Blockade of nicotinic acetylcholine receptors suppresses hippocampal long-term potentiation in wild-type but not ApoE4 targeted replacement mice

Whole-Transcriptome Analysis of Repeated Low-Level Sarin-Exposed Rat Hippocampus and Identification of Cerna Networks to Investigate the Mechanism of Sarin-Induced Cognitive Impairment

Sarin is a potent organophosphorus nerve agent that causes cognitive dysfunction, but its underlying molecular mechanisms are poorly understood. In this study, a rat model of repeated low-level sarin exposure was established using the subcutaneous injection of 0.4 × LD₅₀ for 21 consecutive days. Sarin-exposed rats showed persistent learning and memory impairment and reduced hippocampal dendritic spine density. A whole-transcriptome analysis was applied to study the mechanism of sarin-induced cognitive impairment, and a total of 1035 differentially expressed mRNA (DEmRNA), including 44 DEmiRNA, 305 DElncRNA, and 412 DEcircRNA, were found in the hippocampus of sarin-treated rats. According to Gene Ontology (GO) annotation, Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment, and Protein-Protein Interaction (PPI) analysis, these DERNAs were mainly involved in neuronal synaptic plasticity and were related to the pathogenesis of neurodegenerative diseases. The circRNA/lncRNA-miRNA-mRNA ceRNA network was constructed, in which Circ_Fmn1, miR-741-3p, miR-764-3p, miR-871-3p, KIF1A, PTPN11, SYN1, and MT-CO3 formed one circuit, and Circ_Cacna1c, miR-10b-5p, miR-18a-5p, CACNA1C, PRKCD, and RASGRP1 constituted another circuit. The balance between the two circuits was crucial for maintaining synaptic plasticity and may be the regulatory mechanism by which sarin causes cognitive impairment. Our study reveals the ceRNA regulation mechanism of sarin exposure for the first time and provides new insights into the molecular mechanisms of other organophosphorus toxicants.

Apolipoprotein E (APOE) genotype and the pesticide chlorpyrifos modulate attention, motivation and impulsivity in female mice in the 5-choice serial reaction time task

Organophosphate pesticides - and chlorpyrifos (CPF) in particular - contribute to a wide range of neurobehavioural disorders. Most experimental research focuses on learning and memory processes, while other behaviours remain understudied. The isoforms of the human apolipoprotein E (apoE) confer different cognitive skills on their carriers, but data on this topic are still limited. The current study was performed to assess whether the APOE genotypic variability differently modulates the effects of CPF on attentional performance, inhibitory control and motivation. Human apoE targeted replacement adult female mice (apoE2, apoE3 and apoE4) were trained to stably perform the 5-choice serial reaction time task (5-CSRTT). Animals were then subjected to daily dietary CPF (3.75 mg/kg body weight) for 4 weeks. After CPF exposure, we established a 4-week CPF-free period to assess recovery. All individuals acquired the task, apoE2 mice showed enhanced learning, while apoE4 mice displayed increased premature and perseverative responding. This genotype-dependent lack of inhibitory control was reversed by CPF. Overall, the pesticide induced protracted impairments in sustained attention and motivation, and it reduced anticipatory responding. ApoE3 mice exhibited delayed attentional disruptions throughout the wash-out period. Taken together, these findings provide notable evidence on the emergence of CPF-related attentional and motivational deficits.

Attentional performance, impulsivity, and related neurotransmitter systems in apoE2, apoE3, and apoE4 female transgenic mice

RATIONALE

The apolipoprotein E (apoE) genotype influences cognitive performance in humans depending on age and sex. While the detrimental role of the apoE4 isoform on spatial learning and memory has been well-established in humans and rodents, less is known on its impact on the executive functions.

OBJECTIVES

We aimed to evaluate the effect of apoE isoforms (apoE2, apoE3, apoE4) on visuospatial attention and inhibitory control performance in female transgenic mice, and to determine the neurochemical and neuropharmacological basis of this potential relationship.

METHODS

Female mice carrying apoE2, apoE3, and apoE4 were trained in the five-choice serial reaction time task (5-CSRTT). Upon a stable performance, we manipulated the inter-trial interval and the stimulus duration to elicit impulsive responding and engage attention respectively. We further performed a pharmacological challenge by administering cholinergic and GABAergic agents. Finally, we analyzed the levels of brain amino acids and monoamines by using reversed phase high-performance liquid chromatography (HPLC).

RESULTS

ApoE4 mice showed a deficient inhibitory control as revealed by increased perseveration and premature responding. When attention was challenged, apoE4 mice also showed a higher drop in accuracy. The adverse effect of scopolamine on the task was attenuated in apoE4 mice compared to apoE2 and apoE3. Furthermore, apoE4 mice showed less dopamine in the frontal cortex than apoE2 mice.

CONCLUSIONS

We confirmed that the apoE genotype influences attention and inhibitory control in female transgenic mice. The influence of apoE isoforms in the brain neuromodulatory system may explain the cognitive and behavioral differences attributable to the genotype.

Apolipoprotein E4 reduces evoked hippocampal acetylcholine release in adult mice

Apolipoprotein E4 (apoE4) is the most prevalent genetic risk factor for Alzheimer's disease. We utilized apoE4‐targeted replacement mice (approved by the Tel Aviv University Animal Care Committee) to investigate whether cholinergic dysfunction, which increases during aging and is a hallmark of Alzheimer's disease, is accentuated by apoE4. This revealed that levels of the pre‐synaptic cholinergic marker, vesicular acetylcholine transporter in the hippocampus and the corresponding electrically evoked release of acetylcholine, are similar in 4‐month‐old apoE4 and apolipoprotein E3 (apoE3) mice. Both parameters decrease with age. This decrease is, however, significantly more pronounced in the apoE4 mice. The levels of cholinacetyltransferase (ChAT), acetylcholinesterase (AChE), and butyrylcholinesterase (BuChE) were similar in the hippocampus of young apoE4 and apoE3 mice and decreased during aging. For ChAT, this decrease was similar in the apoE4 and apoE3 mice, whereas it was more pronounced in the apoE4 mice, regarding their corresponding AChE and BuChE levels. The level of muscarinic receptors was higher in the apoE4 than in the apoE3 mice at 4 months and increased to similar levels with age. However, the relative representation of the M1 receptor subtype decreased during aging in apoE4 mice. These results demonstrate impairment of the evoked release of acetylcholine in hippocampus by apoE4 in 12‐month‐old mice but not in 4‐month‐old mice. The levels of ChAT and the extent of the M2 receptor‐mediated autoregulation of ACh release were similar in the adult mice, suggesting that the apoE4‐related inhibition of hippocampal ACh release in these mice is not driven by these parameters.

Evoked ACh release from hippocampal and cortical slices is similar in 4‐month‐old apoE4 and apoE3 mice but is specifically and significantly reduced in hippocampus, but not cortex, of 12‐month‐old apoE4 mice. This effect is accompanied by decreased VAChT levels. These findings show that the hipocampal cholinergic nerve terminals are specifically affected by apoE4 and that this effect is age dependent.

Possible contributions of a novel form of synaptic plasticity in Aplysia to reward, memory, and their dysfunctions in mammalian brain

Recent studies in Aplysia have identified a new variation of synaptic plasticity in which modulatory transmitters enhance spontaneous release of glutamate, which then acts on postsynaptic receptors to recruit mechanisms of intermediate- and long-term plasticity. In this review I suggest the hypothesis that similar plasticity occurs in mammals, where it may contribute to reward, memory, and their dysfunctions in several psychiatric disorders. In Aplysia, spontaneous release is enhanced by activation of presynaptic serotonin receptors, but presynaptic D1 dopamine receptors or nicotinic acetylcholine receptors could play a similar role in mammals. Those receptors enhance spontaneous release of glutamate in hippocampus, entorhinal cortex, prefrontal cortex, ventral tegmental area, and nucleus accumbens. In all of those brain areas, glutamate can activate postsynaptic receptors to elevate Ca²⁺ and engage mechanisms of early-phase long-term potentiation (LTP), including AMPA receptor insertion, and of late-phase LTP, including protein synthesis and growth. Thus, presynaptic receptors and spontaneous release may contribute to postsynaptic mechanisms of plasticity in brain regions involved in reward and memory, and could play roles in disorders that affect plasticity in those regions, including addiction, Alzheimer’s disease, schizophrenia, and attention deficit hyperactivity disorder (ADHD).

Low-dose nicotine facilitates spatial memory in ApoE-knockout mice in the radial arm maze

Here, we investigated the effects of nicotine on spatial memory in ApoE-knockout (ApoE-KO) and wild-type (WT) mice in a radial arm maze. Training occurred on three consecutive days and the test was performed on day 4, with one trial per day. Then on day 4, animals were administered nicotine (0.1, 0.25, 0.5, and 1.0 mg/kg) or the antagonist of nicotinic receptors (nAChRs) mecamylamine (MEC 2 mg/kg) alone or together with 0.1 mg/kg nicotine. The number of errors in the first eight choices was recorded. The results were that 0.1 mg/kg nicotine decreased errors in ApoE-KO mice, while 0.1 and 0.25 mg/kg nicotine reduced errors in WT mice, indicating that lower doses of nicotine elicit a memory improvement. In contrast, 1.0 mg/kg nicotine increased errors in WT mice, but not in ApoE-KO mice. MEC alone had no noticeable effect on errors in either strain of mice. However, co-administration of 0.1 mg/kg nicotine and MEC increased errors and reduced the effects of nicotine in WT mice, but not in ApoE-KO mice. Our study found a biphasic effect of nicotine in WT mice: it improves spatial memory at lower doses and impairs it at a higher dose. In ApoE-KO mice, nicotine improves memory at a low dose and has no effect at a higher dose, suggesting that the ApoE deficiency may influence the efficacy of nicotine. Moreover, a reversal of nicotinic effects with MEC was seen in WT mice, indicating the likelihood of the involvement of nAChRs in the spatial-memory response to nicotine.

Emergence of a seizure phenotype in aged apolipoprotein epsilon 4 targeted replacement mice

The apolipoprotein ε4 allele is the strongest genetic risk factor for late-onset Alzheimer's disease (AD) and is associated with earlier age of onset. The incidence of spontaneous seizures has been reported to be increased in sporadic AD as well as in the early onset autosomal dominant forms of AD. We now report the emergence of a seizure phenotype in aged apolipoprotein E4 (apoE4) targeted replacement (TR) mice but not in age-matched apoE2 TR or apoE3 TR mice. Tonic-clonic seizures developed spontaneously after 5 months of age in apoE4 TR mice and are triggered by mild stress. Female mice had increased seizure penetrance compared to male mice, but had slightly reduced overall seizure severity. The majority of seizures were characterized by head and neck jerks, but 25% of aged apoE4 TR mice had more severe tonic-clonic seizures which occasionally progressed to tonic extension and death. Aged apoE4 TR mice progressed through pentylenetetrazol-induced seizure stages more rapidly than did apoE3 TR and apoE2 TR mice. Electroencephalographic (EEG) recordings revealed more frequent bursts of synchronous theta activity in the hippocampus of apoE4 TR mice than in apoE2 TR or apoE3 TR mice. Cortical EEG recordings also revealed sharp spikes and other abnormalities in apoE4 TR mice. Taken together, these findings demonstrate the emergence of an age-dependent seizure phenotype in old apoE4 TR mice in the absence of human amyloid-β peptide (Aβ) overexpression, suggesting increased central nervous system neural network excitability.

Increased hippocampal default mode synchronization during rest in middle-aged and elderly APOE ε4 carriers: relationships with memory performance

The apolipoprotein (APOE) ε4 allele is a strong genetic risk factor for Alzheimer's disease (AD). Intrinsic fluctuations of brain activity measured by fMRI during rest may be sensitive to AD-related neuropathology. In particular, functional connectivity of the default-mode network (DMN) has gained recent attention as a possible biomarker of disease processes and associated memory decline in AD. Here, we tested the hypothesis of APOE-related alterations in DMN functional connectivity in 95 healthy individuals between 50 and 80 years of age, including 33 carriers of the ε4 allele. Based on previous studies, we hypothesized increased hippocampal DMN synchronization in APOE ε4 carriers. This was supported using independent component analysis in combination with a dual-regression approach for analysis of resting state data. Whole-brain analysis suggested effects also in other areas, including the posterior cingulate cortex, parietal cortex, and parahippocampal regions. DMN synchronization showed a negative correlation with performance on a test of memory functioning, suggesting a neurocognitive significance of the brain activity patterns during rest. Our findings indicate that increased genetic vulnerability for AD is reflected in increased hippocampal DMN synchronization during rest several years before clinical manifestation. We propose that the results reflect ε4-related failure in hippocampal decoupling, which might elevate the total hippocampal metabolic burden and increase the risk of cognitive decline and AD. The results provide an important confirmation of specific genotype effects on intrinsic fluctuations and support the use of functional connectivity indices as imaging-derived endophenotypes in the emerging field of imaging genetics.

Fragile X mice: reduced long-term potentiation and N-Methyl-D-Aspartate receptor-mediated neurotransmission in dentate gyrus

Fragile X syndrome (FXS) is a monogenic mental retardation syndrome that frequently includes autism. The Fmr1-knockout (Fmr1-KO) mouse, like FXS-affected individuals, lacks the fragile X mental retardation protein (FMRP) and models autism as well as FXS. Limited human data and several mouse models have implicated the hippocampal dentate gyrus (DG) in autism. We therefore investigated whether the Fmr1-KO mouse exhibited functional changes in DG. We found diminished medial perforant path-granule cell long-term potentiation (LTP), complementing previous investigations of synaptic plasticity in Fmr1-KO demonstrating impaired LTP in CA1, neocortex, and amygdala and exaggerated long-term depression in CA1. We also found that peak amplitude of NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) was smaller in Fmr1-KO than control. AMPA receptor-mediated EPSCs were comparable in the two strains, yielding a lower NMDA/AMPA ratio in Fmr1-KO mice and suggesting one mechanism by which absent FMRP might contribute to diminished LTP. The clinical hallmarks of autism include both excessive adherence to patterns and impaired detection of socially important patterns. The DG has a putative role in pattern separation (for time, space, and features) that has been attributed to granule cell number, firing rates, adult neurogenesis, and even perforant path LTP. DG also contributes to pattern completion in CA3 via its mossy fiber efferents, whose terminals include abundant FMRP in "fragile X granules." Together with the present data, these observations suggest that DG is a candidate region for further investigation in autism and that the Fmr1-KO model may be particularly apt.

Progressive loss of synaptic integrity in human apolipoprotein E4 targeted replacement mice and attenuation by apolipoprotein E2

Inheritance of the APOE4 allele is a well established genetic risk factor linked to the development of late onset Alzheimer's disease. As the major lipid transport protein in the central nervous system, apolipoprotein (apo) E plays an important role in the assembly and maintenance of synaptic connections. Our previous work showed that 7 month old human apoE4 targeted replacement (TR) mice displayed significant synaptic deficits in the principal neurons of the lateral amygdala, a region that is critical for memory formation and also one of the primary regions affected in Alzheimer's disease, compared to apoE3 TR mice. In the current study, we determined how age and varying APOE genotype affect synaptic integrity of amygdala neurons by comparing electrophysiological and morphometric properties in C57BL6, apoE knockout, and human apoE3, E4 and E2/4 TR mice at 1 month and 7 months. The apoE4 TR mice exhibited the lowest level of excitatory synaptic activity and dendritic arbor compared to other cohorts at both ages, and became progressively worse by 7 months. In contrast, the apoE3 TR mice exhibited the highest synaptic activity and dendritic arbor of all cohorts at both ages. C57BL6 mice displayed virtually identical synaptic activity to apoE3 TR mice at 1 month; however this activity decreased by 7 months. ApoE knockout mice exhibited a similar synaptic activity profile with apoE4 TR mice at 7 months. Consistent with previous reports that APOE2 confers protection, the apoE4-dependent deficits in excitatory activity were significantly attenuated in apoE2/4 TR mice at both ages. These findings demonstrate that expression of human apoE4 contributes to functional deficits in the amygdala very early in development and may be responsible for altering neuronal circuitry that eventually leads to cognitive and affective disorders later in life.

Distinct patterns of brain activity in young carriers of the APOE-epsilon4 allele

The APOE epsilon4 allele is a risk factor for late-life pathological changes that is also associated with anatomical and functional brain changes in middle-aged and elderly healthy subjects. We investigated structural and functional effects of the APOE polymorphism in 18 young healthy APOE epsilon4-carriers and 18 matched noncarriers (age range: 20-35 years). Brain activity was studied both at rest and during an encoding memory paradigm using blood oxygen level-dependent fMRI. Resting fMRI revealed increased "default mode network" (involving retrosplenial, medial temporal, and medial-prefrontal cortical areas) coactivation in epsilon4-carriers relative to noncarriers. The encoding task produced greater hippocampal activation in epsilon4-carriers relative to noncarriers. Neither result could be explained by differences in memory performance, brain morphology, or resting cerebral blood flow. The APOE epsilon4 allele modulates brain function decades before any clinical or neurophysiological expression of neurodegenerative processes.

Middle-aged human apoE4 targeted-replacement mice show retention deficits on a wide range of spatial memory tasks

Apolipoprotein (apo) E4, one of three human apoE (h-apoE) isoforms, has been identified as a major genetic risk factor for Alzheimer's disease and for cognitive deficits associated with aging. However, the biological mechanisms involving apoE in learning and memory processes are unclear. A potential isoform-dependent role of apoE in cognitive processes was studied in human apoE targeted-replacement (TR) mice. These mice express either the human apoE3 or apoE4 gene under the control of endogenous murine apoE regulatory sequences, resulting in physiological expression of h-apoE in both a temporal and spatial pattern similar to humans. Male and female apoE3-TR, apoE4-TR, apoE-knockout and C57BL/6J mice (15-18 months) were tested with spatial memory and avoidance conditioning tasks. Compared to apoE3-TR mice, spatial memory in female apoE4-TR mice was impaired based on their poor performances in; (i) the probe test of the water-maze reference memory task, (ii) the water-maze working memory task and (iii) an active avoidance Y-maze task. Retention performance on a passive avoidance task was also impaired in apoE4-TR mice, but not in other genotypes. These deficits in both spatial and avoidance memory tasks may be related to the anatomical and functional abnormalities previously reported in the hippocampus and the amygdala of apoE4-TR mice. We conclude that the apoE4-TR mice provide an excellent model for understanding the mechanisms underlying apoE4-dependent susceptibility to cognitive decline.

Bidirectional synaptic plasticity in the dentate gyrus of the awake freely behaving mouse

There is significant interest in in vivo synaptic plasticity in mice due to the many relevant genetic mutants now available. Nevertheless, use of in vivo models remains limited. To date long-term potentiation (LTP) has been studied infrequently, and long-term depression (LTD) has not been characterized in the mouse in vivo. Herein we describe protocols and improved methodologies we developed to record hippocampal synaptic plasticity reliably from the dentate gyrus of the awake freely behaving mouse. Seven days prior to recording, we implanted microelectrodes encapsulated within a lightweight, low profile head stage assembly. On the day of recording, we induced either LTP or LTD in the awake freely behaving animal, and monitored subsequent changes in population spike amplitude for at least 24h. Using this protocol we attained 80% success in inducing and maintaining either LTP or LTD. Recording from a chronic implant using this improved methodology is best suited to reveal naturally occurring brain activity and avoids both acute effects of local electrode insertion and drifts in neuronal excitability associated with anesthesia. Ultimately a reliable freely behaving mouse model of bi-directional synaptic plasticity is invaluable for full characterization of genetic models of disease states and manipulations of the mechanisms implicated in learning and memory.

Chronic nicotine cell specifically upregulates functional alpha 4* nicotinic receptors: basis for both tolerance in midbrain and enhanced long-term potentiation in perforant path

Understanding effects of chronic nicotine requires identifying the neurons and synapses whose responses to nicotine itself, and to endogenous acetylcholine, are altered by continued exposure to the drug. To address this problem, we developed mice whose alpha4 nicotinic receptor subunits are replaced by normally functioning fluorescently tagged subunits, providing quantitative studies of receptor regulation at micrometer resolution. Chronic nicotine increased alpha4 fluorescence in several regions; among these, midbrain and hippocampus were assessed functionally. Although the midbrain dopaminergic system dominates reward pathways, chronic nicotine does not change alpha4* receptor levels in dopaminergic neurons of ventral tegmental area (VTA) or substantia nigra pars compacta. Instead, upregulated, functional alpha4* receptors localize to the GABAergic neurons of the VTA and substantia nigra pars reticulata. In consequence, GABAergic neurons from chronically nicotine-treated mice have a higher basal firing rate and respond more strongly to nicotine; because of the resulting increased inhibition, dopaminergic neurons have lower basal firing and decreased response to nicotine. In hippocampus, chronic exposure to nicotine also increases alpha4* fluorescence on glutamatergic axons of the medial perforant path. In hippocampal slices from chronically treated animals, acute exposure to nicotine during tetanic stimuli enhances induction of long-term potentiation in the medial perforant path, showing that the upregulated alpha4* receptors in this pathway are also functional. The pattern of cell-specific upregulation of functional alpha4* receptors therefore provides a possible explanation for two effects of chronic nicotine: sensitization of synaptic transmission in forebrain and tolerance of dopaminergic neuron firing in midbrain.

Egr3, a synaptic activity regulated transcription factor that is essential for learning and memory

Learning and memory depend upon poorly defined synaptic and intracellular modifications that occur in activated neurons. Mitogen activated protein kinase-extracellular regulated kinase (MAPK-ERK) signaling and de novo protein synthesis are essential aspects of enduring memory formation, but the precise effector molecules of MAPK-ERK signaling in neurons are not well defined. Early growth response (Egr) transcriptional regulators are examples of MAPK-ERK regulated genes and Egr1 (zif268) has been widely recognized as essential for some aspects of learning and memory. Here we show that Egr3, a transcriptional regulator closely related to Egr1, is essential for normal hippocampal long-term potentiation (LTP) and for hippocampal and amygdala dependent learning and memory. In the absence of Egr3, the defects in learning and memory appear to be independent of Egr1 since Egr1 protein levels are not altered in amygdala, hippocampus or cortex. Moreover, unlike Egr1-deficient mice which have impairments in late phase hippocampal LTP and consolidation of some forms of long-term hippocampus- and amygdala-dependent memory, Egr3-deficient mice have profound defects in early- and late-phase hippocampal LTP, as well as short-term and long-term hippocampus- and amygdala-dependent learning and memory. Thus, Egr3 has an essential role in learning and memory processing that appears to be partly distinct from the role of Egr1.

Estradiol enhances long term potentiation in hippocampal slices from aged apoE4-TR mice

Hormone replacement therapy to treat or prevent Alzheimer Disease (AD) in postmenopausal women is controversial because it may pose other health risks such as cancer and thromboembolism. ApoE status is thought to influence the nootropic efficacy of hormone therapy, but findings are neither consistent nor well understood. We used a known in vitro memory model (long-term potentiation, LTP) in aged (24-27 month) female targeted replacement mice expressing human apoE3 or E4 to compare the effects of exogenous estradiol. Recording medial perforant path evoked field potentials in dentate gyrus of hippocampal slices, we found that both strains exhibited comparable basal synaptic transmission as assessed by input/output functions and paired pulse depression, and that these measures were not affected by estradiol. Vehicle-treated groups from both strains showed comparable LTP. Estradiol had no effect on LTP in apoE3-TR, but selectively increased LTP magnitude in apoE4-TR. The estradiol induced enhancement of LTP in aged female apoE4-TR is consistent with recent clinical observations that estrogen replacement decreases AD risk in some women with apoE4. Elucidating the mechanism of this selective enhancement may lead to more informed treatment decisions as well as to the development of safer alternatives to hormone therapy.