High Stimulus-Related Information in Barrel Cortex Inhibitory Interneurons.

The manner in which populations of inhibitory (INH) and excitatory (EXC) neocortical neurons collectively encode stimulus-related information is a fundamental, yet still unresolved question. Here we address this question by simultaneously recording with large-scale multi-electrode arrays (of up to 128 channels) the activity of cell ensembles (of up to 74 neurons) distributed along all layers of 3-4 neighboring cortical columns in the anesthetized adult rat somatosensory barrel cortex in vivo. Using two different whisker stimulus modalities (location and frequency) we show that individual INH neurons--classified as such according to their distinct extracellular spike waveforms--discriminate better between restricted sets of stimuli (≤6 stimulus classes) than EXC neurons in granular and infra-granular layers. We also demonstrate that ensembles of INH cells jointly provide as much information about such stimuli as comparable ensembles containing the ~20% most informative EXC neurons, however presenting less information redundancy - a result which was consistent when applying both theoretical information measurements and linear discriminant analysis classifiers. These results suggest that a consortium of INH neurons dominates the information conveyed to the neocortical network, thereby efficiently processing incoming sensory activity. This conclusion extends our view on the role of the inhibitory system to orchestrate cortical activity.

Laminar and Columnar Structure of Sensory-Evoked Multineuronal Spike Sequences in Adult Rat Barrel Cortex In Vivo.

One of the most relevant questions regarding the function of the nervous system is how sensory information is represented in populations of cortical neurons. Despite its importance, the manner in which sensory-evoked activity propagates across neocortical layers and columns has yet not been fully characterized. In this study, we took advantage of the distinct organization of the rodent barrel cortex and recorded with multielectrode arrays simultaneously from up to 74 neurons localized in several functionally identified layers and columns of anesthetized adult Wistar rats in vivo. The flow of activity within neuronal populations was characterized by temporally precise spike sequences, which were repeatedly evoked by single-whisker stimulation. The majority of the spike sequences representing instantaneous responses were led by a subgroup of putative inhibitory neurons in the principal column at thalamo-recipient layers, thus revealing the presence of feedforward inhibition. However, later spike sequences were mainly led by infragranular excitatory neurons in neighboring columns. Although the starting point of the sequences was anatomically confined, their ending point was rather scattered, suggesting that the population responses are structurally dispersed. Our data show for the first time the simultaneous intra- and intercolumnar processing of information at high temporal resolution.

Long-range intralaminar noise correlations in the barrel cortex.

Identifying the properties of correlations in the firing of neocortical neurons is central to our understanding of cortical information processing. It has been generally assumed, by virtue of the columnar organization of the neocortex, that the firing of neurons residing in a certain vertical domain is highly correlated. On the other hand, firing correlations between neurons steeply decline with horizontal distance. Technical difficulties in sampling neurons with sufficient spatial information have precluded the critical evaluation of these notions. We used 128-channel "silicon probes" to examine the spike-count noise correlations during spontaneous activity between multiple neurons with identified laminar position and over large horizontal distances in the anesthetized rat barrel cortex. Eigen decomposition of correlation coefficient matrices revealed that the laminar position of a neuron is a significant determinant of these correlations, such that the fluctuations of layer 5B/6 neurons are in opposite direction to those of layers 5A and 4. Moreover, we found that within each experiment, the distribution of horizontal, intralaminar spike-count correlation coefficients, up to a distance of ∼1.5 mm, is practically identical to the distribution of vertical correlations. Taken together, these data reveal that the neuron's laminar position crucially affects its role in cortical processing. Moreover, our analyses reveal that this laminar effect extends over several functional columns. We propose that within the cortex the influence of the horizontal elements exists in a dynamic balance with the influence of the vertical domain and this balance is modulated with brain states to shape the network's behavior.

LPS-induced microglial secretion of TNFα increases activity-dependent neuronal apoptosis in the neonatal cerebral cortex.

During the pre- and neonatal period, the cerebral cortex reveals distinct patterns of spontaneous synchronized activity, which is critically involved in the formation of early networks and in the regulation of neuronal survival and programmed cell death (apoptosis). During this period, the cortex is also highly vulnerable to inflammation and in humans prenatal infection may have a profound impact on neurodevelopment causing long-term neurological deficits. Using in vitro and in vivo multi-electrode array recordings and quantification of caspase-3 (casp-3)-dependent apoptosis, we demonstrate that lipopolysaccharide-induced inflammation causes rapid alterations in the pattern of spontaneous burst activities, which subsequently leads to an increase in apoptosis. We show that these inflammatory effects are specifically initiated by the microglia-derived pro-inflammatory cytokine tumor necrosis factor α and the chemokine macrophage inflammatory protein 2. Our data demonstrate that inflammation-induced modifications in spontaneous network activities influence casp-3-dependent cell death in the developing cerebral cortex.

Thalamic network oscillations synchronize ontogenetic columns in the newborn rat barrel cortex.

Neocortical areas are organized in columns, which form the basic structural and functional modules of intracortical information processing. Using voltage-sensitive dye imaging and simultaneous multi-channel extracellular recordings in the barrel cortex of newborn rats in vivo, we found that spontaneously occurring and whisker stimulation-induced gamma bursts followed by longer lasting spindle bursts were topographically organized in functional cortical columns already at the day of birth. Gamma bursts synchronized a cortical network of 300-400 µm in diameter and were coherent with gamma activity recorded simultaneously in the thalamic ventral posterior medial (VPM) nucleus. Cortical gamma bursts could be elicited by focal electrical stimulation of the VPM. Whisker stimulation-induced spindle and gamma bursts and the majority of spontaneously occurring events were profoundly reduced by the local inactivation of the VPM, indicating that the thalamus is important to generate these activity patterns. Furthermore, inactivation of the barrel cortex with lidocaine reduced the gamma activity in the thalamus, suggesting that a cortico-thalamic feedback loop modulates this early thalamic network activity.

Sharp-wave-ripple-associated activity in the medial prefrontal cortex supports spatial rule switching.

Previous studies have highlighted an important role for hippocampal sharp-wave ripples in spatial alternation learning, as well as in modulating activity in the medial prefrontal cortex (mPFC). However, the direct influence of hippocampal sharp-wave ripples on mPFC activity during spatial alternation learning has not been investigated. Here, we train Long Evans rats on a three-arm radial maze to perform a sequence of alternations. Three alternation sequences needed to be learned, and while learning a sequence, the activity in the mPFC was inhibited either directly following sharp-wave ripples in the hippocampus (on-time condition) or with a randomized delay (delayed condition). In the on-time condition, the behavioral performance is significantly worse compared to the same animals in the delayed inhibition condition, as measured by a lower correct alternation performance and more perseverative behavior. This indicates that the activity in the mPFC directly following hippocampal sharp-wave ripples is necessary for spatial rule switching.

Control of programmed cell death by distinct electrical activity patterns.

Electrical activity and sufficient supply with survival factors play a major role in the control of apoptosis in the developing cortex. Coherent high-frequency neuronal activity, which efficiently releases neurotrophins, is essential for the survival of immature neurons. We studied the influence of neuronal activity on apoptosis in the developing cortex. Dissociated cultures of the newborn mouse cerebral cortex were grown on multielectrode arrays to determine the activity patterns that promote neuronal survival. Cultures were transfected with a plasmid coding for a caspase-3-sensitive fluorescent protein allowing real-time analysis of caspase-3-dependent apoptosis in individual neurons. Elevated extracellular potassium concentrations (5 and 8 mM), application of 4-aminopyridine or the γ-aminobutyric acid-A receptor antagonist Gabazine induced a shift in the frequency distribution of activity toward high-frequency bursts. Under these conditions, a reduction or delay in caspase-3 activation and an overall increase in neuronal survival could be observed. This effect was dependent on the activity of phosphatidylinositol-3 kinase, as blockade of this enzyme abolished the survival-promoting effect of high extracellular potassium concentrations. Our data indicate that increased network activity can prevent apoptosis in developing cortical neurons.

Single-trial dynamics of hippocampal spatial representations are modulated by reward value.

Reward value is known to modulate learning speed in spatial memory tasks, but little is known about its influence on the dynamical changes in hippocampal spatial representations. Here, we monitored the trial-to-trial changes in hippocampal place cell activity during the acquisition of place-reward associations with varying reward size. We show a faster reorganization and stabilization of the hippocampal place map when a goal location is associated with a large reward. The reorganization is driven by both rate changes and the appearance and disappearance of place fields. The occurrence of hippocampal replay activity largely followed the dynamics of changes in spatial representations. Replay patterns became more selectively tuned toward behaviorally relevant experiences over the course of learning via the refined contributions of specific cell subpopulations. These results suggest that high reward value enhances memory retention by accelerating the formation and stabilization of the hippocampal cognitive map and selectively enhancing its reactivation during learning.

Clustering and control for adaptation uncovers time-warped spike time patterns in cortical networks in vivo.

How information in the nervous system is encoded by patterns of action potentials (i.e. spikes) remains an open question. Multi-neuron patterns of single spikes are a prime candidate for spike time encoding but their temporal variability requires further characterisation. Here we show how known sources of spike count variability affect stimulus-evoked spike time patterns between neurons separated over multiple layers and columns of adult rat somatosensory cortex in vivo. On subsets of trials (clusters) and after controlling for stimulus-response adaptation, spike time differences between pairs of neurons are "time-warped" (compressed/stretched) by trial-to-trial changes in shared excitability, explaining why fixed spike time patterns and noise correlations are seldom reported. We show that predicted cortical state is correlated between groups of 4 neurons, introducing the possibility of spike time pattern modulation by population-wide trial-to-trial changes in excitability (i.e. cortical state). Under the assumption of state-dependent coding, we propose an improved potential encoding capacity.

Three patterns of oscillatory activity differentially synchronize developing neocortical networks in vivo.

Coordinated patterns of electrical activity are important for the early development of sensory systems. The spatiotemporal dynamics of these early activity patterns and the role of the peripheral sensory input for their generation are essentially unknown. We performed extracellular multielectrode recordings in the somatosensory cortex of postnatal day 0 to 7 rats in vivo and observed three distinct patterns of synchronized oscillatory activity. (1) Spontaneous and periphery-driven spindle bursts of 1-2 s in duration and approximately 10 Hz in frequency occurred approximately every 10 s. (2) Spontaneous and sensory-driven gamma oscillations of 150-300 ms duration and 30-40 Hz in frequency occurred every 10-30 s. (3) Long oscillations appeared only every approximately 20 min and revealed the largest amplitude (250-750 microV) and longest duration (>40 s). These three distinct patterns of early oscillatory activity differently synchronized the neonatal cortical network. Whereas spindle bursts and gamma oscillations did not propagate and synchronized a local neuronal network of 200-400 microm in diameter, long oscillations propagated with 25-30 microm/s and synchronized 600-800 microm large ensembles. All three activity patterns were triggered by sensory activation. Single electrical stimulation of the whisker pad or tactile whisker activation elicited neocortical spindle bursts and gamma activity. Long oscillations could be only evoked by repetitive sensory stimulation. The neonatal oscillatory patterns in vivo depended on NMDA receptor-mediated synaptic transmission and gap junctional coupling. Whereas spindle bursts and gamma oscillations may represent an early functional columnar-like pattern, long oscillations may serve as a propagating activation signal consolidating these immature neuronal networks.

Self-organization of repetitive spike patterns in developing neuronal networks in vitro.

The appearance of spontaneous correlated activity is a fundamental feature of developing neuronal networks in vivo and in vitro. To elucidate whether the ontogeny of correlated activity is paralleled by the appearance of specific spike patterns we used a template-matching algorithm to detect repetitive spike patterns in multi-electrode array recordings from cultures of dissociated mouse neocortical neurons between 6 and 15 days in vitro (div). These experiments demonstrated that the number of spiking neurons increased significantly between 6 and 15 div, while a significantly synchronized network activity appeared at 9 div and became the main discharge pattern in the subsequent div. Repetitive spike patterns with a low complexity were first observed at 8 div. The number of repetitive spike patterns in each dataset as well as their complexity and recurrence increased during development in vitro. The number of links between neurons implicated in repetitive spike patterns, as well as their strength, showed a gradual increase during development. About 8% of the spike sequences contributed to more than one repetitive spike patterns and were classified as core patterns. These results demonstrate for the first time that defined neuronal assemblies, as represented by repetitive spike patterns, appear quite early during development in vitro, around the time synchronized network burst become the dominant network pattern. In summary, these findings suggest that dissociated neurons can self-organize into complex neuronal networks that allow reliable flow and processing of neuronal information already during early phases of development.

A Dual Reward-Place Association Task to Study the Preferential Retention of Relevant Memories in Rats.

Memories of past events and common knowledge are critical to flexibly adjust one's future behavior based on prior experiences. The formation and the transformation of these memories into a long-lasting form are supported by a dialogue between populations of neurons in the cortex and the hippocampus. Not all experiences are remembered equally well or equally long. It has been demonstrated experimentally in humans that memory strength positively relates to the behavioral relevance of the associated experience. Behavioral paradigms that test the selective retention of memory in rodents would enable further investigation of the neuronal mechanisms at play. We developed a novel paradigm to follow the repeated acquisition and retrieval of two contextually distinct, yet concurrently learned, food-place associations in rats. We demonstrated the use of this paradigm by varying the amount of reward associated with the two locations. After delays of 2 h or 20 h, rats showed better memory performance for experience associated with large amount of reward. This effect depends on the level of spatial integration required to retrieve the associated location. Thus, this paradigm is suited to study the preferential retention of relevant experiences in rats.

System for recording from multiple flexible polyimide neural probes in freely behaving animals.

OBJECTIVE: Long-term electrophysiological recordings of neural activity in freely behaving animals are indispensable to advance the understanding of complex brain function. It is a technical challenge to chronically monitor the detailed activity across multiple distributed brain regions in freely behaving animals over a period of months. Here we present a new implant for inserting multiple flexible polyimide probes into freely behaving rats for monitoring the brain activity over a long time period. APPROACH: This brain implant integrates multiple flexible probes in small micromanipulator devices that ensure free behaviour of the animal. The probes are micromachined and the positioning mechanism is 3D-printed using stereolithography. Each probe is lowered by a screw-driven shuttle and guided through an exit tip before penetrating the rat's brain. MAIN RESULTS: The brain implant consists of 16 individually lowerable flexible polyimide probes that contain 16 embedded electrodes adding up to a total of 256 recording channels. The total travel distance is 8 mm. The assembly time of the device was only one day. The electrode impedance values had a mean of 335 kΩ and sample standard deviation of 107 kΩ after gold plating, excluding outliers. SIGNIFICANCE: For the first time, hyperdrive-assisted insertion of flexible multichannel probes was demonstrated. Local field potentials and neuronal spiking activity from freely behaving rats were recorded over months.

Activity-dependent regulation of neuronal apoptosis in neonatal mouse cerebral cortex.

A massive neuronal loss during early postnatal development has been well documented in the murine cerebral cortex, but the factors that drive cells into apoptosis are largely unknown. The role of neuronal activity in developmental apoptosis was studied in organotypic neocortical slice cultures of newborn mice. Multielectrode array and whole-cell patch-clamp recordings revealed spontaneous network activity characterized by synchronized burst discharges, which could be blocked by tetrodotoxin and ionotropic glutamate receptor antagonists. The identical neuropharmacological manipulations also caused a significant increase in the number of apoptotic neurons as early as 6 h after the start of drug treatment. Moreover, inhibition of the NMDA receptor subunit NR2A or NR2B induced a differential short-term versus delayed increase in the apoptosis rate, respectively. Activation of L-type, voltage-dependent calcium channels was neuroprotective and could prevent activity-dependent apoptosis during NMDA receptor blockade. Furthermore, this effect involved phosphorylation of cAMP response element-binding protein and activation of the tropomyosin-related kinase (Trk) receptors. Inhibition of electrical synapses and blockade of ionotropic gamma-aminobutyric acid receptors induced specific changes in spontaneous electrical activity patterns, which caused an increase in caspase-3-dependent cell death. Our results demonstrate that synchronized spontaneous network bursts activating ionotropic glutamate receptors promote neuronal survival in the neonatal mouse cerebral cortex.

Post-learning Hippocampal Replay Selectively Reinforces Spatial Memory for Highly Rewarded Locations.

Offline replay of hippocampal neural patterns supports the acquisition of new tasks in novel contexts, but its contribution to consolidation of salient experiences in a familiar context is unknown. Here, we show that in a highly familiar spatial memory task, large rewards selectively enhanced performance for demanding task configurations. The reward-related enhancement was sensitive to ripple-specific disruption, and the proportion of replay events positively correlated with reward size and task demands. Hippocampal replay thus selectively enhances memory of highly rewarded locations in a familiar context.

Short-term facilitation in the anterior cingulate cortex following stimulation of the medial thalamus in the rat.

The present study examined the distribution and localization of synaptic activities (field potentials, multiunit activities and sink source currents) evoked in the anterior cingulate cortex (ACC) by electrical paired pulse stimulation of the ipsilateral medial thalamus (MT). Male Sprague-Dawley rats were anesthetized with halothane (1.0-1.5%), and electrical paired pulses stimuli (100-300 microA, inter-pulse interval, 100 ms) were delivered to the MT. Tungsten microelectrodes and a multichannel Michigan probe were used to record the evoked field potentials and multiunit activities in the ACC. Paired pulse stimulation facilitated field potentials and multiunit activities elicited from several MT nuclei. The second component of the negative field potential (com2) was augmented to about 2.5 times that of the first component (com1), and the integrated multiunit activities were facilitated by about 1.6-fold. Paired stimulation produced an expansion of the maximal negative potential from layer II/III into the deeper layers of the cingulate cortex area 1 (Cg1). Furthermore, the potentiated activity spread into adjacent secondary motor cortex (M2) and prelimbic cortex (PrL). Meanwhile, the area covered by the maximal integrated multiunit activities expanded from layer V (com1) to layers II-V (com2) in M2, Cg1 and PrL. The current source density (CSD) analysis revealed that the short latency sinks were located in layer II/III and layer V/VI. The sink currents were potentiated and expanded to more superficial and to deeper layers when a second pulse was delivered with a 100-ms time delay. Sink currents and the paired pulse facilitation (PPF) were reduced by morphine treatment (5 mg/kg, i.v.), and this effect could be blocked by naloxone. Electrical stimulation at 10 Hz in the MT induced more pronounced c-fos immunolabeling of neurons in the medial prefrontal cortex than did 1-Hz stimulation. The short-term facilitation occurred in the middle layers and expanded to the deeper layers of the ACC. These changes may mediate the effective signal transference in the specific frequency associated with painful responses.

Layer IV of the primary somatosensory cortex has the highest complexity under anesthesia and cortical complexity is modulated by specific thalamic inputs.

The system complexity, as calculated from correlation dimension, embedded in each layer and its modulation by specific inputs and general excitatory state are not yet known. The aims of present study were to estimate the system complexity across the cortical layers by analyzing intracortical EEG signals using a nonlinear analytical method, and to identify how layer-related complexity varies with the alteration of thalamic input and brain state. Male Sprague-Dawley rats were anesthetized under l% halothane. Sixteen channels of evoked or spontaneous EEG signals were recorded simultaneously across the six cortical layers in the somatosensory cortex with a single Michigan probe. The system complexity was assessed by computing correlation dimension, D(2), based on the Nonlinear Time Series Analysis data analysis program. Cortical layer IV exhibited a D(2) value, 3.24, that was significantly higher than that of the other cortical layers. The D(2) values in layers IV and II/III were significantly reduced after reversible deactivation of the ventral posterior lateral thalamic nucleus. D(2) decreased with increases in administered halothane concentration from 0.75% to 2.0%, particularly in layer IV. The present findings suggest that cortical layer IV maintains a higher complexity than the other layers and that the complexity of the mid-cortical layers is subject to regulation from specific thalamic inputs and more sensitive to changes in the general state of brain excitation.

Propagation of spontaneous slow-wave activity across columns and layers of the adult rat barrel cortex in vivo.

During slow-wave sleep, neocortical networks exhibit self-organized activity switching between periods of concurrent spiking (up-states) and periods of network silence (down-states), a phenomenon also occurring under the effects of different anesthetics and in in vitro brain slice preparations. Although this type of ongoing activity has been implicated into important functions such as memory consolidation and learning, the manner in which it propagates across different cortical modules (i.e., columns and layers) has not been fully characterized. In the present study, we investigated this issue by measuring spontaneous activity at large scale in the adult rat barrel cortex under urethane anesthesia by means of voltage-sensitive dye imaging and 128-channel probe recordings. Up to 74 neurons located in all layers of up to four functionally identified barrel-related columns were recorded simultaneously. The spontaneous activity propagated isotropically across the cortical surface with a median speed of ~35 µm/ms. A concomitant radial spread of activation was present from deep to superficial cortical layers. Thus, spontaneous activity occurred rather globally in the barrel cortex, with ≥50 % of the up-states presenting spikes in ≥3 columns and layers. Temporally precise spike sequences, which occurred repeatedly (although sporadically) within the up-states, were typically led by putative excitatory neurons in the infragranular cortical layers. In summary, our data provide for the first time an overall view of the spontaneous slow-wave activity within the barrel cortex circuit, characterizing its propagation across columns and layers at high spatio-temporal resolution.

A method for direct thalamic stimulation in fMRI studies using a glass-coated carbon fiber electrode.

Recent fMRI studies are of interest in exploring long-range interactions between different brain structures and the functional activation of specific brain regions by known neuroanatomical pathways. One of the experimental approaches requires the invasive implantation of an intracranial electrode to excite specific brain structures. In the present report, we describe a procedure for the production of a glass-coated carbon fiber electrode and the use of this electrode for direct activation of the brain in fMRI studies. The glass-coated carbon fiber microelectrode was implanted in the medial thalamus of anaesthetized rats and T2*-weighted gradient echo images in the sagittal plane obtained on a 4.7 T system (Biospec BMT 47/40) during electrical stimulation of the medial thalamus. The image quality obtained using this electrode was acceptable without reduction of the signal-to-noise ratio and image distortion. Cross-correlation analysis showed that the signal intensities of activated areas in the ipsilateral anterior cingulate cortex were significantly increased by about 4-5% during medial thalamus stimulation. The present study shows that glass-coated carbon fiber electrodes are suitable for fMRI studies and can be used to investigate functional thalamocingulate activation.

Reconstruction of network structures from repeating spike patterns in simulated bursting dynamics.

Repeating patterns of spike sequences from a neuronal network have been proposed to be useful in the reconstruction of the network topology. Reverberations in a physiologically realistic model with various physical connection topologies (from random to scale free) have been simulated to study the effectiveness of the pattern-matching method in the reconstruction of network topology from network dynamics. Simulation results show that functional networks reconstructed from repeating spike patterns can be quite different from the original physical networks; even global properties, such as the degree distribution, cannot always be recovered. However, the pattern-matching method can be effective in identifying hubs in the network. Since the form of reverberations is quite different for networks with and without hubs, the form of reverberations together with the reconstruction by repeating spike patterns might provide a reliable method to detect hubs in neuronal cultures.