Synaptic connections of dentate granule cells and hilar neurons: results of paired intracellular recordings and intracellular horseradish peroxidase injections

Adaptive spike threshold dynamics associated with sparse spiking of hilar mossy cells are captured by a simple model

Hilar mossy cells (hMCs) in the dentate gyrus (DG) receive inputs from DG granule cells (GCs), CA3 pyramidal cells and inhibitory interneurons, and provide feedback input to GCs. Behavioural and in vivo recording experiments implicate hMCs in pattern separation, navigation and spatial learning. Our experiments link hMC intrinsic excitability to their synaptically evoked in vivo spiking outputs. We performed electrophysiological recordings from DG neurons and found that hMCs displayed an adaptative spike threshold that increased both in proportion to the intensity of injected currents, and in response to spiking itself, returning to baseline over a long time scale, thereby instantaneously limiting their firing rate responses. The hMC activity is additionally limited by a prominent medium after-hyperpolarizing potential (AHP) generated by small conductance K+ channels. We hypothesize that these intrinsic hMC properties are responsible for their low in vivo firing rates. Our findings extend previous studies that compare hMCs, CA3 pyramidal cells and hilar inhibitory cells and provide novel quantitative data that contrast the intrinsic properties of these cell types. We developed a phenomenological exponential integrate-and-fire model that closely reproduces the hMC adaptive threshold nonlinearities with respect to their threshold dependence on input current intensity, evoked spike latency and long-lasting spike-induced increase in spike threshold. Our robust and computationally efficient model is amenable to incorporation into large network models of the DG that will deepen our understanding of the neural bases of pattern separation, spatial navigation and learning. KEY POINTS: Previous studies have shown that hilar mossy cells (hMCs) are implicated in pattern separation and the formation of spatial memory, but how their intrinsic properties relate to their in vivo spiking patterns is still unknown. Here we show that the hMCs display electrophysiological properties that distinguish them from the other hilar cell types including a highly adaptive spike threshold that decays slowly. The spike-dependent increase in threshold combined with an after-hyperpolarizing potential mediated by a slow K+ conductance is hypothesized to be responsible for the low-firing rate of the hMC observed in vivo. The hMC's features are well captured by a modified stochastic exponential integrate-and-fire model that has the unique feature of a threshold intrinsically dependant on both the stimulus intensity and the spiking history. This computational model will allow future work to study how the hMCs can contribute to spatial memory formation and navigation.

Assessments of dentate gyrus function: discoveries and debates

There has been considerable speculation regarding the function of the dentate gyrus (DG) - a subregion of the mammalian hippocampus - in learning and memory. In this Perspective article, we compare leading theories of DG function. We note that these theories all critically rely on the generation of distinct patterns of activity in the region to signal differences between experiences and to reduce interference between memories. However, these theories are divided by the roles they attribute to the DG during learning and recall and by the contributions they ascribe to specific inputs or cell types within the DG. These differences influence the information that the DG is thought to impart to downstream structures. We work towards a holistic view of the role of DG in learning and memory by first developing three critical questions to foster a dialogue between the leading theories. We then evaluate the extent to which previous studies address our questions, highlight remaining areas of conflict, and suggest future experiments to bridge these theories.

Enhanced Non-Associative Long-Term Potentiation in Immature Granule Cells in the Dentate Gyrus of Adult Rats

The dentate gyrus is one of the few sites of neurogenesis in the adult brain. Integration of new-generated granule cells into the hippocampal circuitry provides a substrate for structural plasticity, fundamental for normal function of adult hippocampus. However, mechanisms of synaptic plasticity that mediate integration of new-generated granule cells into the existing circuitry remain poorly understood. Especially mechanisms of plasticity at GABA-ergic synapses remain elusive. Here, we show that postsynaptic spiking without presynaptic activation can induce heterosynaptic, non-associative plasticity at GABA-ergic inputs to both immature and mature granule cells. In both immature and mature neurons, plastic changes were bidirectional and individual inputs could express long-term potentiation (LTP) or long-term depression (LTD), or do not change. However, properties of non-associative plasticity dramatically change with maturation of newly generated granule cells: while in immature cells there was a clear predominance of non-associative LTP and net potentiation across the inputs, in mature neurons, potentiation and depression were balanced with no net change on average. We conclude that GABA-ergic inputs to granule cells are plastic, and that the rules for induction of non-associative plasticity change with maturation. We propose that potentiation-biased non-associative plasticity of GABA-ergic transmission might help to counter-balance an increase of excitatory drive that is facilitated by enhanced LTP at glutamatergic synapses in maturating granule cells. Such mechanism might help to build a strong GABA-ergic input to surviving active new cells, necessary for normal function of mature granule cells, which operate under a tight inhibitory control and generate sparse spiking activity.

Direct synaptic excitation between hilar mossy cells revealed with a targeted voltage sensor

The dentate gyrus not only gates the flow of information into the hippocampus, it also integrates and processes this information. Mossy cells (MCs) are a major type of excitatory neuron strategically located in the hilus of the dentate gyrus where they can contribute to this processing through networks of synapses with inhibitory neurons and dentate granule cells. Some prior work has suggested that MCs can form excitatory synapses with other MCs, but the role of these synapses in the network activity of the dentate gyrus has received little attention. Here, we investigated synaptic inputs to MCs in mouse hippocampal slices using a genetically encoded hybrid voltage sensor (hVOS) targeted to MCs by Cre-lox technology. This enabled optical recording of voltage changes from multiple MCs simultaneously. Stimulating granule cells and CA3 pyramidal cells activated well-established inputs to MCs and elicited synaptic responses as expected. However, the weak blockade of MC responses to granule cell layer stimulation by DCG-IV raised the possibility of another source of excitation. To evaluate synapses between MCs as this source, single MCs were stimulated focally. Stimulation of one MC above its action potential threshold evoked depolarizing responses in neighboring MCs that depended on glutamate receptors. Short latency responses of MCs to other MCs did not depend on release from granule cell axons. However, granule cells did contribute to the longer latency responses of MCs to stimulation of other MCs. Thus, MCs transmit their activity to other MCs both through direct synaptic coupling and through polysynaptic coupling with dentate granule cells. MC-MC synapses can redistribute information entering the dentate gyrus and thus shape and modulate the electrical activity underlying hippocampal functions such as navigation and memory, as well as excessive excitation during seizures.

Early changes in synaptic and intrinsic properties of dentate gyrus granule cells in a mouse model of Alzheimer's disease neuropathology and atypical effects of the cholinergic antagonist atropine

It has been reported that hyperexcitability occurs in a subset of patients with Alzheimer's disease (AD) and hyperexcitability could contribute to the disease. Several studies have suggested that the hippocampal dentate gyrus (DG) may be an important area where hyperexcitability occurs. Therefore, we tested the hypothesis that the principal DG cell type, granule cells (GCs), would exhibit changes at the single-cell level which would be consistent with hyperexcitability and might help explain it. We used the Tg2576 mouse, where it has been shown that hyperexcitability is robust at 2-3 months of age. GCs from 2 to 3-month-old Tg2576 mice were compared to age-matched wild type (WT) mice. Effects of muscarinic cholinergic antagonism were tested because previously we found that Tg2576 mice exhibited hyperexcitability in vivo that was reduced by the muscarinic cholinergic antagonist atropine, counter to the dogma that in AD one needs to boost cholinergic function. The results showed that GCs from Tg2576 mice exhibited increased frequency of spontaneous excitatory postsynaptic potentials/currents (sEPSP/Cs) and reduced frequency of spontaneous inhibitory synaptic events (sIPSCs) relative to WT, increasing the excitation:inhibition (E:I) ratio. There was an inward NMDA receptor-dependent current that we defined here as a novel synaptic current (nsC) in Tg2576 mice because it was very weak in WT mice. Intrinsic properties were distinct in Tg2576 GCs relative to WT. In summary, GCs of the Tg2576 mouse exhibit early electrophysiological alterations that are consistent with increased synaptic excitation, reduced inhibition, and muscarinic cholinergic dysregulation. The data support previous suggestions that the DG contributes to hyperexcitability and there is cholinergic dysfunction early in life in AD mouse models.

Involvement of Mossy Cells in Sharp Wave-Ripple Activity In Vitro

The role of mossy cells (MCs) of the hippocampal dentate area has long remained mysterious. Recent research has begun to unveil their significance in spatial computation of the hippocampus. Here, we used an in vitro model of sharp wave-ripple complexes (SWRs), which contribute to hippocampal memory formation, to investigate MC involvement in this fundamental population activity. We find that a significant fraction of MCs (∼47%) is recruited into the active neuronal network during SWRs in the CA3 area. Moreover, MCs receive pronounced, ripple-coherent, excitatory and inhibitory synaptic input. Finally, we find evidence for SWR-related synaptic activity in granule cells that is mediated by MCs. Given the widespread connectivity of MCs within and between hippocampi, our data suggest a role for MCs as a hub functionally coupling the CA3 and the DG during ripple-associated computations.

Novelty and Novel Objects Increase c-Fos Immunoreactivity in Mossy Cells in the Mouse Dentate Gyrus

The dentate gyrus (DG) and its primary cell type, the granule cell (GC), are thought to be critical to many cognitive functions. A major neuronal subtype of the DG is the hilar mossy cell (MC). MCs have been considered to play an important role in cognition, but in vivo studies to understand the activity of MCs during cognitive tasks are challenging because the experiments usually involve trauma to the overlying hippocampus or DG, which kills hilar neurons. In addition, restraint typically occurs, and MC activity is reduced by brief restraint stress. Social isolation often occurs and is potentially confounding. Therefore, we used c-fos protein expression to understand when MCs are active in vivo in socially housed adult C57BL/6 mice in their home cage. We focused on c-fos protein expression after animals explored novel objects, based on previous work which showed that MCs express c-fos protein readily in response to a novel housing location. Also, MCs are required for the training component of the novel object location task and novelty-encoding during a food-related task. GluR2/3 was used as a marker of MCs. The results showed that MC c-fos protein is greatly increased after exposure to novel objects, especially in ventral DG. We also found that novel objects produced higher c-fos levels than familiar objects. Interestingly, a small subset of neurons that did not express GluR2/3 also increased c-fos protein after novel object exposure. In contrast, GCs appeared relatively insensitive. The results support a growing appreciation of the role of the DG in novelty detection and novel object recognition, where hilar neurons and especially MCs are very sensitive.

Function of local circuits in the hippocampal dentate gyrus-CA3 system

Anatomical observations, theoretical work and lesioning experiments have supported the idea that the CA3 in the hippocampus is important for encoding, storage and retrieval of memory while the dentate gyrus (DG) is important for the pattern separation of the incoming inputs from the entorhinal cortex. Study of the presumed function of the dentate gyrus in pattern separation has been hampered by the lack of reliable methods to identify different excitatory cell types in the DG. Recent papers have identified different cell types in the DG, in awake behaving animals, with more reliable methods. These studies have revealed each cell type's spatial representation as well as their involvement in pattern separation. Moreover, chronic electrophysiological recording from sleeping and waking animals also provided more insights into the operation of the DG-CA3 system for memory encoding and retrieval. This article will review the local circuit architectures and physiological properties of the DG-CA3 system and discuss how the local circuit in the DG-CA3 may function, incorporating recent physiological findings in the DG-CA3 system.

Parvalbumin+ interneurons obey unique connectivity rules and establish a powerful lateral-inhibition microcircuit in dentate gyrus

Parvalbumin-positive (PV⁺) GABAergic interneurons in hippocampal microcircuits are thought to play a key role in several higher network functions, such as feedforward and feedback inhibition, network oscillations, and pattern separation. Fast lateral inhibition mediated by GABAergic interneurons may implement a winner-takes-all mechanism in the hippocampal input layer. However, it is not clear whether the functional connectivity rules of granule cells (GCs) and interneurons in the dentate gyrus are consistent with such a mechanism. Using simultaneous patch-clamp recordings from up to seven GCs and up to four PV⁺ interneurons in the dentate gyrus, we find that connectivity is structured in space, synapse-specific, and enriched in specific disynaptic motifs. In contrast to the neocortex, lateral inhibition in the dentate gyrus (in which a GC inhibits neighboring GCs via a PV⁺ interneuron) is ~ 10-times more abundant than recurrent inhibition (in which a GC inhibits itself). Thus, unique connectivity rules may enable the dentate gyrus to perform specific higher-order computations.

GABAergic interneurons are known to provide inhibition to allow computational function of neuronal network. Here, Espinoza and colleagues show that connectivity of granule cells and interneurons in the dentate gyrus of mouse hippocampus are consistent with the circuit architecture capable of performing a winners-take-all mechanism.

Advances in understanding hilar mossy cells of the dentate gyrus

Hilar mossy cells (MCs) of the dentate gyrus (DG) distinguish the DG from other hippocampal subfields (CA1-3) because there are two glutamatergic cell types in the DG rather than one. Thus, in the DG, the main cell types include glutamatergic granule cells (GCs) and MCs, whereas in CA1-3, the only glutamatergic cell type is the pyramidal cell. In contrast to GCs, MCs are different in morphology, intrinsic electrophysiological properties, afferent input and axonal projections, so their function is likely to be very different from GCs. Why are MCs necessary to the DG? In past studies, the answer has been unclear because MCs not only excite GCs directly but also inhibit them disynaptically, by exciting GABAergic neurons that project to GCs. Results of new studies are discussed that shed light on this issue. These studies take advantage of recently available transgenic mice with Cre recombinase expression mostly in MCs and techniques such as optogenetics and DREADDs (designer receptors exclusively activated by designer drugs). The recent studies also address in vivo behavioral functions of MCs. Some of the results support past hypotheses whereas others suggest new conceptualizations of how the MCs contribute to DG circuitry and function. While substantial progess has been made, additional research is still needed to clarify the characteristics and functions of these unique cells.

Morphological diversity and connectivity of hippocampal interneurons

The mammalian forebrain is constructed from ensembles of neurons that form local microcircuits giving rise to the exquisite cognitive tasks the mammalian brain can perform. Hippocampal neuronal circuits comprise populations of relatively homogenous excitatory neurons, principal cells and exceedingly heterogeneous inhibitory neurons, the interneurons. Interneurons release GABA from their axon terminals and are capable of controlling excitability in every cellular compartment of principal cells and interneurons alike; thus, they provide a brake on excess activity, control the timing of neuronal discharge and provide modulation of synaptic transmission. The dendritic and axonal morphology of interneurons, as well as their afferent and efferent connections within hippocampal circuits, is central to their ability to differentially control excitability, in a cell-type- and compartment-specific manner. This review aims to provide an up-to-date compendium of described hippocampal interneuron subtypes, with respect to their morphology, connectivity, neurochemistry and physiology, a full understanding of which will in time help to explain the rich diversity of neuronal function.

On How the Dentate Gyrus Contributes to Memory Discrimination

The dentate gyrus (DG) is crucial for behaviorally discriminating similar spatial memories, predicting that DG place cells change ("remap") their relative spatial tuning ("place fields") for memory discrimination. This prediction was never tested, although DG place cells remap across similar environments without memory tasks. We confirm this prior finding but find that DG place fields do not remap across spatial tasks that require DG-dependent memory discrimination. Instead of remapping, place-discriminating discharge is observed transiently among DG place cells, particularly when memory discrimination is most necessary. The DG network may signal memory discrimination by expressing distinctive sub-second network patterns of co-firing at memory discrimination sites. This involves increased coupling of discharge from place cells and interneurons, as was observed during successful, but not failed, behavioral expression of memory discrimination. Instead of remapping, these findings indicate that memory discrimination is signaled by sub-second patterns of correlated discharge within the dentate network.

Sex differences in hippocampal area CA3 pyramidal cells

Numerous studies have demonstrated differences between males and females in hippocampal structure, function, and plasticity. There also are many studies about the different predisposition of a males and females for disorders where the hippocampus plays an important role. Many of these reports focus on area CA1, but other subfields are also very important, and unlikely to be the same as area CA1 based on what is known. Here we review basic studies of male and female structure, function, and plasticity of area CA3 pyramidal cells of adult rats. The data suggest that the CA3 pyramidal cells of males and females are distinct in structure, function, and plasticity. These sex differences cannot be simply explained by the effects of circulating gonadal hormones. This view agrees with previous studies showing that there are substantial sex differences in the brain that cannot be normalized by removing the gonads and depleting peripheral gonadal hormones. Implications of these comparisons for understanding sex differences in hippocampal function and dysfunction are discussed. © 2016 Wiley Periodicals, Inc.

Interactions between Inhibitory Interneurons and Excitatory Associational Circuitry in Determining Spatio-Temporal Dynamics of Hippocampal Dentate Granule Cells: A Large-Scale Computational Study

This paper reports on findings from a million-cell granule cell model of the rat dentate gyrus that was used to explore the contributions of local interneuronal and associational circuits to network-level activity. The model contains experimentally derived morphological parameters for granule cells, which each contain approximately 200 compartments, and biophysical parameters for granule cells, basket cells, and mossy cells that were based both on electrophysiological data and previously published models. Synaptic input to cells in the model consisted of glutamatergic AMPA-like EPSPs and GABAergic-like IPSPs from excitatory and inhibitory neurons, respectively. The main source of input to the model was from layer II entorhinal cortical neurons. Network connectivity was constrained by the topography of the system, and was derived from axonal transport studies, which provided details about the spatial spread of axonal terminal fields, as well as how subregions of the medial and lateral entorhinal cortices project to subregions of the dentate gyrus. Results of this study show that strong feedback inhibition from the basket cell population can cause high-frequency rhythmicity in granule cells, while the strength of feedforward inhibition serves to scale the total amount of granule cell activity. Results furthermore show that the topography of local interneuronal circuits can have just as strong an impact on the development of spatio-temporal clusters in the granule cell population as the perforant path topography does, both sharpening existing clusters and introducing new ones with a greater spatial extent. Finally, results show that the interactions between the inhibitory and associational loops can cause high frequency oscillations that are modulated by a low-frequency oscillatory signal. These results serve to further illustrate the importance of topographical constraints on a global signal processing feature of a neural network, while also illustrating how rich spatio-temporal and oscillatory dynamics can evolve from a relatively small number of interacting local circuits.

A Million-Plus Neuron Model of the Hippocampal Dentate Gyrus: Critical Role for Topography in Determining Spatiotemporal Network Dynamics

GOAL

This paper describes a million-plus granule cell compartmental model of the rat hippocampal dentate gyrus, including excitatory, perforant path input from the entorhinal cortex, and feedforward and feedback inhibitory input from dentate interneurons.

METHODS

The model includes experimentally determined morphological and biophysical properties of granule cells, together with glutamatergic AMPA-like EPSP and GABAergic GABAA-like IPSP synaptic excitatory and inhibitory inputs, respectively. Each granule cell was composed of approximately 200 compartments having passive and active conductances distributed throughout the somatic and dendritic regions. Modeling excitatory input from the entorhinal cortex was guided by axonal transport studies documenting the topographical organization of projections from subregions of the medial and lateral entorhinal cortex, plus other important details of the distribution of glutamatergic inputs to the dentate gyrus. Information contained within previously published maps of this major hippocampal afferent were systematically converted to scales that allowed the topographical distribution and relative synaptic densities of perforant path inputs to be quantitatively estimated for inclusion in the current model.

RESULTS

Results showed that when medial and lateral entorhinal cortical neurons maintained Poisson random firing, dentate granule cells expressed, throughout the million-cell network, a robust nonrandom pattern of spiking best described as a spatiotemporal "clustering." To identify the network property or properties responsible for generating such firing "clusters," we progressively eliminated from the model key mechanisms, such as feedforward and feedback inhibition, intrinsic membrane properties underlying rhythmic burst firing, and/or topographical organization of entorhinal afferents.

CONCLUSION

Findings conclusively identified topographical organization of inputs as the key element responsible for generating a spatiotemporal distribution of clustered firing. These results uncover a functional organization of perforant path afferents to the dentate gyrus not previously recognized: topography-dependent clusters of granule cell activity as "functional units" or "channels" that organize the processing of entorhinal signals. This modeling study also reveals for the first time how a global signal processing feature of a neural network can evolve from one of its underlying structural characteristics.

Differences in paired-pulse inhibition and facilitation in the dentate gyrus and CA3 field between dorsal and ventral rat hippocampus

We studied the processes of inhibition and facilitation in the dentate gyrus (DG) and the CA3 field by examining the effects of paired-pulse stimulation on the evoked population spike (PS) in dorsal (DH) and ventral (VH) hippocampal slices from the adult rat. The antidromic-orthodromic (A-O) and the orthodromic-orthodromic (O-O) paired-pulse stimulation protocols were used at varying inter-pulse intervals (IPI). In the DG, the A-O stimulation produced an early depression of PS lasting 30-40ms which was significantly stronger in the VH compared with DH. The O-O stimulation produced a biphasic pattern of effects, in both dorsal and ventral DG, consisting of an early depression of PS followed by facilitation at relatively longer intervals. In the DH but not the VH the phase of facilitation was followed by a late depression of PS (>200ms). In the CA3 field both A-O and O-O stimulation had a biphasic effect consisting of an early phase of strong depression of similar strength in DH and VH. The depression was followed by a phase of facilitation which was more pronounced with O-O stimulation. The facilitation observed with the O-O stimulation was much stronger in DH than VH and in DH only it was significantly reduced by the antagonist of GABAB receptors CGP52432. Furthermore, the facilitation was insensitive to changes in [Ca(2+)]o in both hippocampal poles. These findings suggest that the dorsal compared with ventral DG is more amenable to fast-frequency input but filters out slow-frequency inputs more reliably while the gating and amplification of the excitatory input in the CA3 circuitry is more prominent in DH than in VH.

Surviving mossy cells enlarge and receive more excitatory synaptic input in a mouse model of temporal lobe epilepsy

Numerous hypotheses of temporal lobe epileptogenesis have been proposed, and several involve hippocampal mossy cells. Building on previous hypotheses we sought to test the possibility that after epileptogenic injuries surviving mossy cells develop into super-connected seizure-generating hub cells. If so, they might require more cellular machinery and consequently have larger somata, elongate their dendrites to receive more synaptic input, and display higher frequencies of miniature excitatory synaptic currents (mEPSCs). To test these possibilities pilocarpine-treated mice were evaluated using GluR2-immunocytochemistry, whole-cell recording, and biocytin-labeling. Epileptic pilocarpine-treated mice displayed substantial loss of GluR2-positive hilar neurons. Somata of surviving neurons were 1.4-times larger than in controls. Biocytin-labeled mossy cells also were larger in epileptic mice, but dendritic length per cell was not significantly different. The average frequency of mEPSCs of mossy cells recorded in the presence of tetrodotoxin and bicuculline was 3.2-times higher in epileptic pilocarpine-treated mice as compared to controls. Other parameters of mEPSCs were similar in both groups. Average input resistance of mossy cells in epileptic mice was reduced to 63% of controls, which is consistent with larger somata and would tend to make surviving mossy cells less excitable. Other intrinsic physiological characteristics examined were similar in both groups. Increased excitatory synaptic input is consistent with the hypothesis that surviving mossy cells develop into aberrantly super-connected seizure-generating hub cells, and soma hypertrophy is indirectly consistent with the possibility of axon sprouting. However, no obvious evidence of hyperexcitable intrinsic physiology was found. Furthermore, similar hypertrophy and hyper-connectivity has been reported for other neuron types in the dentate gyrus, suggesting mossy cells are not unique in this regard. Thus, findings of the present study reveal epilepsy-related changes in mossy cell anatomy and synaptic input but do not strongly support the hypothesis that mossy cells develop into seizure-generating hub cells.

Blockade of excitatory synaptogenesis with proximal dendrites of dentate granule cells following rapamycin treatment in a mouse model of temporal lobe epilepsy

Inhibiting the mammalian target of rapamycin (mTOR) signaling pathway with rapamycin blocks granule cell axon (mossy fiber) sprouting after epileptogenic injuries, including pilocarpine-induced status epilepticus. However, it remains unclear whether axons from other types of neurons sprout into the inner molecular layer and synapse with granule cell dendrites despite rapamycin treatment. If so, other aberrant positive-feedback networks might develop. To test this possibility stereological electron microscopy was used to estimate the numbers of excitatory synapses in the inner molecular layer per hippocampus in pilocarpine-treated control mice, in mice 5 days after pilocarpine-induced status epilepticus, and after status epilepticus and daily treatment beginning 24 hours later with rapamycin or vehicle for 2 months. The optical fractionator method was used to estimate numbers of granule cells in Nissl-stained sections so that numbers of excitatory synapses in the inner molecular layer per granule cell could be calculated. Control mice had an average of 2,280 asymmetric synapses in the inner molecular layer per granule cell, which was reduced to 63% of controls 5 days after status epilepticus, recovered to 93% of controls in vehicle-treated mice 2 months after status epilepticus, but remained at only 63% of controls in rapamycin-treated mice. These findings reveal that rapamycin prevented excitatory axons from synapsing with proximal dendrites of granule cells and raise questions about the recurrent excitation hypothesis of temporal lobe epilepsy.

Long-term potentiation in hilar circuitry modulates gating by the dentate gyrus

The dentate gyrus serves as a gateway to the hippocampus, filtering and processing sensory inputs as an animal explores its environment. The hilus occupies a strategic position within the dentate gyrus from which it can play a pivotal role in these functions. Inputs from dentate granule cells converge on the hilus, and excitatory hilar mossy cells redistribute these signals back to granule cells to transform a pattern of cortical input into a new pattern of output to the hippocampal CA3 region. Using voltage-sensitive dye to image electrical activity in rat hippocampal slices, we explored how long-term potentiation (LTP) of different excitatory synapses modifies the flow of information. Theta burst stimulation of the perforant path potentiated responses throughout the molecular layer, but left responses in the CA3 region unchanged. By contrast, theta burst stimulation of the granule cell layer potentiated responses throughout the molecular layer, as well as in the CA3 region. Theta burst stimulation of the granule cell layer potentiated CA3 responses not only to granule cell layer stimulation but also to perforant path stimulation. Potentiation of responses in the CA3 region reflected NMDA receptor-dependent LTP of upstream synapses between granule cells and mossy cells, with no detectable contribution from NMDA receptor-independent LTP of local CA3 mossy fiber synapses. Potentiation of transmission to the CA3 region required LTP in both granule cell→mossy cell and mossy cell→granule cell synapses. This bidirectional plasticity enables hilar circuitry to regulate the flow of information through the dentate gyrus and on to the hippocampus.

Grid cells and cortical representation

One of the grand challenges in neuroscience is to comprehend neural computation in the association cortices, the parts of the cortex that have shown the largest expansion and differentiation during mammalian evolution and that are thought to contribute profoundly to the emergence of advanced cognition in humans. In this Review, we use grid cells in the medial entorhinal cortex as a gateway to understand network computation at a stage of cortical processing in which firing patterns are shaped not primarily by incoming sensory signals but to a large extent by the intrinsic properties of the local circuit.

Is plasticity of GABAergic mechanisms relevant to epileptogenesis?

Numerous changes in GABAergic neurons, receptors, and inhibitory mechanisms have been described in temporal lobe epilepsy (TLE), either in humans or in animal models. Nevertheless, there remains a common assumption that epilepsy can be explained by simply an insufficiency of GABAergic inhibition. Alternatively, investigators have suggested that there is hyperinhibition that masks an underlying hyperexcitability. Here we examine the status epilepticus (SE) models of TLE and focus on the dentate gyrus of the hippocampus, where a great deal of data have been collected. The types of GABAergic neurons and GABAA receptors are summarized under normal conditions and after SE. The role of GABA in development and in adult neurogenesis is discussed. We suggest that instead of "too little or too much" GABA there is a complexity of changes after SE that makes the emergence of chronic seizures (epileptogenesis) difficult to understand mechanistically, and difficult to treat. We also suggest that this complexity arises, at least in part, because of the remarkable plasticity of GABAergic neurons and GABAA receptors in response to insult or injury.

Hilar mossy cells of the dentate gyrus: a historical perspective

The circuitry of the dentate gyrus (DG) of the hippocampus is unique compared to other hippocampal subfields because there are two glutamatergic principal cells instead of one: granule cells, which are the vast majority of the cells in the DG, and the so-called “mossy cells.” The distinctive appearance of mossy cells, the extensive divergence of their axons, and their vulnerability to excitotoxicity relative to granule cells has led to a great deal of interest in mossy cells. Nevertheless, there is no consensus about the normal functions of mossy cells and the implications of their vulnerability. There even seems to be some ambiguity about exactly what mossy cells are. Here we review initial studies of mossy cells, characteristics that define them, and suggest a practical definition to allow investigators to distinguish mossy cells from other hilar neurons even if all morphological and physiological information is unavailable due to technical limitations of their experiments. In addition, hypotheses are discussed about the role of mossy cells in the DG network, reasons for their vulnerability and their implications for disease.

Mossy cell dendritic structure quantified and compared with other hippocampal neurons labeled in rats in vivo

Mossy cells are likely to contribute to normal hippocampal function and to the pathogenesis of neurologic disorders that involve the hippocampus, including epilepsy. Mossy cells are the least well-characterized excitatory neurons in the hippocampus. Their somatic and dendritic morphology has been described qualitatively but not quantitatively. In the present study rat mossy cells were labeled intracellularly with biocytin in vivo. Somatic and dendritic structure was reconstructed three-dimensionally. For comparison, granule cells, CA3 pyramidal cells, and CA1 pyramidal cells were labeled and analyzed using the same approach. Among the four types of hippocampal neurons, granule cells had the smallest somata, fewest primary dendrites and dendritic branches, and shortest total dendritic length. CA1 pyramidal cells had the most dendritic branches and longest total dendritic length. Mossy cells and CA3 pyramidal cells both had large somata and similar total dendritic lengths. However, mossy cell dendrites branched less than CA3 pyramidal cells, especially close to the soma. These findings suggest that mossy cells have dendritic features that are not identical to any other type of hippocampal neuron. Therefore, electrotonic properties that depend on soma-dendritic structure are likely to be distinct in mossy cells compared to other neurons.

Increased excitatory synaptic input to granule cells from hilar and CA3 regions in a rat model of temporal lobe epilepsy

One potential mechanism of temporal lobe epilepsy is recurrent excitation of dentate granule cells through aberrant sprouting of their axons (mossy fibers), which is found in many patients and animal models. However, correlations between the extent of mossy fiber sprouting and seizure frequency are weak. Additional potential sources of granule cell recurrent excitation that would not have been detected by markers of mossy fiber sprouting in previous studies include surviving mossy cells and proximal CA3 pyramidal cells. To test those possibilities in hippocampal slices from epileptic pilocarpine-treated rats, laser-scanning glutamate uncaging was used to randomly and focally activate neurons in the granule cell layer, hilus, and proximal CA3 pyramidal cell layer while measuring evoked EPSCs in normotopic granule cells. Consistent with mossy fiber sprouting, a higher proportion of glutamate-uncaging spots in the granule cell layer evoked EPSCs in epileptic rats compared with controls. In addition, stimulation spots in the hilus and proximal CA3 pyramidal cell layer were more likely to evoke EPSCs in epileptic rats, despite significant neuron loss in those regions. Furthermore, synaptic strength of recurrent excitatory inputs to granule cells from CA3 pyramidal cells and other granule cells was increased in epileptic rats. These findings reveal substantial levels of excessive, recurrent, excitatory synaptic input to granule cells from neurons in the hilus and proximal CA3 field. The aberrant development of these additional positive-feedback circuits might contribute to epileptogenesis in temporal lobe epilepsy.

Pattern separation in the dentate gyrus: a role for the CA3 backprojection

Many theories of hippocampal function assume that area CA3 of hippocampus is capable of performing rapid pattern storage, as well as pattern completion when a partial version of a familiar pattern is presented, and that the dentate gyrus (DG) is a preprocessor that performs pattern separation, facilitating storage and recall in CA3. The latter assumption derives partly from the anatomical and physiological properties of DG. However, the major output of DG is from a large number of DG granule cells to a smaller number of CA3 pyramidal cells, which potentially negates the pattern separation performed in the DG. Here, we consider a simple CA3 network model, and consider how it might interact with a previously developed computational model of the DG. The resulting "standard" DG-CA3 model performs pattern storage and completion well, given a small set of sparse, randomly derived patterns representing entorhinal input to the DG and CA3. However, under many circumstances, the pattern separation achieved in the DG is not as robust in CA3, resulting in a low storage capacity for CA3, compared to previous mathematical estimates of the storage capacity for an autoassociative network of this size. We also examine an often-overlooked aspect of hippocampal anatomy that might increase functionality in the combined DG-CA3 model. Specifically, axon collaterals of CA3 pyramidal cells project "back" to the DG ("backprojections"), exerting inhibitory effects on granule cells that could potentially ensure that different subpopulations of granule cells are recruited to respond to similar patterns. In the model, addition of such backprojections improves both pattern separation and storage capacity. We also show that the DG-CA3 model with backprojections provides a better fit to empirical data than a model without backprojections. Therefore, we hypothesize that CA3 backprojections might play an important role in hippocampal function.

The interesting interplay between interneurons and adult hippocampal neurogenesis

Adult neurogenesis is a unique form of plasticity found in the hippocampus, a brain region key to learning and memory formation. While many external stimuli are known to modulate the generation of new neurons in the hippocampus, little is known about the local circuitry mechanisms that regulate the process of adult neurogenesis. The neurogenic niche in the hippocampus is highly complex and consists of a heterogeneous population of cells including interneurons. Because interneurons are already highly integrated into the hippocampal circuitry, they are in a prime position to influence the proliferation, survival, and maturation of adult-generated cells in the dentate gyrus. Here, we review the current state of our understanding on the interplay between interneurons and adult hippocampal neurogenesis. We focus on activity- and signaling-dependent mechanisms, as well as research on human diseases that could provide better insight into how interneurons in general might add to our comprehension of the regulation and function of adult hippocampal neurogenesis.

Synaptic reorganization of inhibitory hilar interneuron circuitry after traumatic brain injury in mice

Functional plasticity of synaptic networks in the dentate gyrus has been implicated in the development of posttraumatic epilepsy and in cognitive dysfunction after traumatic brain injury, but little is known about potentially pathogenic changes in inhibitory circuits. We examined synaptic inhibition of dentate granule cells and excitability of surviving GABAergic hilar interneurons 8-13 weeks after cortical contusion brain injury in transgenic mice that express enhanced green fluorescent protein in a subpopulation of inhibitory neurons. Whole-cell voltage-clamp recordings in granule cells revealed a reduction in spontaneous and miniature IPSC frequency after head injury; no concurrent change in paired-pulse ratio was found in granule cells after paired electrical stimulation of the hilus. Despite reduced inhibitory input to granule cells, action potential and EPSC frequencies were increased in hilar GABA neurons from slices ipsilateral to the injury versus those from control or contralateral slices. Furthermore, increased excitatory synaptic activity was detected in hilar GABA neurons ipsilateral to the injury after glutamate photostimulation of either the granule cell or CA3 pyramidal cell layers. Together, these findings suggest that excitatory drive to surviving hilar GABA neurons is enhanced by convergent input from both pyramidal and granule cells, but synaptic inhibition of granule cells is not fully restored after injury. This rewiring of circuitry regulating hilar inhibitory neurons may reflect an important compensatory mechanism, but it may also contribute to network destabilization by increasing the relative impact of surviving individual interneurons in controlling granule cell excitability in the posttraumatic dentate gyrus.

Biocytin-labelling and its impact on late 20th century studies of cortical circuitry

In recognition of the impact that a powerful new anatomical tool, such as the Golgi method, can have, this essay highlights the enormous influence that biocytin-filling has had on modern neuroscience. This method has allowed neurones that have been recorded intracellularly, 'whole-cell' or juxta-cellularly, to be identified anatomically, forming a vital link between functional and structural studies. It has been applied throughout the nervous system and has become a fundamental component of our technical armoury. A comprehensive survey of the applications to which the biocytin-filling approach has been put, would fill a large volume. This essay therefore focuses on one area, neocortical microcircuitry and the ways in which combining physiology and anatomy have revealed rules that help us explain its previously indecipherable variability and complexity.

Effects of adult-generated granule cells on coordinated network activity in the dentate gyrus

Throughout the adult life of most mammals, new neurons are continuously generated in the dentate gyrus of the hippocampal formation. Recent work has documented specific cognitive deficits after elimination of adult hippocampal neurogenesis in rodents, suggesting that these neurons may contribute to information processing in hippocampal circuits. Young adult-born neurons exhibit enhanced excitability and have altered capacity for synaptic plasticity in hippocampal slice preparations in vitro. Still, little is known about the effect of adult-born granule cells on hippocampal activity in vivo. To assess the impact of these new neurons on neural circuits in the dentate, we recorded perforant-path evoked responses and spontaneous network activity from the dentate gyrus of urethane-anesthetized mice whose hippocampus had been focally X-irradiated to eliminate the population of young adult-born granule cells. After X-irradiation, perforant-path responses were reduced in magnitude. In contrast, there was a marked increase in the amplitude of spontaneous γ-frequency bursts in the dentate gyrus and hilus, as well as increased synchronization of dentate neuron firing to these bursts. A similar increase in gamma burst amplitude was also found in animals in which adult neurogenesis was eliminated using the GFAP:TK pharmacogenetic ablation technique. These data suggest that young neurons may inhibit or destabilize recurrent network activity in the dentate and hilus. This unexpected result yields a new perspective on how a modest number of young adult-generated granule cells may modulate activity in the larger population of mature granule cells, rather than acting solely as independent encoding units.

Excitability changes within transverse lamellae of dentate granule cells and their longitudinal spread following orthodromic or antidromic activation

The functional organization of the perforant path input to the dentate gyrus of the exposed hippocampus was studied in adult rabbits anesthetized with urethane and chloralose. Electrical stimulation of perforant path fibers caused excitation of granule cells along narrow, nearly transverse strips (lamellae) of tissue. Stimulation of granule cell axons (mossy fibers) in CA3 caused antidromic activation of granule cells along similar strips. Paired-pulse stimulation revealed marked changes in granule cell excitability both within a lamella (on-line) and for several mm off-line along the septo-temporal axis of the dentate gyrus. After the first pulse, granule cells were inhibited for up to about 100 ms and then facilitated for up to hundreds of ms. Feedback activity along mossy fiber collaterals exciting local inhibitory and excitatory neurons appeared to dominate in producing on- and off-line inhibition and facilitation. Neurons mediating these effects could be inhibitory basket cells and other inhibitory interneurons targeting granule cells on- and off-line. In addition, excitatory mossy cells with far reaching, longitudinally running axons could affect off-line granule cells by exciting them directly or inhibit them indirectly by exciting local inhibitory interneurons. A scheme for dentate gyrus function is proposed whereby information to the dentate gyrus becomes split into interacting transverse strips of neuronal assemblies along which temporal processing occurs. A matrix of neuronal assemblies thus arises within which fragments of events and experiences is stored through the plasticity of synapses within and between the assemblies. Similar fragments may then be recognized at later times allowing memories of the whole to be created by pattern completion at subsequent computational stages in the hippocampus.

Nonrandom local circuits in the dentate gyrus

The dentate hilus has been extensively studied in relation to its potential role in memory and in temporal lobe epilepsy. Little is known, however, about the synapses formed between the two major cell types in this region, glutamatergic mossy cells and hilar interneurons, or the organization of local circuits involving these cells. Using triple and quadruple simultaneous intracellular recordings in rat hippocampal slices, we find that mossy cells evoke EPSPs with high failure rates onto hilar neurons. Mossy cells show profound synapse specificity; 87.5% of their intralamellar connections are onto hilar interneurons. Hilar interneurons also show synapse specificity and preferentially inhibit mossy cells; 81% of inhibitory hilar synapses are onto mossy cells. Hilar IPSPs have low failure rates, are blocked by the GABA(A) receptor antagonist gabazine, and exhibit short-term depression when tested at 17 Hz. Surprisingly, more than half (57%) of the mossy cell synapses we found onto interneurons were part of reciprocal excitatory/inhibitory local circuit motifs. Neither the high degree of target cell specificity, nor the significant enrichment of structured polysynaptic local circuit motifs, could be explained by nonrandom sampling or somatic proximity. Intralamellar hilar synapses appear to function primarily by integrating synchronous inputs and presynaptic burst discharges, allowing hilar cells to respond over a large dynamic range of input strengths. The reciprocal mossy cell/interneuron local circuit motifs we find enriched in the hilus may generate sparse neural representations involved in hippocampal memory operations.

A role for hilar cells in pattern separation in the dentate gyrus: a computational approach

We present a simple computational model of the dentate gyrus to evaluate the hypothesis that pattern separation, defined as the ability to transform a set of similar input patterns into a less-similar set of output patterns, is dynamically regulated by hilar neurons. Prior models of the dentate gyrus have generally fallen into two categories: simplified models that have focused on a single granule cell layer and its ability to perform pattern separation, and large-scale and biophysically realistic models of dentate gyrus, which include hilar cells, but which have not specifically addressed pattern separation. The present model begins to bridge this gap. The model includes two of the major subtypes of hilar cells: excitatory hilar mossy cells and inhibitory hilar interneurons that receive input from and project to the perforant path terminal zone (HIPP cells). In the model, mossy cells and HIPP cells provide a mechanism for dynamic regulation of pattern separation, allowing the system to upregulate and downregulate pattern separation in response to environmental and task demands. Specifically, pattern separation in the model can be strongly decreased by decreasing mossy cell function and/or by increasing HIPP cell function; pattern separation can be increased by the opposite manipulations. We propose that hilar cells may similarly mediate dynamic regulation of pattern separation in the dentate gyrus in vivo, not only because of their connectivity within the dentate gyrus, but also because of their modulation by brainstem inputs and by the axons that "backproject" from area CA3 pyramidal cells.

Presynaptic glycine receptors facilitate spontaneous glutamate release onto hilar neurons in the rat hippocampus

Although glycine receptors are found in most areas of the brain, including the hippocampus, their functional significance remains largely unknown. In the present study, we have investigated the role of presynaptic glycine receptors on excitatory nerve terminals in spontaneous glutamatergic transmission. Spontaneous EPSCs (sEPSCs) were recorded in mechanically dissociated rat dentate hilar neurons attached with native presynaptic nerve terminals using a conventional whole-cell patch recording technique under voltage-clamp conditions. Exogenously applied glycine or taurine significantly increased the frequency of sEPSCs in a concentration-dependent manner. This facilitatory effect of glycine was blocked by 1 microM strychnine, a specific glycine receptor antagonist, but was not affected by 30 microM picrotoxin. In addition, Zn(2+) (10 microM) potentiated the glycine action on sEPSC frequency. Pharmacological data suggested that the activation of presynaptic glycine receptors directly depolarizes glutamatergic terminals resulting in the facilitation of spontaneous glutamate release. Bumetanide (10 microM), a specific Na-K-2C co-transporter blocker, gradually attenuated the glycine-induced sEPSC facilitation, suggesting that the depolarizing action of presynaptic glycine receptors was due to a higher intraterminal Cl(-) concentration. The present results suggest that presynaptic glycine receptors on excitatory nerve terminals might play an important role in the excitability of the dentate gyrus-hilus-CA3 network in physiological and/or pathological conditions.

Targeting the hippocampal mossy fiber synapse for the treatment of psychiatric disorders

It is widely known that new neurons are continuously generated in the dentate gyrus of the hippocampus in the adult mammalian brain. This neurogenesis has been implicated in depression and antidepressant treatments. Recent evidence also suggests that the dentate gyrus is involved in the neuropathology and pathophysiology of schizophrenia and other related psychiatric disorders. Especially, abnormal neuronal development in the dentate gyrus may be a plausible risk factor for the diseases. The synapse made by the mossy fiber, the output fiber of the dentate gyrus, plays a critical role in regulating neuronal activity in its target CA3 area. The mossy fiber synapse is characterized by remarkable activity-dependent short-term synaptic plasticity that is established during the postnatal development and is supposed to be central to the functional role of the mossy fiber. Any defects, including developmental abnormalities, in the dentate gyrus and drugs acting on the dentate gyrus can modulate the mossy fiber-CA3 synaptic transmission, which may eventually affect hippocampal functions. In this paper, I review recent evidence for involvement of the dentate gyrus and mossy fiber synapse in psychiatric disorders and discuss potential importance of drugs targeting the mossy fiber synapse either directly or indirectly in the therapeutic treatments of psychiatric disorders.

Intrinsic connections of the macaque monkey hippocampal formation: I. Dentate gyrus

We have carried out a detailed analysis of the intrinsic connectivity of the Macaca fascicularis monkey hippocampal formation. Here we report findings on the topographical organization of the major connections of the dentate gyrus. Localized anterograde tracer injections were made at various rostrocaudal levels of the dentate gyrus, and we investigated the three-dimensional organization of the mossy fibers, the associational projection, and the local projections. The mossy fibers travel throughout the transverse extent of CA3 at the level of the cells of origin. Once the mossy fibers reach the distal portion of CA3, they change course and travel for 3-5 mm rostrally. The associational projection, originating from cells in the polymorphic layer, terminates in the inner one-third of the molecular layer. The associational projection, though modest at the level of origin, travels both rostrally and caudally from the injection site for as much as 80% of the rostrocaudal extent of the dentate gyrus. The caudally directed projection is typically more extensive and denser than the rostrally directed projection. Cells in the polymorphic layer originate local projections that terminate in the outer two-thirds of the molecular layer. These projections are densest at the level of the cells of origin but also extend several millimeters rostrocaudally. Overall, the topographic organization of the intrinsic connections of the monkey dentate gyrus is largely similar to that of the rat. Such extensive longitudinal connections have the potential for integrating information across much of the rostrocaudal extent of the dentate gyrus.

Neurons born in the adult dentate gyrus form functional synapses with target cells

Adult neurogenesis occurs in the hippocampus and the olfactory bulb of the mammalian CNS. Recent studies have demonstrated that newborn granule cells of the adult hippocampus are postsynaptic targets of excitatory and inhibitory neurons, but evidence of synapse formation by the axons of these cells is still lacking. By combining retroviral expression of green fluorescent protein in adult-born neurons of the mouse dentate gyrus with immuno-electron microscopy, we found output synapses that were formed by labeled terminals on appropriate target cells in the CA3 area and the hilus. Furthermore, retroviral expression of channelrhodopsin-2 allowed us to light-stimulate newborn granule cells and identify postsynaptic target neurons by whole-cell recordings in acute slices. Our structural and functional evidence indicates that axons of adult-born granule cells establish synapses with hilar interneurons, mossy cells and CA3 pyramidal cells and release glutamate as their main neurotransmitter.

Semilunar granule cells: glutamatergic neurons in the rat dentate gyrus with axon collaterals in the inner molecular layer

Synaptic reorganization of the dentate gyrus inner molecular layer (IML) is a pathophysiological process that may facilitate seizures in patients with temporal-lobe epilepsy. Two subtypes of IML neurons were originally described by Ramón y Cajal (1995), but have not been thoroughly studied. We used two-photon imaging, infrared-differential interference contrast microscopy and patch clamp recordings from rat hippocampal slices to define the intrinsic physiology and synaptic targets of spiny, granule-like neurons in the IML, termed semilunar granule cells (SGCs). These neurons resembled dentate granule cells but had axon collaterals in the molecular layer, significantly larger dendritic arborization in the molecular layer, and a more triangular cell body than granule cells. Unlike granule cells, SGCs fired throughout long-duration depolarizing steps and had ramp-like depolarizations during interspike periods. Paired recordings demonstrated that SGCs are glutamatergic and monosynaptically excite both hilar interneurons and mossy cells. Semilunar granule cells appear to represent a distinct excitatory neuron population in the dentate gyrus that may be an important target for mossy fiber sprouting in patients and rodent models of temporal lobe epilepsy.

Glycoprotein M6a is present in glutamatergic axons in adult rat forebrain and cerebellum

Glycoprotein M6a is a neuronally expressed member of the proteolipid protein (PLP) family of tetraspans. In vitro studies suggested a potential role in neurite outgrowth and spine formation and previous investigations have identified M6a as a stress-regulated gene. To investigate whether the distribution of M6a correlates with neuronal structures susceptible to alterations in response to stress, we localized M6a expression in neurons of hippocampal formation, prefrontal cortex and cerebellum using in situ hybridization and confocal immunofluorescence microscopy. In situ hybridization confirmed that M6a is expressed in dentate gyrus and cerebellar granule neurons and in hippocampal and cortical pyramidal neurons. Confocal microscopy localized M6a immunoreactivity to distinct sites within axonal membranes, but not in dendrites or neuronal somata. Moreover, M6a colocalized with synaptic markers of glutamatergic, but not GABAergic nerve terminals. M6a expression in the adult brain is particularly strong in unmyelinated axonal fibers, i.e. cerebellar parallel and hippocampal mossy fibers. In contrast, myelinated axons exhibit only minimal M6a immunoreactivity localized exclusively to terminal regions. The present neuroanatomical data demonstrate that M6a is an axonal component of glutamatergic neurons and that it is localized to distinct sites of the axonal plasma membrane of pyramidal and granule cells.

Anatomic and electrophysiologic evidence for a proconvulsive circuit in the dentate gyrus of reeler mutant mice, an animal model of diffuse cortical malformation

Although cortical malformations (CMs) are often associated with epilepsy, the underlying mechanisms are unknown. The reeler mouse is a model of CM with enhanced susceptibility to epileptiform activity, including the in vitro dentate gyrus, a region normally resistant to seizures. In this study, field potential recordings in hippocampal slices and the Timm stain were used to examine mossy fiber distribution in the dentate gyrus. In artificial cerebrospinal fluid containing bicuculline, 100% of reeler slices and 0% of control slices had spontaneous and antidromic evoked prolonged negative field potential shifts that were blocked by glutamate receptor antagonists. Sections from reeler mice, but not controls, exhibited a dark band of Timm's stain at the molecular layer/granule cell layer border. These data reveal that mossy fiber distribution is altered in reeler mice and coincides with the presence of an abnormal proconvulsive glutamatergic circuit.

Loss of input from the mossy cells blocks maturation of newly generated granule cells

The objective of this work is to check whether the input from the mossy cells to the inner molecular layer is necessary for the integration and maturation of the newly generated granule cells of the dentate gyrus (DG) in mice, and if after status epilepticus the sprouting of the mossy fibers can substitute for this projection. Newly generated cells were labeled by administration of 5-bromo-deoxyuridine either before or after pilocarpine administration. The neuronal loss in the hippocampus after administration of pilocarpine combined with scopolamine and diazepam seemed restricted to the hilar mossy cells. The maturation of the granule cells was studied using immunohistochemistry for calretinin and NeuN in combination with detection of 5-bromo-deoxyuridine. The sprouting of the mossy fibers was detected using Timm staining for zinc-rich boutons. In normal conditions, granule cells took about 2 weeks to lose the immature marker calretinin. After the loss of the mossy cells, newly generated granule cells remained expressing calretinin for more than a month, until the sprouting of the mossy fibers substituted for the projection of the mossy cells in the inner molecular layer of the DG. Therefore, a proper pattern of connectivity is necessary for the normal development and integration of newly generated granule cells in the adult brain. In a changed environment they cannot adapt transforming in other cell types; simply they are unable to mature. The sprouting of the mossy fibers, although aberrant and a probable source of epileptic activity, may be important for the correct physiology of the granule cells, restoring a likeness of normality in their connective environment. The survival of granule cells incorporated as mature neurons was increased after pilocarpine when compared with normal conditions. Thus, it is likely that the reorganization of the circuitry after the loss of the mossy cells facilitates the survival and incorporation of the newly generated granule cells.

The dentate gyrus as a filter or gate: a look back and a look ahead

The idea of the dentate gyrus as a gate or filter at the entrance to the hippocampus, blocking or filtering incoming excitation from the entorhinal cortex, has been an intriguing one. Here we review the historical development of the idea, and discuss whether it may be possible to be more specific in defining this gate. We propose that dentate function can be understood within a context of Hebbian association and competition: hilar mossy cells help the dentate granule cells to recognize incoming entorhinal patterns of activity (Hebbian association), after which patterns that are consistently and repetitively presented to the dentate gyrus are passed through, while random, more transient patterns are blocked (non-associative Hebbian competition). Translamellar inhibition as well as translamellar potentiation can be understood in this context. The dentate-hilar complex thus plays the role of a "pattern excluder", not a pattern completer. The unique role of pattern exclusion may explain the peculiar qualities of dentate granule cells and hilar mossy cells.

Mossy fiber synaptic transmission: communication from the dentate gyrus to area CA3

Communication between the dentate gyrus (DG) and area CA3 of the hippocampus proper is transmitted via axons of granule cells--the mossy fiber (MF) pathway. In this review we discuss and compare the properties of transmitter release from the MFs onto pyramidal neurons and interneurons. An examination of the anatomical connectivity from DG to CA3 reveals a surprising interplay between excitation and inhibition for this circuit. In this respect it is particularly relevant that the major targets of the MFs are interneurons and that the consequence of MF input into CA3 may be inhibitory or excitatory, conditionally dependent on the frequency of input and modulatory regulation. This is further complicated by the properties of transmitter release from the MFs where a large number of co-localized transmitters, including GABAergic inhibitory transmitter release, and the effects of presynaptic modulation finely tune transmitter release. A picture emerges that extends beyond the hypothesis that the MFs are simply "detonators" of CA3 pyramidal neurons; the properties of synaptic information flow from the DG have more subtle and complex influences on the CA3 network.

Ultrastructure and synaptic connectivity of cell types in the adult rat dentate gyrus

The rat hippocampal dentate gyrus is an extensively studied structural component of the limbic system. It is the first station in the classical tri-synaptic circuit of the hippocampus in that its major input arises from the entorhinal cortex via the perforant pathway. The second part of this circuit arises from the projection cells of the dentate gyrus, the granule cells, which send their axons to the pyramidal cells of CA3. Within the dentate gyrus, there also is an extensive inhibitory network of cells that are involved in synchronizing the rhythmic firing of the granule cells. This chapter provides a review of the ultrastructural features and synaptic connectivity of both projection cells and local circuit neurons in the dentate gyrus.

Models, structure, function: the transformation of cortical signals in the dentate gyrus

Our central question is why the hippocampal CA3 region is the only cortical area capable of forming interference-free representations of complex environmental events (episodes), given that apparently all cortical regions have recurrent excitatory circuits with modifiable synapses, the basic substrate for autoassociative memory networks. We review evidence for the radical (but classic) view that a unique transformation of incoming cortical signals by the dentate gyrus and the subsequent faithful transfer of the resulting code by the mossy fibers are absolutely critical for the appropriate association of memory items by CA3 and, in general, for hippocampal function. In particular, at the gate of the hippocampal formation, the dentate gyrus possesses a set of unusual properties, which selectively evolved for the task of code transformation between cortical afferents and the hippocampus. These evolutionarily conserved anatomical features enable the dentate gyrus to translate the noisy signal of the upstream cortical areas into the sparse and specific code of hippocampal formation, which is indispensable for the efficient storage and recall of multiple, multidimensional memory items. To achieve this goal the mossy fiber pathway maximally utilizes the opportunity to differentially regulate its postsynaptic partners. Selective innervation of CA3 pyramidal cells and interneurons by distinct terminal types creates a favorable condition to differentially regulate the short-term and long-term plasticity and the motility of various mossy terminal types. The utility of this highly dynamic system appears to be the frequency-dependent fine-tuning the excitation and inhibition evoked by the large and the small mossy terminals respectively. This will determine exactly which CA3 cell population is active and induces permanent modification in the autoassociational network of the CA3 region.

Hilar mossy cells: functional identification and activity in vivo

Network oscillations are proposed to provide the framework for the ongoing neural computations of the brain. Thus, an important aspect of understanding the functional roles of various cell classes in the brain is to understand the relationship of cellular activity to the ongoing oscillations. While many studies have characterized the firing properties of cells in the hippocampal network including granule cells, pyramidal cells and interneurons, information about the activity of dentate mossy cells in the intact brain is scant. Here we review the currently available information and describe biophysical properties and network-related firing patterns of mossy cells in vivo. These new observations will assist in the extracellular identification of this unique cell type and help elucidate their functional role in behaving animals.

The CA3 "backprojection" to the dentate gyrus

The hippocampus is typically described in the context of the trisynaptic circuit, a pathway that relays information from the perforant path to the dentate gyrus, dentate to area CA3, and CA3 to area CA1. Associated with this concept is the assumption that most hippocampal information processing occurs along the trisynaptic circuit. However, the entorhinal cortex may not be the only major extrinsic input to consider, and the trisynaptic circuit may not be the only way information is processed in hippocampus. Area CA3 receives input from a variety of sources, and may be as much of an "entry point" to hippocampus as the dentate gyrus. The axon of CA3 pyramidal cells targets diverse cell types, and has commissural projections, which together make it able to send information to much more of the hippocampus than granule cells. Therefore, CA3 pyramidal cells seem better designed to spread information through hippocampus than the granule cells. From this perspective, CA3 may be a point of entry that receives information which needs to be "broadcasted," whereas the dentate gyrus may be a point of entry that receives information with more selective needs for hippocampal processing. One aspect of the argument that CA3 pyramidal cells have a widespread projection is based on a part of its axonal arbor that has received relatively little attention, the collaterals that project in the opposite direction to the trisynaptic circuit, "back" to the dentate gyrus. The evidence for this "backprojection" to the dentate gyrus is strong, particularly in area CA3c, the region closest to the dentate gyrus, and in temporal hippocampus. The influence on granule cells is indirect, through hilar mossy cells and GABAergic neurons of the dentate gyrus, and appears to include direct projections in the case of CA3c pyramidal cells of ventral hippocampus. Physiological studies suggest that normally area CA3 does not have a robust excitatory influence on granule cells, but serves instead to inhibit it by activating dentate gyrus GABAergic neurons. Thus, GABAergic inhibition normally controls the backprojection to dentate granule cells, analogous to the way GABAergic inhibition appears to control the perforant path input to granule cells. From this perspective, the dentate gyrus has two robust glutamatergic inputs, entorhinal cortex and CA3, and two "gates," or inhibitory filters that reduce the efficacy of both inputs, keeping granule cells relatively quiescent. When GABAergic inhibition is reduced experimentally, or under pathological conditions, CA3 pyramidal cells activate granule cells reliably, and do so primarily by disynaptic excitation that is mediated by mossy cells. We suggest that the backprojection has important functions normally that are dynamically regulated by nonprincipal cells of the dentate gyrus. Slightly reduced GABAergic input would lead to increased polysynaptic associative processing between CA3 and the dentate gyrus. Under pathological conditions associated with loss of GABAergic interneurons, the backprojection may support reverberatory excitatory activity between CA3, mossy cells, and granule cells, possibly enhanced by mossy fiber sprouting. In this case, the backprojection could be important to seizure activity originating in hippocampus, and help explain the seizure susceptibility of ventral hippocampus.

Unitary IPSPs enhance hilar mossy cell gain in the rat hippocampus

Mechanisms that control neuronal gain allow for adaptive rescaling to synaptic inputs of varying strengths or frequencies. Here, we show that unitary IPSPs (uIPSPs) modulate gain and unitary EPSP (uEPSP)-action potential coupling in mossy cells (MCs) from rat hippocampal slices. Mossy fibre-evoked uEPSCs were large, facilitated and were suppressed by the group II metabotropic glutamate agonist LY354740. Conversely, uIPSCs were smaller, depressed and were not affected by LY354740, but exerted strong inhibitory control over uEPSP-action potential coupling. The IPSC reversal potential was determined by gramicidin perforated patch recordings to be -65.3 +/- 5.0 mV, lying between the resting membrane potential (-75.3 +/- 1.1 mV) and the action potential threshold (-56.5 +/- 2.4 mV). When applied at theta frequency (10 Hz), uIPSPs increased the offset of the MC input-output response to depolarizing current injection, but also increased gain, maximal firing rate and the slope of the depolarization preceding action potentials. These effects were unchanged by the Ca2+ and HCN channel blockers mibefradil and ZD7288, respectively. The height and maximal slope of MC action potentials during tonic depolarization were also increased by uIPSPs, and the decay of uIPSP conductances injected by dynamic clamp at subthreshold membrane potentials was prolonged by TTX. Application of the muscarinic agonist pilocarpine mimicked the effect of IPSPs on MC maximal firing rate, and action potential height and slope, and this was reversed by the GABA(A) antagonist gabazine. Thus, uIPSPs can increase neuronal gain under hyperexcitable conditions, and this effect is probably due to the de-inactivation of a TTX-sensitive voltage-dependent Na+ conductance.

Long- and short-term plasticity at mossy fiber synapses on mossy cells in the rat dentate gyrus

Mossy cells give rise to the commissural and associational pathway of the dentate gyrus, and receive their major excitatory inputs from the mossy fibers of granule cells. Through these feed-back excitatory connections, mossy cells have been suggested to play important roles in both normal signal processing in learning and memory, as well as in seizure propagation. However, the nature of the activity-dependent modifications of the mossy fiber inputs to mossy hilar cells is not well understood. We studied the long- and short-term plasticity properties of the mossy fiber-mossy cell synapse, using the minimal stimulation technique in slices in whole cell recorded mossy cells retrogradely prelabeled with the fluorescent dye DiO from the contralateral dentate gyrus. Following tetanic stimulation, mossy fiber synapses showed significant NMDA receptor-independent long-term potentiation (LTP), associated with increased excitatory postsynaptic currents (EPSC) amplitude and decreased failure rates. Coefficient of variance and failure rate analyses suggested a presynaptic locus of LTP induction. Mossy fiber synapses on mossy cells also showed activity-dependent short-term modification properties, including both frequency-dependent facilitation (stimuli at higher frequencies evoked larger EPSCs with lower failure rates) and burst facilitation (each EPSC in a burst had a larger amplitude and higher probability of occurrence than the preceding EPSCs within the burst). The data show that mossy fiber-mossy cell synapses exhibit both long- and short-term plasticity phenomena that are generally similar to the mossy fiber synapses on CA3 pyramidal cells.