Hippocampal mossy cell function: a speculative view

The dentate gyrus as a filter or gate: a look back and a look ahead

The idea of the dentate gyrus as a gate or filter at the entrance to the hippocampus, blocking or filtering incoming excitation from the entorhinal cortex, has been an intriguing one. Here we review the historical development of the idea, and discuss whether it may be possible to be more specific in defining this gate. We propose that dentate function can be understood within a context of Hebbian association and competition: hilar mossy cells help the dentate granule cells to recognize incoming entorhinal patterns of activity (Hebbian association), after which patterns that are consistently and repetitively presented to the dentate gyrus are passed through, while random, more transient patterns are blocked (non-associative Hebbian competition). Translamellar inhibition as well as translamellar potentiation can be understood in this context. The dentate-hilar complex thus plays the role of a "pattern excluder", not a pattern completer. The unique role of pattern exclusion may explain the peculiar qualities of dentate granule cells and hilar mossy cells.

Hilar mossy cells: functional identification and activity in vivo

Network oscillations are proposed to provide the framework for the ongoing neural computations of the brain. Thus, an important aspect of understanding the functional roles of various cell classes in the brain is to understand the relationship of cellular activity to the ongoing oscillations. While many studies have characterized the firing properties of cells in the hippocampal network including granule cells, pyramidal cells and interneurons, information about the activity of dentate mossy cells in the intact brain is scant. Here we review the currently available information and describe biophysical properties and network-related firing patterns of mossy cells in vivo. These new observations will assist in the extracellular identification of this unique cell type and help elucidate their functional role in behaving animals.

Modeling the dentate gyrus

Computational modeling has become an increasingly useful tool for studying complex neuronal circuits such as the dentate gyrus. In order to effectively apply computational techniques and theories to answer pressing biological questions, however, it is necessary to develop detailed, data-driven models. Development of such models is a complicated process, akin to putting together a jigsaw puzzle with the pieces being such things as cell types, cell numbers, and specific connectivity. This chapter provides a walkthrough for the development of a very large-scale, biophysically realistic model of the dentate gyrus. Subsequently, it demonstrates the utility of a modeling approach in asking and answering questions about both healthy and pathological states involving the modeled brain region. Finally, this chapter discusses some predictions that come directly from the model that can be tested in future experimental approaches.

Loss of synapses in the entorhinal-dentate gyrus pathway following repeated induction of electroshock seizures in the rat

The goal of this study was to answer the question of whether repeated administration of electroconvulsive shock (ECS) seizures causes structural changes in the entorhinal-dentate projection system, whose neurons are known to be particularly vulnerable to seizure activity. Adult rats were administered six ECS seizures, the first five of which were spaced by 24-hr intervals, whereas the last two were only 2 hr apart. Stereological approaches were employed to compare the total neuronal and synaptic numbers in sham- and ECS-treated rats. Golgi-stained material was used to analyze dendritic arborizations of the dentate gyrus granule cells. Treatment with ECS produced loss of neurons in the entorhinal layer III and in the hilus of the dentate gyrus. The number of neurons in the entorhinal layer II, which provides the major source of dentate afferents, and in the granular layer of the dentate gyrus, known to receive entorhinal projections, remained unchanged. Despite this, the number of synapses established between the entorhinal layer II neurons and their targets, dentate granule cells, was reduced in ECS-treated rats. In addition, administration of ECS seizures produced atrophic changes in the dendritic arbors of dentate granule cells. The total volumes of entorhinal layers II, III, and V-VI were also found to be reduced in ECS-treated rats. By showing that treatment with ECS leads to partial disconnection of the entorhinal cortex and dentate gyrus, these findings shed new light on cellular processes that may underlie structural and functional brain changes induced by brief, generalized seizures.

Unitary IPSPs enhance hilar mossy cell gain in the rat hippocampus

Mechanisms that control neuronal gain allow for adaptive rescaling to synaptic inputs of varying strengths or frequencies. Here, we show that unitary IPSPs (uIPSPs) modulate gain and unitary EPSP (uEPSP)-action potential coupling in mossy cells (MCs) from rat hippocampal slices. Mossy fibre-evoked uEPSCs were large, facilitated and were suppressed by the group II metabotropic glutamate agonist LY354740. Conversely, uIPSCs were smaller, depressed and were not affected by LY354740, but exerted strong inhibitory control over uEPSP-action potential coupling. The IPSC reversal potential was determined by gramicidin perforated patch recordings to be -65.3 +/- 5.0 mV, lying between the resting membrane potential (-75.3 +/- 1.1 mV) and the action potential threshold (-56.5 +/- 2.4 mV). When applied at theta frequency (10 Hz), uIPSPs increased the offset of the MC input-output response to depolarizing current injection, but also increased gain, maximal firing rate and the slope of the depolarization preceding action potentials. These effects were unchanged by the Ca2+ and HCN channel blockers mibefradil and ZD7288, respectively. The height and maximal slope of MC action potentials during tonic depolarization were also increased by uIPSPs, and the decay of uIPSP conductances injected by dynamic clamp at subthreshold membrane potentials was prolonged by TTX. Application of the muscarinic agonist pilocarpine mimicked the effect of IPSPs on MC maximal firing rate, and action potential height and slope, and this was reversed by the GABA(A) antagonist gabazine. Thus, uIPSPs can increase neuronal gain under hyperexcitable conditions, and this effect is probably due to the de-inactivation of a TTX-sensitive voltage-dependent Na+ conductance.

Endocannabinoid-Mediated Depolarization-Induced Suppression of Inhibition in Hilar Mossy Cells of the Rat Dentate Gyrus

Hilar mossy cells represent a unique population of local circuit neurons in the hippocampus and dentate gyrus. Here we use electrophysiological techniques in acute preparations of hippocampal slices to demonstrate that depolarization of a single hilar mossy cell can produce robust inhibition of local GABAergic afferents. This depolarization-induced suppression of inhibition (DSI) can be observed as a transient reduction in frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) or as a transient reduction in amplitude of evoked IPSCs (eIPSCs). We find that DSI of eIPSCs as observed in hilar mossy cells is enhanced by activation of muscarinic acetylcholine receptors, blocked by chelation of postsynaptic calcium, and critically dependent on retrograde activation of presynaptic cannabinoid type 1 (CB1) receptors. We further report that activation of CB1 receptors on GABAergic afferents to hilar mossy cells (by either endogenous or exogenous agonists) preferentially inhibits calcium-dependent exocytosis and that endocannabinoid-dependent retrograde signaling in this system is subject to tight spatial constraints.

Bcl-2 over-expression fails to prevent age-related loss of calretinin positive neurons in the mouse dentate gyrus

BACKGROUND

Cognitive performance declines with increasing age. Possible cellular mechanisms underlying this age-related functional decline remain incompletely understood. Early studies attributed this functional decline to age-related neuronal loss. Subsequent studies using unbiased stereological techniques found little or no neuronal loss during aging. However, studies using specific cellular markers found age-related loss of specific neuronal types. To test whether there is age-related loss of specific neuronal populations in the hippocampus, and subsequently, whether over-expression of the B-cell lymphoma protein-2 (Bcl-2) in these neurons could delay possible age-related neuronal loss, we examined calretinin (CR) positive neurons in the mouse dentate gyrus during aging.

RESULT

In normal mice, there was an age-related loss of CR positive cells in the dentate gyrus. At the same region, there was no significant decrease of total numbers of neurons, which suggested that age-related loss of CR positive cells was due to the decrease of CR expression in these cells instead of cell death. In the transgenic mouse line over-expressing Bcl-2 in neurons, there was an age-related loss of CR positive cells. Interestingly, there was also an age-related neuronal loss in this transgenic mouse line.

CONCLUSION

These data suggest an age-related loss of CR positive neurons but not total neuronal loss in normal mice and this age-related neuronal change is not prevented by Bcl-2 over-expression.

Selective spread of neurotropic herpesviruses in the rat hippocampus

Transsynaptically spreading viruses are widely used for tracing neuronal circuits in both the central and peripheral nervous systems. However, viruses are capricious tools with selective spreading properties that can produce false-negative results. Using herpes simplex virus type 1 and two pseudorabies virus strains, we aimed at mapping quantitatively neuronal connections in the rat hippocampus. We found that none of the tested viruses infected CA3 pyramidal neurons across synapses following inoculation into the CA1 area. Combined injections of the viruses with the retrograde tracer cholera toxin B (CTB) resulted in CTB, but not virus labeling of CA3 pyramidal neurons. In contrast, other brain regions known to send inputs to the CA1 (the entorhinal cortex, medial septum and diagonal band of Broca, raphe nuclei) were transsynaptically infected. Our results indicate that Schaffer collaterals of CA3 pyramidal cells lack the appropriate cellular machinery for successful neurotropic herpesvirus infection. After injections of viruses into the dentate gyrus/CA3 area, we found labeling in commissurally projecting mossy cells and their afferent granule cells but not in contralateral CA3 pyramidal cells. Using this unique spreading property, we estimated that single mossy cells receive input from a compact cluster of 30-40 granule cells.

Sex differences in the hilar mossy cells of the guinea-pig before puberty

Numerous sex differences have been detected in the morphology of the dentate and hippocampal neurons and hippocampus-dependent memory functions. The aim of the present study was to ascertain whether the mossy cells, an interneuron population forming a recurrent excitatory circuit with the dentate granule cells, are sexually dimorphic. The brains of juvenile (15-16 days old) and peripubescent (45-46 days old) male and female guinea-pigs were Golgi-Cox stained. Mossy cells were sampled from the hilus in the septal third of the dentate gyrus and their dendritic tree and somata were analyzed. The analysis was separately conducted on mossy cells with soma located in the portions of the hilus that face the upper blade (upper hilus) and lower blade (lower hilus), respectively. The mossy cells in the upper hilus were found to be sexually dimorphic in both juvenile and peripubescent animals. At both ages females had a larger dendritic tree than males. This difference was due to a greater mean branch length and, in peripubescent animals, also to a greater number of branches. In juvenile males, the spines on the proximal dendrites (thorny excrescences) had a greater density than in females. No differences in spine density were present in peripubescent animals. Unlike the mossy cells in the upper hilus, the mossy cells in the lower hilus showed very few sex differences in juvenile animals and no differences in peripubescent animals. The few differences favored females, that had more proximal branches and a greater spine density on the distal dendrites than males. The results show that the mossy cells of the guinea-pig are sexually dimorphic prior to puberty. Extending a previous investigation, the present data provide evidence that sex differences are mainly confined to the dentate region corresponding to the upper blade and upper hilus. The observed segregation of the sexual dimorphism in the upper blade/upper hilus suggests that this region might underlie the sexual dimorphism in hippocampus-dependent memory functions.

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

Longitudinally restricted axonal projections of hippocampal granule cells suggest that transverse segments of the granule cell layer may operate independently (the "lamellar" hypothesis). Longitudinal projections of excitatory hilar mossy cells could be viewed as antithetical to lamellar function, but only if longitudinal impulse flow effectively excites distant granule cells. We, therefore, determined the effect of focal granule cell discharges on granule cells located >2 mm along the longitudinal axis. During perforant pathway stimulation in urethane-anesthetized rats, passive diffusion of the GABA(A) receptor antagonist bicuculline methiodide from the tip of a glass recording electrode evoked granule cell discharges and c-Fos expression in granule cells, mossy cells, and inhibitory interneurons, within a approximately 400 microm radius. This focally evoked activity powerfully suppressed distant granule cell-evoked responses recorded simultaneously approximately 2.5-4.5 mm longitudinally. Three days after kainic acid-induced status epilepticus or prolonged perforant pathway stimulation, translamellar inhibition was intact in rats with <40% hilar neuron loss but was consistently abolished after extensive (>85%) hilar cell loss. Retrograde transport of Fluoro-Gold (FG) from the rostral dentate gyrus revealed that few inhibitory interneurons were among the many retrogradely labeled hilar neurons 2.5-4.5 mm longitudinally. Although many somatostatin-positive hilar interneurons effectively transported FG from the distant septum, few of these neurons transported detectable FG from much closer hippocampal injection sites. Inhibitory basket and chandelier cells also exhibited minimal longitudinal FG transport. These findings suggest that translamellar disinhibition may result from the loss of vulnerable, longitudinally projecting mossy cells and may represent a network-level mechanism underlying postinjury hippocampal dysfunction and epileptic network hyperexcitability.

Recurrent excitation of granule cells with basal dendrites and low interneuron density and inhibitory postsynaptic current frequency in the dentate gyrus of macaque monkeys

Temporal lobe epilepsy is often associated with pathological changes in the dentate gyrus, and such changes may be more common in humans than in some nonprimate species. To examine species-specific characteristics that might predispose the dentate gyrus to epileptogenic damage, we evaluated recurrent excitation of granule cells with and without basal dendrites in macaque monkeys, measured miniature inhibitory postsynaptic currents (mIPSCs) of granule cells in macaque monkeys and compared them to rats, and estimated the granule cell-to-interneuron ratio in macaque monkeys and rats. In hippocampal slices from monkeys, whole-cell patch recording revealed antidromically evoked excitatory PSCs that were four times larger and inhibitory PSCs that were over two times larger in granule cells with basal dendrites than without. These findings suggest that granule cells with basal dendrites receive more recurrent excitation and, to a lesser degree, more recurrent inhibition. Miniature IPSC amplitude was slightly larger in monkey granule cells with basal dendrites than in those without, but mIPSC frequency was similar and only 26% of that reported for rats. In situ hybridization for glutamic acid decarboxylase and immunocytochemistry for somatostatin, parvalbumin, and neuronal nuclei revealed interneuron proportions and distributions in monkeys that were similar to those reported for rats. However, the interneuron-to-granule cell ratio was lower in monkeys (1:28) than in rats (1:11). These findings suggest that in the primate dentate gyrus, recurrent excitation is enhanced and inhibition is reduced compared with rodents. These primate characteristics may contribute to the susceptibility of the human dentate gyrus to epileptogenic injuries.

Differential roles of dorsal hippocampal subregions in spatial working memory with short versus intermediate delay

In order to determine the role of subregions of the hippocampus in spatial working memory, this study combined selective neurotoxic lesions of the hippocampal subregions with a simple delayed nonmatching-to-place task on a radial maze in rats. Lesions of the dentate gyrus or the CA3, but not the CA1, subregion of the hippocampus induced a deficit in the acquisition of the task with short-term delays (i.e., 10 sec) and impaired performance of the task in a novel environment. All subregional lesions produced sustained impairment in performing the task with intermediate-term delays (i.e., 5 min) when rats were tested in a familiar environment. The results suggest a dynamic interaction among the dorsal hippocampal subregions in processing spatial working memory, with the time window (i.e., delay) of a task recognized as an essential controlling factor.

Mouse strain differences in kainic acid sensitivity, seizure behavior, mortality, and hippocampal pathology

Genetic influences contribute to susceptibility to seizures and to excitotoxic injury, but it is unclear if/how these susceptibilities are linked. This study assessed the impact of genetic background on mouse strain seizure susceptibility, seizure phenotype, mortality, and hippocampal histopathology. A subcutaneous (s.c.) kainic acid multiple injection protocol was developed. Five mouse strains were tested: a and b) C57BL/6J and 129/SvJ, strains commonly used in gene targeting experiments; c) C3HeB/FeJ, a strain with reported sensitivity to the convulsant effects of kainic acid (KA); d) 129/SvEms, a strain reportedly susceptible to hippocampal excitotoxic cell death; and e) a mixed genetic background strain (129/SvJXC57BL/6J) from which targeted gene deletion experiments have been carried out. Histopathological features were examined at early (7-10 day), delayed (2-4 month), and late (6-13 month) time points.Mouse background strains can be genetically segregated based on excitotoxin sensitivity, seizure phenotype, mortality, and hippocampal histopathology. When injected with KA, C3HeB/FeJ and C57BL/6J strains were resistant to cell death and synaptic reorganization despite severe behavioral seizures, while 129/SvEms mice developed marked pyramidal cell loss and mossy fiber sprouting despite limited seizure activity. The mixed background 129/SvJXC57BL/6J group exhibited features of both parental strains. In the mouse strains tested, the duration or severity of seizure activity was not predictive of subsequent hippocampal pyramidal cell death and/or synaptic reorganization. Unlike rats, mice exhibiting prolonged high-grade KA-induced seizure activity did not develop subsequent spontaneous behavioral seizures.

Hyperexcitability of hippocampal CA1 neurons after fluid percussion injury of the rat cerebral cortex

Effects of fluid percussion injury (FPI) of the parietal cerebral cortex on the neuronal activity in the temporal region of the rat hippocampal CA1 area were investigated by using optical and extracellular recording techniques. Application of moderate impact (1.5-2.0 atm) to the parietal cerebral cortex enhanced the optical signal of the neuronal activity in the ipsilateral hippocampal CA1 area. The field potential evoked by the Schaffer collaterals had multiple population spikes in the ipsilateral hippocampal CA1 pyramidal cell layer. Bicuculline (15 microM) increased the amplitude and the number of population spikes of the field potential even after the brain injury. These results suggest that FPI produces hyperexcitability of hippocampal CA1 neurons, probably by increasing the activity of the Schaffer collaterals of hippocampal CA3 neurons.

Role of long-term synaptic modification in short-term memory

One way that some types of short-term or working memory may be implemented in the brain is by using autoassociation networks that recirculate information to maintain the firing of a subset of neurons in what is termed an attractor state. We describe how long-term synaptic modification is necessary to set up the appropriate stable attractors, each one of which corresponds to a memory of a particular item. Once the synapses have been modified, any of the short-term memory states may be triggered by an appropriate input which starts the neurons firing in one of the attractors, and then the firing is maintained in that attractor by the already modified synapses, with no further synaptic modification necessary. This analysis leads to the prediction that if this type of implementation is used for working memory, then long-term synaptic modification may be necessary only during an acquisition phase of a task, and once the task has been acquired, the performance of the working memory task should be unimpaired if no further synaptic modification is allowed. We show that a considerable body of research findings on the effects of agents that block synaptic modification on working memory tasks can be understood in this way. Many of the findings are consistent with the hypothesis that blocking synaptic modification in the hippocampus impairs the acquisition, but not the later performance, of hippocampal-dependent working memory tasks.

Storage, recall, and novelty detection of sequences by the hippocampus: elaborating on the SOCRATIC model to account for normal and aberrant effects of dopamine

In order to understand how the molecular or cellular defects that underlie a disease of the nervous system lead to the observable symptoms, it is necessary to develop a large-scale neural model. Such a model must specify how specific molecular processes contribute to neuronal function, how neurons contribute to network function, and how networks interact to produce behavior. This is a challenging undertaking, but some limited progress has been made in understanding the memory functions of the hippocampus with this degree of detail. There is increasing evidence that the hippocampus has a special role in the learning of sequences and the linkage of specific memories to context. In the first part of this paper, we review a model (the SOCRATIC model) that describes how the dentate and CA3 hippocampal regions could store and recall memory sequences in context. A major line of evidence for sequence recall is the "phase precession" of hippocampal place cells. In the second part of the paper, we review the evidence for theta-gamma phase coding. According to a framework that incorporates this form of coding, the phase precession is interpreted as cued recall of a discrete sequence of items from long-term memory. The third part of the paper deals with the issue of how the hippocampus could learn memory sequences. We show that if multiple items can be active within a theta cycle through the action of a short-term "buffer," NMDA-dependent plasticity can lead to the learning of sequences presented at realistic item separation intervals. The evidence for such a buffer function is reviewed. An important underlying issue is whether the hippocampal circuitry is configured differently for learning and recall. We argue that there are indeed separate states for learning and recall, but that both involve theta oscillations, albeit in possibly different forms. This raises the question of how neuromodulatory input might switch the hippocampus between learning and recall states and more generally how different neuromodulatory inputs reconfigure the hippocampus for different functions. In the fifth part of this paper we review our studies of dopamine and dopamine/NMDA interactions in the control of synaptic function. Our results show that dopamine dramatically reduces the direct cortical input to CA1 (the perforant path input), while having little effect on the input from CA3. In order to interpret the functional consequences of this pathway-specific modulation, it is necessary to understand the function of CA1 and the role of dopaminergic input from the ventral tegmental area (VTA). In the sixth part of this paper we consider several possibilities and address the issue of how dopamine hyperfunction or NMDA hypofunction, abnormalities that may underlie schizophrenia, might lead to the symptoms of the disease. Relevant to this issue is the demonstrated role of the hippocampus in novelty detection, a function that is likely to depend on sequence recall by the hippocampus. Novelty signals are generated when reality does not match the expectations generated by sequence recall. One possible site for computing mismatch is CA1, since it receives predictions from CA3 and sensory "reality" via the perforant path. Our data suggest that disruption of this comparison would be expected under conditions of dopamine hyperfunction or NMDA hypofunction. Also relevant is the fact that the VTA, which fires in response to novelty, may both depend on hippocampal-dependent novelty detection processes and, in turn, affect hippocampal function. Through large-scale modeling that considers both the processes performed by the hippocampus and the neuromodulatory loops in which the hippocampus is embedded, it is becoming possible to generate working hypotheses that relate synaptic function and malfunction to behavior.

Mossy cells in epilepsy: rigor mortis or vigor mortis?

Mossy cells are bi-directionally connected through a positive feedback loop to granule cells, the principal cells of the dentate gyrus. This recurrent circuit is strategically placed between the entorhinal cortex and the hippocampal CA3 region. In spite of their potentially pro-convulsive arrangement with granule cells, mossy cells have not been seriously considered to promote seizures, because mossy cells, allegedly one of the most vulnerable cell types in the entire mammalian brain, have long been 'known' to die en masse in epilepsy. However, new data suggest that rumors of the rapid demise of the mossy cells might have been greatly exaggerated.

Molecular and functional analysis of hyperpolarization-activated pacemaker channels in the hippocampus after entorhinal cortex lesion

Differential display of hippocampal tissue after entorhinal cortex lesion (ECL) revealed decreases in mRNA encoding the neuronal hyperpolarization-activated, cyclic nucleotide-gated channel HCN1. In situ hybridization confirmed that hippocampal transcripts of HCN1, but not HCN2/3/4, are down-regulated after ECL. Expression recovered at approximately 21 days after lesion (dal). Immunohistochemistry demonstrated a corresponding regulation of HCN1 protein expression in CA1-CA3 dendrites, hilar mossy cells and interneurons, and granule cells. Patch-clamp recordings in the early phase after lesion from mossy cells and hilar interneurons revealed an increase in the fast time constant of current activation and a profound negative shift in voltage activation of Ih. Whereas current activation recovered at 30 dal, the voltage activation remained hyperpolarized in mossy cells and hilar interneurons. Granule cells, however, were devoid of any detectable somatic Ih currents. Hence, denervation of the hippocampus decreases HCN1 and concomitantly the Ih activity in hilar neurons, and the recovery of h-current activation kinetics occurs parallel to postlesion sprouting.

Dentate hilar mossy cells and somatostatin-containing neurons are immunoreactive for the alpha8 integrin subunit: characterization in normal and kainic acid-treated rats

Integrins are heterodimeric cell surface receptors composed of different alpha and beta subunits that mediate cell-cell and cell-extracellular matrix interactions. They have been implicated in the regulation of neuronal migration, differentiation, process outgrowth, and plasticity. The alpha8 integrin subunit associates exclusively with the beta1 subunit to form a receptor (alpha8beta1) for fibronectin, vitronectin, tenascin, and osteopontin. In a previous study, we demonstrated that hippocampal dentate hilar neurons are immunoreactive for alpha8. The present study identifies the major types of alpha8-immunoreactive hilar neurons and characterizes the effects of kainic acid-induced seizures on alpha8-immunoreactivity in these cells. Examination of the hilus in normal rats revealed alpha8-immunoreactivity in the somatodendritic compartments of large hilar neurons identified as mossy cells, including a subset of dendritic thorny excrescences that were contacted by large mossy fiber terminals. alpha8-immunoreactivity also was found in approximately 71% of somatostatin-containing hilar cells. Kainic acid-induced seizures dramatically and rapidly altered the levels and distribution of alpha8-immunoreactivity in hilar neurons. After 1.5 h of seizures, alpha8-immunoreactivity in their dendrites was reduced greatly. One day after kainic acid treatment, labeling was diminished throughout the somatodendritic compartments of most hilar cells. This decrease appeared to be transient, since alpha8 labeling returned to normal levels in surviving hilar neurons within 2 weeks of treatment. In addition, many alpha8-immunoreactive hilar neurons, particularly in caudal dentate regions, were lost 3-5 weeks after kainic acid treatment. Our findings suggest that alpha8beta1 may mediate adhesive interactions of the dendritic processes of mossy cells and somatostatin-containing hilar neurons with other cellular elements or with extracellular matrix components. They also suggest that alpha8 may be susceptible to activity-dependent proteolysis that could modulate its function in the somatodendritic compartment of these cells.

Enhanced but fragile inhibition in the dentate gyrus in vivo in the kainic acid model of temporal lobe epilepsy: a study using current source density analysis

Temporal lobe epilepsy is related to many structural and physiological changes in the brain. We used kainic acid in rats as an animal model of temporal lobe epilepsy, and studied the neural interactions of the dentate gyrus in urethane-anesthetized rats in vivo. Our initial hypothesis was that sprouting of mossy fibers, the axons of the granule cells, increases proximal dendritic excitatory currents in the inner molecular layer of the dentate gyrus. Extracellular currents were detected in vivo using current source density analysis. Backfiring the mossy fibers in CA3 or orthodromic excitation of the granule cells through the medial perforant path induced a current sink at the inner molecular layer. However, the sink or inferred excitation at the inner molecular layer was not increased in kainic acid-treated rats and the sink actually correlated negatively with the degree of mossy fiber sprouting. It is inferred that the latter sink was mediated mainly by association fibers and not by recurrent mossy fibers. After kainic acid treatment, paired-pulse inhibition of the population spikes in the dentate gyrus was increased. In contrast, reverberant activity that involved looping around an entorhinal-hippocampal circuit was increased in kainic acid-treated rats, compared to control rats. The increase of inhibition in kainic acid-treated rats was readily blocked by a small dose of GABA(A) receptor antagonist bicuculline. The latter dose of bicuculline induced paroxsymal spike bursts in kainic acid-treated but not control rats, demonstrating that the increased inhibition in dentate gyrus was fragile. In conclusion, after kainic acid induced seizures, the dentate gyrus in vivo showed an increase in inhibition that appeared to be fragile. The hypothesized increase in proximal dendritic excitation due to mossy fiber sprouting was not detected. However, the fragile inhibition could explain the seizure susceptibility in patients with temporal lobe epilepsy.

Structural features of mossy cells in the hamster dentate gyrus, with special reference to somatic thorny excrescences

We have recently revealed that large multipolar neurons, presumed mossy cells in the hamster dentate gyrus (DG), were calretinin (CR)-immunoreactive (IR) at the ventral level, although these neurons were CR-negative at the dorsal level. In the present study, we confirmed this identification with several methods and analyzed structural features of hamster mossy cells in detail. Golgi impregnationi and intracellular Lucifer yellow labeling studies revealed that mossy cells in the hamster dentate hilus had extraordinarily prominent thorny excrescences on their somata as well as on their proximal dendrites. Mossy cells exhibited dorsoventral differences in their structural features; proximal dendrites of single mossy cells were fewer, and thorny excrescences were larger and more complicated at the dorsal level than at the ventral level. Electron microscopic serial section three-dimensional reconstructions revealed that somatic thorny excrescences consisted of large and complicated spines, which received numerous asymmetrical synapses from mossy fiber terminals. In addition, our confocal laser scanning microscopic observations also revealed many glutamic acid decarboxylase-immunoreactive punctae abutting the mossy cell somata and dendrites. Our present and previous observations revealed the structural features of hamster mossy cells and their differences along the dorsoventral axis and further indicated that mossy cells were prominently different in their chemical and morphological features among species.

Latent attractors: a model for context-dependent place representations in the hippocampus

Cells throughout the rodent hippocampal system show place-specific patterns of firing called place fields, creating a coarse-coded representation of location. The dependencies of this place code--or cognitive map--on sensory cues have been investigated extensively, and several computational models have been developed to explain them. However, place representations also exhibit strong dependence on spatial and behavioral context, and identical sensory environments can produce very different place codes in different situations. Several recent studies have proposed models for the computational basis of this phenomenon, but it is still not completely understood. In this article, we present a very simple connectionist model for producing context-dependent place representations in the hippocampus. We propose that context dependence arises in the dentate gyrus-hilus (DGH) system, which functions as a dynamic selector, disposing a small group of granule and pyramidal cells to fire in response to afferent stimulus while depressing the rest. It is hypothesized that the DGH system dynamics has "latent attractors," which are unmasked by the afferent input and channel system activity into subpopulations of cells in the DG, CA3, and other hippocampal regions as observed experimentally. The proposed model shows that a minimally structured hippocampus-like system can robustly produce context-dependent place codes with realistic attributes.

Granule cell hyperexcitability in the early post-traumatic rat dentate gyrus: the 'irritable mossy cell' hypothesis

1. Cytochemical and in vitro whole-cell patch clamp techniques were used to investigate granule cell hyperexcitability in the dentate gyrus 1 week after fluid percussion head trauma. 2. The percentage decrease in the number of hilar interneurones labelled with either GAD67 or parvalbumin mRNA probes following trauma was not different from the decrease in the total population of hilar cells, indicating no preferential survival of interneurones with respect to the non-GABAergic hilar cells, i.e. the mossy cells. 3. Dentate granule cells following trauma showed enhanced action potential discharges, and longer-lasting depolarizations, in response to perforant path stimulation, in the presence of the GABAA receptor antagonist bicuculline. 4. There was no post-traumatic alteration in the perforant path-evoked monosynaptic excitatory postsynaptic currents (EPSCs), or in the intrinsic properties of granule cells. However, after trauma, the monosynaptic EPSC was followed by late, polysynaptic EPSCs, which were not present in controls. 5. The late EPSCs in granule cells from fluid percussion-injured rats were not blocked by the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid (APV), but were eliminated by both the non-NMDA glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the AMPA receptor antagonist GYKI 53655. 6. In addition, the late EPSCs were not present in low (0.5 mM) extracellular calcium, and they were also eliminated by the removal of the dentate hilus from the slice. 7. Mossy hilar cells in the traumatic dentate gyrus responded with significantly enhanced, prolonged trains of action potential discharges to perforant path stimulation. 8. These data indicate that surviving mossy cells play a crucial role in the hyperexcitable responses of the post-traumatic dentate gyrus.

Highly specific neuron loss preserves lateral inhibitory circuits in the dentate gyrus of kainate-induced epileptic rats

Patients with temporal lobe epilepsy display neuron loss in the hilus of the dentate gyrus. This has been proposed to be epileptogenic by a variety of different mechanisms. The present study examines the specificity and extent of neuron loss in the dentate gyrus of kainate-treated rats, a model of temporal lobe epilepsy. Kainate-treated rats lose an average of 52% of their GAD-negative hilar neurons (putative mossy cells) and 13% of their GAD-positive cells (GABAergic interneurons) in the dentate gyrus. Interneuron loss is remarkably specific; 83% of the missing GAD-positive neurons are somatostatin-immunoreactive. Of the total neuron loss in the hilus, 97% is attributed to two cell types-mossy cells and somatostatinergic interneurons. The retrograde tracer wheat germ agglutinin (WGA)-apoHRP-gold was used to identify neurons with appropriate axon projections for generating lateral inhibition. Previously, it was shown that lateral inhibition between regions separated by 1 mm persists in the dentate gyrus of kainate-treated rats with hilar neuron loss. Retrogradely labeled GABAergic interneurons are found consistently in sections extending 1 mm septotemporally from the tracer injection site in control and kainate-treated rats. Retrogradely labeled putative mossy cells are found up to 4 mm from the injection site, but kainate-treated rats have fewer than controls, and in several kainate-treated rats virtually all of these cells are missing. These findings support hypotheses of temporal lobe epileptogenesis that involve mossy cell and somatostatinergic neuron loss and suggest that lateral inhibition in the dentate gyrus does not require mossy cells but, instead, may be generated directly by GABAergic interneurons.

Cholinergic innervation of mossy cells in the rat fascia dentata

Hilar mossy cells are the main cells of origin of the commissural/associational projection to the inner molecular layer of the rat fascia dentata. In order to analyze the cholinergic innervation of hilar mossy cells, a light and electron microscopic double-labeling technique was used. Immunolabeling for calcitonin gene-related peptide (CGRP) was employed to identify mossy cells and immunocytochemistry for choline acetyltransferase (ChAT) was used to label cholinergic septohippocampal fibers. Cholinergic boutons were abundant around mossy cell somata and on their proximal dendrites. Electron microscopy confirmed that many of these boutons formed synapses with the CGRP-positive mossy cells. These data demonstrate a direct innervation of hilar mossy cells by cholinergic septohippocampal afferents. This connectivity could contribute to the electrophysiological behavior of mossy cells during theta oscillations.

Muscarinic receptor subtypes involved in hippocampal circuits

Muscarinic receptors modulate hippocampal activity in two main ways: inhibition of synaptic activity and enhancement of excitability of hippocampal cells. Due to the lack of pharmacological tools, it has not been possible to identify the individual receptor subtypes that mediate the specific physiological actions that underlie these forms of modulation. Light and electron microscopic immunocytochemistry using subtype-specific antibodies was combined with lesioning techniques to examine the pre- and postsynaptic location of m1-m4 mAChR at identified hippocampus synapses. The results revealed striking differences among the subtypes, and suggested different ways that the receptors modulate excitatory and inhibitory transmission in distinct circuits. Complementary physiological studies using m1-toxin investigated the modulatory effects of this subtype on excitatory transmission in more detail. The implications of these data for understanding the functional roles of these subtypes are discussed.

Mossy cells in the mouse dentate gyrus: identification in the dorsal hilus and their distribution along the dorsoventral axis

Previously we showed that large multipolar cells immunoreactive for calretinin and subunits 2 and 3 of amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) type glutamate receptors (GluR2/3) clustered in the ventral hilus of the mouse dentate gyrus and revealed that they were mossy cells. Although such large calretinin immunoreactive cells were not seen in the dorsal hilus, our Golgi study revealed the presence of mossy cells in the dorsal hilus. As we observed large intensely GluR2/3 immunoreactive cells in the dorsal hilus, we suggested that these calretinin negative but intensely GluR2/3 positive large cells in the dorsal hilus were also mossy cells. In the present study we confirmed this identification with several methods. The extracellular tracer labeling studies revealed that all of 47 mossy cells identified morphologically were intensely GluR2/3 positive but calretinin negative, whereas none of 22 non-mossy hilar neurons were intensely GluR2/3 positive. Electron microscopically most of intensely GluR2/3 positive somata and dendritic processes showed the characteristic ultrastructural features of mossy cells. Furthermore, the fimbria-fornix-hippocampal commissure transection procedures induced the calretinin expression in some of these dorsal GluR2/3 immunoreactive cells. On the basis of these observations, we concluded that the vast majority of intensely GluR2/3 immunoreactive large cells in the mouse dorsal hilus were mossy cells. Then we evaluated the presumed difference in the distribution of mossy cells along the dorsoventral axis by the disector. The numerical density of mossy cells was about 1.4 times larger at the ventral level than at the dorsal level, indicating that the dorsoventral difference in the distribution of mossy cells in the mouse hilus was far smaller than that previously speculated.

Population dynamics and theta rhythm phase precession of hippocampal place cell firing: a spiking neuron model

O'Keefe and Recce ([1993] Hippocampus 68:317-330) have observed that the spatially selective firing of pyramidal cells in the CA1 field of the rat hippocampus tends to advance to earlier phases of the electroencephalogram theta rhythm as a rat passes through the place field of a cell. We present here a neural network model based on integrate- and-fire neurons that accounts for this effect. In this model, place selectivity in the hippocampus is a consequence of synaptic interactions between pyramidal neurons together with weakly selective external input. The phase shift of neuronal spiking arises in the model as result of asymmetric spread of activation through the network, caused by asymmetry in the synaptic interactions. Several experimentally observed properties of the phase shift effect follow naturally from the model, including 1) the observation that the first spikes a cell fires appear near the theta phase corresponding to minimal population activity, 2) the overall advance is less than 360 degrees, and 3) the location of the rat within the place field of the cell is the primary correlate of the firing phase, not the time the rat has been in the field. The model makes several predictions concerning the emergence of place fields during the earliest stages of exploration in a novel environment. It also suggests new experiments that could provide further constraints on a possible explanation of the phase precession effect.

Type I adenylyl cyclase mutant mice have impaired mossy fiber long-term potentiation

Long-term potentiation (LTP) at the mossy fiber-->CA3 pyramidal cell synapse in the hippocampus is an NMDA-independent form of LTP that requires cAMP-dependent protein kinase (PKA) activity and can be induced by forskolin, a general activator of adenylyl cyclases. Presynaptic Ca2+ influx and elevated cAMP may be obligatory for mossy fiber LTP. Because the Ca2+-stimulated type 1 adenylyl cyclase (AC1) is expressed in the dentate gyrus and CA3 pyramidal cells, it is hypothesized that AC1 may be critical for mossy fiber LTP. To test this hypothesis, we examined several forms of hippocampal LTP in wild-type and AC1 mutant mice. Wild-type and AC1 mutant mice exhibited comparable perforant path LTP recorded in the dentate gyrus as well as decremental LTP at the Schaffer collateral-->CA1 pyramidal cell synapse. Although the mutant mice exhibited normal paired pulse facilitation, mossy fiber LTP was impaired significantly in AC1 mutants. High concentrations of forskolin induced mossy fiber LTP to comparable levels in wild-type and AC1 mutant mice, indicating that signaling components downstream from the adenylyl cyclase, including PKA, ion channels, and secretory machinery, were not affected by disruption of the AC1 gene. These data indicate that coupling of Ca2+ to activation of AC1 is crucial for mossy fiber LTP, most likely via activation of PKA and enhancement of excitatory amino acid secretion.

Specialized electrophysiological properties of anatomically identified neurons in the hilar region of the rat fascia dentata

Because of their strategic position between the granule cell and pyramidal cell layers, neurons of the hilar region of the hippocampal formation are likely to play an important role in the information processing between the entorhinal cortex and the hippocampus proper. Here we present an electrophysiological characterization of anatomically identified neurons in the fascia dentata as studied using patch-pipette recordings and subsequent biocytin-staining of neurons in slices. The resting potential, input resistance (RN), membrane time constant (taum), "sag" in hyperpolarizing responses, maximum firing rate during a 1-s current pulse, spike width, and fast and slow afterhyperpolarizations (AHPs) were determined for several different types of hilar neurons. Basket cells had a dense axonal plexus almost exclusively within the granule cell layer and were distinguishable by their low RN, short taum, lack of sag, and rapid firing rates. Dentate granule cells also lacked sag and were identifiable by their higher RN, longer taum, and lower firing rates than basket cells. Mossy cells had extensive axon collaterals within the hilus and a few long-range collaterals to the inner molecular layer and CA3c and were characterized physiologically by small fast and slow AHPs. Spiny and aspiny hilar interneurons projected primarily either to the inner or outer segment of the molecular layer and had a dense intrahilar axonal plexus, terminating onto somata within the hilus and CA3c. Physiologically, spiny hilar interneurons generally had higher RN values than mossy cells and a smaller slow AHP than aspiny interneurons. The specialized physiological properties of different classes of hilar neurons are likely to be important determinants of their functional operation within the hippocampal circuitry.

Physiological properties of mouse hippocampal mossy cells

Physiological properties of mouse mossy cells were analyzed in slice preparations and compared with those of mouse CA3 pyramidal cells. They had larger input resistances, less spike-frequency adaptation, more anomalous rectification, larger amplitude spontaneous EPSP and higher frequency of spontaneous EPSP than CA3 pyramidal cells. In these respects, they resembled rat mossy cells. However, in contrast to rat mossy cells, mouse mossy cells showed pronounced burst after-hyperpolarization, comparable to that of CA3 pyramidal cells. This study revealed that mouse mossy cells are distinct from CA3 pyramidal cells in their physiological properties and further suggested that they might be less vulnerable to excitotoxic damage than rat mossy cells, as the burst after-hyperpolarization is supposed to be important in maintaining a neuron in a relatively nonexcited state.

Distribution of calretinin immunoreactivity in the mouse dentate gyrus: II. Mossy cells, with special reference to their dorsoventral difference in calretinin immunoreactivity

In our previous study we revealed the presence of clustered large calretinin-immunoreactive multipolar cells in the ventral hilus of the mouse dentate gyrus and indicated that they might be mossy cells, the principal neurons in the dentate hilus. In the present study we confirmed this identification with several methods and analysed further in detail. In Golgi-impregnated samples mossy cells were easily identified by their locations and characteristic thorny excrescences on their proximal dendrites. Golgi-impregnated mossy cells were observed not only in the ventral hilus but also in the dorsal hilus, where no calretinin-immunoreactive large multipolar cells were encountered. Interestingly, mossy cells exhibited dorsoventral differences in the size and complexity of thorny excrescences; mossy cells at the dorsal and middle levels had larger and more complex thorny excrescences, which covered dendritic shafts for a longer distance, while ventral mossy cells had smaller, simpler and shorter thorny excrescences. Confocal laser scanning light microscopic observations at a high magnification showed that the vast majority of calretinin-immunoreactive large neurons in the ventral hilus displayed the thorny excrescences characteristic to mossy cells. Mossy cells identified with the intracellular injection of Lucifer Yellow were calretinin-immunoreactive. Electron microscopic observations clearly revealed that calretinin-immunoreactive elements showed structural features of mossy cells such as thorny excrescences receiving typical synapses from mossy fibre terminals. At the supragranular zone, a well-known target zone of mossy cell axons, a dense calretinin-immunoreactive band was seen, where numerous calretinin-immunoreactive punctae and fibres were packed. Electron microscopic observations revealed that these calretinin-immunoreactive axon terminals in the supragranular zone made asymmetrical synapses on presumed granule cell dendritic spines. Tracer injection studies and lesion experiments indicated that the supragranular calretinin-immunoreactive axon terminals mainly originated from the large calretinin-immunoreactive multipolar cells in the ipsilateral ventral hilus. Fluorescent double immunostaining for calretinin and glutamate receptor 2/3 (GluR2/3) revealed that all large calretinin-immunoreactive hilar cells in the ventral level were GluR2/3-immunoreactive and almost all intensely GluR2/3-immunoreactive hilar cells in the ventral level were calretinin-immunoreactive. In addition intensely GluR2/3-immunoreactive but calretinin-negative large cells were encountered in the dentate hilus at the dorsal level. On the basis of these observations, we concluded that large calretinin-immunoreactive cells in the ventral hilus of the mouse dentate gyrus were really mossy cells and that mossy cells at the dorsal level were calretinin negative. The present study revealed that mouse mossy cells show the dorsoventral difference in the calretinin immunoreactivity and thus they are chemically heterogeneous.

Ultrastructural localization of neurotransmitter immunoreactivity in mossy cell axons and their synaptic targets in the rat dentate gyrus

Electrophysiologically identified and intracellularly biocytin-labeled mossy cells in the dentate hilus of the rat were studied using electron microscopy and postembedding immunogold techniques. Ultrathin sections containing a labeled mossy cell or its axon collaterals were reacted with antisera against the excitatory neurotransmitter glutamate and against the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). From single- and double-immunolabeled preparations, we found that 1) mossy cell axon terminals made asymmetric contacts onto postsynaptic targets in the hilus and stratum moleculare of the dentate gyrus and showed immunoreactivity primarily for glutamate, but never for GABA; 2) in the hilus, glutamate-positive mossy cell axon terminals targeted GABA-positive dendritic shafts of hilar interneurons and GABA-negative dendritic spines; and 3) in the inner molecular layer, the mossy cell axon formed asymmetric synapses with dendritic spines associated with GABA-negative (presumably granule cell) dendrites. The results of this study support the view that excitatory (glutamatergic) mossy cell terminals contact GABAergic interneurons and non-GABAergic neurons in the hilar region and GABA-negative granule cells in the stratum moleculare. This pattern of connectivity is consistent with the hypothesis that mossy cells provide excitatory feedback to granule cells in a dentate gyrus associational network and also activate local hilar inhibitory elements.

Long-term potentiation activates the GAP-43 promoter: selective participation of hippocampal mossy cells

Perforant path long-term potentiation (LTP) in intact mouse hippocampal dentate gyrus increased the neuron-specific, growth-associated protein GAP-43 mRNA in hilar cells 3 days after tetanus, but surprisingly not in granule cells, the perforant path target. This increase was positively correlated with level of enhancement and restricted to central hilar cells on the side of stimulation. Blockade of LTP by puffing DL-aminophosphonovalerate (APV), an N-methyl-D-aspartate (NMDA) receptor blocker into the molecular layer, eliminated LTP-induced GAP-43 mRNA elevation in hilar cells. To determine whether the mRNA elevation was mediated by transcription, LTP was studied in transgenic mice bearing a GAP-43 promoter-lacZ reporter gene. Promoter activity as indexed by Transgene expression (PATE) increased as indicated by blue staining of the lacZ gene product, beta-galactosidase. Potentiation induced a blue band bilaterally in the inner molecular layer of the dentate gyrus along the entire septotemporal axis. Because mossy cells are the only neurons in the central hilar zone that project to the inner molecular layer bilaterally along the entire septotemporal axis and LTP-induced activation of PATE in this zone was confined to the side of stimulation, we concluded that mossy cells were unilaterally activated, increasing synthesis of beta-galactosidase, which was transported bilaterally. Neither granule cells nor pyramidal cells demonstrated increased PATE or increased GAP-43 mRNA levels. These results and recent evidence indicating the necessity of hilar neurons for LTP point to previously unheralded mossy cells as potentially critical for perforant path LTP and the GAP-43 in these cells as important for LTP persistence lasting days.

Mossy cells of the rat dentate gyrus are immunoreactive for calcitonin gene-related peptide (CGRP)

The neuropeptide calcitonin gene-related peptide (CGRP) was localized in the hippocampus and dentate gyrus of the rat by immunocytochemistry at the light and electron microscopic levels. Without colchicine treatment only faint neuropil labelling was found in the inner molecular layer of the dentate gyrus. Following colchicine treatment, a large number of neurons with numerous complex spines along the proximal dendrites were visualized in the hilus of the dentate gyrus, particularly in the ventral areas, and, in addition, staining of the inner molecular layer became stronger. Several CA3c pyramidal cells located adjacent to the hilar region in the ventral hippocampus also appeared to be faintly positive, although in most cases only their axon initial segments were labelled. Outside this region, the subicular end of the CA1 subfield contained occasional CGRP-positive non-pyramidal cells. The hilar CGRP-positive neurons were negative for parvalbumin, calretinin, cholecystokinin and somatostatin, whereas most of them were immunoreactive for GluR2/3 (the AMPA-type glutamate receptor known to be expressed largely by principal cells). Correlated electron microscopy showed that the spines along the proximal dendritic shafts indeed correspond to thorny excrescences engulfed by large complex mossy terminals forming asymmetrical synapses. Pre-embedding immunogold staining demonstrated that CGRP immunoreactivity in the inner molecular layer was confined to axon terminals that form asymmetrical synapses, and the labelling was associated with large dense-core vesicles. The present data provide direct evidence that CGRP is present in mossy cells of the dentate gyrus and to a lesser degree in CA3c pyramidal cells of the ventral hippocampus. These CGRP-containing principal cells terminate largely in the inner molecular layer of the dentate gyrus, and may release the neuropeptide in conjunction with their 'classical' neurotransmitter, glutamate.

Lesions in the dentate hilum and CA2/CA3 regions of the rat hippocampus produce cognitive deficits that correlate with site-specific glial activation

Thirty male rats pressed a lever three times (FR3) when a stimulus light (sD) was off to obtain sucrose pellets. They were then evenly divided into sham Controls versus groups lesioned bilaterally in the hippocampus by stercotaxic injection of ibotenic acid into the dentate hilum (HIL) or the CA2/CA3 region (CA2/3). On measures of recall of the FR3-sD task taken during the initial 30 min of a postlesion test session, the CA2/3 and especially the HIL groups showed significant (p < .05) impairments relative to the Controls. During an ensuing 30-min period, rats were reshaped to criterion, beginning at FR1, and no appreciable intergroup differences were noted on this schedule or at FR1-sD. At FR2-sD, the HIL but not the CA2/3 group showed some impairment relative to Controls. At FR3-sD, both the CA2/3 and HIL groups had impaired task performance. An immunocytochemical index of glial activation showed higher reactivity in CA2/3 or the dentate hilum among CA2/3 or HIL animals, respectively, that was associated with the degree to which they showed an FR3-sD performance deficit.

Network properties of the dentate gyrus in epileptic rats with hilar neuron loss and granule cell axon reorganization

Neuron loss in the hilus of the dentate gyrus and granule cell axon reorganization have been proposed as etiologic factors in human temporal lobe epilepsy. To explore these possible epileptogenic mechanisms, electrophysiological and anatomic methods were used to examine the dentate gyrus network in adult rats that had been treated systemically with kainic acid. All kainate-treated rats, but no age-matched vehicle-treated controls, were observed to have spontaneous recurrent motor seizures beginning weeks to months after exposure to kainate. Epileptic kainate-treated rats and control animals were anesthetized for field potential recording from the dentate gyrus in vivo. Epileptic kainate-treated rats displayed spontaneous positivities ("dentate electroencephalographic spikes") with larger amplitude and higher frequency than those in control animals. After electrophysiological recording, rats were perfused and their hippocampi were processed for Nissl and Timm staining. Epileptic kainate-treated rats displayed significant hilar neuron loss and granule cell axon reorganization. It has been hypothesized that hilar neuron loss reduces lateral inhibition in the dentate gyrus, thereby decreasing seizure threshold. To assess lateral inhibition, simultaneous recordings were obtained from the dentate gyrus in different hippocampal lamellae, separated by 1 mm. The perforant path was stimulated with paired-pulse paradigms, and population spike amplitudes were measured. Responses were obtained from one lamella while a recording electrode in a distant lamella leaked saline or the gamma-aminobutyric acid-A receptor antagonist bicuculline. Epileptic kainate-treated and control rats both showed significantly more paired-pulse inhibition when a lateral lamella was hyperexcitable. To assess seizure threshold in the dentate gyrus, two techniques were used. Measurement of stimulus threshold for evoking maximal dentate activation revealed significantly higher thresholds in epileptic kainate-treated rats compared with controls. In contrast, epileptic kainate-treated rats were more likely than controls to discharge spontaneous bursts of population spikes and to display stimulus-triggered afterdischarges when a focal region of the dentate gyrus was disinhibited with bicuculline. These spontaneous bursts and afterdischarges were confined to the disinhibited region and did not spread to other septotemporal levels of the dentate gyrus. Epileptic kainate-treated rats that displayed spontaneous bursts and/or afterdischarges had significantly larger percentages of Timm staining in the granule cell and molecular layers than epileptic kainate-treated rats that failed to show spontaneous bursts or afterdischarges. In summary, this study reveals functional abnormalities in the dentate gyri of epileptic kainate-treated rats; however, lateral inhibition persists, suggesting that vulnerable hilar neurons are not necessary for generating lateral inhibition in the dentate gyrus.

The calretinin-containing mossy cells survive excitotoxic insult in the gerbil dentate gyrus. Comparison of excitotoxicity-induced neuropathological changes in the gerbil and rat

Our preliminary results showed that mossy fibres do not undergo sprouting after global ischaemia in gerbils, although the pattern of hippocampal cell damage resembled that seen in ischaemic and epileptic rats, where mossy fibre sprouting is known to occur. In order to investigate whether the observed differences in the appearance of mossy fibre sprouting are related to the animal model or species used, this study was undertaken to compare the neuropathological changes induced in gerbils by systemic injection of kainate or by occlusion of carotid arteries with the changes induced in rats by injection of kainate. The pattern of pyramidal cell damage was very similar in each group. Mossy fibre sprouting was present in epileptic rats but not in ischaemic or epileptic gerbils. The number of somatostatin-immunoreactive neurons was decreased in the hilus of epileptic rats and ischaemic gerbils, but not in epileptic gerbils. The analysis of calretinin immunoreactivity in the dentate gyrus revealed differences between the rat and gerbil. The most striking difference between these species was that mossy cells contained calretinin in gerbils but not in rats. Cell counting showed that the calretinin-containing mossy cells had survived both in epileptic and ischaemic gerbils. Therefore, since the mossy cells are known to be highly susceptible to excitotoxic insult in rats and degeneration of these cells is thought to be a key element in the induction of mossy fibre sprouting, we propose that the absence of mossy fibre sprouting in gerbils is related to the survival of the mossy cells.

Axon arbors and synaptic connections of hippocampal mossy cells in the rat in vivo

The axon collateralization patterns and synaptic connections of intracellularly labeled and electrophysiologically identified mossy cells were studied in rat hippocampus. Light microscopic analysis of 11 biocytin-filled cells showed that mossy cell axon arbors extended through an average of 57% of the total septotemporal length of the hippocampus (summated two-dimensional length, not adjusted for tissue shrinkage). Axon collaterals were densest in distant lamellae rather than in lamellae near the soma. Most of the axon was concentrated in the inner one-third of the molecular layer, with the hilus containing an average of only 26% of total axon length and the granule cell layer containing an average of only 7%. Ultrastructural analysis was carried out on three additional intracellularly stained mossy cells, in which axon collaterals and synaptic targets were examined in serial sections of chosen axon segments. In the central and subgranular regions of the hilus, mossy cell axons established a low density of synaptic contacts onto dendritic shafts, neuronal somata, and occasional dendritic spines. Most hilar synapses were made relatively close to the mossy cell somata. At greater distances from the labeled mossy cell (1-2 mm along the septotemporal axis), the axon collaterals ramified predominantly within the inner molecular layer and made a high density of asymmetric synaptic contacts almost exclusively onto dendritic spines. Quantitative measurements indicated that more than 90% of mossy cell synaptic contacts in the ipsilateral hippocampus are onto spines of proximal dendrites of presumed granule cells. These results are consistent with a primary mossy cell role in an excitatory associational network with granule cells of the dentate gyrus.

A genetic test of the effects of mutations in PKA on mossy fiber LTP and its relation to spatial and contextual learning

Using a genetic approach, we assessed the effects of mutations in protein kinase A (PKA) on long-term potentiation (LTP) in the mossy fiber pathway and its relationship to spatial and contextual learning. Ablation by gene targeting of the C beta 1 or the RI beta isoform of PKA produces a selective defect in mossy fiber LTP, providing genetic evidence for the role of these isoforms in the mossy fiber pathway. Despite the elimination of mossy fiber LTP, the behavioral responses to novelty, spatial learning, and conditioning to context are unaffected. Thus, contrary to current theories about hippocampal function, mossy fiber LTP does not appear to be required for spatial or contextual learning. In the absence of mossy fiber LTP, adequate spatial and contextual information might reach the CA1 region via other pathways from the entorhinal cortex.

Electrophysiological effects of Mu-selective opioids on hilar neurons in the hippocampus in vivo

Although mu-selective opioids have been shown to produce dramatic effects on neurons within the CA1 and dentate regions of the rat hippocampus, little is known regarding their effects on neurons within the hilus, a region of potential importance in several disease states. We studied the neurophysiologic responses of hilar neurons recorded extracellularly to electrophoretic [D-Ala2, NMe-Phe4, Gly-ol]-enkephalin (DAMGO) and systemic morphine (MS) in anesthetized rats. We found that hilar cells could be readily divided into two categories, based on their pattern of spontaneous activity and response to perforant path stimulation. Cells that discharged in a bursting-type pattern formed a homogeneous group electrophysiologically. The response of these cells to opioids was dependent on route of administration, with the spontaneous activity of all cells tested increasing following electrophoretically administered DAMGO, and remaining unchanged in response to systemic MS. Cells that discharged in a non-bursting pattern showed some electrophysiologic variation, as well as some differential response to opioids. However, the spontaneous activity in the majority of non-bursting cells increased following electrophoretic administration of DAMGO. In these cells, MS produced similar, although usually less dramatic, effects. Comparison with intracellular data suggests that the bursting cells in our study correlate most closely with hilar "mossy cells," while the non-bursting action potentials were recorded from other cells, primarily putative interneurons. We conclude that mu-selective opioids produce excitation of mossy cells, probably through an indirect mechanism, with the primary site of action occurring on cells in the granule cell layer. This regional excitation may help to mediate the effects of locally administered mu-selective opioids within the dentate gyrus.