Differential expression of synaptophysin and synaptoporin during pre- and postnatal development of the hippocampal network

The closely related synaptic vesicle membrane proteins synaptophysin and synaptoporin are abundant in the hippocampal formation of the adult rat. But the prenatal hippocampal formation contains only synaptophysin, which is first detected at embryonic day 17 (E17) in perikarya and axons of the pyramidal neurons. At E21 synaptophysin immunoreactivity extends into the apical dendrites of these cells and in newly formed terminals contacting these dendrites. The transient presence of synaptophysin in axons and dendrites suggests a functional involvement of synaptophysin in fibre outgrowth of developing pyramidal neurons. Synaptoporin expression parallels the formation of dentate granule cell synaptic contacts with pyramidal neurons: the amount of hippocampal synaptoporin, determined in immunoblots and by synaptoporin immunostaining of developing mossy fibre terminals, increases during the first postnatal week. Moreover, in the adult, synaptoporin is found exclusively in the mossy fibre terminals present in the hilar region of the dentate gyrus and the regio inferior of the cornu ammonis. In contrast, synaptophysin is present in all synaptic fields of the hippocampal formation, including the mossy fibre terminals, where it colocalizes with synaptoporin in the same boutons. Our data indicate that granule neuron terminals differ from all other terminals of the hippocampal formation by the presence of both synaptoporin and synaptophysin. This difference, observed in the earliest synaptic contacts in the postnatal hippocampus and persisting into adult life, suggests distinct functions of synaptoporin in these nerve terminals

Effect of scopolamine-based amnesia on the number of astrocytes in the rat's hippocampus

As neuron-astrocyte interactions play a crucial role in the adult brain, it is thought that astrocytes support learning and memory through specific mechanisms. In this study, the effect of scopolamine based amnesia on the number of astrocytes in rats' hippocampus was studied. Adult male albino Wistar rats were bilaterally cannulated into the CA1 region and animals received saline or different doses of scopolamine (0.5, 1 and 2 mg/ rat, intra - CA1), immediately after training. Then all the rats were sacrificed and coronal sections were taken from the dorsal hippocampal formation of the right cerebral hemispheres and stained with PTAH. The area densities of the astrocytes in dentate gyrus were measured and compared in the all groups (p < 0.05). Data showed that post-training scopolamine (0.5, 1 and 2 μg/rat, intra-CA1) dose-dependently reduced the step-through latency in the inhibitory avoidance task, showing scopolamineinduced amnesia. Also we found different response of astrocytes in different subfields of hippocampal formation. In dentate gyrus the number of astrocytes was increased, but in other areas scopolamine can decreased the density of astrocytes. We concluded that scopolamine can cause amnesia and this phenomenon can have an effect on astrocyte numbers in the rats hippocampal formation

Loss of Immunohistochemical Reactivity in Association With Handling-Induced Dark Neurons in Mouse Brains.

The handling-induced dark neuron is a histological artifact observed in brain samples handled before fixation with aldehydes. To explore associations between dark neurons and immunohistochemical alterations in mouse brains, we examined protein products encoded by Cav3 (neuronal perikarya/neurites), Rbbp4 (neuronal nuclei), Gfap (astroglia), and Aif1 (microglia) genes in adjacent tissue sections. Here, dark neurons were incidental findings from our prior project, studying the effects of age and high-fat diet on metabolic homeostasis in male C57BL/6N mice. Available were brains from 4 study groups: middle-aged/control diet, middle-aged/high-fat diet, old/control diet, and old/high-fat diet. Young/control diet mice were used as baseline. The hemibrains were immersion-fixed with paraformaldehyde and paraffin-embedded. In the hippocampal formation, we found negative correlations between dark neuron hyperbasophilia and immunoreactivity for CAV3, RBBP4, and glial fibrillary acidic protein (GFAP) using quantitative image analysis. There was no significant difference in dark neuron hyperbasophilia or immunoreactivity for any protein examined among all groups. In contrast, in the hippocampal fimbria, old age seemed to be associated with higher immunoreactivity for GFAP and allograft inflammatory factor-1. Our findings suggest that loss of immunohistochemical reactivity for CAV3, RBBP4, and GFAP in the hippocampal formation is an artifact associated with the occurrence of dark neurons. The unawareness of dark neurons may lead to misinterpretation of immunohistochemical reactivity alterations

Electrophysiological Signatures of Spatial Boundaries in the Human Subiculum.

Environmental boundaries play a crucial role in spatial navigation and memory across a wide range of distantly related species. In rodents, boundary representations have been identified at the single-cell level in the subiculum and entorhinal cortex of the hippocampal formation. Although studies of hippocampal function and spatial behavior suggest that similar representations might exist in humans, boundary-related neural activity has not been identified electrophysiologically in humans until now. To address this gap in the literature, we analyzed intracranial recordings from the hippocampal formation of surgical epilepsy patients (of both sexes) while they performed a virtual spatial navigation task and compared the power in three frequency bands (1-4, 4-10, and 30-90 Hz) for target locations near and far from the environmental boundaries. Our results suggest that encoding locations near boundaries elicited stronger theta oscillations than for target locations near the center of the environment and that this difference cannot be explained by variables such as trial length, speed, movement, or performance. These findings provide direct evidence of boundary-dependent neural activity localized in humans to the subiculum, the homolog of the hippocampal subregion in which most boundary cells are found in rodents, and indicate that this system can represent attended locations that rather than the position of one\u27s own body

Hippocampal subregion abnormalities in schizophrenia: A systematic review of structural and physiological imaging studies.

AimThe hippocampus is considered a key region in schizophrenia pathophysiology, but the nature of hippocampal subregion abnormalities and how they contribute to disease expression remain to be fully determined. This study reviews findings from schizophrenia hippocampal subregion volumetric and physiological imaging studies published within the last decade.MethodsThe PubMed database was searched for publications on hippocampal subregion volume and physiology abnormalities in schizophrenia and their findings were reviewed.ResultsThe main replicated findings include smaller CA1 volumes and CA1 hyperactivation in schizophrenia, which may be predictive of conversion in individuals at clinical high risk of psychosis, smaller CA1 and CA4/DG volumes in first-episode schizophrenia, and more widespread smaller hippocampal subregion volumes with longer duration of illness. Several studies have reported relationships between hippocampal subregion volumes and declarative memory or symptom severity.ConclusionsTogether these studies provide support for hippocampal formation circuitry models of schizophrenia. These initial findings must be taken with caution as the scientific community is actively working on hippocampal subregion method improvement and validation. Further improvements in our understanding of the nature of hippocampal formation subregion involvement in schizophrenia will require the collection of structural and physiological imaging data at submillimeter voxel resolution, standardization and agreement of atlases, adequate control for possible confounding factors, and multi-method validation of findings. Despite the need for cautionary interpretation of the initial findings, we believe that improved localization of hippocampal subregion abnormalities in schizophrenia holds promise for the identification of disease contributing mechanisms

The case for a relationship between human memory, hippocampus and corpus callosum

Unilateral brain damage which includes the hippocampus leads to memory impairments consistent with hemispheric specialization on the same side. Damage to the corpus callosum, the major connecting pathway between the left and right hemispheres, also leads to memory impairments. This suggests both hemispheric specialization on the hippocampal level and a critical role for the corpus callosum in memory functions. A complete hippocampal formation is present on either side of the brain but traditionally only one is studied. However, a comparison between the neuronal populations in the hippocampus on both sides revealed asymmetry in connectivity among hippocampal subfields. The profile of memory impairments of commissurotomy (‘split-brain’) patients is described. The results are discussed in terms of a relationship between hippocampus and corpus callosum in humans. As hemispheric specialization evolved, inter-hippocampal connections became less important and the corpus callosum became prominent in memory functions

Exploring the Ventral Hippocampal Formation

The hippocampus (HC) is important for both memory and spatial navigation. These functions are thought to be supported by spatial firing patterns of "place cells" which are active in only certain locations within environments. Downstream from the hippocampus lies the ventral striatum (vStr), a structure involved in the motivational control of behaviour. While hippocampal cells have strong modulation by place, ventral striatal cells have strong modulation by rewards and cues that predict them. Together with the ventral striatum, the hippocampus forms a functional circuit in rats, thought to associate place representations with rewards in a context-, or situationally-, dependent manner. However, within the hippocampal formation there is an intermediate structure in the place-reward circuit between HC and vStr, the ventral subiculum. Due to its intermediate location between HC and vStr, the ventral subiculum is expected to combine aspects of hippocampal place related, and ventral striatal reward-related activity, in a context-dependent manner. However, while ventral subiculum has been implicated in context discrimination by studies performing lesions of that structure, little research exists on how ventral subicular cells are modulated by place, reward, or different contexts. Additionally, while the firing patterns of dorsal hippocampal cells reorganize between different environments, we currently have less evidence of this contextual discrimination in the ventral hippocampal cells that are more directly connected to the ventral subiculum. This project therefore explores ventral subicular cell modulation by place, context, and reward-related stimuli, while examining responses to changes in environmental context in the ventral hippocampus. To explore these structures, 5 male Long-Evans rats were implanted with dual-target implants, 4 of which performed a biconditional discrimination task (Context A: Cue 1 –> reward, Cue 2 –> no reward; Context B: Cue 1 –> no reward, Cue 2 –> reward) while recordings were made from ventral hippocampal formation targets. Local field potentials (LFP) were analyzed for spectral power and event-related signals, while neural firing activity was examined for event-related modulation of activity and evidence of context discrimination. Behavioural results show that as a group, rats learned to perform the task, although not all rats learned equally well. Preliminary evidence is presented for context-modulated event-related activity in ventral subiculum, and context-modulated neural firing rates in ventral hippocampus, as well as evidence for peaks in signal power in the theta (6-10Hz), delta (1-4Hz) and gamma (low gamma ~50Hz) frequency ranges in ventral subicular local field potentials. Thus, a preliminary examination of the ventral hippocampus and ventral subiculum shows functional intermediaries, combining context and reward-related signals

Lateralization of the developing rat hippocampal formation

Differences between the left and right hemisphere of the brain have been observed in humans and rodents (Broca, 1861; Wernicke, 1881), including the hippocampal formation, a region of the brain that is necessary for some forms of learning and memory (Olton and Samuelson, 1976; deToledo-Morrell et al., 1988; Bernasconi-Guastalla et al., 1994; Tabibnia et al., 1994; Poe et al., 2000; Lister et al., 2006; Hanlon et al., 2005; Sommer et al., 2005; Moskal et al., 2006; Thompson et al., 2008; Klur et al., 2009). Although lateralization of the hippocampal formation has been studied in the adult, few have sought to directly examine the development of hippocampal lateralization (Moskal et al., 2006) and none have examined hippocampal lateralization in the embryo. The objective of the study outlined in this dissertation was to characterize the development of hippocampal lateralization in the rat. To achieve this objective, a rat CNS microarray with 1,178 genes representing the majority of ontological categories within the rat genome (Kroes et al., 2006) was used to examine lateralized gene expression in the embryonic rat hippocampal formation: 14 genes were all more highly expressed in the right hippocampus at E18 (Gross et al., 2008; Gross et al., 2010). Database for Annotation Visualization and Integrated Discovery (DAVID) and Gene Set Enrichment Analysis (GSEA) were also used to further investigate whether specific genes differentially expressed at E18 comprised pathways known to be important in the development of the hippocampal formation. Results demonstrated that genes related to structure, transcription and translation, cellular metabolism, glycolysis, and gap junction signaling were more highly expressed in the right hippocampus at E18. Expression of genes corresponding to proteins that comprise the gap junction signaling pathway were further examined using qRT-PCR. Results showed that alpha1a-tubulin, beta3-tubulin, and connexin43 were more highly expressed in the right hippocampus at E18. Using Western blot analysis, alpha1a-tubulin protein levels were also shown to be higher in the right hippocampus at E18. These results indicated that genes related to hippocampal growth and development were more highly expressed in the right hippocampus at E18, and furthermore they suggested that gap junctions may play a critical role in the development of hippocampal lateralization in the embryo. To further characterize the lateralized development of the rat hippocampal formation, the effect of N-methyl-D-aspartate glutamate receptor (NMDAR) mediated synaptic activity lateralized gene expression in the hippocampal formation during early postnatal development in the rat. During normal development, the pattern of lateralized gene expression displays a right-to-left shift in preferential expression between P6 and P9 (Moskal et al., 2006). A reduction in NMDA receptor (NMDAR) mediated synaptic activity using the selective NMDAR antagonist CPP, altered this pattern of lateralized gene expression at P9 (Rahimi et al., 2006, Gross et al., 2007; Claiborne et al., 2010). These data were then analyzed using Significance Analysis for Microarrays, DAVID, and GSEA analyses. The MAPK signaling pathway was enriched in the right hippocampal formation at P9 following CPP injections: these data were corroborated and extended using qRT-PCR. Expression of MAPK14 mRNA was not significantly different between the left and right hippocampal formation at postnatal day 6, nor was it greater in the right HF as compared to the left in saline treated rats at P9; however, MAPK14 mRNA was more highly expressed in the right hippocampal formation at P9 following a reduction in NMDAR activity between P6 and P9. c-Myc was more highly expressed in the right hippocampal formation at P6, and it was not differentially expressed during normal development or following saline control injections between P6 and P9. However, cMyc mRNA expression was significantly greater in the right hippocampal formation in CPP treated rats. These findings indicated that genes involved in the MAPK signaling pathway were upregulated in the right hippocampal formation during early postnatal development following a reduction in NMDAR-mediated synaptic activity. The findings presented in this dissertation are both novel and important: they are the first to demonstrate that lateralized gene expression is present in the embryonic rat hippocampal formation. Furthermore, these findings are the first to show the effect of early experience on the development of hippocampal lateralization in the first postnatal week. The results support the idea that differential gene expression patterns in the hippocampus are likely developmentally regulated and play a key role in the formation and function of that region and that the gene expression patterns can be significantly influenced by factors that modulate synapse plasticity

The similarity of astrocytes number in dentate gyrus and CA3 subfield of rats hippocampus

The dentate gyrus is a part of hippocampal formation that it contains granule cells, which project to the pyramidal cells and interneurons of the CA3 subfield of the hippocampus. Astrocytes play a more active role in neuronal activity, including regulating ion flux currents, energy production, neurotransmitter release and synaptogenesis. Astrocytes are the only cells in the brain that contain the energy molecule glycogen. The close relationship between dentate gyrus and CA3 area can cause the similarity of the number of astrocytes in these areas. In this study 5 male albino wistar rats were used. Rats were housed in large plastic cage in animal house and were maintained under standard conditions, after histological processing, The 7 μm slides of the brains were stained with PTAH staining for showing the astrocytes. This staining is specialized for astrocytes. We showed that the number of astrocytes in different (ant., mid., post) parts of dentate gyrus and CA3 of hippocampus is the same. For example, the anterior parts of two area have the most number of astrocytes and the middle parts of two area have the least number of astrocytes. We concluded that dentate gyrus and CA3 area of hippocampus have the same group of astrocytes. © 2007 Asian Network For Scientific Information