The role of gamma oscillations in central nervous system diseases: Mechanism and treatment

1Neural oscillations are rhythmic fluctuations generated by the activity of local neuron populations or neuron assemblies across brain areas and can be detected by local field potential (LFP), electrocorticography (ECoG), electroencephalography (EEG), and magnetoencephalography (MEG) at frequencies including delta (1-4 Hz), theta (4–8 Hz), alpha (8–12 Hz), beta (15–30 Hz), gamma (30–90 Hz) and high gamma(>50 Hz;; ). They govern the timing of neuronal spikes at the microscale, and at the macroscale, they coordinate the dispersed cortical communication to enable temporal and spatial brain connectivity. According to Pascal Fries’s hypothesis of “communication through coherence (CTC),” effective synaptic communication is dependent on the coordination between presynaptic and postsynaptic groups. Gamma oscillations, characterized as a rapid rhythm, allow excitation in the network to temporarily escape from the following inhibition, so enhancing the effectiveness, precision and selectivity of communication between multiple regions. Electrophysiological data from macaques showed that virtually induced gamma synchronization between primary visual cortex (V1) and higher visual cortex (V4) facilitated sensory transmission to motor responses and shortened their reaction time, hence supporting the CTC hypothesis.

2The discovery of three different types of narrowband gamma rhythm in primary visual cortex (V1), first reported by Han et al., advances the understanding of gamma oscillations information processing. These three gamma rhythms, referred as low gamma (25 to 40 Hz), medium gamma (40 to 65 Hz) and high gamma (65 to 85 Hz), process distinct spatial frequency signals and therefore carry selective aspects of visual information responding to the original stimulus, yet they are actually generated from different neural circuitries. In the issue of multisensory cross-talk, it is widely assumed that the cross-modal matching of sensory signals depends on direct interaction across sensory cortices. Studies of human EEG under gamma (40 Hz) transcranial alternating current stimulation (tACS) further confirmed that corticocortical synchronized gamma oscillations implement to modulate the multisensory communication during a visual–tactile stimuli matching task.

3The synchronous network oscillations in gamma band can be generated through pyramidal-interneuron network gamma (PING) or interneuron network gamma (ING) mechanisms. Gamma power and frequency are modulated by the features of stimulus (direction, speed, contrast, etc.). Meanwhile, the properties of a brain, such as the resting level of GABA and size of cortex, also have an impact. Sensory stimulations and behavioral responses dynamically modulate the functional connectivity between pyramidal cells (PCs) and inhibitory interneurons, including parvalbumin expressing (PV +), somatostatin expressing (SST +) and vasoactive intestinal peptide (VIP) interneurons, so as to take control of the real-time interaction within the local circuits in a flexible way (;; Figure 1). Extensive synaptic connections exist between inhibitory interneurons and their peripheral excitatory neurons, which mediates the short-term plasticity of synaptic inhibition and the consequent endogenous gamma oscillations. The prevailing understanding is that the inhibitory interneuron network characterized by these unique discharges and synaptic activities is essential for the generation of gamma rhythm. Previous studies showed the essential role of PV + and SST + interneurons to sustain gamma activity in cortex and hippocampus (HPC). The frequency of gamma oscillations is determined primarily by GABAA receptor-induced inhibitory postsynaptic currents (IPSCs;;;; ). In ING network, interneurons are mutually inhibited via GABAA receptors to quickly actualize zero-phase synchrony. During the PING mechanism, pyramidal cells elicit rapid excitation to interneurons via AMPA receptor, which consequently provides inhibition via GABAA receptors and triggers gamma-frequency oscillations.

4Recent studies of neural computation have investigated the population dynamics of circuits composed of regularly spiking PCs, fast-spiking PV + and low-threshold spiking SST + neurons, identifying 18 potential motifs organized by diverse connection patterns between the cell types and varied input settings for the SST + cells. In addition to the classic PING and ING gamma oscillations models, the three-cell-type motifs can generate theta-nested PING and beta-nested ING gamma oscillations. Moreover, the dynamic shift of oscillatory pattern in V1 following visual stimulation, from gamma to beta band, is the result of competition between the projective strengths of PV + and SST + interneurons. Collectively, these findings suggest that the multi-interneuron network flexibly modulates PCs firing and consequently controls the onset and switching of brain oscillations.

5In addition to GABAergic transmission, a PING network with asynchronous firing can produce gamma oscillations in response to acetylcholine (Ach) pulses via muscarinic Ach receptors (mAChRs), which enables the dynamic network adaptation during attentional tasks. Cue detection in behavioral attention tasks is dependent on cholinergic-driven gamma oscillations in the frontal cortex, in which both mAChRs and nicotinic Ach receptors (nAChRs) make a distinct contribution. Besides, NMDA receptors in PV + cells that generate relatively slow postsynaptic current are implicated in the control of spontaneous and induced gamma oscillations and have been shown to be the targets for NMDAR blockers (ketamine, MK-801, PCP, etc.) to modulate gamma oscillations. When external stimuli drive the network into a gamma synchronic state, functional NMDA receptors are recruited at the excitatory feedback synapses between CA1 PCs and PV + interneurons to improve and stabilize neuron assemblies. Nav1.1, a subunit of voltage-gated sodium channels, is expressed after neddylation modification in interneurons to maintain the excitability and excitation/inhibition (E/I) balance of GABAergic interneurons. It has been observed that Nav1.1 promotes behavior-dependent gamma oscillations in hAPP-J20 mice, which may be essential for the therapeutic advantages of interneuron transplantation in cognitive disorders.

6Gamma oscillations have been extensively studied in the cortex, HPC, olfactory bulb (OB), amygdala, striatum, and brainstem, respectively, associated with sensation, recognition and memory, locomotion, emotion and sleep-awake control. Since precise synaptic transmission, sufficient energy supply and environmental homeostasis are indispensable for the generation of gamma oscillations, they could represent a sensitive readout of neuronal network dysfunction even before the prodromal symptoms of neurodegeneration. Previous studies have emphasized variable parameters of gamma-band oscillations in different courses of physiology or pathophysiology, including frequency, power density and cross-frequency coupling (Table 1).

7Abnormal gamma oscillations concurrent with neuroinflammation imply a disruption in the microenvironment of the neuronal network of the central nervous system (CNS). In the hippocampal CA3 of aged nfkb-/- mice, a model of low-grade sterile inflammation during the aging process or “inflammaging,” the gamma frequency area power of the oscillations is reduced. As a high-frequency synchronization of neuron populations, gamma oscillations are susceptible to metabolic and oxidative stress since a quick requisition for mitochondria is necessary to meet the energy requirements imposed by neuronal activities. Different levels of network dysfunction are induced by TLR-activated microglia, which is primarily mediated by the excessive release of reactive oxygen and nitrogen species rather than proinflammatory cytokines. In hippocampal slices in situ, IFN-gamma-activated microglia upregulate the expression of inducible nitric oxide synthase (iNOS) and subsequently produce a large amount of NO, resulting in a decrease in the frequency of gamma-band oscillations. NO is an inhibitor of complex IV of the mitochondrial respiratory chain, which stimulates the formation of oxidants and exacerbates neuronal damage. With the combination stimulation of LPS and IFN-gamma, substantial loss of PV + neurons and decreased or absent gamma oscillations are detected in HPC cells. Caliskan and coworkers investigated antibiotics’ effect on behaviorally-relevant networks, showing that antibiotic-induced dysbiosis of the gut microbiota leads to microglia activation and iNOS production, as well as decreased neurotrophic factor (NGF), reduced baseline synaptic transmission, and impaired cholinergic gamma oscillations in hippocampal CA3 and CA1.

8Studies on rodents and clinical trials have demonstrated that anti-inflammatory therapy helps to restore gamma oscillations and improve cognitive performance. However, non-inflammatory proliferation of microglia in hippocampal sections in situ induced by granulocyte macrophage colony-stimulating factor (GM-CSF) infiltration can interfere chronically with gamma oscillations, independent of the production of proinflammatory mediators. These findings indicate the susceptibility of gamma oscillations to the disruption of CNS homeostasis and emphasize the involvement of homeostatic microglia, the resident immune cells for dynamic brain surveillance, in the maintenance of gamma oscillations in the context of various disorders, which may be associated with diverse functions of microglia such as immune response, ROS synthesis, and synaptic remodeling.

9Pain is a perceptual phenomenon formed by the dynamic integration of sensory and contextual processes (cognition, emotion, motivation, etc.), and is dependent on the spatial-temporal coordination of brain oscillations of different frequency. Human EEG reveals that the intensity of pain is encoded uniquely by gamma oscillations in the medial prefrontal cortex (mPFC); yet gamma oscillations are not responsible for tracking the intensity of visual, auditory, or non-nociceptive somatosensory stimulation. In the insula of epileptic patients, notably higher gamma oscillations were detected prior to pain perception. Studies in rodents suggest that gamma oscillations mediate acute pain perception and aversion, as well as hypersensitivity to chronic pain. Electrophysiology provides evidence that gamma-band activities in the primary somatosensory cortex (S1) is coupled with the spike firing of interneurons in S1 superficial layer contralateral to the noxious stimulus. Mice suffering from mechanical injury or inflammatory pain exhibit higher gamma oscillations in S1 and nociception susceptibility. The induction of gamma oscillations in S1 via optogenetic stimulation exacerbates nociceptive hypersensitivity and aversive avoidance..

10The common perspective is that gamma oscillations in cortex and HPC constitute the fundamental network mechanism for routing accurate information flow and communication in the processes of attention, cognition and memory. Most recent research revealed that gamma-frequency communication between entorhinal cortex and dentate gyrus exerts a crucial role in routing task-relevant clues by modulating the recruitment of hippocampal cell assemblies, including gyrus granule cells, mossy cells and CA3 pyramidal cells, whereas disrupting gamma synchronization by optogenetic perturbation of entorhinal cortex leads to learning impairments.

11Reduced rapid (alpha, beta, and gamma) and increased slow (delta and theta) rhythms are general resting-state EEG (rsEEG) metrics in patients with Alzheimer’s disease (AD; ). Studies in humans and rodents provide corroborative evidence of detrimental links between AD neuropathology and gamma oscillations. EEG in neurodegeneration-positive patients provides a demonstration of inverted U-shape curve relationship between amyloid β (Aβ) burden and the power spectral density of gamma oscillations, reflecting the compensatory effect of the brain in the early stage and accelerated lesions after decompensation in the later stage of neurodegeneration. Before the onset of Aβ plaque deposition and cognitive impairment, 5XFAD mice have exhibited abnormalities in sharp wave ripple (SWR)-related slow gamma oscillations (10–50 Hz; ). Aβ oligomer (AβO) can selectively block IPSCs produced by PV + and SST + interneurons by augmenting the initial release of GABA, hence impairing PV-PC and SST-PC interactions at the presynaptic site. This AβO-induced suppression of CA1 PC input may underpin the asynchrony of CA1 PC firing and decrease in gamma power. The power of hippocampal SWR-related slow gamma oscillations significantly decreases in apoE4-KI mice, which can be restored by specific elimination of apoE4 in GABAergic interneurons in forebrain, suggesting the crucial role of SWR-related slow gamma oscillations in apoE4-mediated learning and memory disorders. In addition, the accumulation of 1N3R subtype of tau, detected in the hilar astrocytes in dentate gyrus of AD patients, triggers a drastic relocation of mitochondria in soma-processes of astrocytes and abnormal ATP synthesis, and reduces gamma oscillations in dentate gyrus, leading to the spatial memory impairment.

12Olfactory impairment, manifested as disability to detect and recognize odors, is one of the high incidence symptoms among AD patients. Neuropathology in olfactory processing areas, including OB and entorhinal cortex, can emerge so early that even before the onset of Aβ plague and memory impairment. Along with the aging process, Aβ deposition damages the dendritic synaptic connection between granular cells (GCs) and mitral cells (MCs), resulting in aberrant enhancement of gamma oscillations and olfactory dysfunction. APP/PS1 and 3xTg mice aged 3–5 months (i.e., before the formation of Aβ plaque) have exhibited early olfactory impairment, as the number of olfactory sensory neurons (OSNs) and the amplitude of olfactory potential decreased, causing the loss of glutamatergic innervation from olfactory epithelium (OE) to OB and the consequent abnormalities of mitral/tufted (M/T) cells to trigger GC and other interneurons to release GABA. The power of low-frequency gamma oscillation (40–70 Hz) in OB is abnormally enhanced. Together with these findings, it is reasonable to conceive gamma oscillations as a potential biomarker of preclinical AD.

13The basal ganglia-thalamus-cortex circuit, which shapes voluntary movements, carries information in the form of firing frequencies, especially the gamma band that exists in many network nodes. Both beta event-related desynchronization (ERD) and gamma event-related synchronization (ERS) occur in the internal globus pallidus (GPi) prior to the preparation and execution of self-initiated and internally or externally triggered movements. Human ECoG shows the enhanced phase coupling between subthalamic nucleus (STN) neuronal firing and cortical gamma oscillations before fast reaction. The motor dysfunction of patients with Parkinson’s disease (PD) is interpreted as the imbalance of “antikinetic” beta and “prokinetic” gamma oscillatory patterns in the basal ganglia-thalamus-cortex loop. Bradykinesia, one of the most prominent symptoms of PD, may be caused generally by the insufficient recruitment of gamma-frequency synchronized bursts during movement processing. However, hyperactive gamma oscillations also disrupt motor circuit function. Dopamine depletion induces an aberrant correlation within the GABAergic spinous projection neurons (SPNs) network in the dorsal striatum, sparking off pathologically transient high-frequency gamma (60–100 Hz) oscillations during SPN activation. Levodopa-induced dyskinesia (LID), as reported by Güttler et al., is associated with the constant amplification of narrow-band gamma oscillations in primary motor cortex (M1) of levodopa-treated PD patients.

14During fear expression, the basolateral amygdala (BLA) is synchronized with the HPC and mPFC via theta oscillations. As the central circuit of fear memory consolidation and extinction, the BLA-HPC-mPFC loop establishes an oscillatory mode defined by varying intensities of low/high-frequency gamma oscillations and their cross-frequency coupling with theta oscillations. In the state of anxiety and vigilance, high-frequency gamma oscillations synchronize neural firing in BLA, while mPFC and BLA are strongly entrained in the high-frequency gamma band. Although high-frequency gamma oscillations have no significant impact on the firing rate of BLA neurons, they regulate affection and behavior by dynamically altering the firing rates and synchronization of neuronal assemblies. As a model of early life adversity, maternal separation with early weaning (MSEW) mice demonstrate susceptibility to anxiety and hyperactivity disorders. Upon entering a novel environment, the theta oscillations and theta-gamma cross-frequency coupling in their ventral HPC significantly increase. Furthermore, the loss of PV + and SST + interneurons and increased peripheral networks (PNNs) surrounding PV + neurons are observed in the ventral dentate gyrus, which may underlie the change of neural plasticity caused by early life stress. Angiotensin II (ANG II) is found to disrupt the E/I balance in HPC, particularly the perforant pathway-dentate gyrus (PP-DG) theta-gamma phase-amplitude coupling (PAC), leading to the anxiety-like behaviors of mice, which presumably contributes to the anxiety vulnerability in patients with hypertension.

15Abnormal GABAergic signaling and NMDA receptor hypofunction play a crucial role in the pathophysiology of schizophrenia (SCZ), which disturbs the E/I balance in cortical and subcortical networks and represents aberrant neural oscillations. 40 Hz auditory steady state response (ASSR) is commonly used to evaluate the ability to generate inducible gamma oscillations, and in SCZ patients, the spectrum power and/or phase locking of 40 Hz ASSR are dramatically diminished. When patients were performing cognitive and complex perception tasks, the inducible gamma oscillations in their frontal cortex is abnormally weak, which is partly due to the altered activity of PV + neurons. The expression of dystrobrevin binding protein-1 (dysbindin-1), a potential risk gene for SCZ, is reduced in the brain specimens of SCZ patients. Neuronal firing at gamma-frequency drives the translocation of dysbindin-1 into mitochondria, where it interacts with Drp1 and its receptors and initiates the assembly of Drp1 oligomers that induce mitochondrial fission. Consequently, this molecular depletion could inhibit mitochondrial fission and impair gamma oscillations, whereas stimulating mitochondrial division helps to restore the gamma rhythm.

16Autism spectrum disorder (ASD) is a constellation of restricted behavior, social deficit, cognition and memory impairments. In patients with ASD, reduction in the number of interneurons, the expression of GABA receptor subunits, as well as the level of GABA, indicate an E/I imbalance associated to GABAergic transmission. During the developmental time window of high relevance for formation of synaptic contacts (postnatal day 7–11), briefly activating L2/3 pyramidal neurons in mPFC using optogenetic technique results in the premature maturation of L2/3 dendrites and out-of-balance of neuronal E/I activity, ending up with the decreased gamma oscillations in mPFC and cognitive and social deficits in adulthood. Neuroligins (NLs) are essential for the formation and function of synapses. NL3 R451C-KI mice exhibit lower excitability of fast-spiking interneurons and abnormal gamma oscillations in mPFC, which could be the mechanistic underpinnings of their social novelty deficit. Compared to non-social engagement, the gamma power in the nucleus accumbens spontaneously increases during social interaction; however, this adaptation fails to occur in stressed animals. Cntnap2 KO mice, typified as an ASD phenotype, demonstrate lower pre-somatic inhibition of pyramidal neurons due to the loss of PV + interneurons in HPC, giving rise to the significant decrease of theta-nested gamma oscillations (65–90 Hz) and SWR.

17Despite these advances above, the question of whether gamma oscillations serve a physiological role in cortical processing responding to certain stimulus remains a controversy. Ray and Maunsell argued that gamma oscillations can be conceived as a marker of relatively local and moderate interactions involving excitation and inhibition in the cortex, yet they do not contribute to any advanced cortical functions. Nevertheless, modulating oscillations via sensory stimulation, transcranial electrical stimulation (tES) and deep brain stimulation (DBS), termed oscillation entrainment, have provided considerable evidence that neural oscillations are the critical mechanism operating memory encoding, consolidation, consolidation and retrieval, rather than an epiphenomenon of cognition and memory (;; Table 2).

18Recent studies have explored the positive effect of gamma entrainment on various types and different stages of memory process. During the consolidation of contextual memory, gamma power in BLA increases, while externally boosting gamma synchrony in BLA can strenthen the sequential memory strength. Conversely, memorial consolidation will be impeded by diminished gamma oscillations. 40 Hz optogenetic stimulation of medial septal PV + neurons restores hippocampal low-frequency gamma oscillations amplitude and theta-gamma PAC in AD mice, hence aiding in the repair of spatial memory deficits. Given that AβO accumulation causes injury to interneuron synapses, optogenetic activation of PV + and SST + interneurons in AβO -injected mice selectively restores the decreased peak power of theta and gamma and resynchronizes the spike firing of CA1 PYRs. Consistent with previously established significance of Nav1.1 in gamma oscillations, transplantation of Nav1.1-overexpressing interneurons enhances gamma oscillations and ameliorates the epileptic-like behavior and cognitive deficit of hAPP-J20 mice. Strikingly, using patterned sensory stimuli to induce neural activity and gamma entrainment, termed gamma entrainment using sensory stimulus (GENUS), work from Li-Huei Cai and her colleagues has demonstrated intriguing therapeutic effects in animal models of AD. Their strategies evoking gamma oscillations in visual cortex (VC), auditory cortex (AC), HPC and mPFC via 40Hz LED flickering, auditory stimulation or audiovisual combined stimulation conduce to improve the cognitive and spatial memory deficits in AD model mice.

19Although optogenetic and sensory stimulation have demonstrated a surprising neuroprotection effect, relatively little is understood about how gamma entrainment mediates the decrease in Aβ deposition. Studies from Tsai’s team show that optogenetic gamma stimulation or GENUS activates microglia to a phagocytotic state, moderately dilates the vessel diameter, and increases the co-localization of lipoprotein receptor-related protein 1(LRP1) and Aβ, which mediates Aβ phagocytosis by the endothelium. Besides, the levels of the cleavage intermediates and endosomal processing of Aβ precursor protein (APP) in CA1 neurons are also reduced following 40 Hz stimulation, suggesting that gamma entrainment helps to alleviate Aβ burden through both diminished amyloid generation by neurons and increased amyloid endocytosis by microglia. However, recent evidence supports that optogenetic gamma stimulation can improve memory in the absence of Aβ clearance. Paradoxically, Wilson et al. observed that optogenetic activation of PV + neurons in the basal forebrain induced gamma entrainment in the cortex of 5xFAD mice, which instead increases the burden of Aβ1–42 in frontal cortex. Hence, it is speculated that different methods, such as optogenetic, visual or auditory stimulation, are likely to entrain gamma oscillations within a complex neurocircuit while crossing various brain areas, despite the fact that cellular and molecular underpinnings of gamma entrainment in memory improvement remain a mystery that is vital for further understanding and application of gamma entrainment.

20To date there is little agreement on whether visual stimulation can effectively induce endogenous gamma entrainment in humans. Applying 40 Hz transcranial magnetic stimulation (tMS) on AD patients enhanced gamma-band power in the left temporal parietal cortex and improved cognitive and executive function through boosting local, remote, and dynamic integrative activities within brain regions. Patients with mild cognitive impairment (MCI) receiving 1-hour daily 40 Hz audiovisual stimulation for 4–8 weeks validated GENUS as a safe and tolerable treatment, while the participants demonstrated improved connectivity between posterior cingulate cortex and precuneus lobe, and decreased level of several cytokines such as TGF-α, MIP-1β, IL-5 and TNF-like weak inducer of apoptosis (TWEAK) in cerebrospinal fluid, though Aβ and tau load remained unchanged. Likewise, 40 Hz tACS treatment had no significant influence on Aβ burden but a moderate decrease of p-Tau in the targeted temporal lobe. Using 125Hz fast-gamma magnetic stimulation (FGMS) in patients with AD and MCI, the results showed no significant amelioration in cognition and depression. Given the limited sample size, non-uniformed treatment settings and evaluation criteria in the clinical trials at present, further large-scale studies are warranted to confirm a stable and reliable phenotype of the therapeutic effect of gamma entrainment on human cognitive and memory disorders.

21A recent study revealed that pallidotomy, one of the oldest therapies for motor impairments such as rigidity and bradykinesia, enhances the gamma oscillations in M1, which implies the effectiveness of modulating network rhythm in PD treatment. The advent of DBS and tACS has made a major breakthrough in the treatment of PD, though there is limited information available about their impact on oscillation entrainment, a possible therapeutic mechanism revealed recently. Recordings from the basal ganglia and cortex of PD patients showed that DBS inhibits the antikinetic beta (13–30 Hz) while reinforces the prokinetic gamma rhythm (60–90 Hz;; ). Gamma entrainment via tACS improves motor impairment in individuals with PD, and the increase of motor amplitude during tACS is interrelated to the modulation of GABAA activity in M1. Notably, aberrant enhancement of beta-gamma PAC in premotor, primary motor and somatosensory cortex is associated with the severity of clinical PD symptoms. Future research will have to clarify the influence of interventions such as DBS and dopaminergic therapy on cross-frequency coupling.

22Post-stroke survivors have high incidence of disabilities related to self-care and mobility. Although their gamma oscillations recorded by magnetoencephalogram (MEG) at resting state remains normal, the auditory gamma entrainment is attenuated, indicating a specific defect in the reserve of gamma oscillations. Gamma oscillations reserve in the affected cerebral hemisphere demonstrates a high correlation with the rehabilitation of patients, which suggests that gamma oscillations are probably accompanied with the whole rehabilitation process of stroke. Few studies have investigated the effect of gamma entrainment on ischemic lesions. In the acute phase after stroke, activation of inhibitory interneurons in M1 through 40 Hz stimulation reduces the incidence of spreading depolarizations (SDs) while alleviating brain edema and lesion volume, and increasing cerebral blood flow, all of which contributes to an improvement in the motor performance of post-stroke mice.

23Previous studies have examined gamma oscillations in the neural circuits related to emotion and social behavior, though few attempts have been made to determine the effect of gamma entrainment on mental disorders. Work from our group demonstrated that 40 Hz visual stimulation down-regulates the expression of histone deacetylase 3 (HDAC3) and cyclooxygenase-1 (COX1) in the ischemic cortex and EP2 in the amygdala, which ameliorates the susceptibility to anxiety and depression in mice exposed to post-stroke stress. 40 Hz nested at 8 Hz optogenetic activation of PV + interneurons in mPFC of NL3 R451C-KI mice effectively improves the social novelty preference, while constant 40 Hz stimulation has no significant effect. A number of clinical trials showed that tES and tMS is presumably a therapeutic operation for ASD patients by modulating gamma oscillations to be normalized. Intriguingly, light therapy for major depressive disorder (MDD), as reported by Huang et al., performs an antidepressant effect through activating the thalamic ventral lateral geniculate nucleus and intergeniculate leaflet and lateral habenula (retina-vLGN/IGL-LHb) circuit, which provides a theoretical basis for the application of sensory stimulation in depression treatment.

24So far, there is limited research investigating the mechanism of therapeutic gamma entrainment at the level of cell and molecular biology. It appears that gamma entrainment exerts an overall neuroprotective impact on neurons and glial cells, especially microglia, yet the neuron-glia crosstalk underlying gamma entrainment might be rather complicated. Moreover, it is astonishing to find that gamma entrainment is capable to drive changes of gene expression and protein phosphorylation.

2540 Hz visual stimulation significantly reduced neuronal loss in V1, CA1, somatosensory cortex and cingulate cortex of mouse models of neurodegeneration, as well as the loss of CA1 excitatory neurons in ischemic stroke mice, yet the mechanism of this effect is not well understood. Studies from Adaikkan et al. showed that GENUS down-regulates the expression of inflammatory genes and reduces DNA damage. The administration of GENUS protects P301S and CK-p25 mice from suffering severe neuronal loss and brain atrophy. Intriguingly, the rescued neuronal survival after ischemia is independent of any change of cerebral blood flow or microglia response, implying that visual stimulation may exert a direct effect on neurons. As Zhang et al. hypothesized, enhancement of CA3-CA1 excitatory synaptic transmission under gamma entrainment sends pro-survival signals to the CA1 neurons, so making CA1 neurons more resistant to reperfusion-induced neuronal death.

26Synchronized synaptic activity of neuronal populations are the major microevents underlying neural oscillations measured at the level of LFP. The activity-dependent accommodation of neuronal connections, also known as synaptic plasticity, is closely allied with neural oscillations.

27Regulated by metabotropic glutamate receptor 5 (mGlu5), synaptic plasticity plays a pivotal role in hippocampal theta and gamma oscillations induced by high-frequency afferent stimulation. This oscillatory response not only represents the amplitude and persistence of synaptic efficiency, but is internally associated with the successful generation of hippocampal long-term potentiation (LTP; ). PV + interneurons mediate LTP for E→I synapses (LTPE→I) via γCaMKII and maintain theta and gamma oscillations, which is essential for the establishment of long-term memory dependent on HPC. Accumulation of AβO1–42 selectively demages the synaptic transmission between CA1 PC and PV + interneurons and disturbs theta-nested gamma oscillations. Meanwhile, AβO1–42 interferes with the disinhibition of SST + interneurons on the proximal dendrites of CA1 PC, hence impairing the spike timing-dependent LTP induced by nested gamma oscillation. Optogenetic activation of PV + and SST + interneurons rescues the gamma oscillations and oscillation-induced LTP.

28In the brain-wide crosstalk between network oscillations and synaptic plasticity, it is worthwhile to investigate whether enhanced theta and gamma could facilitate the expression of genes related to synaptic connections. Multiple genes involved in synaptic, intracellular, and vesicle-mediated transport (Syn, vGlut1, etc.) are upregulated by GENUS, and GENUS might be able to modulate the phosphorylation of synaptic proteins. In the scenario of cerebral ischemia, 40 Hz visual stimulation restores the density of spines in CA1, particularly the mature stubby-shaped spines, while increasing the expression of regulator of G-protein signaling 12 (RGS12) and boosting the LTP of CA3-CA1 synapses via the RGS12-N type voltage-gated calcium channel (N-VGCC) pathway. This reveals that gamma entrainment in the brain can modulate protein expression and synaptic plasticity. Clinical researches demonstrated that gamma-band tACS reversed the long-term depression (LTD)-like effect but enhanced the LTP-like plasticity in M1 by inhibiting GABAergic interneurons. Whether gamma tACS regulates synaptic metaplasticity and the activity of cortical GABAergic neurons by a steady or unstable manner remains elusive.

29Although neuron assemblies are the terminal apparatus for generating neural oscillations, non-neuronal cells play an important role in supporting energetic metabolism, manipulating synaptic plasticity and preserving microenvironmental homeostasis. Moreover, the notion of glial transmitter system has emphasized the value of glial cells in network oscillations and information processing. Using cholinergic agonists in hippocampal slices, Lee et al. investigated gamma oscillations and found that the transient increase of calcium concentration in astrocytes precedes the onset of oscillations and that the release of astrocyte vesicles is necessary for the maintenance (but not the initiation) of cholinergic-induced gamma oscillations, as well as normal cognition and memory in animals. Astrocyte specific S100 calcium binding protein B (S100B) in mPFC enhances theta-gamma PAC in vivo and improves cognitive flexibility, indicating that astrocytes might participate in the complicated signaling that constitutes the neural circuits of advanced functions.

30Furthermore, PV + interneurons in mPFC recruit astrocytes to support the generation of gamma oscillations and to rectify decision-making behavior via activating GABABR. In mice performing T-maze cognitive tasks, selective depletion of the mPFC astrocytes GABABR (GFAP/PFCΔGb) decreases gamma oscillations and impairs decision-making and working memory, whereas optogenetic activation of astrocytes (but not GABAergic interneurons) rescues their cognitive deficits. In addition, astrocyte atrophy is observed in various AD model mice. Astrocyte markers glial fibrillary acidic protein (GFAP) and S100B are up-regulated in AC and CA1 of 5xFAD mice treated with GENUS, which might contribute to Aβ clearance and cognition improvement.

31During brain development, microglia modulate synaptic plasticity bidirectionally through spine formation and elimination, while simultaneously serving as dynamic neuron activity monitors. In cortical organoids, periodic oscillatory events and NMDA-induced gamma oscillations can be recorded, which is attributable in part to microglia-like cells that support the maturation and differentiation of neurons. In a mature brain, the interaction between homeostatic microglia and synapses increases neuronal activity and contributes to the synchronized firing of local neuronal populations, which is impaired by LPS-activated microglia.

32As aforementioned, neuroinflammation and oxidative stress driven by microglia can disrupt normal gamma oscillations (see section “Central nervous system perturbation: inflammation and oxidative stress”). Minocycline, a microglia inhibitor, alleviates neuroinflammation and synaptic loss, which consequently restores gamma oscillations in prefrontal circuit and improves cognitive deficit. Since most studies have focused on the destructive outcome of overactivated microglia, the neuroprotection effect of activated microglia is reported recently. LPS-activated microglia replace the inhibitory presynaptic terminals of cortical neurons by synaptic stripping, which increases gamma-band synchrony of cortical neuron firing. Consequently, the increased neuronal activity results in activation of Ca2+-mediated CaM kinase IV, phosphorylation of CREB, and a rise in the level of anti-apoptotic and neurotrophic molecules, which contributes to prevent the death of cortical neurons after inflammation stress. Together, microglia can play either a restorative or destructive role, depending upon the different exposures and relative balance between neuroprotective versus neurotoxic factors.

33Microglia play a dual role in the pathophysiology of AD, that is, they can actively uptake and degrade Aβ, but long-term activated microglia tend to produce neurotoxicity and pro-inflammatory substances. GENUS is demonstrated to transform microglia to a phagocytic state with enlarged soma and shortened processes. Compared to GENUS, acute 40 Hz pulse transcranially delivered focused ultrasound (tFUS) can activate microglia more widely encompassing with Aβ plaque. Besides, healthy mice exposed to 40 Hz visual stimulation showed an upregulation of NF-κB and MAPK phosphorylation signaling that is discriminated from the immune response to acute neuroinflammation. After GENUS, the number of rod-shaped microglia as well as the level of CD40 and C1q are decreased, suggesting a possible anti-inflammatory effect of GENUS. Our findings revealed the neuroinflammatory mechanism of post-stroke anxiety that HDAC3 is up-regulated in activated microglia in ischemic cortex, which mediates the deacetylation and nuclear translocation of p65 that activates NF-κB pathway and evokes the expression of downstream molecules COX1 and PGE2. PGE2 subsequently interacts with EP2 in amygdala to increase the susceptibility of animals to stress exposure after ischemic stroke. It is noteworthy that we found 40 Hz visual stimulation effective to inhibit the activation of cortical microglia, down-regulate HDAC3/COX1/PGE2/EP2 signaling, and rescue the anxiety-like behavior of animals, indicating that GENUS might be a powerful and non-invasive intervention to manipulate microglia immune response. However, GENUS has no significant impact on the number, morphology and immune markers of microglia in elderly C57BL/6J mice (absence of the phenotype observed in P301S and CK-p25 mice). Likewise, few microglial responses are observed in the animal model of ischemic stroke. Thus, whether GENUS is capable to modulate microglia may vary depending upon the disease status or genetic background.

34In addition, a number of studies have questioned whether the stimulation frequency 40 Hz, a key metric reported in most researches, is required to manipulate microglia. Visual stimulation at 60 Hz (rather than 40 Hz) increases the expression of CD68 and MMP-9 in microglia in V1 and evokes the degradation of PNNs, representing a non-invasive method to induce the interaction between microglia and PV + neurons and remodel PNN by network oscillation entrainment. 1070 nm light stimulation at a pulse frequency of 10 Hz (but not 40 Hz) activates microglia while increasing the co-localization of microglia and Aβ in APP/PS1 mice. Future studies will be necessary to adequately clarify the mechanisms of oscillatory entrainment operating microglia response, for instance by using single-cell sequencing to determine whether GENUS drives a significant change in microglial gene expression profiles.

35For several decades, the relationship of gamma oscillations and advanced brain functions has been gradually understood. Gamma entrainment demonstrates therapeutic efficacy in a variety of neuropsychiatric diseases, especially cognitive and memory disorders (Figure 2). Optogenetic technology and even more cutting-edge techniques permit us to customize the activation of brain regions and targeted cells of interest, and investigate the phenomenon of gamma oscillations in either physiological or pathophysiological states, as well as the methods to rescue aberrant gamma rhythm. Cortical organoid provides us a flexible model to observe and manipulate the neural oscillations dynamically at a macroscale and neurotransmitter signals at a microscale during different periods of brain development. Further researches are warranted to validate the phenotype of gamma oscillations in neurological diseases, the mechanism of neuroprotection offered by gamma entrainment and its reliability in clinical treatment. Resolving these outstanding challenges will promote to define gamma entrainment as a non-invasive, cost-effective, and practical therapy, and accelerate its clinical application in the diagnosis and treatment of CNS diseases in the future.

36AG, SW, and AH contributed to search the literature and write the main text. CQ, YL, XL, and JW provided helpful discussions and/or comments in preparing this manuscript. QW and BD critically revised the manuscript. All authors contributed to the article and approved the submitted version.