Transient effect of mossy fiber stimulation on spatial firing of CA3 neurons in familiar and novel environments

Hippocampal mossy fibers have long been proposed to impose new patterns to learn onto CA3 neurons during new memory formation. However, inconsistent with this theory, we found in our previous study that mossy fiber stimulation induces only transient changes in CA3 spatial firing in a familiar environment. Here, we tested whether mossy fiber stimulation affects CA3 spatial firing differently between familiar and novel environments. We compared spatial firing of CA3 neurons before and after optogenetic stimulation of mossy fibers in freely behaving mice in a familiar and three sets of novel environments. We found that CA3 neurons are more responsive to mossy fiber stimulation in the novel than familiar environments. However, we failed to obtain evidence for long‐lasting effect of mossy fiber stimulation on spatial firing of CA3 neurons in both the familiar and novel environments. Our results provide further evidence against the view that mossy fibers carry teaching signals.     

Effects of vigabatrin on epileptiform abnormal discharges in hippocampal CA3 neurons of spontaneously epileptic rats (SER)

Vigabatrin, a γ-amino butyric acid (GABA) transaminase inhibitor, is known to inhibit partial epilepsy in humans. The spontaneously epileptic rat (SER), a double mutant (zi/zi, tm/tm), exhibits both tonic convulsion and absence-like seizures from the age of 8 weeks. Hippocampal CA3 pyramidal neurons in SER show a long-lasting depolarization shift with accompanying repetitive firing when a single stimulus is delivered to the mossy fibers in slice preparations. The effects of vigabatrin on the abnormal excitability of hippocampal CA3 pyramidal neurons in SER were examined to elucidate the mechanism underlying the antiepileptic action of the drug. Intracellular recordings were performed in 24 hippocampal slice preparations of 20 SER aged 8–17 weeks old. Bath application of vigabatrin (1 mM) inhibited the depolarizing shifts with repetitive firing induced by mossy fiber stimulation in 15 min without affecting the first spike and resting membrane potentials in hippocampal CA3 neurons of SER. A higher dose of vigabatrin (10 mM) sometimes inhibited the first spike. However, vigabatrin at doses up to 10 mM did not significantly affect the single action potential elicited by stimulation of the mossy fibers in the hippocampal CA3 neurons of age-matched Wistar rats. In addition, application of vigabatrin (10 mM) did not significantly affect the firing induced by depolarizing pulse applied in the CA3 neurons of the SER, nor the miniature excitatory postsynaptic potential (mEPSP) recorded in the CA3 neurons of SER. The inhibitory effect of vigabatrin (1 mM) on the mossy fiber stimulation-induced depolarization shift with repetitive firing was blocked by concomitant application of bicuculline (10 μM), a GABA_A receptor antagonist. These findings strongly suggested that GABA increased by inhibition of GABA transaminase with vigabatrin inhibits abnormal excitation of hippocampal CA3 neurons of SER via GABA_A receptors, although the possibility that the drug acted directly on the GABA_A receptors of CA3 neurons could not be completely excluded.     

Modulation of abnormal synaptic transmission in hippocampal CA3 neurons of spontaneously epileptic rats (SERs) by levetiracetam

Levetiracetam (LEV) inhibits partial refractory epilepsy in human, and both convulsive and absence-like seizures in the spontaneously epileptic rat (SER). Two-thirds of hippocampal CA3 neurons in SER show a long-lasting depolarization shift, with accompanying repetitive firing upon mossy fiber stimulation. This abnormal excitability is probably attributable to abnormalities in the L-type Ca(2+) channels. We performed electrophysiological studies to elucidate the mechanism underlying the antiepileptic effects of LEV via intracellular recording from the hippocampal CA3 neurons in slice preparations of SER and non-epileptic Wistar rats. LEV (100 μM) inhibited the depolarization shift with repetitive firing by mossy fiber stimulation (MFS), without affecting the first spike in SER CA3 neurons. At a higher dose (1mM), LEV suppressed the first spike in all SER neurons (including the CA3 neurons which showed only a single action potential by MFS), while the single action potential of Wistar rat CA3 neurons remained unaffected. SER CA3 neurons with MFS-induced abnormal firing exhibited a higher number of repetitive spikes when a depolarization pulse was applied in the SER CA3 neurons. LEV (100 μM, 1mM) reduced the repetitive firing induced by a depolarization pulse applied without affecting Ca(2+) spike in SER neurons. LEV is known not to bind glutamate and gamma-aminobutyric acid (GABA) receptors. These findings suggest that the therapeutic concentration of LEV inhibits abnormal firing of the CA3 neurons by modulating abnormal synaptic transmission and abnormal Na(+) channels in SER.     

Abnormal Excitability of Hippocampal CA3 Pyramidal Neurons of Spontaneously Epileptic Rats (SER), a Double Mutant

The spontaneously epileptic rat (SER:zi/zi, tm/tm), a double mutant, shows both tonic convulsions and absence-like seizures characterized by low-voltage fast waves and by 5-7 Hz spike and wave-like complexes in the cerebral cortical and hippocampal EEG, respectively. Characteristics of hippocampal CA3 pyramidal neurons were examined to determine whether these neurons are abnormally excitable. When a single stimulus was given to the mossy fiber, there was repetitive firing and a depolarization shift in neurons of mature SER (over 12 weeks old), in which epileptic seizures had fully developed. However, in young SER (7-8 weeks old) and littermates (zi/zi, tm/+), which did not show any seizures, only a single spike was elicited with each single stimulation of the mossy fiber. Intracellular recording showed that the resting membrane potential was not significantly different among young and mature SER and littermates, but a long-lasting (100-200 ms) depolarizing shift accompanied by repetitive firing was observed following a single stimulation of the mossy fiber in half of the CA3 neurons of mature SER. Furthermore, the input impedance of the CA3 neurons in mature SER was lower than that in young SER and in littermates. These results indicate that SER hippocampal CA3 neurons become abnormally excitable in conjunction with the development of epileptic seizures.     

Stimulation-induced status epilepticus: role of the hippocampal mossy fibers in the seizures and associated neuropathology

A study of seizure activity and neuronal cell death produced by intracerebroventricular kainic acid had suggested that seizures conveyed by the hippocampal mossy fibers are more damaging to CA3 pyramidal cells than seizures conveyed by other pathways. To test this idea, the effects of a unilateral mossy fiber lesion were determined on seizure activity and neuronal degeneration provoked by repetitive electrical stimulation of the hippocampal fimbria in unanesthetized rats. Fimbrial stimulation resulted in self-sustained status epilepticus accompanied by neuronal degeneration in several brain regions, including area CA3 of the hippocampal formation. A unilateral mossy fiber lesion more readily attenuated the electrographic and behavioral seizures provoked by fimbrial stimulation than those provoked by kainic acid. If status epilepticus developed in the presence of a mossy fiber lesion, denervated CA3 pyramidal cells were still destroyed, although similar lesions protect these neurons from kainic acid-induced status epilepticus. Thus the two models of status epilepticus employ somewhat different seizure circuitries and neurodegenerative mechanisms. Seizures which involve the mossy fiber projection are not necessarily more damaging to CA3 pyramidal cells than seizures which do not.     

Fetal Hippocampal Cells Grafted to Kainate-Lesioned CA3 Region of Adult Hippocampus Suppress Aberrant Supragranular Sprouting of Host Mossy Fibers

Selective lesion of the rat hippocampus using an intracerebroventricular administration of kainic acid (KA) represents an animal model for studying both lesion recovery and temporal lobe epilepsy. This KA lesion leads initially to loss of CA3 hippocampal neurons, the postsynaptic target of mossy fibers, and later results in aberrant mossy fiber sprouting into the dentate supragranular layer (DSGL). Because of the close association of this aberrant mossy fiber sprouting with an increase in the seizure susceptibility of the dentate gyrus, delayed therapeutic strategies capable of suppressing the sprouting of mossy fibers into the DSGL are of significant importance. We hypothesize that neural grafting can restore the disrupted hippocampal mossy fiber circuitry in this model through the establishment of appropriate mossy fiber projections onto grafted pyramidal neurons and that these appropriate projections will lead to reduced inappropriate sprouting into the DSGL. Large grafts of Embryonic Day 19 hippocampal cells were transplanted into adult hippocampus at 4 days post-KA lesion. Aberrant mossy fiber sprouting was quantified after 3–4 months survival using three different measures of Timm's staining density. Grafts located near the degenerated CA3 cell layer showed dense ingrowth of host mossy fibers compared to grafts elsewhere in the hippocampus. Aberrant mossy fiber sprouting throughout the dentate gyrus was dramatically and specifically reduced in animals with grafts near the degenerated CA3 cell layer compared to “lesion only” animals and those with ectopic grafts away from the CA3 region. These results reveal the capability of appropriately placed fetal hippocampal grafts to restore disrupted hippocampal mossy fiber circuitry by attracting sufficient host mossy fibers to suppress the development of aberrant circuitry in hippocampus. Thus, providing an appropriate postsynaptic target at early postlesion periods significantly facilitates lesion recovery. The graft-induced long-term suppression of aberrant sprouting shown here may provide a new avenue for amelioration of hyperexcitability that occurs following hippocampal lesions.     

Suppression by topiramate of epileptiform burst discharges in hippocampal CA3 neurons of spontaneously epileptic rat in vitro

Topiramate, a novel antiepileptic drug, inhibits the seizures of spontaneously epileptic rat (SER), a double mutant (zi/zi, tm/tm) which exhibits both tonic convulsion and absence-like seizures from the age of 8-weeks. Hippocampal CA3 pyramidal neurons in SER show a long-lasting depolarization shift with accompanying repetitive firing when a single electrostimulation is delivered to the mossy fibers in vitro. The effects of topiramate on the excitability of CA3 pyramidal neurons in SER were examined to elucidate the mechanism underlying the antiepileptic action. Intracellular recordings were performed in 23 hippocampal slice preparations of 16 SER aged 8–17 weeks. Topiramate (10–100 μM) dose-dependently inhibited the depolarizing shifts with repetitive firing induced by mossy fiber stimulation without affecting the first spike and resting membrane potentials in hippocampal CA3 neurons of SER. Higher dose of topiramate (100 μM) sometimes inhibited the first spike, and decreased excitatory postsynaptic potentials in the SER CA3 neurons. However, topiramate up to 100 μM did not affect the single action potential elicited by the stimulation in the hippocampal CA3 neurons of age-matched Wistar rat devoid of the seizure. Application of topiramate (100 μM) did not significantly affect the firing induced by depolarizing pulse applied in the CA3 neurons of the SER. In addition, topiramate (100 μM) had no effects on the Ca²⁺ spike induced by intracellularly applied depolarizing pulse in the presence of tetrodotoxin and tetraethylammonium. In contrast, a dose-dependent inhibition of depolarization and repetitive firing induced by bath application of glutamate in CA3 pyramidal neurons was obtained with topiramate (10–100 μM). Furthermore, topiramate (100 μM) decreased the number of miniature postsynaptic potential of CA3 pyramidal neurons of SER. In patch clamp whole cell recording using acutely dissociated hippocampal CA3 neurons from SER aged 8-weeks and age-matched normal Wistar rats, there were no remarkable effects on voltage dependent Ca²⁺ current with topiramate up to 300 μM in either animal; the current was completely blocked by Cd²⁺ at a concentration of 1 mM. These findings suggest that topiramate inhibits release of glutamate from the nerve terminals and/or abnormal firing of the CA3 pyramidal neurons of SER by mainly blocking glutamate receptors in the neurons.     

Mossy fiber sprouting and pyramidal cell dispersion in the hippocampal CA2 region in a mouse model of temporal lobe epilepsy

Dentate granule cells and the hippocampal CA2 region are resistant to cell loss associated with mesial temporal lobe epilepsy (MTLE). It is known that granule cells undergo mossy fiber sprouting in the dentate gyrus which contributes to a recurrent, proepileptogenic circuitry in the hippocampus. Here it is shown that mossy fiber sprouting also targets CA2 pyramidal cell somata and that the CA2 region undergoes prominent structural reorganization under epileptic conditions. Using the intrahippocampal kainate mouse model for MTLE and the CA2‐specific markers Purkinje cell protein 4 (PCP4) and regulator of G‐Protein signaling 14 (RGS14), it was found that during epileptogenesis CA2 neurons survive and disperse in direction of CA3 and CA1 resulting in a significantly elongated CA2 region. Using transgenic mice that express enhanced green fluorescent protein (eGFP) in granule cells and mossy fibers, we show that the recently described mossy fiber projection to CA2 undergoes sprouting resulting in aberrant large, synaptoporin‐expressing mossy fiber boutons which surround the CA2 pyramidal cell somata. This opens up the potential for altered synaptic transmission that might contribute to epileptic activity in CA2. Indeed, intrahippocampal recordings in freely moving mice revealed that epileptic activity occurs concomitantly in the dentate gyrus and in CA2. Altogether, the results call attention to CA2 as a region affected by MTLE‐associated pathological restructuring. © 2015 Wiley Periodicals, Inc.     

Connections of the hippocampal formation in humans: I. The mossy fiber pathway

The hippocampal formation has been one of the most extensively studied cortical regions in rats, yet little is known about the anatomical connections of the hippocampus in primates, especially humans. With the use of an antibody against the calcium-binding protein, calbindin-D28K, in normal autopsy tissue and the neuronal tracers biocytin or biotinylated dextrans in in vitro slice preparations from tissue removed during surgery for intractable epilepsy, we examined the human hippocampal mossy fiber pathway. The injections of biocytin into the dentate granule cell layer labeled neurons in a Golgi-like manner, revealing the presence of basal dendrites on about 30% of the granule cells. The granule cell axons, the mossy fibers, initially formed a diffuse plexus of fibers in the polymorphic layer before organizing into fiber fascicles in the hilar pyramidal region. These fiber fascicles were much more prominent rostrally than caudally. Within the hilus and proximal portions of the extrahilar CA3 field, the mossy fibers ran through the pyramidal cell layer, and while near the transition to field CA2, the fibers turned superficially and crossed the pyramidal layer to run in the stratum lucidum. All of these features, seen following injections of tracer into hippocampal slices from the brains of epileptics, were confirmed by calbindin-staining of mossy fibers in normal brains. Biocytin-labeled mossy fiber axons revealed two characteristic types of enlargements: small varicosities and larger expansions. The expansions were found throughout the neuropil and were highly irregular, diaminobenzidine-dense profiles that had pleiomorphic modes of attachment to the parent axon. Electron microscopic images of these biocytin labeled expansions revealed that they were large synaptic boutons bearing asymmetric synapses. This study indicates that the human mossy fiber pathway shows some minor deviations from the rodent brain but little difference from monkeys. We argue that these changes mirror a phylogenetic growth of the CA3 pyramidal neurons (subfield CA3c) into the hilus rather than an evolutionary change of the mossy fiber pathway. This growth of subfield CA3c and the increase in mossy fibers running through the pyramidal layer (and a presumed accompanying increase in proximal basal dendritic contacts) may reflect a growing role of the projection from the dentate granule cells to subfield CA3c and from there to field CA1 in the primate hippocampus. J. Comp. Neurol. 385:325–351, 1997. © 1997 Wiley-Liss, Inc.     

Inhibition of presynaptic activity by zinc released from mossy fiber terminals during tetanic stimulation

Zinc exists in high densities in the giant boutons of hippocampal mossy fibers. On the basis of the evidence that zinc decreases extracellular glutamate concentration in the hippocampus, the presynaptic action of zinc released from mossy fibers during high-frequency (tetanic) stimulation was examined using hippocampal slices. The increase in zinc-specific fluorescent signals was observed in both extracellular and intracellular compartments in the mossy fiber terminals during the delivery of tetanic stimuli (100 Hz, 1 sec) to the dentate granule cell layer, suggesting that zinc released from mossy fibers is immediately retaken up by mossy fibers. When mossy fiber terminals were preferentially double-stained with zinc and calcium indicators and tetanic stimuli (100 Hz, 1 sec) were delivered to the dentate granule cell layer, the increase in calcium orange signal during the stimulation was enhanced in mossy fiber terminals by addition of CaEDTA, a membrane-impermeable zinc chelator, and was suppressed by addition of zinc. The decrease in FM4-64 signal (vesicular exocytosis) during tetanic stimulation (10 Hz, 180 sec), which induced mossy fiber long-term potentiation, was also enhanced in mossy fiber terminals by addition of CaEDTA and was suppressed by addition of zinc. The present study demonstrates that zinc released from mossy fibers may be a negative-feedback factor against presynaptic activity during tetanic stimulation.     

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

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

Roles of afadin in the formation of the cellular architecture of the mouse hippocampus and dentate gyrus

The hippocampal formation with tightly packed neurons, mainly at the dentate gyrus, CA3, CA2, and CA1 regions, constitutes a one-way neural circuit, which is associated with learning and memory. We previously showed that the cell adhesion molecules nectins and its binding protein afadin play roles in the formation of the mossy fiber synapses which are formed between the mossy fibers of the dentate gyrus granule cells and the dendrites of the CA3 pyramidal cells. We showed here that in the afadin-deficient hippocampal formation, the dentate gyrus granules cells and the CA3, CA2, and CA1 pyramidal cells were abnormally located; the mossy fiber trajectory was abnormally elongated; the CA3 pyramidal cells were abnormally differentiated; and the densities of the presynaptic boutons on the mossy fibers and the apical dendrites of the CA3 pyramidal cells were decreased. These results indicate that afadin plays roles not only in the formation of the mossy fiber synapses but also in the formation of the cellular architecture of the hippocampus and the dentate gyrus.     

Adverse effects of maternal ethanol consumption on development of dorsal hippocampus in rat offspring

We examined the laminar structure and distribution of mossy fiber terminal fields in the dorsal hippocampus, an important area for spatial learning, in rats exposed to ethanol during gestational days 10-21. Pyramidal cells in the CA3a subfield were loosely packed compared to control rats. Aberrant infra- and intrapyramidal mossy fibers were found in the CA3 region, especially in the CA3a subfield, throughout the dorsal hippocampus of ethanol-exposed rats. Aberrant mossy fiber terminals were observed more frequently in the rostral than the caudal level of the dorsal hippocampus. At the most caudal level of the dorsal hippocampus, disarrangement of pyramidal cells was seen in the CA3c subfield along with disturbed mossy fiber terminals. Immunohistochemical studies revealed that neural cell adhesion molecule (NCAM) was not related to aberrant distribution of mossy fiber terminals after prenatal exposure to ethanol. Parvalbumin immunoreactivity was increased in the dorsal hippocampus of ethanol-exposed rats compared with control rats. Abnormal development of the dorsal hippocampus induced by prenatal ethanol exposure may be associated with the defect of spatial memory seen in fetal alcohol syndrome children and their animal models.     

Extensive target cell loss during development results in mossy fibers in the regio superior (CA1) of the rat hippocampal formation

The axons of dentate granule cells (mossy fibers) have been reported to appear in the regio superior (CA1) of the rat hippocampal formation following destruction of the pyramidal cells in the regio inferior (CA3). We undertook the present experiments to confirm this finding and to determine the requirements for this dramatic neuronal rearrangement. We found that extensive (greater than 80%) loss of CA3 cells, as well as the presence of surviving CA1 neurons within a narrow period of development (postnatal days 3-5) is necessary, however apparently not sufficient, for the appearance of CA1 mossy fibers. That the absence of normal target cells during a restricted period of mossy fiber development will lead to their association with novel targets suggests that much of the specificity of this developing connection depends on the presence of normal targets during a critical period.     

Evidence of functional mossy fiber sprouting in hippocampal formation of kainic acid-treated rats

In the rat hippocampal formation, degeneration of CA4-derived afferent fibers provokes the growth of mossy fiber collaterals into the fascia dentata. These aberrant fibers subsequently form granule cell-granule cell synapses. The hippocampal slice preparation was employed to determine whether these recurrent connections are electrophysiologically functional. Hippocampal slices were prepared 12 to 21 days after the bilateral destruction of CA4 neurons with either intracerebroventricular or intravenous kainic acid (KA). In slices from control rats, antidromic stimulation of the mossy fibers elicited a single population spike in the granular layer of the fascia dentata. In contrast, when slices from some KA-treated rats were similarly tested, antidromic stimulation elicited multiple population spikes. This effect was not reproduced by blocking inhibitory transmission with bicuculline methiodide. Slices from other KA-treated rats fired a single population spike, but an antidromic conditioning volley increased the amplitude of a subsequent antidromic population spike by 5 to 15%. In slices from control rats, on the other hand, an antidromic conditioning volley always either decreased or failed to alter the amplitude of an antidromic test response. Superfusion with Ca2+-free medium containing 3.8 mM Mg2+ reversibly abolished all effects of KA administration. Abnormal responses to antidromic stimulation correlated with the loss of CA4 neurons and the growth of supragranular mossy fiber collaterals in the same animals. These results suggest that supragranular mossy fiber collateral sprouts form a functional recurrent excitatory circuit. These aberrant connections may further compromise hippocampal function already disrupted by neuronal degeneration, such as by facilitating seizure activity.     

Postnatal development of CA3 pyramidal neurons and their afferents in the Ammon's horn of rhesus monkeys

Previous studies described the postnatal development of CA3 pyramidal neurons and their afferents in the rat. However, the postnatal development of the primate hippocampus was not previously studied. Thus, pyramidal neurons of the CA3 area of the monkey hippocampus were analyzed postnatally in the present study. At birth, a few thorny excrescences, the complex spines postsynaptic to mossy fibers, were found on the proximal segments of both apical and basal dendrites, whereas distal dendrites displayed pedunculate spines. Thorny excrescences increased in number until the third month. A continuous increase in the number of spines per unit length along the distal dendrites was observed during the first 12 months. The ultrastructural features of somata and dendrites of pyramidal cells in newborn monkeys were similar to those of adults. The analysis of the afferents to the CA3 pyramidal neurons was limited to the development of mossy fibers, the axons of granule cells, and myelinated axons in the alveus, stratum oriens, and stratum lacunosum-moleculare. At birth, most mossy fiber terminals were densely packed with synaptic vesicles and formed mainly axospinous synapses with CA3 pyramidal cells. By 1 month of age, the number of mitochondria and embedded spines increased to mature amounts. In the first postnatal month, degenerating axons and axon terminals were frequently observed in the mossy fiber bundles in stratum lucidum. The proportion of myelinated axons increased simultaneously in all three examined layers. At birth most axons were unmyelinated, whereas at 7 months of age the proportion of myelinated axons was similar to that found in adults. The present study indicates that most pyramidal neurons of the CA3 region in monkeys are in an advanced stage of development at the time of birth. Thus, mossy fibers from granule cells in the dentate gyrus have established mature-looking synapses, and the thorny excrescences of pyramidal cells that are postsynaptic to mossy fibers are also adult-like. Nevertheless, several of the adult features, such as the spine density of distal dendrites of pyramidal neurons and the myelination of afferent axons, develop during an extended period of time in the first year. The significance of this early anatomical maturation in a brain region involved in memory function is consistent with recent behavioral data that show a rapid postnatal maturation of limbic-dependent recognition memory in rhesus monkeys. © 1995 Wiley-Liss, Inc.     

Hippocampal mossy fiber distribution and long-term potentiation in two inbred mouse strains

We studied long-term potentiation in the inbred mouse strains DBA/2 and C3H/He known to be different in both hippocampal mossy fiber distribution and several aspects of learning. Tetanic stimulation of mossy fibers resulted in a significantly stronger increase of the population spike amplitude in the CA3 pyramidal cell layer of C3H mice. This result suggests that the extent of the CA3 hippocampal mossy fiber projection correlates with synaptic efficacy in mice.     

Combined administration of levetiracetam and valproic acid attenuates age‐related hyperactivity of CA3 place cells,reduces place field area,and increases spatial information content in aged rat hippocampus

Learning and memory deficits associated with age‐related mild cognitive impairment have long been attributed to impaired processing within the hippocampus. Hyperactivity within the hippocampal CA3 region that is associated with aging is mediated in part by a loss of functional inhibitory interneurons and thought to underlie impaired performance in spatial memory tasks, including the abnormal tendency in aged animals to pattern complete spatial representations. Here, we asked whether the spatial firing patterns of simultaneously recorded CA3 and CA1 neurons in young and aged rats could be manipulated pharmacologically to selectively reduce CA3 hyperactivity and thus, according to hypothesis, the associated abnormality in spatial representations. We used chronically implanted high‐density tetrodes to record the spatial firing properties of CA3 and CA1 units during animal exploration for food in familiar and novel environments. Aged CA3 place cells have higher firing rates, larger place fields, less spatial information content, and respond less to a change from a familiar to a novel environment than young CA3 cells. We also find that the combination of levetiracetam (LEV) + valproic acid (VPA), previously shown to act as a cognitive enhancer in tests of spatial memory, attenuate CA3 place cell firing rates, reduce place field area, and increase spatial information content in aged but not young adult rats. This is consistent with drug enhancing the specificity of neuronal firing with respect to spatial location. Contrary to expectation, however, LEV + VPA reduces place cell discrimination between novel and familiar environments, i.e., spatial correlations increase, independent of age even though drug enhances performance in cognitive tasks. The results demonstrate that spatial information content, or the number of bits of information encoded per action potential, may be the key correlate for enhancement of spatial memory by LEV + VPA. © 2015 Wiley Periodicals, Inc.     

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. © 2008 Wiley‐Liss, Inc.