Quantitative immunohistochemical analysis of VIP-neurons in the rat visual cortex

Synaptic configuration and reconfiguration in the neocortex are spatiotemporally selective

Brain computation relies on the neural networks. Neurons extend the neurites such as dendrites and axons, and the contacts of these neurites that form chemical synapses are the biological basis of signal transmissions in the central nervous system. Individual neuronal outputs can influence the other neurons within the range of the axonal spread, while the activities of single neurons can be affected by the afferents in their somatodendritic fields. The morphological profile, therefore, binds the functional role each neuron can play. In addition, synaptic connectivity among neurons displays preference based on the characteristics of presynaptic and postsynaptic neurons. Here, the author reviews the "spatial" and "temporal" connection selectivity in the neocortex. The histological description of the neocortical circuitry depends primarily on the classification of cell types, and the development of gene engineering techniques allows the cell type-specific visualization of dendrites and axons as well as somata. Using genetic labeling of particular cell populations combined with immunohistochemistry and imaging at a subcellular spatial resolution, we revealed the "spatial selectivity" of cortical wirings in which synapses are non-uniformly distributed on the subcellular somatodendritic domains in a presynaptic cell type-specific manner. In addition, cortical synaptic dynamics in learning exhibit presynaptic cell type-dependent "temporal selectivity": corticocortical synapses appear only transiently during the learning phase, while learning-induced new thalamocortical synapses persist, indicating that distinct circuits may supervise learning-specific ephemeral synapse and memory-specific immortal synapse formation. The selectivity of spatial configuration and temporal reconfiguration in the neural circuitry may govern diverse functions in the neocortex.

Vasoactive Intestinal Polypeptide-Immunoreactive Interneurons within Circuits of the Mouse Basolateral Amygdala

In cortical structures, principal cell activity is tightly regulated by different GABAergic interneurons (INs). Among these INs are vasoactive intestinal polypeptide-expressing (VIP+) INs, which innervate preferentially other INs, providing a structural basis for temporal disinhibition of principal cells. However, relatively little is known about VIP+ INs in the amygdaloid basolateral complex (BLA). In this study, we report that VIP+ INs have a variable density in the distinct subdivisions of the mouse BLA. Based on different anatomical, neurochemical, and electrophysiological criteria, VIP+ INs could be identified as IN-selective INs (IS-INs) and basket cells expressing CB1 cannabinoid receptors. Whole-cell recordings of VIP+ IS-INs revealed three different spiking patterns, none of which was associated with the expression of calretinin. Genetic targeting combined with optogenetics and in vitro recordings enabled us to identify several types of BLA INs innervated by VIP+ INs, including other IS-INs, basket and neurogliaform cells. Moreover, light stimulation of VIP+ basket cell axon terminals, characterized by CB1 sensitivity, evoked IPSPs in ∼20% of principal neurons. Finally, we show that VIP+ INs receive a dense innervation from both GABAergic inputs (although only 10% from other VIP+ INs) and distinct glutamatergic inputs, identified by their expression of different vesicular glutamate transporters.

In conclusion, our study provides a wide-range analysis of single-cell properties of VIP+ INs in the mouse BLA and of their intrinsic and extrinsic connectivity. Our results reinforce the evidence that VIP+ INs are structurally and functionally heterogeneous and that this heterogeneity could mediate different roles in amygdala-dependent functions.

SIGNIFICANCE STATEMENT We provide the first comprehensive analysis of the distribution of vasoactive intestinal polypeptide-expressing (VIP+) interneurons (INs) across the entire mouse amygdaloid basolateral complex (BLA), as well as of their morphological and physiological properties. VIP+ INs in the neocortex preferentially target other INs to form a disinhibitory network that facilitates principal cell firing. Our study is the first to demonstrate the presence of such a disinhibitory circuitry in the BLA. We observed structural and functional heterogeneity of these INs and characterized their input/output connectivity. We also identified several types of BLA INs that, when inhibited, may provide a temporal window for principal cell firing and facilitate associative plasticity, e.g., in fear learning.

Differential Inputs to the Perisomatic and Distal-Dendritic Compartments of VIP-Positive Neurons in Layer 2/3 of the Mouse Barrel Cortex

The recurrent network composed of excitatory and inhibitory neurons is fundamental to neocortical function. Inhibitory neurons in the mammalian neocortex are molecularly diverse, and individual cell types play unique functional roles in the neocortical microcircuit. Recently, vasoactive intestinal polypeptide-positive (VIP+) neurons, comprising a subclass of inhibitory neurons, have attracted particular attention because they can disinhibit pyramidal cells through inhibition of other types of inhibitory neurons, such as parvalbumin- (PV+) and somatostatin-positive (SOM+) inhibitory neurons, promoting sensory information processing. Although VIP+ neurons have been reported to receive synaptic inputs from PV+ and SOM+ inhibitory neurons as well as from cortical and thalamic excitatory neurons, the somatodendritic localization of these synaptic inputs has yet to be elucidated at subcellular spatial resolution. In the present study, we visualized the somatodendritic membranes of layer (L) 2/3 VIP+ neurons by injecting a newly developed adeno-associated virus (AAV) vector into the barrel cortex of VIP-Cre knock-in mice, and we determined the extensive ramification of VIP+ neuron dendrites in the vertical orientation. After immunohistochemical labeling of presynaptic boutons and postsynaptic structures, confocal laser scanning microscopy revealed that the synaptic contacts were unevenly distributed throughout the perisomatic (<100 μm from the somata) and distal-dendritic compartments (≥100 μm) of VIP+ neurons. Both corticocortical and thalamocortical excitatory neurons preferentially targeted the distal-dendritic compartment of VIP+ neurons. On the other hand, SOM+ and PV+ inhibitory neurons preferentially targeted the distal-dendritic and perisomatic compartments of VIP+ neurons, respectively. Notably, VIP+ neurons had few reciprocal connections. These observations suggest different inhibitory effects of SOM+ and PV+ neuronal inputs on VIP+ neuron activity; inhibitory inputs from SOM+ neurons likely modulate excitatory inputs locally in dendrites, while PV+ neurons could efficiently interfere with action potential generation through innervation of the perisomatic domain of VIP+ neurons. The present study, which shows a precise configuration of site-specific inputs, provides a structural basis for the integration mechanism of synaptic inputs to VIP+ neurons.

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.

Resurfacing of goat articular cartilage by chondrocytes derived from bone marrow

The feasibility of using cartilaginous implants containing bone marrow derived chondrocytes in biological resurfacing procedures for correcting defects in articular cartilage was examined in goats. The experimental protocol included bone marrow aspiration, mesenchymal cell culturing, cell proliferation, favorable conditions inducing chondrogenic differentiation, and implantation of autogeneic and allogeneic cells. Autogeneic implant transplantations proved to be the best source for regeneration and repair of defective articular surfaces with use of densitometric computed image analysis of histochemical and immunohistochemical parameters on tissue sections. Allogeneic chondrocyte enriched cultures derived from bone marrow evoke a typical immune response in the host, expressed by the formation of fibrosis and progressive joint arthrosis. In the current study, a biological resurfacing procedure is described in detail for large mammals of similar weight and size as humans. Autogeneic mesenchymal cells derived from a bone marrow aspiration are the best cell source and when embedded in hyaluronic acid based adhesive glue make an excellent cartilaginous implant. The reparative regenerated cartilaginous tissue outcome within the defects appear different than neighboring normal articular cartilage shortly after surgery. Whether in the long term the cartilaginous remodeling process will shape the cartilage such that it more closely resembles the original articular cartilage is not known.

Maffucci's syndrome--the result of neural abnormalities? Evidence of mitogenic neurotransmitters present in enchondromas and soft tissue hemangiomas

BACKGROUND

Maffucci's syndrome (MS) is distinguished by the enigmatic association of benign cartilaginous bone tumors and soft tissue hemangiomas.

METHODS

This study was conducted to define the distribution of nerves and neuropeptides around these tumors. Results were measured by quantitative image analysis of immunohistochemical staining. Four types of tissues were compared: connective tissues around normal muscles, solitary hemangiomas, MS hemangiomas, and MS enchondromas (the last two from a single patient).

RESULTS

The number of nerves was found to be quadrupled in both types of hemangiomas as compared to normal connective tissue. A unique feature of MS tissues is the presence of an increased number of nerve fibers not only in the lesions but also in histologically normal margins of resection surrounding the lesions. Furthermore, hemangiomas of both types were found to contain a significantly higher number of calcitonin gene-related peptide-, substance P-, and methionine enkephalin-positive fibers than did normal muscle or its related fibroconnective tissue. These neuropeptides are mitogens, and their presence stimulates the growth of the abnormal blood vessels. Enchondroma fragments from an MS patient contained numerous methionine enkephalin-positive nerves. This neuropeptide is known to act as a growth factor in cartilage proliferation.

CONCLUSIONS

A neural abnormality of the neuropeptidergic nervous system seems to relate to the abnormal tumors seen in MS.

Differentiation of transmitter phenotypes in rat cerebral cortex

Cortical neurons differ in their neurochemical properties. Projection neurons use excitatory amino acids as transmitters, most local interneurons contain the inhibitory transmitter GABA, and specific subtypes of local circuit neurons express distinct neuropeptides. How this cellular diversity is generated during development is not known. We have been studying the transmitter differentiation of cortical neurons in different in vitro systems using immunohistochemical techniques. Transmitter phenotypes of cortical neurons were examined in slice cultures, i.e. in the absence of extrinsic cortical connections, and in dissociated cortical cell cultures, i.e. in the absence of extrinsic and intrinsic cortical connections. The expression of vasoactive intestinal polypeptide in cortical interneurons occurred normally in slice cultures prepared from neonatal rats between birth and 2 days of age, but was strongly impaired in dissociated cell cultures prepared at the same time. These results suggest that the intact cortical environment present in the slice cultures exerts crucial influences for neuropeptide differentiation. In contrast, the transmitters glutamate and GABA were expressed normally in the appropriate cell types and similar in proportions in dissociated cell cultures prepared from cortices at embryonic day 19. Only cells dissociated during S-phase failed to express glutamate and GABA in vitro. When cells were kept for 24 h after mitosis in a cortical slice preparation in vitro, however, they later expressed their appropriate transmitter phenotypes. Thus, signals from the local cortical environment that act early in the cell cycle are required for the specification of transmitter phenotypes of cortical neurons.

Vasoactive intestinal polypeptide immunoreactive structures in the mouse barrel field

Immunohistochemistry for vasoactive intestinal polypeptide was carried out in tangentially cut vibratome sections of the barrel cortex in adult mice. Sections through layer IV have revealed an association between the cytoarchitectonically visible modular organization of barrels and the distribution of immunoreactive axon terminals. These terminals are preferentially localized in the side region of a barrel, whereas the hollow shows a relative scarcity of these structures as shown with image analysis. This finding is the first direct demonstration of a modular distribution of vasoactive intestinal polypeptide-containing axon terminals in the neocortex.

Matching localization of vasoactive intestinal polypeptide (VIP) and VIP-receptor at pre- and postsynaptic sites in the mouse visual cortex

Vibratome sections of the mouse occipital cortex were processed by double label immunohistochemistry to demonstrate the localization of the receptor for vasoactive intestinal polypeptide and the peptide itself. The receptor was found to be distributed in the cytoplasm and major dendrites of numerous cortical cells, mainly pyramidal neurons. Vasoactive intestinal polypeptide, on the other hand, occurred in a population of non-pyramidal neurons and axonal boutons. Image analysis revealed a close spatial association of peptide-containing presynaptic terminals with receptor-containing cells. Ultrastructurally, these connections represented symmetrical axo-somatic and axo-dendritic synapses. Our findings demonstrate a matching histological localization of vasoactive intestinal polypeptide and its receptor in the brain.

Formation and preservation of cortical layers in slice cultures

During cortical development, neurons generated at the same time in the ventricular zone migrate out into the cortical plate and form a cortical layer (Berry and Eayrs, 1963, Nature 197:984-985; Berry and Rogers, 1965, J. Anat. 99:691-709). We have been studying both the formation and maintenance of cortical layers in slice cultures from rat cortex. The bromodeoxyuridine (BrdU) method was used to label cortical neurons on their birthday in vivo. When slice cultures were prepared from animals at different embryonic and postnatal ages, all cortical layers that have already been established in vivo remained preserved for several weeks in vitro. In slice cultures prepared during migration in the cortex, cells continued to migrate towards the pial side of the cortical slice, however, migration ceased after about 1 week in culture. Thus, cortical cells reached their final laminar position only in slice cultures from postnatal animals, whereas in embryonic slice, migrating cells became scattered throughout the cortex. Previous studies demonstrated that radial glia fibers are the major substrate for migrating neurons (Rakic, 1972, J. Comp. Neurol. 145:61-84; Hatten and Mason, 1990, Experientia 46:907-916). Using antibodies directed against the intermediate filament Vimentin, radial glial cells were detected in all slice cultures where cell migration did occur. Comparable to the glia development in vivo, radial glial fibers disappeared and astrocytes containing the glia fibrillary-associated protein (GFAP) differentiated in slice cultures from postnatal cortex, after the neurons have completed their migration. In contrast, radial glial cells were detected over the whole culture period, and very few astrocytes differentiated in embryonic slices, where cortical neurons failed to finish their migration. The results of this study indicate that the local environment is sufficient to sustain the layered organization of the cortex and support the migration of cortical neurons. In addition, our results reveal a close relationship between cell migration and the developmental status of glial cells.

Neuropeptidergic innervation of intramuscular hemangiomas

Intramuscular hemangiomas are idiopathic lesions which are either tumoral or developmental in origin. A close association of abnormal blood vessels with nerve fibers is found and may suggest that nerves have a primary inciting role in the development of these lesions. In the current study, the number of nerve fibers in different zones around the tumors, as well as the type of neuropeptides present in these fibers, was quantitatively assessed by computer-assisted image analysis of immunohistochemical staining of histological slides. The number of nerve fibers as determined by positive staining by anti-protein S-100 antibodies was found to be elevated in the immediate vicinity of the abnormal blood vessels. The density of the nerve fibers rapidly declined with increasing distance from the hemangiomas, reaching normal values at distances of over 2 mm. Furthermore, hemangiomas contain a significantly higher number of calcitonin gene-related peptide (CGRP), substance P, and Met-enkephalin-positive fibers. The most significant rise in number is that of CGRP-positive fibers. This neuropeptide is a known mitogen, which could be responsible for the growth of the hemangiomatous blood vessels. Substance P is a nociceptive neurotransmitter and its presence can explain the pain which often accompanies even tiny intramuscular hemangiomas.

Developmental gradients of vasoactive intestinal polypeptide (VIP)-containing neurons in the rat visual cortex detected by image analysis

The postnatal development of vasoactive intestinal polypeptide-immunoreactive (VIP-IR) neurons was followed by computer-assisted image analysis in the rat visual cortex. The laminar distribution of all VIP-IR structures was measured. These structures were subdivided into cell somata, dendritic profiles and axonal boutons and measured separately. VIP-IR neurons were first seen on postnatal day 1, mostly in the upper half of the presumptive visual cortex. A localization of cell bodies similar to that in the adult is reached between days 12 and 16. VIP-IR dendrites have a protracted growth period as compared to perikarya, involving a developmental gradient from an even distribution to a concentration in the upper cortical layers. This is due to the formation of dendritic terminal arbors after the second postnatal week. Scattered VIP-IT axonal boutons appear on day 3 in the midportion of the presumptive visual cortex. Their typical laminar distribution in layers II, IV and lower VI was observed after day 12. Our results suggest that the biochemically detected sharp increase in VIP levels after the second postnatal week is due to the maturation of cell processes as a morphological basis of neuronal connectivity.

In situ hybridization reveals VIP precursor mRNA-containing neurons in monkey and rat neocortex

The cDNAs encoding monkey vasoactive intestinal polypeptide (VIP) and PHM-27 and rat VIP and PHI-27 were cloned and used to generate antisense RNA probes. Using in situ hybridization, neurons expressing the VIP/PHM or VIP/PHI precursor mRNAs were localized in monkey and rat somatic sensory and visual cortex. In both neocortical areas of both species, labeled cells were observed in all 6 layers as well as the subcortical white matter.

Neocortical VIP neurons are increased in the hemisphere containing focal cerebrocortical microdysgenesis in New Zealand Black mice

Twenty to forty percent of New Zealand Black mice, a strain that develops severe autoimmune disease and learning deficits, exhibit focal unilateral collections of ectopic neurons and glia in layer I of the neocortex with underlying laminar dysplasia. This type of anomaly traditionally has been considered to represent disordered neuronal migration. In an attempt to further characterize these abnormalities, we compared counts of immunohistochemically-stained VIP-neurons in cortical regions containing ectopias and in adjacent cortex to homologous regions of the opposite hemisphere. There was an overall increase in the number of these neurons in the hemisphere containing the ectopias, which resulted from an increase in the number of VIP neurons both in the column of cortex within and underlying the ectopias and in the medially adjoining columns. We concluded that the presence of ectopias in the cerebral cortex not only represent abnormal migration, but also an increase in the number of at least one subset of neurons.

Development of vasoactive intestinal polypeptide (VIP)-containing neurons in organotypic slice cultures from rat visual cortex

Using immunohistochemistry we have been studying the postnatal maturation of vasoactive intestinal polypeptide (VIP)-positive neurons in organotypic slice cultures from rat visual cortex. The development in vitro is compared with the occurrence of VIP-containing cells in vivo, where they are first observed around postnatal day 5. A further increase in number and morphological maturation occurs within the following 3 weeks. In cultures prepared from 1- or 2-day-old rats, i.e. before VIP is expressed in vivo, VIP-containing neurons appear after about 5 days and gradually increase in number over the next 2 weeks. Thus the time course of postnatal expression of VIP in vitro and the morphology of VIP-immunoreactive neurons in culture closely matches the situation in vivo. These observations suggest that the maturation of VIP-containing neurons occurs independently of cortical afferents and that the intrinsic connectivity and activity is sufficient for their postnatal maturation. Therefore organotypic slice cultures should be a suitable system to study mechanisms of neurochemical maturation in the cortex.

Distribution of glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes in the rat brain. I. Forebrain

In the first of two papers dealing with the distribution of glial fibrillary acidic protein-(GFAP)-immunoreactive elements in the rat brain, the localization of immunostaining in the forebrain is systematically described. While the limbic cortex was found to contain intensely stained, evenly distributed astrocytes, the neocortex showed clearly stratified GFAP-staining, with substantially less immunoreactivity occurring in the middle layers than in the areas close to the brain surface or the white matter. A remarkably regular staining pattern was observed in the hippocampus and dentate gyrus. The striatum remained unstained in sharp contrast to the pallidum. In the diencephalon, the main thalamic nuclei were poor in GFAP-labelled elements in contrast to the internuclear border zones. In the hypothalamus, nuclei were conspicuous by their GFAP-staining. A consistent differential staining pattern was obtained in the epithalamic structures. The observed distributional pattern of diencephalic GFAP-immunoreactivity is thought to be due to different regional proliferation of the embryonic neuroepithelium of the diencephalon. The uneven distribution of GFAP-immunoreactivity in the forebrain is explained on a mainly developmental basis.