Lateralization of CA1 assemblies in the absence of CA3 input

A multi-channel implantable micro-magnetic stimulator for synergistic magnetic neuromodulation

Micro-magnetic stimulation (μMS) is an emerging technology in magnetic neuromodulation. However, for larger brain structures with complex neural pathways, such as deep brain neural clusters, traditional implantable single-point μMS devices are immobile and incapable of multi-regional magnetic modulation. While multi-channel μMS can effectively address this limitation, its large size, difficulty in implantation, and unclear synergistic modulation patterns restrict its application. To tackle these challenges, this study designs a 4 × 4 array micro-coil structure targeted at the deep hippocampal region of the mouse brain. Numerical simulations were performed to analyze the coupling coefficients among the micro-coils and the distribution of the electromagnetic field in the structure, indicating that, with optimized parameters, the effective magnetic stimulation threshold can be achieved. Based on this, a multi-channel μMS device was fabricated, solving key issues such as waterproofing, biocompatibility, and dual-brain-region implantation of both stimulation and recording electrodes. A multi-point synergistic magnetic stimulation protocol was developed. After determining the synergistic magnetic stimulation parameters and effective target positions through in vitro experiments, real-time monitoring of calcium signal changes in the CA1 region of the hippocampus in mice during synergistic magnetic stimulation was performed. The results demonstrate that synergistic magnetic stimulation significantly enhances synaptic plasticity and calcium signal activity. This validates the feasibility of the implantable multi-channel micro-magnetic stimulator.

Nitrate ameliorates alcohol-induced cognitive impairment via oral microbiota

Alcohol use is associated with cognitive impairment and dysregulated inflammation. Oral nitrate may benefit cognitive impairment in aging through altering the oral microbiota. Similarly, the beneficial effects of nitrate on alcohol-induced cognitive decline and the roles of the oral microbiota merit investigation. Here we found that nitrate supplementation effectively mitigated cognitive impairment induced by chronic alcohol exposure in mice, reducing both systemic and neuroinflammation. Furthermore, nitrate restored the dysbiosis of the oral microbiota caused by alcohol consumption. Notably, removing the oral microbiota led to a subsequent loss of the beneficial effects of nitrate. Oral microbiota from donor alcohol use disordered humans who had been taking the nitrate intervention were transplanted into germ-free mice which then showed increased cognitive function and reduced neuroinflammation. Finally, we examined 63 alcohol drinkers with varying levels of cognitive impairment and found that lower concentrations of nitrate metabolism-related bacteria were associated with higher cognitive impairment and lower nitrate levels in plasma. These findings highlight the protective role of nitrate against alcohol-induced cognition impairment and neuroinflammation and suggest that the oral microbiota associated with nitrate metabolism and brain function may form part of a "microbiota-mouth-brain axis".

Integration and competition between space and time in the hippocampus

Episodic memory is organized in both spatial and temporal contexts. The hippocampus is crucial for episodic memory and has been demonstrated to encode spatial and temporal information. However, how the representations of space and time interact in the hippocampal memory system is still unclear. Here, we recorded the activity of hippocampal CA1 neurons in mice in a variety of one-dimensional navigation tasks while systematically varying the speed of the animals. For all tasks, we found neurons simultaneously represented space and elapsed time. There was a negative correlation between the preferred space and lap duration, e.g., the preferred spatial position shifted more toward the origin when the lap duration became longer. A similar relationship between the preferred time and traveled distance was also observed. The results strongly suggest a competitive and integrated representation of space-time by single hippocampal neurons, which may provide the neural basis for spatiotemporal contexts.

CA3 Circuit Model Compressing Sequential Information in Theta Oscillation and Replay

The hippocampus plays a critical role in the compression and retrieval of sequential information. During wakefulness, it achieves this through theta phase precession and theta sequences. Subsequently, during periods of sleep or rest, the compressed information reactivates through sharp-wave ripple events, manifesting as memory replay. However, how these sequential neuronal activities are generated and how they store information about the external environment remain unknown. We developed a hippocampal cornu ammonis 3 (CA3) computational model based on anatomical and electrophysiological evidence from the biological CA3 circuit to address these questions. The model comprises theta rhythm inhibition, place input, and CA3-CA3 plastic recurrent connection. The model can compress the sequence of the external inputs, reproduce theta phase precession and replay, learn additional sequences, and reorganize previously learned sequences. A gradual increase in synaptic inputs, controlled by interactions between theta-paced inhibition and place inputs, explained the mechanism of sequence acquisition. This model highlights the crucial role of plasticity in the CA3 recurrent connection and theta oscillational dynamics and hypothesizes how the CA3 circuit acquires, compresses, and replays sequential information.

Uncoordinated sleep replay across hemispheres in the zebra finch

Bilaterally organized brain regions are often simultaneously active in both humans1,2,3 and animal models,4,5,6,7,8,9 but the extent to which the temporal progression of internally generated dynamics is coordinated across hemispheres and how this coordination changes with brain state remain poorly understood. To address these issues, we investigated the zebra finch courtship song (duration: 0.5-1.0 s), a highly stereotyped complex behavior10,11 produced by a set of bilaterally organized nuclei.12,13,14 Unilateral lesions to these structures can eliminate or degrade singing,13,15,16,17 indicating that both hemispheres are required for song production.18 Additionally, previous work demonstrated broadly coherent and symmetric bilateral premotor signals during song.9 To precisely track the temporal evolution of activity in each hemisphere, we recorded bilaterally in the song production pathway. We targeted the robust nucleus of the arcopallium (RA) in the zebra finch, where population activity reflects the moment-to-moment progression of the courtship song during awake vocalizations19,20,21,22,23,24 and sleep, where song-related network dynamics reemerge in "replay" events.24,25 We found that activity in the left and right RA is synchronized within a fraction of a millisecond throughout song. In stark contrast, the two hemispheres displayed largely independent replay activity during sleep, despite shared interhemispheric arousal levels. These findings demonstrate that the degree of bilateral coordination in the zebra finch song system is dynamically modulated by behavioral state.

A local circuit-basis for spatial navigation and memory processes in hippocampal area CA1

The hippocampus is a multi-stage neural circuit that is critical for memory formation. Its distinct anatomy has long inspired theories that rely on local interactions between neurons within each subregion in order to perform serial operations important for memory encoding and storage. These local computations have received less attention in CA1 area, the primary output node of the hippocampus, where excitatory neurons are thought to be only very sparsely interconnected. However, recent findings have demonstrated the power of local circuitry in CA1, with evidence for strong functional interactions among excitatory neurons, regulation by diverse inhibitory microcircuits, and novel plasticity rules that can profoundly reshape the hippocampal ensemble code. Here we review how these properties expand the dynamical repertoire of CA1 beyond the confines of feedforward processing, and what implications they have for hippocampo-cortical functions in memory formation.

Effect of learning on slow gamma propagation between hippocampus and cortex in the wild-type and AD mice

Slow gamma oscillations (20-50 Hz) have been suggested to coordinate information transfer between brain structures involved in memory formation. Whereas the involvement of slow gamma in memory processing was studied by means of correlation between the gamma power and the occurrence of a given event (sharp wave ripples (SWRs), cortical transients), our approach consists of the analysis of the transmission of slow gamma itself. We use the method based on Granger causality principle-direct Directed Transfer Function, which allows to determine directed propagation of brain activity, including bidirectional flows. Four cortical sites along with CA1 ipsi- and contralateral were recorded in behaving wild-type and APP/PS1 mice before and after learning session of a spatial memory task. During slow wave sleep propagation of slow gamma was bidirectional, forming multiple loops of interaction which involved both CA1 and some of cortical sites. In episodes coincident with SWRs the number and strength of connectivity pathways increased in both groups compared to episodes without SWRs. The effect of learning was expressed only in APP/PS1 mice and consisted in strengthening of the slow gamma transmission from hippocampus to cortex as well as between both CA1 which may serve more efficient transmission of information from impaired CA1.

The expanded circuitry of hippocampal ripples and replay

The place cells and well-defined oscillatory population rhythms of the rodent hippocampus have served as a powerful model system in linking cells and circuits to memory function. While the initial three decades of place cell research primarily focused on the activity of neurons during exploration, the last twenty-five years have seen growing interest in the physiology of the hippocampus at rest. During slow-wave sleep and quiet wakefulness the hippocampus exhibits sharp-wave ripples (SWRs), short high-frequency, high-amplitude oscillations, that organize the reactivation or 'replay' of sequences of place cells, and interventions that disrupt SWRs impair learning. While the canonical model of SWRs generation have emphasized CA3 input to CA1 as the source of excitatory drive, recent work suggests there are multiple circuits, including the CA2 region, that can both influence, generate and organize SWRs, both from the oscillatory and information content perspectives in a task and state-dependent manner. This extended circuitry and its function must be considered for a true understanding of the role of the hippocampus in off-line processes such as planning and consolidation.

Theta and gamma oscillations in the rat hippocampus support the discrimination of object displacement in a recognition memory task

Episodic memory depends on the recollection of spatial and temporal aspects of past experiences in which the hippocampus plays a critical role. Studies on hippocampal lesions in rodents have shown that dentate gyrus (DG) and CA3 are necessary to detect object displacement in memory tasks. However, the understanding of real-time oscillatory activity underlying memory discrimination of subtle and pronounced displacements remains elusive. Here, we chronically implanted microelectrode arrays in adult male Wistar rats to record network oscillations from DG, CA3, and CA1 of the dorsal hippocampus while animals executed an object recognition task of high and low spatial displacement tests (HD: 108 cm, and LD: 54 cm, respectively). Behavioral analysis showed that the animals discriminate between stationary and displaced objects in the HD but not LD conditions. To investigate the hypothesis that theta and gamma oscillations in different areas of the hippocampus support discrimination processes in a recognition memory task, we compared epochs of object exploration between HD and LD conditions as well as displaced and stationary objects. We observed that object exploration epochs were accompanied by strong rhythmic activity in the theta frequency (6-12 Hz) band in the three hippocampal areas. Comparison between test conditions revealed higher theta band power and higher theta-gamma phase-amplitude coupling in the DG during HD than LD conditions. Similarly, direct comparison between displaced and stationary objects within the HD test showed higher theta band power in CA3 during exploration of displaced objects. Moreover, the discrimination index between displaced and stationary objects directly correlated with CA1 gamma band power in epochs of object exploration. We thus conclude that theta and gamma oscillations in the dorsal hippocampus support the successful discrimination of object displacement in a recognition memory task.

Optogenetic stimulation of CA3 pyramidal neurons restores synaptic deficits to improve spatial short-term memory in APP/PS1 mice

The hippocampal CA3 region, that is involved in the encoding and retrieval of spatial memory, is found to be synaptically impaired in the early-onset of Alzheimer's disease (AD). It is reported optogenetic manipulation of DG or CA1 can rescue the memory impairment of APP/PS1 mice, however, how CA3 region contributes to AD-related deficits in cognitive function is still unknown. Our work shows optogenetic stimulation of CA3 pyramidal neurons (PNs) significantly restores the impaired spatial short-term memory of APP/PS1 mice. This enhances the anatomical synaptic density/strength and synaptic plasticity as well as activates astrocytes. Chemogenetic inhibiting the activity of CA3 astrocytes reverses the effect of optogenetic stimulation of CA3 PNs that leads to reduced anatomical synaptic density/strength, decreased synaptic protein and AMPA receptors GluA3/4, thus disrupting the cognitive restoration of APP/PS1 mice. These results reveal the molecular mechanism of optogenetic activation of CA3 PNs on restoration of the spatial short-term memory of APP/PS1 mice and unveil a potential strategy of manipulating CA3 for AD treatment.