Integration of the CA2 region in the hippocampal network during epileptogenesis

The CA2 pyramidal cells are mostly resistant to cell death in mesial temporal lobe epilepsy (MTLE) with hippocampal sclerosis, but they are aberrantly integrated into the epileptic hippocampal network via mossy fiber sprouting. Furthermore, they show increased excitability in vitro in hippocampal slices obtained from human MTLE specimens or animal epilepsy models. Although these changes promote CA2 to contribute to epileptic activity (EA) in vivo, the role of CA2 in the epileptic network within and beyond the sclerotic hippocampus is still unclear. We used the intrahippocampal kainate mouse model for MTLE, which recapitulates most features of the human disease including pharmacoresistant epileptic seizures and hippocampal sclerosis, with preservation of dentate gyrus (DG) granule cells and CA2 pyramidal cells. In vivo recordings with electrodes in CA2 and the DG showed that EA occurs at high coincidence between the ipsilateral DG and CA2 and current source density analysis of silicon probe recordings in dorsal ipsilateral CA2 revealed CA2 as a local source of EA. Cell-specific viral tracing in Amigo2-icreERT2 mice confirmed the preservation of the axonal projection from ipsilateral CA2 pyramidal cells to contralateral CA2 under epileptic conditions and indeed, EA propagated from ipsi- to contralateral CA2 with increasing likelihood with time after KA injection, but always at lower intensity than within the ipsilateral hippocampus. Furthermore, we show that CA2 presents with local theta oscillations and like the DG, shows a pathological reduction of theta frequency already from 2 days after KA onward. The early changes in activity might be facilitated by the loss of glutamic acid decarboxylase 67 (Gad67) mRNA-expressing interneurons directly after the initial status epilepticus in ipsi- but not contralateral CA2. Together, our data highlight CA2 as an active player in the epileptic network and with its contralateral connections as one possible router of aberrant activity.     

Epileptiform activity induced by low Mg in cultured rat hippocampal slices

Organotypic cultured slices of the rat hippocampus undergo synaptic reorganization. Besides the establishment of reciprocal connections between area CA1 and the dentate gyrus (DG), collateral excitatory connections between granule cells are formed which are similar to those appearing in several epilepsy models and in the DG from patients with temporal lobe epilepsy. We studied the characteristics of epileptiform activity induced by low Mg²⁺ perfusion in cultured hippocampal slices using extra- and intracellular recordings. With low Mg²⁺ perfusion synchronous seizure like events (SLEs) were readily observed in the DG and areas CA3 and CA1. Also, the isolated DG was able to display seizure like activity. Intracellular recordings revealed long lasting depolarization shifts in granule cells of the DG and pyramidal cells of areas CA3 and CA1. The SLEs, lasting 2–3 s, could be recorded for at least 3 h in areas CA1 and CA3. However, approximately an hour after perfusion with low Mg²⁺, the epileptiform activity disappeared in the DG and responses to single pulse hilar stimulation progressively deteriorated. These responses returned to control values 1 week after reincubating the cultures. Interestingly, no deterioration of stimulus induced responses was observed in the isolated DG after exposure to low Mg²⁺.     

The hippocampal CA3 region can generate two distinct types of sharp wave‐ripple complexes,in vitro

Hippocampal sharp wave‐ripples (SPW‐Rs) occur during slow wave sleep and behavioral immobility and are thought to play an important role in memory formation. We investigated the cellular and network properties of SPW‐Rs with simultaneous laminar multielectrode and intracellular recordings in a rat hippocampal slice model, using physiological bathing medium. Spontaneous SPW‐Rs were generated in the dentate gyrus (DG), CA3, and CA1 regions. These events were characterized by a local field potential gradient (LFPg) transient, increased fast oscillatory activity and increased multiple unit activity (MUA). Two types of SPW‐Rs were distinguished in the CA3 region based on their different LFPg and current source density (CSD) pattern. Type 1 (T1) displayed negative LFPg transient in the pyramidal cell layer, and the associated CSD sink was confined to the proximal dendrites. Type 2 (T2) SPW‐Rs were characterized by positive LFPg transient in the cell layer, and showed CSD sinks involving both the apical and basal dendrites. In both types, consistent with the somatic CSD source, only a small subset of CA3 pyramidal cells fired, most pyramidal cells were hyperpolarized, while most interneurons increased firing rate before the LFPg peak. Different neuronal populations, with different proportions of pyramidal cells and distinct subsets of interneurons were activated during T1 and T2 SPW‐Rs. Activation of specific inhibitory cell subsets—with the possible leading role of perisomatic interneurons—seems to be crucial to synchronize distinct ensembles of CA3 pyramidal cells finally resulting in the expression of different SPW‐R activities. This suggests that the hippocampus can generate dynamic changes in its activity stemming from the same excitatory and inhibitory circuits, and so, might provide the cellular and network basis for an input‐specific and activity‐dependent information transmission. © 2014 The Authors Hippocampus Published by Wiley Periodicals, Inc.     

Pattern separation in the dentate gyrus: A role for the CA3 backprojection

Many theories of hippocampal function assume that area CA3 of hippocampus is capable of performing rapid pattern storage, as well as pattern completion when a partial version of a familiar pattern is presented, and that the dentate gyrus (DG) is a preprocessor that performs pattern separation, facilitating storage and recall in CA3. The latter assumption derives partly from the anatomical and physiological properties of DG. However, the major output of DG is from a large number of DG granule cells to a smaller number of CA3 pyramidal cells, which potentially negates the pattern separation performed in the DG. Here, we consider a simple CA3 network model, and consider how it might interact with a previously developed computational model of the DG. The resulting “standard” DG‐CA3 model performs pattern storage and completion well, given a small set of sparse, randomly derived patterns representing entorhinal input to the DG and CA3. However, under many circumstances, the pattern separation achieved in the DG is not as robust in CA3, resulting in a low storage capacity for CA3, compared to previous mathematical estimates of the storage capacity for an autoassociative network of this size. We also examine an often‐overlooked aspect of hippocampal anatomy that might increase functionality in the combined DG‐CA3 model. Specifically, axon collaterals of CA3 pyramidal cells project “back” to the DG (“backprojections”), exerting inhibitory effects on granule cells that could potentially ensure that different subpopulations of granule cells are recruited to respond to similar patterns. In the model, addition of such backprojections improves both pattern separation and storage capacity. We also show that the DG‐CA3 model with backprojections provides a better fit to empirical data than a model without backprojections. Therefore, we hypothesize that CA3 backprojections might play an important role in hippocampal function. © 2010 Wiley Periodicals, Inc.     

Activity dynamics and behavioral correlates of CA3 and CA1 hippocampal pyramidal neurons

The CA3 and CA1 pyramidal neurons are the major principal cell types of the hippocampus proper. The strongly recurrent collateral system of CA3 cells and the largely parallel‐organized CA1 neurons suggest that these regions perform distinct computations. However, a comprehensive comparison between CA1 and CA3 pyramidal cells in terms of firing properties, network dynamics, and behavioral correlations is sparse in the intact animal. We performed large‐scale recordings in the dorsal hippocampus of rats to quantify the similarities and differences between CA1 (n > 3,600) and CA3 (n > 2,200) pyramidal cells during sleep and exploration in multiple environments. CA1 and CA3 neurons differed significantly in firing rates, spike burst propensity, spike entrainment by the theta rhythm, and other aspects of spiking dynamics in a brain state‐dependent manner. A smaller proportion of CA3 than CA1 cells displayed prominent place fields, but place fields of CA3 neurons were more compact, more stable, and carried more spatial information per spike than those of CA1 pyramidal cells. Several other features of the two cell types were specific to the testing environment. CA3 neurons showed less pronounced phase precession and a weaker position versus spike‐phase relationship than CA1 cells. Our findings suggest that these distinct activity dynamics of CA1 and CA3 pyramidal cells support their distinct computational roles. © 2012 Wiley Periodicals, Inc.     

Long term effects of stress on hippocampal function: Emphasis on early life stress paradigms and potential involvement of neuropeptide Y

The brain is both central in orchestrating the response to stress, and, a very sensitive target when such response is not controlled. In fact, stress has long been associated with the onset and/or exacerbation of several neuropsychiatric disorders such as anxiety, depression, and drug addiction. The hippocampus is a key brain region involved in the response to stress, not only due to its anatomical connections with the hypothalamic‐pituitary‐adrenal axis but also as a major target of stress mediators. The hippocampal dentate gyrus (DG)‐CA3 circuit, composed of DG granule cells axons (mossy fibers) synapsing onto CA3 pyramidal cells, plays an essential role in memory encoding and retrieval, functions that are vulnerable to stress. Although naturally excitatory, this circuit is under the inhibitory control of GABAergic interneurons that maintain the excitation/inhibition balance. One subgroup of such interneurons produces neuropeptide Y (NPY), which has emerged as a promising endogenous stress “resilience molecule” due to its anxiolytic and anti‐epileptic properties. Here we examine existing evidence that reveals a potential role for hilar NPY+ interneurons in mediating stress‐induced changes in hippocampal function. We will focus specifically on rodent models of early life stress (ELS), defined as adverse conditions during the early postnatal period that can have profound consequences for neurodevelopment. Collectively, these findings suggest that the long‐lasting effects of ELS might stem from the loss of GABAergic NPY+ cells, which then can lead to reduced inhibition in the DG‐CA3 pathway. Such change might then lead to hyperexcitability and concomitant hippocampal‐dependent behavioral deficits.     

3‐Hz subthreshold oscillations of CA2 neurons In vivo

The CA2 region is unique in the hippocampus; it receives direct synaptic innervations from several hypothalamic nuclei and expresses various receptors of neuromodulators, including adenosine, vasopressin, and oxytocin. Furthermore, the CA2 region may have distinct brain functions, such as the control of instinctive and social behaviors; however, little is known about the dynamics of the subthreshold membrane potentials of CA2 neurons in vivo. We conducted whole‐cell current‐clamp recordings from CA2 pyramidal cells in urethane‐anesthetized mice and monitored the intrinsic fluctuations in their membrane potentials. The CA2 pyramidal cells emitted spontaneous action potentials at mean firing rates of ～0.8 Hz. In approximately half of the neurons, the subthreshold membrane potential oscillated at ～3 Hz. In two neurons, we obtained simultaneous recordings of local field potentials from the CA1 stratum radiatum and demonstrated that the 3‐Hz oscillations of CA2 neurons were not correlated with CA1 field potentials. In tetrodotoxin‐perfused acute hippocampal slices, the membrane potentials of CA2 pyramidal cells were not preferentially entrained to 3‐Hz sinusoidal current inputs, which suggest that intracellular 3‐Hz oscillations reflect the neuronal dynamics of the surrounding networks. © 2016 Wiley Periodicals, Inc.     

Human cerebrospinal fluid promotes spontaneous gamma oscillations in the hippocampus in vitro

Gamma oscillations (30–80 Hz) are fast network activity patterns frequently linked to cognition. They are commonly studied in hippocampal brain slices in vitro, where they can be evoked via pharmacological activation of various receptor families. One limitation of this approach is that neuronal activity is studied in a highly artificial extracellular fluid environment, as provided by artificial cerebrospinal fluid (aCSF). Here, we examine the influence of human cerebrospinal fluid (hCSF) on kainate‐evoked and spontaneous gamma oscillations in mouse hippocampus. We show that hCSF, as compared to aCSF of matched electrolyte and glucose composition, increases the power of kainate‐evoked gamma oscillations and induces spontaneous gamma activity in areas CA3 and CA1 that is reversed by washout. Bath application of atropine entirely abolished hCSF‐induced gamma oscillations, indicating critical contribution from muscarinic acetylcholine receptor‐mediated signaling. In separate whole‐cell patch clamp recordings from rat hippocampus, hCSF increased theta resonance frequency and strength in pyramidal cells along with enhancement of h‐current (I_h) amplitude. We found no evidence of intrinsic gamma frequency resonance at baseline (aCSF) among fast‐spiking interneurons, and this was not altered by hCSF. However, hCSF increased the excitability of fast‐spiking interneurons, which likely contributed to gamma rhythmogenesis. Our findings show that hCSF promotes network gamma oscillations in the hippocampus in vitro and suggest that neuromodulators distributed in CSF could have significant influence on neuronal network activity in vivo.     

Orthogonal wave propagation of epileptiform activity in the planar mouse hippocampus in vitro

Purpose: In vitro brain preparations have been used extensively to study the generation and propagation of epileptiform activity. Transverse and longitudinal slices of the rodent hippocampus have revealed various patterns of propagation. Yet intact connections between the transverse and longitudinal pathways should generate orthogonal (both transverse and longitudinal) propagation of seizures involving the entire hippocampus. This study utilizes the planar unfolded mouse hippocampus preparation to reveal simultaneous orthogonal epileptiform propagation and to test a method of arresting propagation. Methods: This study utilized an unfolded mouse hippocampus preparation. It was chosen due to its preservation of longitudinal neuronal processes, which are thought to play an important role in epileptiform hyperexcitability. 4‐Aminopyridine (4‐AP), microelectrodes, and voltage‐sensitive dye imaging were employed to investigate tissue excitability. Key Findings: In 50‐μm 4‐AP, stimulation of the stratum radiatum induced transverse activation of CA3 cells but also induced a longitudinal wave of activity propagating along the CA3 region at a speed of 0.09 m/s. Without stimulation, a wave originated at the temporal CA3 and propagated in a temporal–septal direction could be suppressed with glutamatergic receptor antagonists. Orthogonal propagation traveled longitudinally along the CA3 pathway, secondarily invading the CA1 region at a velocity of 0.22 ± 0.024 m/s. Moreover, a local lesion restricted to the CA3 region could arrest wave propagation. Significance: These results reveal a complex two‐dimensional epileptiform wave propagation pattern in the hippocampus that is generated by a combination of synaptic transmission and axonal propagation in the CA3 recurrent network. Epileptiform propagation block via a transverse selective CA3 lesion suggests a potential surgical technique for the treatment of temporal lobe epilepsy.     

Recurrent connections between CA2 pyramidal cells

Recurrent excitatory synapses have theoretically been shown to play roles in memory storage and associative learning and are well described to occur in the CA3 region of the hippocampus. Here, we report that the CA2 region also contains recurrent excitatory monosynaptic couplings. Using dual whole‐cell patch‐clamp recordings from CA2 pyramidal cells in mouse hippocampal slices under differential interference contrast microscopic controls, we evaluated monosynaptic excitatory connections. Unitary excitatory postsynaptic potentials occurred in 1.4% of 502 cell pairs. These connected pairs were preferentially located in the superficial layer and proximal part (CA2b) of the CA2 region. These results indicate that recurrent excitatory circuits are denser in the CA2 region than in the CA1 region, as well as in the CA3 region.     

Scn2a sodium channel mutation results in hyperexcitability in the hippocampus in vitro

PURPOSE: To investigate in vitro, the cellular network activity of the hippocampus in Q54 mice that display spontaneous seizures because of a gain-of-function mutation of the Scn2a sodium channel gene. METHODS: Extacellular recordings were obtained from CA1 and CA3 pyramidal neurons in hippocampal slices prepared from Q54 transgenic and nontransgenic littermates (WT) under physiologic conditions as well as during periods of orthodromic stimulation of the Schaffer collaterals. Cerebral spinal fluid samples were analyzed and cresyl violet histology of the hippocampus was conducted. RESULTS: Increased spontaneous extracellular activity was found in both CA1 and CA3 regions of Q54 hippocampal slices. Q54 slices also demonstrated significantly greater spontaneous and afterdischarge activity as well as population spike amplitude and duration following tetanic stimulus in comparison to WT slices. Frequency analysis of tetanically stimulated recordings indicated high-frequency components (100 and 200 Hz) unique to Q45 slices. Analysis of cresyl violet histology supports healthy Q54 slices up to 10 weeks, while Q54 cerebral spinal fluid shows elevated osmolarity. CONCLUSION: Evidence for hyperexcitability and increased synaptic efficacy in Q54 mice was found by observing spontaneous activity as well as evoked activity. Response to tetanic stimulation included unique high-frequency oscillations, and resulted in an increased population spike amplitude and duration. Histological assessment shows equivalent neuronal development in both experimental groups. The data support the hypothesis that modified Scn2a channels in Q54 mice result in network hyperexcitability of the hippocampus necessary for the development and maintenance of temporal lobe seizures.     

The race to learn: Spike timing and STDP can coordinate learning and recall in CA3

The CA3 region of the hippocampus has long been proposed as an autoassociative network performing pattern completion on known inputs. The dentate gyrus (DG) region is often proposed as a network performing the complementary function of pattern separation. Neural models of pattern completion and separation generally designate explicit learning phases to encode new information and assume an ideal fixed threshold at which to stop learning new patterns and begin recalling known patterns. Memory systems are significantly more complex in practice, with the degree of memory recall depending on context‐specific goals. Here, we present our spike‐timing separation and completion (STSC) model of the entorhinal cortex (EC), DG, and CA3 network, ascribing to each region a role similar to that in existing models but adding a temporal dimension by using a spiking neural network. Simulation results demonstrate that (a) spike‐timing dependent plasticity in the EC‐CA3 synapses provides a pattern completion ability without recurrent CA3 connections, (b) the race between activation of CA3 cells via EC‐CA3 synapses and activation of the same cells via DG‐CA3 synapses distinguishes novel from known inputs, and (c) modulation of the EC‐CA3 synapses adjusts the learned versus test input similarity required to evoke a direct CA3 response prior to any DG activity, thereby adjusting the pattern completion threshold. These mechanisms suggest that spike timing can arbitrate between learning and recall based on the novelty of each individual input, ensuring control of the learn‐recall decision resides in the same subsystem as the learned memories themselves. The proposed modulatory signal does not override this decision but biases the system toward either learning or recall. The model provides an explanation for empirical observations that a reduction in novelty produces a corresponding reduction in the latency of responses in CA3 and CA1. © 2010 Wiley‐Liss, Inc.     

Chronic administration of resveratrol prevents morphological changes in prefrontal cortex and hippocampus of aged rats

Resveratrol may induce its neuroprotective effects by reducing oxidative damage and chronic inflammation apart from improving vascular function and activating longevity genes, it also has the ability to promote the activity of neurotrophic factors. Morphological changes in dendrites of the pyramidal neurons of the prefrontal cortex (PFC) and hippocampus have been reported in the brain of aging humans, or in humans with neurodegenerative diseases such as Alzheimer's disease. These changes are reflected particularly in the decrement of both the dendritic tree and spine density. Here we evaluated the effect of resveratrol on the dendrites of pyramidal neurons of the PFC (Layers 3 and 5), CA1‐ and CA3‐dorsal hippocampus (DH) as well as CA1‐ventral hippocampus, dentate gyrus (DG), and medium spiny neurons of the nucleus accumbens of aged rats. 18‐month‐old rats were administered resveratrol (20 mg/kg, orally) daily for 60 days. Dendritic morphology was studied by the Golgi‐Cox stain procedure, followed by Sholl analysis on 20‐month‐old rats. In all resveratrol‐treated rats, a significant increase in dendritic length and spine density in pyramidal neurons of the PFC, CA1, and CA3 of DH was observed. Interestingly, the enhancement in dendritic length was close to the soma in pyramidal neurons of the PFC, whereas in neurons of the DH and DG, the increase in dendritic length was further from the soma. Our results suggest that resveratrol induces modifications of dendritic morphology in the PFC, DH, and DG. These changes may explain the therapeutic effect of resveratrol in aging and in Alzheimer's disease. Synapse, 2016. © 2016 Wiley Periodicals, Inc. Synapse 70:206–217, 2016. © 2016 Wiley Periodicals, Inc.     

Synaptic reorganization in explanted cultures of rat hippocampus

Due to loss of afferent innervation, synaptic reorganization occurs in organotypic hippocampal slice cultures. With extra- and intracellular recordings, we confirm that the excitatory loop from the dentate gyrus (DG) to CA3 and further to CA1 is preserved. However, hilar stimulation evoked antidromic population spikes in the DG which were followed by a population postsynaptic potential (PPSP); intracellularly, an antidromic spike with a broad shoulder or EPSP/IPSP sequences were induced. Synaptic responses were blocked by glutamate receptor antagonists. Stimulation of CA1 induced a PPSP in DG. Dextranamine stained pyramidal cells of CA1 were shown to project to DG. After removal of area CA3, DG's and mossy fibers' (MF) stimulation still elicited PPSPs and EPSP/IPSP sequences in area CA1 which disappeared when a cut was made through the hippocampal fissure. During bicuculline perfusion, hilar stimulation caused EPSPs in granule cells and spontaneous and evoked repetitive firing appeared even after its isolation from areas CA3 and CA1. Collateral excitatory synaptic coupling between granule cells was confirmed by paired recordings. Besides the preservation of the trisynaptic pathway in this preparation, new functional synaptic contacts appear, presumably due to MF collateral sprouting and formation of pathways between areas CA1 and DG.     

荧光金逆行示踪观察匹罗卡品致痫大鼠CA1区锥体细胞突触联系变化的研究

目的通过荧光金(FG)逆行示踪观察氯化锂-匹罗卡品致痫模型大鼠慢性自发发作期海马CA1区锥体细胞之间的突触联系变化。方法 SD大鼠2 0只随机分为实验组和对照组。癫痫持续状态后6 0 d左右,利用立体定位仪在活体内注射逆行性示踪剂FG至海马CA1区,术后常规喂养5~7 d后灌注取材。激光扫描共聚焦显微镜下观察FG的分布。结果 7只实验组大鼠中有5只可见有FG标记的锥体细胞,对照组未见。实验组中有2只大鼠在海马下托亦可见有FG标记的锥体细胞,而对照组未见。结论颞叶癫痫大鼠海马CA1区锥体细胞之间和下托至CA1区有异常兴奋性突触联系,其可能是构成异常兴奋性回路的解剖学基础。     

Deafferentation enhances neurogenesis in the young and middle aged hippocampus but not in the aged hippocampus

Increased neurogenesis in the dentate gyrus (DG) after brain insults such as excitotoxic lesions, seizures, or stroke is a well known phenomenon in the young hippocampus. This plasticity reflects an innate compensatory response of neural stem cells (NSCs) in the young hippocampus to preserve function or minimize damage after injury. However, injuries to the middle‐aged and aged hippocampi elicit either no or dampened neurogenesis response, which could be due to an altered plasticity of NSCs and/or the hippocampus with age. We examined whether the plasticity of NSCs to increase neurogenesis in response to a milder injury such as partial deafferentation is preserved during aging. We quantified DG neurogenesis in the hippocampus of young, middle‐aged, and aged F344 rats after partial deafferentation. A partial deafferentation of the left hippocampus without any apparent cell loss was induced via administration of Kainic acid (0.5 μg in 1.0 μl) into the right lateral ventricle of the brain. In this model, degeneration of CA3 pyramidal neurons and dentate hilar neurons in the right hippocampus results in loss of commissural axons which leads to partial deafferentation of the dendrites of dentate granule cells and CA1‐CA3 pyramidal neurons in the left hippocampus. Quantification of newly born cells that are added to the dentate granule cell layer at postdeafferentation days 4–15 using 5′‐bromodeoxyuridine (BrdU) labeling revealed greatly increased addition of newly born cells (～three fold increase) in the deafferented young and middle‐aged hippocampi but not in the deafferented aged hippocampus. Measurement of newly born neurons using doublecortin (DCX) immunostaining also revealed similar findings. Analyses using BrdU‐DCX dual immunofluorescence demonstrated no changes in neuronal fate‐choice decision of newly born cells after deafferentation, in comparison to the age‐matched naive hippocampus in all age groups. Thus, the plasticity of hippocampal NSCs to increase DG neurogenesis in response to a milder injury such as partial hippocampal deafferentation is preserved until middle age but lost at old age. © 2010 Wiley‐Liss, Inc.     

Local circuit synaptic interactions between CA1 pyramidal cells and interneurons in the kainate-lesioned hyperexcitable hippocampus.

Following kainate (KA)-induced lesions of subfield CA3--a lesion relevant to human temporal lobe epilepsy--remaining pyramidal cells in CA1 display synchronous hyperexcitability associated with a loss of synaptic inhibition. Despite this loss, inhibitory interneurons in CA1 remain viable, and the density and function of GABAergic receptors on the CA1 pyramidal cells are maintained at approximately normal levels. To further evaluate inhibition in this system, the authors examined interactions between pyramidal cells and inhibitory interneurons in paired intracellular recordings. Recordings were carried out in rat hippocampal slices 2-4 weeks following bilateral intraventricular KA injections. The frequency of synaptic interactions between CA1 basket cells and pyramidal cells was lower in hyperexcitable slices than in controls; both synapses in the recurrent inhibitory circuit appeared to be involved. No recurrent excitatory interactions were seen between pyramidal cell pairs in lesioned or normal slices. The weakened interconnections between pyramidal cells and interneurons are consistent with the decreased inhibition previously found in this model. Unexpectedly, strong stimulation, which may directly activate local inhibitory circuitry, was effective in reducing hyperexcitability in KA-lesioned slices. These data suggest that development of recurrent excitatory connections among CA1 hippocampal pyramidal cells contribute little to tissue excitability, and support the hypothesis that a functional uncoupling between inhibitory interneurons and CA1 pyramidal cells is responsible for the seizure-like activity typical of KA-lesioned hippocampus. The data are also consistent with the hypothesis that in the KA model, the structural circuitry needed for inhibition in CA1 is maintained, and can be functionally activated by appropriate stimuli.     

Regional and time‐dependent neuroprotective effect of hypothermia following oxygen‐glucose deprivation

The neuroprotective effect of hypothermia has been demonstrated in in vivo and in vitro models of cerebral ischemia. In regard to the hippocampus, previous studies have mainly focused on CA1 pyramidal neurons, which are very vulnerable to ischemia. But the dentate gyrus (DG), in which neuronal proliferation occurs, can also be damaged by ischemia. In this study, we explored the neuroprotective effect of postischemic hypothermia in different areas of the hippocampus after mild or severe ischemia. Organotypic hippocampal slice cultures were prepared from 6‐ to 8‐day‐old rats and maintained for 12 days. Cultures were exposed to 25 or 35 min of oxygen and glucose deprivation (OGD). Neuronal damage was quantified after 6, 24, 48, and 72 h by propidium iodide fluorescence. Mild hypothermia (33°C) was induced 1 h after the end of OGD and was maintained for a period of 24 h. Short OGD produced delayed neuronal damage in the CA1 area and in the DG and to a lesser extend in the CA3 area. Damage in CA1 pyramidal cells was totally prevented by hypothermia whereas neuroprotection was limited in the DG. Thirty‐five‐minute OGD induced more rapid and more severe cell death in the three regions. In this case, hypothermia induced 1 h after OGD was unable to protect CA1 pyramidal cells whereas hypothermia induced during OGD was able to prevent cell loss. This study provides evidence that neuroprotection by hypothermia is limited to specific areas and depends on the severity of the ischemia. © 2014 Wiley Periodicals, Inc.     

Pattern of neuronal loss in the rat hippocampus following experimental cardiac arrest-induced ischemia.

The pattern of neuronal loss in the rat hippocampus following 10-min-long cardiac arrest-induced global ischemia was analyzed using the unbiased, dissector morphometric technique and hierarchical sampling. On the third day after ischemia, the pyramidal layer of sector CA1 demonstrated significant (27%) neuronal loss (P<0.05). At this time, no neuronal loss was observed in other cornu Ammonis sectors or the granular layer of the dentate gyrus. On the 14th postischemic day, further neuronal loss in the sector CA1 pyramidal layer was noticed. At this time, this sector contained 31% fewer pyramidal neurons than on the third day (P<0.05) and 58% fewer than in the control group (P<0.01). On the 14th day, neuronal loss in other hippocampal subdivisions also was observed. The pyramidal layer of sector CA3 contained 36% fewer neurons than in the control group (P<0.05), whereas the granular layer of the dentate gyrus contained 40% fewer (P<0.05). The total number of pyramidal neurons in sector CA2 remained unchanged. After the 14th day, no significant alterations in the total number of neurons were observed in any subdivision of the hippocampus until the 12th month of observation. Unbiased morphometric analysis emphasizes the exceptional susceptibility of sector CA1 pyramidal neurons to hypoxia/ischemia but also demonstrates significant neuronal loss in sector CA3 and the dentate granular layer, previously considered 'relatively resistant'. The different timing of neuronal dropout in sectors CA1 and CA3 and the dentate gyrus may implicate the existence of region-related properties, which determine earlier or later reactions to ischemia. However, the hippocampus has a unique, unidirectional system of intrinsic connections, whereby the majority of dentate granular neuron projections target the sector CA3 pyramidal neurons, which in turn project mostly to sector CA1. As a result, the early neuronal dropout in sector CA1 may result in retrograde transynaptic degeneration of neurons in other areas. The lack of neuronal loss in sector CA2 can be explained by the resistance of this sector to ischemia/hypoxia and the fact that this sector is not included in the major chain of intrahippocampal connections and hence is not affected by retrograde changes.