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The neurodegeneration observed in Alzheimer's disease has been associated with synaptic dismantling and progressive decrease in neuronal activity. We tested this hypothesis in vivo by using two-photon Ca2+ imaging in a mouse model of Alzheimer's disease. Although a decrease in neuronal activity was seen in 29% of layer 2/3 cortical neurons, 21% of neurons displayed an unexpected increase in the frequency of spontaneous Ca2+ transients. These "hyperactive" neurons were found exclusively near the plaques of amyloid beta-depositing mice. The hyperactivity appeared to be due to a relative decrease in synaptic inhibition. Thus, we suggest that a redistribution of synaptic drive between silent and hyperactive neurons, rather than an overall decrease in synaptic activity, provides a mechanism for the disturbed cortical function in Alzheimer's disease.

Calcium ions generate versatile intracellular signals that control key functions in all types of neurons. Imaging calcium in neurons is particularly important because calcium signals exert their highly specific functions in well-defined cellular subcompartments. In this Primer, we briefly review the general mechanisms of neuronal calcium signaling. We then introduce the calcium imaging devices, including confocal and two-photon microscopy as well as miniaturized devices that are used in freely moving animals. We provide an overview of the classical chemical fluorescent calcium indicators and of the protein-based genetically encoded calcium indicators. Using application examples, we introduce new developments in the field, such as calcium imaging in awake, behaving animals and the use of calcium imaging for mapping single spine sensory inputs in cortical neurons in vivo. We conclude by providing an outlook on the prospects of calcium imaging for the analysis of neuronal signaling and plasticity in various animal models.

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Abstract.(1) A preparation is described which allows patch clamp recordings to be made on mammalian central nervous system (CNS) neurones in situ. (2) A vibrating tissue slicer was used to cut thin slices in which individual neurones could be identified visually. Localized cleaning of cell somata with physiological saline freed the cell membrane, allowing the formation of a high resistance seal between the membrane and the patch pipette. (3) The various configurations of the patch clamp technique were used to demonstrate recording of membrane potential, whole cell currents and single channel currents from neurones and isolated patches. (4) The patch clamp technique was used to record from neurones filled with fluorescent dyes. Staining was achieved by filling cells during recording or by previous retrograde labelling. (5) Thin slice cleaning and patch clamp techniques were shown to be applicable to the spinal cord and almost any brain region and to various species. These techniques are also applicable to animals of a wide variety of postnatal ages, from newborn to adult.

Alzheimer's disease (AD) is characterized by a progressive dysfunction of central neurons. Recent experimental evidence indicates that in the cortex, in addition to the silencing of a fraction of neurons, other neurons are hyperactive in amyloid-β (Aβ) plaqueenriched regions. However, it has remained unknown what comes first, neuronal silencing or hyperactivity, and what mechanisms might underlie the primary neuronal dysfunction. Here we examine the activity patterns of hippocampal CA1 neurons in a mouse model of AD in vivo using two-photon Ca 2+ imaging. We found that neuronal activity in the plaque-bearing CA1 region of older mice is profoundly altered. There was a marked increase in the fractions of both silent and hyperactive neurons, as previously also found in the cortex. Remarkably, in the hippocampus of young mice, we observed a selective increase in hyperactive neurons already before the formation of plaques, suggesting that soluble species of Aβ may underlie this impairment. Indeed, we found that acute treatment with the γ-secretase inhibitor LY-411575 reduces soluble Aβ levels and rescues the neuronal dysfunction. Furthermore, we demonstrate that direct application of soluble Aβ can induce neuronal hyperactivity in wild-type mice. Thus, our study identifies hippocampal hyperactivity as a very early functional impairment in AD transgenic mice and provides direct evidence that soluble Aβ is crucial for hippocampal hyperactivity.brain disease | in vivo imaging A lzheimer's disease (AD) is associated with multiple neuronal dysfunctions, including impairments underlying the storage and processing of information in the brain (1). One of the major functional defects in AD is a massive decrease in neuronal activity (2, 3). This generalized silencing of brain circuits generated the synaptic failure hypothesis (4). Interestingly, more recent studies revealed a more complex picture of the neuronal defects in AD, demonstrating a mix of both hypoactivity and hyperactivity in various brain regions. For example, in transgenic mice overexpressing both mutant human amyloid precursor protein (APP) and mutant human presenilin 1 (PS1), half of the neurons in layer 2/3 of the cortex were functionally impaired, with a decrease in neuronal activity in 29% of the neurons (termed "silent" neurons) and a profound increase in more than 20% of neurons (termed "hyperactive" neurons) (5). Interestingly, the increase in hyperactive neurons was most prominent in the vicinity of plaques. Such alterations of cortical activity were not observed in predepositing transgenic mice or in wild-type mice, indicating that the changes in neuronal activity were temporally correlated with the histological pathology. Consistently, resting Ca 2+ levels in cortical dendrites of APP/PS1 transgenic mice were substantially increased in the area surrounding plaques (6). Furthermore, APP transgenic mice exhibited nonconvulsive seizure activity in cortex and hippocampus, which was associated with GABAergic sprouting, enhanced synaptic inhibition, and syna...

Long-term potentiation (LTP) is a cellular mechanism that potentially underlies learning and memory. To test the hypothesis that LTP is involved in activity-dependent synapse formation, we combined whole-cell recordings and confocal microscopy to investigate hippocampal glutamatergic synapses at their earliest stages of development. Here we report that, during the first postnatal week, the hippocampal glutamatergic network becomes gradually functional owing to the transformation of precursor, pure NMDA (N-methyl-D-aspartate)-receptor-based synaptic contacts into conducting AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate)/NMDA-re cep tor-type synapses. This functional synapse induction is caused by an associative form of LTP, so it is input-specific and easily triggered experimentally by pairing presynaptic stimulation with postsynaptic depolarization. Our results challenge previous views that LTP occurs in the hippocampus only at later stages of development and that its induction requires dendritic spines. They also provide direct evidence that LTP is important for the activity-dependent formation of conducting glutamatergic synapses in the developing mammalian brain.

Hyperpolarization-activated cation (HCN) channels are believed to be involved in the generation of cardiac pacemaker depolarizations as well as in the control of neuronal excitability and plasticity. The contributions of the four individual HCN channel isoforms (HCN1±4) to these diverse functions are not known. Here we show that HCN2-de®cient mice exhibit spontaneous absence seizures. The thalamocortical relay neurons of these mice displayed a near complete loss of the HCN current, resulting in a pronounced hyperpolarizing shift of the resting membrane potential, an altered response to depolarizing inputs and an increased susceptibility for oscillations. HCN2-null mice also displayed cardiac sinus dysrhythmia, a reduction of the sinoatrial HCN current and a shift of the maximum diastolic potential to hyperpolarized values. Mice with cardiomyocytespeci®c deletion of HCN2 displayed the same dysrhythmia as mice lacking HCN2 globally, indicating that the dysrhythmia is indeed caused by sinoatrial dysfunction. Our results de®ne the physiological role of the HCN2 subunit as a major determinant of membrane resting potential that is required for regular cardiac and neuronal rhythmicity.