Physiology and pharmacology of unitary synaptic connections between pairs of cells in areas CA3 and CA1 of rat hippocampal slice cultures

Differences in the consolidation by spontaneous and evoked ripples in the presence of active dendrites

Ripples are a typical form of neural activity in hippocampal neural networks associated with the replay of episodic memories during sleep as well as sleep-related plasticity and memory consolidation. The emergence of ripples has been observed both dependent as well as independent of input from other brain areas and often coincides with dendritic spikes. Yet, it is unclear how input-evoked and spontaneous ripples as well as dendritic excitability affect plasticity and consolidation. Here, we use mathematical modeling to compare these cases. We find that consolidation as well as the emergence of spontaneous ripples depends on a reliable propagation of activity in feed-forward structures which constitute memory representations. This propagation is facilitated by excitable dendrites, which entail that a few strong synapses are sufficient to trigger neuronal firing. In this situation, stimulation-evoked ripples lead to the potentiation of weak synapses within the feed-forward structure and, thus, to a consolidation of a more general sequence memory. However, spontaneous ripples that occur without stimulation, only consolidate a sparse backbone of the existing strong feed-forward structure. Based on this, we test a recently hypothesized scenario in which the excitability of dendrites is transiently enhanced after learning, and show that such a transient increase can strengthen, restructure and consolidate even weak hippocampal memories, which would be forgotten otherwise. Hence, a transient increase in dendritic excitability would indeed provide a mechanism for stabilizing memories.

Structure and function of the hippocampal CA3 module

The hippocampal formation is crucial for learning and memory, with submodule CA3 thought to be the substrate of pattern completion. However, the underlying synaptic and computational mechanisms of this network are not well understood. Here, we perform circuit reconstruction of a CA3 module using three dimensional (3D) electron microscopy data and combine this with functional connectivity recordings and computational simulations to determine possible CA3 network mechanisms. Direct measurements of connectivity schemes with both physiological measurements and structural 3D EM revealed a high connectivity rate, multi-fold higher than previously assumed. Mathematical modelling indicated that such CA3 networks can robustly generate pattern completion and replay memory sequences. In conclusion, our data demonstrate that the connectivity scheme of the hippocampal submodule is well suited for efficient memory storage and retrieval.

Dorsal hippocampus to nucleus accumbens projections drive reinforcement via activation of accumbal dynorphin neurons

The hippocampus is pivotal in integrating emotional processing, learning, memory, and reward-related behaviors. The dorsal hippocampus (dHPC) is particularly crucial for episodic, spatial, and associative memory, and has been shown to be necessary for context- and cue-associated reward behaviors. The nucleus accumbens (NAc), a central structure in the mesolimbic reward pathway, integrates the salience of aversive and rewarding stimuli. Despite extensive research on dHPC→NAc direct projections, their sufficiency in driving reinforcement and reward-related behavior remains to be determined. Our study establishes that activating excitatory neurons in the dHPC is sufficient to induce reinforcing behaviors through its direct projections to the dorso-medial subregion of the NAc shell (dmNAcSh). Notably, dynorphin-containing neurons specifically contribute to dHPC-driven reinforcing behavior, even though both dmNAcSh dynorphin- and enkephalin-containing neurons are activated with dHPC stimulation. Our findings unveil a pathway governing reinforcement, advancing our understanding of the hippocampal circuity's role in reward-seeking behaviors.

Dense 4D nanoscale reconstruction of living brain tissue

Three-dimensional (3D) reconstruction of living brain tissue down to an individual synapse level would create opportunities for decoding the dynamics and structure-function relationships of the brain's complex and dense information processing network; however, this has been hindered by insufficient 3D resolution, inadequate signal-to-noise ratio and prohibitive light burden in optical imaging, whereas electron microscopy is inherently static. Here we solved these challenges by developing an integrated optical/machine-learning technology, LIONESS (live information-optimized nanoscopy enabling saturated segmentation). This leverages optical modifications to stimulated emission depletion microscopy in comprehensively, extracellularly labeled tissue and previous information on sample structure via machine learning to simultaneously achieve isotropic super-resolution, high signal-to-noise ratio and compatibility with living tissue. This allows dense deep-learning-based instance segmentation and 3D reconstruction at a synapse level, incorporating molecular, activity and morphodynamic information. LIONESS opens up avenues for studying the dynamic functional (nano-)architecture of living brain tissue.

Neuroprotective effects of glucagon-like peptide-1 (GLP-1) analogues in epilepsy and associated comorbidities

Epilepsy is a common neurological condition induced by losing equilibrium of different pathway as well as neurotransmitters that affects over 50 million people globally. Furthermore, long-term administration of anti-seizure medications has been associated with psychological adverse effects. Also, epilepsy has been related to an increased prevalence of obesity and called type 2 diabetes mellitus. On the other hand, GLP-1 receptors are located throughout the brain, including the hippocampus, which have been associated to majority of neurological conditions, such as epilepsy and psychiatric disorders. Moreover, the impact of different GLP-1 analogues on diverse neurotransmitter systems and associated cellular and molecular pathways as a potential therapeutic target for epilepsy and associated comorbidities has piqued curiosity. In this regard, the anticonvulsant effects of GLP-1 analogues have been investigated in various animal models and promising results such as anticonvulsants as well as cognitive improvements have been observed. For instance, GLP-1 analogues like liraglutide in addition to their possible anticonvulsant benefits, could be utilized to alleviate mental cognitive problems caused by both epilepsy and anti-seizure medication side effects. In this review and growing protective function of GLP-1 in epilepsy induced by disturbed neurotransmitter pathways and the probable mechanisms of action of GLP-1 analogues as well as the GLP-1 receptor in these effects have been discussed.

Dendritic Branch-constrained N-Methyl-d-Aspartate Receptor-mediated Spikes Drive Synaptic Plasticity in Hippocampal CA3 Pyramidal Cells

N-methyl-d-aspartate receptor-mediated ( spikes can be causally linked to the induction of synaptic long-term potentiation (LTP) in hippocampal and cortical pyramidal cells. However, it is unclear if they regulate plasticity at a local or global scale in the dendritic tree. Here, we used dendritic patch-clamp recordings and calcium imaging to investigate the integrative properties of single dendrites of hippocampal CA3 cells. We show that local hyperpolarization of a single dendritic segment prevents NMDA spikes, their associated calcium transients, as well as LTP in a branch-specific manner. This result provides direct, causal evidence that the single dendritic branch can operate as a functional unit in regulating CA3 pyramidal cell plasticity.

Phase precession in the human hippocampus and entorhinal cortex

Knowing where we are, where we have been, and where we are going is critical to many behaviors, including navigation and memory. One potential neuronal mechanism underlying this ability is phase precession, in which spatially tuned neurons represent sequences of positions by activating at progressively earlier phases of local network theta oscillations. Based on studies in rodents, researchers have hypothesized that phase precession may be a general neural pattern for representing sequential events for learning and memory. By recording human single-neuron activity during spatial navigation, we show that spatially tuned neurons in the human hippocampus and entorhinal cortex exhibit phase precession. Furthermore, beyond the neural representation of locations, we show evidence for phase precession related to specific goal states. Our findings thus extend theta phase precession to humans and suggest that this phenomenon has a broad functional role for the neural representation of both spatial and non-spatial information.

Local Design Principles at Hippocampal Synapses Revealed by an Energy-Information Trade-Off

Synapses across different brain regions display distinct structure-function relationships. We investigated the interplay of fundamental design constraints that shape the transmission properties of the excitatory CA3-CA1 pyramidal cell connection, a prototypic synapse for studying the mechanisms of learning in the mammalian hippocampus. This small synapse is characterized by probabilistic release of transmitter, which is markedly facilitated in response to naturally occurring trains of action potentials. Based on a physiologically motivated computational model of the rat CA3 presynaptic terminal, we show how unreliability and short-term dynamics of vesicular release work together to regulate the trade-off of information transfer versus energy use. We propose that individual CA3-CA1 synapses are designed to operate near the maximum possible capacity of information transmission in an efficient manner. Experimental measurements reveal a wide range of vesicular release probabilities at hippocampal synapses, which may be a necessary consequence of long-term plasticity and homeostatic mechanisms that manifest as presynaptic modifications of the release probability. We show that the timescales and magnitude of short-term plasticity (STP) render synaptic information transfer nearly independent of differences in release probability. Thus, individual synapses transmit optimally while maintaining a heterogeneous distribution of presynaptic strengths indicative of synaptically-encoded memory representations. Our results support the view that organizing principles that are evident on higher scales of neural organization percolate down to the design of an individual synapse.

Differential role of pre- and postsynaptic neurons in the activity-dependent control of synaptic strengths across dendrites

Neurons receive a large number of active synaptic inputs from their many presynaptic partners across their dendritic tree. However, little is known about how the strengths of individual synapses are controlled in balance with other synapses to effectively encode information while maintaining network homeostasis. This is in part due to the difficulty in assessing the activity of individual synapses with identified afferent and efferent connections for a synapse population in the brain. Here, to gain insights into the basic cellular rules that drive the activity-dependent spatial distribution of pre- and postsynaptic strengths across incoming axons and dendrites, we combine patch-clamp recordings with live-cell imaging of hippocampal pyramidal neurons in dissociated cultures and organotypic slices. Under basal conditions, both pre- and postsynaptic strengths cluster on single dendritic branches according to the identity of the presynaptic neurons, thus highlighting the ability of single dendritic branches to exhibit input specificity. Stimulating a single presynaptic neuron induces input-specific and dendritic branchwise spatial clustering of presynaptic strengths, which accompanies a widespread multiplicative scaling of postsynaptic strengths in dissociated cultures and heterosynaptic plasticity at distant synapses in organotypic slices. Our study provides evidence for a potential homeostatic mechanism by which the rapid changes in global or distant postsynaptic strengths compensate for input-specific presynaptic plasticity.

Imaging pathological activities of human brain tissue in organotypic culture

BACKGROUND

Insights into human brain diseases may emerge from tissue obtained after operations on patients. However techniques requiring transduction of transgenes carried by viral vectors cannot be applied to acute human tissue.

NEW METHOD

We show that organotypic culture techniques can be used to maintain tissue from patients with three different neurological syndromes for several weeks in vitro. Optimized viral vector techniques and promoters for transgene expression are described.

RESULTS

Region-specific differences in neuronal form, firing pattern and organization as well as pathological activities were maintained over 40-50 days in culture. Both adeno-associated virus and lentivirus based vectors were persistently expressed from ∼10 days after application, providing 30-40 days to exploit genetically expressed constructs. Different promoters, including hSyn, e/hSyn, CMV and CaMKII, provided cell-type specific transgene expression. The Ca probe GCaMP let us explore epileptogenic synchrony and a FRET-based probe was used to follow activity of the kinase mTORC1.

COMPARISON WITH EXISTING METHODS

The use of a defined culture medium, with low concentrations of amino acids and no growth factors, permitted organotypic culture of tissue from humans aged 3-62 years. Epileptic activity was maintained and excitability changed relatively little until ∼6 weeks in culture.

CONCLUSIONS

Characteristic morphology and region-specific neuronal activities are maintained in organotypic culture of tissue from patients diagnosed with mesial temporal lobe epilepsy, cortical dysplasia and cortical glioblastoma. Viral vector techniques permit expression of probes for long-term measurements of multi-cellular activity and intra-cellular signaling.

Methodological standards for in vitro models of epilepsy and epileptic seizures. A TASK1-WG4 report of the AES/ILAE Translational Task Force of the ILAE

In vitro preparations are a powerful tool to explore the mechanisms and processes underlying epileptogenesis and ictogenesis. In this review, we critically review the numerous in vitro methodologies utilized in epilepsy research. We provide support for the inclusion of detailed descriptions of techniques, including often ignored parameters with unpredictable yet significant effects on study reproducibility and outcomes. In addition, we explore how recent developments in brain slice preparation relate to their use as models of epileptic activity.

Photomarking Relocalization Technique for Correlated Two-Photon and Electron Microcopy Imaging of Single Stimulated Synapses

Synapses learn and remember by persistent modifications of their internal structures and composition but, due to their small size, it is difficult to observe these changes at the ultrastructural level in real time. Two-photon fluorescence microscopy (2PM) allows time-course live imaging of individual synapses but lacks ultrastructural resolution. Electron microscopy (EM) allows the ultrastructural imaging of subcellular components but cannot detect fluorescence and lacks temporal resolution. Here, we describe a combination of procedures designed to achieve the correlated imaging of the same individual synapse under both 2PM and EM. This technique permits the selective stimulation and live imaging of a single dendritic spine and the subsequent localization of the same spine in EM ultrathin serial sections. Landmarks created through a photomarking method based on the 2-photon-induced precipitation of an electrodense compound are used to unequivocally localize the stimulated synapse. This technique was developed to image, for the first time, the ultrastructure of the postsynaptic density in which long-term potentiation was selectively induced just seconds or minutes before, but it can be applied for the study of any biological process that requires the precise relocalization of micron-wide structures for their correlated imaging with 2PM and EM.

Dietary Supplementation of Hericium erinaceus Increases Mossy Fiber-CA3 Hippocampal Neurotransmission and Recognition Memory in Wild-Type Mice

Hericium erinaceus (Bull.) Pers. is a medicinal mushroom capable of inducing a large number of modulatory effects on human physiology ranging from the strengthening of the immune system to the improvement of cognitive functions. In mice, dietary supplementation with H. erinaceus prevents the impairment of spatial short-term and visual recognition memory in an Alzheimer model. Intriguingly other neurobiological effects have recently been reported like the effect on neurite outgrowth and differentiation in PC12 cells. Until now no investigations have been conducted to assess the impact of this dietary supplementation on brain function in healthy subjects. Therefore, we have faced the problem by considering the effect on cognitive skills and on hippocampal neurotransmission in wild-type mice. In wild-type mice the oral supplementation with H. erinaceus induces, in behaviour test, a significant improvement in the recognition memory and, in hippocampal slices, an increase in spontaneous and evoked excitatory synaptic current in mossy fiber-CA3 synapse. In conclusion, we have produced a series of findings in support of the concept that H. erinaceus induces a boost effect onto neuronal functions also in nonpathological conditions.

Infectious Progression of Canine Distemper Virus from Circulating Cerebrospinal Fluid into the Central Nervous System

In the current study, we generated recombinant chimeric canine distemper viruses (CDVs) by replacing the hemagglutinin (H) and/or phosphoprotein (P) gene in an avirulent strain expressing enhanced green fluorescent protein (EGFP) with those of a mouse-adapted neurovirulent strain. An in vitro experimental infection indicated that the chimeric CDVs possessing the H gene derived from the mouse-adapted CDV acquired infectivity for neural cells. These cells lack the CDV receptors that have been identified to date (SLAM and nectin-4), indicating that the H protein defines infectivity in various cell lines. The recombinant viruses were administered intracerebrally to 1-week-old mice. Fatal neurological signs of disease were observed only with a recombinant CDV that possessed both the H and P genes of the mouse-adapted strain, similar to the parental mouse-adapted strain, suggesting that both genes are important to drive virulence of CDV in mice. Using this recombinant CDV, we traced the intracerebral propagation of CDV by detecting EGFP. Widespread infection was observed in the cerebral hemispheres and brainstems of the infected mice. In addition, EGFP fluorescence in the brain slices demonstrated a sequential infectious progression in the central nervous system: CDV primarily infected the neuroependymal cells lining the ventricular wall and the neurons of the hippocampus and cortex adjacent to the ventricle, and it then progressed to an extensive infection of the brain surface, followed by the parenchyma and cortex. In the hippocampal formation, CDV spread in a unidirectional retrograde pattern along neuronal processes in the hippocampal formation from the CA1 region to the CA3 region and the dentate gyrus. Our mouse model demonstrated that the main target cells of CDV are neurons in the acute phase and that the virus spreads via neuronal transmission pathways in the hippocampal formation.

IMPORTANCE

CDV is the etiological agent of distemper in dogs and other carnivores, and in many respects, the pathogenesis of CDV infection in animals resembles that of measles virus infection in humans. We successfully generated a recombinant CDV containing the H and P genes from a mouse-adapted neurovirulent strain and expressing EGFP. The recombinant CDV exhibited severe neurovirulence with high mortality, comparable to the parental mouse-adapted strain. The mouse-infectious model could become a useful tool for analyzing CDV infection of the central nervous system subsequent to passing through the blood-cerebrospinal fluid barrier and infectious progression in the target cells in acute disease.

Neuroprotective levels of IGF-1 exacerbate epileptogenesis after brain injury

Exogenous Insulin-Like Growth Factor-1 (IGF-1) is neuroprotective in animal models of brain injury, and has been considered as a potential therapeutic. Akt-mTOR and MAPK are downstream targets of IGF-1 signaling that are activated after brain injury. However, both brain injury and mTOR are linked to epilepsy, raising the possibility that IGF-1 may be epileptogenic. Here, we considered the role of IGF-1 in development of epilepsy after brain injury, using the organotypic hippocampal culture model of post-traumatic epileptogenesis. We found that IGF-1 was neuroprotective within a few days of injury but that long-term IGF-1 treatment was pro-epileptic. Pro-epileptic effects of IGF-1 were mediated by Akt-mTOR signaling. We also found that IGF-1 - mediated increase in epileptic activity led to neurotoxicity. The dualistic nature of effects of IGF-1 treatment demonstrates that anabolic enhancement through IGF-1 activation of mTOR cascade can be beneficial or harmful depending on the stage of the disease. Our findings suggest that epilepsy risk may need to be considered in the design of neuroprotective treatments for brain injury.

Brain (Hyper)Excitability Revealed by Optimal Electrical Stimulation of GABAergic Interneurons

BACKGROUND

Neurological disorders are often characterized by an excessive and prolonged imbalance between neural excitatory and inhibitory processes. An ubiquitous finding among these disorders is the disrupted function of inhibitory GABAergic interneurons.

OBJECTIVE

The objective is to propose a novel stimulation procedure able to evaluate the efficacy of inhibition imposed by GABAergic interneurons onto pyramidal cells from evoked responses observed in local field potentials (LFPs).

METHODS

Using a computational modeling approach combined with in vivo and in vitro electrophysiological recordings, we analyzed the impact of electrical extracellular, local, bipolar stimulation (ELBS) on brain tissue. We implemented the ELBS effects in a neuronal population model in which we can tune the excitation-inhibition ratio and we investigated stimulation-related parameters. Computer simulations led to sharp predictions regarding: i) the shape of evoked responses as observed in local field potentials, ii) the type of cells (pyramidal neurons and interneurons) contributing to these field responses and iii) the optimal tuning of stimulation parameters (intensity and frequency) to evoke meaningful responses. These predictions were tested in vivo (mouse). Neurobiological mechanisms were assessed in vitro (hippocampal slices).

RESULTS

Appropriately-tuned ELBS allows for preferential activation of GABAergic interneurons. A quantitative neural network excitability index (NNEI) is proposed. It is computed from stimulation-induced responses as reflected in local field potentials. NNEI was used in four patients with focal epilepsy. Results show that it can readily reveal hyperexcitable brain regions.

CONCLUSION

Well-tuned ELBS and NNEI can be used to locally probe brain regions and quantify the (hyper)excitability of the underlying brain tissue.

Bifurcation analysis of a two-compartment hippocampal pyramidal cell model

The Pinsky-Rinzel model is a non-smooth 2-compartmental CA3 pyramidal cell model that has been used widely within the field of neuroscience. Here we propose a modified (smooth) system that captures the qualitative behaviour of the original model, while allowing the use of available, numerical continuation methods to perform full-system bifurcation and fast-slow analysis. We study the bifurcation structure of the full system as a function of the applied current and the maximal calcium conductance. We identify the bifurcations that shape the transitions between resting, bursting and spiking behaviours, and which lead to the disappearance of bursting when the calcium conductance is reduced. Insights gained from this analysis, are then used to firstly illustrate how the irregular spiking activity found between bursting and stable spiking states, can be influenced by phase differences in the calcium and dendritic voltage, which lead to corresponding changes in the calcium-sensitive potassium current. Furthermore, we use fast-slow analysis to investigate the mechanisms of bursting and show that bursting in the model is dependent on the intermediately slow variable, calcium, while the other slow variable, the activation gate of the afterhyperpolarisation current, does not contribute to setting the intraburst dynamics but participates in setting the interburst interval. Finally, we discuss how some of the described bifurcations affect spiking behaviour, during sharp-wave ripples, in a larger network of Pinsky-Rinzel cells.

A Unified Dynamic Model for Learning, Replay, and Sharp-Wave/Ripples

Hippocampal activity is fundamental for episodic memory formation and consolidation. During phases of rest and sleep, it exhibits sharp-wave/ripple (SPW/R) complexes, which are short episodes of increased activity with superimposed high-frequency oscillations. Simultaneously, spike sequences reflecting previous behavior, such as traversed trajectories in space, are replayed. Whereas these phenomena are thought to be crucial for the formation and consolidation of episodic memory, their neurophysiological mechanisms are not well understood. Here we present a unified model showing how experience may be stored and thereafter replayed in association with SPW/Rs. We propose that replay and SPW/Rs are tightly interconnected as they mutually generate and support each other. The underlying mechanism is based on the nonlinear dendritic computation attributable to dendritic sodium spikes that have been prominently found in the hippocampal regions CA1 and CA3, where SPW/Rs and replay are also generated. Besides assigning SPW/Rs a crucial role for replay and thus memory processing, the proposed mechanism also explains their characteristic features, such as the oscillation frequency and the overall wave form. The results shed a new light on the dynamical aspects of hippocampal circuit learning.

SIGNIFICANCE STATEMENT

During phases of rest and sleep, the hippocampus, the "memory center" of the brain, generates intermittent patterns of strongly increased overall activity with high-frequency oscillations, the so-called sharp-wave/ripples. We investigate their role in learning and memory processing. They occur together with replay of activity sequences reflecting previous behavior. Developing a unifying computational model, we propose that both phenomena are tightly linked, by mutually generating and supporting each other. The underlying mechanism depends on nonlinear amplification of synchronous inputs that has been prominently found in the hippocampus. Besides assigning sharp-wave/ripples a crucial role for replay generation and thus memory processing, the proposed mechanism also explains their characteristic features, such as the oscillation frequency and the overall wave form.

Cooperation and competition of gamma oscillation mechanisms

Oscillations of neuronal activity in different frequency ranges are thought to reflect important aspects of cortical network dynamics. Here we investigate how various mechanisms that contribute to oscillations in neuronal networks may interact. We focus on networks with inhibitory, excitatory, and electrical synapses, where the subnetwork of inhibitory interneurons alone can generate interneuron gamma (ING) oscillations and the interactions between interneurons and pyramidal cells allow for pyramidal-interneuron gamma (PING) oscillations. What type of oscillation will such a network generate? We find that ING and PING oscillations compete: The mechanism generating the higher oscillation frequency "wins"; it determines the frequency of the network oscillation and suppresses the other mechanism. For type I interneurons, the network oscillation frequency is equal to or slightly above the higher of the ING and PING frequencies in corresponding reduced networks that can generate only either of them; if the interneurons belong to the type II class, it is in between. In contrast to ING and PING, oscillations mediated by gap junctions and oscillations mediated by inhibitory synapses may cooperate or compete, depending on the type (I or II) of interneurons and the strengths of the electrical and chemical synapses. We support our computer simulations by a theoretical model that allows a full theoretical analysis of the main results. Our study suggests experimental approaches to deciding to what extent oscillatory activity in networks of interacting excitatory and inhibitory neurons is dominated by ING or PING oscillations and of which class the participating interneurons are.

Evolution of Network Synchronization during Early Epileptogenesis Parallels Synaptic Circuit Alterations

In secondary epilepsy, a seizure-prone neural network evolves during the latent period between brain injury and the onset of spontaneous seizures. The nature of the evolution is largely unknown, and even its completeness at the onset of seizures has recently been challenged by measures of gradually decreasing intervals between subsequent seizures. Sequential calcium imaging of neuronal activity, in the pyramidal cell layer of mouse hippocampal in vitro preparations, during early post-traumatic epileptogenesis demonstrated rapid increases in the fraction of neurons that participate in interictal activity. This was followed by more gradual increases in the rate at which individual neurons join each developing seizure, the pairwise correlation of neuronal activities as a function of the distance separating the pair, and network-wide measures of functional connectivity. These data support the continued evolution of synaptic connectivity in epileptic networks beyond the latent period: early seizures occur when recurrent excitatory pathways are largely polysynaptic, while ongoing synaptic remodeling after the onset of epilepsy enhances intranetwork connectivity as well as the onset and spread of seizure activity.

Effects of Hydrogen Sulfide on Modulation of Theta-Gamma Coupling in Hippocampus in Vascular Dementia Rats

Our previous study showed that hydrogen sulfide (H2S) could alleviate the cognitive deficits in vascular dementia (VD) rats associated with the improvement of synaptic plasticity. Neural oscillations are reported to interact with each other through either identical-frequency or cross-frequency coupling. This study examined whether impaired neural couplings could be alleviated by H2S in the hippocampal CA3-CA1 of VD rats and explored its possible mechanism. A VD rat model was established by two-vessel occlusion. Sodium hydrosulfide (NaHS), a kind of H2S donor, was administered intraperitoneally (5.6 mg/kg/day) for 3 weeks. Local field potentials were simultaneously collected in the hippocampal CA3 and CA1. The effects of NaHS on the modulation of theta-gamma coupling were evaluated by using the measurements of both phase-phase coupling and phase-amplitude coupling, while several other approaches including behavior, electrophysiology, western blot, immunofluorescence staining were also employed. The results showed that NaHS significantly prevented spatial learning and memory impairments (p < 0.01). NaHS considerably alleviated the impairment of neural coupling in VD rats in an identical-frequency rhythm and between cross-frequency bands. Moreover, the expression of cystathionine-β-synthase (CBS) was markedly attenuated in VD rats. NaHS elevated the expression of CBS to maintain the intrinsic balance of H2S. Interestingly, it was observed that NaHS increased the protein expression of N-methyl-D-aspartic acid receptor 2A (NMDAR2A) in VD rats. In conclusion, the data suggest that NaHS played the neuroprotective role partly via modulating the expression of NMDAR2A in order to alleviate the impairments of neural couplings in VD rats.

Electrophysiological and morphological characterization of neuronal microcircuits in acute brain slices using paired patch-clamp recordings

The combination of patch clamp recordings from two (or more) synaptically coupled neurons (paired recordings) in acute brain slice preparations with simultaneous intracellular biocytin filling allows a correlated analysis of their structural and functional properties. With this method it is possible to identify and characterize both pre- and postsynaptic neurons by their morphology and electrophysiological response pattern. Paired recordings allow studying the connectivity patterns between these neurons as well as the properties of both chemical and electrical synaptic transmission. Here, we give a step-by-step description of the procedures required to obtain reliable paired recordings together with an optimal recovery of the neuron morphology. We will describe how pairs of neurons connected via chemical synapses or gap junctions are identified in brain slice preparations. We will outline how neurons are reconstructed to obtain their 3D morphology of the dendritic and axonal domain and how synaptic contacts are identified and localized. We will also discuss the caveats and limitations of the paired recording technique, in particular those associated with dendritic and axonal truncations during the preparation of brain slices because these strongly affect connectivity estimates. However, because of the versatility of the paired recording approach it will remain a valuable tool in characterizing different aspects of synaptic transmission at identified neuronal microcircuits in the brain.

Oscillation-induced signal transmission and gating in neural circuits

Reliable signal transmission constitutes a key requirement for neural circuit function. The propagation of synchronous pulse packets through recurrent circuits is hypothesized to be one robust form of signal transmission and has been extensively studied in computational and theoretical works. Yet, although external or internally generated oscillations are ubiquitous across neural systems, their influence on such signal propagation is unclear. Here we systematically investigate the impact of oscillations on propagating synchrony. We find that for standard, additive couplings and a net excitatory effect of oscillations, robust propagation of synchrony is enabled in less prominent feed-forward structures than in systems without oscillations. In the presence of non-additive coupling (as mediated by fast dendritic spikes), even balanced oscillatory inputs may enable robust propagation. Here, emerging resonances create complex locking patterns between oscillations and spike synchrony. Interestingly, these resonances make the circuits capable of selecting specific pathways for signal transmission. Oscillations may thus promote reliable transmission and, in co-action with dendritic nonlinearities, provide a mechanism for information processing by selectively gating and routing of signals. Our results are of particular interest for the interpretation of sharp wave/ripple complexes in the hippocampus, where previously learned spike patterns are replayed in conjunction with global high-frequency oscillations. We suggest that the oscillations may serve to stabilize the replay.

Rhythmic activity in the brain is ubiquitous, its functions are debated. Here we show that it may contribute to the reliable transmission of information within brain areas. We find that its effect is particularly strong if we take nonlinear coupling into account. This experimentally found neuronal property implies that inputs which arrive nearly simultaneously can have a much stronger impact than expected from the sum of their individuals strengths. In such systems, rhythmic activity supports information transmission even if its positive and negative part exactly cancels all the time. Further, the information transmission can adapt to the oscillation frequency to optimally benefit from it. Finally, we show that rhythms with different frequencies may enable or disable communication channels, and are thus suitable for the steering of information flow.

Ex vivo cultured neuronal networks emit in vivo-like spontaneous activity

Spontaneous neuronal activity is present in virtually all brain regions, but neither its function nor spatiotemporal patterns are fully understood. Ex vivo organotypic slice cultures may offer an opportunity to investigate some aspects of spontaneous activity, because they self-restore their networks that collapsed during slicing procedures. In hippocampal networks, we compared the levels and patterns of in vivo spontaneous activity to those in acute and cultured slices. We found that the firing rates and excitatory synaptic activity in the in vivo hippocampus are more similar to those in slice cultures compared to acute slices. The soft confidence-weighted algorithm, a machine learning technique without human bias, also revealed that hippocampal slice cultures resemble the in vivo hippocampus in terms of the overall tendency of the parameters of spontaneous activity.

Paired whole cell recordings in organotypic hippocampal slices

Pair recordings involve simultaneous whole cell patch clamp recordings from two synaptically connected neurons, enabling not only direct electrophysiological characterization of the synaptic connections between individual neurons, but also pharmacological manipulation of either the presynaptic or the postsynaptic neuron. When carried out in organotypic hippocampal slice cultures, the probability that two neurons are synaptically connected is significantly increased. This preparation readily enables identification of cell types, and the neurons maintain their morphology and properties of synaptic function similar to that in native brain tissue. A major advantage of paired whole cell recordings is the highly precise information it can provide on the properties of synaptic transmission and plasticity that are not possible with other more crude techniques utilizing extracellular axonal stimulation. Paired whole cell recordings are often perceived as too challenging to perform. While there are challenging aspects to this technique, paired recordings can be performed by anyone trained in whole cell patch clamping provided specific hardware and methodological criteria are followed. The probability of attaining synaptically connected paired recordings significantly increases with healthy organotypic slices and stable micromanipulation allowing independent attainment of pre- and postsynaptic whole cell recordings. While CA3-CA3 pyramidal cell pairs are most widely used in the organotypic slice hippocampal preparation, this technique has also been successful in CA3-CA1 pairs and can be adapted to any neurons that are synaptically connected in the same slice preparation. In this manuscript we provide the detailed methodology and requirements for establishing this technique in any laboratory equipped for electrophysiology.

Mossy fiber-evoked subthreshold responses induce timing-dependent plasticity at hippocampal CA3 recurrent synapses

Dentate granule cells exhibit exceptionally low levels of activity and rarely elicit action potentials in targeted CA3 pyramidal cells. It is thus unclear how such weak input from the granule cells sustains adequate levels of synaptic plasticity in the targeted CA3 network. We report that subthreshold potentials evoked by mossy fibers are sufficient to induce synaptic plasticity between CA3 pyramidal cells, thereby complementing the sparse action potential discharge. Repetitive pairing of a CA3-CA3 recurrent synaptic response with a subsequent subthreshold mossy fiber response induced long-term potentiation at CA3 recurrent synapses in rat hippocampus in vitro. Reversing the timing of the inputs induced long-term depression. The underlying mechanism depends on a passively conducted giant excitatory postsynaptic potential evoked by a mossy fiber that enhances NMDA receptor-mediated current at active CA3 recurrent synapses by relieving magnesium block. The resulting NMDA spike generates a supralinear depolarization that contributes to synaptic plasticity in hippocampal neuronal ensembles implicated in memory.

Recurrent synapses and circuits in the CA3 region of the hippocampus: an associative network

In the CA3 region of the hippocampus, pyramidal cells excite other pyramidal cells and interneurons. The axons of CA3 pyramidal cells spread throughout most of the region to form an associative network. These connections were first drawn by Cajal and Lorente de No. Their physiological properties were explored to understand epileptiform discharges generated in the region. Synapses between pairs of pyramidal cells involve one or few release sites and are weaker than connections made by mossy fibers on CA3 pyramidal cells. Synapses with interneurons are rather effective, as needed to control unchecked excitation. We examine contributions of recurrent synapses to epileptiform synchrony, to the genesis of sharp waves in the CA3 region and to population oscillations at theta and gamma frequencies. Recurrent connections in CA3, as other associative cortices, have a lower connectivity spread over a larger area than in primary sensory cortices. This sparse, but wide-ranging connectivity serves the functions of an associative network, including acquisition of neuronal representations as activity in groups of CA3 cells and completion involving the recall from partial cues of these ensemble firing patterns.

Repetitive magnetic stimulation induces functional and structural plasticity of excitatory postsynapses in mouse organotypic hippocampal slice cultures

Repetitive transcranial magnetic stimulation (rTMS) is a noninvasive brain stimulation technique that can alter cortical excitability in human subjects for hours beyond the stimulation period. It thus has potential as a therapeutic tool in neuropsychiatric disorders associated with alterations in cortical excitability. However, rTMS-induced neural plasticity remains insufficiently understood at the cellular level. To learn more about the effects of repetitive magnetic stimulation (rMS), we established an in vitro model of rMS using mouse organotypic entorhino-hippocampal slice cultures. We assessed the outcome of a high-frequency (10 Hz) rMS protocol on functional and structural properties of excitatory synapses in mature hippocampal CA1 pyramidal neurons. Whole-cell patch-clamp recordings, immunohistochemistry, and time-lapse imaging techniques revealed that rMS induces a long-lasting increase in glutamatergic synaptic strength, which is accompanied by structural remodeling of dendritic spines. The effects of rMS on spine size were predominantly seen in small spines, suggesting differential effects of rMS on subpopulations of spines. Furthermore, our data indicate that rMS-induced postsynaptic changes depend on the NMDA receptor-mediated accumulation of GluA1-containing AMPA receptors. These results provide first experimental evidence that rMS induces coordinated functional and structural plasticity of excitatory postsynapses, which is consistent with a long-term potentiation of synaptic transmission.

Developmental shift of short-term synaptic plasticity in cortical organotypic slices

Although short-term synaptic plasticity (STP) is ubiquitous in neocortical synapses its functional role in neural computations is not well understood. Critical to elucidating the function of STP will be to understand how STP itself changes with development and experience. Previous studies have reported developmental changes in STP using acute slices. It is not clear, however, to what extent the changes in STP are a function of local ontogenetic programs or the result of the many different sensory and experience-dependent changes that accompany development in vivo. To address this question we examined the in vitro development of STP in organotypic slices cultured for up to 4 weeks. Paired recordings were performed in L5 pyramidal neurons at different stages of in vitro development. We observed a shift in STP in the form of a decrease in the paired-pulse ratio (PPR) (less depression) from the second to fourth week in vitro. This shift in STP was not accompanied by a change in initial excitatory postsynaptic potential (EPSP) amplitude. Fitting STP to a quantitative model indicated that the developmental shift is consistent with presynaptic changes. Importantly, despite the change in the PPR we did not observe changes in the time constant governing STP. Since these experiments were conducted in vitro our results indicate that the shift in STP does not depend on in vivo sensory experience. Although sensory experience may shape STP, we suggest that developmental shifts in STP are at least in part ontogenetically determined.

A candidate mechanism underlying the variance of interictal spike propagation

Synchronous activation of neural networks is an important physiological mechanism, and dysregulation of synchrony forms the basis of epilepsy. We analyzed the propagation of synchronous activity through chronically epileptic neural networks. Electrocorticographic recordings from epileptic patients demonstrate remarkable variance in the pathways of propagation between sequential interictal spikes (IISs). Calcium imaging in chronically epileptic slice cultures demonstrates that pathway variance depends on the presence of GABAergic inhibition and that spike propagation becomes stereotyped following GABA receptor blockade. Computer modeling suggests that GABAergic quenching of local network activations leaves behind regions of refractory neurons, whose late recruitment forms the anatomical basis of variability during subsequent network activation. Targeted path scanning of slice cultures confirmed local activations, while ex vivo recordings of human epileptic tissue confirmed the dependence of interspike variance on GABA-mediated inhibition. These data support the hypothesis that the paths by which synchronous activity spreads through an epileptic network change with each activation, based on the recent history of localized activity that has been successfully inhibited.

Interictal spikes precede ictal discharges in an organotypic hippocampal slice culture model of epileptogenesis

In organotypic hippocampal slice cultures, principal neurons form aberrant excitatory connections with other principal cells in response to slicing induced deafferentation, similar to mechanisms underlying epileptogenesis in posttraumatic epilepsy. To investigate the consequences of this synaptogenesis, the authors recorded field-potential activity from area CA3 during perfusion with the complete growth medium used during incubation. At 7 days in vitro, slice cultures only displayed multiunit activity. At 14 days in vitro, the majority displayed population bursts reminiscent of interictal-like spikes, but sustained synchronous activity was rare. Band-pass filtering of interictal discharges revealed fast ripple-like complexes, similar to in vivo recordings. Spontaneous ictal-like activity became progressively more prevalent with age: at 21 days in vitro, 50% of organotypic hippocampal slice cultures displayed long-lasting, ictal-like discharges that could be suppressed by phenytoin, whereas interictal activity was not suppressed. The fraction of cultures displaying ictal events continually increased with incubation time. Quantification of population spike activity throughout epileptogenesis using automatic detection and clustering algorithms confirmed the appearance of interictal-like activity before ictal-like discharges and also revealed high-frequency pathologic multiunit activity in slice cultures at 14 to 17 days in vitro. These experiments indicate that interictal-like spikes precede the appearance of ictal-like activity in a reduced in vitro preparation. Epileptiform activity in cultures resembled in vivo epilepsy, including sensitivity to anticonvulsants and steadily increasing seizure incidence over time, although seizure frequency and rate of epileptogenesis were higher in vitro. Organotypic hippocampal slice cultures comprise a useful model system for investigating mechanisms of epileptogenesis as well as developing antiepileptic and antiepileptogenic drugs.

A computer model of unitary responses from associational/commissural and perforant path synapses in hippocampal CA3 pyramidal cells

Despite the central position of CA3 pyramidal cells in the hippocampal circuit, the experimental investigation of their synaptic properties has been limited. Recent slice experiments from adult rats characterized AMPA and NMDA receptor unitary synaptic responses in CA3b pyramidal cells. Here, excitatory synaptic activation is modeled to infer biophysical parameters, aid analysis interpretation, explore mechanisms, and formulate predictions by contrasting simulated somatic recordings with experimental data. Reconstructed CA3b pyramidal cells from the public repository NeuroMorpho.Org were used to allow for cell-specific morphological variation. For each cell, synaptic responses were simulated for perforant pathway and associational/commissural synapses. Means and variability for peak amplitude, time-to-peak, and half-height width in these responses were compared with equivalent statistics from experimental recordings. Synaptic responses mediated by AMPA receptors are best fit with properties typical of previously characterized glutamatergic receptors where perforant path synapses have conductances twice that of associational/commissural synapses (0.9 vs. 0.5 nS) and more rapid peak times (1.0 vs. 3.3 ms). Reanalysis of passive-cell experimental traces using the model shows no evidence of a CA1-like increase of associational/commissural AMPA receptor conductance with increasing distance from the soma. Synaptic responses mediated by NMDA receptors are best fit with rapid kinetics, suggestive of NR2A subunits as expected in mature animals. Predictions were made for passive-cell current clamp recordings, combined AMPA and NMDA receptor responses, and local dendritic depolarization in response to unitary stimulations. Models of synaptic responses in active cells suggest altered axial resistivity and the presence of synaptically activated potassium channels in spines.

Spike-time precision and network synchrony are controlled by the homeostatic regulation of the D-type potassium current

Homeostatic plasticity of neuronal intrinsic excitability (HPIE) operates to maintain networks within physiological bounds in response to chronic changes in activity. Classically, this form of plasticity adjusts the output firing level of the neuron through the regulation of voltage-gated ion channels. Ion channels also determine spike timing in individual neurons by shaping subthreshold synaptic and intrinsic potentials. Thus, an intriguing hypothesis is that HPIE can also regulate network synchronization. We show here that the dendrotoxin-sensitive D-type K+ current (ID) disrupts the precision of AP generation in CA3 pyramidal neurons and may, in turn, limit network synchronization. The reduced precision is mediated by the sequence of outward ID followed by inward Na+ current. The homeostatic downregulation of ID increases both spike-time precision and the propensity for synchronization in iteratively constructed networks in vitro. Thus, network synchronization is adjusted in area CA3 through activity-dependent remodeling of ID.

Afferent-specific properties of interneuron synapses underlie selective long-term regulation of feedback inhibitory circuits in CA1 hippocampus

Hebbian long-term potentiation (LTP) develops at specific synapses onto hippocampal CA1 oriens/alveus interneurons (OA-INs), suggesting selective regulation of distinct input pathways. Afferent-specific properties at interneuron synapses have been characterized extensively in CA3 stratum lucidum cells, but given interneuron diversity these rules of transmission and plasticity may not hold in other interneuron types. Here, we used paired recordings and demonstrate that CA2/3 pyramidal cell (PC) feedforward and CA1 PC feedback synapses onto OA-INs show distinct AMPA receptor rectification and Ca(2+) permeability, short-term plasticity and mGluR2/3-mediated inhibition. Only feedback synapses undergo Hebbian LTP. OA-IN firing during repeated synaptic stimulation displays onset-transient or late-persistent responses consistent with activation of feedforward and feedback inputs, respectively. Input-output functions are preserved after theta-burst stimulation, but late-persistent responses selectively show mGluR1-dependent long-term increases. Thus, cell type- and afferent-specific rules of transmission and plasticity underlie distinct OA-IN input-output functions, providing selective long-term regulation in feedback inhibitory networks.

Synchronization of GABAergic Inputs to CA3 Pyramidal Cells Precedes Seizure-Like Event Onset in Juvenile Rat Hippocampal Slices

Here we address how dynamics of glutamatergic and GABAergic synaptic input to CA3 pyramidal cells contribute to spontaneous emergence and evolution of recurrent seizure-like events (SLEs) in juvenile (P10-13) rat hippocampal slices bathed in low-[Mg2+] artificial cerebrospinal fluid. In field potential recordings from the CA3 pyramidal layer, a short epoch of high-frequency oscillation (HFO; 400–800 Hz) was observed during the first 10 ms of SLE onset. GABAergic synaptic input currents to CA3 pyramidal cells were synchronized and coincided with HFO, whereas the glutamatergic input lagged by ∼10 ms. If the intracellular [Cl−] remained unperturbed (cell-attached recordings) or was set high with whole cell electrode solution, CA3 pyramidal cell firing peaked with HFO and GABAergic input. By contrast, with low intracellular [Cl−], spikes of CA3 pyramidal cells lagged behind HFO and GABAergic input. This temporal arrangement of HFO, synaptic input sequence, synchrony of GABAergic currents, and pyramidal cell firing emerged gradually with preictal discharges until the SLE onset. Blockade of GABAA receptor-mediated currents by picrotoxin reduced the inter-SLE interval and the number of preictal discharges and did not block recurrent SLEs. Our data suggest that dynamic changes of the functional properties of GABAergic input contribute to ictogenesis and GABAergic and glutamatergic inputs are both excitatory at the instant of SLE onset. At the SLE onset GABAergic input contributes to synchronization and recruitment of pyramidal cells. We conjecture that this network state is reached by an activity-dependent shift in GABA reversal potential during the preictal phase.

Paired-recordings from synaptically coupled cortical and hippocampal neurons in acute and cultured brain slices

Analysis of synaptic transmission, synaptic plasticity, axonal processing, synaptic timing or electrical coupling requires the simultaneous recording of both the pre- and postsynaptic compartments. Paired-recording technique of monosynaptically connected neurons is also an appropriate technique to probe the function of small molecules (calcium buffers, peptides or small proteins) at presynaptic terminals that are too small to allow direct whole-cell patch-clamp recording. We describe here a protocol for obtaining, in acute and cultured slices, synaptically connected pairs of cortical and hippocampal neurons, with a reasonably high probability. The protocol includes four main stages (acute/cultured slice preparation, visualization, recording and analysis) and can be completed in approximately 4 h.

A method for chronic stimulation of cortical organotypic cultures using implanted electrodes

The neural mechanisms underlying some forms of learning and memory require hours or days to be expressed; however it has proven difficult to study these slowly developing forms of plasticity in reduced preparations due to the short-term nature of acute slice preparations and the fact that most culture preparations lack exposure to structured external input, which plays a critical role in normal cortical development and plasticity. To address this limitation, we developed a method for chronic stimulation of organotypic slice cultures using implanted microelectrodes. This method imparts the ability to apply patterned stimulation to cortical tissue for hours or days, and allows intracellular electrophysiological recordings before and after the stimulation. Importantly, the permanent implantation of the electrodes in the tissue assures that the same neuronal pathways are being excited both during the chronic stimulation while the cultures are in the incubator and while recording in the testing phase. This technique establishes a reduced model for studying experience-dependent plasticity.

Theta phase-specific codes for two-dimensional position, trajectory and heading in the hippocampus

Temporal coding is a means of representing information by the time, as opposed to the rate, at which neurons fire. Evidence of temporal coding in the hippocampus comes from place cells, whose spike times relative to theta oscillations reflect a rat's position while running along stereotyped trajectories. This arises from the backwards shift in cell firing relative to local theta oscillations (phase precession). Here we demonstrate phase precession during place-field crossings in an open-field foraging task. This produced spike sequences in each theta cycle that disambiguate the rat's trajectory through two-dimensional space and can be used to predict movement direction. Furthermore, position and movement direction were maximally predicted from firing in the early and late portions of the theta cycle, respectively. This represents the first direct evidence of a combined representation of position, trajectory and heading in the hippocampus, organized on a fine temporal scale by theta oscillations.

Release-dependent variations in synaptic latency: a putative code for short- and long-term synaptic dynamics

In the cortex, synaptic latencies display small variations ( approximately 1-2 ms) that are generally considered to be negligible. We show here that the synaptic latency at monosynaptically connected pairs of L5 and CA3 pyramidal neurons is determined by the presynaptic release probability (Pr): synaptic latency being inversely correlated with the amplitude of the postsynaptic current and sensitive to manipulations of Pr. Changes in synaptic latency were also observed when Pr was physiologically regulated in short- and long-term synaptic plasticity. Paired-pulse depression and facilitation were respectively associated with increased and decreased synaptic latencies. Similarly, latencies were prolonged following induction of presynaptic LTD and reduced after LTP induction. We show using the dynamic-clamp technique that the observed covariation in latency and synaptic strength is a synergistic combination that significantly affects postsynaptic spiking. In conclusion, amplitude-related variation in latency represents a putative code for short- and long-term synaptic dynamics in cortical networks.

Specific inhibitory synapses shift the balance from feedforward to feedback inhibition of hippocampal CA1 pyramidal cells

Feedforward and feedback inhibition are two fundamental modes of operation widespread in the nervous system. We have functionally identified synaptic connections between rat CA1 hippocampal interneurons of the stratum oriens (SO) and interneurons of the stratum lacunosum moleculare (SLM), which can act as feedback and feedforward interneurons, respectively. The unitary inhibitory postsynaptic currents (uIPSCs) detected with K-gluconate-based patch solution at -50 mV had an amplitude of 20.0 +/- 2.0 pA, rise time 2.2 +/- 0.2 ms, decay time 25 +/- 2.2 ms, jitter 1.4 +/- 0.2 ms (average +/- SEM, n = 39), and were abolished by the gamma-aminobutyric acid (GABA)(A) receptor antagonist 2-(3-carboxypropyl)-3-amino-6-methoxyphenyl-pyridazinium bromide (SR 95531). Post hoc anatomical characterization revealed that all but one of the identified presynaptic neurons were oriens-lacunosum moleculare (O-LM) cells, whereas the postsynaptic neurons were highly heterogeneous, including neurogliaform (n = 4), basket (n = 4), Schaffer collateral-associated (n = 10) and perforant path-associated (n = 9) cells. We investigated the short-term plasticity expressed at these synapses, and found that stimulation at 10-40 Hz resulted in short-term depression of uIPSCs. This short-term plasticity was determined by presynaptic factors and was not target-cell specific. We found that the feedforward inhibition elicited by the direct cortical input including the perforant path onto CA1 pyramidal cells was modulated through the inhibitory synapses we have characterized. Our data show that the inhibitory synapses between interneurons of the SO and SLM shift the balance between feedback and feedforward inhibition onto CA1 pyramidal neurons.

Imbalance between excitation and inhibition among synaptic connections of CA3 pyramidal neurons in cultured hippocampal slices

A fundamental property of small neuronal ensembles is their ability to be selectively activated by distinct stimuli. One cellular mechanism by which neurons achieve this input selectivity is by modulating the temporal dynamics of excitation and inhibition. We explored the interplay of excitation and inhibition in synapses between pyramidal neurons of cornu ammonis field 3 of the hippocampal formation (CA3) in cultured rat hippocampal slices, where activation of a single excitatory cell can readily recruit local interneurons. Simultaneous whole-cell recordings from pairs of CA3 pyramidal neurons revealed that the strength of connections was neither uniform nor balanced. Rather, stimulation of presynaptic neurons elicited distinct combinations of excitatory postsynaptic current-inhibitory postsynaptic current (EPSC-IPSC) amplitudes in the postsynaptic neurons. EPSC-IPSC sequences with small EPSCs had large IPSCs and sequences that contained large EPSCs had small IPSCs. In addition to differences in the amplitudes of the responses, the kinetics of the EPSCs were also different, creating distinct temporal dynamics of excitation and inhibition. Weaker EPSCs had significantly slower kinetics and were efficiently occluded by IPSCs, thereby further limiting their contribution to depolarizing the postsynaptic membrane. Our data suggest that hippocampal pyramidal cells may use an imbalance between excitation and inhibition as a filter to enhance selectivity toward preferential excitatory connections.

Pathway specificity of dendritic spine morphology in identified synapses onto rat hippocampal CA1 neurons in organotypic slices

The output of the hippocampus is largely determined by interaction of the three excitatory pathways that impinge on CA1 pyramidal neurons. These synapses, formed by axons of: (1) CA3 pyramidal neurons; (2) neurons of the entorhinal cortex (EC); and (3) neighboring CA1 neurons, are all potentially plastic. Here, we take advantage of the accessibility of the organotypic slice preparation to identify the type of spines with which each of these pathways forms synapses, at different developmental stages. Recent reports have shown that morphology of dendritic spines is activity-dependent with large mushroom spines being thought to represent stronger synaptic connections than thin or stubby spines. Although in a wide range of preparations, mushroom spines represent only 15% of spines across the whole dendritic tree, we find that this proportion is highly pathway specific. Thus in organotypic slices, the axons of CA3 neurons form synapses with mushroom spines on CA1 neurons in approximately 50% of cases, whereas this spine type is rare (<10%) in either of the other two pathways. This high proportion of mushroom spines only occurs after spontaneous excitatory activity in the CA1 cells increases over the second week in vitro. Previous studies suggest that pathway specificity also occurs in vivo. In tissue fixed in vivo, it is the synapses of distal apical dendrites thought to be formed by axons originating in the EC that are richer in mushroom spines. Hence, contrary to previous suggestions, the proportion of mushroom spines is clearly not an intrinsic property of the pathway but rather a characteristic dependent on the environment. We suggest that this is most likely a result of the previous activity of the synapses. The fact that, despite the large differences in pathway specificity between preparations, the overall proportion of different spine types remains unchanged, suggests a strong influence of homeostasis across the network.

A brief period of epileptiform activity strengthens excitatory synapses in the rat hippocampus in vitro

PURPOSE

We examined here whether a very short period of epileptiform activity could produce lasting modifications of synaptic strength and network properties in the rat hippocampus in vitro.

METHODS

Synaptic transmission at CA3-CA1 and at CA3-CA3 pyramidal cell synapses was monitored in hippocampal slice cultures before and after a very brief episode of epileptiform activity (20-180 s) induced with bicuculline methochloride.

RESULTS

We show here that a brief period of epileptiform activity induces long-lasting potentiation of glutamatergic transmission at CA3-CA1 and at CA3-CA3 pyramidal cell synapses. This potentiation also was observed at synapses formed by pairs of monosynaptically connected neurons. It was dependent on N-methyl-d-aspartate (NMDA) receptors, occluded classic long-term potentiation, and could be depotentiated by low-frequency stimulation at 3 Hz. Recruitment of polysynaptic pathways within area CA3 was facilitated after epileptiform activity indicating that the induced potentiation enhanced overall hippocampal network excitability.

CONCLUSIONS

These changes in synaptic transmission may contribute to the genesis of epilepsy and to seizure-associated memory deficits.

Non-NMDA and NMDA receptors are involved in suprathreshold excitation of network of frog tectal neurons by a single retinal ganglion cell

NMDA receptors play an important functional role in the neuron excitability and plasticity. The conditions and consequences of their activation are of interest for many neuroscientists. This investigation was designed to explore an activation of the NMDA receptors of frog tectal neurons in vivo by a burst of spikes of individual retinotectal fiber. We show that: (1) the NMDA receptors of tectal neurons can be activated by an intense burst discharge of an individual ganglion cell (likely darkness detector) at physiological conditions. (2) Activation of the NMDA receptors is achieved, primarily, due to temporal summation and frequency facilitation of the fast non-NMDA synaptic potentials. However, it is very likely that spatial summation of the fast retinotectal synaptic potentials with excitatory synaptic potentials of recurrent connections contributes to elicit the NMDA response. (3) The activation of NMDA receptors causes a higher level of activity of tectal neuron network. The suprathreshold excitation of efferent tectal neurons is characteristic for this level. Therefore, the burst discharge of only single retinal ganglion cell can activate the tectobulbospinal tract and lead to the motor reaction.

Optical current source density analysis in hippocampal organotypic culture shows that spreading depression occurs with uniquely reversing currents

Spreading depression (SD) involves current flow through principal neurons, but the pattern of current flow over the expanse of susceptible tissues or individual principal neurons remains undefined. Accordingly, tissue and single cell maps made from digital imaging of voltage-sensitive dye changes in hippocampal organotypic cultures undergoing SD were processed via optical current source density analysis to reveal the currents associated with pyramidal neurons. Two distinctive current flow patterns were seen. The first was a trilaminar pattern (420 microm2) that developed with the onset of SD in CA3 pyramidal neurons, in which SD most often began. This initial pattern comprised a somatic current sink with current sources to either side in the dendrites that lasted for seconds extending into the first aspect of the classical "inverted saddle" interstitial direct current waveform of SD. Next, the somatic sink backpropagated at a speed of millimeters per minute into the proximal dendrites, resulting in a reversal of the initial current flow pattern to its second orientation, namely dendritic sinks associated with a somatic source. The latter persisted for the remainder of SD in CA3 and was the only pattern seen in CA1, in which SD was rarely initiated. This backpropagating SD current flow resembles that of activity-dependent synaptic activation. Retrograde and associative signaling via principal neuron current flow is a key means to affect tissue function, including synaptic activation and, by extension, perhaps SD. Such current-related postsynaptic signaling might not only help explain SD but also neuroprotection and migraine, two phenomena increasingly recognized as being related to SD.