The identification of thalamocortical axon terminals in barrels of mouse Sml cortex using immunohistochemistry of anterogradely transported lectin (Phaseolus vulgaris-leucoagglutinin)

Routes of the thalamus through the history of neuroanatomy

The most distant roots of neuroanatomy trace back to antiquity, with the first human dissections, but no document which would identify the thalamus as a brain structure has reached us. Claudius Galenus (Galen) gave to the thalamus the name 'thalamus nervorum opticorum', but later on, other names were used (e.g., anchae, or buttocks-like). In 1543, Andreas Vesalius provided the first quality illustrations of the thalamus. During the 19th century, tissue staining techniques and ablative studies contributed to the breakdown of the thalamus into subregions and nuclei. The next step was taken using radiomarkers to identify connections in the absence of lesions. Anterograde and retrograde tracing methods arose in the late 1960s, supporting extension, revision, or confirmation of previously established knowledge. The use of the first viral tracers introduced a new methodological breakthrough in the mid-1970s. Another important step was supported by advances in neuroimaging of the thalamus in the 21th century. The current review follows the history of the thalamus through these technical revolutions from Antiquity to the present day.

Quantitative 3D Ultrastructure of Thalamocortical Synapses from the "Lemniscal" Ventral Posteromedial Nucleus in Mouse Barrel Cortex

Thalamocortical synapses from “lemniscal” neurons of the dorsomedial portion of the rodent ventral posteromedial nucleus (VPMdm) are able to induce with remarkable efficacy, despite their relative low numbers, the firing of primary somatosensory cortex (S1) layer 4 (L4) neurons. To which extent this high efficacy depends on structural synaptic features remains unclear. Using both serial transmission (TEM) and focused ion beam milling scanning electron microscopy (FIB/SEM), we 3D-reconstructed and quantitatively analyzed anterogradely labeled VPMdm axons in L4 of adult mouse S1. All VPMdm synapses are asymmetric. Virtually all are established by axonal boutons, 53% of which contact multiple (2–4) elements (overall synapse/bouton ratio = 1.6). Most boutons are large (mean 0.47 μm³), and contain 1–3 mitochondria. Vesicle pools and postsynaptic density (PSD) surface areas are large compared to others in rodent cortex. Most PSDs are complex. Most synapses (83%) are established on dendritic spine heads. Furthermore, 15% of the postsynaptic spines receive a second, symmetric synapse. In addition, 13% of the spine heads have a large protrusion inserted into a membrane pouch of the VPMdm bouton. The unusual combination of structural features in VPMdm synapses is likely to contribute significantly to the high efficacy, strength, and plasticity of these thalamocortical synapses.

Multifaceted Changes in Synaptic Composition and Astrocytic Involvement in a Mouse Model of Fragile X Syndrome

Fragile X Syndrome (FXS), a common inheritable form of intellectual disability, is known to alter neocortical circuits. However, its impact on the diverse synapse types comprising these circuits, or on the involvement of astrocytes, is not well known. We used immunofluorescent array tomography to quantify different synaptic populations and their association with astrocytes in layers 1 through 4 of the adult somatosensory cortex of a FXS mouse model, the FMR1 knockout mouse. The collected multi-channel data contained approximately 1.6 million synapses which were analyzed using a probabilistic synapse detector. Our study reveals complex, synapse-type and layer specific changes in the neocortical circuitry of FMR1 knockout mice. We report an increase of small glutamatergic VGluT1 synapses in layer 4 accompanied by a decrease in large VGluT1 synapses in layers 1 and 4. VGluT2 synapses show a rather consistent decrease in density in layers 1 and 2/3. In all layers, we observe the loss of large inhibitory synapses. Lastly, astrocytic association of excitatory synapses decreases. The ability to dissect the circuit deficits by synapse type and astrocytic involvement will be crucial for understanding how these changes affect circuit function, and ultimately defining targets for therapeutic intervention.

An Ultrastructural Study of the Thalamic Input to Layer 4 of Primary Motor and Primary Somatosensory Cortex in the Mouse

The traditional classification of primary motor cortex (M1) as an agranular area has been challenged recently when a functional layer 4 (L4) was reported in M1. L4 is the principal target for thalamic input in sensory areas, which raises the question of how thalamocortical synapses formed in M1 in the mouse compare with those in neighboring sensory cortex (S1). We identified thalamic boutons by their immunoreactivity for the vesicular glutamate transporter 2 (VGluT2) and performed unbiased disector counts from electron micrographs. We discovered that the thalamus contributed proportionately only half as many synapses to the local circuitry of L4 in M1 compared with S1. Furthermore, thalamic boutons in M1 targeted spiny dendrites exclusively, whereas ∼9% of synapses were formed with dendrites of smooth neurons in S1. VGluT2⁺ boutons in M1 were smaller and formed fewer synapses per bouton on average (1.3 vs 2.1) than those in S1, but VGluT2⁺ synapses in M1 were larger than in S1 (median postsynaptic density areas of 0.064 μm² vs 0.042 μm²). In M1 and S1, thalamic synapses formed only a small fraction (12.1% and 17.2%, respectively) of all of the asymmetric synapses in L4. The functional role of the thalamic input to L4 in M1 has largely been neglected, but our data suggest that, as in S1, the thalamic input is amplified by the recurrent excitatory connections of the L4 circuits. The lack of direct thalamic input to inhibitory neurons in M1 may indicate temporal differences in the inhibitory gating in L4 of M1 versus S1.SIGNIFICANCE STATEMENT Classical interpretations of the function of primary motor cortex (M1) emphasize its lack of the granular layer 4 (L4) typical of sensory cortices. However, we show here that, like sensory cortex (S1), mouse M1 also has the canonical circuit motif of a core thalamic input to the middle cortical layer and that thalamocortical synapses form a small fraction (M1: 12%; S1: 17%) of all asymmetric synapses in L4 of both areas. Amplification of thalamic input by recurrent local circuits is thus likely to be a significant mechanism in both areas. Unlike M1, where thalamocortical boutons typically form a single synapse, thalamocortical boutons in S1 usually formed multiple synapses, which means they can be identified with high probability in the electron microscope without specific labeling.

Glutamatergic thalamopallial projections in the pigeon identified by retrograde labeling and expression of vGluT2 mRNA

A neocortical hypothesis as to homology of certain nuclear components of the avian brain proposes that the entopallium and field L2 are homologous to layer 4 of mammalian extrastriate and auditory neocortex, respectively. However, the hypothesis lacks support from the neurochemistry of thalamopallial projections. We investigated whether these projections are glutamatergic by injecting cholera toxin B into either the entopallium or field L2 in combination with in situ hybridization. Retrogradely labeled neurons in nucleus rotundus and nucleus ovoidalis were found to express vesicular glutamate transporter 2 mRNA, showing that the thalamopallial projections are glutamatergic. The results are consistent with the neocortical hypothesis.

Cortical hypoexcitation defines neuronal responses in the immediate aftermath of traumatic brain injury

Traumatic brain injury (TBI) from a blow to the head is often associated with complex patterns of brain abnormalities that accompany deficits in cognitive and motor function. Previously we reported that a long-term consequence of TBI, induced with a closed-head injury method modelling human car and sporting accidents, is neuronal hyper-excitation in the rat sensory barrel cortex that receives tactile input from the face whiskers. Hyper-excitation occurred only in supra-granular layers and was stronger to complex than simple stimuli. We now examine changes in the immediate aftermath of TBI induced with same injury method. At 24 hours post-trauma significant sensorimotor deficits were observed and characterisation of the cortical population neuronal responses at that time revealed a depth-dependent suppression of neuronal responses, with reduced responses from supragranular layers through to input layer IV, but not in infragranular layers. In addition, increased spontaneous firing rate was recorded in cortical layers IV and V. We postulate that this early post-injury suppression of cortical processing of sensory input accounts for immediate post-trauma sensory morbidity and sets into train events that resolve into long-term cortical hyper-excitability in upper sensory cortex layers that may account for long-term sensory hyper-sensitivity in humans with TBI.

Ultra high-resolution fMRI and electrophysiology of the rat primary somatosensory cortex

High-resolution functional-magnetic-resonance-imaging (fMRI) has been used to study brain functions at increasingly finer scale, but whether fMRI can accurately reflect layer-specific neuronal activities is less well understood. The present study investigated layer-specific cerebral-blood-volume (CBV) fMRI and electrophysiological responses in the rat cortex. CBV fMRI at 40×40 μm in-plane resolution was performed on an 11.7-T scanner. Electrophysiology used a 32-channel electrode array that spanned the entire cortical depth. Graded electrical stimulation was used to study activations in different cortical layers, exploiting the notion that most of the sensory-specific neurons are in layers II-V and most of the nociceptive-specific neurons are in layers V-VI. CBV response was strongest in layer IV of all stimulus amplitudes. Current source density analysis showed strong sink currents at cortical layers IV and VI. Multi-unit activities mainly appeared at layers IV-VI and peaked at layer V. Although our measures showed scaled activation profiles during modulation of stimulus amplitude and failed to detect specific recruitment at layers V and VI during noxious electrical stimuli, there appears to be discordance between CBV fMRI and electrophysiological peak responses, suggesting neurovascular uncoupling at laminar resolution. The technique implemented in the present study offers a means to investigate intracortical neurovascular function in the normal and diseased animal models at laminar resolution.

Shifts in developmental timing, and not increased levels of experience-dependent neuronal activity, promote barrel expansion in the primary somatosensory cortex of rats enucleated at birth

Birth-enucleated rodents display enlarged representations of whiskers (i.e., barrels of the posteromedial subfield) in the primary somatosensory cortex. Although the historical view maintains that barrel expansion is due to incremental increases in neuronal activity along the trigeminal pathway during postnatal development, recent evidence obtained in experimental models of intramodal plasticity challenges this view. Here, we re-evaluate the role of experience-dependent neuronal activity on barrel expansion in birth-enucleated rats by combining various anatomical methods and sensory deprivation paradigms. We show that barrels in birth-enucleated rats were already enlarged by the end of the first week of life and had levels of metabolic activity comparable to those in control rats at different ages. Dewhiskering after the postnatal period of barrel formation did not prevent barrel expansion in adult, birth-enucleated rats. Further, dark rearing and enucleation after barrel formation did not lead to expanded barrels in adult brains. Because incremental increases of somatosensory experience did not promote barrel expansion in birth-enucleated rats, we explored whether shifts of the developmental timing could better explain barrel expansion during the first week of life. Accordingly, birth-enucleated rats show earlier formation of barrels, accelerated growth of somatosensory thalamocortical afferents, and an earlier H4 deacetylation. Interestingly, when H4 deacetylation was prevented with a histone deacetylases inhibitor (valproic acid), barrel specification timing returned to normal and barrel expansion did not occur. Thus, we provide evidence supporting that shifts in developmental timing modulated through epigenetic mechanisms, and not increased levels of experience dependent neuronal activity, promote barrel expansion in the primary somatosensory cortex of rats enucleated at birth.

Organization of myelin in the mouse somatosensory barrel cortex and the effects of sensory deprivation

In rodents, the barrel cortex is a specialized area within the somatosensory cortex that processes signals from the mystacial whiskers. We investigated the normal development of myelination in the barrel cortex of mice, as well as the effects of sensory deprivation on this pattern. Deprivation was achieved by trimming the whiskers on one side of the face every other day from birth. In control mice, myelin was not present until postnatal day 14 and did not show prominence until postnatal day 30; adult levels of myelination were reached by the end of the second postnatal month. Unbiased stereology was used to estimate axon density in the interbarrel septal region and barrel walls as well as the barrel centers. Myelin was significantly more concentrated in the interbarrel septa/barrel walls than in the barrel centers in both control and sensory-deprived conditions. Sensory deprivation did not impact the onset of myelination but resulted in a significant decrease in myelinated axons in the barrel region and decreased the amount of myelin ensheathing each axon. Visualization of the oligodendrocyte nuclear marker Olig2 revealed a similar pattern of myelin as seen using histochemistry, but with no significant changes in Olig2+ nuclei following sensory deprivation. Consistent with the anatomical results showing less myelination, local field potentials revealed slower rise times following trimming. Our results suggest that myelination develops relatively late and can be influenced by sensory experience.

Synaptic properties of thalamic input to the subgranular layers of primary somatosensory and auditory cortices in the mouse

The classification of synaptic inputs is an essential part of understanding brain circuitry. In the present study, we examined the synaptic properties of thalamic inputs to pyramidal neurons in layers 5a, 5b, and 6 of primary somatosensory (S1) and auditory (A1) cortices in mouse thalamocortical slices. Stimulation of the ventral posterior medial nucleus and the ventral division of the medial geniculate body resulted in three distinct response classes, two of which have never been described before in thalamocortical projections. Class 1A responses included synaptic depression and all-or-none responses, while Class 1B responses exhibited synaptic depression and graded responses. Class 1C responses are characterized by mixed facilitation and depression as well as graded responses. Activation of metabotropic glutamate receptors was not observed in any of the response classes. We conclude that Class 1 responses can be broken up into three distinct subclasses, and that thalamic inputs to the subgranular layers of cortex may combine with other, intracortical inputs to drive their postsynaptic target cells. We also integrate these results with our recent, analogous study of thalamocortical inputs to granular and supragranular layers (Viaene et al., 2011).

Properties of the thalamic projection from the posterior medial nucleus to primary and secondary somatosensory cortices in the mouse

Primary somatosensory cortex (S1) receives two distinct classes of thalamocortical input via the lemniscal and paralemniscal pathways, the former via ventral posterior medial nucleus (VPM), and the latter, from the posterior medial nucleus (POm). These projections have been described as parallel thalamocortical pathways. Although the VPM thalamocortical projection has been studied in depth, several details of the POm projection to S1 are unknown. We studied the synaptic properties and anatomical features in the mouse of the projection from POm to all layers of S1 and to layer 4 of secondary somatosensory cortex (S2). Neurons in S1 responded to stimulation of POm with what has been termed Class 2 properties (paired-pulse facilitation, small initial excitatory postsynaptic potentials (EPSPs), a graded activation profile, and a metabotropic receptor component; thought to be modulatory), whereas neurons in layer 4 of S2 responded with Class 1A properties (paired-pulse depression, large initial EPSPs, an all-or-none activation profile, and no metabotropic receptor component, thought to be a main information input). Also, labeling from POm produced small boutons in S1, whereas both small and large boutons were found in S2. Our data suggest that the lemniscal and paralemniscal projections should not be thought of as parallel information pathways to S1 and that the paralemniscal projection may instead provide modulatory inputs to S1.

Synaptic properties of thalamic input to layers 2/3 and 4 of primary somatosensory and auditory cortices

We studied the synaptic profile of thalamic inputs to cells in layers 2/3 and 4 of primary somatosensory (S1) and auditory (A1) cortices using thalamocortical slices from mice age postnatal days 10-18. Stimulation of the ventral posterior medial nucleus (VPM) or ventral division of the medial geniculate body (MGBv) resulted in two distinct classes of responses. The response of all layer 4 cells and a minority of layers 2/3 cells to thalamic stimulation was Class 1, including paired-pulse depression, all-or-none responses, and the absence of a metabotropic component. On the other hand, the majority of neurons in layers 2/3 showed a markedly different, Class 2 response to thalamic stimulation: paired-pulse facilitation, graded responses, and a metabotropic component. The Class 1 and Class 2 response characteristics have been previously seen in inputs to thalamus and have been described as drivers and modulators, respectively. Driver input constitutes a main information bearing pathway and determines the receptive field properties of the postsynaptic neuron, whereas modulator input influences the response properties of the postsynaptic neuron but is not a primary information bearing input. Because these thalamocortical projections have comparable properties to the drivers and modulators in thalamus, we suggest that a driver/modulator distinction may also apply to thalamocortical projections. In addition, our data suggest that thalamus is likely to be more than just a simple relay of information and may be directly modulating cortex.

Dimensions of a projection column and architecture of VPM and POm axons in rat vibrissal cortex

This is the first article in a series of 3 studies that investigate the anatomical determinants of thalamocortical (TC) input to excitatory neurons in a cortical column of rat primary somatosensory cortex (S1). S1 receives 2 major types of TC inputs, lemiscal and paralemniscal. Lemiscal axons arise from the ventral posteromedial nucleus (VPM) of the thalamus, whereas paralemniscal fibers originate in the posteromedial nucleus (POm). While these 2 TC projections are largely complementary in L4, overlap in other cortical layers is still a matter of debate. VPM and POm axons were specifically labeled in the same rat by virus-mediated expression of different fluorescent proteins. We show that columnar and septal projection patterns are maintained throughout most of the cortical depth with a lower degree of separation in infragranular layers, where TC axons form bands along rows. Finally, we present anatomical dimensions of "TC projection domains" for a standard column in S1.

Cell type-specific thalamic innervation in a column of rat vibrissal cortex

This is the concluding article in a series of 3 studies that investigate the anatomical determinants of thalamocortical (TC) input to excitatory neurons in a cortical column of rat primary somatosensory cortex (S1). We used viral synaptophysin-enhanced green fluorescent protein expression in thalamic neurons and reconstructions of biocytin-labeled cortical neurons in TC slices to quantify the number and distribution of boutons from the ventral posterior medial (VPM) and posteromedial (POm) nuclei potentially innervating dendritic arbors of excitatory neurons located in layers (L)2–6 of a cortical column in rat somatosensory cortex. We found that 1) all types of excitatory neurons potentially receive substantial TC input (90–580 boutons per neuron); 2) pyramidal neurons in L3–L6 receive dual TC input from both VPM and POm that is potentially of equal magnitude for thick-tufted L5 pyramidal neurons (ca. 300 boutons each from VPM and POm); 3) L3, L4, and L5 pyramidal neurons have multiple (2–4) subcellular TC innervation domains that match the dendritic compartments of pyramidal cells; and 4) a subtype of thick-tufted L5 pyramidal neurons has an additional VPM innervation domain in L4. The multiple subcellular TC innervation domains of L5 pyramidal neurons may partly explain their specific action potential patterns observed in vivo. We conclude that the substantial potential TC innervation of all excitatory neuron types in a cortical column constitutes an anatomical basis for the initial near-simultaneous representation of a sensory stimulus in different neuron types.

Laterodorsal nucleus of the thalamus: A processor of somatosensory inputs

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

Development of 5-HT(1B), SERT and thalamo-cortical afferents in early nutrionally restricted rats: an emerging explanation for delayed barrel formation

Barrel formation is delayed in nutritionally restricted rats. The underlying cause of such delay is yet unclear. Because barrels appear upon the arrival of somatosensory thalamo-cortical afferents and the reorientation of the dendritic arborizations of cortical spiny stellate neurons, it is likely that at least one of these processes is altered by nutritional restriction. Also, the serotoninergic afferent system has been implicated in regulating barrel segregation and growth during early postnatal life. We then evaluated the pattern of immunostaining of the serotonin transporter (SERT) and of the serotonin receptor 1B (5-HT(1B)), as well as the growth and arrival time of somatosensory thalamo-cortical afferents, to infer the contribution of these elements in the delayed formation of barrels observed in nutritionally restricted rats. It was found that the rates of development and the segregation of thalamo-cortical fibers were normal in nutritionally restricted rats. SERT, but not 5-HT(1B) immunoreactivity, was decreased in the primary somatosensory cortex during barrel specification. The availability of both proteins in nutritionally restricted rats was lower than that observed in their well fed counterparts at later developmental times. It is concluded that the delayed formation of barrels observed in nutritionally restricted rats is due to a retarded reorientation of dendritic arbors of cortical neurons. This might happen as a secondary effect of decreasing the availability of SERT and/or increasing the availability of 5-HT(1B) receptor early in postnatal life.

Reflections on the specificity of synaptic connections

The principal focus of this treatise is the specificity of synaptic connectivity in the mammalian central nervous system. The occurrence of stereotypical patterns of connection at the macro level (e.g., the general consistency with which axonal pathways impinge on and originate within specific cortical areas and layers) implies that the cerebral cortex is a highly ordered structure. Order is seen also at the more micro level of synaptic connectivity, for instance, in the contrasting synaptic patterns of spiny vs. non-spiny neurons. Quantitative electron microscopic studies of synapses between identified neurons and correlative anatomical/electrophysiological investigations indicate that the high degree of order characterizing many aspects of cortical organization is mirrored by an equally ordered arrangement of synaptic connections between specific types of neurons. The recognition of recurring synaptic patterns has generated increased support for the notion of synaptic specificity as opposed to randomness, and we have begun now to understand the role of specificity in cortical function. At the core of cortical processing lie myriad possibilities for computation provided by the wealth of synaptic connections involving each neuron. Specificity, by limiting possibilities for connection, imposes an order on synaptic interactions even as processes of dynamic selection or synaptic remodeling ensure the constant formation and dissolution of cortical circuits. Collectively, these operations make maximal use of the richness of cortical synaptic connections to produce a highly flexible system, irrespective of the degree of hard-wiring, mutability, randomness or specificity that obtains for cortical wiring at any particular time. A brief, historical account of developments leading to our current understanding of cortical synaptic organization will precede the presentation of evidence for synaptic specificity.

Short-term sensory learning does not alter parvalbumin neurons in the barrel cortex of adult mice: A double-labeling study

We have previously reported that a classical conditioning paradigm involving stimulation of a row of facial vibrissae produced expansion of the cortical representation of the activated vibrissae ("trained row"), this was demonstrated by labeling with 2-deoxyglucose in layer IV of the barrel cortex. We have also shown that functional reorganization of the primary somatosensory cortex is accompanied by an increase in the density of small GABAergic cells and glutamate decarboxylase 67-positive neurons in the hollows of barrels representing the "trained row." GABA neurons of the rat neocortex co-localize with calcium-binding proteins [parvalbumin, carletinin, calbindin D28k] and neuropeptides (vasoactive intestinal polypeptide, somatostatin). In the present study we have examined GABAergic parvalbumin-positive, interneurons in the cortical representation of "trained" facial vibrissae after short-term aversive training, in order to determine whether the observed changes in glutamate decarboxylase 67-positive neurons are accompanied by changes in parvalbumin-positive neurons. Using double immunofluorescent staining, it was found that (i) all parvalbumin-positive neurons in the barrel hollows were glutamate decarboxylase 67-positive, (ii) following aversive training density of glutamate decarboxylase 67-positive neurons in barrel hollows increased significantly compared with controls and (iii) density glutamate decarboxylase 67-positive/parvalbumin-positive neurons in "trained" barrel hollows did not change compared with controls. This study is the first to demonstrate that the density of double-labeled glutamate decarboxylase 67-positive/parvalbumin-positive neurons does not alter during cortical plasticity, thus suggesting that some other population (i.e. parvalbumin negative) of GABAergic interneurons is involved in learning-dependent changes in layer IV of the barrel cortex.

Commissural propriospinal connections between the lateral aspects of laminae III-IV in the lumbar spinal cord of rats

It has been established that there is a strong functional link between sensory neural circuits on the two sides of the spinal cord. In one of our recent studies we provided a morphological confirmation of this functional phenomenon, presenting evidence for the presence of a direct commissural connection between the lateral aspects of the dorsal horn on the two sides of the lumbar spinal cord. By using a combination of neural tracing and immunocytochemical detection of neural markers like vesicular glutamate transporters, glutamic acid decarboxylase, glycine transporter, and met-enkephalin (which are characteristic of various subsets of excitatory and inhibitory neurons), we investigated here the distribution, synaptic relations, and neurochemical characteristics of the commissural axon terminals. We found that the cells of origin of commissural fibers in the lateral aspect of the dorsal horn were confined to laminae III-IV and projected to the corresponding area of the contralateral gray matter. Most of the commissural axon terminals established synaptic contacts with dendrites. Axospinous or axosomatic synaptic contacts were found in limited numbers. We demonstrated that interactions among commissural neurons also exist. More than three-fourths of the labeled axon terminals were immunostained for glutamic acid decarboxylase and/or glycine transporter, but none of them showed positive immunoreaction for met-enkephalin and vesicular glutamate transporters. The results indicate that there is a substantial reciprocal commissural synaptic interaction between the lateral aspects of laminae III-IV on the two sides of the lumbar spinal cord and that this pathway may transmit both inhibitory and excitatory signals to their postsynaptic targets.

Regional pattern of metabolic activation is reflected in the sleep EEG after sleep deprivation combined with unilateral whisker stimulation in mice

Regional differences in EEG slow wave activity (SWA) during sleep after sleep deprivation (SD) may be a consequence of differential metabolic activation of cortical areas. We investigated the relationship between the regional EEG dynamics and 2-deoxyglucose (DG) uptake after SD in mice. Six hours' SD were combined with natural unilateral whisker stimulation in an enriched environment to selectively activate the barrel cortex and motor areas. As expected, an interhemispheric asymmetry of 2-DG uptake was found in the barrel cortex immediately after SD. To test whether sleep contributes to recovery of the asymmetry, the stimulation was followed by either undisturbed sleep or by an additional SD. The asymmetry vanished after recovery sleep but also after the additional period of wakefulness without stimulation. In addition, relative 2-DG uptake in the primary motor cortex and retrosplenial area was significantly higher immediately after the SD than after the additional sleep or wakefulness, whereas no other region differed between the groups. Whisker stimulation elicited a greater increase in EEG SWA during non rapid eye movement sleep in the stimulated hemisphere than in the control hemisphere; this increase lasted for 10 h. Within a hemisphere, the initial increase in SWA was higher in the frontal than in the parietal derivation. We conclude that the regional SWA differences during sleep are use-dependent and may be related to the regional pattern of metabolism during the previous waking episode. However, the regional metabolic recovery is not dependent on sleep, and is not directly reflected in changes in SWA during sleep.

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

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

Locus coeruleus activation shortens synaptic drive while decreasing spike latency and jitter in sensorimotor cortex. Implications for neuronal integration

Chronic recording of locus coeruleus (LC) neurons in rat and monkey have pointed out that brief, phasic LC discharges, but not sustained activity, are specifically related to salient stimuli and attention. However, the sensory consequences of phasic activation of the noradrenergic system by a brief conditioning stimulation of the LC have not been fully investigated. This study examined the effect of LC activation on synaptic and neuronal responses to a tactile stimulus in the sensorimotor cortex of the anaesthetized rat, by analysing the fine temporal structure of sensory discharges and current source-density profiles recorded from the same electrodes. LC stimulation, with minimal EEG effects, consistently reduced the synaptic input in layers IV and V-VI, by decreasing the amplitude and duration of short-latency current sinks, but not the slope of their early rising phase. Simultaneously, most multiple and single unit excitatory responses were shortened by the suppression of their late component after 25-30 ms, whereas robust temporal facilitation of the early discharge was found for spike latency mean and variance, spike timing and synchronization to the stimulus, but leaving the number of spikes unaffected. These two apparently opposite effects on the synaptic drive and neuronal response are reminiscent of the noradrenergic depression of afferent synaptic potentials observed with an increased neuronal excitability in vitro. They are interpreted as a noradrenergic sharpening of thalamocortical processing consistent with a presumed role of synchronous discharges in perception that would depend on activated states, particularly when LC activity is correlated with vigilance or attention.

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.

Receptive field scatter, topography and map variability in different layers of the hindpaw representation of rat somatosensory cortex

We recorded neurons extracellularly in layers II/III, IV, and V of the hindpaw representation of primary somatosensory cortex in anesthetized rats and studied laminar features of receptive fields (RFs) and representational maps. On average, RFs were smallest in layer IV and largest in layer V; however, for individual penetrations we found substantial deviations from this rule. Within the hindpaw representation, a distinct rostrocaudal gradient of RF size was present in all layers. While layer V RFs were generally largest independent of this gradient, layer IV RFs recorded caudally representing the proximal portions of the paw were larger than layer II/III RFs recorded rostrally representing the digits. The individual scatter of the locations of RFs across laminar groups was in the range of several millimeters, corresponding to about 25% of the average RF diameter. The cutaneous representations of the hindpaw in extragranular layers were confined to the areal extent defined by responsive sites in layer IV. Comparison between RFs determined quantitatively and by handplotting showed a reliable correspondence. Repeated measurements of RFs revealed spontaneous fluctuations of RF size of no more than 5% of the initial condition over an observation period of several hours. The topography and variability of cortical maps of the hindpaw representation were studied with a quantitative interpolation method taking into account the geometric centers of RFs and the corresponding cortical recording sites. On average, the overall topography in terms of preservation of neighborhood relations was present in all layers, although some individual maps showed severe distortions of topography. Factors contributing to map variability were overall position of the representation on the cortical surface, internal topography and spatial extent. Interindividual variability of map layout was always highest in the digit representations. Local topographic orderliness was lowest in layer V, but comparable in layers II/III and IV. Within layer IV, the lowest orderliness was observed in the digit representations. Our data emphasize a substantial variability of RF size, overlap and position across layers and within layers. At the level of representational maps, we found a similar degree of variability that often co-varied across layers, with little evidence for significant layer specificity. Laminar differences are likely to arise from the specific input-output pattern, layer-specific cell types and the connectivity between different layers. Our findings emphasizing similarities in the variability across layers support the notion of tightly coupled columnar interactions between different layers.

Quantitative analysis of synaptic distribution along thalamocortical axons in adult mouse barrels

Quantitative data on thalamocortical synapses in adult mouse barrels have been obtained largely by using lesion-nduced degeneration to label thalamic afferents. By the time degenerating axons can be identified with the electron microscope, they have broken up into many separate pieces, making it impossible to assess the distribution of synapses along unbroken lengths of afferent. Here, this deficiency is rectified by examining intact lengths of axon labeled by the injection of biotinylated dextran amine into ipsilateral thalamus. Serial thin section reconstructions were analyzed to determine the numbers of synapses per axon length made with dendritic spines vs. shafts and the locations of synapses with respect to axonal varicosities. Results for seven axonal segments from six mice showed an average of 0.2 synapses/microm; 80% were made with spines and 20% with dendritic shafts. Just over two-thirds of axonal varicosities formed one synapse; most of the remainder formed two and rarely three, whereas 8% formed none. Although most synapses occurred at varicosities (88%), more than 12% were made at cylindrically shaped regions of the reconstructed axonal segments. These results serve as a caveat for the use of light microscopy to quantify synapses, wherein the usual approach is to equate one varicosity with one synapse. For thalamocortical afferents to mouse barrels, equating one varicosity with one synapse would prove to be incorrect more than 30% of the time and would exclude the roughly 12% of synaptic connections made at cylindrical regions of thalamocortical afferents.

Synaptic patterns of thalamocortical afferents in mouse barrels at postnatal day 11

This study focuses on the synaptic output patterns of thalamocortical axons in mouse barrel cortex at postnatal day (P) 11. Axons were labeled by biotinylated dextran amine transported anterogradely following injection in vivo into the ventrobasal thalamus. Labeled axons in the posteromedial barrel subfield were examined by light and electron microscopy and then reconstructed in three dimensions to assess the spatial distribution of their synapses. Thalamocortical axons form asymmetrical synapses, both at varicosities and along cylindrical portions of the axons; usually, only one synapse occurs per site, contrasting with the case in the adult, in which multiple synapses are typical. At P11, varicosities without synapses are common. As in adult barrels, approximately 80% of synapses formed by thalamocortical axons are with dendritic spines; 20% are with dendritic shafts. The similarity in the distribution of thalamocortical synapses onto spines vs. dendrites in developing and mature barrels indicates that adult synaptic patterns already are specified at a very early stage of thalamocortical synaptogenesis.

Differences in somatosensory processing in S1 barrel cortex between normal and monoamine oxidase A knockout (Tg8) adult mice

Spatio-temporal processing of whisker information was analysed in vivo for single neurons in D2 barrel columns of S1 cortex in Tg8 mutant mice, which lack barrels. Findings were compared with normal C3H mice of the same genetic background. The topographical organization of functional columns was similar in Tg8 and normal mice. Response magnitudes (RMs) to D2 principal whisker deflections in D2 columns for Tg8 were similar to normals for layers I-III and layer IV cells but short latency responses (>10 ms post-stimulus) were twice the magnitude of normal mice. The surrounding whiskers D1 and D3 yielded smaller RMs in layer IV of mutants than normal mice whereas RMs in layers I-III were equipotent (P>0.5). Modal latencies were shorter in Tg8 mice in all layers. Latency distributions for whisker D2 responses in both laminae were bimodal in normal mice, peaking at 6-8 and 12 ms post-stimulus, but unimodal in Tg8 mice in both laminae, peaking at 6-8 ms. Hence, despite an absence of barrels, segregation of columns is enhanced in layer IV and sensory processing is faster in layers I-IV compared with normal mice. This contrasts with adenylyl cyclase knockout mice where both an absence of barrels and enhanced surrounding whisker responses have been observed. These findings suggest that factors other than barrels and clustering of thalamo-cortical terminals define receptive field geometry.

Noradrenergic modulation of tactile responses in rat cortex. Current source-density and unit analyses

This study describes the noradrenergic modulation of tactile afferent information in the sensorimotor cortex of urethane-anesthetized rats. Synaptic and spike responses to a mechanical stimulation of the hand palm were evaluated by means of current source-density analysis and unit activity recording in all cortical layers. Results showed that activation of the locus coeruleus decreased and shortened afferent synaptic excitation in supragranular, but not in deep layers. On the average, unit responses exhibited facilitated latency, moderately increased amplitude, enhanced postexcitatory inhibition and synchronization of responses across layers. The apparent paradox of this global phasic facilitation correlated with a decrease in input synaptic currents was discussed according to hypotheses which might explain its functional significance.

Malformation of the functional organization of somatosensory cortex in adult ephrin-A5 knock-out mice revealed by in vivo functional imaging

The molecular mechanisms that coordinate the functional organization of the mammalian neocortex are largely unknown. We tested the involvement of a putative guidance label, ephrin-A5, in the functional organization of the somatosensory cortex by quantifying the functional representations of individual whiskers in vivo in adult ephrin-A5 knock-out mice, using intrinsic signal optical imaging. In wild-type mice ephrin-A5 is expressed in a gradient in the somatosensory cortex during development. In adult ephrin-A5 knock-out mice, we found a spatial gradient of change in the amount of cortical territory shared by individual whisker functional representations across the somatosensory cortex, as well as a gradient of change in the distance between the functional representations. Both gradients of change were in correspondence with the developmental expression gradient of ephrin-A5 in wild-type mice. These changes involved malformations of the cortical spacing of the thalamocortical components, without concurrent malformations of the intracortical components of individual whisker functional representations. Overall, these results suggest that a developmental guidance label, such as ephrin-A5, is involved in establishing certain spatial relationships of the functional organization of the adult neocortex, and they underscore the advantage of investigating gene manipulation using in vivo functional imaging.

Propriospinal afferent and efferent connections of the lateral and medial areas of the dorsal horn (laminae I-IV) in the rat lumbar spinal cord

The different subdivisions along the mediolateral extent of the superficial dorsal horn of the spinal cord are generally regarded as identical structures that execute the function of sensory information processing without any significant communication with other regions of the spinal gray matter. In contrast to this standing, here we endeavor to show that neural assemblies along the mediolateral extent of laminae I-IV cannot be regarded as identical structures. After injecting Phaseolus vulgaris leucoagglutinin and biotinylated dextran amine into various areas of the superficial dorsal horn (laminae I-IV) at the level of the lumbar spinal cord in rats, we have demonstrated that the medial and lateral areas of the superficial dorsal horn show the following distinct features in their propriospinal afferent and efferent connections: 1) A 300- to 400-microm-long section of the medial aspects of laminae I-IV projects to and receives afferent fibers from a three segment long compartment of the spinal dorsal gray matter, whereas the same length of the lateral aspects of laminae I-IV projects to and receives afferent fibers from the entire rostrocaudal extent of the lumbar spinal cord. 2) The medial aspects of laminae I-IV project extensively to the lateral areas of the superficial dorsal horn. In contrast to this, the lateral areas of laminae I-IV, with the exception of a few fibers at the segmental level, do not project back to the medial territories. 3) There is a substantial direct commissural connection between the lateral aspects of laminae I-IV on the two sides of the lumbar spinal cord. The medial part of laminae I-IV, however, does not establish any direct connection with the gray matter on the opposite side. 4) The lateral aspects of laminae I-IV appear to be the primary source of fibers projecting to the ipsi- and contralateral ventral horns and supraspinal brain centers. Projecting fibers arise from the medial subdivision of laminae I-IV in a substantially lower number. The findings indicate that the medial and lateral areas of the superficial spinal dorsal horn of rats may play different roles in sensory information processing.

Synaptic efficacy and reliability of excitatory connections between the principal neurones of the input (layer 4) and output layer (layer 5) of the neocortex

A prerequisite for the understanding of how a cortical column functions is a description of small and defined neuronal circuits consisting of only a few identified neurones. Here we summarise, with particular reference to the barrel cortex, the morphological and physiological properties of two synaptic connections, namely those between pairs of spiny neurones in layer 4 and pairs of pyramidal cells in layer 5. While layer 4 spiny neurones are the cortical input neurones that amplify and relay incoming excitation from the periphery, layer 5 pyramidal cells integrate neuronal activity both within and across cortical columns and subsequently distribute it to both cortical and subcortical brain regions.

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.

Development, neurochemical properties, and axonal projections of a population of last-order premotor interneurons in the white matter of the chick lumbosacral spinal cord

There is general agreement that last-order premotor interneurons-a set of neurons that integrate activities generated by the spinal motor apparatus, sensory information and volleys arising from higher motor centres, and transmit the integrated signals to motoneurons through monosynaptic contacts-play crucial roles in the initiation and maintenance of spinal motor activities. Here, we demonstrate the development, neurochemical properties, and axonal projections of a unique group of last-order premotor interneurons within the ventrolateral aspect of the lateral funiculus of the chick lumbosacral spinal cord. Neurons expressing immunoreactivity for neuron-specific enolase were first detected in the ventrolateral white matter at embryonic day 9 (E9). The numbers of immunoreactive neurons were significantly increased at E10-E12, while most of them were gradually concentrated in small segmentally arranged nuclei (referred to as major nuclei of Hofmann) protruding from the white matter in a necklace like fashion dorsal to the ventral roots. The major nuclei of Hofmann became more prominent at E12-E16, but substantial numbers of cells were still located within the ventrolateral white matter (referred to as minor nucleus of Hofmann). The distribution of immunoreactive neurons achieved by E16 was maintained during later developmental stages and was also characteristic of adult animals. After injection of Phaseolus vulgaris-leucoagglutinin unilaterally into the minor nucleus of Hofmann, labeled fibres were detected in the ventrolateral white matter ipsilateral to the injection site. Ascending and descending fibres were revealed throughout the entire rostro-caudal length of the lumbosacral spinal cord. Axon terminals were predominantly found within the lateral motor column and the ventral regions of lamina VII ipsilateral to the injection site. Several axon varicosities made close appositions with somata and dendrites of motoneurons, which were identified as synaptic contacts in a consecutive electron microscopic study. With the postembedding immunogold method, 21 of 97 labeled terminals investigated were immunoreactive for glycine and 2 of them showed immunoreactivity for gamma-aminobutyric acid (GABA). The axon trajectories of neurons within the minor nucleus of Hofmann suggest that some of these cells might represent a population of last-order premotor interneurons. J. Exp. Zool. 286:157-172, 2000.

GABA immunoreactivity in mouse barrel field after aversive and appetitive classical conditioning training involving facial vibrissae

We have previously reported that a classical conditioning paradigm involving stimulation of a row of facial vibrissae produced an expansion of the cortical representation of the "trained row", labeled with 2-deoxyglucose (2DG), in layer IV of the barrel field. The present study has examined the pattern of GABA immunoreactivity (GABA-IR) in the cortical representation of row B of the facial vibrissae after (i) 3 days of aversive training, and (ii) 2 months of appetitive training, where stimulation of row B of vibrissae on one side of the snout was used as a conditioned stimulus. The most notable observation was a greater density of GABA-IR cells concentrated in the hollows of the "trained row" B barrels compared to the hollows in the barrel field of the opposite hemisphere in the same mouse. After aversive training, we noted a 2-fold increase in the density of GABA-IR neurons in the hollows of row B; after reward training, the increase amounted to 49%. In contrast, GABA-IR was unchanged in the control groups, which received only stimulation of vibrissae without the unconditioned stimulus. The classification of labeled neurons according to size revealed that the increase in density of GABA-IR neurons was confined to the small (12-15 microm) diameter group. We concluded that the GABAergic system undergoes up-regulation, after both associative learning paradigms, and that the population of small, GABAergic neurons plays an active role in use-dependent plasticity.

Modular distribution of vasoactive intestinal polypeptide in the rat barrel cortex: changes induced by neonatal removal of vibrissae

The distribution of vasoactive intestinal polypeptide-immunoreactive neuronal structures in the barrel cortex (posteromedial barrel subfield) of adult rats was analysed after unilateral removal of the vibrissal follicles of row C in neonatal rats. The hypothesis was tested whether the distribution of vasoactive intestinal polypeptide-immunoreactive structures depends on the normal anatomical organization of the specific sensory input. After three months survival the distribution of the vasoactive intestinal polypeptide-immunoreactive structures was morphometrically evaluated. This approach revealed alterations in the contralateral posteromedial barrel subfield, where the disappearance of barrel row C and a substantial increase in size mainly of barrel row D, but also of other rows could be detected. Increase in row D included both barrels and the interspace (septal segments between barrels in one row). As vasoactive intestinal polypeptide immunoreactivity of the barrel field was found previously to be localized in synaptic boutons involved in symmetric synapses, our present findings suggest that (i) the interspace is enriched in inhibitory vasoactive intestinal polypeptide-immunoreactive synapses as opposed to the excitatory thalamocortical input reaching the barrel hollow, (ii) the spatial distribution of the vasoactive intestinal polypeptide system in the barrel cortex is closely associated with the neuronal organization of the sensory input and reacts with a considerable plasticity to lesion-induced changes of the input, and (iii) the compensatory barrel hypertrophy in a row neighbouring the deafferented row involves an increasing number of vasoactive intestinal polypeptide-immunoreactive synapses per barrel.

Thalamocortical synapses

Thalamocortical synapses inform the cerebral neocortex about the external and internal worlds. The thalamus produces myriad thalamocortical pathways that vary in morphological, physiological, pharmacological and functional properties. All these features are of great importance for understanding how information is acquired, integrated, processed, stored and retrieved by the thalamocortical system. This paper reviews the properties of the afferents from thalamus to cortex, and identifies some of the gaps in our knowledge of thalamocortical pathways.

Distribution of GABAergic elements postsynaptic to ventroposteromedial thalamic projections in layer IV of rat barrel cortex

The spatial synaptic pattern formed by boutons, originating in the ventroposteromedial thalamic nucleus, with GABAergic neurons in the rat barrel cortex was mapped. The aim was to shed light on the structural basis by which inhibitory circuits may be activated at the first stage of cortical information processing. The thalamic afferent projection was labelled by anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L), whereas the GABAergic targets in layer IV of the rat barrel cortex was visualized by postembedding GABA immunogold-labelling or by pre-embedding parvalbumin immunocytochemistry. In the first set of experiments, we mapped barrels, contained in single ultrathin sections, by means of a computer-controlled electron microscope stage in their entire layer IV representation. From a total of 1199 asymmetric PHA-L-labelled synapses, only 98 were on GABAergic elements, mainly on dendritic shafts. This corresponded to 8.2% of all synapses counted. These synapses on GABAergic targets were essentially homogeneously distributed without a reliable relationship to barrel subdivisions, i.e., hollow versus wall; or layer IVa versus layer IVb. In the second part of the study, we demonstrated that parvalbumin-containing neurons represent the major GABAergic cell type targetted by thalamic afferents in layer IV of the barrel cortex, since all parvalbumin-positive cells investigated received multiple synaptic contacts (up to eight synapses per neuron) from the ventroposteromedial thalamic nucleus. These results imply that interneurons responsible for perisomatic inhibition (basket and chandelier cells known to contain parvalbumin) are likely to be strongly excited by thalamic afferents, despite the relatively low proportion of thalamic synapses on GABAergic elements compared to spines of principal cells, and participate in the early stages of cortical sensory information processing.

Upregulation of BDNF mRNA expression in the barrel cortex of adult mice after sensory stimulation

Upregulation of brain-derived neurotrophic factor (BDNF) mRNA expression by neuronal activity has been reported in cultured hippocampal cells and in different in vivo excitotoxic paradigms. The aim of the present study was to determine whether sensory stimulation of the whisker-to-barrel pathway alters BDNF mRNA expression in the cortex and, if so, to evaluate the specificity of this effect. To this end, a set of mystacial whiskers was unilaterally stimulated by mechanical deflection, and the expression of BDNF mRNA was analyzed in the barrel cortex by in situ hybridization (ISH) using a 35S-labeled antisense BDNF riboprobe and emulsion autoradiography. A clear-cut and specific upregulation of the BDNF mRNA expression was found at the level of the somatosensory cortex after the increased peripheral stimulation. In the barrel cortex of control mice, BDNF mRNA was present in a few cells in layers II/III and VI, whereas it was almost undetectable in layer IV. After 6 hr of whisker stimulation, increased levels of BDNF mRNA were found in layers II to VI of the contralateral barrel cortex. In layer IV, BDNF upregulation was confined to the barrels corresponding to the stimulated follicles. ISH combined with immunocytochemistry against the three calcium-binding proteins parvalbumin, calretinin, and calbindin-D28K revealed that BDNF mRNA-expressing cells do not belong to the GABAergic cell population of the barrel cortex. The present results support a role for BDNF in activity-dependent modifications of the adult cerebral cortex.

Innervation of VIP-immunoreactive neurons by the ventroposteromedial thalamic nucleus in the barrel cortex of the rat

We investigated the synaptic terminals of fibers originating in the ventroposteromedial thalamic nucleus (VPM) and projecting to the main input layers (IV/III) of the rat posteromedial barrel subfield. It was our aim to determine whether or not the subpopulation of vasoactive intestinal polypeptide (VIP)-immunoreactive neurons in these layers are directly innervated by the sensory thalamus. Anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) and immunohistochemistry for VIP were combined for correlated light and electron microscopic examination. Columns of cortical tissue were well defined by barrel-like patches of PHA-L-labeled fibers and boutons in layers IV and III. Within these columns VIP-immunoreactive perikarya were located mainly in supragranular layers. Marked perikarya were also seen in infragranular layers, but their immunoreactivity was often weaker. Granular layer IV, which is the main terminal field for thalamic fibers, contained fewer VIP neurons than supragranular layers. In the light microscope, however, PHA-L-labeled fibers appeared to contact the somata or proximal dendrites of 60-86% of the layer IV VIP neurons . By contrast, only 18-35% of the VIP neurons in the supragranular layers, which receive a moderately dense projection from the VPM, appeared to be contacted. PHA-L-labeled boutons were seen close to 13-25% of infragranular VIP-positive cells. Electron microscopy showed that thalamic fibers formed at most four asymmetric synapses on a single layer IV, VIP-positive neuron. Although the proportion of VIP-positive neurons with labeled synapses was lower in supragranular layers, most of them shared multiple asymmetric synapses with labeled thalamic fibers. Up to six labeled synapses were seen on individual VIP neurons in layer III. We conclude that subpopulations of VIP-immunoreactive neurons, located in layers IV, III, and II are directly innervated by the VPM. These neurons may be involved in the initial stages of cortical processing of sensory information from the large, mystacial vibrissae. Since VIP is known to be colocalized with the inhibitory transmitter GABA, it is likely that VIP neurons participate in the shaping of the receptive fields in the barrel cortex.

Thalamic afferents of the rat barrel cortex: a light- and electron-microscopic study using Phaseolus vulgaris leucoagglutinin as an anterograde tracer

Anterograde tracers, Phaseolus vulgaris leucoagglutinin (PHA-L) and horseradish peroxidase (HRP), were used to study the thalamocortical afferents of the posteromedial barrel subfield (PMBSF) in rat primary somatosensory cortex (SI) at both light- and electron-microscopic levels. The PMBSF, also known as the barrel cortex, can be subdivided into barrel and interbarrel areas on the basis of cytoarchitectonic characteristics. Restricted injections confined to either the ventroposterior medial (VPM) or the rostral part of the posterior (Pom) nucleus allowed us to study and compare their projection patterns to the barrel cortex. We found that the interbarrel area receives inputs exclusively from the Pom, whereas the barrel area receives inputs from both the Pom and VPM. The laminar distributions of these two projections are largely segregated. After an injection of PHA-L or HRP into the VPM, labeled bouton-like swellings are found in layer VI and in layers IV through I of the barrel area, with the highest concentration in layer IV. On the other hand, after an injection of PHA-L or HRP into the Pom, labeled bouton-like swellings are distributed from upper layer V to layer I of the interbarrel area, as well as in layers V and I of the barrel area. Ultrastructural analysis showed that labeled bouton-like swellings of the VPM and the Pom pathways make synaptic contacts onto cortical neurons, and that these contacts are asymmetrical. Therefore, the VPM and the Pom projections are complementary to each other in the barrel cortex, and together they provide thalamic inputs to most layers of both the barrel and interbarrel areas. The differential patterns of terminations of the VPM and the Pom projections in the barrel cortex suggest that they may be involved in different types of cortical processing. Furthermore, our present findings may provide the anatomical basis for two parallel thalamocortical pathways, which previous physiological studies have indicated are each concerned with particular submodalities of somatic information.

The termination pattern and postsynaptic targets of rubrospinal fibers in the rat spinal cord: a light and electron microscopic study

The spinal course, termination pattern, and postsynaptic targets of the rubrospinal tract, which is known to contribute to the initiation and execution of movements, were studied in the rat at the light and electron microscopic levels by using the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) in combination with calbindin-D28k (CaBP), gamma-aminobutyric acid (GABA), and glycine immunocytochemistry. After injections of PHA-L unilaterally into the red nucleus, labelled fibers and terminals were detected at cervical, thoracic, and lumbar segments of the spinal cord. Most of the descending fibers were located in the dorsolateral funiculus contralateral to the injection site, but axons descending ipsilaterally were also revealed. Rubrospinal axon terminals were predominantly found in laminae V-VI and in the dorsal part of lamina VII at all levels and on both sides of the spinal cord, but stained collaterals were also seen in the ventrolateral aspect of Clark's column and in the ventral regions of lamina VII on both sides. The proportion of axonal varicosities revealed on the ipsilateral side varied at different segments and represented 10-28% of the total number of labelled boutons. Most of the labelled boutons were engaged in synaptic contacts with dendrites. Of the 137 rubrospinal boutons investigated, only 2 were found to establish axosomatic synaptic junctions in the lumbar spinal cord contralateral to the PHA-L injection. With the postembedding immunogold method, 80.8% of dendrites establishing synaptic contacts with rubrospinal terminals did not show immunoreactivity for either GABA or glycine, whereas 19.2% of them were immunoreactive for both amino acids. Rubrospinal axons made multiple contacts with CaBP-immunoreactive neurons in laminae V-VI. Synaptic contacts between rubrospinal terminals and CaBP-immunoreactive dendrites were identified at the electron microscopic level, and all CaBP-containing postsynaptic dendrites investigated were negative for both GABA and glycine. The results suggest that rubrospinal terminals establish synaptic contacts with both excitatory and inhibitory interneurons in the rat spinal cord, and a population of excitatory interneurons receiving monosynaptic rubrospinal input is located in laminae V-VI.

Vibrissectomy induced changes in GAP-43 immunoreactivity in the adult rat barrel cortex

Within the rat primary somatosensory cortex, neurons responding principally to movement of each individual mystacial vibrissa are grouped together in structures termed barrels. Previous studies have examined changes in the area of cortex showing increased 2-deoxyglucose uptake in response to vibrissal stimulation. These studies have shown that chronic removal of all but the central (C3) vibrissa in adult rats induces an enlarged representation of the remaining C3 barrel in the contralateral cortex. This increase is prevented by cortical norepinephrine depletion. The major question raised by such studies is whether such plasticity is due to structural rearrangement or unmasking of otherwise silent synapses. In this study, antibodies to GAP-43, a presynaptic protein whose synthesis is related to neuronal development and regeneration, were used to investigate this issue. In adult rat brain, tangential sections through layer IV of the barrel receptor field normally show moderate levels of GAP-43 immunoreactivity (GAP-IR) in the inter-barrel septa and low levels within the barrels themselves. The present study examined changes in the pattern of GAP-IR from 1 to 8 weeks after vibrissectomy with sparing of C3 as an index of possible physical reorganization of cortical circuits. Quantitative analysis of the cortices of animals with unilateral vibrissectomy with sparing of C3 showed that the area of low GAP-IR within the barrels surrounding C3 was decreased at 1 week (8.4% shrinkage; P less than 0.01) and 8 weeks (12.0% shrinkage; P less than 0.015), relative to the cortex ipsilateral to the surgery. Both bilateral vibrissectomy with sparing of C3 and ibotenic acid lesions of the ventrobasal thalamus produced similar results. Some evidence was also seen that the area of low GAP-IR in the C3 barrel shrank to a similar degree after such manipulations. Cortical norepinephrine depletion had no apparent effect on vibrissectomy-induced GAP-IR changes. These results suggest that removal of vibrissal input to the adult rat barrel cortex produces transynaptic induction of axonal sprouting within the barrel cortex.

Thalamo-cortical processing of vibrissal information in the rat. II. spatiotemporal convergence in the thalamic ventroposterior medial nucleus (VPm) and its relevance to generation of receptive fields of S1 cortical "barrel" neurones

One hundred and twenty-six cells, sampled in the vicinity of the D1 barreloid in the ventroposterior medial nucleus of the thalamus, were tested for magnitude and latency of response to brief deflections (3 ms; 1.14 degrees) of vibrissae in adult rats under controlled conditions of light urethane anaesthesia. Similar results were achieved for D1 and non-D1-dominant cells. D1-dominant cells (N = 76) responded to the centre-receptive field (D1) vibrissa with a mean of 1.08 spikes per stimulus at modal latencies of 3-12 ms (inter-quartile range 4-5 ms) and to surrounding vibrissae with a mean of 0.26 spikes per stimulus at latencies of 3-41 ms (interquartile range 5-8 ms). Surround-receptive fields showed extensive overlap but were reduced and finally eliminated by deepening anaesthesia. A cell-by-cell analysis showed no correlation between latency and response magnitude for responses to surround vibrissae. Response magnitudes to the surround- and centre-receptive field inputs for D1-dominant barrel cells were some 2.5- and 1.7-fold greater, respectively, than for thalamic cells under identical experimental conditions. The latencies to centre- and surround-receptive field inputs for D1-dominant barrel cells were 2.5 and 10-20 ms later than for thalamus, respectively. These data on a mismatch of latencies for surround- and centre-receptive fields in thalamus and cortex support the notion that surround-receptive fields of cortical barrel cells are almost entirely constructed intracortically during light anaesthesia (Armstrong-James et al., '91), although it is argued that surround-receptive fields of thalamic cells conceivably could be relayed in other cortical states or serve a role in plasticity.

Uptake of Phaseolus vulgaris leucoagglutinin (PHA-L) by axons of passage

Iontophoretic injection of Phaseolus vulgaris leucoagglutinin (PHA-L) into the spinal trigeminal tract, the trapezoid body or the inferior cerebellar peduncle produced extensive labelling of axons of passage. Anterograde transport labelled axon terminals more than 13 mm distant and retrograde transport labelled cells more than 3 mm distant. Axons of passage were labelled with both PHA-L and biotinylated PHA-L, and were observed following injection with different micropipette tip sizes, different durations of iontophoresis, and with continuous or pulsed iontophoretic current. The results suggest that while different injection parameters may influence the number of axons that are labelled, the possibility of labelling axons of passage should be considered whenever PHA-L is used.

Basal ganglia and cerebellum receive different somatosensory information in rats

There are two great subcortical circuits that relay sensory information to motor structures in the mammalian brain. One pathway relays via the pontine nuclei and cerebellum, and the other relays by way of the basal ganglia. We studied the cells of origin of these two major pathways from the posteromedial barrel subfield of rats, a distinct region of the somatosensory cortex that contains the sensory representation of the large whiskers. We injected tracer substances into the caudate putamen or the pontine nuclei and charted the location of retrogradely filled cortical cells. In preliminary studies, we used double-labeling techniques to determine whether the cells of origin of these two pathways send axon collaterals to other subcortical targets. Lamina V of the rat posteromedial barrel subfield contains two distinct populations of subcortically projecting neurons, which are organized into distinct sublamina. . Corticopontine cells are located exclusively in sublamina Vb, the deeper of two sublamina revealed by cytochrome oxidase staining. Corticostriate cells are located almost exclusively in the more superficial sublamina Va. Experiments using double-labeling fluorescent tracers demonstrate that about one-quarter of the corticopontine cells send a collateral branch to the superior colliculus. Other studies have shown that cells in Vb are activated at very short latency after vibrissal stimulation; hence, they would seem to be an appropriate relay for the rapid transmission of sensory information to the cerebellum for use in sensory guidance of movement.