Intrinsic firing patterns and whisker-evoked synaptic responses of neurons in the rat barrel cortex

Controlling synaptic input patterns in vitro by dynamic photo stimulation

Recent experimental and theoretical work indicates that both the intensity and the temporal structure of synaptic activity strongly modulate the integrative properties of single neurons in the intact brain. However, studying these effects experimentally is complicated by the fact that, in experimental systems, network activity is either absent, as in the acute slice preparation, or difficult to monitor and to control, as in in vivo recordings. Here, we present a new implementation of neurotransmitter uncaging in acute brain slices that uses functional projections to generate tightly controlled, spatio-temporally structured synaptic input patterns in individual neurons. For that, a set of presynaptic neurons is activated in a precisely timed sequence through focal photolytic release of caged glutamate with the help of a fast laser scanning system. Integration of synaptic inputs can be studied in postsynaptic neurons that are not directly stimulated with the laser, but receive input from the targeted neurons through intact axonal projections. Our new approach of dynamic photo stimulation employs functional synapses, accounts for their spatial distribution on the dendrites, and thus allows study of the integrative properties of single neurons with physiologically realistic input. Data obtained with our new technique suggest that, not only the neuronal spike generator, but also synaptic transmission and dendritic integration in neocortical pyramidal cells, can be highly reliable.

Computing with thalamocortical ensembles during different behavioural states

A series of recent studies have indicated that ensembles of neurones, distributed within the neural structures that form the primary thalamocortical loop (TCL) of the trigeminal component of the rat somatosensory system, change the way they respond to similar tactile stimuli, according to both the behavioural strategy employed by animals to gather information and the animal's internal brain states. These findings suggest that top-down influences, which are more likely to play a role during active discrimination than during passive whisker stimulation, may alter the pattern of neuronal firing within both the distinct layers of the primary somatosensory cortex (S1) and the ventral posterior medial nucleus (VPM). We propose that through this physiological process, which involves concurrent dynamic modulations at both cellular and circuit levels in the TCL, rats can either optimize the detection of novel or hard to sense stimuli or they can analyse complex patterns of multi-whisker stimulation, during natural exploration of their surrounding environment.

CoCoDat: a database system for organizing and selecting quantitative data on single neurons and neuronal microcircuitry

We present a novel database system for organizing and selecting quantitative experimental data on single neurons and neuronal microcircuitry that has proven useful for reference-keeping, experimental planning and computational modelling. Building on our previous experience with large neuroscientific databases, the system takes into account the diversity and method-dependence of single cell and microcircuitry data and provides tools for entering and retrieving published data without a priori interpretation or summarizing. Data representation is based on the framework suggested by biophysical theory and enables flexible combinations of data on membrane conductances, ionic and synaptic currents, morphology, connectivity and firing patterns. Innovative tools have been implemented for data retrieval with optional relaxation of search criteria along the conceptual dimensions of brain region, cortical layer, cell type and subcellular compartment. The relaxation procedures help to overcome the traditional trade-off between exact, non-interpreted data representation in the original nomenclature and convenient data retrieval. We demonstrate the use of these tools for the construction, tuning and validation of a multicompartmental model of a layer V pyramidal cell from the rat barrel cortex. CoCoDat is freely available at . Its application is scalable from offline use by individual researchers via local laboratory networks to a federation of distributed web sites in platform-independent XML format using Axiope tools.

Disruption of layer 4 development alters laminar processing in ferret somatosensory cortex

Treatment with the anti-mitotic agent methylazoxymethanol (MAM) on embryonic day 33 (E33) in ferrets changes features of somatosensory cortex. These include dramatic reduction of cells in layer 4, and altered distributions of thalamocortical afferent terminations and GABA(A) receptors. To determine the effect of the relative absence of layer 4 on processing of sensory stimuli we used current source-density profiles to assess laminar activity patterns. Nearly synchronous activation occurs across all layers in treated animals, which contrasts with the normal cortical activation pattern of initial sinks in layer 4. This change after MAM treatment is consistent with the absence of layer 4 cells and widespread termination of thalamocortical afferents. Using periodic stimulation at 'flutter' frequency, layer 4 neurons in normal somatosensory cortex fire reproducibly to the stimulus rate; the capacity for entrainment is best for layer 4 and weaker in the extragranular layers. The capacity to encode periodic sensory stimuli is disrupted in MAM-treated somatosensory cortex; after an initial response to the onset of periodic stimuli, neurons in all cortical layers show weak entrainment. Neural responses to sensory drive in E33 MAM-treated cortex are also embedded in levels of neural activity substantially above those in normal somatosensory cortex. Sustained stimulation additionally reveals different capacities in each layer for improved signal-to-noise ratios, with layer 4 neurons in normal animals exhibiting the most improved signaling over time. We conclude that normal thalamic terminations, an intact layer 4 and subsequent intracortical processing are integral to proper encoding of stimulus features.

Functional Properties of Fast Spiking Interneurons and Their Synaptic Connections With Pyramidal Cells in Primate Dorsolateral Prefrontal Cortex

Recent studies suggest that fast-spiking (FS) interneurons of the monkey dorsolateral prefrontal cortex (DLPFC) exhibit task-related firing during working-memory tasks. To gain further understanding of the functional role of FS neurons in monkey DLPFC, we described the in vitro electrophysiological properties of FS interneurons and their synaptic connections with pyramidal cells in layers 2/3 of areas 9 and 46. Extracellular spike duration was found to distinguish FS cells from non-FS interneuron subtypes. However, a substantial overlap in extracellular spike duration between these populations would make classification of individual interneurons difficult. FS neurons could be divided into two main morphological groups, chandelier and basket neurons, with very similar electrophysiological properties but significantly different horizontal spread of the axonal arborization. In paired cell recordings, unitary inhibitory postsynaptic potentials (IPSPs) elicited by FS neurons in pyramidal cells had rapid time course, small amplitude at resting membrane potential, and were mediated by GABAAreceptors. Repetitive FS neuron stimulation, partially mimicking the sustained firing of interneurons in vivo, produced short-term depression of the unitary IPSPs, present at connections made by both basket and chandelier neurons and due at least in part to presynaptic mechanisms. These results suggest that FS neurons and their synaptic connections with pyramidal cells have homogeneous physiological properties. Thus different functional roles of basket and chandelier neurons in the DLPFC in vivo must arise from the distinct properties of the interneuronal axonal arborization or from a different functional pattern of excitatory and inhibitory connections with other components of the DLPFC neuronal network.

Backpropagating action potentials in neurones: measurement, mechanisms and potential functions

Here we review some properties and functions of backpropagating action potentials in the dendrites of mammalian CNS neurones. We focus on three main aspects: firstly the current techniques available for measuring backpropagating action potentials, secondly the morphological parameters and voltage gated ion channels that determine action potential backpropagation and thirdly the potential functions of backpropagating action potentials in real neuronal networks.

Integration of synaptic responses to neighboring whiskers in rat barrel cortex in vivo

Characterizing input integration at the single-cell level is a critical step to understanding cortical function, particularly when sensory stimuli are represented over wide cortical areas and single cells exhibit large receptive fields. To study synaptic integration of sensory inputs, we made intracellular recordings from the barrel cortex of anesthetized rats in vivo. For each cell, we deflected the principal whisker (PW) either alone or preceded by the deflection of a single adjacent whisker (AW) at an interval of 20 or 3 ms. At the 20-ms interval in all cases, prior AW deflection significantly suppressed the PW-evoked spike output and caused the underlying synaptic response to reach a peak Vm less depolarized than that arising from PW deflection alone. The decrease in peak Vm was not attributed to hyperpolarizing inhibition but to a divisive reduction in PW-evoked PSP amplitude. The reduction in amplitude was not a result of shunting inhibition but was mostly a result of removal of the synaptic drive, or disfacilitation. When the AW-PW interval was shortened to 3 ms, spike suppression was observed in a subset of the cells studied. In most cases, a divisive reduction in synaptic response amplitude was offset by summation with the preceding AW-evoked depolarization. To determine whether suppression is a general feature of synaptic integration by barrel cortex neurons, we also characterized the interaction of responses evoked by local electrical stimulation. In contrast to the whisker data, we found that responses to paired stimulation at the same intervals produced more spikes and reached a peak Vm more depolarized than the individual responses alone, suggesting that whisker-evoked suppression is not a result of postsynaptic mechanisms. Instead, we propose that cross-whisker response suppression depends on sensory-specific mechanisms at cortical and subcortical levels.

Effects of neonatal C-fiber depletion on the integration of paired-whisker inputs in rat barrel cortex

In the present study we used computer-controlled mechanical displacement of paired whiskers in normal and C-fiber-depleted rats to quantitatively examine the role of C-fibers in the receptive field properties of barrel cortical cells. In rodents when adjacent whiskers are stimulated prior to the main whisker responses to the main whisker are inhibited, the degree of inhibition being a function of the inter-deflection intervals. The adjacent-whisker-evoked inhibition of barrel cells in normal and C-fiber-depleted rats using neonatal capsaicin treatment were examined by stimulation of the adjacent whisker zero, 10, 20, 30, 50 and 100 ms prior to the main whisker deflection. C-fiber depletion reduced the suppressive effect of paired whisker stimulation at all of the tested inter-stimulus intervals without changing response latencies. The main effect was observed during the later phase of response (about 13-17 ms from stimulus onset) and not during the initial responses (7-12 ms). These results suggest that the inhibitory receptive field properties of low-threshold mechanical somatosensory cells are influenced by C-fibers.

Synaptic transformations underlying highly selective auditory representations of learned birdsong

Stimulus-specific neuronal responses are a striking characteristic of several sensory systems, although the synaptic mechanisms underlying their generation are not well understood. The songbird nucleus HVC (used here as a proper name) contains projection neurons (PNs) that fire temporally sparse bursts of action potentials to playback of the bird's own song (BOS) but are essentially silent when presented with other acoustical stimuli. To understand how such remarkable stimulus specificity emerges, it is necessary to compare the auditory-evoked responsiveness of the afferents of HVC with synaptic responses in identified HVC neurons. We found that inactivating the interfacial nucleus of the nidopallium (NIf) could eliminate all auditory-evoked subthreshold activity in both HVC PN types, consistent with NIf serving as the major auditory afferent of HVC. Simultaneous multiunit extracellular recordings in NIf and intracellular recordings in HVC revealed that NIf population activity and HVC subthreshold responses were similar in their selectivity for BOS and that NIf spikes preceded depolarizations in all HVC cell types. These results indicate that information about the BOS as well as other auditory stimuli is transmitted synaptically from NIf to HVC. Unlike HVC PNs, however, HVC-projecting NIf neurons fire throughout playback of BOS as well as non-BOS stimuli. Therefore, temporally sparse BOS-evoked firing and enhanced BOS selectivity, manifested as an absence of suprathreshold responsiveness to non-BOS stimuli, emerge in HVC. The transformation to a sparse auditory representation parallels differences in NIf and HVC activity patterns seen during singing, which may point to a common mechanism for encoding sensory and motor representations of song.

Neuronal mechanisms mediating the variability of somatosensory evoked potentials during sleep oscillations in cats

The slow oscillation (SO) generated within the corticothalamic system is composed of active and silent states. The studies of response variability during active versus silent network states within thalamocortical system of human and animals provided inconsistent results. To investigate this inconsistency, we used electrophysiological recordings from the main structures of the somatosensory system in anaesthetized cats. Stimulation of the median nerve (MN) elicited cortical responses during all phases of SO. Cortical responses to stimulation of the medial lemniscus (ML) were virtually absent during silent periods. At the ventral-posterior lateral (VPL) level, ML stimuli elicited either EPSPs in isolation or EPSPs crowned by spikes, as a function of membrane potential. Response to MN stimuli elicited compound synaptic responses and spiked at any physiological level of membrane potential. The responses of dorsal column nuclei neurones to MN stimuli were of similar latency, but the latencies of antidromic responses to ML stimuli were variable. Thus, the variable conductance velocity of ascending prethalamic axons was the most likely cause of the barrages of synaptic events in VPL neurones mediating their firing at different level of the membrane potential. We conclude that the preserved ability of the somatosensory system to transmit the peripheral stimuli to the cerebral cortex during all the phases of sleep slow oscillation is based on the functional properties of the medial lemniscus and on the intrinsic properties of the thalamocortical cells. However the reduced firing ability of the cortical neurones during the silent state may contribute to impair sensory processing during sleep.

Optical imaging of the spatiotemporal dynamics of cerebral blood flow and oxidative metabolism in the rat barrel cortex

Oxidative metabolism and cerebral blood flow (CBF) are two of the most important measures in neuroimaging. However, results from concurrent imaging of the two with high spatial and temporal resolution have never been published. We used flavoprotein autofluorescence (AF) and laser speckle imaging (LSI) in the anaesthetized rat to map oxidative metabolism and CBF in response to single vibrissa stimulation. Autofluorescence responses reflecting oxidative metabolism demonstrated a fast increase with a delay of 0.1 s. The sign-reversed speckle contrast reflecting CBF started to rise with a delay of 0.6 s and reached its maximum 1.4 s after the stimulation offset. The fractional signal changes were 2.0% in AF and 9.7% in LSI. Pixelwise modelling revealed that CBF maps spread over an area up to 2.5-times larger than metabolic maps. The results provide evidence that the increase in cerebral oxidative metabolism in response to sensory stimulation is considerably faster and more localized than the CBF response. This suggests that future developments in functional imaging concentrating on the metabolic response promise an increased spatial resolution.

Spatiotemporal characteristics of neuronal sensory integration in the barrel cortex of the rat

In primary sensory cortices, neuronal responses to a stimulus presented as part of a rapid sequence often differ from responses to an isolated stimulus. It has been reported that sequential stimulation of two whiskers produces facilitatory modulations of barrel cortex neuronal responses. These results are at odds with the well-known suppressive interaction that has been usually described. Herein, we have examined the dependency of response modulation on the spatiotemporal pattern of stimulation by varying the spatial arrangement of the deflected vibrissae, the temporal frequency of stimulation, and the time interval between whisker deflections. Extracellular recordings were made from primary somatosensory cortex of anesthetized rats. Two contralateral whiskers were stimulated at 0.5 and 8 Hz at intervals ranging from 0 to +/-30 ms. Response interactions were assessed during stimulation of the principal and adjacent whiskers, first from the same row and second from the same arc. When tested at 0.5 Hz, 59% of single units showed a statistically significant suppressive interaction, whereas response facilitation was found in only 6% of cells. In contrast, at 8 Hz, a significant supralinear summation was observed in 19% of the cells, particularly for stimulations along an arc rather than along a row. Multi-unit recordings showed similar results. These observations indicate that most of the interactions in the barrel cortex during two-whisker stimulation are suppressive. However, facilitation can be revealed when stimuli are applied at a physiological frequency and could be the basis for internal representations of the spatiotemporal pattern of the stimulus.

Cortical synaptic integration in vivo is disrupted by amyloid-beta plaques

The accumulation of amyloid-beta protein into plaques is a characteristic feature of Alzheimer's disease. However, the contribution of amyloid-beta plaques to neuronal dysfunction is unknown. We compared intracellular recordings from neocortical pyramidal neurons in vivo in APP-Sw (Tg2576 transgenic mice overexpressing amyloid precursor protein with the Swedish mutation) transgenic mice to age-matched nontransgenic cohorts at ages either before or after deposition of cortical plaques. We show that the evoked synaptic response of neurons to transcallosal stimuli is severely impaired in cortex containing substantial plaque accumulation, with an average 2.5-fold greater rate of response failure and twofold reduction in response precision compared with age-matched nontransgenic controls. This effect correlated with the presence of amyloid-beta plaques and alterations in neuronal process geometry. Responses of neurons in younger APP-Sw animals, before plaque accumulation, were similar to those in nontransgenic controls. In all cases, spontaneous membrane potential dynamics were similar, suggesting that overall levels of synaptic innervation were not affected by plaques. Our results show that plaques disrupt the synchrony of convergent inputs, reducing the ability of neurons to successfully integrate and propagate information.

Synaptic responses to whisker deflections in rat barrel cortex as a function of cortical layer and stimulus intensity

To study the synaptic and spike responses of barrel cortex neurons as a function of cortical layer and stimulus intensity, we recorded intracellularly in vivo from barbiturate anesthetized rats while increasing the velocity-acceleration of the whisker deflection. Granular (Gr; layer 4) cells had the EPSP with the shortest peak and onset latency, whereas supragranular (SGr; layers 2-3) cells had the EPSP with longest duration and slowest rate of rise. Infragranular (Igr; layers 5-6) cells had intermediate values, and thus each layer was unique. The spike response peak of Gr cells was followed by IGr and then by SGr cells. In all cells, depolarization reduced the duration and amplitude of the response, but only in Gr cells did it reveal an early IPSP that cut short the EPSP. This early IPSP was associated with a large decrease in input resistance and an apparent reversal potential below spike threshold; consequently, synaptic integration in Gr cells was limited to the initial 5-7 msec of the response. In contrast, in SGr and IGr cells, results suggest an overlap in time of the EPSP and IPSP, with a small drop in input resistance and an apparent reversal potential above spike threshold, facilitating input integration for up to 20 msec. Decreasing stimulus intensity (velocity-acceleration) reduced the amplitude and increased the peak latency of the response without altering its synaptic composition. We propose that layer 4 circuits are better suited to perform coincidence detection, whereas supra and infragranular circuits are better designed for input integration.

Derivation and analysis of basic computational operations of thalamocortical circuits

Shared anatomical and physiological features of primary, secondary, tertiary, polysensory, and associational neocortical areas are used to formulate a novel extended hypothesis of thalamocortical circuit operation. A simplified anatomically based model of topographically and nontopographically projecting ("core" and "matrix") thalamic nuclei, and their differential connections with superficial, middle, and deep neocortical laminae, is described. Synapses in the model are activated and potentiated according to physiologically based rules. Features incorporated into the models include differential time courses of excitatory versus inhibitory postsynaptic potentials, differential axonal arborization of pyramidal cells versus interneurons, and different laminar afferent and projection patterns. Observation of the model's responses to static and time-varying inputs indicates that topographic "core" circuits operate to organize stored memories into natural similarity-based hierarchies, whereas diffuse "matrix" circuits give rise to efficient storage of time-varying input into retrievable sequence chains. Examination of these operations shows their relationships with well-studied algorithms for related functions, including categorization via hierarchical clustering, and sequential storage via hash- or scatter-storage. Analysis demonstrates that the derived thalamocortical algorithms exhibit desirable efficiency, scaling, and space and time cost characteristics. Implications of the hypotheses for central issues of perceptual reaction times and memory capacity are discussed. It is conjectured that the derived functions are fundamental building blocks recurrent throughout the neocortex, which, through combination, gives rise to powerful perceptual, motor, and cognitive mechanisms.

Effect of subthreshold up and down states on the whisker-evoked response in somatosensory cortex

Changes in spontaneous activity within the cortex recognized by subthreshold fluctuations of the membrane potential of cortical neurons modified the response of cortical neurons to sensory stimuli. Sensory stimuli occurring in the hyperpolarized "down" state evoked a larger depolarization and were more effective in evoking action potentials than stimuli occurring in the depolarized "up" state. Direct electrical stimulation of the thalamus showed the same dependence on the cell's state at the time of the stimulus, ruling out a strictly thalamic mechanism. Stimuli were more effective at triggering action potentials in the down state even during moderate de- or hyperpolarization of the somatic membrane potential. The postsynaptic potential (PSP) evoked from the down state was larger than the up state PSP but achieved about the same peak membrane potential, which was also near the reversal potential of the PSP (about -51 mV). Chloride loading shifted the reversal potentials of both the up state and the whisker-evoked PSP toward a more depolarized membrane potential. In addition, the threshold for action potentials evoked from the down state was lower than for spikes evoked in the up state. Thus the larger PSP from the down state may be caused by its larger driving force, and the state dependence of action potential generation in response to whisker stimulation may in part be related to a shift in threshold. Different mechanisms are therefore responsible for the state-dependence of PSP amplitude and the spike frequency response to the whisker stimulus.

Chandelier cells control excessive cortical excitation: characteristics of whisker-evoked synaptic responses of layer 2/3 nonpyramidal and pyramidal neurons

Chandelier cells form inhibitory axo-axonic synapses on pyramidal neurons with their characteristic candlestick-like axonal terminals. The functional role of chandelier cells is still unclear, although the preferential loss of this cell type at epileptic loci suggests a role in epilepsy. Here we report an examination of whisker- and spontaneous activity-evoked responses in chandelier cells and other fast-spiking nonpyramidal neurons and regular-spiking pyramidal neurons in layer 2/3 of the barrel cortex. Fast-spiking nonpyramidal neurons, including chandelier cells, basket cells, neurogliaform cells, double bouquet cells, net basket cells, bitufted cells, and regular-spiking pyramidal neurons all respond to stimulation of multiple whiskers on the contralateral face. Whisker stimulation, however, evokes small, delayed EPSPs preceded by an earlier IPSP and no action potentials in chandelier cells, different from other nonpyramidal and pyramidal neurons. In addition, chandelier cells display a larger receptive field with lower acuity than other fast-spiking nonpyramidal neurons and pyramidal neurons. Notably, simultaneous dual whole-cell in vivo recordings show that chandelier cells, which rarely fire action potentials spontaneously, fire more robustly than other types of cortical neurons when the overall cortical excitation increases. Thus, chandelier cells may not process fast ascending sensory information but instead may be reserved to prevent excessive excitatory activity in neuronal networks.

Effects of neonatal C-fiber depletion on discrimination of principal and adjacent whisker stimulation within rat individual cortical barrels

Controlled mechanical displacement was used to stimulate single whiskers in normal and C-fiber depleted rats to quantitatively examine the role of C-fibers in the response properties of barrel cortical cells. C-fiber depletion using neonatal capsaicin treatment increased the barrel single-unit response magnitude to deflection of both principal and adjacent whiskers while there was not any significant difference in the barrel cells' spontaneous activity. Capsaicin treatment increased the neural response duration of adjacent whisker stimulation but did not change that to the principal whisker deflection. There was no difference in response latencies of principal or adjacent whisker displacement between the normal and C-fiber-depleted groups. The efficiency of neural code for differentiation of principal and adjacent whiskers was measured by ROC analysis, which reflects the performance of an ideal observer in this discrimination using cells' firing rate. No significant difference was found in the performance of neurons in capsaicin-treated and control groups in distinguishing principal and adjacent whisker deflections from each other. These results suggest that neonatal C-fiber depletion causes an expansion of barrel cells receptive field but it does not affect the discrimination of individual whisker stimulation by the barrel cells.

Frequency-dependent processing in the vibrissa sensory system

The vibrissa sensory system is a key model for investigating principles of sensory processing. Specific frequency ranges of vibrissa motion, generated by rodent sensory behaviors (e.g., active exploration or resting) and by stimulus features, characterize perception by this system. During active exploration, rats typically sweep their vibrissae at approximately 4-12 Hz against and over tactual surfaces, and during rest or quiescence, their vibrissae are typically still (<1 Hz). When a vibrissa is swept over an object, microgeometric surface features (e.g., grains on sandpaper) likely create higher frequency vibrissa vibrations that are greater than or equal to several hundred Hertz. In this article, I first review thalamic and cortical neural responses to vibrissa stimulation at 1-40 Hz. I then propose that neural dynamics optimize the detection of stimuli in low-frequency contexts (e.g., 1 Hz) and the discrimination of stimuli in the whisking frequency range. In the third section, I describe how the intrinsic biomechanical properties of vibrissae, their ability to resonate when stimulated at specific frequencies, may promote detection and discrimination of high-frequency inputs, including textured surfaces. In the final section, I hypothesize that distinct low- and high-frequency processing modes may exist in the somatosensory cortex (SI), such that neural responses to stimuli at 1-40 Hz do not necessarily predict responses to higher frequency inputs. In total, these studies show that several frequency-specific mechanisms impact information transmission in the vibrissa sensory system and suggest that these properties play a crucial role in perception.

Precise Development of Functional and Anatomical Columns in the Neocortex

Sensory cortex is ordered into columns, each tuned to a subset of peripheral stimuli. To identify the principles underlying the construction of columnar architecture, we monitored the development of circuits in the rat barrel cortex, using laser-scanning photostimulation analysis of synaptic connectivity, reconstructions of axonal arbors, and in vivo whole-cell recording. Circuits impinging onto layer 2/3 neurons from layers 4 and 2/3 developed in a monotonic, precise progression, with little evidence for transient hyperinnervation at the level of cortical columns. Consistent with this, synaptic currents measured in layer 2/3 neurons at PND 8, just after these neurons ceased to migrate, revealed already spatially well-tuned receptive fields.

Neural Correlates of Vibrissa Resonance

The array of vibrissae on a rat's face is the first stage of a high-resolution tactile sensing system. Recently, it was discovered that vibrissae (whiskers) resonate when stimulated at specific frequencies, generating several-fold increases in motion amplitude. We investigated the neural correlates of vibrissa resonance in trigeminal ganglion and primary somatosensory cortex (SI) neurons (regular and fast spiking units) by presenting low-amplitude, high-frequency vibrissa stimulation. We found that somatosensory neurons showed band-pass tuning and enhanced sensitivity to small amplitude stimuli, reflecting the resonance amplification of vibrissa motion. Further, a putative somatotopic map of frequency selectivity was observed in SI, with isofrequency columns extending along the representations of arcs of vibrissae, in agreement with the gradient in vibrissa resonance across the vibrissa pad. These findings suggest several parallels between frequency processing in the vibrissa system and the auditory system and have important implications for detection and discrimination of tactile information.

Shared and private variability in the auditory cortex

The high variability of cortical sensory responses is often assumed to impose a major constraint on efficient computation. In the auditory cortex, however, response variability can be very low. We have used in vivo whole cell patch-clamp methods to study the trial-to-trial variability of the subthreshold fluctuations in membrane potential underlying tone-evoked responses in the auditory cortex of anesthetized rats. Using methods adapted from classical quantal analysis, we partitioned this subthreshold variability into a private component (which includes synaptic, thermal, and other sources local to the recorded cell) and a shared component arising from network interactions. Here we report that this private component is remarkably small, usually about 1-3 mV, as quantified by the variance divided by the mean of the ensemble of tone-evoked response heights. The shared component can be much larger, and shows more heterogeneity across the population, ranging from about 0 to 10 mV. The remarkable fact that, at least 5 synapses from the auditory periphery, this variability remains so small raises the possibility that the intervening neural circuitry is organized so as to prevent private noise from accumulating as neural signals propagate to the cortex.

Rapid arrival and integration of ascending sensory information in layer 1 nonpyramidal neurons and tuft dendrites of layer 5 pyramidal neurons of the neocortex

Ascending sensory inputs arriving in layer 1 of the neocortex carry crucial signals for detecting salient information; but how the inputs are processed in layer 1 is unknown. Using a whole-cell in vivo recording technique targeting nonpyramidal neurons in layer 1 and tuft dendrites of layer 5 pyramidal neurons in layers 1-2, we examined the processing of these ascending sensory inputs in the barrel cortex. Here, we show that local circuit and deeper-layer-projecting neurons in layer 1, as well as tuft dendrites and somata of layer 5 pyramidal neurons, respond to multiple whiskers (6-15) with robust EPSPs. Remarkably, the latency for primary whisker-evoked responses is as short as approximately 5-7 msec in layer 1 neurons and tuft dendrites of layer 5 pyramidal neurons. In addition, the latency for primary whisker-evoked responses in tuft dendrites of layer 5 pyramidal neurons is approximately 1 msec shorter than that in somata. These results indicate that ascending sensory inputs arrive in layers 1 and 4 concurrently, which provides a neural mechanism for rapid integration and coincident detection of salient sensory information.

Sub- and suprathreshold receptive field properties of pyramidal neurones in layers 5A and 5B of rat somatosensory barrel cortex

Layer 5 (L5) pyramidal neurones constitute a major sub- and intracortical output of the somatosensory cortex. This layer 5 is segregated into layers 5A and 5B which receive and distribute relatively independent afferent and efferent pathways. We performed in vivo whole-cell recordings from L5 neurones of the somatosensory (barrel) cortex of urethane-anaesthetized rats (aged 27-31 days). By delivering 6 deg single whisker deflections, whisker pad receptive fields were mapped for 16 L5A and 11 L5B neurones located below the layer 4 whisker-barrels. Average resting membrane potentials were -75.6 +/- 1.1 mV, and spontaneous action potential (AP) rates were 0.54 +/- 0.14 APs s(-1). Principal whisker (PW) evoked responses were similar in L5A and L5B neurones, with an average 5.0 +/- 0.6 mV postsynaptic potential (PSP) and 0.12 +/- 0.03 APs per stimulus. The layer 5A sub- and suprathreshold receptive fields (RFs) were more confined to the principle whisker than those of layer 5B. The basal dendritic arbors of layer 5A and 5B cells were located below both layer 4 barrels and septa, and the cell bodies were biased towards the barrel walls. Responses in both L5A and L5B developed slowly, with onset latencies of 10.1 +/- 0.5 ms and peak latencies of 33.9 +/- 3.3 ms. Contralateral multi-whisker stimulation evoked PSPs similar in amplitude to those of PW deflections; whereas, ipsilateral stimulation evoked smaller and longer latency PSPs. We conclude that in L5 a whisker deflection is represented in two ways: focally by L5A pyramids and more diffusely by L5B pyramids as a result of combining different inputs from lemniscal and paralemniscal pathways. The relevant output evoked by a whisker deflection could be the ensemble activity in the anatomically defined cortical modules associated with a single or a few barrel-columns.

Cocaine-induced vs. behaviour-related alterations of spontaneous and evoked discharge of somatosensory cortical neurons

While the abuse potential of cocaine stems mainly from its ability to increase dopaminergic transmission in limbic regions, drug actions on other monoamine-innervated circuits may contribute to the development and maintenance of cocaine addiction. Previous extracellular recordings in anaesthetized rats revealed a facilitatory influence of cocaine on primary sensory pathways, which could influence the processing of drug-related stimuli during the development of cocaine addiction. We further analysed these sensory effects of cocaine in freely behaving rats (n = 9). Using an array of eight microelectrodes chronically implanted in infragranular layers of primary somatosensory cortex, we recorded the basal activity of 40 single- and 64 multiunits and their response to electrical stimulation of the whisker pad before and after incremental doses of cocaine (0.25-2 mg/kg i.v.). Both spontaneous and cocaine-induced explorations were associated with elevated basal firing of the cortical neurons and suppression of their short-latency excitation and postexcitatory inhibition in response to the whisker-pad stimulation. In addition to exploration-related alterations, the administration of cocaine enhanced the long-latency rebound excitation induced by the whisker-pad stimulation. This component of the sensory response, which is more labile and does not seem to convey information about the physical characteristics of the stimulus, may participate in the processing of drug-related sensory stimuli.

Whisker movements evoked by stimulation of single pyramidal cells in rat motor cortex

Neuronal activity in the motor cortex is understood to be correlated with movements, but the impact of action potentials (APs) in single cortical neurons on the generation of movement has not been fully determined. Here we show that trains of APs in single pyramidal cells of rat motor cortex can evoke long sequences of small whisker movements. For layer-5 pyramids, we find that evoked rhythmic movements have a constant phase relative to the AP train, indicating that single layer-5 pyramids can reset the rhythm of whisker movements. Action potentials evoked in layer-6 pyramids can generate bursts of rhythmic whisking, with a variable phase of movements relative to the AP train. An increasing number of APs decreases the latency to onset of movement, whereas AP frequency determines movement direction and amplitude. We find that the efficacy of cortical APs in evoking whisker movements is not dependent on background cortical activity and is greatly enhanced in waking rats. We conclude that in vibrissae motor cortex sparse AP activity can evoke movements.

Tone-evoked excitatory and inhibitory synaptic conductances of primary auditory cortex neurons

In primary auditory cortex (AI) neurons, tones typically evoke a brief depolarization, which can lead to spiking, followed by a long-lasting hyperpolarization. The extent to which the hyperpolarization is due to synaptic inhibition has remained unclear. Here we report in vivo whole cell voltage-clamp measurements of tone-evoked excitatory and inhibitory synaptic conductances of AI neurons of the pentobarbital-anesthetized rat. Tones evoke an increase of excitatory synaptic conductance, followed by an increase of inhibitory synaptic conductance. The synaptic conductances can account for the gross time course of the typical membrane potential response. Synaptic excitation and inhibition have the same frequency tuning. As tone intensity increases, the amplitudes of synaptic excitation and inhibition increase, and the latency of synaptic excitation decreases. Our data indicate that the interaction of synaptic excitation and inhibition shapes the time course and frequency tuning of the spike responses of AI neurons.

Synaptic dynamics control the timing of neuronal excitation in the activated neocortical microcircuit

It is well established that sensory stimulation results in the activity of multiple functional columns in the neocortex. The manner in which neurones within each column are active in relation to each other is, however, not known. Multiple whole-cell recordings in activated neocortical slices from rat revealed diverse correlation profiles of excitatory synaptic input to different types of neurones. The specific correlation profile between any two neurones could be predicted by the settings of synaptic depression and facilitation at the input synapses. Simulations further showed that patterned activity is essential for synaptic dynamics to impose the temporal dispersion of excitatory input. We propose that synaptic dynamics choreograph neuronal activity within the neocortical microcircuit in a context-dependent manner.

Development of intrinsic properties and excitability of layer 2/3 pyramidal neurons during a critical period for sensory maps in rat barrel cortex

The development of layer 2/3 sensory maps in rat barrel cortex (BC) is experience dependent with a critical period around postnatal days (PND) 10-14. The role of intrinsic response properties of neurons in this plasticity has not been investigated. Here we characterize the development of BC layer 2/3 intrinsic responses to identify possible sites of plasticity. Whole cell recordings were performed on pyramidal cells in acute BC slices from control and deprived rats, over ages spanning the critical period (PND 12, 14, and 17). Vibrissa trimming began at PND 9. Spiking behavior changed from phasic (more spike frequency adaptation) to regular (less adaptation) with age, such that the number of action potentials per stimulus increased. Changes in spiking properties were related to the strength of a slow Ca(2+)-dependent afterhyperpolarization. Maturation of the spiking properties of layer 2/3 pyramidal neurons coincided with the close of the critical period and was delayed by deprivation. Other measures of excitability, including I-f curves and passive membrane properties, were affected by development but unaffected by whisker deprivation.

Sequential activation of microcircuits underlying somatosensory-evoked potentials in rat neocortex

Evoked cortical field potentials are widely used in neurophysiological studies into cortical functioning, but insight in the underlying neural mechanisms is severely hampered by ambiguities in the interpretation of the field potentials. The present study aimed at identifying the precise relationships between the primary evoked cortical field potential (the positive-negative [P1-N1]response) and the temporal and spatial sequence in which different local cortical micro-circuits are recruited. We electrically stimulated the median nerve and recorded field potentials using a 12-channel depth probe in somatosensory cortex of ketamine anesthetized rats. Current source density analysis was used and a grand average was constructed based on all individual animals taking into account individual differences in cortical layering. Manipulation of stimulus strength, selective averaging of single trial responses, and double-pulse stimulation, were used to help disentangle overlapping dipoles and to determine the sequence of neuronal events. We discriminated three phases in the generation of the P1-N1 wave. In the first phase, specific thalamic afferents depolarize both layer III and layer V pyramidal cells. In the second phase, superficial pyramidal cells are depolarized via supragranular intracortical projections. In the third phase, population spikes are generated in layer Vb pyramidal cells, associated with a distinct fast (approximately 1 ms) sink/source configuration. Axon-collaterals of layer Vb pyramidal cells produce an enhanced activation of the supragranular pyramidal cells in layer I-II, which generates N1.

Nonlinear encoding of tactile patterns in the barrel cortex

Cells in the rodent barrel cortex respond to vibrissa deflection with a brief excitatory component and a longer suppressive component. The response to a given deflection is thus scaled because of suppression induced by a preceding deflection, causing the neuronal response to be linked to the temporal properties of the peripheral stimulus. A paired-deflection stimulus was used to characterize the postexcitatory suppression and a 3-deflection stimulus was used to investigate the nonlinear response to patterns of whisker deflections in barbiturate-anesthetized Sprague-Dawley rats. The postexcitatory suppression was not dependent on a sensory-evoked action potential to the first deflection, implying that it is likely a subthreshold property of the network. The suppression induced by a deflection served to suppress both the excitatory and suppressive components of a subsequent neuronal response, thus effectively disinhibiting it. Two different response properties were observed in the recorded cells. Approximately 65% responded to a vibrissa deflection with an excitatory component followed by a suppressive component and 35% responded with excitation, suppression, and a subsequent rebound in excitation. Based on these observations of postexcitatory dynamics, a prediction method was used to estimate neuronal responses to more complex stimulus trains. Using the 2nd-order representation obtained from the paired-deflection stimulus, responses to general periodic deflection patterns were well predicted. A higher cutoff frequency was predicted for rebound cells compared with cells not exhibiting rebound excitation, consistent with experimental observations. The method also predicted the response of neurons to a random aperiodic deflection pattern. Therefore the temporal structure of cortical dynamics after a single deflection dictates the response to complex temporal patterns, which are more representative of stimuli encountered under natural conditions.

Synaptic basis of persistent activity in prefrontal cortex in vivo and in organotypic cultures

Persistent activity is observed in many cortical and subcortical brain regions, and may subserve a variety of functions. Within the prefrontal cortex (PFC), neurons transiently maintain information in working memory via persistent activity patterns; however, the mechanisms involved are largely unknown. The present study used intracellular recordings from deep layer PFC neurons in vivo and patch-clamp recordings from PFC neurons in organotypic brain slice cultures to examine the ionic mechanisms underlying persistent activity states evoked by various inputs. Persistent activity had consistent features regardless of the initiating stimulus; it was driven by non-NMDA glutamate receptors yet consisted of an initial GABA mediated component, followed by a prolonged synaptically mediated inward current that maintained the sustained depolarization on which rode many asynchronous GABA-mediated events. The stereotyped nature of the multiple-component persistent activity pattern reported here might be a common feature of interconnected cortical networks but within PFC could be related to the persistent activity required for working memory.

Nonlinear integration of sensory responses in the rat barrel cortex: an intracellular study in vivo

To study integration of converging sensory inputs on single cortical neurons, we performed intracellular recordings in vivo in the barrel cortex of the barbiturate-anesthetized rat. We deflected the principal whisker (PW) for each cell either alone or preceded (at 20, 50, and 100 msec) by the deflection of a small number of remote whiskers (RWs) far from the PW. The synaptic responses to both the PW and the RW were similar qualitatively and consisted of excitation followed by inhibition that comprised an early and a late component. The RW response was of smaller amplitude and more often subthreshold for action potential generation. The main effect of the RW deflection was a suppression of the subsequent response to the PW that was most pronounced at the 20 msec interval and decreased progressively at the 50 and 100 msec intervals. Suppression of the spike output of the cell was not caused by hyperpolarization (subtractive inhibition) but by a reduction in the EPSP amplitude (divisive inhibition), resulting in a highly sublinear summation of the two responses. The small decrease in input resistance caused by the RW responses is not consistent with synaptic shunting as the main cause of the reduction of the EPSP amplitude. Instead, our results suggest that suppression results from a decrease in the amount of synaptic input triggered by the PW, particularly the early excitation. We suggest that this process involves a reduction in reverberant granular cell excitation that is induced by PW deflection.

Dynamic receptive fields of reconstructed pyramidal cells in layers 3 and 2 of rat somatosensory barrel cortex

Whole-cell voltage recordings were made in vivo from subsequently reconstructed pyramidal neurons (n = 30) in layer 3 (L3) and layer 2 (L2) of the barrel cortex of urethane-anaesthetised rats. Average resting membrane potentials were well below (15-40 mV) action potential (AP) initiation threshold. The average spontaneous AP activity (0.068 +/- 0.22 APs s-1) was low. Principal whisker (PW) deflections evoked postsynaptic potentials (PSPs) in almost all cells of a PW column but evoked AP activity (0.031 +/- 0.056 APs per PW stimulus 6 deg deflection) was low indicating 'sparse' coding by APs. Barrel-related cells (n = 16) have their soma located above a barrel and project their main axon through the barrel whereas septum-related cells (n = 8) are located above and project their main axon through the septum between barrels. Both classes of cell had broad subthreshold receptive fields (RFs) which comprised a PW and several (> 8) surround whiskers (SuW). Barrel-related cells had shorter PSP onset latencies (9.6 +/- 4.6 ms) and larger amplitude PW stimulus responses (9.1 +/- 4.5 mV) than septum-related cells (23.3 +/- 16.5 ms and 5.0 +/- 2.8 mV, respectively). The dendritic fields of barrel-related cells were restricted, in the horizontal plane, to the PW column width. Their axonal arbors projected horizontally into several SuW columns, preferentially those representing whiskers of the same row, suggesting that they are the major anatomical substrate for the broad subthreshold RFs. In barrel-related cells the response time course varied with whisker position and subthreshold RFs were highly dynamic, expanding in size from narrow single-whisker to broad multi-whisker RFs, elongated along rows within 10-150 ms following a deflection. The response time course in septum-related cells was much longer and almost independent of whisker position. Their broad subthreshold RF suggests that L2/3 cells integrate PSPs from several barrel columns. We conclude that the lemniscal (barrel-related) and paralemniscal (septum-related) afferent inputs remain anatomically and functionally segregated in L2/3.

Spatiotemporal effects of microstimulation in rat neocortex: a parametric study using multielectrode recordings

Using microstimulation to imprint meaningful activity patterns into intrinsically highly interconnected neuronal substrates is hampered by activation of fibers of passage leading to a spatiotemporal "blur" of activity. The focus of the present study was to characterize the shape of this blur in the neocortex to arrive at an estimate of the resolution with which signals can be transmitted by multielectrode stimulation. The horizontal spread of significant unit activity evoked by near-threshold focal electrical stimulation (charge transfer 0.8-4.8 nC) and multielectrode recording in the face representation of the primary somatosensory cortex of ketamine anesthetized rats was determined to be about 1,350 microm. The evoked activity inside this range consisted in a sequence of fast excitatory response followed by an inhibition lasting >100 ms. These 2 responses could not be separated by varying the intensity of stimulation while a slow excitatory rebound after the inhibitory response was restricted to higher stimulus intensities (>2.4 nC). Stimulation frequencies of 20 and 40 Hz evoked repetitive excitatory response standing out against a continuous background of inhibition. At 5- and 10-Hz stimulation, the inhibitory response showed a complex interaction pattern attributed to highly sublinear superposition of individual inhibitory responses. The present data help to elucidate the neuronal underpinnings of behavioral effects of microstimulation. Furthermore, they provide essential information to determine spatiotemporal constraints for purposeful multielectrode stimulation in the neocortex.

The origin of cortical surround receptive fields studied in the barrel cortex

The neocortex is thought to be organized into functional columns of neurons, each of which processes an element of a larger representation. In the barrel cortex, the thalamic input to the column preferentially terminates in a barrel. To study the extent and nature of functional connections between columns, we measured the degree to which whisker responses are relayed between columns in the barrel cortex. Inactivating a single barrel by iontophoresis of the GABA(A) agonist muscimol abolished the representation of that barrel's whisker in neighboring barrels. Reactivating a single barrel by iontophoresis of the GABA(A) antagonist bicuculline while the rest of the cortex was blocked by muscimol led to single whisker receptive fields. Under the same conditions, septal cells tended to exhibit multiwhisker receptive fields. These studies demonstrate that the surround receptive fields of barrel cells are generated by intracortical transmission and that many septal cells derive a component of their surround receptive field from the thalamus.

Hyperpolarization-activated current Ih disconnects somatic and dendritic spike initiation zones in layer V pyramidal neurons

Layer V pyramidal cells of the somatosensory cortex operate with two spike initiation zones. Subthreshold depolarizations are strongly attenuated along the apical dendrite linking the somatic and distal dendritic spike initiation zones. Sodium action potentials, on the other hand, are actively back-propagating from the axon hillock into the apical tuft. There they can interact with local excitatory input leading to the generation of calcium action potentials. We investigated if and how back-propagating sodium action potentials alone, without concomitant excitatory dendritic input, can initiate calcium action potentials in the distal dendrite. In acute slices of the rat somatosensory cortex, layer V pyramidal cells were studied under current-clamp with simultaneous recordings from the soma and the apical dendrite. A train of four somatic action potentials had to reach high frequencies to induce calcium action potentials in the dendrite ("critical frequency," CF approximately 100 Hz). Depolarization in the dendrite reduced the CF, while hyperpolarization increased it. The CF depended on the presence of the hyperpolarization-activated current Ih: blockade with 20 microM 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyridinium chloride (ZD7288) reduced the CF to 68% of control. If the neurons were stimulated with noisy current injections, leading to in-vivo-like irregular spiking, no calcium action potentials were induced in the dendrite. However, after Ih channel blockade, calcium action potentials were frequently seen. These data suggest that Ih prevents initiation of the dendritic calcium action potential by proximal input alone. Dendritic calcium action potentials may therefore represent a unique signature for coincident somatic and dendritic activation.

The barrel cortex--integrating molecular, cellular and systems physiology

A challenge for neurobiology is to integrate information across many levels of research, ranging from behaviour and neuronal networks to cells and molecules. The rodent whisker signalling pathway to the primary somatosensory neocortex with its remarkable somatotopic barrel map is emerging as a key system for such integrative studies.

Targeted whole-cell recordings in the mammalian brain in vivo

While electrophysiological recordings from visually identified cell bodies or dendrites are routinely performed in cell culture and acute brain slice preparations, targeted recordings from the mammalian nervous system are currently not possible in vivo. The "blind" approach that is used instead is somewhat random and largely limited to common neuronal cell types. This approach prohibits recordings from, for example, molecularly defined and/or disrupted populations of neurons. Here we describe a method, which we call TPTP (two-photon targeted patching), that uses two-photon imaging to guide in vivo whole-cell recordings to individual, genetically labeled cortical neurons. We apply this technique to obtain recordings from genetically manipulated, parvalbumin-EGFP-positive interneurons in the somatosensory cortex. We find that both spontaneous and sensory-evoked activity patterns involve the synchronized discharge of electrically coupled interneurons. TPTP applied in vivo will therefore provide new insights into the molecular control of neuronal function at the systems level.

Band-pass response properties of rat SI neurons

Rats typically employ 4- to 12-Hz "whisking" movements of their vibrissae during tactile exploration. The intentional sampling of signals in this frequency range suggests that neural processing of tactile information may be differentially engaged in this bandwidth. We examined action potential responses in rat primary somatosensory cortex (SI) to a range of frequencies of vibrissa motion. Single vibrissae were mechanically deflected with 5-s pulse trains at rates </=40 Hz. As previously reported, vibrissa deflection evoked robust neural responses that consistently adapted to stimulus rates >/=3 Hz. In contrast with this low-pass feature of the response, several other characteristics of the response revealed bandpass response properties. While average evoked response amplitudes measured 0-35 ms after stimulus onset typically decreased with increasing frequency, the later components of the response (>15 ms post stimulus) were augmented at frequencies between 3 and 10 Hz. Further, during the steady state, both rate and temporal measures of neural activity, measured as total spike rate or vector strength (a measure of temporal fidelity of spike timing across cycles), showed peak signal values at 5-10 Hz. A minimal biophysical network model of SI layer IV, consisting of an excitatory and inhibitory neuron and thalamocortical input, captured the spike rate and vector strength band-pass characteristics. Further analyses in which specific elements were selectively removed from the model suggest that slow inhibitory influences give rise to the band-pass peak in temporal precision, while thalamocortical adaptation can account for the band-pass peak in spike rate. The presence of these band-pass characteristics may be a general property of thalamocortical circuits that lead rodents to target this frequency range with their whisking behavior.

Voltage-gated Na+ channels enhance the temporal filtering properties of electrosensory neurons in the torus

Regenerative processes enhance postsynaptic potential (PSP) amplitude and behaviorally relevant temporal filtering in more than one-third of electrosensory neurons in the torus semicircularis of Eigenmannia. Data from in vivo current-clamp intracellular recordings indicate that these "regenerative PSPs" can be divided in two groups based on their half-amplitude durations: constant duration (CD) and variable duration (VD) PSPs. CD PSPs have half-amplitude durations of between 20 and 60 ms that do not vary in relation to stimulus periodicity. In contrast, the half-amplitude durations of VD PSPs vary in relation to stimulus periodicity and range from approximately 10 to 500 ms. Injection of 0.1 nA sinusoidal current through the recording electrode demonstrated that CD PSPs and not VD PSPs can be elicited by voltage fluctuations alone. In addition, CD PSPs were blocked by intracellular application of either QX-314 or QX-222, whereas VD PSPs were not. These in vivo data suggest, therefore, that CD PSPs are mediated by voltage-dependent Na+ conductances.

Topography and synaptic shaping of direction selectivity in primary auditory cortex

The direction of frequency-modulated (FM) sweeps is an important temporal cue in animal and human communication. FM direction-selective neurons are found in the primary auditory cortex (A1), but their topography and the mechanisms underlying their selectivity remain largely unknown. Here we report that in the rat A1, direction selectivity is topographically ordered in parallel with characteristic frequency (CF): low CF neurons preferred upward sweeps, whereas high CF neurons preferred downward sweeps. The asymmetry of 'inhibitory sidebands', suppressive regions flanking the tonal receptive field (TRF) of the spike response, also co-varied with CF. In vivo whole-cell recordings showed that the direction selectivity already present in the synaptic inputs was enhanced by cortical synaptic inhibition, which suppressed the synaptic excitation of the non-preferred direction more than that of the preferred. The excitatory and inhibitory synaptic TRFs had identical spectral tuning, but with inhibition delayed relative to excitation. The spectral asymmetry of the synaptic TRFs co-varied with CF, as had direction selectivity and sideband asymmetry, and thus suggested a synaptic mechanism for the shaping of FM direction selectivity and its topographic ordering.

Imaging spatiotemporal dynamics of surround inhibition in the barrels somatosensory cortex

Sensory processing and its perception require that local information would also be available globally. Indeed, in the mammalian neocortex, local excitation spreads over large distances via the long-range horizontal connections in layer 2/3 and may spread over an entire cortical area if excitatory polysynaptic pathways are also activated. Therefore, a balance between local excitation and surround inhibition is required. Here we explore the spatiotemporal aspects of cortical depolarization and hyperpolarization of rats anesthetized with urethane. New voltage-sensitive dyes (VSDs) were used for high-resolution real-time visualization of the cortical responses to whisker deflections and cutaneous stimulations of the whisker pad. These advances facilitated imaging of ongoing activity and evoked responses even without signal averaging. We found that the motion of a single whisker evoked a cortical response exhibiting either one or three phases. During a triphasic response, there was first a cortical depolarization in a small cortical region the size of a single cortical barrel. Subsequently, this depolarization increased and spread laterally in an oval manner, preferentially along rows of the barrel field. During the second phase, the amplitude of the evoked response declined rapidly, presumably because of recurrent inhibition. Subsequently, the third phase exhibiting a depolarization rebound was observed and clear, and approximately 16 Hz oscillations were detected. Stimulus conditions revealing a net surround hyperpolarization during the second phase were also found. By using new, improved VSD, the present findings shed new light on the spatial parameters of the intricate spatiotemporal cortical interplay of inhibition and excitation.

Submillisecond synchronization of fast electrical oscillations in neocortex

Fast electrical oscillations (FOs; >200 Hz) in the sensory neocortex can be recorded in a variety of species, including humans, and may reflect extremely fast integration of sensory information. This report demonstrates that, in the whisker representation of rat cortex, multivibrissa stimulation produces propagating FO field potential patterns and time-locked unit activity that are sensitive to submillisecond delays in interstimulus intervals. We propose that FOs may be produced by synchronized population spikes and their subthreshold sequelas in cortical pyramidal cells. FOs serve to accurately mark stimulus onset as a phase-encoded excitatory signal, producing phase-sensitive interactions that, in the context of exploratory whisking, may extract features of an object under exploration.

Spatiotemporal dynamics of sensory responses in layer 2/3 of rat barrel cortex measured in vivo by voltage-sensitive dye imaging combined with whole-cell voltage recordings and neuron reconstructions

The spatiotemporal dynamics of the sensory response in layer 2/3 of primary somatosensory cortex evoked by a single brief whisker deflection was investigated by simultaneous voltage-sensitive dye (VSD) imaging and whole-cell (WC) voltage recordings in the anesthetized rat combined with reconstructions of dendritic and axonal arbors of L2/3 pyramids. Single and dual WC recordings from pyramidal cells indicated a strong correlation between the local VSD population response and the simultaneously measured subthreshold postsynaptic potential changes in both amplitude and time course. The earliest VSD response was detected 10-12 msec after whisker deflection centered above the barrel isomorphic to the stimulated principal whisker. It was restricted horizontally to the size of a single barrel-column coextensive with the dendritic arbor of barrel-column-related pyramids in L2/3. The horizontal spread of excitation remained confined to a single barrel-column with weak whisker deflection. With intermediate deflections, excitation spread into adjacent barrel-columns, propagating twofold more rapidly along the rows of the barrel field than across the arcs, consistent with the preferred axonal arborizations in L2/3 of reconstructed pyramidal neurons. Finally, larger whisker deflections evoked excitation spreading over the entire barrel field within approximately 50 msec before subsiding over the next approximately 250 msec. Thus the subthreshold cortical map representing a whisker deflection is dynamic on the millisecond time scale and strongly depends on stimulus strength. The sequential spatiotemporal activation of the excitatory neuronal network in L2/3 by a simple sensory stimulus can thus be accounted for primarily by the columnar restriction of L4 to L2/3 excitatory connections and the axonal field of barrel-related pyramids.

Quantitative comparison between functional imaging and single-unit spiking in rat somatosensory cortex

The profile of activity across rat somatosensory cortex on stimulation of a single whisker was examined using both intrinsic signal imaging and electrophysiological recording. In the same animals, under sodium pentobarbital anesthesia, the intrinsic signal response to a 5-Hz stimulation of whisker C2 was recorded through a thinned skull. Subsequently, the thinned skull was removed, and individual cortical neurons were recorded at multiple locations and in all cortical layers in response to the same whisker stimulation paradigm. The amplitude of the evoked response obtained with both techniques was quantified across the cortical surface with respect to distance (<or=1.6 mm) from the peak intrinsic signal activity. Cortical neurons were rated as having a significant or nonsignificant whisker-evoked response as compared with a baseline period of spontaneous firing; a minority of neurons exhibited a small but significant increase in neuronal spiking even at long distances (>1.6 mm) from the optically determined peak of activity. Overall, this analysis shows a significant correlation between the two techniques in terms of the profile of evoked activity across the cortical surface. Furthermore, this data set affords a detailed and quantitative comparison between the two activity-dependent techniques-one measuring an intrinsic decrease in light reflectance based largely on metabolic changes and one measuring neuronal firing patterns. Studies such as this, comparing directly between imaging and detailed electrophysiology, may influence the interpretation of the extent of the activated area as assessed with in vivo functional imaging techniques.

Postnatal development of GABAergic signalling in the rat lateral geniculate nucleus: presynaptic dendritic mechanisms

Diverse forms of GABAergic inhibition are found in the mature brain. To understand how this diversity develops, we studied the changes in morphology of inhibitory interneurons and changes in interneuron-mediated synaptic transmission in the rat dorsal lateral geniculate nucleus (dLGN). We found a steady expansion of the dendritic tree of interneurons over the first three postnatal weeks. During this period, the area around a thalamocortical cell from which GABA(A) inhibition could be elicited also expanded. Dendritic branching and burst firing in interneurons evolved more slowly. The distal dendrites of interneurons began to branch extensively after the third week, and at the same time burst firing appeared. The appearance of burst firing and an elaborated dendritic tree were accompanied by a pronounced GABA(B) inhibition of thalamocortical cells. Thus, GABA inhibition of thalamocortical cells developed from one type of GABA(A) inhibition (spatially restricted) in the young animal into two distinct types of GABA(A) inhibition (short- and long-range) and GABA(B) inhibition in the adult animal. The close temporal relationships between the development of the diverse forms of inhibition and the postnatal changes in morphology of local GABAergic interneurons in the dLGN suggest that postnatal dendritic maturation is an important presynaptic factor for the developmental time course of the various types of feedforward inhibition in thalamus.

Long-term in vivo imaging of experience-dependent synaptic plasticity in adult cortex

Do new synapses form in the adult cortex to support experience-dependent plasticity? To address this question, we repeatedly imaged individual pyramidal neurons in the mouse barrel cortex over periods of weeks. We found that, although dendritic structure is stable, some spines appear and disappear. Spine lifetimes vary greatly: stable spines, about 50% of the population, persist for at least a month, whereas the remainder are present for a few days or less. Serial-section electron microscopy of imaged dendritic segments revealed retrospectively that spine sprouting and retraction are associated with synapse formation and elimination. Experience-dependent plasticity of cortical receptive fields was accompanied by increased synapse turnover. Our measurements suggest that sensory experience drives the formation and elimination of synapses and that these changes might underlie adaptive remodelling of neural circuits.

Properties of mEPSCs recorded in layer II neurones of rat barrel cortex

Voltage-clamp recordings from layer II neurones in somatosensory cortex of rats aged between 12 and 17 days showed a high frequency of spontaneous postsynaptic currents (sPSCs), which on average was 33 +/- 13 Hz (s.d.). sPSCs were mediated largely by glutamatergic AMPA receptors. Their rates and amplitudes were independent of blocking sodium channels with 1 microM tetrodotoxin (TTX). Most of them, therefore, represent genuine miniature excitatory postsynaptic currents (mEPSCs). The rise time of the fastest (10 %) mEPSCs was 288 +/- 86 micros (10-90 %) and the half-width was 1073 +/- 532 micros. The amplitude was -5.9 +/- 1.1 pA with a coefficient of variation (CV) of 0.44 +/- 0.14. The rate of mEPSCs was very temperature sensitive with a Q(10) (33-37 degrees C) of 8.9 +/- 0.9. Due to this temperature sensitivity, we estimated that the microscope lamp contributed an increase in temperature of about 4 degrees C to the tissue in the focal volume of the condenser. Cell-type differences in the rate of mEPSCs were found between pyramidal/multipolar and bipolar cells. The latter had a frequency of about a third of that seen in the other cell groups. Recordings in layer II are ideally suited to investigate mechanisms of spontaneous transmitter release.

Signaling of layer 1 and whisker-evoked Ca2+ and Na+ action potentials in distal and terminal dendrites of rat neocortical pyramidal neurons in vitro and in vivo

Dendritic regenerative potentials play an important role in integrating and amplifying synaptic inputs. To understand how distal synaptic inputs are integrated and amplified, we made multiple simultaneous (double, triple, or quadruple) and sequential (4-12 paired) recordings from different locations of single tufted layer 5 pyramidal neurons in the cortex in vitro and studied the spatial and temporal properties of their dendritic regenerative potential initial zone. Recordings from the soma and from trunk, primary, secondary, tertiary, and quaternary tuft branches of the apical dendrite of these neurons reveal a spatially restricted low-threshold zone approximately 550-900 microm from the soma for Ca2+-dependent regenerative potentials. Dendritic regenerative potentials initiated in this zone have a clearly defined threshold and a refractory period, and they can propagate actively along the dendrite before evoking somatic action potentials. The detailed biophysical characterization of this dendritic action potential initiation zone allowed for the further investigation of dendritic potentials in the intact brain and their roles in information processing. By making whole-cell recordings from the soma and varied locations along the apical dendrite of 53 morphologically identified layer 5 pyramidal neurons in anesthetized rats, we found that three of the dendritic potentials characterized in vitro could be induced by spontaneous or whisker inputs in vivo. Thus layer 5 pyramidal neurons of the rat neocortex have a spatially restricted low-threshold zone in the apical dendrite, the activation or interaction of which with the axonal action potential initiation zone is responsible for multiple forms of regenerative potentials critical for integrating and amplifying sensory and modulatory inputs.