Spontaneous and synaptic input from granule cells and the perforant path to dentate basket cells in the rat hippocampus

Disynaptic effect of hilar cells on pattern separation in a spiking neural network of hippocampal dentate gyrus

We study the disynaptic effect of the hilar cells on pattern separation in a spiking neural network of the hippocampal dentate gyrus (DG). The principal granule cells (GCs) in the DG perform pattern separation, transforming similar input patterns into less-similar output patterns. In our DG network, the hilus consists of excitatory mossy cells (MCs) and inhibitory HIPP (hilar perforant path-associated) cells. Here, we consider the disynaptic effects of the MCs and the HIPP cells on the GCs, mediated by the inhibitory basket cells (BCs) in the granular layer; MC → BC → GC and HIPP → BC → GC. The MCs provide disynaptic inhibitory input (mediated by the intermediate BCs) to the GCs, which decreases the firing activity of the GCs. On the other hand, the HIPP cells disinhibit the intermediate BCs, which leads to increasing the firing activity of the GCs. In this way, the disynaptic effects of the MCs and the HIPP cells are opposite. We investigate change in the pattern separation efficacy by varying the synaptic strength K ( BC , X ) [from the pre-synaptic X (= MC or HIPP) to the post-synaptic BC]. Thus, sparsity for the firing activity of the GCs is found to improve the efficacy of pattern separation, and hence the disynaptic effects of the MCs and the HIPP cells on the pattern separation become opposite ones. In the combined case when simultaneously changing both K ( BC , MC ) and K ( BC , HIPP ) , as a result of balance between the two competing disynaptic effects of the MCs and the HIPP cells, the efficacy of pattern separation is found to become the highest at their original default values where the activation degree of the GCs is the lowest. We also note that, while the GCs perform pattern separation, sparsely synchronized rhythm is found to appear in the population of the GCs. Hence, we examine quantitative association between population and individual firing behaviors in the sparsely synchronized rhythm and pattern separation. They are found to be strongly correlated. Consequently, the better the population and individual firing behaviors in the sparsely synchronized rhythm are, the more pattern separation efficacy becomes enhanced.

Population and individual firing behaviors in sparsely synchronized rhythms in the hippocampal dentate gyrus

We investigate population and individual firing behaviors in sparsely synchronized rhythms (SSRs) in a spiking neural network of the hippocampal dentate gyrus (DG). The main encoding granule cells (GCs) are grouped into lamellar clusters. In each GC cluster, there is one inhibitory (I) basket cell (BC) along with excitatory (E) GCs, and they form the E-I loop. Winner-take-all competition, leading to sparse activation of the GCs, occurs in each GC cluster. Such sparsity has been thought to enhance pattern separation performed in the DG. During the winner-take-all competition, SSRs are found to appear in each population of the GCs and the BCs through interaction of excitation of the GCs with inhibition of the BCs. Sparsely synchronized spiking stripes appear successively with the population frequencyf p ( = 13.1 Hz) in the raster plots of spikes. We also note that excitatory hilar mossy cells (MCs) control the firing activity of the GC-BC loop by providing excitation to both the GCs and the BCs. SSR also appears in the population of MCs via interaction with the GCs (i.e., GC-MC loop). Population behaviors in the SSRs are quantitatively characterized in terms of the synchronization measures. In addition, we investigate individual firing activity of GCs, BCs, and MCs in the SSRs. Individual GCs exhibit random spike skipping, leading to a multi-peaked inter-spike-interval histogram, which is well characterized in terms of the random phase-locking degree. In this case, population-averaged mean-firing-rate (MFR) < f i ( GC ) > is less than the population frequency f p . On the other hand, both BCs and MCs show "intrastripe" burstings within stripes, together with random spike skipping. Thus, the population-averaged MFR ⟨ f i ( X ) ⟩ ( X = MC and BC) is larger than f p , in contrast to the case of the GCs. MC loss may occur during epileptogenesis. With decreasing the fraction of the MCs, changes in the population and individual firings in the SSRs are also studied. Finally, quantitative association between the population/individual firing behaviors in the SSRs and the winner-take-all competition is discussed.

Dynamical origin for winner-take-all competition in a biological network of the hippocampal dentate gyrus

We consider a biological network of the hippocampal dentate gyrus (DG). Computational models suggest that the DG would be a preprocessor for pattern separation (i.e., a process transforming a set of similar input patterns into distinct nonoverlapping output patterns) which could facilitate pattern storage and retrieval in the CA3 area of the hippocampus. The main encoding cells in the DG are the granule cells (GCs) which receive the input from the entorhinal cortex (EC) and send their output to the CA3. We note that the activation degree of GCs is very low (∼5%). This sparsity has been thought to enhance the pattern separation. We investigate the dynamical origin for winner-take-all (WTA) competition which leads to sparse activation of the GCs. The whole GCs are grouped into lamellar clusters. In each cluster, there is one inhibitory (I) basket cell (BC) along with excitatory (E) GCs. There are three kinds of external inputs into the GCs: the direct excitatory EC input; the indirect feedforward inhibitory EC input, mediated by the HIPP (hilar perforant path-associated) cells; and the excitatory input from the hilar mossy cells (MCs). The firing activities of the GCs are determined via competition between the external E and I inputs. The E-I conductance ratio R_{E-I}^{(con)}^{*} (given by the time average of the ratio of the external E to I conductances) may represent well the degree of such external E-I input competition. It is thus found that GCs become active when their R_{E-I}^{(con)}^{*} is larger than a threshold R_{th}^{*}, and then the mean firing rates of the active GCs are strongly correlated with R_{E-I}^{(con)}^{*}. In each cluster, the feedback inhibition from the BC may select the winner GCs. GCs with larger R_{E-I}^{(con)}^{*} than the threshold R_{th}^{*} survive, and they become winners; all the other GCs with smaller R_{E-I}^{(con)}^{*} become silent. In this way, WTA competition occurs via competition between the firing activity of the GCs and the feedback inhibition from the BC in each cluster. Finally, we also study the effects of MC death and adult-born immature GCs on the WTA competition.

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.

Dendrites of dentate gyrus granule cells contribute to pattern separation by controlling sparsity

The hippocampus plays a key role in pattern separation, the process of transforming similar incoming information to highly dissimilar, nonverlapping representations. Sparse firing granule cells (GCs) in the dentate gyrus (DG) have been proposed to undertake this computation, but little is known about which of their properties influence pattern separation. Dendritic atrophy has been reported in diseases associated with pattern separation deficits, suggesting a possible role for dendrites in this phenomenon. To investigate whether and how the dendrites of GCs contribute to pattern separation, we build a simplified, biologically relevant, computational model of the DG. Our model suggests that the presence of GC dendrites is associated with high pattern separation efficiency while their atrophy leads to increased excitability and performance impairments. These impairments can be rescued by restoring GC sparsity to control levels through various manipulations. We predict that dendrites contribute to pattern separation as a mechanism for controlling sparsity. © 2016 The Authors Hippocampus Published by Wiley Periodicals, Inc.

3,4-methylenedioxymethamphetamine increases excitability in the dentate gyrus: role of 5HT2A receptor-induced PGE2 signaling

3,4-methylenedioxymethamphetamine (MDMA) is a widely abused psychostimulant, which causes release of serotonin in various forebrain regions. Recently, we reported that MDMA increases extracellular glutamate concentrations in the dentate gyrus, via activation of 5HT2A receptors. We examined the role of prostaglandin signaling in mediating the effects of 5HT2A receptor activation on the increases in extracellular glutamate and the subsequent long-term loss of parvalbumin interneurons in the dentate gyrus caused by MDMA. Administration of MDMA into the dentate gyrus of rats increased PGE2 concentrations which was prevented by coadministration of MDL100907, a 5HT2A receptor antagonist. MDMA-induced increases in extracellular glutamate were inhibited by local administration of SC-51089, an inhibitor of the EP1 prostaglandin receptor. Systemic administration of SC-51089 during injections of MDMA prevented the decreases in parvalbumin interneurons observed 10 days later. The loss of parvalbumin immunoreactivity after MDMA exposure coincided with a decrease in paired-pulse inhibition and afterdischarge threshold in the dentate gyrus. These changes were prevented by inhibition of EP1 and 5HT2A receptors during MDMA. Additional experiments revealed an increased susceptibility to kainic acid-induced seizures in MDMA-treated rats, which could be prevented with SC51089 treatments during MDMA exposure. Overall, these findings suggest that 5HT2A receptors mediate MDMA-induced PGE2 signaling and subsequent increases in glutamate. This signaling mediates parvalbumin cell losses as well as physiologic changes in the dentate gyrus, suggesting that the lack of the inhibition provided by these neurons increases the excitability within the dentate gyrus of MDMA-treated rats. We hypothesized that the widely abused psychostimulant MDMA causes a loss of parvalbumin (PV) cells and increases excitability in the dentate gyrus. MDMA increases serotonin (5HT) release and activates 5HT2A receptors. The increased activation of 5HT2A receptors promotes the production of prostaglandin E2 (PGE2) and subsequent activation of EP1 receptors in the dentate gyrus. EP1 receptor activation leads to eventual excitotoxicity and loss of PV interneurons resulting in reduced inhibition and lowered seizure threshold resulting in increased seizure susceptibility.

Newly generated neurons at 2 months post-status epilepticus are functionally integrated into neuronal circuitry in mouse hippocampus

Emerging evidence has linked chronic temporal lobe epilepsy to dramatically reduced neurogenesis in the dentate gyrus. However, the profile of different components of neurogenesis in the chronically epileptic hippocampus is still unclear, especially the incorporation of newly generated cells. To address the issue, newly generated cells in the sub-granular zone of the dentate gyrus were labeled by the proliferation marker bromodeoxyuridine (BrdU) or retroviral vector expressing green fluorescent protein 2 months after pilocarpine-induced status epilepticus. The newly generated neurons that extended axons to CA3 area or integrated into memory circuits were visualized by cholera toxin B subunit retrograde tracing, and detecting activation of BrdU(+) cells following a recall of spatial memory test at the chronic stage of TLE. We found that the microenvironment was still able to sustain significant neuronal differentiation of newly generated cells at 2 months post-status epilepticus time-point, and newly added neurons into granular cell layer were still able to integrate into neuronal circuitry, both anatomically and functionally. Quantified analyses of BrdU(+) or Ki-67(+) cells demonstrated that there was a reduced proliferation of progenitor cells and diminished survival of newly generated cells in the epileptic hippocampus. Both decreased levels of neurotrophic factors in the surrounding milieu and cell loss in the CA3 area might contribute the decreased production of new cells and their survival following chronic epilepsy. These results suggest that decreased neurogenesis in the chronically epileptic hippocampus 2 months post status epilepticus is not associated with altered integration of newly generated neurons, and that developing strategies to augment hippocampal neurogenesis in chronic epilepsy might be protective.

MDMA-induced loss of parvalbumin interneurons within the dentate gyrus is mediated by 5HT2A and NMDA receptors

MDMA is a widely abused psychostimulant which causes a rapid and robust release of the monoaminergic neurotransmitters dopamine and serotonin. Recently, it was shown that MDMA increases extracellular glutamate concentrations in the dorsal hippocampus, which is dependent on serotonin release and 5HT2A/2C receptor activation. The increased extracellular glutamate concentration coincides with a loss of parvalbumin-immunoreactive (PV-IR) interneurons of the dentate gyrus region. Given the known susceptibility of PV interneurons to excitotoxicity, we examined whether MDMA-induced increases in extracellular glutamate in the dentate gyrus are necessary for the loss of PV cells in rats. Extracellular glutamate concentrations increased in the dentate gyrus during systemic and local administration of MDMA. Administration of the NMDA receptor antagonist, MK-801, during systemic injections of MDMA, prevented the loss of PV-IR interneurons seen 10 days after MDMA exposure. Local administration of MDL100907, a selective 5HT2A receptor antagonist, prevented the increases in glutamate caused by reverse dialysis of MDMA directly into the dentate gyrus and prevented the reduction of PV-IR. These findings provide evidence that MDMA causes decreases in PV within the dentate gyrus through a 5HT2A receptor-mediated increase in glutamate and subsequent NMDA receptor activation.

Redistribution of ionotropic glutamate receptors detected by laser microdissection of the rat dentate gyrus 48 h following LTP induction in vivo

The persistence and input specificity of long-term potentiation (LTP) make it attractive as a mechanism of information storage. In its initial phase, both in vivo and in vitro studies have shown that LTP is associated with increased membrane localization of AMPA receptor subunits, but the molecular basis of LTP maintenance over the long-term is still unclear. We have previously shown that expression of AMPA and NMDA receptor subunits is elevated in whole homogenates prepared from dentate gyrus 48 h after LTP induction in vivo. In the present study, we utilized laser microdissection (LMD) techniques to determine whether AMPA and NMDA receptor upregulation occurs specifically in the stimulated regions of the dentate gyrus dendritic arbor. Receptor proteins GluN1, GluA1 and GluA2, as well as postsynaptic density protein of 95 kDa and tubulin were detected by Western blot analysis in microdissected samples. Gradients of expression were observed for GluN1 and GluA2, decreasing from the inner to the outer zones of the molecular layer, and were independent of LTP. When induced at medial perforant path synapses, LTP was associated with an apparent specific redistribution of GluA1 and GluN1 to the middle molecular layer that contains these synapses. These data indicate that glutamate receptor proteins are delivered specifically to dendritic regions possessing LTP-expressing synapses, and that these changes are preserved for at least 48 h.

Frequency-tuned distribution of inhibition in the dentate gyrus

Granule cells (GCs) of the dentate gyrus use sparse encoding to perform redundancy reduction, pattern separation, and novelty detection. One likely candidate mechanism to enforce low spiking activity is feedforward inhibition, in which the cortical excitatory drive from the perforant path (PP) recruits GABAergic interneurons that then inhibit GCs. Little is known, however, about how PP drive is balanced between GCs versus inhibitory neurons. In simultaneous recordings of GCs and fast-spiking (FS) interneurons from C57BL/6 mice, we find that focal PP stimulation preferentially recruits spiking in FS interneurons over GCs, because GCs require a larger excitatory synaptic current density to reach spike threshold. Blocking inhibition reversed this relationship, revealing a stronger intrinsic coupling between the PP and GCs versus FS interneurons and showing that inhibition can sparsify the output of the dentate gyrus by tightly regulating GC spike probability. Moreover, this regulation is dynamic, because the spiking profile of FS interneurons was frequency tuned, displaying bursting behavior in response to PP stimulation near theta rhythm frequency (∼10 Hz). The later spikes in the bursts were part of the feedback inhibitory pathway because they were driven by late EPSCs, were blocked by an inhibitor of synaptic output from GCs, and shared the same frequency dependence as GC spiking. Therefore, the temporal content of signals arriving via the PP determines whether a FS interneuron participates in only feedforward (one spike) or both feedforward and feedback (burst) inhibition.

Dysfunction of the dentate basket cell circuit in a rat model of temporal lobe epilepsy

Temporal lobe epilepsy is common and difficult to treat. Reduced inhibition of dentate granule cells may contribute. Basket cells are important inhibitors of granule cells. Excitatory synaptic input to basket cells and unitary IPSCs (uIPSCs) from basket cells to granule cells were evaluated in hippocampal slices from a rat model of temporal lobe epilepsy. Basket cells were identified by electrophysiological and morphological criteria. Excitatory synaptic drive to basket cells, measured by mean charge transfer and frequency of miniature EPSCs, was significantly reduced after pilocarpine-induced status epilepticus and remained low in epileptic rats, despite mossy fiber sprouting. Paired recordings revealed higher failure rates and a trend toward lower amplitude uIPSCs at basket cell-to-granule cell synapses in epileptic rats. Higher failure rates were not attributable to excessive presynaptic inhibition of GABA release by activation of muscarinic acetylcholine or GABA(B) receptors. High-frequency trains of action potentials in basket cells generated uIPSCs in granule cells to evaluate readily releasable pool (RRP) size and resupply rate of recycling vesicles. Recycling rate was similar in control and epileptic rats. However, quantal size at basket cell-to-granule cell synapses was larger and RRP size smaller in epileptic rats. Therefore, in epileptic animals, basket cells receive less excitatory synaptic drive, their pools of readily releasable vesicles are smaller, and transmission failure at basket cell-to-granule cell synapses is increased. These findings suggest dysfunction of the dentate basket cell circuit could contribute to hyperexcitability and seizures.

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.

Relationship of hippocampal theta and gamma oscillations to potentiation of synaptic transmission

In the hippocampus in vivo, both synaptic plasticity and network activity are closely interdependent. We have found that immediately after an attempt to induce long-term potentiation (LTP), changes in theta (5-10 Hz) and gamma (30-100 Hz) activity correlate tightly with the occurrence of LTP, suggesting that tetanisation-driven activation of sensory inputs synchronises the activity of granule cells and interneurons, and thus, facilitates the encoding of acquired stimuli. This results in increase of theta and gamma power, and elevates the probability that afferent stimuli both coincide with the peak of theta cycle and reach their post-synaptic target within the gamma time-window (of 10-30 ms). Both these mechanisms can effectively shift the direction, of tetanisation-induced changes in synaptic weight, towards potentiation and induction of LTP. Here, we discuss our findings in the context of possible mechanisms that link theta and gamma oscillations with LTP induction, as well as their role in information processing and formation of memories.

Electric fields due to synaptic currents sharpen excitatory transmission

The synaptic response waveform, which determines signal integration properties in the brain, depends on the spatiotemporal profile of neurotransmitter in the synaptic cleft. Here, we show that electrophoretic interactions between AMPA receptor-mediated excitatory currents and negatively charged glutamate molecules accelerate the clearance of glutamate from the synaptic cleft, speeding up synaptic responses. This phenomenon is reversed upon depolarization and diminished when intracleft electric fields are weakened through a decrease in the AMPA receptor density. In contrast, the kinetics of receptor-mediated currents evoked by direct application of glutamate are voltage-independent, as are synaptic currents mediated by the electrically neutral neurotransmitter GABA. Voltage-dependent temporal tuning of excitatory synaptic responses may thus contribute to signal integration in neural circuits.

Expression of the neuronal calcium sensor visinin-like protein-1 in the rat hippocampus

Visinin-like protein-1 (VILIP-1) belongs to the neuronal calcium sensor (NCS) family of EF-hand Ca(2+)-binding proteins which are involved in a variety of Ca(2+)-dependent signal transduction processes in neurons. VILIP-1 has been implicated in the pathology of CNS disorders including Alzheimer's disease and schizophrenia, but its expression has also been found to be regulated following induction of hippocampal synaptic plasticity underlying learning and memory processes. VILIP-1 is strongly expressed in different populations of principal and non-principal neurons in the rat hippocampus. VILIP-1-containing interneurons are morphologically and neurochemically heterogeneous. On the basis of co-localizing markers, VILIP-1 is rarely present in perisomatic inhibitory parvalbumin containing cells. However, VILIP-1 is frequently expressed in mid-proximal dendritic inhibitory cells characterized by calbindin immunoreactivity, and most strongly co-expressed in calretinin-positive disinhibitory interneurons. Partial co-localization of the metabotropic glutamate receptor mGluR1alpha with VILIP-1 was often found in interneurons located in the stratum oriens of the hippocampal CA1 region and in hilar interneurons. Partial co-localization of alpha4beta2 nicotinic acetylcholine receptor with VILIP-1 was seen in stratum oriens interneurons and particularly at the border of the hilus in the dentate gyrus, where VILIP-1 also strongly co-localized with calretinin. We speculate that depending on the regulation of the expression of VILIP-1 in hippocampal pyramidal cells or defined types of interneurons, it may have different effects on hippocampal synaptic plasticity and network activity in health and disease.

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.

GABAergic Excitation Promotes Neuronal Differentiation in Adult Hippocampal Progenitor Cells

Hippocampal activity influences neurogenesis in the adult dentate gyrus; however, little is known about the involvement of the hippocampal circuitry in this process. In the subgranular zone of the adult dentate gyrus, neurogenesis involves a series of differentiation steps from radial glia-like stem/progenitor (type-1) cells, to transiently amplifying neuronal progenitor (type-2) cells, to postmitotic neurons. In this study, we conducted GFP-targeted recordings of progenitor cells in fresh hippocampal slices from nestin-GFP mice and found that neuronal progenitor (type-2) cells receive active direct neural inputs from the hippocampal circuitry. This input was GABAergic but not glutamatergic. The GABAergic inputs depolarized type-2 cells because of their elevated [Cl(-)](i). This excitation initiated an increase of [Ca(2+)](i) and the expression of NeuroD. A BrdU-pulse labeling study with GABA(A)-R agonists demonstrated the promotion of neuronal differentiation via this GABAergic excitation. Thus, it appears that GABAergic inputs to hippocampal progenitor cells promote activity-dependent neuronal differentiation.

Enhanced Excitability Compensates for High-Pressure-Induced Depression of Cortical Inputs to the Hippocampus

High pressure (>1.0 MPa) induces the high-pressure neurological syndrome (HPNS) characterized by increased excitability of the CNS and cognitive impairments involving memory disorders. The perforant-path transfer of cortical information to the hippocampal formation is important for memory acquisition. High pressure may alter information transfer in this connection. We used rat corticohippocampal slices for studying the effect of pressure on the transfer function between synaptic inputs from the medial perforant path (MPP) and spike generation by granule cells (GC) of the dentate gyrus. High pressure (10.1 MPa) reduced single MPP field excitatory postsynaptic potential (fEPSP) amplitude and slope by nearly 50%. Field antidromic action potentials (AAPs) elicited by stimulation of GC axons, and population spike (PS) generation by the pressure-depressed MPP fEPSP were not significantly altered at hyperbaric conditions. Nevertheless the relationship PS/fEPSP increased at high pressure, indicating dendritic hyperexcitability in the GC. PSs elicited by paired-pulse MPP fEPSPs at 10- to 200-ms interstimulus intervals and PS generated by trains of five fEPSPs at 25 Hz were also not affected in spite of severe pressure-induced synaptic depression. Similarly, trains of AAPs at 25–50 Hz were not significantly changed. Trains of fEPSPs at higher frequency (50 Hz), however, induced additional spikes at high pressure, indicating pressure disruption of the regular low-pass filter properties of the DG. Such effect was closely mimicked by partial blockade of GABAA inhibition. High pressure depresses synaptic activity while increases excitability in the neuronal dendrites but not in the axons. This mechanism, allowing neuronal communication at low input signals, may partially cope with pressure effects at the low frequency range (<25 Hz) but losses reliability at higher frequencies (>50 Hz).

Role of mossy fiber sprouting and mossy cell loss in hyperexcitability: a network model of the dentate gyrus incorporating cell types and axonal topography

Mossy cell loss and mossy fiber sprouting are two characteristic consequences of repeated seizures and head trauma. However, their precise contributions to the hyperexcitable state are not well understood. Because it is difficult, and frequently impossible, to independently examine using experimental techniques whether it is the loss of mossy cells or the sprouting of mossy fibers that leads to dentate hyperexcitability, we built a biophysically realistic and anatomically representative computational model of the dentate gyrus to examine this question. The 527-cell model, containing granule, mossy, basket, and hilar cells with axonal projections to the perforant-path termination zone, showed that even weak mossy fiber sprouting (10-15% of the strong sprouting observed in the pilocarpine model of epilepsy) resulted in the spread of seizure-like activity to the adjacent model hippocampal laminae after focal stimulation of the perforant path. The simulations also indicated that the spatially restricted, lamellar distribution of the sprouted mossy fiber contacts reported in in vivo studies was an important factor in sustaining seizure-like activity in the network. In contrast to the robust hyperexcitability-inducing effects of mossy fiber sprouting, removal of mossy cells resulted in decreased granule cell responses to perforant-path activation in agreement with recent experimental data. These results indicate the crucial role of mossy fiber sprouting even in situations where there is only relatively weak mossy fiber sprouting as is the case after moderate concussive experimental head injury.

D-Serine differently modulates NMDA receptor function in rat CA1 hippocampal pyramidal cells and interneurons

The organization of the neuronal hippocampal network depends on the tightly regulated interaction between pyramidal cells (PCs) and interneurons (Ints). NMDA receptor (NMDAR) activation requires the binding of glutamate and co-activation of the 'glycine site'. It has been reported that D-serine is a more potent endogenous agonist than glycine for that site. While many studies have focused on NMDAR function in PCs, little is known regarding the modulation of NMDARs in Ints. We studied the modulatory effect of D-serine on NMDAR EPSCs in PCs and in stratum radiatum Ints using whole-cell patch-clamp recording in rat acute hippocampal slices. We found that D-serine enhances NMDAR function and differently modulates NMDAR currents in both cell types. The augmentation of NMDAR currents by D-serine was significantly larger in PCs compared with Ints. Moreover, we found differences in the kinetics of NMDAR currents in PCs and Ints. Our findings indicate that regulation of NMDAR through the 'glycine site' depends on the cell types. We speculate that the observed differences arise from assemblies of diverse NMDAR subunits. Overall, our data suggest that D-serine may be involved in regulation of the excitation-inhibition balance in the CA1 hippocampal region.

Modulation of network behaviour by changes in variance in interneuronal properties

Interneurones are important regulators of neuronal networks. The conventional approach to interneurones is to focus on the mean values of various parameters. Here we tested the hypothesis that changes in the variance of interneuronal properties (e.g. in the degree of scattering of parameter values of individual cells around the population mean) may modify the behaviour of networks. Biophysically based multicompartmental models of principal cells and interneurones showed that changes in the variance in the electrophysiological and anatomical properties of interneurones significantly alter the input-output functions, rhythmicity and synchrony of principal cells, even if the mean values were unchanged. In most cases, increased heterogeneity in interneurones resulted in stronger inhibition of principal cell firing; however, there were parameter ranges where increased interneuronal variance decreased the inhibition of principal cells. Electrophysiological recordings showed that the variance in the resting membrane potential of CA1 stratum oriens interneurones persistently increased following experimental complex febrile seizures in developing rats, without a change in the mean resting membrane potential, indicating that lasting alterations in interneuronal heterogeneity can take place in real neuronal systems. These computational and experimental data demonstrate that modifications in interneuronal population variance influence the behaviour of neuronal networks, and suggest a physiological role for interneuronal diversity. Furthermore, the results indicate that interneuronal heterogeneity can change in neurological diseases, and raise the possibility that neuromodulators may act by regulating the variance of key parameters in interneuronal populations.

Glutamate receptor involvement in dentate granule cell epileptiform activity evoked by mossy fiber stimulation

In many persons with temporal lobe epilepsy, dentate granule cells form an interconnected synaptic network. This recurrent mossy fiber circuit mediates reverberating excitation that may facilitate seizure propagation by synchronizing granule cell discharge. The involvement of specific glutamate receptors in granule cell epileptiform activity evoked by stimulating the mossy fibers was investigated with use of rat hippocampal slices superfused with bicuculline, with or without increasing [K+](o) to 6 mM. The occurrence of short-latency mossy fiber-evoked granule cell epileptiform activity in slices from pilocarpine-treated rats correlated with the presence and extent of recurrent mossy fiber growth. Blockade of AMPA receptors nearly abolished the orthodromic component of the response; subsequent antagonism of kainate receptors as well appeared to have no further action. Antagonism of NMDA receptors reduced the duration of epileptiform discharge, but increased the amplitude of population spikes within the evoked burst. Thus AMPA and NMDA, but perhaps not kainate, receptors play an important role in this type of epileptiform activity. Activation of type II metabotropic glutamate receptors, which inhibits the release of glutamate from mossy fiber boutons, reduced the magnitude of epileptiform discharge. This action was reversed by a partial agonist of these receptors. However, neither an agonist nor an antagonist of type III metabotropic glutamate receptors significantly altered the response. Considering the importance of synchronous granule cell discharge for seizure propagation from the entorhinal cortex to the hippocampus, agonists of type II metabotropic glutamate receptors may be useful in suppressing such discharge both experimentally and clinically.

Loss of hilar mossy cells in Ammon's horn sclerosis

PURPOSE

Hilar mossy cells represent an important excitatory subpopulation of the hippocampal formation. Several studies have identified this cell type as particularly vulnerable to seizure activity in rat models of limbic epilepsy. Here we have subjected hilar mossy cell loss in the hippocampus of patients with chronic temporal lobe epilepsy (TLE) to a systematic morphological and immunohistochemical analysis.

METHODS

Hippocampal specimens from 30 TLE patients were included; 21 patients presented with segmental neuronal cell loss [Ammon's horns clerosis (AHS)] and 8 with focal lesions (tumors, scars, malformations) not involving the hippocampus proper. In one additional TLE patient, no histopathological alteration could be observed. Surgical specimens from tumor patients without epilepsy (n = 2) and nonepileptic autopsy brains (n = 8) were used as controls. Hilar mossy cells in the human hippocampus were visualized using a novel polycloncal antiserum directed against the metabotropic glutamate receptor subtype mGluR7b or by intracellular Lucifer Yellow injection, confocal laser scanning microscopy, and three-dimensional morphological reconstruction.

RESULTS

Compared with controls, a significant loss of mGluR7 immunoreactive mossy cells was observed in patients with AHS (p < 0.05). In contrast, TLE patients with focal lesions but structurally intact hippocampus demonstrated only a discrete, nonsignificant reduction of this neuronal subpopulation. This observation was confirmed by analysis of 62 randomly injected hilar neurons from AHS patients, in which we were unable to detect neurons with a morphology like that of hilar mossy cells.

CONCLUSION

Our present data indicate significant hilar mossy cell loss in TLE patients with AHS. In contrast, hilar mossy cells appear to be less vulnerable in patients with lesion-associated TLE. Although the significance of mGluR7 immunoreactivity in mossy cells remains to be studied, loss of this cell population is compatible with alterations in hippocampal networks and regional hyperexcitability as pathogenic mechanism of AHS and TLE.

Mossy fiber-granule cell synapses in the normal and epileptic rat dentate gyrus studied with minimal laser photostimulation

Dentate granule cells become synaptically interconnected in the hippocampus of persons with temporal lobe epilepsy, forming a recurrent mossy fiber pathway. This pathway may contribute to the development and propagation of seizures. The physiology of mossy fiber-granule cell synapses is difficult to characterize unambiguously, because electrical stimulation may activate other pathways and because there is a low probability of granule cell interconnection. These problems were addressed by the use of scanning laser photostimulation in slices of the caudal hippocampal formation. Glutamate was released from a caged precursor with highly focused ultraviolet light to evoke action potentials in a small population of granule cells. Excitatory synaptic currents were recorded in the presence of bicuculline. Minimal laser photostimulation evoked an apparently unitary excitatory postsynaptic current (EPSC) in 61% of granule cells from rats that had experienced pilocarpine-induced status epilepticus followed by recurrent mossy fiber growth. An EPSC was also evoked in 13-16% of granule cells from the control groups. EPSCs from status epilepticus and control groups had similar peak amplitudes ( approximately 30 pA), 20-80% rise times (approximately 1.2 ms), decay time constants ( approximately 10 ms), and half-widths (approximately 8 ms). The mean failure rate was high (approximately 70%) in both groups, and in both groups activation of N-methyl-D-aspartate receptors contributed a small component to the EPSC. The strong similarity between responses from the status epilepticus and control groups suggests that they resulted from activation of a similar synaptic population. No EPSC was recorded when the laser beam was focused in the dentate hilus, suggesting that indirect activation of hilar mossy cells contributed little, if at all, to these results. Recurrent mossy fiber growth increases the density of mossy fiber-granule cell synapses in the caudal dentate gyrus by perhaps sixfold, but the new synapses appear to operate very similarly to preexisting mossy fiber-granule cell synapses.

Molecular neuropathology of human mesial temporal lobe epilepsy

With the recent progress in surgical treatment modalities, human brain tissue from patients with intractable focal epilepsies will increasingly become available for studies on the molecular pathology, electrophysiological changes and pathogenesis of human focal epilepsies. An inherent problem for studies on human temporal lobe epilepsy (TLE) is the lack of suitable controls. Strategies to alleviate this obstacle include the use of human post mortem samples, hippocampus from experimental animals and, in particular, the comparative analysis of surgical specimens from patients with Ammon's horn sclerosis (AHS) and with focal temporal lesions but anatomically preserved hippocampal structures. In this review we focus on selected aspects of the molecular neuropathology of TLE: (1) the potential impact of persisting calretinin-immunoreactive neurons with Cajal-Retzius cell morphology, (2) astrocytic tenascin-C induction and redistribution as potential regulator of aberrant axonal sprouting and (3) alterations of Ca2+ -mediated hippocampal signalling pathways. The diverse and complex changes described so far in human TLE specimens require a systematic interdisciplinary approach to distinguish primary, epileptogenic alterations and secondary, compensatory mechanisms in the pathogenesis of human temporal lobe epilepsies.

Recurrent mossy fiber pathway in rat dentate gyrus: synaptic currents evoked in presence and absence of seizure-induced growth

A common feature of temporal lobe epilepsy and of animal models of epilepsy is the growth of hippocampal mossy fibers into the dentate molecular layer, where at least some of them innervate granule cells. Because the mossy fibers are axons of granule cells, the recurrent mossy fiber pathway provides monosynaptic excitatory feedback to these neurons that could facilitate seizure discharge. We used the pilocarpine model of temporal lobe epilepsy to study the synaptic responses evoked by activating this pathway. Whole cell patch-clamp recording demonstrated that antidromic stimulation of the mossy fibers evoked an excitatory postsynaptic current (EPSC) in approximately 74% of granule cells from rats that had survived >10 wk after pilocarpine-induced status epilepticus. Recurrent mossy fiber growth was demonstrated with the Timm stain in all instances. In contrast, antidromic stimulation of the mossy fibers evoked an EPSC in only 5% of granule cells studied 4-6 days after status epilepticus, before recurrent mossy fiber growth became detectable. Notably, antidromic mossy fiber stimulation also evoked an EPSC in many granule cells from control rats. Clusters of mossy fiber-like Timm staining normally were present in the inner third of the dentate molecular layer at the level of the hippocampal formation from which slices were prepared, and several considerations suggested that the recorded EPSCs depended mainly on activation of recurrent mossy fibers rather than associational fibers. In both status epilepticus and control groups, the antidromically evoked EPSC was glutamatergic and involved the activation of both AMPA/kainate and N-methyl-D-aspartate (NMDA) receptors. EPSCs recorded in granule cells from rats with recurrent mossy fiber growth differed in three respects from those recorded in control granule cells: they were much more frequently evoked, a number of them were unusually large, and the NMDA component of the response was generally much more prominent. In contrast to the antidromically evoked EPSC, the EPSC evoked by stimulation of the perforant path appeared to be unaffected by a prior episode of status epilepticus. These results support the hypothesis that recurrent mossy fiber growth and synapse formation increases the excitatory drive to dentate granule cells and thus facilitates repetitive synchronous discharge. Activation of NMDA receptors in the recurrent pathway may contribute to seizure propagation under depolarizing conditions. Mossy fiber-granule cell synapses also are present in normal rats, where they may contribute to repetitive granule cell discharge in regions of the dentate gyrus where their numbers are significant.

Temporal patterns and depolarizing actions of spontaneous GABAA receptor activation in granule cells of the early postnatal dentate gyrus

Whole cell patch-clamp recordings were used to investigate the properties of the gamma-aminobutyric acid type A (GABAA) receptor-mediated spontaneous synaptic events in immature granule cells of the developing, early postnatal day (P0-P6) rat dentate gyrus. With Cs-gluconate-filled whole cell patch pipettes at 0 mV in control medium, spontaneous inhibitory postsynaptic currents (sIPSCs) occurred in prominent bursts (peak amplitude of the bursts 406.9 +/- 58.4 pA; intraburst IPSC frequency 71.0 +/- 12.4 Hz) at 0.05 +/- 0.02 Hz in every immature granule cell younger than P7. Between the bursts of IPSCs, lower frequency (1.7 +/- 0.7 Hz), interburst IPSCs could be observed. Bicuculline and picrotoxin as well as the intracellularly applied chloride-channel blockers CsF- and 4,4'-diisothiocyanatostilbene-2, 2'-disulfonic acid (DIDS) abolished the intraburst as well as the interburst IPSCs, indicating that the IPSCs were mediated by GABAA receptor channels. The bursts of IPSCs, but not the interburst IPSCs, were blocked by the simultaneous application of the glutamate receptor antagonists 2-amino-5-phosphovaleric acid and 6-cyano-7-nitroquinoxaline-2,3-dione, indicating the importance of the glutamatergic excitatory drive onto the interneurons in the early postnatal dentate gyrus. The spontaneously occurring excitatory postsynaptic currents in immature granule cells, observable after the intracellular blockade of GABAA receptor channels with CsF- and DIDS, appeared exclusively as single events at low frequencies, i.e., they did not occur in prominent bursts. Gramicidin-based perforated patch-clamp recordings determined that the reversal potential for the burst of IPSCs (-46.6 +/- 3.1 mV) was more depolarized than the resting membrane potential (-54.2 +/- 4.2 mV) but more hyperpolarized than the action potential threshold (-41. 8 +/- 1.7 mV). The depolarizing action of the bursts of synaptic events most often evoked only a single action potential per burst. Simultaneous whole cell patch recordings, with KCl-filled patch pipettes at -60 mV in current clamp from pairs of immature granule cells of the developing dentate gyrus, determined that the bursts of IPSPs took place in a similar temporal pattern but with imperfect synchrony in neighboring granule cells (average lag between the onsets of the bursts between granule cell pairs 77.7 +/- 8.6 ms). These results show that the spontaneous activation of GABAA receptors in immature dentate granule cells displays unique properties that are distinct from the temporal patterns and biophysical features of spontaneous GABAA receptor activation taking place in the developing Ammon's horn and in the adult dentate gyrus.

Interneurones are not so dormant in temporal lobe epilepsy: a critical reappraisal of the dormant basket cell hypothesis

One axiom at the basis of epilepsy research is that there exists an imbalance between excitation and inhibition. This abnormality can be achieved by an increase of excitation on principal cells, a decreased inhibition (i.e. disinhibition) or both. This review focuses on dysfunction of inhibition, and in particular on the 'dormant basket cell hypothesis'. This hypothesis states that, (1) interneurones are functionally disconnected from excitatory afferents, resulting in hyperexcitability of principal neurones and loss of paired pulse inhibition, (2) when properly activated, interneurones can still perform their task, i.e. suppress epileptiform activity and restore paired pulse inhibition. The aim of this review is to discuss the evidence in support of the 'dormant basket cell hypothesis'. We will first discuss the rationale underlying the hypothesis and the criteria needed to validate the hypothesis. We will then show that, (1) the key experimental data offered in support of the hypothesis (Bekenstein and Lothman, 1993. Dormancy of inhibitory interneurones in a model of temporal lobe epilepsy. Science 259, 97-100; Sloviter, 1991. Permanently altered hippocampal structure, excitability, and inhibition after experimental status epilepticus in the rat: the 'dormant basket cell' hypothesis and its relevance to temporal lobe epilepsy. Hippocampus 1, 41-66) are difficult to interpret, and (2) recent recordings from interneurones in epileptic tissue argue against the hypothesis. The 'dormant basket cell hypothesis' is then discussed in the broader context of disinhibition.

Neurocalcin-immunoreactive cells in the rat hippocampus are GABAergic interneurons

Neurocalcin (NC) is a recently described calcium-binding protein isolated and characterized from bovine brain. NC belongs to the neural calcium-sensor proteins defined by the photoreceptor cell-specific protein recoverin that have been proposed to be involved in the regulation of calcium-dependent phosphorylation in signal transduction pathways. We analyzed the distribution and morphology of the NC-immunoreactive (IR) neurons in the rat dorsal hippocampus and the coexistence of NC with GABA and different neurochemical markers which label perisomatic inhibitory cells [parvalbumin (PV) and cholecystokinin (CCK)], mid-proximal dendritic inhibitory cells [calbindin D28k (CB)], distal dendritic inhibitory cells [somatostatin (SOM) and neuropeptide Y (NPY)], and interneurons specialized to innervate other interneurons [calretinin (CR) and vasoactive intestinal polypeptide (VIP)]. NC-IR cells were present in all layers of the dentate gyrus and hippocampal fields. In the dentate gyrus, NC-IR cells were concentrated in the granule cell layer, especially in the hilar border, whereas in the CA fields they were most frequently found in the stratum radiatum. NC-IR cells were morphologically heterogeneous and exhibited distinctive features of non-principal cells. In the dentate gyrus, pyramidal-like, multipolar and fusiform (horizontal and vertical) cells were found. In the CA3 region most NC-IR cells were multipolar, but vertical and horizontal fusiform cells also appeared. In the CA1 region, where NC-IR cells showed most frequently vertically arranged dendrites, multipolar, bitufted and fusiform (vertical and horizontal) cells could be distinguished. All the NC-IR cells were found to be GABA-IR in all hippocampal layers and regions, and they represented about 19% of the GABA-positive cells. NC/CB, NC/CR and NC/VIP double-labeled cells were found in all hippocampal regions, and represented 29%, 24% and 18% of the NC-IR cells, respectively. NC and CCK did not coexist in the dentate gyrus; however, 9% of the NC-IR cells in the CA fields also contained CCK. No coexistence of NC with PV, SOM or NPY was found in any hippocampal region. We conclude that NC is exclusively expressed by interneurons in the rat hippocampus. NC-IR cells are a morphologically and neurochemically heterogeneous subset of GABAergic non-principal cells, which, on the basis of the known termination pattern of the colocalizing markers, are also functionally heterogeneous and are mainly involved in feed-forward dendritic inhibition in the commissural-associational and Schaffer collateral termination zones (CB containing cells), in innervation of other interneurons (CR- and VIP-containing cells), and in perisomatic inhibition (CCK-containing cells). NC is never present in perisomatic inhibitory PV-containing cells, or in feed-back distal dendritic inhibitory SOM/NPY-containing cells.

Physiological unmasking of new glutamatergic pathways in the dentate gyrus of hippocampal slices from kainate-induced epileptic rats

In humans with temporal lobe epilepsy and kainate-treated rats, the mossy fibers of the dentate granule cells send collateral axons into the inner molecular layer. Prior investigations on kainate-treated rats demonstrated that abnormal hilar-evoked events can occasionally be observed in slices with mossy fiber sprouting when gamma-aminobutyric acid-A (GABAA)-mediated inhibition is blocked with bicuculline. However, these abnormalities were observed infrequently, and it was unknown whether these rats were epileptic. Wuarin and Dudek reported that in slices from kainate-induced epileptic rats (3-13 mo after treatment), hilar stimulation evoked abnormal events in most slices with mossy fiber sprouting exposed simultaneously to bicuculline and elevated extracellular potassium concentration [K+]o. Using the same rats, extracellular recordings were obtained from granule cells in hippocampal slices to determine whether 1) hilar stimulation could evoke abnormal events in slices with sprouting in normal artificial cerebrospinal fluid (ACSF), 2) adding only bicuculline could unmask hilar-evoked abnormalities and glutamate-receptor antagonists could block these events, and 3) increasing only [K+]o could unmask these abnormalities. In normal ACSF, hilar stimulation evoked abnormal field potentials in 27% of slices with sprouting versus controls without sprouting (i.e., saline-treated or only 2-4 days after kainate treatment). In bicuculline (10 microM) alone, hilar stimulation triggered prolonged field potentials in 84% of slices with sprouting, but not in slices from the two control groups. Addition of the N-methyl-D-aspartate (NMDA) receptor antagonist, DL-2-amino-5-phosphonopentanoic acid (AP5), either blocked the bursts or reduced their probability of occurrence. The alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX), always eliminated the epileptiform bursts. In kainate-treated rats with sprouting, but not in saline-treated controls, abnormal hilar-evoked responses were also revealed in 6-9 mM [K+]o. Additionally, 63% of slices with sprouting generated spontaneous bursts lasting 1-40 s in ACSF containing 9 mm [K+]o; similar bursts were not observed in controls. These results indicate that 1) mossy fiber sprouting is associated with new glutamatergic pathways, and although NMDA receptors are important for propagation through these circuits, AMPA receptor activation is crucial, 2) modest elevations of [K+]o, in a range that would have relatively little effect on granule cells, can unmask these new excitatory circuits and generate epileptiform bursts, and 3) this new circuitry underlies an increased electrographic seizure susceptibility when inhibition is depressed or membrane excitability is increased.

Instantaneous perturbation of dentate interneuronal networks by a pressure wave-transient delivered to the neocortex

Whole-cell patch-clamp recordings and immunocytochemical experiments were performed to determine the short- and long-term effects of lateral fluid percussion head injury on the perisomatic inhibitory control of dentate granule cells in the adult rat, with special reference to the development of trauma-induced hyperexcitability. One week after the delivery of a single, moderate (2.0-2.2 atm) mechanical pressure wave to the neocortex, the feed-forward inhibitory control of dentate granule cell discharges was compromised, and the frequency of miniature IPSCs was decreased. Consistent with the electrophysiological data, the number of hilar parvalbumin (PV)- and cholecystokinin (CCK)-positive dentate interneurons supplying the inhibitory innervation of the perisomatic region of granule cells was decreased weeks and months after head injury. The initial injury to the hilar neurons took place instantaneously after the impact and did not require the recruitment of active physiological processes. Furthermore, the decrease in the number of PV- and CCK-positive hilar interneurons was similar to the decrease in the number of the AMPA-type glutamate receptor subunit 2/3-immunoreactive mossy cells, indicating that the pressure wave-transient causes injurious physical stretching and bending of most cells that are large and not tightly packed in a cell layer. These results reveal for the first time that moderate pressure wave-transients, triggered by traumatic head injury episodes, impact the dentate neuronal network in a unique temporal and spatial pattern, resulting in a net decrease in the perisomatic control of granule cell discharges.

Interneurons of the dentate-hilus border of the rat dentate gyrus: morphological and electrophysiological heterogeneity

Interneurons located near the border of the dentate granule cell layer and the hilus were studied in hippocampal slices using whole-cell current clamp and biocytin staining. Because these interneurons exhibit both morphological and electrophysiological diversity, we asked whether passive electrotonic parameters or repetitive firing behavior correlated with axonal distribution. Each interneuron was distinguished by a preferred axonal distribution in the molecular layer or granule cell layer, and four groups could be discerned, the axons of which arborized in (1) the granule cell layer, (2) the inner molecular layer, (3) the outer molecular layer, and (4) diffusely in the molecular layer. In our sample, interneurons with axons arborizing diffusely in the molecular layer were most frequent, and those with axons restricted to the granule cell layer were least frequent. Resting potential, input resistance, time constant, electrotonic length, and spike frequency adaptation (SFA) were not significantly different among the four groups, and the variability in SFA between cells with similar axonal distributions was striking. Clear differences in action potential morphology and afterhyperpolarizations, however, emerged when nonadapting interneurons were compared with those exhibiting SFA. Interneurons exhibiting SFA had characteristically broader spikes, progressive slowing of action potential repolarization during repetitive firing, and slow afterhyperpolarizations that distinguished them from nonadapting interneurons. We propose that the variability in repetitive firing behavior and morphology exhibited by each of these interneurons makes each interneuron unique and may provide a high level of fine tuning of inhibitory control critical to information processing in the dentate.

In contrast to kindled seizures, the frequency of spontaneous epilepsy in the limbic status model correlates with greater aberrant fascia dentata excitatory and inhibitory axon sprouting, and increased staining for N-methyl-D-aspartate, AMPA and GABA(A) receptors

This study determined whether there were differences in hippocampal neuron loss and synaptic plasticity by comparing rats with spontaneous epilepsy after limbic status epilepticus and animals with a similar frequency of kindled seizures. At the University of Virginia, Sprague-Dawley rats were implanted with bilateral ventral hippocampal electrodes and treated as follows; no stimulation (electrode controls; n=5): hippocampal stimulation without status (stimulation controls; n=5); and limbic status from continuous hippocampal stimulation (n=12). The limbic status group were electrographically monitored for a minimum of four weeks. Four rats had no recorded chronic seizures (status controls), and all three control groups showed no differences in hippocampal pathology and were therefore incorporated into a single group (controls). Eight limbic status animals eventually developed chronic epilepsy (spontaneous seizures) and an additional eight rats were kindled to a similar number and frequency of stage 5 seizures (kindled) as the spontaneous seizures group. At the University of California (UCLA) the hippocampi were processed for: (i) Niss1 stain for densitometric neuron counts; (ii) neo-Timm's histochemistry for mossy fiber sprouting; and (iii) immunocytochemical staining for glutamate decarboxylase, N-methyl-D-aspartate receptor subunit 2, AMPA receptor subunit 1 and the GABA(A) receptor. In the fascia dentata inner and outer molecular layers the neo-Timm's stain and immunoreactivity was quantified as gray values using computer image analysis techniques. Statistically significant results (P<0.05) showed the following. Compared to controls and kindled animals, rats with spontaneous seizures had: (i) lower neuron counts for the fascia dentata hilus, CA3 and CA1 stratum pyramidale; (ii) greater supragranular inner molecular layer mossy fiber staining; and (iii) greater glutamate decarboxylase immunoreactivity in both molecular layers. Greater supragranular excitatory mossy fiber and GABAergic axon sprouting correlated with: (i) increases in N-methyl-D-aspartate receptor subunit 2 inner molecular layer staining; (ii) more AMPA receptor subunit 1 immunoreactivity in both molecular layers; and (iii) greater outer than inner molecular layer GABA(A) immunoreactivity. Furthermore, in contrast to kindled animals, rats with spontaneous seizures showed that increasing seizure frequency per week and the total number of natural seizures positively correlated with greater Timm's and GABAergic axon sprouting, and with increases in N-methyl-D-aspartate receptor subunit 2 and AMPA receptor subunit 1 receptor staining. In this rat limbic status model these findings indicate that chronic seizures are associated with hippocampal neuron loss, reactive axon sprouting and increases in excitatory receptor plasticity that differ from rats with an equal frequency of kindled seizures and controls. The hippocampal pathological findings in the limbic status model are similar to those in humans with hippocampal sclerosis and mesial temporal lobe epilepsy, and support the hypothesis that synaptic reorganization of both excitatory and inhibitory systems in the fascia dentata is an important pathophysiological mechanism that probably contributes to or generates chronic limbic seizures.

Regulation of excitatory input to inhibitory interneurons of the dentate gyrus during hypoxia

The role of metabotropic glutamate receptors (mGluRs) and adenosine receptors in hypoxia-induced suppression of excitatory synaptic input to interneurons residing at the granule cell-hilus border in the dentate gyrus was investigated with the use of whole cell electrophysiological recording techniques in thin (250 microns) slices of immature rat hippocampus. Minimal stimulation evoked glutamatergic excitatory postsynaptic currents (EPSCs) in dentate interneurons in 68 +/- 4% (mean +/- SE) of trials during stimulation in the dentate granule cell layer (GCL) and 48 +/- 3% of trials during stimulation in CA3. Hypoxic episodes, produced by switching the perfusing solution from 95% O2-5% CO2 to a solution containing 95% N2-5% CO2 for 3-5 min, rapidly and reversibly decreased the synaptic reliability, or probability of evoking an EPSC, from either input without reducing EPSC amplitude, consistent with a presynaptic suppression of transmitter release. The mGluR antagonist (+)-alpha-methyl-4-carboxyphenylglycine [(+) MCPG; 500 microM] did not alter synaptic reliability or mean EPSC amplitude in either pathway. However, (+) MCPG significantly attenuated hypoxic suppression of input from both pathways, suggesting that mGluRs activated by release of glutamate partially mediate hypoxic suppression of EPSCs to dentate interneurons. The mGluR agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD; 100 microM) rapidly decreased the reliability of excitatory transmission from both the GCL (19 +/- 5% of control) and CA3 (39 +/- 15% of control). ACPD also increased the frequency of spontaneous EPSCs and evoked a slow inward current in dentate interneurons. Exogenous adenosine (10-300 microM) decreased synaptic reliability for both pathways and reduced the frequency of spontaneous EPSCs, but did not cause a decrease in the mean amplitude of evoked EPSCs, consistent with a presynaptic suppression of excitatory input to dentate interneurons. Conversely, the selective adenosine A1 receptor antagonists 8-cyclopentyl-1,3-dipropylxanthine (200 nM to 1 microM) and N6-cyclopentyl-9-methyladenine (1 microM) enhanced excitatory input to dentate interneurons by increasing synaptic reliability for both the GCL and CA3 inputs. Adenosine A1 receptor antagonists did not, however, reduce hypoxic suppression of excitatory input to dentate interneurons. These results indicate that hypoxia induces a presynaptic inhibition of excitatory input to dentate interneurons mediated in part by activation of mGluRs, but not adenosine A1 receptors, whereas both mGluRs and adenosine A1 receptors can depress excitatory input to dentate interneurons during normoxic stimulation. Regulation of excitatory input to dentate interneurons provides a mechanism to shape excitatory input to the hippocampus under both normal and pathological conditions.