The neuropathological correlates of Alzheimer's disease (AD) include amyloid-beta (Abeta) plaques and neurofibrillary tangles. To study the interaction between Abeta and tau and their effect on synaptic function, we derived a triple-transgenic model (3xTg-AD) harboring PS1(M146V), APP(Swe), and tau(P301L) transgenes. Rather than crossing independent lines, we microinjected two transgenes into single-cell embryos from homozygous PS1(M146V) knockin mice, generating mice with the same genetic background. 3xTg-AD mice progressively develop plaques and tangles. Synaptic dysfunction, including LTP deficits, manifests in an age-related manner, but before plaque and tangle pathology. Deficits in long-term synaptic plasticity correlate with the accumulation of intraneuronal Abeta. These studies suggest a novel pathogenic role for intraneuronal Abeta with regards to synaptic plasticity. The recapitulation of salient features of AD in these mice clarifies the relationships between Abeta, synaptic dysfunction, and tangles and provides a valuable model for evaluating potential AD therapeutics as the impact on both lesions can be assessed.

In the mammalian neocortex, the EEG reflects the state of behavioral arousal. The EEG undergoes a transformation, known as activation, during the transition from sleep to waking. Abundant evidence indicates the involvement of the neurotransmitter acetylcholine (ACh) in EEG activation; however, the cellular basis of this involvement remains unclear. We have used electrophysiological techniques with in vivo and in vitro preparations to demonstrate actions of endogenous ACh on neurons in auditory neocortex. In vivo stimulation of the nucleus basalis (NB), a primary source of neocortical ACh, (1) elicited EEG activation via cortical muscarinic receptors, (2) depolarized cortical neurons, and (3) produced a change in subthreshold membrane potential fluctuations from large-amplitude, slow (1–5 Hz) oscillations to low- amplitude, fast (20–40 Hz) oscillations. The NB-mediated change in pattern of membrane potential fluctuations resulted in a shift of spike discharge pattern from phasic to tonic. Stimulation of afferents in the in vitro neocortex elicited cholinergic actions on putative layer 5 pyramidal neurons. Acting via muscarinic receptors, endogenous ACh (1) reduced slow, rhythmic burst discharge and facilitated higher- frequency, single-spike discharge in burst-generating neurons, and (2) facilitated the appearance and magnitude of intrinsic membrane potential oscillations. These in vivo and in vitro observations suggest that neocortical activation results from muscarinic modulation of intrinsic neural oscillations and firing modes. Rhythmic-bursting pyramidal neurons in layer 5 may act as cortical pacemakers; if so, then modifying their discharge characteristics could alter local cortical networks. Larger, intercortical networks could also be modified, due to the widespread projections of NB neurons. Thus, NB cholinergic neurons may play a critical role in producing different states of neocortical function.

To facilitate an understanding of auditory thalamocortical mechanisms, we have developed a mouse brain-slice preparation with a functional connection between the ventral division of the medial geniculate (MGv) and the primary auditory cortex (ACx). Here we present the basic characteristics of the slice in terms of physiology (intracellular and extracellular recordings, including current source density analysis), pharmacology (including glutamate receptor involvement), and anatomy (gross anatomy, Nissl, parvalbumin immunocytochemistry, and tract tracing with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate). Thalamocortical transmission in this preparation (the "primary" slice) involves both alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid/kainate and N-methyl-D-aspartate-type glutamate receptors that appear to mediate monosynaptic inputs to layers 3-4 of ACx. MGv stimulation also initiates disynaptic inhibitory postsynaptic potentials and longer-duration intracortical, polysynaptic activity. Important differences between responses elicited by MGv versus conventional columnar ("on-beam") stimulation emphasize the necessity of thalamic activation to infer thalamocortical mechanisms. We also introduce a second slice preparation, the "shell" slice, obtained from the brain region immediately ventral to the primary slice, that may contain a nonprimary thalamocortical pathway to temporal cortex. In the shell slice, stimulation of the thalamus or the region immediately ventral to it appears to produce fast activation of synapses in cortical layer 1 followed by robust intracortical polysynaptic activity. The layer 1 responses may result from orthodromic activation of nonprimary thalamocortical pathways; however, a plausible alternative could involve antidromic activation of corticotectal neurons and their layer 1 collaterals. The primary and shell slices will provide useful tools to investigate mechanisms of information processing in the ACx.

To examine the basis of frequency receptive fields in auditory cortex (ACx), we have recorded intracellular (whole cell) and extracellular (local field potential, LFP) responses to tones in anesthetized rats. Frequency receptive fields derived from excitatory postsynaptic potentials (EPSPs) and LFPs from the same location resembled each other in terms of characteristic frequency (CF) and breadth of tuning, suggesting that LFPs reflect local synaptic (including subthreshold) activity. Subthreshold EPSP and LFP receptive fields were remarkably broad, often spanning five octaves (the maximum tested) at moderate intensities (40-50 dB above threshold). To identify receptive-field features that are generated intracortically, we microinjected the GABA(A) receptor agonist muscimol (0.2-5.1 mM, 1-5 microl) into ACx. Muscimol dramatically reduced LFP amplitude and reduced receptive-field bandwidth, implicating intracortical contributions to these features but had lesser effects on CF response threshold or onset latency, suggesting minimal loss of thalamocortical input. Reversal of muscimol's inhibition preferentially at the recording site by diffusion from the recording pipette of the GABA(A) receptor antagonist picrotoxin (0.01-100 microM) disinhibited responses to CF stimuli more than responses to spectrally distant, non-CF stimuli. We propose that thalamocortical and intracortical pathways preferentially contribute to responses evoked by CF and non-CF stimuli, respectively, and that intracortical projections linking frequency representations determine the breadth of receptive fields in primary ACx. Broad, subthreshold receptive fields may distinguish ACx from subcortical auditory relay nuclei, promote integrated responses to spectrotemporally complex stimuli, and provide a substrate for plasticity of cortical receptive fields and maps.

Soluble amyloid ␤ oligomers (A␤Os) interfere with synaptic function and bind with high affinity to synapses, but the mechanism underlying A␤O synaptic targeting is not known. Here, we show that the accumulation of synthetic or native Alzheimer's disease (AD)-brain oligomers at synapses is regulated by synaptic activity. Electrical or chemical stimulation increased A␤O synaptic localization and enhanced oligomer formation at synaptic terminals, whereas inhibition with TTX blocked A␤O synaptic localization and reduced A␤O synaptic load. The zinc-binding 8-OH-quinoline clioquinol markedly reduced A␤O synaptic targeting, which was also reduced in brain sections of animals deficient in the synaptic vesicle zinc transporter ZnT3, indicating that vesicular zinc released during neurotransmission is critical for A␤O synaptic targeting. Oligomers were not internalized in recycled vesicles but remained at the cell surface, where they colocalized with NR2B NMDA receptor subunits. Furthermore, NMDA antagonists blocked A␤O synaptic targeting, implicating excitatory receptor activity in oligomer formation and accumulation at synapses. In AD brains, oligomers of different size colocalized with synaptic markers in hippocampus and cortex, where oligomer synaptic accumulation correlated with synaptic loss.

Extracellular recordings of 209 neurons were obtained with carbon fiber-containing multibarrel micropipettes. The cells were isolated in the primary somatosensory cortex of cats anesthetized with barbiturate and classified according to the nature of their response to natural stimuli, the nature of the surrounding multiunit responses to the same stimuli, the response to thalamic stimulation, and their depth in the cortex. To study factors controlling the excitability of somatosensory neurons, their receptive fields were examined in the presence of iontophoretically administered gamma-aminobutyric acid (GABA), glutamate, and bicuculline methiodide (BMI). Even when the neurons were depolarized to perithreshold levels with glutamate, or when local inhibitory influences mediated by GABA were antagonized by BMI, the apparent specificity for one class of afferent input was maintained. Neurons responding to stimulation of either cutaneous or deep receptors maintained their modality specificity, and neurons in cutaneous rapidly adapting regions never took on slowly adapting properties. When ejected at currents that did not elicit action potentials, glutamate lowered the threshold for activation by cutaneous stimuli but did not enlarge the receptive field. With larger ejecting currents, the neurons developed an on-going discharge, but even at these higher doses, glutamate did not produce an increase in the receptive-field size. Some neurons in regions of cortex exhibiting slowly adapting multiunit responses were relatively insensitive to glutamate. These cells required four to five times more glutamate to evoke discharges than did most neurons. Other cells, previously unresponsive to somatic stimuli, could be shown to possess distinct cutaneous receptive fields when either glutamate or BMI was ejected in their vicinity. Iontophoretically administered BMI altered the firing pattern of somatosensory neurons, causing them to discharge in bursts of 3-15 impulses. BMI enlarged the receptive-field size of neurons in regions displaying rapidly adapting multiunit background discharges but not in those regions with slowly adapting multiunit discharges. This differential effect of BMI, suggesting that GABA controls receptive-field size in rapidly adapting regions, also indicates that neurons in rapidly adapting regions differ pharmacologically from those in other submodality regions. In all cortical regions, BMI blocked the poststimulus inhibitory period that normally followed thalamic stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

The neurotransmitters acetylcholine (ACh) and glutamate have been separately implicated in synaptic plasticity during development of sensory neocortex. Here we show that these neurotransmitters can, in fact, act synergistically via their actions at nicotinic ACh receptors (nAChRs) and NMDA receptors, respectively. To determine how activation of nAChRs modifies glutamatergic EPSPs, we made whole-cell recordings from visualized pyramidal neurons in slices of rat auditory cortex. Pulsed (pressure) ejection of nicotine onto apical dendrites selectively enhanced EPSPs mediated by NMDA receptors without affecting AMPA/kainate (AMPA/KA) receptor-mediated EPSPs. The enhancement occurred during a transient, postnatal period of heightened cholinergic function [neurons tested on postnatal day 8-16 (P8-16)], and not in the mature cortex (>P19). Three related findings indicated the mechanism of action: (1) The specific alpha7 nAChR antagonist methyllycaconitine citrate (MLA) blocked the effect of nicotine; (2) pulsed nicotine did not enhance postsynaptic depolarizations induced by iontophoretically applied NMDA; and (3) bath exposure to nicotine for several minutes produced apparent nAChR desensitization and precluded enhancement of EPSPs by pulsed nicotine. Together, the data suggest that nicotine acts at rapidly desensitizing, presynaptic alpha7 nAChRs to increase glutamate release onto postsynaptic NMDA receptors. The synergistic actions mediated by alpha7 nAChRs and NMDA receptors may contribute to experience-dependent synaptic plasticity in sensory neocortex during early postnatal life.

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