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.

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.

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.

Amyloid-beta1-42 reduces neuronal excitability in mouse dentate gyrus.

Amyloid-beta (Abeta) is causally implicated in Alzheimer's disease and neuroplasticity failure has acquired validity as a possible mechanism of early AD pathogenesis. We have previously demonstrated that oligomeric Abeta(1-42) inhibits LTP in the dentate gyrus of rat hippocampal slices. We now show, using whole cell recordings in hippocampal granule cells, that oligomeric Abeta(1-42) decreases neuronal excitability. In particular, Abeta(1-42) application was associated with a decrease in the number of action potentials fired in response to current injection, and with an increase in the amplitude of the afterhyperpolarization. Reduced excitability may underlie the Abeta-mediated impairment in neuroplasticity, and ultimately may contribute to the memory loss in Alzheimer disease.

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

Both impaired nicotinic neurotransmission and the inheritance of apoE4 are associated with increased risk for Alzheimer disease (AD) as well as other deficiencies in memory-related behavior. Long-term potentiation (LTP), a cellular model of memory, is known to be altered by nicotinic agents. Recent studies also support an emergent role for apoE in LTP. We compared the effects of mecamylamine, a nonspecific antagonist of nicotinic acetylcholine receptors (nAChRs), on basal synaptic transmission and LTP in hippocampal slices from wild-type (wt) mice and targeted replacement mice expressing human apoE4 (apoE4-TR). Field excitatory postsynaptic potentials (EPSPs) were recorded in the dentate gyrus (DG) in response to medial perforant path activation, and theta burst stimulation was used to induce LTP. Bath application of mecamylamine (3 microM) did not alter input-output relationships or paired pulse depression in either mouse strain. Under control conditions, apoE4-TR mice showed significantly less LTP than wt mice (17.5% +/- 3.2%, n = 9, vs. 30.1% +/- 3.9%, n = 11, P < 0.02). Mecamylamine reduced LTP in wt mice to a level that was similar to control levels for apoE4-TR mice (15.7% +/- 3.4%, n = 9), whereas apoE4-TR showed no further reduction of LTP (16.6% +/- 3.7%, n = 8) by mecamylamine. Thus mice expressing human apoE4 differ from wt mice both in their capacity for LTP and in the effect on LTP of nicotinic cholinergic blockade. It is possible that nicotinic neurotransmission is already compromised in apoE4-TR mice and, hence, that interference with the integrity of this cholinergic system represents a mechanism by which inheritance of the apoE4 allele contributes to cognitive risk.

AMPA and NMDA receptor-mediated currents in developing dentate gyrus granule cells.

Granule cells (GCs) of the hippocampal dentate gyrus (DG) undergo postnatal neurogenesis such that cells at different maturational stages can be studied within an anatomically restricted region and a narrow animal age epoch. Using whole cell patch clamp recordings in hippocampal slices, we have previously found that input resistance (IR) correlates inversely with morphometric indicators of GC maturity. Using IR as an index of maturity we measured developmental changes in synaptic currents mediated by N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in GCs from 5- to 12-day rats. Peak NMDA and AMPA EPSC amplitudes increased, and the NMDA/AMPA ratio reversed with advancing cell age. NMDA EPSCs showed a maturational decrease in rise time but no change in decay time, whereas AMPA EPSCs showed neither rise nor decay time changes with development. Ifenprodil, a high affinity selective inhibitor of NR1/NR2B diheteromeric NMDA receptors, blocked approximately 50% of the peak amplitude of evoked NMDA EPSCs in all tested GCs regardless of their maturity and did not affect the measured kinetic properties. These data suggest that development of glutamatergic synapses follows distinct schedules. AMPA receptors possessed mature kinetics and became the dominant glutamatergic current within the age epoch studied, whereas NMDA receptors showed maturational changes in rise times but not decay kinetics. The reported modifications of EPSC properties are consistent with changes in receptor and synapse number, and relative quantities of AMPA and NMDA receptors. Changes in the subunit composition that determines NMDA decay kinetics may occur beyond the early neonatal period.

ApoE isoform-specific effects on LTP: blockade by oligomeric amyloid-beta1-42.

Amyloid-beta1-42 (Abeta1-42) is crucial to Alzheimer disease (AD) pathogenesis but the conformation of the toxic Abeta species remains uncertain. AD risk is increased by apolipoprotein E4 (apoE4) and decreased by apoE2 compared with the apoE3 isoform, but whether inheritance of apoE4 represents a gain of negative or a loss of protective function is also unresolved. Using hippocampal slices from apoE knockout (apoE-KO) and human apoE2, E3, and E4 targeted replacement (apoE-TR) mice, we found that oligomeric Abeta1-42 inhibited long-term potentiation (LTP) with a hierarchy of susceptibility mirroring clinical AD risk (apoE4-TR > apoE3-TR = apoE-KO > apoE2-TR), and that comparable doses of unaggregated Abeta1-42 did not affect LTP. These data provide a novel link among apoE isoform, Abeta1-42, and a functional cellular model of memory. In this model, apoE4 confers a gain of negative function synergistic with Abeta1-42, apoE2 is protective, and the apoE-Abeta interaction is specific to oligomeric Abeta1-42.

Larger deficits in brain networks for response inhibition than for visual selective attention in attention deficit hyperactivity disorder (ADHD).

BACKGROUND: Brain activation differences between 12 control and 12 attention deficit hyperactivity disorder (ADHD) children (9- to 12-year-olds) were examined on two cognitive tasks during functional magnetic resonance imaging (fMRI). METHOD: Visual selective attention was measured with the visual search of a conjunction target (red triangle) in a field of distracters and response inhibition was measured with a go/no-go task. RESULTS: There were limited group differences in the selective attention task, with control children showing significantly greater intensity of activation in a small area of the superior parietal lobule region of interest. There were large group differences in the response inhibition task, with control children showing significantly greater intensity of activation in fronto-striatal regions of interest including the inferior, middle, superior and medial frontal gyri as well as the caudate nucleus and globus pallidus. CONCLUSION: The widespread hypoactivity for the ADHD children on the go/no-go task is consistent with the hypothesis that response inhibition is a specific deficit in attention deficit hyperactivity disorder.

ApoE isoform affects LTP in human targeted replacement mice.

Inheritance of the epsilon4 allele for apolipoprotein E (apoE) increases the risk of Alzheimer disease and memory impairment, whereas epsilon2 decreases these risks compared with the most common epsilon3 allele, but the mechanism for these effects is unknown. Long-term potentiation (LTP) is an experimentally induced increase in synaptic efficacy that models memory. Using hippocampal slices from wild type (WT), apoE knockout (apoE-KO), and targeted replacement mice expressing human apoE2, E3, or E4 (apoE-TR) we found that although all strains had comparable basal synaptic transmission, LTP was significantly greater in WT and apoE3-TR than in apoE-KO, apoE2-TR or apoE4-TR. This novel system may be used to investigate the mechanisms of apoE isoform dependent modulation of susceptibility to memory impairment.

Brain-behavior correlation in children depends on the neurocognitive network.

We examined brain-behavior correlations in 12 children (age range 9.3 to 11.7 years) during a selective attention task that required the visual search of a conjunction of features and during a response inhibition task that required the inhibition of a pre-potent response during "no-go" blocks. We found that the association between performance in these tasks and brain activation as measured by functional magnetic resonance imaging (fMRI) depended on the neurocognitive network. Specifically, better performance during the no-go task was associated with greater activation in the response inhibition network including the prefrontal cortex and basal ganglia. In contrast, better performance during the visual search task was associated with less activation in the selective attention network including superior parietal lobule and lateral premotor cortex. These results show that the relation of performance to the magnitude of neural activation is complex and may display differential relationships based on the cognitive domain, anatomical region, and perhaps also developmental stage.

Neural development of selective attention and response inhibition.

Brain activation differences between 12 children (9- to 12-year-olds) and 12 adults (20- to 30-year-olds) were examined on two cognitive tasks during functional magnetic resonance imaging (fMRI). Spatial selective attention was measured with the visual search for a conjunction target (red triangle) in a field of distracters and response inhibition was measured with a go no-go task. There were small developmental differences in the selective attention task, with children showing greater activation than adults in the anterior cingulate and thalamus. There were large developmental differences in the response inhibition task, with children showing greater activation than adults in a fronto-striatal network including middle cingulate, medial frontal gyrus, medial aspects of bilateral superior frontal gyrus, and the caudate nucleus on the left. Children also showed greater bilateral activation for the response inhibition task in posterior cingulate, thalamus and the hippocampo-amygdaloid region. The extensive developmental differences on the response inhibition task are consistent with the prolonged maturation of the fronto-striatal network.

Soluble oligomers of beta amyloid (1-42) inhibit long-term potentiation but not long-term depression in rat dentate gyrus.

The dementia in Alzheimer disease (AD) is usually attributed to widespread neuronal loss in conjunction with the pathologic hallmarks of intracellular neurofibrillary tangles and extracellular plaques containing amyloid (A beta) in fibrillar form. Recently it has been demonstrated that non-fibrillar assemblies of A beta possess electrophysiologic activity, with the corollary that they may produce dementia by disrupting neuronal signaling prior to cell death. We therefore examined the effects of soluble oligomers of A beta(1-42) on long-term potentiation (LTP) and long-term depression (LTD), two cellular models of memory, in the dentate gyrus of rat hippocampal slices. Compared with vehicle controls, slices pre-incubated 60 min in the presence of A beta-derived diffusible ligands (ADDLs) showed no differences in threshold intensity to evoke a synaptic response, slope of field excitatory post-synaptic potentials (EPSPs), or the input/output function. Tetanus-induced LTP and reversal of LTD were strongly inhibited in ADDLs-treated slices whereas LTD was unaffected. These data suggest that soluble non-fibrillar amyloid may contribute to the pathogenesis of AD both by impairing LTP/memory formation at the cellular level and by creating 'neuroplasticity imbalance' manifested by unopposed LTD in the setting of impaired capacity for neural repair via reversal of LTD or LTP.