Local intra- and interlaminar connections in mouse barrel cortex

Focal injections of horseradish peroxidase (HRP) in dimethylsulfoxide (DMSO) were targeted into mouse somatosensory cortex, in vitro, with a template. Injections were made at different depths and in different locations in the whisker-barrel-defined somatosensory map in order to determine quantitative connectivity patterns within and between barrel-defined cortical columns. Cortices were sectioned in a plane parallel to the pia at 75 microns. Data were collected directly from microscope slides by computer. Data are presented as: 1) Plots of computer-mapped HRP reaction product density in neurons and cell locations for each section in relation to barrel boundaries; 2) histograms of label in cortical layers related to individual barrel-defined columns; 3) polar plots of relative amounts of label within individual barrel columns in sections through each barrel column; 4) vectors which represent HRP reaction product density as a function of direction and distance from the injection site; 5) statistical analysis of the shape of the label distribution pattern in the plane of the cortex as a function of injection site depth; and 6) probability of labeling of any other barrel column given a labeled barrel column. The principal findings are: 1) The pattern of label distribution, after an injection directly above or directly below an individual barrel, is hour-glass shaped with the waist of the hour-glass in layer IV. 2) Connections within barrel cortex are asymmetrical. Barrel-related columns within a row are more strongly interconnected than those in different rows. 3) Connections of the small barrels associated with whiskers on the upper lip are strongest with other small barrels, but strong connections also exist between these small barrels and the larger barrels. 4) The pattern of intracortical connections in SII is not asymmetrical; interlaminar connections in SII are fundamentally different from those in barrel cortex. 5) Quantitative intracortical projection patterns are highly consistent with functional data on intracortical processing of whisker information. As such, the quantitative data clearly indicate the spatial extent and relative magnitude of populations of neurons involved in intracortical processing of sensory information. The spatial arrangements of these intracortical connections, in conjunction with known developmental events, make it highly likely that the distribution of intracortical axons in mouse barrel cortex is sculpted in part by experience.     

Mouse Barrel Cortex Functionally Compensates for Deprivation Produced by Neonatal Lesion of Whisker Follicles

In the murine somatosensory pathway, the metabolic whisker map in barrel cortex derived with the autoradiographic deoxyglucose method is spatially in register with the morphological whisker map represented by the barrels. The barrel cortex of adult mice, in which we had removed three whisker follicles from the middle row of whiskers shortly after birth, contained a disorganized zone surrounded by enlarged barrels with partially disrupted borders. With the fully quantitative autoradiographic deoxyglucose method, we investigated in barrel cortex of such mice the magnitude and the pattern of metabolic responses evoked by the deflection of whiskers. Most remarkably, the simultaneous deflection of six whiskers neighbouring the lesion activated not only the territory of the corresponding barrels, but also the unspecifiable area intercalated between the clearly identified barrels. This metabolic whisker map, unpredictable from the morphological 'barrel' map, may reflect a functional compensation for the deficit in input.     

Metabolic barrel representations with various patterns of neonatal whisker deafferentation in rats

With various patterns of whisker deafferentation, C3 whisker stimulation produced divergently shaped metabolic barrel representations in layer IV of the primary somatosensory cortex. Whisker deafferentation results in functional and structural reorganization of the barrels in the primary somatosensory cortex. The present study examines the alteration of the metabolic barrel representations in layer IV with various configurations of selective whisker deafferentation in neonates, using [14C]2-deoxyglucose autoradiography. The deafferentation was produced by unilateral ablation of whiskers, leaving certain follicles intact. Configurations of intact follicles included: (I) row C follicles; (II) B3, C3, and D3 follicles; (III) B3, B4, C3, and C4 follicles; (IV) C2, C3, D2, and D3 follicles. The metabolic C3 barrel representations in layer IV after the deafferentations were found to have expanded only toward the barrel sites in which the corresponding whiskers were ablated, with no expansion toward the neighboring barrels. Expansion toward row D was significantly more pronounced than expansion toward row B, and expansion toward the C2 barrel was significantly more pronounced than expansion toward the C4 barrel. From these results, it can be inferred that asymmetric intrinsic structural connections are reflected in the functional metabolic barrel representation under the condition of neural plasticity in the barrel cortex following whisker deafferentation.     

Postnatal growth of intrinsic connections in mouse barrel cortex.

Surprisingly little is known about the development of connections within a functional area of the cerebral cortex. We examined the postnatal growth of connections in mouse barrel cortex during the second and third weeks after birth, coinciding with the period of rapid synaptogenesis that occurs just after the barrels first form. A barrel is a group of neurons in layer 4 of somatosensory cortex that is part of a cortical column. Each whisker/barrel column is linked anatomically and functionally to a homotopic whisker on the contralateral face. Radial groups of cortical neurons were labeled with the neuronal tracer biotinylated dextran amine in mice ranging in age from postnatal day 8 (P8; P0 is the date of birth) to adulthood. The spatial distributions of retrogradely labeled neurons in different laminae were analyzed. The barrel map in layer 4 was used as a template to compare quantitative data from different animals and to account for substantial changes in barrel and barrel field size during development. Intrinsic projections 1) innervate increasingly more distant targets within barrel cortex up to 3 weeks of age; 2) continue to form in targets after 3 weeks, effectively strengthening existing connections; 3) follow a timetable for growth that is layer-specific; 4) link more distant barrel columns in layer 4 from neurons that are found preferentially in the barrel side and the septa between barrels; and 5) form over the shortest distances between the barrel columns. These data indicate that intrinsic connections in mouse barrel cortex develop by the progressive addition of neuronal connections rather than by sculpting preliminary connections. We describe statistically significant changes in connectivity during development that may be applied to model and assess the development of connections after a variety of experimental perturbations, such as to the environment and/or the genome.     

2-DG uptake patterns related to single vibrissae during exploratory behaviors in the hamster trigeminal system

Stimulation of one or several whiskers activates discrete foci throughout the trigeminal (V) neuraxis. These foci contribute to patterns, corresponding to the patterns of vibrissae, that have been directly related to aggregates of cells and axon terminals in the “barrel” cortex. Here, we combine high-resolution, 2-deoxyglucose (2DG) mapping and cytochrome oxidase (CO) staining to determine whether the known pattern of V primary afferent projections is sufficient to deduce the functional activation of their targets during exploratory behavior. Four adult hamsters had all of their large mystacial vibrissae trimmed acutely, except for C3 on the left, and B2 and D4 on the right; in two others, the left C3 and right A1 and E4 whiskers were spared. After fasting overnight, 2DG was injected and the animals behaved freely in the dark for 45 minutes. The brainstem, thalamus, and cortices were sectioned, then processed for both CO staining and 2DG autoradiography. Image-processing microscopy was used to separate the autoradiographic silver grains from the histochemical staining. CO patches were patterned in a whiskerlike fashion in the full rostrocaudal extent of V nucleus principalis and in caudal portions of spinal V subnuclei interpolaris and caudalis, but absent in subnucleus oralis. 2DG silver grains were densest above those CO patches in the pattern corresponding to the active whiskers. There were no consistent 2DG foci in subnuclei oralis or rostral caudalais. In these same cases, prominent 2DG labeling was restricted to the appropriate barrels in the contralateral cortex. Only one case, however, displayed a clear and appropriate region of heightened 2DG uptake in contralateral ventroposteromedial thalamus (VPM) and the adjacent part of the reticular thalamic nucleus. Patterns of increased glucose utilization with single whisker stimulation are well matched to the CO patterns that mirror distributions of neurons associated with a vibrissa in the V brainstem complex, thalamus, and cortex. Single whiskers are represented by relatively homogeneous longitudinal columns of 2DG labeling in the V brainstem nuclei. The columns are not continuous through the axial extent of the V brainstem complex; rather, they occur separately within principalis, interpolaris, and caudalis. While whisker columns were consistently labeled in interpolaris and caudalis in all animals, the labeling was increasingly variable in principalis, barrel cortex, and VPM, respectively. This suggests that the behaving animal can and does significantly modulate activity in this major, synaptically secure pathway. © 1993 Wiley-Liss, Inc.     

Plasticity in the barrel cortex of the adult mouse: effects of chronic stimulation upon deoxyglucose uptake in the behaving animal.

We investigated experience-dependent regulation of neuronal activity in the whisker-to-barrel pathway of the adult mouse using the autoradiographic deoxyglucose (DG) method. Animals were placed in the Lausanne whisker stimulator, and three of their whisker follicles were passively stimulated for a period of 1, 2, or 4 d. After this period, mice received a dose of DG and were placed in a cage containing a pile of wooden sticks. Mice that underwent the same procedure except the passive stimulation served as controls. Patterns of stimulus-dependent DG uptake were studied in the somatosensory cortex and in the trigeminal sensory brainstem complex. DG uptake in the barrels corresponding to the passively stimulated whiskers was lower than in controls. This decrease was present throughout the radial extent of a barrel column and was observed in all passively stimulated animals. Quantitative analysis confirmed these observations and, furthermore, showed a statistically significant decrease in DG uptake in barrels neighboring the passively stimulated ones. In half of the animals, the brainstem nuclei showed a decreased DG uptake in the representation of the passively stimulated whiskers, whereas in the other animals the pattern of DG uptake was as in controls. We propose that the signs of cortical plasticity are due to a mechanism that operates in layer IV and functions as a gate for peripheral sensory activity to enter cortical circuitry.     

Barreloids in adult rat thalamus: Three-dimensional architecture and relationship to somatosensory cortical barrels

Histochemical staining for cytochrome oxidase (CO) and axonal transport of horseradish peroxidase (HRP) were used to investigate thalamocortical connections in the vibrissa-barrel system of adult rats. CO staining revealed that the medial division of the ventrobasal thalamic nucleus (VBm) consists of intensely stained rod-like configurations, containing thalamocortical projection neurons and intervening neuropil, separated by lighter-stained septa. CO-dark rods span the thickness of VBm, are arranged in a pattern of rows and arcs that resembles the distribution of vibrissae on the mystacial pad, and are similar to the cytoarchitectonic structures termed “barreloids” in the mouse thalamus. Based upon the dimensions of CO-dark structures and the numerical density of neurons in VBm we estimated that a barreloid in the rat may contain 250-300 neurons. HRP injections into lamina IV of the somatosensory cortex led to retrograde labeling of neurons within one or more barreloids. When injections were centered within the CO-dark hollows of cortical barrels about 95% of retrogradely labeled neurons were located in the barreloid that is isomorphic to the injected barrel; up to 5% of labeled neurons were located within a single adjacent barreloid. Barrel hollow injections that also included a barrel side yielded a larger proportion of labeled neurons in non-isomorphic barreloids. Interestingly, such extra-barreloid labeling was topologically consistent in that HRP-labeled neurons were distributed among barreloids that corresponded to cortical barrels nearest the injected barrel side. Injections into the septa between barrels similarly resulted in labeling within barreloids that corresponded to cortical barrels flanking the septal injection site. Following lamina IV injections the density of labeled neurons tended to be highest in the ventrolateral one-half to two-thirds of VBm. Retrograde labeling of neurons in the dorsomedial one-third to one-half of VBm was more often observed after HRP injections at the lamina V/VI border. Thus, barreloid neurons may be heterogeneous with respect to their laminar pattern of terminations within the somatosensory cortex. Some HRP injections in the cortex resulted in orthograde labeling of corticothalamic axons in the barreloids. When observed, labeled corticothalamic axons arborized principally within the barreloid isomorphic to the injected barrel column. Indeed, terminal labeling was densest in the vicinity of neurons retrogradely labeled by the same injection.     

Functional organization in cortical barrels of normal and vibrissae-damaged mice: a (3H) 2-deoxyglucose study

The large mystacial vibrissae on the faces of rodents have punctate representations in all stations in the central trigeminal pathway, including layer IV of the somatosensory cortex (SmI). The cortical whisker correlates, multicellular units termed barrels, are not present at birth, and damage to the vibrissae during the first postnatal week results in altered adult cytoarchitectonics. The anatomical effects of vibrissae damage in the cortex have been well documented; here, we investigated the functional organization of altered SmI barrels with a high-resolution 2-deoxyglucose (2-DG) technique (Durham et al., '81, J. Neurosci. 1:519). The middle row of vibrissae was cauterized in 1-, 2-, 3-, 4-, or 5-day-old mice, and the animals were allowed to survive to sexual maturity. Various combinations of vibrissae were clipped acutely 24 hours prior to injection of 2-4 mCi of (3H)2-DG. Mice actively explored an empty cage for 60 minutes, stimulating the remaining vibrissae. The mice then were perfused and their brains prepared for paraffin histology and emulsion autoradiography. In tangential sections through layer IV, patterns of neuropil and cell body labeling were analyzed with respect to barrel cytoarchitecture in normal and vibrissae-damaged mice. In both control and experimental animals, patterns of neuropil and cell somata label corresponded exactly to barrel boundaries, whether normal or altered by vibrissae damage. Only those barrels for which vibrissae were intact had high levels of label, with anterior barrels more heavily labeled. Many neurons in the septa between these barrels and the adjacent barrels were labeled also. We found slightly higher neuropil label in the cortical zone corresponding to the damaged zone on the face in animals lesioned at any time. These data indicate that physiological somatotopy in vibrissae-damaged animals matches the anatomical cytoarchitecture.     

Septal columns in rodent barrel cortex: functional circuits for modulating whisking behavior

In rodents, each mystacial whisker is represented in the granular layer of primary somatosensory (SI) cortex by a compact cluster of cells known as a barrel, and barrels are separated from each other by domains that are called septa. Vertical columns of neurons aligned with each barrel act as a functional assembly to process information from a "principal" whisker, but a functional role has not been identified for vertical columns of neurons that are aligned with the septa. To determine whether these septal columns provide the main source of projections to primary motor (MI) cortex, we placed retrograde tracers in MI cortex and analyzed the location of the retrogradely labeled neurons with respect to the septal and barrel compartments of SI barrel cortex. In cases in which SI barrel cortex was sectioned tangentially, retrogradely labeled neurons in the extragranular layers of SI were plotted and superimposed onto reconstructions of the layer IV barrel field. In each of these cases, most labeled neurons were located above or below the septal regions of layer IV. When SI barrel cortex was sectioned coronally, we observed multiple columns of labeled SI neurons that were vertically aligned with the septal zones of layer IV. These results indicate that columns of neurons that are vertically aligned with the septa, or septal columns, are functionally linked by virtue of their projections to MI cortex. We hypothesize that these septal columns represent an interconnected and functionally distinct circuit that transmits information to MI and other brain regions involved in motor control.     

Sexual dimorphism in the vomeronasal system of the rabbit

We have previously shown that a classical aversive conditioning paradigm involving stimulation of a row of facial vibrissae (whiskers) in the mouse produced expansion of the cortical representation of the activated vibrissae ("trained row"). This was demonstrated by labeling with 2-deoxyglucose (2DG) in layer IV of the barrel cortex. We have also shown that functional reorganization of the S1 cortex is accompanied by increases in the density of small GABAergic cells, and in GAD67 mRNA in the hollows of barrels representing the "trained row". The aim of this study was to determine whether GAD67-positive puncta (boutons) are affected by learning. Unbiased optical disector counting was applied to sections from the mouse barrel cortex that had been immunostained using a polyclonal antibody against GAD67. Quantification of the numerical density of GAD67-positive boutons was performed for four groups of mice: those that had been given aversive conditioning, pseudoconditioned mice with random application of the unconditioned stimulus, mice that had received only whisker stimulation, and naive animals. This study is the first to demonstrate that learning-dependent modification of mature somatosensory cortex is associated with a 50% increase in GAD67-positive boutons in the hollows of "trained" barrels compared with those of control barrels. Sensory learning seems to mobilize the activity of the inhibitory transmission system in the cortical region where plastic changes were previously detected by 2DG labeling.     

Somatotopic organization of rat thalamocortical slices

The thalamocortical slice is widely employed for in vitro studies of cortical circuits. This preparation was developed in order to preserve anatomical and functional connectivity between the ventrobasal thalamus and somatosensory (whisker/barrel) cortex of young mice, and thalamocortical slice experiments have contributed significantly to our understanding of the thalamocortical synapse. Cortical somatotopy within thalamocortical slices, however, has not been characterized, and this greatly limits their use in studies that require identification of cortical areas associated with particular regions of the sensory periphery. To address this shortcoming we used electrophysiological recording and neuroanatomical labeling techniques in rats to mark the position of functionally defined whisker barrels, in vivo. We subsequently processed the brains in a plane appropriate for TC slices and characterized the location of somatotopically identified barrels in relation to other aspects of slice topology. We found that barrels associated with the large mobile whiskers occupy a particular location in TC slices, but that there are certain constraints to studying this portion of the barrelfield in vitro.     

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.     

Topography of cortical projections to the dorsolateral neostriatum in rats: multiple overlapping sensorimotor pathways

In rodents, the whisker representation in primary somatosensory (SI) cortex projects to the dorsolateral neostriatum, but the location of these projections has never been characterized with respect to layer IV barrels and their intervening septa. To address this issue, we injected a retrograde tracer into the dorsolateral neostriatum and then reconstructed the location of the labeled corticostriatal neurons with respect to the cytochrome oxidase (CO)-labeled barrels in SI. When the tracer was restricted to a small focal site in the neostriatum, the retrogradely labeled neurons formed elongated strips that were parallel to the curvilinear orientation of layer IV barrel rows. After larger tracer injections, labeled neurons were distributed uniformly across layer V and were aligned with both the barrel and septal compartments. Labeled projections from the contralateral SI barrel cortex, however, were much fewer in number and were disproportionately associated with the septal compartments. A comparison of the labeling patterns in the ipsilateral and contralateral hemispheres revealed symmetric, mirror-image distributions that extended across primary motor cortex (MI) and multiple somatosensory cortical regions, including the secondary somatosensory (SII) cortex, the parietal ventral (PV) and parietal rhinal (PR) areas, and the posteromedial (PM) region. Examination of the thalamus revealed labeled neurons in the intralaminar nuclei, in the medial part of the posterior nucleus (POm), and in the ventrobasal complex. These results indicate that the dorsolateral neostriatum integrates sensorimotor information from multiple sensorimotor representations in the thalamus and cortex.     

Spatial-temporal distribution of whisker-evoked activity in rat somatosensory cortex and the coding of stimulus location.

Rats use their facial vibrissae ("whiskers") to locate and identify objects. To learn about the neural coding of contact between whiskers and objects, we investigated the representation of single-vibrissa deflection by populations of cortical neurons. Microelectrode arrays, arranged in a geometric 10 x 10 grid, were inserted into the thalamo-recipient layers of "barrel cortex" (the vibrissal region of somatosensory cortex) in urethane-anesthetized rats, and neuronal activity across large sets of barrel-columns was measured. Typically, 5 msec after deflection of a whisker a 0.2 mm(2) focus of activity emerged. It rapidly expanded, doubling in size by 7 msec, before retracting and disappearing 28-59 msec after stimulus onset. The total territory engaged by the stimulus ranged from 0.5 to 2.9 mm(2) (2-11 barrels). Stimulus site dictated the domain of activity. To quantify the coding of whisker location, we applied the population d' measure of discriminability. Activity patterns elicited by two whiskers were highly discriminable at the initial cortical response; peak discriminability typically occurred within 16 msec of stimulus onset. To determine how widely information about stimulus location was distributed, we measured population d' while excluding response data from the on-center electrodes of the two tested whiskers. Response patterns remained discriminable, indicating that information about stimulus location was distributed across barrel cortex. Taken together, these results show that single-whisker deflections are represented in a multicolumn region constrained by barrel cortex map topography. The nature of this coding allows information about stimulus location to be coded extremely rapidly and unambiguously by one to two spikes per neuron.     

Spatial organization of thalamocortical and corticothalamic projection systems in the rat SmI barrel cortex

Axonal tracing techniques were used to examine the distribution of corticothalamic projection neurons in relation to the organization of the thalamocortical recipient zones in the whisker representation of the rat first somatic sensory cortex. Following injection of horseradish peroxidase into the physiologically defined vibrissa area in the ventrobasal complex of the thalamus, labeling in the cortex had a columnar appearance. Dense patches of anterograde labeling were located within the centers of the layer IV barrels and extended superficially through lamina III; the septa between barrels contained considerably less reaction product. Retrogradely labeled neurons were observed in lower layer V and layer VI where they were concentrated preferentially deep to the barrel centers. Regions deep to the septa displayed less overall labeling and a lower relative number of thalamic projecting neurons. Zones having the larger numbers of retrogradely labeled cells also contained terminallike labeling of either corticothalamic or thalamocortical origin. Following an injection that included the posterior group medial to the ventrobasal complex, anterograde labeling in layer IV was located largely in the septa. In conjunction with previous findings concerning the origin and termination of other projection systems in the barrel cortex, these results suggest that a vibrissal column contains a central core zone intimately linked with the ventrobasal thalamus that is bounded by narrower regions of more diverse inputs and outputs that form an interface between adjacent cortical columns.     

Barrels and septa: separate circuits in rat barrels field cortex.

The neural circuitry within sensory cortex determines its functional properties, and different solutions have evolved for integrating the activity that arises from an array of sensory inputs to cortex. In rodent, circumscribed receptors, such as whiskers, are represented in somatic sensory (S-I) cortex in islands of cells in layer IV called "barrels" surrounded by narrow channels that separate barrels called "septa." These two cortical domains were previously shown to receive sensory inputs through parallel subcortical pathways. Here, by using small biocytin injections, we demonstrate that distinct intrinsic and corticocortical circuitries arise from barrel and septal columns. The intracortical S-I projections originating from barrel columns are rather short-ranged, terminating for the most part within the far boundaries of the most immediate neighboring barrel columns, whereas corticocortical projections reach the second somatosensory (S-II) cortex. In contrast, the intrinsic projections arising from septal columns extend two to three barrels' distance along the row of whisker representation, producing terminals preferentially in other septal columns. Septal corticocortical projections terminate in the dysgranular cortex anterior to E-row barrels and in the posteromedial parietal cortex in addition to S-II. Whereas layer IV barrels are largely isolated from lateral connections, septa are the main conduits of intracortical projections arising from neighboring barrel and septal columns. These results indicate that the two subcortical pathways from whiskers to cortex continue as two distinct partially segregated pathways in cortex.     

Large-scale plasticity in barrel cortex following repeated whisker trimming in young adult hamsters

Using the 2DG/immunostaining method [McCasland, J.S., Graczyk, G.M., 2000. Metabolic mapping-Unit 1.6. In: Gerfen, C.R. (Ed.), Current Protocols in Neuroscience. Wiley, New York, pp 1.6.1-1.6.15], we have previously demonstrated large-scale plasticity in whisker/barrel fields of young adult hamsters subject to follicle ablation on postnatal day 7 (P7) [Somatosens. Motor Res. 13 (1996) 245]. This plasticity occurs after the barrel field has formed, but before neuronal differentiation and synaptogenesis are complete. The present study tested for similar large-scale plasticity following whisker deprivation in young adult hamsters, when neuronal and synaptic development are more mature. Beginning around P40, animals had all whiskers except row C trimmed on alternating days for periods ranging from 1 h to 2 weeks, after which they were administered (3)H 2DG (i.p.) and allowed to explore a fresh empty cage. Autoradiograms from these animals showed a clear expansion in the zone of heavy 2DG labeling with continued whisker trimming. Hamsters with row C spared overnight showed markedly higher labeling in the row C barrels, as expected. After 2 weeks of repeated trimming, the pattern of 2DG labeling in the barrel field ranged from complete activation of all large-whisker columns, as in a previous study of P7 follicle ablation, down to a more localized activation of rows B, C, and D. Intermediate periods of trimming produced more localized label in the region of row C. There was a clear trend toward larger areas of activation with longer periods of trimming. Because inhibitory neurons are strongly activated in all cases, this large-scale neuronal plasticity must take place in the presence of strong inhibition. The data show that simple trimming of all but a few whiskers in normally reared adults leads to abnormally widespread metabolic labeling encompassing virtually the entire barrel field. Taken together, our findings suggest that a large-scale synaptic reorganization occurs in barrel fields deprived of normal sensory input in the adult as well as during postnatal development.     

The evolution of whisker‐mediated somatosensation in mammals: Sensory processing in barrelless S1 cortex of a marsupial,Monodelphis domestica

Movable tactile sensors in the form of whiskers are present in most mammals, but sensory coding in the cortical whisker representation has been studied almost exclusively in mice and rats. Many species that possess whiskers lack the modular “barrel” organization found in the primary somatosensory cortex (S1) of mice and rats, but it is unclear how whisker‐related input is represented in these species. We used single‐unit extracellular recording techniques to characterize receptive fields and response properties in S1 of Monodelphis domestica (short‐tailed opossum), a nocturnal, terrestrial marsupial that shared its last common ancestor with placental mammals over 160 million years ago. Short‐tailed opossums lack barrels and septa in S1 but show active whisking behavior similar to that of mice and rats. Most neurons in short‐tailed opossum S1 exhibited multiwhisker receptive fields, including a single best whisker (BW) and lower magnitude responses to the deflection of surrounding whiskers. Mean tuning width was similar to that reported for mice and rats. Both symmetrical and asymmetrical receptive fields were present. Neurons tuned to ventral whiskers tended to show broad tuning along the rostrocaudal axis. Thus, despite the absence of barrels, most receptive field properties were similar to those reported for mice and rats. However, unlike those species, S1 neuronal responses to BW and surround whisker deflection showed comparable latencies in short‐tailed opossums. This dissimilarity suggests that some aspects of barrel cortex function may not generalize to tactile processing across mammalian species and may be related to differences in the architecture of the whisker‐to‐cortex pathway. J. Comp. Neurol. 524:3587–3613, 2016. © 2016 Wiley Periodicals, Inc.     

Activity-dependent regulation of glutamic acid decarboxylase in the rat barrel cortex: effects of neonatal versus adult sensory deprivation.

Immunocytochemical techniques were used to study the effects of tactual deprivation on glutamic acid decarboxylase (GAD) containing neurons in rat somatosensory barrel cortex. In normal rats GAD immunoreactive neurons and puncta are present in all laminae, with dense patches of GAD immunoreactive puncta centered on the barrels in lamina IV. Trimming whiskers of adult rats leads to a reversible decrease of GAD immunoreactivity in barrels corresponding to trimmed hairs. Intensity of GAD staining also is reversibly altered in supragranular laminae of nondeprived barrel columns flanked by deprived barrels. This indicates that GAD levels in the barrel cortex ordinarily fluctuate with changes in sensory input. By contrast, animals whose whiskers are trimmed from birth have normal GAD staining in both deprived and nondeprived barrels. Moreover, if trimmed whiskers of neonatally deprived animals are allowed to grow to normal lengths and are retrimmed later in adulthood GAD staining is not affected. Thus early tactual deprivation disrupts mechanisms that permit modulation of transmitter enzyme levels in cortical neurons following changes in sensory experience.     

Experience‐dependent increase in spine calcium evoked by backpropagating action potentials in layer 2/3 pyramidal neurons in rat somatosensory cortex

In spines on basal dendrites of layer 2/3 pyramidal neurons in somatosensory barrel cortex, calcium transients evoked by back‐propagating action potentials (bAPs) were investigated (i) along the length of the basal dendrite, (ii) with postnatal development and (iii) with sensory deprivation during postnatal development. Layer 2/3 pyramidal neurons were investigated at three different ages. At all ages [postnatal day (P)8, P14, P21] the bAP‐evoked calcium transient amplitude increased with distance from the soma with a peak at around 50 μm, followed by a gradual decline in amplitude. The effect of sensory deprivation on the bAP‐evoked calcium was investigated using two different protocols. When all whiskers on one side of the rat snout were trimmed daily from P8 to P20‐24 there was no difference in the bAP‐evoked calcium transient between cells in the contralateral hemisphere, lacking sensory input from the whisker, and cells in the ipsilateral barrel cortex, with intact whisker activation. When, however, only the D‐row whiskers on one side were trimmed the distribution of bAP‐evoked calcium transients in spines was shifted towards larger amplitudes in cells located in the deprived D‐column. In conclusion, (i) the bAP‐evoked calcium transient gradient along the dendrite length is established at P8, (ii) the calcium transient increases in amplitude with age and (iii) this increase is enhanced in layer 2/3 pyramidal neurons located in a sensory‐deprived barrel column that is bordered by non‐deprived barrel columns.