Gamma neuromodulation improves episodic memory and its associated network in amnestic mild cognitive impairment: a pilot study

Gamma oscillation modulation for cognitive impairment: A systematic review

BackgroundGamma oscillation modulation has emerged as a potential non-invasive treatment to counteract cognitive impairment in Alzheimer's disease (AD) and mild cognitive impairment (MCI). Non-invasive brain stimulation techniques like transcranial alternating current stimulation (tACS), gamma sensory stimulation (GSS), and transcranial magnetic stimulation (TMS) show promise in supporting specific cognitive functions.ObjectiveTo review and evaluate the efficacy of gamma oscillation modulation techniques in benefiting cognitive functions among individuals with AD and MCI.MethodsA systematic review was conducted, analyzing studies from 2015 to 2023 across databases such as PubMed, Web of Science, and Scopus. Inclusion criteria focused on studies involving tACS, GSS, or TMS applied to older adults with MCI or AD. A total of 438 articles were screened, of which 10 met the eligibility criteria.ResultsFindings suggest that gamma tACS, especially targeting the precuneus and dorsolateral prefrontal cortex, benefits episodic memory and cognitive performance. GSS also showed potential in supporting memory and attention, while TMS exhibited inconsistent but promising results when applied to the angular gyrus. However, heterogeneity in study designs and small sample sizes limit the generalizability of these outcomes.ConclusionsGamma oscillation modulation offers potential cognitive benefits for patients with AD and MCI, particularly in memory support. Further studies with larger samples and well-designed protocols are needed to confirm its therapeutic efficacy and optimize intervention parameters.

TRanscranial AlterNating current stimulation FOR patients with mild Alzheimer's Disease (TRANSFORM-AD): a randomized controlled clinical trial

BACKGROUND

The mechanistic effects of gamma transcranial alternating current stimulation (tACS) on hippocampal gamma oscillation activity in Alzheimer's Disease (AD) remains unclear. This study aimed to clarify beneficial effects of gamma tACS on cognitive functioning in AD and to elucidate effects on hippocampal gamma oscillation activity.

METHODS

This is a double-blind, randomized controlled single-center trial. Participants with mild AD were randomized to tACS group or sham group, and underwent 30 one-hour sessions of either 40 Hz tACS or sham stimulation over consecutive 15 days. Cognitive functioning, structural magnetic resonance imaging (MRI), and simultaneous electroencephalography-functional MRI (EEG-fMRI) were evaluated at baseline, the end of the intervention and at 3-month follow-up from the randomization.

RESULTS

A total of 46 patients were enrolled (23 in the tACS group, 23 in the sham group). There were no group differences in the change of the primary outcome, 11-item cognitive subscale of the Alzheimer's Disease Assessment Scale (ADAS-Cog) score after intervention (group*time, p = 0.449). For secondary outcomes, compared to the control group, the intervention group showed significant improvement in MMSE (group*time, p = 0.041) and MoCA scores (non-parametric test, p = 0.025), which were not sustained at 3-month follow-up. We found an enhancement of theta-gamma coupling in the hippocampus, which was positively correlated with improvements of MMSE score and delayed recall. Additionally, fMRI revealed increase of the local neural activity in the hippocampus.

CONCLUSION

Effects on the enhancement of theta-gamma coupling and neural activity within the hippocampus suggest mechanistic models for potential therapeutic mechanisms of tACS.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT03920826; Registration Date: 2019-04-19.

Transcranial alternating current stimulation (tACS) at gamma frequency: an up-and-coming tool to modify the progression of Alzheimer's Disease

The last decades have witnessed huge efforts devoted to deciphering the pathological mechanisms underlying Alzheimer's Disease (AD) and to testing new drugs, with the recent FDA approval of two anti-amyloid monoclonal antibodies for AD treatment. Beyond these drug-based experimentations, a number of pre-clinical and clinical trials are exploring the benefits of alternative treatments, such as non-invasive stimulation techniques on AD neuropathology and symptoms. Among the different non-invasive brain stimulation approaches, transcranial alternating current stimulation (tACS) is gaining particular attention due to its ability to externally control gamma oscillations. Here, we outline the current knowledge concerning the clinical efficacy, safety, ease-of-use and cost-effectiveness of tACS on early and advanced AD, applied specifically at 40 Hz frequency, and also summarise pre-clinical results on validated models of AD and ongoing patient-centred trials.

Evaluating the efficacy and safety of Alzheimer's disease drugs: A meta-analysis and systematic review

BACKGROUND

Alzheimer's disease (AD) is a progressive neurodegenerative disorder. Dementia severity was assessed mainly through cognitive function, psychobehavioral symptoms, and daily living ability. Currently, there are not many drugs that can be selected to treat mild to moderate AD, and the value of drugs remains controversial.

OBJECTIVE

The aim of this study is to quantitatively evaluate the efficacy and safety of cholinesterase inhibitors (ChEIs), memantine, and sodium oligomannate (GV-971) in the treatment of patients with AD. Additionally, molecular docking analysis will be used to investigate the binding affinities of donepezil, galantamine, rivastigmine, and memantine with key receptor proteins associated with AD, including beta-amyloid (Abeta), microtubule-associated protein (MAP), apolipoprotein E4 (APOE4), and Mitofusin-2 (MFN2), to further validate the results of the meta-analysis.

METHODS

We obtained clinical trials characterized by randomization, placebo control, and double-blinded methodologies concerning ChEIs, memantine, and GV-971. Statistical analysis was performed using Review Manager Version 5.4 software. Molecular docking was also conducted to evaluate the results.

RESULTS

All drugs improved the cognitive function, with the effect value ranging from -1.23 (95% CI -2.17 to -0.30) for 20 mg memantine to -3.29 (95% CI -4.14 to -2.45) for 32 mg galantamine. Although 32 mg galanthamine and GV-971 did not improve the clinicians' Global Impression of Change scale, other drugs showed significant results compared with placebo. On NPI, only 10 mg of donepezil and 24 mg of galantamine had improvement effects. On ADCS/ADL, only 20 mg memantine and 900 mg GV-971 had no significant difference from the placebo. Donepezil 5 mg and GV-971 900 mg did not increase the drug withdrawal rates due to various reasons or adverse reactions when compared to the placebo. Donepezil demonstrated superior binding to the protein and exhibited greater efficacy compared to other drugs.

CONCLUSION

ChEIs, memantine, and GV-971 all can slow the progression of AD but have different effects on respective assessments. Donepezil and GV-971 were relatively well tolerated.

Gamma oscillations and episodic memory

Enhanced gamma oscillatory activity (30-80 Hz) accompanies the successful formation and retrieval of episodic memories. While this co-occurrence is well documented, the mechanistic contributions of gamma oscillatory activity to episodic memory remain unclear. Here, we review how gamma oscillatory activity may facilitate spike timing-dependent plasticity, neural communication, and sequence encoding/retrieval, thereby ensuring the successful formation and/or retrieval of an episodic memory. Based on the evidence reviewed, we propose that multiple, distinct forms of gamma oscillation can be found within the canonical gamma band, each of which has a complementary role in the neural processes listed above. Further exploration of these theories using causal manipulations may be key to elucidating the relevance of gamma oscillatory activity to episodic memory.

Functional brain activity in patients with amnestic mild cognitive impairment: an rs-fMRI study

Background

Amnestic mild cognitive impairment (aMCI) is an early stage of Alzheimer's disease (AD). Regional homogeneity (ReHo) and amplitude of low-frequency fluctuation (ALFF) are employed to explore spontaneous brain function in patients with aMCI. This study applied ALFF and ReHo indicators to analyze the neural mechanism of aMCI by resting-state functional magnetic resonance imaging (rs-fMRI).

Methods

Twenty-six patients with aMCI were included and assigned to the aMCI group. The other 26 healthy subjects were included as a healthy control (HC) group. Rs-fMRI was performed for all participants in both groups. Between-group comparisons of demographic data and neuropsychological scores were analyzed using SPSS 25.0. Functional imaging data were analyzed using DPARSF and SPM12 software based on MATLAB 2017a. Gender, age, and years of education were used as covariates to obtain ALFF and ReHo indices.

Results

Compared with HC group, ALFF decreased in the left fusiform gyrus, left superior temporal gyrus, and increased in the left cerebellum 8, left inferior temporal gyrus, left superior frontal gyrus (BA11), and right inferior temporal gyrus (BA20) in the aMCI group (p < 0.05, FWE correction). In addition, ReHo decreased in the right middle temporal gyrus and right anterior cuneiform lobe, while it increased in the left middle temporal gyrus, left inferior temporal gyrus, cerebellar vermis, right parahippocampal gyrus, left caudate nucleus, right thalamus, and left superior frontal gyrus (BA6) (p < 0.05, FWE correction). In the aMCI group, the ALFF of the left superior frontal gyrus was negatively correlated with Montreal Cognitive Assessment (MoCA) score (r = -0.437, p = 0.026), and the ALFF of the left superior temporal gyrus was positively correlated with the MoCA score (r = 0.550, p = 0.004). The ReHo of the right hippocampus was negatively correlated with the Mini-Mental State Examination (MMSE) score (r = -0.434, p = 0.027), and the ReHo of the right middle temporal gyrus was positively correlated with MMSE score (r = 0.392, p = 0.048).

Conclusion

Functional changes in multiple brain regions rather than in a single brain region have been observed in patients with aMCI. The abnormal activity of multiple specific brain regions may be a manifestation of impaired central function in patients with aMCI.