Modulation of receptive field properties of thalamic somatosensory neurons by the depth of anesthesia

Auditory evoked bursts in mouse visual cortex during isoflurane anesthesia

General anesthesia is not a uniform state of the brain. Ongoing activity differs between light and deep anesthesia and cortical response properties are modulated in dependence of anesthetic dosage. We investigated how anesthesia level affects cross-modal interactions in primary sensory cortex. To examine this, we continuously measured the effects of visual and auditory stimulation during increasing and decreasing isoflurane level in the mouse visual cortex and the subiculum (from baseline at 0.7 to 2.5 vol % and reverse). Auditory evoked burst activity occurred in visual cortex after a transition during increase of anesthesia level. At the same time, auditory and visual evoked bursts occurred in the subiculum, even though the subiculum was unresponsive to both stimuli previous to the transition. This altered sensory excitability was linked to the presence of burst suppression activity in cortex, and to a regular slow burst suppression rhythm (∼0.2 Hz) in the subiculum. The effect disappeared during return to light anesthesia. The results show that pseudo-heteromodal sensory burst responses can appear in brain structures as an effect of an anesthesia induced state change.

Correlated input reveals coexisting coding schemes in a sensory cortex

As in other sensory modalities, one function of the somatosensory system is to detect coherence and contrast in the environment. To investigate the neural bases of these computations, we applied different spatiotemporal patterns of stimuli to rat whiskers while recording multiple neurons in the barrel cortex. Model-based analysis of the responses revealed different coding schemes according to the level of input correlation. With uncorrelated stimuli on 24 whiskers, we identified two distinct functional categories of neurons, analogous in the temporal domain to simple and complex cells of the primary visual cortex. With correlated stimuli, however, a complementary coding scheme emerged: two distinct cell populations, similar to reinforcing and antagonist neurons described in the higher visual area MT, responded specifically to correlations. We suggest that similar context-dependent coexisting coding strategies may be present in other sensory systems to adapt sensory integration to specific stimulus statistics.

Role of cortical reorganization on the effect of 5-HT pharmacotherapy for spinal cord injury

Cortical reorganization or expansion of the intact cortical regions into the deafferented cortex after complete spinal transection in neonatally spinalized rats was shown to be essential for increases in weight-supported stepping at adulthood. The novel somatotopic organization identified in these animals can be induced by exercise or spinal transplants that bridge the site of injury. However, the role of cortical reorganization in increased weight-supported (WS) stepping after pharmacotherapy is unknown. For the neonatally spinalized rat model, the 5-HT(2C) receptor agonist 1-(m-chlorophenyl)-piperazine hydrochloride (mCPP) increases the number of WS steps taken when administered to adult rats spinalized as neonates (mCPP+) though not all animals showed this effect (mCPP-). Since no differences in the behavior of the animals off-drug has been demonstrated, it is unclear why acute administration of 5-HT affects only a subset of animals. One possibility is that differences in cortical organization between mCPP+ and mCPP- may contribute to the differences in the functional effect of mCPP. To test this, we recorded from single neurons in the deafferented hindlimb sensorimotor cortex during passive sensory stimulation of the cutaneous surface of the forepaws and during active sensorimotor stimulation of the forepaws while the animals locomoted on a motorized treadmill. Our results show that neurons recorded from mCPP+ animals increased their responsiveness to both passive and active stimulation off-drug in comparison to neurons from mCPP- animals. These data suggest that differences in the cortical organization of mCPP+ compared to mCPP- animals may be at least partially responsible for the effect of a 5-HT(2C) receptor agonist on functional outcome.

Spontaneous persistent activity in entorhinal cortex modulates cortico-hippocampal interaction in vivo

Persistent activity is thought to mediate working memory during behavior. Can it also occur during sleep? We found that the membrane potential of medial entorhinal cortex layer III (MECIII) neurons, a gateway between neocortex and hippocampus, showed spontaneous, stochastic persistent activity in vivo in mice during Up-Down state oscillations (UDS). This persistent activity was locked to the neocortical Up states with a short delay, but persisted over several cortical UDS cycles. Lateral entorhinal neurons did not show substantial persistence, and current injections similar to those used in vitro failed to elicit persistence in vivo, implicating network mechanisms. Hippocampal CA1 neurons' spiking activity was reduced during neocortical Up states, but was increased during MECIII persistent states. These results provide, to the best of our knowledge, the first direct evidence for persistent activity in MECIII neurons in vivo and reveal its contribution to cortico-hippocampal interaction that could be involved in working memory and learning of long behavioral sequences during behavior, and memory consolidation during sleep.

Surround suppression and sparse coding in visual and barrel cortices

During natural vision the entire retina is stimulated. Likewise, during natural tactile behaviors, spatially extensive regions of the somatosensory surface are co-activated. The large spatial extent of naturalistic stimulation means that surround suppression, a phenomenon whose neural mechanisms remain a matter of debate, must arise during natural behavior. To identify common neural motifs that might instantiate surround suppression across modalities, we review models of surround suppression and compare the evidence supporting the competing ideas that surround suppression has either cortical or sub-cortical origins in visual and barrel cortex. In the visual system there is general agreement lateral inhibitory mechanisms contribute to surround suppression, but little direct experimental evidence that intracortical inhibition plays a major role. Two intracellular recording studies of V1, one using naturalistic stimuli (Haider et al., 2010), the other sinusoidal gratings (Ozeki et al., 2009), sought to identify the causes of reduced activity in V1 with increasing stimulus size, a hallmark of surround suppression. The former attributed this effect to increased inhibition, the latter to largely balanced withdrawal of excitation and inhibition. In rodent primary somatosensory barrel cortex, multi-whisker responses are generally weaker than single whisker responses, suggesting multi-whisker stimulation engages similar surround suppressive mechanisms. The origins of suppression in S1 remain elusive: studies have implicated brainstem lateral/internuclear interactions and both thalamic and cortical inhibition. Although the anatomical organization and instantiation of surround suppression in the visual and somatosensory systems differ, we consider the idea that one common function of surround suppression, in both modalities, is to remove the statistical redundancies associated with natural stimuli by increasing the sparseness or selectivity of sensory responses.

Thalamic POm projections to the dorsolateral striatum of rats: potential pathway for mediating stimulus-response associations for sensorimotor habits

The dorsolateral part of the striatum (DLS) represents the initial stage for processing sensorimotor information in the basal ganglia. Although the DLS receives much of its input from the primary somatosensory (SI) cortex, peripheral somesthetic stimulation activates the DLS at latencies that are shorter than the response latencies recorded in the SI cortex. To identify the subcortical regions that transmit somesthetic information directly to the DLS, we deposited small quantities of retrograde tracers at DLS sites that displayed consistent time-locked responses to controlled whisker stimulation. The neurons that were retrogradely labeled by these injections were located mainly in the sensorimotor cortex and, to a lesser degree, in the amygdala and thalamus. Quantitative analysis of neuronal labeling in the thalamus indicated that the strongest thalamic input to the whisker-sensitive part of the DLS originates from the medial posterior nucleus (POm), a somesthetic-related region that receives inputs from the spinal trigeminal nucleus. Anterograde tracer injections in POm confirmed that this thalamic region projects to the DLS neuropil. In subsequent experiments, simultaneous recordings from POm and the DLS during whisker stimulation showed that POm consistently responds before the DLS. These results suggest that POm could transmit somesthetic information to the DLS, and this modality-specific thalamostriatal pathway may cooperate with the thalamostriatal projections that originate from the intralaminar nuclei.

Coding of apparent motion in the thalamic nucleus of the rat vibrissal somatosensory system

While exploring objects, rats make multiple contacts using their whiskers, thereby generating complex patterns of sensory information. The cerebral structures that process this information in the somatosensory system show discrete patterns of anatomically distinct units, each corresponding to one whisker. Moreover, the feedforward and feedback connections are remarkably topographic, with little cross-whisker divergence before reaching the cortical network. Despite this parallel design, information processing from several whiskers has been reported in subcortical nuclei. Here, we explored whether sensory neurons in the ventral posterior medial nucleus (VPM) of the thalamus encode emergent properties of complex multiwhisker stimulations. Using a 24-whisker stimulator, we tested the responses of VPM neurons to sequences of caudal deflections that generated an apparent motion in eight different directions across the whiskerpad. Overall, 45% of neurons exhibited an evoked increase in firing rate significantly selective to the direction of apparent motion of the global stimulus. Periods of suppression of firing rate were often observed, but were generally not selective. Global motion selectivity of VPM neurons could occur regardless of the extent and spatial organization of their receptive fields, and of their selectivity for the direction of motion of their principal whisker. To investigate whether the global selectivity could be due to corticothalamic feedback connections, we inactivated the barrel cortex while repeating the stimulation protocol. For most VPM neurons, the direction selectivity decreased but was still present. These results suggest that nonlinear processing of stimuli from different whiskers emerges in subcortical nuclei and is amplified by the corticofugal feedback.

Trial-to-trial correlation between thalamic sensory response and global EEG activity

Thalamic gating of sensory inputs to the cortex varies with behavioral conditions, such as sleep-wake cycles, or with different stages of anesthesia. Behavioral conditions in turn are accompanied by stereotypic spectral content of the EEG. In the rodent somatosensory system, the receptive field size of the ventral posteromedial thalamic nucleus (VPM) shrinks when anesthesia is deepened. Here we examined whether evoked thalamic responses are correlated with global EEG activity on a fine time scale of a few seconds. Trial-by-trial analysis of responses of VPM cells to whisker stimulation in lightly anesthetized rats indicated that increased EEG power in the delta band (1-4 Hz) was accompanied by a small, but highly significant, reduction in spontaneous and evoked thalamic firing. The opposite effect was found for the gamma EEG band (30-50 Hz). These significant correlations were not accompanied by an apparent change in the size of the receptive fields and were not EEG phase-related. The correlation between EEG and firing rate was observed only in neurons that responded to multiple whiskers and was higher for the non-principal whiskers. Importantly, the contributions of the two EEG bands to the modulation of VPM responses were to a large extent independent of each other. Our findings suggest that information conveyed by different whiskers can be rapidly modulated according to the global brain activity.

Functional role of exercise-induced cortical organization of sensorimotor cortex after spinal transection

Spinal cord transection silences neuronal activity in the deafferented cortex to cutaneous stimulation of the body and untreated animals show no improvement in functional outcome (weight-supported stepping) with time after lesion. However, adult rats spinalized since neonates that receive exercise therapy exhibit greater functional recovery and exhibit more cortical reorganization. This suggests that the change in the somatotopic organization of the cortex may be functionally relevant. To address this issue, we chronically implanted arrays of microwire electrodes into the infragranular layers of the hindlimb somatosensory cortex of adult rats neonatally transected at T8/T9 that received exercise training (spinalized rats) and of normal adult rats. Multiple, single neuron activity was recorded during passive sensory stimulation, when the animals were anesthetized, and during active sensorimotor stimulation during treadmill-induced locomotion when the animal was awake and free to move. Our results demonstrate that cortical neurons recorded from the spinalized rats that received exercise 1) had higher spontaneous firing rates, 2) were more likely to respond to both sensory and sensorimotor stimulations of the forelimbs, and also 3) responded with more spikes per stimulus than those recorded from normal rats, suggesting expansion of the forelimb map into the hindlimb map. During treadmill locomotion the activity of neurons recorded from neonatally spinalized rats was greater during weight-supported steps on the treadmill compared with the neuronal activity during nonweight supported steps. We hypothesize that this increased activity is related to the ability of the animal to take weight supported steps and that, therefore, these changes in cortical organization after spinal cord injury are relevant for functional recovery.

Olfactory cortex generates synchronized top-down inputs to the olfactory bulb during slow-wave sleep

The olfactory cortex is functionally isolated from the external odor world during slow-wave sleep. However, the neuronal activity pattern in the olfactory cortex and its functional roles during slow-wave sleep are not well understood. Here, we demonstrate in freely behaving rats that the anterior piriform cortex, a major area of the olfactory cortex, repeatedly generates sharp waves that are accompanied by synchronized discharges of numerous cortical neurons. Olfactory cortex sharp waves occurred relatively independently of hippocampal sharp waves. Current source density analysis showed that sharp wave generation involved the participation of recurrent association fiber synapses to pyramidal cells in the olfactory cortex. During slow-wave sleep, the olfactory bulb showed sharp waves that were in synchrony with olfactory cortex sharp waves, indicating that olfactory cortex sharp waves drove synchronized top-down inputs to the olfactory bulb. Based on these results, we speculate that the olfactory cortex sharp waves may play a role in the reorganization of bulbar neuronal circuits during slow-wave sleep.

Trial-to-trial variability in the responses of neurons carries information about stimulus location in the rat whisker thalamus

From the perspective of neural coding, the considerable trial-to-trial variability in the responses of neurons to sensory stimuli is puzzling. Trial-to-trial response variability is typically interpreted in terms of "noise" (i.e., it represents either intrinsic noise of the system or information unrelated to the stimuli). However, trial-to-trial response variability can be considerably different across stimuli, suggesting that it could also provide an important contribution to the information conveyed by the neural responses about the stimuli. To test this hypothesis, we addressed the problem of discriminating stimulus location from the spike-count responses of neurons recorded in the ventro-postero-medial (VPM) nucleus of the thalamus in anesthetized rats. Using a recently developed information theory approach, we verified that differences between stimuli in the trial-to-trial spike-count variability of the responses provided an important contribution to the overall information carried by the neurons. In addition, we found that the relatively reliable (sub-Poisson) firing regime of our VPM neurons was not only more informative, but also more redundant between neurons compared with a more variable (Poisson) firing regime with the same total number of spikes. The typical increase in trial-to-trial response variability from the periphery to the cortex could therefore serve as a strategy to reduce redundancy between neurons and promote efficient sparse coding distributed in large populations of neurons. Overall, our data suggest that the trial-to-trial response variability plays a critical role in establishing the trade-off between total information and redundancy between neurons in population codes.

Spinal direct current stimulation modulates the activity of gracile nucleus and primary somatosensory cortex in anaesthetized rats

Afferent somatosensory activity from the spinal cord has a profound impact on the activity of the brain. Here we investigated the effects of spinal stimulation using direct current, delivered at the thoracic level, on the spontaneous activity and on the somatosensory evoked potentials of the gracile nucleus, which is the main entry point for hindpaw somatosensory signals reaching the brain from the dorsal columns, and of the primary somatosensory cortex in anaesthetized rats. Anodal spinal direct current stimulation (sDCS) increased the spontaneous activity and decreased the amplitude of evoked responses in the gracile nucleus, whereas cathodal sDCS produced the opposite effects. At the level of the primary somatosensory cortex, the changes in spontaneous activity induced by sDCS were consistent with the effects observed in the gracile nucleus, but the changes in cortical evoked responses were more variable and state dependent. Therefore, sDCS can modulate in a polarity-specific manner the supraspinal activity of the somatosensory system, offering a versatile bottom-up neuromodulation technique that could potentially be useful in a number of clinical applications.

Anisotropic distribution of thalamocortical boutons in barrels

A characteristic feature of the somatosensory cortex in rodents is the presence of discrete cellular aggregates in layer 4 (barrels) that process input from the mystacial vibrissae. Just like thalamic cells that relay vibrissal information to the barrels, barrel cells display directional preference to whisker motion. The present study examined whether the projection of single thalamic cells into a barrel is consistent with the existence of an orderly map of direction preference. The direction preference of single thalamic cells was assessed, and axonal projections were visualized after juxtacellular labeling with biotinylated dextran. Results show that the terminal field of individual thalamic neurons in a barrel is markedly anisotropic and that the location of boutons with respect to the somatotopic map is either positively or negatively correlated with the angular tuning of the thalamic neuron. These results indicate that angular tuning is not represented across a systematic map with fixed anteroposterior/mediolateral coordinates in a barrel. The actual significance of the direction-dependent segregation of thalamocortical terminals in barrels may only come to light in the context of active sensing.

Cross-sensory modulation of primary sensory cortex is developmentally regulated by early sensory experience

The presence of cross-sensory influences on neuronal responses in primary sensory cortex has been observed previously using several different methods. To test this idea in rat S1 barrel cortex, we hypothesized that auditory stimuli combined with whisker stimulation ("cross-sensory" stimuli) may modify response levels to whisker stimulation. Since the brain has been shown to have a remarkable capacity to be modified by early postnatal sensory activity, manipulating postnatal sensory experiences would be predicted to alter the degree of cross-sensory interactions. To test these ideas, we raised rats with or without whisker deprivation and with or without postnatal exposure to repeated auditory clicks. We recorded extracellular responses under urethane anesthesia from barrel cortex neurons in response to principal whisker stimulation alone, to auditory click stimulation alone, or to a cross-sensory stimulus. The responses were compared statistically across different stimulus conditions and across different rearing groups. Barrel neurons did not generate action potentials in response to auditory click stimuli alone in any rearing group. However, in cross-sensory stimulus conditions the response magnitude was facilitated in the 0-15 ms post-whisker-stimulus epoch in all rearing conditions, whereas modulation of response magnitude in a later 15-30 ms post-whisker-stimulus epoch was significantly different in each rearing condition. The most significant cross-sensory effect occurred in rats that were simultaneously whisker deprived and click reared. We conclude that there is a modulatory type of cross-sensory auditory influence on normal S1 barrel cortex, which can be enhanced by early postnatal experiences.

Progressive motor cortex functional reorganization following 6-hydroxydopamine lesioning in rats

Many studies have attempted to correlate changes of motor cortex activity with progression of Parkinson's disease, although results have been controversial. In the present study we used intracortical microstimulation (ICMS) combined with behavioral testing in 6-hydroxydopamine hemilesioned rats to evaluate the impact of dopamine depletion on movement representations in primary motor cortex (M1) and motor behavior. ICMS allows for motor-effective stimulation of corticofugal neurons in motor areas so as to obtain topographic movements representations based on movement type, area size, and threshold currents. Rats received unilateral 6-hydroxydopamine in the nigrostriatal bundle, causing motor impairment. Changes in M1 were time dependent and bilateral, although stronger in the lesioned than the intact hemisphere. Representation size and threshold current were maximally impaired at 15 d, although inhibition was still detectable at 60-120 d after lesion. Proximal forelimb movements emerged at the expense of the distal ones. Movement lateralization was lost mainly at 30 d after lesion. Systemic L-3,4-dihydroxyphenylalanine partially attenuated motor impairment and cortical changes, particularly in the caudal forelimb area, and completely rescued distal forelimb movements. Local application of the GABA(A) antagonist bicuculline partially restored cortical changes, particularly in the rostral forelimb area. The local anesthetic lidocaine injected into the M1 of the intact hemisphere restored movement lateralization in the lesioned hemisphere. This study provides evidence for motor cortex remodeling after unilateral dopamine denervation, suggesting that cortical changes were associated with dopamine denervation, pathogenic intracortical GABA inhibition, and altered interhemispheric activity.

Comprehensive mapping of whisker-evoked responses reveals broad, sharply tuned thalamocortical input to layer 4 of barrel cortex

Cortical neurons are organized in columns, distinguishable by their physiological properties and input-output organization. Columns are thought to be the fundamental information-processing modules of the cortex. The barrel cortex of rats and mice is an attractive model system for the study of cortical columns, because each column is defined by a layer 4 (L4) structure called a barrel, which can be clearly visualized. A great deal of information has been collected regarding the connectivity of neurons in barrel cortex, but the nature of the input to a given L4 barrel remains unclear. We measured this input by making comprehensive maps of whisker-evoked activity in L4 of rat barrel cortex using recordings of multiunit activity and current source density analysis of local field potential recordings of animals under light isoflurane anesthesia. We found that a large number of whiskers evoked a detectable response in each barrel (mean of 13 suprathreshold, 18 subthreshold) even after cortical activity was abolished by application of muscimol, a GABA(A) agonist. We confirmed these findings with intracellular recordings and single-unit extracellular recordings in vivo. This constitutes the first direct confirmation of the hypothesis that subcortical mechanisms mediate a substantial multiwhisker input to a given cortical barrel.

Diverse patterns of odor representation by neurons in the anterior piriform cortex of awake mice

The mammalian piriform cortex receives direct synaptic input from the olfactory bulb and is likely the locus for the formation of odor percept. It remains unclear how individual cortical neurons encode olfactory information in unanesthetized animals. By single-cell recordings from head-restrained awake mice, we studied the odor response profiles of individual neurons in the anterior piriform cortex (aPCX). Neurons were juxtacellularly labeled, and their cell types were determined by their morphology and neurotransmitter phenotypes. We found a considerable level of variability in selectivity patterns among pyramidal neurons (PNs). Approximately one-quarter of PNs were broadly activated by structurally dissimilar odorants, whereas the excitations to the rest of PNs were highly selective. Broad inhibition was only observed from a subpopulation of PNs. GABAergic neurons displayed nonselective excitatory responses to test odorants and rarely exhibited inhibition. In contrast, non-GABAergic nonpyramidal neurons in the deep layer tended to be strongly inhibited by multiple different odorants. Our findings suggest that odor representation is accomplished by both broadly tuned and narrow-tuned PNs in the aPCX of awake animals. In addition, various types of interneurons may play different roles in the intracortical processing of olfactory information.

Central gating of fly optomotor response

We study the integration of multisensory and central input at the level of an identified fly motoneuron, the ventral cervical nerve motoneuron (VCNM) cell, which controls head movements of the animal. We show that this neuron receives input from a central neuron signaling flight activity, from two identified wide-field motion-sensitive neurons, from the wind-sensitive Johnston organ on the antennae, and from the campaniform sensillae of the halteres. We find that visual motion alone leads to only subthreshold responses. Only when it is combined with flight activity or wind stimuli does the VCNM respond to visual motion by modulating its spike activity in a directionally selective way. This nonlinear enhancement of visual responsiveness in the VCNM by central activity is reflected at the behavioral level, when compensatory head movements are measured in response to visual motion. While head movements of flies have only a small amplitude when flies are at rest, the response amplitude is increased by a factor of 30-40 during flight.

Phasic and tonic patterns of locus coeruleus output differentially modulate sensory network function in the awake rat

Neurons of the nucleus locus coeruleus (LC) discharge with phasic bursts of activity superimposed on highly regular tonic discharge rates. Phasic bursts are elicited by bottom-up input mechanisms involving novel/salient sensory stimuli and top-down decision making processes; whereas tonic rates largely fluctuate according to arousal levels and behavioral states. Although it is generally believed that these two modes of activity differentially modulate information processing in LC targets, the unique role of phasic versus tonic LC output on signal processing in cells, circuits, and neural networks of waking animals is not well understood. In the current study, simultaneous recordings of individual neurons within ventral posterior medial thalamus and barrel field cortex of conscious rats provided evidence that each mode of LC output produces a unique modulatory impact on single neuron responsiveness to sensory-driven synaptic input and representations of sensory information across ensembles of simultaneously recorded cells. Each mode of LC activation specifically modulated the relationship between sensory-stimulus intensity and the subsequent responses of individual neurons and neural ensembles. Overall these results indicate that phasic versus tonic modes of LC discharge exert fundamentally different modulatory effects on target neuronal circuits within the rodent trigeminal somatosensory system. As such, each mode of LC output may differentially influence signal processing as a means of optimizing behaviorally relevant neural computations within this sensory network. Likely the ability of the LC system to differentially regulate neural responses and local circuit operations according to behavioral demands extends to other brain regions including those involved in higher cognitive functions.

Environmental enrichment differentially modifies specific components of sensory-evoked activity in rat barrel cortex as revealed by simultaneous electrophysiological recordings and optical imaging in vivo

Environmental enrichment of laboratory animals leads to multi-faceted changes to physiology, health and disease prognosis. An important and under-appreciated factor in enhancing cognition through environmental manipulation may be improved basic sensory function. Previous studies have highlighted changes in cortical sensory map plasticity but have used techniques such as electrophysiology, which suffer from poor spatial resolution, or optical imaging of intrinsic signals, which suffers from low temporal resolution. The current study attempts to overcome these limitations by combining voltage-sensitive dye imaging with somatosensory-evoked potential (SEP) recordings: the specific aim was to investigate sensory function in barrel cortex using multi-frequency whisker stimulation under urethane anaesthesia. Three groups of rats were used that each experienced a different level of behavioural or environmental enrichment. We found that enrichment increased all SEP response components subsequent to the initial thalamocortical input, but only when evoked by single stimuli; the thalamocortical component remained unchanged across all animal groups. The optical signal exhibited no changes in amplitude or latency between groups, resembling the thalamocortical component of the SEP response. Permanent and extensive changes to housing conditions conferred no further enhancement to sensory function above that produced by the milder enrichment of regular handling and behavioural testing, a finding with implications for improvements in animal welfare through practical changes to animal husbandry.

Effects of urethane anaesthesia on sensory processing in the rat barrel cortex revealed by combined optical imaging and electrophysiology

The spatiotemporal dynamics of neuronal assemblies evoked by sensory stimuli have not yet been fully characterised, especially the extent to which they are modulated by prevailing brain states. In order to examine this issue, we induced different levels of anaesthesia, distinguished by specific electroencephalographic indices, and compared somatosensory-evoked potentials (SEPs) with voltage-sensitive dye imaging (VSDI) responses in the rat barrel cortex evoked by whisker deflection. At deeper levels of anaesthesia, all responses were reduced in amplitude but, surprisingly, only VSDI responses exhibited prolonged activation resulting in a delayed return to baseline. Further analysis of the optical signal demonstrated that the reduction in response amplitude was constant across the area of activation, resulting in a global down-scaling of the population response. The manner in which the optical signal relates to the various neuronal generators that produce the SEP signal is also discussed. These data provide information regarding the impact of anaesthetic agents on the brain, and show the value of combining spatial analyses from neuroimaging approaches with more traditional electrophysiological techniques.

Neonatal sensory deprivation and the development of cortical function: unilateral and bilateral sensory deprivation result in different functional outcomes

The normal development of sensory perception in mammals depends on appropriate sensory experience between birth and maturity. Numerous reports have shown that trimming some or all of the large mystacial vibrissa (whiskers) on one side of the face after birth has a detrimental effect on the maturation of cortical function. The objective of the present study was to understand the differences that occur after unilateral whisker trimming compared with those that occur after bilateral deprivation. Physiological deficits produced by bilateral trimming (BD) of all whiskers for 2 mo after birth were compared with the deficits produced by unilateral trimming (UD) for the same period of time using extracellular recording under urethan anesthesia from single cells in rat barrel cortex. Fast spiking (FSUs) and regular spiking (RSUs) units were separated and their properties compared in four subregions identified by histological reconstructions of the electrode penetrations, namely: layer IV barrel and septum, and layers II/III above a barrel and above a septum. UD upregulated responses in layer IV septa and in layers II/III above septa and perturbed the timing of responses to whisker stimuli. After BD, nearly all responses were decreased, and poststimulus latencies were increased. Circuit changes are proposed as an argument for how inputs arising from the spared whiskers project to the undeprived cortex and, via commissural fibers, could upregulate septal responses after UD. Following BD, more global neural deficits create a signature difference in the outcome of UD and BD in rat barrel cortex.

The Matrix: a new tool for probing the whisker-to-barrel system with natural stimuli

The whisker to barrel system in rodents has become one of the major models for the study of sensory processing. Several tens of whiskers (or vibrissae) are distributed in a regular manner on both sides of the snout. Many tactile discrimination tasks using this system need multiple contacts with more than one whisker to be solved. With the aim of mimicking those multi-whisker stimuli during electrophysiological recordings, we developed a novel mechanical stimulator composed of 24 independent multi-directional piezoelectric benders adapted to the five rows and the five caudal arcs of the rat whisker pad. The most widely used technology for producing mechanical deflections of the whiskers is based on piezoelectric benders that display a non-linear behavior when driven with high frequency input commands and, if not compensated, show high unwanted ringing at particular resonance frequencies. If not corrected, this non-linear behavior precludes the application of high frequency deflections and the study of cortical responses to behaviorally relevant stimuli. To cope with the ringing problem, a mechanical and a software based solutions have been developed. With these corrections, the upper bound of the linear range of the bender is increased to 1 kHz. This new device allows the controlled delivery of large scale natural patterns of whisker deflections characterized by rapid high frequency vibrations of multiple whiskers.

Modulation of visual responses by behavioral state in mouse visual cortex

Studies of visual processing in rodents have conventionally been performed on anesthetized animals, precluding examination of the effects of behavior on visually evoked responses. We have now studied the response properties of neurons in primary visual cortex of awake mice that were allowed to run on a freely rotating spherical treadmill with their heads fixed. Most neurons showed more than a doubling of visually evoked firing rate as the animal transitioned from standing still to running, without changes in spontaneous firing or stimulus selectivity. Tuning properties in the awake animal were similar to those measured previously in anesthetized animals. Response magnitude in the lateral geniculate nucleus did not increase with locomotion, demonstrating that the striking change in responsiveness did not result from peripheral effects at the eye. Interestingly, some narrow-spiking cells were spontaneously active during running but suppressed by visual stimuli. These results demonstrate powerful cell-type-specific modulation of visual processing by behavioral state in awake mice.

Corticofugal control of vibrissa-sensitive neurons in the interpolaris nucleus of the trigeminal complex

Trigeminal sensory nuclei that give rise to ascending pathways of vibrissal information are heavily linked by intersubnuclear connections. This is the case, for instance, of the principal trigeminal nucleus, which receives strong inhibitory input from the caudal sector of the interpolaris subnucleus. Because this inhibitory input can gate the relay of sensory messages through the lemniscal pathway, a central issue in vibrissal physiology is how brain regions that project to the interpolaris control the activity of inhibitory cells. In the present study, we examined how corticotrigeminal neurons of the primary and second somatosensory cortical areas control the excitability of interpolaris cells. Results show that these two cortical areas exert a differential control over the excitability of projection cells and intersubnuclear interneurons, and that this control also involves the recruitment of inhibitory cells in the caudalis subnucleus. These results provide a basic circuitry for a mechanism of disinhibition through which the cerebral cortex can control the relay of sensory messages in the lemniscal pathway. It is proposed that top-down control of brainstem circuits is prompted by motor strategies, expectations, and motivational states of the animal.

Origins of cortical layer V surround receptive fields in the rat barrel cortex

Layer IV of the barrel cortex contains an anatomical map of the contralateral whisker pad, which serves as a useful reference in relating receptive field properties of cells to the cortical columns in which they reside. Recent studies have shown that the degree to which the surround receptive fields of layer IV cells are generated intracortically or subcortically depends on whether they lie in barrel or septal columns. To investigate whether this is true for layer V cells, we blocked intracortical activity in the barrel cortex by infusing muscimol from the cortical surface and measuring spike responses to sensory stimulation in the presence of locally iontophoresed bicuculline. Layer V cells beneath barrels had small subcortically generated single- or double-whisker center receptive fields and larger intracortically generated six to seven whisker surround receptive fields. Conversely, septally located cells received multiwhisker input from both subcortical and cortical sources. Most properties of layer Va and layer Vb cells were very similar. However, layer Vb barrel neurons showed a relative lack of phasic inhibition evoked from sensory input compared with layer Va cells. The direct thalamic input to the layer V cells was not sufficient to evoke a sensory response in the absence of input from superficial layers. These findings suggest that despite the apparent overt similarity of layer V receptive fields, barrel and septal subdivisions process different sources of information within the barrel cortex.

Feedforward inhibition determines the angular tuning of vibrissal responses in the principal trigeminal nucleus

Trigeminal neurons that relay vibrissal messages to the thalamus receive input from first-order afferents that are tuned to different directions of whisker motion. This raises the question of how directional tuning is maintained in central relay stations of the whisker system. In the present study we performed a detailed analysis of the angular tuning properties of cells in the principal trigeminal nucleus of the rat. We found that stimulus direction systematically influences response latency, so that the degree of directional tuning and the preferred deflection angle computed with first-spike latency yielded results nearly similar to those obtained with spike counts. Furthermore, we found that inhibition sharpens directional selectivity, and that pharmacological blockade of inhibition markedly decreases the angular tuning of cellular responses. These results indicate that the angular tuning of cells in the first relay station of the vibrissal system is determined by fast feedforward inhibition, which shapes excitatory inputs at the very beginning of synaptic integration.

The effects of hypertonic saline and nicotinamide on sensorimotor and cognitive function following cortical contusion injury in the rat

Hypertonic saline (HTS) is an accepted treatment for traumatic brain injury (TBI). However, the behavioral and cognitive consequences following HTS administration have not thoroughly been examined. Recent preclinical evidence has suggested that nicotinamide (NAM) is beneficial for recovery of function following TBI. The current study compared the behavioral and cognitive consequences of HTS and NAM as competitive therapeutic agents for the treatment of TBI. Following controlled cortical impact (CCI), bolus administrations of NAM (500 mg/kg), 7.5% HTS, or 0.9% saline Vehicle (1.0 mL/kg) were given at 2, 24, and 48 h post-CCI. Behavioral results revealed that animals treated with NAM and HTS showed significant improvements in beam walk and locomotor placing compared to the Vehicle group. The Morris water maze (MWM) retrograde amnesia test was conducted on day 12 post-CCI and showed that all groups had significant retention of memory compared to injured, Vehicle-treated animals. Working memory was also assessed on days 8-20 using the MWM. The NAM and Vehicle groups quickly acquired the task; however, HTS animals showed no acquisition of this task. Histological examinations revealed that the HTS-treated animals lost significantly more cortical tissue than either the NAM or Vehicle-treated animals. HTS-treated animals showed a greater loss of hippocampal tissue compared to the other groups. In general, NAM showed a faster rate of recovery than HTS without this associated tissue loss. The results of this study reiterate the strengths of NAM following injury and show concerns with bolus administrations of HTS due to the differential effects on cognitive performance and apparent tissue loss.

Early frequency-dependent information processing and cortical control in the whisker pathway of the rat: electrophysiological study of brainstem nuclei principalis and interpolaris

The rat facial whiskers form a high-resolution sensory apparatus for tactile information coding and are used by these animals for the exploration and perception of their environment. Previous work on the rat vibrissae system obtained evidence for vibration-based feature extraction by the whiskers, texture classification by the cortical neurons, and "low-pass", "high-pass", and "band-pass" filtering properties in both thalamic and cortical neurons. However, no data are available for frequency-dependent information processing in the brainstem sensory trigeminal complex (STC), the first relay station of the vibrissae pathway. In the present paper, we studied the frequency-dependent processing characteristics of the STC nuclei that mainly project to the thalamus, nuclei principalis, and interpolaris. This is the first time that STC nuclei have been studied together via a wide range of stimulation frequencies (1-40 Hz), four different and complementary metrics, and the same experimental protocol. Moreover, the role of corticofugal projection to these nuclei as well as the influence of input from the whiskers has been analyzed. We show that both nuclei perform frequency-dependent coding of tactile information: low pass and band-pass filtering occurs for the spiking rate in short post-stimuli time intervals, high-pass and band-pass filtering occurs for the spiking rate in long trains of stimuli, and an increase of response latencies and low pass filtering occurs for phase-locked stimuli. These information-processing characteristics are neither imposed by the sensorimotor cortex nor introduced by the afferent fibres. The sensorimotor cortex exerts a distinct modulatory effect on each nucleus.

Optical imaging of digit topography in individual awake and anesthetized squirrel monkeys

Topographic maps and columnar structures are fundamental to cortical sensory information processing. Most of the knowledge about detailed topographic maps and columnar structure comes mainly from experiments conducted on anesthetized animals. Towards the goal of evaluating whether topographic maps change with respect to behavioral demands, we used intrinsic signal optical imaging in alert monkeys to examine the spatial specificity of cortical topographic representation. Specifically, the somatotopies of neighboring distal finger pad representation in areas 3b and 1 were examined in the same awake and anesthetized squirrel monkey. In comparison to the anesthetized animal, we found larger cortical activation sizes in the alert animal in area 3b, where activation widths were found to overlap with even non-adjacent digits. This may suggest that in the alert animal, there is less inhibition across the somatotopic map within area 3b.

Exercise induces cortical plasticity after neonatal spinal cord injury in the rat

Exercise-induced cortical plasticity is associated with improved functional outcome after brain or nerve injury. Exercise also improves functional outcomes after spinal cord injury, but its effects on cortical plasticity are not known. The goal of this investigation was to study the effect of moderate exercise (treadmill locomotion, 3 min/d, 5 d/week) on the somatotopic organization of forelimb and hindlimb somatosensory cortex (SI) after neonatal thoracic transection. We used adult rats spinalized as neonates because some of these animals develop weight-supported stepping, and, therefore, the relationship between cortical plasticity and stepping could also be examined. Acute, single-neuron mapping was used to determine the percentage of cortical cells responding to cutaneous forelimb stimulation in normal, spinalized, and exercised spinalized rats. Multiple single-neuron recording from arrays of chronically implanted microwires examined the magnitude of response of these cells in normal and exercised spinalized rats. Our results show that exercise not only increased the percentage of responding cells in the hindlimb SI but also increased the magnitude of the response of these cells. This increase in response magnitude was correlated with behavioral outcome measures. In the forelimb SI, neonatal transection reduced the percentage of responding cells to forelimb stimulation, but exercise reversed this loss. This restoration in the percentage of responding cells after exercise was accompanied by an increase in their response magnitude. Therefore, the increase in responsiveness of hindlimb SI to forelimb stimulation after neonatal transection and exercise may be due, in part, to the effect of exercise on the forelimb SI.

Design and evaluation of a low-cost respiratory monitoring device for use with anaesthetized animals

This report describes a simple, non-invasive electronic device that employs a compact accelerometer integrated circuit to transduce movements in the chest wall of an anaesthetized animal into an analogue signal that can be used to calculate the rate and relative depth of respiration. The device requires amplification by signal processing hardware/software which are common to most experimental laboratories. We assessed the sensitivity of the device by its ability to detect changes in respiratory patterns produced by modulating the depth of anaesthesia in isoflurane-anaesthetized Wistar rats. It is widely accepted that many anaesthetic agents affect respiratory patterns, especially respiratory rate (RR), which is often used as an important index of anaesthetic depth. Respiratory parameters obtained with the device were compared with concurrently recorded electroencephalographic and cardiac measures. Different concentrations of anaesthetic agent produced four depths of anaesthesia, identified using established electroencephalographic criteria. The accelerometer was attached easily and securely to the location of maximal chest wall movement and produced a strong respiratory signal that was detectable in all four anaesthetic stages. Deepening the anaesthesia produced a gradual decrease in RR, a decrease in dominant spectral frequency of the electroencephalogram (EEG) but no change in the heart rate. There was a significant correlation between RR and the dominant spectral frequency of the EEG, indicating that one useful application of the monitor could be to identify anaesthetic stages. The results demonstrate that respiratory parameters can be recorded using a simply constructed, low-cost device and suggest an application in the monitoring of anaesthetic depth.

Spike timing, spike count, and temporal information for the discrimination of tactile stimuli in the rat ventrobasal complex

The aim of this work was to investigate the role of spike timing for the discrimination of tactile stimuli in the thalamic ventrobasal complex of the rat. We applied information-theoretic measures and computational experiments on neurophysiological data to test the ability of single-neuron responses to discriminate stimulus location and stimulus dynamics using either spike count (40 ms bin size) or spike timing (1 ms bin size). Our main finding is not only that spike timing provides additional information over spike count alone, but specifically that the temporal aspects of the code can be more informative than spike count in the rat ventrobasal complex. Virtually all temporal information--i.e., information exclusively related to when the spikes occur--is conveyed by first spikes, arising mostly from latency differences between the responses to different stimuli. Although the imprecision of first spikes (i.e., the jitter) is highly detrimental for the information conveyed by latency differences, jitter differences can contribute to temporal information, but only if latency differences are close to zero. We conclude that temporal information conveyed by spike timing can be higher than spike count information for the discrimination of somatosensory stimuli in the rat ventrobasal complex.

Septal neurons in barrel cortex derive their receptive field input from the lemniscal pathway

Barrel-related circuits in the somatosensory cortex of rodents process vibrissal information conveyed through the lemniscal pathway. Yet, the origin of vibrissal input to interbarrrel regions (septa) remains an unsettled issue. A recurring proposal that never received conclusive experimental support is that septa-related circuits process paralemniscal inputs conveyed through the posterior group of the thalamus. Here we show that the receptive field of septal cells is independent of paralemniscal inputs, and that septal cells derive their receptive field input from neurons in the dorsal part of the thalamic barreloids. This result provides the missing piece of evidence for a separate pathway of vibrissal information that projects to septal columns of the barrel cortex.

Early bilateral sensory deprivation blocks the development of coincident discharge in rat barrel cortex

Several theories have proposed a functional role for synchronous neuronal firing in generating the neural code of a sensory perception. Synchronous neural activity develops during a critical postnatal period of cortical maturation, and severely reducing neural activity in a sensory pathway during this period could interfere with the development of coincident discharge among cortical neurons. Loss of such synchrony could provide a fundamental mechanism for the degradation of acuity shown in behavioral studies. We tested the hypothesis that synchronous discharge of barrel cortex neurons would fail to develop after sensory deprivation produced by bilateral whisker trimming from birth to postnatal day 60. By studying the correlated discharge of cortical neuron pairs, we found evidence for strong correlated firing in control animals, and this synchrony was almost absent among pairs of cortical barrel neurons in deprived animals. The degree of synchrony impairment was different in subregions of rat barrel cortex. The model that best fits the data is that cortical neurons receiving direct inputs from the primary sensory (lemniscal) pathway show the greatest decrement in synchrony following sensory deprivation, while neurons with diverse inputs from other areas of thalamus and cortex are relatively less affected in this dimension of cortical function.

Astrocytic calcium signaling: mechanism and implications for functional brain imaging

Astrocytes are electrically non-excitable cells that, on a slow time scale of seconds, integrate synaptic transmission by dynamic increases in cytosolic Ca2+. A number of groups have recently shown that astrocytic Ca2+ signaling regulates vascular tones and that astrocytes play a central role in functional hyperemia by Ca2+ -dependent release of Prostaglandin E2 (PGE2). Astrocytes are, however, not simple detectors of excitatory transmission, since a number of neuromodulator and hormones trigger elevations in astrocytic Ca2+ independently of synaptic transmission. Furthermore, astrocytes exhibit ex vivo intrinsic Ca2+ excitability, or spontaneous increases in Ca2+ that are not triggered by receptor activation. The notion that astrocytes can regulate vascular tone independently of synaptic transmission challenges the notion that changes in the blood oxygenation level dependent (BOLD) signal is directly proportional to neuronal activity and may thus require a reevaluation of the large body of data accumulated using functional magnetic resonance imaging (fMRI).

Behavioral state regulation of dendrodendritic synaptic inhibition in the olfactory bulb

Behavioral states regulate how information is processed in local neuronal circuits. Here, we asked whether dendrodendritic synaptic interactions in the olfactory bulb vary with brain and behavioral states. To examine the state-dependent change of the dendrodendritic synaptic transmission, we monitored changes in field potential responses in the olfactory bulb of urethane-anesthetized and freely behaving rats. In urethane-anesthetized rats, granule-to-mitral dendrodendritic synaptic inhibition was larger and longer when slow waves were present in the electroencephalogram (slow-wave state) than during the fast-wave state. The state-dependent alternating change in the granule-to-mitral inhibition was regulated by the cholinergic system. In addition, the frequency of the spontaneous oscillatory activity of local field potentials and periodic discharges of mitral cells in the olfactory bulb shifted in synchrony with shifts in the neocortical brain state. Freely behaving rats showed multilevel changes in dendrodendritic synaptic inhibition that corresponded to diverse behavioral states; the inhibition was the largest during slow-wave sleep state, and successively smaller during light sleep, awake immobility, and awake moving states. These results provide evidence that behavioral state-dependent global changes in cholinergic tone modulate dendrodendritic synaptic inhibition and the information processing mode in the olfactory bulb.

Prenatal exposure to benzo(a)pyrene impairs later-life cortical neuronal function

Prenatal exposure to environmental contaminants, such as benzo(a)pyrene [B(a)P] has been shown to impair brain development. The overarching hypothesis of our work is that glutamate receptor subunit expression is crucial for cortical evoked responses and that prenatal B(a)P exposure modulates the temporal developmental expression of glutamatergic receptor subunits in the somatosensory cortex. To characterize prenatal B(a)P exposure on the development of cortical function, pregnant Long Evans rats were exposed to low-level B(a)P (300 microg/kg BW) by oral gavage on gestational days 14-17. At this exposure dose, there was no significant effect of B(a)P on (1) the number of pups born per litter, (2) the pre-weaning growth curves and (3) initial and final brain to body weight ratios. Control and B(a)P-exposed offspring were profiled for B(a)P metabolites in plasma and whole brain during the pre-weaning period. No detectable levels of metabolites were found in the control offspring. However, a time-dependent decrease in total metabolite concentration was observed in B(a)P-exposed offspring. On PND100-120, cerebrocortical mRNA expression was determined for the glutamatergic NMDA receptor subunit (NR2B) in control and B(a)P-exposed offspring. Neural activity was also recorded from neurons in primary somatic sensory (barrel) cortex. Semiquantitative PCR from B(a)P-exposed offspring revealed a significant 50% reduction in NR2B mRNA expression in B(a)P-exposed offspring relative to controls. Recordings from B(a)P-exposed offspring revealed that N-methyl-d-aspartate (NMDA) receptor-dependent neuronal activity in barrel cortex evoked by whisker stimulation was also significantly reduced (70%) as compared to controls. Analysis showed that the greatest deficit in cortical neuronal responses occurred in the shorter latency epochs from 5 to 20 ms post-stimulus. The results suggest that in utero exposure to benzo(a)pyrene results in diminished mRNA expression of the NMDA NR2B receptor subunit to result in late life deficits in cortical neuronal activity in the offspring. The findings from this study lead to a strong prediction that in utero exposure to benzo(a)pyrene at a time when synapses are first formed and adjusted in strength by activity in the sensory pathways will produce a strong negative effect on brain function in offspring progeny.

Tactile stimulation evokes long-term synaptic plasticity in the granular layer of cerebellum

Several forms of long-term synaptic plasticity [long-term potentiation (LTP) and long-term depression (LTD)] have been reported in the cerebellar circuit in vitro, but their determination in vivo was still lacking in most cases. Here we show that, in the urethane-anesthetized rat, appropriate patterns of facial tactile stimulation as well as intracerebellar electrical stimulation can induce LTP and LTD in local field potentials recorded from the granular layer of Crus-IIa. LTD prevailed in control conditions, whereas LTP prevailed during local application of gabazine. No relevant plasticity was observed when gabazine and APV were coapplied. The pharmacological and kinetic properties of LTP and LTD in vivo were compatible with those reported in the granule cell layer in vitro (Mapelli and D'Angelo, 2007), suggesting that NMDA receptor-dependent plasticity was generated at the mossy fiber-granule cell synapse under the inhibitory control of the Golgi cell circuit. Interestingly, LTP and LTD were able to regulate the response latency to tactile stimulation, as expected from computational modeling of the expression mechanisms (Nieus et al., 2006). This result suggests that LTP and LTD could regulate the spatiotemporal pattern of granular layer responses to mossy fiber inputs.

Behavioral states, network states, and sensory response variability

We review data demonstrating that single-neuron sensory responses change with the states of the neural networks (indexed in terms of spectral properties of local field potentials) in which those neurons are embedded. We start with broad network changes--different levels of anesthesia and sleep--and then move to studies demonstrating that the sensory response plasticity associated with attention and experience can also be conceptualized as functions of network state changes. This leads naturally to the recent data that can be interpreted to suggest that even brief experience can change sensory responses via changes in network states and that trial-to-trial variability in sensory responses is a nonrandom function of network fluctuations, as well. We suggest that the CNS may have evolved specifically to deal with stimulus variability and that the coupling with network states may be central to sensory processing.

Mechanical resonance enhances the sensitivity of the vibrissa sensory system to near-threshold stimuli

The representation of high-frequency sensory information is a crucial problem faced by the nervous system. Rodent facial vibrissae constitute a high-resolution sensory system, capable of discriminating and detecting subtle changes in tactual input. During active sensing, the mechanical properties of vibrissae may play a key role in filtering sensory information and translating it into neural activity. Previous studies have shown that rat vibrissae resonate, conferring frequency specificity to trigeminal ganglion (NV) and primary somatosensory cortex (SI) neurons during suprathreshold sensory stimulation. In addition to frequency specificity, a further potential impact of vibrissa resonance is enhancement of sensitivity to near-threshold stimuli through signal amplification. To examine the effect of resonance on peri-threshold inputs (<or=80 microm at the vibrissa tip), we recorded NV and SI neurons during stimulation at multiple amplitudes and frequencies, and generated minimal amplitude tuning curves. Several novel findings emerged from this study. First, vibrissa resonance significantly lowered the threshold for evoked neural activity, in many cases by an order of magnitude compared to stimuli presented at off-resonance frequencies. When stimulated at the fundamental resonance frequency, motions as small as 8 microm at the vibrissa tip, corresponding to angular deflections of less than 0.2 degrees, drove neural firing in the periphery and cortex. Second, a closer match between vibrissal and neural frequency tuning was found for lower amplitude motions. Third, simultaneous paired recordings demonstrated that the minimal amplitude of resonant vibrissa stimulation required to evoke responses in SI increased significantly for recordings outside the primary vibrissa barrel column, providing additional evidence for somatotopically localized frequency columns. These data demonstrate that resonant amplification can increase the sensitivity of the vibrissa sensory system to an ecologically relevant range of low-amplitude, high-frequency stimuli.

Vibrissal responses of thalamic cells that project to the septal columns of the barrel cortex and to the second somatosensory area

The rodent somatosensory cortex contains barrel-related and septa-related circuits representing two separate streams of vibrissa information processing that differ in their response patterns and anatomical connections. Whereas barrel-related circuits process lemniscal inputs that transit through the thalamic barreloids, septa-related circuits process paralemniscal inputs and inputs that are relayed through the ventral lateral part of the ventral posterior medial nucleus (VPMvl). Septa-projecting thalamic afferents also target the secondary somatosensory cortical area. Although a number of studies have examined response properties in the lemniscal pathway, and demonstrated that barreloids receive feedback from specific sets of corticothalamic and reticular thalamic neurons, such information is currently lacking for the VPMvl. In the present study, we show that in sharp contrast to the relay cells of the barreloids VPMvl neurons exhibit large multiwhisker receptive fields that are independent of input from the principal trigeminal nucleus. Results also suggest that the topography of receptive fields and response properties in VPMvl rely on converging input from neurons of the interpolaris trigeminal nucleus. Tracer injection and single-cell labeling further reveal that the VPMvl receives input from specific populations of reticular thalamic and corticothalamic neurons. Together, these results confirm the status of the VPMvl as a thalamic relay of an independent parallel pathway of vibrissa information processing. They further indicate that a sensory pathway does not merely consist on a three-neuron chain that links the vibrissae to the cerebral cortex, but that it also involves specific sets of topographically related corticothalamic and reticular thalamic projections.

Cortical transformation of wide-field (multiwhisker) sensory responses

In the barrel cortex of rodents, cells respond to a principal whisker (PW) and more weakly to several adjacent whiskers (AWs). Here we show that compared with PW responses, simultaneous wide-field stimulation of the PW and several AWs enhances short-latency responses and suppresses long-latency responses. Multiwhisker enhancement and suppression is first seen at the level of the cortex in layer 4 and not in the ventroposterior medial thalamus. Within the cortex, enhancement is manifested as a reduction in spike latency in layer 4 but also as an increase in spike probability in layer 2/3. Intracellular recordings revealed that multiwhisker enhancement of short-latency responses is caused by synaptic summation that can be explained by synaptic cooperativity (i.e., convergence of synaptic inputs activated by different whiskers). Conversely, multiwhisker suppression of long-latency responses is due to increased recruitment of inhibition in cortical cells. Interestingly, the ability to differentiate multiwhisker and PW responses is lost during rapid sensory adaptation caused by high-frequency whisker stimulation. The results reveal that simultaneous and temporally dispersed wide-field sensory inputs are discriminated at the level of single cells in barrel cortex with high temporal resolution, but the ability to compute this difference is highly dynamic and dependent on the level of adaptation in the thalamocortical network.

State-dependent bidirectional modification of somatic inhibition in neocortical pyramidal cells

Cortical pyramidal neurons alter their responses to input signals depending on behavioral state. We investigated whether changes in somatic inhibition contribute to these alterations. In layer 5 pyramidal neurons of rat visual cortex, repetitive firing from a depolarized membrane potential, which typically occurs during arousal, produced long-lasting depression of somatic inhibition. In contrast, slow membrane oscillations with firing in the depolarized phase, which typically occurs during slow-wave sleep, produced long-lasting potentiation. The depression is mediated by L-type Ca2+ channels and GABA(A) receptor endocytosis, whereas potentiation is mediated by R-type Ca2+ channels and receptor exocytosis. It is likely that the direction of modification is mainly dependent on the ratio of R- and L-type Ca2+ channel activation. Furthermore, somatic inhibition was stronger in slices prepared from rats during slow-wave sleep than arousal. This bidirectional modification of somatic inhibition may alter pyramidal neuron responsiveness in accordance with behavioral state.

Altered neurovascular coupling during information-processing states

Brain imaging techniques rely on changes in blood flow, volume and oxygenation to infer the loci and magnitude of changes in activity. Although progress has been made in understanding the link between stimulus-evoked neural activity and haemodynamics, the extent to which neurovascular-coupling relationships remain constant during different states of baseline cortical activity is poorly understood. Optical imaging spectroscopy, laser Doppler flowmetry and electrophysiology were used to measure haemodynamics and neural activity in the barrel cortex of anaesthetized rats. The responses to stimulation of the whisker pad were recorded during quiescence and cortical desynchronization produced by stimulation of the brainstem. Cortical desynchronization was accompanied by increases in baseline blood flow, volume and oxygenation. Haemodynamic responses to low-frequency whisker stimuli (1 Hz) were attenuated during arousal compared with that observed during quiescence. During arousal it was possible to increase stimulus-evoked haemodynamics by increasing the frequency of the stimulus. Neural responses to low-frequency stimuli were also attenuated but to a far lesser extent than the reduction in the accompanying haemodynamics. In contrast, neuronal activity evoked by high-frequency stimuli (40 Hz) was enhanced during arousal, but induced haemodynamic responses of a similar magnitude compared with that observed for the same high-frequency stimulus presented during quiescence. These data suggest that there may be differences in stimulus-evoked neural activity and accompanying haemodynamics during different information-processing states.

Laterodorsal nucleus of the thalamus: A processor of somatosensory inputs

The laterodorsal (LD) nucleus of the thalamus has been considered a "higher order" nucleus that provides inputs to limbic cortical areas. Although its functions are largely unknown, it is often considered to be involved in spatial learning and memory. Here we provide evidence that LD is part of a hitherto unknown pathway for processing somatosensory information. Juxtacellular and extracellular recordings from LD neurons reveal that they respond to vibrissa stimulation with short latency (median = 7 ms) and large magnitude responses (median = 1.2 spikes/stimulus). Most neurons (62%) had large receptive fields, responding to six and more individual vibrissae. Electrical stimulation of the trigeminal nucleus interpolaris (SpVi) evoked short latency responses (median = 3.8 ms) in vibrissa-responsive LD neurons. Labeling produced by anterograde and retrograde neuroanatomical tracers confirmed that LD neurons receive direct inputs from SpVi. Electrophysiological and neuroanatomical analyses revealed also that LD projects upon the cingulate and retrosplenial cortex, but has only sparse projections to the barrel cortex. These findings suggest that LD is part of a novel processing stream involved in spatial orientation and learning related to somatosensory cues.