CA1 Field Research Articles

Presynaptic LTP in Blockade of NMDA2B Receptors

The aim of the present work was to study the dynamics of presynaptic modification in conditions of blockade of GABA receptor NR2B subunits, which are key for consolidation of LTP. This was addressed by studying the developmental features of NMDA-dependent long-term potentiation (LTP) in field CA1 of living rat hippocampal slices incubated with ifenprodil by testing with paired-pulse stimuli throughout the recording period before and for 1 h after high-frequency stimulation of Schaffer collaterals (100 Hz, 1 sec). Perfusion of slices with ifenprodil had no effect on the baseline characteristics of CA3–CA1 synapses in terms of the relationship between the amplitudes of the first and second responses and stimulus intensity, i.e., paired-pulse facilitation. LTP was signifi cantly less marked in slices incubated with ifenprodil, while potentiation of the second response in the pair was essentially the same in the control and experimental groups. In control slices, potentiation was accompanied by a significant decrease in PPF immediately after tetanization. This effect was not seen in slices incubated with ifenprodil; furthermore, a transient increase in PPF was seen 15 min after tetanization. In both groups, early LTP was increased proportionately to the decrease in PPF. In the absence of presynaptic modifications, LTP was weakened, including in control slices, and an increase in PPF in experimental slices was accompanied by temporary depression of focal potentials. Ifenprodil had no obvious effect on the maintenance of LTP. It is suggested that detection of the adverse influence of ifenprodil on the process of consolidation requires complex analysis taking account of all the factors operating. The results obtained here support a tight link between LTP and modifications of pairedpulse plasticity and provide grounds for suggesting that NMDA receptor NR2B subunits are directly related to the involvement of the presynaptic component in long-term synaptic potentiation.

Effects of Homocysteine and its Derivatives on Spontaneous Network Activity in the Hippocampus of Neonatal Rat Pups

Homocysteine is a sulfur-containing amino acid, which at high concentrations has neurotoxic effects and induces impairments to the development of the nervous system. Homocysteine is rapidly oxidized in the plasma, forming disulfide bonds with proteins and other low molecular weight thiols; it also undergoes transformation into the into homocysteine thiolactone. On chronic exposure, the neurotoxicity of homocysteine is therefore mediated mainly by its derivatives. The aim of the present work was to investigate the effects of homocysteine and its derivatives – homocystine and homocysteine thiolactone – on spontaneous network activity in the hippocampus of rats in the first week after birth. Giant depolarizing potentials (GDP) and multiple action potentials (MAP) were recorded using an extracellular electrode in hippocampal field CA3. All three study compounds were found to induce increases in the frequency of GDP and MAP at concentrations of 100 and 500 μM, homocystine producing the most significant increase in neuron network activity. The effects of homocysteine, homocystine, and homocysteine thiolactone on the spontaneous network activity of neurons were completely eliminated on blockade of NMDA and AMPA receptors. Thus, homocysteine and its derivatives lead to increased spontaneous network activity of hippocampal neurons in neonatal rats, which can induce impairments to the formation of the neural networks of the hippocampus in conditions of chronic hyperhomocysteinemia, and could also induce hyperexcitability and the risk of developing epilepsy in the postnatal period.

Neurodegeneration, Myelin Loss and Glial Response in the Three-Vessel Global Ischemia Model in Rat.

(1) Background: Although myelin disruption is an integral part of ischemic brain injury, it is rarely the subject of research, particularly in animal models. This study assessed for the first time, myelin and oligodendrocyte loss in a three-vessel model of global cerebral ischemia (GCI), which causes hippocampal damage. In addition, we investigated the relationships between demyelination and changes in microglia and astrocytes, as well as oligodendrogenesis in the hippocampus; (2) Methods: Adult male Wistar rats (n = 15) underwent complete interruption of cerebral blood flow for 7 min by ligation of the major arteries supplying the brain or sham-operation. At 10 and 30 days after the surgery, brain slices were stained for neurodegeneration with Fluoro-Jade C and immunohistochemically to assess myelin content (MBP+ percentage of total area), oligodendrocyte (CNP+ cells) and neuronal (NeuN+ cells) loss, neuroinflammation (Iba1+ cells), astrogliosis (GFAP+ cells) and oligodendrogenesis (NG2+ cells); (3) Results: 10 days after GCI significant myelin and oligodendrocyte loss was found only in the stratum oriens and stratum pyramidale. By the 30th day, demyelination in these hippocampal layers intensified and affected the substratum radiatum. In addition to myelin damage, activation and an increase in the number of microglia and astrocytes in the corresponding layers, a loss of the CA1 pyramidal neurons, and neurodegeneration in the neocortex and thalamus was observed. At a 10-day time point, we observed rod-shaped microglia in the substratum radiatum. Parallel with ongoing myelin loss on the 30th day after ischemia, we found significant oligodendrogenesis in demyelinated hippocampal layers; (4) Conclusions: Our study showed that GCI-simulating cardiac arrest in humans—causes not only the loss of pyramidal neurons in the CA1 field, but also the myelin loss of adjacent layers of the hippocampus.

The entorhinal cortex of the monkey: VI. Organization of projections from the hippocampus, subiculum, presubiculum, and parasubiculum.

The organization of projections from the macaque monkey hippocampus, subiculum, presubiculum, and parasubiculum to the entorhinal cortex was analyzed using anterograde and retrograde tracing techniques. Projections exclusively originate in the CA1 field of the hippocampus and in the subiculum, presubiculum, and parasubiculum. The CA1 and subicular projections terminate most densely in Layers V and VI of the entorhinal cortex, with sparser innervation of the deep portion of Layers III and II. Entorhinal projections from CA1 and the subiculum are topographically organized such that a rostrocaudal axis of origin is related to a medial-to-lateral axis of termination. A proximodistal axis of origin in CA1 and distoproximal axis in subiculum are related to a rostrocaudal axis of termination in the entorhinal cortex. The presubiculum sends a dense, bilateral projection to caudal parts of the entorhinal cortex. This projection terminates most densely in Layer III with sparser termination in Layers I, II, and V. The same parts of entorhinal cortex receive a dense projection from the parasubiculum. This projection terminates in Layers III and II. Both presubicular and parasubicular projections demonstrate the same longitudinal topographic organization as the projections from CA1 and the subiculum. These studies demonstrate that: (a) hippocampal and subicular inputs to the entorhinal cortex in the monkey are organized similar to those described in nonprimate species; (b) the topographic organization of the projections from the hippocampus and subicular areas matches that of the reciprocal projections from the entorhinal cortex to the hippocampus and the subicular areas.

Ih, GIRK, and KCNQ/Kv7 channels differently modulate sharp wave - ripples in the dorsal and ventral hippocampus

Sharp waves and ripples (SPW-Rs) are endogenous transient patterns of hippocampus local network activity implicated in several functions including memory consolidation, and they are diversified between the dorsal and the ventral hippocampus. Ion channels in the neuronal membrane play important roles in cell and local network function. In this study, using transverse slices and field potential recordings from the CA1 field of rat hippocampus we show that GIRK and KCNQ2/3 potassium channels play a higher role in modulating SPW-Rs in the dorsal hippocampus, while Ih and other KCNQ (presumably KCNQ5) channels, contribute to shaping SPW-R activity more in the ventral than in dorsal hippocampus. Specifically, blockade of Ih channels by ZD 7288 reduced the rate of occurrence of SPW-Rs and increased the generation of SPW-Rs in the form of clusters in both hippocampal segments, while enhanced the amplitude of SPW-Rs only in the ventral hippocampus. Most effects of ZD 7288 appeared to be independent of NMDA receptors' activity. However, the effects of blockade of NMDA receptors depended on the functional state of Ih channels in both hippocampal segments. Blockade of GIRK channels by Tertiapin-Q increased the rate of occurrence of SPW-Rs only in the dorsal hippocampus and the probability of clusters in both segments of the hippocampus. Blockade of KCNQ2/3 channels by XE 991 increased the rate of occurrence of SPW-Rs and the probability of clusters in the dorsal hippocampus, and only reduced the clustered generation of SPW-Rs in the ventral hippocampus. The blocker of KCNQ1/2 channels, that also enhances KCNQ5 channels, UCL 2077, increased the probability of clusters and the power of the ripple oscillation in the ventral hippocampus only. These results suggest that GIRK, KCNQ and Ih channels represent a key mechanism for modulation of SPW-R activity which act differently in the dorsal and ventral hippocampus, fundamentally supporting functional diversification along the dorsal-ventral axis of the hippocampus.

Three-dimensional synaptic organization of the human hippocampal CA1 field.

The hippocampal CA1 field integrates a wide variety of subcortical and cortical inputs, but its synaptic organization in humans is still unknown due to the difficulties involved studying the human brain via electron microscope techniques. However, we have shown that the 3D reconstruction method using Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) can be applied to study in detail the synaptic organization of the human brain obtained from autopsies, yielding excellent results. Using this technology, 24,752 synapses were fully reconstructed in CA1, revealing that most of them were excitatory, targeting dendritic spines and displaying a macular shape, regardless of the layer examined. However, remarkable differences were observed between layers. These data constitute the first extensive description of the synaptic organization of the neuropil of the human CA1 region.

Whole-Brain Map of Long-Range Monosynaptic Inputs to Different Cell Types in the Amygdala of the Mouse

The amygdala, which is involved in various behaviors and emotions, is reported to connect with the whole brain. However, the long-range inputs of distinct cell types have not yet been defined. Here, we used a retrograde trans-synaptic rabies virus to generate a whole-brain map of inputs to the main cell types in the mouse amygdala. We identified 37 individual regions that projected to neurons expressing vesicular glutamate transporter 2, 78 regions to parvalbumin-expressing neurons, 104 regions to neurons expressing protein kinase C-δ, and 89 regions to somatostatin-expressing neurons. The amygdala received massive projections from the isocortex and striatum. Several nuclei, such as the caudate-putamen and the CA1 field of the hippocampus, exhibited input preferences to different cell types in the amygdala. Notably, we identified several novel input areas, including the substantia innominata and zona incerta. These findings provide anatomical evidence to help understand the precise connections and diverse functions of the amygdala.

Delayed Effects of Surgery during Early Pregnancy on Brain Development in OXYS Rats

The aim of the present work was to study the effects of stress induced by surgical interventions in early pregnancy on neuron density in the prefrontal cortex and hippocampal fields CA1 and CA3 and on neurogenesis in the dentate fascia of the hippocampus in the adult offspring of OXYS rats. Female OXTS rats were paired with fertile males of the same strain and, 96 h after detection of spermatozoids in vaginal smears, underwent a surgical procedure imitating embryo transplantation. The offspring of these females (OXYS-PS) were studied in comparison with intact OXYS and WAG rats aged five months. The density of pyramidal neurons in the prefrontal cortex of intact OXYS rats was significantly greater than that in rats of the WAG control strain. Neuron density in hippocampal field CA1 in intact OXYS rats was smaller, while that in field CA3 was greater, than that in WAG rats. No significant differences in the intensity of neurogenesis were seen between intact WAG and OXYS rats. In rats of the experimental group, OXYS-PS, the density of pyramidal neurons was lower in the prefrontal cortex and hippocampal field CA1 than in both control strains, while in field CA3 it was lower only in comparison with intact OXYS rats. In addition, rats of the OXYS-PS group showed a higher level of neurogenesis in the dentate gyrus of the hippocampus than both control strains.

Hippocampal CA1 and CA3 neural recording in the human brain: validation of depth electrode placement through high-resolution imaging and electrophysiology.

Intracranial human brain recordings typically utilize recording systems that do not distinguish individual neuron action potentials. In such cases, individual neurons are not identified by location within functional circuits. In this paper, verified localization of singly recorded hippocampal neurons within the CA3 and CA1 cell fields is demonstrated. Macro-micro depth electrodes were implanted in 23 human patients undergoing invasive monitoring for identification of epileptic seizure foci. Individual neurons were isolated and identified via extracellular action potential waveforms recorded via macro-micro depth electrodes localized within the hippocampus. A morphometric survey was performed using 3T MRI scans of hippocampi from the 23 implanted patients, as well as 46 normal (i.e., nonepileptic) patients and 26 patients with a history of epilepsy but no history of depth electrode placement, which provided average dimensions of the hippocampus along typical implantation tracks. Localization within CA3 and CA1 cell fields was tentatively assigned on the basis of recording electrode site, stereotactic positioning of the depth electrode in comparison with the morphometric survey, and postsurgical MRI. Cells were selected as candidate CA3 and CA1 principal neurons on the basis of waveform and firing rate characteristics and confirmed within the CA3-to-CA1 neural projection pathways via measures of functional connectivity. Cross-correlation analysis confirmed that nearly 80% of putative CA3-to-CA1 cell pairs exhibited positive correlations compatible with feed-forward connection between the cells, while only 2.6% exhibited feedback (inverse) connectivity. Even though synchronous and long-latency correlations were excluded, feed-forward correlation between CA3-CA1 pairs was identified in 1071 (26%) of 4070 total pairs, which favorably compares to reports of 20%-25% feed-forward CA3-CA1 correlation noted in published animal studies. This study demonstrates the ability to record neurons in vivo from specified regions and subfields of the human brain. As brain-machine interface and neural prosthetic research continues to expand, it is necessary to be able to identify recording and stimulation sites within neural circuits of interest.

Preconditioning by Moderate Hypoxia Increases the Amount of Corticosteroid Receptors in the Rat Brain in a Model of Depression

Abstract—The involvement of corticosteroid brain receptors in the antidepressant-like effects of moderate hypobaric hypoxia was studied in a rat model of depression (the learned helplessness paradigm) using immunohistochemistry. The formation of a depressive-like state is accompanied by a significant deficiency in glucocorticoid receptors in the brain which is most pronounced in the dentate gyrus of the hippocampus, as well as by a considerable shift in the relative balance between gluco- and mineralocorticoid receptors towards the latter. The use of hypoxic preconditioning prior to stress exposure in the learned helplessness paradigm prevented the development of depression and eliminated receptor deficiency by post-stress day 10 in the hypothalamus and during the first day of the observation in the neocortex. In the hippocampus of preconditioned animals, stress adaptation was accompanied by an acute and consistent increase in glucocorticoid receptor immunoreactivity, with a relative balance of the two receptor types in the CA1 field, and considerable glucocorticoid receptor prevalence in the dentate gyrus. It may be assumed that deficiency and imbalance of hippocampal corticosteroid receptors is one of the most important molecular mechanisms of glucocorticoid regulation dysfunction in stress-induced psychopathologies, whereas the post-stress activation of receptor expression contributes to the manifestation of the antidepressant-like effects of moderate hypoxia. The data obtained expand the current views on the endocrine mechanisms of the formation and prevention of depressive disorders and increase the translation potential of moderate hypoxia as a drug-free strategy for their treatment.

Dorsal-Ventral Differences in Modulation of Synaptic Transmission in the Hippocampus.

Functional diversification along the longitudinal axis of the hippocampus is a rapidly growing concept. Modulation of synaptic transmission by neurotransmitter receptors may importantly contribute to specialization of local intrinsic network function along the hippocampus. In the present study, using transverse slices from the dorsal and the ventral hippocampus of adult rats and recordings of evoked field postsynaptic excitatory potentials (fEPSPs) from the CA1 stratum radiatum, we aimed to compare modulation of synaptic transmission between the dorsal and the ventral hippocampus. We found that transient heterosynaptic depression (tHSD, <2 s), a physiologically relevant phenomenon of regulation of excitatory synaptic transmission induced by paired stimulation of two independent inputs to stratum radiatum of CA1 field, has an increased magnitude and duration in the ventral hippocampus, presumably contributing to increased input segregation in this segment of the hippocampus. GABAB receptors, GABAA receptors, adenosine A1 receptors and L-type voltage-gated calcium channels appear to contribute differently to tHSD in the two hippocampal segments; GABABRs play a predominant role in the ventral hippocampus while both GABABRs and A1Rs play important roles in the dorsal hippocampus. Activation of GABAB receptors by an exogenous agonist, baclofen, robustly and reversibly modulated both the initial fast and the late slow components of excitatory synaptic transmission, expressed by the fEPSPslope and fEPSP decay time constant (fEPSPτ), respectively. Specifically, baclofen suppressed fEPSP slope more in the ventral than in the dorsal hippocampus and enhanced fEPSPτ more in the dorsal than in the ventral hippocampus. Also, baclofen enhanced paired-pulse facilitation in the two hippocampal segments similarly. Blockade of GABAB receptors did not affect basal paired-pulse facilitation in either hippocampal segment. We propose that the revealed dorsal-ventral differences in modulation of synaptic transmission may provide a means for specialization of information processing in the local neuronal circuits, thereby significantly contributing to diversifying neuronal network functioning along the dorsal-ventral axis of hippocampus.

Medial temporal lobe volume is associated with neuronal loss but not with hippocampal microinfarcts despite their high frequency in aging brains

Medial temporal lobe (MTL) atrophy is an important marker for the clinical diagnosis of Alzheimer's disease at its prodromal stages. Several brain lesions have been associated with MTL atrophy including hippocampal sclerosis, neurodegenerative neuronal loss, and vascular pathology. To better explore the relationship between MTL volume on MRI and age-related degenerative and microvascular hippocampal pathology, we compared MTL volume on postmortem whole brain MRI and stereological estimates of the total number of neurons, cortical microinfarcts (CMIs), and neurofibrillary tangles (NFTs) in a consecutive autopsy series of 21 older individuals (11 females and 10 males, mean age 83.3 ± 5.8; range: 74–93 years, 7 demented and 14 nondemented). Our results revealed a very high percentage of cases with hippocampal CMIs (52%), particularly in the CA1 field. MTL volume was closely related to neuronal loss in both the CA1 area of the hippocampus (p = 0.0109) and the entorhinal cortex (p = 0.0272). MTL volume was not related to total CMI volume or to the total number of NFTs in our sample. In conclusion, hippocampal CMIs are very common in old age. MTL volume is determined essentially by the number of neurons in the hippocampus and does not appear to be related to the presence of NFTs or CMIs in this region.

Cognitive Decline and Modulation of Alzheimer's Disease-Related Genes After Inhibition of MicroRNA-101 in Mouse Hippocampal Neurons.

MicroRNAs have emerged as regulators of brain development and function. Reduction of miR-101 expression has been reported in rodent hippocampus during ageing, in the brain of Alzheimer's disease (AD) patients and in AD animal models. In this study, we investigated the behavioral and molecular consequences of inhibition of endogenous miR-101 in 4-5-month-old C57BL/6J mice, infused with lentiviral particles expressing a miR-101 sponge (pLSyn-miR-101 sponge) in the CA1 field of the hippocampus. The sponge-infected mouse model showed cognitive impairment. The pLSyn-miR-101 sponge-infected mice were unable to discriminate either a novel object location or a novel object as assessed by object place recognition (OPR) and novel object recognition (NOR) tasks, respectively. Moreover, the sponge-infected mice evaluated for contextual memory in inhibitory avoidance task showed shorter retention latency compared to control pLSyn mice. These cognitive impairment features were associated with increased hippocampal expression of relevant miR-101 target genes, amyloid precursor protein (APP), RanBP9 and Rab5 and overproduction of amyloid beta (Aβ) 42 levels, the more toxic species of Aβ peptide. Notably, phosphorylation-dependent AMP-activated protein kinase (AMPK) hyperactivation is associated with AD pathology and age-dependent memory decline, and we found AMPK hyperphosphorylation in the hippocampus of pLSyn-miR-101 sponge mice. This study demonstrates that mimicking age-associated loss of miR-101 in hippocampal neurons induces cognitive decline and modulation of AD-related genes in mice.

Functional and Morphological Changes in the Pyramidal Layer of the Hippocampus in Rats with Encephalopathy Induced by Prolonged Exposure to Gravitational Overloading

Studies of morphofunctional changes in the hippocampus using a model of impaired cerebral hemodynamics are needed for understanding the mechanisms of formation of psychoneurological deficits. The aim of the present work was to assess CNS damage and the characteristics of the expression of one of the main regulators of apoptosis – Bcl-2 – in the hippocampus of rats with impaired cerebral hemodynamics induced by repeated gravitational overloading. The study was carried out on Wistar rats which were subjected to gravitational overloading (9 g) in the caudocranial direction for two 5-min periods each day for 28 days. Psychoneurological impairments in the animals were assessed using the McGraw scale and the open field, adhesion, conditioned passive avoidance reaction (CPAR), and escape extrapolation (EET) tests. Brain sections were stained by the Nissl method and the specific numbers of neurons with signs of damage were counted. Immunohistochemical studies of the level of Bcl-2 expression assessed the relative areas of immunoreactive material in the pyramidal layer in all hippocampal fields. Gravitational overloading induced impairments to the animals’ behavior, which was expressed as low movement activity and signs of retrograde amnesia, decreased fi ne motor function, and increases in psychoneurological deficit. Histological studies identified destructive changes to vessels and increases in the specific number of pyramidal neurons with damage in all fields of the hippocampus. Fields CA2 and CA4 showed the greatest increases in the numbers of damaged neurons, along with marked Bcl-2 expression.

Laminarin Pretreatment Provides Neuroprotection against Forebrain Ischemia/Reperfusion Injury by Reducing Oxidative Stress and Neuroinflammation in Aged Gerbils.

Laminarin is a polysaccharide isolated from brown algae that has various biological and pharmacological activities, such as antioxidant and anti-inflammatory properties. We recently reported that pretreated laminarin exerted neuroprotection against transient forebrain ischemia/reperfusion (IR) injury when we pretreated with 50 mg/kg of laminarin once a day for seven days in adult gerbils. However, there have been no studies regarding a neuroprotective effect of pretreated laminarin against IR injury in aged animals and its related mechanisms. Therefore, in this study, we intraperitoneally inject laminarin (50 mg/kg) once a day to aged gerbils for seven days before IR (5-min transient ischemia) surgery and examine the neuroprotective effect of laminarin treatment and the mechanisms in the gerbil hippocampus. IR injury in vehicle-treated gerbils causes loss (death) of pyramidal neurons in the hippocampal CA1 field at five days post-IR. Pretreatment with laminarin effectively protects the CA1 pyramidal neurons from IR injury. Regarding the laminarin-treated gerbils, production of superoxide anions, 4-hydroxy-2-nonenal expression and pro-inflammatory cytokines [interleukin(IL)-1β and tumor necrosis factor-α] expressions are significantly decreased in the CA1 pyramidal neurons after IR. Additionally, laminarin treatment significantly increases expressions of superoxide dismutase and anti-inflammatory cytokines (IL-4 and IL-13) in the CA1 pyramidal neurons before and after IR. Taken together, these findings indicate that laminarin can protect neurons from ischemic brain injury in an aged population by attenuating IR-induced oxidative stress and neuroinflammation.

Retrograde and anterograde contextual fear amnesia induced by selective elimination of layer IV-Va neurons in the granular retrosplenial cortex (A29).

Differences in cytoarchitectural organization and connectivity distinguishes granular (or area 29, A29) and dysgranular (or area 30, A30) subdivisions of the retrosplenial cortex (RSC). Although increasing evidence supports the participation of RSC in contextual fear learning and memory, the contribution of each RSC subdivision remains unknown. Here we used orchiectomized rats and intraperitoneal (i.p.) injections of saline (control) or 5mg/kg MK801, to trigger selective degeneration of pyramidal neurons in layers IV-Va of A29 (A29MK801 neurons). These treatments were applied 3days before or two days after contextual fear conditioning, and contextual fear memory was evaluated by scoring freezing in the conditioned context five days after training. Afterwards, brains were fixed and c-Fos and Egr-1 expression were assessed as surrogates of neuronal activity elicited by the recall in A29, A30 and in limbic areas. We found that eliminating A29MK801 neurons after training reduces conditioned freezing to 43.1±9.9% respect to control rats. This was associated with a significant reduction of c-Fos and Egr-1 expression in A30, but not in other limbic areas. On the other hand, eliminating A29MK801 neurons before training caused a mild but significant reduction of conditioned freezing to 79.7±6.8%, which was associated to enhanced expression of c-Fos in A29, A30 and CA1 field of hippocampus, while Egr-1 expression in caudomedial (CEnt) entorhinal cortex was not depressed as in control animals. These observations show that severeness of amnesia differs according to whether A29MK801 neurons were eliminated before or after conditioning, likely because loss of A29MK801 neurons after conditioning disrupt memory engram while their elimination before training allow recruitment of other neurons in A29 for partial compensation of contextual fear learning and memory. These observations add further support for the critical role of A29MK801 neurons in contextual fear learning and memory by connecting limbic structures with A30.

Functional Connectivity of Hippocampal CA3 Predicts Neurocognitive Aging via CA1-Frontal Circuit.

The CA3 and CA1 principal cell fields of the hippocampus are vulnerable to aging, and age-related dysfunction in CA3 may be an early seed event closely linked to individual differences in memory decline. However, whether the differential vulnerability of CA3 and CA1 is associated with broader disruption in network-level functional interactions in relation to age-related memory impairment, and more specifically, whether CA3 dysconnectivity contributes to the effects of aging via CA1 network connectivity, has been difficult to test. Here, using resting-state fMRI in a group of aged rats uncontaminated by neurodegenerative disease, aged rats displayed widespread reductions in functional connectivity of CA3 and CA1 fields. Age-related memory deficits were predicted by connectivity between left CA3 and hippocampal circuitry along with connectivity between left CA1 and infralimbic prefrontal cortex. Notably, the effects of CA3 connectivity on memory performance were mediated by CA1 connectivity with prefrontal cortex. We additionally found that spatial learning and memory were associated with functional connectivity changes lateralized to the left CA3 and CA1 divisions. These results provide novel evidence that network-level dysfunction involving interactions of CA3 with CA1 is an early marker of poor cognitive outcome in aging.

Impact of Monosodium Glutamate and Corticosterone in the Hippocampus: Glucocorticoid Regulation and Caspase‐3 mediated Microvascular and Neuronal Apoptosis

Does monosodium glutamate (MSG) trigger stress circuitry axis and induce apoptosis in the hippocampus? If so, does it mimic the effect of corticosterone (CORT)? In order to test this hypothesis, we put forth a Glutamate model and CORT‐ induced model system in rats to study the modality of glucocorticoid response and apoptotic signaling cascade. Young adult (2‐month‐old) male Wistar rats weighing about 50–100 g body weight were divided into 4 groups. Group I control rats were injected with saline (NaCl 0.9%) and Group II rats with MSG at a dose of 4 mg/kg body weight. Group III control was ethanol‐saline injected and Group IV was CORT injected at 3 mg/kg body weight. Rats were injected subcutaneously with test materials or vehicles for 7 days and maintained for 8, 15 and 30 days and were sacrificed by decapitation. The MSG model showed an enhanced glucocorticoid receptor (GR) (Nr3c1) gene expression in the hippocampus on day 8, as an immediate response to stress, though not significant. This was followed by a down regulation on day 15 and day 30. Bax, a pro‐apoptotic factor gene showed an insignificant upregulated expression in the hippocampus on day 8, 15 and 30. This indicates the activation of Bax‐ dependent apoptotic pathways at the mRNA level. In the CORT model, GR (Nr3c1) gene expression in the hippocampus showed a very significant downregulation on day 8. Noticed an insignificant activation on day 15 and 30. Bax gene showed a very significant downregulation on day 8, an immediate time point. Day 15 and Day 30 displayed an insignificant up and down regulation. Immunohistochemical analysis revealed the Caspase‐3 mediated death program activation in the microvasculature and neurons of the hippocampus upon MSG and CORT exposure. In the hippocampus field CA1, CA2 and CA3 and dentate gyrus displayed prominent Caspase‐3 immunoreactivity in the microvessels, suggesting potential blood‐brain barrier (BBB) breach. A leaky BBB can transcend vasculotoxic and neurotoxic proteins to invade the brain. Probable events like this can pave way for the neurons and glial cells to be exposed to high levels of glutamate circulating in the blood, which can lead to receptor ‐ mediated neurotoxicity. Neuronal apoptosis is also activated in the hippocampus upon exposure to MSG and CORT. Our results suggest the potential disruption in the brain neural connectivity and neural synchronization upon exposure to MSG and CORT. Recent reports also state that BBB dysfunction and neuronal loss can be attributed and linked to neurodegenerative disorders like Alzheimer’s. Furthermore, MSG mimics the CORT in triggering‐Bax and Caspase dependent apoptotic signaling cascade, to suggest that MSG is a stressor and a neurotoxin. Our results raise concern over the use of MSG as a food additive which needs public awakening and awareness.Support or Funding InformationCUSAT Postdoctoral Fellowship‐Dr. Smita Mathew. U.O. No.Ac. B3/PDF/2016‐17(b) dt. 10.5.2017

Ultrastructural Alterations in Granular Neurons of the Dentate Fascia Caused by Intrahippocampal Injection of Beta-Amyloid 1-42.

The deposition of beta-amyloid (Aβ) in the brain is detected in Alzheimer's disease and during ageing. Until now, ultrastructural studies of changes caused by Aβ in the dentate gyrus are very scarce. The effects of Aβ 1-42 injection into the CA1 field of rat hippocampus were studied by electron microscopy. In 2 weeks after injection of aggregated Aβ in low concentrations, destructive changes were seen in the structure of dentate gyrus cells, which consisted in a decrease in the number of dentate gyrus neurons and axo-dendritic synapses. These changes were accompanied by enlargement of the endoplasmic reticulum cisterns and widening of the active zones of synapses. Thus, injection of aggregated Aβ 1-42 into the hippocampus led to irreversible (a decrease in the number of neurons and axo-dendritic synapses, agglutination of synthetic vesicles) and adaptive changes (an increase in the sizes of endoplasmic reticulum cisterns and active zones of synapses) in dentate gyrus neurons aimed at the maintenance of functional activity of the nervous system.

SYNCHRONIZATION OF EPILEPTIFORM ACTIVITY BETWEEN CA1 AND CA3 HIPPOCAMPAL FIELDS UNDER SYNAPTIC AND NON-SYNAPTIC CONDITIONS IN RAT BRAIN SLICES

Relevance. Over-synchronization of neuronal activity results in epileptic-like discharges that can lead to seizures and status epilepticus. Understanding mechanisms of neural net synchronization could provide new insights into the treatment of epileptic disorders.&#x0D; Objective: to compare the levels of synchronization between CA3 and CA1 hippocampal zones during epileptiform activity induced under synaptic and non-synaptic conditions.&#x0D; Materials and Methods. Transverse brain slices were obtained from 12-14 days old rats. For induction of epileptiform activity common pro-epileptic agents were used: bicuculline and 4-aminopiridine. Nonsynaptic epileptiform activity was induced by perfusion brain slices with low-Ca2+ and Cd2+-containing artificial cerebrospinal fluid (aCSF). Simultaneous extracellular recordings of field potentials were obtained from the CA3 and CA1 pyramidal cell layer with extracellular glass microelectrodes (2–3 MΩ). Signals were then low-pass filtered (kHz), amplified using a 2-channel differential amplifier M1800, digitized at 10 kHz using analog-to-digital converter. The level of synchronization between CA3 and CA1 was evaluated using cross-correlation analysis. &#x0D; Results: Perfusion hippocampal slices with bicuculline and 4-aminopyridine induced epileptiform activity with high level of synchronization between CA3 and CA1 hippocampal zones. Removing Ca2+ from extracellular solution as well as adding CdCl2 to the perfusion aCSF induced epileptiform activity that was not synchronized between hippocampal CA3 and CA1 fields. &#x0D; Conclusions: Synaptic interaction account for high level of CA3-CA1 synchronization induced by pro-epileptic agents bicuculline and 4-aminopiridine. Under non-synaptic conditions, local cellular interactions induce epileptiform activity with no synchronization between CA3 and CA1 hippocampal zones.