Growth of hippocampal mossy fibers: a lesion and coculture study of organotypic slice cultures

In hippocampal slice cultures, the mossy fibers from the dentate granule cells project as normally to cells in the dentate hilus (CA4) and the hippocampal CA3 pyramidal cells. After lesions in vivo and intracerebral transplantation, the mossy fibers can alter their normal distribution within CA3 and even contact CA1 pyramidal cells. The present study examined whether similar changes could be induced in the more simple, virtual two-dimensionally organized slice cultures. For this purpose slices of 7-day-old hippocampi were prepared and subjected to one of the two following manipulations: (1) transection of the mossy fiber layer in CA3 or (2) rearrangement of the geometrical relations between the dentate granule cells in their potential targets (CA3 and CA1) by coculturing dentate slices with CA3 or CA1 slices. Two to 8 weeks later the distribution of the mossy fiber system was visualized by histochemical Timm sulphide silver staining of the terminals. The distribution of the mossy fiber system observed in previous studies of ordinary hippocampal slice cultures was confirmed. In addition, mossy fibers were found to cross the cuts through the mossy fiber layer with formation of a reduced number of characteristic Timm-stained terminals in CA3 distal to the lesion. Close proximity and contiguity of the cut surfaces were important for such growth to occur. Significantly fewer mossy fiber terminals were found when separate slices of dentate and CA3 tissue were joined and grown as cocultures. Similar apposition of slices of dentate and CA1 tissue only rarely resulted in the ingrowth of mossy fibers into CA1. The Timm-stained mossy fiber terminals were then of subnormal size. The results show the potentials of the slice culture technique in supplementing lesion and transplant studies in situ. The growth of mossy fibers across a transection of their pathway is thus a new observation, difficult to demonstrate in the brain. The limited growth in the cocultures of aberrant mossy fibers into Ca1 does, on the other hand, emphasize the importance of close structural contact for the formation of nerve connections, and such contact is apparently more easily obtained in the brain. When the growth of the mossy fibers and that of the cholinergic septohippocampal fibers are compared, it is evident that the cholinergic axons grow better both in vitro and in vivo after lesions and transplantation.     

Beta-adrenergic receptors in the hippocampal and retrohippocampal regions of rats and guinea pigs: Autoradiographic and immunohistochemical studies

Species differences in the distribution of β-adrenergic receptors in the hippocampal and retrohippocampal regions of rats and guinea pigs were examined using in vitro autoradiographic techniques. β₁-receptors were found in the hippocampal area CA1 of both species, although guinea pigs had significantly lower receptor densities in comparison to rats. In guinea pigs, β₂-adrenergic receptors were predominant in hippocmpal area CA1. Hippocampal area CA3 had very low levels of β₁- and β₂-receptors in both species. The retrohippocampal area was also found to have a distinct topographic distribution of β-receptors. In rats, the subiculum and parasubiculum (layers II-III) were heavily labeled for β₁-receptors; in contrast, guinea pigs had few receptors in these regions. β₂-receptors were particularly prominent in the parasubicular region in rats. The entorhinal cortex laminae was found to contain β-receptors in both rats and guinea pigs. Immunohistochemical techniques were used to compare the pattern of catecholaminergic innervation with the receptor distribution within each hippocampal subregion. Despite the general lack of β-receptors in area CA3, abundant catecholamine immunoreactive fibers were observed in CA3 of rat and guinea pig hippocampus. Significant species differences were found in the distribution of hippocampal β-adrenergic receptor subtypes, and moreover, in both species the distribution of hippocampal β-adrenergic receptors did not coincide with the pattern of hippocampal adrenergic innervation. © 1993 Wiley-Liss, Inc.     

Secretoneurin: A marker in rat hippocampal pathways

Secretoneurin is a 33-amino acid peptide, generated in brain by proteolytic processing of secretogranin II. The distribution of secretoneurin-like immunoreactivity and secretogranin II mRNA was investigated in the hippocampus of the rat. Secretogranin II mRNA was found in high concentrations throughtout the granule cell and pyramidal cell layers and in many local neurons, notably in the hilus of the dentate gyrus. The general distributional pattern of secretoneurin-like immunoreactivity was characterized by a prominent staining in the area of the terminal field of mossy fibers with an obvious staining in the infrapyramidal area of CA3 and a strongly immunopositive band in the inner third of the molecular layer of the dentate gyrus. Lesions of the granule cells by local injection of colchicine significantly reduced secretoneurin-like immunoreactivity in the terminal field of mossy fibers, but not in the inner molecular layer of the dentate gyrus. On the other hand, destruction of interneurons of the dentate gyrus (mossy cells and certain γ-aminobutyric acid-ergic interneurons) by kainic acid—induced seizures was associated with a reduction of secretoneurin-like immunoreactivity in the inner molecular layer of the dentate gyrus. However, 30 days after kainic acid—induced seizures, a strongly secretoneurin-immunoreactive band reappeared in this area, which at this late time point is due to sprouting of mossy fibers collaterals. Our experiments suggest a widespread distribution of secretoneurin-like immunoreactivity in neurons of the hippocampal formation with a preferential localization in excitatory pathways including associational/commissural fibers originating from secretoneurin-containing mossy cells. J. Comp. Neurol. 377:29-40, 1997. © 1997 Wiley-Liss, Inc.     

The comparative immunocytochemical distribution of 28 kDa cholecalcin (CaBP) in the hippocampus of rat, guinea pig and hedgehog

The distribution of 28 kDa cholecalcin (calcium-binding protein, CaBP) in the hippocampal formation of the rat, guinea pig and European hedgehog was examined by immunocytochemistry. The extension of the mossy fibers (the axons of the granule cells of the dentate gyrus) was also studied using the Timm's sulfide-silver method. Cholecalcin was present in all mossy fibers. In the rat, only those pyramidal cells not reached by the labeled mossy fibers displayed cholecalcin immunoreactivity. Immunocytochemical staining of the hedgehog hippocampus showed that contacts between cholecalcin-containing mossy fibers and cholecalcin-containing pyramidal cells are possible. Consequently, the protein is probably not involved in the control of mossy fiber extension. Strikingly, no guinea pig pyramidal cells showed cholecalcin immunoreactivity. The possible involvement of cholecalcin in the differential excitability of pyramidal cells in the CA3 and CA1 areas of the hippocampus could therefore be tested in a comparative study of rat, guinea pig and hedgehog.     

Target regulation of sympathetic sprouting in the rat hippocampal formation

Sympathetic fibers appear in the rat hippocampal formation after lesions of the medial septal nucleus. The distribution of sympathetic axons correlates most closely with the distribution of dentate granule cells and their axons. To test the hypothesis that granule cells are the targets of sympathetic fibers, neurotoxins were injected to destroy either the granule cells (colchicine) or the pyramidal cells (kainic acid) in the hippocampal formation 1 month prior to placing septal lesions. Sympathetic fibers appeared in animals with kainic acid lesions but not in regions lacking granule cells in animals treated with colchicine. These results support the hypothesis that the granule cells of the dentate gyrus are the targets of sprouting sympathetic fibers.     

Calcium-binding protein (calbindin-D28k) and parvalbumin immunocytochemistry: localization in the rat hippocampus with specific reference to the selective vulnerability of hippocampal neurons to seizure activity

Two neuronal calcium-binding proteins, calbindin-D28k (CaBP) and parvalbumin (PV), were localized in the normal rat hippocampus by using immunocytochemical methods to determine 1) their location and 2) whether a correlation exists between the presence of these two calcium-binding proteins and the selective vulnerability of different hippocampal neuronal populations to experimental seizure activity. CaBP-like immunoreactivity (CaBP-LI) is present in all dentate granule cells and some, but not all, CA1 and CA2 pyramidal cells. Some CA1 pyramidal cells lack CaBP-LI, and those that do are lightly stained compared to the dentate granule cells. CA3 pyramidal cells appear to contain neither CaBP- nor PV-LI, and no granule or pyramidal cells exhibit PV-LI. CaBP-LI is present in distinct populations of dentate and hippocampal interneurons but absent from others. In area dentata, CaBP-LI is present in a small number of interneurons of the molecular and granule cell layers and in a small population of presumed basket cells in or below the granule cell layer. Conversely, more presumed dentate basket cells exhibit PV-LI than CaBP-LI. In the hilus of area dentata, few cells are CaBP- or PV-immunoreactive. The hilar somatostatin/neuropeptide Y (NPY)-immunoreactive cells and hilar mossy cells, two distinct and large populations, lack CaBP- and PV-LI. In the CA3 region, CaBP-LI is present in a relatively small number of interneurons in each stratum. PV-immunoreactive interneurons in area CA3 are more numerous. In area CA1, CaBP-LI is present in many interneurons in strata radiatum and lacunosum-moleculare. Some, but relatively fewer, CaBP-positive interneurons are present in strata pyramidale and oriens. Conversely, PV-immunoreactive interneurons are numerous in strata pyramidale and oriens but rare in strata radiatum and lacunosum-moleculare. Staining with the particulate chromagen benzidine hydrochloride revealed a previously undescribed dense band of CaBP-LI in the inner dentate molecular layer, a lamina enriched with kainate-displaceable glutamate-binding sites and innervated by the apparently excitatory ipsilateral associational/commissural (IAC) pathway that originates in the CaBP-negative hilar mossy cells. Bilateral electrical stimulation of the perforant path was performed in order to destroy the hilar mossy cells and to determine if this band of CaBP-LI is normally present within the mossy cell terminals. Perforant path stimulation that destroyed hilar mossy cells throughout the dorsal portions of both hippocampi did not abolish the dense CaBP-like immunoreactivity in the inner molecular layer.     

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.     

Intracellular recording and labeling of mossy cells and proximal CA3 pyramidal cells in macaque monkeys

Little is known about the morphological characteristics and intracellular electrophysiological properties of neurons in the primate hippocampus and dentate gyrus. We have therefore begun a program of studies using intracellular recording and biocytin labeling in hippocampal slices from macaque monkeys. In the current study, we investigated mossy cells and proximal CA3 pyramidal cells. As in rats, macaque mossy cells display fundamentally different traits than proximal CA3 pyramidal cells. Interestingly, macaque mossy cells and CA3 pyramidal neurons display some morphological differences from those in rats. Macaque monkey mossy cells extend more dendrites into the molecular layer of the dentate gyrus, have more elaborate thorny excrescences on their proximal dendrites, and project more axon collaterals into the CA3 region. In macaques, three types of proximal CA3 pyramidal cells are found: classical pyramidal cells, neurons with their dendrites confined to the CA3 pyramidal cell layer, and a previously undescribed cell type, the "dentate" CA3 pyramidal cell, whose apical dendrites extend into and ramify within the hilus, granule cell layer, and molecular layer of the dentate gyrus. The basic electrophysiological properties of mossy cells and proximal CA3 cells are similar to those reported for the rodent. Mossy cells have a higher frequency of large amplitude spontaneous depolarizing postsynaptic potentials, and proximal CA3 pyramidal cells are more likely to discharge bursts of action potentials. Although mossy cells and CA3 pyramidal cells in macaque monkeys display many morphological and electrophysiological features described in rodents, these findings highlight significant species differences, with more heterogeneity and the potential for richer interconnections in the primate hippocampus.     

Localization of enkephalin-like immunoreactivity to identified axonal and neuronal populations of the rat hippocampus

The distribution of enkephalin-like immunoreactivity in the hippocampal formation of the rat was analyzed. Two specific projection systems are described. The first emerges from the hilus of the dentate gyrus and appears to terminate with notably large boutons on the proximal apical and, to a lesser extent, basal dendrites of hippocampal regio inferior pyramidal cells. This projection corresponds in source, position, and character to the hippocampal mossy fiber system. The second axonal population enters the temporal hippocampal formation from the medial wall of the subicular complex and follows the hippocampal fissure to occupy stratum lacunosum-moleculare of the hippocampus proper and the distal third of the dentate gyrus molecular layer; this pattern corresponds to the distribution of afferent input from the lateral entorhinal cortex and/or perirhinal area. Lesions of the hilus or retrohippocampal area caused a selective depletion of immunoreactivity in the mossy fiber fields and molecular layers of the dentate gyrus, respectively. Enkephalin-like immunoreactivity was found within the somata of three types of hippocampal neurons: (1) granule cells of the dentate gyrus, (2) occasional pyramidal shaped cells of field CA1 stratum pyramidale, and (3) varied scattered interneurons. Of this last group, two types of interneurons were consistently seen. The first occupy the border between stratum radiatum and stratum lacunosum-moleculare and extend processes at right angles to the long axis of the pyramidal cell dendrites, whereas the second lie within stratum radiatum of field CA1 and extend processes in alignment with the long axis of the pyramidal cell dendrites. Cells containing enkephalin-like immunoreacactivity were also observed in the subiculum and retrohippocampal region, most notably including layers II and III of the lateral entorhinal cortex-perirhinal area—the probable source of extrinsic immunoreactive input to the hippocampal formation. Intraventricular colchicine treatment intensified the immunoreactive staining of some hippocampal neurons but did not reveal any cell types not seen to be labeled in untreated rats.     

The hippocampal mossy fiber system of the rat studied with retrograde tracing techniques. Correlation between topographic organization and neurogenetic gradients

The topographical organization of the hippocampal mossy fiber system, which connects the dentate granule cells with the pyramidal cells of the regio inferior, has been examined in rats with retrograde tracing methods. Following the application of the fluorescent dye True Blue to different parts of the mossy fiber layer in the hippocampal regio inferior, retrogradely labeled granule cells were observed in the dentate fascia. The distribution of the labeled cells within the dentate granule cell layer indicates that all mossy fibers have an almost parallel, slightly descending course in regio inferior near the dentate hilus. In the ventricular part of regio inferior, particularly toward the transition to the regio superior, the mossy fibers are sorted out according to the position of their parent cell bodies within the granular layer. Near the transition to regio superior the fibers from lateral granule cells extend both septally and temporally over a longer distance than the fibers from more medial cells. Similarly, the fibers coming from the superficial cells extend both septally and temporally over a longer distance than those from the deep cells. The mossy fibers arising from a specific septotemporal level of the dentate fascia innervate a segment of the regio inferior that extends approximately 180 μm above to approximately 1,600 μm below the level of origin. Similar results were obtained following injections of Nuclear Yellow and horseradish peroxidase. Since previous studies have demonstrated that the granule cells are formed along gradients from lateral to medial and from superficial to deep, there appears to be a correlation between the formation, and hence the position, of the granule cells and the topography of their projection into the regio inferior.     

Cellular and connective organization of slice cultures of the rat hippocampus and fascia dentata

This study examined the cellular and connective organization of hippocampal tissue taken from 6-8-day-old rats and cultured by the roller tube technique for 3-6 weeks. In the cultures containing the fascia dentata and the hippocampus proper (CA1, CA3, CA4) the main cell and neuropil layers were organotypically organized when observed in ordinary cell stains. The normal distribution of smaller cell populations of AChE-positive neurons and somatostatin-reactive neurons was demonstrated by histochemical and immunohistochemical methods. Both cell types were mainly confined to str. oriens of CA3 and CA1 and the dentate hilus (CA4). Individual dentate granule cells and hippocampal pyramidal cells were injected with lucifer yellow and HRP, revealing great stability of the dendritic patterns of these cells in the culture condition. The same was found for the axonal branching and termination of HRP-filled mossy fibers arising from an HRP-injected granule cell. The preservation of organotypic afferent patterns in the cultures was also shown by Timm staining of the terminal distribution of the mossy fiber system. Mossy fiber terminals, with characteristic ultrastructural features verified in the electron microscope, were thus found in the hilus (CA4) and along the CA3 pyramidal cell layer onto the CA3-CA1 transition. Depending on the amount of dentate tissue relative to CA3 the terminals could stop before reaching CA1 (small fascia dentata) or take up additional intra and infrapyramidal locations along CA3 (small CA3). In cultures with a gap in the CA3 pyramidal cell layer some mossy fiber terminals were found in contact with the CA3 pyramidal cells beyond the gap. In all cultures there was an aberrant projection of supragranular mossy fibers. This projection is analogous to the one known from lesion and transplant studies to form in the absence of the entorhinal perforant path input to the dentate molecular layer. Also, in accordance with these studies the Timm staining pattern of the outer parts of the dentate molecular layer and the entire molecular layer of the hippocampus was altered corresponding to the spread of afferents normally confined to the inner zone of the dentate and str. radiatum of CA3 and CA1. Possibly as a consequence of the lack of normal targets for projections from CA1, this subfield contained an unusually dense Timm staining suggestive of autoinnervation.(ABSTRACT TRUNCATED AT 400 WORDS)     

Protein kinase C subspecies distinguish major cell types in rat hippocampus: an immunocytochemical and in situ hybridization histochemical study.

This study compares the distribution of protein kinase C (PKC) alpha and beta with the distribution of PKC epsilon in the hippocampal formation of rats by immunocytochemistry and in situ hybridization histochemistry. Alpha and PKC beta are members of the group A PKC genes that were first described; PKC epsilon is a member of the group B PKC genes that were more recently identified by molecular cloning. A combination of all three gene products and their mRNAs overlapped in their distributions in dentate granule cells and pyramidal and nonpyramidal neurons. However, each subspecies predominated in one of the major cell types. PKC alpha-immunoreactivity and mRNA were most intense in CA2-3 pyramidal cells and dendrites, whereas PKC beta-immunoreactivity and mRNA were most intense in CA1 pyramidal cells and dendrites. PKC epsilon-immunoreactivity and mRNA were concentrated in dentate granule cells and CA3 pyramidal cells. Furthermore, PKC epsilon-immunoreactivity was detectable in mossy fibers. Each subspecies labeled different kinds of interneurons that were particularly numerous in, but not restricted to, the hilus. These data support the contention that different subtypes of hippocampal neurons are distinguished by the expression of different combinations of PKC subspecies under resting conditions.     

Substance P innervation of the rat hippocampal formation

Light and electron microscopic substance P (SP) immunostaining was performed on hippocampal sections of colchicine-pretreated, control, untreated fimbria-fornix-transected (5 days), as well as perforant path-stimulated Sprague-Dawley rats fixed in 5% acrolein. Numerous SP-immunoreactive neurons could be observed in the stratum oriens of the Ammon's horn and subiculum, fewer were seen in the dentate hilar area and stratum radiatum of CA2 and CA3, and even fewer were seen at the border between the CA1 strata radiatum and the lacunosum moleculare of CA1 subfield. A higher dose of colchicine resulted in SP immunoreactivity in a large population of granule cells and mossy axon terminals. The entire CA2 region, the stratum oriens of CA1, CA3, and the subiculum were densely innervated by SP-containing axon terminals. A homogeneous SP innervation was found in the stratum radiatum of CA1. Only a few SP fibers were seen adjacent to the granule cell layer. Symmetric axosomatic contacts were seen between SP-containing boutons and somata in the stratum oriens of the Ammon's horn. However, throughout the hippocampal formation, the majority of SP-containing axons formed axodendritic symmetric synapses. A dense population of SP-immunoreactive boutons that formed axodendritic asymmetric synapses was observed in the strata oriens and radiatum of the CA3a and CA2 regions, and a few were found in the supragranular and subgranular layers of the dentate gyrus. Fimbria-fornix transection resulted in a marked loss of SP fibers in the strata oriens, pyramidale, and radiatum of the CA3a and CA2 subfields. In perforant pathway-stimulated animals, a population of granule cells and a large number of mossy axon terminals were immunoreactive for SP. These observations suggest two sources of SP innervation to the hippocampal formation: one arising from intrinsic sources (interneurons and granule cells) and one arising from extrinsic sources, most likely the supramammillary region. J. Comp. Neurol. 384:41–58, 1997. © 1997 Wiley-Liss, Inc.     

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

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

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

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

Reduction of chromogranin B-like immunoreactivity in distinct subregions of the hippocampus from individuals with schizophrenia

Synaptic disturbances may play a key role in the pathophysiology of schizophrenia. This study was designed to further investigate possible synaptic alterations in the brains of chronic schizophrenic patients. Chromogranin B was applied as a marker for large dense core vesicles and synapsin I as a protein associated with the synaptic vesicle membrane. The distribution and density of chromogranin B-and synapsin I-like immunoreactivity in subregions of the hippocampus was compared between controls (n = 16) and patients with schizophrenia (n = 17).The overall distribution of hippocampal chromogranin B- and synapsin I-like immunoreactivity was similar in controls and in schizophrenic patients with the highest densities in the terminal field of mossy fibers and in the inner molecular layer of the dentate gyrus. In schizophrenic hippocampi, a significant reduction in the density of chromogranin B-like immunoreactivity was found in the CA4 and CA3 but not in the CA1 area of the dentate gyrus based on computerized image analysis. The loss of immunoreactivity was localized to mossy fibers and terminals surrounding hilar interneurons. Double-labelling immunohistochemistry revealed that synapsin I was co-expressed with chromogranin B in these neuronal structures and was also significantly reduced in schizophrenic hippocampi.The present study demonstrates an area-specific reduction of chromogranin B which is paralleled by a decrease of synapsin I. The loss of presynaptic proteins involved in distinct steps of exocytosis may cause complex synaptic disturbances in specific hippocampal subregions resulting in an imbalanced neurotransmitter availability in schizophrenic patients.     

Aging impairs axonal sprouting response of dentate granule cells following target loss and partial deafferentation.

Compared to other brain regions, the hippocampus shows considerable susceptibility to the aging process. Aging may impair the compensatory plastic response of hippocampal neurons following lesions, target loss, and/or deafferentation. We hypothesize that sprouting of dentate granule cell axons (mossy fibers) in response to target loss and partial deafferentation diminishes with age. We quantified mossy fiber sprouting into the dentate supragranular layer (DSGL) following intracerebroventricular kainic acid administration in young adult, middle-aged, and aged rats, using Timm's histochemical method. Mossy fiber ingrowth into the DSGL was assessed in the septal hippocampus at 2- and 4 months postlesion by measuring both the average width and the relative density of sprouted terminals. Kainic acid lesions produced degeneration of CA3 pyramids with sparing of CA1 and dentate granule cells in all age groups. Although young adults demonstrated robust DSGL mossy fiber sprouting, sprouting was significantly reduced in both middle-aged and aged rats. Compared to the case in young adults, the overall sprouting in middle-aged animals was reduced by 52% at 2 months and 50% at 4 months postlesion, whereas in aged rats the sprouting was reduced by 53% at 2 months and 64% at 4 months postlesion. Aged animals also showed an overall reduction of 28% compared to middle-aged animals at 4 months postlesion. Dramatically reduced sprouting in aged animals may represent a deficit in recognition of target loss and partial deafferentation by aged granule cells and/or an impaired up-regulation of factors that stimulate neurite outgrowth in the aged brain.     

Peripheral sympathetic ingrowth can alter metabolic activity within the hippocampal formation

After cholinergic denervation of the hippocampal formation, peripheral sympathetic fibers arising from the superior cervical ganglia grow into the dentate gyrus and CA3 region; the functional significance of sympathetic ingrowth into the hippocampal formation is unknown. Utilizing electrical stimulation of the preganglionic trunk in combination with 2-deoxy-D-[3H]-glucose fine-grained autoradiograms, we demonstrated an alteration of hippocampal glucose metabolism, suggesting that this neuronal rearrangement makes functional connections within the hippocampus.     

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.     

Enhanced acetylcholinesterase staining in hippocampal area CA3 after lesion of granule cells by infusion of colchicine

Unilateral infusion of colchicine into the lateral ventricle produced relatively selective destruction of dentate granule cells in the ipsilateral dorsal hippocampal formation of the rat. Timms silver sulfide stain is markedly reduced in the mossy fiber layer on the colchicine treated side but is normal contralaterally. After colchicine treatment, an increase in acetylcholinesterase staining is apparent in the apical dendritic zone of CA3 pyramidal cells. This enhanced staining is localized in the proximal apical dendritic layer of CA3, a region normally occupied by the mossy fiber terminals of dentate granule cells. These results suggest that cholinergic fibers proliferate in CA3 after granule cell lesion and may participate in reinnervation of the denervated area.