Infrabarrels Are Layer 6 Circuit Modules in the Barrel Cortex that Link Long-Range Inputs and Outputs.

The rodent somatosensory cortex includes well-defined examples of cortical columns-the barrel columns-that extend throughout the cortical depth and are defined by discrete clusters of neurons in layer 4 (L4) called barrels. Using the cell-type-specific Ntsr1-Cre mouse line, we found that L6 contains infrabarrels, readily identifiable units that align with the L4 barrels. Corticothalamic (CT) neurons and their local axons cluster within the infrabarrels, whereas corticocortical (CC) neurons are densest between infrabarrels. Optogenetic experiments showed that CC cells received robust input from somatosensory thalamic nuclei, whereas CT cells received much weaker thalamic inputs. We also found that CT neurons are intrinsically less excitable, revealing that both synaptic and intrinsic mechanisms contribute to the low firing rates of CT neurons often reported in vivo. In summary, infrabarrels are discrete cortical circuit modules containing two partially separated excitatory networks that link long-distance thalamic inputs with specific outputs.

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.

Altered excitatory-inhibitory balance within somatosensory cortex is associated with enhanced plasticity and pain sensitivity in a mouse model of multiple sclerosis.

BACKGROUND: Chronic neuropathic pain is a common symptom of multiple sclerosis (MS). MOG35-55-induced experimental autoimmune encephalomyelitis (EAE) has been used as an animal model to investigate the mechanisms of pain in MS. Previous studies have implicated sensitization of spinal nociceptive networks in the pathogenesis of pain in EAE. However, the involvement of supraspinal sites of nociceptive integration, such as the primary somatosensory cortex (S1), has not been defined. We therefore examined functional, structural, and immunological alterations in S1 during the early stages of EAE, when pain behaviors first appear. We also assessed the effects of the antidepressant phenelzine (PLZ) on S1 alterations and nociceptive (mechanical) sensitivity in early EAE. PLZ has been shown to restore central nervous system (CNS) tissue concentrations of GABA and the monoamines (5-HT, NA) in EAE. We hypothesized that PLZ treatment would also normalize nociceptive sensitivity in EAE by restoring the balance of excitation and inhibition (E-I) in the CNS. METHODS: We used in vivo flavoprotein autofluorescence imaging (FAI) to assess neural ensemble responses in S1 to vibrotactile stimulation of the limbs in early EAE. We also used immunohistochemistry (IHC), and Golgi-Cox staining, to examine synaptic changes and neuroinflammation in S1. Mechanical sensitivity was assessed at the clinical onset of EAE with Von Frey hairs. RESULTS: Mice with early EAE exhibited significantly intensified and expanded FAI responses in S1 compared to controls. IHC revealed increased vesicular glutamate transporter (VGLUT1) expression and disrupted parvalbumin+ (PV+) interneuron connectivity in S1 of EAE mice. Furthermore, peri-neuronal nets (PNNs) were significantly reduced in S1. Morphological analysis of excitatory neurons in S1 revealed increased dendritic spine densities. Iba-1+ cortical microglia were significantly elevated early in the disease. Chronic PLZ treatment was found to normalize mechanical thresholds in EAE. PLZ also normalized S1 FAI responses, neuronal morphologies, and cortical microglia numbers and attenuated VGLUT1 reactivity-but did not significantly attenuate the loss of PNNs. CONCLUSIONS: These findings implicate a pro-excitatory shift in the E-I balance of the somatosensory CNS, arising early in the pathogenesis EAE and leading to large-scale functional and structural plasticity in S1. They also suggest a novel antinociceptive effect of PLZ treatment.

NrCAM deletion causes topographic mistargeting of thalamocortical axons to the visual cortex and disrupts visual acuity.

NrCAM is a neural cell adhesion molecule of the L1 family that has been linked to autism spectrum disorders, a disease spectrum in which abnormal thalamocortical connectivity may contribute to visual processing defects. Here we show that NrCAM interaction with neuropilin-2 (Npn-2) is critical for semaphorin 3F (Sema3F)-induced guidance of thalamocortical axon subpopulations at the ventral telencephalon (VTe), an intermediate target for thalamic axon sorting. Genetic deletion of NrCAM or Npn-2 caused contingents of embryonic thalamic axons to misproject caudally in the VTe. The resultant thalamocortical map of NrCAM-null mutants showed striking mistargeting of motor and somatosensory thalamic axon contingents to the primary visual cortex, but retinogeniculate targeting and segregation were normal. NrCAM formed a molecular complex with Npn-2 in brain and neural cells, and was required for Sema3F-induced growth cone collapse in thalamic neuron cultures, consistent with a vital function for NrCAM in Sema3F-induced axon repulsion. NrCAM-null mice displayed reduced responses to visual evoked potentials recorded from layer IV in the binocular zone of primary visual cortex (V1), particularly when evoked from the ipsilateral eye, indicating abnormal visual acuity and ocularity. These results demonstrate that NrCAM is required for normal maturation of cortical visual acuity, and suggest that the aberrant projection of thalamic motor and somatosensory axons to the visual cortex in NrCAM-null mutant mice impairs cortical functions.

Distribution of large terminal inputs from the primary and secondary somatosensory cortices to the dorsal thalamus in the rodent.

The present study was undertaken to determine the precise projection pattern from the primary (S1) and secondary (S2) somatosensory cortices to the posterior nuclear proper (POm) and ventroposterior thalamic nuclei (VP). The POm was previously shown to receive large boutons arising exclusively from layer V of the S1 barrel region. This descending input was proposed to play a key role, namely, as a driver, in shaping the receptive property of POm neurons. To determine whether other body parts and the S2 also contribute such unique inputs to the dorsal thalamus, anterograde neuroanatomical tracers were focally deposited in the S1 and S2 forepaw and whisker regions of rats and C57BL6-Tg (GFPm)/Thy1 transgenic mice. Our major findings were that, 1) irrespective of body representations, both the S1 and the S2 provided corticothalamic large terminals to the POm with comparable morphological characteristics and 2) descending large terminals were also noted in particular subzones within the VP, including boundary and caudal areas. We concluded, based on these findings, that the rodent VP has three partitions: the rostral VP innervated by small corticothalamic terminals, the caudal VP with both corticothalamic small and large terminals, and a surrounding shell region, which also contained large terminals. Furthermore, assuming that the large terminal has a driver's role, we propose that particular subzones in the VP may play a role as a multiple-order thalamic relay so that they can simultaneously coordinate with first- and higher-order relays in the thalamocortical circuitry for processing somatosensory information.

Critical period plasticity is disrupted in the barrel cortex of FMR1 knockout mice.

Alterations in sensory processing constitute prominent symptoms of fragile X syndrome; however, little is known about how disrupted synaptic and circuit development in sensory cortex contributes to these deficits. To investigate how the loss of fragile X mental retardation protein (FMRP) impacts the development of cortical synapses, we examined excitatory thalamocortical synapses in somatosensory cortex during the perinatal critical period in Fmr1 knockout mice. FMRP ablation resulted in dysregulation of glutamatergic signaling maturation. The fraction of silent synapses persisting to later developmental times was increased; there was a temporal delay in the window for synaptic plasticity, while other forms of developmental plasticity were not altered in Fmr1 knockout mice. Our results indicate that FMRP is required for the normal developmental progression of synaptic maturation, and loss of this important RNA binding protein impacts the timing of the critical period for layer IV synaptic plasticity.

Vesicular glutamate transporters define two sets of glutamatergic afferents to the somatosensory thalamus and two thalamocortical projections in the mouse.

The ventral posterior nucleus of the thalamus (VP) receives two major sets of excitatory inputs, one from the ascending somatosensory pathways originating in the dorsal horn, dorsal column nuclei, and trigeminal nuclei, and the other originating from the cerebral cortex. Both systems use glutamate as neurotransmitter, as do the thalamocortical axons relaying somatosensory information from the VP to the primary somatosensory cortex (SI). The synapses formed by these projection systems differ anatomically, physiologically, and in their capacity for short-term synaptic plasticity. Glutamate uptake into synaptic vesicles and its release at central synapses depend on two isoforms of vesicular glutamate transporters, VGluT1 and VGluT2. Despite ample evidence of their complementary distribution, some instances exist of co-localization in the same brain areas or at the same synapses. In the thalamus, the two transcripts coexist in cells of the VP and other nuclei but not in the posterior or intralaminar nuclei. We show that the two isoforms are completely segregated at VP synapses, despite their widespread expression throughout the dorsal and ventral thalamus. We present immunocytochemical, ultrastructural, gene expression, and connectional evidence that VGluT1 in the VP is only found at corticothalamic synapses, whereas VGluT2 is only found at terminals made by axons originating in the spinal cord and brainstem. By contrast, the two VGluT isoforms are co-localized in thalamocortical axon terminals targeting layer IV, but not in those targeting layer I, suggesting the presence of two distinct projection systems related to the core/matrix pattern of organization of thalamocortical connectivity described in other mammals.

Development of structural and functional connectivity in the thalamocortical somatosensory pathway in the wallaby.

Neuronal activity is implicated as a driving force in the development of sensory systems. In order for it to play a developmental role, however, the pathways involved must be capable of transmitting this activity. The relationship between afferent arrival, synapse formation and the onset of chemical neurotransmission has been examined using the advantageous model of a marsupial mammal, the wallaby (Macropus eugenii), to determine at what stage activity has the capacity to influence cortical development. It is known that thalamocortical afferents arrive in the somatosensory cortex on postnatal day (P)15 and that their growth cones reach to the base of the compact cell zone of the cortical plate. However, electronmicroscopy showed that thalamocortical synapses were absent at this stage. Glutamatergic responses were recorded in the cortex following stimulation of the thalamus in slices at this time but only in magnesium-free conditions. The responses were mediated entirely by N-methyl-d-aspartate (NMDA) receptors. From P28, responses could be recorded in normal magnesium and comprised a dominant NMDA-mediated component and a non-NMDA mediated component. At this time thalamocortical synapses were first identified and they were in the cortical plate. By P63 the non-NMDA-mediated component had increased relative to the NMDA-mediated component, and by P70 layer IV began to emerge and contained thalamocortical synapses. By P76 a fast non-NMDA-mediated peak dominated the response. This coincides with the appearance of cortical whisker-related patches and the onset in vivo of responses to peripheral stimulation of the whiskers.

Functional thalamocortical synapse reorganization from subplate to layer IV during postnatal development in the reeler-like mutant rat (shaking rat Kawasaki).

Transient synapse formation between thalamic axons and subplate neurons is thought to be important in thalamocortical targeting. Shaking rat Kawasaki (SRK), having reversed cortical layering similarly observed in reeler mouse, provides an interesting model system to test this idea. The spatial and temporal pattern of excitation was investigated using optical recording with voltage-sensitive dyes in thalamocortical slice preparations from SRK. At postnatal day 0 (P0), a strong optical response was elicited within the superplate of the SRK in the cell layer corresponding to subplate in wild-type (WT) rats. By P3, this response rapidly descended into deep cortical layers comprised of layer IV cells, as identified with 5-bromo-2'-deoxyuridine birthdating at embryonic day 17. During the first 3 postnatal days, both the subplate and cortical plate responses were present, but by P7, the subplate response was abolished. Tracing individual axons in SRK revealed that at P0-P3, a large number of thalamocortical axons reach the superplate, and by P7-P10, the ascending axons develop side branches into the lower or middle cortical layers. Synaptic currents were also demonstrated in WT subplate cells and in SRK superficial cortical cells using whole-cell recording. These currents were elicited monosynaptically, because partial AMPA current blockade did not modify the latencies. These results suggest that the general developmental pattern of synapse formation between thalamic axons and subplate (superplate) neurons in WT and SRK is very similar, and individual thalamic arbors in cortex are considerably remodeled during early postnatal development to find layer IV equivalent neurons.

Intracellularly labeled pyramidal neurons in the cortical areas projecting to the spinal cord. II. Intra- and juxta-columnar projection of pyramidal neurons to corticospinal neurons.

Intra- or juxta-columnar connections of pyramidal neurons to corticospinal neurons in rat motorsensory cortices were examined with brain slices by combining intracellular staining with Golgi-like retrograde labeling of corticospinal neurons. Of 108 intracellularly labeled pyramidal neurons, 27 neurons were selected for morphological analysis by successful staining of their axonal arborizations and sufficient retrograde labeling of corticospinal neurons. Many varicosities of local axon collaterals of each pyramidal neuron were closely apposed to the dendrites of corticospinal neurons, suggesting the convergent projections of layer II-VI pyramidal neurons to corticospinal neurons. Particularly, the varicosities of a layer IV star-pyramidal neuron made two- to three-fold more appositions to the dendrites of corticospinal neurons than those of a pyramidal neuron in the other layers. Fifteen appositions were examined electron-microscopically and 60% of them made asymmetric axospinous synapses. The present results together with those of the preceding report suggest that thalamic inputs are conveyed to corticospinal neurons preferentially via layer IV star-pyramidal neurons with phasic response properties, and thereby might contribute to the initiation or switching of movement. In contrast, inputs with tonic response properties from the other layers seem to be integrated in corticospinal neurons, and might be useful in maintaining the activity of corticospinal neurons.

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.

Effect of activity blockade on changes in vibrissae-related patterns in the rat's primary somatosensory cortex induced by serotonin depletion.

Depletion of cortical serotonin (5-HT) during development results in a decrease in the size of the patches of thalamocortical afferents representing the mystacial vibrissae in lamina IV of the primary somatosensory cortex (SI). We previously suggested that this change may be due to a reduction in 5-HT-induced suppression of thalamocortical activity in these animals. The present experiments directly tested the role that modulation of activity may play in the morphologic changes observed after reducing cortical 5-HT concentrations. Serotonin was depleted from the cortex by systemic administration of 5,7-dihydroxytryptamine (5,7-DHT, 100 mg/kg) on the day of birth in animals that also had either tetrodotoxin (TTX)-impregnated or control implants placed unilaterally over the developing SI on this day. Other rat pups were treated with TTX-impregnated or control implants alone. Administration of 5,7-DHT reduced cortical serotonin levels and this effect was not significantly modified by the presence of either control or TTX-impregnated cortical implants. Administration of 5,7-DHT reduced the cross-sectional area of the cortical patches, demonstrated by acetylcholinesterase, corresponding to the vibrissae by 19.9% (P < 0.05). A similar reduction was observed in the animals treated with both 5,7-DHT and TTX-impregnated implants. Treatment with TTX-impregnated implants alone resulted in a 3.1% increase in patch size (P > 0.05). None of the treatments significantly altered the overall area of the part of SI devoted to the representation of the long mystacial vibrissae. These results suggest that the effects of 5-HT depletion on the size of the cortical patches representing the long vibrissae are independent of activity that can be blocked by administration of TTX.

Long-term depression at thalamocortical synapses in developing rat somatosensory cortex.

Sensory experience during an early critical period guides the development of thalamocortical circuits in many cortical areas. This process has been hypothesized to involve long-term potentiation (LTP) and long-term depression (LTD) at thalamocortical synapses. Here, we show that thalamocortical synapses in rat barrel cortex can express LTD, and that LTD is most readily induced during a developmental period that is similar to the critical period for thalamocortical plasticity in vivo. Thalamocortical LTD is homosynaptic and dependent on activation of N-methyl-D-aspartate (NMDA) receptors. The age-related decline of LTD is not due to changes in inhibition nor to changes in NMDA receptor voltage dependence. Minimal stimulation experiments indicate that, unlike thalamocortical LTP, thalamocortical LTD is not associated with a significant change in failure rate. The existence of LTD and its developmental time course suggest that LTD, like LTP, may contribute to the refinement of thalamocortical inputs in vivo.

Individual axon morphology and thalamocortical topography in developing rat somatosensory cortex.

The morphology of individual thalamocortical axons in developing rat primary somatosensory cortex was studied using lipophilic tracers. Anterograde labeling with lipophilic dyes demonstrated a topographical organization of thalamocortical projections exiting the thalamus as early as embryonic day (E) 16; retrograde labeling studies demonstrated topography of these projections as they reached the cortex as early as E18. At E17, axons course tangentially within the intermediate zone and turn or branch near the deepest layer of cortex (layer VIb), suggesting the presence of guidance cues in this region. Axons appear to grow and branch progressively within layers VIb and VIa during the following days; axons in the intermediate zone may give rise to radially directed branches. Individual axons appear to grow steadily and progressively into the cortex, with the leading front of axons at the transition zone between the cortical plate (CP) and the differentiating cortical layers. At birth (P0), thalamocortical axons extend radially through layers VIa and V and emit branches within these layers; some axons reach the CP. By P1, layer IV has begun to differentiate and axons begin to form a few simple branches in the vicinity of the layer IV cells. Over the ensuing week, axons generate more branches within layer IV, but the tangential extent of individual axon arbors does not exceed the width of a barrel. By P7, individual axons overlap within barrel clusters, and individual axons span the width of a cluster. These observations indicate that thalamic afferents develop by progressive growth of arbors that remain spatially restricted, rather than by overbranching and retracting arbors.

Extent of intracortical arborization of thalamocortical axons as a determinant of representational plasticity in monkey somatic sensory cortex.

The extent of intracortical arborization of individual thalamocortical axons in area 3b of the somatic sensory cortex and the degree of overlap in the cortical projections of relay cells in the ventral posterior nucleus of the thalamus were examined in macaque monkeys. Paired intracortical deposits of Fast blue (FB) and Diamidino yellow (DY) separated by 100-1500 microns were made by inserting crystals of dye into the tracks of tungsten microelectrodes used to record receptive field data on area 3b cells. Each injection gave retrograde labeling of one or more clusters of cells extending in elongated anteroposterior arrays through the ventral posterior medial (VPM) or ventral posterior lateral (VPL) nucleus. Double-labeled cells were only found when the distance between the centers of the dye deposits was less than 600 microns. With interdeposit distances greater than 600 microns, most clusters of retrogradely labeled cells had a majority of cells labeled by FB or DY. However, even with interdeposit distances of 1-1.5 mm the labeled clusters also contained significant numbers of cells labeled with the other dye. These results and an accompanying regression analysis indicate that the extent of intracortical arborization of single thalamocortical axons in area 3b is no greater than 600 microns. However, adjoining cells in the same part of the thalamic body representation can project to cortical targets as discrepant as 1.5 mm. It is proposed that the fine grain of the cortical representation depends upon inputs from the majority population of each thalamic cell cluster.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of four types of synapse on physiologically identified relay neurons in the ventral posterior thalamic nucleus of the cat.

This study was aimed at providing quantitative data on the thalamic circuitry that underlies the central processing of somatosensory information. Four physiologically identified thalamocortical relay neurons in the ventral posterior lateral nucleus (VPL) of the cat thalamus were injected with horseradish peroxidase and subjected to quantitative electron microscopy after pre- or postembedding immunostaining for gamma-aminobutyric acid to reveal synaptic terminals of thalamic inhibitory neurons. The four cells all had rapidly adapting responses to light mechanical stimuli applied to their receptive fields, which were situated on hairy or glabrous skin or related to a joint. Their dendritic architecture was typical of cells previously described as type I relay cells in VPL, and they lacked dendritic appendages. Terminals ending in synapses on the injected cells were categorized as RL (ascending afferent), F (inhibitory), PSD (presynaptic dendrite), and RS (mainly corticothalamic) types and were quantified in reconstructions of serial thin sections. RL and F terminals formed the majority of the synapses on proximal dendrites (approximately 50% each). The number of synapses formed by RL terminals declined on intermediate dendrites, but those formed by F terminals remained relatively high, declining to moderate levels (20-30%) on distal dendrites. RS terminals formed moderate numbers of the synapses on intermediate dendrites and the majority (> 60%) of the synapses on distal dendrites. Synapses formed by PSDs were concentrated on intermediate dendrites and were few in number (approximately 6%). They formed synaptic triads with F terminals and rarely with RL terminals. On somata, only a few synapses were found, all made by F terminals. The total number of synapses per cell was calculated to be 5,584-8,797, with a density of 0.6-0.9 per micrometer of dendritic length. Of the total, RL terminals constituted approximately 15%, F terminals approximately 35%, PSD terminals approximately 5%, and RS terminals approximately 50%. These results provide the first quantitative assessment of the synaptic architecture of thalamic somatic sensory relay neurons and show the basic organizational pattern exhibited by representatives of the physiological type of relay neurons most commonly encountered in the VPL nucleus.

The axonal arborization of single thalamic reticular neurons in the somatosensory thalamus of the rat.

This study describes the axonal projections of single neurons of the thalamic reticular complex within the somatosensory thalamic nuclei in rats. Experiments were performed under urethane anaesthesia and reticular cells were labelled by extracellular microiontophoretic applications of biocytin. The axonal arborization of 25 thalamic reticular cells projecting to the ventrobasal (VB) nucleus and/or to the posterior thalamic (Po) complex were reconstructed from serial horizontal sections. Reticular cells labelled with biocytin display somatodendritic features similar to those reported previously. Their cell body is fusiform and their dendrites bear few spines and show a high degree of streaming along the horizontal curved axis of the nucleus. In most cells, axon-like beaded processes stem out from dendrites but, contrary to previous descriptions, no intrareticular axonal collateral was observed. The axonal arborization of most thalamic reticular cells is confined within the limits of a single thalamic nucleus; only two neurons were seen projecting to both the VB and the Po nuclei. In VB, termination fields form short rods (diameter approximately 150 microns, length approximately 200-300 microns) densely packed with grape-like boutons and varicosities; termination fields in Pro are larger, much less dense, and they are contained within a horizontal slab of tissue (thickness approximately 200 microns, mediolateral width approximately 400 microns, rostrocaudal length approximately 1 mm. By charting the position of all labelled cells within the thickness of the thalamic reticular complex, a strip-like arrangement was revealed. Cells projecting to Po occupy the innermost portion of the nucleus whereas those projecting to the ventral-posteromedial and ventral-posterolateral nuclei are located respectively in the middle and in the outer tiers of the nucleus. This strip-like reciprocity was confirmed by separate biocytin injections performed in VB and in Po. These results show that inhibition of reticular origin is distributed within the rat dorsal thalamus in a highly specific manner, most likely according to a principle of reciprocity within the somatotopic representation of the body.

Early development of the somatotopic map and barrel patterning in rat somatosensory cortex.

Several lines of evidence implicate a crucial role for thalamic afferents from the ventroposterior nucleus (VP) in the development of barrels and their characteristic pattern in the primary somatosensory cortex (S1) of rodents. We sought to determine the stage in development when VP thalamocortical afferents are first distributed in a periphery-related pattern and the sequence of events that culminate in a mature pattern. Using acetylcholinesterase (AChE) histochemistry, an early marker for VP thalamocortical afferents, and the anterograde axon tracer DiI, we show that VP thalamocortical afferents become distributed into a periphery-related pattern earlier than was previously reported, including their parcellation into a barrel-related pattern that mirrors the distribution of sensory hairs on the face. The earliest periphery-related patterning observed is transiently present in the deep cortical layers prior to the emergence of layer 4, the layer in which barrels later develop. AChE histochemistry reveals a clear sequence of maturation of the barrel pattern in the distribution of VP afferents: An initially patternless distribution of AChE-reactive afferents is followed by their distribution in a nascent trigeminal representation, from which rows subsequently emerge; barrel-related clusters of afferents then emerge from the rows. This process begins before birth, and the transition from row-related to barrel-related distributions of VP afferents is evident during the first postnatal day (P0). This demonstration of a periphery-related pattern in developing rat S1 precedes by about 2 days that revealed by any other marker reported to delineate barrels. These findings confirm that VP thalamocortical afferents are the first barrel component to have a periphery-related pattern and support the hypothesis that thalamocortical afferents provide to immature S1 the patterning information that initiates the formation of barrels and their characteristic array. Furthermore because these findings show an earlier onset for barrel formation than was previously realized, they necessitate a reevaluation of conclusions drawn from experiments examining developmental plasticity in barrel patterning.

Distribution and synaptic localization of immunocytochemically identified NMDA receptor subunit proteins in sensory-motor and visual cortices of monkey and human.

NMDA receptors are composed of multiple receptor subunit proteins, of which NMDAR1 appears to be a critical component for normal receptor function (Nakanishi, 1992). In this study, quantitative immunocytochemical methods were used at the light and electron microscopic levels to localize NMDAR1 subunits in the primary motor (M1) and somatic sensory (S1) cortex of monkeys, and in the primary visual cortices (V1) of monkey and human. Three principal features of NMDAR1 subunit organization were examined in detail in the monkey cortex: (1) the laminar and cellular distribution patterns, relying in part on double-labeling paradigms with the calcium-binding proteins parvalbumin (PV) and calretinin (CR) as markers for discrete subpopulations of GABAergic interneurons; (2) the codistribution of NMDAR1 subunits with non-NMDA ionotropic receptor subunits; (3) a quantitative assessment of the percentages of asymmetrical synapses in layers II/III, IV, and V/VI that were NMDAR1 immunoreactive. In monkey M1, S1, and V1, NMDAR 1 immunoreactivity was present in all layers, localized primarily to large numbers of pyramidal cell somata and proximal apical dendrites, to presumptive spiny stellate cells in layer IV of V1, and to the vast majority (approximately 80-90%) of PV-immunoreactive cells. By contrast, NMDAR1 immunoreactivity was present in only a very small percentage of the CR-immunoreactive cells (approximately 6-9%). Colocalization with non-NMDA receptor subunits showed that all cells (100%) that contained GluR2/3 subunits were also NMDAR1 immunoreactive. In addition, the complete codistribution of GluR5/6/7 subunits with GluR2/3 subunits suggests, indirectly, that all GluR5/6/7-immunoreactive cells are also NMDAR1 immunoreactive. The laminar and cellular distribution patterns of immunostaining in human V1 were very similar to those in monkey V1. Electron microscopy of monkey sections confirmed an extensive dendritic and synaptic localization of NMDAR1 subunits. Labeling of synapses was present on asymmetrical postsynaptic densities associated with both dendritic shafts and spines. In supragranular layers of V1, a greater percentage of asymmetrical synapses were NMDAR1 immunopositive (44%) in comparison to layer IVC beta (34%) or deep layers (19%). In contrast, in area 3b of S1, the percentage of labeled synapses was greatest in layer IV (45%) in comparison to superficial (26%) and deep (37%) layers, while in M1, the percentages of labeled synapses were similar between superficial (46%) and deep (40%) layers. Taken together, these data indicate that NMDAR1-immunoreactive cells in neocortex represent a morphologically, functionally, and neurochemically heterogeneous population.(ABSTRACT TRUNCATED AT 400 WORDS)