Autoradiographic studies of the commissural and ipsilateral association connection of the hippocampus and detentate gyrus of the rat. I. The commissural connections

A narrative review of the anatomy and function of the white matter tracts in language production and comprehension

Much is known about the role of cortical areas in language processing. The shift towards network approaches in recent years has highlighted the importance of uncovering the role of white matter in connecting these areas. However, despite a large body of research, many of these tracts' functions are not well-understood. We present a comprehensive review of the empirical evidence on the role of eight major tracts that are hypothesized to be involved in language processing (inferior longitudinal fasciculus, inferior fronto-occipital fasciculus, uncinate fasciculus, extreme capsule, middle longitudinal fasciculus, superior longitudinal fasciculus, arcuate fasciculus, and frontal aslant tract). For each tract, we hypothesize its role based on the function of the cortical regions it connects. We then evaluate these hypotheses with data from three sources: studies in neurotypical individuals, neuropsychological data, and intraoperative stimulation studies. Finally, we summarize the conclusions supported by the data and highlight the areas needing further investigation.

Mossy Cells in the Dorsal and Ventral Dentate Gyrus Differ in Their Patterns of Axonal Projections

Mossy cells (MCs) of the dentate gyrus (DG) are a major group of excitatory hilar neurons that are important for regulating activity of dentate granule cells. MCs are particularly intriguing because of their extensive longitudinal connections within the DG. It has generally been assumed that MCs in the dorsal and ventral DG have similar patterns of termination in the inner one-third of the dentate molecular layer. Here, we demonstrate that axonal projections of MCs in these two regions are considerably different. MCs in dorsal and ventral regions were labeled selectively with Cre-dependent eYFP or mCherry, using two transgenic mouse lines (including both sexes) that express Cre-recombinase in MCs. At four to six weeks following unilateral labeling of MCs in the ventral DG, a dense band of fibers was present in the inner one-fourth of the molecular layer and extended bilaterally throughout the rostral-caudal extent of the DG, replicating the expected distribution of MC axons. In contrast, following labeling of MCs in the dorsal DG, the projections were more diffusely distributed. At the level of transfection, fibers were present in the inner molecular layer, but they progressively expanded into the middle molecular layer and, most ventrally, formed a distinct band in this region. Optical stimulation of these caudal fibers expressing ChR2 demonstrated robust EPSCs in ipsilateral granule cells and enhanced the effects of perforant path stimulation in the ventral DG. These findings suggest that MCs in the dorsal and ventral DG differ in the distribution of their axonal projections and possibly their function.SIGNIFICANCE STATEMENT Mossy cells (MCs), a major cell type in the hilus of the dentate gyrus (DG), are unique in providing extensive longitudinal and commissural projections throughout the DG. Although it has been assumed that all MCs have similar patterns of termination in the inner molecular layer of the DG, we discovered that the axonal projections of dorsal and ventral MCs differ. While ventral MC projections exhibit the classical pattern, with dense innervation in the inner molecular layer, dorsal MCs have a more diffuse distribution and expand into the middle molecular layer where they overlap and interact with innervation from the perforant path. These distinct locations and patterns of axonal projections suggest that dorsal and ventral MCs may have different functional roles.

Generation of Granule Cell Dendritic Morphologies by Estimating the Spatial Heterogeneity of Dendritic Branching

Biological realism of dendritic morphologies is important for simulating electrical stimulation of brain tissue. By adding point process modeling and conditional sampling to existing generation strategies, we provide a novel means of reproducing the nuanced branching behavior that occurs in different layers of granule cell dendritic morphologies. In this study, a heterogeneous Poisson point process was used to simulate branching events. Conditional distributions were then used to select branch angles depending on the orthogonal distance to the somatic plane. The proposed method was compared to an existing generation tool and a control version of the proposed method that used a homogeneous Poisson point process. Morphologies were generated with each method and then compared to a set of digitally reconstructed neurons. The introduction of a conditionally dependent branching rate resulted in the generation of morphologies that more accurately reproduced the emergent properties of dendritic material per layer, Sholl intersections, and proximal passive current flow. Conditional dependence was critically important for the generation of realistic granule cell dendritic morphologies.

Interdependence between dorsal and ventral hippocampus during spatial navigation

INTRODUCTION

The hippocampus is linked to the formation and retrieval of episodic memories and spatial navigation. In rats, it is an elongated structure divided into dorsal (septal) and ventral (temporal) regions paralleling the respective division in the posterior and anterior hippocampus in humans. The dorsal hippocampus has been suggested to be more important for spatial processing and the ventral to processing anxiety-based behaviors. Far less is known regarding the degree to which these different regions interact during information processing. The anatomical connectivity suggests a flow of information between the dorsal and ventral regions; conversely, there are also commissural connections to the contralateral hippocampus. The current study examined the extent to which information from the dorsal hippocampus interacts with processing in the ipsilateral and contralateral ventral hippocampus following the acquisition of a spatial task.

METHODS

Rats were well-trained on a spatial reference version of the water maze, followed by muscimol inactivation of different hippocampal subregions in a within-animal repeated design. Various combinations of bilateral, ipsilateral, and contralateral infusions were used.

RESULTS

Combined dorsal and ventral inactivation produced a severe impairment in spatial performance. Inactivation of only the dorsal or ventral regions resulted in intermediate impairment with performance levels falling between controls and combined inactivation. Performance was impaired during contralateral inactivation and was almost equivalent to bilateral dorsal and ventral hippocampus inactivation, while ipsilateral inactivation resulted in little impairment.

CONCLUSIONS

Taken together, results indicate that for spatial processing, the hippocampus functions as a single integrated structure along the longitudinal axis.

Advances in understanding hilar mossy cells of the dentate gyrus

Hilar mossy cells (MCs) of the dentate gyrus (DG) distinguish the DG from other hippocampal subfields (CA1-3) because there are two glutamatergic cell types in the DG rather than one. Thus, in the DG, the main cell types include glutamatergic granule cells (GCs) and MCs, whereas in CA1-3, the only glutamatergic cell type is the pyramidal cell. In contrast to GCs, MCs are different in morphology, intrinsic electrophysiological properties, afferent input and axonal projections, so their function is likely to be very different from GCs. Why are MCs necessary to the DG? In past studies, the answer has been unclear because MCs not only excite GCs directly but also inhibit them disynaptically, by exciting GABAergic neurons that project to GCs. Results of new studies are discussed that shed light on this issue. These studies take advantage of recently available transgenic mice with Cre recombinase expression mostly in MCs and techniques such as optogenetics and DREADDs (designer receptors exclusively activated by designer drugs). The recent studies also address in vivo behavioral functions of MCs. Some of the results support past hypotheses whereas others suggest new conceptualizations of how the MCs contribute to DG circuitry and function. While substantial progess has been made, additional research is still needed to clarify the characteristics and functions of these unique cells.

Epilepsy-associated alterations in hippocampal excitability

The hippocampus exhibits a wide range of epilepsy-related abnormalities and is situated in the mesial temporal lobe, where limbic seizures begin. These abnormalities could affect membrane excitability and lead to overstimulation of neurons. Multiple overlapping processes refer to neural homeostatic responses develop in neurons that work together to restore neuronal firing rates to control levels. Nevertheless, homeostatic mechanisms are unable to restore normal neuronal excitability, and the epileptic hippocampus becomes hyperexcitable or hypoexcitable. Studies show that there is hyperexcitability even before starting recurrent spontaneous seizures, suggesting although hippocampal hyperexcitability may contribute to epileptogenesis, it alone is insufficient to produce epileptic seizures. This supports the concept that the hippocampus is not the only substrate for limbic seizure onset, and a broader hyperexcitable limbic structure may contribute to temporal lobe epilepsy (TLE) seizures. Nevertheless, seizures also occur in conditions where the hippocampus shows a hypoexcitable phenotype. Since TLE seizures most often originate in the hippocampus, it could therefore be assumed that both hippocampal hypoexcitability and hyperexcitability are undesirable states that make the epileptic hippocampal network less stable and may, under certain conditions, trigger seizures.

Critical appraisal of pathology transmission in the α-synuclein fibril model of Lewy body disorders

Lewy body disorders are characterized by the emergence of α-synucleinopathy in many parts of the central and peripheral nervous systems, including in the telencephalon. Dense α-synuclein+ pathology appears in regio inferior of the hippocampus in both Parkinson's disease and dementia with Lewy bodies and may disturb cognitive function. The preformed α-synuclein fibril model of Parkinson's disease is growing in use, given its potential for seeding the self-propagating spread of α-synucleinopathy throughout the mammalian brain. Although it is often assumed that the spread occurs through neuroanatomical connections, this is generally not examined vis-à-vis the uptake and transport of tract-tracers infused at precisely the same stereotaxic coordinates. As the neuronal connections of the hippocampus are historically well defined, we examined the first-order spread of α-synucleinopathy three months following fibril infusions centered in the mouse regio inferior (CA2+CA3), and contrasted this to retrograde and anterograde transport of the established tract-tracers FluoroGold and biotinylated dextran amines (BDA). Massive hippocampal α-synucleinopathy was insufficient to elicit memory deficits or loss of cells and synaptic markers in this model of early disease processes. However, dense α-synuclein+ inclusions in the fascia dentata were negatively correlated with memory capacity. A modest compensatory increase in synaptophysin was evident in the stratum radiatum of cornu Ammonis in fibril-infused animals, and synaptophysin expression correlated inversely with memory function in fibril but not PBS-infused mice. No changes in synapsin I/II expression were observed. The spread of α-synucleinopathy was somewhat, but not entirely consistent with FluoroGold and BDA axonal transport, suggesting that variables other than innervation density also contribute to the materialization of α-synucleinopathy. For example, layer II entorhinal neurons of the perforant pathway exhibited somal α-synuclein+ inclusions as well as retrogradely labeled FluoroGold+ somata. However, some afferent brain regions displayed dense retrograde FluoroGold label and no α-synuclein+ inclusions (e.g. medial septum/diagonal band), supporting the selective vulnerability hypothesis. The pattern of inclusions on the contralateral side was consistent with specific spread through commissural connections (e.g. stratum pyramidale of CA3), but again, not all commissural projections exhibited α-synucleinopathy (e.g. hilar mossy cells). The topographical extent of inclusions is displayed here in high-resolution images that afford viewers a rich opportunity to dissect the potential spread of pathology through neural circuitry. Finally, the results of this expository study were leveraged to highlight the challenges and limitations of working with preformed α-synuclein fibrils.

Sensorimotor Intervention Recovers Noradrenaline Content in the Dentate Gyrus of Cortical Injured Rats

Nowadays, a consensus has been reached that designates the functional and structural reorganization of synapses as the primary mechanisms underlying the process of recovery from brain injury. We have reported that pontine noradrenaline (NA) is increased in animals after cortical ablation (CA). The aim of the present study was to explore the noradrenergic and morphological response after sensorimotor intervention (SMI) in rats injured in the motor cortex. We used male Wistar adult rats allocated in four conditions: sham-operated, injured by cortical ablation, sham-operated with SMI and injured by cortical ablation with SMI. Motor and somatosensory performance was evaluated prior to and 20 days after surgery. During the intervening period, a 15-session, SMI program was implemented. Subsequently, total NA analysis in the pons and dentate gyrus (DG) was performed. All groups underwent histological analysis. Our results showed that NA content in the DG was reduced in the injured group versus control, and this reduction was reverted in the injured group that underwent SMI. Moreover, injured rats showed reduction in the number of granule cells in the DG and decreased dentate granule cell layer thickness. Notably, after SMI, the loss of granule cells was reverted. Locus coeruleus showed turgid cells in the injured rats. These results suggest that SMI elicits biochemical and structural modifications in the hippocampus that could reorganize the system and lead the recovery process, modulating structural and functional plasticity.

Interactions between Inhibitory Interneurons and Excitatory Associational Circuitry in Determining Spatio-Temporal Dynamics of Hippocampal Dentate Granule Cells: A Large-Scale Computational Study

This paper reports on findings from a million-cell granule cell model of the rat dentate gyrus that was used to explore the contributions of local interneuronal and associational circuits to network-level activity. The model contains experimentally derived morphological parameters for granule cells, which each contain approximately 200 compartments, and biophysical parameters for granule cells, basket cells, and mossy cells that were based both on electrophysiological data and previously published models. Synaptic input to cells in the model consisted of glutamatergic AMPA-like EPSPs and GABAergic-like IPSPs from excitatory and inhibitory neurons, respectively. The main source of input to the model was from layer II entorhinal cortical neurons. Network connectivity was constrained by the topography of the system, and was derived from axonal transport studies, which provided details about the spatial spread of axonal terminal fields, as well as how subregions of the medial and lateral entorhinal cortices project to subregions of the dentate gyrus. Results of this study show that strong feedback inhibition from the basket cell population can cause high-frequency rhythmicity in granule cells, while the strength of feedforward inhibition serves to scale the total amount of granule cell activity. Results furthermore show that the topography of local interneuronal circuits can have just as strong an impact on the development of spatio-temporal clusters in the granule cell population as the perforant path topography does, both sharpening existing clusters and introducing new ones with a greater spatial extent. Finally, results show that the interactions between the inhibitory and associational loops can cause high frequency oscillations that are modulated by a low-frequency oscillatory signal. These results serve to further illustrate the importance of topographical constraints on a global signal processing feature of a neural network, while also illustrating how rich spatio-temporal and oscillatory dynamics can evolve from a relatively small number of interacting local circuits.

Prenatal and postnatal development of synapses and acetylcholinesterase staining in the dentate gyrus of the rhesus monkey

Morphogenesis, distribution of cholinergic enzyme acetylcholinesterase and synaptogenesis in the dentate gyrus of the rhesus monkey during the pre- and postnatal periods of development were examined using histological, histochemical and ultrastructural methods. The pattern of neuronal differentiation in the dentate gyrus demonstrated distinct superficial-to-deep and lateral-to-medial gradients. The histochemical reaction for acetylcholinesterase was present on gestation day 120 as minimal staining in the supragranular band and in the inner one-third of the dentate molecular layer. At term, the laminar distribution of the enzyme assumed mature pattern although considerable enhancement in staining intensity was achieved postnatally. At term and at 9 months of postnatal age, the most pronounced enzyme activity was found in the supragranular band and in the inner one-third of the molecular layer. Synaptogenesis in the dentate molecular layer was characterized by the early formation of axo-dendritic contacts on dendritic trunks and branches followed by the appearance of synapses on simple and complex spines. Spines were detected infrequently on gestation day 132. On day 148, they ranged in morphology from short stubby protrusions to pedunculated, triangular processes. The majority of the spines exhibited flat postsynaptic surfaces. Complex, synapse-bearing U- and W-shaped spines were observed rarely at this age but appeared more frequently at term and at 15 months of postnatal age. However, at all ages, including 15 months postnatally, synapses on flat-surfaced simple spines predominated. Most synapses were of the asymmetric variety. With certain exceptions, these features of development of the rhesus dentate gyrus resemble the reported patterns of postnatal ontogenesis of this structure in the rat. However, the ingrowth of cholinergic afferents and the major modifications in synapse structure occur prenatally in the rhesus monkey during the second half of the gestation period. This temporal difference between the two species should receive consideration in the planning of neuroplasticity experiments designed to explore lesion-induced adaptations in afferent growth and synaptogenesis in the rhesus dentate gyrus.

Structural plasticity in the dentate gyrus- revisiting a classic injury model

The adult brain is in a continuous state of remodeling. This is nowhere more true than in the dentate gyrus, where competing forces such as neurodegeneration and neurogenesis dynamically modify neuronal connectivity, and can occur simultaneously. This plasticity of the adult nervous system is particularly important in the context of traumatic brain injury or deafferentation. In this review, we summarize a classic injury model, lesioning of the perforant path, which removes the main extrahippocampal input to the dentate gyrus. Early studies revealed that in response to deafferentation, axons of remaining fiber systems and dendrites of mature granule cells undergo lamina-specific changes, providing one of the first examples of structural plasticity in the adult brain. Given the increasing role of adult-generated new neurons in the function of the dentate gyrus, we also compare the response of newborn and mature granule cells following lesioning of the perforant path. These studies provide insights not only to plasticity in the dentate gyrus, but also to the response of neural circuits to brain injury.

Low frequency stimulation of ventral hippocampal commissures reduces seizures in a rat model of chronic temporal lobe epilepsy

PURPOSE

To investigate the effects of low frequency stimulation (LFS) of a fiber tract for the suppression of spontaneous seizures in a rat model of human temporal lobe epilepsy.

METHODS

Stimulation electrodes were implanted into the ventral hippocampal commissure (VHC) in a rat post-status epilepticus (SE) model of human temporal lobe epilepsy (n = 7). Two recording electrodes were placed in the CA3 regions bilaterally and neural data were recorded for a minimum of 6 weeks. LFS (60 min train of 1 Hz biphasic square wave pulses, each 0.1 ms in duration and 200 μA in amplitude, followed by 15 min of rest) was applied to the VHC for 2 weeks, 24 h a day.

KEY FINDINGS

The baseline mean seizure frequency of the study animals was 3.7 seizures per day. The seizures were significantly reduced by the application of LFS in every animal (n = 7). By the end of the 2-week period of stimulation, there was a significant, 90% (<1 seizure/day) reduction of seizure frequencies (p < 0.05) and a 57% reduction during the period following LFS (p < 0.05) when compared to baseline. LFS also resulted in a significant reduction of hippocampal interictal spike frequency (71%, p < 0.05), during 2 weeks of LFS session. The hippocampal histologic analysis showed no significant difference between rats that received LFS and SE induction and those that had received only SE-induction. None of the animals showed any symptomatic hemorrhage, infection, or complication.

SIGNIFICANCE

Low frequency stimulation applied at a frequency of 1 Hz significantly reduced both the excitability of the neural tissue as well as the seizure frequency in a rat model of human temporal lobe epilepsy. The results support the hypothesis that LFS of fiber tracts can be an effective method for the suppression of spontaneous seizures in a temporal lobe model of epilepsy in rats and could lead to the development of a new therapeutic modality for human patients with temporal lobe epilepsy.

Low frequency stimulation decreases seizure activity in a mutation model of epilepsy

PURPOSE

To investigate brain electrical activity in Q54 mice that display spontaneous seizures because of a gain-of-function mutation of the Scn2a sodium channel gene, and to evaluate the efficacy of low frequency deep brain stimulation (DBS) for seizure frequency reduction.

METHODS

Electroencephalography (EEG), electromyography (EMG), and hippocampal deep electrodes were implanted into Q54 mice expressing an epileptic phenotype (n = 6). Chronic six channel recordings (wideband, 0.1-300 Hz) were stored 24 h a day for more than 12 days. Low frequency stimulation (LFS) (3 Hz, square wave, biphasic, 100 μs, 400 μA) was applied to the ventral hippocampal commissure (VHC) in alternating 5 min cycles (on or off) 24 h a day for a period of 4 days.

RESULTS

LFS (3 Hz) resulted in a significant reduction in seizure frequency and duration (21% and 35%, p < 0.05), when applied to the VHC of epileptic Q54 mice (n = 6). Seizure frequency was not directly affected by stimulation state ("on" vs. "off").

CONCLUSION

LFS applied at a frequency of 3 Hz significantly reduced seizure frequency and duration in the Q54 model. Furthermore, the reduction of seizure frequency and duration by LFS was not immediate but had a delayed and lasting effect, supporting complex, indirect mechanisms of action.

Dynamic gene and protein expression patterns of the autism-associated met receptor tyrosine kinase in the developing mouse forebrain

The establishment of appropriate neural circuitry depends on the coordination of multiple developmental events across space and time. These events include proliferation, migration, differentiation, and survival-all of which can be mediated by hepatocyte growth factor (HGF) signaling through the Met receptor tyrosine kinase. We previously found a functional promoter variant of the MET gene to be associated with autism spectrum disorder, suggesting that forebrain circuits governing social and emotional function may be especially vulnerable to developmental disruptions in HGF/Met signaling. However, little is known about the spatiotemporal distribution of Met expression in the forebrain during the development of such circuits. To advance our understanding of the neurodevelopmental influences of Met activation, we employed complementary Western blotting, in situ hybridization, and immunohistochemistry to comprehensively map Met transcript and protein expression throughout perinatal and postnatal development of the mouse forebrain. Our studies reveal complex and dynamic spatiotemporal patterns of expression during this period. Spatially, Met transcript is localized primarily to specific populations of projection neurons within the neocortex and in structures of the limbic system, including the amygdala, hippocampus, and septum. Met protein appears to be principally located in axon tracts. Temporally, peak expression of transcript and protein occurs during the second postnatal week. This period is characterized by extensive neurite outgrowth and synaptogenesis, supporting a role for the receptor in these processes. Collectively, these data suggest that Met signaling may be necessary for the appropriate wiring of forebrain circuits, with particular relevance to the social and emotional dimensions of behavior.

Localization of putative transmitters in the hippocampal formation: with a note on the connections to septum and hypothalamus

Biochemical assays on microdissected samples, denervation studies, subcellular fractionation, and light and electron microscopic autoradiography of high affinity uptake have been performed to study the cellular localization of transmitter candidates in the rat hippocampal formation. High affinity uptake of glutamate and aspartate is localized in the terminals of several excitatory systems, such as the entorhino-dentate fibres (perforant path), mossy fibres (from granular cells) and pyramidal cell axons. Thus, in stratum radiatum and oriens of CA1, 85% of glutamate and asparate uptake and 40% of glutamate and aspartate content are lost after lesions of ipsilateral plus commissural fibres from CA3/CA4. Hippocampal efferents also take up aspartate and glutamate, since these activities are heavily reduced in the lateral septum and mamillary bodies after transection of fimbria and the dorsal fornix. The synthesis (by glutamic acid decarboxylase), content and high affinity uptake of gamma-aminobutyrate (GABA) are not reduced after lesions of these or other projection fibre systems. A localization in intrinsic neurons is confirmed by a selective loss of glutamic acid decarboxylase after local injections of kainic acid. Peak concentrations of the enzyme occur near the pyramidal and granular cell bodies, corresponding to the site of the inhibitory basket cell terminals, and in the outer parts of the molecular layers. Some 85% of glutamic acid decarboxylase is situated in 'nerve ending particles'. Acetylcholine synthesis (by choline acetyltransferase) disappears after lesions of septo-hippocampal fibres. Since 80% of the hippocampal choline acetyltransferase is in 'nerve ending particles', the characteristic topographical distribution of this enzyme should reflect the distribution of cholinergic septo-hippocampal afferents. Serotonin, noradrenaline, dopamine and histamine are located/synthesized in afferent fibre systems. Some monoamine-containing afferents to the hippocampal formation pass via the septal area, others via the amygdala. The hippocampal formation also contains nerve elements reacting with antibodies against neuroactive peptides, such as enkephalin, substance P, somatostatin and gastrin/cholecystokinin.

In vivo roles for matrix metalloproteinase-9 in mature hippocampal synaptic physiology and plasticity

Extracellular proteolysis is an important regulatory nexus for coordinating synaptic functional and structural plasticity, but the identity of such proteases is incompletely understood. Matrix metalloproteinases (MMPs) have well-known, mostly deleterious roles in remodeling after injury or stroke, but their role in nonpathological synaptic plasticity and function in intact adult brains has not been extensively investigated. Here we address the role of MMP-9 in hippocampal synaptic plasticity using both gain- and loss-of-function approaches in urethane-anesthetized adult rats. Acute blockade of MMP-9 proteolytic activity with inhibitors or neutralizing antibodies impairs maintenance, but not induction, of long-term potentiation (LTP) at synapses formed between Schaffer-collaterals and area CA1 dendrites. LTP is associated with significant increases in levels of MMP-9 and proteolytic activity within the potentiated neuropil. By introducing a novel application of gelatin-substrate zymography in vivo, we find that LTP is associated with significantly elevated numbers of gelatinolytic puncta in the potentiated neuropil that codistribute with immunolabeling for MMP-9 and for markers of synapses and dendrites. Such increases in proteolytic activity require NMDA receptor activation. Exposing intact area CA1 neurons to recombinant-active MMP-9 induces a slow synaptic potentiation that mutually occludes, and is occluded by, tetanically evoked potentiation. Taken together, our data reveal novel roles for MMP-mediated proteolysis in regulating nonpathological synaptic function and plasticity in mature hippocampus.

Extrinsic afferent systems to the dentate gyrus

The dentate gyrus is the first stage of the intrahippocampal, excitatory, trisynaptic loop, and a primary target of the majority of entorhinal afferents that terminate in a laminar fashion on granule cell dendrites and carry sensory information of multiple modalities about the external world. The electric activity of the trisynaptic pathway is controlled mainly by different types of local, GABAergic interneurons, and subcortical and commissural afferents. In this chapter we will outline the origin and postsynaptic targets in the dentate gyrus of chemically identified subcortical inputs. These systems are afferents originating from the medial septum/diagonal band of Broca GABAergic and cholinergic neurons, neurochemically distinct types of neurons located in the supramammillary area, serotonergic fibers from the median raphe, noradrenergic afferents from the pontine nucleus, locus ceruleus, dopamine axons originating in the ventral tegmental area, and the commissural projection system. Because of the physiological implications, these afferents are discussed in the context of the glutamatergic innervation of the dentate gyrus. One common feature of the extrinsic dentate afferent systems is that they originate from a relatively small number of neurons. However, the majority of these afferents are able to exert a powerful control over the electrical activity of the hippocampus. This strong influence is due to the fact that the majority of the extrinsic afferents terminate on a relatively small, but specific, populations of neurons that are able to control large areas of the hippocampal formation.

Spatial learning with unilateral and bilateral hippocampal networks

This study investigated the ability of animals to learn both reference memory and delayed matching-to-place variants of the watermaze after large lesions of the hippocampus that deliberately spared only small remnants of the structure. Groups were created that had differing blocks of residual tissue in the septal pole of the hippocampus (15% or 30% of total volume), located either unilaterally (30 or 50% on one side, 0% on the other) or bilaterally (30 + 30%). These groups were capable of learning the reference memory task, as indexed by normal spatially focused searching in a probe trial, but their rate of learning was slower than that of sham-lesioned rats. An impairment in the rate of learning was also seen in the delayed match-to-place task, where one-trial memory was observed only at the shortest (5 s) intertrial interval in the lesioned groups with the largest sparing. In both tasks performance was proportional to the volume of hippocampus spared and independent of whether this was unilaterally or bilaterally located. The findings are compatible with distributed processing accounts of hippocampal memory storage.

Polysynaptic olfactory pathway to the ipsi- and contralateral entorhinal cortex mediated via the hippocampus

Interactions between olfactory cortices and the hippocampus support sensory discrimination and spatial learning functions. The olfactory input accesses the hippocampal formation via a polysynaptic pathway mediated by the lateral and rostral entorhinal cortex (EC). We recently demonstrated that following repetitive stimulation of the lateral olfactory tract (LOT) at 2-8 Hz, a delayed response (onset at circa 60 ms) was evoked in the caudal portion of the EC, identified as medial EC, that does not receive a direct olfactory input. By performing simultaneous laminar profile analysis in the EC and in different hippocampal subfields, we conclusively demonstrate that the delayed EC response evoked by repetitive ipsilateral LOT stimulation is headed by the sequential activation of the dentate gyrus and the CA3/CA1 subfields in the septal and temporal hippocampus. Repetitive stimulation of the contralateral LOT also induced an EC response that peaked at 76.28+/-2.42 ms (n=15). Current source density analysis and time-delay analysis of simultaneous field potential laminar profiles performed from the EC and from DG, CA3 and CA1 hippocampal subfields suggested that the contralateral EC response is mainly carried by an intrahippocampal CA3-CA3 commissural pathway. Contralateral LOT stimulation also induced a later EC component (delay >100 ms) generated in the superficial layers, mediated either by local associative interactions or by extrahippocampal circuits. The opportunity to activate the ipsi- and contralateral olfactory pathways in the same experiment and to record field potentials profiles simultaneously in different structures of both hemispheres in the isolated guinea-pig brain confirms that this preparation is unique and is particularly suitable for investigating the system physiology of the limbic region. The present study demonstrates that patterned stimulation of the olfactory input that mimics sniffing patterns during odor discrimination induces a diffuse activation of both ipsi- and contralateral hippocampi and ECs. The findings contribute to the understanding the physiological mechanisms that underlie associative interactions between olfactory and non-olfactory cortical inputs converging into the mesial temporal region.

Topographic specificity of functional connections from hippocampal CA3 to CA1

The hippocampus is a cortical region thought to play an important role in learning and memory. Most of our knowledge about the detailed organization of hippocampal circuitry responsible for these functions is derived from anatomical studies. These studies present an incomplete picture, however, because the functional character and importance of connections are often not revealed by anatomy. Here, we used a physiological method (photostimulation with caged glutamate) to probe the fine pattern of functional connectivity between the CA3 and CA1 subfields in the mouse hippocampal slice preparation. We recorded intracellularly from CA1 and CA3 pyramidal neurons while scanning with photostimulation across the entire CA3 subfield with high spatial resolution. Our results show that, at a given septotemporal level, nearby CA1 neurons receive synaptic inputs from neighboring CA3 neurons. Thus, the CA3 to CA1 mapping preserves neighbor relations.

Comparison of commissural sprouting in the mouse and rat fascia dentata after entorhinal cortex lesion

Reactive axonal sprouting occurs in the fascia dentata after entorhinal cortex lesion. This sprouting process has been described extensively in the rat, and plasticity-associated molecules have been identified that might be involved in its regulation. To demonstrate causal relationships between these candidate molecules and the axonal reorganization process, it is reasonable to analyze knockout and transgenic animals after entorhinal cortex lesion, and because gene knockouts are primarily generated in mice, it is necessary to characterize the sprouting response after entorhinal cortex lesion in this species. In the present study, Phaseolus vulgaris-leucoagglutinin (PHAL) tracing was used to analyze the commissural projection to the inner molecular layer in mice with longstanding entorhinal lesions. Because the commissural projection to the fascia dentata is neurochemically heterogeneous, PHAL tracing was combined with immunocytochemistry for calretinin, a marker for commissural/associational mossy cell axons. Using both techniques singly as well as in combination (double-immunofluorescence) at the light or electron microscopic level, it could be shown that in response to entorhinal lesion mossy cell axons leave the main commissural fiber plexus, invade the denervated middle molecular layer, and form asymmetric synapses within the denervated zone. Thus, the commissural sprouting response in mice has a considerable translaminar component. This is in contrast to the layer-specific commissural sprouting observed in rats, in which the overwhelming majority of mossy cell axons remain within their home territory. These data demonstrate an important species difference in the commissural/associational sprouting response between rats and mice that needs to be taken into account in future studies.

Distribution of a lysosomal enzyme in the adult brain by axonal transport and by cells of the rostral migratory stream

A portion of the lysosomal enzymes produced by cells is secreted, diffuses through extracellular spaces, and can be taken up by distal cells via mannose-6-phosphate receptor-mediated endocytosis. This provides the basis for treating lysosomal storage diseases, many of which affect the CNS. Normal enzyme secreted from a cluster of genetically corrected cells has been shown to reverse storage lesions in a zone of surrounding brain tissue in mouse disease models. However, low levels of enzyme activity and reduction of storage lesions also have been observed at sites in the brain that may not be explained by a contiguous gradient of secreted enzyme diffusing away from the genetically corrected cells. No direct evidence for alternative mechanisms of enzyme transport has been shown, and little is understood about the intracellular movement of lysosomal enzymes in neurons. We investigated whether axonal transport could occur, by expressing an eukaryotic lysosomal enzyme that can be visualized in tissue sections (beta-glucuronidase) in brain structures that have defined axonal connections to other structures. This resulted in the transfer of enzyme to, and a reversal of storage lesions in, neurons that project to the gene expression site, but not in nearby structures that would have been corrected if the effect had been mediated by diffusion. In addition, transduction of cells in the subventricular zone resulted in the uptake of beta-glucuronidase by cells entering the rostral migratory stream. Gene transfer to specific neuronal circuits or cells in migratory pathways may facilitate delivery to the global brain lesions found in these disorders.

Commissurally projecting inhibitory interneurons of the rat hippocampal dentate gyrus: a colocalization study of neuronal markers and the retrograde tracer Fluoro-gold

Improved methods for detecting neuronal markers and the retrograde tracer Fluoro-Gold (FG) were used to identify commissurally projecting neurons of the rat hippocampus. In addition to the dentate hilar mossy cells and CA3 pyramidal cells shown previously to transport retrograde tracers after injection into the dorsal hippocampus, FG-positive interneurons of the dentate granule cell layer and hilus were detected in numbers greater than previously reported. FG labeling of interneurons was variable among animals, but was as high as 96% of hilar somatostatin-positive interneurons, 84% of parvalbumin-positive cells of the granule cell layer and hilus combined, and 33% of hilar calretinin-positive cells. By comparison, interneurons of the dentate molecular layer and all hippocampal subregions were conspicuously FG-negative. Whereas hilar mossy cells and CA3 pyramidal cells were FG-labeled throughout the longitudinal axis, FG-positive interneurons exhibited a relatively homotopic distribution. "Control" injections of FG into the neocortex, septum, and ventral hippocampus demonstrated that the homotopic labeling of dentate interneurons was injection site-specific, and that the CA1-CA3 interneurons unlabeled by contralateral hippocampal FG injection were nonetheless able to transport FG from the septum. These data suggest a hippocampal organizing principle according to which virtually all commissurally projecting hippocampal neurons share the property of being monosynaptic targets of dentate granule cells. Because granule cells innervate their exclusively ipsilateral target cells in a highly lamellar pattern, these results suggest that focal granule cell excitation may result in commissural inhibition of the corresponding "twin" granule cell lamella, thereby lateralizing and amplifying the influence of the initiating discharge.

Retrograde amnesia and consolidation: anatomical and lesion considerations

The four papers in this issue of Hippocampus dealing with retrograde amnesia, together with relevant animal studies in the literature, are reviewed from the perspective of the anatomical location of the lesion and extent of damage to the brain. In order to evaluate the underlying damage in these and related prospective experimental studies, it is necessary to consider both the lesion techniques that were used as well as the care with which the resulting damage was determined. Both temporally graded and flat, ungraded retrograde amnesia have been reported, as well a lack of effects, following damage to structures in the medial temporal area. Most research has centered around damage to the hippocampus, but differences in selectivity of the lesions and behavioral testing procedures preclude any definite conclusions regarding the precise nature of the involvement of this structure. With a greater appreciation for the importance of the locus and extent of the damage, together with the kind of information being processed, it should be possible to obtain a better understanding of the neural substrates underlying retrograde amnesia.

Physiology of the entorhinal and perirhinal projections to the hippocampus studied by current source density analysis

Evoked field potentials and current-source-density analysis were used to study the olfactory, entorhinal, and perirhinal projections to the hippocampus. In urethane-anesthetized rats, various structures were electrically stimulated, and evoked potentials were mapped using glass micropipettes or multichannel silicon probes. Stimulation of the olfactory bulb, lateral olfactory tract, piriform cortex, amygdala-entorhinal transition, lateral entorhinal cortex, or lateral perforant path (LPP) evoked an outer molecular layer sink (inferred distal dendritic excitation) in the dentate gyrus, with progressively decreasing onset latency. Medial perforant path (MPP) stimulation evoked a middle molecular layer sink (mid-dendritic excitation) in the dentate gyrus. LPP and MPP were also inferred to monosynaptically excite the distal dendrites of CA3, often resulting in a population spike in CA3. CA3 spiking, in turn, was often followed by excitation at the inner molecular layer of the dentate gyrus. LPP and MPP evoked distal dendritic sinks but no population spikes in CA1. Stimulation of the perirhinal cortex activated a sink in the subiculum/CA1 border without activating the dentate gyrus. In addition, reverberatory activity through a hippocampal-entorhinal-hippocampal pathway may be activated by MPP or CA3 stimulation. It is suggested that the parallel projections of the entorhinal and perirhinal inputs to the distal dendrites of hippocampal principal neurons enhance local and distributed processing as characterized by CA3 to dentate gyrus feedback, and hippocampal-entorhinal reverberation.

Disconnecting hippocampal projections to the anterior thalamus produces deficits on tests of spatial memory in rats

A disconnection procedure was used to test whether projections from the hippocampus to the anterior thalamic nuclei (AT), via the fimbria-fornix (FX), form functional components of a spatial memory system. The behavioural effects of combined unilateral lesions in the AT and FX were compared when they were either in contralateral hemispheres (AT-FX Contra) or the same hemisphere (AT-FX Ipsi). Other groups received bilateral FX lesions and Sham surgeries. Expt 1 demonstrated that none of these lesions affected performance of an object recognition task, while performance of an object location task, which tests the subjects' preference for an object that has changed location, was impaired in the AT-FX Contra and FX groups. In a T-maze alternation task, however, the FX group was severely impaired while both the AT-FX Ipsi and AT-FX Contra lesion groups showed only a mild impairment. In order to test whether spared crossed projections might support spatial performance in the AT-FX Contra group we then examined the effects of a combined AT-FX Contra lesion coupled with transection of the hippocampal commissure. This combination of lesions produced a severe disruption in spatial memory performance in the water maze, radial arm maze and T-maze, which was significantly greater than that produced by ipsilateral and contralateral AT-FX lesions alone. These results support the notion that disconnection of the AT from their hippocampal inputs produces impairments on a range of spatial memory tasks, but indicate that there are an array of different routes that can subserve this function.

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

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

Molecular mechanisms that underlie structural and functional changes at the postsynaptic membrane during synaptic plasticity

The synaptic plasticity that is addressed in this review follows neurodegeneration in the brain and thus has both structural as well as functional components. The model of neurodegeneration that has been selected is the kainic acid lesioned hippocampus. Degeneration of the CA3 pyramidal cells results in a loss of the Schaffer collateral afferents innervating the CA1 pyramidal cells. This is followed by a period of structural plasticity where new synapses are formed. These are associated with changes in the numbers and shapes of spines as well as changes in the morphometry of the dendrites. It is suggested that this synaptogenesis is responsible for an increase in the ratio of NMDA to AMPA receptors mediating excitatory synaptic transmission at these synapses. Changes in the temporal and spatial properties of these synapses resulted in an altered balance between LTP and LTD. These properties together with a reduction in the inhibitory drive increased the excitability of the surviving CA1 pyramidal cells which in turn triggered epileptiform bursting activity. In this review we discuss the insights that may be gained from studies of the underlying molecular machinery. Developments in one of the collections of the cogs in this machinery has been summarized through recent studies characterizing the roles of neural recognition molecules in synaptic plasticity in the adult nervous systems of vertebrates and invertebrates. Such investigations of neural cell adhesion molecules, cadherins and amyloid precursor protein have shown the involvement of these molecules on the morphogenetic level of synaptic changes, on the one hand, and signal transduction effects, on the other. Further complex cogs are found in the forms of the low-density lipoprotein receptor (LDL-R) family of genes and their ligands play pivotal roles in the brain development and in regulating the growth and remodelling of neurones. Evidence is discussed for their role in the maintenance of cognitive function as well as Alzheimer's. The molecular mechanisms responsible for the clustering and maintenance of transmitter receptors at postsynaptic sites are the final cogs in the machinery that we have reviewed. Postsynaptic densities (PSD) from excitatory synapses have yielded many cytoskeletal proteins including actin, spectrin, tubulin, microtubule-associated proteins and calcium/calmodulin-dependent protein kinase II. Isolated PSDs have also been shown to be enriched in AMPA, kainate and NMDA receptors. However, recently, a new family of proteins, the MAGUKs (for membrane-associated guanylate kinase) has emerged. The role of these proteins in clustering different NMDA receptor subunits is discussed. The MAGUK proteins are also thought to play a role in synaptic plasticity mediated by nitric oxide (NO). Both NMDA and non-NMDA receptors are highly clustered at excitatory postsynaptic sites in cortical and hippocampal neurones but have revealed differences in their choice of molecular components. Both GABAA and glycine (Gly) receptors mediate synaptic inhibition in the brain and spinal cord. Whilst little is known about how GABAA receptors are localized in the postsynaptic membrane, considerable progress has been made towards the elucidation of the molecular mechanisms underlying the formation of Gly receptors. It has been shown that the peripheral membrane protein gephyrin plays a pivotal role in the formation of Gly receptor clusters most likely by anchoring the receptor to the subsynaptic cytoskeleton. Evidence for the distribution as well as function of gephyrin and Gly receptors is discussed. Postsynaptic membrane specializations are complex molecular machinery subserving a multitude of functions in the proper communication between neurones. Despite the fact that only a few key players have been identified it will be a fascinating to watch the story as to how they contribute to structural and functional plasticity unfold.

Lesion-induced plasticity of central neurons: sprouting of single fibres in the rat hippocampus after unilateral entorhinal cortex lesion

In response to a central nervous system trauma surviving neurons reorganize their connections and form new synapses that replace those lost by the lesion. A well established in vivo system for the analysis of this lesion-induced plasticity is the reorganization of the fascia dentata following unilateral entorhinal cortex lesions in rats. After general considerations of neuronal reorganization following a central nervous system trauma, this review focuses on the sprouting of single fibres in the rat hippocampus after entorhinal lesion and the molecular factors which may regulate this process. First, the connectivity of the fascia dentata in control animals is reviewed and previously unknown commissural fibers to the outer molecular layer and entorhinal fibres to the inner molecular layer are characterized. Second, sprouting of commissural and crossed entorhinal fibres after entorhinal cortex lesion is described. Single fibres sprout by forming additional collaterals, axonal extensions, boutons, and tangle-like axon formations. It is pointed out that the sprouting after entorhinal lesion mainly involves unlesioned fibre systems terminating within the layer of fibre degeneration and is therefore layer-specific. Third, molecular changes associated with axonal growth and synapse formation are considered. In this context, the role of adhesion molecules, glial cells, and neurotrophic factors for the sprouting process are discussed. Finally, an involvement of sprouting processes in the formation of neuritic plaques in Alzheimer's disease is reviewed and discussed with regard to the axonal tangle-like formations observed after entorhinal cortex lesion.

Evidence for site selection during synaptogenesis: the surface distribution of synaptic sites in photoreceptor terminals of the files Musca and Drosophila

1. Photoreceptor terminals in the flies Musca domestica and Drosophila melanogaster have been reconstructed in three dimensions from serial EM to reveal the surface distributions of afferent tetrad synapses. 2. The terminals are cylindrical and surround two target cells; they have synaptic sites distributed along their length and around their circumference, except for a strip along the face that lies furthest away from the target cells. 3. Over their inner faces, the terminals have presynaptic sites that are distributed evenly. 4. The distribution of sites in maps plotted from reconstructed membrane surfaces was examined by quadrat analyses. The frequency of sites per quadrat division was not Poissonian, i.e. was non-random. Thus, some form of site selection must exist during synaptogenesis. 5. The sites were shown by variance ratio analysis to be regular (evenly dispersed, not clustered). This suggests that some form of interaction exists, so as to reduce the probability that a synapse will form close to an already existing synaptic site. 6. Distances between nearest-neighbour pairs of synapses had a closest minimum spacing of about 0.8 micron in Musca that was violated by about 5% of pairs, whereas the corresponding distances were about 0.2 micron shorter in Drosophila, which had 13% of pairs situated closer together than 0.8 micron. 7. During synaptogenesis, either initially in the pupa or later in the adult, the probability that a synapse will form is therefore effectively zero within these distances from an existing synaptic site, perhaps through an inhibitory influence exerted by the latter. The nearest-neighbour distances are normally distributed. 8. Unlike the distribution of presynaptic sites, the distribution of postsynaptic sites over the surfaces of the dendrites of the target cells is not even. Although not studied in detail, the corresponding nearest-neighbour distances are much smaller, as little as 0.1 micron. Thus the wider spacing seen between sites over the receptor terminals is a function of the presynaptic cells, and not of their postsynaptic partners, and implies the existence of interactions between synaptic sites.

Glucocorticoids and the hippocampus. Developmental interactions facilitating the expression of behavioral inhibition

When threatened, the rapid induction of fear and anxiety responses is adaptive. This article summarizes the current knowledge of the neurobiological development of behavioral inhibition, a prominent response occurring in fear and anxiety-provoking situations. In the rat, behavioral inhibition as exemplified by freezing first appears near the end of the second postnatal week. This emergence of freezing coincides with the developmental period marked by the rapid increase in plasma concentrations of glucocorticoids. Studies show that removal of glucocorticoids at this time severely impairs the age-dependent appearance of freezing. This behavioral impairment produced by adrenalectomy, however, is prevented by exogenous glucocorticoid administration. The effectiveness of glucocorticoids in facilitating the development of freezing appears to be caused by its actions in the hippocampus. In particular, glucocorticoids appear to play a vital role in the postnatal cellular development of the hippocampal dentate gyrus. Doses of glucocorticoids shown to reverse the behavioral inhibitory deficits occurring after adrenalectomy are ineffective when hippocampal dentate granule neurons are destroyed by neurotoxins. Notably, site-specific administration of glucocorticoids to the dorsal hippocampus is successful in promoting the occurrence of freezing in the adrenalectomized rat pup. It is hypothesized that glucocorticoids exert their behavioral inhibitory effects by influencing the development of the septohippocampal cholinergic system. Support for this hypothesis is derived from work demonstrating the importance of glucocorticoids on nerve growth factor systems that play a critical role in septohippocampal cholinergic survival.

Heterogeneity of the commissural projection to the rat dentate gyrus: a Phaseolus vulgaris leucoagglutinin tracing study

The commissural and associational projections to the rat dentate gyrus are believed to be anatomically homologous fiber systems. They are often referred to as the so-called commissural/ associational system of the dentate gyrus. However, whereas characteristic laminar termination patterns within the molecular layer of the dentate gyrus have been described for the different cells of origin of the associational projection, the axons of the different cell types of commissural neurons have long been believed to terminate exclusively within the inner molecular layer. Only recently, a previously unknown commissural projection to the outer molecular layer of the dentate gyrus was described and the question was raised whether the commissural fibers could exhibit a heterogeneity similar to that of the associational projections. Using the anterograde tracer Phaseolus vulgaris leucoagglutinin, which labels individual axons and their collaterals, we have studied the termination pattern of commissural axons in the dentate gyrus of the septal hippocampus. At least four different commissural fiber types could be revealed on the basis of their laminar termination pattern: fibers to the inner molecular layer (type 1), fibers to the outer molecular layer (type 2), fibers terminating throughout the molecular layer (type 3), and fibers terminating in both the granule cell layer and the molecular layer (type 4). These observations demonstrate a previously underestimated heterogeneity of the commissural projection. In addition, there is a great deal of parallelism between the different commissural and associational fibers, pointing to a coordinated action of the two systems in the two hippocampi.

A novel entorhinal projection to the rat dentate gyrus: direct innervation of proximal dendrites and cell bodies of granule cells and GABAergic neurons

Entorhinal fibers to the fascia dentata originating from layer II stellate neurons are known to terminate exclusively in the outer two thirds of the molecular layer, where they innervate distal dendritic portions of dentate neurons. Using anterograde tracing with Phaseolus vulgaris leucoagglutinin, we unraveled a previously unknown entorhinal projection that directly innervates proximal dendritic portions and somata of granule cells and GABAergic neurons. This projection originates from neurons located in entorhinal layers IV-VI of the medial entorhinal area. These fibers enter the outer two thirds of the molecular layer, traverse the inner molecular layer (IML) and granule cell layer, where they form numerous boutons, and finally arborize subjacent to the granule cells. Correlated light and electron microscopy revealed that the boutons formed by these fibers establish asymmetric synapses on dendrites in the IML, on spines and somata of granule cells, and on spineless dendrites subjacent to the granule cell layer. Postembedding immunogold staining indicates that this entorhino-dentate projection is not GABAergic and that it also terminates on GABAergic inhibitory neurons. These data demonstrate that not all entorhino-dentate fibers display a similar high laminar specificity for the outer molecular layer (OML). Although fibers from the superficial layers of the entorhinal cortex terminate exclusively in the OML, entorhinal fibers arising from deeper layers are not confined to laminar boundaries. Finally, the possibility that these supposedly excitatory entorhinal afferents may monosynaptically activate proximal dendrites and somata of dentate neurons needs to be incorporated into contemporary concepts of the hippocampal network.

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.

Induction of heat-shock protein-70 messenger RNA and protein following systemic kainate injection in the rat: evidence of protein axonal transport

Induction of heat-shock protein-70 was studied using in situ hybridization and immunohistochemistry in the adult rat at different times following intraperitoneal injection of kainate. Marked expression of heat-shock protein-70 messenger RNA was observed during the first 24 h, followed by residual signal at 48 h. Inducible heat-shock protein-70 protein was found in sensitive areas not subjected to early cell death, including the lateral, dorsal and cingular cortices, lateral amygdala, thalamus and hippocampus at 12, 24 and 48 h after injection. In the hippocampus, inducible heat-shock protein-70 immunoreactivity was contained in the soma and proximal dendritic region of neurons of CA3, hilus and CA1 at 12 h, and within the entire dendritic arbor at 24 h. Heat-shock protein-70 immunoreactivity decreased in the cell bodies at four days, but delicate immunostaining appeared in the dorsal fornix, fimbria, and ventral and dorsal hippocampal commissures, as well as in the strata oriens and radiatum of CA3, and part of the stratum radiatum of CA1. Inducible heat-shock protein-70 immunoreactivity at day 7 was mainly localized in the strata oriens and radiatum of CA1 and CA3, and inner one-third of the molecular layer of the dentate gyrus, in which the ipsilateral and commissural hippocampal pathways terminate. These findings show that, in the hippocampus, inducible heat-shock protein-70 is synthesized in the cytoplasm of neurons and subsequently transported at slow rates (about 2-5 mm/day) through the axons to appropriate terminals in the ipsilateral and contralateral hippocampus. A similar pattern is observed for sensitive neurons (heat-shock protein-70 immunoreactive) in the neocortex and thalamus, and labelling of corticocortical, corticostriatal and intrathalamic (between the dorsal and the reticular nuclei) fibres. Since inducible heat-shock protein-70 keeps native proteins unfolded to prevent abnormal configuration following diverse insults, heat-shock protein-70 is proposed as a marker of transient impaired assembly of native proteins in sensitive neurons and axons following intraperitoneal kainate injection.

What does the hippocampus really do?

Much of the evidence used to implicate the hippocampus in learning and memory has been obtained from clinical cases and/or experimental studies with animals where the damage is extensive and includes more than just the hippocampus. When the damage is limited to the cells that comprise the hippocampus (CA1-CA3 pyramidal cells, hilar and granule cells in the dentate gyrus) the effect on behavior in the rat is more limited than what is usually reported. Selective, axon-sparing ibotenic acid lesions of the hippocampus were used in the experiments that are reviewed to study the effects of removing the hippocampus on: (1) the acquisition of spatial and non-spatial information; (2) complex, non-spatial representational learning; and (3) acquisition and utilization of contextual information. The results indicated that rats with the hippocampus removed were impaired on those tasks that require the utilization of spatial and contextual information but performed like controls in learning about and handling (even complex) non-spatial information. Future research utilizing selective lesions of the hippocampus and sensitive behavioral testing techniques should help clarify the extent to which the impairments in the acquisition of spatial information and the ability to utilize contextual, background cues can be reduced to a single, underlying learning process.

Behaviour of small inhibitory interneurons in early postnatal mouse cerebellar microexplant cultures: a video time-lapse analysis

The aim of this work was to investigate how the environment of the neuropil determines the positioning and differentiation of neurons that are postsynaptic to them. We investigated how stellate and basket cells, the small inhibitory interneurons of the cerebellar cortex, find their perpendicular orientation to the direction of fasciculated granule cell axons. Cultures of early postnatal mouse cerebellar microexplants showing this cellular behaviour in vitro were analysed by video time-lapse cinematography and evaluated by morphometry. The small interneurons were first detectable when they migrated, intermingled with granule cells, away from the explant along the radial fascicles of granule cell neurites. During migration some cells suddenly changed their orientation by extending neurites in perpendicular orientation to the radial fascicles. These cells were all GABA-immunoreactive and expressed the cytoskeletal markers tau in the thin axon-like process and MAP2 in the thicker dendrite-like arborizations at the opposite pole of the cell body. After having translocated in perpendicular orientation, these neurons were again able to turn back to move along the radial neurite bundles to another position. Furthermore, while in perpendicular orientation, the processes of these cells repelled each other upon contact of their growth cones, leading to equal spacing between the cell bodies with time in culture.

Bilateral gliosis in unilaterally lesioned septohippocampal system: changes in GFAP immunoreactivity and content

Unilateral damage to the lateral fimbria led to a bilateral gliosis in the septum and hippocampus. The gliosis was manifested by an increase in GFAP staining, accompanied by an increased number of glial fibrillary acidic protein (GFAP)(+) cells and GFAP content; the latter however was not visible in the contralateral septum. In general, the contralateral reaction appeared weaker than the ipsilateral one. The pattern of contralateral increase in GFAP-immunoreactivity (IR) matched almost exactly that observed on the ipsilateral side in the hippocampus (the most evident increase was seen in the oriens and pyramidal layers of cornu Ammonis 3 and in polymorphic area of gyrus dentatus). In the septum the bilateral increase in GFAP-IR was mainly visible in the dorsolateral quadrant of the structure; however in the ipsilateral side it spread over the whole half of the structure. The astrocytic responses in the septum and hippocampus were not equivalent: they differed mainly with regard to the increase of GFAP(+) cells (over 300% of control in the anterior part of the septum and only about 120% in the dorsal hippocampus). The differences between the percentage increases of other gliotic indices: GFAP-IR and GFAP content. Various possibilities that may account for the occurrence of contralateral gliosis are discussed, the most plausible being the contribution of interhemispheric and intraseptal links and the action of some diffusible agents. We suggest that bilateral gliosis may have an impact on compensatory postlesion processes, possibly by providing trophic support to impaired neurons.

Phaseolus vulgaris-leucoagglutinin tracing of commissural fibers to the rat dentate gyrus: evidence for a previously unknown commissural projection to the outer molecular layer

Numerous studies have shown a lamina-specific termination of commissural fibers to the dentate gyrus in the inner molecular layer. However, the exact course and arborization pattern of individual fibers remained unknown. In this study, the commissural fiber tract to the dentate gyrus of the rat has been studied using the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L), which labels individual axons and their collaterals. Following iontophoretic application of the tracer, anterogradely labeled fibers were followed through the posterior basal fornix and medial fimbria where they formed a dense fiber bundle. Labeled fibers then entered the dentate gyrus close to the medial blade of the granule cell layer where they separated and traversed the hilus. Only in those cases where the injection also involved CA3 pyramidal cells could axons arborizing in the hilus be observed. Typically, fibers that continued into the molecular layer did not arborize in the hilus. Upon their entrance into the molecular layer, these fibers changed direction, gave off several collaterals, and followed a new path parallel to the granule cell layer where they preferentially formed en passant contacts. These commissural fibers to the inner molecular layer terminated in a wide septotemporal (longitudinal) extension. However, a considerable number of fibers reached the outer molecular layer where some of them formed extensive arborizations. Moreover, these commissural fibers to the outer molecular layer appeared to be restricted to the hippocampal lamella, corresponding to the level of the contralateral injection site. These data suggest the existence of three commissural projections to the rat dentate gyrus: (1) commissural fibers to the hilus arising from CA3 neurons, (2) commissural fibers to the inner molecular layer, and, (3) commissural fibers to the outer molecular layer.

Unscheduled brain DNA synthesis, long-term potentiation, and depression at the perforant path-granule cell synapse in the rat

We investigated the effect of long-term potentiation (LTP) of the perforant path-granule cell synapse, on the synthesis of DNA in the target area and in polysynaptically stimulated hippocampal (CA3/CA1) and cortical areas (entorhinal, temporal, and occipital cortices) in the rat. The contralateral nonstimulated side was used as a control. The degree of LTP was indexed by the field EPSP and population spike amplitude recorded in the dentate area of the stimulated side before and after high frequency stimulation (250 Hz, 250 ms) every 30 min. DNA synthesis was evaluated in tissue homogenates after a 3-h period of incorporation of 3H-thymidine. DNA synthesis was significantly lower in the stimulated side in the hippocampal cortex CA3/CA1 (-25%), and in the entorhinal cortex (-50%), but not in the dentate area. In addition, the occurrence of preparations without expression of LTP allowed the analysis of unscheduled brain DNA synthesis (UBDS) in a supposedly long-term depression (LTD) subgroup. UBDS was higher in the group without LTP (no-LTP group) than in that with a significant LTP expression (LTP-group) on both sides of the brain. Furthermore, correlative analyses revealed that UBDS covaried with LTP of the EPSP (but not of population spike) in the dentate area and in extratarget hippocampal subregions on both sides and in dorsal cortex on the stimulated side. Further, regional crosscorrelation analyses revealed a high degree of coupling among brain sites following LTP.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain RNA synthesis, long-term potentiation and depression at the perforant path-granule cell synapse in the guinea pig

The effects of long-term changes in synaptic efficacy at the perforant path-granule cell synapse on the de-novo synthesis of ribonucleic acid (RNA) were investigated in hippocampal and cortical areas in anaesthetized Guinea pig preparations. Two experiments were run with stimulating and recording microelectrodes aimed at the perforant bundle and dentate gyrus hilus on both sides. In Experiment 1, a low-frequency (LFS; 0.02 Hz, 3 h) or high-frequency stimulation (HFS; 400 Hz, 250 ms) was delivered to the left perforant bundle with the contralateral side as control. In Experiment 2, animals received LFS or HFS trains with implanted nonstimulated animals used as controls. The latency and amplitude of the field postsynaptic potentials (FPSP) and population spike (POPS) were monitored under baseline conditions and following stimulation over a 3 h period. In addition, two HFS groups were tested with few (HFS-F: every 15 min) or several test stimuli (HFS-S: every 3 min). In both experiments RNA synthesis was determined by measuring the amount of 3H-5,6-uridine incorporated into the RNA 3 h after bilateral intraventricular injection. In Exp. 1 the LFS group showed a higher synthesis of RNA than both HFS groups. The rate of RNA synthesis did not differ between the stimulated and nonstimulated side. In Exp. 2 the HFS groups showed a decreased RNA synthesis. In the HFS-F group, it pertained to the dorsal dentate area, CA1, subiculum, cingulate and dorsal cortices bilaterally, and to the ventral dentate area and CA3 on the nonstimulated side. In contrast, the HFS-S group showed decreased RNA synthesis at the dorsal dentate area and dorsal cortex on the stimulated side, and at CA1, subiculum, and cingulate cortex bilaterally. The decrease was stronger in the HFS-F than in the HFS-S group. Moreover, the subgroup with a low (0-60%) and that with a high (61-240%) level of long-term potentiation of FPSP revealed lower and higher RNA synthesis, respectively, both in homosynaptic target areas, and in heterosynaptic sites. Further, correlative analyses between FPSP, POPS and RNA synthesis revealed a complex pattern, depending upon the type of stimulation and on the brain side. Finally, cross-correlation analyses revealed a high degree of coupling among brain sites in the stimulated groups, indicating distributed covariant changes in RNA synthesis across different brain sites. Thus, changes in synaptic efficacy covary with changes in RNA synthesis, and presumably exert a modulatory role on gene expression.

Connection matrix of the hippocampal formation: I. The dentate gyrus

The hippocampal formation presents a special opportunity for realistic neural modeling since its structure, connectivity, and physiology are better understood than that of other cortical components. A review of the quantitative neuroanatomy of the rodent dentate gyrus (DG) is presented in the context of the development of a computational model of its connectivity. The DG is a three-layered folded sheet of neural tissue. This sheet is represented as a rectangle, having a surface area of 37 mm2 and a septotemporal length of 12 mm. Points, representing cell somata, are distributed in the model rectangle in a roughly uniform fashion. Synaptic connectivity is generated by assigning each presynaptic cell a spatial zone representing its axonal arbor. For each postsynaptic cell, a list of potential presynaptic cells is compiled, based on which arbor zones the given postsynaptic cell falls within. An appropriate number of presynaptic inputs are then selected at random. The principal cells of the DG, the granule cells, are represented in the model, as are non-principal cells, including basket cells, chandelier cells, mossy cells, and GABAergic peptidergic polymorphic (GPP) cells. The neurons of layer II of the entorhinal cortex are included also. The DG receives its main extrinsic input from these cells via the perforant path. The basket cells, chandelier cells, and GPP cells receive perforant path and granule cell input and exert both feedforward and feedback inhibition onto the granule cells. Mossy cells receive converging input from granule cells and send their output back primarily to distant septotemporal levels, where they contact both granule cells and non-principal cells. To permit numerical simulations, the model must be scaled down while preserving its anatomical structure. A variety of methods for doing this exist. Hippocampal allometry provides valuable clues in this regard.

Glutamate dehydrogenase in astrocytes of the rat dentate gyrus following lesion of the entorhinal cortex

Applying quantitative microscopic histochemistry, the activity of the mitochondrial glutamate dehydrogenase which is localized in astrocytes was determined in the molecular layer of the dentate gyrus of the rat hippocampus. This hippocampal region contains the important terminations of the glutamatergic perforant path. For comparison, determinations of the mitochondrial succinate dehydrogenase were performed, which is localized preferentially in terminals and dendrites. Two age groups of animals were examined: young adults (three months old) and aged subjects (24 months old). Both age groups were divided into controls, and animals killed three, 21 and 90 days following unilateral electrolytic lesion of the entorhinal cortex. The post-lesional shrinkage of the terminal field of the perforant path, ipsilateral to the lesion side, was determined and considered in the evaluation of enzymatic data. Statistic analysis revealed that ipsilateral to the lesion side there was a significant decrease of glutamate and succinate dehydrogenase activities in the terminal field of the perforant path three, 21 and 90 days following lesion. It is reasonable to assume that the decrease of succinate dehydrogenase activity (50-60%) was caused by the loss of mitochondria localized in degenerating terminals, whereas the decrease of glutamate dehydrogenase activity (20-30%) was related to the decrease of glutamatergic transmission following lesion. In the terminal field of the perforant path contralateral to the lesion side both significant increases and decreases of enzyme activities were measured following lesion. From these results it is concluded that the hippocampus contralateral to the lesion side cannot be considered as an appropriate intraindividual control. The comparison between young and aged animals showed no differences in the demonstration of glutamate dehydrogenase and only restricted differences in the activity level of succinate dehydrogenase post-lesion. Therefore, it is reasonable to assume that the post-lesional reactivity of the enzymes studied was very similar in both age groups.

The organization of the embryonic and early postnatal murine hippocampus. II. Development of entorhinal, commissural, and septal connections studied with the lipophilic tracer DiI

We have analyzed the early development of the main hippocampal afferents in the mouse. Following injections of the lipophilic tracer 1-1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) in the entorhinal cortex, entorhinal axons were observed for the first time in the hippocampus at E15, in the white matter. At E17, entorhinal fibers arborized within the stratum lacunosum-moleculare. At subsequent stages entorhinal axons formed dense networks that were restricted to their appropriate termination zone in the lacunosum-moleculare. The first axons invading the fascia dentata were noticed at E19, their density increasing at later stages. These axons were mainly present in the outer molecular layer. This onset of entorhinohippocampal projections was corroborated by retrograde labeling data after injections in the hippocampus. Commissural fibers first entered the contralateral hippocampus at E18, their number increasing at the following stages. Commissural axons arborized within the stratum oriens and radiatum in the hippocampus proper. In the fascia dentata, the earliest commissural fibers were seen at P2, terminating in the inner zone of the molecular layer and in the hilus. We conclude that developing entorhinal and commissural axons show a high degree of laminar specificity from the earliest stages of formation, which is compatible with the notion that distinct subsets of early maturing neurons populating the hippocampal plexiform layers may attract particular fiber systems. Hippocamposeptal fibers develop at E15, before the first septal fibers can be detected in the hippocampus. These early hippocamposeptal fibers originated from nonpyramidal neurons and terminated in the medial septal area, which is the main source of septal afferents to the hippocampus. In contrast, septohippocampal fibers were not seen in the hippocampus until E17. At perinatal stages, the hippocamposeptal connection reshapes, sending axons to the dorsolateral septal area as the innervation of the medial septum becomes less conspicuous. This sequence suggests that hippocampal neurons pioneer the formation of septohippocampal connections.

Organization of the embryonic and early postnatal murine hippocampus. I. Immunocytochemical characterization of neuronal populations in the subplate and marginal zone

Immunocytochemical techniques were used to characterize the neuronal populations in the hippocampal subplate and marginal zone from embryonic day 13 (E13) to postnatal day 5 (P5). Sections were processed for the visualization of microtubule-associated protein 2 (MAP2) and other antigens such as neurotransmitters, neuropeptides, calcium-binding proteins and a synaptic antigen (Mab SMI81). At E13-E14, only the ventricular zone and the primitive plexiform layer were recognized. Some cells in the later stratum displayed MAP2-, gamma-aminobutyric acid (GABA)- and calretinin immunoreactivities. From E15 onwards, the hippocampal and dentate plates became visible. Neurons in the plexiform layers were immunoreactive at E15-E16, whereas the hippocampal and dentate plates showed immunostaining two or three days later. Between E15 and E19 the following populations were distinguished in the plexiform layers: the subventricular zone displayed small neurons that reacted with MAP2 and GABA antibodies; the subplate (prospective stratum oriens) was poorly populated by MAP2- and GABA-positive cells; the inner marginal zone (future stratum radiatum) was heavily populated by multipolar GABAergic cells; the outer marginal zone (stratum lacunosum-moleculare) displayed horizontal neurons that showed glutamate- and calretinin immunoreactivities, their morphology being reminiscent of neocortical Cajal-Retzius cells. Thus, each plexiform layer was populated by a characteristic neuronal population whose distribution did not overlap. Similar segregated neuronal populations were also found in the developing dentate gyrus. At perinatal stages, small numbers of neurons in the plexiform layers began to express calbindin D-28K and neuropeptides. During early postnatal stages, neurons in the subplate and inner marginal zones were transformed into resident cells of the stratum oriens and radiatum, respectively. In contrast, calretinin-positive neurons in the stratum lacunosum-moleculare disappeared at postnatal stages. At E15-E19, SMI81-immunoreactive fibers were observed in the developing white matter, subplate and outer marginal zone, which suggests that these layers are sites of early synaptogenesis. At P0-P5, SMI81 immunoreactivity became homogeneously distributed within the hippocampal layers. The present results show that neurons in the hippocampal subplate and marginal zones have a more precocious morphological and neurochemical differentiation than the neurons residing in the principal cell layers. It is suggested that these early maturing neurons may have a role in the targeting of hippocampal afferents, as subplate cells do in the developing neocortex.