A light and electron microscopic analysis of the mossy fibers of the rat dentate gyrus

Altered patterning of dentate granule cell mossy fiber inputs onto CA3 pyramidal cells in limbic epilepsy

Impaired gating by hippocampal dentate granule cells may promote the development of limbic epilepsy by facilitating seizure spread through the hippocampal trisynaptic circuit. The second synapse in this circuit, the dentate granule cell≫CA3 pyramidal cell connection, may be of particular importance because pathological changes occurring within the dentate likely exert their principal effect on downstream CA3 pyramids. Here, we utilized GFP-expressing mice and immunolabeling for the zinc transporter ZnT-3 to reveal the pre- and postsynaptic components of granule cell≫CA3 pyramidal cell synapses following pilocarpine-epileptogenesis. Confocal analyses of these terminals revealed that while granule cell presynaptic giant boutons increased in size and complexity 1 month after status epilepticus, individual thorns making up the postsynaptic thorny excrescences of the CA3 pyramidal cells were reduced in number. This reduction, however, was transient, and 3 months after status, thorn density recovered. This recovery was accompanied by a significant change in the distribution of thorns along pyramidal cells dendrites. While thorns in control animals tended to be tightly clustered, thorns in epileptic animals were more evenly distributed. Computational modeling of thorn distributions predicted an increase in the number of boutons required to cover equivalent numbers of thorns in epileptic vs. control mice. Confirming this prediction, ZnT-3 labeling of presynaptic giant boutons apposed to GFP-expressing thorns revealed a near doubling in bouton density, while the number of individual thorns per bouton was reduced by half. Together, these data provide clear evidence of novel plastic changes occurring within the epileptic hippocampus.

Impairment of synaptic plasticity in hippocampus is exacerbated by methylprednisolone in a rat model of traumatic brain injury

Patients with traumatic brain injury (TBI) exhibit impaired cognitive capability that is exacerbated with methylprednisolone (MP). Since long-term potentiation (LTP) is a leading cellular model underlying learning, we hypothesize that MP disturbs the electrophysiological character in the hippocampus by decreasing the number of interneurons post-traumatically in the dentate gyrus (DG) and cornu ammonis3 (CA3) regions, resulting in learning deficits. To test this hypothesis, we investigated the alterations of learning abilities and correlated the alternation with hippocampal synaptic plasticity in rats receiving lateral fluid percussion injury (FPI) and being treated with MP. We found that MP aggravates the spatial learning deficiency and changes in the excitability of the DG and cornu ammonis1 (CA1) areas in rats subjected to FPI. The functional and electrophysiological deficits are associated with a decrease in the number of parvalbumin-immunoreactive (PV-IR) and cholecystokinin-immunoreactive (CCK-IR) GABAergic cells. The data suggest that MP therapy may decrease the number of DG interneurons in post-traumatic hippocampus, resulting in the aggravated deficits of learning ability induced by TBI.

Passive and active shaping of unitary responses from associational/commissural and perforant path synapses in hippocampal CA3 pyramidal cells

Although associational/commissural (A/C) and perforant path (PP) inputs to CA3b pyramidal cells play a central role in hippocampal mnemonic functions, the active and passive processes that shape A/C and PP AMPA and NMDA receptor-mediated unitary EPSP/EPSC (AMPA and NMDA uEPSP/uEPSC) have not been fully characterized yet. Here we find no differences in somatic amplitude between A/C and PP for either AMPA or NMDA uEPSPs. However, larger AMPA uEPSCs were evoked from proximal than from distal A/C or PP. Given the space-clamp constraints in CA3 pyramidal cells, these voltage clamp data suggest that the location-independence of A/C and PP AMPA uEPSP amplitudes is achieved in part through the activation of voltage dependent conductances at or near the soma. Moreover, similarity in uEPSC amplitudes for distal A/C and PP points to the additional participation of unclamped active conductances. Indeed, the pharmacological blockade of voltage-dependent conductances eliminates the location-independence of these inputs. In contrast, the location-independence of A/C and PP NMDA uEPSP/uEPSC amplitudes is maintained across all conditions indicating that propagation is not affected by active membrane processes. The location-independence for A/C uEPSP amplitudes may be relevant in the recruitment of CA3 pyramidal cells by other CA3 pyramidal cells. These data also suggest that PP excitation represents a significant input to CA3 pyramidal cells. Implication of the passive data on local synaptic properties is further investigated in the companion paper with a detailed computational model.

Spike-timing-dependent plasticity in hippocampal CA3 neurons

Synaptic plasticity of different inputs converging onto CA3 pyramidal neurons is central to theories of hippocampal function. The mossy fibre (MF) input to these neurons is thought to exhibit plasticity that is in nearly all aspects fundamentally different from plasticity in other brain regions: in particular, when induced by high frequency presynaptic stimulation, plasticity at these synapses is independent of NMDA receptor (NMDAR) activation and presynaptically expressed. Here, we show that different stimulation protocols that depend on the close timing of MF activity and postsynaptic spikes induce bidirectional plasticity in CA3 neurons in 3-week-old rats. Long-term potentiation (LTP) is observed when an excitatory postsynaptic potential (EPSP), evoked by MF stimulation, precedes a single postsynaptic action potential (AP) or a brief AP burst by 10 ms. Instead, timing-dependent long-term depression (LTD) requires the pairing of a single AP to an EPSP with a delay of 30 ms. The pairing of APs to synaptic activity is required for plasticity induction, since the application of unpaired APs or EPSPs did not alter synaptic strength. Furthermore, our results demonstrate that both timing-dependent LTP and LTD critically depend on the activation of NMDARs. Specifically blocking postsynaptic NMDARs prevents plasticity, demonstrating that NMDARs important to spike-timing-dependent plasticity in CA3 neurons are required at postsynaptic sites. In summary, this study shows that the close timing of APs to MF excitatory synaptic input can alter synaptic efficacy in CA3 neurons in a bidirectional manner.

Age- and hormone-regulation of opioid peptides and synaptic proteins in the rat dorsal hippocampal formation

Circulating estrogen levels and hippocampal-dependent cognitive functions decline with aging. Moreover, the responses of hippocampal synaptic structure to estrogens differ between aged and young rats. We recently reported that estrogens increase levels of post-synaptic proteins, including PSD-95, and opioid peptides leu-enkephalin and dynorphin in the hippocampus of young animals. However, the influence of ovarian hormones on synaptic protein and opioid peptide levels in the aging hippocampus is understudied. Here, young (3- to 5-month-old), middle-aged (9- to 12-month-old), and aged (about 22-month-old) female rats were ovariectomized and then, 4 weeks later, subcutaneously implanted with a silastic capsule containing vehicle or 17β-estradiol. After 48 h, rats were subcutaneously injected with progesterone or vehicle and sacrificed 1 day later. Coronal sections through the dorsal hippocampus were processed for quantitative peroxidase immunohistochemistry of leu-enkephalin, dynorphin, synaptophysin, and PSD-95. With age, females showed opposing changes in leu-enkephalin and dynorphin levels in the mossy fiber pathway, particularly within the hilus, and regionally specific changes in synaptic protein levels. 17β-estradiol, with or without progesterone, altered leu-enkephalin levels in the dentate gyrus and synaptophysin levels in the CA1 of young but not middle-aged or aged females. Additionally, 17β-estradiol decreased synaptophysin levels in the CA3 of middle-aged females. Our results support and extend previous findings indicating 17β-estradiol modulation of hippocampal opioid peptides and synaptic proteins while demonstrating regional and age-specific effects. Moreover, they lend credence to the "window of opportunity" hypothesis during which hormone replacement can modulate hippocampal structure and circuitry to improve cognitive outcomes.

Critical involvement of postsynaptic protein kinase activation in long-term potentiation at hippocampal mossy fiber synapses on CA3 interneurons

Hippocampal mossy fiber (MF) synapses on area CA3 lacunosum-moleculare (L-M) interneurons are capable of undergoing a Hebbian form of NMDA receptor (NMDAR)-independent long-term potentiation (LTP) induced by the same type of high-frequency stimulation (HFS) that induces LTP at MF synapses on pyramidal cells. LTP of MF input to L-M interneurons occurs only at synapses containing mostly calcium-impermeable (CI)-AMPA receptors (AMPARs). Here, we demonstrate that HFS-induced LTP at these MF-interneuron synapses requires postsynaptic activation of protein kinase A (PKA) and protein kinase C (PKC). Brief extracellular stimulation of PKA with forskolin (FSK) alone or in combination with 1-Methyl-3-isobutylxanthine (IBMX) induced a long-lasting synaptic enhancement at MF synapses predominantly containing CI-AMPARs. However, the FSK/IBMX-induced potentiation in cells loaded with the specific PKA inhibitor peptide PKI(6-22) failed to be maintained. Consistent with these data, delivery of HFS to MFs synapsing onto L-M interneurons loaded with PKI(6-22) induced posttetanic potentiation (PTP) but not LTP. Hippocampal sections stained for the catalytic subunit of PKA revealed abundant immunoreactivity in interneurons located in strata radiatum and L-M of area CA3. We also found that extracellular activation of PKC with phorbol 12,13-diacetate induced a pharmacological potentiation of the isolated CI-AMPAR component of the MF EPSP. However, HFS delivered to MF synapses on cells loaded with the PKC inhibitor chelerythrine exhibited PTP followed by a significant depression. Together, our data indicate that MF LTP in L-M interneurons at synapses containing primarily CI-AMPARs requires some of the same signaling cascades as does LTP of glutamatergic input to CA3 or CA1 pyramidal cells.

EphA4 signaling in juveniles establishes topographic specificity of structural plasticity in the hippocampus

The formation and loss of synapses is involved in learning and memory. Distinct subpopulations of permanent and plastic synapses coexist in the adult brain, but the principles and mechanisms underlying the establishment of these distinctions remain unclear. Here we show that in the hippocampus, terminal arborizations (TAs) with high plasticity properties are specified at juvenile stages, and account for most synapse turnover of adult mossy fibers. Out of 9-12 giant terminals along CA3, distinct subpopulations of granule neurons revealed by mouse reporter lines exhibit 0, 1, or >2 TAs. TA specification involves a topographic rule based on cell body position and EphA4 signaling. Upon disruption of EphA4 signaling or PSA-NCAM in juvenile circuits, single-TA mossy fibers establish >2 TAs, suggesting that intra-axonal competition influences plasticity site selection. Therefore, plastic synapse specification in juveniles defines sites of synaptic remodeling in the adult, and hippocampal circuit plasticity follows unexpected topographic principles.

Structural plasticity of dentate granule cell mossy fibers during the development of limbic epilepsy

Altered granule cell>>CA3 pyramidal cell synaptic connectivity may contribute to the development of limbic epilepsy. To explore this possibility, granule cell giant mossy fiber bouton plasticity was examined in the kindling and pilocarpine models of epilepsy using green fluorescent protein-expressing transgenic mice. These studies revealed significant increases in the frequency of giant boutons with satellite boutons 2 days and 1 month after pilocarpine status epilepticus, and increases in giant bouton area at 1 month. Similar increases in giant bouton area were observed shortly after kindling. Finally, both models exhibited plasticity of mossy fiber giant bouton filopodia, which contact GABAergic interneurons mediating feedforward inhibition of CA3 pyramids. In the kindling model, however, all changes were fleeting, having resolved by 1 month after the last evoked seizure. Together, these findings demonstrate striking structural plasticity of granule cell mossy fiber synaptic terminal structure in two distinct models of adult limbic epileptogenesis. We suggest that these plasticities modify local connectivities between individual mossy fiber terminals and their targets, inhibitory interneurons, and CA3 pyramidal cells potentially altering the balance of excitation and inhibition during the development of epilepsy.

Overexpression of GAP-43 reveals unexpected properties of hippocampal mossy fibers

The mossy fiber (MF) system targets the apical dendrites of CA3 pyramidal cells in the stratum lucidum (SL). In mice overexpressing the growth-associated protein GAP-43 there is an apparent ectopic growth of these MFs into the stratum oriens (SO) targeting the basal dendrites of these same pyramidal cells (Aigner et al. (1995) Cell 83:269-278). This is the first evidence to our knowledge that links increased GAP-43 expression with growth of central axons. Here we studied the Aigner et al. transgenic mice but were unable to confirm such growth into SO. However, using quantitative methods we did observe enhanced growth within the regions normally targeted by MFs, for example, the SL in the CA3a region. These contrasting results led us to study MFs with double-immunostaining using an immunohistochemical marker for MFs, the zinc transporter, ZnT3, to visualize the colocalization of transgenic GAP-43 within MFs. Unexpectedly, using both fluorescence and confocal microscopy, we were unable to detect colocalization of GAP-43-positive axons with ZnT3-positive MF axons within the MF pathways, either in the region of the MF axons or in the SL, where MF terminals are abundant. In contrast, the plasma membrane-associated presynaptic marker SNAP-25 did colocalize with transgenic GAP-43-positive terminals in the SL. Synaptophysin, the vesicle-associated presynaptic terminal marker, colocalized with ZnT3 but did not appear to colocalize with GAP-43. The present findings raise important questions about the properties of granule cells and the MF mechanisms that differentially regulate axonal remodeling in the adult hippocampus: (1) Because there appears to be at least two populations of granule cells defined by their differential protein expression, this points to the existence of an intrinsic heterogeneity of granule cell expression beyond that contributed by adult neurogenesis; (2) Giventhe present evidence that growth is induced in mice overexpressing GAP-43 in adjacent non-GAP-43 containing MFs, the potential exists for a heretofore unexplored interaxonal communication mechanism.

A symphony of signals conducts early and late stages of adult neurogenesis

Neurogenesis is continually occurring in two regions within the mammalian central nervous system (CNS) and increasing evidence suggests that it is important for selective learning and memory. How this plasticity is maintained in isolated niches within mature networks has been extensively studied in recent years, and a large body of evidence has accumulated describing many different regulatory factors and points of regulation. In this review, we attempt to organize the current research by summarizing findings affecting early neurogenesis: during proliferation, fate commitment and migration, versus late neurogenesis: including dendritic development, synaptic integration, and survival. We discuss the roles of three different classes of factors regulating early and late phases of neurogenesis: intrinsic factors, extrinsic factors, and neurotransmitters. Finally, we suggest that neurotransmitters may act upstream from extracellular other factors and cell-intrinsic mechanisms by coupling network activity to the niche microenvironment and intracellular machinery to ultimately regulate neurogenesis.

Origins of an intrinsic hippocampal EEG pattern

Sharp waves (SPWs) are irregular waves that originate in field CA3 and spread throughout the hippocampus when animals are alert but immobile or as a component of the sleep EEG. The work described here used rat hippocampal slices to investigate the factors that initiate SPWs and govern their frequency. Acute transection of the mossy fibers reduced the amplitude but not the frequency of SPWs, suggesting that activity in the dentate gyrus may enhance, but is not essential for, the CA3 waves. However, selective destruction of the granule cells and mossy fibers by in vivo colchicine injections profoundly depressed SPW frequency. Reducing mossy fiber release with an mGluR2 receptor agonist or enhancing it with forskolin respectively depressed or increased the incidence of SPWs. Collectively, these results indicate that SPWs can be triggered by constitutive release from the mossy fibers. The waves were not followed by large after-hyperpolarizing potentials and their frequency was not strongly affected by blockers of various slow potassium channels. Antagonists of GABA-B mediated IPSCs also had little effect on incidence. It appears from these results that the spacing of SPWs is not dictated by slow potentials. However, modeling work suggests that the frequency and variance of large mEPSCs from the mossy boutons can account for the temporal distribution of the waves. Together, these results indicate that constitutive release from the mossy fiber terminal boutons regulates the incidence of SPWs and their contribution to information processing in hippocampus.

Quantitative morphometry of electrophysiologically identified CA3b interneurons reveals robust local geometry and distinct cell classes

The morphological and electrophysiological diversity of inhibitory cells in hippocampal area CA3 may underlie specific computational roles and is not yet fully elucidated. In particular, interneurons with somata in strata radiatum (R) and lacunosum-moleculare (L-M) receive converging stimulation from the dentate gyrus and entorhinal cortex as well as within CA3. Although these cells express different forms of synaptic plasticity, their axonal trees and connectivity are still largely unknown. We investigated the branching and spatial patterns, plus the membrane and synaptic properties, of rat CA3b R and L-M interneurons digitally reconstructed after intracellular labeling. We found considerable variability within but no difference between the two layers, and no correlation between morphological and biophysical properties. Nevertheless, two cell types were identified based on the number of dendritic bifurcations, with significantly different anatomical and electrophysiological features. Axons generally branched an order of magnitude more than dendrites. However, interneurons on both sides of the R/L-M boundary revealed surprisingly modular axodendritic arborizations with consistently uniform local branch geometry. Both axons and dendrites followed a lamellar organization, and axons displayed a spatial preference toward the fissure. Moreover, only a small fraction of the axonal arbor extended to the outer portion of the invaded volume, and tended to return toward the proximal region. In contrast, dendritic trees demonstrated more limited but isotropic volume occupancy. These results suggest a role of predominantly local feedforward and lateral inhibitory control for both R and L-M interneurons. Such a role may be essential to balance the extensive recurrent excitation of area CA3 underlying hippocampal autoassociative memory function.

Dentate gyrus and spatial behaviour

This article reviews evidence from studies employing colchicine-induced granule cell loss in the adult rat brain, and irradiation-induced hypoplasia of the neonatal dentate gyrus, on the performance of spatial and non-spatial behavioral tasks. The general picture emerging from this analysis reveals that the dentate gyrus granule cells are critically involved in spatial behavior, particularly when this requires the adoption of place strategies. This notion also provides an explanation for the behavioral effects of dentate gyrus granule cell loss seen in apparently non-spatial tasks.

Wnt signaling mediates experience-related regulation of synapse numbers and mossy fiber connectivities in the adult hippocampus

We investigated how experience regulates the structure of a defined neuronal circuit in adult mice. Enriched environment (EE) produced a robust and reversible increase in hippocampal stratum lucidum synapse numbers, mossy fiber terminal (LMT) numbers, and spine plus synapse densities at LMTs, whereas a distinct mechanism depending on Rab3a promoted LMT volume growth. In parallel, EE increased postsynaptic CA3 pyramidal neuron Wnt7a/b levels. Inhibiting Wnt signaling through locally applied sFRP-1 suppressed the effects of EE on synapse numbers and further reduced synapse numbers in control mice. Wnt7 applied to CA3 mimicked the effects of EE on synapse and LMT numbers. CA3 Wnt7a/b levels were enhanced by excitatory activity and reduced by sFRP-1. Synapse numbers and Wnt7a/b levels peaked in mice aged 6-12 months; a decline in aged mice was reversed by EE. Therefore, behavioral experience specifically regulates adult global stratum lucidum synapse numbers and hippocampal network structure through Wnt signaling.

The anatomy of memory: an interactive overview of the parahippocampal-hippocampal network

Converging evidence suggests that each parahippocampal and hippocampal subregion contributes uniquely to the encoding, consolidation and retrieval of declarative memories, but their precise roles remain elusive. Current functional thinking does not fully incorporate the intricately connected networks that link these subregions, owing to their organizational complexity; however, such detailed anatomical knowledge is of pivotal importance for comprehending the unique functional contribution of each subregion. We have therefore developed an interactive diagram with the aim to display all of the currently known anatomical connections of the rat parahippocampal-hippocampal network. In this Review, we integrate the existing anatomical knowledge into a concise description of this network and discuss the functional implications of some relatively underexposed connections.

Bidirectional Hebbian plasticity at hippocampal mossy fiber synapses on CA3 interneurons

Hippocampal area CA3 is critically involved in the formation of nonoverlapping neuronal subpopulations ("pattern separation") to store memory representations as distinct events. Efficient pattern separation relies on the strong and sparse excitatory input from the mossy fibers (MFs) to pyramidal cells and feedforward inhibitory interneurons. However, MF synapses on CA3 pyramidal cells undergo long-term potentiation (LTP), which, if unopposed, will degrade pattern separation because MF activation will now recruit additional CA3 pyramidal cells. Here, we demonstrate MF LTP in stratum lacunosum-moleculare (L-M) interneurons induced by the same stimulation protocol that induces MF LTP in pyramidal cells. This LTP was NMDA receptor (NMDAR) independent and occurred at MF Ca(2+)-impermeable AMPA receptor synapses. LTP was prevented by with voltage clamping the postsynaptic cell soma during high-frequency stimulation (HFS), intracellular injections of the Ca(2+) chelator BAPTA (20 mm), or bath applications of the L-type Ca(2+) channel blocker nimodipine (10 microm). We propose that MF LTP in L-M interneurons preserves the sparsity of pyramidal cell activation, thus allowing CA3 to maintain its role in pattern separation. In the presence of the mGluR1alpha antagonist LY367385 [(S)-(+)-a-amino-4-carboxy-2-methylbenzeneacetic acid] (100 microm), the same HFS that induces MF LTP in naive slices triggered NMDAR-independent MF LTD. This LTD, like LTP, required activation of the L-type Ca(2+) channel and also was induced after blockade of IP(3) receptors with heparin (4 mg/ml) or the selective depletion of receptor-gated Ca(2+) stores with ryanodine (10 or 100 microm). We conclude that L-M interneurons are endowed with Ca(2+) signaling cascades suitable for controlling the polarity of MF long-term plasticity induced by joint presynaptic and postsynaptic activities.

Target-cell specificity of kainate autoreceptor and Ca2+-store-dependent short-term plasticity at hippocampal mossy fiber synapses

Presynaptic kainate receptors (KARs) modulate transmission between dentate granule cells and CA3 pyramidal neurons. Whether presynaptic KARs affect other synapses made by granule cell axons [mossy fibers (MFs)], on hilar mossy cells or interneurons, is not known. Nor is it known whether glutamate release from a single MF is sufficient to activate these receptors. Here, we monitor Ca(2+) in identified MF boutons traced from granule cell bodies. We show that a single action potential in a single MF activates both presynaptic KARs and Ca(2+) stores, contributing to use-dependent facilitation at MF-CA3 pyramidal cell synapses. Rapid local application of kainate to the giant MF bouton has no detectable effect on the resting Ca(2+) but facilitates action-potential-evoked Ca(2+) entry through a Ca(2+) store-dependent mechanism. Localized two-photon uncaging of the Ca(2+) store receptor ligand IP(3) directly confirms the presence of functional Ca(2+) stores at these boutons. In contrast, presynaptic Ca(2+) kinetics at MF synapses on interneurons or mossy cells are insensitive to KAR blockade, to local kainate application or to photolytic release of IP(3). Consistent with this, postsynaptic responses evoked by activation of a single MF show KAR-dependent paired-pulse facilitation in CA3 pyramidal cells, but not in interneurons or mossy cells. Thus, KAR-Ca(2+) store coupling acts as a synapse-specific, short-range autoreceptor mechanism.

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

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

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

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

Hippocampal dynorphin immunoreactivity increases in response to gonadal steroids and is positioned for direct modulation by ovarian steroid receptors

The hippocampal formation (HF) is involved in modulating learning related to drug abuse. While HF-dependent learning is regulated by both endogenous opioids and estrogen, the interaction between these two systems is not well understood. The mossy fiber (MF) pathway formed by dentate gyrus (DG) granule cell axons is involved in some aspects of learning and contains abundant amounts of the endogenous opioid peptide dynorphin (DYN). To examine the influence of ovarian steroids on DYN expression, we used quantitative light microscopic immunocytochemistry to measure DYN levels in normal cycling rats as well as in two established models of hormone-treated ovariectomized (OVX) rats. Rats in estrus had increased levels of DYN-immunoreactivity (ir) in the DG and certain CA3 lamina compared with rats in proestrus or diestrus. OVX rats exposed to estradiol for 24 h showed increased DYN-ir in the DG and CA3, while those with 72 h estradiol exposure showed increases only in the DG. Six hours of estradiol exposure produced no change in DYN-ir. OVX rats chronically implanted with medroxyprogesterone also showed increased DYN-ir in the DG and CA3. Next, dual-labeling electron microscopy (EM) was used to evaluate the subcellular relationships of estrogen receptor (ER) alpha-, ERbeta and progestin receptor (PR) with DYN-labeled MFs. ERbeta-ir was in some DYN-labeled MF terminals and smaller terminals, and had a subcellular association with the plasmalemma and small synaptic vesicles. In contrast, ERalpha-ir was not in DYN-labeled terminals, although some DYN-labeled small terminals synapsed on ERalpha-labeled dendritic spines. PR labeling was mostly in CA3 axons, some of which were continuous with DYN-labeled terminals. These studies indicate that ovarian hormones can modulate DYN in the MF pathway in a time-dependent manner, and suggest that hormonal effects on the DYN-containing MF pathway may be directly mediated by ERbeta and/or PR activation.

Place cells, grid cells, and the brain's spatial representation system

More than three decades of research have demonstrated a role for hippocampal place cells in representation of the spatial environment in the brain. New studies have shown that place cells are part of a broader circuit for dynamic representation of self-location. A key component of this network is the entorhinal grid cells, which, by virtue of their tessellating firing fields, may provide the elements of a path integration-based neural map. Here we review how place cells and grid cells may form the basis for quantitative spatiotemporal representation of places, routes, and associated experiences during behavior and in memory. Because these cell types have some of the most conspicuous behavioral correlates among neurons in nonsensory cortical systems, and because their spatial firing structure reflects computations internally in the system, studies of entorhinal-hippocampal representations may offer considerable insight into general principles of cortical network dynamics.

Altered morphology of hippocampal dentate granule cell presynaptic and postsynaptic terminals following conditional deletion of TrkB

Dentate granule cells play a critical role in the function of the entorhinal-hippocampal circuitry in health and disease. Dentate granule cells are situated to regulate the flow of information into the hippocampus, a structure required for normal learning and memory. Correspondingly, impaired granule cell function leads to memory deficits, and, interestingly, altered granule cell connectivity may contribute to the hyperexcitability of limbic epilepsy. It is important, therefore, to understand the molecular determinants of synaptic connectivity of these neurons. Brain-derived neurotrophic factor and its receptor TrkB are expressed at high levels in the dentate gyrus (DG) of the hippocampus, and are implicated in regulating neuronal development, neuronal plasticity, learning, and the development of epilepsy. Whether and how TrkB regulates granule cell structure, however, is incompletely understood. To begin to elucidate the role of TrkB in regulating granule cell morphology, here we examine conditional TrkB knockout mice crossed to mice expressing green fluorescent protein in subsets of dentate granule cells. In stratum lucidum, where granule cell mossy fiber axons project, the density of giant mossy fiber boutons was unchanged, suggesting similar output to CA3 pyramidal cell targets. However, filopodial extensions of giant boutons, which contact inhibitory interneurons, were increased in number in TrkB knockout mice relative to wildtype controls, predicting enhanced feedforward inhibition of CA3 pyramidal cells. In knockout animals, dentate granule cells possessed fewer primary dendrites and enlarged dendritic spines, indicative of disrupted excitatory synaptic input to the granule cells. Together, these findings demonstrate that TrkB is required for development and/or maintenance of normal synaptic connectivity of the granule cells, thereby implying an important role for TrkB in the function of the granule cells and hippocampal circuitry.

Ovarian steroids modulate leu-enkephalin levels and target leu-enkephalinergic profiles in the female hippocampal mossy fiber pathway

In the hippocampal formation (HF), the enkephalin opioids and estrogen are each known to modulate learning and cognitive performance relevant to drug abuse. Within the HF, leu-enkephalin (LENK) is most prominent in the mossy fiber (MF) pathway formed by the axons of dentate gyrus (DG) granule cells. To examine the influence of ovarian steroids on MF pathway LENK levels, we used quantitative light microscopic immunocytochemistry to evaluate LENK levels in normal cycling rats and in estrogen-treated ovariectomized rats. Rats in estrus had increased levels of LENK-immunoreactivity (ir) in the DG hilus compared to rats in diestrus or proestrus. Rats in estrus and proestrus had higher levels of LENK-ir in CA3a-c compared to rats in diestrus. Ovariectomized (OVX) rats 24 h (but not 6 or 72 h) after estradiol benzoate (EB; 10 microg) administration had increased LENK-ir in the DG hilus and CA3c. Electron microscopy showed a larger proportion of LENK-labeled small terminals and axons in the DG hilus compared to CA3 which may have contributed to region-specific changes in LENK-ir densities. Next we evaluated the subcellular relationships of estrogen receptor (ER) alpha, ERbeta and progestin receptor (PR) with LENK-labeled MF pathway profiles using dual-labeling electron microscopy. ERbeta-ir colocalized in some LENK-labeled MF terminals and smaller terminals while PR-ir was mostly in CA3 axons, some of which also showed colocalization with LENK. ERalpha-ir was in dendritic spines, but no colocalization with LENK-labeled profiles was observed. The present studies indicate that estrogen can modulate LENK in subregions of the MF pathway in a dose-and time-dependent manner. These effects might be triggered by direct activation of ERbeta or PR in LENK-containing terminals.

What is the mammalian dentate gyrus good for?

In the mammalian hippocampus, the dentate gyrus (DG) is characterized by sparse and powerful unidirectional projections to CA3 pyramidal cells, the so-called mossy fibers (MF). The MF form a distinct type of synapses, rich in zinc, that appear to duplicate, in terms of the information they convey, what CA3 cells already receive from entorhinal cortex layer II cells, which project both to the DG and to CA3. Computational models have hypothesized that the function of the MF is to enforce a new, well-separated pattern of activity onto CA3 cells, to represent a new memory, prevailing over the interference produced by the traces of older memories already stored on CA3 recurrent collateral connections. Although behavioral observations support the notion that the MF are crucial for decorrelating new memory representations from previous ones, a number of findings require that this view be reassessed and articulated more precisely in the spatial and temporal domains. First, neurophysiological recordings indicate that the very sparse dentate activity is concentrated on cells that display multiple but disorderly place fields, unlike both the single fields typical of CA3 and the multiple regular grid-aligned fields of medial entorhinal cortex. Second, neurogenesis is found to occur in the adult DG, leading to new cells that are functionally added to the existing circuitry, and may account for much of its ongoing activity. Third, a comparative analysis suggests that only mammals have evolved a DG, despite some of its features being present also in reptiles, whereas the avian hippocampus seems to have taken a different evolutionary path. Thus, we need to understand both how the mammalian dentate operates, in space and time, and whether evolution, in other vertebrate lineages, has offered alternative solutions to the same computational problems.

Coincidence detection of convergent perforant path and mossy fibre inputs by CA3 interneurons

We performed whole-cell recordings from CA3 s. radiatum (R) and s. lacunosum-moleculare (L-M) interneurons in hippocampal slices to examine the temporal aspects of summation of converging perforant path (PP) and mossy fibre (MF) inputs. PP EPSPs were evoked from the s. lacunosum-moleculare in area CA1. MF EPSPs were evoked from the medial extent of the suprapyramidal blade of the dentate gyrus. Summation was strongly supralinear when examining PP EPSP with MF EPSP in a heterosynaptic pair at the 10 ms ISI, and linear to sublinear at longer ISIs. This pattern of nonlinearities suggests that R and L-M interneurons act as coincidence detectors for input from PP and MF. Summation at all ISIs was linear in voltage clamp mode demonstrating that nonlinearities were generated by postsynaptic voltage-dependent conductances. Supralinearity was not detected when the first EPSP in the pair was replaced by a simulated EPSP injected into the soma, suggesting that the conductances underlying the EPSP boosting were located in distal dendrites. Supralinearity was selectively eliminated with either Ni2+ (30 microm), mibefradil (10 microm) or nimodipine (15 microm), but was unaffected by QX-314. This pharmacological profile indicates that supralinearity is due to recruitment of dendritic T-type Ca2+channels by the first subthreshold EPSP in the pair. Results with the hyperpolarization-activated (Ih) channel blocker ZD 7288 (50 microm) revealed that Ih restricted the time course of supralinearity for coincidently summed EPSPs, and promoted linear to sublinear summation for asynchronous EPSPs. We conclude that coincidence detection results from the counterbalanced activation of T-type Ca2+ channels and inactivation of Ih.

GABA Actions in Hippocampal Area CA3 During Postnatal Development: Differential Shift From Depolarizing to Hyperpolarizing in Somatic and Dendritic Compartments

γ-Aminobutyric acid type A receptor (GABAA-R) activation leads to depolarization of pyramidal cells during the first postnatal week and produces hyperpolarization from the second week. However, immunohistochemical evidence has suggested that during the second and third postnatal weeks the NKCC1 cotransporter relocates from the soma to the dendrites of CA3 pyramidal cells. We hypothesized that this leads to depolarizing responses in apical dendrites. Here we show that the activation of GABAA-R in the distal dendrites of CA3 pyramidal cells at P15 by restricted application of muscimol or synaptic activation by stimulation of interneurons in stratum radiatum (SR) causes depolarizing postsynaptic potentials (PSPs), which are blocked by NKCC1 cotransporter antagonists. By contrast, activation of proximal GABAA-R by muscimol application or by stimulation of interneurons in s. oriens (SO) leads to hyperpolarizing PSPs. Activation of the dentate gyrus (DG) in the presence of glutamatergic blockers evokes hyperpolarizing responses during the second postnatal week; however, the reversal potential of the DG-evoked inhibitory (I)PSPs is more depolarized than that of IPSPs evoked by activation of SO interneurons. Despite the shift of GABA action from depolarizing to hyperpolarizing, DG-evoked field potentials (f-PSPs) recorded in s. lucidum/radiatum (SL/R) do not change in polarity until the third week. Current source density analysis yielded results consistent with depolarizing actions of GABA in the dendritic compartment. Our data suggest that GABAergic input to apical dendrites of pyramidal cells of CA3 evokes depolarizing PSPs long after synaptic inhibition has become hyperpolarizing in the somata, in the axon initial segments and in basal dendrites.

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

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

Target cell-specific involvement of presynaptic mitochondria in post-tetanic potentiation at hippocampal mossy fiber synapses

Previous studies indicate that boutons from the same axon exhibit distinct Ca2+ dynamics depending on the postsynaptic targets. Mossy fibers of hippocampal granule cells innervate synaptic targets via morphologically distinct boutons. We investigated mitochondrial involvement in the generation of post-tetanic residual Ca2+ (Ca(res)) at large and small en passant mossy fiber boutons (MFBs). Mitochondria limited the [Ca2+]i build-up during high-frequency stimulation (HFS) at large MFBs, but not at small MFBs. The amount of Ca(res), quantified as a time integral of residual [Ca2+]i, was significantly larger at large MFBs than at small MFBs, and that at large MFBs was substantially attenuated by inhibitors of mitochondrial Ca2+ uniporter and mitochondrial Na+/Ca2+ exchanger (mitoNCX). In contrast, blockers of mitoNCX had no effect on the Ca(res) at small MFBs. Post-tetanic Ca(res) has been proposed as a mechanism for post-tetanic potentiation (PTP). We examined mitochondrial involvement in PTP at mossy fiber synapses on hilar mossy cells (MF-->MC synapse) and on hilar interneurons (MF-->HI synapse), which are presumably innervated via large and small MFBs, respectively. Consistent with the differential contribution of mitochondria to Ca(res) at large and small MFBs, mitoNCX blockers significantly reduced the PTP at the MF-->MC synapse, but not at the MF-->HI synapse. In contrast, protein kinase C (PKC) inhibitors significantly reduced the PTP at MF-->HI synapse, but not at the MF-->MC synapse. These results indicate that mitochondria- and PKC-dependent PTP are expressed at distinct hilar mossy fiber synapses depending on postsynaptic targets.

Differential mechanisms of transmission and plasticity at mossy fiber synapses

The last few decades have seen the hippocampal formation at front and center in the field of synaptic transmission. However, much of what we know about hippocampal short- and long-term plasticity has been obtained from research at one particular synapse; the Schaffer collateral input onto principal cells of the CA1 subfield. A number of recent studies, however, have demonstrated that there is much to be learned about target-specific mechanisms of synaptic transmission by study of the lesser known synapse made between the granule cells of the dentate gyrus; the so-called mossy fiber synapse, and its targets both within the hilar region and the CA3 hippocampus proper. Indeed investigation of this synapse has provided an embarrassment of riches concerning mechanisms of transmission associated with feedforward excitatory and inhibitory control of the CA3 hippocampus. Importantly, work from a number of labs has revealed that mossy fiber synapses possess unique properties at both the level of their anatomy and physiology, and serve as an outstanding example of a synapse designed for target-specific compartmentalization of synaptic transmission. The purpose of the present review is to highlight several aspects of this synapse as they pertain to a novel mechanism of bidirectional control of synaptic plasticity at mossy fiber synapses made onto hippocampal stratum lucidum interneurons. It is not my intention to pour over all that is known regarding the mossy fiber synapse since many have explored this topic exhaustively in the past and interested readers are directed to other fine reviews (Henze et al., 2000; Urban et al., 2001; Lawrence and McBain, 2003; Bischofberger et al., 2006; Nicoll and Schmitz, 2005).

Patch-clamp recording from mossy fiber terminals in hippocampal slices

Rigorous analysis of synaptic transmission in the central nervous system requires access to presynaptic terminals. However, cortical terminals have been largely inaccessible to presynaptic patch-clamp recording, due to their small size. Using improved patch-clamp techniques in brain slices, we recorded from mossy fiber terminals in the CA3 region of the hippocampus, which have a diameter of 2-5 microm. The major steps of improvement were the enhanced visibility provided by high-numerical aperture objectives and infrared illumination, the development of vibratomes with minimal vertical blade vibrations and the use of sucrose-based solutions for storage and cutting. Based on these improvements, we describe a protocol that allows us to routinely record from hippocampal mossy fiber boutons. Presynaptic recordings can be obtained in slices from both rats and mice. Presynaptic recordings can be also obtained in slices from transgenic mice in which terminals are labeled with enhanced green fluorescent protein.

Asymmetry in enhanced neurogenesis in the rostral dentate gyrus following kainic acid-induced status epilepticus in adult rats

Neurogenesis in the suprapyramidal and infrapyramidal blades of the rostral dentate gyrus was investigated following kainic acid (KA)-induced status epilepticus (SE) in adult rats. Rats were injected with KA (14 mg/kg, i.p.) or saline, with convulsions terminated by an intraperitoneal injection of diazepam. Five days after the induction of SE, the rats were injected with 5-bromo-2-deoxyuridine-5-monophosphate (BrdU; 75 mg/kg, i.p.), a marker of cell division. One day after the BrdU injection, the numbers of BrdU-labeled cells in the supra- and infrapyramidal blades were significantly higher in the KA-injected rats compared to the saline-injected rats. In the saline-injected rats, the number of BrdU-labeled cells in the infrapyramidal blade was greater than in the suprapyramidal blade. Twenty-eight days after the BrdU injection, the number of BrdU-labeled cells remained significantly higher in the KA-injected rats than the saline-injected rats, but only in the infrapyramidal blade. In addition, when the extent of cell death was examined with Fluoro-Jade B (a marker of dead and dying cells) 3 days after the induction of SE, degenerating cells were more numerous in the infrapyramidal blade than in the suprapyramidal blade. Our results suggest that there is an asymmetry of neurogenesis and cell death in the rostral dentate gyrus of rats following KA-induced SE.

Remodeling of hippocampal mossy fibers is selectively induced seven days after the acquisition of a spatial but not a cued reference memory task

Relating storage of specific information to a particular neuromorphological change is difficult because behavioral performance factors are not readily disambiguated from underlying cognitive processes. This issue is addressed here by demonstrating robust reorganization of the hippocampal mossy fiber terminal field (MFTF) when adult rats learn the location of a hidden platform but not when rats learn to locate a visible platform. Because the latter task requires essentially the same behavioral performance as the former, the observed MFTF growth is seen as the consequence of specific input-dependent hippocampal activity patterns selectively generated by processing of extramaze but not intramaze cues. Successful performance on the hidden platform task requires formation of spatial memory. Increased MFTFs in hidden platform-trained rats are observed 7 d but not 2 d after training nor in swim controls. These results suggest that structural plasticity of the mossy fiber:CA3 circuit may contribute to the maintenance of long-lasting memory but not to the initial storage of the spatial context.

Action-potential-independent GABAergic tone mediated by nicotinic stimulation of immature striatal miniature synaptic transmission

Stimulation of presynaptic nicotinic acetylcholine receptors (nAChRs) increases the frequency of miniature excitatory synaptic activity (mEPSCs) to a point where they can promote cell firing in hippocampal CA3 neurons. We have evaluated whether nicotine regulation of miniature synaptic activity can be extended to inhibitory transmission onto striatal medium spiny projection neurons (MSNs) in acute brain slices. Bath application of micromolar nicotine typically induced 12-fold increases in the frequency of miniature inhibitory synaptic currents (mIPSCs). Little effect was observed on the amplitude of mIPSCs or mEPSCs under these conditions. Nicotine stimulation of mIPSCs was dependent on entry of extracellular calcium because removal of calcium from perfusate was able to block its action. To assess the potential physiological significance of the nicotine-stimulated increase in mIPSC frequency, we also examined the nicotine effect on evoked IPSCs (eIPSCs). eIPSCs were markedly attenuated by nicotine. This effect could be attributed to two potential mechanisms: transmitter depletion due to extremely high mIPSC rates and/or a reduction in presynaptic excitability associated with nicotinic depolarization. Treatment with low concentrations of K(+) was able to in part mimic nicotine's stimulatory effect on mIPSCs and inhibitory effect on eIPSCs. Current-clamp recordings confirmed a direct depolarizing action of nicotine that could dampen eIPSC activity leading to a switch to striatal inhibitory synaptic transmission mediated by tonic mIPSCs.

Axon pruning in the developing vertebrate hippocampus

During early development of the central nervous system (CNS), there is an exuberant outgrowth of projections which later need to be refined to achieve precise connectivity. One widely used strategy for this refinement is axon pruning. Axon pruning has also been suggested to be involved in creating more diverse connection patterns between different species. An understanding of the mechanism of pruning, however, has been elusive in the CNS. Recent studies have focused on a stereotyped pruning event that occurs within the mossy fibers of the developing vertebrate hippocampus. In the following discussion, we will review the cellular and molecular factors that are known to regulate pruning in the hippocampus and highlight some advantages this system presents for future studies on pruning in the developing CNS.

Learning-induced axonal remodeling: Evolutionary divergence and conservation of two components of the mossy fiber system within Rodentia

Damage to the hippocampal formation results in profound impairments in spatial navigation in rats and mice leading to the widely accepted assumption that the hippocampal cellular and molecular memory mechanisms of both genera are conserved. Recently our group has shown in two rat strains that hippocampal-dependent training in the water maze specifically induces robust 'sprouting' of granule cell suprapyramidal mossy fiber axon terminal fields. Here we sought to investigate whether the pronounced remodeling of adult hippocampal circuitry observed in the rat is also present in the mouse motivated by the thought that subsequent studies using genetically-engineered mice could then be implemented to explore the molecular mechanisms underlying training-dependent axonal growth in adult rodents. However, in contrast to Wistar rats, no changes in the Timm's-stained area of mossy fiber terminal fields (MFTFs) were observed in C57BL/6J or 129Sv/EmsJ inbred wild-type mice after water maze training. Neither extending the duration of training nor scaling down the size of the apparatus was able to induce sprouting in mouse mossy fiber pathways. Though there may be similarities in the ultimate output of the hippocampus of rats and mice as inferred from lesion studies, the current results, as well as differences in learning and memory characteristics between the two genera, suggest that the way in which the component circuitry functions is likely to be different; a not too surprising conclusion given the substantial evolutionary distance between them (>20 million years). The present findings afford an opportunity for uncovering linkages between evolutionarily significant alterations in hippocampal circuitry in relation to genera-specific information storage requirements.

Ectopic granule cells of the rat dentate gyrus

Granule cells of the mammalian dentate gyrus normally form a discrete layer, and virtually all granule cells migrate to this location. Exceptional granule cells that are positioned incorrectly, in 'ectopic' locations, are rare. Although the characteristics of such ectopic granule cells appear similar in many respects to granule cells located in the granule cell layer, their rare occurrence has limited a full evaluation of their structure and function. More information about ectopic granule cells has been obtained by studying those that develop after experimental manipulations that increase their number. For example, after severe seizures, the number of ectopic granule cells located in the hilus increases dramatically. These experimentally-induced ectopic granule cells may not be equivalent to normal ectopic granule cells necessarily, but the vastly increased numbers have allowed much more information to be obtained. Remarkably, the granule cells that are positioned ectopically develop intrinsic properties and an axonal projection that are similar to granule cells that are located normally, i.e., in the granule cell layer. However, dendritic structure and synaptic structure/function appear to differ. These studies have provided new insight into a rare type of granule cell in the dentate gyrus, and the plastic characteristics of dentate granule cells that appear to depend on the location of the cell body.

The dentate gyrus: fundamental neuroanatomical organization (dentate gyrus for dummies)

The dentate gyrus is a simple cortical region that is an integral portion of the larger functional brain system called the hippocampal formation. In this review, the fundamental neuroanatomical organization of the dentate gyrus is described, including principal cell types and their connectivity, and a summary of the major extrinsic inputs of the dentate gyrus is provided. Together, this information provides essential information that can serve as an introduction to the dentate gyrus--a "dentate gyrus for dummies."

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

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

Morphological development and maturation of granule neuron dendrites in the rat dentate gyrus

The first granule neurons in the dentate gyrus are born during late embryogenesis in the rodent, and the primary period of granule cell neurogenesis continues into the second postnatal week. On the day of birth in the rat, the oldest granule neurons are visible in the suprapyramidal blade and exhibit rudimentary dendrites extending into the molecular layer. Here we describe the morphological development of the dendritic trees between birth and day 14, and we then review the process of dendritic remodeling that occurs after the end of the second week. Data indicate that the first adult-like granule neurons are present on day 7, and, furthermore, physiological recordings demonstrate that some granule neurons are functional at this time. Taken together, these results suggest that the dentate gyrus may be incorporated into the hippocampal circuit as early as the end of the first week. The dendritic trees of the granule neurons, however, continue to increase in size until day 14. After that time, the dendritic trees of the oldest granule neurons are sculpted and refined. Some dendrites elongate while others are lost, resulting in a conservation of total dendritic length. We end this chapter with a review of the quantitative aspects of granule cell dendrites in the adult rat and a discussion of the relationship between the morphology of a granule neuron and the location of its cell body within stratum granulosum and along the transverse axis of the dentate gyrus.

The dentate mossy fibers: structural organization, development and plasticity

Hippocampal mossy fibers are the axons of the dentate granule cells and project to hippocampal CA3 pyramidal cells and mossy cells of the dentate hilus (CA4) as well as a number of interneurons in the two areas. Besides their role in hippocampal function, studies of which are still evolving and taking interesting turns, the mossy fibers display a number of unique features with regard to axonal projections, terminal structures and synaptic contacts, development and variations among species and strains, as well as to normal occurring and lesion-induced plasticity and neural transplantation. These features are the topic of this review, which will use the mossy fiber system of the rat as basis and reference in its aim to provide an up-to-date, yet historically based guide to students in the field.

Plastic processes in the dentate gyrus: a computational perspective

The dentate gyrus has the capacity for numerous types of synaptic plasticity that use diverse mechanisms and are thought essential for the storage of information in the hippocampus. Here we review the various forms of synaptic plasticity that involve afferents and efferents of the dentate gyrus, and, from a computational perspective, relate how these plastic processes might contribute to sparse, orthogonal encoding, and the selective recall of information within the hippocampus.

Managing knowledge in neuroscience

Processing text from scientific literature has become a necessity due to the burgeoning amounts of information that are fast becoming available, stemming from advances in electronic information technology. We created a program, NeuroText ( http://senselab.med.yale.edu/textmine/neurotext.pl ), designed specifically to extract information relevant to neuroscience-specific databases, NeuronDB and CellPropDB ( http://senselab.med.yale.edu/senselab/ ), housed at the Yale University School of Medicine. NeuroText extracts relevant information from the Neuroscience literature in a two-step process: each step parses text at different levels of granularity. NeuroText uses an expert-mediated knowledge base and combines the techniques of indexing, contextual parsing, semantic and lexical parsing, and supervised and non-supervised learning to extract information. The constrains, metadata elements, and rules for information extraction are stored in the knowledge base. NeuroText was created as a pilot project to process 3 years of publications in Journal of Neuroscience and was subsequently tested for 40,000 PubMed abstracts. We also present here a template to create domain non-specific knowledge base that when linked to a text-processing tool like NeuroText can be used to extract knowledge in other fields of research.

Unmasking recurrent excitation generated by mossy fiber sprouting in the epileptic dentate gyrus: an emergent property of a complex system

Seizure-induced sprouting of the mossy fiber pathway in the dentate gyrus has been observed nearly universally in experimental models of limbic epilepsy and in the epileptic human hippocampus. The observation of progressive mossy fiber sprouting induced by kindling demonstrated that even a few repeated seizures are sufficient to alter synaptic connectivity and circuit organization. As it is now recognized that seizures induce synaptic reorganization in hippocampal and cortical pathways, the implications of seizure-induced synaptic reorganization for circuit properties and function have been subjects of intense interest. Detailed anatomical characterization of the sprouted mossy fiber pathway has revealed that the overwhelming majority of sprouted synapses in the inner molecular layer of the dentate gyrus form recurrent excitatory connections, and are thus likely to contribute to recurrent excitation and potentially to enhanced susceptibility to seizures. Nevertheless, difficulties in detecting functional abnormalities in circuits reorganized by mossy fiber sprouting and the fact that some sprouted axons appear to form synapses with inhibitory interneurons have been cited as evidence that sprouting may not contribute to seizure susceptibility, but could form recurrent inhibitory circuits and be a compensatory response to prevent seizures. Quantitative analysis of the synaptic connections of the sprouted mossy fiber pathway, assessment of the functional features of sprouted circuitry using reliable physiological measures, and the perspective of complex systems analysis of neural circuits strongly support the view that the functional effects of the recurrent excitatory circuits formed by mossy fiber sprouting after seizures or injury emerge only conditionally and intermittently, as observed with spontaneous seizures in human epilepsy. The recognition that mossy fiber sprouting is induced after hippocampal injury and seizures and contributes conditionally to emergence of recurrent excitation has provided a conceptual framework for understanding how injury and seizure-induced circuit reorganization may contribute to paroxysmal network synchronization, epileptogenesis, and the consequences of repeated seizures, and thus has had a major influence on understanding of fundamental aspects of the epilepsies.

Injury-induced alterations in CNS electrophysiology

Mild to moderate cases of traumatic brain injury (TBI) are very common, but are not always associated with the overt pathophysiogical changes seen following severe trauma. While neuronal death has been considered to be a major factor, the pervasive memory, cognitive and motor function deficits suffered by many mild TBI patients do not always correlate with cell loss. Therefore, we assert that functional impairment may result from alterations in surviving neurons. Current research has begun to explore CNS synaptic circuits after traumatic injury. Here we review significant findings made using in vivo and in vitro models of TBI that provide mechanistic insight into injury-induced alterations in synaptic electrophysiology. In the hippocampus, research now suggests that TBI regionally alters the delicate balance between excitatory and inhibitory neurotransmission in surviving neurons, disrupting the normal functioning of synaptic circuits. In another approach, a simplified model of neuronal stretch injury in vitro, has been used to directly explore how injury impacts the physiology and cell biology of neurons in the absence of alterations in blood flow, blood brain barrier integrity, or oxygenation associated with in vivo models of brain injury. This chapter discusses how these two models alter excitatory and inhibitory synaptic transmission at the receptor, cellular and circuit levels and how these alterations contribute to cognitive impairment and a reduction in seizure threshold associated with human concussive brain injury.

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

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