Dynamic aspects of interactions between astrocytes, neurons and the vasculature have recently been in the neuroscience spotlight. It has emerged that not only neurons but also astrocytes are organized into networks. Whereas neuronal networks exchange information through electrical and chemical synapses, astrocytes are interconnected through gap junction channels that are regulated by extra- and intracellular signals and allow exchange of information. This intercellular communication between glia has implications for neuroglial and gliovascular interactions and hence has added another level of complexity to our understanding of brain function.

SUMMARY High-frequency ripple oscillations, observed most prominently in the hippocampal CA1 pyramidal layer, are associated with memory consolidation. The cellular and network mechanisms underlying the generation, frequency control, and spatial coherence of the rhythm are poorly understood. Using multisite optogenetic manipulations in freely behaving rodents, we found that depolarization of a small group of nearby pyramidal cells was sufficient to induce high-frequency oscillations, whereas closed-loop silencing of pyramidal cells or activation of parvalbumin-(PV) or somatostatin-immunoreactive interneurons aborted spontaneously occurring ripples. Focal pharmacological blockade of GABAA receptors abolished ripples. Localized PV inter-neuron activation paced ensemble spiking, and simultaneous induction of high-frequency oscillations at multiple locations resulted in a temporally coherent pattern mediated by phase-locked inter-neuron spiking. These results constrain competing models of ripple generation and indicate that temporally precise local interactions between excitatory and inhibitory neurons support ripple generation in the intact hippocampus.

Both circuit and single-cell properties contribute to network rhythms. In vitro, pyramidal cells exhibit theta-band membrane potential (subthreshold) resonance, but whether and how sub-threshold resonance translates into spiking resonance in behaving animals is unknown. Here, we used optogenetic activation to trigger spiking in pyramidal cells or parvalbumin immunoreactive interneurons (PV) in the hippocampus and neocortex of freely-behaving rodents. Individual directly-activated pyramidal cells exhibited narrow-band spiking centered on a wide range of frequencies. In contrast, PV photoactivation indirectly induced theta band-limited, excess post-inhibitory spiking in pyramidal cells (resonance). PV-inhibited pyramidal cells and interneurons spiked at PV-inhibition troughs, similar to CA1 cells during spontaneous theta oscillations. Pharmacological blockade of hyperpolarization-activated (Ih) currents abolished theta resonance. Inhibition-induced theta-band spiking was replicated in a pyramidal cell-interneuron model that included Ih. Thus, PV interneurons mediate pyramidal cell spiking resonance in intact cortical networks, favoring transmission at theta frequency.

Monitoring representative fractions of neurons from multiple brain circuits in behaving animals is necessary for understanding neuronal computation. Here, we describe a system that allows high-channel-count recordings from a small volume of neuronal tissue using a lightweight signal multiplexing headstage that permits free behavior of small rodents. The system integrates multishank, high-density recording silicon probes, ultraflexible interconnects, and a miniaturized microdrive. These improvements allowed for simultaneous recordings of local field potentials and unit activity from hundreds of sites without confining free movements of the animal. The advantages of large-scale recordings are illustrated by determining the electroanatomic boundaries of layers and regions in the hippocampus and neocortex and constructing a circuit diagram of functional connections among neurons in real anatomic space. These methods will allow the investigation of circuit operations and behavior-dependent interregional interactions for testing hypotheses of neural networks and brain function.

The addition of the hepatitis delta virus genomic ribozyme to the 3' end sequence of a Sendai virus defective interfering RNA (DI-H4) allowed the reproducible and efficient replication of this RNA by the viral functions expressed from cloned genes when the DI RNA was synthesized from plasmid. Limited nucleotide additions or deletions (+7 to-7 nucleotides) in the DI RNA sequence were then made at five different sites, and the different RNA derivatives were tested for their abilities to replicate. Efficient replication was observed only when the total nucleotide number was conserved, regardless of the modifications, or when the addition of a total of 6 nucleotides was made. The replicated RNAs were shown to be properly enveloped into virus particles. It is concluded that, to form a proper template for efficient replication, the Sendai virus RNA must contain a total number of nucleotides which is a multiple of 6. This was interpreted as the need for the nucleocapsid protein to contact exactly 6 nucleotides.

We have recovered infectious Sendai virus (SeV) from full‐length cDNA (FL‐3) by transfecting this cDNA and pGEM plasmids expressing the nucleocapsid protein (NP), phosphoprotein and large proteins into cells infected with a vaccinia virus which expresses T7 RNA polymerase. These cells were then injected into chicken eggs, in which SeV grows to very high titers. FL‐3 was marked with a BglII site in the leader region and an NsiI site (ATGCAT) in the 5′ nontranslated region of the NP gene, creating a new, out‐of‐frame, 5′ proximal AUG. All the virus stocks generated eventually removed this impediment to NP expression, by either point mutation or recombination between FL‐3 and pGEM‐NP. The recovery system was found to be highly recombinogenic. Even in the absence of selective pressure, one in 20 of the recombinant SeV generated had exchanged the NP gene of FL‐3 with that of pGEM‐NP. When a fifth plasmid containing a new genomic 3′ end without the presumably deleterious BglII site was included as another target for recombination, the new genomic 3′ end was found in the recombinant SeV in 12 out of 12 recoveries. Using this approach, a novel copy‐back nondefective virus was generated which interferes with wild‐type virus replication.

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High-frequency ripple oscillations, observed most prominently in the hippocampal CA1 pyramidal layer, are associated with memory consolidation. The cellular and network mechanisms underlying the generation of the rhythm and the recruitment of spikes from pyramidal neurons are still poorly understood. Using intracellular, sharp electrode recordings in freely moving, drug-free mice, we observed consistent large depolarizations in CA1 pyramidal cells during sharp wave ripples, which are associated with ripple frequency fluctuation of the membrane potential ("intracellular ripple"). Despite consistent depolarization, often exceeding pre-ripple spike threshold values, current pulse-induced spikes were strongly suppressed, indicating that spiking was under the control of concurrent shunting inhibition. Ripple events were followed by a prominent afterhyperpolarization and spike suppression. Action potentials during and outside ripples were orthodromic, arguing against ectopic spike generation, which has been postulated by computational models of ripple generation. These findings indicate that dendritic excitation of pyramidal neurons during ripples is countered by shunting of the membrane and postripple silence is mediated by hyperpolarizing inhibition.