Faculty Bibliography

The specific connectivity among principal cells and interneurons determines the flow of activity in neuronal networks. To elucidate the connections between hippocampal principal cells and various classes of interneurons, CA3 pyramidal cells were intracellularly labelled with biocytin in anaesthetized rats and the three-dimensional distribution of their axon collaterals was reconstructed. The sections were double-stained for substance P receptor (SPR)- or metabotropic glutamate receptor 1alpha (mGluR-1alpha)-immunoreactivity to investigate interneuron targets of the CA3 pyramidal cells. SPR-containing interneurons represent a large portion of the GABAergic population, including spiny and aspiny classes. Axon terminals of CA3 pyramidal cells contacted SPR-positive interneuron dendrites in the hilus and in all hippocampal strata in both CA3 and CA1 regions (7.16% of all boutons). The majority of axons formed single contacts (87.5%), but multiple contacts (up to six) on single target neurons were also found. CA3 pyramidal cell axon collaterals innervated several types of morphologically different aspiny SPR-positive interneurons. In contrast, spiny SPR-interneurons or mGluR-1alpha-positive interneurons in the hilus, CA3 and CA1 regions were rarely contacted by the filled pyramidal cells. These findings indicate a strong target selection of CA3 pyramidal cells favouring the activation of aspiny classes of interneurons

Recent technological advances now allow for simultaneous recording of large populations of anatomically distributed neurons in behaving animals. The free software package described here was designed to help neurophysiologists process and view recorded data in an efficient and user-friendly manner. This package consists of several well-integrated applications, including NeuroScope (http://neuroscope.sourceforce.net), an advanced viewer for electrophysiological and behavioral data with limited editing capabilities, Klusters (http://klusters.sourceforge.net), a graphical cluster cutting application for manual and semi-automatic spike sorting, NDManager (GPL,see http://www.gnu.org/licenses/gpl.html), an experimental parameter and data processing manager. All of these programs are distributed under the GNU General Public License (GPL, see ), which gives its users legal permission to copy, distribute and/or modify the software. Also included are extensive user manuals and sample data, as well as source code and documentation

Both episodic memory and spatial navigation require temporal encoding of the relationships between events or locations. In a linear maze, ordered spatial distances between sequential locations were represented by the temporal relations of hippocampal place cell pairs within cycles of theta oscillation in a compressed manner. Such correlations could arise due to spike 'phase precession' of independent neurons driven by common theta pacemaker or as a result of temporal coordination among specific hippocampal cell assemblies. We found that temporal correlation between place cell pairs was stronger than predicted by a pacemaker drive of independent neurons, indicating a critical role for synaptic interactions and precise timing within and across cell assemblies in place sequence representation. CA1 and CA3 ensembles, identifying spatial locations, were active preferentially on opposite phases of theta cycles. These observations suggest that interleaving CA3 neuronal sequences bind CA1 assemblies representing overlapping past, present, and future locations into single episodes

Cannabinoids impair hippocampus-dependent memory in both humans and animals, but the network mechanisms responsible for this effect are unknown. Here we show that the cannabinoids Delta(9)-tetrahydrocannabinol and CP55940 decreased the power of theta, gamma and ripple oscillations in the hippocampus of head-restrained and freely moving rats. These effects were blocked by a CB1 antagonist. The decrease in theta power correlated with memory impairment in a hippocampus-dependent task. By simultaneously recording from large populations of single units, we found that CP55940 severely disrupted the temporal coordination of cell assemblies in short time windows (<100 ms) yet only marginally affected population firing rates of pyramidal cells and interneurons. The decreased power of local field potential oscillations correlated with reduced temporal synchrony but not with firing rate changes. We hypothesize that reduced spike timing coordination and the associated impairment of physiological oscillations are responsible for cannabinoid-induced memory deficits

Although extracellular unit recording is typically used for the detection of spike occurrences, it also has the theoretical ability to report about what are typically considered intracellular features of the action potential. We address this theoretical ability by developing a model system that captures features of experimentally recorded simultaneous intracellular and extracellular recordings of CA1 pyramidal neurons. We use the line source approximation method of Holt and Koch to model the extracellular action potential (EAP) voltage resulting from the spiking activity of individual neurons. We compare the simultaneous intracellular and extracellular recordings of CA1 pyramidal neurons recorded in vivo with model predictions for the same cells reconstructed and simulated with compartmental models. The model accurately reproduces both the waveform and the amplitude of the EAPs, although it was difficult to achieve simultaneous good matches on both the intracellular and extracellular waveforms. This suggests that accounting for the EAP waveform provides a considerable constraint on the overall model. The developed model explains how and why the waveform varies with electrode position relative to the recorded cell. Interestingly, each cell's dendritic morphology had very little impact on the EAP waveform. The model also demonstrates that the varied composition of ionic currents in different cells is reflected in the features of the EAP

Cytochrome c (CC) immunoreactivity was quantified in functionally distinct rat hippocampal inhibitory neuron populations using double immunocytochemistry and laser scanning confocal microscopy to measure the CC expression level as well as the amount of mitochondria within the cells, which is a sign of neuronal activity. The CC signal showed a similar distribution to cytochrome c oxidase histochemical staining. Strongly stained somata, dendrites and axon terminal clouds were dispersed over the low intensity neuropil staining. The staining was granular and electron microscopic investigation confirmed that the signal was localized in mitochondria. Intensively labeled neurons, showing the morphological features of inhibitory cells, were most frequently found in the principal cell layers, stratum oriens of the CA1-3 areas, stratum lucidum and hilus. These neurons contained not only a higher number of mitochondria than the principal cells but the intensity of the mitochondrial staining was evidently stronger. Among the examined interneuronal populations, parvalbumin-immunoreactive neurons were intensively labeled for CC. Calbindin D28k- (CB), somatostatin- and cholecystokinin-labeled cells showed heterogeneous CC levels, whereas calretinin-immunoreactive cells never showed a strong CC signal. CB cells in stratum oriens and alveus layers, lucidum and the hilus were strongly labeled for CC. CB cells in such regions are known to project to the medial septum and contain somatostatin. We have demonstrated that the CA1 interneurons that project to the medial septum (hippocampo-septal neurons) express a high level of CC. Thus, similar to the parvalbumin-containing basket and axo-axonic cells, the hippocampo-septal neurons potentially have a high average activity level

Brain systems communicate by means of neuronal oscillations at multiple temporal and spatial scales. In anesthetized rats, we find that neocortical 'slow' oscillation engages neurons in prefrontal, somatosensory, entorhinal, and subicular cortices into synchronous transitions between UP and DOWN states, with a corresponding bimodal distribution of their membrane potential. The membrane potential of hippocampal granule cells and CA3 and CA1 pyramidal cells lacked bimodality, yet it was influenced by the slow oscillation in a region-specific manner. Furthermore, in both anesthetized and naturally sleeping rats, the cortical UP states resulted in increased activity of dentate and most CA1 neurons, as well as the highest probability of ripple events. Yet, the CA3-CA1 network could self-organize into gamma bursts and occasional ripples during the DOWN state. Thus, neo/paleocortical and hippocampal networks periodically reset, self-organize, and temporally coordinate their cell assemblies via the slow oscillation

Two thin-film microelectrode arrays with integrated circuitry have been developed for extracellular neural recording in behaving animals. An eight-site probe for simultaneous neural recording and stimulation has been designed that includes on-chip amplifiers that can be individually bypassed, allowing direct access to the iridium sites for electrical stimulation. The on-probe amplifiers have a gain of 38.9 dB, an upper-cutoff frequency of 9.9 kHz, and an input-referred noise of 9.2 microV rms integrated from 100 Hz to 10 kHz. The low-frequency cutoff of the amplifier is tunable to allow the recording of field potentials and minimize stimulus artifact. The amplifier consumes 68 microW from +/- 1.5 V supplies and occupies 0.177 mm2 in 3 microm features. In vivo recordings have shown that the preamplifiers can record single-unit activity 1 ms after the onset of stimulation on sites as close as 20 microm to the stimulating electrode. A second neural recording array has been developed which multiplexes 32 neural signals onto four output data leads. Providing gain on this array eliminates the need for bulky headmounted circuitry and reduces motion artifacts. The time-division multiplexing circuitry has crosstalk between consecutive channels of less than 6% at a sample rate of 20 kHz per channel. Amplified, time-division-multiplexed multichannel neural recording allows the large-scale recording of neuronal activity in freely behaving small animals with minimum number of interconnect leads

Oscillatory spike timing in the hippocampus is regarded as a temporal coding mechanism for space, but the underlying mechanisms are poorly understood. To contrast the predictions of the different models of phase precession, we transiently turned off neuronal discharges for up to 250 ms and reset the phase of theta oscillations by stimulating the commissural pathway in rats. After recovery from silence, phase precession continued. The phase of spikes for the first theta cycle after the perturbation was more advanced than the phase of spikes for the last theta cycle just before the perturbation. These findings indicate that phase advancement that emerges within hippocampal circuitry may be updated at the beginning of each theta cycle by extrahippocampal inputs

Five key topics have been reverberating in hippocampal-entorhinal cortex (EC) research over the past five decades: episodic and semantic memory, path integration ('dead reckoning') and landmark ('map') navigation, and theta oscillation. We suggest that the systematic relations between single cell discharge and the activity of neuronal ensembles reflected in local field theta oscillations provide a useful insight into the relationship among these terms. In rats trained to run in direction-guided (1-dimensional) tasks, hippocampal cell assemblies discharge sequentially, with different assemblies active on opposite runs, i.e., place cells are unidirectional. Such tasks do not require map representation and are formally identical with learning sequentially occurring items in an episode. Hebbian plasticity, acting within the temporal window of the theta cycle, converts the travel distances into synaptic strengths between the sequentially activated and unidirectionally connected assemblies. In contrast, place representations by hippocampal neurons in 2-dimensional environments are typically omnidirectional, characteristic of a map. Generation of a map requires exploration, essentially a dead reckoning behavior. We suggest that omnidirectional navigation through the same places (junctions) during exploration gives rise to omnidirectional place cells and, consequently, maps free of temporal context. Analogously, multiple crossings of common junction(s) of episodes convert the common junction(s) into context-free or semantic memory. Theta oscillation can hence be conceived as the navigation rhythm through both physical and mnemonic space, facilitating the formation of maps and episodic/semantic memories