Artificial association of memory events by optogenetic stimulation of hippocampal CA3 cell ensembles

Engram and behavior: How memory is stored in the brain

During the processing of information in humans, activated neurons behave in a specific way. The activity of these neurons leaves traces on the neurons, such as changes in synaptic or intrinsic properties. Formation of the memory traces is associated with molecular changes in the neurons. Hence, monitoring collective neural activities and following the trace of neural activities are important to neuroscience research. This collective or group of neurons is described as a 'neural ensemble', while the neural trace is described as a 'neural engram'. Both terms have been used and studied by neuroscientists for a long time. In this article, we discuss the development of these concepts, current research methods, and future areas of development.

Periaqueductal gray activates antipredatory neural responses in the amygdala of foraging rats

Pavlovian fear conditioning research suggests that the interaction between the dorsal periaqueductal gray (dPAG) and basolateral amygdala (BLA) acts as a prediction error mechanism in the formation of associative fear memories. However, their roles in responding to naturalistic predatory threats, characterized by less explicit cues and the absence of reiterative trial-and-error learning events, remain unexplored. In this study, we conducted single-unit recordings in rats during an 'approach food-avoid predator' task, focusing on the responsiveness of dPAG and BLA neurons to a rapidly approaching robot predator. Optogenetic stimulation of the dPAG triggered fleeing behaviors and increased BLA activity in naive rats. Notably, BLA neurons activated by dPAG stimulation displayed immediate responses to the robot, demonstrating heightened synchronous activity compared to BLA neurons that did not respond to dPAG stimulation. Additionally, the use of anterograde and retrograde tracer injections into the dPAG and BLA, respectively, coupled with c-Fos activation in response to predatory threats, indicates that the midline thalamus may play an intermediary role in innate antipredatory-defensive functioning.

Periaqueductal gray activates antipredatory neural responses in the amygdala of foraging rats

Pavlovian fear conditioning research suggests that the interaction between the dorsal periaqueductal gray (dPAG) and basolateral amygdala (BLA) acts as a prediction error mechanism in the formation of associative fear memories. However, their roles in responding to naturalistic predatory threats, characterized by less explicit cues and the absence of reiterative trial-and-error learning events, remain unexplored. In this study, we conducted single-unit recordings in rats during an 'approach food-avoid predator' task, focusing on the responsiveness of dPAG and BLA neurons to a rapidly approaching robot predator. Optogenetic stimulation of the dPAG triggered fleeing behaviors and increased BLA activity in naive rats. Notably, BLA neurons activated by dPAG stimulation displayed immediate responses to the robot, demonstrating heightened synchronous activity compared to BLA neurons that did not respond to dPAG stimulation. Additionally, the use of anterograde and retrograde tracer injections into the dPAG and BLA, respectively, coupled with c-Fos activation in response to predatory threats, indicates that the midline thalamus may play an intermediary role in innate antipredatory defensive functioning.

Efficient encoding of aversive location by CA3 long-range projections

Memorizing locations that are harmful or dangerous is a key capability of all organisms and requires an integration of affective and spatial information. In mammals, the dorsal hippocampus mainly processes spatial information, while the intermediate to ventral hippocampal divisions receive affective information via the amygdala. However, how spatial and aversive information is integrated is currently unknown. To address this question, we recorded the activity of hippocampal long-range CA3 axons at single-axon resolution in mice forming an aversive spatial memory. We show that intermediate CA3 to dorsal CA3 (i-dCA3) projections rapidly overrepresent areas preceding the location of an aversive stimulus due to a spatially selective addition of new place-coding axons followed by spatially non-specific stabilization. This sequence significantly improves the encoding of location by the i-dCA3 axon population. These results suggest that i-dCA3 axons transmit a precise, denoised, and stable signal indicating imminent danger to the dorsal hippocampus.

Requirement for hippocampal CA3 NMDA receptors in artificial association of memory events stored in CA3 cell ensembles

The N-methyl-D-aspartate receptors (NRs) in hippocampal CA3 are crucial for the synaptic transmission and plasticity within the CA3 recurrent circuit, which supports the hippocampal functions, such as pattern completion, and reverberatory association of sensory inputs. Previous study showed that synchronous activation of distinct cell populations in CA3, which correspond to distinct events, associated independent events, suggesting that the recurrent circuit expressing NRs in CA3 mediates the artificial association of memory events stored in CA3 ensembles. However, it is still unclear whether CA3 NRs are crucial for the artificial association of memory events stored in the CA3 ensembles. Here we report that the triple transgenic mice (cfos-tTA/KA1-Cre/NR1 flox/flox), which specifically lack NRs in the CA3 cell ensembles, showed impairment in artificial association between two events, which in control mice triggered artificial association. This result indicates that NRs in the hippocampal CA3 are required for the artificial association of memory events stored in the CA3 cell ensembles.

Memory reactivations during sleep

Neuronal activities that occur during awake periods are often reactivated again during sleep, to consolidate recently encoded memories, a process known as consolidation. In recent years, advanced tools, specially optical techniques and in-vivo live Ca²⁺ imaging, have revealed a deeper understanding to the offline periods' neuronal activities and their correspondence to later awake behavioral outputs. Recently, there is a growing consensus that sleep is more of an active process. Sleep has been associated with various functions, memory updating, future imaginations of possible familiar scenarios, decision making and planning by replaying past memories. Also, boosting insightful thoughts, creative thinking and problem solving by forming new associations and connections that were not present in awake states. Sleep activities have been directly associated with many "EUREKA" or "AHA" moments. Here, we describe recent views on memory reactivations during sleep and their implications on learning and memory.

Sevoflurane-induced amnesia is associated with inhibition of hippocampal cell ensemble activity after learning

General anesthesia could induce amnesia, however the mechanism remains unclear. We hypothesized that suppression of neuronal ensemble activity in the hippocampus by anesthesia during the post-learning period causes retrograde amnesia. To test this hypothesis, two experiments were conducted with sevoflurane anesthesia (2.5%, 30 min): a hippocampus-dependent memory task, the context pre-exposure facilitation effect (CPFE) procedure to measure memory function and in vivo calcium imaging to observe neural activity in hippocampal CA1 during context exploration and sevoflurane/home cage session. Sevoflurane treatment just after context pre-exposure session impaired the CPFE memory, suggesting sevoflurane induced retrograde amnesia. Calcium imaging showed sevoflurane treatment prevented neuronal activity in CA1. Further analysis of neuronal activity with non-negative matrix factorization, which extracts neural ensemble activity based on synchronous activity, showed that sevoflurane treatment reduced the reactivation of neuronal ensembles between during context exploration just before and one day after sevoflurane inhalation. These results suggest that sevoflurane treatment immediately after learning induces amnesia, resulting from suppression of reactivation of neuronal ensembles.

Interictal-period-activated neuronal ensemble in piriform cortex retards further seizure development

Epileptic networks are characterized as having two states, seizures or more prolonged interictal periods. However, cellular mechanisms underlying the contribution of interictal periods to ictal events remain unclear. Here, we use an activity-dependent labeling technique combined with genetically encoded effectors to characterize and manipulate neuronal ensembles recruited by focal seizures (FS-Ens) and interictal periods (IP-Ens) in piriform cortex, a region that plays a key role in seizure generation. Ca²⁺ activities and histological evidence reveal a disjointed correlation between the two ensembles during FS dynamics. Optogenetic activation of FS-Ens promotes further seizure development, while IP-Ens protects against it. Interestingly, both ensembles are functionally involved in generalized seizures (GS) due to circuit rearrangement. IP-Ens bidirectionally modulates FS but not GS by controlling coherence with hippocampus. This study indicates that the interictal state may represent a seizure-preventing environment, and the interictal-activated ensemble may serve as a potential therapeutic target for epilepsy.

A short-term memory trace persists for days in the mouse hippocampus

Active recall of short-term memory (STM) is known to last for a few hours, but whether STM has long-term functions is unknown. Here we show that STM can be optogenetically retrieved at a time point during which natural recall is not possible, uncovering the long-term existence of an STM engram. Moreover, re-training within 3 days led to natural long-term recall, indicating facilitated consolidation. Inhibiting offline CA1 activity during non-rapid eye movement (NREM) sleep, N-methyl-D-aspartate receptor (NMDAR) activity, or protein synthesis after first exposure to the STM-forming event impaired the future re-exposure-facilitated consolidation, which highlights a role of protein synthesis, NMDAR and NREM sleep in the long-term storage of an STM trace. These results provide evidence that STM is not completely lost within hours and demonstrates a possible two-step STM consolidation, first long-term storage as a behaviorally inactive engram, then transformation into an active state by recurrence within 3 days.

Short-term memory (STM) forms a protein synthesis-, NMDAR- and NREM sleep-dependent engram which lasts at least 3 days in the mouse hippocampus following a novel object location task, suggesting that STM is not completely lost within hours.

MicroLED neural probe for effective in vivo optogenetic stimulation

The MicroLED probe enables optogenetic control of neural activity in spatially separated brain regions. Understanding its heat generation characteristics is important. In this study, we investigated the temperature rise (ΔT) characteristics in the brain tissue using a MicroLED probe. The ΔT strongly depended on the surrounding environment of the probe, including the differences between the air and the brain, and the area touching the brain tissue. Through animal experiments, we suggest an in situ temperature monitoring method using temperature dependence on electrical characteristics of the MicroLED. Finally, optical stimulation by MicroLEDs proved effective in controlling optogenetic neural activity in animal models.

Formation and computational implications of assemblies in neural circuits

In the brain, patterns of neural activity represent sensory information and store it in non-random synaptic connectivity. A prominent theoretical hypothesis states that assemblies, groups of neurons that are strongly connected to each other, are the key computational units underlying perception and memory formation. Compatible with these hypothesised assemblies, experiments have revealed groups of neurons that display synchronous activity, either spontaneously or upon stimulus presentation, and exhibit behavioural relevance. While it remains unclear how assemblies form in the brain, theoretical work has vastly contributed to the understanding of various interacting mechanisms in this process. Here, we review the recent theoretical literature on assembly formation by categorising the involved mechanisms into four components: synaptic plasticity, symmetry breaking, competition and stability. We highlight different approaches and assumptions behind assembly formation and discuss recent ideas of assemblies as the key computational unit in the brain. Abstract figure legend Assembly Formation. Assemblies are groups of strongly connected neurons formed by the interaction of multiple mechanisms and with vast computational implications. Four interacting components are thought to drive assembly formation: synaptic plasticity, symmetry breaking, competition and stability. This article is protected by copyright. All rights reserved.

Formation of False Context Fear Memory Is Regulated by Hypothalamic Corticotropin-Releasing Factor in Mice

Traumatic events frequently produce false fear memories. We investigated the effect of hypothalamic corticotropin-releasing factor (CRF) knockdown (Hy-Crf-KD) or overexpression (Hy-CRF-OE) on contextual fear memory, as fear stress-released CRF and hypothalamic-pituitary-adrenal axis activation affects the memory system. Mice were placed in a chamber with an electric footshock as a conditioning stimulus (CS) in Context A, then exposed to a novel chamber without CS, as Context B, at 3 h (B-3h) or 24 h (B-24h). The freezing response in B-3h was intensified in the experimental mice, compared to control mice not exposed to CS, indicating that a false fear memory was formed at 3 h. The within-group freezing level at B-24h was higher than that at B-3h, indicating that false context fear memory was enhanced at B-24h. The difference in freezing levels between B-3h and B-24h in Hy-Crf-KD mice was larger than that of controls. In Hy-CRF-OE mice, the freezing level at B-3h was higher than that of control and Hy-Crf-KD mice, while the freezing level in B-24h was similar to that in B-3h. Locomotor activity before CS and freezing level during CS were similar among the groups. Therefore, we hypothesized that Hy-Crf-KD potentiates the induction of false context fear memory, while Hy-CRF-OE enhances the onset of false fear memory formation.

The shallow cognitive map hypothesis: A hippocampal framework for thought disorder in schizophrenia

Memories are not formed in isolation. They are associated and organized into relational knowledge structures that allow coherent thought. Failure to express such coherent thought is a key hallmark of Schizophrenia. Here we explore the hypothesis that thought disorder arises from disorganized Hippocampal cognitive maps. In doing so, we combine insights from two key lines of investigation, one concerning the neural signatures of cognitive mapping, and another that seeks to understand lower-level cellular mechanisms of cognition within a dynamical systems framework. Specifically, we propose that multiple distinct pathological pathways converge on the shallowing of Hippocampal attractors, giving rise to disorganized Hippocampal cognitive maps and driving conceptual disorganization. We discuss the available evidence at the computational, behavioural, network, and cellular levels. We also outline testable predictions from this framework, including how it could unify major chemical and psychological theories of schizophrenia and how it can provide a rationale for understanding the aetiology and treatment of the disease.

Apoptosis, rather than neurogenesis, induces significant hippocampal-dependent learning and memory impairment in chronic low Cd2+ exposure

Cadmium (Cd), a ubiquitous toxic heavy metal, with the intractable trait of low degradation, can induce multiple organ damage. Whereas, far less is known about its neurotoxicity and the specific mechanism in the chronic low Cd exposure. To investigate the chronic neurotoxicity of Cd²⁺ , we traced its effects for up to 30 months in mice which were exposed to Cd²⁺ by drinking the mimicking Cd-polluted water. We found the toxicity of chronic Cd exposure was a process associated with the transition from autophagy to apoptosis, and the switch of autophagy-apoptosis was Cd dose-dependent with the threshold of [Cd²⁺ ] 0.04 mg/L. Furthermore, JNK was found to be a hub molecule orchestrated the switch of autophagy-apoptosis by interacting with Sirt1 and p53. At last, the hippocampus-dependent learning and memory was damaged by continuous neuron apoptosis rather than deficit of neurogenesis. Therefore, elucidation of the effect, process, and potential molecular mechanism of the chronic low Cd²⁺ exposure is important for controlling of the environmental-pollutant Cd.

Neuronal Ensembles Organize Activity to Generate Contextual Memory

Contextual learning is a critical component of episodic memory and important for living in any environment. Context can be described as the attributes of a location that are not the location itself. This includes a variety of non-spatial information that can be derived from sensory systems (sounds, smells, lighting, etc.) and internal state. In this review, we first address the behavioral underpinnings of contextual memory and the development of context memory theory, with a particular focus on the contextual fear conditioning paradigm as a means of assessing contextual learning and the underlying processes contributing to it. We then present the various neural centers that play roles in contextual learning. We continue with a discussion of the current knowledge of the neural circuitry and physiological processes that underlie contextual representations in the Entorhinal cortex-Hippocampal (EC-HPC) circuit, as the most well studied contributor to contextual memory, focusing on the role of ensemble activity as a representation of context with a description of remapping, and pattern separation and completion in the processing of contextual information. We then discuss other critical regions involved in contextual memory formation and retrieval. We finally consider the engram assembly as an indicator of stored contextual memories and discuss its potential contribution to contextual memory.

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.

Sevoflurane Increases Hippocampal Theta Oscillations and Impairs Memory Via TASK-3 Channels

Sevoflurane can induce memory impairment during clinical anesthesia; however, the underlying mechanisms are largely unknown. TASK-3 channels are one of the potential targets of sevoflurane. Accumulating evidence supports a negative role of intracranial theta rhythms (4–12 Hz) in memory formation. Here, we investigated whether TASK-3 channels contribute to sevoflurane-induced memory impairment by regulating hippocampal theta rhythms. In this study, the memory performance of mice was tested by contextual fear conditioning and inhibitory avoidance experiments. The hippocampal local field potentials (LFPs) were recorded from chronically implanted electrodes located in CA3 region. The results showed that sevoflurane concentration-dependently impaired the memory function of mice, as evidenced by the decreased time mice spent on freezing and reduced latencies for mice to enter the shock compartment. Our electrophysiological results revealed that sevoflurane also enhanced the spectral power of hippocampal LFPs (1–30 Hz), particularly in memory-related theta rhythms (4–12 Hz). These effects were mitigated by viral-mediated knockdown of TASK-3 channels in the hippocampal CA3 region. The knockdown of hippocampal TASK-3 channels significantly reduced the enhancing effect of sevoflurane on hippocampal theta rhythms and alleviated sevoflurane-induced memory impairment. Our data indicate that sevoflurane can increase hippocampal theta oscillations and impair memory function via TASK-3 channels.

Complementary roles of differential medial entorhinal cortex inputs to the hippocampus for the formation and integration of temporal and contextual memory (Systems Neuroscience)

In humans and rodents, the entorhinal cortical (EC)-hippocampal (HPC) circuit is crucial for the formation and recall of memory, preserving both spatial information and temporal information about the occurrence of past events. Both modeling and experimental studies have revealed circuits within this network that play crucial roles in encoding space and context. However, our understanding about the time-related aspects of memory is just beginning to be understood. In this review, we first describe updates regarding recent anatomical discoveries for the EC-HPC network, as several important neural circuits critical for memory formation have been discovered by newly developed neural tracing technologies. Second, we examine the complementary roles of multiple medial entorhinal cortical inputs, including newly discovered circuits, into the hippocampus for the temporal and spatial aspects of memory. Finally, we will discuss how temporal and contextual memory information is integrated in HPC cornu ammonis 1 cells. We provide new insights into the neural circuit mechanisms for anatomical and functional segregation and integration of the temporal and spatial aspects of memory encoding in the EC-HPC networks.

Dimensions and mechanisms of memory organization

Memory formation is dynamic in nature, and acquisition of new information is often influenced by previous experiences. Memories sharing certain attributes are known to interact so that retrieval of one increases the likelihood of retrieving the other, raising the possibility that related memories are organized into associative mnemonic structures of interconnected representations. Although the formation and retrieval of single memories have been studied extensively, very little is known about the brain mechanisms that organize and link related memories. Here we review studies that suggest the existence of mnemonic structures in humans and animal models. These studies suggest three main dimensions of experience that can serve to organize related memories: time, space, and perceptual/conceptual similarities. We propose potential molecular, cellular, and systems mechanisms that might support organization of memories according to these dimensions.

High-Frequency Stimulation of Ventral CA1 Neurons Reduces Amygdala Activity and Inhibits Fear

The hippocampus can be divided into distinct segments that make unique contributions to learning and memory. The dorsal segment supports cognitive processes like spatial learning and navigation while the ventral hippocampus regulates emotional behaviors related to fear, anxiety and reward. In the current study, we determined how pyramidal cells in ventral CA1 respond to spatial cues and aversive stimulation during a context fear conditioning task. We also examined the effects of high and low frequency stimulation of these neurons on defensive behavior. Similar to previous work in the dorsal hippocampus, we found that cells in ventral CA1 expressed high-levels of c-Fos in response to a novel spatial environment. Surprisingly, however, the number of activated neurons did not increase when the environment was paired with footshock. This was true even in the subpopulation of ventral CA1 pyramidal cells that send direct projections to the amygdala. When these cells were stimulated at high-frequencies (20 Hz) we observed feedforward inhibition of basal amygdala neurons and impaired expression of context fear. In contrast, low-frequency stimulation (4 Hz) did not inhibit principal cells in the basal amygdala and produced an increase in fear generalization. Similar results have been reported in dorsal CA1. Therefore, despite clear differences between the dorsal and ventral hippocampus, CA1 neurons in each segment appear to make similar contributions to context fear conditioning.

Reset of hippocampal-prefrontal circuitry facilitates learning

The ability to rapidly adapt to novel situations is essential for survival, and this flexibility is impaired in many neuropsychiatric disorders¹. Thus, understanding whether and how novelty prepares, or primes, brain circuitry to facilitate cognitive flexibility has important translational relevance. Exposure to novelty recruits the hippocampus and medial prefrontal cortex (mPFC)² and may prime hippocampal-prefrontal circuitry for subsequent learning-associated plasticity. Here we show that novelty resets the neural circuits that link the ventral hippocampus (vHPC) and the mPFC, facilitating the ability to overcome an established strategy. Exposing mice to novelty disrupted a previously encoded strategy by reorganizing vHPC activity to local theta (4-12 Hz) oscillations and weakening existing vHPC-mPFC connectivity. As mice subsequently adapted to a new task, vHPC neurons developed new task-associated activity, vHPC-mPFC connectivity was strengthened, and mPFC neurons updated to encode the new rules. Without novelty, however, mice adhered to their established strategy. Blocking dopamine D1 receptors (D1Rs) or inhibiting novelty-tagged cells that express D1Rs in the vHPC prevented these behavioural and physiological effects of novelty. Furthermore, activation of D1Rs mimicked the effects of novelty. These results suggest that novelty promotes adaptive learning by D1R-mediated resetting of vHPC-mPFC circuitry, thereby enabling subsequent learning-associated circuit plasticity.

DeepLabStream enables closed-loop behavioral experiments using deep learning-based markerless, real-time posture detection

In general, animal behavior can be described as the neuronal-driven sequence of reoccurring postures through time. Most of the available current technologies focus on offline pose estimation with high spatiotemporal resolution. However, to correlate behavior with neuronal activity it is often necessary to detect and react online to behavioral expressions. Here we present DeepLabStream, a versatile closed-loop tool providing real-time pose estimation to deliver posture dependent stimulations. DeepLabStream has a temporal resolution in the millisecond range, can utilize different input, as well as output devices and can be tailored to multiple experimental designs. We employ DeepLabStream to semi-autonomously run a second-order olfactory conditioning task with freely moving mice and optogenetically label neuronal ensembles active during specific head directions.

De novo proteomic methods for examining the molecular mechanisms underpinning long-term memory

Memory formation is a fundamental function of the nervous system that enables the experience-based adaptation of behaviour. The formation, recall and updating of long-term memory (LTM) requires new protein synthesis through its direct involvement in neuronal processes, such as long-term potentiation (LTP), long-term depression (LTD) and synaptic scaling. We discuss the advantages and limitations of several emerging techniques which enable the tagging of newly synthesised proteins, including stable isotope labelling with amino acids in cell culture (SILAC), puromycin labelling, and non-canonical amino acid (NCAA) labelling. We further present how these methods allow for the identification and visualisation of proteins which are newly synthesised during different stages of memory formation. These emerging techniques will continue to expand our understanding of how memories are formed, consolidated and retrieved.

An Integrated Index: Engrams, Place Cells, and Hippocampal Memory

The hippocampus and its extended network contribute to encoding and recall of episodic experiences. Drawing from recent anatomical, physiological, and behavioral studies, we propose that hippocampal engrams function as indices to mediate memory recall. We broaden this idea to discuss potential relationships between engrams and hippocampal place cells, as well as the molecular, cellular, physiological, and circuit determinants of engrams that permit flexible routing of information to intra- and extrahippocampal circuits for reinstatement, a feature critical to memory indexing. Incorporating indexing into frameworks of memory function opens new avenues of study and even therapies for hippocampal dysfunction.

New era of optogenetics: from the central to peripheral nervous system

Optogenetics has recently gained recognition as a biological technique to control the activity of cells using light stimulation. Many studies have applied optogenetics to cell lines in the central nervous system because it has the potential to elucidate neural circuits, treat neurological diseases and promote nerve regeneration. There have been fewer studies on the application of optogenetics in the peripheral nervous system. This review introduces the basic principles and approaches of optogenetics and summarizes the physiology and mechanism of opsins and how the technology enables bidirectional control of unique cell lines with superior spatial and temporal accuracy. Further, this review explores and discusses the therapeutic potential for the development of optogenetics and its capacity to revolutionize treatment for refractory epilepsy, depression, pain, and other nervous system disorders, with a focus on neural regeneration, especially in the peripheral nervous system. Additionally, this review synthesizes the latest preclinical research on optogenetic stimulation, including studies on non-human primates, summarizes the challenges, and highlights future perspectives. The potential of optogenetic stimulation to optimize therapy for peripheral nerve injuries (PNIs) is also highlighted. Optogenetic technology has already generated exciting, preliminary evidence, supporting its role in applications to several neurological diseases, including PNIs.

Ultrastructure of light-activated axons following optogenetic stimulation to produce late-phase long-term potentiation

Analysis of neuronal compartments has revealed many state-dependent changes in geometry but establishing synapse-specific mechanisms at the nanoscale has proven elusive. We co-expressed channelrhodopsin2-GFP and mAPEX2 in a subset of hippocampal CA3 neurons and used trains of light to induce late-phase long-term potentiation (L-LTP) in area CA1. L-LTP was shown to be specific to the labeled axons by severing CA3 inputs, which prevented back-propagating recruitment of unlabeled axons. Membrane-associated mAPEX2 tolerated microwave-enhanced chemical fixation and drove tyramide signal amplification to deposit Alexa Fluor dyes in the light-activated axons. Subsequent post-embedding immunogold labeling resulted in outstanding ultrastructure and clear distinctions between labeled (activated), and unlabeled axons without obscuring subcellular organelles. The gold-labeled axons in potentiated slices were reconstructed through serial section electron microscopy; presynaptic vesicles and other constituents could be quantified unambiguously. The genetic specification, reliable physiology, and compatibility with established methods for ultrastructural preservation make this an ideal approach to link synapse ultrastructure and function in intact circuits.

Disinhibition-assisted long-term potentiation in the prefrontal-amygdala pathway via suppression of somatostatin-expressing interneurons

Significance: Natural brain adaptations often involve changes in synaptic strength. The artificial manipulations can help investigate the role of synaptic strength in a specific brain circuit not only in various physiological phenomena like correlated neuronal firing and oscillations but also in behaviors. High- and low-frequency stimulation at presynaptic sites has been used widely to induce long-term potentiation (LTP) and depression. This approach is effective in many brain areas but not in the basolateral amygdala (BLA) because the robust local GABAergic tone inside BLA restricts synaptic plasticity. Aim: We aimed at identifying the subclass of GABAergic neurons that gate LTP in the BLA afferents from the dorsomedial prefrontal cortex (dmPFC). Approach: Chemogenetic or optogenetic suppression of specific GABAergic neurons in BLA was combined with high-frequency stimulation of the BLA afferents as a method for LTP induction. Results: Chemogenetic suppression of somatostatin-positive interneurons (Sst-INs) enabled the ex vivo LTP by high-frequency stimulation of the afferent but the suppression of parvalbumin-positive interneurons (PV-INs) did not. Moreover, optogenetic suppression of Sst-INs with Arch also enabled LTP of the dmPFC-BLA synapses, both ex vivo and in vivo. Conclusions: These findings reveal that Sst-INs but not PV-INs gate LTP in the dmPFC-BLA pathway and provide a method for artificial synaptic facilitation in BLA.