1 Introduction
Parkinson’s disease (PD) is recognized as the second most prevalent neurodegenerative disorder worldwide, just next to Alzheimer’s disease. Over the past 25 years, the global prevalence of PD has double [
1]. It is estimated that the total number of patients will exceed 12.9 million by 2040, making it the most common neurodegenerative movement disorder [
1]. The main pathological features are progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta of the midbrain and abnormal aggregation of α-synuclein [
2,
3]. This leads to motor symptoms such as tremor and rigidity, as well as induces non-motor symptoms such as depression and cognitive impairment, significantly affecting the quality of life of patients [
4–
6]. With the global population aging and environmental risk exposure increasing, the burden of PD on patients, families, and societal healthcare system has risen sharply.
Currently, clinical treatment mainly relies on dopamine replacement therapy and neurosurgery. Although these methods can alleviate motor symptoms, they do not halt neurodegeneration and offer limited improvement for non-motor symptoms. Besides, it comes with some side effects of the drugs and surgical risks [
7]. As a non-invasive and novel therapeutic approach, photobiomodulation (PBM) holds great promise for neuroprotection and functional repair through mechanisms such as regulating cellular metabolism, repairing mitochondrial function and stimulating neurons regeneration, potentially serving as an effective therapy alternative to traditional treatment methods [
8,
9]. However, the therapeutic effect of PBM is highly influenced by the optimization of irradiation parameters, including the duration of light exposure, the dosage of light, the specific time of each day to performing light irradiation, etc [
10–
12]. Therefore, exploring safe and effective therapeutic parameters of PBM is essential.
Among all these optical parameters, the light spectrum is the most essential one. Presently, PBM technology has gradually evolved from early laser light sources to more affordable and safer light-emitting diodes (LEDs) [
13]. Human tissues have typically broadband absorption, such as the absorption spectra of porphyrin in heme and cytochrome c oxidase. In the past early days, the laser light sources were widely used in PBM. However, the linear emission of laser can only cover a tiny fraction of it. As for the LED chip light sources that were popularly used at present, their narrow emission band can also cove a small part of it. However, the phosphor-converted LED light sources feature broadband emission, which can better match the absorption of human tissues. The regulation of cellular energy metabolism by light is one of most widely recognized mechanism in PBM. Specifically, light acts on cytochrome c oxidase (CCO) in mitochondria, increasing the permeability of the mitochondrial membrane and triggering a transient rise in reactive oxygen species. This process activates mitochondrial signaling pathways, associated with neuroprotection and cell survival. The nitric oxide released from the photodissociation and resynthesis of CCO can induce vasodilation and boost blood flow. Meanwhile, the elevated oxygen consumption promotes adenosine triphosphate (ATP) production, thereby supplying energy to cells. Moreover, numerous studies on Parkinson’s disease and Alzheimer’s disease have demonstrated that near-infrared light with wavelengths longer than 800 nm have special effects on neurons, in terms of modulating microglial phenotypes to produce anti-inflammatory activity, protecting neurons, repairing damaged nerve cells, stimulating synaptogenesis and neuronal regeneration, restoring neuronal function, and improving cognition and memory [
14–
18]. Nevertheless, from the perspective of precision medicine, a single monochromatic wavelength will be better and the narrower spectral bandwidth with higher spectral purity will exert better therapeutic effect, if the cellular and molecular mechanisms of pathogenesis have been well understood. However, extensive studies using laser previously have proven that the combined wavelengths yield better outcomes than a single one [
19–
23]. Additionally, PD arises from multiple pathogenic factors, involving not only energy metabolism and cell proliferation but also neurotransmitter regulation and signal transduction. From this viewpoint, the broadband light will produce superior therapeutic effect on PD.
In this study, an acute PD model was established by intraperitoneally injecting paraquat (PQ) into mice and then the mice were treated with PBM approach. Herein, 3 types of light sources, i.e., the LED chip with narrow emission band in region of 620−690 nm with a peak at 670 nm, the phosphor-converted LED with emission peak at 840 nm in wavelength range of 700–1100 nm, and their combination (narrowband 670 + broadband 840). We hypothesized that the combination light, by integrating the narrowband 670 nm light to provide energy for promoting neurons survival and proliferation and by integrating the broadband 840 nm to repair neuron morphologies and restore neuronal function, will produce better effect on improving the pathological features than either wavelength alone in the PQ-induced acute PD mice.
2 Experimental
2.1 Research design
This is a randomized controlled animal study, following the CONSORT guidelines for animal research and was approved by the Animal Ethics Committee of Anhui University of Chinese Medicine (Approval Number: AHUCM-mouse-2021082). This study aims to optimize the light wavelengths and spectral formulations for PBM therapy against Parkinson’s disease through investing the physiologic effects of different light. Herein, three types of light sources were employed in the experiment, including a LED-chip light with a peak at 670 nm, a phosphor-converted LED light with an emission peak 840 nm, and their combination. Thirty mice were randomly assigned to six groups in average, with 5 mice in each group. The PBM safety was first assessed on organ histology and serum biomarkers. Then, the morphology of hippocampal cells (including CA3, CA1, and dentate gyrus) as well as the expression of 4-HNE, MBP, and DAT/TH in the substantia nigra region were evaluated as the primary efficacy outcomes.
2.2 Experimental animals and breeding conditions
Thirty male Balb/c mice, aged 6−8 weeks and weighing approximately 20 g, were selected for the experiment and purchased from Beijing Huafu Kang Biotechnology Co., Ltd. (License No. SCXK (Jing) 2019-0008). All mice were raised in specific pathogen-free (SPF) grade animal rooms (temperature 20°C−25°C, humidity 40%−60%), with a 12-h light/dark cycle, light time 8:00−20:00), and freely fed standard mouse feed and sterile deionized water. Before employing PBM treatment, the mice were acclimatized for 14 days to adapt to the housing conditions. The mental state, diet and activity of the mice were observed daily to ensure the stability of their physiologic state. In addition, no mice were dead or dropped out throughout the entire experiment.
2.3 Randomization and grouping
Prior to the experiment, all mice were numbered and labeled for individual identification. By drawing the number from an opaque container through a lottery method, then, the mice were randomly divided into groups: the control group (P−L−, healthy mice, no paraquat exposure or PBM treatment), the light safety assessment group (P−L+ , healthy mice, no paraquat exposure, with 670 nm + 840 nm combined PBM treatment), the PD model group (P+L−, PD mice, paraquat exposure, no PBM treatment), the 670 nm PBM group (P+L+ (670 nm), PD mice, paraquat exposure, with narrowband 670 nm PBM treatment), treatment the 840 nm PBM group (P+L+ (840 nm), PD mice, paraquat exposure, with broadband 840 nm PBM treatment), and the combined PBM group (P+L+ (670 nm + 840 nm), PD mice, paraquat exposure, with 670 nm + 840 nm combined PBM treatment).
2.4 Acute Parkinson’s disease mouse model establishing
The PD model was induced with paraquat (purity ≥ 98%, purchased from Sigma-Aldrich), which was dissolved in sterile physiologic saline to prepare a solution. On the 1st, 8th, and 15th days after adaptive rearing of mice, PQ solution was administered by intraperitoneal injection at a dose of 10 mg/kg [
24]. The control group (P−L−/P−L+) was intraperitoneally injected with the same volume of sterile normal saline at the same time points.
2.5 Photobiomodulation treatment
All PBM procedures were performed by the same trained researchers. Two LED lamps were fixed on the ceiling of each mouse cage, as can be discerned from the pictures displayed in Fig. 1 for the experimental scene photos. The exterior of each mouse cage was wrapped with aluminum foil to prevent light leakage. During light irradiation, all mice without hair removal were free to move in the cage. Each session lasted 30 min, twice daily from 9:00−11:00 and 17:00−19:00, providing 9 J/cm2 per session and 18 J/cm2 per day. The treatment was initiated on the 16th day succeeding to the next day of the model establishment and continued for 14 days. The detailed parameters of the photobiomodulation treatment are shown in Table 1.
2.6 Blood and organ safety assessment
The heart, liver, spleen, lung and kidney sections of mice from both the control group and light safety assessment groups were examined using hematoxylin-eosin (H&E) staining. The sections were dewaxed with xylene I and II for 10 min each, hydrated with graded ethanol (from 100% to 70%) for 5 min each, and rinsed with distilled water for 5 min. Stain with hematoxylin for 10 min, then rinse with tap water, differentiate with 1% hydrochloric acid ethanol for 5 s, followed by bluing with 0.5% ammonia water for 30 s and rinsing for 5 min. Eosin staining for 1 min, followed by rapid rinsing with tap water and sequential dehydration with gradient ethanol ranging from 70% to 100% for 1 min. Xylene I and II each were cleared transparent for 1 min. Mounting medium was added to mount the sections, and after solidification, the sections were observed and photographed under a light microscope.
Besides, the levels of blood indicators, including α-glutamyl transferase (ALT/GPT), aspartate aminotransferase (AST/GOT), alkaline phosphatase (AKP), creatinine (CRE), blood urea nitrogen (BUN), total bilirubin (TBIL), and total protein (TP), were analyzed using commercial ELISA kits from Nanjing Jiancheng Bioengineering Institute, with the respective catalog numbers for each indicator: C009-2-1, C010-2-1, A059-2, C011-2-1, C013-2-1, C019-1-1, and A045-4.
2.7 Histological analysis and immunohistochemistry
Brain samples were fixed with 4% paraformaldehyde, paraffin-embedded and sectioned coronally. H&E staining was performed to observe the pathological changes in the striatum and substantia nigra (SN) regions. The effects of PBM on neurons in the SN region of mice were also examined through immunofluorescence staining. Following antigen retrieval, sections were incubated overnight with primary antibodies: (1) Rabbit polyclonal antibody against 4-Hydroxynonenal (4-HNE; Bioss, bs-6313R, 1:200); (2) Rabbit monoclonal antibody against Tyrosine Hydroxylase (TH; Zenbio, R381285, 1:200); (3) Rabbit polyclonal antibody to SLC6A3 (Affinity, DF4529, RRID: AB_2836880, 1:200); (4) Rabbit polyclonal antibody to Myelin Basic Protein (MBP; Affinity, AF4085, RRID: AB_2835364, 1:200). A fluorescently labeled goat anti-rabbit IgG (H + L) (Cy3; Biosharp, BL058A, 1:200) secondary antibody was subsequently incubated. After incubation in the dark, the cell nuclei were restained with DAPI (Beyotime, C1006). Immunofluorescence images were collected by the TG panoramic tissue cell quantitative analysis system (TissueGnostics, TissueFAXS Plus S).
2.8 Statistical analysis and blinded implementation
Quantitative image analysis was performed with ImageJ software (v1.52a) to observe the morphological parameters and fluorescence intensity of neurons. Statistical analysis was performed in GraphPad Prism software (v8.3.0). Comparisons among multiple groups were made with the control group (P−L−) as the reference. In this work, no pre-treatment baseline measurements were collected for individual mice and thus, all reported data are post-treatment endpoints. One-way analysis of variance (ANOVA) combined with Dunnett’s post hoc test was used to evaluate the differences between groups, and the effect size (η2) is reported. All experimental results are presented as mean ± standard deviation (SD), and the significance level was set at p < 0.05. All charts were drawn using OriginPro 2024 (v10.1). The treatment operators were aware of the groupings but did not participate in any subsequent evaluations. The researchers who are responsible for data collecting and analysis, including the mouse dissection, brain sectioning, histological staining and micrograph acquisition for morphological observation of brain tissue, image quantification, and statistical analysis, are blinded to each other.
3 Results
3.1 Light sources and spectra
The (In0.88Cr0.12)Ga3O6 phosphor utilized in this study was home-synthesized by the high-temperature solid-state reaction. Its emission and excitation spectra are presented in Fig. 2a. More details about the phosphor will be reported elsewhere. The emission spectra of the narrowband LED chip light source (full width at half maximum, FWHM = 27 nm) and the broadband phosphor-converted LED light source (FWHM = 178 nm), as well as the absorption spectrum of CCO, are presented in Fig. 2b.
3.2 Examining the effects of PBM on paraquat-induced toxicity
To assess PBM light safety, we conducted histological examination of mouse heart, liver, spleen, lung and kidney tissues (Fig. 3). No obvious tissue damage was observed in mice exposed to the combined 670 nm narrowband and 840 nm broadband light as relative to normal controls, verifying the biosafety of this phototherapy.
To further explore PBM effects on hepatic and renal functions, we detected seven serum biochemical indicators in the six groups of mice (Figs. 4a−g, Tables S1–S7), including alanine aminotransferase (ALT/GPT), creatinine (CRE), alkaline phosphatase (AKP), blood urea nitrogen (BUN), aspartate aminotransferase (AST/GOT), total bilirubin (TBIL), and total protein (TP). No significant differences were found between control and light-treated groups, proving the combined light causes no hepatic or renal injury. PQ exposure markedly elevated these serum indexes, indicating obvious organ toxicity. Notably, 670 nm, 840 nm and combined PBM treatments effectively reversed such abnormal elevations. Since ALT positively correlates with hepatic inflammation, all PBM regimens alleviated liver inflammation, with 670 nm light exhibiting the optimal efficacy. As a key renal function marker, CRE was also significantly downregulated by PBM, which effectively ameliorated paraquat-induced renal dysfunction. Overall, the single and the combined 670 nm or 840 nm light are biologically safe, and exert differential regulatory effects on functional serum indexes, providing preliminary supportive evidence for its clinical application.
3.3 PBM treatment improving the morphology of neurons in the CA3 area of the hippocampus
The CA3 region of the hippocampus is essential for cognition [
27]. Considering its vital role in memory and vulnerability to epilepsy and neurodegeneration, we analyzed neuronal morphology (Fig. 5) and pyramidal cell layer thickness (Fig. 6, Table S8) across groups. No obvious difference in layer thickness was seen between control and P−L+ groups, yet neurons in the P−L+ group presented tighter arrangement, revealing that PBM optimizes neuronal morphology (Figs. 5a and 5b). Paraquat treatment (P+L−) distinctly increased pyramidal layer thickness and nuclear vacuolation, verifying its hippocampal neurotoxicity (Fig. 5c). All PBM treatments reversed these pathological changes and restored neuronal compactness. Specifically, 670 nm light enlarged neuronal cell bodies (Fig. 5d), while 840 nm light diminished vacuolated neurons and promoted elongated neuronal shape (Fig. 5f). The combined 670 nm + 840 nm light exerted the best protective effect, sustaining compact neuronal arrangement and normal pyramidal layer thickness (Fig. 5e). Spectrum analysis of CCO showed that 670 nm light matches the mitochondrial Cu
B oxidase absorption site, and 840 nm-centered broad-spectrum light covers all four characteristic CCO absorption peaks (Fig. 2b) [
28]. Other pathways including aquaporin-related regulation also facilitate the superior efficacy of combined light therapy. Furthermore, no such obviously neuroprotective effect as that of PBM was detected in the pyramidal layer of the CA1 region of the hippocampus and the granular cells of the dentate gyrus (Fig. S1).
3.4 PBM treatment enhances neuron survival and growth in the substantia nigra
Given that progressive loss of SN dopaminergic neurons is the core pathological hallmark of PD, we conducted H&E staining on mouse SN tissues (Fig. 7). Compared with the control group, mice receiving combined 670 nm + 840 nm light showed higher neuronal density, larger cell bodies, thicker axons and intact synapses, demonstrating this light regimen facilitates SN neuronal and synaptic growth in normal mice (Figs. 7a and 7b). PD model mice displayed obvious neuronal loss, nuclear pyknosis and synaptic damage (Fig. 7c), whereas PBM treatments effectively alleviated paraquat-induced neuronal deficiency and morphological atrophy. Specifically, 670 nm light markedly enlarged neuronal somata, 840 nm light promoted synaptic branching, and their combination exerted synergistic effects on increasing cell volume and facilitating neurite outgrowth (Figs. 7d−7f). Consistent with the results in hippocampal CA3 region, 670 nm preferentially boosted neurite development, 840 nm favored neurogenesis and synaptogenesis, and combined light integrated their respective advantages. Quantitative statistics of normal and motor neurons in SN confirmed elevated neuronal counts in light-only group relative to controls. Paraquat drastically reduced neuronal quantity, which was significantly restored by PBM treatment. Among all regimens, 670 nm + 840 nm combined light achieved the optimal therapeutic effect, followed by single 670 nm and 840 nm irradiation (Figs. 7g and 7h, Tables S9 and S10). Notably, all experimental mice retained complete body hair, implying photons can penetrate human skin and hair to deliver energy to damaged lesions. This work further clarifies the underlying action mechanism of PBM.
3.5 PBM treatment significantly improving oxidative damage, promoting myelin regeneration, and effectively regulating neuronal function
To verify whether PBM exerts neuronal protection against PQ-triggered oxidative stress, we conducted 4-HNE staining in the SN (Fig. 8a). PQ notably elevated 4-HNE expression relative to normal controls. Single-wavelength PBM using the narrow 670 nm and broad 840 nm reduced 4-HNE-positive cells by 10.88% and 9.54%, respectively (Fig. 8b, Table S11), while the combined 670 nm + 840 nm irradiation achieved the maximum reduction of 12.94%. These data confirm that PBM modulates redox balance and alleviates lipid peroxidation injury. We further detected myelin regeneration via MBP staining (Fig. 8a). MBP expression was markedly downregulated in paraquat-induced PD mice, and was distinctly rescued after PBM treatment, with combined light showing the strongest efficacy (Fig. 8c, Table S12). Collectively, PBM upregulates MBP expression to facilitate myelin regeneration and restore impaired nerve conduction structures.
As shown in Fig. 9a, TH, the rate-limiting enzyme for dopamine synthesis, serves as a typical marker reflecting dopaminergic neuronal activity. Compared with the control group, TH fluorescence intensity rose by 31.24% in PD model mice (Fig. 9b, Table S13), suggesting dopaminergic neurons entered a pathological compensatory state with elevated TH expression. Such abnormal compensation could be suppressed by PBM treatment, especially under 840 nm broadband light, which reduced TH level by about 11.80% and effectively relieved this pathological compensatory response. Regarding dopamine reuptake, DAT expression in PD mice decreased by 30.47% relative to normal mice. All three PBM regimens notably restored DAT expression (Fig. 9c, Table S14), among which combined 670 nm + 840 nm light exerted the optimal effect. In this group, DAT expression recovered to 86.05% of the normal level, with a 23.76% increase versus the PD group, demonstrating that combined light substantially facilitates DAT expression in surviving dopaminergic neurons.
4 Discussion
The central pathological hallmark of PD is the progressive degeneration of dopaminergic neurons. Existing therapeutic approaches are largely ineffective at slowing disease progression or preventing ongoing neurodegeneration. PBM has emerged as a non-invasive treatment with a favorable safety. In this work, the PBM safety of the three types of the light, i.e., the narrow 670 nm, the broad 840 nm, and their combination were confirmed at first, through histological analyses of cardiac, hepatic, splenic, pulmonary, and renal tissues and by testing serum hepatic and renal function biomarkers. All results exhibited the variant lights have no toxicity. Instead, wavelength-specific modulation of markers such as AKP and BUN was observed, supporting the neuroprotective benefits of PBM. This favorable efficacy-safety profile indicates a promising potential for clinical translation, which is also supported by preliminary clinical studies [
29–
32].
Mechanistically, CCO within mitochondria serves as the primary photoreceptor, absorbing light at wavelengths between 600 nm and 850 nm (Fig. 2b). This absorption facilitates activation of the electron transport chain, promotes ATP synthesis, and modulates oxidative stress responses [
33]. These mechanisms have been extensively investigated across multiple neurodegenerative contexts [
34–
36]. But as for the pathways of light absorption and conversion, it should be mentioned that the mice were cultured without hair being trimmed and the mice were subjected to whole-body light irradiation. The evident therapeutic effects observed in this study suggests that the light or photons may be absorbed by skin or hair, triggered biochemical reactions within the body, and then utilized to repair injured neurons.
Through employing the paraquat-induced PD mouse model to evaluate the neuroprotective and therapeutic efficacy [
37], the results obtained in this work indicated that PBM of all lights supported neuronal survival and growth in the CA3 region and SN, meanwhile reducing interneuronal spacing and nuclear vacuolization. Specifically, the narrowband 670 nm light likely promoted neurite outgrowth, whereas the broadband 840 nm light likely enhanced synaptogenesis, as can be discerned from Figs. 5−7. Moreover, the combined-light irradiation engaged complementary signaling pathways in addition to their individual effect, yielding superior therapeutic outcomes.
The narrow 670 nm light since of the CCO mechanism plays the role of enhancing ATP production and providing energy for neuronal repair [
33]. In contrast, the broad spectrum of 840 nm light may activate multiple signaling pathways, initiating antioxidant defense and regulating dopaminergic function, synaptic plasticity, and myelin regeneration, as shown in Fig. 10. Similar effects have been documented for other near-infrared wavelengths, including the 810 nm [
38−
40], 808 nm [
41], and 1025 nm [
42]. Consistent with Troshev et al. [
43], we observed that TH expression in PD model mice was higher than in the control group, which was attributed to compensatory upregulation. This compensatory mechanism is a key feature in both experimental PD models and patient progression, involving not only the dopaminergic system but also non-dopaminergic pathways [
44]. Following PBM treatment, the compensatory TH increase was suppressed and DAT expression rose, suggesting rebalancing of dopamine synthesis and reuptake, possibly via reduced oxidative stress and improved mitochondrial function. During combined-light irradiation, the energy supply from 670 nm light potentiates the repair pathways triggered by 840 nm light, while the energy demand further mobilizes metabolism, forming a positive feedback loop that restores dopaminergic homeostasis and promotes MBP expression [
45,
46].
The morphological improvements in Figs. 5−9 imply functional neurorestoration following PBM. The 4-HNE is a lipid peroxidation product related to mitophagy. It can regulate autophagic flux in a concentration-dependent manner with its level rising, while excessive accumulation can induce autophagic apoptosis [
47–
49]. Immunofluorescence analysis of SN tissue sections revealed an elevated level of 4-HNE, while the PBM treatment effectively reduced 4-HNE expression and attenuated autophagic dysregulation (Fig. 8). Additionally, the decreased MBP immunoreactivity (Fig. 8) suggested structural myelin deficits within the nigrostriatal pathway—a phenomenon linked to oligodendrocyte maturation deficits and known to precede neuronal loss [
50]. PBM mediated MBP upregulation may facilitate remyelination via oligodendrocyte activation or enhanced neural microenvironments [
51], though direct evidence for these mechanisms requires further investigation. In PD, dopaminergic neuron loss in the SN leads to marked dopamine depletion. Although levodopa replacement therapy remains a clinical cornerstone, it is frequently associated with adverse effects such as motor fluctuations [
52]. We further observed the compensatory upregulation of TH (Fig. 9), the rate-limiting enzyme in dopamine biosynthesis [
45,
53]. The remaining neurons are forced to upregulate the expression of the synthase TH to maintain the output, so as to make up for the deficiency of dopamine. However, this kind of compensation is inefficient and unsustainable. Notably, PBM alleviates this compensatory TH response, which helps to reduce the metabolic load on neurons and restore them from an overactive pathological state to a normal one. At the synaptic level, DAT expression was significantly reduced in PD models (Fig. 9), indicating the impaired synaptic dopamine reuptake [
46]. Given that DAT serves as the key receptor for paraquat entry [
54], we speculate that its toxic mechanism is a vicious cycle. Probably, paraquat initially invades dopaminergic nerve terminals via DAT and then triggers severe oxidative stress. This stress, in turn, downregulates DAT expression and damages synaptic structures, ultimately leading to neurodegeneration. Significantly, our study demonstrates that PBM administration counteracted this damage by restoring DAT expression. This finding suggests that the therapeutic benefit of PBM stems not only from regulating dopamine synthesis but also from reinforcing the reuptake capacity of dopamine, thereby optimizing the dynamic balance of neurotransmitters within the synaptic cleft. Through a dual strategy that curtails excessive synthesis and promotes recycling, PBM effectively improves dopaminergic neurotransmission and alleviates neuronal dysfunction. These coordinated molecular alterations underscore a key therapeutic feature of PBM: multi-targets synergistic modulation.
Based on the concept of precision medicine, monochromatic light with high spectral purity is theoretically supposed to achieve better therapeutic efficacy. Nevertheless, our experimental results demonstrate that combined light yields superior treatment, which can be attributed to the synergistic regulatory effect generated by multi-wavelength light targeting multiple pathological sites. This strategy shows promise for personalized optical therapy based on individual PD pathology, offering a preliminary foundation for clinical translation.
It should be noted that this study was conducted exclusively in male mice and based on short-term outcomes. In addition, the sample size is relatively small, with five mice per group. Nevertheless, the significant differences observed among different groups provide preliminary evidence to approve the efficacy and safety of PBM. Future studies, by incorporating larger cohorts, both sexes, and longer-term evaluations, will be essential to validate these findings and explore potential sex-dependent effects.
5 Conclusion
In summary, the acute PD model was successfully established by injecting paraquat into mice belly. Treated with three types of the light, the narrowband 670 nm, the broadband 840 nm, and their combination, all of them show evident therapeutic effects and the combined light is most effective. The PBM at power of 5 mW/cm2 and the light dosage of 9 J/cm2/time and two times/day were verified safe; and the physiologic effects of these lights and their possible mechanisms on PD treatment were explored. The analyses on morphologies of neurons in the striatum CA3 area of the hippocampus show that the PBM treatment could significantly improve the morphologies damage caused by the toxicity of paraquat. The narrowband 670 nm light could efficiently increase the neuron volume, while the broadband 840 nm light tends to reducing the vacuolated neurons and prolonging their morphology. The combined 670 nm + 840 nm light remarkably shortens the intercellular spaces of neurons in the CA3 area and can make the neurons arrangement densely without dispersion. Besides, PBM treatment evidently promote the survival and growth of normal neurons and motor neurons. The narrowband 670 nm light is efficient on increasing neuron bodies while the broadband 840 nm light is more efficient on promoting synaptic branches. The synergistical effect of the combined light improve neuron volume and neurite growth, restore neurons number to the maximum. In term of oxidative stress inhibition, PBM can efficiently reduce the oxidative stress level of neurons. The proportion of 4-Hydroxynonenal positive cells in the substantia nigra of PD model mice was 29.75% higher than that of the normal group, but it was reduced 14.39% in the PBM group by the combined light. In term of myelin repair, the expression of myelin basic protein in PD model mice was 29.21% lower than that of the normal group. However, the expression of MBP in the PBM group increased 18.28% by the combined light, indicating its advantageous role in myelin regeneration. In term of dopamine system regulation, PD model mice have compensatory increases in tyrosine hydroxylase due to neuronal degeneration (31.24% higher than the normal group). Yet, the broadband 840 nm light exhibits most efficient inhibition of this compensation (11.80% reduction). At the same time, the expression of dopamine transporter in PD model mice was 30.47% lower than that of the normal group, and the combined light restore it to 86.05% of the normal group (23.76% higher than the PD group). The combination light of 670 nm + 840 nm integrates the advantages of both the narrowband 670 nm and broadband 840 nm light, achieving the best performance in neuroprotective, morphology repair, functional regulation, and neurons regeneration.
Through above systematic analyses on the morphological and molecular biological data, we incline to think that PBM improve the pathological phenotype of PD may through the synergistic mechanisms of: energy metabolism repair - oxidative stress regulation - neural function remodeling. The wavelength-dependent efficacy observed in this work provides a preliminary reference for future research and clinical exploration of PBM. This non-invasive and safe-treatment approach of PBM with multi-pathway regulatory characteristics is expected to become an important supplement to the comprehensive treatment system for PD, also promisingly promote a paradigm shift in the treatment of neurodegenerative diseases from symptom control to pathological mechanism intervention.
Besides, the mice were cultured without hair being trimmed herein. The pathways of light absorption and transformation are fascinating and warrant further extensive research. In addition, behavioral validation remains necessary in future studies and further richens the spectrum engineering to PBM.
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