Translamellar disinhibition in the rat hippocampal dentate gyrus after seizure-induced degeneration of vulnerable hilar neurons

Rapid sequential clustering of NMDARs, CaMKII, and AMPARs upon activation of NMDARs at developing synapses

Rapid, synapse-specific neurotransmission requires the precise alignment of presynaptic neurotransmitter release and postsynaptic receptors. How postsynaptic glutamate receptor accumulation is induced during maturation is not well understood. We find that in cultures of dissociated hippocampal neurons at 11 days in vitro (DIV) numerous synaptic contacts already exhibit pronounced accumulations of the pre- and postsynaptic markers synaptotagmin, synaptophysin, synapsin, bassoon, VGluT1, PSD-95, and Shank. The presence of an initial set of AMPARs and NMDARs is indicated by miniature excitatory postsynaptic currents (mEPSCs). However, AMPAR and NMDAR immunostainings reveal rather smooth distributions throughout dendrites and synaptic enrichment is not obvious. We found that brief periods of Ca2+ influx through NMDARs induced a surprisingly rapid accumulation of NMDARs within 1 min, followed by accumulation of CaMKII and then AMPARs within 2-5 min. Postsynaptic clustering of NMDARs and AMPARs was paralleled by an increase in their mEPSC amplitudes. A peptide that blocked the interaction of NMDAR subunits with PSD-95 prevented the NMDAR clustering. NMDAR clustering persisted for 3 days indicating that brief periods of elevated glutamate fosters permanent accumulation of NMDARs at postsynaptic sites in maturing synapses. These data support the model that strong glutamatergic stimulation of immature glutamatergic synapses results in a fast and substantial increase in postsynaptic NMDAR content that required NMDAR binding to PSD-95 or its homologues and is followed by recruitment of CaMKII and subsequently AMPARs.

Neuronal activity dynamics in the dentate gyrus during early epileptogenesis

Epileptogenesis is a complex process involving a multitude of changes at the molecular, cellular and network level. Previous studies have identified several key alterations contributing to epileptogenesis and the development of hyper-excitability in different animal models, but only a few have focused on the early stages of this process. For post status epilepticus (SE) temporal lobe epilepsy in particular, understanding network dynamics during the early phases might be crucial for developing accurate preventive treatments to block the development of chronic spontaneous seizures. In this study, we used a viral vector mediated approach to examine activity of neurons in the dentate gyrus of the hippocampus during early epileptogenesis. We find that while granule cells are active 8 h after SE and then gradually decrease their activity, Calretinin-positive mossy cells and Neuropeptide Y-positive GABAergic interneurons in the hilus show a delayed activation pattern starting at 24 and peaking at 48 h after SE. These data suggest that indirect inhibition of granule cells by mossy cells through recruitment of local GABAergic interneurons could be an important mechanisms of excitability control during early epileptogenesis, and contribute to our understanding of the complex role of these cells in normal and pathological conditions.

Hippocampal interlamellar cell-cell connectome that counts

The hippocampus is regarded as a cognition hub, particularly for learning and memory. Previously, neuronal mechanisms underlying various cognitive functions are delineated with the lamellar hippocampal circuitry, dentate gyrus-CA3 or CA2-CA1, within the transverse plane. More recently, interlamellar (often referred to as longitudinal) projections have received intensive attention to help understand signal convergence and divergence in cognition and behavior. Signal propagation along the longitudinal axis is evidenced by axonal arborization patterns and synaptic responses to electro- and photo-stimulation, further demonstrating that information flow is more enriched in the longitudinal plane than the transverse plane. Here, we review the significance of longitudinal connections for cognition, discuss a putative circuit mechanism of place coding, and suggest the reconceptualization of the hippocampal circuitry.

Dynamics of longitudinal dentate gyrus axons associated with seizure

KEY POINTS

Axon collaterals of DG granule neurons project towards neighbouring DG granule cell layer Longitudinal axons in the DG-DG circuit possess denser synapses than transverse axons in the DG-CA3 circuit The size of varicosities of the longitudinal axons, but not transverse ones, is regulated by seizures as measured behaviourally Varicosity size of DG-DG axons can be a symptomatic marker of DG-related brain diseases ABSTRACT: The hippocampus network has captured the attention of neuroscientists as a model for understanding cognition and behaviour. Previously, we have identified interlamellar, namely longitudinal, axons between CA1 pyramidal neurons analogous to recurrent connections between CA3 pyramidal neurons. Currently it is unknown whether longitudinal axons of DG granule neurons are present and how they are associated with the behavioural symptoms of seizure. We found longitudinal axons projections from DG granule cells extending to neighbouring DG granule cell layers. These DG-DG axons have more numerous varicosities and are thinner than the DG-CA3 axons, suggesting heavy synaptic formation along a longitudinal axis. Furthermore, the size of varicosities in the DG-DG but not DG-CA3 axons is regulated by seizures as measured behaviourally. The results suggest that the dynamics of longitudinal DG axons is a symptomatic marker of DG-related brain diseases.

Mossy Cells in the Dorsal and Ventral Dentate Gyrus Differ in Their Patterns of Axonal Projections

Mossy cells (MCs) of the dentate gyrus (DG) are a major group of excitatory hilar neurons that are important for regulating activity of dentate granule cells. MCs are particularly intriguing because of their extensive longitudinal connections within the DG. It has generally been assumed that MCs in the dorsal and ventral DG have similar patterns of termination in the inner one-third of the dentate molecular layer. Here, we demonstrate that axonal projections of MCs in these two regions are considerably different. MCs in dorsal and ventral regions were labeled selectively with Cre-dependent eYFP or mCherry, using two transgenic mouse lines (including both sexes) that express Cre-recombinase in MCs. At four to six weeks following unilateral labeling of MCs in the ventral DG, a dense band of fibers was present in the inner one-fourth of the molecular layer and extended bilaterally throughout the rostral-caudal extent of the DG, replicating the expected distribution of MC axons. In contrast, following labeling of MCs in the dorsal DG, the projections were more diffusely distributed. At the level of transfection, fibers were present in the inner molecular layer, but they progressively expanded into the middle molecular layer and, most ventrally, formed a distinct band in this region. Optical stimulation of these caudal fibers expressing ChR2 demonstrated robust EPSCs in ipsilateral granule cells and enhanced the effects of perforant path stimulation in the ventral DG. These findings suggest that MCs in the dorsal and ventral DG differ in the distribution of their axonal projections and possibly their function.SIGNIFICANCE STATEMENT Mossy cells (MCs), a major cell type in the hilus of the dentate gyrus (DG), are unique in providing extensive longitudinal and commissural projections throughout the DG. Although it has been assumed that all MCs have similar patterns of termination in the inner molecular layer of the DG, we discovered that the axonal projections of dorsal and ventral MCs differ. While ventral MC projections exhibit the classical pattern, with dense innervation in the inner molecular layer, dorsal MCs have a more diffuse distribution and expand into the middle molecular layer where they overlap and interact with innervation from the perforant path. These distinct locations and patterns of axonal projections suggest that dorsal and ventral MCs may have different functional roles.

Pharmaco-genetic inhibition of pyramidal neurons retards hippocampal kindling-induced epileptogenesis

AIMS

Pharmaco-genetics emerges as a new promising approach for epileptic seizures. Whether it can modulate epileptogenesis is still unknown.

METHODS

Here, parvalbumin neurons and pyramidal neurons of the seizure focus were transfected with engineered excitatory Gq-coupled human muscarinic receptor hM3Dq and engineered inhibitory Gi-coupled human muscarinic receptor hM4Di, respectively. And their therapeutic value in mouse hippocampal kindling-induced epileptogenesis was tested.

RESULTS

Pharmaco-genetic activating parvalbumin neurons limitedly retarded the progression of behavioral seizure stage and afterdischarge duration (ADD) during epileptogenesis induced by kindling. Activating parvalbumin neurons delayed seizure development only in the early stage, but accelerated it in late stages. On the contrary, pharmaco-genetic inhibiting pyramidal neurons robustly retarded the progression of seizure stages and ADDs, which greatly delayed seizure development in both early and late stages. Although both pharmaco-genetic therapeutics efficiently alleviated the severity of acute kindling-induced seizures, pharmaco-genetic inhibiting pyramidal neurons were able to reverse the enhanced synaptic plasticity during epileptogenesis, compared with that of pharmaco-genetic activating parvalbumin neurons.

CONCLUSION

Our results demonstrated that pharmaco-genetic inhibiting pyramidal neurons retard hippocampal kindling-induced epileptogenesis and reverse the enhanced synaptic plasticity during epileptogenesis, compared with that of pharmaco-genetic activating parvalbumin neurons. It suggests that pharmaco-genetics targeting pyramidal neurons may be a promising treatment for epileptogenesis.

Bestrophin1-mediated tonic GABA release from reactive astrocytes prevents the development of seizure-prone network in kainate-injected hippocampi

Tonic extrasynaptic GABA_A receptor (GABA_A R) activation is under the tight control of tonic GABA release from astrocytes to maintain the brain's excitation/inhibition (E/I) balance; any slight E/I balance disturbance can cause serious pathological conditions including epileptic seizures. However, the pathophysiological role of tonic GABA release from astrocytes has not been tested in epileptic seizures. Here, we report that pharmacological or genetic intervention of the GABA-permeable Bestrophin-1 (Best1) channel prevented the generation of tonic GABA inhibition, disinhibiting CA1 pyramidal neuronal firing and augmenting seizure susceptibility in kainic acid (KA)-induced epileptic mice. Astrocyte-specific Best1 over-expression in KA-injected Best1 knockout mice fully restored the generation of tonic GABA inhibition and effectively suppressed seizure susceptibility. We demonstrate for the first time that tonic GABA from reactive astrocytes strongly contributes to the compensatory shift of E/I balance in epileptic hippocampi, serving as a good therapeutic target against altered E/I balance in epileptic seizures.

Targeted hippocampal GABA neuron ablation by Stable Substance P-saporin causes hippocampal sclerosis and chronic epilepsy in rats

Cryptogenic temporal lobe epilepsy develops in the absence of identified brain injuries, infections, or structural malformations, and in these cases, an unidentified pre-existing abnormality may initiate febrile seizures, hippocampal sclerosis, and epilepsy. Although a role for GABAergic dysfunction in epilepsy is intuitively obvious, no causal relationship has been established. In this study, hippocampal GABA neurons were targeted for selective elimination to determine whether a focal hippocampal GABAergic defect in an otherwise normal brain can initiate cryptogenic temporal lobe epilepsy with hippocampal sclerosis. We used Stable Substance P-saporin conjugate (SSP-saporin) to target rat hippocampal GABA neurons, which selectively and constitutively express the neurokinin-1 receptors that internalize this neurotoxin. Bilateral and longitudinally extensive intrahippocampal microinjections of SSP-saporin caused no obvious behavioral effects for several days. However, starting ~4 days postinjection, rats exhibited episodes of immobilization, abnormal flurries of "wet-dog" shakes, and brief focal motor seizures characterized by facial automatisms and forepaw clonus. These clinically subtle behaviors stopped after ~4 days. Convulsive status epilepticus did not develop, and no deaths occurred. Months later, chronically implanted rats exhibited spontaneous focal motor seizures and extreme hippocampal sclerosis. These data suggest that hippocampal GABAergic dysfunction is epileptogenic and can produce the defining features of cryptogenic temporal lobe epilepsy.

Acute Stress Facilitates LTD Induction at Glutamatergic Synapses in the Hippocampal CA1 Region by Activating μ-Opioid Receptors on GABAergic Neurons

Acute stress impairs recall memory through the facilitation of long-term depression (LTD) of hippocampal synaptic transmission. The endogenous opioid system (EOS) plays essential roles in stress-related emotional and physiological responses. Specifically, behavioral studies have shown that the impairment of memory retrieval induced by stressful events involves the activation of opioid receptors. However, it is unclear whether signaling mediated by μ-opioid receptors (μRs), one of the three major opioid receptors, participates in acute stress-related hippocampal LTD facilitation. Here, we examined the effects of a single elevated platform (EP) stress exposure on excitatory synaptic transmission and plasticity at the Schaffer collateral-commissural (SC) to CA1 synapses by recording electrically evoked field excitatory postsynaptic potentials and population spikes of hippocampal pyramidal neurons in anesthetized adult mice. EP stress exposure attenuated GABAergic feedforward and feedback inhibition of CA1 pyramidal neurons and facilitated low-frequency stimulation (LFS)-induced long-term depression (LTD) at SC-CA1 glutamatergic synapses. These effects were reproduced by exogenously activating μRs in unstressed mice. The specific deletion of μRs on GABAergic neurons (μR_GABA) not only prevented the EP stress-induced memory impairment but also reversed the EP stress-induced attenuation of GABAergic inhibition and facilitation of LFS-LTD. Our results suggest that acute stress endogenously activates μR_GABA to attenuate hippocampal GABAergic signaling, thereby facilitating LTD induction at excitatory synapses and eliciting memory impairments.

No latency to dentate granule cell epileptogenesis in experimental temporal lobe epilepsy with hippocampal sclerosis

OBJECTIVE

To determine when spontaneous granule cell epileptiform discharges first occur after hippocampal injury, and to identify the postinjury "latent" period as either a "silent" gestational state of epileptogenesis or a subtle epileptic state in gradual transition to a more obvious epileptic state.

METHODS

Nonconvulsive status epilepticus evoked by perforant path stimulation in urethane-sedated rats produced selective and extensive hippocampal injury and a "latent" period that preceded the onset of the first clinically obvious epileptic seizures. Continuous granule cell layer depth recording and video monitoring assessed the time course of granule cell hyperexcitability and the onset/offset times of spontaneous epileptiform discharges and behavioral seizures.

RESULTS

One day postinjury, granule cells in awake rats were hyperexcitable to afferent input, and continuously generated spontaneous population spikes. During the ~2-4 week "latent" period, granule cell epileptiform discharges lasting ~30 seconds caused subtle focal seizures characterized by immobilization and facial automatisms that were undetected by behavioral assessment alone but identified post hoc. Granule cell layer epileptiform discharge duration eventually tripled, which caused the first clinically obvious seizure, ending the "latent" period. Behavioral seizure duration was linked tightly to spontaneous granule cell layer events. Granule cell epileptiform discharges preceded all behavioral seizure onsets, and clonic behaviors ended abruptly within seconds of the termination of each granule cell epileptiform discharge. Noninjurious hippocampal excitation produced no evidence of granule cell hyperexcitability or epileptogenesis.

SIGNIFICANCE

The latent period in this model is a subtle epileptic state in transition to a more clinically obvious epileptic state, not a seizure-free "gestational" state when an unidentified epileptogenic mechanism gradually develops. Based on the onset/offset times of electrographic and behavioral events, granule cell behavior may be the prime determinant of seizure onset, phenotype, duration, and offset in this model of hippocampal-onset epilepsy. Extensive hippocampal neuron loss could be the primary epileptogenic 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.

Epilepsy-associated alterations in hippocampal excitability

The hippocampus exhibits a wide range of epilepsy-related abnormalities and is situated in the mesial temporal lobe, where limbic seizures begin. These abnormalities could affect membrane excitability and lead to overstimulation of neurons. Multiple overlapping processes refer to neural homeostatic responses develop in neurons that work together to restore neuronal firing rates to control levels. Nevertheless, homeostatic mechanisms are unable to restore normal neuronal excitability, and the epileptic hippocampus becomes hyperexcitable or hypoexcitable. Studies show that there is hyperexcitability even before starting recurrent spontaneous seizures, suggesting although hippocampal hyperexcitability may contribute to epileptogenesis, it alone is insufficient to produce epileptic seizures. This supports the concept that the hippocampus is not the only substrate for limbic seizure onset, and a broader hyperexcitable limbic structure may contribute to temporal lobe epilepsy (TLE) seizures. Nevertheless, seizures also occur in conditions where the hippocampus shows a hypoexcitable phenotype. Since TLE seizures most often originate in the hippocampus, it could therefore be assumed that both hippocampal hypoexcitability and hyperexcitability are undesirable states that make the epileptic hippocampal network less stable and may, under certain conditions, trigger seizures.

Commonalities in epileptogenic processes from different acute brain insults: Do they translate?

The most common forms of acquired epilepsies arise following acute brain insults such as traumatic brain injury, stroke, or central nervous system infections. Treatment is effective for only 60%-70% of patients and remains symptomatic despite decades of effort to develop epilepsy prevention therapies. Recent preclinical efforts are focused on likely primary drivers of epileptogenesis, namely inflammation, neuron loss, plasticity, and circuit reorganization. This review suggests a path to identify neuronal and molecular targets for clinical testing of specific hypotheses about epileptogenesis and its prevention or modification. Acquired human epilepsies with different etiologies share some features with animal models. We identify these commonalities and discuss their relevance to the development of successful epilepsy prevention or disease modification strategies. Risk factors for developing epilepsy that appear common to multiple acute injury etiologies include intracranial bleeding, disruption of the blood-brain barrier, more severe injury, and early seizures within 1 week of injury. In diverse human epilepsies and animal models, seizures appear to propagate within a limbic or thalamocortical/corticocortical network. Common histopathologic features of epilepsy of diverse and mostly focal origin are microglial activation and astrogliosis, heterotopic neurons in the white matter, loss of neurons, and the presence of inflammatory cellular infiltrates. Astrocytes exhibit smaller K+ conductances and lose gap junction coupling in many animal models as well as in sclerotic hippocampi from temporal lobe epilepsy patients. There is increasing evidence that epilepsy can be prevented or aborted in preclinical animal models of acquired epilepsy by interfering with processes that appear common to multiple acute injury etiologies, for example, in post-status epilepticus models of focal epilepsy by transient treatment with a trkB/PLCγ1 inhibitor, isoflurane, or HMGB1 antibodies and by topical administration of adenosine, in the cortical fluid percussion injury model by focal cooling, and in the albumin posttraumatic epilepsy model by losartan. Preclinical studies further highlight the roles of mTOR1 pathways, JAK-STAT3, IL-1R/TLR4 signaling, and other inflammatory pathways in the genesis or modulation of epilepsy after brain injury. The wealth of commonalities, diversity of molecular targets identified preclinically, and likely multidimensional nature of epileptogenesis argue for a combinatorial strategy in prevention therapy. Going forward, the identification of impending epilepsy biomarkers to allow better patient selection, together with better alignment with multisite preclinical trials in animal models, should guide the clinical testing of new hypotheses for epileptogenesis and its prevention.

Dentate Mossy Cell and Pattern Separation

Three studies by Danielson et al. (2017), GoodSmith et al. (2017), and Sensai and Buzsáki (2017) distinguish in vivo firing properties of dentate mossy cells from granule cells during behavior. Robust spatial remapping of mossy cells, in contrast to sparse firing of granule cells, suggests differential involvement in pattern separation.

Depolarized GABAergic Signaling in Subicular Microcircuits Mediates Generalized Seizure in Temporal Lobe Epilepsy

Secondary generalized seizure (sGS) is a major source of disability in temporal lobe epilepsy (TLE) with unclear cellular/circuit mechanisms. Here we found that clinical TLE patients with sGS showed reduced volume specifically in the subiculum compared with those without sGS. Further, using optogenetics and extracellular electrophysiological recording in mouse models, we found that photoactivation of subicular GABAergic neurons retarded sGS acquisition by inhibiting the firing of pyramidal neurons. Once sGS had been stably acquired, photoactivation of GABAergic neurons aggravated sGS expression via depolarized GABAergic signaling. Subicular parvalbumin, but not somatostatin subtype GABAergic, neurons were easily depolarized in sGS expression. Finally, photostimulation of subicular pyramidal neurons genetically targeted with proton pump Arch, rather than chloride pump NpHR3.0, alleviated sGS expression. These results demonstrated that depolarized GABAergic signaling in subicular microcircuit mediates sGS in TLE. This may be of therapeutic interest in understanding the pathological neuronal circuitry underlying sGS. VIDEO ABSTRACT.

Dentate gyrus autonomous ictal activity in the status epilepticus rat model of epilepsy

The dentate gyrus (DG) as part of the hippocampal formation is believed to serve as a gatekeeper with strong inhibitory properties against uncontrolled propagation of neuronal activity from the entorhinal cortex and neocortical structures. In temporal lobe epilepsy, the DG becomes hyperexcitable and loses its gate function, enabling propagation of ictal activity into downstream structures such as CA3 and CA1 areas. Furthermore, the DG, apart from facilitating propagation, may also be able to autonomously generate ictal activity, but this point has remained open so far. To tackle this question, we used intrinsic optical imaging in combination with electrophysiological recordings in brain slice preparations from rats in which status epilepticus had been induced electrically several weeks prior to measurements. Upon omission of Mg⁺⁺ from the artificial cerebrospinal fluid, in 15 out of 33 slices (45.4%) from 9 out of 13 epileptic animals (69.2%), spontaneous and autonomous ictal activity, mostly seizure-like events (SLE), was observed in the DG. This activity manifested independently from SLE generated in adjacent cortices and never occurred in slices from control animals. SLE generated in the DG differed from those with origin in the entorhinal or temporal cortex by longer latency to the first event after Mg⁺⁺ omission (p<0.001), a higher SLE frequency (p<0.05), higher amplitude (p<0.001) and a longer SLE duration (p<0.05). We conclude that in epilepsy, the DG, in addition to facilitated gating of activity from upstream structures, can serve as an independent generator of ictal activity.

Mesial Temporal Lobe Epilepsy: Pathogenesis, Induced Rodent Models and Lesions

Mesial temporal lobe epilepsy (MTLE), the most common epilepsy in adults, is generally intractable and is suspected to be the result of recurrent excitation or inhibition circuitry. Recurrent excitation and the development of seizures have been associated with aberrant mossy fiber sprouting in the hippocampus. Of the animal models developed to investigate the pathogenesis of MTLE, post-status epilepticus models have received the greatest acceptance because they are characterized by a latency period, the development of spontaneous motor seizures, and a spectrum of lesions like those of MTLE. Among post-status epilepticus models, induction of systemic kainic acid or pilocarpine-induced epilepsy is less labor-intensive than electrical-stimulation models and these models mirror the clinicopathologic features of MTLE more closely than do kindling, tetanus toxin, hyperthermia, post-traumatic, and perinatal hypoxia/ischemia models. Unfortunately, spontaneous motor seizures do not develop in kindling or adult hyperthermia models and are not a consistent finding in tetanus toxin-induced or perinatal hypoxia/ischemia models. This review presents the mechanistic hypotheses for seizure induction, means of model induction, and associated pathology, especially as compared to MTLE patients. Animal models are valuable tools not only to study the pathogenesis of MTLE, but also to evaluate potential antiepileptogenic drugs.

Observations on hippocampal mossy cells in mink (Neovison vison) with special reference to dendrites ascending to the granular and molecular layers

Detailed knowledge about the neural circuitry connecting the hippocampus and entorhinal cortex is necessary to understand how this system contributes to spatial navigation and episodic memory. The two principal cell types of the dentate gyrus, mossy cells and granule cells, are interconnected in a positive feedback loop, by which mossy cells can influence information passing from the entorhinal cortex via granule cells to hippocampal pyramidal cells. Mossy cells, like CA3 pyramidal cells, are characterized by thorny excrescences on their proximal dendrites, postsynaptic to giant terminals of granule cell axons. In addition to disynaptic input from the entorhinal cortex and perforant path via granule cells, mossy cells may also receive monosynaptic input from the perforant path via special dendrites ascending to the molecular layer. We here report qualitative and quantitative descriptions of Golgi-stained hippocampal mossy cells in mink, based on light microscopic observations and three-dimensional reconstructions. The main focus is on the location, branching pattern, and length of dendrites, particularly those ascending to the granular and molecular layers. In mink, the latter dendrites are more numerous than in rat, but fewer than in primates. They form on average 12% (and up to 29%) of the total dendritic length, and appear to cover the terminal fields of both the lateral and medial perforant paths. In further contrast to rat, the main mossy cell dendrites in mink branch more extensively with distal dendrites encroaching upon the CA3 field. The dendritic arbors extend both along and across the septotemporal axis of the dentate gyrus, not conforming to the lamellar pattern of the hippocampus. The findings suggest that the afferent input to the mossy cells becomes more complex in species closer to primates.

Aberrant neuronal synaptic connectivity in CA1 area of the hippocampus from pilocarpine-induced epileptic rats observed by fluorogold

In this study, we observed synaptic connectivity among neurons in CA1 region of pilocarpine-induced chronic seizures in rats. Twenty healthy male Sprague-Dawley rats were divided randomly into an epilepsy group (n = 10) and a control group (n = 10). Approximately 60 days after status epilepticus (SE) , Fluorogold (FG) was injected into the CA1 area of the hippocampus in vivo. Somatostatin (SS) expression was observed using immunofluorescence. The distribution of FG-positive and FG/SS double-labeled neurons was observed using a confocal microscope. FG-labeled pyramidal cells could be seen remotely from the FG-injected site in the CA1 area and in the subiculum in the experimental group. FG/SS double-labeled interneurons were distributed remotely from the FG-injected site in the CA1 area in the epileptic rats. These changes suggest aberrant neuronal connectivity in CA1 region, which may lead to the formation of aberrant excitatory and inhibitory circuitry, and may play an important role in the generation or compensation for temporal lobe epilepsy.

Reorganization of supramammillary-hippocampal pathways in the rat pilocarpine model of temporal lobe epilepsy: evidence for axon terminal sprouting

In mesial temporal lobe epilepsy (MTLE), spontaneous seizures likely originate from a multi-structural epileptogenic zone, including several regions of the limbic system connected to the hippocampal formation. In this study, we investigate the structural connectivity between the supramammillary nucleus (SuM) and the dentate gyrus (DG) in the model of MTLE induced by pilocarpine in the rat. This hypothalamic nucleus, which provides major extracortical projections to the hippocampal formation, plays a key role in the regulation of several hippocampus-dependent activities, including theta rhythms, memory function and emotional behavior, such as stress and anxiety, functions that are known to be altered in MTLE. Our findings demonstrate a marked reorganization of DG afferents originating from the SuM in pilocarpine-treated rats. This reorganization, which starts during the latent period, is massive when animals become epileptic and continue to evolve during epilepsy. It is characterized by an aberrant distribution and an increased number of axon terminals from neurons of both lateral and medial regions of the SuM, invading the entire inner molecular layer of the DG. This reorganization, which reflects an axon terminal sprouting from SuM neurons, could contribute to trigger spontaneous seizures within an altered hippocampal intrinsic circuitry.

The brain, seizures and epilepsy throughout life: understanding a moving target

The human brain is a tremendously complex and still enigmatic three-dimensional structure, composed of countless interconnected neurons and glia. The temporal evolution of the brain throughout life provides a fourth dimension, one that influences every element of the brain's function in health and disease. This temporal evolution contributes to the probability of seizure generation and to the type and the nature of these seizures. The age-specific properties of the brain also influence the consequences of seizures on neuronal structure and behavior. These, in turn, govern epileptic activity and cognitive and emotional functions, contributing to the diverse consequences of seizures and epilepsy throughout life.

Hilar mossy cell degeneration causes transient dentate granule cell hyperexcitability and impaired pattern separation

Although excitatory mossy cells of the hippocampal hilar region are known to project both to dentate granule cells and to interneurons, it is as yet unclear whether mossy cell activity's net effect on granule cells is excitatory or inhibitory. To explore their influence on dentate excitability and hippocampal function, we generated a conditional transgenic mouse line, using the Cre/loxP system, in which diphtheria toxin receptor was selectively expressed in mossy cells. One week after injecting toxin into this line, mossy cells throughout the longitudinal axis were degenerated extensively, theta wave power of dentate local field potentials increased during exploration, and deficits occurred in contextual discrimination. By contrast, we detected no epileptiform activity, spontaneous behavioral seizures, or mossy-fiber sprouting 5-6 weeks after mossy cell degeneration. These results indicate that the net effect of mossy cell excitation is to inhibit granule cell activity and enable dentate pattern separation.

Hilar mossy cell circuitry controlling dentate granule cell excitability

Glutamatergic hilar mossy cells of the dentate gyrus can either excite or inhibit distant granule cells, depending on whether their direct excitatory projections to granule cells or their projections to local inhibitory interneurons dominate. However, it remains controversial whether the net effect of mossy cell loss is granule cell excitation or inhibition. Clarifying this controversy has particular relevance to temporal lobe epilepsy, which is marked by dentate granule cell hyperexcitability and extensive loss of dentate hilar mossy cells. Two diametrically opposed hypotheses have been advanced to explain this granule cell hyperexcitability—the “dormant basket cell” and the “irritable mossy cell” hypotheses. The “dormant basket cell” hypothesis proposes that mossy cells normally exert a net inhibitory effect on granule cells and therefore their loss causes dentate granule cell hyperexcitability. The “irritable mossy cell” hypothesis takes the opposite view that mossy cells normally excite granule cells and that the surviving mossy cells in epilepsy increase their activity, causing granule cell excitation. The inability to eliminate mossy cells selectively has made it difficult to test these two opposing hypotheses. To this end, we developed a transgenic toxin-mediated, mossy cell-ablation mouse line. Using these mutants, we demonstrated that the extensive elimination of hilar mossy cells causes granule cell hyperexcitability, although the mossy cell loss observed appeared insufficient to cause clinical epilepsy. In this review, we focus on this topic and also suggest that different interneuron populations may mediate mossy cell-induced translamellar lateral inhibition and intralamellar recurrent inhibition. These unique local circuits in the dentate hilar region may be centrally involved in the functional organization of the dentate gyrus.

Apolipoprotein E4 causes age- and sex-dependent impairments of hilar GABAergic interneurons and learning and memory deficits in mice

Apolipoprotein (apo) E4 is the major genetic risk factor for Alzheimer's disease (AD). ApoE4 has sex-dependent effects, whereby the risk of developing AD is higher in apoE4-expressing females than males. However, the mechanism underlying the sex difference, in relation to apoE4, is unknown. Previous findings indicate that apoE4 causes age-dependent impairments of hilar GABAergic interneurons in female mice, leading to learning and memory deficits. Here, we investigate whether the detrimental effects of apoE4 on hilar GABAergic interneurons are sex-dependent using apoE knock-in (KI) mice across different ages. We found that in female apoE-KI mice, there was an age-dependent depletion of hilar GABAergic interneurons, whereby GAD67- or somatostatin-positive–but not NPY- or parvalbumin-positive–interneuron loss was exacerbated by apoE4. Loss of these neuronal populations was correlated with the severity of spatial learning deficits at 16 months of age in female apoE4-KI mice; however, this effect was not observed in female apoE3-KI mice. In contrast, we found an increase in the numbers of hilar GABAergic interneurons with advancing age in male apoE-KI mice, regardless of apoE genotype. Moreover, male apoE-KI mice showed a consistent ratio of hilar inhibitory GABAergic interneurons to excitatory mossy cells approximating 1.5 that is independent of apoE genotype and age, whereas female apoE-KI mice exhibited an age-dependent decrease in this ratio, which was exacerbated by apoE4. Interestingly, there are no apoE genotype effects on GABAergic interneurons in the CA1 and CA3 subregions of the hippocampus as well as the entorhinal and auditory cortexes. These findings suggest that the sex-dependent effects of apoE4 on developing AD is in part attributable to inherent sex-based differences in the numbers of hilar GABAergic interneurons, which is further modulated by apoE genotype.

Updating the lamellar hypothesis of hippocampal organization

Andersen et al. (1971) proposed that excitatory activity in the entorhinal cortex propagates topographically to the dentate gyrus, and on through a "trisynaptic circuit" lying within transverse hippocampal "slices" or "lamellae." In this way, a relatively simple structure might mediate complex functions in a manner analogous to the way independent piano keys can produce a nearly infinite variety of unique outputs. The lamellar hypothesis derives primary support from the "lamellar" distribution of dentate granule cell axons (the mossy fibers), which innervate dentate hilar neurons and area CA3 pyramidal cells and interneurons within the confines of a thin transverse hippocampal segment. Following the initial formulation of the lamellar hypothesis, anatomical studies revealed that unlike granule cells, hilar mossy cells, CA3 pyramidal cells, and Layer II entorhinal cells all form axonal projections that are more divergent along the longitudinal axis than the clearly "lamellar" mossy fiber pathway. The existence of pathways with "translamellar" distribution patterns has been interpreted, incorrectly in our view, as justifying outright rejection of the lamellar hypothesis (Amaral and Witter, 1989). We suggest that the functional implications of longitudinally projecting axons depend not on whether they exist, but on what they do. The observation that focal granule cell layer discharges normally inhibit, rather than excite, distant granule cells suggests that longitudinal axons in the dentate gyrus may mediate "lateral" inhibition and define lamellar function, rather than undermine it. In this review, we attempt a reconsideration of the evidence that most directly impacts the physiological concept of hippocampal lamellar organization.

A method for recording evoked local field potentials in the primate dentate gyrus in vivo

Recording evoked local field potentials (LFPs) in the hippocampus in vivo has yielded us useful information about the neural mechanisms of learning and memory. Although this technique has been used in studies of the hippocampus of rodents, lagomorphs, and felines, it has not yet been applied to the primate hippocampus. Here, we report a method for recording evoked LFPs in the hippocampus of monkeys. A stimulation electrode and a recording electrode were implanted in the perforant pathway and dentate gyrus, respectively, under the guidance of electrophysiological recording. With a low stimulus intensity just above the threshold, the potential appeared as a slow positive-wave component, which was regarded as field excitatory postsynaptic potential (putative fEPSP); as stimulation intensity increased, the fEPSP amplitude increased, followed by a sharp negative component which was regarded as putative population spike. When the coordinates of the recording or stimulation electrode were moved stepwise, we observed a systematic change in the waveforms of evoked LFPs; this change corresponded to the structural arrangement through which the electrode passed. In a test for short-term synaptic plasticity by paired-pulse stimulation, potentials evoked by the second pulse were influenced by the first one in a manner dependent on interpulse intervals. In a test for long-term synaptic plasticity by high-frequency stimulation, the slopes of the fEPSPs and the area of population spikes were increased for more than 1 h. These results indicate that the method developed in the present study is useful for testing theories of hippocampal functions in primates.

Defining "epileptogenesis" and identifying "antiepileptogenic targets" in animal models of acquired temporal lobe epilepsy is not as simple as it might seem

The "latent period" between brain injury and clinical epilepsy is widely regarded to be a seizure-free, pre-epileptic state during which a time-consuming cascade of molecular events and structural changes gradually mediates the process of "epileptogenesis." The concept of the "latent period" as the duration of "epileptogenesis" implies that epilepsy is not an immediate result of brain injury, and that anti-epileptogenic strategies need to target delayed secondary mechanisms that develop sometime after an initial injury. However, depth recordings made directly from the dentate granule cell layers in awake rats after convulsive status epilepticus-induced injury have now shown that whenever perforant pathway stimulation-induced status epilepticus produces extensive hilar neuron loss and entorhinal cortical injury, hyperexcitable granule cells immediately generate spontaneous epileptiform discharges and focal or generalized behavioral seizures. This indicates that hippocampal injury caused by convulsive status epilepticus is immediately epileptogenic and that hippocampal epileptogenesis requires no delayed secondary mechanism. When latent periods do exist after injury, we hypothesize that less extensive cell loss causes an extended period during which initially subclinical focal seizures gradually increase in duration to produce the first clinical seizure. Thus, the "latent period" is suggested to be a state of "epileptic maturation," rather than a prolonged period of "epileptogenesis," and therefore the antiepileptogenic therapeutic window may only remain open during the first week after injury, when some delayed cell death may still be preventable. Following the perhaps unavoidable development of the first focal seizures ("epileptogenesis"), the most fruitful therapeutic strategy may be to interrupt the process of "epileptic maturation," thereby keeping focal seizures focal. This article is part of the Special Issue entitled 'New Targets and Approaches to the Treatment of Epilepsy'.

Depolarising and hyperpolarising actions of GABA(A) receptor activation on gonadotrophin-releasing hormone neurones: towards an emerging consensus

The gonadotrophin-releasing hormone (GnRH) neurones represent the final output neurones of a complex neuronal network that controls fertility. It is now appreciated that GABAergic neurones within this network provide an important regulatory influence on GnRH neurones. However, the consequences of direct GABA(A) receptor activation on adult GnRH neurones have been controversial for nearly a decade now, with both hyperpolarising and depolarising effects being reported. This review provides: (i) an overview of GABA(A) receptor function and its investigation using electrophysiological approaches and (ii) re-examines the past and present results relating to GABAergic regulation of the GnRH neurone, with a focus on mouse brain slice data. Although it remains difficult to reconcile the results of the early studies, there is a growing consensus that GABA can act through the GABA(A) receptor to exert both depolarising and hyperpolarising effects on GnRH neurones. The most recent studies examining the effects of endogenous GABA release on GnRH neurones indicate that the predominant action is that of excitation. However, we are still far from a complete understanding of the effects of GABA(A) receptor activation upon GnRH neurones. We argue that this will require not only a better understanding of chloride ion homeostasis in individual GnRH neurones, and within subcellular compartments of the GnRH neurone, but also a more integrative view of how multiple neurotransmitters, neuromodulators and intrinsic conductances act together to regulate the activity of these important cells.

Short-term synaptic plasticity in the dentate gyrus of monkeys

The hippocampus plays an important role in learning and memory. Synaptic plasticity in the hippocampus, short-term and long-term, is postulated to be a neural substrate of memory trace. Paired-pulse stimulation is a standard technique for evaluating a form of short-term synaptic plasticity in rodents. However, evidence is lacking for paired-pulse responses in the primate hippocampus. In the present study, we recorded paired-pulse responses in the dentate gyrus of monkeys while stimulating to the medial part of the perforant path at several inter-pulse intervals (IPIs) using low and high stimulus intensities. When the stimulus intensity was low, the first pulse produced early strong depression (at IPIs of 10–30 ms) and late slight depression (at IPIs of 100–1000 ms) of field excitatory postsynaptic potentials (fEPSPs) generated by the second pulse, interposing no depression IPIs (50–70 ms). When the stimulus intensity was high, fEPSPs generated by the second pulse were depressed by the first pulse at all IPIs except for the longest one (2000 ms). Population spikes (PSs) generated by the second pulse were completely blocked or strongly depressed at shorter IPIs (10–100 or 200 ms, respectively), while no depression or slight facilitation occurred at longer IPIs (500–2000 ms). Administration of diazepam slightly increased fEPSPs, while it decreased PSs produced by the first pulse. It also enhanced the facilitation of PSs produced by the second stimulation at longer IPIs. The present results, in comparison with previous studies using rodents, indicate that paired-pulse responses of fEPSPs in the monkey are basically similar to those of rodents, although paired-pulse responses of PSs in the monkey are more delayed than those in rodents and have a different sensitivity to diazepam.

Classic hippocampal sclerosis and hippocampal-onset epilepsy produced by a single "cryptic" episode of focal hippocampal excitation in awake rats

In refractory temporal lobe epilepsy, seizures often arise from a shrunken hippocampus exhibiting a pattern of selective neuron loss called "classic hippocampal sclerosis." No single experimental injury has reproduced this specific pathology, suggesting that hippocampal atrophy might be a progressive "endstage" pathology resulting from years of spontaneous seizures. We posed the alternative hypothesis that classic hippocampal sclerosis results from a single excitatory event that has never been successfully modeled experimentally because convulsive status epilepticus, the insult most commonly used to produce epileptogenic brain injury, is too severe and necessarily terminated before the hippocampus receives the needed duration of excitation. We tested this hypothesis by producing prolonged hippocampal excitation in awake rats without causing convulsive status epilepticus. Two daily 30-minute episodes of perforant pathway stimulation in Sprague-Dawley rats increased granule cell paired-pulse inhibition, decreased epileptiform afterdischarge durations during 8 hours of subsequent stimulation, and prevented convulsive status epilepticus. Similarly, one 8-hour episode of reduced-intensity stimulation in Long-Evans rats, which are relatively resistant to developing status epilepticus, produced hippocampal discharges without causing status epilepticus. Both paradigms immediately produced the extensive neuronal injury that defines classic hippocampal sclerosis, without giving any clinical indication during the insult that an injury was being inflicted. Spontaneous hippocampal-onset seizures began 16-25 days postinjury, before hippocampal atrophy developed, as demonstrated by sequential magnetic resonance imaging. These results indicate that classic hippocampal sclerosis is uniquely produced by a single episode of clinically "cryptic" excitation. Epileptogenic insults may often involve prolonged excitation that goes undetected at the time of injury.

Stereological estimation of numerical densities of glutamatergic principal neurons in the mouse hippocampus

Recent studies have emphasized functional dissociations between dorsal and ventral hippocampus in learning, emotion, and affect. A rigorous quantitative analysis concerning lamellar cytoarchitecture would be important for promoting further research on the regional differentiation of the hippocampus. Here, we stereologically estimated the numerical densities (NDs) of glutamatergic principal neurons in the mouse hippocampus and encountered the significant differences along the dorsoventral axis. In the CA1 region, the NDs of CA1 pyramidal neurons were almost three times higher at the dorsal level (447.5 x 10(3)/mm(3)) than at the ventral level (180.5 x 10(3)/mm(3)); meanwhile, along the transverse axis, the NDs were significantly higher in the proximal portion than in the distal portion both at the dorsal and ventral levels. An EF-hand calcium-binding protein, calbindin D28K, was expressed in approximately 45% of CA1 pyramidal neurons both at the dorsal and ventral level. In the CA3 region, there were no significant differences in the NDs along the dorsoventral and transverse axes (dorsal, 165.2 x 10(3)/mm(3); ventral, 172.4 x 10(3)/mm(3)). In the dentate gyrus (DG), the NDs of granule cells were significantly higher at the dorsal level (916.7 x 10(3)/mm(3)) than at the ventral level (788.9 x 10(3)/mm(3)). The significant differences were observed only in the suprapyramidal blade, but not in the infrapyramidal blade. Then, we calculated the total neuron numbers contained in a 300-microm-thick hypothetical transverse slice of the hippocampus and found that the ratios of GABAergic to glutamatergic neuron numbers were two to three times higher in the ventral slice than in the dorsal slice. The ratios of numbers of eight GABAergic neuron subtypes to principal cells indicate structural dissociations in the neural network between dorsal and ventral slices. These findings provide an essential quantitative basis for elucidating mechanisms of distinct neural circuits underlying various hippocampal functions.

Transplant of GABAergic Precursors Restores Hippocampal Inhibitory Function in a Mouse Model of Seizure Susceptibility

Defects in GABAergic function can cause epilepsy. In the last years, cell-based therapies have attempted to correct these defects with disparate success on animal models of epilepsy. Recently, we demonstrated that medial ganglionic eminence (MGE)-derived cells grafted into the neonatal normal brain migrate and differentiate into functional mature GABAergic interneurons. These cells are able to modulate the local level of GABA-mediated synaptic inhibition, which suggests their suitability for cell-based therapies. However, it is unclear whether they can integrate in the host circuitry and rescue the loss of inhibition in pathological conditions. Thus, as proof of principle, we grafted MGE-derived cells into a mouse model of seizure susceptibility caused by specific elimination of GABAergic interneuron subpopulations in the mouse hippocampus after injection of the neurotoxic saporin conjugated to substance P (SSP-Sap). This ablation was associated with significant decrease in inhibitory postsynaptic currents (IPSC) on CA1 pyramidal cells and increased seizure susceptibility induced by pentylenetetrazol (PTZ). Grafting of GFP+ MGE-derived cells in SSP-Sap-treated mice repopulates the hippocampal ablated zone with cells expressing molecular markers of mature interneurons. Interestingly, IPSC kinetics on CA1 pyramidal cells of ablated hippocampus significantly increased after transplantation, reaching levels similar to the normal mice. More importantly, this was associated with reduction in seizure severity and decrease in postseizure mortality induced by PTZ. Our data show that MGE-derived cells fulfill most of the requirements for an appropriate cell-based therapy, and indicate their suitability for neurological conditions where a modulation of synaptic inhibition is needed, such as epilepsy.

Nicotinamide prevents the long-term effects of perinatal asphyxia on apoptosis, non-spatial working memory and anxiety in rats

There is no established treatment for the long-term effects produced by perinatal asphyxia. Thus, we investigated the neuroprotection provided by nicotinamide against the effects elicited by perinatal asphyxia on hippocampus and behaviour observed at 30-90 days of age. Asphyxia was induced by immersing foetuses-containing uterine horns, removed from ready-to-deliver rats into a water bath at 37 degrees C for 20 min. Caesarean-delivered siblings were used as controls. Saline or nicotinamide (0.8 mmol/kg, i.p.) was administered to control and asphyxia-exposed animals 24, 48, and 72 h after birth. The animals were examined for morphological changes in hippocampus, focusing on delayed cell death and mossy fibre sprouting, and behaviour, focusing on cognitive behaviour and anxiety. At the age of 30-45 days, asphyxia-exposed rats displayed (1) increased apoptosis, assessed in whole hippocampus by nuclear Hoechst staining, and (2) increased mossy fibre sprouting, restricted to the stratum oriens of dorsal hippocampus, assessed by Timm's staining. Rats from the same cohorts displayed (3) deficits in non-spatial working memory, assessed by a novel object recognition task, and (4) increased anxiety, assessed by an elevated plus-maze test when examined at the age of 90 days. Nicotinamide prevented the effects elicited by perinatal asphyxia on apoptosis, working memory, and anxiety.

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.

Hippocampal injury, atrophy, synaptic reorganization, and epileptogenesis after perforant pathway stimulation-induced status epilepticus in the mouse

Prolonged dentate granule cell discharges produce hippocampal injury and chronic epilepsy in rats. In preparing to study this epileptogenic process in genetically altered mice, we determined whether the background strain used to generate most genetically altered mice, the C57BL/6 mouse, is vulnerable to stimulation-induced seizure-induced injury. This was necessary because C57BL/6 mice are reportedly resistant to the neurotoxic effects of kainate-induced seizures, which we hypothesized to be related to strain differences in kainate's effects, rather than genetic differences in intrinsic neuronal vulnerability. Bilateral perforant pathway stimulation-induced granule cell discharge for 4 hours under urethane anesthesia produced degeneration of glutamate receptor subunit 2 (GluR2)-positive hilar mossy cells and peptide-containing interneurons in both FVB/N (kainate-vulnerable) and C57BL/6 (kainate-resistant) mice, indicating no strain differences in neuronal vulnerability to seizure activity. Granule cell discharge for 2 hours in C57BL/6 mice destroyed most GluR2-positive dentate hilar mossy cells, but not peptide-containing hilar interneurons, indicating that mossy cells are the neurons most vulnerable to this insult. Stimulation for 24 hours caused extensive hippocampal neuron loss and injury to the septum and entorhinal cortex, but no other detectable damage. Mice stimulated for 24 hours developed hippocampal sclerosis, granule cell mossy fiber sprouting, and chronic epilepsy, but not the granule cell layer hypertrophy (granule cell dispersion) produced by intrahippocampal kainate. These results demonstrate that perforant pathway stimulation in mice reliably reproduces the defining features of human mesial temporal lobe epilepsy with hippocampal sclerosis. Experimental studies in transgenic or knockout mice are feasible if electrical stimulation is used to produce controlled epileptogenic insults.

Hippocampal epileptogenesis in animal models of mesial temporal lobe epilepsy with hippocampal sclerosis: the importance of the "latent period" and other concepts

Prolonged chemoconvulsant-induced status epilepticus in rats has long been promoted as an animal model of mesial temporal lobe epilepsy with hippocampal sclerosis, under the assumption that these animals involve: (1) pathology similar to that of the human neurologic condition; (2) a seizure-free, "preepileptic" latent period of several weeks duration after injury, during which a secondary epileptogenic process gradually develops; and (3) a chronic epileptic state in which the hippocampus, in general, and the dentate gyrus, in particular, becomes a source of the spontaneous behavioral seizures that define these animals as "epileptic." Retrospective analysis suggests that all of these assumptions are in doubt. Neuropathologic studies have shown that prolonged status epilepticus causes greater extrahippocampal than hippocampal damage, and does not produce classic hippocampal sclerosis. In vivo electrophysiologic studies suggest that the hippocampus of these animals may not be "epileptic." Most importantly, studies using continuous video monitoring to detect spontaneous behavioral seizures indicate that these rats become epileptic soon after insult, before any delayed secondary processes have time to develop. High mortality, significant variability, and the lack of an extended "therapeutic window" after brain injury suggest the need to develop animal models that more closely resemble the human neurologic condition.

Nitric oxide regulates activity-dependent neuroprotective protein (ADNP) in the dentate gyrus of the rodent model of kainic acid-induced seizure

The dentate gyrus (DG) of the normal rat brain contains activity-dependent neuroprotective protein (ADNP) which is widely distributed in the cytoplasm of neurons and astrocytes. Treatment with nitric oxide (NO) synthase (NOS) inhibitor N(G)-nitro-L: -arginine methyl ester (L: -NAME) caused a decrease in ADNP expression in granule cells which persisted 3 days post-treatment. However, treatment with neuronal-specific NOS inhibitor, 7-nitroindazole (7-NI), or soluble guanylyl cyclase inhibitor, ODQ, did not change ADNP expression in the DG. We have previously shown that kainic acid (KA)-induced seizure increases neuronal NOS in neurons and inducible NOS in glia cells and suppresses ADNP in the hippocampus (Cosgrave et al., Neurobiol Dis 30(3):281-292, 2008). In the DG, L: -NAME treatment prior to KA causes ADNP synthesis in granule cells by 3 h which was later restricted to the subgranular zone by 3 days. 7-NI and ODQ had no effect. Double immunostaining for neuronal marker NeuN and ADNP revealed a significant decrease of both ADNP(+) neurons and of total neuron numbers (NeuN(+)) in the hilus of animals having KA-induced seizure that had been pretreated with L: -NAME implying that NO and ADNP may act together to protect hilar neurons. Overall, these observations suggest that NO regulates ADNP in the DG under both basal and pathophysiological conditions.

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.

Minimal latency to hippocampal epileptogenesis and clinical epilepsy after perforant pathway stimulation-induced status epilepticus in awake rats

Hippocampal epileptogenesis is hypothesized to involve secondary mechanisms triggered by initial brain injury. Chemoconvulsant-induced status epilepticus has been used to identify secondary epileptogenic mechanisms under the assumption that a seizure-free, preepileptic "latent period" exists that is long enough to accommodate delayed mechanisms. The latent period is difficult to assess experimentally because early spontaneous seizures may be caused or influenced by residual chemoconvulsant that masks the true duration of the epileptogenic process. To avoid the use of chemoconvulsants and determine the latency to hippocampal epileptogenesis and clinical epilepsy, we developed an electrical stimulation-based method to evoke hippocampal discharges in awake rats and produce hippocampal injury and hippocampal-onset epilepsy reliably. Continuous video monitoring and granule cell layer recording determined whether hippocampal epileptogenesis develops immediately or long after injury. Bilateral perforant pathway stimulation for 3 hours evoked granule cell epileptiform discharges and convulsive status epilepticus with minimal lethality. Spontaneous stage 3-5 behavioral seizures reliably developed within 3 days poststimulation, and all 72 spontaneous behavioral seizures recorded in 10 animals were preceded by spontaneous granule cell epileptiform discharges. Histological analysis confirmed a reproducible pattern of limited hippocampal and extrahippocampal injury, including an extensive bilateral loss of hilar neurons throughout the hippocampal longitudinal axis. These results indicate that hippocampal epileptogenesis after convulsive status epilepticus is an immediate network defect coincident with neuron loss or other early changes. We hypothesize that the latent period is directly related and inversely proportional to the extent of neuron loss in brain regions involved in seizure initiation, spread, and clinical expression.

Dynamics of evoked local field potentials in the hippocampus of epileptic rats with spontaneous seizures

A change in neuronal network excitability within the hippocampus is one of the hallmarks of temporal lobe epilepsy (TLE). In the dentate gyrus (DG), however, neuronal loss and mossy fiber sprouting are associated with enhanced inhibition rather than progressive hyperexcitability. The aim of this study was to investigate how alterations in excitability take place in association with spontaneous seizures expressed in the DG before, during, and after a seizure. For this purpose, we used freely moving rats that had developed spontaneous seizures after a kainate-induced status epilepticus (SE). Continuous EEG was recorded in the DG during several days along with local field potentials (LFPs) that were evoked every 15-30 s by applying paired-pulse stimuli to the angular bundle. Input-output relations showed increased paired pulse depression in epileptic compared with control rats, suggesting a rather strong inhibition in the DG during the interictal state. A characteristic pattern of changes in intrinsic excitability was observed during the ictal period: an increase in the population spike (PS) amplitude, mostly during the early phase of a seizure and often followed by a decrease of the main evoked potential amplitude. The paired-pulse extracellular postsynaptic potential (fEPSP) ratio increased during the seizure and did slowly recover to preictal levels after the seizure ended. Although clear changes in excitability occurred during and after seizure activity, changes of LFP parameters were more subtle before seizure onset; a significant reduction of LFP and PS amplitudes was observed that started 1-2 min in advance in approximately 33% of the cases; in approximately 18%, an increase of LFP/PS amplitude was observed; in the other cases, no significant change was observed. Taken together, these results provide evidence that, in this experimental model, DG physiology is more likely to follow the already ongoing seizure activity rather than to contribute to its generation.

[Changes in spontaneous epileptic activity after selective intrahippocampal transection in a model of chronic mesial temporal lobe epilepsy]

Drug-resistant mesial temporal lobe epilepsy with hippocampal sclerosis is associated with anatomical, ultrastructural and functional changes that facilitate generation and spread of epileptic seizures. Intrahippocampal circuits are modified in their transversal lamellar and longitudinal translamellar organization. Neuronal death and the neuroplasticity of surviving cells contribute to major phenomena: an increased hyperexcitability of the hippocampal formation and an increased synchronization of its principal cells. Selective disruption of the epileptic networks that are involved in mesial temporal lobe epilepsy may have a therapeutic effect. We present here the preliminary results of a selective intrahippocampal transection in a chronic model of mesial temporal lobe epilepsy after focal injection of kainic acid in adult mice. A complete transection of the hippocampal formation (including dentate gyrus and hippocampus proper, sparing the fimbria) results in a blockade of ictal activities spread from the generator, a reduction in their frequency and an increase in their duration. In contrast, after a transection sparing the dentate gyrus and hilus, no modification was noted. In this model of mesial temporal lobe epilepsy, longitudinally projecting axonal circuits of the dentate gyrus and hilus appear to be implicated in generation, propagation and interruption of ictal activities within hippocampal formation.

Unilateral hippocampal CA3-predominant damage and short latency epileptogenesis after intra-amygdala microinjection of kainic acid in mice

Mesial temporal lobe epilepsy is the most common, intractable seizure disorder in adults. It is associated with an asymmetric pattern of hippocampal neuron loss within the endfolium (hilus and CA3) and CA1, with limited pathology in extra-hippocampal regions. We previously developed a model of focally-evoked seizure-induced neuronal death using intra-amygdala kainic acid (KA) microinjection and characterized the acute hippocampal pathology. Here, we sought to characterize the full extent of hippocampal and potential extra-hippocampal damage in this model, and the temporal onset of epileptic seizures. Seizure damage assessed at four stereotaxic levels by FluoroJade B staining was most prominent in ipsilateral hippocampal CA3 where it extended from septal to temporal pole. Minor but significant neuronal injury was present in ipsilateral CA1. Extra-hippocampal neuronal damage was generally limited in extent and restricted to the lateral septal nucleus, injected amygdala and select regions of neocortex ipsilateral to the seizure elicitation side. Continuous surface EEG recorded with implanted telemetry units in freely-moving mice detected spontaneous, epileptic seizures by five days post-KA in all mice. Epileptic seizure number averaged 1-4 per day. Hippocampi from epileptic mice 15 days post-KA displayed unilateral CA3 lesions, astrogliosis and increased neuropeptide Y immunoreactivity suggestive of mossy fiber rearrangement. These studies characterize a mouse model of unilateral hippocampal-dominant neuronal damage and short latency epileptogenesis that may be suitable for studying the cell and molecular pathogenesis of human mesial temporal lobe epilepsy.

Toward an integrative perspective on hippocampal function: from the rapid encoding of experience to adaptive behavior

The mammalian hippocampus has been associated with learning and memory, as well as with many other behavioral processes. In this article, these different perspectives are brought together, and it is pointed out that integration of diverse functional domains may be a key feature enabling the hippocampus to support not only the encoding and retrieval of certain memory representations, but also their translation into adaptive behavior. The hippocampus appears to combine: (i) sensory afferents and synaptic mechanisms underlying certain types of rapid learning; and (ii) links to motivational, emotional, executive, and sensorimotor functions. Recent experiments are highlighted, indicating that the induction of hippocampal synaptic plasticity is required to encode rapidly aspects of experience, such as places, into memory representations; subsequent retrieval of these representations requires transmission through the previously modified hippocampal synapses, but no further plasticity. In contrast, slow incremental place learning may not absolutely require hippocampal contributions. The neocortical sensory inputs, especially visuo-spatial information, necessary for hippocampus-dependent rapid learning, are preferentially associated with the septal to intermediate hippocampus. In contrast, connectivity with the prefrontal cortex and subcortical sites, which link the hippocampus to motivational, emotional, executive, and sensorimotor functions, is primarily associated with the intermediate to temporal hippocampus. A model of functional differentiation and integration along the septo-temporal axis of the hippocampus is proposed, describing key hippocampal contributions to adaptive behavior based on information encoded during a single or a few past experiences.