Mossy Cells Research Articles

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

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

Essential role of Rac1 and Rac3 GTPases in neuronal development.

Rac GTPases are members of the Rho family regulating the actin cytoskeleton and implicated in neuronal development. Ubiquitous Rac1 and neuron-specific Rac3 GTPases are coexpressed in the developing mammalian brain. We used Cre-mediated conditional deletion of Rac1 in neurons combined with knockout of neuron-specific Rac3 to study the role of these GTPases in neural development. We found that lack of both genes causes motor behavioral defects, epilepsy, and premature death of mice. Deletion of either GTPase does not produce evident phenotypes. Double-knockout mice show specific defects in the development of the hippocampus. Selective impairment of the dorsal hilus of double-knockout animals is associated with alteration in the formation of the hippocampal circuitry. Axonal pathways to and from the dorsal hilus are affected because of the deficit of hilar mossy cells. Moreover, analysis of Rac function in hippocampal cultures shows that spine formation is strongly hampered only in neurons lacking both Rac proteins. These findings show for the first time that both Rac1 and Rac3 are important for the development of the nervous system, wherein they play complementary roles during late stages of neuronal and brain development.

Mossy cells of the hippocampal dentate gyrus are not selectively vulnerable in human mesial temporal lobe epilepsy

Event Abstract Back to Event Mossy cells of the hippocampal dentate gyrus are not selectively vulnerable in human mesial temporal lobe epilepsy Hajnalka Ábrahám1*, Laszlo Seress1, Zsolt Horvath1, Tamas Doczi1, Jozsef Janszky2, Joyce Klemm3, Richard Byrne3 and Roy A. Bakay3 1 Department of Neurosurgery, University of Pécs, Hungary 2 Department of Neurology , University of Pécs, Hungary 3 Deparment of Neurosurgery, Rush Presbyterian-St. Luke’s Medical Center, United States Hippocampal sclerosis is a common pathology in temporal lobe epilepsy. It is generally believed that there are epilepsy-prone and epilepsy-resistant neurons in the hippocampal formation. Mossy cells of the hippocampal dentate gyrus are considered to be one of the most vulnerable neurons. Since selective histological marker for mossy cells was not available, it was previously difficult to assess whether mossy cells are among the remaining hilar neurons in the epileptic hippocampus. It has recently been described that the cocaine- and amphetamine-regulated transcript peptide (CART) is a reliable marker for human mossy cells. Therefore, we performed histological evaluation of resected epileptic hippocampi using CART immunocytochemistry to identify mossy cells. Cytoarchitecture and neuronal loss was determined with NeuN immunohistochemistry and Nissl staining. In most cases, the number of mossy cells was reduced and the thorny excrescences were smaller in epileptic hippocampi than in controls. Significant loss of pyramidal cells and partial loss of granule cells were accompanied by mossy cell loss. However, in many cases large numbers of mossy cells were still present in the dentate gyrus when most pyramidal neurons of Ammon’s horn were lost suggesting that mossy cells may not be more sensitive for epileptic seizures than the hippocampal pyramidal neurons. Thus, mossy cells are vulnerable to seizures, but a group of them appears to survive even decades of epilepsy. Supported by ETT grant 219/2006 (J. Janszky.) and OTKA grants F 68720 (J. Janszky), PD 73384 (H. Ábrahám) and by Bolyai Scholarship (H. Ábrahám). Conference: 12th Meeting of the Hungarian Neuroscience Society, Budapest, Hungary, 22 Jan - 24 Jan, 2009. Presentation Type: Poster Presentation Topic: Pathophysiology and neurology - degenerative disorders Citation: Ábrahám H, Seress L, Horvath Z, Doczi T, Janszky J, Klemm J, Byrne R and Bakay RA (2009). Mossy cells of the hippocampal dentate gyrus are not selectively vulnerable in human mesial temporal lobe epilepsy. Front. Syst. Neurosci. Conference Abstract: 12th Meeting of the Hungarian Neuroscience Society. doi: 10.3389/conf.neuro.01.2009.04.135 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 04 Mar 2009; Published Online: 04 Mar 2009. * Correspondence: Hajnalka Ábrahám, Department of Neurosurgery, University of Pécs, Pecs, Hungary, hajnalka.abraham@aok.pte.hu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Hajnalka Ábrahám Laszlo Seress Zsolt Horvath Tamas Doczi Jozsef Janszky Joyce Klemm Richard Byrne Roy A Bakay Google Hajnalka Ábrahám Laszlo Seress Zsolt Horvath Tamas Doczi Jozsef Janszky Joyce Klemm Richard Byrne Roy A Bakay Google Scholar Hajnalka Ábrahám Laszlo Seress Zsolt Horvath Tamas Doczi Jozsef Janszky Joyce Klemm Richard Byrne Roy A Bakay PubMed Hajnalka Ábrahám Laszlo Seress Zsolt Horvath Tamas Doczi Jozsef Janszky Joyce Klemm Richard Byrne Roy A Bakay Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Computational Influence of Adult Neurogenesis on Memory Encoding

Adult neurogenesis in the hippocampus leads to the incorporation of thousands of new granule cells into the dentate gyrus every month, but its function remains unclear. Here, we present computational evidence that indicates that adult neurogenesis may make three separate but related contributions to memory formation. First, immature neurons introduce a degree of similarity to memories learned at the same time, a process we refer to as pattern integration. Second, the extended maturation and change in excitability of these neurons make this added similarity a time-dependent effect, supporting the possibility that temporal information is included in new hippocampal memories. Finally, our model suggests that the experience-dependent addition of neurons results in a dentate gyrus network well suited for encoding new memories in familiar contexts while treating novel contexts differently. Taken together, these results indicate that new granule cells may affect hippocampal function in several unique and previously unpredicted ways.

Computational modeling of reduced excitability in the dentate gyrus of betaIV-spectrin mutant mice

Event Abstract Back to Event Computational modeling of reduced excitability in the dentate gyrus of betaIV-spectrin mutant mice Peter Jedlicka1*, Raphael Winkels1, Felix K. Weise1, Christian Schultz2, Thomas Deller1 and Stephan W. Schwarzacher1 1 Goethe University, NeuroScience Center, Clinical Neuroanatomy, Germany 2 Justus Liebig University, Institute of Anatomy and Cell Biology, Germany The submembrane cytoskeletal meshwork of the axon contains the scaffolding protein betaIV-spectrin. It provides mechanical support for the axon and anchors membrane proteins. Quivering (qv3j) mice lack functional betaIV-spectrin and have reduced voltage-gated sodium channel (VGSC) immunoreactivity at the axon initial segment and nodes of Ranvier. Because VGSCs are critically involved in action potential generation and conduction, we hypothesized that qv3j mice should also show functional deficits at the network level. To test this hypothesis, we investigated granule cell function in the dentate gyrus of anesthetized qv3j mice after electrical stimulation of the perforant path in vivo. This revealed an impaired input-output (IO) relationship between stimulus intensity and granule cell population spikes and an enhanced paired-pulse inhibition (PPI) of population spikes, indicating a reduced ability of granule cells to generate action potentials and decreased network excitability. In contrast, the IO curve for evoked field excitatory postsynaptic potentials (fEPSPs) and paired-pulse facilitation of fEPSPs were unchanged, suggesting normal excitatory synaptic transmission at perforant path-granule cell synapses in qv3j mutants.To better understand the influence of betaIVspectrin and VGSC density changes on the dentate gyrus network activity, we employed computational modeling approach. We used a recently developed and highly detailed computational model of the dentate gyrus network (Santhakumar et al., J Neurophysiol 93:437–453, 2005). The network model is based on realistic morphological and electrophysiological data and consists of perforant path inputs and connections of granule, mossy, basket and hilar cells. The role of VGSCs in network excitability was analyzed by systematically varying their densities in axosomatic compartments. This in silico approach confirmed that the loss of VGSCs is sufficient to explain the electrophysiological changes observed in qv3j mice. Computer simulations of the IO and PPI test indicated that in the dentate circuit with altered VGSCs, network excitability decreases owing to impaired spike-generator properties of granule cells and subsequent relative increase of GABAergic inhibitory control over granule cell firing.Taken together, our in vivo and in silico data demonstrate that the destabilization of VGSC clustering in qv3j mice leads to a reduced spike-generating ability of granule cells and considerably decreased network excitability in the dentate circuit. This provides the first evidence that betaIV-spectrin is required for normal granule cell firing and for physiological levels of network excitability in the mouse dentate gyrus in vivo. Conference: Bernstein Conference on Computational Neuroscience, Frankfurt am Main, Germany, 30 Sep - 2 Oct, 2009. Presentation Type: Poster Presentation Topic: Information processing in neurons and networks Citation: Jedlicka P, Winkels R, Weise FK, Schultz C, Deller T and Schwarzacher SW (2009). Computational modeling of reduced excitability in the dentate gyrus of betaIV-spectrin mutant mice. Front. Comput. Neurosci. Conference Abstract: Bernstein Conference on Computational Neuroscience. doi: 10.3389/conf.neuro.10.2009.14.054 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 26 Aug 2009; Published Online: 26 Aug 2009. * Correspondence: Peter Jedlicka, Goethe University, NeuroScience Center, Clinical Neuroanatomy, Frankfurt, Germany, jedlicka@em.uni-frankfurt.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Peter Jedlicka Raphael Winkels Felix K Weise Christian Schultz Thomas Deller Stephan W Schwarzacher Google Peter Jedlicka Raphael Winkels Felix K Weise Christian Schultz Thomas Deller Stephan W Schwarzacher Google Scholar Peter Jedlicka Raphael Winkels Felix K Weise Christian Schultz Thomas Deller Stephan W Schwarzacher PubMed Peter Jedlicka Raphael Winkels Felix K Weise Christian Schultz Thomas Deller Stephan W Schwarzacher Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Comparison of the ectopic sprouting of mossy fibers and the number of mossy cells in pilocarpine-treated rats

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

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

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.

Patterns of hippocampal neuronal loss and axon reorganization of the dentate gyrus in the mouse pilocarpine model of temporal lobe epilepsy

The patterns of hippocampal neuronal loss and rewiring of the dentate gyrus (DG) were studied in the mouse model of temporal lobe epilepsy at 2 months after pilocarpine-induced status epilepticus (PISE). NeuN immunocytochemistry showed two patterns of neuronal damage, i.e., type 1 with partial loss of pyramidal neurons in CA3 area and type 2 with almost compete loss of CA3 pyramidal neurons. Anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) demonstrated that, at different rostrocaudal segments of the hippocampus, associational and commissural connections in the DG changed differently between mice with type 1 vs. type 2 neuronal loss. Calretinin (CR)-immunopositive mossy cells in ventral hilus and its fibers in inner molecular layer of bilateral DG remained in mice with type 1 but almost disappeared in mice with type 2 neuronal loss, which was further supported by retrograde labeling with cholera toxin subunit B (CTB) showing colocalization of CTB with CR in the ventral hilus of bilateral DG in mice with type 1 neuronal loss, which was lost in those with type 2 neuronal loss. Furthermore, the sprouted PHA-L-immunopositive en passant and terminal boutons from the DG were found in CA1 area to contact with surviving calbindin-, CR-, and parvalbumin-immunopositive neurons. The present study therefore suggests that different patterns of neuronal loss in CA3 area may be linked to different axon reorganizations in the DG.

Nonrandom Local Circuits in the Dentate Gyrus

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

Postmortem changes in the neuroanatomical characteristics of the primate brain: Hippocampal formation

Comparative studies of the structural organization of the brain are fundamental to our understanding of human brain function. However, whereas brains of experimental animals are fixed by perfusion of a fixative through the vasculature, human or ape brains are fixed by immersion after varying postmortem intervals. Although differential treatments might affect the fundamental characteristics of the tissue, this question has not been evaluated empirically in primate brains. Monkey brains were either perfused or acquired after varying postmortem intervals before immersion-fixation in 4% paraformaldehyde. We found that the fixation method affected the neuroanatomical characteristics of the monkey hippocampal formation. Soma size was smaller in Nissl-stained, immersion-fixed tissue, although overall brain volume was larger as compared to perfusion-fixed tissue. Nonphosphorylated high-molecular-weight neurofilament immunoreactivity was lower in CA3 pyramidal neurons, dentate mossy cells, and the entorhinal cortex, whereas it was higher in the mossy fiber pathway in immersion-fixed tissue. Serotonin-immunoreactive fibers were well stained in perfused tissue but were undetectable in immersion-fixed tissue. Although regional immunoreactivity patterns for calcium-binding proteins were not affected, intracellular staining degraded with increasing postmortem intervals. Somatostatin-immunoreactive clusters of large axonal varicosities, previously reported only in humans, were observed in immersion-fixed monkey tissue. In addition, calretinin-immunoreactive multipolar neurons, previously observed only in rodents, were found in the rostral dentate gyrus in both perfused and immersion-fixed brains. In conclusion, comparative studies of the brain must evaluate the effects of fixation on the staining pattern of each marker in every structure of interest before drawing conclusions about species differences.

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

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

Predominant expression of phospholipase Cβ1 in telencephalic principal neurons and cerebellar interneurons, and its close association with related signaling molecules in somatodendritic neuronal elements

Upon activation of receptors coupled to the Gq subclass of G proteins, phospholipase C (PLC)beta hydrolyses membrane phospholipid to yield a pair of second messengers, inositol 1,4,5-trisphosphate and 1,2-diacylglycerol. Of four PLCbeta isoforms, PLCbeta1 is transcribed predominantly in the telencephalon and its gene inactivation in mice impairs metabotropic glutamate receptor- and muscarinic acetylcholine receptor-dependent hippocampal oscillations, endocannabinoid production in the hippocampus and barrel formation in the somatosensory cortex. Here we examined cellular and subcellular distributions of PLCbeta1 in adult mouse brains. In the telencephalon, high levels of PLCbeta1 were observed in principal neurons, including pyramidal cells in the cortex and hippocampus, granule cells and mossy cells in the dentate gyrus, and medium spiny neurons in the caudate-putamen, whereas most interneurons had low levels of or were negative for PLCbeta1 and, instead, expressed PLCbeta4. By immunofluorescence, tiny clusters of PLCbeta1 were distributed in somatodendritic compartments of principal neurons and positioned close to those of metabotropic glutamate receptor 5, muscarinic acetylcholine receptor M1 and diacylglycerol lipase-alpha, respectively. Immunoelectron microscopy revealed that PLCbeta1 was often associated with the smooth endoplasmic reticulum, cell membrane or postsynaptic density. In particular, it was highly accumulated at the perisynapse of dendritic spines forming asymmetrical synapses. In the cerebellum, PLCbeta1 was generally low but was enriched in axons and dendrites of basket cells. These results suggest that PLCbeta1 is the key effector in telencephalic principal neurons and cerebellar interneurons. Furthermore, the well-orchestrated molecular arrangement appears to be the anatomical basis for the specificity, efficiency and convergence of the neuronal phosphoinositide signaling system.

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.

Tactile Stimulation Evokes Long-Term Synaptic Plasticity in the Granular Layer of Cerebellum

Several forms of long-term synaptic plasticity [long-term potentiation (LTP) and long-term depression (LTD)] have been reported in the cerebellar circuit in vitro, but their determination in vivo was still lacking in most cases. Here we show that, in the urethane-anesthetized rat, appropriate patterns of facial tactile stimulation as well as intracerebellar electrical stimulation can induce LTP and LTD in local field potentials recorded from the granular layer of Crus-IIa. LTD prevailed in control conditions, whereas LTP prevailed during local application of gabazine. No relevant plasticity was observed when gabazine and APV were coapplied. The pharmacological and kinetic properties of LTP and LTD in vivo were compatible with those reported in the granule cell layer in vitro (Mapelli and D'Angelo, 2007), suggesting that NMDA receptor-dependent plasticity was generated at the mossy fiber-granule cell synapse under the inhibitory control of the Golgi cell circuit. Interestingly, LTP and LTD were able to regulate the response latency to tactile stimulation, as expected from computational modeling of the expression mechanisms (Nieus et al., 2006). This result suggests that LTP and LTD could regulate the spatiotemporal pattern of granular layer responses to mossy fiber inputs.

Calretinin expression in hilar mossy cells of the hippocampal dentate gyrus of nonhuman primates and humans

Mossy cells, the major excitatory neurons of the hilus of the dentate gyrus constitutively express calretinin in several rodent species, including mouse and hamster, but not in rats. Several studies suggest that mossy cells of the monkey dentate gyrus are calretinin-positive, but others have reported mossy cells in monkeys to be devoid of detectable calretinin-like immunoreactivity. In the present study, the hilar region was investigated throughout the entire longitudinal extent of the hippocampal dentate gyrus in both Old World and New World monkeys, as well as in humans. In the examined four monkey species, mossy cells were found to be calretinin-positive at the uncal pole and at variable length within the main body of the dentate gyrus but not in the tail part. The associational pathway, formed by axons of mossy cells in the inner dentate molecular layer was calretinin-positive in more caudal sections, suggesting that mossy cell axon terminals may contain calretinin, whereas mossy cell somata may contain calretinin in a concentration too low to be detected by immunocytochemistry. In contrast, human mossy cells appear to be devoid of calretinin immunoreactivity in both their somata and their axon terminals. Taken together, mossy cells of nonhuman primates and humans exhibit different expression pattern for calretinin whereas they show similarities in neurochemical content, such as the cocaine and amphetamine-related transcript peptide.

Adenosine A2A Receptors Are Essential for Long-Term Potentiation of NMDA-EPSCs at Hippocampal Mossy Fiber Synapses

The physiological conditions under which adenosine A2A receptors modulate synaptic transmission are presently unclear. We show that A2A receptors are localized postsynaptically at synapses between mossy fibers and CA3 pyramidal cells and are essential for a form of long-term potentiation (LTP) of NMDA-EPSCs induced by short bursts of mossy fiber stimulation. This LTP spares AMPA-EPSCs and is likely induced and expressed postsynaptically. It depends on a postsynaptic Ca2+ rise, on G protein activation, and on Src kinase. In addition to A2A receptors, LTP of NMDA-EPSCs requires the activation of NMDA and mGluR5 receptors as potential sources of Ca2+ increase. LTP of NMDA-EPSCs displays a lower threshold for induction as compared with the conventional presynaptic mossy fiber LTP; however, the two forms of LTP can combine with stronger induction protocols. Thus, postsynaptic A2A receptors may potentially affect information processing in CA3 neuronal networks and memory performance.

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.

Nonprincipal neurons and CA2 pyramidal cells, but not mossy cells are immunoreactive for calcitonin gene-related peptide in the mouse hippocampus

The distribution and morphological features of calcitonin gene-related peptide (CGRP) positive neurons in the mouse hippocampus were immunohistochemically analyzed, focusing on their differences between mice and rats. In contrast with those in the rat dentate gyrus, the mossy cell somata and their axon terminals in the mouse dentate gyrus were CGRP negative even after intraventricular colchicine injection. In the rat CA1–CA2–CA3 regions, there were two types of CGRP positive neurons, some of the CA3 pyramidal cells and relatively few nonprincipal neurons. In the mouse CA1–CA2–CA3 regions, there were also two types of CGRP positive neurons. The majority were scattered throughout layers and abundant in number when compared with those in the rat hippocampus. They were regarded as nonprincipal neurons by their distribution, structural features and glutamic acid decarboxylase 67 (GAD67) immunoreactivity. The minor group was clustered in the stratum pyramidale of the CA2 region. They extended thick apical dendritic shafts into the stratum radiatum, were GAD67 negative, and thus were regarded as the CA2 pyramidal cells. The CGRP positive nonprincipal neurons were apparently heterogeneous and further characterized immunohistochemically. Although there were significant regional differences in the chemical properties of the CGRP positive nonprincipal neurons, in the whole hippocampus, over 40% of CGRP positive nonprincipal neurons were also positive for parvalbumin, about 15% were positive for somatostatin and about 20% were positive for cholecystokinin, respectively. The present study clearly showed that there were prominent species differences between the mouse and rat hippocampus in the CGRP immunoreactivities.