Treatment strategies targeting excess hippocampal activity benefit aged rats with cognitive impairment

Response of the medial temporal lobe network in amnestic mild cognitive impairment to therapeutic intervention assessed by fMRI and memory task performance

Studies of individuals with amnestic mild cognitive impairment (aMCI) have detected hyperactivity in the hippocampus during task-related functional magnetic resonance imaging (fMRI). Such elevated activation has been localized to the hippocampal dentate gyrus/CA3 (DG/CA3) during performance of a task designed to detect the computational contributions of those hippocampal circuits to episodic memory. The current investigation was conducted to test the hypothesis that greater hippocampal activation in aMCI represents a dysfunctional shift in the normal computational balance of the DG/CA3 regions, augmenting CA3-driven pattern completion at the expense of pattern separation mediated by the dentate gyrus. We tested this hypothesis using an intervention based on animal research demonstrating a beneficial effect on cognition by reducing excess hippocampal neural activity with low doses of the atypical anti-epileptic levetiracetam. In a within-subject design we assessed the effects of levetiracetam in three cohorts of aMCI participants, each receiving a different dose of levetiracetam. Elevated activation in the DG/CA3 region, together with impaired task performance, was detected in each aMCI cohort relative to an aged control group. We observed significant improvement in memory task performance under drug treatment relative to placebo in the aMCI cohorts at the 62.5 and 125 mg BID doses of levetiracetam. Drug treatment in those cohorts increased accuracy dependent on pattern separation processes and reduced errors attributable to an over-riding effect of pattern completion while normalizing fMRI activation in the DG/CA3 and entorhinal cortex. Similar to findings in animal studies, higher dosing at 250 mg BID had no significant benefit on either task performance or fMRI activation. Consistent with predictions based on the computational functions of the DG/CA3 elucidated in basic animal research, these data support a dysfunctional encoding mechanism detected by fMRI in individuals with aMCI and therapeutic intervention using fMRI to detect target engagement in response to treatment.

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Patients with aMCI show increased fMRI activation in DG/CA3 relative to controls.

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Low dose levetiracetam treatment decreases excess DG/CA3 activation in aMCI.

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Low dose levetiracetam treatment normalizes decreased entorhinal activation in aMCI.

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Low dose levetiracetam treatment improves task related memory performance in aMCI.

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Targeting excess hippocampal activity has therapeutic potential in amnestic MCI.

Resveratrol prevents age-related memory and mood dysfunction with increased hippocampal neurogenesis and microvasculature, and reduced glial activation

Greatly waned neurogenesis, diminished microvasculature, astrocyte hypertrophy and activated microglia are among the most conspicuous structural changes in the aged hippocampus. Because these alterations can contribute to age-related memory and mood impairments, strategies efficacious for mitigating these changes may preserve cognitive and mood function in old age. Resveratrol, a phytoalexin found in the skin of red grapes having angiogenic and antiinflammatory properties, appears ideal for easing these age-related changes. Hence, we examined the efficacy of resveratrol for counteracting age-related memory and mood impairments and the associated detrimental changes in the hippocampus. Two groups of male F344 rats in late middle-age having similar learning and memory abilities were chosen and treated with resveratrol or vehicle for four weeks. Analyses at ~25 months of age uncovered improved learning, memory and mood function in resveratrol-treated animals but impairments in vehicle-treated animals. Resveratrol-treated animals also displayed increased net neurogenesis and microvasculature, and diminished astrocyte hypertrophy and microglial activation in the hippocampus. These results provide novel evidence that resveratrol treatment in late middle age is efficacious for improving memory and mood function in old age. Modulation of the hippocampus plasticity and suppression of chronic low-level inflammation appear to underlie the functional benefits mediated by resveratrol.

Parvalbumin and neuropeptide Y expressing hippocampal GABA-ergic inhibitory interneuron numbers decline in a model of Gulf War illness

Cognitive dysfunction is amongst the most conspicuous symptoms in Gulf War illness (GWI). Combined exposure to the nerve gas antidote pyridostigmine bromide (PB), pesticides and stress during the Persian Gulf War-1 (PGW-1) are presumed to be among the major causes of GWI. Indeed, our recent studies in rat models have shown that exposure to GWI-related (GWIR) chemicals and mild stress for 4 weeks engenders cognitive impairments accompanied with several detrimental changes in the hippocampus. In this study, we tested whether reduced numbers of hippocampal gamma-amino butyric acid (GABA)-ergic interneurons are among the pathological changes induced by GWIR-chemicals and stress. Animals were exposed to low doses of GWIR-chemicals and mild stress for 4 weeks. Three months after this exposure, subpopulations of GABA-ergic interneurons expressing the calcium binding protein parvalbumin (PV), the neuropeptide Y (NPY) and somatostatin (SS) in the hippocampus were stereologically quantified. Animals exposed to GWIR-chemicals and stress for 4 weeks displayed reduced numbers of PV-expressing GABA-ergic interneurons in the dentate gyrus and NPY-expressing interneurons in the CA1 and CA3 subfields. However, no changes in SS+ interneuron population were observed in the hippocampus. Furthermore, GABA-ergic interneuron deficiency in these animals was associated with greatly diminished hippocampus neurogenesis. Because PV+ and NPY+ interneurons play roles in maintaining normal cognitive function and neurogenesis, and controlling the activity of excitatory neurons in the hippocampus, reduced numbers of these interneurons may be one of the major causes of cognitive dysfunction and reduced neurogenesis observed in GWI. Hence, strategies that improve inhibitory neurotransmission in the hippocampus may prove beneficial for reversing cognitive dysfunction in GWI.

P301L tau expression affects glutamate release and clearance in the hippocampal trisynaptic pathway

Individuals at risk of developing Alzheimer's disease (AD) often exhibit hippocampal hyperexcitability. A growing body of evidence suggests that perturbations in the glutamatergic tripartite synapse may underlie this hyperexcitability. Here, we used a tau mouse model of AD (rTg(TauP301L)4510) to examine the effects of tau pathology on hippocampal glutamate regulation. We found a 40% increase in hippocampal vesicular glutamate transporter, which packages glutamate into vesicles, and has previously been shown to influence glutamate release, and a 40% decrease in hippocampal glutamate transporter 1, the major glutamate transporter responsible for removing glutamate from the extracellular space. To determine whether these alterations affected glutamate regulation in vivo, we measured tonic glutamate levels, potassium-evoked glutamate release, and glutamate uptake/clearance in the dentate gyrus, cornu ammonis 3(CA3), and cornu ammonis 1(CA1) regions of the hippocampus. P301L tau expression resulted in a 4- and 7-fold increase in potassium-evoked glutamate release in the dentate gyrus and CA3, respectively, and significantly decreased glutamate clearance in all three regions. Both release and clearance correlated with memory performance in the hippocampal-dependent Barnes maze task. Alterations in mice expressing P301L were observed at a time when tau pathology was subtle and before readily detectable neuron loss. These data suggest novel mechanisms by which tau may mediate hyperexcitability. Pre-synaptic vesicular glutamate transporters (vGLUTs) package glutamate into vesicles before exocytosis into the synaptic cleft. Once in the extracellular space, glutamate acts on glutamate receptors. Glutamate is removed from the extracellular space by excitatory amino acid transporters, including GLT-1, predominantly localized to glia. P301L tau expression increases vGLUT expression and glutamate release, while also decreasing GLT-1 expression and glutamate clearance.

Hippocampal injury-induced cognitive and mood dysfunction, altered neurogenesis, and epilepsy: can early neural stem cell grafting intervention provide protection?

Damage to the hippocampus can occur through many causes including head trauma, ischemia, stroke, status epilepticus, and Alzheimer's disease. Certain changes such as increased levels of neurogenesis and elevated concentrations of multiple neurotrophic factors that ensue in the acute phase after injury seem beneficial for restraining hippocampal dysfunction. However, many alterations that arise in the intermediate to chronic phase after injury such as abnormal migration of newly born neurons, aberrant synaptic reorganization, progressive loss of inhibitory gamma-amino butyric acid positive interneurons including those expressing reelin, greatly declined neurogenesis, and sustained inflammation are detrimental. Consequently, the net effect of postinjury plasticity in the hippocampus remains inadequate for promoting significant functional recovery. Hence, ideal therapeutic interventions ought to be efficient for restraining these detrimental changes in order to block the propensity of most hippocampal injuries to evolve into learning deficits, memory dysfunction, depression, and temporal lobe epilepsy. Neural stem cell (NSC) grafting into the hippocampus early after injury appears alluring from this perspective because several recent studies have demonstrated the therapeutic value of this intervention, especially for preventing/easing memory dysfunction, depression, and temporal lobe epilepsy development in the chronic phase after injury. These beneficial effects of NSC grafting appeared to be mediated through considerable modulation of aberrant hippocampal postinjury plasticity with additions of new inhibitory gamma-amino butyric acid positive interneurons and astrocytes secreting a variety of neurotrophic factors and anticonvulsant proteins. This review presents advancements made in NSC grafting therapy for treating hippocampal injury in animal models of excitotoxic injury, traumatic brain injury, Alzheimer's disease, and status epilepticus; potential mechanisms of functional recovery mediated by NSC grafts placed early after hippocampal injury; and issues that need to be resolved prior to considering clinical application of NSC grafting for hippocampal injury.

Reassessing the effects of histone deacetylase inhibitors on hippocampal memory and cognitive aging

Converging results link histone acetylation dynamics to hippocampus-dependent memory, including evidence that histone deacetylase inhibitor (HDACi) administration enhances long-term memory. Previously, we demonstrated that aging disrupts the coordinated epigenetic response to recent experience observed in the young adult hippocampus. Here, we extended that work to test the cognitive effects of a novel, brain-penetrant HDACi (EVX001688; EVX) that we confirmed yields robust, relatively long lasting dose-dependent increases in histone acetylation in the hippocampus. In young rats, acute systemic EVX administration, scheduled to yield elevated histone acetylation levels during training in a contextual fear conditioning (CFC) task, had no effect on memory retention at 24 h at any dose examined (10, 30, or 60 mg/kg). Pretraining injection of another HDACi, sodium butyrate, also failed to affect fear memory, and CFC training itself had no influence on hippocampal histone acetylation at 1 hour in mice or two strains of rats. EVX administration before water maze training in young rats yielded a modest effect such that the middle dose produced marginally better 24-h retention than either the low or high dose, but only a small numerical benefit relative to vehicle. Guided by those findings, a final experiment tested the influence of pretraining EVX treatment on age-related spatial memory impairment. The results, revealing no effect on performance, are consistent with the idea that effective procognitive HDACi treatments in aging may require intervention aimed at restoring coordinated epigenetic regulation rather than bulk increases in hippocampal histone acetylation.

Regional-specific effects of ovarian hormone loss on synaptic plasticity in adult human APOE targeted replacement mice

The human apolipoprotein ε4 allele (APOE4) has been implicated as one of the strongest genetic risk factors associated with Alzheimer’s disease (AD) and in influencing normal cognitive functioning. Previous studies have demonstrated that mice expressing human apoE4 display deficits in behavioral and neurophysiological outcomes compared to those with apoE3. Ovarian hormones have also been shown to be important in modulating synaptic processes underlying cognitive function, yet little is known about how their effects are influenced by apoE. In the current study, female adult human APOE targeted replacement (TR) mice were utilized to examine the effects of human APOE genotype and long-term ovarian hormone loss on synaptic plasticity in limbic regions by measuring dendritic spine density and electrophysiological function. No significant genotype differences were observed on any outcomes within intact mice. However, there was a significant main effect of genotype on total spine density in apical dendrites in the hippocampus, with post-hoc t-tests revealing a significant reduction in spine density in apoE3 ovariectomized (OVX) mice compared to sham operated mice. There was also a significant main effect of OVX on the magnitude of LTP, with post-hoc t-tests revealing a decrease in apoE3 OVX mice relative to sham. In contrast, apoE4 OVX mice showed increased synaptic activity relative to sham. In the lateral amygdala, there was a significant increase in total spine density in apoE4 OVX mice relative to sham. This increase in spine density was consistent with a significant increase in spontaneous excitatory activity in apoE4 OVX mice. These findings suggest that ovarian hormones differentially modulate synaptic integrity in an apoE-dependent manner within brain regions that are susceptible to neurophysiological dysfunction associated with AD.

Hilar interneuron vulnerability distinguishes aged rats with memory impairment

Hippocampal interneuron populations are reportedly vulnerable to normal aging. The relationship between interneuron network integrity and age-related memory impairment, however, has not been tested directly. That question was addressed in the present study using a well-characterized model in which outbred, aged, male Long-Evans rats exhibit a spectrum of individual differences in hippocampal-dependent memory. Selected interneuron populations in the hippocampus were visualized for stereological quantification with a panel of immunocytochemical markers, including glutamic acid decarboxylase-67 (GAD67), somatostatin, and neuropeptide Y. The overall pattern of results was that, although the numbers of GAD67- and somatostatin-positive interneurons declined with age across multiple fields of the hippocampus, alterations specifically related to the cognitive outcome of aging were observed exclusively in the hilus of the dentate gyrus. Because the total number of NeuN-immunoreactive hilar neurons was unaffected, the decline observed with other markers likely reflects a loss of target protein rather than neuron death. In support of that interpretation, treatment with the atypical antiepileptic levetiracetam at a low dose shown previously to improve behavioral performance fully restored hilar SOM expression in aged, memory-impaired rats. Age-related decreases in GAD67- and somatostatin-immunoreactive neuron number beyond the hilus were regionally selective and spared the CA1 field of the hippocampus entirely. Together these findings confirm the vulnerability of hippocampal interneurons to normal aging and highlight that the integrity of a specific subpopulation in the hilus is coupled with age-related memory impairment.

Behaviorally activated mRNA expression profiles produce signatures of learning and enhanced inhibition in aged rats with preserved memory

Aging is often associated with cognitive decline, but many elderly individuals maintain a high level of function throughout life. Here we studied outbred rats, which also exhibit individual differences across a spectrum of outcomes that includes both preserved and impaired spatial memory. Previous work in this model identified the CA3 subfield of the hippocampus as a region critically affected by age and integral to differing cognitive outcomes. Earlier microarray profiling revealed distinct gene expression profiles in the CA3 region, under basal conditions, for aged rats with intact memory and those with impairment. Because prominent age-related deficits within the CA3 occur during neural encoding of new information, here we used microarray analysis to gain a broad perspective of the aged CA3 transcriptome under activated conditions. Behaviorally-induced CA3 expression profiles differentiated aged rats with intact memory from those with impaired memory. In the activated profile, we observed substantial numbers of genes (greater than 1000) exhibiting increased expression in aged unimpaired rats relative to aged impaired, including many involved in synaptic plasticity and memory mechanisms. This unimpaired aged profile also overlapped significantly with a learning induced gene profile previously acquired in young adults. Alongside the increased transcripts common to both young learning and aged rats with preserved memory, many transcripts behaviorally-activated in the current study had previously been identified as repressed in the aged unimpaired phenotype in basal expression. A further distinct feature of the activated profile of aged rats with intact memory is the increased expression of an ensemble of genes involved in inhibitory synapse function, which could control the phenotype of neural hyperexcitability found in the CA3 region of aged impaired rats. These data support the conclusion that aged subjects with preserved memory recruit adaptive mechanisms to retain tight control over excitability under both basal and activated conditions.

Neural stem cell- and neurogenesis-related gene expression profiles in the young and aged dentate gyrus

Hippocampal neurogenesis, important for memory and mood function, wanes greatly in old age. Studies in rat models have implied that this decrease is not due to loss of neural stem cells (NSCs) in the subgranular zone of the dentate gyrus (DG) but rather due to an increased quiescence of NSCs. Additional studies have suggested that changes in the microenvironment, particularly declines in the concentrations of neurotrophic factors, underlie this change. In this study, we compared the expression of 84 genes that are important for NSC proliferation and neurogenesis between the DG of young (4 months old) and aged (24 months old) Fischer 344 rats, using a quantitative real-time polymerase chain reaction array. Interestingly, the expression of a vast majority of genes that have been reported previously to positively or negatively regulate NSC proliferation was unaltered with aging. Furthermore, most genes important for cell cycle arrest, regulation of cell differentiation, growth factors and cytokine levels, synaptic functions, apoptosis, cell adhesion and cell signaling, and regulation of transcription displayed stable expression in the DG with aging. The exceptions included increased expression of genes important for NSC proliferation and neurogenesis (Stat3 and Shh), DNA damage response and NF-kappaB signaling (Cdk5rap3), neuromodulation (Adora1), and decreased expression of a gene important for neuronal differentiation (HeyL). Thus, age-related decrease in hippocampal neurogenesis is not associated with a decline in the expression of selected genes important for NSC proliferation and neurogenesis in the DG.

Seizures and epileptiform activity in the early stages of Alzheimer disease

IMPORTANCE

Epileptic activity associated with Alzheimer disease (AD) deserves increased attention because it has a harmful impact on these patients, can easily go unrecognized and untreated, and may reflect pathogenic processes that also contribute to other aspects of the illness. We report key features of AD-related seizures and epileptiform activity that are instructive for clinical practice and highlight similarities between AD and transgenic animal models of the disease.

OBJECTIVE

To describe common clinical characteristics and treatment outcomes of patients with amnestic mild cognitive impairment (aMCI) or early AD who also have epilepsy or subclinical epileptiform activity.

DESIGN

Retrospective observational study from 2007 to 2012. SETTING Memory and Aging Center, University of California, San Francisco.

PATIENTS

We studied 54 patients with a diagnosis of aMCI plus epilepsy (n = 12), AD plus epilepsy (n = 35), and AD plus subclinical epileptiform activity (n = 7).

MAIN OUTCOMES AND MEASURES

Clinical and demographic data, electroencephalogram (EEG) readings, and treatment responses to antiepileptic medications.

RESULTS

Patients with aMCI who had epilepsy presented with symptoms of cognitive decline 6.8 years earlier than patients with aMCI who did not have epilepsy (64.3 vs 71.1 years; P = .02). Patients with AD who had epilepsy presented with cognitive decline 5.5 years earlier than patients with AD who did not have epilepsy (64.8 vs 70.3 years; P = .001). Patients with AD who had subclinical epileptiform activity also had an early onset of cognitive decline (58.9 years). The timing of seizure onset in patients with aMCI and AD was nonuniform (P < .001), clustering near the onset of cognitive decline. Epilepsies were most often complex partial seizures (47%) and more than half were nonconvulsive (55%). Serial or extended EEG monitoring appeared to be more effective than routine EEG at detecting interictal and subclinical epileptiform activity. Epileptic foci were predominantly unilateral and temporal. Of the most commonly prescribed antiepileptics, treatment outcomes appeared to be better for lamotrigine and levetiracetam than for phenytoin.

CONCLUSIONS AND RELEVANCE

Common clinical features of patients with aMCI- or AD-associated epilepsy at our center included early age at onset of cognitive decline, early incidence of seizures in the disease course, unilateral temporal epileptic foci detected by serial/extended EEG, transient cognitive dysfunction, and good seizure control and tolerability with lamotrigine and levetiracetam. Careful identification and treatment of epilepsy in such patients may improve their clinical course.

Integrity of mGluR-LTD in the associative/commissural inputs to CA3 correlates with successful aging in rats

The impact of aging on cognitive capabilities varies among individuals ranging from significant impairment to preservation of function on par with younger adults. Research on the neural basis for age-related memory decline has focused primarily on the CA1 region of the hippocampus. However, recent studies in elderly human and rodents indicate that individual differences in cognitive aging are more strongly tied to functional alterations in CA3 circuits. To examine synaptic plasticity in the CA3 region, we used aged rats behaviorally characterized in a hippocampal-dependent task to evaluate the status of long-term potentiation and long-term depression (LTP and LTD) in the associative/commissural pathway (A/C → CA3), which provides the majority of excitatory input to CA3 pyramidal neurons. We found that, unlike in CA1 synapses, in A/C → CA3 LTP is minimally affected by age. However, two forms of LTD, involving NMDA and metabotropic glutamate receptors (mGluR), are both greatly reduced in age-impaired rats. Age-unimpaired rats, in contrast, had intact mGluR LTD. These findings indicate that the integrity of mGluR-LTD at A/C → CA3 inputs may play a crucial role in maintaining the performance of CA3 circuitry in aging.

Perturbations of neural circuitry in aging, mild cognitive impairment, and Alzheimer's disease

Alzheimer's disease (AD) is a global public health threat that continues to rise as the proportion of the population over the age of 60 rapidly increases. Aging and dementia are both associated with cognitive decline and share some features in terms of structural and functional alterations in neural circuitry. In this review, we attempt to highlight the network perturbations that occur in "typical" aging and emphasize how they may differ from those that manifest in dementia. We focus in particular on neuroimaging studies of the medial temporal lobe (MTL) network, which is involved in episodic memory and is known to change both with age and with AD pathology. We propose a temporal model of structural and functional alterations in the MTL along the aging-dementia continuum. The earliest changes are synaptic in nature and are detectable in particularly vulnerable white matter pathways such as the perforant path. These are followed by structural degradation in the transentorhinal region and subsequently neurodegeneration of the hippocampus as a result of accumulating pathology as well as deafferentation from entorhinal input. We believe that testing this model explicitly is an important direction for future research, particularly in the context of biomarker discovery and clinical trial design.

A behavioral paradigm to evaluate hippocampal performance in aged rodents for pharmacological and genetic target validation

Aged-related cognitive ability is highly variable, ranging from unimpaired to severe impairments. The Morris water maze (a reliable tool for assessing memory) has been used to distinguish aged rodents that are superior learners from those that are learning impaired. This task, however, is not practical for pre- and post-pharmacological treatment, as the memory of the task is long lasting. In contrast, the object location memory task, also a spatial learning paradigm, results in a less robust memory that decays quickly. We demonstrate for the first time how these two paradigms can be used together to assess hippocampal cognitive impairments before and after pharmacological or genetic manipulations in rodents. Rats were first segregated into superior learning and learning impaired groups using the object location memory task, and their performance was correlated with future outcome on this task and on the Morris water maze. This method provides a tool to evaluate the effect of treatments on cognitive impairment associated with aging and neurodegenerative disorders.

Effects of levetiracetam, an antiepileptic drug, on memory impairments associated with aging and Alzheimer's disease in mice

Emerging evidence suggests that elevated hippocampal activation may be important for disrupting cognitive functions in aged subjects as well as patients with Alzheimer's disease (AD). Therefore, reducing deleterious overactivity of the hippocampus may have therapeutic benefits. This study was designed to compare the effects of levetiracetam, an antiepileptic drug, on memory deficits associated with normal aging and AD in mouse models. Pretraining administration of levetiracetam ameliorated memory impairments of aged C57BL/6 mice (17-20months of age) in the contextual fear conditioning paradigm. Acute levetiracetam immediately after training was also efficacious in rescuing contextual memory decline in aged mice, whereas administration at a later posttraining interval (3h) had no effect. These results suggest that suppressing overexcitation with acute levetiracetam around the time of acquisition or early consolidation may be sufficient to reverse memory decline associated with aging. In contrast, pretraining administration of levetiracetam was not able to rescue memory deficits in 5XFAD transgenic mice harboring amyloid plaque pathologies at moderate (6-8months old) or massive (12-15months old) levels, differentiating between normal aging- and AD-related memory impairments in the responsiveness to acute levetiracetam treatment.

Hippocampal Gαq/₁₁ but not Gαo-coupled receptors are altered in aging

Normal aging may limit the signaling efficacy of certain GPCRs by disturbing the function of specific Gα-subunits and leading to deficient modulation of intracellular functions that subserve synaptic plasticity, learning and memory. Evidence suggests that Gαq/₁₁ is more sensitive to the effects of aging relative to other Gα-subunits, including Gαo. To test this hypothesis, the functionality of Gαq/₁₁ and Gαo were compared in the hippocampus of young (6 months) and aged (24 months) F344 × BNF₁ hybrid rats assessed for spatial learning ability. Basal GTPγS-binding to Gαq/₁₁ was significantly elevated in aged rats relative to young and but not reliably associated with spatial learning. mAChR stimulation of Gαq/₁₁ with oxotremorine-M produced equivocal GTPγS-binding between age groups although values tended to be lower in the aged hippocampus and were inversely related to basal activity. Downstream Gαq/₁₁ function was measured in hippocampal subregion CA₁ by determining changes in [Ca(2+)]i after mAChR and mGluR (DHPG) stimulation. mAChR-stimulated peak change in [Ca(2+)]i was lower in aged CA₁ relative to young while mGluR-mediated integrated [Ca(2+)]i responses tended to be larger in aged. GPCR modulation of [Ca(2+)]i was observed to depend on intracellular stores to a greater degree in aged than young. In contrast, measures of Gαo-mediated GTPγS-binding were stable across age, including basal, mAChR-, GABABR (baclofen)-stimulated levels. Overall, the data indicate that aging selectively modulates the activity of Gαq/₁₁ within the hippocampus leading to deficient modulation of [Ca(2+)]i following stimulation of mAChRs but these changes are not related to spatial learning.

Apolipoprotein E4 causes age- and sex-dependent impairments of hilar GABAergic interneurons and learning and memory deficits in mice

Apolipoprotein (apo) E4 is the major genetic risk factor for Alzheimer's disease (AD). ApoE4 has sex-dependent effects, whereby the risk of developing AD is higher in apoE4-expressing females than males. However, the mechanism underlying the sex difference, in relation to apoE4, is unknown. Previous findings indicate that apoE4 causes age-dependent impairments of hilar GABAergic interneurons in female mice, leading to learning and memory deficits. Here, we investigate whether the detrimental effects of apoE4 on hilar GABAergic interneurons are sex-dependent using apoE knock-in (KI) mice across different ages. We found that in female apoE-KI mice, there was an age-dependent depletion of hilar GABAergic interneurons, whereby GAD67- or somatostatin-positive–but not NPY- or parvalbumin-positive–interneuron loss was exacerbated by apoE4. Loss of these neuronal populations was correlated with the severity of spatial learning deficits at 16 months of age in female apoE4-KI mice; however, this effect was not observed in female apoE3-KI mice. In contrast, we found an increase in the numbers of hilar GABAergic interneurons with advancing age in male apoE-KI mice, regardless of apoE genotype. Moreover, male apoE-KI mice showed a consistent ratio of hilar inhibitory GABAergic interneurons to excitatory mossy cells approximating 1.5 that is independent of apoE genotype and age, whereas female apoE-KI mice exhibited an age-dependent decrease in this ratio, which was exacerbated by apoE4. Interestingly, there are no apoE genotype effects on GABAergic interneurons in the CA1 and CA3 subregions of the hippocampus as well as the entorhinal and auditory cortexes. These findings suggest that the sex-dependent effects of apoE4 on developing AD is in part attributable to inherent sex-based differences in the numbers of hilar GABAergic interneurons, which is further modulated by apoE genotype.

Levetiracetam suppresses neuronal network dysfunction and reverses synaptic and cognitive deficits in an Alzheimer's disease model

In light of the rising prevalence of Alzheimer's disease (AD), new strategies to prevent, halt, and reverse this condition are needed urgently. Perturbations of brain network activity are observed in AD patients and in conditions that increase the risk of developing AD, suggesting that aberrant network activity might contribute to AD-related cognitive decline. Human amyloid precursor protein (hAPP) transgenic mice simulate key aspects of AD, including pathologically elevated levels of amyloid-β peptides in brain, aberrant neural network activity, remodeling of hippocampal circuits, synaptic deficits, and behavioral abnormalities. Whether these alterations are linked in a causal chain remains unknown. To explore whether hAPP/amyloid-β-induced aberrant network activity contributes to synaptic and cognitive deficits, we treated hAPP mice with different antiepileptic drugs. Among the drugs tested, only levetiracetam (LEV) effectively reduced abnormal spike activity detected by electroencephalography. Chronic treatment with LEV also reversed hippocampal remodeling, behavioral abnormalities, synaptic dysfunction, and deficits in learning and memory in hAPP mice. Our findings support the hypothesis that aberrant network activity contributes causally to synaptic and cognitive deficits in hAPP mice. LEV might also help ameliorate related abnormalities in people who have or are at risk for AD.

Neural stem cell grafting counteracts hippocampal injury-mediated impairments in mood, memory, and neurogenesis

The hippocampus is vital for functions such as mood and memory. Hippocampal injury typically leads to mood and memory impairments associated with reduced and aberrant neurogenesis in the dentate gyrus. We examined whether neural stem cell (NSC) grafting after hippocampal injury would counteract impairments in mood, memory, and neurogenesis. We expanded NSCs from the anterior subventricular zone (SVZ) of postnatal F344 rat pups expressing the human placental alkaline phosphatase and grafted them into the hippocampus of young adult F344 rats at 5 days after an injury inflicted through a unilateral intracerebroventricular administration of kainic acid. Analyses through forced swim, water maze, and novel object recognition tests revealed significant impairments in mood and memory function in animals that underwent injury and sham-grafting surgery. In contrast, animals that received SVZ-NSC grafts after injury exhibited mood and memory function comparable to those of naïve control animals. Graft-derived cells exhibited excellent survival and pervasive migration, and they differentiated into neurons, subtypes of inhibitory GABAergic interneurons, astrocytes, oligodendrocytes, and oligodendrocyte progenitors. Significant fractions of graft-derived cells also expressed beneficial neurotrophic factors such as the glial cell line-derived neurotrophic factor, brain-derived neurotrophic factor, fibroblast growth factor, and vascular endothelial growth factor. Furthermore, SVZ-NSC grafting counteracted the injury-induced reductions and abnormalities in neurogenesis by both maintaining a normal level of NSC activity in the subgranular zone and providing protection to reelin+ interneurons in the dentate gyrus. These results underscore that early SVZ-NSC grafting intervention after hippocampal injury is efficacious for thwarting mood and memory dysfunction and abnormal neurogenesis.

Characterization of CpG island DNA methylation of impairment-related genes in a rat model of cognitive aging

Cognitive abilities, particularly memory formation, vary substantially in the elderly, with some individuals exhibiting dramatic decline with age while others maintain function well into late life. Epigenetic modifications suggest an intriguing mechanism to account for the range of cognitive outcomes in aging as they are responsive to environmental influences and affect gene transcription in cognitively relevant brain regions. Leveraging a well-characterized rat model of neurocognitive aging that recapitulates the range of outcomes seen in humans, we previously identified gene expression profiles in the CA3 subregion of the hippocampus that distinguish between young and aged subjects as well as between impaired and preserved spatial memory function. To investigate the influence of epigenetics on these profiles, we examined genomic CpG DNA methylation in the promoter regions of three neurophysiologically relevant genes (Gabra5, Hspa5 and Syn1) whose expression levels decrease with age and correlate with spatial memory performance. Consistent with mRNA decreases, DNA methylation increased in aged rats relative to young in CpG dense regions of all target promoters examined. However, no correlation with cognition was found. Focused analysis of the Gabra5 gene found that methylation changes were limited to the CpG island and varied substantially across individual CpGs. Methylation at one CpG correlated with learning and demonstrated a significant difference between memory impaired aged rats and those with intact learning. These data provide evidence that broad age-dependent DNA methylation changes occur in CpG dense promoter regions of cognitively relevant genes but suggest that methylation at single CpGs may be more pertinent to individual cognitive differences.

Reduction of hippocampal hyperactivity improves cognition in amnestic mild cognitive impairment

Elevated hippocampal activation is observed in conditions that confer risk for Alzheimer's disease, including amnestic mild cognitive impairment (aMCI). Studies in relevant animal models have indicated that overactivity in selective hippocampal circuits contributes to cognitive impairment. Here, we tested the effect of reducing hippocampal activation in aMCI. Under placebo treatment, hippocampal activation in the dentate gyrus/CA3 was elevated in aMCI patients compared to a healthy control group. By using a low dose of the antiepileptic levetiracetam hippocampal activation in aMCI was reduced to a level that did not differ from the control group. Compared to aMCI memory performance under placebo, performance in the scanning task was significantly improved under drug treatment. Contrary to the view that greater hippocampal activation might serve a beneficial function, these results support the view that increased hippocampal activation in aMCI is a dysfunctional condition and that targeting excess hippocampal activity has therapeutic potential.

Selective GABA(A) α5 positive allosteric modulators improve cognitive function in aged rats with memory impairment

A condition of excess activity in the hippocampal formation is observed in the aging brain and in conditions that confer additional risk during aging for Alzheimer's disease. Compounds that act as positive allosteric modulators at GABA(A) α5 receptors might be useful in targeting this condition because GABA(A) α5 receptors mediate tonic inhibition of principal neurons in the affected network. While agents to improve cognitive function in the past focused on inverse agonists, which are negative allosteric modulators at GABA(A) α5 receptors, research supporting that approach used only young animals and predated current evidence for excessive hippocampal activity in age-related conditions of cognitive impairment. Here, we used two compounds, Compound 44 [6,6-dimethyl-3-(3-hydroxypropyl)thio-1-(thiazol-2-yl)-6,7-dihydro-2-benzothiophen-4(5H)-one] and Compound 6 [methyl 3,5-diphenylpyridazine-4-carboxylate], with functional activity as potentiators of γ-aminobutyric acid at GABA(A) α5 receptors, to test their ability to improve hippocampal-dependent memory in aged rats with identified cognitive impairment. Improvement was obtained in aged rats across protocols differing in motivational and performance demands and across varying retention intervals. Significant memory improvement occurred after either intracereboventricular infusion with Compound 44 (100 μg) in a water maze task or systemic administration with Compound 6 (3 mg/kg) in a radial arm maze task. Furthermore, systemic administration improved behavioral performance at dosing shown to provide drug exposure in the brain and in vivo receptor occupancy in the hippocampus. These data suggest a novel approach to improve neural network function in clinical conditions of excess hippocampal activity. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Clinical trials: new opportunities

Human cognitive aging has been too long neglected and underappreciated for its critical importance to quality of life in old age. The articles in this session present novel approaches to improving cognitive function in normal aging persons with drugs and interventions that are based on findings in epidemiology, studies in aged animals, and in vitro research. In addition, since aging is the primary risk factor for Alzheimer's disease, these studies also have implications as interventions for prevention and treatment. As a field of research, new knowledge regarding the causes and mechanisms of cognitive aging are ripe for translation into human studies, with the application of this knowledge leading the development of interventions and therapeutics for the prevention of cognitive decline in old age and Alzheimer's disease.

[Pharmacological treatment strategies targeting cognitive impairment associated with aging]

Amnestic mild cognitive impairment (aMCI) is associated with increased activation in the CA3-dentate region of hippocampus. Excess CA3 activity also occurs in aged rats with memory impairment. Therapies to counter such excess activity might include antiepileptics or agonists for GABA(A) α5 receptors, which regulate tonic inhibition. Use of GABA(A) α5 agonists may seem unexpected because GABA(A) α5 inverse agonists were developed as cognitive enhancers. We found that inverse agonists, while yielding benefit in normal young adult rats, are not effective in treating memory loss in aged rats. Instead, aged rats showed improved memory after treatment with selective GABA(A) α5 agonists and with certain antiepileptics. These benefits of treatment are consistent with the concept that excess activity in the CA3 of the hippocampus is a dysfunctional condition contributing to age-associated memory impairment. Because excess hippocampal activation is also observed in aMCI, our findings support the use of antiepileptic or GABA(A) α5 agonist therapy in aMCI. Such therapy, in addition to memory improvement, may also have disease modifying potential because hippocampal overactivity in aging/MCI predicts further cognitive decline and conversion to Alzheimer's disease.

Alzheimer mechanisms and therapeutic strategies

There are still no effective treatments to prevent, halt, or reverse Alzheimer's disease, but research advances over the past three decades could change this gloomy picture. Genetic studies demonstrate that the disease has multiple causes. Interdisciplinary approaches combining biochemistry, molecular and cell biology, and transgenic modeling have revealed some of its molecular mechanisms. Progress in chemistry, radiology, and systems biology is beginning to provide useful biomarkers, and the emergence of personalized medicine is poised to transform pharmaceutical development and clinical trials. However, investigative and drug development efforts should be diversified to fully address the multifactoriality of the disease.

Episodic memory on the path to Alzheimer's disease

This review is focused on specific circuits of the medial temporal lobe that have become better understood in recent years for their computational properties contributing to episodic memory and to memory impairment associated with aging and other risk for AD. The layer II neurons in the entorhinal cortex and their targets in the dentate gyrus and CA3 region of hippocampus comprise a system that rapidly encodes representations that are distinct from prior memories. Frank neuron loss in the entorhinal cortex is specific for AD, and related structural and functional changes across the network comprised of the entorhinal cortex and the dentate/CA3 regions hold promise for predicting progression on the path to AD.

Electrical and Pharmacological Stimuli Reveal a Greater Susceptibility for CA3 Network Excitability in Hippocampal Slices from Aged vs. Adult Fischer 344 Rats

Clinical data and experimental studies in rats have shown that the aged CNS is more susceptible to the proconvulsive effects of the excitotoxic glutamate analogues kainate (KA) and domoate (DA), which bind high-affinity receptors localized at mossy fiber (MF) synapses in the CA3 subregion of the hippocampus. Although decreased renal clearance appears to play a role in the hypersensitivity of the aged hippocampus to systemically-administered DA, it is unclear if the excitability of the CA3 network is also altered with age. Therefore, this study monitored CA3 field potential activity in hippocampal slices from aged and adult male Fischer 344 rats in response to electrical and pharmacological network stimulation targeted to the MF-CA3 circuit. Network challenges with repetitive hilar stimulation or bath application of nanomolar concentrations of KA more readily elicited excitable network activity (e.g. population spike facilitation, multiple population spikes, and epileptiform bursts) in slices from aged vs. adult rats, although basal network excitability was comparable between age groups. Additionally, exposure to 200 nM KA often abolished epileptiform activity and revealed theta or gamma oscillations instead. However, slices from aged rats were less sensitive to the rhythmogenic effects of 200 nM KA. Taken together, these findings suggest that aging decreases the capacity of the CA3 network to constrain the spread of excitability during focal excitatory network challenges.

Can small molecules provide truly effective enhancement of cognition? Current achievements and future directions

INTRODUCTION

The prevalence of age-related diseases that implicate a deterioration of cognitive abilities is increasing. Moreover, cognitive decline occurs in numerous CNS disorders affecting patients at younger ages as well, resulting in reduced functional ability and quality of life. Despite the existence of few medications treating cognition, the need for efficacious treatment options to alleviate, halt or even prevent cognitive decline is generally unmet to date. Consequently, extensive research efforts are undertaken to identify medications that can effectively enhance cognition.

AREAS COVERED

This review covers ongoing clinical trials for cognition and reflects on efforts undertaken to increase the success rates of procognitive drug treatment. The review discusses ways to optimize the drug development process for cognition enhancing agents at the preclinical to clinical interface and provides concrete examples.

EXPERT OPINION

The existing efficacy readouts addressing cognition in preclinical research offer little translational validity to the clinical situation. In order to identify truly efficacious drug candidates, biomarkers need to be developed that directly address conserved mechanisms underlying cognitive performances. To this end, technologies such as neuroimaging or electroencephalography constitute promising entry points for identifying both the cognitive domain and the patient population most responsive to drug treatment.

High-resolution structural and functional MRI of hippocampal CA3 and dentate gyrus in patients with amnestic Mild Cognitive Impairment

Functional magnetic resonance imaging (fMRI) studies have observed hyperactivity in the hippocampal region in individuals with Mild Cognitive Impairment (MCI). However, the actual source of such hyperactivity is not well understood. Studies of aged rats observed similar hyperactive signals in the CA3 region of the hippocampus that correlated with spatial memory deficits and, in particular, with their ability to represent novel environments as being distinct from familiar ones (pattern separation). In this study, we tested the hypothesis that patients with amnestic MCI (aMCI) have deficits in pattern separation, along with hyperactive fMRI BOLD activity in the CA3 region of the hippocampus. We used high-resolution fMRI during a continuous recognition task designed to emphasize pattern separation. We conducted hippocampal subfield-level region of interest analyses to test for dysfunctional activity in aMCI patients. We found that patients showed impaired performance on trials that taxed their pattern separation abilities. We also observed hyperactive BOLD signals in the CA3/dentate and hypoactive signals in the entorhinal cortex during the separation condition. In a high-resolution morphometric analysis of hippocampal subfields, aMCI patients also had smaller CA3/dentate and CA1 volumes (no difference in the subiculum). The CA3/dentate region bilaterally also exhibited the largest shape deformations in aMCI patients, suggesting that this locus is affected early in the course of the disease. These findings suggest that structural and functional changes in the CA3/dentate region of the hippocampus contribute to the deficits in episodic memory that are observed in patients with aMCI. The functional hyperactivity may be evidence for a dysfunctional encoding mechanism, consistent with the predictions of computational models of hippocampal learning.