Modelling cognitive dysfunctions with bilateral injections of ibotenic acid into the rat entorhinal cortex

A map of spatial navigation for neuroscience

Spatial navigation has received much attention from neuroscientists, leading to the identification of key brain areas and the discovery of numerous spatially selective cells. Despite this progress, our understanding of how the pieces fit together to drive behavior is generally lacking. We argue that this is partly caused by insufficient communication between behavioral and neuroscientific researchers. This has led the latter to under-appreciate the relevance and complexity of spatial behavior, and to focus too narrowly on characterizing neural representations of space-disconnected from the computations these representations are meant to enable. We therefore propose a taxonomy of navigation processes in mammals that can serve as a common framework for structuring and facilitating interdisciplinary research in the field. Using the taxonomy as a guide, we review behavioral and neural studies of spatial navigation. In doing so, we validate the taxonomy and showcase its usefulness in identifying potential issues with common experimental approaches, designing experiments that adequately target particular behaviors, correctly interpreting neural activity, and pointing to new avenues of research.

Etoricoxib inhibits peripheral inflammation and alters immune responses in intracerebroventricular colchicine injected rats

The present study was designed to investigate the effectiveness of etoricoxib induced inhibition of neuroinflammation by studying the peripheral inflammatory markers and select immune parameters in intracerebroventricular colchicine injected rats (ICIR). Results showed time dependent upregulation of the inflammatory markers in the serum along with alterations of peripheral immune parameters in ICIR and dose-dependent recovery was observed upon administration of etoricoxib to ICIR; most of these effects were greater with the longer duration of study. The present study indicates that colchicine induced neuroinflammation may cause systemic inflammation and alteration of immune responses which are mediated by increased cox- 2 activity.

Protection against alcohol-induced neuronal and cognitive damage by the PPARγ receptor agonist pioglitazone

Binge alcohol drinking has emerged as a typical phenomenon in young people. This pattern of drinking, repeatedly leading to extremely high blood and brain alcohol levels and intoxication is associated with severe risks of neurodegeneration and cognitive damage. Mechanisms involved in excitotoxicity and neuroinflammation are pivotal elements in alcohol-induced neurotoxicity. Evidence has demonstrated that PPARγ receptor activation shows anti-inflammatory and neuroprotective properties. Here we examine whether treatment with the PPARγ agonist pioglitazone is beneficial in counteracting neurodegeneration, neuroinflammation and cognitive damage produced by binge alcohol intoxication. Adult Wistar rats were subjected to a 4-day binge intoxication procedure, which is commonly used to model excessive alcohol consumption in humans. Across the 4-day period, pioglitazone (0, 30, 60mg/kg) was administered orally twice daily at 12-h intervals. Degenerative cells were detected by fluoro-jade B (FJ-B) immunostaining in brain regions where expression of pro-inflammatory cytokines was also determined. The effects of pioglitazone on cognitive function were assessed in an operant reversal learning task and the Morris water maze task. Binge alcohol exposure produced selective neuronal degeneration in the hippocampal dentate gyrus and the adjacent entorhinal cortex. Pioglitazone reduced FJ-B positive cells in both regions and prevented alcohol-induced expression of pro-inflammatory cytokines. Pioglitazone also rescued alcohol-impaired reversal learning in the operant task and spatial learning deficits in the Morris water maze. These findings demonstrate that activation of PPARγ protects against neuronal and cognitive degeneration elicited by binge alcohol exposure. The protective effect of PPARγ agonist appears to be linked to inhibition of pro-inflammatory cytokines.

Memantine combined with environmental enrichment improves spatial memory and alleviates Alzheimer's disease-like pathology in senescence-accelerated prone-8 (SAMP8) mice

Memantine is a N-methyl-D-aspartate (NMDA) receptor antagonist approved for the treatment of moderate to severe Alzheimer's disease (AD). Environmental enrichment (EE) has shown significant beneficial effects on functional improvement in AD. In this study, we sought to determine whether combining these two distinct therapies would yield greater benefit than either drug used alone. We investigated the effect of memantine combined with EE on spatial learning and memory and AD-like pathology in a widely used AD model, the senescence-accelerated prone mice (SAMP8). The SAMP8 mice were randomly assigned to enriched housing (EH) or standard housing (SH), where either memantine (20 mg/kg) or saline was given by gastric lavage once daily continuously for eight weeks. Our results showed that, when provided separately, memantine and EE significantly improved spatial learning and memory by shortening escape latencies and increasing the frequency of entrance into the target quadrant. When combined, memantine and EE showed additive effect on learning and memory as evidenced by significant shorter escape latencies and higher frequency of target entrance than either drug alone. Consistent with the behavior results, pathological studies showed that both memantine and EE significantly reduced hippocampal CA1 neurofibrilliary tangles (NFTs) as well as amyloid beta precursor protein (APP) levels. Combining both therapies synergistically lessened NFTs and APP expression compared to either drug alone in SAMP8 mice, indicating that the combination of memantine with EE could offer a novel and efficient therapeutic strategy for the treatment of AD.

Alcohol-induced neurodegeneration, suppression of transforming growth factor-beta, and cognitive impairment in rats: prevention by group II metabotropic glutamate receptor activation

BACKGROUND

Glutamatergic neurotransmission has been implicated in mechanisms of alcohol-induced neurodegeneration and cognitive impairment, but the underlying mechanism remains unknown. Here, we examined whether the group II metabotropic glutamate receptor agonist LY379268 prevents neuronal death and learning deficits in a rat model of binge-like exposure to alcohol.

METHODS

Following 4-day binge alcohol exposure concurrent with LY379268 or vehicle treatment, Fluoro-Jade B and transforming growth factor-beta (TGF-beta) staining were carried out, and reversal learning in the Morris water maze was assessed.

RESULTS

Fluoro-Jade B staining indicating neurodegeneration was most extensive in the ventral hippocampus and the entorhinal cortex (EC). LY379268 was potently neuroprotective in the EC but not in the dentate gyrus of the hippocampus. In parallel, binge alcohol exposure suppressed TGF-beta expression in both the EC and dentate gyrus, whereas LY379268 increased TGF-beta in the EC only. Finally, neuroprotective effects of LY379268 were accompanied by prevention of deficits in spatial reversal learning.

CONCLUSIONS

Our data support a neuroprotective role for group II metabotropic glutamate receptor agonists and TGF-beta in alcohol-induced neurodegeneration.

Neuroprotective effects of vinpocetine and its major metabolite cis-apovincaminic acid on NMDA-induced neurotoxicity in a rat entorhinal cortex lesion model

Vinpocetine (ethyl-apovincaminate, Cavinton), a synthetic derivative of the Vinca minor alkaloid vincamine, has been used now for decades for prevention and treatment of cerebrovascular diseases predisposing to development of dementia. Both vinpocetine and its main metabolite cis-apovincaminic acid (cAVA) exert a neuroprotective type of action. Bilateral N-methyl-D-aspartate (NMDA)-induced neurodegeneration in the entorhinal cortex of rat was used as a dementia model to confirm the neuroprotective action of these compounds in vivo. NMDA-lesioned rats were treated 60 min before lesion and throughout 3 postoperative days with a 10 mg/kg intraperitoneal dose of vinpocetine or cAVA. Behavioral tests started after termination of drug treatment and consisted of novel object recognition, social discrimination, and spontaneous alternation in a Y-maze, and spatial learning in the Morris water maze. At the end of behavioral testing brains were perfused with fixative and the size of the excitotoxic neuronal lesion and that of microglial activation around the lesion were assayed quantitatively on brain sections immunostained for neuron-specific nuclear protein (NeuN) and integrin CD11b, respectively. Entorhinal NMDA lesions impaired recognition of novel objects and the new social partner, and suppressed spontaneous alternation and spatial learning performance in the Morris maze. Both vinpocetine and cAVA effectively attenuated the behavioral deficits, and significantly decreased lesion size and the region of microglia activation. Both lesion-induced attention deficit and learning disabilities were markedly alleviated by vinpocetine and cAVA. The morphological findings corroborated the behavioral observations and indicated reduced lesion size and microglia activation especially after vinpocetine treatment which supports an in vivo neuroprotective mode of action of vinpocitine and a less potent action of cAVA.

Linking hippocampal structure and function to memory performance in an aging population

BACKGROUND

Hippocampal atrophy and reductions in basal cerebral blood volume (CBV), a hemodynamic correlate of brain function, occur with cognitive impairment in Alzheimer disease, but whether these are early or late changes remains unclear. Magnetic resonance imaging is used to assess structure and function in the hippocampal formation.

OBJECTIVE

To estimate differences in the associations of hippocampal and entorhinal cortex volumes and CBV with memory function in the early and late stages of cognitive impairment by relating these measures to memory function in persons with and without dementia who underwent detailed brain imaging and neuropsychological assessment.

DESIGN

Multivariate regression analyses were used to relate entorhinal cortex volume, entorhinal cortex CBV, hippocampal volume, and hippocampal CBV to measurements of memory performance. The same measures were related to language function as a reference cognitive domain.

SETTING

Community-based cohort.

PARTICIPANTS

Two hundred thirty-one elderly Medicare recipients (aged > or =65 years) residing in northern Manhattan, New York.

MAIN OUTCOME MEASURES

Values for entorhinal cortex volume, hippocampal volume, entorhinal cortex CBV, and hippocampal CBV and their relation to memory performance.

RESULTS

No association was noted between entorhinal cortex volume or hippocampal CBV and memory. Decreased hippocampal volume was strongly associated with worse performance in total recall, and lower entorhinal cortex CBV was associated with lower performance in delayed recall. Excluding persons with Alzheimer disease, the association of entorhinal cortex CBV with memory measures was stronger, whereas the association between hippocampal volume and total recall became nonsignificant.

CONCLUSIONS

In the early stages of Alzheimer disease or in persons without dementia with worse memory ability, functional and metabolic hippocampal hypofunction contributes to memory impairment, whereas in the later stages, functional and structural changes play a role.

Entorhinal cortex and cognition

Understanding the function of the entorhinal cortex (EC) has been an important subject over the years, not least because of its cortical intermediary to and from the hippocampus proper, and because of electrophysiological advances which have started to reveal the physiology in behaving animals. Clearly, a lot more needs to be done but is clear to date that EC is not merely a throughput station providing all hippocampal subfields with sensory information, but that processing within EC contributes significantly to attention, conditioning, event and spatial cognition possibly by compressing representations that overlap in time. These are transmitted to the hippocampus, where they are differentiated again and returned to EC. Preliminary evidence for such a role, but also their possible pitfalls are summarised.

Memory performance is related to amyloid and tau pathology in the hippocampus

OBJECTIVE

To determine the relation of amyloid and tau pathology in the hippocampal formation to decline in memory and other cognitive functions in Alzheimer's disease (AD).

METHODS

Regression models were used to relate semiquantitative measurements of amyloid plaques, neurofibrillary tangles (NFTs) and neuropil threads (NTs) at autopsy with antemortem performance in memory, abstract/visuospatial and language domains in two independent samples (n = 41, n = 66) that had repeated neuropsychological measurements before death.

RESULTS

In both groups, the number of NFTs in the entorhinal cortex, subiculum and CA1 region was inversely associated with memory performance at the last visit before death. However, the number of amyloid plaques and NTs in the entorhinal cortex was also inversely related to poor memory function. Moreover, as the number of plaques or NTs increased in any region of the hippocampal formation, there was a more rapid decline in memory performance over time; a similar decline was associated with increasing numbers of NFTs in the CA1 or subiculum. In contrast, there was no association between amyloid plaques, NFTs or NTs in the frontal or parietal lobe and performance in memory, nor was there an association between plaques, NFTs or NTs in the hippocampal formation and cognitive functions unrelated to memory.

DISCUSSION

This study implicates both amyloid deposition and tau pathology in the hippocampus as an early and late cause of decline in memory function over time in AD. Memory performance appears to be specifically related to the amount of amyloid plaques, NFTs and NTs in the entorhinal cortex and hippocampus.

Delay-dependent involvement of the rat entorhinal cortex in habituation to a novel environment

Evidence has accumulated that the entorhinal cortex (EC) is involved in memory operations underlying formation of a long-term memory. Because entorhinal-lesioned rats are impaired for long delays in delayed matching and non-matching to sample tasks, it has been proposed that EC contributes to the maintenance of information in short-term memory. In the present study, we asked whether such a time-limited role applies also when learning complex spatial information in a novel environment. We therefore examined the effects of EC lesions on habituation in an object exploration task in which a delay of either 4min or 10min is imposed between successive sessions. EC-lesioned rats exhibited a deficit in habituation at 10min but not 4min delays. Following habituation, reactions to spatial change (object configuration) and non-spatial change (novel object) were also examined. EC-lesioned rats were impaired in detecting the spatial change but were able to detect a non-spatial change, irrespective of the delay. Overall, the results suggest that EC is involved in maintaining a large amount of novel, multidimensional information in short-term memory therefore enabling formation of long-term memory. Switching to a novelty detection mode would then allow the animal to rapidly adapt to environmental changes. In this mode, EC would preferentially process spatial information rather than non-spatial information.

Effects of repeated dizocilpine treatment on adult rat behavior after neonatal lesions of the entorhinal cortex

Disturbed cortical development is implicated in some psychiatric diseases, e.g. in schizophrenia. Additionally, N-methyl-d-aspartate (NMDA) receptor antagonists like ketamine or phencyclidine have been reported to exacerbate schizophrenic symptoms. We here investigated the effects of neonatal entorhinal cortex (EC) lesions on adult rat behavior before and after repeated high-dose treatment with the NMDA antagonist dizocilpine, in order to combine these etiopathogenetical factors in an animal model. Bilateral neonatal (postnatal day 7) lesions were induced by microinjection of ibotenic acid (1.3 microg/0.2 microl PBS) into the EC. Naive and sham-lesioned rats served as controls. Adult rats were tested for behavioral flexibility on a cross maze, for locomotor activity in the open field and for sensorimotor gating using prepulse inhibition (PPI) of startle. Rats were then treated with dizocilpine (0.5 mg/kg b.i.d. for 7 days) and retested 1 week after withdrawal using the same behavioral tests as before. PPI was additionally measured after acute low-dose challenge with dizocilpine (0.15 mg/kg). EC lesions reduced behavioral flexibility as shown by impaired switching between spatial (allocentric) and non-spatial (egocentric) maze strategies. High-dose dizocilpine treatment disturbed switching to the egocentric strategy in all groups, which added to the effect of EC lesions. Neonatal EC lesions did not alter locomotor activity or PPI, but high-dose dizocilpine treatment reduced motor activity of all groups without changing PPI. The combination of neonatal EC lesions and adult dizocilpine treatment does not lead to super-additive effects on behavior. However, both treatments may serve to model certain aspects of psychiatric symptoms.

Beta-amyloid pathology in the entorhinal cortex of rats induces memory deficits: implications for Alzheimer's disease

Alzheimer's disease is characterized by the presence of senile plaques in the brain, composed mainly of aggregated amyloid-beta peptide (Abeta), which plays a central role in the pathogenesis of Alzheimer's disease and is a potential target for therapeutic intervention. Amyloid plaques occur in an increasing number of brain structures during the progression of the disease, with a heavy load in regions of the temporal cortex in the early phases. Here, we investigated the cognitive deficits specifically associated with amyloid pathology in the entorhinal cortex. The amyloid peptide Abeta(1-42) was injected bilaterally into the entorhinal cortex of rats and behavioral performance was assessed between 10 and 17 days after injection. We found that parameters of motor behavior in an open-field as well as spatial working memory tested in an alternation task were normal. In contrast, compared with naive rats or control rats injected with saline, rats injected with Abeta(1-42) showed impaired recognition memory in an object recognition task and delayed acquisition in a spatial reference memory task in a water-maze, despite improved performance with training in this task and normal spatial memory in a probe test given 24 h after training. This profile of behavioral deficits after injection of Abeta(1-42) into the entorhinal cortex was similar to that observed in another group of rats injected with the excitotoxic drug, N-methyl-d-aspartate. Immunohistochemical analysis after behavioral testing revealed that Abeta(1-42) injection induced a reactive astroglial response and plaque-like deposits in the entorhinal cortex. These results show that experimentally-induced amyloid pathology in the entorhinal cortex induces selective cognitive deficits, resembling those observed in early phases of Alzheimer's disease. Therefore, injection of protofibrillar-fibrillar Abeta(1-42) into the entorhinal cortex constitutes a promising animal model for investigating selective aspects of Alzheimer's disease and for screening drug candidates designed against Abeta pathology.

Effects of the mGluR2/3 agonist LY354740 on computerized tasks of attention and working memory in marmoset monkeys

RATIONALE

LY354740 is a recently developed metabotropic glutamatergic receptor 2 and 3 (mGluR2/3) agonist. A high density of mGluR2 has been reported in terminal fields of the perforant path in rodents and humans, suggesting its involvement in cognitive functions mediated by the temporal lobe, including memory. A small number of in vivo studies in rodents have assessed the effects of LY354740 on memory tasks, reporting the induction of impaired memory for spatial orientation in a water maze task and for delayed match and non-match to position in an operant version of these tasks.

OBJECTIVE

In the present primate study, we used radioautography to describe the distribution and intensity of (3)H-LY354740 binding in the hippocampal formation of the common marmoset monkey (Callithrix jacchus) relative to the rat. In the major, in vivo part of the study, the effects of systemic LY354740 on computerized tasks of attention and memory were investigated.

METHODS

Adult common marmosets were trained to perform a five-choice serial reaction time (5-CSRT) task and a concurrent delayed match-to-position (CDMP) task from the Cambridge Neuropsychological Automated test Battery (CANTAB). Filter tests of LY354740 effects on motor dexterity and motivation for reward revealed high inter-individual variation in sensitivity; therefore, on the 5-CSRT, subjects were tested at a dose range of 3--10 mg/kg, and on the CDMP, subjects were tested at 1--3 or 3--10 mg/kg.

RESULTS

Radioautography revealed a relatively low level of (3)H-LY354740 binding in the marmoset hippocampal formation compared to the rat. Despite low binding, LY354740 reduced sustained-attention accuracy in the 5-CSRT, and reduced accuracy in two stages of the CDMP.

CONCLUSIONS

The current study provides novel evidence for the importance of mGluR2/3 in the regulation of primate cognitive functioning.

Spatial discrimination deficits by excitotoxic lesions in the Morris water escape task

The effects of the cholinesterase inhibitors (ChEI) metrifonate and donepezil were assessed on spatial performance of rats with bilateral lesions of the entorhinal cortex (EC), which is thought to model early changes in the brains of patients suffering from Alzheimer's disease. In the present study, we found that spatial discrimination deficits in rats, induced by bilateral ibotenic acid (IBO) lesions of the EC region can partially be antagonised by treatment with the cholinesterase inhibitors metrifonate (30 mg kg(-1)) and donepezil (0.3 and 3 mg kg(-1)). Performance was improved in the spatial discrimination task compared with that of the EC-lesioned control group. It is concluded that the rat with bilateral EC lesions is a suitable deficit model for the assessment of effects of putative Alzheimer therapeutics.

Entorhinal cortex lesioning promotes neurogenesis in the hippocampus of adult mice

Hippocampal neurogenesis in adult mammals is influenced by many factors. Lesioning of the entorhinal cortex is a standard model used to study injury and repair in the hippocampus. Here we use bromodeoxyuridine (BrdU) labeling combined with immunohistochemical identification using cell type specific markers to follow the fate of neural progenitors in the hippocampus following entorhinal cortex lesioning in mice. We show that unilateral entorhinal cortex lesioning does not alter the rate of neural progenitor proliferation in the ipsilateral dentate gyrus during the first 3 days after lesioning. However it enhances cell survival at 42 days post-lesioning leading to an increased number of beta-III tubulin and calbindin-immunoreactive neurons being produced. By contrast, when BrdU was administered 21 days post-lesioning, the number of surviving cells 21 days later was similar on the lesioned and non-lesioned sides. Thus, acutely entorhinal cortex lesioning promotes neurogenesis by enhancing survival of either neural progenitors or their progeny. However, this stimulus to neurogenesis is not sustained into the recovery period.

Entorhinal cortex lesions impair the use of distal but not proximal landmarks during place navigation in the rat

Rats with entorhinal cortex lesions were trained in two versions of the place navigation task in the Morris water maze. In the distal condition, they had to locate the hidden platform on the basis of remote landmarks, while in the proximal condition, they had to rely only on a configuration of proximal objects, placed directly in the pool. Entorhinal rats were impaired in using distal landmarks but were able to use proximal landmarks to navigate toward the platform. These results suggest that the use of distal and proximal landmarks during navigation involves activation of different neural structures. They also suggest, in agreement with previous data, that there are two distinct landmark-processing systems, one devoted to the processing of proximal landmarks and the other to the processing of distal landmarks.

Ibotenic acid and thioibotenic acid: a remarkable difference in activity at group III metabotropic glutamate receptors

In this study, we have determined and compared the pharmacological profiles of ibotenic acid and its isothiazole analogue thioibotenic acid at native rat ionotropic glutamate (iGlu) receptors and at recombinant rat metabotropic glutamate (mGlu) receptors expressed in mammalian cell lines. Thioibotenic acid has a distinct pharmacological profile at group III mGlu receptors compared with the closely structurally related ibotenic acid; the former is a potent (low microm) agonist, whereas the latter is inactive. By comparing the conformational energy profiles of ibotenic and thioibotenic acid with the conformations preferred by the ligands upon docking to mGlu1 and models of the other mGlu subtypes, we propose that unlike other subtypes, group III mGlu receptor binding sites require a ligand conformation at an energy level which is prohibitively expensive for ibotenic acid, but not for thioibotenic acid. These studies demonstrate how subtle differences in chemical structures can result in profound differences in pharmacological activity.

Chronic cognitive effects of bilateral subdural haematomas in the rat

Humans suffering from subdural haematomas often show long-term cognitive dysfunctions. For identifying putative, recovery-enhancing therapeutics, animal models need to be developed in which recovery of function can be measured. For investigating whether and which type of recovery, i.e. spontaneous or training-induced recovery, or continuous partial retardation, is present in the rat model for bilateral subdural haematomas, spatial navigation abilities were assessed in the Morris water escape task in independent groups of rats at 1, 2, 4, 8, or 18 weeks after surgery. Complete spontaneous recovery seemed to occur at 8 weeks after injury. However, at 18 weeks after injury, the subdural haematoma caused a renewed deterioration of water maze performance, which was of a lesser degree than the impairments observed immediately after injury. This second phase performance deterioration was accompanied by an increase in generalised astrocyte reactivity. The rat subdural haematoma model provides an interesting tool for investigating spontaneous recovery processes of spatial navigation (8 weeks after injury), but also for progressive brain dysfunctions, considering the second phase of behavioural impairments seen at 18 weeks after injury.

Androgens protect against apolipoprotein E4-induced cognitive deficits

Compared with apolipoprotein (apo) E2 and E3, apoE4 increases the risk of Alzheimer's disease (AD), but it remains unknown how apoE4 affects neuronal function. ApoE4 interacts with female gender, further increasing the risk of AD and decreasing treatment response. Female mice are also more susceptible to apoE4-induced impairments of spatial learning and memory than male mice. To assess the role of sex steroids in this process, we studied mice deficient in mouse apoE (Apoe(-/-)) and expressing human apoE4 or apoE3 in the brain at comparable levels. Even brief periods of androgen treatment improved the memory deficits of female apoE4 mice. Female apoE3 mice had no memory deficits and did not benefit from the treatment. ApoE4 male mice, which performed normally in a water-maze test at baseline, developed prominent deficits in spatial learning and memory after blockade of androgen receptors (ARs), whereas apoE3 male mice did not. Untreated apoE4 mice had significantly lower cytosolic AR levels in the neocortex than wild-type, Apoe(-/-), and apoE3 mice. Improved memory in androgen-treated female apoE4 mice was associated with increased cytosolic AR levels. Our findings suggest that apoE4 contributes to cognitive decline by reducing AR levels in the brain, and that stimulating AR-dependent pathways can reverse apoE4-induced cognitive deficits.

Extracellular signal-regulated kinase activity in the entorhinal cortex is necessary for long-term spatial memory

Lesion studies have provided evidence that the entorhinal cortex (EC) participates in spatial memory. However, the molecular cascades that underlie memory-associated changes in the EC and its specific role in spatial memory, however, have not been clearly delineated. Recently, it has been shown that activation of extracellular signal-regulated kinase (Erk, a mitogen-activated protein kinase family member) in the dorsal hippocampus is necessary for spatial memory. To examine whether similar mechanisms are used for spatial memory storage in the EC, Erk activity was inhibited after training in the Morris water maze. Bilateral infusion of the mitogen-activated protein kinase kinase inhibitor PD098059 into the EC immediately after training resulted in a memory deficit observed during a retention test performed 48 h later. This deficit was abolished with pretraining in a different water maze in which animals were able to learn the general task requirements and the appropriate search strategies. The absence of a deficit indicates that Erk activity in the EC may be involved in storing the task requirements or the search strategies. The findings presented in this article are consistent with the idea that the EC is involved in spatial memory and indicate that Erk activity is necessary for memory consolidation in this structure.

Memory impairment in temporal lobe epilepsy: the role of entorhinal lesions

Temporal lobe epilepsy (TLE) patients are frequently afflicted with deficits in spatial and other forms of declarative memory. This impairment is likely associated with the medial temporal lobe, which suffers widespread damage in the disease. Physiological and lesion studies, as well as examinations of the complex connectivity of the medial temporal lobe in animals and humans, have identified the entorhinal cortex (EC) as a key structure in the function and dysfunction of this brain region. Lesions in EC layer III, which normally provides monosynaptic input to area CA1 of the hippocampus, frequently occur in TLE and may be causally related to the memory impairments seen in the disease. Lesions that are initially largely restricted to EC layer III can be produced in rats by focal intra-entorhinal injections of 'indirect excitotoxins' such as aminooxyacetic acid or gamma-acetylenic GABA. These animals eventually show more extensive neurodegeneration in temporal lobe structures and, after a latent period, exhibit spontaneously recurring seizure activity. These progressive features, which may mimic events that occur in TLE, provide new opportunities to explore the role of the EC in memory deficits associated with TLE. These animals will also be useful for evaluating new treatment strategies that focus on the prevention of pathological events in the EC.