Gating of hippocampal activity, plasticity, and memory by entorhinal cortex long-range inhibition

Emergence of Coordinated Activity in the Developing Entorhinal-Hippocampal Network

Correlated activity in the entorhinal–hippocampal neuronal networks, supported by oscillatory and intermittent population activity patterns is critical for learning and memory. However, when and how correlated activity emerges in these networks during development remains largely unknown. Here, we found that during the first postnatal week in non-anaesthetized head-restrained rats, activity in the superficial layers of the medial entorhinal cortex (MEC) and hippocampus was highly correlated, with intermittent population bursts in the MEC followed by early sharp waves (eSPWs) in the hippocampus. Neurons in the superficial MEC layers fired before neurons in the dentate gyrus, CA3 and CA1. eSPW current-source density profiles indicated that perforant/temporoammonic entorhinal inputs and intrinsic hippocampal connections are co-activated during entorhinal–hippocampal activity bursts. Finally, a majority of the entorhinal–hippocampal bursts were triggered by spontaneous myoclonic body movements, characteristic of the neonatal period. Thus, during the neonatal period, activity in the entorhinal cortex (EC) and hippocampus is highly synchronous, with the EC leading hippocampal activation. We propose that such correlated activity is embedded into a large-scale bottom-up circuit that processes somatosensory feedback resulting from neonatal movements, and that it is likely to instruct the development of connections between neocortex and hippocampus.

Direct Medial Entorhinal Cortex Input to Hippocampal CA3 Is Crucial for eEF2K Inhibitor-Induced Neuronal Oscillations in the Mouse Hippocampus

The hippocampal formation plays a vital role in memory formation and takes part in the control of the default neuronal network activity of the brain. It also represents an important structure to analyze drug-induced effects on subregion-specific synchronization of neuronal activity. However, the consequences of an altered functional state of synapses for subregion-specific synchronization of neuronal microcircuits remain to be fully understood. Therefore, we analyzed the direct interaction of neuronal microcircuits utilizing a genetically encoded calcium sensor (GCaMP6s) and local field potential (LFP) recording in acute hippocampal-entorhinal brain slices in response to a modulator of synaptic transmission. We observed that application of the eukaryotic elongation factor-2 kinase (eEF2K) inhibitor A484954, induced a large-scale synchronization of neuronal activity within different regions of the hippocampal formation. This effect was confirmed by the recording of extracellular LFPs. Further, in order to understand if the synchronized activity depended on interconnected hippocampal areas, we lesioned adjacent regions from each other. These experiments identified the origin of A484954-induced synchronized activity in the hippocampal CA3 subfield localized near the hilus of the dentate gyrus. Remarkably, the synchronization of neuronal activity in the hippocampus required an intact connection with the medial entorhinal cortex (MEC). In line with this observation, we detected an increase in neuronal activity in the MEC area after application of A484954. In summary, inhibition of eEF2K alters the intrinsic activity of interconnected neuronal microcircuits dominated by the MEC-CA3 afferents.

Most hippocampal CA1 pyramidal cells in rabbits increase firing during awake sharp-wave ripples and some do so in response to external stimulation and theta

Hippocampus forms neural representations of real-life events including multimodal information of spatial and temporal context. These representations, i.e., organized sequences of neuronal firing, are repeated during following rest and sleep, especially when so-called sharp-wave ripples (SPW-Rs) characterize hippocampal local field potentials. This SPW-R -related replay is thought to underlie memory consolidation. Here, we set out to explore how hippocampal CA1 pyramidal cells respond to the conditioned stimulus during trace eyeblink conditioning and how these responses manifest during SPW-Rs in awake adult female New Zealand White rabbits. Based on reports in rodents, we expected SPW-Rs to take place in bursts, possibly according to a slow endogenous rhythm. In awake rabbits, half of all SPW-Rs took place in bursts, but no endogenous slow rhythm appeared. Conditioning trials suppressed SPW-Rs while increasing theta for a period of several seconds. As expected based on previous findings, only a quarter of the putative CA1 pyramidal cells increased firing in response to the conditioned stimulus. Compared with other cells, rate-increasing cells were more active during spontaneous epochs of hippocampal theta while response profile during conditioning did not affect firing during SPW-Rs. Taken together, CA1 pyramidal cell firing during SPW-Rs is not limited to cells that fired during the preceding experience. Furthermore, the importance of possible reactivations taking place during theta epochs on memory consolidation warrants further investigation.NEW & NOTEWORTHY We studied hippocampal sharp-wave ripples and theta and CA1 pyramidal cell activity during trace eyeblink conditioning in rabbits. Conditioning trials suppressed ripples while increasing theta for a period of several seconds. A quarter of the cells increased firing in response to the conditioned stimulus and fired extensively during endogenous theta as well as ripples. The role of endogenous theta epochs in off-line memory consolidation should be studied further.

Heterosynaptic Plasticity Determines the Set Point for Cortical Excitatory-Inhibitory Balance

Excitation in neural circuits must be carefully controlled by inhibition to regulate information processing and network excitability. During development, cortical inhibitory and excitatory inputs are initially mismatched but become co-tuned or balanced with experience. However, little is known about how excitatory-inhibitory balance is defined at most synapses or about the mechanisms for establishing or maintaining this balance at specific set points. Here we show how coordinated long-term plasticity calibrates populations of excitatory-inhibitory inputs onto mouse auditory cortical pyramidal neurons. Pairing pre- and postsynaptic activity induced plasticity at paired inputs and different forms of heterosynaptic plasticity at the strongest unpaired synapses, which required minutes of activity and dendritic Ca²⁺ signaling to be computed. Theoretical analyses demonstrated how the relative rate of heterosynaptic plasticity could normalize and stabilize synaptic strengths to achieve any possible excitatory-inhibitory correlation. Thus, excitatory-inhibitory balance is dynamic and cell specific, determined by distinct plasticity rules across multiple excitatory and inhibitory synapses.

Long-Lasting Somatic Modifications of Convergent Dendritic Inputs in Hippocampal Neurons

Integrated neural inputs from different dendrites converge at the soma for action potential generation. However, it is unclear how the convergent dendritic inputs interact at the soma and whether they can be further modified there. We report here an entirely new plasticity rule in hippocampal neurons in which repetitive pairing of subthreshold excitatory inputs from proximal apical and basal dendrites at a precise interval induces persistent bidirectional modifications of the two dendritic inputs. Strikingly, the modification of the dendritic inputs specially occurs at soma in the absence of somatic action potential and requires activation of somatic N-methyl-D-aspartate receptors (NMDARs). Once induced, the somatic modification can also be observed in other unpaired dendritic inputs upon their arrival at the soma. We further reveal that the soma can employ an active mechanism to potentiate the dendritic inputs by promoting sustained activation of somatic NMDARs and subsequent down-regulating of the fast inactivating A-type potassium current (IA) at the soma. Thus, the input-timing-dependent somatic plasticity we uncovered here is in sharp contrast to conventional forms of synaptic plasticity that occur at the dendrites and is important to somatic action potential generation.

Synaptic organisation and behaviour-dependent activity of mGluR8a-innervated GABAergic trilaminar cells projecting from the hippocampus to the subiculum

In the hippocampal CA1 area, the GABAergic trilaminar cells have their axon distributed locally in three layers and also innervate the subiculum. Trilaminar cells have a high level of somato-dendritic muscarinic M2 acetylcholine receptor, lack somatostatin expression and their presynaptic inputs are enriched in mGluR8a. But the origin of their inputs and their behaviour-dependent activity remain to be characterised. Here we demonstrate that (1) GABAergic neurons with the molecular features of trilaminar cells are present in CA1 and CA3 in both rats and mice. (2) Trilaminar cells receive mGluR8a-enriched GABAergic inputs, e.g. from the medial septum, which are probably susceptible to hetero-synaptic modulation of neurotransmitter release by group III mGluRs. (3) An electron microscopic analysis identifies trilaminar cell output synapses with specialised postsynaptic densities and a strong bias towards interneurons as targets, including parvalbumin-expressing cells in the CA1 area. (4) Recordings in freely moving rats revealed the network state-dependent segregation of trilaminar cell activity, with reduced firing during movement, but substantial increase in activity with prolonged burst firing (> 200 Hz) during slow wave sleep. We predict that the behaviour-dependent temporal dynamics of trilaminar cell firing are regulated by their specialised inhibitory inputs. Trilaminar cells might support glutamatergic principal cells by disinhibition and mediate the binding of neuronal assemblies between the hippocampus and the subiculum via the transient inhibition of local interneurons.

Cholecystokinin-Expressing Interneurons of the Medial Prefrontal Cortex Mediate Working Memory Retrieval

Distinct components of working memory are coordinated by different classes of inhibitory interneurons in the PFC, but the role of cholecystokinin (CCK)-positive interneurons remains enigmatic. In humans, this major population of interneurons shows histological abnormalities in schizophrenia, an illness in which deficient working memory is a core defining symptom and the best predictor of long-term functional outcome. Yet, CCK interneurons as a molecularly distinct class have proved intractable to examination by typical molecular methods due to widespread expression of CCK in the pyramidal neuron population. Using an intersectional approach in mice of both sexes, we have succeeded in labeling, interrogating, and manipulating CCK interneurons in the mPFC. Here, we describe the anatomical distribution, electrophysiological properties, and postsynaptic connectivity of CCK interneurons, and evaluate their role in cognition. We found that CCK interneurons comprise a larger proportion of the mPFC interneurons compared with parvalbumin interneurons, targeting a wide range of neuronal subtypes with a distinct connectivity pattern. Phase-specific optogenetic inhibition revealed that CCK, but not parvalbumin, interneurons play a critical role in the retrieval of working memory. These findings shine new light on the relationship between cortical CCK interneurons and cognition and offer a new set of tools to investigate interneuron dysfunction and cognitive impairments associated with schizophrenia.SIGNIFICANCE STATEMENT Cholecystokinin-expressing interneurons outnumber other interneuron populations in key brain areas involved in cognition and memory, including the mPFC. However, they have proved intractable to examination as experimental techniques have lacked the necessary selectivity. To the best of our knowledge, the present study is the first to report detailed properties of cortical cholecystokinin interneurons, revealing their anatomical organization, electrophysiological properties, postsynaptic connectivity, and behavioral function in working memory.

Optogenetic activation of parvalbumin and somatostatin interneurons selectively restores theta-nested gamma oscillations and oscillation-induced spike timing-dependent long-term potentiation impaired by amyloid β oligomers

Background

Abnormal accumulation of amyloid β_1–42 oligomers (AβO_1–42), a hallmark of Alzheimer’s disease, impairs hippocampal theta-nested gamma oscillations and long-term potentiation (LTP) that are believed to underlie learning and memory. Parvalbumin-positive (PV) and somatostatin-positive (SST) interneurons are critically involved in theta-nested gamma oscillogenesis and LTP induction. However, how AβO_1–42 affects PV and SST interneuron circuits is unclear. Through optogenetic manipulation of PV and SST interneurons and computational modeling of the hippocampal neural circuits, we dissected the contributions of PV and SST interneuron circuit dysfunctions on AβO_1–42-induced impairments of hippocampal theta-nested gamma oscillations and oscillation-induced LTP.

Results

Targeted whole-cell patch-clamp recordings and optogenetic manipulations of PV and SST interneurons during in vivo-like, optogenetically induced theta-nested gamma oscillations in vitro revealed that AβO_1–42 causes synapse-specific dysfunction in PV and SST interneurons. AβO_1–42 selectively disrupted CA1 pyramidal cells (PC)-to-PV interneuron and PV-to-PC synapses to impair theta-nested gamma oscillogenesis. In contrast, while having no effect on PC-to-SST or SST-to-PC synapses, AβO_1–42 selectively disrupted SST interneuron-mediated disinhibition to CA1 PC to impair theta-nested gamma oscillation-induced spike timing-dependent LTP (tLTP). Such AβO_1–42-induced impairments of gamma oscillogenesis and oscillation-induced tLTP were fully restored by optogenetic activation of PV and SST interneurons, respectively, further supporting synapse-specific dysfunctions in PV and SST interneurons. Finally, computational modeling of hippocampal neural circuits including CA1 PC, PV, and SST interneurons confirmed the experimental observations and further revealed distinct functional roles of PV and SST interneurons in theta-nested gamma oscillations and tLTP induction.

Conclusions

Our results reveal that AβO_1–42 causes synapse-specific dysfunctions in PV and SST interneurons and that optogenetic modulations of these interneurons present potential therapeutic targets for restoring hippocampal network oscillations and synaptic plasticity impairments in Alzheimer’s disease.

Continual Learning in a Multi-Layer Network of an Electric Fish

Distributing learning across multiple layers has proven extremely powerful in artificial neural networks. However, little is known about how multi-layer learning is implemented in the brain. Here, we provide an account of learning across multiple processing layers in the electrosensory lobe (ELL) of mormyrid fish and report how it solves problems well known from machine learning. Because the ELL operates and learns continuously, it must reconcile learning and signaling functions without switching its mode of operation. We show that this is accomplished through a functional compartmentalization within intermediate layer neurons in which inputs driving learning differentially affect dendritic and axonal spikes. We also find that connectivity based on learning rather than sensory response selectivity assures that plasticity at synapses onto intermediate-layer neurons is matched to the requirements of output neurons. The mechanisms we uncover have relevance to learning in the cerebellum, hippocampus, and cerebral cortex, as well as in artificial systems.

Structured inhibitory activity dynamics in new virtual environments

Inhibition plays a powerful role in regulating network excitation and plasticity; however, the activity of defined interneuron types during spatial exploration remain poorly understood. Using two-photon calcium imaging, we recorded hippocampal CA1 somatostatin- and parvalbumin-expressing interneurons as mice performed a goal-directed spatial navigation task in new visual virtual reality (VR) contexts. Activity in both interneuron classes was strongly suppressed but recovered as animals learned to adapt the previously learned task to the new spatial context. Surprisingly, although there was a range of activity suppression across the population, individual somatostatin-expressing interneurons showed consistent levels of activity modulation across exposure to multiple novel environments, suggesting context-independent, stable network roles during spatial exploration. This work reveals population-level temporally dynamic interneuron activity in new environments, within which each interneuron shows stable and consistent activity modulation.

Degeneracy in hippocampal physiology and plasticity

Degeneracy, defined as the ability of structurally disparate elements to perform analogous function, has largely been assessed from the perspective of maintaining robustness of physiology or plasticity. How does the framework of degeneracy assimilate into an encoding system where the ability to change is an essential ingredient for storing new incoming information? Could degeneracy maintain the balance between the apparently contradictory goals of the need to change for encoding and the need to resist change towards maintaining homeostasis? In this review, we explore these fundamental questions with the mammalian hippocampus as an example encoding system. We systematically catalog lines of evidence, spanning multiple scales of analysis that point to the expression of degeneracy in hippocampal physiology and plasticity. We assess the potential of degeneracy as a framework to achieve the conjoint goals of encoding and homeostasis without cross-interferences. We postulate that biological complexity, involving interactions among the numerous parameters spanning different scales of analysis, could establish disparate routes towards accomplishing these conjoint goals. These disparate routes then provide several degrees of freedom to the encoding-homeostasis system in accomplishing its tasks in an input- and state-dependent manner. Finally, the expression of degeneracy spanning multiple scales offers an ideal reconciliation to several outstanding controversies, through the recognition that the seemingly contradictory disparate observations are merely alternate routes that the system might recruit towards accomplishment of its goals.

Regulation of Hippocampal Memory by mTORC1 in Somatostatin Interneurons

Translational control of long-term synaptic plasticity via Mechanistic Target Of Rapamycin Complex 1 (mTORC1) is crucial for hippocampal learning and memory. The role of mTORC1 is well characterized in excitatory principal cells but remains largely unaddressed in inhibitory interneurons. Here, we used cell-type-specific conditional knock-out strategies to alter mTORC1 function selectively in somatostatin (SOM) inhibitory interneurons (SOM-INs). We found that, in male mice, upregulation and downregulation of SOM-IN mTORC1 activity bidirectionally regulates contextual fear and spatial memory consolidation. Moreover, contextual fear learning induced a metabotropic glutamate receptor type 1 (mGluR1)-mediated late long-term potentiation (LTP) of excitatory input synapses onto hippocampal SOM-INs that was dependent on mTORC1. Finally, the induction protocol for mTORC1-mediated late-LTP in SOM-INs regulated Schaffer collateral pathway LTP in pyramidal neurons. Therefore, mTORC1 activity in somatostatin interneurons contributes to learning-induced persistent plasticity of their excitatory synaptic inputs and hippocampal memory consolidation, uncovering a role of mTORC1 in inhibitory circuits for memory.SIGNIFICANCE STATEMENT Memory consolidation necessitates synthesis of new proteins. Mechanistic Target Of Rapamycin Complex 1 (mTORC1) signaling is crucial for translational control involved in long-term memory and in late long-term potentiation (LTP). This is well described in principal glutamatergic pyramidal cells but poorly understood in GABAergic inhibitory interneurons. Here, we show that mTORC1 activity in somatostatin interneurons, a major subclass of GABAergic cells, is important to modulate long-term memory strength and precision. Furthermore, mTORC1 was necessary for learning-induced persistent LTP at excitatory inputs of somatostatin interneurons that depends on type I metabotropic glutamatergic receptors in the hippocampus. This effect was consistent with a newly described role of these interneurons in the modulation of LTP at Schaffer collateral synapses onto pyramidal cells.

Control of synaptic plasticity in deep cortical networks

Humans and many other animals have an enormous capacity to learn about sensory stimuli and to master new skills. However, many of the mechanisms that enable us to learn remain to be understood. One of the greatest challenges of systems neuroscience is to explain how synaptic connections change to support maximally adaptive behaviour. Here, we provide an overview of factors that determine the change in the strength of synapses, with a focus on synaptic plasticity in sensory cortices. We review the influence of neuromodulators and feedback connections in synaptic plasticity and suggest a specific framework in which these factors can interact to improve the functioning of the entire network.

Somatostatin-Expressing Interneurons Form Axonal Projections to the Contralateral Hippocampus

Conscious memories are critically dependent upon bilateral hippocampal formation, and interhemispheric commissural projections made by mossy cells and CA3 pyramidal cells. GABAergic interneurons also make long-range axonal projections, but little is known regarding their commissural, inter-hippocampal connections. We used retrograde and adeno-associated viral tracing, immunofluorescence and electron microscopy, and in vitro optogenetics to assess contralateral projections of neurochemically defined interneuron classes. We found that contralateral-projecting interneurons were 24-fold less common compared to hilar mossy cells, and mostly consisted of somatostatin- and parvalbumin-expressing types. Somatostatin-expressing cells made denser contralateral axonal projections than parvalbumin-expressing cells, although this was typically 10-fold less than the ipsilateral projection density. Somatostatin-expressing cells displayed a topographic-like innervation according to the location of their somata, whereas parvalbumin-expressing cells mostly innervated CA1. In the dentate gyrus molecular layer, commissural interneuron post-synaptic targets were predominantly putative granule cell apical dendrites. In the hilus, varicosities in close vicinity to various interneuron subtypes, as well as mossy cells, were observed, but most contralateral axon varicosities had no adjacent immunolabeled structure. Due to the relative sparsity of the connection and the likely distal dendritic location of their synapses, commissural projections made by interneurons were found to be weak. We postulate that these projections may become functionally active upon intense network activity during tasks requiring increased memory processing.

Allocentric representations of space in the hippocampus

The hippocampal-entorhinal system is essential for navigation and memory. The first description of spatially tuned place cell activity in area CA1 of the hippocampus suggested that spatial representations are not centered on self, but are rather allocentric. This idea is supported by extensive neurophysiological data, including temporally coordinated sequential activity during theta phase precession and sharp wave ripples. CA1 pyramidal neurons represent other information as well, such as objects, time, and events. Additionally, our recent research revealed that CA1 place cells jointly represent the spatial location of self and a conspecific, further supporting the idea of allocentric spatial representations by CA1 place cells. The neural mechanisms underlying CA1 spatial representations have long remained a mystery, but recent research examining circuit dynamics and synaptic plasticity suggests that the temporal relationships of inputs from entorhinal cortex layer III and CA3 could be critical for generating spatially tuned CA1 activity. Here, I review studies of the hippocampal representations of space and other features, and discuss the related networks and synaptic mechanisms supporting the representations of these features.

Cortical Circuits of Callosal GABAergic Neurons

Anatomical studies have shown that the majority of callosal axons are glutamatergic. However, a small proportion of callosal axons are also immunoreactive for glutamic acid decarboxylase, an enzyme required for gamma-aminobutyric acid (GABA) synthesis and a specific marker for GABAergic neurons. Here, we test the hypothesis that corticocortical parvalbumin-expressing (CC-Parv) neurons connect the two hemispheres of multiple cortical areas, project through the corpus callosum, and are a functional part of the local cortical circuit. Our investigation of this hypothesis takes advantage of viral tracing and optogenetics to determine the anatomical and electrophysiological properties of CC-Parv neurons of the mouse auditory, visual, and motor cortices. We found a direct inhibitory pathway made up of parvalbumin-expressing (Parv) neurons which connects corresponding cortical areas (CC-Parv neurons → contralateral cortex). Like other Parv cortical neurons, these neurons provide local inhibition onto nearby pyramidal neurons and receive thalamocortical input. These results demonstrate a previously unknown long-range inhibitory circuit arising from a genetically defined type of GABAergic neuron that is engaged in interhemispheric communication.

Brainstem nucleus incertus controls contextual memory formation

Hippocampal pyramidal cells encode memory engrams, which guide adaptive behavior. Selection of engram-forming cells is regulated by somatostatin-positive dendrite-targeting interneurons, which inhibit pyramidal cells that are not required for memory formation. Here, we found that γ-aminobutyric acid (GABA)-releasing neurons of the mouse nucleus incertus (NI) selectively inhibit somatostatin-positive interneurons in the hippocampus, both monosynaptically and indirectly through the inhibition of their subcortical excitatory inputs. We demonstrated that NI GABAergic neurons receive monosynaptic inputs from brain areas processing important environmental information, and their hippocampal projections are strongly activated by salient environmental inputs in vivo. Optogenetic manipulations of NI GABAergic neurons can shift hippocampal network state and bidirectionally modify the strength of contextual fear memory formation. Our results indicate that brainstem NI GABAergic cells are essential for controlling contextual memories.

Architects of neuronal wiring

Synapse formation is steered to neuronal subcompartments by molecular signals

Precise excitation-inhibition balance controls gain and timing in the hippocampus

Excitation-inhibition (EI) balance controls excitability, dynamic range, and input gating in many brain circuits. Subsets of synaptic input can be selected or 'gated' by precise modulation of finely tuned EI balance, but assessing the granularity of EI balance requires combinatorial analysis of excitatory and inhibitory inputs. Using patterned optogenetic stimulation of mouse hippocampal CA3 neurons, we show that hundreds of unique CA3 input combinations recruit excitation and inhibition with a nearly identical ratio, demonstrating precise EI balance at the hippocampus. Crucially, the delay between excitation and inhibition decreases as excitatory input increases from a few synapses to tens of synapses. This creates a dynamic millisecond-range window for postsynaptic excitation, controlling membrane depolarization amplitude and timing via subthreshold divisive normalization. We suggest that this combination of precise EI balance and dynamic EI delays forms a general mechanism for millisecond-range input gating and subthreshold gain control in feedforward networks.

The Role of AMPARs in the Maturation and Integration of Caudal Ganglionic Eminence-Derived Interneurons into Developing Hippocampal Microcircuits

In the hippocampal CA1, caudal ganglionic eminence (CGE)-derived interneurons are recruited by activation of glutamatergic synapses comprising GluA2-containing calcium-impermeable AMPARs and exert inhibitory regulation of the local microcircuit. However, the role played by AMPARs in maturation of the developing circuit is unknown. We demonstrate that elimination of the GluA2 subunit (GluA2 KO) of AMPARs in CGE-derived interneurons, reduces spontaneous EPSC frequency coupled to a reduction in dendritic glutamatergic synapse density. Removal of GluA1&2&3 subunits (GluA1-3 KO) in CGE-derived interneurons, almost completely eliminated sEPSCs without further reducing synapse density, but increased dendritic branching. Moreover, in GluA1-3 KOs, the number of interneurons invading the hippocampus increased in the early postnatal period but converged with WT numbers later due to increased apoptosis. However, the CCK-containing subgroup increased in number, whereas the VIP-containing subgroup decreased. Both feedforward and feedback inhibitory input onto pyramidal neurons was decreased in GluA1-3 KO. These combined anatomical, synaptic and circuit alterations, were accompanied with a wide range of behavioural abnormalities in GluA1-3 KO mice compared to GluA2 KO and WT. Thus, AMPAR subunits differentially contribute to numerous aspects of the development and maturation of CGE-derived interneurons and hippocampal circuitry that are essential for normal behaviour.

Resolving the Micro-Macro Disconnect to Address Core Features of Seizure Networks

Current drug treatments for epilepsy attempt to broadly restrict excitability to mask a symptom, seizures, with little regard for the heterogeneous mechanisms that underlie disease manifestation across individuals. Here, we discuss the need for a more complete view of epilepsy, outlining how key features at the cellular and microcircuit level can significantly impact disease mechanisms that are not captured by the most common methodology to study epilepsy, electroencephalography (EEG). We highlight how major advances in neuroscience tool development now enable multi-scale investigation of fundamental questions to resolve the currently controversial understanding of seizure networks. These findings will provide essential insight into what has emerged as a disconnect between the different levels of investigation and identify new targets and treatment options.

Cholecystokinin release triggered by NMDA receptors produces LTP and sound-sound associative memory

Memory is stored in neural networks via changes in synaptic strength mediated in part by NMDA receptor (NMDAR)-dependent long-term potentiation (LTP). Here we show that a cholecystokinin (CCK)-B receptor (CCKBR) antagonist blocks high-frequency stimulation-induced neocortical LTP, whereas local infusion of CCK induces LTP. CCK^-/- mice lacked neocortical LTP and showed deficits in a cue-cue associative learning paradigm; and administration of CCK rescued associative learning deficits. High-frequency stimulation-induced neocortical LTP was completely blocked by either the NMDAR antagonist or the CCKBR antagonist, while application of either NMDA or CCK induced LTP after low-frequency stimulation. In the presence of CCK, LTP was still induced even after blockade of NMDARs. Local application of NMDA induced the release of CCK in the neocortex. These findings suggest that NMDARs control the release of CCK, which enables neocortical LTP and the formation of cue-cue associative memory.

Neuronal firing rates diverge during REM and homogenize during non-REM

Neurons fire at highly variable intrinsic rates and recent evidence suggests that low- and high-firing rate neurons display different plasticity and dynamics. Furthermore, recent publications imply possibly differing rate-dependent effects in hippocampus versus neocortex, but those analyses were carried out separately and with potentially important differences. To more effectively synthesize these questions, we analyzed the firing rate dynamics of populations of neurons in both hippocampal CA1 and frontal cortex under one framework that avoids the pitfalls of previous analyses and accounts for regression to the mean (RTM). We observed several consistent effects across these regions. While rapid eye movement (REM) sleep was marked by decreased hippocampal firing and increased neocortical firing, in both regions firing rate distributions widened during REM due to differential changes in high- versus low-firing rate cells in parallel with increased interneuron activity. In contrast, upon non-REM (NREM) sleep, firing rate distributions narrowed while interneuron firing decreased. Interestingly, hippocampal interneuron activity closely followed the patterns observed in neocortical principal cells rather than the hippocampal principal cells, suggestive of long-range interactions. Following these undulations in variance, the net effect of sleep was a decrease in firing rates. These decreases were greater in lower-firing hippocampal neurons but also higher-firing frontal cortical neurons, suggestive of greater plasticity in these cell groups. Our results across two different regions, and with statistical corrections, indicate that the hippocampus and neocortex show a mixture of differences and similarities as they cycle between sleep states with a unifying characteristic of homogenization of firing during NREM and diversification during REM.

Temperoammonic Stimulation Depotentiates Schaffer Collateral LTP via p38 MAPK Downstream of Adenosine A1 Receptors

We previously found that low-frequency stimulation of direct temperoammonic (TA) inputs to hippocampal area CA1 depotentiates previously established long-term potentiation in the Schaffer collateral (SC) pathway through complex signaling involving dopamine, endocannabinoids, neuregulin-1, GABA, and adenosine, with adenosine being the most distal modulator identified to date. In the present studies, we examined mechanisms contributing to the effects of adenosine in hippocampal slices from male albino rats. We found that extracellular conversion of ATP to adenosine via an ectonucleotidase contributes significantly to TA-mediated SC depotentiation and the depotentiation resulting from block of adenosine transport. Adenosine-mediated SC depotentiation does not involve activation of c-Jun N-terminal protein kinase, serine phosphatases, or nitric oxide synthase, unlike homosynaptic SC depotentiation. Rather, adenosine-induced depotentiation is inhibited by specific antagonists of p38 MAPK, but not by a structural analog that does not inhibit p38. Additionally, using antagonists with relative selectivity for p38 subtypes, it appears that TA-induced SC depotentiation most likely involves p38 MAPK β. These findings have implications for understanding the role of adenosine and other extrahippocampal and intrahippocampal modulators in regulating SC synaptic function and the contributions of these modulators to the cognitive dysfunction associated with neuropsychiatric illnesses.SIGNIFICANCE STATEMENT Low-frequency stimulation of temperoammonic (TA) inputs to stratum lacunosum moleculare of hippocampal area CA1 heterosynaptically depotentiates long-term potentiation of Schaffer collateral (SC) synapses. TA-induced SC depotentiation involves complex signaling including dopamine, endocannabinoids, GABA, and adenosine, with adenosine serving as the most downstream messenger in the cascade identified to date. The present results indicate that TA-induced depotentiation requires intact inputs from entorhinal cortex and that adenosine ultimately drives depotentiation via activation of p38 MAPK. These studies have implications for understanding the cognitive dysfunction of psychiatric illnesses and certain abused drugs.

Is plasticity of synapses the mechanism of long-term memory storage?

It has been 70 years since Donald Hebb published his formalized theory of synaptic adaptation during learning. Hebb's seminal work foreshadowed some of the great neuroscientific discoveries of the following decades, including the discovery of long-term potentiation and other lasting forms of synaptic plasticity, and more recently the residence of memories in synaptically connected neuronal assemblies. Our understanding of the processes underlying learning and memory has been dominated by the view that synapses are the principal site of information storage in the brain. This view has received substantial support from research in several model systems, with the vast majority of studies on the topic corroborating a role for synapses in memory storage. Yet, despite the neuroscience community's best efforts, we are still without conclusive proof that memories reside at synapses. Furthermore, an increasing number of non-synaptic mechanisms have emerged that are also capable of acting as memory substrates. In this review, we address the key findings from the synaptic plasticity literature that make these phenomena such attractive memory mechanisms. We then turn our attention to evidence that questions the reliance of memory exclusively on changes at the synapse and attempt to integrate these opposing views.

Selective Activation of Cholecystokinin-Expressing GABA (CCK-GABA) Neurons Enhances Memory and Cognition

Cholecystokinin-expressing GABAergic (CCK-GABA) neurons are perisomatic inhibitory cells that have been argued to regulate emotion and sculpt the network oscillations associated with cognition. However, no study has selectively manipulated CCK-GABA neuron activity during behavior in freely-moving animals. To explore the behavioral effects of activating CCK-GABA neurons on emotion and cognition, we utilized a novel intersectional genetic mouse model coupled with a chemogenetic approach. Specifically, we generated triple transgenic CCK-Cre;Dlx5/6-Flpe;RC::FL-hM3Dq (CCK-GABA/hM3Dq) mice that expressed the synthetic excitatory hM3Dq receptor in CCK-GABA neurons. Results showed that clozapine-N-oxide (CNO)-mediated activation of CCK-GABA neurons did not alter open field (OF) or tail suspension (TS) performance and only slightly increased anxiety in the elevated plus maze (EPM). Although CNO treatment had only modestly affected emotional behavior, it significantly enhanced multiple cognitive and memory behaviors including social recognition, contextual fear conditioning, contextual discrimination, object recognition, and problem-solving in the puzzle box. Collectively, these findings suggest that systemic activation of CCK-GABA neurons minimally affects emotion but significantly enhances cognition and memory. Our results imply that CCK-GABA neurons are more functionally diverse than originally expected and could serve as a potential therapeutic target for the treatment of cognitive/memory disorders.

Adeno-associated viral overexpression of neuroligin 2 in the mouse hippocampus enhances GABAergic synapses and impairs hippocampal-dependent behaviors

The cell adhesion molecule neuroligin2 (NLGN2) regulates GABAergic synapse development, but its role in neural circuit function in the adult hippocampus is unclear. We investigated GABAergic synapses and hippocampus-dependent behaviors following viral-vector-mediated overexpression of NLGN2. Transducing hippocampal neurons with AAV-NLGN2 increased neuronal expression of NLGN2 and membrane localization of GABAergic postsynaptic proteins gephyrin and GABA_ARγ2, and presynaptic vesicular GABA transporter protein (VGAT) suggesting trans-synaptic enhancement of GABAergic synapses. In contrast, glutamatergic postsynaptic density protein-95 (PSD-95) and presynaptic vesicular glutamate transporter (VGLUT) protein were unaltered. Moreover, AAV-NLGN2 significantly increased parvalbumin immunoreactive (PV⁺) synaptic boutons co-localized with postsynaptic gephyrin⁺ puncta. Furthermore, these changes were demonstrated to lead to cognitive impairments as shown in a battery of hippocampal-dependent mnemonic tasks and social behaviors.

A Dendritic Substrate for the Cholinergic Control of Neocortical Output Neurons

The ascending cholinergic system dynamically regulates sensory perception and cognitive function, but it remains unclear how this modulation is executed in neocortical circuits. Here, we demonstrate that the cholinergic system controls the integrative operations of neocortical principal neurons by modulating dendritic excitability. Direct dendritic recordings revealed that the optogenetic-evoked release of acetylcholine (ACh) transformed the pattern of dendritic integration in layer 5B pyramidal neurons, leading to the generation of dendritic plateau potentials which powerfully drove repetitive action potential output. In contrast, the synaptic release of ACh did not positively modulate axo-somatic excitability. Mechanistically, the transformation of dendritic integration was mediated by the muscarinic ACh receptor-dependent enhancement of dendritic R-type calcium channel activity, a compartment-dependent modulation which decisively controlled the associative computations executed by layer 5B pyramidal neurons. Our findings therefore reveal a biophysical mechanism by which the cholinergic system controls dendritic computations causally linked to perceptual detection.

Subregion-specific Protective Effects of Fluoxetine and Clozapine on Parvalbumin Expression in Medial Prefrontal Cortex of Chronically Isolated Rats

Dysregulation of GABAergic system is becoming increasingly associated with depression, psychiatric disorder that imposes severe clinical, social and economic burden. Special attention is paid to the fast-spiking parvalbumin-positive (PV+) interneurons, GABAergic neurons which are highly susceptible to redox dysregulation and oxidative stress and implicated in a variety of psychiatric diseases. Here we analyzed the number of PV+ and cleaved caspase-3-positive (CC3+) cells in the rat medial prefrontal cortical (mPFC) subregions following chronic social isolation (CSIS), an animal model of depression and schizophrenia. Also, we examined potential protective effects of antidepressant fluoxetine (FLX) and atypical antipsychotic clozapine (CLZ) on the number of these cells in mPFC subregions, when applied parallel with CSIS in doses that correspond to therapeutically effective ones in patients. Immunofluorescence analysis revealed decreased number of PV+ cells in cingulate cortex area 1, prelimbic area (PrL), infralimbic area (IL) and dorsal peduncular cortex of the mPFC in isolated rats, which coincided with depressive- and anxiety-like behaviors. In addition, CSIS-induced increase in the number of CC3+ cells was detected in aforementioned subregions of mPFC. Treatments with either FLX or CLZ prevented behavioral changes, decrease in PV+ and increase in CC3+ cell numbers in PrL and IL subregions in isolated rats. These results indicate the importance of intact GABAergic signaling in these areas for resistance against CSIS-induced behavioral changes, as well as subregion-specific protective effects of FLX and CLZ in mPFC of CSIS rats.

Chemogenetic Tools for Causal Cellular and Neuronal Biology

Chemogenetic technologies enable selective pharmacological control of specific cell populations. An increasing number of approaches have been developed that modulate different signaling pathways. Selective pharmacological control over G protein-coupled receptor signaling, ion channel conductances, protein association, protein stability, and small molecule targeting allows modulation of cellular processes in distinct cell types. Here, we review these chemogenetic technologies and instances of their applications in complex tissues in vivo and ex vivo.

Negative effects of interictal spikes on theta rhythm in human temporal lobe epilepsy

Interictal spike is a biomarker of epilepsy that can occur frequently between seizures. Its potential effects on brain oscillations, especially on theta rhythm (4-8 Hz) that is related to a variety of cognitive processes, remain controversial. Using local field potentials recorded from patients with temporal lobe epilepsy (TLE), we investigated here the impact of spikes on theta rhythm immediately after spikes and during the prolonged periods (lasting 4-36 s) between adjacent spikes. Local field potentials (LFPs) were recorded in different epileptogenic areas including the anterior hippocampus (aH) and the entorhinal cortex (EC) as well as in the extended propagation pathway. We found that interictal spikes had a significant inhibitory effect on theta rhythm. Power of theta rhythm was reduced immediately after spikes, and the inhibitory effect on theta rhythm might sustain during the prolonged between-spike periods. The inhibitory effect was more severe when the epileptogenic areas involved both the aH and EC compared to that involved only a single structure. These observations suggest that interictal spikes have a significant negative impact on theta rhythm and may thus play a role in theta-related cognition changes in patients with TLE.

Early Appearance and Spread of Fast Ripples in the Hippocampus in a Model of Cortical Traumatic Brain Injury

Fast ripples (FRs; activity of >250 Hz) have been considered as a biomarker of epileptic activity in the hippocampus and entorhinal cortex; it is thought that they signal the focus of seizure generation. Similar high-frequency network activity has been produced in vitro by changing extracellular medium composition, by using pro-epileptic substances, or by electrical stimulation. Here we study the propagation of these events between different subregions of the male rat hippocampus in a recently introduced experimental model of FRs in entorhinal cortex-hippocampal slices in vitro By using a matrix of 4096 microelectrodes, the sites of initiation, propagation pathways, and spatiotemporal characteristics of activity patterns could be studied with unprecedented high resolution. To this end, we developed an analytic tool based on bidimensional current source density estimation, which delimits sinks and sources with a high precision and evaluates their trajectories using the concept of center of mass. With this methodology, we found that FRs can arise almost simultaneously at noncontiguous sites in the CA3-to-CA1 direction, underlying the spatial heterogeneity of epileptogenic foci, while continuous somatodendritic waves of activity develop. An unexpected, yet important propagation route is the propagation of activity from CA3 into the hilus and dentate gyrus. This pathway may cause reverberating activation of both regions, supporting sustained pathological network events and altered information processing in hippocampal networks.SIGNIFICANCE STATEMENT Fast ripples (FRs) have been considered as a biomarker of epileptic activity and may signal the focus of seizure generation. In a model of traumatic brain injury in the rat, FRs appear in the hippocampus within a couple of hours after an extrahippocampal, cortical lesion. We analyzed the origin and dynamics of the FRs in the hippocampus using massive electrophysiological recordings, allowing an unprecedented high spatiotemporal resolution. We show that FRs originate in distinct and noncontiguous locations within the CA3 region and uncover, with high precision, the extent and dynamics of their current density. This activity propagates toward CA1 but also backpropagates to the hilus and the dentate gyrus, suggesting activation of defined microcircuits that can sustain recurrent excitation.

Specificity, Versatility, and Continual Development: The Power of Optogenetics for Epilepsy Research

Optogenetics is a powerful and rapidly expanding set of techniques that use genetically encoded light sensitive proteins such as opsins. Through the selective expression of these exogenous light-sensitive proteins, researchers gain the ability to modulate neuronal activity, intracellular signaling pathways, or gene expression with spatial, directional, temporal, and cell-type specificity. Optogenetics provides a versatile toolbox and has significantly advanced a variety of neuroscience fields. In this review, using recent epilepsy research as a focal point, we highlight how the specificity, versatility, and continual development of new optogenetic related tools advances our understanding of neuronal circuits and neurological disorders. We additionally provide a brief overview of some currently available optogenetic tools including for the selective expression of opsins.

Molecular Mechanisms of Synaptic Specificity: Spotlight on Hippocampal and Cerebellar Synapse Organizers

Synapses and neural circuits form with exquisite specificity during brain development to allow the precise and appropriate flow of neural information. Although this property of synapses and neural circuits has been extensively investigated for more than a century, molecular mechanisms underlying this property are only recently being unveiled. Recent studies highlight several classes of cell-surface proteins as organizing hubs in building structural and functional architectures of specific synapses and neural circuits. In the present mini-review, we discuss recent findings on various synapse organizers that confer the distinct properties of specific synapse types and neural circuit architectures in mammalian brains, with a particular focus on the hippocampus and cerebellum.

Generalization of Conditioned Auditory Fear is Regulated by Maternal Effects on Ventral Hippocampal Synaptic Plasticity

Maternal care shapes individual differences in fear-associated neural circuitry. In rats, maternal licking and grooming (LG) in early life regulates ventral hippocampal (VH) function and plasticity in adulthood, but its consequent effect on the regulation of fear memories remains unknown. We report an effect of maternal care on generalization of learned fear, such that offspring of high LG mothers express generalized fear responses when confronted with neutral stimuli following auditory fear conditioning. These animals simultaneously display a reduction in the magnitude of VH long-term potentiation (LTP) expressed and reduced input-output transformation in Schaffer collateral synapses. Inhibition of VH-LTP during learning specifically increases fear generalization in offspring of low LG mothers during recall, suggesting a role for VH synaptic plasticity in the specification of fear memories. These findings suggest that rearing by low LG dams enhances the efficacy of fear-related neural systems to support accurate encoding of fear memories through effects on the VH.

Cognitive benefits of lithium chloride in APP/PS1 mice are associated with enhanced brain clearance of β-amyloid

Epidemiological evidence suggests that people with bipolar disorder prescribed lithium exhibit a lower risk of Alzheimer's disease (AD) relative to those prescribed other mood-stabilizing medicines. Lithium chloride (LiCl) reduces brain β-amyloid (Aβ) levels, and the brain clearance of Aβ is reduced in AD. Therefore, the purpose of this study was to assess whether the cognitive benefits of LiCl are associated with enhanced brain clearance of exogenously-administered Aβ. The brain clearance of intracerebroventricularly (icv) administered ¹²⁵I-Aβ₄₂ was assessed in male Swiss outbred mice administered daily oral NaCl or LiCl (300 mg/kg for 21 days). LiCl exhibited a 31% increase in the brain clearance of ¹²⁵I-Aβ₄₂ over 10 min, which was associated with a 1.6-fold increase in brain microvascular expression of the blood-brain barrier efflux transporter low density lipoprotein receptor-related protein 1 (LRP1) and increased cerebrospinal fluid (CSF) bulk-flow. 8-month-old female wild type (WT) and APP/PS1 mice were also administered daily NaCl or LiCl for 21 days, which was followed by cognitive assessment by novel object recognition and water maze, and measurement of soluble Aβ₄₂, plaque-associated Aβ₄₂, and brain efflux of ¹²⁵I-Aβ₄₂. LiCl treatment restored the long-term spatial memory deficit observed in APP/PS1 mice as assessed by the water maze (back to similar levels of escape latency as WT mice), but the short-term memory deficit remained unaffected by LiCl treatment. While LiCl did not affect plaque-associated Aβ₄₂, soluble Aβ₄₂ levels were reduced by 49.9% in APP/PS1 mice receiving LiCl. The brain clearance of ¹²⁵I-Aβ₄₂ decreased by 27.8% in APP/PS1 mice, relative to WT mice, however, LiCl treatment restored brain ¹²⁵I-Aβ₄₂ clearance in APP/PS1 mice to a rate similar to that observed in WT mice. These findings suggest that the cognitive benefits and brain Aβ₄₂ lowering effects of LiCl are associated with enhanced brain clearance of Aβ₄₂, possibly via brain microvascular LRP1 upregulation and increased CSF bulk-flow, identifying a novel mechanism of protection by LiCl for the treatment of AD.

Extended Interneuronal Network of the Dentate Gyrus

Local interneurons control principal cells within individual brain areas, but anecdotal observations indicate that interneuronal axons sometimes extend beyond strict anatomical boundaries. Here, we use the case of the dentate gyrus (DG) to show that boundary-crossing interneurons with cell bodies in CA3 and CA1 constitute a numerically significant and diverse population that relays patterns of activity generated within the CA regions back to granule cells. These results reveal the existence of a sophisticated retrograde GABAergic circuit that fundamentally extends the canonical interneuronal network.

Layer 5 Callosal Parvalbumin-Expressing Neurons: A Distinct Functional Group of GABAergic Neurons

Previous studies have shown that parvalbumin-expressing neurons (CC-Parv neurons) connect the two hemispheres of motor and sensory areas via the corpus callosum, and are a functional part of the cortical circuit. Here we test the hypothesis that layer 5 CC-Parv neurons possess anatomical and molecular mechanisms which dampen excitability and modulate the gating of interhemispheric inhibition. In order to investigate this hypothesis we use viral tracing to determine the anatomical and electrophysiological properties of layer 5 CC-Parv and parvalbumin-expressing (Parv) neurons of the mouse auditory cortex (AC). Here we show that layer 5 CC-Parv neurons had larger dendritic fields characterized by longer dendrites that branched farther from the soma, whereas layer 5 Parv neurons had smaller dendritic fields characterized by shorter dendrites that branched nearer to the soma. The layer 5 CC-Parv neurons are characterized by delayed action potential (AP) responses to threshold currents, lower firing rates, and lower instantaneous frequencies compared to the layer 5 Parv neurons. Kv1.1 containing K⁺ channels are the main source of the AP repolarization of the layer 5 CC-Parv and have a major role in determining both the spike delayed response, firing rate and instantaneous frequency of these neurons.

Functional implications of inhibitory synapse placement on signal processing in pyramidal neuron dendrites

A rich literature describes inhibitory innervation of pyramidal neurons in terms of the distinct inhibitory cell types that target the soma, axon initial segment, or dendritic arbor. Less attention has been devoted to how localization of inhibition to specific parts of the pyramidal dendritic arbor influences dendritic signal detection and integration. The effect of inhibitory inputs can vary based on their placement on dendritic spines versus shaft, their distance from the soma, and the branch order of the dendrite they inhabit. Inhibitory synapses are also structurally dynamic, and the implications of these dynamics depend on their dendritic location. Here we consider the heterogeneous roles of inhibitory synapses as defined by their strategic placement on the pyramidal cell dendritic arbor.

Entorhinal fast-spiking speed cells project to the hippocampus

Our location in space is represented by a spectrum of space and direction-responsive cell types in medial entorhinal cortex and hippocampus. Many cells in these areas respond also to running speed. The presence of local speed-tuned cells is considered a requirement for position to be encoded in a self-motion–dependent manner; however, whether and how speed-responsive cells in entorhinal cortex and hippocampus are functionally connected have not been determined. The present study shows that a large proportion of entorhinal speed cells are fast-spiking with properties similar to those of GABAergic interneurons and that outputs from a subset of these cells, particularly the parvalbumin-expressing subset, form a component of the medial entorhinal input to the hippocampus.

The mammalian positioning system contains a variety of functionally specialized cells in the medial entorhinal cortex (MEC) and the hippocampus. In order for cells in these systems to dynamically update representations in a way that reflects ongoing movement in the environment, they must be able to read out the current speed of the animal. Speed is encoded by speed-responsive cells in both MEC and hippocampus, but the relationship between the two populations has not been determined. We show here that many entorhinal speed cells are fast-spiking putative GABAergic neurons. Using retrograde viral labeling from the hippocampus, we find that a subset of these fast-spiking MEC speed cells project directly to hippocampal areas. This projection contains parvalbumin (PV) but not somatostatin (SOM)-immunopositive cells. The data point to PV-expressing GABAergic projection neurons in MEC as a source for widespread speed modulation and temporal synchronization in entorhinal–hippocampal circuits for place representation.

Control of recollection by slow gamma dominating mid-frequency gamma in hippocampus CA1

Behavior is used to assess memory and cognitive deficits in animals like Fmr1-null mice that model Fragile X Syndrome, but behavior is a proxy for unknown neural events that define cognitive variables like recollection. We identified an electrophysiological signature of recollection in mouse dorsal Cornu Ammonis 1 (CA1) hippocampus. During a shocked-place avoidance task, slow gamma (SG) (30–50 Hz) dominates mid-frequency gamma (MG) (70–90 Hz) oscillations 2–3 s before successful avoidance, but not failures. Wild-type (WT) but not Fmr1-null mice rapidly adapt to relocating the shock; concurrently, SG/MG maxima (SG_dom) decrease in WT but not in cognitively inflexible Fmr1-null mice. During SG_dom, putative pyramidal cell ensembles represent distant locations; during place avoidance, these are avoided places. During shock relocation, WT ensembles represent distant locations near the currently correct shock zone, but Fmr1-null ensembles represent the formerly correct zone. These findings indicate that recollection occurs when CA1 SG dominates MG and that accurate recollection of inappropriate memories explains Fmr1-null cognitive inflexibility.

Behavior is often used as proxy to study memory and cognitive deficits in animals like Fmr1-KO mice that model Fragile X Syndrome, the most prevalent single-gene cause of intellectual disability and autism. However, it is unclear what neural events define cognitive variables like recollection of memory and cognitive inflexibility. We identified a signature of recollection in the local field potentials of mouse dorsal CA1 hippocampus. When mice on a rotating platform avoided an invisible, fixed shock zone, slow gamma (30–50 Hz) oscillations dominated mid-frequency gamma (70–90 Hz) oscillations (SG_dom) 2–3 s before mice successfully avoided the shock zone. Wild-type but not Fmr1-KO mice adapt to relocating the shock zone; concurrently, SG_dom decreases in wild-type but not in cognitively inflexible Fmr1-KO mice. During SG_dom, principal cell ensembles represent distant locations; during place avoidance, these are avoided places in the shock zone vicinity. During shock relocation, wild-type ensembles encode distant locations near the currently correct shock zone, but Fmr1-KO ensembles manifest representational inflexibility, encoding the formerly correct zone. These findings suggest evidence for competition amongst CA1 inputs for CA1 information-processing modes and indicate that recollection occurs when CA1 slow gamma dominates mid-frequency gamma and that accurate recollection of inappropriate memories explains Fmr1-KO cognitive inflexibility.

Studies of cortical connectivity using optical circuit mapping methods

An important consideration when probing the function of any neuron is to uncover the source of synaptic input onto the cell, its intrinsic physiology and efferent targets. Over the years, electrophysiological approaches have generated considerable insight into these properties in a variety of cortical neuronal subtypes and circuits. However, as researchers explore neuronal function in greater detail, they are increasingly turning to optical techniques to bridge the gap between local network interactions and behaviour. The application of optical methods has increased dramatically over the past decade, spurred on by the optogenetic revolution. In this review, we provide an account of recent innovations, providing researchers with a primer detailing circuit mapping strategies in the cerebral cortex. We will focus on technical aspects of performing neurotransmitter uncaging and channelrhodopsin-assisted circuit mapping, with the aim of identifying common pitfalls that can negatively influence the collection of reliable data.

Towards a circuit-level understanding of hippocampal CA1 dysfunction in Alzheimer's disease across anatomical axes

The hippocampus has been a primary region of study with regards to synaptic and functional changes in Alzheimer’s disease (AD) due to its involvement in early stages, specifically area CA1. However, most work in this area has treated CA1 as a homogeneous structure comprised of uniform neural circuits. Yet, there is a plethora of evidence that CA1 varies in its structure and function across anatomical axes. Here I review the heterogeneity of the functional and circuit architecture of hippocampal area CA1 across three primary anatomical axes. I also summarize evidence that AD differentially affects these subregions, as well as hypotheses as to why this may occur.

Medial and Lateral Entorhinal Cortex Differentially Excite Deep versus Superficial CA1 Pyramidal Neurons

Although hippocampal CA1 pyramidal neurons (PNs) were thought to comprise a uniform population, recent evidence supports two distinct sublayers along the radial axis, with deep neurons more likely to form place cells than superficial neurons. CA1 PNs also differ along the transverse axis with regard to direct inputs from entorhinal cortex (EC), with medial EC (MEC) providing spatial information to PNs toward CA2 (proximal CA1) and lateral EC (LEC) providing non-spatial information to PNs toward subiculum (distal CA1). We demonstrate that the two inputs differentially activate the radial sublayers and that this difference reverses along the transverse axis, with MEC preferentially targeting deep PNs in proximal CA1 and LEC preferentially exciting superficial PNs in distal CA1. This differential excitation reflects differences in dendritic spine numbers. Our results reveal a heterogeneity in EC-CA1 connectivity that may help explain differential roles of CA1 PNs in spatial and non-spatial learning and memory.