Acute application of Centella asiatica extract enhanced AMPAR-mediated postsynaptic currents in rat entorhinal cortex.

Centella asiatica is notable for its wide range of biological activities beneficial to human health, particularly its cognitive enhancement and neuroprotective effects. The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors are ionotropic glutamate receptors mediating fast excitatory neurotransmission essential in long-term potentiation widely thought to be the cellular mechanism of learning and memory. The method of whole-cell patch-clamp was used to study the effect of the acute application of Centella asiatica extract on the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-mediated spontaneous excitatory postsynaptic currents in the entorhinal cortex of rat brain slices. The respective low dose of test compounds significantly increased the amplitude of spontaneous excitatory postsynaptic currents while having no significant effects on the frequency. The findings suggested that Centella asiatica extract increased the response of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors at the postsynaptic level, revealing the potential role of Centella asiatica in modulating the glutamatergic responses in the entorhinal cortex of rat brain slices to produce cognitive enhancement effects.

The effects of developmental alcohol exposure on the neurobiology of spatial processing.

The consumption of alcohol during gestation is detrimental to the developing central nervous system. One functional outcome of this exposure is impaired spatial processing, defined as sensing and integrating information pertaining to spatial navigation and spatial memory. The hippocampus, entorhinal cortex, and anterior thalamus are brain regions implicated in spatial processing and are highly susceptible to the effects of developmental alcohol exposure. Some of the observed effects of alcohol on spatial processing may be attributed to changes at the synaptic to circuit level. In this review, we first describe the impact of developmental alcohol exposure on spatial behavior followed by a summary of the development of brain areas involved in spatial processing. We then provide an examination of the consequences of prenatal and early postnatal alcohol exposure in rodents on hippocampal, anterior thalamus, and entorhinal cortex-dependent spatial processing from the cellular to behavioral level. We conclude by highlighting several unanswered questions which may provide a framework for future investigation.

Differential expression of entorhinal cortex and hippocampal subfields α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors enhanced learning and memory of rats following administration of Centella asiatica.

Centella asiatica (CA) is a widely used traditional herb, notably for its cognitive enhancing effect and potential to increase synaptogenesis. The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) and N-methyl-D-aspartate receptors (NMDARs) mediate fast excitatory neurotransmission with key roles in long-term potentiation which is believed to be the cellular mechanism of learning and memory. Improved learning and memory can be an indication to the surface expression level of these receptors. Our previous study demonstrated that administration of CA extract improved learning and memory and enhanced expression of AMPAR GluA1 subunit while exerting no significant effects on GABAA receptors of the hippocampus in rats. Hence, to further elucidate the effects of CA, this study investigated the effects of CA extract in recognition memory and spatial memory, and its effects on AMPAR GluA1 and GluA2 subunit and NMDAR GluN2 A and GluN2B subunit expression in the entorhinal cortex (EC) and hippocampal subfields CA1 and CA3. The animals were administered with saline, 100 mg/kg, 300 mg/kg, and 600 mg/kg of CA extract through oral gavage for 14 days, followed by behavioural analysis through Open Field Test (OFT), Novel Object Recognition Task (NORT), and Morris Water Maze (MWM) and lastly morphological and immunohistochemical analysis of the surface expression of AMPAR and NMDAR subunits were performed. The results showed that 14 days of administration of 600 mg/kg of CA extract significantly improved memory assessed through NORT while 300 mg/kg of CA extract significantly improved memory of the animals assessed through MWM. Immunohistochemical analysis revealed differential modulation effects on the expressions of receptor subunits across CA1, CA3 and EC. The CA extract at the highest dose (600 mg/kg) significantly enhanced the expression of AMPAR subunit GluA1 and GluA2 in CA1, CA3 and EC, and NMDAR subunit GluN2B in CA1 and CA3 compared to control. At 300 mg/kg, CA significantly increased expression of AMPAR GluA1 in CA1 and EC, and GluA2 in CA1, CA3 and EC while 100 mg/kg of CA significantly increased expression of only AMPAR subunit GluA2 in CA3 and EC. Expression of NMDAR subunit GluN2 A was significantly reduced in the CA3 (at 100, 300, and 600 mg/kg) while no significant changes of subunit expression was observed in CA1 and EC compared to control. The results suggest that the enhanced learning and memory observed in animals administered with CA was mainly mediated through increased expression of AMPAR GluA1 and GluA2 subunits and differential expression of NMDAR GluN2 A and GluN2B subunits in the hippocampal subfields and EC. With these findings, the study revealed a new aspect of cognitive enhancing effect of CA and its therapeutic potentials through modulating receptor subunit expression.

Sex-dependent co-occurrence of hypoxia and ß-amyloid plaques in hippocampus and entorhinal cortex is reversed by long-term treatment with ubiquinol and ascorbic acid in the 3 × Tg-AD mouse model of Alzheimer's disease.

Structural and functional abnormalities in the cerebral microvasculature have been observed in Alzheimer's disease (AD) patients and animal models. One cause of hypoperfusion is the thickening of the cerebrovascular basement membrane (CVBM) due to increased collagen-IV deposition around capillaries. This study investigated whether these and other alterations in the cerebrovascular system associated with AD can be prevented by long-term dietary supplementation with the antioxidant ubiquinol (Ub) stabilized with Kaneka QH P30 powder containing ascorbic acid (ASC) in a mouse model of advanced AD (3 × Tg-AD mice, 12 months old). Animals were treated from prodromal stages of disease (3 months of age) with standard chow without or with Ub + ASC or ASC-containing vehicle and compared to wild-type (WT) mice. The number of ß-amyloid (Aß) plaques in the hippocampus and entorhinal cortex was higher in female than in male 3 × Tg-AD mice. Extensive regions of hypoxia were characterized by a higher plaque burden in females only. This was abolished by Ub + ASC and, to a lesser extent, by ASC treatment. Irrespective of Aß burden, increased collagen-IV deposition in the CVBM was observed in both male and female 3 × Tg-AD mice relative to WT animals; this was also abrogated in Ub + ASC- and ASC-treated mice. The chronic inflammation in the hippocampus and oxidative stress in peripheral leukocytes of 3 × Tg-AD mice were likewise reversed by antioxidant treatment. These results provide strong evidence that long-term antioxidant treatment can mitigate plasma oxidative stress, amyloid burden, and hypoxia in the AD brain parenchyma.

Medial entorhinal cortex and medial septum contribute to self-motion-based linear distance estimation.

Path integration is a navigation strategy that requires animals to integrate self-movements during exploration to determine their position in space. The medial entorhinal cortex (MEC) has been suggested to play a pivotal role in this process. Grid cells, head-direction cells, border cells as well as speed cells within the MEC collectively provide a dynamic representation of the animal position in space based on the integration of self-movements. All these cells are strongly modulated by theta oscillations, thus suggesting that theta rhythmicity in the MEC may be essential for integrating and coordinating self-movement information during navigation. In this study, we first show that excitotoxic MEC lesions, but not dorsal hippocampal lesions, impair the ability of rats to estimate linear distances based on self-movement information. Next, we report similar deficits following medial septum inactivation, which strongly impairs theta oscillations in the entorhinal-hippocampal circuits. Taken together, these findings demonstrate a major role of the MEC and MS in estimating distances to be traveled, and point to theta oscillations within the MEC as a neural mechanism responsible for the integration of information generated by linear self-displacements.

Anticonvulsant effect of Rhynchophylline involved in the inhibition of persistent sodium current and NMDA receptor current in the pilocarpine rat model of temporal lobe epilepsy.

Rhynchophylline (RIN) is a significant active component isolated from the Chinese herbal medicine Uncaria rhynchophylla. Several studies have demonstrated that RIN has a significant anticonvulsant effect in many types of epilepsy models in vivo. However, the mechanisms of the anticonvulsant effect remain elusive. Using combined methods of behavioral testing, immunofluorescence and electrophysiological recordings, we characterized the anticonvulsant effect of RIN in a pilocarpine-induced status epilepticus (SE) rat model of temporal lobe epilepsy (TLE) and investigated the underlying cellular mechanisms. In one set of experiments, rats received RIN treatment prior to pilocarpine injection. In a second set of experiments, rats received RIN treatment following the onset of stage 3 seizures. Pretreatment and posttreatment with RIN effectively reduced the seizure severity in the acute phase of TLE. Furthermore, RIN protected medial entorhinal cortex (mEC) layer III neurons from neuronal death and terminated spontaneous epileptiform discharge of mEC layer II neurons in SE-experienced rats. Whole-cell voltage-clamp recordings indicated that RIN inhibited neuronal hyperexcitability via inhibition of the persistent sodium current (INaP) and NMDA receptor current. Immunofluorescence experiments also demonstrated that RIN rectified the pilocarpine-induced upregulation of Nav1.6 and NR2B protein expression. In conclusion, our results identified RIN as an anticonvulsant agent that inhibited ictal discharge via INap and NMDA receptor current inhibition.

Bioenergetic Impairment in Animal and Cellular Models of Alzheimer's Disease: PARP-1 Inhibition Rescues Metabolic Dysfunctions.

Amyloid-beta peptide accumulation in the brain is one of the main hallmarks of Alzheimer's disease. The amyloid aggregation process is associated with the generation of free radical species responsible for mitochondrial impairment and DNA damage that in turn activates poly(ADP-ribose)polymerase 1 (PARP-1). PARP-1 catalyzes the poly(ADP-ribosylation), a post-translational modification of proteins, cleaving the substrate NAD+ and transferring the ADP-ribose moieties to the enzyme itself or to an acceptor protein to form branched polymers of ADP-ribose. In this paper, we demonstrate that a mitochondrial dysfunction occurs in Alzheimer's transgenic mice TgCRND8, in SH-SY5Y treated with amyloid-beta and in 7PA2 cells. Moreover, PARP-1 activation contributes to the functional energetic decline affecting cytochrome oxidase IV protein levels, oxygen consumption rates, and membrane potential, resulting in cellular bioenergetic deficit. We also observed, for the first time, an increase of pyruvate kinase 2 expression, suggesting a modulation of the glycolytic pathway by PARP-1. PARP-1 inhibitors are able to restore both mitochondrial impairment and pyruvate kinase 2 expression. The overall data here presented indicate a pivotal role for this enzyme in the bioenergetic network of neuronal cells and open new perspectives for investigating molecular mechanisms underlying energy charge decline in Alzheimer's disease. In this scenario, PARP-1 inhibitors might represent a novel therapeutic intervention to rescue cellular energetic metabolism.

Neuroinflammation and neurodegeneration in adult rat brain from binge ethanol exposure: abrogation by docosahexaenoic acid.

Evidence that brain edema and aquaporin-4 (AQP4) water channels have roles in experimental binge ethanol-induced neurodegeneration has stimulated interest in swelling/edema-linked neuroinflammatory pathways leading to oxidative stress. We report here that neurotoxic binge ethanol exposure produces comparable significant effects in vivo and in vitro on adult rat brain levels of AQP4 as well as neuroinflammation-linked enzymes: key phospholipase A2 (PLA2) family members and poly (ADP-ribose) polymerase-1 (PARP-1). In adult male rats, repetitive ethanol intoxication (3 gavages/d for 4 d, ∼ 9 g/kg/d, achieving blood ethanol levels ∼ 375 mg/dl; "Majchrowicz" model) significantly increased AQP4, Ca+2-dependent PLA2 GIVA (cPLA2), phospho-cPLA2 GIVA (p-cPLA2), secretory PLA2 GIIA (sPLA2) and PARP-1 in regions incurring extensive neurodegeneration in this model--hippocampus, entorhinal cortex, and olfactory bulb--but not in two regions typically lacking neurodamage, frontal cortex and cerebellum. Also, ethanol reduced hippocampal Ca+2-independent PLA2 GVIA (iPLA2) levels and increased brain "oxidative stress footprints" (4-hydroxynonenal-adducted proteins). For in vitro studies, organotypic cultures of rat hippocampal-entorhinocortical slices of adult age (∼ 60 d) were ethanol-binged (100 mM or ∼ 450 mg/dl) for 4 d, which augments AQP4 and causes neurodegeneration (Collins et al. 2013). Reproducing the in vivo results, cPLA2, p-cPLA2, sPLA2 and PARP-1 were significantly elevated while iPLA2 was decreased. Furthermore, supplementation with docosahexaenoic acid (DHA; 22:6n-3), known to quell AQP4 and neurodegeneration in ethanol-treated slices, blocked PARP-1 and PLA2 changes while counteracting endogenous DHA reduction and increases in oxidative stress footprints (3-nitrotyrosinated proteins). Notably, the PARP-1 inhibitor PJ-34 suppressed binge ethanol-dependent neurodegeneration, indicating PARP upstream involvement. The results with corresponding models support involvement of AQP4- and PLA2-associated neuroinflammatory pro-oxidative pathways in the neurodamage, with potential regulation by PARP-1 as well. Furthermore, DHA emerges as an effective inhibitor of these binge ethanol-dependent neuroinflammatory pathways as well as associated neurodegeneration in adult-age brain.

Differential regulation of ABCA1 and ABCG1 gene expressions in the remodeling mouse hippocampus after entorhinal cortex lesion and liver-X receptor agonist treatment.

Entorhinal cortex lesioning (ECL) causes an extensive deafferentation of the hippocampus that is classically followed by a compensatory reinnervation, where apolipoprotein E, the main extracellular lipid-carrier in the CNS, has been shown to play a crucial role by shuttling cholesterol to reconstructing neurons terminals. Hence, we investigated whether the ATP-binding cassette (ABC) transporters -A1 and -G1, known to regulate cellular cholesterol efflux and lipidation of the apolipoprotein E-containing lipoprotein complex are actively involved in this context of brain׳s plastic response to neurodegeneration and deafferentation. We assessed ABCA1 and ABCG1 mRNA and protein levels throughout the degenerative phase and the reinnervation process and evaluated the associated cholinergic sprouting following ECL in the adult mouse brain. We subsequently tested the effect of the pharmacological activation of the nuclear receptor LXR, prior to versus after ECL, on hippocampal ABCA1 and G1 expression and on reinnervation. ECL induced a time-dependent up-regulation of ABCA1, but not G1, that coincided with a significant increase in acetylcholine esterase (AChE) activity in the ipsilateral hippocampus. Pre-ECL, but not post-ECL i.p. treatment with the LXR agonist TO901317 also led to a significant increase solely in hippocampal ABCA1 expression, paralleled by increases in both AchE and synaptophysin protein levels in the deafferented hippocampus. Thus, ABCA1 and -G1 are differentially regulated in the lesioned brain and upon treatment with an LXR agonist. Further, TO901317-induced up-regulation of ABCA1 appears to be more beneficial in a prevention (pre-lesion) than rescue (post-lesion) treatment; both findings support a central role for ABC transporters in brain plasticity.

Dietary intake of unsaturated fatty acids modulates physiological properties of entorhinal cortex neurons in mice.

Dietary lipids modify brain fatty acid profile, but evidence of their direct effect on neuronal function is sparse. The enthorinal cortex (EC) neurons connecting to the hippocampus play a critical role in learning and memory. Here, we have exposed mice to diets based on canola:soybean oils (40 : 10, g/kg) or safflower : corn oils (25 : 25, g/kg) to investigate the relationship between the lipid profile of brain fatty acids and the intrinsic properties of EC neurons. Consumption of canola : soybean oil-enriched diet led to the increase of the monounsaturated fatty acid oleic acid and to a decrease of arachidonic acid in ethanolamine glycerophospholipids of the white matter. We also found an important rise in docosahexaenoic acid (DHA) within ethanolamine glycerophospholipids and phosphatidylserine of gray matter. The canola:soybean oil treatment led to a shorter duration of action potential (-21%), a reduction in the duration of postsynaptic response (-21%) and increased firing activity (+43%). Data from additional experiments with animals fed DHA alone or DHA with canola oil suggested that dietary monounsaturated fatty acid may have contributed to these effects on EC neuron physiology. Since neuronal function within the enthorhinal-hippocampal loop is critical to learning and memory processes, the present data may provide a functional basis for the beneficial cognitive effects of canola oil-based diets.

DHA improves cognition and prevents dysfunction of entorhinal cortex neurons in 3xTg-AD mice.

Defects in neuronal activity of the entorhinal cortex (EC) are suspected to underlie the symptoms of Alzheimer's disease (AD). Whereas neuroprotective effects of docosahexaenoic acid (DHA) have been described, the effects of DHA on the physiology of EC neurons remain unexplored in animal models of AD. Here, we show that DHA consumption improved object recognition (↑12%), preventing deficits observed in old 3xTg-AD mice (↓12%). Moreover, 3xTg-AD mice displayed seizure-like akinetic episodes, not detected in NonTg littermates and partly prevented by DHA (↓50%). Patch-clamp recording revealed that 3xTg-AD EC neurons displayed (i) loss of cell capacitance (CC), suggesting reduced membrane surface area; (ii) increase of firing rate versus injected current (F-I) curve associated with modified action potentials, and (iii) overactivation of glutamatergic synapses, without changes in synaptophysin levels. DHA consumption increased CC (↑12%) and decreased F-I slopes (↓21%), thereby preventing the opposite alterations observed in 3xTg-AD mice. Our results indicate that cognitive performance and basic physiology of EC neurons depend on DHA intake in a mouse model of AD.

A cholinergic-dependent role for the entorhinal cortex in trace fear conditioning.

Trace conditioning is considered a model of higher cognitive involvement in simple associative tasks. Studies of trace conditioning have shown that cortical areas and the hippocampal formation are required to associate events that occur at different times. However, the mechanisms that bridge the trace interval during the acquisition of trace conditioning remain unknown. In four experiments with fear conditioning in rats, we explored the involvement of the entorhinal cortex (EC) in the acquisition of fear under a trace-30 s protocol. We first determined that pretraining neurotoxic lesions of the EC selectively impaired trace-, but not delay-conditioned fear as evaluated by freezing behavior. A local cholinergic deafferentation of the EC using 192-IgG-saporin did not replicate this deficit, presumably because cholinergic interneurons were spared by the toxin. However, pretraining local blockade of EC muscarinic receptors with the M1 antagonist pirenzepine yielded a specific and dose-dependent deficit in trace-conditioned responses. The same microinjections performed after conditioning were without effect on trace fear responses. These effects of blocking M1 receptors are consistent with the notion that conditioned stimulus (CS)-elicited, acetylcholine-dependent persistent activities in the EC are needed to maintain a representation of a tone CS across the trace interval during the acquisition of trace conditioning. This function of the EC is consistent with recent views of this region as a short-term stimulus buffer.

Binge ethanol-induced neurodegeneration in rat organotypic brain slice cultures: effects of PLA2 inhibitor mepacrine and docosahexaenoic acid (DHA).

Using rat organotypic hippocampal-entorhinal cortical (HEC) slice cultures, we examined whether phospholipase A2 (PLA2) activity is involved in binge alcohol (ethanol)-induced neurodegeneration, and whether docosahexaenoic acid (DHA; 22:6n-3), a fish oil-enriched fatty acid that is anti-inflammatory in brain damage models, is neuroprotective. Assessed with propidium iodide and lactate dehydrogenase (LDH) leakage, neurodamage from ethanol (6 days 100 mM ethanol with four withdrawal periods) was prevented by the PLA2 pan-inhibitor, mepacrine. Also, ethanol-dependent neurodegeneration-particularly in the entorhinal region-was significantly ameliorated by DHA supplementation (25 microM); however, adrenic acid, a 22:4n-6 analog, was ineffective. Consistent with PLA2 activation, [(3)H] liberation was approximately fivefold greater in [(3)H]arachidonic acid-preloaded HEC slice cultures during ethanol withdrawal compared to controls, and DHA supplementation suppressed [(3)H] release to control levels. DHA might antagonize PLA2 activity directly or suppress upstream activators (e.g., oxidative stress); however, other DHA mechanisms could be important in subdueing ethanol-induced PLA2-dependent and independent neuroinflammatory processes.

[The anticonvulsive effect of 4,4-bis(hydroxymethyl)-2-phenyl-2-oxazoline]. / Efecto anticonvulsionante de la 2-fenil-4,4-bis (hidroximetil)-2-oxazolina.

INTRODUCTION: Certain compounds belonging to the family of the 2-aryl oxazolines have been reported to act on the central nervous system with a number of different effects and applications, which make them useful as depressants, anaesthetics, anticonvulsants, and so on. AIMS: Our aim was to study the possible effect of 4,4-bis(hydroxymethyl)-2-phenyl-2-oxazoline (OX), obtained by chemical synthesis using microwaves, in two experimental models of epilepsy. MATERIALS AND METHODS: Two models were used: one involving (repeated stimulation) electroconvulsive shock in mice and the other consisted in inducing audiogenic seizures in Mongolian gerbils. Recordings were performed of the potentials in the dentate gyrus (DG) generated in response to electrical stimulation of the entorhinal cortex in anaesthetised gerbils, using the stereotactic technique. RESULTS: A 150 mg/kg dose of OX lowered the number of electrical pulses required to induce the tonic seizures triggered by the electroshock, as well as their duration. This same dose blocked the seizures induced by audiogenic stimuli in the gerbils and significantly reduced their severity (degrees of seizures) and occurrence. OX diminished, in a dose-dependent manner, the amplitude of the excitatory post-synaptic potential and that of the population spike, triggered by stimulating the entorhinal cortex in the DG. CONCLUSIONS: OX acts as an antiepileptic agent and its mechanism of action could be related to the inhibiting effect it exerts on the entorhinal cortex-DG synapses in the hippocampus.

Electrical and chemical long-term depression do not attenuate low-Mg2+-induced epileptiform activity in the entorhinal cortex.

PURPOSE: Low-frequency electrical and magnetic stimulation of cortical brain regions has been shown to reduce cortical excitability and to decrease the susceptibility to seizures in humans and in vivo models of epilepsy. The induction of long-term depression (LTD) or depotentiation of a seizure-related long-term potentiation has been proposed to be part of the underlying mechanism. With the low-Mg(2+)-model of epilepsy, this study investigated the effect of electrical LTD, chemical LTD, and depotentiation on the susceptibility of the entorhinal cortex to epileptiform activity. METHODS: The experiments were performed on isolated entorhinal cortex slices obtained from adult Wistar rats and mice. With extracellular recording techniques, we studied whether LTD induced by (a) three episodes of low-frequency paired-pulse stimulation (3 x 900 paired pulses at 1 Hz), and by (b) bath-applied N-methyl-D-aspartate (NMDA, 20 microM) changes time-to-onset, duration, and frequency of seizure-like events (SLEs) induced by omitting MgSO(4) from the artificial cerebrospinal fluid. Next we investigated the consequences of depotentiation on SLEs themselves by applying low-frequency stimulation after onset of low-Mg(2+)-induced epileptiform activity. RESULTS: LTD, induced either by low-frequency stimulation or by bath-applied NMDA, had no effect on time-to-onset, duration, and frequency of SLEs compared with unconditioned slices. Low-frequency stimulation after onset of SLEs did not suppress but induced SLEs that lasted for the time of stimulation and were associated with a simultaneous increase of the extracellular K(+) concentration. CONCLUSIONS: Our study demonstrates that neither conditioning LTD nor brief low-frequency stimulation decreases the susceptibility of the entorhinal cortex to low-Mg(2+)-induced epileptiform activity. The present study does not support the hypothesis that low-frequency brain stimulation exerts its anticonvulsant effect via the induction of LTD or depotentiation.

Reduced prepulse inhibition in rats with entorhinal cortex lesions.

The relationship between the entorhinal cortex and prepulse inhibition (PPI) as well as dopaminergic participation in this relationship were examined. PPI is an operational measure of sensorimotor gating in which a robust response to a startling auditory pulse stimulus is inhibited when the stimulus is preceded by a weak prepulse. PPI can be measured in various species and is reduced in several neuropsychiatric disorders and in dopamine-activated rats. The entorhinal cortex was damaged bilaterally using ibotenic acid, and acoustic startle experiments were performed during treatment with haloperidol or saline on day 21 after the ibotenic acid injection. Neither this injection nor haloperidol affected the amplitude of the startle movement. Bilateral entorhinal cortex lesions reduced PPI, while haloperidol partially restored it. The entorhinal cortex and the sensorimotor gating system therefore may be related via dopaminergic circuits, possibly including the nucleus accumbens. Further, as the entorhinal cortex provides the major extrinsic synaptic input to the rat hippocampus, disease involvement of this region may severely affect cognition in various disorders including schizophrenia.

A neuronal glutamate transporter contributes to neurotransmitter GABA synthesis and epilepsy.

The predominant neuronal glutamate transporter, EAAC1 (for excitatory amino acid carrier-1), is localized to the dendrites and somata of many neurons. Rare presynaptic localization is restricted to GABA terminals. Because glutamate is a precursor for GABA synthesis, we hypothesized that EAAC1 may play a role in regulating GABA synthesis and, thus, could cause epilepsy in rats when inactivated. Reduced expression of EAAC1 by antisense treatment led to behavioral abnormalities, including staring-freezing episodes and electrographic (EEG) seizures. Extracellular hippocampal and thalamocortical slice recordings showed excessive excitability in antisense-treated rats. Patch-clamp recordings of miniature IPSCs (mIPSCs) conducted in CA1 pyramidal neurons in slices from EAAC1 antisense-treated animals demonstrated a significant decrease in mIPSC amplitude, indicating decreased tonic inhibition. There was a 50% loss of hippocampal GABA levels associated with knockdown of EAAC1, and newly synthesized GABA from extracellular glutamate was significantly impaired by reduction of EAAC1 expression. EAAC1 may participate in normal GABA neurosynthesis and limbic hyperexcitability, whereas epilepsy can result from a disruption of the interaction between EAAC1 and GABA metabolism.

Low levels of estrogen significantly diminish axonal sprouting after entorhinal cortex lesions in the mouse.

This study tested the hypothesis that estrogen enhances axonal sprouting in the hippocampal formation in the female mouse. The entorhinal cortex was unilaterally lesioned with ibotenic acid in control mice and in ovariectomized mice that were treated with a high dose of, a moderate dose of, or zero estrogen supplementation pellets. Four weeks later the density of staining for synaptophysin immunoreactivity and acetylcholinesterase (AChE) histochemistry was measured in the molecular layer of the dentate gyrus. In control mice, lesions of the lateral part of the entorhinal cortex increased synaptophysin and acetylcholinesterase staining (i.e., indicative of axonal sprouting) in the outer one-third of the molecular layer of the dentate gyrus. Mice receiving high and moderate estrogen supplementation displayed the same sprouting response; however, in ovariectomized mice the sprouting response was significantly reduced (to nearly nothing). Thus, in ovariectomized compared with control mice the lesion-induced sprouting response is severely blunted, and this effect is reversed by estrogen supplementation. Together, these findings suggest that estrogen plays a prominent role in promoting neuronal plasticity and remodeling in the dentate gyrus.

Nucleus accumbens, entorhinal cortex and latent inhibition: a neural network model.

A neural network model of classical conditioning (Schmajuk, Lam, and Gray, J. Exp. Psychol.: Anim. Behav. Process, 22, 1996, 321-349) is applied to the description of the neural substrates of latent inhibition. Experimental data suggest that latent inhibition might be controlled by a circuit that involves the hippocampus, the entorhinal cortex, the nucleus accumbens, and the mesolimbic dopaminergic projection from the ventral tegmental area to the accumbens. By mapping different nodes and connections in the model onto this brain circuit, computer simulations demonstrate that, in most cases, the model provides a good quantitative description of: (1) the impairment of latent inhibition by lesions of the shell of the nucleus accumbens; (2) the restoration of latent inhibition by haloperidol following lesions of the shell; (3) the preservation of latent inhibition by lesions of the core of the nucleus accumbens; (4) the facilitation of latent inhibition by combined shell core lesions and by core lesions with extended conditioning; (5) the impairment of latent inhibition following lesions of the entorhinal cortex or the hippocampus; and (6) the restoration of latent inhibition by haloperidol following lesions of the entorhinal cortex and ventral subiculum. In addition, the model is able to describe neural activity in the nucleus accumbens.

Chronic intracerebroventricular exposure to beta-amyloid(1-40) impairs object recognition but does not affect spontaneous locomotor activity or sensorimotor gating in the rat.

This study examined the cognitive effects of chronic in vivo exposure to beta-amyloid(1-40) via the intracerebroventricular route on two distinct paradigms. The first test evaluated a form of early attentional control referred to as sensorimotor gating in which an antecedent weak prepulse stimulus modulates the reactivity to a subsequent startle-eliciting stimulus. The second test utilized the spontaneous preference for a novel object over that of a familiar one in rats as a measure of object recognition memory. We found that beta-amyloid exposure leads to a severe deficit in the object memory test but spares sensorimotor gating. Moreover, unlike the water maze deficit induced by beta-amyloid (Nag et al., in preparation), the deficit on object recognition was resistant to amelioration by systemic physostigmine treatment at a dose of 0.06 mg/kg per day intraperitoneally. The present results add to previous reports that beta-amyloid exposure can lead to deficits on hippocampal lesion sensitive tasks, suggesting that dysfunction of the rhinal cortices in addition to that of the septohippocampal system is implicated in beta-amyloid-induced behavioral impairments. It therefore lends support to the hypothesis that beta-amyloid exposure can lead to severe impairment across multiple memory systems.