Hippocampal Entorhinal Cortex Research Articles

Exploring structural brain changes in children with neonatal brachial plexus palsy: a voxel-based morphometry analysis

BACKGROUND: Obstetric brachial plexus palsy (OBPP) is paralysis of the upper limb resulting from nerve injury during vaginal delivery. Although current treatment approaches frequently lead to complete reinnervation of the limb, some patients show long-term motor deficits. These impairments may result from upper limb disuse that causes structural brain changes. AIM: This study aimed to compare deep gray matter volumes between children with OBPP and healthy controls. METHODS: We analyzed the structural magnetic resonance imaging results of 46 children with OBPP (n=24, mean age — 10.20, of whom 12 were girls) and healthy age-matched controls (n=22, mean age — 9.63, of whom 10 were girls) using a voxel-based morphometry technique in SPM12 package (Statistical Parametric Mapping) in MATLAB R2019b. To minimize false discoveries, we used a stringent procedure to control the family-wise error rate. RESULTS: We found volumetric brain differences between children with OBPP and healthy controls (all FWE-corrected p 0.005). Children with OBPP had significantly lower gray matter volumes in the left amygdala, bilateral hippocampus, and right entorhinal cortex. CONCLUSION: Integrating our findings with previous work, we speculate that the amygdala–hippocampus–entorhinal cortex complex might play a significant role in motor disorders.

CSF neurodegeneration biomarkers predict MTL atrophy over time in cognitively healthy older adults

AbstractBackgroundStructural decline in the medial temporal lobe is an unspecific biomarker of neurodegeneration, assessed in normal older adults (OA) and Alzheimer’s Disease (AD). We investigated, in OA, whether CSF biomarkers of neurodegeneration (t‐tau, NFL, Ng, FABP3) predict longitudinal brain atrophy, whether this association is moderated by core AD biomarkers of beta‐amyloid (Aβ) and phosphorylated tau (p‐tau) and whether it is related to specific cellular and molecular mechanisms by using neurogenetic profiles.MethodLongitudinal structural MRI up to 7 years and baseline CSF biomarkers levels were collected in 189 OA (mean age = 74.8 years). Hippocampus (HC) and entorhinal cortex (EC) were selected as regions of interest. Linear mixed and generalized additive mixed models (GAMM) were used to assess the effects of Biomarker, Time, and Time×Biomarker on EC thickness and HC volume. Similar models were used to assess the interaction with core AD biomarkers. Correlation between cortical decline maps, associated with CSF biomarkers, and gene‐expression maps (Allen Human Brain Atlas https://human.brain‐map.org/) for specific cellular components, was assessed with BrainSMASH package (https://brainsmash.readthedocs.io/en/latest/)ResultHigher NFL, FABP3, and t‐tau CSF values were associated with steeper EC thinning and more HC atrophy (p<.05) over time (Figure 1a). GAMM showed significant non‐linear interactions with Time (except NG): NFL and FABP3 on EC (edf = 3.15, 3.81) and NFL and t‐tau on HC (edf = 3.09, 2.77) (Figure 1b). Aβ+ status (<550 pg/mL) did not significantly moderate the association with any biomarker (all tests p>.08) (Figure 2a). The relationships between CSF NFL with EC thinning (t = ‐3.27, p = 0.001) and HC atrophy (t = ‐2.91, p = 0.004), and FABP3 with EC thinning (t = ‐1.98, p = .05) remained significant, independently of p‐tau. The spatial pattern of cortical atrophy associated with the CSF biomarkers of neurodegeneration (Figure 2b) overlapped with distinct neurogenetic profiles: t‐tau with axonal growth cone (r = ‐.50, punc = .002); NFL with cortical cytoskeleton (r = ‐.61, pfdr<.001) and Ng with the dendritic shaft (r = ‐.40, punc<.009) components.ConclusionCSF NFL and FABP3 predict HC atrophy and EC thinning independently of Aβ/p‐tau. The neurodegenerative biomarkers may improve the accuracy of individual predictions of neurodegenerative changes in OA and identify those with a higher risk of future cognitive decline.

Spatial memory deficits in Alzheimer's disease and their connection to cognitive maps' formation by place cells and grid cells.

Whenever we navigate through different contexts, we build a cognitive map: an internal representation of the territory. Spatial navigation is a complex skill that involves multiple types of information processing and integration. Place cells and grid cells, collectively with other hippocampal and medial entorhinal cortex neurons (MEC), form a neural network whose activity is critical for the representation of self-position and orientation along with spatial memory retrieval. Furthermore, this activity generates new representations adapting to changes in the environment. Though there is a normal decline in spatial memory related to aging, this is dramatically increased in pathological conditions such as Alzheimer's disease (AD). AD is a multi-factorial neurodegenerative disorder affecting mainly the hippocampus-entorhinal cortex (HP-EC) circuit. Consequently, the initial stages of the disease have disorientation and wandering behavior as two of its hallmarks. Recent electrophysiological studies have linked spatial memory deficits to difficulties in spatial information encoding. Here we will discuss map impairment and remapping disruption in the HP-EC network, as a possible circuit mechanism involved in the spatial memory and navigation deficits observed in AD, pointing out the benefits of virtual reality as a tool for early diagnosis and rehabilitation.

A hippocampal-entorhinal cortex neuronal network for dynamical mechanisms of epileptic seizure.

Temporal lobe epilepsy (TLE) is thought to be associated with neuronal hyperexcitability in the hippocampal-entorhinal cortical (EC) circuit. Due to the complexity of the hippocampal-EC network connections, the biophysical mechanisms of the different circuits in epilepsy generation and propagation are still not fully established. In this work, we propose a hippocampal-EC neuronal network model to explore the mechanism of epileptic generation. We demonstrate that enhanced excitability of pyramidal neurons in cornu ammonis 3 (CA3) can drive hippocampal-EC to produce a transition from background to seizure state and cause exaggerated phase-amplitude coupling (PAC) phenomenon of theta modulated high-frequency oscillations (HFO) in CA3, cornu ammonis 1 (CA1), dentate gyrus, and EC. Interestingly, PAC strength indirectly responds to the degree of CA3 pyramidal (PY) neuron hyperexcitability, suggesting that PAC can be used as a potential marker of seizures. Furthermore, we find that enhanced synaptic connectivity of mossy cells to granule cells and CA3 PY neurons drives the system to produce epileptic discharges. These two channels may play a key role in mossy fiber sprouting. In particular, the PAC phenomenon of delta-modulated HFO and theta-modulated HFO can be generated according to the different degrees of moss fiber sprouting. Finally, the results suggest that hyperexcitability of stellate cells in EC can lead to seizures, which supports the argument that EC can act as an independent source of seizures. Overall, these results highlight the key role of different circuits in seizures, providing a theoretical basis and new insights into the generation and propagation of TLE.

L-Theanine Reduces Epileptiform Activity In Brain Slices

L-Theanine Reduces Epileptiform Activity In Brain Slices ABSTRACT Objective: L-theanine is one of the main amino acid of tea plant which can cross the blood brain barrier. In the central nervous system, L-theanine has certain effects on a number of neurotransmitter systems. In this study we tested whether acute application of L-theanine could induce any seizure like or inhibitory activity in the brain slice. Materials and Methods: Effects of L-theanine on epileptiform activity were investigated in two different brain regions. Horizontal hippocampal-entorhinal cortex slices were obtained from 30-35 days old C57BL/6 mice. Extracellular field potentials were recorded from medial entorhinal cortex (EC) and CA3 region of hippocampus. Epileptiform activity was induced by application of 4 Aminopyridine (4AP, 100 µM) in the brain slices and 50 µM L-theanine was applied. From the recordings, the duration and frequencies of ictal like events as well as frequency and amplitude of inter-ictal like activities were calculated. Results: L-theanine alone did not initiate any synchronized activity. Although, bath application of L-theanine after 4AP induces epileptiform acitivities, did not significantly alter the duration and frequency of ictal discharges, L-theanine attenuated the interictal discharges. In EC the frequencies of interictal discharges were significantly decreased. In CA3, the amplitudes of interictal activities were significantly reduced. Conclusion: The present findings suggest that L-theanine do not induce any seizure like event but rather had a suppressive effect on ongoing epileptiform activity. Key Words: Theanine, 4 Aminopyridine, Brain Slice, Epileptiform activity, CA3, Entorhinal cortex

Phoenixin-14 reduces the frequency of interictal-like events in mice brain slices

Phoenixin-14 (PNX-14) has a wide bioactivity in the central nervous system. Its role in the hypothalamus has been investigated, and it has been reported that it is involved in the regulation of excitability in hypothalamic neurons. However, its role in the regulation of excitability in entorhinal cortex and the hippocampus is unknown. In this study, we investigated whether i. PNX-14 induces any synchronous discharges or epileptiform activity and ii. PNX-14 has any effect on already initiated epileptiform discharges. We used 350 µm thick acute horizontal hippocampal-entorhinal cortex slices obtained from 30- to 35-day-old mice. Extracellular field potential recordings were evaluated in the entorhinal cortex and hippocampus CA1 region. Bath application of PNX-14 did not initiate any epileptiform activity or abnormal discharges. 4-Aminopyridine was applied to induce epileptiform activity in the slices. We found that 200nM PNX-14 reduced the frequency of interictal-like events in both the entorhinal cortex and hippocampus CA1 region which was induced by 4-aminopyridine. Furthermore, PNX-14 led to a similar suppression in the total power of local field potentials of 1-120Hz. The frequency or the duration of the ictal events was not affected. These results exhibited for the first time that PNX-14 has a modulatory effect on synchronized neuronal discharges which should be considered in future therapeutic approaches.

Navigating for reward.

An organism's survival can depend on its ability to recall and navigate to spatial locations associated with rewards, such as food or a home. Accumulating research has revealed that computations of reward and its prediction occur on multiple levels across a complex set of interacting brain regions, including those that support memory and navigation. However, how the brain coordinates the encoding, recall and use of reward information to guide navigation remains incompletely understood. In this Review, we propose that the brain's classical navigation centres - the hippocampus and the entorhinal cortex - are ideally suited to coordinate this larger network by representing both physical and mental space as a series of states. These states may be linked to reward via neuromodulatory inputs to the hippocampus-entorhinal cortex system. Hippocampal outputs can then broadcast sequences of states to the rest of the brain to store reward associations or to facilitate decision-making, potentially engaging additional value signals downstream. This proposal is supported by recent advances in both experimental and theoretical neuroscience. By discussing the neural systems traditionally tied to navigation and reward at their intersection, we aim to offer an integrated framework for understanding navigation to reward as a fundamental feature of many cognitive processes.

Normal and Abnormal Sharp Wave Ripples in the Hippocampal-Entorhinal Cortex System: Implications for Memory Consolidation, Alzheimer's Disease, and Temporal Lobe Epilepsy

The appearance of hippocampal sharp wave ripples (SWRs) is an electrophysiological biomarker for episodic memory encoding and behavioral planning. Disturbed SWRs are considered a sign of neural network dysfunction that may provide insights into the structural connectivity changes associated with cognitive impairment in early-stage Alzheimer's disease (AD) and temporal lobe epilepsy (TLE). SWRs originating from hippocampus have been extensively studied during spatial navigation in rodents, and more recent studies have investigated SWRs in the hippocampal-entorhinal cortex (HPC-EC) system during a variety of other memory-guided behaviors. Understanding how SWR disruption impairs memory function, especially episodic memory, could aid in the development of more efficacious therapeutics for AD and TLE. In this review, we first provide an overview of the reciprocal association between AD and TLE, and then focus on the functions of HPC-EC system SWRs in episodic memory consolidation. It is posited that these waveforms reflect rapid network interactions among excitatory projection neurons and local interneurons and that these waves may contribute to synaptic plasticity underlying memory consolidation. Further, SWRs appear altered or ectopic in AD and TLE. These waveforms may thus provide clues to understanding disease pathogenesis and may even serve as biomarkers for early-stage disease progression and treatment response.

Rhythmic Activity in the Hippocampus and Entorhinal Cortex is Impaired in a Model of Kainate Neurotoxicity in Rats in Free Behavior

The hippocampus and medial entorhinal cortex (MEC) interact by means of bidirectional connections and play an important role in the processing, memorization, and reproduction of information. Data obtained in healthy animals show that the θ and γ oscillations are critical activities necessary for the interaction of the hippocampus and the MEC in signal processing. At the same time, these structures are among the most vulnerable parts of the brain to hyperactivation leading to excitotoxic damage and neuron death. In the present study toxicity was provoked by systemic administration of kainic acid (KA), inducing the development of status epilepticus. In control rats given physiological saline and rats given injections of KA, local field potentials were recorded simultaneously in hippocampal field CA1 and the MEC during exploratory behavior in an open field. A clearly apparent θ rhythm (4–10 Hz) was observed, along with a slow γ rhythm (25–50 Hz) and a fast γ rhythm (55–100 Hz) in the hippocampus and MEC of animals of both groups. Movement of control animals to the center of the open field was accompanied by an increase in the frequency of the θ rhythm and a decrease in the frequency of the fast γ rhythm in the hippocampus; the MEC showed a decrease in the power of the slow γ rhythm. This was not seen in rats given KA. This group also showed impairment to the phase-amplitude modulation of MEC activity by the hippocampal θ rhythm: changes in this modulation on movement of animals from the peripheral zones to the center of the open field were significantly less marked than in controls. There was also a significant increase in θ coherence between the hippocampus and MEC for all locations of the animal in the open field. Changes in the characteristics of rhythms in hippocampus-entorhinal interactions are potential biomarkers for impairments to the coding of spatial information and its retrieval from memory due to status epilepticus and often leading to the development of a convulsive focus in the temporal structures of the brain.

Processing of Hippocampal Network Activity in the Receiver Network of the Medial Entorhinal Cortex Layer V.

The interplay between hippocampus and medial entorhinal cortex (mEC) is of key importance for forming spatial representations. Within the hippocampal-entorhinal loop, the hippocampus receives context-specific signals from layers II/III of the mEC and feeds memory-associated activity back into layer V (LV). The processing of this output signal within the mEC, however, is largely unknown. We characterized the activation of the receiving mEC network by evoked and naturally occurring output patterns in mouse hippocampal-entorhinal cortex slices. Both types of glutamatergic neurons (mEC LVa and LVb) as well as fast-spiking inhibitory interneurons receive direct excitatory input from the intermediate/ventral hippocampus. Connections between the two types of excitatory neurons are sparse, and local processing of hippocampal output signals within mEC LV is asymmetric, favoring excitation of far projecting LVa neurons over locally projecting LVb neurons. These findings suggest a new role for mEC LV as a bifurcation gate for feedforward (telencephalic) and feedback (entorhinal-hippocampal) signal propagation.SIGNIFICANCE STATEMENT Patterned network activity in hippocampal networks plays a key role in the formation and consolidation of spatial memories. It is, however, largely unclear how information is transferred to the neocortex for long-term engrams. Here, we elucidate the propagation of network activity from the hippocampus to the medial entorhinal cortex. We show that patterned output from the hippocampus reaches both major cell types of deep entorhinal layers. These cells are, however, only weakly connected, giving rise to two parallel streams of activity for local and remote signal propagation, respectively. The relative weight of both pathways is regulated by local inhibitory interneurons. Our data reveal important insights into the hippocampal-neocortical dialogue, which is of key importance for memory consolidation in the mammalian brain.

Prenatal betamethasone exposure increases corticotropin-releasing hormone expression along with increased hippocampal slice excitability in the developing hippocampus

BackgroundThe objective of this study was to determine whether prenatal exposure to betamethasone alters hippocampal expression of corticotropin-releasing hormone (CRH) and resultant hippocampal circuit excitability. MethodsReal time (RT)-PCR and western blots were used to determine CRH mRNA and protein expression levels, respectively, in hippocampal extracts of two-week old rat pups prenatally primed with betamethasone or saline on gestational day 15. The data were compared to changes in epileptiform activity induced by kainic acid (KA) or depletion of [Mg2+]0 in combined hippocampus-entorhinal cortex slices. ResultsRT-PCR analysis showed 3-fold increased levels of CRH mRNA in hippocampal extracts from prenatally betamethasone-primed pups compared to saline controls (p < 0.05), but no changes in mRNA expression of CRH receptors (1 and 2). Changes in CRH protein isoform ratio in hippocampal extracts suggest 30 % increase in mature CRH levels in betamethasone-primed hippocampi (p < 0.05). No changes in mRNA expression in CRH feedback loop associated genes, GR and FKBP51, were found. Compared to saline-exposed pups, slices from betamethasone-primed pups had faster onset of epileptiform-like activity (inter-ictal discharges and seizure-like-events) after bath application of 4 μM KA (p < 0.05) suggesting a “more hyperexcitable” state. The epileptiform-like activity after KA application was significantly reduced following bath application of a CRH R2 antagonist (p < 0.05) but CRH R1 antagonist had no effect (p > 0.05). Also in the low-Mg2+-induced epileptiform activity, there was increased excitability, in the form of enhanced inter-ictal discharges, in slices from betamethasone primed compared to saline exposed rat pups (p < 0.05). ConclusionsOur study suggests a possible mechanistic link to prenatal betamethasone priming-induced increase in postnatal hippocampal excitability that involves enhanced expression of CRH acting at CRH R2. This is important in regards to the links between prenatal stress/corticosteroid-exposure and syndromes, such as epilepsy, autism spectrum disorders and other psychiatric disorders associated with neuronal hyperexcitability.

Electrical Stimulation of the Lateral Entorhinal Cortex Causes a Frequency-Specific BOLD Response Pattern in the Rat Brain

Although deep brain stimulation of the entorhinal cortex has recently shown promise in the treatment of early forms of cognitive decline, the underlying neurophysiological processes remain elusive. Therefore, the lateral entorhinal cortex (LEC) was stimulated with trains of continuous 5 Hz and 20 Hz pulses or with bursts of 100 Hz pulses to visualize activated neuronal networks, i.e., neuronal responses in the dentate gyrus and BOLD responses in the entire brain were simultaneously recorded. Electrical stimulation of the LEC caused a wide spread pattern of BOLD responses. Dependent on the stimulation frequency, BOLD responses were only triggered in the amygdala, infralimbic, prelimbic, and dorsal peduncular cortex (5 Hz), or in the nucleus accumbens, piriform cortex, dorsal medial prefrontal cortex, hippocampus (20 Hz), and contralateral entorhinal cortex (100 Hz). In general, LEC stimulation caused stronger BOLD responses in frontal cortex regions than in the hippocampus. Identical stimulation of the perforant pathway, a fiber bundle projecting from the entorhinal cortex to the dentate gyrus, hippocampus proper, and subiculum, mainly elicited significant BOLD responses in the hippocampus but rarely in frontal cortex regions. Consequently, BOLD responses in frontal cortex regions are mediated by direct projections from the LEC rather than via signal propagation through the hippocampus. Thus, the beneficial effects of deep brain stimulation of the entorhinal cortex on cognitive skills might depend more on an altered prefrontal cortex than hippocampal function.

Mathematical model of Na-K-Cl homeostasis in ictal and interictal discharges.

Despite big experimental data on the phenomena and mechanisms of the generation of ictal and interictal discharges (IDs and IIDs), mathematical models that can describe the synaptic interactions of neurons and the ionic dynamics in biophysical detail are not well-established. Based on experimental recordings of combined hippocampal-entorhinal cortex slices from rats in a high-potassium and a low-magnesium solution containing 4-aminopyridine as well as previous observations of similar experimental models, this type of mathematical model has been developed. The model describes neuronal excitation through the application of the conductance-based refractory density approach for three neuronal populations: two populations of glutamatergic neurons with hyperpolarizing and depolarizing GABAergic synapses and one GABAergic population. The ionic dynamics account for the contributions of voltage-gated and synaptic channels, active and passive transporters, and diffusion. The relatively slow dynamics of potassium, chloride, and sodium ion concentrations determine the transitions from pure GABAergic IIDs to IDs and GABA-glutamatergic IIDs. The model reproduces different types of IIDs, including those initiated by interneurons; repetitive IDs; tonic and bursting modes of an ID composed of clustered IID-like events. The simulations revealed contributions from different ionic channels to the ion concentration dynamics before and during ID generation. The proposed model is a step forward to an optimal mathematical description of the mechanisms of epileptic discharges.

7,8-Dihydroxyflavone potentiates ongoing epileptiform activity in mice brain slices

In the central nervous system, Tropomyosin-receptor-kinase B (TrkB) signaling is involved in neuronal survival, differentiation as well as in regulation of synaptic transmission and excitability. As its powerful potential to modulate neuronal functions, TrkB pathway is an attractive target for novel drugs and treatment of common neurological disorders. 7,8-Dihydroxyflavone (DHF), a TrkB receptor agonist, has similar properties with neurotrophin Brain Derived Neurotropic Factor (BDNF). DHF is reported to have a number of beneficial effects in neuroprotection, against depression and improving learning and memory. However, the outcome of acute application of DHF on the excitability of neuronal circuits is not clear. Especially the effects of DHF on synchronized epileptiform activity are not known. In this study, we investigated whether DHF induces epileptiform activity in brain slices and DHF has any effect on already initiated epileptiform discharges. We used acute horizontal hippocampal-entorhinal cortex slices obtained from 30 to 35 days of mice. Extracellular field potential recordings were obtained from entorhinal cortex (EC) and hippocampus CA1 region. DHF did not initiate any epileptiform activity or abnormal discharges. However, DHF increased the frequency of 4 aminopyridine (4AP) induced ictal and interictal events in both EC and CA1. The duration of induced ictal charges were also prolonged upon DHF application. In a number of slices, both EC and CA1, DHF led to ictogenesis. These results suggest that the acute activation of TrkB by DHF has a powerful potential on synchronized neuronal discharges which should be considered in future therapeutical approaches.

Curcumin reduces development of seizurelike events in the hippocampal-entorhinal cortex slice culture model for epileptogenesis.

Inhibition of the mammalian target of rapamycin (mTOR) pathway could be antiepileptogenic in temporal lobe epilepsy (TLE), possibly via anti-inflammatory actions. We studied effects of the mTOR inhibitor rapamycin and the anti-inflammatory compound curcumin-also reported to inhibit the mTOR pathway-on epileptogenesis and inflammation in an invitro organotypic hippocampal-entorhinal cortex slice culture model. Brain slices containing hippocampus and entorhinal cortex were obtained from 6-day-old rat pups and maintained in culture for up to 3weeks. Rapamycin or curcumin was added to the culture medium from day 2 invitro onward. Electrophysiological recordings revealed epileptiformlike activity that developed over 3weeks. In week 3, spontaneous seizurelike events (SLEs) could be detected using whole cell recordings from CA1 principal neurons. The percentage of recorded CA1 neurons displaying SLEs was lower in curcumin-treated slice cultures compared to vehicle-treated slices (25.8% vs 72.5%), whereas rapamycin did not reduce SLE occurrence significantly (52%). Western blot for phosphorylated-S6 (pS6) and phosphorylated S6K confirmed that rapamycin inhibited the mTOR pathway, whereas curcumin only lowered pS6 expression at one phosphorylation site. Real-time quantitative polymerase chain reaction results indicated a trend toward lower expression of inflammatory markers IL-1β and IL-6 and transforming growth factor β after 3weeks of treatment with rapamycin and curcumin compared to vehicle. Our results show that curcumin suppresses SLEs in the combined hippocampal-entorhinal cortex slice culture model and suggest that its antiepileptogenic effects should be further investigated in experimental models of TLE.

Polysomnography, brain volumetry, and mismatch negativity as early biomarkers of amnestic mild cognitive impairment progression

BackgroundMild cognitive impairment (MCI) is a heterogenous disorder in which a proportion of patients follow stationary or regressive courses while others undergo clinical progression to dementia.MethodsThis study was conducted on 60 amnestic mild cognitive impairment (amMCI) and 20 healthy control subjects submitted to baseline Montreal Cognitive Assessment (MoCA) scale, one-night polysomnography (PSG), hippocampal/entorhinal cortex (HPC/ERC) MRI volumetry, and auditory mismatch negativity (MMN). Fifty-six amMCI subjects continued the study and underwent follow-up MoCA scale 1 year after their baseline evaluation, 17 showed amMCI progression (≥ 3 points decrease in MoCA scale), and 39 had stationary or regressive courses.ResultsProgressive amMCI patients showed reduced sleep efficiency and shortened rapid eye movement (REM) sleep in PSG, decreased HPC/ERC–MRI volumetry and reduced amplitudes with delayed latencies of the MMN evoked potentials.ConclusionsPSG shortened REM sleep, MRI–HPC/ERC volumes reduction, and low amplitude delayed auditory MMN are valuable non-invasive screening predictors of amMCI progression.

A Comparison of Different Slicing Planes in Preservation of Major Hippocampal Pathway Fibers in the Mouse.

The hippocampus plays a critical role in learning and memory and higher cognitive functions, and its dysfunction has been implicated in various neuropathological disorders. Electrophysiological recording undertaken in live brain slices is one of the most powerful tools for investigating hippocampal cellular and network activities. The plane for cutting the slices determines which afferent and/or efferent connections are best preserved, and there are three commonly used slices: hippocampal-entorhinal cortex (HEC), coronal and transverse. All three slices have been widely used for studying the major afferent hippocampal pathways including the perforant path (PP), the mossy fibers (MFs) and the Schaffer collaterals (SCs). Surprisingly, there has never been a systematic investigation of the anatomical and functional consequences of slicing at a particular angle. In the present study, we focused on how well fiber pathways are preserved from the entorhinal cortex (EC) to the hippocampus, and within the hippocampus, in slices generated by sectioning at different angles. The postmortem neural tract tracer 1,1′-dioctadecyl-3,3,3′3′-tetramethylindocarbocyanine perchlorate (DiI) was used to label afferent fibers to hippocampal principal neurons in fixed slices or whole brains. Laser scanning confocal microscopy was adopted for imaging DiI-labeled axons and terminals. We demonstrated that PP fibers were well preserved in HEC slices, MFs in both HEC and transverse slices and SCs in all three types of slices. Correspondingly, field excitatory postsynaptic potentials (fEPSPs) could be consistently evoked in HEC slices when stimulating PP fibers and recorded in stratum lacunosum-moleculare (sl-m) of area CA1, and when stimulating the dentate granule cell layer (gcl) and recording in stratum lucidum (sl) of area CA3. The MF evoked fEPSPs could not be recorded in CA3 from coronal slices. In contrast to our DiI-tracing data demonstrating severely truncated PP fibers in coronal slices, fEPSPs could still be recorded in CA1 sl-m in this plane, suggesting that an additional afferent fiber pathway other than PP might be involved. The present study increases our understanding of which hippocampal pathways are best preserved in the three most common brain slice preparations, and will help investigators determine the appropriate slices to use for physiological studies depending on the subregion of interest.

Engrams and circuits crucial for systems consolidation of a memory.

Episodic memories initially require rapid synaptic plasticity within the hippocampus for their formation and are gradually consolidated in neocortical networks for permanent storage. However, the engrams and circuits that support neocortical memory consolidation have thus far been unknown. We found that neocortical prefrontal memory engram cells, which are critical for remote contextual fear memory, were rapidly generated during initial learning through inputs from both the hippocampal-entorhinal cortex network and the basolateral amygdala. After their generation, the prefrontal engram cells, with support from hippocampal memory engram cells, became functionally mature with time. Whereas hippocampal engram cells gradually became silent with time, engram cells in the basolateral amygdala, which were necessary for fear memory, were maintained. Our data provide new insights into the functional reorganization of engrams and circuits underlying systems consolidation of memory.

Reorganization of anterior and posterior hippocampal networks associated with memory performance in mesial temporal lobe epilepsy.

To investigate the pattern of functional demarcation of hippocampal network and its relationship with memory performance in mesial temporal lobe epilepsy (mTLE) with unilateral hippocampal sclerosis. Resting state fMRI data were acquired from fifteen left mTLE patients, fourteen right mTLE patients and twenty healthy subjects. We explore the hippocampal-cortical alterations and corresponding inter-hemispheric functional connectivity (FC) across anterior and posterior hippocampal networks. The association between FC and memory performance was assessed. Left mTLE showed increased intra-hemispheric FC in anterior hippocampal networks, including left anterior hippocampal-entorhinal cortex and right anterior hippocampal-orbitofrontal cortex, and decreased inter-hemispheric FC between anterior hippocampus, entorhinal cortex and posterior cingulate cortex. Right mTLE was associated with extensive reduction in inter-hemispheric FC along the areas of anterior and posterior hippocampal networks. Intra-hemispheric FC between left anterior hippocampus and entorhinal cortex was positively correlated with verbal memory in left mTLE. Inter-hemispheric FC between posterior parahippocampal gyrus was negatively correlated with verbal memory in right mTLE. Our findings suggested that left and right mTLE exhibit different neural reorganization patterns of anterior and posterior hippocampal networks associated with verbal memory. The findings may facilitate the characterization of mTLE associated with memory deficit.

Microglial-derived miRNA let-7 and HMGB1 contribute to ethanol-induced neurotoxicity via TLR7

BackgroundToll-like receptor (TLR) signaling is emerging as an important component of neurodegeneration. TLR7 senses viral RNA and certain endogenous miRNAs to initiate innate immune responses leading to neurodegeneration. Alcoholism is associated with hippocampal degeneration, with preclinical studies linking ethanol-induced neurodegeneration with central innate immune induction and TLR activation. The endogenous miRNA let-7b binds TLR7 to cause neurodegeneration.MethodsTLR7 and other immune markers were assessed in postmortem human hippocampal tissue that was obtained from the New South Wales Tissue Bank. Rat hippocampal-entorhinal cortex (HEC) slice culture was used to assess specific effects of ethanol on TLR7, let-7b, and microvesicles.ResultsWe report here that hippocampal tissue from postmortem human alcoholic brains shows increased expression of TLR7 and increased microglial activation. Using HEC slice culture, we found that ethanol induces TLR7 and let-7b expression. Ethanol caused TLR7-associated neuroimmune gene induction and initiated the release let-7b in microvesicles (MVs), enhancing TLR7-mediated neurotoxicity. Further, ethanol increased let-7b binding to the danger signaling molecule high mobility group box-1 (HMGB1) in MVs, while reducing let-7 binding to classical chaperone protein argonaute (Ago2). Flow cytometric analysis of MVs from HEC media and analysis of MVs from brain cell culture lines found that microglia were the primary source of let-7b and HMGB1-containing MVs.ConclusionsOur results identify that ethanol induces neuroimmune pathology involving the release of let-7b/HMGB1 complexes in microglia-derived microvesicles. This contributes to hippocampal neurodegeneration and may play a role in the pathology of alcoholism.