Stimulation Of Perforant Pathway Research Articles

Hemodynamic responses in the rat hippocampus are simultaneously controlled by at least two independently acting neurovascular coupling mechanisms

We combined electrical perforant pathway stimulation with electrophysiological and fMRI recordings in the hippocampus to investigate the effects of neuronal afterdischarges (nAD) on subsequent fMRI BOLD signals in the presence of isoflurane and medetomidine. These two drugs already alter basal hemodynamics in the hippocampus, with isoflurane being mildly vasodilatory and medetomidine being mildly vasoconstrictive. The perforant pathway was stimulated once for 8 seconds with either continuous 20 Hz pulses (continuous stimulation) or 8 bursts of 20 high-frequency pulses (burst stimulation). Burst stimulation in the presence of medetomidine elicited long-lasting nAD that coincided with a brief positive BOLD response and a subsequent long-lasting decrease in BOLD signals. Under isoflurane, this stimulation elicited only short-lasting nAD and only a short-lasting decline in BOLD signals. In contrast, continuous stimulation under isoflurane and medetomidine caused a similar duration of nAD. Under isoflurane, this caused only a sharp and prolonged decline in BOLD signals, whereas under medetomidine, again, only a brief positive BOLD response was elicited, followed by a shorter and moderate decline in BOLD signals. Our results suggest that nAD simultaneously activate different neurovascular coupling mechanisms that then independently alter local hemodynamics in the hippocampus, resulting in an even more complex neurovascular coupling mechanism.

Acute neuroinflammation leads to disruption of neuronal chloride regulation and consequent hyperexcitability in the dentate gyrus

Neuroinflammation is a salient part of diverse neurological and psychiatric pathologies that associate with neuronal hyperexcitability, but the underlying molecular and cellular mechanisms remain to be identified. Here, we show that peripheral injection of lipopolysaccharide (LPS) renders the dentate gyrus (DG) hyperexcitable to perforant pathway stimulation invivo and increases the internal spiking propensity of dentate granule cells (DGCs) invitro 24h post-injection (hpi). In parallel, LPS leads to a prominent downregulation of chloride extrusion via KCC2 and to the emergence of NKCC1-mediated chloride uptake in DGCs under experimental conditions optimized to detect specific changes in transporter efficacy. These data show that acute neuroinflammation leads to disruption of neuronal chloride regulation, which unequivocally results in a loss of GABAergic inhibition in the DGCs, collapsing the gating function of the DG. The present work provides a mechanistic explanation for neuroinflammation-driven hyperexcitability and consequent cognitive disturbance.

Insulin-induced long-term potentiation in the dentate gyrus of hippocampal formation

The discovery that brain areas involving in learning and memory express receptors for insulin hormone, led to the idea that insulin signaling may have a role in regulating cognitive function. Although previous studies have shown a role for insulin in regulation of the threshold of plasticity induction, no study has addressed whether insulin can induce a chemical plasticity per se. Young-adult male rats that are fed with standard diets with or without carbohydrate syrup (sucrose or high-fructose corn syrups) were enrolled in this study. Extracellular field potentials were recorded from the dentate gyrus in response to perforant pathway stimulation at 0.033 Hz in anesthetized rats. The slope of field excitatory postsynaptic potentials (fEPSPs) and the amplitude of population spike (PS) were measured 15 min after a 60-min infusion of insulin (500 nM), NT157 (an IRS inhibitor, 6 μM), alone or together, or physiological saline. mRNA expressions of insulin signaling proteins were measured by rt-PCR in the whole hippocampus. We did not observe any appreciable change in the fEPSP slope and the PS amplitude before and after saline infusion. However, intra-hippocampal insulin application results in the induction of LTP of fEPSP and of PS in the dentate gyrus. Insulin infusion together with NT157 inhibited fEPSP-LTP, but not PS-LTP, and rats that are fed with carbohydrate syrup did not express synaptic LTP. In rats that additional carbohydrate syrup is not given, insulin-induced LTP was accompanied with an increase in PI3K-mRNA, AKT-mRNA, and GSK-3β-mRNA which was not observed when co-administered with NT157. The GSK-3β-mRNA and IRS1-mRNA levels were found to be lower in rats that received supplemental carbohydrate and that not express insulin-induced synaptic LTP, compared to the rats expressing synaptic LTP and fed by standard diet. The results obtained provide a mechanistic link between insulin and synaptic plasticity. We concluded that insulin not only functions as a modulator of synaptic plasticity but also acts as a chemical inducer of LTP.

Latent toxoplasmosis impairs learning and memory yet strengthens short-term and long-term hippocampal synaptic plasticity at perforant pathway-dentate gyrus, and Schaffer collatterals-CA1 synapses

Investigating long-term potentiation (LTP) in disease models provides essential mechanistic insight into synaptic dysfunction and relevant behavioral changes in many neuropsychiatric and neurological diseases. Toxoplasma (T) gondii is an intracellular parasite causing bizarre changes in host’s mind including losing inherent fear of life-threatening situations. We examined hippocampal-dependent behavior as well as in vivo short- and long-term synaptic plasticity (STP and LTP) in rats with latent toxoplasmosis. Rats were infected by T. gondii cysts. Existence of REP-529 genomic sequence of the parasite in the brain was detected by RT-qPCR. Four and eight weeks after infection, spatial, and inhibitory memories of rats were assessed by Morris water maze and shuttle box tests, respectively. Eight weeks after infection, STP was assessed in dentate gyrus (DG) and CA1 by double pulse stimulation of perforant pathway and Shaffer collaterals, respectively. High frequency stimulation (HFS) was applied to induce LTP in entorhinal cortex-DG (400 Hz), and CA3-CA1 (200 Hz) synapses. T. gondii infection retarded spatial learning and memory performance at eight weeks post-infection period, whereas inhibitory memory was not changed. Unlike uninfected rats that normally showed paired-pulse depression, the infected rats developed paired-pulse facilitation, indicating an inhibitory synaptic network disruption. T. gondii-infected rats displayed strengthened LTP of both CA1-pyramidal and DG-granule cell population spikes. These data indicate that T. gondii disrupts inhibition/excitation balance and causes bizarre changes to the post-synaptic neuronal excitability, which may ultimately contribute to the abnormal behavior of the infected host.

Fingolimod Administration Following Hypoxia Induced Neonatal Seizure Can Restore Impaired Long-term Potentiation and Memory Performance in Adult Rats

Neonatal seizures commonly caused by hypoxia can lead to long-term neurological outcomes. Early inflammation plays an important role in the pathology of these outcomes. Therefore, in the current study, we explored the long-term effects of Fingolimod (FTY720), an analog of sphingosine and potent sphingosine 1-phosphate (S1P) receptors modulator, as an anti-inflammatory and neuroprotective agent in attenuating anxiety, memory impairment, and possible alterations in gene expression of hippocampal inhibitory and excitatory receptors following hypoxia-induced neonatal seizure (HINS). Seizure was induced in 24 male and female pups (6 in each experimental group) at postnatal day 10 (P10) by premixed gas (5% oxygen/ 95% nitrogen) in a hypoxic chamber for 15 minutes. Sixty minutes after the onset of hypoxia, FTY720 (0.3 mg/kg) or saline (100 µl) was administered for 12 days (from P10 up to P21). Anxiety-like behavior and hippocampal memory function were assessed at P90 by elevated plus maze (EPM) and novel object recognition (NOR), respectively. Long-term potentiation (LTP) was recorded from hippocampal dentate gyrus region (DG) following stimulation of perforant pathway (PP). In addition, the hippocampal concentration of superoxide dismutase activity (SOD), malondialdehyde (MDA), and thiol as indices of oxidative stress were evaluated. Finally, the gene expression of NR2A subunit of N-Methyl-D-aspartic acid (NMDA) receptor, GluR2 subunit of (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) AMPA receptor and γ2 subunit of γ-Aminobutyric acid (GABAA) receptor were assessed at P90 by the quantitative real-time PCR. FTY720 significantly reduced later-life anxiety-like behavior, ameliorated object recognition memory and increased the amplitude and slope of the field excitatory postsynaptic potential (fEPSP) in the rats following HINS. These effects were associated with restoration of the hippocampal thiol content to the normal values and the regulatory role of FTY720 in the expression of hippocampal GABA and glutamate receptors subunits. In conclusion, FTY720 could restore the dysregulated gene expression of excitatory and inhibitory receptors. It also increased the reduced hippocampal thiol content, which was accompanied with attenuation of HINS-induced anxiety, reduced the impaired hippocampal related memory, and prevented hippocampal LTP deficits in later life following HINS.

Benchmarking the proteomic profile of animal models of mesial temporal epilepsy.

ObjectivesWe compared the proteomic signatures of the hippocampal lesion induced in three different animal models of mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE+HS): the systemic pilocarpine model (PILO), the intracerebroventricular kainic acid model (KA), and the perforant pathway stimulation model (PPS).MethodsWe used shotgun proteomics to analyze the proteomes and find enriched biological pathways of the dorsal and ventral dentate gyrus (DG) isolated from the hippocampi of the three animal models. We also compared the proteomes obtained in the animal models to that from the DG of patients with pharmacoresistant MTLE+HS.ResultsWe found that each animal model presents specific profiles of proteomic changes. The PILO model showed responses predominantly related to neuronal excitatory imbalance. The KA model revealed alterations mainly in synaptic activity. The PPS model displayed abnormalities in metabolism and oxidative stress. We also identified common biological pathways enriched in all three models, such as inflammation and immune response, which were also observed in tissue from patients. However, none of the models could recapitulate the profile of molecular changes observed in tissue from patients.SignificanceOur results indicate that each model has its own set of biological responses leading to epilepsy. Thus, it seems that only using a combination of the three models may one replicate more closely the mechanisms underlying MTLE+HS as seen in patients.

Brief neuronal afterdischarges in the rat hippocampus lead to transient changes in oscillatory activity and to a very long-lasting decline in BOLD signals without inducing a hypoxic state

The effects of hippocampal neuronal afterdischarges (nAD) on hemodynamic parameters, such as blood-oxygen-level-dependent (BOLD) signals) and local cerebral blood volume (CBV) changes, as well as neuronal activity and metabolic parameters in the dentate gyrus, was investigated in rats by combining in vivo electrophysiology with functional magnetic resonance imaging (fMRI) or 1H-nuclear magnetic resonance spectroscopy (1H-NMRS). Brief electrical high-frequency pulse-burst stimulation of the right perforant pathway triggered nAD, a seizure-like activity, in the right dentate gyrus with a high incidence, a phenomenon that in turn caused a sustained decrease in BOLD signals for more than 30 min. The decrease was associated with a reduction in CBV but not with signs of hypoxic metabolism. nAD also triggered transient changes mainly in the low gamma frequency band that recovered within 20 min, so that the longer-lasting altered hemodynamics reflected a switch in blood supply rather than transient changes in ongoing neuronal activity. Even in the presence of reduced baseline BOLD signals, neurovascular coupling mechanisms remained intact, making long-lasting vasospasm unlikely. Subsequently generated nAD did not further alter the baseline BOLD signals. Similarly, nAD did not alter baseline BOLD signals when acetaminophen was previously administered, because acetaminophen alone had already caused a similar decrease in baseline BOLD signals as observed after the first nAD. Thus, at least two different blood supply states exist for the hippocampus, one low and one high, with both states allowing similar neuronal activity. Both acetaminophen and nAD switch from the high to the low blood supply state. As a result, the hemodynamic response function to an identical stimulus differed after nAD or acetaminophen, although the triggered neuronal activity was similar.

Electrophysiological study of the interactive role of the cannabinoid breakdown inhibitors and L-type calcium channels on granular neurons in the hippocampal dentate gyrus in rats

ABSTRACT The interaction between L-type voltage-dependent Ca2+ channels and the endocannabinoid system (eCs) in synaptic plasticity is controversial. In the present research, the impact of acute administration of URB597, as an endocannabinoid breakdown inhibitor, was evaluated after chronic injection of verapamil, as a Ca2+ channels blocker, on inducing long-term potentiation (LTP) in the rat’s hippocampal dentate gyrus (DG). Treatment of male Wistar rats was done using intraperitoneal(i.p) injection of verapamil hydrochloride (n = 8) and saline (n = 10), as the solvent of verapamil once a day within 13 days. Anesthetization was done by i.p injection of urethane and the rats were located in the stereotaxic apparatus for surgery, electrode implantation, and field potential recording. After observing a steady-state baseline response, saline or URB597 were injected (n = 9). Measurement of the population spike (PS) amplitude and slope of field excitatory postsynaptic potentials (fEPSPs) in the DG region was performed as a result of perforant pathway (PP) stimulation. Our treatments could inhibit LTP. Our results indicated that the chronic administration of verapamil produced a significant decrease in the slope of fEPSP and PS amplitude. Also, acute URB597 administration decreased the slope of fEPSP and PS amplitude compared to the saline group. Moreover, URB597 administration in combination with chronic administration of verapamil produced a greater decrease in fEPSP slope and PS amplitude than the saline group. These findings indicated that verapamil and URB597 disrupted LTP induction in the DG. Moreover, an interaction was observed between Ca2+ channels and eCs. Therefore, the eCs possibly play a selective role in synaptic plasticity.

P2. Unsupervised anomaly detection for diagnosing brain disorders from EEG recordings – Results from a rodent epilepsy model

We present a general machine learning (ML) framework for diagnosing brain disorders using EEG recordings. It could detect any systematic deviations from the baseline (BL) EEG recordings. We test the approach using a rodent model of an acquired mesial temporal lobe epilepsy (mTLE) via electrical perforant pathway stimulation (PPS) [1] followed by usually a latent period of several weeks of epileptogenesis (EPG) progression without any seizures. Identifying ongoing EPG during this latent period prior to the first spontaneous seizure might enable early intervention. Previous supervised approaches have proven that deep neural networks can distinguish intracranial EEG signals between the BL period prior to PPS and the EPG period after PPS [2]. Here, we formulate the problem as an unsupervised anomaly detection task, which allows learning the statistics of the input data without the need for any labels as in supervised approaches. Specifically, we first train an autoencoder (AE) to learn to encode EEG data from the BL period. We then apply the trained AE on data from the EPG period. We expect that activity patterns systematically deviating from those observed during BL should be characterized by higher reconstruction errors (REs). Indeed, our results show that REs averaged over one hour gradually increase as a function of time after the PPS (Fig. 1). We also find that the distribution of REs from the BL and the EPG phases are significantly different (Fig. 1B). EEG segments with particularly high REs exhibit spikes and high frequency oscillations (Fig. 1B, right inset). Interestingly, there are an increased large number of segments with unusually low REs (Fig. 1B, left inset), which exhibits EEG slowing and delta rhythm (1.5–3 Hz). In conclusion, we showed that unsupervised ML with AEs allows the automatic detection of systematic drifts in brain activity, which may facilitate rapid EEG recordings for early diagnosis of brain disorders and open the door for early interventions. References Costard, L.S., Neubert, V., Venø, M.T., Su, J., Kjems, J., Connolly, N.M., Prehn, J.H., Schratt, G., Henshall, D.C., Rosenow, F. and Bauer, S., 2019. Electrical stimulation of the ventral hippocampal commissure delays experimental epilepsy and is associated with altered microRNA expression. Brain Stimulation, 12(6), pp. 1390-1401. Lu, D., Bauer, S., Neubert, V., Costard, L.S., Rosenow, F. and Triesch, J., 2020, September. Towards Early Diagnosis of Epilepsy from EEG Data. In Machine Learning for Healthcare Conference (pp. 80-96). PMLR. Fig. 1. A. The reconstruction error evolution throughout the whole recording period of rat #1B. The distribution of the reconstruction errors from the BL and the EPG phases from the recordings of rat #1. We also show some typical EPG segments whose reconstruction errors are below the 10th percentile and over the 90th percentile, highlighted with a solid and a dashed black box, respectively. Other stimulated rats show a similar trend.

Optogenetic stimulation of entorhinal cortex reveals the implication of insulin signaling in adult rat’s hippocampal neurogenesis

Adult neurogenesis in the hippocampal dentate gyrus plays a critical role in learning and memory. Projections originating from entorhinal cortex, known as the perforant pathway, provide the main input to the dentate gyrus and promote neurogenesis. However, neuromodulators and molecular changes mediating neurogenic effects of this pathway are not yet fully understood. Here, by means of an optogenetic approach, we investigated neurogenesis and synaptic plasticity in the hippocampus of adult rats induced by stimulation of the perforant pathway. The lentiviruses carrying hChR2 (H134R)-mCherry gene under the control of the CaMKII promoter were injected into the medial entorhinal cortex region of adult rats. After 21 days, the entorhinal cortex region was exposed to the blue laser (473 nm) for five consecutive days (30 min/day). The expression of synaptic plasticity and neurogenesis markers in the hippocampus were evaluated using molecular and histological approaches. In parallel, the changes in the gene expression of insulin and its signaling pathway, trophic factors, and components of mitochondrial biogenesis were assessed. Our results showed that optogenetic stimulation of the entorhinal cortex promotes hippocampal neurogenesis and synaptic plasticity concomitant with the increased levels of insulin mRNA and its signaling markers, neurotrophic factors, and activation of mitochondrial biogenesis. These findings suggest that effects of perforant pathway stimulation on the hippocampus, at least in part, are mediated by insulin increase in the dentate gyrus and subsequently activation of its downstream signaling pathway.

Enrichment of Circular RNA Expression Deregulation at the Transition to Recurrent Spontaneous Seizures in Experimental Temporal Lobe Epilepsy.

Mesial temporal lobe epilepsy (mTLE) is a common form of epilepsy and is characterized by recurrent spontaneous seizures originating from the temporal lobe. The majority of mTLE patients develop pharmacoresistance to available anti-epileptic drugs (AEDs) while exhibiting severe pathological changes that can include hippocampal atrophy, neuronal death, gliosis and chronic seizures. The molecular mechanisms leading to mTLE remain incompletely understood, but are known to include defects in post-transcriptional gene expression regulation, including in non-coding RNAs (ncRNAs). Circular RNAs (circRNAs) are a class of recently rediscovered ncRNAs with high levels of expression in the brain and proposed roles in diverse neuronal processes. To explore a potential role for circRNAs in epilepsy, RNA-sequencing (RNA-seq) was performed on hippocampal tissue from a rat perforant pathway stimulation (PPS) model of TLE at different post-stimulation time points. This analysis revealed 218 differentially expressed (DE) circRNAs. Remarkably, the majority of these circRNAs were changed at the time of the occurrence of the first spontaneous seizure (DOFS). The expression pattern of two circRNAs, circ_Arhgap4 and circ_Nav3, was further validated and linked to miR-6328 and miR-10b-3p target regulation, respectively. This is the first study to examine the regulation of circRNAs during the development of epilepsy. It reveals an intriguing link between circRNA deregulation and the transition of brain networks into the state of spontaneous seizure activity. Together, our results provide a molecular framework for further understanding the role and mechanism-of-action of circRNAs in TLE.

Persistent changes in extracellular lactate dynamics following synaptic potentiation

A diverse array of neurometabolic coupling mechanisms exist within the brain to ensure that sufficient metabolite availability is present to meet both acute and chronic energetic demands.Excitatory synaptic activity, which produces the majority of the brain’s energetic demands, triggers a rapid metabolic response including a characteristic shift towards aerobic glycolysis. Herein, astrocytically derived lactate appears to serve as an important metabolite to meet the extensive metabolic needs of activated neurons. Despite a wealth of literature characterizing lactate’s role in mediating these acute metabolic needs, the extent to which lactate supports chronic energetic demands of neurons remains unclear. We hypothesized that synaptic potentiation, a ubiquitous brain phenomenon that can produce chronic alterations in synaptic activity, could necessitate persistent alterations in brain energetics. In freely-behaving rats, we induced long-term potentiation (LTP) of synapses within the dentate gyrus through high-frequency electrical stimulation (HFS) of the medial perforant pathway. Before, during, and after LTP induction, we continuously recorded extracellular lactate concentrations within the dentate gyrus to assess how changes in synaptic strength alter local glycolytic activity. Synaptic potentiation 1) altered the acute response of extracellular lactate to transient neuronal activation as evident by a larger initial dip and subsequent overshoot and 2) chronically increased local lactate availability. Although synapses were potentiated immediately following HFS, observed changes in lactate dynamics were only evident beginning ~24 h later. Once observed, however, both synaptic potentiation and altered lactate dynamics persisted for the duration of the experiment (~72 h). Persistent alterations in synaptic strength, therefore, appear to be associated with metabolic plasticity in the form of persistent augmentation of glycolytic activity.

Dynamic oxygen changes during status epilepticus and subsequent endogenous kindling.

ObjectiveBrain tissue oxygen (partial oxygen pressure [pO2]) levels are tightly regulated to stay within the normoxic zone, with deviations on either side resulting in impaired brain function. Whereas pathological events such as ischemic attacks and brief seizures have previously been shown to result in pO2 levels well below the normoxic zone, oxygen levels during prolonged status epilepticus (SE) and the subsequent endogenous kindling period are unknown.MethodsWe utilized two models of acquired temporal lobe epilepsy in rats: intrahippocampal kainic acid infusion and prolonged perforant pathway stimulation. Local tissue oxygen was measured in the dorsal hippocampus using an optode during and for several weeks following SE.ResultsWe observed hyperoxia in the hippocampus during induced SE in both models. Following termination of SE, 88% of rats initiated focal self‐generated spiking activity in the hippocampus within the first 7 days, which was associated with dynamic oxygen changes. Self‐generated and recurring epileptiform activity subsequently organized into higher‐frequency bursts that became progressively longer and were ultimately associated with behavioral seizures that became more severe with time and led to postictal hypoxia.SignificanceInduced SE and self‐generated recurrent epileptiform activity can have profound and opposing effects on brain tissue oxygenation that may serve as a biomarker for ongoing pathological activity in the brain.

The role of ongoing neuronal activity for baseline and stimulus-induced BOLD signals in the rat hippocampus

To understand how ongoing neuronal activity affects baseline BOLD signals, neuronal and resultant fMRI responses were simultaneously recorded in the right hippocampus of male rats during continuous low-frequency (2 or 4 Hz) pulse stimulation of the right perforant pathway. Despite continuously increased neuronal activity, BOLD signals only transiently increased in the hippocampus and subsequently returned to either the initial level (2 Hz) or even to a consistently lower level (4 Hz). Whereas the initially transient increase in BOLD signals coincided with an increased spiking of granule cells, the subsequent reduction of BOLD signals was independent of granule cell spiking activity but coincided with persistent inhibition of granule cell excitability, i.e., with reduced postsynaptic activity and prolonged population spike latency. The decline in BOLD signals occurred in the presence of an elevated local cerebral blood volume (CBV), thus the reduction of granule cell excitability is attended by high oxygen consumption. When previous or current stimulations lessen baseline BOLD signals, subsequent short stimulation periods only elicited attenuated BOLD responses, even when actual spiking activity of granule cells was similar. Thus, the quality of stimulus-induced BOLD responses critically depends on the current existing inhibitory activity, which closely relates to baseline BOLD signals. Thus, a meaningful interpretation of stimulus-induced BOLD responses should consider slowly developing variations in baseline BOLD signals; therefore, baseline correction tools should be cautiously used for fMRI data analysis.

Depotentiation of Long-Term Potentiation Is Associated with Epitope-Specific Tau Hyper-/Hypophosphorylation in the Hippocampus of Adult Rats.

It is well-known that some kinases which are involved in the induction of synaptic plasticity probably modulate tau phosphorylation. However, how depression of potentiated synaptic strength contributes to tau phosphorylation is unclear because of the lack of experiments in which depotentiation of LTP was induced. Field excitatory postsynaptic potential (fEPSP) and population spike (PS) were recorded from the dentate gyrus in response to the perforant pathway stimulation. To induce LTP, high-frequency stimulation (HFS) was used, while, for depotentiation of LTP, low-frequency stimulation (LFS) consisting of 900 pulses at 1Hz was applied 5min after tetanization. In some experiments, a neutral protocol at 0.033Hz was applied throughout the experiment without any induction of synaptic plasticity. One-hertz depotentiation protocol was able to decrease fEPSP slope which was previously increased by HFS, whereas no significant change in fEPSP slope and PS amplitude was observed in neutral protocol experiments. Relative to saline infusion, LTP was lower in magnitude and was more reversed by subsequent LFS in the presence of ERK1/2 inhibitor. Western blot experiments indicated that tau protein was hyperphosphorylated at ser416 epitope but rather hypophosphorylated at thr231 epitope in the whole hippocampus upon depotentiation of LTP. These changes concomitantly occurred with a notable increase in the levels of total tau and in the levels of phosphorylated form of the extracellular signal-regulated protein kinases 1 and 2 (ERK1/2). ERK1/2 inhibition resulted in a decrease in phosphorylation of tau at p416Tau when ERK1/2 was inhibited. These findings indicate that some forms of long-term plastic changes might be related with epitope-specific tau phosphorylation and ERK1/2 activation in the hippocampus. Therefore, we emphasize that tau may be crucial for physiological learning as well as Alzheimer's disease pathology.

Early minor stimulation of microglial TLR2 and TLR4 receptors attenuates Alzheimer's disease-related cognitive deficit in rats: behavioral, molecular, and electrophysiological evidence.

At early stages of Alzheimer's disease (AD), soluble amyloid beta (Aβ) accumulates in brain while microglia are in resting state. Microglia can recognize Aβ long after formation of plaques and release neurotoxic mediators. We examined impact of early minor activation of microglia by Toll-like receptors (TLRs) 2 and 4 agonists on Alzheimer's disease-related disturbed synaptic function and spatial memory in rats. Microglial BV-2 cells were treated by 0.1, 1, and 10μg/mL of the TLRs ligands lipopolysaccharide, monophosphoryl lipid A (MPL), and Pam3Cys for 24 hours. Culture medium was then changed with media containing 1-μM Aβ. Tumour necrosis factor (TNF)-α and CCL3 levels were measured in the supernatant, 24 hours thereafter. One μg of TLRs ligands which was able to release low level of TNF-α and CCL3, was administered intracerebroventricularly (i.c.v) to adult male rats every 3days for 24days. At the half of the treatment period, Aβ1-42 was infused i.c.v (0.075μg/hour) for 2 weeks. Finally, the following factors were measured: memory performance by Morris water maze, postsynaptic potentials of dentate gyrus following perforant pathway stimulation, hippocampal inflammatory cytokines interleukin 1 (IL-1)βand TNF-α, anti-inflammatory cytokines IL-10 and TGF-1β, microglia marker arginase 1, Aβ deposits, and the receptor involved in Aβ clearance, formyl peptide receptor 2 (FPR2). TLRs ligands caused dose-dependent release of TNF-α and CCL3 by BV-2 cells. Aβ-treated cells did not release TNF-α and CCL3, whereas those pretreated with MPL and Pam3Cys significantly released these cytokines in response to Aβ. Low-dose TLRs ligands improved the disturbance in spatial and working memory; restored the impaired long-term potentiation induced by Aβ; decreased TNF-α, and Aβ deposits; enhanced TGF-1β, IL-10, and arginase 1 in the hippocampus of Aβ-treated rats; and increased polarization of hippocampal microglia to the anti-inflammatory phenotype. The ligands increased formyl peptide receptor 2 in both BV-2 cells and hippocampus/cortex of Aβ-treated rats. Microglia can sense/clear soluble Aβ by early low-dose MPL and Pam3Cys and safeguard synaptic function and memory in rats.

Late effect of dopamine D1/5 receptor activation on stimulus-induced BOLD responses in the hippocampus and its target regions depends on the history of previous stimulations

fMRI was used to study late effects of dopamine D1/5 receptor activation on hippocampal signal processing and signal propagation to several target regions. The dopamine D1/5 receptor agonists SKF83959 and SKF38393 were intraperitoneally applied without, immediately before or 7 days after electrical stimulation of the right perforant pathway with bursts of high-frequency pulses. Control animals received a 0.9% NaCl solution. One day after D1/5 receptor activation, the perforant pathway was stimulated and the induced BOLD responses in the right hippocampus and its target regions, left hippocampus (l-HC) and medial prefrontal cortex (mPFC), were measured. Depending on the temporal relation between dopamine receptor activation and the first perforant pathway stimulation the induced BOLD response pattern differed. When applied without concurrent perforant pathway stimulation, the agonists caused region-selective increases in the induced BOLD responses: the effect of SKF83959 was evident in the mPFC whereas that of SKF38393 was confined to the l-HC. When applied in conjunction with perforant pathway stimulation, either agonist caused increased BOLD responses in both regions. In contrast, when applied 7 days after perforant pathway stimulation, neither SKF83959 nor SKF38393 modified the BOLD responses in the mPFC or l-HC 1day later. These findings suggest that (i) activation of dopamine D1/5 receptors alone is sufficient to modify stimulus-induced BOLD responses in target regions of the right hippocampus 24h later, and (ii), the history of previous stimulations crucially affects the impact of dopamine receptor activation on stimulus-induced BOLD responses.

Potent Anti-seizure Effects of Locked Nucleic Acid Antagomirs Targeting miR-134 in Multiple Mouse and Rat Models of Epilepsy

Current anti-epileptic drugs (AEDs) act on a limited set of neuronal targets, are ineffective in a third of patients with epilepsy, and do not show disease-modifying properties. MicroRNAs are small noncoding RNAs that regulate levels of proteins by post-transcriptional control of mRNA stability and translation. MicroRNA-134 is involved in controlling neuronal microstructure and brain excitability and previous studies showed that intracerebroventricular injections of locked nucleic acid (LNA), cholesterol-tagged antagomirs targeting microRNA-134 (Ant-134) reduced evoked and spontaneous seizures in mouse models of status epilepticus. Translation of these findings would benefit from evidence of efficacy in non-status epilepticus models and validation in another species. Here, we report that electrographic seizures and convulsive behavior are strongly reduced in adult mice pre-treated with Ant-134 in the pentylenetetrazol model. Pre-treatment with Ant-134 did not affect the severity of status epilepticus induced by perforant pathway stimulation in adult rats, a toxin-free model of acquired epilepsy. Nevertheless, Ant-134 post-treatment reduced the number of rats developing spontaneous seizures by 86% in the perforant pathway stimulation model and Ant-134 delayed epileptiform activity in a rat ex vivo hippocampal slice model. The potent anticonvulsant effects of Ant-134 in multiple models may encourage pre-clinical development of this approach to epilepsy therapy.

The role of the mesolimbic dopamine system in the formation of blood-oxygen-level dependent responses in the medial prefrontal/anterior cingulate cortex during high-frequency stimulation of the rat perforant pathway

Several human functional magnetic resonance imaging studies point to an activation of the mesolimbic dopamine system during reward, addiction and learning. We previously found activation of the mesolimbic system in response to continuous but not to discontinuous perforant pathway stimulation in an experimental model that we now used to investigate the role of dopamine release for the formation of functional magnetic resonance imaging responses. The two stimulation protocols elicited blood-oxygen-level dependent responses in the medial prefrontal/anterior cingulate cortex and nucleus accumbens. Inhibition of dopamine D1/5 receptors abolished the formation of functional magnetic resonance imaging responses in the medial prefrontal/anterior cingulate cortex during continuous but not during discontinuous pulse stimulations, i.e. only when the mesolimbic system was activated. Direct electrical or optogenetic stimulation of the ventral tegmental area caused strong dopamine release but only electrical stimulation triggered significant blood-oxygen level-dependent responses in the medial prefrontal/anterior cingulate cortex and nucleus accumbens. These functional magnetic resonance imaging responses were not affected by the D1/5 receptor antagonist SCH23390 but reduced by the N-methyl-D-aspartate receptor antagonist MK801. Therefore, glutamatergic ventral tegmental area neurons are already sufficient to trigger blood-oxygen-level dependent responses in the medial prefrontal/anterior cingulate cortex and nucleus accumbens. Although dopamine release alone does not affect blood-oxygen-level dependent responses it can act as a switch, permitting the formation of blood-oxygen-level dependent responses.

Removing entorhinal cortex input to the dentate gyrus does not impede low frequency oscillations, an EEG-biomarker of hippocampal epileptogenesis.

Following prolonged perforant pathway stimulation (PPS) in rats, a seizure-free “latent period” is observed that lasts around 3 weeks. During this time, aberrant neuronal activity occurs, which has been hypothesized to contribute to the generation of an “epileptic” network. This study was designed to 1) examine the pathological network activity that occurs in the dentate gyrus during the latent period, and 2) determine whether suppressing this activity by removing the main input to the dentate gyrus could stop or prolong epileptogenesis. Immediately following PPS, continuous video-EEG monitoring was used to record spontaneous neuronal activity and detect seizures. During the latent period, low frequency oscillations (LFOs), occurring at a rate of approximately 1 Hz, were detected in the dentate gyrus of all rats that developed epilepsy. LFO incidence was apparently random, but often decreased in the hour preceding a spontaneous seizure. Bilateral transection of the perforant pathway did not impact the incidence of hippocampal LFOs, the latency to epilepsy, or hippocampal neuropathology. Our main findings are: 1) LFOs are a reliable biomarker of hippocampal epileptogenesis, and 2) removing entorhinal cortex input to the hippocampus neither reduces the occurrence of LFOs nor has a demonstrable antiepileptogenic effect.