Organization of the mossy fiber system of the rat studied in extended hippocampi. II. Experimental analysis of fiber distribution with silver impregnation methods

The fibro- and cyto-architecture demarcating the border between the dentate gyrus and CA3 in sheep (Ovis aries) and domestic pig (Sus scrofa domesticus)

The hippocampal formation is essential for spatial navigation and episodic memory. The anatomical structure is largely similar across mammalian species, apart from the deep polymorphic layer of the dentate gyrus and the adjacent part of cornu ammonis 3 (CA3) which feature substantial variations. In rodents, the polymorphic layer has a triangular cross‐section abutting on the end of the CA3 pyramidal layer, while in primates it is long and band‐shaped capping the expanded CA3 end, which here lacks a distinct pyramidal layer. This structural variation has resulted in a confusing nomenclature and unclear anatomical criteria for the definition of the dentate‐ammonic border. Seeking to clarify the border, we present here a light microscopic investigation based on Golgi‐impregnated and Timm–thionin‐stained sections of the Artiodactyla sheep and domestic pig, in which the dentate gyrus and CA3 end have some topographical features in common with primates. In short, the band‐shaped polymorphic layer coincides with the Timm‐positive mossy fiber collateral plexus and the Timm‐negative subgranular zone. While the soma and excrescence‐covered proximal dendrites of the mossy cells are localized within the plexus, the peripheral mossy cell dendrites extend outside the plexus, both into the granular and molecular layers, and the CA3. The main mossy fibers leave the collateral plexus in a scattered formation to converge gradually through the CA3 end in between the dispersed pyramidal cells, which are of three subtypes, as in monkey, with the classical apical subtype dominating near the hidden blade, the nonapical subtype near the exposed blade, and the dentate subtype being the only pyramidal cells that extend dendrites into the dentate gyrus. In agreement with our previous study in mink, the findings show that the border between the dentate gyrus and the CA3 end can be more accurately localized by the mossy fiber system than by cyto‐architecture alone.

Measuring Sharp Waves and Oscillatory Population Activity With the Genetically Encoded Calcium Indicator GCaMP6f

GCaMP6f is among the most widely used genetically encoded calcium indicators for monitoring neuronal activity. Applications are at both the cellular and population levels. Here, we explore two important and under-explored issues. First, we have tested if GCaMP6f is sensitive enough for the detection of population activity with sparse firing, similar to the sensitivity of the local field potential (LFP). Second, we have tested if GCaMP6f is fast enough for the detection of fast network oscillations critical for the encoding and consolidation of memory. We have focused this study on the activity of the hippocampal network including sharp waves (SWs), carbachol-induced theta oscillations, and interictal-like spikes. We compare simultaneous LFP and optical GCaMP6f fluorescent recordings in Thy1-GCaMP6f mouse hippocampal slices. We observe that SWs produce a clear population GCaMP6f signal above noise with an average magnitude of 0.3% ΔF/F. This population signal is highly correlated with the LFP, albeit with a delay of 40.3 ms (SD 10.8 ms). The population GCaMP6f signal follows the LFP evoked by 20 Hz stimulation with high fidelity, while electrically evoked oscillations up to 40 Hz were detectable with reduced amplitude. GCaMP6f and LFP signals showed a large amplitude discrepancy. The amplitude of GCaMP6f fluorescence increased by a factor of 28.9 (SD 13.5) between spontaneous SWs and carbachol-induced theta bursts, while the LFP amplitude increased by a factor of 2.4 (SD 1.0). Our results suggest that GCaMP6f is a useful tool for applications commonly considered beyond the scope of genetically encoded calcium indicators. In particular, population GCaMP6f signals are sensitive enough for detecting synchronous network events with sparse firing and sub-threshold activity, as well as asynchronous events with only a nominal LFP. In addition, population GCaMP6f signals are fast enough for monitoring theta and beta oscillations (<25 Hz). Faster calcium indicators (e.g., GCaMP7) will further improve the frequency response for the detection of gamma band oscillations. The advantage of population optical over LFP recordings are that they are non-contact and free from stimulation artifacts. These features may be particularly useful for high-throughput recordings and applications sensitive to stimulus artifact, such as monitoring responses during continuous stimulation.

Neurons Generated by Mouse ESCs with Hippocampal or Cortical Identity Display Distinct Projection Patterns When Co-transplanted in the Adult Brain

The capability of generating neural precursor cells with distinct types of regional identity in vitro has recently opened new opportunities for cell replacement in animal models of neurodegenerative diseases. By manipulating Wnt and BMP signaling, we steered the differentiation of mouse embryonic stem cells (ESCs) toward isocortical or hippocampal molecular identity. These two types of cells showed different degrees of axonal outgrowth and targeted different regions when co-transplanted in healthy or lesioned isocortex or in hippocampus. In hippocampus, only precursor cells with hippocampal molecular identity were able to extend projections, contacting CA3. Conversely, isocortical-like cells were capable of extending long-range axonal projections only when transplanted in motor cortex, sending fibers toward both intra- and extra-cortical targets. Ischemic damage induced by photothrombosis greatly enhanced the capability of isocortical-like cells to extend far-reaching projections. Our results indicate that neural precursors generated by ESCs carry intrinsic signals specifying axonal extension in different environments.

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Wnt signaling induces hippocampal fate in neuralized mouse ESCs

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Transplanted cortical and hippocampal neurons target distinct regions in adult brain

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Photothrombotic lesion favors neurite elongation of cortical transplanted cells

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Cortical cell transplantation improves the motor performance after ischemic damage

Aging affects new cell production in the adult hippocampus: A quantitative anatomic review

In the last century, cognitive impairment in elderly people was considered as the consequence of neuronal death. However, later analyses indicated that age-related reduction in neuron number was limited to specific regions of the central nervous system, and was irrelevant to brain dysfunction in both humans and non-human animals. Recent studies have indicated that progressive diminution of neural plasticity across an individual's life span may underlie age-related brain dysfunction. To date, various factors have been shown to contribute to neural plasticity. In particular, substantial data supports the importance of production of new cells in the adult brain: the rate of hippocampal neurogenesis wanes radically during aging; similarly, white matter homeostasis via oligodendrogenesis is also affected by aging. This review briefly summarizes quantitative studies on adult hippocampal neurogenesis and oligodendrogenesis. Although the hippocampus is traditionally recognized as the memory center of the brain, it has started to emerge as an integrator of cognition and emotion. One of the current research highlights is that diverse functions of the hippocampus are topographically embedded along its longitudinal and transverse axes. Here we discuss alterations in adult neurogenesis and oligodendrogenesis during aging from a topographic view point. The quantitative anatomic approach to age-related alterations in production of new cells in the hippocampus may give a novel insight into how brain functions suffer from aging.

Updating the lamellar hypothesis of hippocampal organization

Andersen et al. (1971) proposed that excitatory activity in the entorhinal cortex propagates topographically to the dentate gyrus, and on through a "trisynaptic circuit" lying within transverse hippocampal "slices" or "lamellae." In this way, a relatively simple structure might mediate complex functions in a manner analogous to the way independent piano keys can produce a nearly infinite variety of unique outputs. The lamellar hypothesis derives primary support from the "lamellar" distribution of dentate granule cell axons (the mossy fibers), which innervate dentate hilar neurons and area CA3 pyramidal cells and interneurons within the confines of a thin transverse hippocampal segment. Following the initial formulation of the lamellar hypothesis, anatomical studies revealed that unlike granule cells, hilar mossy cells, CA3 pyramidal cells, and Layer II entorhinal cells all form axonal projections that are more divergent along the longitudinal axis than the clearly "lamellar" mossy fiber pathway. The existence of pathways with "translamellar" distribution patterns has been interpreted, incorrectly in our view, as justifying outright rejection of the lamellar hypothesis (Amaral and Witter, 1989). We suggest that the functional implications of longitudinally projecting axons depend not on whether they exist, but on what they do. The observation that focal granule cell layer discharges normally inhibit, rather than excite, distant granule cells suggests that longitudinal axons in the dentate gyrus may mediate "lateral" inhibition and define lamellar function, rather than undermine it. In this review, we attempt a reconsideration of the evidence that most directly impacts the physiological concept of hippocampal lamellar organization.

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

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

Dentate gyrus and spatial behaviour

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

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

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

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

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

The dentate mossy fibers: structural organization, development and plasticity

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

Dentate gyrus-selective colchicine lesion and performance in temporal and spatial tasks

The effects of multiple-site, intradentate, colchicine injections on the performance of a temporal, 'differential reinforcement of low rates of responding' (DRL-20s) task and a spatial, 'delayed non-matching-to-place' (DNMTP) task in a plus-maze were investigated in rats trained in both tasks prior to the lesion. Quantitative analysis revealed a greater than 86% reduction in the dentate gyrus (DG) of the colchicine-injected rats compared to the sham-operated controls. Dentate gyrus damage rendered rats less efficient than sham-operated controls in the performance of the DRL-20s task. The DRL inter-response time (IRT) distribution for the DG-lesioned rats and the sham-operated controls was similar; however, while the distribution peak for the control rats was 20s, it was 16s for the DG-lesioned rats, indicating that the latter rats underestimated time. Performance of the DG-lesioned rats was also disrupted in the DNMTP task. However, DG-lesioned rats recovered control levels of performance during repeated training with an intertrial interval equal to 3s. An increase in intertrial interval in lesioned and sham-operated controls disrupted performance in both groups; however, while DG-lesioned rats performed at chance levels when the intertrial interval was increased to 4min or longer, the sham-operated controls performed at chance levels only when the intertrial interval was increased to 16min. These results seem most parsimoniously interpreted following the cognitive map theory of hippocampal function.

Hippocampal mossy fibre terminal field size is differentially affected in a rat model of risk-taking behaviour

Individual differences in novelty-induced exploratory activity identify rats which can serve as a model of human sensation-seeking, risk-taking behaviour. Experimentally naïve rats, when exposed to mild stress of a novel environment, exhibit variability in their exploratory activity. Some rats display high rates of locomotor reactivity to novelty (high responders (HR)), and others display low rates (low responders (LR)). The LRHR phenotype is a reliable predictor of drug-taking behaviour and is linked to differences in hippocampal glucocorticoid receptor mRNA expression. In this study, we investigated whether the LRHR phenotype is associated with differences in the quantitative morphology of the hippocampal field CA3, dentate gyrus molecular layer, granule cell layer and mossy fibres. LRs and HRs showed no significant differences in the volumes of CA3 and dentate molecular layer volume or the number of dentate granule cells. However, LRs had a significantly larger suprapyramidal mossy fibre terminal field volume when compared to HRs. The infrapyramidal mossy fibres did not differ between phenotypes. Also, we found a LRHR phenotype-independent significant negative correlation between molecular layer volume per granule cell and the total number of granule cells. These findings implicate the SP-MF in vulnerability for risk-taking behaviour, and we propose that LR and HR hippocampi may differ in the way novelty information is processed.

Overexpression of calbindin D28k in dentate gyrus granule cells alters mossy fiber presynaptic function and impairs hippocampal-dependent memory

Calcium is a key signaling ion for induction of synaptic plasticity processes that are believed to influence cognition. Mechanisms regulating activity-induced increases in neuronal calcium and related synaptic modifications are not fully understood. Moreover, involvement of specific synapses in discrete aspects of spatial learning remains to be elucidated. We used herpes simplex amplicons to overexpress calbindin D(28k) (CaBP) selectively in dentate gyrus (DG) granule cells. We then examined the effects on hippocampal network activity by recording evoked synaptic responses in vivo and in vitro and analyzing hippocampal-dependent behavior. Relative to Lac-Z- and sham-infected controls, CaBP overexpression increased mossy fiber (MF-CA3) excitatory postsynaptic potentials and reduced paired-pulse facilitation (PPF), suggesting an increase in presynaptic strength. Additionally, CaBP overexpression reduced long-term potentiation (LTP), caused a frequency-dependent inhibition of post-tetanic potentiation (PTP), and impaired spatial navigation. Thus, increasing CaBP levels selectively in the DG disrupts MF-CA3 presynaptic function and impairs spatial cognition. The results demonstrate the power of gene delivery in the study of the neural substrates of learning and memory and suggest that mossy fiber synaptic plasticity is critical for long-term spatial memory.

Hierarchical model of the population dynamics of hippocampal dentate granule cells

A hierarchical modeling approach is used as the basis for a mathematical representation of the population activity of hippocampal dentate granule cells. Using neural field equations, the variation in time and space of dentate granule cell activity is derived from the summed synaptic potential and summed action potential responses of a population of granule cells evoked by monosynaptic excitatory input from entorhinal cortical afferents. In this formulation of the problem, we have considered a two-level hierarchy: the synapses of entorhinal cortical axons define the first level of organization, and dentate granule cells, which include these synapses, define the second, higher level of organization. The model is specified by two state field variables, for membrane potential and for synaptic efficacy, respectively, with both evolving according to different time scales. The two state field variables introduce new parameters, physiological and anatomical, which characterize the dentate from the point of view of neuronal and synaptic populations: (1) a set of geometrical constraints corresponding to the morphological properties of granule cells and anatomical characteristics of entorhinal-dentate connections; and (2) a set of neuronal parameters corresponding to physiological mechanisms. Assuming no interaction between granule cells, i.e., neither ephaptic nor synaptic coupling, the model is shown to be mathematically tractable and allows solution of the field equations leading to the determination of activity. This treatment leads to the definition of two state variables, volume of stimulated synapses and firing time, which describe observed activity. Numerical simulations are used to investigate the populational characterization of the dentate by individual parameters: (1) the relationship between the conditions of stimulation of active perforant path fibers, e.g., stimulating intensity, and activity in the granule cell layer; and (2) the influence of geometry on the generation of activity, i.e., the influence of neuron density and synaptic density-connectivity. As an example application of the model, the granule cell population spike is reconstructed and compared with experimental data.

Granule neurons generated during development extend divergent axon collaterals to hippocampal area CA3

Most excitatory intrahippocampal pathways are characterized by significant, highly ordered projections into the long, or septotemporal, hippocampal axis. However, the mossy fiber system, the excitatory projection by which the dentate gyrus projects to hippocampal area CA3, is considered an exception, being organized to innervate exclusively transversely oriented cortical layers of the hippocampus. In the present study, the anatomy of the rat mossy fiber system was investigated using axonal tracing techniques, with an emphasis on determining its projection pattern into the long hippocampal axis. To this end, we used dual ipsilateral retrograde tracer injections to determine whether individual granule cells extend divergent axon collaterals to septotemporally distinct levels of hippocampal area CA3. We combined this technique with the fluorescent immunohistochemical detection of 5-bromo-2'-deoxyuridine (BrdU), a marker of granule cell precursors and their progeny, to address whether the divergence of mossy fiber collaterals within area CA3 might by related to ontogenic gradients in granule cell genesis. We observed single granule neurons that had retrogradely transported both tracers, indicating that they had axon collaterals passing through or terminating within the distinct levels of area CA3 into which tracer had been injected. By using BrdU labeling, we identified divergent granule neurons that were produced during embryonic and postnatal development. We observed no adult-generated granule neurons with significantly diverging mossy fiber collaterals. However, many fewer cells were labeled with BrdU in adult-exposed animals. Because of this smaller sample, we cannot rule out the possibility that small numbers of divergent adult-generated granule cells exist. We conclude that a proportion of the hippocampal mossy fiber projection extends septotemporally divergent axon collaterals to hippocampal area CA3.

Preservation of perisomatic inhibitory input of granule cells in the epileptic human dentate gyrus

Temporal lobe epilepsy is known to be associated with hyperactivity that is likely to be generated or amplified in the hippocampal formation. The majority of granule cells, the principal cells of the dentate gyrus, are found to be resistant to damage in epilepsy, and may serve as generators of seizures if their inhibition is impaired. Therefore, the parvalbumin-containing subset of interneurons, known to provide the most powerful inhibitory input to granule cell somata and axon initial segments, were examined in human control and epileptic dentate gyrus. A strong reduction in the number of parvalbumin-containing cells was found in the epileptic samples especially in the hilar region, although in some patches of the granule cell layer parvalbumin-positive terminals that form vertical clusters characteristic of axo-axonic cells were more numerous than in controls. Analysis of the postsynaptic target elements of parvalbumin-positive axon terminals showed that they form symmetric synapses with somata, dendrites, axon initial segments and spines as in the control, but the ratio of axon initial segment synapses was increased in the epileptic tissue (control: 15.9%, epileptic: 31.3%). Furthermore, the synaptic coverage of granule cell axon initial segments increased more than three times (control: 0.52, epileptic: 2.10 microm synaptic length/100 microm axon initial segment membrane) in the epileptic samples, whereas the amount of somatic symmetric synapses did not change significantly. Although the number of parvalbumin-positive interneurons is decreased, the perisomatic inhibitory input of dentate granule cells is preserved in temporal lobe epilepsy. Basket and axo-axonic cell terminals - whether positive or negative for parvalbumin - are present, moreover, the axon collaterals targeting axon initial segments sprout in the epileptic dentate gyrus. We suggest that perisomatic inhibitory interneurons survive in epilepsy, but their somadendritic compartment and partly the axon loses parvalbumin or immunoreactivity for parvalbumin. The hyperinnervation of axon initial segments might be a compensatory change in the inhibitory network, but at the same time may lead to a more effective synchronization of granule cell firing that could contribute to the generation or amplification of epileptic seizures.

Multiple parallel memory systems in the brain of the rat

A theory of multiple parallel memory systems in the brain of the rat is described. Each system consists of a series of interconnected neural structures. The "central structures" of the three systems described are the hippocampus, the matrix compartment of the dorsal striatum (caudate-putamen), and the amygdala. Information, coded as neural signals, flows independently through each system. All systems have access to the same information from situations in which learning occurs, but each system is specialized to represent a different kind of relationship among the elements (stimulus events, responses, reinforcers) of the information that flows through it. The speed and accuracy with which a system forms a coherent representation of a learning situation depend on the correspondence between the specialization of the system and the relationship among the elements of the situation. The coherence of these stored representations determines the degree of control exerted by each system on behavior in the situation. Although they process information independently the systems interact in at least two ways: by simultaneous parallel influence on behavioral output and by directly influencing each other. These interactions can be cooperative (leading to similar behaviors) or competitive (leading to different behaviors). Experimental findings consistent with these ideas, mostly from experiments with rats, are reviewed.

Novel expression of AMPA-receptor subunit GluR1 on mossy cells and CA3 pyramidal neurons in the human epileptogenic hippocampus

Previous immunocytochemical investigations performed in our laboratory on the human hippocampus surgically resected for the treatment of mesial temporal lobe epilepsy (MTLE) have demonstrated an increased expression of the AMPA-receptor subunit GluR1 on neurons in the hilus and area CA3. Light microscopically, many of these neurons exhibited peculiar filamentous extensions and grape-like excrescences that protruded from their somata and proximal dendrites, suggesting that these neurons may be mossy cells and CA3 pyramidal neurons, respectively. The present electron microscopic study was carried out to further characterize these cells. The filamentous extensions were identified as dendrites from which spines often protruded, and the grape-like excrescences represented clusters of closely associated dendrites and spines. A variety of synapses were formed by the GluR1-positive profiles. These arrangements ranged from simple contacts between a single unlabelled axon terminal and a single labelled postsynaptic element, to complex contacts involving multiple unlabelled axon terminals and labelled postsynaptic elements. Many of the axon terminals involved in these arrangements were mossy fibre boutons. Thus, a large proportion of the GluR1-positive neurons were identified as hilar mossy cells and CA3 pyramidal neurons, cells hitherto thought to be absent or greatly reduced in the MTLE hippocampus. Taken together, these data suggest the presence of a highly efficient excitatory circuit involving AMPA receptors, mossy cells and CA3 pyramidal neurons in the sclerotic hippocampus. Such a circuit could be critically involved in the genesis and maintenance of temporal lobe epilepsy.

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.

Recurrent neonatal seizures: relationship of pathology to the electroencephalogram and cognition

Seizures in preterm infants are associated with a high risk of neurological sequelae. In the neonatal rat recurrent seizures have been associated with long-term changes in cerebral excitability and cognition as well as sprouting of mossy fiber terminals in the granule cell layer of the dentate gyrus and hippocampal CA3 subfield. To evaluate the relationship between seizure-induced morphological changes and cognitive function we subjected newborn rats to 55 seizures with flurothyl during the first 12 days of life. During adolescence rats with prior recurrent seizures were compared with controls in electroencephalographic power and performance in the Morris water maze and open field test. Rats subjected to recurrent seizures had marked impairment in water maze performance and never reached the level of learning seen in controls despite a total of 54 trials. Recurrent seizures were also associated with an overall reduction in spectral power which was most pronounced in the theta range. On histological examination rats with recurrent neonatal seizures had sprouting of mossy fiber terminals in CA3 and the granule cell layer of the dentate gyrus without any accompanying cell loss. Sprouting in CA3, but not the granule cell layer of the dentate gyrus, correlated with water maze performance. This study demonstrates that recurrent neonatal seizures can result in profound impairment of water maze performance and reduction of electroencephalographic power despite the lack of discernible cell loss and that this cognitive impairment correlates with mossy fiber sprouting in CA3.

Dentate gyrus-selective colchicine lesion and disruption of performance in spatial tasks: difficulties in "place strategy" because of a lack of flexibility in the use of environmental cues?

The effects of intradentate colchicine injections on the performance of tasks requiring spatial working and reference memory are controversial. Multiple-site colchicine injections (7 microg/microl; via a drawn micropipette) throughout the dentate gyrus (DG) of rats (nine sites in each hemisphere, 0.06 microl at each site) selectively destroy about 90% of the DG granule cells, as revealed by quantitative stereological estimates; stereology also revealed minor neuronal losses in the CA4 (33%) and CA1 (23%) subfields, but lack of damage to the CA3 hippocampal subfield. Spatial reference and working memory were assessed in Morris' water maze; in the reference memory task, the rats were required to learn a single, fixed location for the platform over several days of training; in the working memory task, animals were required to learn a new platform location every day, in a matching-to-place procedure. Compared to sham-operated controls, lesioned rats showed significant disruption in acquisition of the reference memory water maze task; however, the data reveal that these rats did acquire relevant information about the task, probably based on guidance and orientation strategies. In a subsequent probe test, with the platform removed, lesioned rats showed disruption in precise indexes of spatial memory (e.g., driving search towards the surroundings of the former platform location), but not in less precise indexes of spatial location. Finally, the lesioned rats showed no improvement in the match-to-place procedure, suggesting that their working memory for places was disrupted. Thus, although capable of acquiring relevant information about the task, possibly through guidance and/or orientation strategies, DG-lesioned rats exhibit a marked difficulty in place strategies. This is particularly evident when these rats are required to deal with one-trial place learning in a familiar environment, such as in the working memory version of the water maze task, which requires flexibility in the use of previously acquired information.

Mossy fiber sprouting after recurrent seizures during early development in rats

In some children, epilepsy is a catastrophic condition, leading to significant intellectual and behavioral impairment, but little is known about the consequences of recurrent seizures during development. In the present study, we evaluated the effects of 15 daily pentylenetetrazol-induced convulsions in immature rats beginning at postnatal day (P) 1, 10, or 60. In addition, we subjected another group of P10 rats to twice daily seizures for 15 days. Both supragranular and terminal sprouting in the CA3 hippocampal subfield was assessed in Timm-stained sections by using a rating scale and density measurements. Prominent sprouting was seen in the CA3 stratum pyramidale layer in all rats having 15 daily seizures, regardless of the age when seizures began. Based on Timm staining in control P10, P20, and P30 rats, the terminal sprouting in CA3 appears to be new growth of axons and synapses as opposed to a failure of normal regression of synapses. In addition to CA3 terminal sprouting, rats having twice daily seizures had sprouting noted in the dentate supragranular layer, predominately in the inferior blade of the dentate, and had a decreased seizure threshold when compared with controls. Cell counting of dentate granule cells, CA3, CA1, and hilar neurons, with unbiased stereological methods demonstrated no differences from controls in rats with daily seizures beginning at P1 or P10, whereas adult rats with daily seizures had a significant decrease in CA1 neurons. Rats that received twice daily seizures on P10-P25 had an increase in dentate granule cells. This study demonstrates that, like the mature brain, immature animals have neuronal reorganization after recurrent seizures, with mossy fiber sprouting in both the CA3 subfield and supragranular region. In the immature brain, repetitive seizures also result in granule cell neurogenesis without loss of principal neurons. Although the relationship between these morphological changes after seizures during development and subsequent cognitive impairment is not yet clear, our findings indicate that during development recurrent seizures can result in significant alterations in cell number and axonal growth.

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

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

Imaging free zinc in synaptic terminals in live hippocampal slices

Some glutamatergic synapses in the mammalian central nervous system exhibit high levels of free ionic zinc in their synaptic vesicles. The precise role of this vesicular zinc remains obscure, despite suggestive evidence for zinc as a neuromodulator. As a step towards elucidating the role of free zinc in the brain we have developed a method for imaging zinc release in live brain slices. A newly synthesized zinc-sensitive fluorescent probe, N-(6-methoxy-8-quinolyl)-p-carboxybenzoylsulphonamide (TFLZn), was used to monitor intracellular zinc in live rat hippocampal slices. The dye loaded into the zinc-rich synaptic vesicles of the mossy fibre terminals in the hippocampal formation. Direct electrical stimulation of the mossy fibre pathway diminished the fluorescence in the mossy fibre terminals, consistent with a stimulus-dependent zinc release. The synaptic release of zinc was followed by the rapid replenishment of the zinc levels in vesicles from an as yet unidentified intracellular zinc source. Furthermore, we present evidence that zinc may play a role in a form of long-term potentiation exhibited by the mossy fibre pathway.

Delayed cell death in the contralateral hippocampus following kainate injection into the CA3 subfield

A model of epileptic cell death has been developed employing unilateral injections of kainic acid, a glutamate agonist, into the CA3 subfield of the hippocampus. The contralateral hippocampus, where neuronal damage is induced by hyperactivity in afferent pathways, served as the model structure. The pattern of cell death in this model was shown earlier to correspond to the vulnerable regions in human temporal lobe epilepsy. In the present time-course study we demonstrated that the different subpopulations of vulnerable cells in the contralateral hippocampus of the rat degenerate at different times following kainate injection. Spiny calretinin-containing cells in the hilus and CA3 stratum lucidum disappear at 12-24 h, other types of hilar neurons and CA3c pyramidal cells show shrinkage and argyrophilia at two days, whereas CA1 pyramidal cells degenerate at three days postinjection. The majority of cells destined to die showed a transient expression of the heatshock protein 72, approximately one day (for hilar-CA3c) or two days (for CA1) before degeneration. Parvalbumin-immunoreactivity transiently disappeared from the soma and dendrites of interneurons between the first and the fourth day. The results suggest that seizure-induced cell death is delayed, therefore acute oedema, even if it occurs, is insufficient to kill neurons. The only exception is the population of calretinin-containing interneurons degenerating at 12-24 h. The further one day delay between hilar-CA3c and CA1 cell death is likely to be due to differences in the relative density of glutamate receptor types (kainate versus NMDA) and the source of afferent input of these subfields. Thus, simple pharmacotherapy targeting only one of the excitotoxic mechanisms (i.e. acute oedema of calretinin cells versus delayed death of hilar-CA3c and CA1 cells at different time points) is likely to fail.

Ultrastructural plasticity of the dentate gyrus granule cells following recurrent limbic seizures: II. Alterations in somatic synapses

Hilus lesion-induced recurrent limbic seizures cause a dramatic increase in the numbers of somatic spines on dentate gyrus granule cells in the adult rat. Somatic spines are maximally increased 3 h after the initiation of seizures at which time many of these spines form synapses. The present quantitative electron microscopic study assessed the numbers and types of synapses present on the granule cell perikarya and somatic spines of control and experimental seizure rats with the goal of determining if newly elaborated somatic spines arise at the site of pre-existing synapses or are associated with new innervation. Experimental rats were sacrificed 5 h after hilar lesion placement (or 3 h after seizure onset). In both control and hilus-lesioned (HL) rats, 15-20% of the somatic spines could be seen to form synaptic contacts within a single plane of section; these synapses were almost exclusively of the asymmetric type. With the increased incidence of spines in experimental-seizure rats, there was a 6.25-fold greater number of spine synapses in HL versus control rats. There was, in addition, a 60% decrease in the number of asymmetric synapses occurring directly on the granule cell perikarya but no change in the total (spine plus somatic) number of asymmetric synapses. Although few asymmetric synapses were associated with spines in control tissue, 60-70% of asymmetric synapses were associated with spines in experimental-seizure tissue. In addition, in hilus lesion rats symmetric somatic synapses were increased by 20% on cells in deep stratum granulosum resulting in a dissolution of the superficial-to-deep innervation gradient present in the untreated rat. These findings support the conclusion that spines induced by seizure activity form at the site of pre-existing asymmetric synapses on the granule cells and demonstrate that brief seizure episodes can rapidly induce marked changes in innervation patterns in the adult brain.

Temporal onset of synapsin I gene expression coincides with neuronal differentiation during the development of the nervous system

Synapsin I is the best characterized member of a family of nerve terminal-specific phosphoproteins implicated in the regulation of neurotransmitter release. During development, the expression of synapsin I correlates temporally and topographically with synapse formation, and recent physiological studies (Lu et al. [1992] Neuron 8:521-529.) have suggested that synapsin I may participate in the functional maturation of synapses. To better understand the temporal relationship between synapsin I gene expression and particular cellular events during neuronal development, we have used in situ hybridization histochemistry to localize synapsin I mRNA throughout the rat central and peripheral nervous systems during embryonic and postnatal development. From the earliest embryonic time points assayed (E12), the expression of the synapsin I gene was detectable in both the central and peripheral nervous systems. While, in general, levels of synapsin I mRNAs were high in utero, synapsin I cDNA probes revealed specific patterns of hybridization in different regions of the embryonic nervous system. To determine precisely the temporal onset of expression of the synapsin I gene during neuronal development, we examined in detail the appearance of synapsin I mRNA during the well characterized postnatal development of granule cells of the rat cerebellum and hippocampus. In both regions, the onset of synapsin I gene expression correlated with the period of stem cell commitment to terminal differentiation. Finally, our data demonstrate that, in a second phase, synapsin I gene expression increases to a maximum for a given neuronal population during a particular phase of differentiation, i.e., synaptogenesis.

Blockade of NMDA receptors increases cell death and birth in the developing rat dentate gyrus

Excitatory input regulates cell birth and survival in many systems. The granule cell population of the rat dentate gyrus is formed primarily during the postnatal period. Excitatory afferents enter the dentate gyrus and begin to form synapses with granule cells during the first postnatal week, the time of maximal cell birth and death. In order to determine whether excitatory input plays a role in the regulation of cell birth and survival in the developing granule cell layers and their germinal regions, the subependymal layer and hilus, we treated rat pups with the N-methyl D-aspartate (NMDA) receptor antagonists MK-801, CGP 37849, or CGP 43487 during the first postnatal week and examined the numbers of 3H-thymidine-labeled cells, pyknotic cells, and healthy cells in these regions. In order to determine the cell type that was affected, sections from brains of MK-801-treated rats were processed for 3H-thymidine autoradiography combined with immunohistochemistry for the marker of radial glia, vimentin, and the marker of mature astrocytes, glial fibrillary acidic protein (GFAP). Within the dentate gyrus, NMDA receptor blockade resulted in the following changes: (1) the density of 3H-thymidine-labeled cells was increased, (2) the density of pyknotic cells was increased, (3) the density of 3H-thymidine-labeled pyknotic cells was increased, and (4) the density of healthy cells was decreased. The infrapyramidal blade/hilus showed changes throughout its extent, whereas the suprapyramidal blade showed changes only at the rostral level. No change in the numbers of 3H-thymidine-labeled vimentin-immunoreactive or GFAP-immunoreactive cells was observed in the dentate gyrus with MK-801 treatment, indicating that glia are not primarily affected by NMDA receptor blockade. Blockade of NMDA receptors resulted in gross morphologic changes in the dentate gyrus; in most cases, the infrapyramidal blade was indistinguishable from the hilus. Moreover, in several brains of animals treated with CGP 37849 or CGP 43487 on postnatal day (P)5, an abnormal aggregation of cells was observed ventral to the normal location of the infrapyramidal blade. This cellular cluster contained many pyknotic and 3H-thymidine-labeled cells and may represent cells that normally comprise the infrapyramidal blade. Dramatic changes to the subependymal layer were also seen following NMDA receptor blockade. The cross-sectional area of this region was significantly increased with MK-801, CGP 37849, or CGP 43487 treatment and contained a high density of 3H-thymidine-labeled cells and 3H-thymidine-labeled pyknotic cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Divergence of hippocampal mossy fibers

By connecting the fascia dentata with the hippocampus proper, the axons of the granule cells, the mossy fibers, represent an important element of the main excitatory, trisynaptic pathway of the hippocampal formation. In this review the various synaptic connections of the mossy fibers are discussed. It turns out that the mossy fibers do not only establish synapses with the pyramidal neurons of regio inferior as traditionally assumed, but a variety of local circuit neurons as well as projection cells are also contacted by the mossy fibers. Thus there is an underestimated divergence of the impulse flow within the "trisynaptic" pathway at the level of the mossy fibers. Similarly, the pattern of afferent input to the granule cells, especially that of GABAergic neurons, is more complex than previously assumed. In this respect the concept of a unidirectional "trisynaptic" pathway certainly is an oversimplification. In particular, the hilus of the fascia dentata, that the mossy fibers traverse on their way to regio inferior, is often neglected in this concept. The hilar region comprises a large variety of morphologically and functionally distinct neuronal types that, to a large extent, are targets of hilar mossy fiber collaterals. By focusing on the mossy fiber system, an attempt is made in this review to summarize new data on hippocampal circuitries that have been accumulated since the original description of the trisynaptic pathway. This concept, which originally comprised the synapses of the perforant path fibers on dentate granule cells, the mossy fiber synapses on CA3 pyramidal neurons, and the synapses of the Schaffer collaterals on CA1 pyramidal cells, has been of great heuristic value but needs to be modified in view of recent morphological and physiological data.

Adrenal steroids regulate postnatal development of the rat dentate gyrus: II. Effects of glucocorticoids and mineralocorticoids on cell birth

Unlike the majority of mammalian brain regions, the rat dentate gyrus undergoes maximal cell birth and cell death during the same developmental time period. Granule cell birth and death peak at the end of the first postnatal week. We have found that manipulations of glucocorticoid levels during the stress hyporesponsive period profoundly influence the density of pyknotic cells in the dentate gyrus while apparently not affecting the density of healthy cells. This raises the possibility that glucocorticoids are regulating processes in addition to cell death, i.e., cell birth. In order to determine whether increases in circulating glucocorticoids or mineralocorticoids affect the birth of cells in the developing dentate gyrus, 3H-thymidine autoradiography was performed on brains of rat pups treated with either corticosterone or aldosterone during the first postnatal week. Quantitative analysis of 3H-thymidine-labelled cells revealed significant decreases in the density of labelled cells in the granule cell layers with both corticosterone and aldosterone treatment. In these same brains, significant decreases in the density of pyknotic cells were also observed in the granule cell layers. However, no changes in the numbers of 3H-thymidine-labelled pyknotic cells were observed with any treatment. Increases in circulating corticosterone or aldosterone resulted in significant increases in the density of both 3H-thymidine-labelled and pyknotic cells in the hilus. These results suggest that dentate gyrus cell birth and cell death are related and that these processes are regulated by adrenal steroids.

Naturally occurring cell death in the developing dentate gyrus of the rat

The rat dentate gyrus is a unique brain structure in that most of its neurons are born postnatally. Cell death is known to be an important phenomenon in brain development and yet it is at present unknown whether the dentate gyrus undergoes a period of naturally occurring cell death. In order to determine whether or not cell death plays a role in the development of the dentate gyrus, we examined the density of degenerating cells and healthy cells in the suprapyramidal and infrapyramidal granule cell blades and the hilus during the postnatal period. Light microscopic examination of Nissl-stained brain tissue revealed substantial numbers of pyknotic cells throughout the dentate gyrus during the first postnatal week. Quantitative analysis of the suprapyramidal blade showed a peak in the density of pyknotic cells at the end of the first postnatal week. This peak in the density of degenerating cells coincided with a significant decrease in the density of healthy cells in this region. No rostrocaudal gradient in cell death was observed for the suprapyramidal blade. However, cell death in the suprapyramidal blade proceeded along superficial to deep as well as lateral to medial gradients. Within the infrapyramidal blade/hilus, cell death occurred at different times depending on the rostrocaudal level of the dentate gyrus. Peak density of pyknotic cells was observed the day after birth in the rostral part of the infrapyramidal blade/hilus while pyknosis did not reach a peak in the middle and temporal thirds of this region until the end of the first postnatal week. Cell death in the infrapyramidal blade proceeded in a superficial to deep and lateral to medial direction. These results indicate that the dentate gyrus undergoes a significant period of naturally occurring cell death during the early postnatal period.

Prolonged sojourn of developing pyramidal cells in the intermediate zone of the hippocampus and their settling in the stratum pyramidale

In radiograms of rat embryos that received a single dose of [3H]thymidine between days E16 and E20 and were killed 24 hours after the injection, the heavily labeled cells (those that ceased to multiply soon after the injection) form a horizontal layer in the intermediate zone of the hippocampus, called the inferior band. The fate of these heavily labeled cells was traced in radiograms of the dorsal hippocampus in embryos that received [3H]thymidine on day E18 and were killed at different intervals thereafter. Two hours after injection the labeled proliferative cells are located in the Ammonic neuroepithelium. The heavily labeled cells that leave the neuroepithelium and aggregate in the inferior band 1 day after the injection become progressively displaced toward the stratum pyramidale 2-3 days later, and penetrate the stratum pyramidale of the CA1 region on the 4th day. In the stratum pyramidale of the CA3 region, farther removed from the Ammonic neuroepithelium, the heavily labeled cells are still sojourning in the intermediate zone 4 days after labeling. Observations in methacrylate sections suggest that two morphogenetic features of the developing hippocampus may contribute to the long sojourn of young pyramidal cells in the intermediate zone: the way in which the stratum pyramidale forms and the way in which the alveolar channels develop. The stratum pyramidale of the CA1 region forms before that of the CA3 region, which is the reverse of the neurogenetic gradient in the production of pyramidal cells. We hypothesize that this is so because the pyramidal cells destined to settle in the CA3 region, which will be contacted by granule cells axons (the mossy fibers), have to await the formation of the granular layer on days E21-E22. Concordant with this is the observation that the hippocampal intermediate zone, which contains the sojourning young pyramidal cells, greatly enlarges between days E16 and E20, then suddenly diminishes and disappears by day E22. The other factor that may contribute to the prolonged sojourn of pyramidal cells, specifically those destined to settle in the CA1 region, is the pattern of alveolar channel development. This transient extracellular matrix begins to form several days after the onset of pyramidal cell neurogenesis, grows in a direction opposite to the settling of pyramidal cells in the stratum pyramidale, and does not reach the subicular end of Ammon's horn until day E21.(ABSTRACT TRUNCATED AT 400 WORDS)

Labeling of the neurons of origin of zinc-containing pathways by intraperitoneal injections of sodium selenite

Intraperitoneal injections of sodium selenite result in the formation of zinc-selenium complexes in zinc-containing axonal boutons ("Timm stainable boutons"), and the zinc-selenium precipitate can be rendered visible in histological sections by silver enhancement. In this work we present evidence, in the rat, that zinc-selenium precipitates formed in vivo after intraperitoneal injections of sodium selenite are translocated by colchicine-sensitive retrograde transport to neural perikarya when animals are allowed to survive 12-24 h after the selenite administration. Silver enhancement renders the perikaryal precipitates visible and thus demonstrates the perikarya of all zinc-containing neurons in the CNS simultaneously. Large populations of zinc-containing neurons identified by the method are found in layers II, III, and VI of all neocortical areas, in the superficial and deep layers of the prepyriform areas and, with a high degree of regional differentiation, in the retrosplenial, entorhinal, para- and presubicular cortices, the hippocampal formation and the amygdaloid complex. Zinc-containing cells were absent from the caudate-putamen, nucleus accumbens and septal complex. Labeled zinc-containing cells are absent in non-telencephalic parts of the brain. The findings indicate that the zinc-containing circuitry of the brain mainly serves in telencephalic information processing.

Comparative quantitative study of the hippocampal region of two closely related species of wild mice: interspecific and intraspecific variations in volumes of hippocampal components

To investigate the structural changes in the hippocampal region (subiculum, Ammon's horn, and area dentata) associated with speciation, the volumes of homologous components of this region of the forebrain were compared in the two closely related murine rodent species, Apodemus flavicollis (yellow-necked wood mice) and A. sylvaticus (long-tailed field mice), and in two geographically separated groups of A. sylvaticus. With the exception of the mossy fiber zone, no significant differences were found in the relative sizes of the components of Ammon's horn. Significant interspecific differences were found in the deep subiculum, in the intermediate (medial perforant path) and deep (commissural-associational) zones of the molecular layer of the area dentata, and in the hilus. A significant intraspecific difference was found in the combined intermediate and superficial (medial and lateral perforant path) zone of the dentate molecular layer. Differences found in the relative size of the hilus and the mossy fiber zone of Ammon's horn were complementary in that the combined volumes of these zones, which are both terminal fields of dentate granule cells, did not differ in the species. This finding provides evidence that the distribution of the terminal field of a projection system can be altered while the size of the terminal field is maintained. Within the hippocampal region, components of Ammon's horn appear well suited for quantitative comparative studies that span taxonomic units beyond the species level. In agreement with previous quantitative studies, differences in the volumes of components of Ammon's horn found in species of different orders are more likely to reflect phylogenetic trends rather than changes resulting from specializations of the particular species used. This is not true for the subiculum and the components of the area dentata. Experimentally induced changes in the connectivity of the hippocampal region are discussed in terms of the structural changes which may be responsible for the quantitative differences observed between the two species studied here.

Excessive intra- and supragranular mossy fibers in the dentate gyrus of tottering (tg/tg) mice

In Timm's sulfide silver preparations, intragranular and supragranular mossy fiber staining is found to be much more prevalent in the temporal dentate gyrus of the spontaneous epileptic mouse, tottering, than at matching levels in unaffected littermate controls. This aberrant distribution of mossy fibers may be due to the spontaneous seizures affecting this mutant.

Neurophysiological characterization of commissurally projecting dentate neurons in the rat

Commissural neurons in the dentate hilus and in the deep dentate granule cell layer were recorded intracellularly in vivo, in conjunction with combined injection of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) at sites of electrical stimulation. Two hilar neurons responded with short latency antidromic spikes to stimulation of the contralateral dentate infrapyramidal molecular layer, but did not show any synaptic potentials, suggesting that these neurons do not receive commissural hilar input, either directly or indirectly, from the stimulating sites. On the other hand, 3 dentate-hilar border neurons responded to the contralateral hilar stimulation with antidromic spikes, excitatory postsynaptic potentials (EPSPs), orthodromic spikes, and inhibitory PSPs (IPSPs), suggesting a rich synaptic interaction both commissurally and locally in this region. No direct commissural inhibition was observed in any of the cells. PHA-L injection at the stimulation site indicated that commissural hilar axon terminals project to a limited region of the contralateral molecular layer in a lamellar fashion, and have only a sparse distribution in the contralateral hilus. The results indicate that rapidly conducting commissural neurons in the dentate gyrus are themselves inhibited in an indirect manner by commissural fibers.

Hippocampal granule cells are necessary for normal spatial learning but not for spatially-selective pyramidal cell discharge

The effects of massive destruction of granule cells of the fascia dentata on the spatial and temporal firing characteristics of pyramidal cells in the CA1 and CA3 subfields of the hippocampus were examined in freely moving rats. Microinjections of the neurotoxin colchicine were made at a number of levels along the septo-temporal axis of the dentate gyri of both hemispheres, resulting in destruction of over 75% of the granule cells. By contrast there was relatively little damage to the pyramidal cell fields. As assessed by three different behavioral tests, the colchicine treatment resulted in severe spatial learning deficits. Single units were recorded from the CA1 and CA3 subfields using the stereotrode recording method while the animals performed a forced choice behavioral task on the radial 8-arm maze. Considering the extent of damage to the dentate gyrus, which has hitherto been considered to be the main source of afferent information to the CA fields, there was remarkably little effect on the spatial selectivity of "place cell" discharge on the maze, as compared to recordings from control animals. There was, however, a change in the temporal firing characteristics of these cells, which was manifested primarily as an increase in the likelihood of burst discharge. The main conclusion derived from these findings is that most of the spatial information exhibited by hippocampal pyramidal cells is likely to be transmitted from the cortex by routes other than the traditional "trisynaptic circuit". These routes may include the direct projections from entorhinal layers II and III to CA3 and CA1, respectively.

Projections from the medial cortex in the brain of lizards: correlation of anterograde and retrograde transport of horseradish peroxidase with Timm staining

Efferent projections of the medial cortex of the lizards Podarcis hispanica and Gallotia stehlinii were studied by examining the transport of horseradish peroxidase; results were correlated with those from Timm-stained sections. Two efferent systems were found. The first reaches the distal part of the outer plexiform layer in the medial, dorsomedial, and dorsal cortices, i.e., zones that are negative to Timm staining, and possibly originates from horizontal fusiform neurons. The second reaches the Timm-positive zones in the cortex and septum and is topographically arranged: the vertical portion of the intermediate and caudal medial cortex and the entire rostral medial cortex project to the inner two-thirds of the outer plexiform layer of the dorsomedial cortex and of the medial subfield of the dorsal cortex; to the paraventricular zone of the inner plexiform layer of the medial cortex; and bilaterally to the dorsal part of the dorsal precommissural septum. The dorsal part of the intermediate and caudal medial cortex and the ventralmost folded part of its caudal edge project rostrally to the juxtasomatic zone of the outer plexiform layer and the entire inner plexiform layer of the intermediate and lateral subfields of the dorsal cortex and to the ventral part of the dorsal septum. In its intense Timm reaction and its ultrastructural properties, as reported in earlier studies, the Timm-positive fiber system of the lizard brain shows a close resemblance to the mossy fiber system of the mammalian hippocampus.

Evidence that granule cells generated in the dentate gyrus of adult rats extend axonal projections

Fully mature rats were injected intraperitoneally with 3H-TdR on postnatal day (P) 100. After an additional 28-32 days, a retrograde fluorescent tracer, either FB or DY, was injected into the regio inferior of the hippocampal formation to label granule cells of the dentate gyrus through their mossy fiber axons. Examination of autoradiographs from these brains reveals that 3H-TdR labeled cells within the granule cell layer of the dentate gyrus are often labeled with the retrograde tracer as well. This indicates that within the mature hippocampal formation, newly generated dentate granule cells are capable of extending axonal projections for considerable distances.

The time of origin of somatostatin-immunoreactive neurons in the rat hippocampal formation

Experiments utilizing a combination of [3H]thymidine autoradiography and immunohistochemistry were conducted to determine the time of origin of somatostatin-immunoreactive (SSIR) neurons in the hippocampal formation of the rat. A quantitative and topographic description of neurogenesis in this peptide-containing neuronal system was generated using a computer-aided system to plot the position of labeled cells. Dissected and 'flattened' hippocampal preparations were used to facilitate the analysis of spatial gradients of SSIR cell development. The results indicate that most SSIR hippocampal cells are generated during a short embryonic period which extends from the 12th through the 15th day of gestation (E12-E15). Within this period of development, the distribution of SSIR cells follows a spatial gradient along the transverse or subiculo-dentate axis of the hippocampus. The earliest formed SSIR neurons, generated on E12 and E13, are preferentially distributed to the subiculum, those generated on E14 are most commonly observed throughout the CA1-CA3 fields of the hippocampus and SSIR neurons which become postmitotic on E15 are more heavily represented in the hilar region of the dentate gyrus than cells born at other stages of development. There was no clear-cut neurogenic gradient along the septotemporal axis of the hippocampus. These results indicate that somatostatin cells in the rat hippocampal formation are generated during the same prenatal period when glutamic acid decarboxylase (GAD)-positive neurons become postmitotic. These studies also suggest that quantitative developmental analyses of chemically specific cell types can reveal prominent features of cortical ontogeny that are not readily apparent in standard [3H]thymidine preparations.

Ultrastructural organization of normal and transplanted rat fascia dentata: I. A qualitative analysis of intracerebral and intraocular grafts

Few studies have dealt with the general ultrastructure and synaptic organization of grafted brain tissue. This study was therefore performed to extend current light microscopic observations on intracerebral and intraocular grafts of hippocampal tissue to the ultrastructural level. Blocks of tissue containing the hippocampus and fascia dentata from day 21 embryonic rats were grafted into the brain of developing and adult rats and to the anterior eye chamber of adult rats. After 100 or 200 days of survival the recipient rat brains or eyes were processed for electron microscopy. Tissue containing the graft dentate molecular layer and adjacent granule cell layer was selected for ultrastructural analysis, together with a few samples of the hilus and CA3. Normal dentate tissue was analyzed as control. At the light microscopic level most intracerebral and intraocular grafts displayed an organotypic organization with clearly recognizable cell and neuropil layers. Under the electron microscope the grafted granule cells had normal-appearing dendrites bearing the normal types of spines and forming the normal types of synapses. This was the case even in the absence of the normal major extrinsic afferents like the perforant path. The graft dentate granule cells formed axons and terminals with characteristics of the normal mossy fiber system in the hilus and CA3, in addition to aberrant supragranular mossy fiber terminals known from light microscopic studies of dentate transplants. Abnormal structures included a few dendritic growth cones and an increased occurrence of polyribosomes in spines. Their occurrence indicates ongoing dendritic plasticity even 100 days after transplantation. There was also an increased density of glial elements, particularly in the intraocular grafts. In some of these grafts the granule cells displayed immature traits in terms of nuclear indentations. Dentate interneurons of the basket cell type were present in both the intracerebral and the intraocular grafts. We conclude that grafted dentate granule cells, in different surroundings and without the normal, major perforant path input, can develop a basically normal cellular morphology, which includes the normal ultrastructural characteristics of the dendrites with spines and synapses, and the mossy fibers and its terminals.

Fine structure and synaptic connections of identified neurons in the rat fascia dentata

A survey is given of the synaptic connections of identified neurons in the rat fascia dentata based on our own Golgi/electron microscopic and light and electron microscopic immunocytochemical findings as well as on results obtained from the literature. The report largely deals with the dominating cell type in the region, the dentate granule cell. Of the various types of hilar cells, the GABAergic neurons, particularly the inhibitory basket cells, are taken into account. Differences in fine structure between granule cells and basket cells as well as mutual synaptic connections between these two types of dentate neurons are elaborated. This survey may provide a basis for further neurophysiological and pharmacological studies on these cells.

Possible pathogenic mechanisms on trimethyltin-induced lesions in the hippocampus of adult and neonatal rats : An overview

The extent of trimethyltin (TMT) induced lesions in the rat hippocampal formation was reviewed. Adult rats were treated with a single dose of 6.0 mg TMT/kg body wt and were sacrificed between 3-60 d following exposure. In the hippocampal formation, the granule cells of fascia dentata showed early changes, which subsided considerably at a later time of the intoxication. On the other hand, destruction of the pyramidal neurons in the Ammon's horn became more pronounced with time, resulting in an extensive destruction of this structure. It is interesting to note that the CA3 neurons in the septal portion of the Ammon's horn were more vulnerable than those located more temporally, whereas the reverse pattern was observed for the dentate granule cells as well as for the CA1,2 neurons of the Ammon's horn. Special stain for zinc (Timm's method) also revealed a progressive depletion of zinc in the mossy fibers. When neonatal rats were treated at various times with a single injection of TMT, rapid and progressive destruction of the Ammon's horn was observed in animals injected between postnatal day (PND) 5-15. The progression of neuronal involvement was CA3b →CA3a, b →CA3(a,b,c)→CA3+CA2→entire Ammon's horn (CA1,2,3). This pattern of pathological lesion was in good concert with morphological development and functional maturity of the hippocampal formation. Destruction of the Ammon's horn neurons was proposed to be the result of hyperexcitation of the dentate granule neurons under the influence of TMT. Other possible mechanisms are also discussed.

On the numbers of neurons in fields CA1 and CA3 of the hippocampus of Sprague-Dawley and Wistar rats

In a previous study it was found that there are significant differences in the numbers of granule cells in the dentate gyrus of adult Sprague-Dawley and Wistar rats and also that the continued postnatal addition of new cells to the dentate gyrus has quite different consequences in the two strains. We have now extended these observations to the two major cytoarchitectonic fields of the hippocampus (the regio superior or field CA1; and the regio inferior or field CA3). The mean number of pyramidal neurons in field CA1 of 1-month-old Sprague-Dawley rats is 420,000 (+/- 60,000 S.E.), while Wistar rats at the same age have 320,000 (+/- 20,000). The numbers of neurons in field CA3 in the two strains are: 330,000 (+/- 30,000) and 210,000 (+/- 20,000), respectively. Whether these strain differences reflect specific differences in the neural organization of the hippocampal formation in the two strains, or are related to more general differences in total body weight or brain weight, is unknown. Since during the first two days postnatally we estimate that there are between 358,000 and 491,000 cells in field CA1 of Sprague-Dawley rats, it would seem that there is no significant naturally-occurring neuronal death in this hippocampal field. This may be due to the extensive collateral projections of the hippocampal pyramidal neurons.

Kainate binding sites in the hippocampal mossy fibers: localization and plasticity

The regional distribution of high affinity binding sites for kainic acid has been determined in rat hippocampi by quantitative autoradiography. Selective lesions were made in order to determine the exact localization of these sites in the mossy fiber system, and to evaluate whether the sprouting and synaptic reorganization of the mossy fibers are associated with alterations in the distribution of these binding sites. The results show that kainate binding sites in the stratum lucidum are more vulnerable to destruction of the granules and their mossy fibers by intrahippocampal colchicine injections than to destruction of the CA3/CA4 pyramidal cells by injection of kainate into the amygdala. This suggests that a substantial proportion of the kainate binding sites is associated with the mossy fiber terminals (i.e. the presynaptic elements). Furthermore, in keeping with an earlier study, destruction of the pyramidal neurons of CA3 by intracerebral kainate produced a dark Timm positive band in the supragranular zone which is due to the sprouting of mossy fibers. This was associated with an increase in the density of kainate binding sites, which further stresses the parallelism between the distribution of these sites and mossy fiber terminals.