Local intra- and interlaminar connections in mouse barrel cortex

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.

Sensorimotor corticocortical projections from rat barrel cortex have an anisotropic organization that facilitates integration of inputs from whiskers in the same row

We used a dual anterograde-tracing paradigm to characterize the organization of corticocortical projections from primary somatosensory (SI) barrel cortex. In one group of rats, biotinylated dextran amine (BDA) and Fluoro-Ruby (FR) were injected into separate barrel columns that occupied the same row of barrel cortex; in the other group, the tracers were deposited into barrel columns residing in different rows. The labeled corticocortical terminals in the primary motor (MI) and secondary somatosensory (SII) cortices were plotted, and digital reconstructions of these plots were quantitatively analyzed. In all cases, labeled projections from focal tracer deposits in SI barrel cortex terminated in elongated, row-like strips of cortex that corresponded to the whisker representations of the MI or SII cortical areas. When both tracers were injected into separate parts of the same SI barrel row, FR- and BDA-labeled terminals tended to merge into a single strip of labeled MI or SII cortex. By comparison, when the tracers were placed in different SI barrel rows, both MI and SII contained at least two row-like FR- and BDA-labeled strips that formed mirror image representations of the SI injection sites. Quantitative analysis of these labeling patterns revealed three major findings. First, labeled overlap in SII was significantly greater for projections from the same barrel row than for projections from different barrel rows. Second, in the infragranular layers of MI but not in the supragranular layers, labeled overlap was significantly higher for projections from the same SI barrel row. Finally, in all layers of SII and in the infragranular layers of MI, the amount of labeled overlap was proportional to the proximity of the tracer injection sites. These results indicate that SI projections to MI and SII have an anisotropic organization that facilitates the integration of sensory information received from neighboring barrels that represent whiskers in the same row.

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.

Development of columnar topography in the excitatory layer 4 to layer 2/3 projection in rat barrel cortex

The excitatory feedforward projection from layer (L) 4 to L2/3 in rat primary somatosensory (S1) cortex exhibits precise, columnar topography that is critical for columnar processing of whisker inputs. Here, we characterize the development of axonal topography in this projection using single-cell reconstructions in S1 slices. In the mature projection [postnatal day (P) 14-26], axons of L4 cells extending into L2/3 were confined almost entirely to the home barrel column, consistent with previous results. At younger ages (P8-11), however, axonal topography was significantly less columnar, with a large proportion of branches innervating neighboring barrel columns representing adjacent whisker rows. Mature topography developed from this initial state by targeted axonal growth within the home column and by growth of barrel columns themselves. Raising rats with all or a subset of whiskers plucked from P8-9, manipulations that induce reorganization of functional whisker maps and synaptic depression at L4 to L2/3 synapses, did not alter normal anatomical development of L4 to L2/3 axons. Thus, development of this projection does not require normal sensory experience after P8, and deprivation-induced reorganization of whisker maps at this age is unlikely to involve physical remodeling of L4 to L2/3 axons.

Circuit analysis of experience-dependent plasticity in the developing rat barrel cortex

Sensory deprivation during a critical period reduces spine motility and disrupts receptive field structure of layer 2/3 neurons in rat barrel cortex. To determine the locus of plasticity, we used laser scanning photostimulation, allowing us to rapidly map intracortical synaptic connectivity in brain slices. Layer 2/3 neurons differed in their spatial distributions of presynaptic partners: neurons directly above barrels received, on average, significantly more layer 4 input than those above the septa separating barrels. Complementary connectivity was found in deprived cortex: neurons above septa were now strongly coupled to septal regions, while connectivity between barrel regions and layer 2/3 was reduced. These results reveal competitive interactions between barrel and septal circuits in the establishment of precise intracortical circuits.

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.

Plasticity of cortical projections after stroke

Ischemic stroke produces cell death and disability, and a process of repair and partial recovery. Plasticity within cortical connections after stroke leads to partial recovery of function after the initial injury. Physiologically, cortical connections after stroke become hyperexcitable and more susceptible to the induction of LTP Stroke produces changes in the distribution and laterality of sensory, motor, and language representations within the brain that correlate with functional recovery. Anatomically, ischemic lesions induce axonal sprouting within local, intracortical projections and long distance, interhemispheric projections. This postischemic axonal sprouting establishes substantially new patterns of cortical connections with de-afferented or partially damaged brain areas. Axonal sprouting after ischemic lesions is induced by a transient pattern of synchronous, low-frequency neuronal activity in a network of cortical areas connected to the infarct. This pattern of neuronal activity that induces axonal sprouting in the adult after ischemic lesions resembles that seen in the developing brain during axonal elongation and synaptogenesis. Thus, stroke induces a process of remapping and reconnection within the adult brain through changes in neuronal activity that may involve a reactivation of developmental programs in areas connected to the infarct.

Developmental patterns of cadherin expression and localization in relation to compartmentalized thalamocortical terminations in rat barrel cortex

The wiring of synaptic circuitry during development is remarkably precise, but the molecular interactions that enable such precision remain largely to be defined. Cadherins are cell adhesion molecules hypothesized to play roles in axon growth and synaptic targeting during development. We previously showed that N-cadherin localizes to ventrobasal (VB) thalamocortical synapses in rat somatosensory (barrel) cortex during formation of the whisker-map in layer IV (Huntley and Benson [1999] J. Comp. Neurol. 407:453-471). Such specific association of N-cadherin with one identified afferent pathway raises the prediction that other cadherins are expressed in barrel cortex and that these are, in some combination, also differentially associated with distinct inputs. Here, we first show that N-cadherin and three other classic cadherins (cadherin-6, -8, and -10) are expressed contemporaneously in barrel cortex with relative levels of postnatal expression that are highest during the first 2 weeks, when afferent and intrinsic circuitries are forming and synaptogenesis is maximal. Each displayed distinct, but partly overlapping laminar patterns of expression that changed over time. Cadherin-8 probe hybridization formed a particularly striking pattern of intermittent, columnar patches extending from layer V through layer III, which was first detectable at approximately postnatal day 3. The patches were centered precisely over regions of dysgranular layer IV and, in the whisker barrel field, over barrel septa. This pattern is similar to that formed by the terminal distribution of thalamocortical afferents arising from the posterior nucleus (POm), suggesting cadherin-8 association with the POm thalamocortical synaptic circuit. Consistent with this, cadherin-8 mRNAs were enriched in the POm nucleus, and cadherin-8 immunolabeling in layer IV was enriched in barrel septa and codistributed with labeled POm thalamocortical synaptic-like puncta. The striking molecular parcellation of at least two different cadherins to the two, converging thalamic pathways that terminated in non-overlapping barrel center and septal compartments in layer IV strongly suggested that cadherins provide requisite molecular recognition and targeting that enable precise construction of thalamocortical and other synaptic circuitry.

Long-term depression and long-term potentiation in horizontal connections of the barrel cortex

Synaptic plasticity of horizontally orientated connections between barrels, in the barrel cortex of adult mice, was studied in slice preparations cut across rows of barrels. Field potentials were evoked in the middle of one barrel column (in layer IV or V) and recorded in the neighbouring barrel (in layer IV and V). In layer IV, long-term depression (LTD) by 26.5 +/- 5% was first induced by a low-frequency stimulation (2 Hz) applied for 10 min. After 30 min, theta-burst stimulation was delivered to previously depressed connections, resulting in long-term potentiation (LTP) by 28.8 +/- 11.8%. When theta-burst stimulation was delivered without an earlier low-frequency stimulation, no LTP was induced. Similar results were obtained in layer V connections (LTD: 40.6 +/- 12.5%; LTP: 26.9 +/- 12.5%). In layer IV, the application of 100 micro m d,l-2-amino-5-phosphonovaleric acid (APV), an antagonist of NMDA receptors, blocked the induction of both LTD and LTP. These experiments show that a potential for synaptic plasticity is retained in granular and infragranular layers of adult mice.

Anatomical pathways and molecular mechanisms for plasticity in the barrel cortex

The barrel cortex has yielded a wealth of information about cortical plasticity in recent years. Barrel cortex is one of the few cortical areas studied so far where plasticity can be examined from birth through to adulthood. This review looks at plasticity mechanisms in three periods of life: early post-natal development, adolescence and adulthood. Separate consideration is given to depression and potentiation mechanisms. Plasticity can be induced in barrel cortex by whisker deprivation. Single whisker experience leads to expansion of the area of cortex responding to the spared whisker. In early post-natal life, plasticity occurs in thalamocortical pathways, while later in adolescence, intracortical pathways become more important. Ablation of the spared whisker's barrel prevents expression of plasticity in the cortex. A row of lesions between the spared and an adjacent barrel prevents expression of plasticity in the adjacent barrel. This evidence, together with latency of response data and an analysis of pathways capable of inducing long-term potentiation (LTP) within barrel cortex, leads to the view that horizontal and/or diagonal pathways between barrels are responsible for plasticity expression. The mouse has become the most commonly mutated mammalian species and has a well-developed barrel cortex. Therefore, mutations can be used to study the role of particular molecules in experience-dependent plasticity of barrel cortex. Through this work, it has become clear that the major post-synaptic density protein, alpha-CaMKII, and its T286 autophosphorylation site are essential for experience-dependent plasticity. This points to a major role for excitatory transmission in cortical plasticity and raises the possibility that LTP like mechanisms are involved. Furthermore, transgenic mice carrying a reporter gene for CRE have provided evidence that CRE-mediated gene expression is also involved in barrel cortex plasticity. This view is supported by studies on alpha/delta CREB knockouts, and provides a starting point for studying the role of gene expression in experience-dependent cortical plasticity.

Spatial integration of vascular changes with neural activity in mouse cortex

The authors evaluated representations of discretely activated, neighboring brain regions using real-time optical intrinsic signals by transcranial imaging with 540-nm and 610-nm broadband illumination of the mouse barrel cortex. Iron filings were glued to two neighboring whiskers (C2 + D2) that were stimulated magnetically, singly and together. Real-time images were collected, averaged, and analyzed statistically. Postmortem filling of arteries with fluorescent beads was shown in relation to histochemical staining of barrels to accurately relate surface changes to functional cortical columns. Significant optical intrinsic signal changes are related to overlapping distributions of arterioles that feed the two separate areas. Activation of adjacent and interacting cortical columns leads not only to increased magnitude of vascular responses in those columns, but also to wider spatial extent of absorption changes occurring principally in areas of cortex fed by vessels upstream of the active cortex. The localization of changing hemoglobin absorption around upstream blood vessels and their vascular domains suggests that propagated vasodilation of upstream parent vessels is greater when vasodilatory signals from separate areas of active cortex converge on common arterioles that feed them.

Functional independence of layer IV barrels

Our aim was to investigate the patterns of functional inputs and outputs from individual barrels in the mouse somatosensory cortex, and to test the hypothesis that individual barrels in layer IV are functionally independent of direct inputs from neighboring barrels. In a mouse in vitro slice preparation of the barrel cortex, we recorded voltage-sensitive dye signals evoked in response to microstimulation of a single barrel. Activity propagated from the stimulated barrel to the supragranular layers, where it spread to activate several barrel columns. However, in no instance did activity propagate directly from the stimulated barrel to neighboring barrels. Neither suppression of GABAergic inhibition, nor activation of N-methyl-D-aspartate receptors, revealed direct interbarrel interactions. By contrast, microstimulation in the supra- or infragranular layers resulted in direct propagation of activity to neighboring barrel columns. We conclude that the neurons within individual barrels are functionally independent of direct inputs from neighboring barrels. This suggests that the response properties of layer IV barrel neurons are shaped primarily by their presynaptic thalamic afferents and by intrabarrel interactions, and that these responses are independent of direct inputs from neighboring barrels.

Functionally independent columns of rat somatosensory barrel cortex revealed with voltage-sensitive dye imaging

Whisker movement is somatotopically represented in rodent neocortex by electrical activity in clearly defined barrels, which can be visualized in living brain slices. The functional architecture of this part of the cortex can thus be mapped in vitro with respect to its physiological input and compared with its anatomical architecture. The spatial extent of excitation was measured at high temporal resolution by imaging optical signals from voltage-sensitive dye evoked by stimulation of individual barrels in layer 4. The optical signals correlated closely with subthreshold EPSPs recorded simultaneously from excitatory neurons in layer 4 and layer 2/3, respectively. Excitation was initially (<2 msec) limited to the stimulated barrel and subsequently (>3 msec) spread in a columnar manner into layer 2/3 and then subsided in both layers after approximately 50 msec. The lateral extent of the response was limited to the cortical column defined structurally by the barrel in layer 4. Two experimental interventions increased the spread of excitation. First, blocking GABA(A) receptor-mediated synaptic inhibition caused excitation to spread laterally throughout wide regions of layer 2/3 and layer 5 but not into neighboring barrels, suggesting that the local excitatory connections within layer 4 are restricted to single barrels and that inhibitory neurons control spread in supragranular and infragranular layers. Second, NMDA receptor-dependent increase of the spread of excitation was induced by pairing repetitive stimulation of a barrel column with coincident stimulation of layer 2/3 in a neighboring column. Such plasticity in the spatial extent of excitation in a barrel column could underlie changes in cortical map structure induced by alterations of sensory experience.

Figuring space by time

Sensory information is encoded both in space and in time. Spatial encoding is based on the identity of activated receptors, while temporal encoding is based on the timing of activation. In order to generate accurate internal representations of the external world, the brain must decode both types of encoded information, even when processing stationary stimuli. We review here evidence in support of a parallel processing scheme for spatially and temporally encoded information in the tactile system and discuss the advantages and limitations of sensory-derived temporal coding in both the tactile and visual systems. Based on a large body of data, we propose a dynamic theory for vision, which avoids the impediments of previous dynamic theories.

Organization of corticostriatal projections from the vibrissal representations in the primary motor and somatosensory cortical areas of rodents

To characterize corticostriatal projections from rodent sensorimotor cortex, the anterograde tracers biotinylated dextran amine (BDA) and fluororuby (FR) were injected into the whisker representations of the primary motor (MI) and somatosensory (SI) cortices. Reconstructions of labeled terminals and their beaded varicosities in the neostriatum and thalamus were analyzed quantitatively to determine the degree of labeled overlap in both of these subcortical structures. Corticostriatal projections from the vibrissal representation in MI were more extensive than corresponding projections from SI. Both cortical areas sent dense projections to the dorsolateral neostriatum, but the MI vibrissal representation also projected to regions located more rostrally and medially. Despite these differences, both MI and SI projected to overlapping parts of the dorsolateral neostriatum. Tracer injections in both cortical areas also produced dense anterograde and retrograde labeling in the medial sector of the posterior complex of the thalamus (POm). Because POm is somatotopically organized and has reciprocal connections with both SI and MI cortices, the amount of labeled overlap in POm was used to indicate whether the tracers were injected into corresponding whisker representations of MI and SI. We found that the proportion of labeled overlap in the neostriatum was highly correlated with the amount of labeled overlap in POm. These results indicate that the rodent neostriatum receives convergent projections from corresponding regions in MI and SI cortex. Furthermore, the thalamocortical projections of the POm indicate that it may modulate corticostriatal outputs from corresponding representations in MI and SI.

New patterns of intracortical projections after focal cortical stroke

Cortical strokes alter functional maps but associated changes in connections have not been documented. The neuroanatomical tracer biotinylated dextran amine (BDA) was injected into cortex bordering infarcts 3 weeks after focal strokes in rat whisker barrel (somatosensory) cortex. The mirror locus in the opposite hemisphere was injected as a control. After 1 week of survival, brains were processed for cytochrome oxidase (CO)-, Nissl-, and BDA-labeled neurons. Cortex bordering the infarct (peri-infarct cortex) had abnormal CO and Nissl structure. BDA-labeled neurons were plotted and projections were analyzed quantitatively. Animals with small strokes had intracortical projections, arising from peri-infarct cortex, not seen in normal hemispheres: the overall orientation was statistically significantly different from and rotated 157 degrees relative to the controls. Compared to the controls, significantly fewer cells were labeled in the thalamus. Thus, after focal cortical stroke, the peri-infarct cortex is structurally abnormal, loses thalamic connections, and develops new horizontal cortical connections by axonal sprouting.

Voltage-sensitive dye versus intrinsic signal optical imaging: comparison of optically determined functional maps from rat barrel cortex

Using intrinsic and voltage-sensitive dye optical imaging methods, somatosensory-evoked neural activity and the consequent metabolic activity were visualized in the barrel cortex at high temporal and spatial resolution. We compared maps of neural and metabolic activity from the perspective of spatial distribution in the cortex. There was good agreement between the two functional maps, if the extent of metabolic activity before a prominent increase in cerebral blood volume (CBV) was assessed. This result indicates that oxygen consumption occurs before CBV changes, in approximately the same cortical area as that in which the preceding neural activity was evoked. This also suggests that the intrinsic signal reflects subthreshold synaptic activity, as well as spiking activity, which is similar to the dye-related signals.

Developmental changes in trial-to-trial variations in whisker barrel responses studied using intrinsic optical imaging: comparison between normal and de-whiskered rats

We used an intrinsic optical imaging technique to examine postnatal developmental changes in the rat barrel response to a single whisker movement. We compared the optical response patterns between control and de-whiskered rats, from which whiskers were removed except for the D1 whisker just after birth. Barrel responses were evoked by D1-whisker movement stimulation, and the intrinsic optical signals were detected from the somatosensory cortex through the dura mater. In the control rats, the area of the barrel response decreased gradually as postnatal development proceeded from 2 to 7 wk, until reaching the adult pattern. On the other hand, in the de-whiskered rats, the barrel response area did not change during development and showed a larger size than in the control rats. We also compared the trial-to-trial variations in the barrel responses between the two groups. In the control rats, trial-to-trial variations in the optical responses were observed under the same conditions of whisker stimulation, and the extent of the variations decreased with postnatal development up to 7 wk. In the de-whiskered rats, trial-to-trial variations were also observed, but the extent was larger and unchanged during development. In both groups, the positions of the response area were the same with respect to the bregma. These results suggest that the decrease in the area and variations in the optical responses are caused by interactions of the corresponding whisker barrel with neighboring barrels and that these interactions are necessary for the developmental stabilization of the intracortical horizontal connections, which are widespread and have high plasticity in early postnatal periods.

Temporal frequency of whisker movement. II. Laminar organization of cortical representations

Part of the information obtained by rodent whiskers is carried by the frequency of their movement. In the thalamus of anesthetized rats, the whisker frequency is represented by two different coding schemes: by amplitude and spike count (i.e., response amplitudes and spike counts decrease as a function of frequency) in the lemniscal thalamus and by latency and spike count (latencies increase and spike counts decrease as a function of frequency) in the paralemniscal thalamus (see accompanying paper). Here we investigated neuronal representations of the whisker frequency in the primary somatosensory ("barrel") cortex of the anesthetized rat, which receives its input from both the lemniscal and paralemniscal thalamic nuclei. Single and multi-units were recorded from layers 2/3, 4 (barrels only), 5a, and 5b during vibrissal stimulation. Typically, the input frequency was represented by amplitude and spike count in the barrels of layer 4 and in layer 5b (the "lemniscal layers") and by latency and spike count in layer 5a (the "paralemniscal layer"). Neurons of layer 2/3 displayed a mixture of the two coding schemes. When the pulse width of the stimulus was reduced from 50 to 20 ms, the latency coding in layers 5a and 2/3 was dramatically reduced, while the spike-count coding was not affected; in contrast, in layers 4 and 5b, the latencies remained constant, but the spike counts were reduced with 20-ms stimuli. The same effects were found in the paralemniscal and lemniscal thalamic nuclei, respectively (see accompanying paper). These results are consistent with the idea that thalamocortical loops of different pathways, although terminating within the same cortical columns, perform different computations in parallel. Furthermore, the mixture of coding schemes in layer 2/3 might reflect an integration of lemniscal and paralemniscal outputs.

Vertical bias in dendritic trees of non-pyramidal neocortical neurons expressing GAD67-GFP in vitro

The neocortical neuropil has a strong vertical (orthogonal to pia) orientation, constraining the intracortical flow of information and forming the basis for the functional parcellation of the cortex into semi-independent vertical columns or 'modules'. Apical dendrites of excitatory pyramidal neurons are a major component of this vertical neuropil, but the extent to which inhibitory, GABAergic neurons conform to this structural and functional design is less well documented. We used a gene gun to transfect organotypic slice cultures of mouse and rat neocortex with the enhanced green fluorescent protein (eGFP) gene driven by the promoter for glutamic acid decarboxylase 67 (GAD67), an enzyme expressed exclusively in GABAergic cells. Many GAD67-GFP expressing cells were highly fluorescent, and their dendritic morphologies and axonal patterns, revealed in minute detail, were characteristic of GABAergic neurons. We traced 150 GFP-expressing neurons from confocal image stacks, and estimated the degree of vertical bias in their dendritic trees using a novel computational metric. Over 70% of the neurons in our sample had dendritic trees with a highly significant vertical bias. We conclude that GABAergic neurons make an important contribution to the vertical neocortical neuropil, and are likely to integrate synaptic inputs from axons terminating within their own module.

Physiological and anatomical organization of multiwhisker response interactions in the barrel cortex of rats

To understand the physiological properties and anatomical organization of the spatiotemporal interaction of the responses to multiwhisker stimulation in neurons of the rat barrel cortex, single-unit recordings of 114 neurons were performed across all layers (layer II/III, n = 39; IV, n = 33; V/VI, n = 42) of the posteromedial barrel subfield of the primary somatosensory cortex of anesthetized rats. Two neighboring principal and adjacent whiskers (PW and AW, respectively) in the same row were deflected rostrally or caudally at varying interstimulus intervals (ISIs). In 37% of the neurons, the response to the combined stimulus was significantly larger than the sum of the responses to stimulation of the individual whiskers. In instances in which response facilitation was observed, selectivity was noted for the combination (75%) of the PW with a particular AW or for a particular direction (60%) of whisker deflection. The direction bias of the responses to multiwhisker stimulation was well correlated with that of the sum of the responses to single whisker stimulation (r = 0.83; p < 0.001). The pattern and magnitude of the response interaction in the neurons of the superficial layers were closely related to the location of the recorded cell in the barrel columns. Multiwhisker stimulation at short ISIs (</=3 msec) evoked prominent response facilitation in cells located close to the border between two columns (p < 0.05, one-way ANOVA), where two excitatory inputs were expected to arrive at the same time. Our results suggest that the spatiotemporal patterns of multiwhisker stimulation, such as whisker combination, direction of deflection, and timing, are expressed as different magnitudes of response interaction, which depends on the proximity of cells to home and adjacent barrel columns.

Upregulation of cAMP response element-mediated gene expression during experience-dependent plasticity in adult neocortex

Gene transcription is thought to be essential for memory consolidation and long-lasting changes in synaptic function. In particular, the signal transduction pathways that activate the transcription factor cAMP response element binding protein (CREB) have been implicated in the process of synaptic potentiation. To study the involvement of this pathway in neocortical plasticity within the barrel cortex, we have used a strain of mice carrying a LacZ reporter gene with six cAMP response elements (CREs) upstream of a minimal promoter. Removal of all but one facial whisker results in the expansion of the spared whisker's functional representation within somatosensory cortex. Under the same conditions of whisker deprivation, we observed a strong (eightfold compared with baseline) and highly place-specific upregulation of CRE-mediated gene transcription in layer IV of the spared whisker barrel. Reporter gene upregulation occurred rapidly after deprivation (16 hr) and was only observed under experimental conditions capable of inducing whisker response potentiation. LacZ expression in layer IV was accompanied by an increase in responsiveness of a subpopulation of layers II/III cells to spared whisker stimulation as determined by in vivo single-unit recording. Given that CREB is involved in the expression of plasticity in superficial layers (Glazewski et al., 1999), and yet CRE-mediated gene expression occurs in layer IV, it is likely that the molecular events initiating plasticity occur presynaptically to the cells that exhibit changes in their receptive field properties.

Dopamine D(3) receptor is selectively and transiently expressed in the developing whisker barrel cortex of the rat

The rodent primary somatosensory cortex (SI) contains a map of the body surface, the most conspicuous part of which are "barrels," neuronal aggregates in layer IV that receive somatotopic projections from whiskers on the rodent's snout. We report that the D(3) dopamine receptor (D(3)R) is selectively and transiently expressed in SI during the first 2 weeks of postnatal development. D(3)R binding sites and mRNA overlap completely and are limited to layer IV of SI. D(3)R/mRNA are organized in a pattern corresponding to somatotopic representations of the body (e.g., whiskers, jaws, paws, etc.) with the highest expression in the barrel field. D(3) mRNA is first detected at postnatal day (P)4, increases rapidly until P7-10, and sharply decreases after P14. D(3)R binding sites are detectable at P6, peak at P14, and decline afterwards. D(1), D(2), D(4), or D(5) mRNAs display dissimilar expression pattern. D(1) mRNA is mostly confined to infragranular layers throughout the cortex. D(4) mRNA expression in layer IV rises by 4 weeks postnatal, when D(3)R expression is virtually undetectable. Quantitative analysis of D(3) mRNA expression demonstrates that the proportion of D(3) mRNA-positive cells decreases between P7 and P14, whereas mRNA concentration per cell remains stable. Moreover, D(3)R number continues to rise, whereas mRNA levels begin to decline. Thus, a process limiting D(3)R expression to fewer cells may occur that also induces changes in post-transcriptional regulation of D(3)R expression in remaining cells. These findings indicate that dopamine acting via D(3)R may play an important role in the development or function of the SI.

Corticofugal axons from adjacent 'barrel' columns of rat somatosensory cortex: cortical and thalamic terminal patterns

The cortical representations of the vibrissae of the rat form a matrix in which each whisker has its own area of cortex, called a 'barrel'. The afferent pathways from the periphery travel first to the trigeminal nuclei and thence via the ventroposteromedial thalamus (VPM) to the cortical barrels have been described in detail. We have studied the output from barrels by filling adjacent areas of the primary somatosensory cortex (SI) with either Phaseolus vulgaris leucoagglutinin (PHA-L) or biotinylated dextran amine (BDA) and demonstrating the course and terminations of the axons that arise within the barrel fields. The method not only dramatically illustrates the previously described corticothalamic pathway to VPM but also demonstrates a strict topography in the cortical afferents to the thalamic reticular nucleus (RT). Cells supplying the RT projection are found below the barrels in layer IV. Connections to the posterior thalamus, on the other hand, have no discernible topography and are derived from cortical areas surrounding the barrels. Thus the outputs of these 'septal' areas return to the region from which they receive thalamic input. The corticocortical connections are also visible in the same material. Contralateral cortical connections arise from the cells of the septa between barrels. The projections to secondary somatosensory area (SII) are mirror images of the barrel pattern in SI with rather more overlap but nonetheless a recognisable topography.

Consistency behind trial-to-trial variation in intrinsic optical responses to single-whisker movement in the rat D1-barrel cortex

We examined consistent characteristics behind the trial-to-trial variati on in intrinsic optical imaging of single barrel cortical responses to D 1-whisker movement in 2-5-week postnatal (2-5 W) and adul t (>9-weeks) Wistar rats, and we identified the effective are a of the neural response. The extent/size, configuration and orientation of the intrinsic optical response area varied from trial-to-trial with the same whisker stimulation. We argue that the trial-to-trial variation was due to cortical blood circulation related to the barrel neural activity. Subsequently, interpolating a family of the traces of the optical response area imaged with repeated stimulation for each animal, we extracted a centered circular area from the trial-to-trial response for each animal. Although the trial-to-trial variation decreased gradually with age, the spatial extent of the interpolated response area was consistently about 660 microm in diameter, in agreement with that measured morphologically and/or histochemically. A possible interpretation is that the optically defined area appears to image the actual effective single-barrel response area, as a first approximation. Furthermore, the constancy of the extracted area independent of age suggests that the barrel cortex is, in fact, virtually mature by 2 weeks of age. The extracted area was also nearly independent of the frequency (>/=5 Hz) of whisker movement.

Thalamic-evoked synaptic interactions in barrel cortex revealed by optical imaging

We used optical imaging of voltage-sensitive dye signals to study the spatiotemporal spread of activity in the mouse barrel cortex, evoked by stimulation of thalamocortical afferents in an in vitro slice preparation. Stimulation of the thalamus, at low current intensity, results in activity largely restricted to a single barrel, and to the border between layers Vb and VI. Low concentrations of the GABA(A) receptor antagonist bicuculline increase the amplitude of the optical signals, without affecting their spatiotemporal propagation. Higher concentrations of bicuculline result in paroxysmal activity, which propagates via intracolumnar and intercolumnar excitatory pathways. Enhancing the activity of NMDA receptors, by removing Mg(2+) from the extracellular solution, dramatically alters the spatiotemporal pattern of excitation: activity spreads to supragranular and infragranular layers and adjacent barrel columns. This enhanced propagation is suppressed by the NMDA receptor antagonist AP5. A similar enhancement of activity propagation can be produced by stimulating the thalamus with a short, high-frequency pulse train. Application of AP5 suppresses the frequency-dependent spread of activity. These findings indicate that the spatiotemporal spread of activity in the barrel cortex is altered by varying the temporal patterns of thalamic inputs, via an NMDA receptor-mediated mechanism, and suggest that a similar process occurs during repetitive whisking activity.

Corticostriatal projections from rat barrel cortex have an anisotropic organization that correlates with vibrissal whisking behavior

To elucidate the detailed organization of corticostriatal projections from rodent somatosensory cortex, the anterograde tracers biotinylated dextran amine (BDA) and fluoro-ruby (FR) were injected into separate parts of the whisker "barrel" representation. In one group of rats, the two tracers were injected into different barrel columns residing in the same row; in the other group of rats, the tracers were deposited into barrel columns residing in different rows. Reconstructions of labeled axonal varicosities in the neostriatum and ventrobasal thalamus were analyzed quantitatively to compare the extent of overlapping projections to these subcortical structures. For both groups of animals, corticostriatal projections terminated in densely packed clusters that occupied curved lamellar-shaped regions along the dorsolateral edge of the neostriatum. When the tracers were injected into different whisker barrel rows, the distribution of BDA- and FR-labeled terminals in the neostriatum followed a crude somatotopic organization in which the amount of overlap was approximately the same as in the ventrobasal thalamus. When both tracers were injected into the same whisker barrel row, however, the amount of corticostriatal overlap was significantly higher than the amount of overlap observed in the ventrobasal thalamus. These results indicate that corticostriatal projections from whisker barrel cortex have an anisotropic organization that correlates with the pattern of vibrissal movements during whisking behavior.

Functional neocortical microcircuitry demonstrated with intrinsic signal optical imaging in vitro

Intrinsic signal optical imaging was used to record the changes in light transmittance evoked by electrical stimulation in slices prepared from sensorimotor cortex of young adult rats. The spatial characteristics of the optical signal evoked by stimulation of layer II/III, IV, V, or VI were clearly different. Layer IV and V stimulation elicited a radially-oriented region of increased light transmittance which was "hourglass" shaped: its tangential extent was greatest in layers II/III and layer V, and least in layer IV. Layer VI stimulation also elicited a radially-oriented signal but the tangential extent of this signal was the same across layers II-VI--that is, it was column-shaped. Upper layer stimulation produced a signal whose tangential extent was much greater in the upper layers than its radial extent to the deeper layers. The spatial form of the stimulus-evoked intrinsic signal was not dependent on the cytoarchitectonic area in which it was elicited. The tangential and radial distribution of the signal evoked by stimulation of different layers appears to reflect the connectivity of cortex, particularly the horizontal connectivity present in layers II/III, V, and VI, and the interlaminar connections that exist between layers II/III and V and from layers VI to IV. The spatial characteristics of the intrinsic signal were independent of the strength of stimulation used. The idea that inhibitory mechanisms restrict the tangential extent of the signal was evaluated in experiments in which the intrinsic signal was recorded before and after the addition of 10 microM bicuculline methiodide. In all slices studied in this way (n = 12), bicuculline methiodide drastically increased the tangential extent of the signal. In 4/12 slices, the tangential spread of the signal was asymmetric with respect to the stimulus site. Asymmetric spread of the signal occurred for both layer V and layer VI stimulation and, in 2/4 of those cases, could be attributed to a cytoarchitectonic border whose presence appeared to restrict the spread of the signal across the border. Although increasing stimulation strength did not change the spatial characteristics of the radially-oriented signal evoked by layer V or VI stimulation, at maximal stimulus intensity the signal evoked from these layers was often accompanied by a band of decreased light transmittance in the most superficial layers (layers I and II). It is concluded that in vitro intrinsic optical signal imaging allows one to image a response attributable to activation of local subsets of cortical connections. In addition, the opposite effects of high-intensity deep layer stimulation on the superficial layers vs layers III-VI of the same column raise the possibility that the most superficial layers may respond differently to repetitive input drive than the rest of the cortical column.

Temporal characteristics of response integration evoked by multiple whisker stimulations in the barrel cortex of rats

We investigated the responses of 114 cells in the barrel cortex of rats to describe the temporal characteristics of excitatory interactions among neurons serving two vibrissae. To examine these interactions, the principal whisker and one adjacent whisker in the same row were stimulated simultaneously or serially at various interstimulus intervals (ISIs). In 37% of the cells tested, combined stimulation of two whiskers exhibited response facilitation; the response to the combined stimulus was larger than the sum of the responses to stimulation of the individual whiskers. The occurrence and magnitude of the facilitation were strongly dependent on the ISI. The ISI capable of producing facilitation for a particular cell was tuned to a narrow range (mean +/- SD, 5.3 +/- 2.3 msec). The ISI that evoked the maximal facilitation was 1.3 +/- 1.3, 3.4 +/- 2.3, and 2.8 +/- 4.5 msec for neurons in layers II/III, IV, and V/VI, respectively. These ISIs corresponded to the difference in latencies between the responses to the individual stimulations of the principal and adjacent whiskers. A significant response facilitation was observed in the regular-spiking cells but not in the fast-spiking cells. When the ISI was longer than the range that evoked facilitation, a suppression of the response to the second whisker stimulation was observed. Facilitation was observed predominantly in layer II/III cells (69%) and to a lesser extent in cells of layers IV (15%) and V/VI (24%). Our results suggest that, in the barrel cortex, the temporal relationships among tactile stimuli are coded by facilitatory and inhibitory interactions among neurons located in neighboring barrel columns.

Cortical columnar processing in the rat whisker-to-barrel system

Controlled whisker stimulation and single-unit recordings were used to elucidate response transformations that occur during the processing of tactile information from ventral posterior medial thalamus (VPM) through cortical columns in the rat whisker/barrel cortex. Whiskers were either deflected alone, using punctate ramp-and-hold stimuli, or in combination with a random noise vibration applied simultaneously to two or more neighboring whiskers. Quantitative data were obtained from five anatomically defined groups of neurons based on their being located in: VPM, layer IV barrels, layer IV septa, supragranular laminae, and infragranular laminae. Neurons in each of these populations displayed characteristic properties related to their response latency and time course, relative magnitudes of responses evoked by stimulus onset versus offset, strength of excitatory responses evoked by the noise stimulus, and/or the degree to which the noise stimulus, when applied to neighboring whiskers, suppressed or facilitated responses evoked by the columnar whisker. Results indicate that within layer IV itself there are at least two anatomically distinct networks, barrel and septum, that independently process afferent information, transforming thalamic input in similar but quantitatively distinguishable ways. Transformed signals are passed on to circuits in supragranular and infragranular laminae. In the case of supragranular neurons, evidence suggests that circuits there function in a qualitatively different fashion from those in layer IV, diminishing response differentials between weak and strong inputs, rather than enhancing them. Compared to layer IV, the greater heterogeneity of receptive field properties in nongranular layers suggests the existence of multiple, operationally distinct local circuits in the output layers of the cortical column.

Neonatal whisker clipping alters intracortical, but not thalamocortical projections, in rat barrel cortex

Retrograde axonal transport of cholera toxin B subunit (CTB) was used to compare the development of intracortical and thalamocortical connections in normal rats with those in rats in which all of the whiskers were trimmed continuously from birth. In normal animals, injections of CTB into a single barrel column resulted in an asymmetrical labeling of cells that were distributed preferentially within columns related to the same row in which the injection was placed. This anisotropy in the patterns of intracortical connections was not observed in whisker-clipped animals. In these animals, there was a significant reduction in the mean number of labeled cells in the infragranular layers, and labeled cells were distributed symmetrically around the injection site. The same injections of CTB also labeled thalamocortical neurons in the ventrobasal thalamus. Analysis of the distribution of these cells revealed that, in both control and experimental animals, the vast majority of labeled cells were restricted to a homologous (i.e., corresponding to the injected cortical barrel) thalamic barreloid. These findings indicate that manipulations of sensory experience alter patterns of intracortical, but not thalamocortical, connections.

Spatiotemporal organization of fast (>200 Hz) electrical oscillations in rat Vibrissa/Barrel cortex

A 64-channel electrode array was used to study the spatial and temporal characteristics of fast (>200 Hz) electrical oscillations recorded from the surface of rat cortex in both awake and anesthetized animals. Transient vibrissal displacements were effective in evoking oscillatory responses in the vibrissa/barrel field and were tightly time-locked to stimulus onset, coinciding with the earliest temporal components of the coincident slow-wave response. Vibrissa-evoked fast oscillations exhibited modality specificity and were earliest and of largest amplitude over the cortical barrel, which corresponded to the vibrissa stimulated, spreading to sequentially engage neighboring barrels over subsequent oscillatory cycles. The response was enhanced after paired-vibrissal stimulation and was sensitive to time delays between movement of separate vibrissae. These data suggest that spatiotemporal interactions between fast oscillatory bursts in the barrel field may play a role in rapidly integrating information from the vibrissal array during the earliest cortical response to somatosensory stimulation.

Functional independence of layer IV barrels in rodent somatosensory cortex

Layer IV of rodent primary somatosensory cortex is characterized by an array of whisker-related groups of neurons, known as "barrels." Neurons within each barrel respond best to a particular whisker on the contralateral face, and, on deflection of adjacent whiskers, display relatively weak excitation followed by strong inhibition. A prominent hypothesis for the processing of vibrissal information within layer IV is that the multiwhisker receptive fields of barrel neurons reflect interconnections among neighboring barrels. An alternative view is that the receptive field properties of barrel neurons are derived from operations performed on multiwhisker, thalamic inputs by local circuitry within each barrel, independently of neighboring barrels. Here we report that adjacent whisker-evoked excitation and inhibition within a barrel are unaffected by ablation of the corresponding adjacent barrel. In supragranular neurons, on the other hand, excitatory responses to the ablated barrel's associated whisker are substantially reduced. We conclude that the layer IV barrels function as an array of independent parallel processors, each of which individually transforms thalamic afferent input for subsequent processing by horizontally interconnected circuits in other layers.

Anatomical correlates of representational map reorganization induced by partial vibrissectomy in the barrel cortex of adult mice

We examined the potential for changes in cortical connectivity to accompany long-term plastic changes in functional cortical representations of mystacial vibrissae. Plasticity in the barrel cortex of young adult mice was evoked by vibrissectomy that spared row C of whiskers. We found that 2-deoxyglucose brain mapping causes a progressive expansion of cortical representation of the spared vibrissae. Two months after vibrissectomy, when the width of the cortical map of the spared row of vibrissae doubled, living cortical slices of the barrel cortex were injected with fluorescent dextrans. The injections were centered on spared, deprived and control vibrissal columns. The injections labeled three intracortical projection systems: (i) local connections from one vibrissal column to neighboring columns; (ii) long-range projections running in the septa and walls of the barrels and spanning several barrels; and (iii) very-long-range fibers running horizontally in the lower part of layer V. The local, short-range projection system was analysed following small injections into the centers of columns in layers III and IV. We found that injections into spared barrels labeled axons extending for significantly greater distances in all layers (except layer V), and labeled cell bodies situated significantly further, than after injections into deprived or control barrels. Also, the total axonal density labeled by injections into the spared barrel was higher by 70% than for the deprived or control barrels. Alterations of topographical maps in adult somatosensory cortex may occur immediately after functional denervation, but may also increase with time, as in the case of our experimental situation. Our results indicate that persistent, long-term plastic change can remodel connectivity in the barrel cortex.

Distribution of tactile learning and its neural basis

The brain's sensory processing systems are modified during perceptual learning. To learn more about the spatial organization of learning-related modifications, we trained rats to utilize the sensory signal from a single intact whisker to carry out a behavioral task. Once a rat had mastered the task, we clipped its "trained" whisker and attached a "prosthetic" one to a different whisker stub. We then tested the rat to determine how quickly it could relearn the task by using the new whisker. We observed that rats were immediately able to use the prosthetic whisker if it were attached to the stub of the trained whisker but not if it were attached to a different stub. Indeed, the greater the distance between the trained and prosthetic whisker, the more trials were needed to relearn the task. We hypothesized that this "transfer" of learning between whiskers might depend on how much the representations of individual whiskers overlap in primary somatosensory cortex. Testing this hypothesis by using 100-electrode cortical recordings, we found that the overlap between the cortical response patterns of two whiskers accounted well for the transfer of learning between them: The correlation between the electrophysiological and behavioral data was very high (r = 0.98). These findings suggest that a topographically distributed memory trace for sensory-perceptual learning may reside in primary sensory cortex.

Enlargement of cortical vibrissa representation in the surround of an ischemic cortical lesion

It has been shown that cortical lesions are associated with an increase of excitability in surrounding brain regions, and with a downregulation of GABA(A) receptors. In the present study we investigated whether this increased excitability affects the cortical map of inputs represented in areas surrounding the lesioned brain area. Focal lesions with a diameter of 2-2.5 mm were induced photochemically in the hindlimb area at the border of the primary somatosensory cortex of the rat. One week after lesioning, the cortical representation of the B3 vibrissa was studied using 14C-deoxyglucose (DG) autoradiography. In all animals mechanical stimulation of the B3 vibrissa produced a column-shaped DG-labeling in the somatosensory cortex, corresponding to the B3-barrel with a maximum of the glucose uptake in layer IV. In control animals without cortical lesions (n=6), stimulation increased the glucose uptake rate by 50.8+/-10.5% in layer IV. In lesioned animals (n=6) maximum DG-uptake in layer IV (54.8+/-8.6%) did not differ significantly from that in controls. However, as compared to control animals, lesioned animals showed also increased glucose uptake within the activated column in layers II/II (51.+/-11.1%, lesioned animals; 31.8+/-11.2%, controls; P<0.05, lesioned vs. control) and V (47.5+/-5.8%, lesioned animals, 28.8+/-10.5%, controls; P<0.05, lesioned vs. control). The diameter of the metabolically activated B3-barrel area of layer IV was expanded from 461.8+/-77.6 microm in control animals to 785.5+/-103.6 microm; P<0.01) in lesioned animals. Lesioned animals also showed expansion of the activated area in layers II/III (890.4+/-134.8 microm, lesioned animals; 430.6+/-95.1 microm, controls; P<0.01) and layer V (1117.5+/-163.6 microm, lesioned animals; 648.7+/-114.1 microm, controls; P<0.01). The depth profile of the activation columns showed a maximum in layer IV in control animals, which was expanded towards layers II/III and layer V in lesioned animals. It is concluded that cortical lesions alter the representational area of neighboring afferent inputs through disinhibition or 'unmasking' of pre-existing silent or ineffectual intracortical synapses. The present observations raise the possibility that the brain supports recovery from lesions by decreasing GABAergic inhibition, thereby facilitating a remapping of the cortical representation in neighboring brain areas.

Contribution of supragranular layers to sensory processing and plasticity in adult rat barrel cortex

Contribution of supragranular layers to sensory processing and plasticity in adult rat barrel cortex. J. Neurophysiol. 80: 3261-3271, 1998. In mature rat primary somatic sensory cortical area (SI) barrel field cortex, the thalamic-recipient granular layer IV neurons project especially densely to layers I, II, III, and IV. A prior study showed that cells in the supragranular layers are the fastest to change their response properties to novel changes in sensory inputs. Here we examine the effect of removing supragranular circuitry on the responsiveness and synaptic plasticity of cells in the remaining layers. To remove the layer II + III (supragranular) neurons from the circuitry of barrel field cortex, N-methyl--aspartate (NMDA) was applied to the exposed dura over the barrel cortex, which destroys those neurons by excitotoxicity without detectable damage to blood vessels or axons of passage. Fifteen days after NMDA treatment, the first responsive cells encountered were 400-430 micrometers below the pial surface. In separate cases triphenyltetrazolium chloride (TTC), a vital dye taken up by living cells, was absent from the lesion area. Cytochrome oxidase (CO) activity was absent in the first few tangential sections through the barrel field in all cases before arriving at the CO-dense barrel domains. These findings indicate that the lesions were quite consistent from animal to animal. Controls consisted of applying vehicle without NMDA under similar conditions. Responses of D2 barrel cells were assessed for spontaneous activity and level of response to stimulation of the principal D2 whisker and four surround whiskers D1, D3, C2, and E2. In two additional groups of animals treated in the same way, sensory plasticity was assessed by trimming all whiskers except D2 and either D1 or D3 (called Dpaired) for 7 days before recording cortical responses. Such whisker pairing normally potentiates D2 barrel cell responses to stimulation of the two intact whiskers (D2 + Dpaired). After NMDA lesions, cortical cells still responded to all whiskers tested. Cells in lesioned cortex showed reduced response amplitude compared with sham-operated controls to all D-row whiskers. In-arc surround whisker (C2 or E2) responses were normal. Spontaneous activity did not change significantly in any remaining layer at the time tested. Modal latencies to stimulation of principal D2 or surround D1 or D3 whiskers showed no significant change after lesioning. These findings indicate that there is a reasonable preservation of the response properties of layer IV, V, VI neurons after removal of layer II-III neurons in this way. Whisker pairing plasticity in layer IV-VI D2 barrel column neurons occurred in both lesioned and sham animals but was reduced significantly in lesioned animals compared with controls. The response bias generated by whisker trimming (Dpaired/Dcut + Dpaired ratio) was less pronounced in NMDA-lesioned than sham-lesioned animals. Proportionately fewer neurons in layer IV (52 vs. 64%) and in the infragranular layers (55 vs. 68%) exhibited a clear response bias to paired whiskers. We conclude that receptive-field plasticity can occur in layers IV-VI of barrel cortex in the absence of the supragranular layer circuitry. However, layer I-III circuitry does play a role in normal receptive-field generation and is required for the full expression of whisker pairing plasticity in granular and infragranular layer cells.

Sensory-evoked high-frequency (gamma-band) oscillating potentials in somatosensory cortex of the unanesthetized rat

A 64-channel epipial electrode array was used to investigate high-frequency (gamma-band) oscillations in somatosensory cortex of the unanesthetized and unrestrained rat. Oscillations were evoked by manual stimulation of the vibrissae and mystacial pad. Stimulation of the contralateral vibrissae resulted in a significant increase in gamma-power during 128-ms epochs taken just following stimulus onset compared to the prestimulus baseline. Stimulation of the ipsilateral vibrissae was completely ineffective in evoking gamma-oscillations in any animals. Sensory evoked gamma-oscillations were constrained to primary (SI) and secondary (SII) somatosensory cortex. When averaged to an arbitrary reference of peak times in one of the channels, these oscillations exhibited a systematic temporal organization, propagating from the rostral portion of SI to the barrel field proper, and finally to SII. These spatiotemporal characteristics were probably produced by intracortical pathways within rodent somatosensory cortex. The rostrocaudal propagation of gamma-oscillations within the barrel field may also reflect whisking patterns observed when the vibrissae are used as a sensory array, suggesting that synchronized gamma-oscillations may play a role in assembling punctate afferent information provided by the vibrissae into a coherent representation of a somatosensory stimulus.

Functional development of the vibrissae somatosensory system of the rat: (14C) 2-deoxyglucose metabolic mapping study

Functional development of the rat whisker somatosensory system was studied by using the (14C) 2-deoxyglucose (2DG) metabolic mapping technique. Restrained rat pups had their left mystacial vibrissae stroked for 30 minutes and their brains harvested, sectioned, and autoradiographed from the level of the lower medulla to the frontal cortex. Subjects were tested at postnatal days (PNDs) 0-9 and 21. At birth, all subjects exhibited a significant increase of 2DG uptake in the left spinal trigeminal nuclei, the principal trigeminal sensory nucleus, and a portion of the right ventral posteromedial thalamic nucleus. The primary somatosensory cortex exhibited significant 2DG uptake contralateral to stimulation by PND 6, followed by the secondary somatosensory cortex at PND 7. The pattern of 2DG uptake in the somatosensory cortices became more intense and well defined by PND 9. Given that the somatosensory system develops in an orderly fashion from the periphery to higher brain structures, the present results show that brain structures mediating whisker sensory input are not metabolically active until projections from lower somatosensory centers are established. Neurons become responsive to whisker stimulation in the subcortical structures at birth and in the somatosensory cortex a few days later. This cortical activity follows the organization of the upper tier of thalamocortical fibers into a "barrelfield." Moreover, there is a gradual enhancement in functional activity of the vibrissa neurons at different somatosensory nuclei as rats mature. The present study elucidates the time course of functional development in the rat somatosensory system.

Nonlinear processing of tactile information in the thalamocortical loop

Rats explore tangible objects in a manner such that, at any given moment in time, multiple facial whiskers simultaneously contact the surface of the object. Although both thalamic and cortical neurons responsible for processing such tactile information have large, multiwhisker receptive fields, it remains unclear what kinds of computations can be carried out by these neuronal populations when behaviorally relevant multiwhisker stimuli are used. By simultaneously recording the activity of up to 78 cortical and thalamic neurons per animal, we observed that the magnitude of sensory responses and the spatial spread of ensemble activity increased in a nonlinear fashion according to the extent and spatial orientation of the multiwhisker stimuli. Supralinear responses were seen more frequently with vertically than with horizontally oriented stimuli. These data suggest that thalamocortical interactions in the rat somatosensory system can generate complex spatial transformations of multiwhisker stimuli that go beyond the classic inhibitory interactions previously observed.

Developmental expression of GABA(A) receptor subunit and GAD genes in mouse somatosensory barrel cortex

In situ hybridization histochemistry with radioactive cRNA probes was used to study patterns of gene expression for alpha1, alpha2, alpha4, alpha5, beta1, beta2, and gamma2 subunit mRNAs of typeAgamma aminobutyric acid (GABA(A)) receptors and for 67-kDa glutamic acid decarboxylase (GAD67) mRNA in mouse barrel cortex during the period (postnatal days 1-12; P1-P12) when thalamocortical innervation of layer IV barrels is occurring. The alpha1, beta2, and gamma2 subunit mRNAs increased substantially with age, especially in layers V and VI, and throughout the period studied, invariably had the same laminar-specific patterns of expression. All three mRNAs were highly expressed in the dense cortical plate at P1. In layer IV after differentiation of barrels, they were expressed in cells of both barrel walls and hollows but especially in the walls. The alpha2, alpha4, alpha5, and beta1 subunit mRNAs were expressed at lower levels and had different laminar patterns of distribution; alpha2 and alpha4 showed switches between layers over time; alpha5 was invariably associated with the subplate or its derivative, beta1 with layer IV. Levels of alpha2 mRNA did not change over time; alpha4 and beta1 mRNAs increased and alpha5 decreased. GAD67 mRNA was highest in layer I at P1 and progressively increased in other layers. These results suggest that postnatal development of GABA(A) receptors is mainly directed at the production of receptors assembled from alpha1, beta2, and gamma2 subunits, with beta1 contributing in layer IV. Other subunits may be associated with receptors involved in trophic actions of GABA during development and may give GABA(A) receptor-mediated responses in the developing cortex their particular physiological profile.

Distributed representation of vibrissa movement in the upper layers of somatosensory cortex revealed with voltage-sensitive dyes

We have identified large-scale patterns of electrical activity in circuits that occur in response to stimulation of peripheral receptors. Our focus was on primary (S1) vibrissal cortex of anesthetized rat, and we used optical techniques in conjunction with voltage-sensitive dyes to measure depolarization of the upper layers of cortex. Displacement of one vibrissa produced a field of activity that extends over very many cortical columns in S1. There are multiple, focal maxima within this field. A global maximum is located near the center of the field of activity, and, as determined electrically and histologically, this site maps to the cortical column appropriate for the deflected vibrissa. The amplitude of this component attains a steady-state value under continuous stimulation. Additional temporal characteristics are revealed by the response to a single displacement; the signal was triphasic and began with a prompt depolarization that was followed by a transient phase of inhibition and a final phase of long-lasting depolarization. The somatotopy of the other, satellite maxima in the field of activity were established through the reconstruction of the fields of activity produced by individual stimulation of other vibrissae. Local maxima for one vibrissa were seen to overlie the global maximum found for stimulation of nearest- and next-nearest-neighbor vibrissae. In contrast to the amplitude of the global maxima, the amplitude associated with the local maxima was not maintained with either continuous or infrequent but repetitive stimulation. Finally, the field of activity induced by alternate deflection of two neighboring vibrissae was suppressed in amplitude in comparison to the summed amplitudes of the signals elicited by deflection of each vibrissa alone. We suggest that these patterns of activity are a manifestation of the dynamic interaction among neighboring cortical columns.

Rapid optical imaging of whisker responses in the rat barrel cortex

Videomicroscopy was used to image 'intrinsic' responses over the rat barrel cortex through a closed cranial window during controlled whisker stimulation. With a Macintosh IIfx running Image 1.49 VDM, video frames from a CCD camera were captured and averaged before, during and after whisker stimulation. The technique presented here is a functional imaging modality--using conventional videomicroscopic equipment, a small computer, and public domain NIH Image software--with a temporal resolution of 33 ms. Images can be obtained directly from the CCD camera or recorded to videotape for post hoc analysis. Pixel by pixel comparison of prestimulation images to images obtained during stimulation revealed changes in the reflectance characteristics of cortex and vessels overlying the barrel field. Imaged responses superimposed on barrel histology to map intrinsic signal matched barrels of the stimulated whiskers in every case. Video imaging of the rat barrel cortex provides a useful method for rapid targeting for other experimental protocols and has potential for analyzing localized responses to physiologic stimuli in vivo.

LCBF changes in rat somatosensory cortex during whisker stimulation monitored by dynamic H2 clearance

We evaluated increases in local cerebral blood flow (LCBF) localized to single activated cortical columns by H2 clearance methods. The rat whisker-barrel cortex is a model for cortical function and neural processing in active explorative behaviors. Up to four 30-40 microns Pt wire electrodes were inserted in or near the rat whisker-barrel cortex. Electrode positions were mapped by postmortem histology. H2 was generated electrochemically by constant current from one electrode and detected by one or more other electrodes 300-500 microns away. Changes in LCBF produced inverse changes in PH2. Shifts during steady H2 generation were calibrated against standard H2 inhalation clearance curves at rest and during inhalation of 7.5% CO2 for 1 min for quantitative estimates of LCBF. Contralateral whisker stimulation at 3 Hz, 1 min duration and delivered every 2 min produced the largest increases in LCBF. LCBF responses were detected in approximately 1 s. Stimulation of single whiskers produced the largest responses when an electrode was in the corresponding barrel. These results indicate that increased neural activity in a single cortical column produces blood flow responses primarily in that column.

Differential effect of whisker trimming on excitatory and inhibitory transmission in primary somatosensory cortex of the adult rat in vivo

The effects of sensory deprivation on excitatory and inhibitory activity in the primary somatosensory cortex were studied in the adult rat. Excitatory and inhibitory transmission generated by whisker stimulation, and neuronal responsiveness to iontophoretically applied excitatory amino acids were recorded. Whisker input deprivation, through whisker trimming for a median of 24 days, resulted in a significant decrease in excitatory transmission to surround whisker stimulation. In contrast, the response magnitude to principal whisker stimulation remained unchanged. However, the response latencies to principal whisker and surround whisker stimulation were significantly reduced, which led to altered temporal response distributions in deprived cells. Neurons deprived of sensory input were significantly less responsive to glutamate, N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionate and kainate. Following deprivation, no change was observed in cortical inhibitory transmission measured 30-200 ms post-stimulus. These results show that excitatory transmission (including excitatory amino acid receptor function) is altered by adult whisker deprivation.

Patterning of local intracortical projections within the vibrissae representation of rat primary somatosensory cortex

Anterograde and retrograde tracing with biotinylated dextran amine and Phaseolus vulgaris leukoagglutinin was used to assess projection patterns within the vibrissae representation of the rat's primary somatosensory cortex (S-I). Large and small injections of either tracer into the center of the vibrissae representation yielded dense anterograde and retrograde labelling throughout much of the tangential extent of the vibrissae representation within S-I. In all layers, the pattern and extent of retrograde and anterograde label was in rough congruence. The organization of this labelling varied across cortical layers. In layers II and III, labelled fibers extended away from injection sites in all directions and yielded a uniform pattern, which decreased in density with increasing distance from the tracer injection. There was a tendency for labelling to be more extensive along the representation of the row of vibrissae follicles that included the injection site than across rows. There was also a tendency for anterograde labelling to be more extensive in the direction of the representation of follicles more rostral on the face than that injected. In lamina IV, both labelled fibers and cells were restricted for the most part to the septa regions between the barrels. However, a small number of retrogradely labelled neurons were also located in the barrels (approximately one-ninth of the number found in the septa). The pattern observed in laminae II-III was repeated in layers V and VI. In these laminae, there was no evidence of a pattern of intracortical connections related to the vibrissae representation in overlying lamina IV.