Cognitive dysfunction is a common symptom observed in stroke survivors that significantly affect their quality of life, and early recognition of PSCI is necessary for assessing the need for rehabilitation [36]. In the present study, we found rTMS promoted structural remodeling after cerabral ischemia. We also demonstrated that 10 Hz rTMS attenuated the cognition deficits and motor impairments in MCAO rats, which was associated with the activation of SDF-1α/CXCR4 axis. However, the rTMS-induced functional changes were significantly inhibited by AMD3100. To our knowledge, this is the first study utilizing DTI in vivo to investigate the neuroprotective effects of rTMS on cerebral ischemia.
rTMS, a novel neurological tool for its therapeutic benefits on recovery from stroke, is a promising approach for the treatment of PSCI. In recent years, rTMS has been increasingly applied in clinical to aid cognitive impairment resulting from normal brain aging, mild cognitive impairment (MCI), Alzheimer's disease (AD) and other dementia [37–42]. Besides, studies have concerned the effect of rTMS on PSCI other than motor dysfunction after stroke and more reviews of rTMS for PSCI began to published from last year [43, 44]. The main theoretical mechanisms of rTMS on PSCI proposed in published clinical studies are based on modulating the excitability of the cortical areas and foster plasticity changes in the cerebral cortex [45]. In vivo, many mechanisms have been reported to elucidate therapeutic effects of rTMS on PSCI. Wu T et al. put forward that rTMS could improve PSCI by modulating the levels of specifc miRs [46]. RNA sequencing and bioinformatics analysis results suggested that the modulation of the processes of synaptic plasticity-related synapses and chemical synaptic transmission in the hippocampus might be a possible underlying mechanism of rTMS in PSCI [47]. Besides, Hong Y et al. demonstrated that rTMS could modulate astrocytes polarization in cerebral ischemic models to promote PSCI recovery [48]. In addition, evidences that rTMS may improve PSCI by enhancing neurogenesis were also reported [27]. Most studies suggested that rTMS exerts a positive effect on cognitive impairment. Consistent with those positive results, we used a rat model of MCAO to confirm that rTMS improves cognitive impairment. In addition, the results of our study confirmed that 10 Hz rTMS is safe and effective for improving PSCI.
Whereas few studies have reported opposite results, claiming that rTMS has no evident treatment effect in PSCI patients [49, 50]. Therefore, further explorations of the underlying mechanisms are required to help better elaborate these effects of rTMS on PSCI. Studies have long acknowledged that structural plasticity changes in brain is closely related to the formation of new neural networks in the recovery of stroke [51]. And lots of researches have confirmed that cognitive repairment is associated with the destruction and remodeling of white matter connections [52, 53, 54]. White matter tracts are composed of a dense array of axons, which allow for rapid communication between various regions of the central and peripheral nervous system [55]. It is sensitive to reduced local blood flow and represented as demyelination, swelling of myelin and destruction of axons. Ischemic stroke generated tissue infarction and propagation of damage to adjacent cells. White matter lesions are typical manifestations of damage and severely destructed after cerebral ischemia [56]. According to previous studies, white matter lesions contributed to cognitive impairment after stroke, and those with more severe white matter lesions were at greater risk of PSCI [4, 57]. Among the published thesis, Duru, A.D., et al., group analysised correlation between white matter changes in thalamic stroke and obtained positive results [58]. Yang, L., et al., considered white matter damage was significantly associated with PSCI and monitored the hypothesis in pMCAO rats by DTI [28]. Aleksonis et al. explored the relationship between cognitive function and white matter integrity by analyzing the correlation between FA and cognitive function, which found that the higher of FA is, the better of the performance in information processing speed, cognitive flexibility and delayed memory [59]. Furthermore, a recent study exhibited that the improvement of white matter lesions is a promising strategy for PSCI treatments [60]. These previous results suggested that reparation of white matter structures adjacent to the stroke lesion may be vitally important to the response to rehabilitation interventions in PSCI.
As preceding stated, rTMS is currently the method of choice to non-invasively induce neural activity in the brain. The electric field it induced is strongest in the underlying white matter and may excite bends of myelinated axons in the juxtacortical white matter below the gyral crown [61]. Auriat, A.M., et al., earliest reviewed that rTMS treatment improved microstructures in motor related white matter brain regions that correlated with the behavioral recovery in stroke patients [62]. Another study suggested that white matter, modulating in contralesional cortico-cerebellar pathways and in interhemispheric connections, may reflect contralesional compensation facilitated by the excitatory rTMS [63]. Recently, one study indicated that motor function improvements are associated with the extent of white matter injury in motor function-related areas and sensory-motor coordination-related areas and white matter nerve fibre structures are involved in functions improvement following rTMS [64]. Hence, white matter structural improvements may be a potential cause of benefits from rTMS applied over the ischemic stroke.
DTI, as a non-invasive approach to display and analyze white matter integrity, can provide us with information on this. DTT, also known as fiber tractography, is a noninvasive imaging method to show the travel and spatial distribution of brain white matter fiber bundles three-dimensionally in vivo, which can reflect the structure of the whole brain and the connectivity of different brain regions [65]. Therefore, DTI and DTT are persuasive indexes to depict structural changes after rTMS and to elaborate the therapeutic mechanism of rTMS in stroke recovery [63]. In fact, DTI technology has been widely used to analyze the correlation between white matter changes and functional responses in the stroke. A DTI experiment showed that acupuncture combined with rTMS significantly promoted white matter tracts reconstruction and enhanced ADL in stroke patients compared to acupuncture alone [66]. Another preliminary DTI study concluded that rTMS-treated stroke patients represented better improvements in the motor function and motor-related white matter lesions [67].
However, much less is known about the role of white matter lesions in rTMS promoting PSCI recovery. Only one recent research indicated that rTMS improves cognitive function and attenuates white matter lesions via Luxol Fast Blue (LFB) staining in rats after cerebral ischemia [68]. In this experiment, we discovered changes in white matter structures adjacent to the area of injury post-rTMS using DTI in MCAO rats. DTT images showed that white matter fibers in the ischemic hemisphere were sparser than that of the contralateral hemisphere with abnormal morphology, partial curl, and were even missing in the rats of the model group on the 1st, 7th and 14th days after stroke. The statistical results showed that the mean tract length and density were significantly decreased after MCAO and they were increased by treatment with rTMS on the 14th day. The results showed that rTMS could reduce nerve fiber injury and protect the integrity of white matter nerve fiber bundles after ischemic stroke. And the structural impairments in white matters induced by rTMS represented potential means for the recovery of cognitive functions following ischemic injury.
In addition, dynamic expression changes in the SDF-1α/CXCR-4 axis in brain tissues following rTMS intervention may help to explain the possible mechanism. SDF-1α is constitutively expressed in the normal nervous system, and may be significantly induced by ischemia, a hypothesis supported by our results. A correlation between the neurobehavioral recovery and the increased SDF-1α and CXCR-4 expression has been described in Yin, L., et al.’ research [69]. Furthermore, cognitive functional recovery could arise from the elevated levels of SDF-1α and CXCR4 in ischemia rats [70]. Additionally, our prior study has shown that 10 Hz rTMS could significantly increase the protein expression levels of SDF-1α and CXCR4 in striatum in MCAO rats,which contributed to the restoration of motor function [23]. Evidence is accumulating that SDF-1α is involved in axonal pathfinding, outgrowth, and branching, while axonal sprouting occurs from the intact cortex into the peri-injured cortex and striatum [71]. As we known, white matter lesions are thought to represent regions of axon loss, demyelination and swelling of myelin. Hence, rTMS regulate the structural plasticity and improve the behaviour recovery by up-regulating the level of SDF-1α/CXCR4 axis in the ischemic brain tissues after stroke is theoretically possible. In the current study, we observed that the protein and mRNA levels of SDF-1α and CXCR4 are higher in the ipsilateral brain tissues after rTMS. The up-regulation of SDF-1α/CXCR4 axis by rTMS could potentially reduce cerebral infarct volumes and attenuate white matter lesion and assist cognitive rehabilitation after stroke.
The major aim of the present study was to investigate the mechanisms underlying the neuroprotective efficacy of rTMS in PSCI. Our results incorporate several lines of evidence demonstrating that treatment with rTMS relieved structural impairments in the ischemic brain tissues, perhaps because of the SDF-1α/CXCR4 signalling pathway. First, the results of T2WI showed that applying 10 Hz rTMS to MCAO rats ameliorated infarct volume. Second, the results of DTI showed that applying 10 Hz rTMS to MCAO rats attenuated white matter lesion. Then, mNSS and MWM showed that applying 10 Hz rTMS to MCAO rats induced behaviour improvements and these positive effects were abolished by the CXCR4 inhibitor AMD3100. Finally, WB and PCR tests revealed that treatment with 10 Hz rTMS up-regulated SDF-1α/CXCR4 signalling expressions, which is critical to above effections. All of our data supported the notion that rTMS is an efficient therapeutic treatment for PSCI and that the effects of rTMS on these symptoms may involve increasing SDF-1α/CXCR4 levels in the ischemic brain tissues. To our knowledge, the present findings are novel in that no one has previously described how these white matter structures change on DTI in response of rTMS to PSCI.
Taken together, the neurorehabilitation action of rTMS on the focal cerebral ischemia is associated with the activation of SDF-1α/CXCR4 axis. This could mediate, at least in part, the beneficial effects of rTMS on structural plasticity and PSCI in the transient focal cerebral ischemia brain. The findings are novel in that no one has previously described how these white matter structures change on DTI in response to rTMS in vivo. Our study is expected to provide new insights into the effects of rTMS on neurological deficits after ischemic stroke. Further studies with a large number of patients and appropriate animal models are warranted.