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SUMMARYThe medial prefrontal cortex and amygdala are involved in the regulation of social behavior and associated with psychiatric diseases but their detailed neurophysiological mechanisms at a network level remain unclear. We recorded local field potentials (LFPs) from the dorsal medial PFC (dmPFC) and basolateral amygdala (BLA) while mice engaged on social behavior. We found that in wild-type mice, both the dmPFC and BLA increased 4–7 Hz oscillation power and decreased 30–60 Hz power when they needed to attend to another target mouse. In mouse models with reduced social interactions, dmPFC 4–7 Hz power further increased especially when they exhibited social avoidance behavior. In contrast, dmPFC and BLA decreased 4–7 Hz power when wild-type mice socially approached a target mouse. Frequency-specific optogenetic manipulations of replicating social approach-related LFP patterns restored social interaction behavior in socially deficient mice. These results demonstrate a neurophysiological substrate of the prefrontal cortex and amygdala related to social behavior and provide a unified pathophysiological understanding of neuronal population dynamics underlying social behavioral deficits.

The ventromedial prefrontal cortex (vmPFC) plays key roles in higher cognitive abilities, including mental representations and the regulation of emotion. Previous studies have reported that vmPFC activity is altered in depressed human patients, highlighting this subregion as a major site of dysfunction in neuropsychiatric diseases. To examine how neuronal activity at spike levels in the vmPFC is altered by social defeat stress, we performed electrophysiological multiunit recordings along the dorsoventral axis of the mPFC of freely moving mice. Chronic social defeat stress-susceptible mice showing an impairment in social interaction exhibited significant reductions in the overall spike frequencies of neurons in the vmPFC, but not in the dorsal mPFC. Analysis of local field potentials revealed that the vmPFC generated spatially constrained 20–40 Hz events lasting hundreds of milliseconds, with an average event frequency of 0.05 Hz; during these events, a subset of neurons were transiently inhibited. The frequency of 20–40 Hz events in the vmPFC was reduced in defeated stress-susceptible animals, and this decrease was reversed by systemic ketamine administration. The novel neurophysiological correlates of stress-induced changes in the vmPFC advance the understanding of the neural basis of stress-induced dysregulation of social behavior.

The composition of cellulosomal carbohydrate-active enzymes (CAZymes) secreted from a cellulolytic bacterium Clostridium thermocellum varies depending on the cellulosic substrate used during cultivation. C. thermocellum detects the polysaccharides in cellulosic material via anti-sigma factors and expresses the appropriate CAZyme gene via alternative sigma factors, SigIs. Previous studies on the regulation of CAZyme gene expression via SigIs in C. thermocellum have been conducted in vitro or in a heterologous host, because of the limited genetic tools available for C. thermocellum. To characterize the in vivo function of SigIs, in the present study, we established a sigI7 gene expression strain of C. thermocellum. Transcriptome analysis of this strain revealed that SigI7 induced the expression of cellulosomal CAZyme genes and cellulosomal scaffold genes. However, there was a decrease in the degradation ability of the exoproteome from the sigI7 expression strain; the product of the downregulated gene, Clo1313_1002, rescued the activity of the C. thermocellum exoproteome from the sigI7 expression strain. In this study, we demonstrate the in vivo function of SigI7 and discuss the CAZymes that are important for cellulosic biomass degradation by C. thermocellum.

Neuronal activity is highly sensitive to changes in oxygen tension. In this study, we examined the impact of hypoxic/ischemic conditions on neuronal ensemble activity patterns in the mouse brain using in vivo extracellular electrophysiological recordings from up to 8 sites in the thalamus, dorsal hippocampus, and neocortex, while cerebral hypoperfusion was induced by unilateral carotid artery occlusion. After a few minutes, the occlusion triggered a rapid change in the power of the local field oscillations. In the hippocampus, but not in the neocortex, the absolute power changes at all frequency ranges (relative to the baseline) became less pronounced with time, and no significant changes were observed 30min after the occlusion-induced hypoperfusion. We also tested whether continuous hypoperfusion induced by the occlusion for up to 1 week alters neuronal activity. In the hippocampus and the thalamus, the chronic occlusion did not lead to a reduction in the power of the local field oscillations. These results indicate that certain neuronal populations have the ability to maintain internal neurophysiological homeostasis against continuous hypoperfusion.

The mechanism of response of hippocampal neurons to a specific feature in sensory stimuli is not fully understood, although the hippocampus is well known to contribute to the formation of episodic memory in the multisensory world. Using in-vivo voltage-clamp recordings from awake mice, we found that sound pulses induced a transient increase in inhibitory, but not excitatory, conductance in hippocampal CA1 pyramidal cells. In local field potentials, sound pulses induced a phase resetting of the θ oscillations, one of the major oscillatory states of the hippocampus. Repetitive sound pulses at 7 Hz (θ rhythm) increased the θ oscillation power, an effect that was abolished by a surgical fimbria-fornix lesion. Thus, tone-induced inhibition is likely of subcortical origin. It may segment hippocampal neural processing and render temporal boundaries in continuously ongoing experiences.

Spontaneous neuronal activity is present in virtually all brain regions, but neither its function nor spatiotemporal patterns are fully understood. Ex vivo organotypic slice cultures may offer an opportunity to investigate some aspects of spontaneous activity, because they self-restore their networks that collapsed during slicing procedures. In hippocampal networks, we compared the levels and patterns of in vivo spontaneous activity to those in acute and cultured slices. We found that the firing rates and excitatory synaptic activity in the in vivo hippocampus are more similar to those in slice cultures compared to acute slices. The soft confidence-weighted algorithm, a machine learning technique without human bias, also revealed that hippocampal slice cultures resemble the in vivo hippocampus in terms of the overall tendency of the parameters of spontaneous activity.

Astrocytes are thought to detect neuronal activity in the form of intracellular calcium elevations; thereby, astrocytes can regulate neuronal excitability and synaptic transmission. Little is known, however, about how the astrocyte calcium signal regulates the activity of neuronal populations. In this study, we addressed this issue using functional multineuron calcium imaging in hippocampal slice cultures. Under normal conditions, CA3 neuronal networks exhibited temporally correlated activity patterns, occasionally generating large synchronization among a subset of cells. The synchronized neuronal activity was correlated with astrocyte calcium events. Calcium buffering by an intracellular injection of a calcium chelator into multiple astrocytes reduced the synaptic strength of unitary transmission between pairs of surrounding pyramidal cells and caused desynchronization of the neuronal networks. Uncaging the calcium in the astrocytes increased the frequency of neuronal synchronization. These data suggest an essential role of the astrocyte calcium signal in the maintenance of basal neuronal function at the circuit level.

Sensory stimuli not only activate specific populations of cortical neurons but can also silence other populations. However, it remains unclear whether neuronal silencing per se leads to memory formation and behavioral expression. Here we show that mice can report optogenetic inactivation of auditory neuron ensembles by exhibiting fear responses or seeking a reward. Mice receiving pairings of footshock and silencing of a neuronal ensemble exhibited a fear response selectively to the subsequent silencing of the same ensemble. The valence of the neuronal silencing was preserved for at least 30 d and was susceptible to extinction training. When we silenced an ensemble in one side of auditory cortex for conditioning, silencing of an ensemble in another side induced no fear response. We also found that mice can find a reward based on the presence or absence of the silencing. Neuronal silencing was stored as working memory. Taken together, we propose that neuronal silencing without explicit activation in the cerebral cortex is enough to elicit a cognitive behavior.

The hippocampus is involved in episodic memory, which is composed of subjective experiences in the multisensory world; however, little is known about the subthreshold membrane potential responses of individual hippocampal neurons to sensory stimuli. Using in-vivo whole-cell patch-clamp recordings from hippocampal CA1 neurons in awake mice, we found that almost all hippocampal neurons exhibited a hyperpolarization of 1-2 mV immediately after the onset of a sound. This large-scale hyperpolarization was unaffected by the duration or pitch of the tone. The response was abolished by general anesthesia and a surgical fimbria-fornix lesion.