Connectional architecture of the prefrontal cortex in the marmoset brain

The primate prefrontal cortex (PFC) has greatly expanded to evolve specialized architecture, but its roles in top-down brain control remain enigmatic. Based on connectomics mapping of the marmoset PFC, we characterized two contrasting features of corticocortical and corticostriatal projections. One is the "focalness" of projections, exemplified by multiple columnar axonal terminations in the cortical layers and the other is the "widespreadness" of weaker projections, whose patterns consisted of several common motifs representing the framework of PFC connectivity. We clarified the topographic rules of distribution for these features, which should constrain how PFC neurons can coordinate to control the target regions as populations. These features are observed only primitively in rodents and are considered critical in understanding the roles of the PFC in neuropsychiatric disorders.

Changes in the spatiotemporal pattern of spontaneous activity across a cortical column after noise trauma

In the auditory modality, noise trauma has often been used to investigate cortical plasticity as it causes cochlear hearing loss. One limitation of these past studies, however, is that the effects of noise trauma have been mostly documented at the granular layer, which is the main cortical recipient of thalamic inputs. Importantly, the cortex is composed of six different layers each having its own pattern of connectivity and specific role in sensory processing. The present study aims at investigating the effects of acute and chronic noise trauma on the laminar pattern of spontaneous activity in primary auditory cortex of the anesthetized guinea pig. We show that spontaneous activity is dramatically altered across cortical layers after acute and chronic noise-induced hearing loss. First, spontaneous activity was globally enhanced across cortical layers, both in terms of firing rate and amplitude of spike-triggered average of local field potentials. Second, current source density on (spontaneous) spike-triggered average of local field potentials indicates that current sinks develop in the supra- and infragranular layers. These latter results suggest that supragranular layers become a major input recipient and that the propagation of spontaneous activity over a cortical column is greatly enhanced after acute and chronic noise-induced hearing loss. We discuss the possible mechanisms and functional implications of these changes.

Layer-Specific Intracortical Amplification Shortens the Lifetime of Thalamocortical Repetition Suppression in Auditory Cortex

Neural adaptation in sensory cortex serves important sensory functions, and is altered by various neurophsychiatric diseases. Although adaptation is a widely studied phenomenon, much remains unknown about its underlying mechanisms on a cortical circuit level. Here, we investigated repetition suppression as fundamental aspect of adaptation by layer-specific current source density analyses of synaptic mass activities in primary auditory cortex of anesthetized Mongolian gerbils (Meriones unguiculatus). We disentangled different synaptic contributions to repetition suppression in different cortical layers, and separated thalamocortical from intracortical inputs by cortical silencing with GABAA-agonist muscimol. We systematically varied stimulus onset intervals and employed statistically robust model fitting based on bootstrapping to determine the full suppression kinetics of different synaptic responses in the steady state. Whereas thalamocortical input to granular and infragranular layers was governed by longer lasting repetition suppression, most likely reflecting depression of thalamocortical synapses, intracortical amplification in granular layers shortened the lifetime of suppression by re-enhancing granular responses mainly through synchronization of synaptic events. With increasing latency, the shorter lasting suppression kinetics observed in granular layers at early latencies (<100ms) passed on to deeper layers replacing the longer lasting infragranular suppression kinetics. Granular circuit dynamics can therefore actively shape neural adaptation across cortical layers.

Double-μPeriscope, a tool for multilayer optical recordings, optogenetic stimulations or both

Intelligent behavior and cognitive functions in mammals depend on cortical microcircuits made up of a variety of excitatory and inhibitory cells that form a forest-like complex across six layers. Mechanistic understanding of cortical microcircuits requires both manipulation and monitoring of multiple layers and interactions between them. However, existing techniques are limited as to simultaneous monitoring and stimulation at different depths without damaging a large volume of cortical tissue. Here, we present a relatively simple and versatile method for delivering light to any two cortical layers simultaneously. The method uses a tiny optical probe consisting of two microprisms mounted on a single shaft. We demonstrate the versatility of the probe in three sets of experiments: first, two distinct cortical layers were optogenetically and independently manipulated; second, one layer was stimulated while the activity of another layer was monitored; third, the activity of thalamic axons distributed in two distinct cortical layers was simultaneously monitored in awake mice. Its simple-design, versatility, small-size, and low-cost allow the probe to be applied widely to address important biological questions.

SELENOT deficiency alters projection neuron migration during corticogenesis in mice

During corticogenesis, projection neurons migrate along the radial glial axis to form cortical layers, the alteration of which is associated with functional deficits in adulthood. As byproducts of cell metabolism, reactive oxygen species act as second messengers to contribute to neurodevelopment; however, free radical excess may impede this process. SELENOT is a thioredoxin-like enzyme of the endoplasmic reticulum abundantly expressed during embryogenesis whose gene disruption in the brain leads to neuroblast cell demise due to increased free radical levels. To determine the potential contribution of SELENOT to the establishment of cortical networks, we analyzed first its expression profile in the neocortex at different stages of development. These studies revealed the widespread expression of SELENOT in all cortical layers, and its continous increase throughout mouse lifespan. In addition, we disrupted the SELENOT gene in the cortex using in utero electroporation and Nes-Cre/lox knockout. SELENOT deficiency altered neuroblast migration polarity, at the level of radial scaffolding, and projection neuron positionning. These results indicate that SELENOT plays a crucial role during neurodevelopment by sustaining projection neuron migration.

Gliogenic Potential of Single Pallial Radial Glial Cells in Lower Cortical Layers

During embryonic development, progenitor cells are progressively restricted in their potential to generate different neural cells. A specific progenitor cell type, the radial glial cells, divides symmetrically and then asymmetrically to produce neurons, astrocytes, oligodendrocytes, and NG2-glia in the cerebral cortex. However, the potential of individual progenitors to form glial lineages remains poorly understood. To further investigate the cell progeny of single pallial GFAP-expressing progenitors, we used the in vivo genetic lineage-tracing method, the UbC-(GFAP-PB)-StarTrack. After targeting those progenitors in embryonic mice brains, we tracked their adult glial progeny in lower cortical layers. Clonal analyses revealed the presence of clones containing sibling cells of either a glial cell type (uniform clones) or two different glial cell types (mixed clones). Further, the clonal size and rostro-caudal cell dispersion of sibling cells differed depending on the cell type. We concluded that pallial E14 neural progenitors are a heterogeneous cell population with respect to which glial cell type they produce, as well as the clonal size of their cell progeny.

Brain state and cortical layer-specific mechanisms underlying perception at threshold

Identical stimuli can be perceived or go unnoticed across successive presentations, producing divergent behavioral readouts despite similarities in sensory input. We hypothesized that fluctuations in neurophysiological states in the sensory neocortex, which could alter cortical processing at the level of neural subpopulations, underlies this perceptual variability. We analyzed cortical layer-specific electrophysiological activity in visual area V4 during a cued attention task. We find that hit trials are characterized by a larger pupil diameter and lower incidence of microsaccades, indicative of a behavioral state with increased arousal and perceptual stability. Target stimuli presented at perceptual threshold evoke elevated multi-unit activity in V4 neurons in hit trials compared to miss trials, across all cortical layers. Putative excitatory and inhibitory neurons are strongly positively modulated in the input (IV) and deep (V & VI) layers of the cortex during hit trials. Excitatory neurons in the superficial cortical layers exhibit lower variability in hit trials. Deep layer neurons are less phase-locked to low frequency rhythms in hits. Hits are also characterized by greater interlaminar coherence between the superficial and deep layers in the pre-stimulus period, and a complementary pattern between the input layer and both the superficial and deep layers in the stimulus-evoked period. Taken together, these results indicate that a state of elevated levels of arousal and perceptual stability allow enhanced processing of sensory stimuli, which contributes to hits at perceptual threshold.

Audiovisual task switching rapidly modulates sound encoding in mouse auditory cortex

In everyday behavior, sensory systems are in constant competition for attentional resources, but the cellular and circuit-level mechanisms of modality-selective attention remain largely uninvestigated. We conducted translaminar recordings in mouse auditory cortex (AC) during an audiovisual (AV) attention shifting task. Attending to sound elements in an AV stream reduced both pre-stimulus and stimulus-evoked spiking activity, primarily in deep layer neurons. Despite reduced spiking, stimulus decoder accuracy was preserved, suggesting improved sound encoding efficiency. Similarly, task-irrelevant probe stimuli during intertrial intervals evoked fewer spikes without impairing stimulus encoding, indicating that these attention influences generalized beyond training stimuli. Importantly, these spiking reductions predicted trial-to-trial behavioral accuracy during auditory attention, but not visual attention. Together, these findings suggest auditory attention facilitates sound discrimination by filtering sound-irrelevant spiking in AC, and that the deepest cortical layers may serve as a hub for integrating extramodal contextual information.

Studying Synaptically Evoked Cortical Responses ex vivo With Combination of a Single Neuron Recording (Whole-Cell) and Population Voltage Imaging (Genetically Encoded Voltage Indicator)

In a typical electrophysiology experiment, synaptic stimulus is delivered in a cortical layer (1–6) and neuronal responses are recorded intracellularly in individual neurons. We recreated this standard electrophysiological paradigm in brain slices of mice expressing genetically encoded voltage indicators (GEVIs). This allowed us to monitor membrane voltages in the target pyramidal neurons (whole-cell), and population voltages in the surrounding neuropil (optical imaging), simultaneously. Pyramidal neurons have complex dendritic trees that span multiple cortical layers. GEVI imaging revealed areas of the brain slice that experienced the strongest depolarization on a specific synaptic stimulus (location and intensity), thus identifying cortical layers that contribute the most afferent activity to the recorded somatic voltage waveform. By combining whole-cell with GEVI imaging, we obtained a crude distribution of activated synaptic afferents in respect to the dendritic tree of a pyramidal cell. Synaptically evoked voltage waves propagating through the cortical neuropil (dendrites and axons) were not static but rather they changed on a millisecond scale. Voltage imaging can identify areas of brain slices in which the neuropil was in a sustained depolarization (plateau), long after the stimulus onset. Upon a barrage of synaptic inputs, a cortical pyramidal neuron experiences: (a) weak temporal summation of evoked voltage transients (EPSPs); and (b) afterhyperpolarization (intracellular recording), which are not represented in the GEVI population imaging signal (optical signal). To explain these findings [(a) and (b)], we used four voltage indicators (ArcLightD, chi-VSFP, Archon1, and di-4-ANEPPS) with different optical sensitivity, optical response speed, labeling strategy, and a target neuron type. All four imaging methods were used in an identical experimental paradigm: layer 1 (L1) synaptic stimulation, to allow direct comparisons. The population voltage signal showed paired-pulse facilitation, caused in part by additional recruitment of new neurons and dendrites. “Synaptic stimulation” delivered in L1 depolarizes almost an entire cortical column to some degree.