopreserved for short (1 week) and long duration (3 months), compared to freshly isolated tissues. In conclusion, this optimized protocol shows a promising potential to standardize TG nerve culture and generate a high-quality corneal nerve model for drug testing and neurotoxicity studies.

Attenuation Aβ1-42-induced neurotoxicity in neuronal cell by 660nm and 810nm LED light irradiation

Oligomeric amyloid-β 1-42 (Aβ1-42) has a close correlation with neurodegenerative disorder especially Alzheimer's disease (AD). It induces oxidative stress and mitochondrial damage in neurons. Therefore, it is used to generate AD-like in vitro model for studying neurotoxicity and neuroprotection against amyloid-β. A low-level light therapy (LLLT) is a non-invasive method that has been used to treat several neurodegenerative disorders. In this study, the red wavelength (660nm) and near infrared wavelength (810nm) at energy densities of 1, 3, and 5 J/cm2 were used to modulate biochemical processes in the neural cells. The exposure of Aβ1-42 resulted in cell death, increased intracellular reactive oxygen species (ROS), and retracted neurite outgrowth. We showed that both of LLLT wavelengths could protect neurons form Aβ1-42-induced neurotoxicity in a biphasic manner. The treatment of LLLT at 3 J/cm2 potentially alleviated cell death and recovered neurite outgrowth. In addition, the treatment of LLLT following Aβ1-42 exposure could attenuate the intracellular ROS generation and Ca2+ influx. Interestingly, both wavelengths could induce minimal level of ROS generation. However, they did not affect cell viability. In addition, LLLT also stimulated Ca2+ influx, but not altered mitochondrial membrane potential. This finding indicated LLLT may protect neurons through the stimulation of secondary signaling messengers such as ROS and Ca2+. The increase of these secondary messengers was in a functional level and did not harmful to the cells. These results suggested the use of LLLT as a tool to modulate the neuronal toxicity following Aβ1-42 accumulation in AD's brain.

Lipopolysaccharide-induced Trigeminal Ganglion Nerve Fiber Damage is Associated with Autophagy Inhibition

OBJECTIVE

This study aimed to determine whether lipopolysaccharide (LPS) induces the loss of corneal nerve fibers in cultured trigeminal ganglion (TG) cells, and the underlying mechanism of LPS-induced TG neurite damage.

METHODS

TG neurons were isolated from C57BL/6 mice, and the cell viability and purity were maintained for up to 7 days. Then, they were treated with LPS (1 µg/mL) or the autophagy regulator (autophibib and rapamycin) alone or in combination for 48 h, and the length of neurites in TG cells was examined by the immunofluorescence staining of the neuron-specific protein β3-tubulin. Afterwards, the molecular mechanisms by which LPS induces TG neuron damage were explored.

RESULTS

The immunofluorescence staining revealed that the average length of neurites in TG cells significantly decreased after LPS treatment. Importantly, LPS induced the impairment of autophagic flux in TG cells, which was evidenced by the increase in the accumulation of LC3 and p62 proteins. The pharmacological inhibition of autophagy by autophinib dramatically reduced the length of TG neurites. However, the rapamycin-induced activation of autophagy significantly lessened the effect of LPS on the degeneration of TG neurites.

CONCLUSION

LPS-induced autophagy inhibition contributes to the loss of TG neurites.

Neurite growth induced by red light-caused intracellular reactive oxygen species production through cytochrome c oxidase activation

The applications of red-light photobiomodulation (PBM) to enhance neurite growth have been proposed for many years. However, the detailed mechanisms require further studies. In the present work we used a focused red-light spot to illuminate the junction of the longest neurite and the soma of a neuroblastoma cell (N2a), and demonstrated enhanced neurite growth at 620 nm and 760 nm with adequate illumination energy fluences. In contrast, 680 nm light showed no effect on neurite growth. The neurite growth was accompanied with the increase of intracellular reactive oxygen species (ROS). Using Trolox to reduce the ROS level, this red light-induced neurite growth was hindered. Suppressing the activities of cytochrome c oxidase (CCO) by using either a small-molecule inhibitor or siRNA abrogated the red light-induced neurite growth. These results suggest that red light-induced ROS production through the activation of CCO could be beneficial for neurite growth.

Trigeminal ganglion itself can be a viable target to manage trigeminal neuralgia

Excruciating trigeminal neuralgia (TN) management is very difficult and severely affects the patient's quality of life. Earlier studies have shown that the trigeminal ganglion (TG) comprises several receptors and signal molecules that are involved in the process of peripheral sensitization, which influences the development and persistence of neuropathic pain. Targeting TG can modulate this sensitization pathway and mediate the pain-relieving effect. So far,there are few studies in which modulation approaches to TG itself have been suggested so far. "Trigeminal ganglion modulation" and "trigeminal neuralgia" were used as search phrases in the Scopus Index and PubMed databases to discover articles that were pertinent to the topic. In this review, we address the role of the trigeminal ganglion in TN and underlying molecules and neuropeptides implicated in trigeminal pain pathways in processing pathological orofacial pain. We also reviewed different modulation approaches in TG for TN management. Furthermore, we discuss the prospect of targeting trigeminal ganglion to manage such intractable pain.

Excess intracellular ATP causes neuropathic pain following spinal cord injury

Intractable neuropathic pain following spinal cord injury (NP-SCI) reduces a patient's quality of life. Excessive release of ATP into the extracellular space evokes neuroinflammation via purinergic receptor. Neuroinflammation plays an important role in the initiation and maintenance of NP. However, little is known about whether or not extracellular ATP cause NP-SCI. We found in the present study that excess of intracellular ATP at the lesion site evokes at-level NP-SCI. No significant differences in the body weight, locomotor function, or motor behaviors were found in groups that were negative and positive for at-level allodynia. The intracellular ATP level at the lesion site was significantly higher in the allodynia-positive mice than in the allodynia-negative mice. A metabolome analysis revealed that there were no significant differences in the ATP production or degradation between allodynia-negative and allodynia-positive mice. Dorsal horn neurons in allodynia mice were found to be inactivated in the resting state, suggesting that decreased ATP consumption due to neural inactivity leads to a build-up of intracellular ATP. In contrast to the findings in the resting state, mechanical stimulation increased the neural activity of dorsal horn and extracellular ATP release at lesion site. The forced production of intracellular ATP at the lesion site in non-allodynia mice induced allodynia. The inhibition of P2X4 receptors in allodynia mice reduced allodynia. These results suggest that an excess buildup of intracellular ATP in the resting state causes at-level NP-SCI as a result of the extracellular release of ATP with mechanical stimulation.

The use of photobiomodulation therapy for the prevention of chemotherapy-induced peripheral neuropathy: a randomized, placebo-controlled pilot trial (NEUROLASER trial)

PURPOSE

The purpose of this study was to investigate the effectiveness of photobiomodulation (PBM) therapy for the prevention of chemotherapy-induced peripheral neuropathy (CIPN) in breast cancer patients.

METHODS

A prospective, randomized placebo-controlled pilot trial (NEUROLASER) was set up with 32 breast cancer patients who underwent chemotherapy (ClinicalTrials.gov; NCT03391271). Patients were randomized to receive PBM (n = 16) or placebo treatments (n = 16) (2 × /week) during their chemotherapy. The modified Total Neuropathy Score (mTNS), six-minute walk test (6MWT), Numeric pain Rating Scale (NRS), and Functional Assessment of Cancer Therapy/Gynecologic Oncology Group Taxane (FACT/GOG-Taxane) were used to evaluate the severity of CIPN and the patients' quality of life (QoL). Outcome measures were collected at the first chemotherapy session, 6 weeks after initiation of chemotherapy, at the final chemotherapy session, and 3 weeks after the end of chemotherapy (follow-up).

RESULTS

The mTNS score increased significantly over time in both the control and the PBM group. A significantly higher score for FACT/GOG-Taxane was observed in the PBM group during chemotherapy compared to the control group. Questions of the FACT/GOG-Taxane related to sensory peripheral neuropathy symptoms showed a significant increase in severeness over time in the control group, whereas they remained constant in the PBM group. At follow-up, a (borderline) significant difference was observed between both groups for the 6MWT and patients' pain level, in benefit of the PBM group.

CONCLUSIONS

This NEUROLASER trial shows promising results concerning the prevention of CIPN with PBM in breast cancer patients. Furthermore, a better QoL was observed when treated with PBM.