Connectivity of neuronal populations within and between areas of primate somatosensory cortex.

Functions of the cerebral cortex emerge via interactions of horizontally distributed neuronal populations within and across areas. However, the connectional underpinning of these interactions is not well understood. The present study explores the circuitry of column-size cortical domains within the hierarchically organized somatosensory cortical areas 3b and 1 using tract tracing and optical intrinsic signal imaging (OIS). The anatomical findings reveal that feedforward connections exhibit high topographic specificity, while intrinsic and feedback connections have a more widespread distribution. Both intrinsic and inter-areal connections are topographically oriented across the finger representations. Compared to area 3b, the low clustering of connections and small cortical magnification factor supports that the circuitry of area 1 scaffolds a sparse functional representation that integrates peripheral information from a large area that is fed back to area 3b. Fast information exchange between areas is ensured by thick axons forming a topographically organized, reciprocal pathway. Moreover, the highest density of projecting neurons and groups of axon arborization patches corresponds well with the size and locations of the functional population response reported by OIS. The findings establish connectional motifs at the mesoscopic level that underpin the functional organization of the cerebral cortex.

What makes the prefrontal cortex so appealing in the era of brain imaging? a network analytical perspective.

It is thought that the prefrontal cortex (PFC) subserves cognitive control processes by coordinating the flow of information in the cerebral cortex. In the network of cortical areas the central position of the PFC makes difficult to dissociate processing and the cognitive function mapped to this region, especially when using whole brain imaging techniques, which can detect frequently activated regions. Accordingly, the present study showed particularly high rate of increase of published studies citing the PFC and imaging as compared to other fields of the neurosciences on the PubMed. Network measures used to characterize the role of the areas in signal flow indicated specialization of the different regions of the PFC in cortical processing. Notably, areas of the dorsolateral PFC and the anterior cingulate cortex, which received the highest number of citations, were identified as global convergence points in the network. These prefrontal regions also had central position in the dominant cluster consisted exclusively by the associational areas of the cortex. We also present findings relevant to models suggesting that control processes of the PFC are depended on serial processing, which results in bottleneck effects. The findings suggest that PFC is best understood via its role in cortical information processing.

Perisynaptic aggrecan-based extracellular matrix coats in the human lateral geniculate body devoid of perineuronal nets.

The extracellular matrix surrounds different neuronal compartments in the mature nervous system. In a variety of vertebrates, most brain regions are loaded with a distinct type of extracellular matrix around the somatodendritic part of neurons, termed perineuronal nets. The present study reports that chondrotin sulfate proteoglycan-based matrix is structured differently in the human lateral geniculate body. Using various chondrotin sulfate proteoglycan-based extracellular matrix antibodies, we show that perisomatic matrix labeling is rather weak or absent, whereas dendrites are contacted by axonal coats appearing as small, oval structures. Confocal laser scanning microscopy and electron microscopy demonstrated that these typical structures are associated with synaptic loci on dendrites. Using multiple labelings, we show that different chondrotin sulfate proteoglycan components of the extracellular matrix do not associate exclusively with neuronal structures but possibly associate with glial structures as well. Finally, we confirm and extend previous findings in primates that intensity differences of various extracellular matrix markers between magno- and parvocellular layers reflect functional segregation between these layers in the human lateral geniculate body.

Layer-specific activity of tissue non-specific alkaline phosphatase in the human neocortex.

The ectoenzyme tissue non-specific alkaline phosphatase (TNAP) is mostly known for its role in bone mineralization. However, in the severe form of hypophosphatasia, TNAP deficiency also results in epileptic seizures, suggesting a role of this enzyme in brain functions. Accordingly, TNAP activity was shown in the neuropil of the cerebral cortex in diverse mammalian species. However in spite of its clinical significance, the neuronal localization of TNAP has not been investigated in the human brain. By using enzyme histochemistry, we found an unprecedented pattern of TNAP activity appearing as an uninterrupted layer across diverse occipital-, frontal- and temporal lobe areas of the human cerebral cortex. This marked TNAP-active band was localized infragranulary in layer 5 as defined by quantitative comparisons on parallel sections stained by various techniques to reveal the laminar pattern. On the contrary, TNAP activity was localized in layer 4 of the primary visual and somatosensory cortices, which is consistent with earlier observations on other species. This result suggests that the expression of TNAP in the thalamo-recipient granular layer is an evolutionary conserved feature of the sensory cortex. The observations of the present study also suggest that diverse neurocognitive functions share a common cerebral cortical mechanism depending on TNAP activity in layer 5. In summary, the present data point on the distinctive role of layer 5 in cortical computation and neurological disorders caused by TNAP dysfunctions in the human brain.

Morphometric characterization of synapses in the primate prefrontal cortex formed by afferents from the mediodorsal thalamic nucleus.

The main thalamic afferentation of the prefrontal cortex (PFC) originates in the mediodorsal nucleus (MD). Although it is suggested that this pathway is affected in schizophrenia, there is a lack of functional and structural data regarding its synaptic organization. The scope of this study was to characterize the ultrastructural features of thalamocortical synapses formed by afferents from the MD by applying anterograde tract tracing, immunohistochemical detection of parvalbumin (PV, a probable marker of thalamocortical endings), and quantitative electron microscopic techniques to the PFC of the macaque monkey. Our findings indicate that anterogradely-labeled and PV-immunoreactive boutons exhibit similar ultrastructural properties, characterized by their larger size, higher incidence of release sites and a higher occurrence of mitochondria when compared to non-labeled, excitatory-like endings in the middle layers of the PFC. Although most of the contacts were made on spines in both cases, PV-immunopositive axon terminals apparently targeted dendritic shafts at about twice the frequency found for anterogradely-labeled afferents from the MD (20.5% and 9.5%, respectively). This result suggests diversity among thalamocortical and/or PV-immunoreactive axon terminals of the PFC. In accordance with studies in other cortical areas, our findings suggest that corollary discharge through the mediodorsal thalamocortical projection is also adapted to synaptic transmission with high efficacy and probably exhibits marked short-term temporal dynamics in the PFC.

Cross-modal plasticity of the corticothalamic circuits in rats enucleated on the first postnatal day.

Reorganization of the reciprocal corticothalamic connections was studied as a possible anatomical substrate of the cross-modal compensation of the missing visual input of the visual cortex by somatosensory-evoked activities in neonatally enucleated rats. The use of quantitative retrograde tract-tracing techniques revealed that the contribution of the lateral posterior thalamic nucleus (LP) is significantly increased following enucleation, while that of the dorsolateral geniculate and the lateral dorsal nuclei is decreased in the thalamocortical afferentation of a region in visual cortical area 17. In contrast with the control rats, a dense terminal arborization of afferents was labelled in the LP after the injection of anterograde tracer into the barrel cortex of the enucleated rats. The injection of anterograde tracer into the visual cortex also demonstrated a massive afferentation into the LP of the enucleated rats. Visual and somatosensory corticothalamic afferents exhibited similar ultrastructural features in the LP after enucleation, but their synaptic organizations differed as regards the diameter of the postsynaptic dendrites. Taken together with the previous observations, these results suggest a central role for the LP in the transmission of the somatosensory-evoked activities to the visual cortex after early blindness.

Contralateral cortical projection to the mediodorsal thalamic nucleus: origin and synaptic organization in the rat.

The origin of the corticothalamic projections to the contralateral mediodorsal nucleus, the collateralization of cortical fibers and their synaptic organization in the ipsi- and contralateral mediodorsal nuclei were investigated in adult rats with double retrograde fluorescent and anterograde tracing. After tracer injections in the mediodorsal nuclei on either side, neurons were retrogradely labeled in all the areas of the contralateral prefrontal cortex in which ipsilateral labeling was also observed. Contralateral corticothalamic cells accounted for 15% of the labeled neurons in the orbital and agranular insular areas, while their proportion was lower (3%) in the anterior cingulate cortex. Up to 70% of the contralateral cortical neurons were double labeled by bilateral injections in the mediodorsal nuclei. At the electron microscopic level, unilateral injections of biotinylated dextran-amine in the orbitofrontal cortex resulted in anterograde labeling of small terminals and a few large boutons in the ipsilateral mediodorsal nucleus, while only small boutons were identified contralaterally. The diameter of postsynaptic dendritic profiles contacted by labeled small cortical endings was significantly larger in the ipsilateral mediodorsal nucleus than contralaterally. These findings demonstrate that dense contralateral cortical projections to the mediodorsal nucleus derive from the orbital and agranular insular areas, and that crossed corticothalamic afferents are mostly formed by collaterals of the ipsilateral connections. Our observations also point out the heterogeneity of corticothalamic boutons in the rat mediodorsal nucleus and morphological differences in the synaptic organization of prefrontal fibers innervating the two sides, indicating that ipsilateral cortical afferents may be more proximally distributed than crossed cortical fibers on dendrites of mediodorsal neurons.

Light and electron microscopic demonstration of mGluR5 metabotropic glutamate receptor immunoreactive neuronal elements in the rat cerebellar cortex.

The cellular and subcellular localization of the mGluR5 metabotropic glutamate receptor subtype was studied in the rat cerebellar cortex, by using the preembedding immunoperoxidase and immunogold techniques. Light microscopic observations revealed an abundant, intense labeling of neurons in the granular layer as well as in the molecular layer. Lugaro and Golgi cells exhibited an intense mGluR5 immunoreactivity, while only a fraction of the neurons in the molecular layer were found to be mGluR5 immunopositive. In addition to a dense plexus of immunoreactive dendrites in the molecular layer of the cerebellar cortex, the mGluR5 immunopositive Golgi cell dendrites resembling axons at the light microscopic level were also labeled in the granular layer. At the ultrastructural level, mGluR5 immunoreactivity was present in neuronal elements postsynaptic to axon terminals of different morphology. By using a pre-embedding immunogold method, it was found that mGluR5 immunoreactivity is accumulated at the plasma membranes extrasynaptically as well as at the periphery of the postsynaptic specializations, mainly of the parallel fiber synaptic contacts. These findings provide morphological evidence that mGluR5 is expressed by a population of neurons in the cerebellar cortex and can synaptically be activated via the parallel fiber system.

Immunocytochemical visualization of the mGluR1a metabotropic glutamate receptor at synapses of corticothalamic terminals originating from area 17 of the rat.

Pre-embedding immunogold histochemistry was combined with Phaseolus vulgaris leucoagglutinin anterograde tract tracing in order to analyse the relationship between the subcellular localization of the GluR1a metabotropic glutamate receptors and the distribution of corticothalamic synapses in the dorsal lateral geniculate nucleus (dLGN) and the lateral posterior nucleus (LP) of the rat. The injection of the tracer into area 17 labelled two types of corticothalamic terminals: (i) the small boutons constituting the majority of the labelled fibres which form asymmetrical synapses both in the dLGN and LP; and (ii) the giant terminals typically participating in glomerulus-like synaptic arrangements and found exclusively in the lateral posterior nucleus. The small corticothalamic terminals often established synapses with mGluR1a-immunopositive dendrites, with immunometal particles concentrated at the periphery of their postsynaptic membranes. In contrast, the synapses formed by giant boutons in the lateral posterior nucleus were always mGluR1a-immunonegative. We conclude that the corticothalamic fibres forming the small synaptic terminals are the most likely candidates for the postulated mGluR-mediated modulation of visual information flow by corticothalamic feedback mechanisms.

Synaptic reorganisation of the mediodorsal thalamic nucleus in adult rat following chronic prefrontal cortical lesions.

Chronic changes in the thalamic mediodorsal nucleus (MD) after unilateral lesions of the prefrontal cortex were studied with the aid of quantitative light, and electron microscopic immunohistochemistry. Three months after the lesions, although the size of MD ipsilateral to the lesion did not change considerably, the neuronal density was significantly reduced. Conversely, as demonstrated by quantitative electron microscopy, the density of GABA immunostained axon terminals significantly increased in the lesioned side. It is suggested, that as MD does not contain GABA cells, the reactive hyperinnervation of the MD by GABA-containing axons is of extrinsic origin. This finding is also the morphological evidence of the potential of GABA-ergic nerve cells for reactive (induced) axonal sprouting.

Cellular and subcellular localization of the mGluR5a metabotropic glutamate receptor in rat spinal cord.

The cellular and subcellular distribution of the mGluR5a metabotropic glutamate receptor was studied in the spinal cord of the rat using an antibody raised against a mGluR5a-specific carboxy-terminal peptide. Strong mGluR5a-immunoreactivity (mGluR5a-ir) was found in the laminae I-II of the dorsal horn, which gradually decreased towards the deeper layers. At the electron microscopical level, mGluR5a-ir was present exclusively in neuronal somata and dendrites. Immunometal labelling revealed that mGluR5a-ir is concentrated at the periphery of postsynaptic densities of asymmetrical synapses or localized extrasynaptically at dendritic and somatic membranes. The mGluR5a-immunoreactive dendritic profiles were often targeted by synaptic boutons with the morphological characteristics of C-fibre terminals. These observations provide evidence for mGluR5a being involved in the nociceptive transmission at the dorsal horn.

A Combined golgi and postembedding GABA and glutamate electron microscopic study of the nucleus dorsomedialis thalami of the rat.

The synaptic organisation of neurons in the nucleus medialis dorsalis (MD) was investigate by combined Golgi, and post-embedding gamma-aminobutyric acid (GABA) and/or glutamate (GLU) immunogold methods. The morphological features of the impregnated neurons in the MD (detailed by Kuroda et al. 1992a) were similar to thalamocortical relay cells present in other thalamic nuclei. GABAergic cell bodies could not be found. Distal dendritic branches of gold toned neurons established synaptic contacts with small and medium size glutamate as well as GABA-immunoreactive axonal endings. Large glutamate and GABA-positive axon terminals formed synaptic contacts with proximal dendrites. GABAergic axon terminals exhibited symmetric synaptic contacts and contained pleomorphic vesicles. Glutamate immunoreactive terminals formed asymmetric synapses and contained spheroid vesicles, although, some large glutamate immunoreactive endings contained pleomorphic vesicles. Synaptic glomerulus-like complexes could also be observed with the participation of glutamate-, and GABA-positive large axon terminals synapsing with Golgi impregnated proximal dendrites.