Multimodal spatiotemporal phenotyping of human retinal organoid development

Organoids generated from human pluripotent stem cells provide experimental systems to study development and disease, but quantitative measurements across different spatial scales and molecular modalities are lacking. In this study, we generated multiplexed protein maps over a retinal organoid time course and primary adult human retinal tissue. We developed a toolkit to visualize progenitor and neuron location, the spatial arrangements of extracellular and subcellular components and global patterning in each organoid and primary tissue. In addition, we generated a single-cell transcriptome and chromatin accessibility timecourse dataset and inferred a gene regulatory network underlying organoid development. We integrated genomic data with spatially segmented nuclei into a multimodal atlas to explore organoid patterning and retinal ganglion cell (RGC) spatial neighborhoods, highlighting pathways involved in RGC cell death and showing that mosaic genetic perturbations in retinal organoids provide insight into cell fate regulation.

A gene expression atlas of embryonic neurogenesis in Drosophila reveals complex spatiotemporal regulation of lncRNAs

Cell type specification during early nervous system development in Drosophila melanogaster requires precise regulation of gene expression in time and space. Resolving the programs driving neurogenesis has been a major challenge owing to the complexity and rapidity with which distinct cell populations arise. To resolve the cell type-specific gene expression dynamics in early nervous system development, we have sequenced the transcriptomes of purified neurogenic cell types across consecutive time points covering crucial events in neurogenesis. The resulting gene expression atlas comprises a detailed resource of global transcriptome dynamics that permits systematic analysis of how cells in the nervous system acquire distinct fates. We resolve known gene expression dynamics and uncover novel expression signatures for hundreds of genes among diverse neurogenic cell types, most of which remain unstudied. We also identified a set of conserved long noncoding RNAs (lncRNAs) that are regulated in a tissue-specific manner and exhibit spatiotemporal expression during neurogenesis with exquisite specificity. lncRNA expression is highly dynamic and demarcates specific subpopulations within neurogenic cell types. Our spatiotemporal transcriptome atlas provides a comprehensive resource for investigating the function of coding genes and noncoding RNAs during crucial stages of early neurogenesis.

Summary: DIV-MARIS, an adapted technique for examining stage- and cell type-specific gene expression, reveals a complex network of mRNAs and lncRNAs expressed in specific cell types during early Drosophila embryonic nervous system development.

The Drosophila embryo at single-cell transcriptome resolution

By the onset of morphogenesis, Drosophila embryos consist of about 6000 cells that express distinct gene combinations. Here, we used single-cell sequencing of precisely staged embryos and devised DistMap, a computational mapping strategy to reconstruct the embryo and to predict spatial gene expression approaching single-cell resolution. We produced a virtual embryo with about 8000 expressed genes per cell. Our interactive Drosophila Virtual Expression eXplorer (DVEX) database generates three-dimensional virtual in situ hybridizations and computes gene expression gradients. We used DVEX to uncover patterned expression of transcription factors and long noncoding RNAs, as well as signaling pathway components. Spatial regulation of Hippo signaling during early embryogenesis suggests a mechanism for establishing asynchronous cell proliferation. Our approach is suitable to generate transcriptomic blueprints for other complex tissues.

Cell fixation and preservation for droplet-based single-cell transcriptomics

BACKGROUND

Recent developments in droplet-based microfluidics allow the transcriptional profiling of thousands of individual cells in a quantitative, highly parallel and cost-effective way. A critical, often limiting step is the preparation of cells in an unperturbed state, not altered by stress or ageing. Other challenges are rare cells that need to be collected over several days or samples prepared at different times or locations.

METHODS

Here, we used chemical fixation to address these problems. Methanol fixation allowed us to stabilise and preserve dissociated cells for weeks without compromising single-cell RNA sequencing data.

RESULTS

By using mixtures of fixed, cultured human and mouse cells, we first showed that individual transcriptomes could be confidently assigned to one of the two species. Single-cell gene expression from live and fixed samples correlated well with bulk mRNA-seq data. We then applied methanol fixation to transcriptionally profile primary cells from dissociated, complex tissues. Low RNA content cells from Drosophila embryos, as well as mouse hindbrain and cerebellum cells prepared by fluorescence-activated cell sorting, were successfully analysed after fixation, storage and single-cell droplet RNA-seq. We were able to identify diverse cell populations, including neuronal subtypes. As an additional resource, we provide 'dropbead', an R package for exploratory data analysis, visualization and filtering of Drop-seq data.

CONCLUSIONS

We expect that the availability of a simple cell fixation method will open up many new opportunities in diverse biological contexts to analyse transcriptional dynamics at single-cell resolution.

Resequencing of VAX1 in patients with nonsyndromic cleft lip with or without cleft palate

BACKGROUND

Nonsyndromic cleft lip with or without cleft palate (NSCL/P) is one of the most common of all congenital anomalies, and has a multifactorial etiology involving both environmental and genetic factors. Recent genome-wide association studies (GWAS) identified strong association between a locus on chromosome 10q25.3 and NSCL/P in European samples. One gene at 10q25.3, the ventral anterior homeobox 1 (VAX1) gene, is considered a strong candidate gene for craniofacial malformations. The purpose of the present study was to provide further evidence that VAX1 is the causal gene at the 10q25.3 locus through identification of an excess of rare mutations in patients with NSCL/P.

METHODS

The 5'UTR, complete coding regions, and adjacent splice sites of the two known VAX1 isoforms were sequenced in 384 patients with NSCL/P and 384 controls of Central European descent. Observed variants were investigated with respect to familial cosegregation or de novo occurrence, and in silico analyses were performed to identify putative effects on the transcript or protein level.

RESULTS

Eighteen single-base variants were found, 15 of them rare and previously unreported. In the long VAX1 isoform, predicted functionally relevant variants were observed more often in NSCL/P cases, although this difference was not significant (p = 0.17). Analysis of family members demonstrated incomplete cosegregation in most pedigrees.

CONCLUSION

Our data do not support the hypothesis that highly penetrant rare variants in VAX1 are a cause of NSCL/P. To determine whether VAX1 is the causative gene at 10q25.3 further research, in particular into the biologic function of its long isoform, is warranted. Birth Defects Research (Part A), 2012.

Neuronal activity and TrkB ligands influence Kv3.1b and Kv3.2 expression in developing cortical interneurons

Among the GABAergic neocortical interneurons, fast-spiking (FS) basket and chandelier cells are essential mediators for feed-forward inhibition, network synchrony and oscillations. The FS properties are in part mediated by the voltage-gated potassium channels Kv3.1b/3.2 which allow the fast repolarization of the membrane necessary for firing non-adapting action potentials at high frequencies. It has been recently reported that the FS phenotype fails to mature in BDNF knockout mice suggesting a role for neurotrophins. We now describe the role of neuronal activity and neurotrophins for Kv3.1b/3.2 expression using organotypic cultures of rat visual cortex as model system. Chronic activity deprivation from 2 days in vitro (DIV) prevented the postnatal developmental increase of Kv3.2, but not Kv3.1b mRNA expression. However, chronic activity deprivation failed to alter Kv3.1b and marginally delayed Kv3.2 protein expression. Activity deprivation by glutamate receptor blockade from 10 to 20 DIV reduced both mRNAs, whereas deprivation with tetrodotoxin (TTX) reduced both mRNAs and the Kv3.2 protein. Thalamic and cortical afferents in cocultures failed to alter the expression. BDNF and NT4 supplemented from 2 DIV onwards increased the expression of Kv3.1b, but not Kv3.2 mRNA in young cultures. Only NT4 increased the expression of both mRNAs later in development. Kv3 protein levels were not changed by exogenous tropomyosin-related kinase B (TrkB) ligands, but the levels decreased upon inhibiting the MAPK signaling suggesting a role for endogenous factors and in particular MEK2 signaling for translation. The results show that Kv3.1b/3.2 expression is differentially controlled by neuronal activity and neurotrophic factors.

Characterization of pretectal-nuclear-complex afferents to the pulvinar in the cat

We investigated anatomical and physiological properties of the projection from the pretectal nuclear complex (PNC) to the ipsilateral lateral posterior-pulvinar complex in the cat. After Phaseolus vulgaris leucoagglutinin injections into the PNC, the majority (70%) of anterogradely labeled terminals was localized in the pulvinar proper, the remaining 30% were scattered in the lateral and medial portions of the LP. No PNC neuron retrogradely labeled from the pulvinar was found to also express glutamic acid decarboxylase (GAD) mRNA, although a large number of neurons carrying the GAD label were found in close vicinity. In contrast, 69% of retrogradely labeled PNC cells also displayed glutamate-like immunoreactivity. Twenty-six out of 96 (27%) visually responsive pulvinar neurons were orthodromically activated by electrical stimulation of the ipsilateral PNC at latencies between 1 and 10 ms (median 1.9 ms). All orthodromically activated neurons responded well to the onset and offset of large visual stimuli and to sudden stimulus shifts. Whenever a saccadic eye movement was executed, these neurons were also activated, except during saccades in darkness. The comparison of saccade-evoked response with responses to visual stimuli that elicit similar retinal image shifts revealed that pretectorecipient pulvinar neurons also seem to receive a saccade-related non-visual input. All response properties correspond to those of a specific class of pulvinar neurons that have been termed "SV" neurons because they respond to visual stimulation as well as during saccades. They also closely resemble response properties of PNC neurons that project to the ipsilateral pulvinar. The results support the proposal that PNC cells not only directly activate their postsynaptic target neurons in the pulvinar, but that they also provide a visual input to these neurons that greatly contributes to their response characteristics.

Transgenic activation of Ras in neurons promotes hypertrophy and protects from lesion-induced degeneration

Ras is a universal eukaryotic intracellular protein integrating extracellular signals from multiple receptor types. To investigate its role in the adult central nervous system, constitutively activated V12-Ha-Ras was expressed selectively in neurons of transgenic mice via a synapsin promoter. Ras-transgene protein expression increased postnatally, reaching a four- to fivefold elevation at day 40 and persisting at this level, thereafter. Neuronal Ras was constitutively active and a corresponding activating phosphorylation of mitogen-activated kinase was observed, but there were no changes in the activity of phosphoinositide 3-kinase, the phosphorylation of its target kinase Akt/PKB, or expression of the anti-apoptotic proteins Bcl-2 or Bcl-X(L). Neuronal Ras activation did not alter the total number of neurons, but induced cell soma hypertrophy, which resulted in a 14.5% increase of total brain volume. Choline acetyltransferase and tyrosine hydroxylase activities were increased, as well as neuropeptide Y expression. Degeneration of motorneurons was completely prevented after facial nerve lesion in Ras-transgenic mice. Furthermore, neurotoxin-induced degeneration of dopaminergic substantia nigra neurons and their striatal projections was greatly attenuated. Thus, the Ras signaling pathway mimics neurotrophic effects and triggers neuroprotective mechanisms in adult mice. Neuronal Ras activation might become a tool to stabilize donor neurons for neural transplantation and to protect neuronal populations in neurodegenerative diseases.

Specification of neuropeptide Y phenotype in visual cortical neurons by leukemia inhibitory factor

Building the complex mammalian neocortex requires appropriate numbers of neurochemically specified neurons. It is not clear how the highly diverse cortical interneurons acquire their distinctive phenotypes. The lack of genetic determination implicates environmental factors in this selection and specification process. We analysed, in organotypic visual cortex cultures, the specification of neurons expressing neuropeptide Y (NPY), a potent anticonvulsant. Endogenous brain-derived neurotrophic factor and neurotrophin 4/5 play no role in early NPY phenotype specification. Rather, the decision to express NPY is made during a period of molecular plasticity during which differentiating neurons with the potential to express NPY compete for the cytokine leukemia inhibitory factor which is produced in the cortex, but is negatively regulated by thalamic afferences. The neurons that fail in this competition are parvalbuminergic basket and chandelier neurons, which express NPY transiently, but will not acquire a permanent NPY expression. They switch into a facultative NPY expression mode, and remain responsive to the neurotrophins which modulate NPY expression later in development.

Cellular reactivity to mechanical axonal injury in an organotypic in vitro model of neurotrauma

An in vitro model of traumatic brain injury is described that is based on organotypic cocultures (OTCs) of rat neocortex and thalamus connected by reciprocal axonal projections. Localized mechanical compression of this projection was inflicted with a mechanical device, and the effects on cell viability, axonal morphology, and protein expression levels were analyzed. Within 24 h after insult, major cell damage occurred in infragranular cortical layers containing the corticothalamic projection neurons and in thalamic regions adjacent to the mechanical impact as was assessed through the use of the vital stain Syto 21, and propidium iodide labeling. A small, but significant number of calretinin-positive interneurons in cortical and thalamic areas displayed symptoms of injury. Axonal elements, as revealed by neurofilament (NF-H/M) immunohistochemistry, in the corticothalamic transition zone displayed pathomorphological changes, such as axonal bulbs and swellings, already 4 h after insult. Densitometric analysis revealed that MAP-2a,b expression was not significantly changed within 4 h after injury. A significant reduction in MAP-2a,b amount was evident at 20 h after injury in thalamus (by 31.6%) and cortex (by 30%) maintained for 12 days in vitro (DIV), but not in OTCs aged 20 DIV. The axonally localized form MAP-2c significantly increased in cortex of 12-DIV OTCs at 4 and 20 h after insult (65.6% and 33.4%, respectively). MAP-2c levels in cortex of 20 DIV initially increased by 47.7% and declined below control values 20 h after injury. Thalamic areas revealed a delay in MAP-2c reactivity, in that expression was significantly elevated only at 20 h after injury (by 84.4% in 12-DIV and by 39.6% in 20-DIV OTCs, respectively). These data may reflect the regenerative ability of juvenile, but not of older neurons in response to mechanical axonal injury.

Development of neuronal activity and activity-dependent expression of brain-derived neurotrophic factor mRNA in organotypic cultures of rat visual cortex

We have analyzed in organotypic rat visual cortex cultures the way in which expression of brain-derived neurotrophic factor (BDNF) mRNA depends on synaptically generated spontaneous bioelectric activity (SBA) as monitored by recordings of pyramidal cells. SBA was initially low, but from the fourth week onwards 83% of the neurons fired action potentials at 0.2-1.2 impulses/s in a well-balanced state of excitation and inhibition. BDNF mRNA expression increased during the second week to a level surprisingly similar to the adult visual cortex in vivo, despite the fact that activity rates in vitro were approximately 10-fold lower than rates reported in vivo. Thus, SBA generated by a cortical neuronal network in the absence of sensory input is sufficient to elicit and maintain BDNF expression. The transient BDNF peak occurring after eye opening in vivo did not occur in vitro. A blockade of SBA seems not to alter the expression of neurotrophin (NT)-3 and -4/5, and tyrosine kinase receptor C and B mRNA. However, BDNF expression remained extremely low. A recovery of SBA after a period of blockade concurred with a transient hyperexcitability. BDNF immediately increased, driven by calcium influx through voltage-gated channels in synergy with NMDA receptors. Expression transiently reached high levels in neurons of supragranular layers. Infragranular neurons, although firing action potentials, recovered BDNF expression much slower. After 5 days in vitro recovery, the network had de novo established a balanced state of excitation and inhibition. Distribution and expression level of BDNF mRNA had returned to control. Even in 'adult' cultures an acute blockade of SBA downregulated BDNF, and a subsequent recovery of SBA restored BDNF expression. We conclude that BDNF mRNA expression depends on and responds with a fast kinetic to changes of the SBA. Steady-state levels do not depend on the absolute levels of activity, but more likely on the balance between excitation and inhibition, suggesting a role for BDNF in activity homeostasis.

The paleocortical ventricle is the origin of reelin-expressing neurons in the marginal zone of the foetal human neocortex

The subpial granular layer (SGL) is a transient cell layer in the cortical marginal zone during the period of neuronal migration into the cortical plate. The origin of the SGL has been studied by immunocytochemistry for calretinin (CR) and reelin in human foetuses from 11 to 40 gestational weeks (GW). At 11 GW, the paleocortical ventricle, a rostral dilatation of the lateral ventricle, gives rise to two fountainheads: a medial fountainhead provides neurons for the marginal zone (MZ) of the rostral cortex and rostral hippocampal rudiment, while multiple cell streams migrate from a lateral fountainhead into the MZ of the paleocortex and insula. The latero-medial gradient of neuronal packing density in the neocortical MZ indicates that migration extends farther into the neocortex. Neurons express CR already in the retrobulbar ventricular zone; they express reelin only as they approach the MZ of the paleocortex and rostral archicortex. At 16/17 GW, large numbers of CR-immunoreactive granule cells originate from the same fountainheads, and then direct medially, toward the surface of the anterior perforated substance, and laterally, into the paleocortical MZ, from where they continue into the neocortical SGL following a ventrolateral to dorsomedial gradient. From 13 to 18 GW, reelin is expressed by a subpopulation of granule cells and by Cajal-Retzius-like neurons. By 22 GW, the paleocortical ventricle undergoes regression and no longer supplies the SGL. Our results show that the paleocortical ventricle gives rise to a stream of neurons which extends over the cortical MZ as the subpial granular layer. The fact that SGL derivatives express reelin suggests that this transient cell layer may play a significant role in the establishment of the complex cytoarchitecture of the cerebral cortex.

Patterns of spontaneous activity and morphology of interneuron types in organotypic cortex and thalamus-cortex cultures

The physiological and morphological properties of interneurons in infragranular layers of rat visual cortex have been studied in organotypic cortex monocultures and thalamus-cortex co-cultures using intracellular recordings and biocytin injections. Cultures were prepared at the day of birth and maintained for up to 20 weeks. Twenty-nine interneurons of different types were characterized, in addition to 170 pyramidal neurons. The cultures developed a considerable degree of synaptically driven "spontaneous" bioelectric activity without epileptiform activity. Interneurons in cortex monocultures and thalamus-cortex co-cultures had the same physiological and morphological properties, and also pyramidal cell properties were not different in the two culture conditions. All interneurons and the majority of pyramidal cells displayed synaptically driven action potentials. The physiological group of fast-spiking interneurons included large basket cells, columnar basket cells (two cells with an arcade axon) and horizontally bitufted cells. The physiological group of slow-spiking interneurons included Martinotti cells and a "long-axon" cell. Analyses of the temporal patterns of activity revealed that fast-spiking interneurons have higher rates of spontaneous activity than slow-spiking interneurons and pyramidal cells. Furthermore, fast-spiking interneurons fired spontaneous bursts of action potentials in the gamma frequency range. We conclude from these findings that physiological and morphological properties of interneurons in organotypic mono- and co-cultures match those of interneurons characterized in vivo or in acute slice preparations, and they maintain in long-term cultures a well-balanced state of excitation and inhibition. This suggests that cortex-intrinsic or cell-autonomous mechanisms are sufficient for the expression of cell type-specific electrophysiological properties in the absence of afferents or sensory input.

Sensory impairments and delayed regeneration of sensory axons in interleukin-6-deficient mice

Interleukin-6 (IL-6) is a multifunctional cytokine mediating inflammatory or immune reactions. Here we investigated the possible role of IL-6 in the intact or lesioned peripheral nervous system using adult IL-6 gene knockout (IL-6(-/-)) mice. Various sensory functions were tested by applying electrophysiological, morphological, biochemical, and behavioral methods. There was a 60% reduction of the compound action potential of the sensory branch of IL-6(-/-) mice as compared with the motor branch in the intact sciatic nerve. Cross sections of L5 DRG of IL-6(-/-) mice showed a shift in the relative size distribution of the neurons. The temperature sensitivity of IL-6(-/-) mice was also significantly reduced. After crush lesion of the sciatic nerve, its functional recovery was delayed in IL-6(-/-) mice as analyzed from a behavioral footprint assay. Measurements of compound action potentials 20 d after crush lesion showed that there was a very low level of recovery of the sensory but not of the motor branch of IL-6(-/-) mice. Similar results of sensory impairments were obtained with mice showing slow Wallerian degeneration (Wlds) and a delayed lesion-induced recruitment of macrophages. However, in contrast to WldS mice, in IL-6(-/-) mice we observed the characteristic lesion-induced invasion of macrophages and the upregulation of low-affinity neurotrophin receptor p75 (p75LNTR) mRNA levels identical to those of IL-6(+/+) mice. Thus, the mechanisms leading to the common sensory deficiencies were different between IL-6(-/-) and WldS mice. Altogether, the results suggest that interleukin-6 is essential to modulate sensory functions in vivo.

Expression of TrkB and TrkC but not BDNF mRNA in neurochemically identified interneurons in rat visual cortex in vivo and in organotypic cultures

The mammalian visual cortex contains morphologically diverse populations of interneurons whose neurochemical properties are believed to be regulated by neurotrophic factors. This requires the expression of neurotrophin receptors. We have analysed whether brain-derived neurotrophic factor (BDNF), its receptor trkB and the NT-3 receptor trkC are expressed in interneurons of rat visual cortex in vivo, and in organotypic visual cortex cultures, paying particular attention to the subsets of neuropeptidergic neurons. In situ hybridization in combination with immunofluorescence for calcium-binding proteins and neuropeptides revealed that BDNF is not expressed in interneurons in vivo or in vitro. For the neurotrophin receptors we found in vivo at postnatal day 70 (P70) that approximately 80% of the parvalbumin-immunoreactive (-ir), but only 50% of the intensely calbindin-ir, and only 20% of the calretinin-ir neurons express trkB. Double labelling with neuropeptides revealed that approximately 50% of the neuropeptide Y-ir and approximately 50% of the somatostatin-ir neurons express trkB in a laminar-specific way. Only 25% of the vasoactive intestinal polypeptide (VIP)-ir neurons coexpress trkB. The coexpression of neuropeptide Y with trkB, but not with BDNF or trkC, was confirmed with a double in situ hybridization. In contrast, the percentages differed in the immature cortex; at P14 70% of the NPY-ir neurons and 46% of the calretinin-ir neurons revealed trkB expression, while the ratio for calbindin-ir cells was fairly constant (59%). From the interneuron populations studied, only 12% of the parvalbumin-ir neurons expressed trkC. A triple labelling revealed that some neurons coexpressed both trk mRNAs, while others had only trkC. The analysis of interneurons in organotypic cultures yielded very similar results. The results indicate that trkB ligands synthesized by pyramidal neurons influence neuropeptide or calcium-binding protein expression in a paracrine or transsynaptic manner. However, in contrast to current belief, in the adult only about half of all interneurons appear responsive to trkB ligands. Although the proportion is higher in the immature cortex, not all of the interneurons appear neurotrophin-receptive. With regard to the presence or absence of neurotrophin receptors, the molecular heterogeneity of GABAergic interneurons in the visual cortex is higher than currently assumed, and the responsiveness to neurotrophins changes with development in a cell type-specific way.

GABAergic inhibition in the rat pontine nuclei is exclusively extrinsic: evidence from an in situ hybridization study for GAD67 mRNA

As clearly indicated by our electrophysiological work, GABAergic inhibition plays a powerful role in the pontine nuclei (PN), the major link between cerebral cortex and the cerebellum. Using the technique of in situ hybridization for the mRNA encoding for the gamma-aminobutyric acid (GABA)-synthesizing isoenzyme glutamic acid decarboxylase67 (GAD67), we demonstrate here the total absence of potentially GABAergic neurons from the rat PN. This negative finding supports the notion that GABAergic inhibition in the PN of rats, unlike that of higher mammals, is exclusively based on extrapontine GABAergic afferents.

Epigenetic factors regulate the NPY expression in rat cortical neurons

The NPY phenotype expressed in a subset of rat neocortical neurons is influenced by a variety of epigenetic factors. In the present study, we analyzed the role of synaptically driven spontaneous bioelectric (action potential) activity (SBA) and neurotrophic factors. Our model systems are organotypic monocultures of visual cortex which either grow as spontaneously active cultures or as activity-blocked cultures to which neurotrophic factors can be applied via the medium. NPY mRNA expressing neurons are detected by in situ hybridization and are quantified as a percentage of all neurons. In spontaneously active cultures, about 7% of all neurons express NPY mRNA. This expression is regulated by SBA, because expression is reduced to about 2% by different activity blockade paradigms. When putative NPY neurons differentiate under activity blockade, they are unable to restitute the NPY expression during a subsequent period of SBA. A restitution of the NPY phenotype in 6-7% of the neurons after a transient blockade of activity is only possible when neurons were initially allowed to differentiate in the presence of SBA. We then analyzed whether neurotrophic factors known to promote NPY expression can do so in the absence of SBA. Neurotrophin-4/5 and leukemia inhibitory factor, but not brain-derived neurotrophic factor and neurotrophin-3, stimulate the NPY phenotype in the absence of SBA. In situ hybridization in combination with immuno-fluorescence reveals that NPY-ir neurons express the receptors trkB or LIFRbeta, but not trkC. This coexpression pattern explains why neurotrophin-4/5 and leukemia inhibitory factor are efficient regulators of the NPY-expression. Our results suggest that the NPY expression in neocortical neurons depends on epigenetic factors: spontaneous activity and neurotrophic factors modulate the expression and are thus involved in shaping the neurochemical architecture of the cerebral cortex.

Postnatal expression pattern of calcium-binding proteins in organotypic thalamic cultures and in the dorsal thalamus in vivo

The present study describes the postnatal expression of calbindin, calretinin and parvalbumin and glutamic acid decarboxylase (GAD) and microtubule-associated protein 2 (MAP2) in organotypic monocultures of rat dorsal thalamus compared to the thalamus in vivo. Cultures were maintained for up to 7 weeks. Cortex-conditioned medium improved the survival of thalamic cultures. MAP2-immunoreactive material was present in somata and dendrites of small and large-sized neurons throughout the cultures. Parvalbumin immunoreactivity was present in larger multipolar or bitufted neurons along the edge of a culture. These neurons also displayed strong parvalbumin mRNA and GAD mRNA expression, and GABA immunoreactivity. They likely corresponded to cells of the nucleus reticularis thalami. Parvalbumin mRNA, but neither parvalbumin protein nor GAD mRNA, was expressed in neurons with large somata within the explant. They likely represented relay cells. GAD mRNA, but not parvalbumin mRNA, was expressed in small neurons within the explants. Small neurons also displayed calbindin- and calretinin-immunoreactivity. The small neurons likely represented local circuit neurons. The time course of expression of the calcium-binding proteins revealed that all were present at birth with the predicted molecular weights. A low, but constant parvalbumin expression was observed in vitro without the developmental increase seen in vivo, which most likely represented parvalbumin from afferent sources. In contrast, the explantation transiently downregulated the calretinin and calbindin expression, but the neurons recovered the expression after 14 and 21 days, respectively. In conclusion, thalamic monocultures older than three weeks represent a stable neuronal network containing well differentiated neurons of the nucleus reticularis thalami, relay cells and local circuit neurons.

Characterization of a directional selective inhibitory input from the medial terminal nucleus to the pretectal nuclear complex in the rat

The receptive field properties of neurons in the medial terminal nucleus of the accessory optic system (MTN) that project to the ipsilateral nucleus of the optic tract (NOT) and dorsal terminal nucleus (DTN), as identified by antidromic electrical activation, were analysed in the anaesthetized rat. The great majority (88%) of MTN neurons that were antidromically activated from NOT and DTN preferred downward directed movement of large visual stimuli while the remaining cells preferred upward directed stimulus movement. Distinct retrograde tracer injections into the NOT/DTN and the ipsilateral inferior olive (IO) revealed that no MTN neurons project to both targets. MTN neurons projecting to the ipsilateral NOT/DTN were predominantly found in the ventral part of the MTN, whereas those projecting to the IO were found in the dorsal part of the MTN. In situ hybridization for glutamic acid decarboxylase (GAD) mRNA was used as a marker for GABAergic neurons. Up to 98% of MTN neurons retrogradely labelled from the ipsilateral NOT/DTN also expressed GAD mRNA. Earlier studies have shown that MTN neurons that prefer upward directed stimulus movements are segregated from MTN neurons that prefer downward directed stimulus movements. It also has been demonstrated that directionally selective neurons in the NOT/DTN prefer horizontal stimulus movements and receive an inhibitory input from ipsilateral MTN. Our results indicate that this input is mediated by GABAergic cells in the ventral part of MTN, which to a large extent prefer downward directed stimulus movements, and that the great majority of MTN neurons that prefer upward directed stimulus movements project to other targets one of which possibly is the IO.

NT-4/5 and LIF, but not NT-3 and BDNF, promote NPY mRNA expression in cortical neurons in the absence of spontaneous bioelectrical activity

Epigenetic factors are known to influence the differentiation of neocortical neurons. The present study analyses the role of spontaneous bioelectrical activity (SBA) and neurotrophic factors on the expression of neuropeptide Y (NPY) in rat visual cortical neurons using organotypic monocultures prepared from newborn animals and in situ hybridization to detect the NPY messenger ribonucleic acid (mRNA). Spontaneously active cortex cultures display NPY mRNA expression in about 7% of all cortical neurons from 10 days in vitro (DIV) on. Blocking the SBA by chronic application of 10 mM Mg2+ for 3-30 DIV reduces the percentage of NPY neurons to about 2%. Allowing an initial phase of SBA (1-20 DIV) followed by an SBA blockade (for 21-50 DIV) results in 2% labelled neurons, indicating a dramatic reduction of NPY mRNA expression in the absence of SBA. Surprisingly, the reverse experiment (a period of SBA blockade for 1-20 DIV followed by a period of SBA recovery for 21-40 DIV) does not cause an upregulation of NPY mRNA expression. However, allowing cultures to differentiate as spontaneously active cultures, then applying a transient period of SBA blockade which is followed by a second period of SBA, does rescue the NPY mRNA expression in 7% of the cortical neurons. We conclude that SBA is a main trigger for NPY mRNA expression and it is particularly important during an early postnatal period of differentiation. We then analysed whether neurotrophic factors known to modulate cortical neuropeptide expression are able to do so in the absence of SBA. Supplementing chronically blocked cultures with the neurotrophins, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-4/5 (NT-4/5) and the cytokine, leukaemia inhibitory factor (LIF), reveals that BDNF and NT-3 are unable to increase the percentage of NPY neurons. In contrast, LIF and NT-4/5 increase the percentage of NPY neurons to 4 and 6-7%, respectively. Moreover, neurons treated with NT-4/5 display a very high level of NPY mRNA expression in somata and in the dendritic trees. The data suggest a complex interplay and a hierarchy of epigenetic factors in regulating the neurochemical architecture of the developing neocortex.

Visual activity is required to maintain the phenotype of supragranular NPY neurons in rat area 17

Visual activity governs the functional maturation of the mammalian visual cortex. We report here, that visual experience is required for stabilizing the phenotype of a subset of cortical interneurons. Neurons expressing neuropeptide Y mRNA (NPY neurons) display a transiently higher expression in the early postnatal visual areas 18a and 17 that is followed by a phenotype restriction during the second postnatal month: about 50% of the NPY neurons in supragranular and infragranular layers of area 18a, and in infragranular layers of area 17 gradually stop the NPY expression. In contrast, the expression remains unchanged in supragranular layers of area 17. Dark rearing rats from birth to up to 100 days does neither prevent the developmental onset of NPY mRNA expression, nor does it prevent the phenotype restriction from occurring. In contrast, in dark reared animals NPY neurons in supragranular layers of area 17 now also undergo a phenotype restriction. Returning animals to light after variable periods of darkness results in an upregulation of NPY mRNA expression selectively in neurons in supragranular layers of area 17. These neurons acquire a constitutive expression during the second postnatal month. This suggests that the phenotypic specification of a distinct subset of cortical interneurons is regulated by visual experience which thus influences on the maturation of the neurochemical architecture of area 17.

Phenotype specification of cortical neurons during a period of molecular plasticity

The transient expression of neuropeptide transmitters is a common feature of the developing cortex. We have now analysed the role of cortical afferents in shaping the neurochemical architecture of rat visual cortex using organotypic cultures. Deafferented cortex monocultures prepared from newborn rats reveal a constant NPY mRNA expression in 6-8% of all cortical neurons up to 90 days in vitro (DIV). In contrast, afferent thalamocortical and corticocortical axonal innervation elicits a progressive reduction in the percentage of NPY mRNA expressing neurons from initially 6-8% in 30DIV cocultures to 2-3% and 3-4% respectively in 60DIV cocultures, which is maintained for up to 90DIV. This phenotype restriction is not observed in only efferently connected corticocollicular cocultures. Further, axonal innervation does not change the percentage of GAD mRNA-expressing neurons, which remains at 13% in mono- and cocultures. When feeding thalamocortical cocultures with monoculture-conditioned medium between 3-20DIV followed by normal medium up to 60DIV, the phenotype restriction fails to occur in the cocultured cortex. We conclude that cortex-derived factors secreted into the medium by a monoculture suppress the phenotype-restricting capacity of the afferents, but only when present within the first 14DIV during the period of formation of axonal connections. To elucidate the nature of the cortex-derived factors, brain-derived neurotrophic factor was applied to the medium. When applied for the first 14DIV, it does not prevent the phenotype restriction from occurring. This suggests that epigenetic factors such as axonal innervation and cortex-derived factors other than brain-derived neurotrophic factor govern a phenotype decision in neocortical neurons during a period of molecular plasticity.

Biochemical and histological analysis of two Müller cell antibodies in developing and adult cat and rat central nervous system

We investigated the binding characteristics of two monoclonal antibodies, 4F3 and 3F8, which in the retina specifically stain Müller cells, both with protein blots and immunohistochemically in sections of various regions of the central nervous system of neonatal and adult cats and rats. Clear differences emerged between the two antibodies. In addition, some species-specific as well as developmental differences within the staining pattern of each individual antibody were evident. The epitopes recognized by 4F3 lay mainly in the 57-63 kDa range. Histologically, 4F3 labelled mainly glia cells: oligodendrocytes and astrocytes in optic nerve, astrocytes in neocortex and cerebellum, Bergmann glia in the cerebellum and radial glia in neonatal animals. This was confirmed by double-immunofluorescence with the astrocyte marker GFAP. By contrast, 3F8 epitopes lay mainly in the 47-49 kDa range. Histologically, 3F8 labelled oligodendrocytes in the optic nerve, but only neurons in cerebellum and neocortex as confirmed by double-labelling with neuronal markers. Neither 4F3 nor 3F8 recognized GFAP or vimentin. These results clearly indicate (1) that the two antibodies identify new epitopes/molecules, (2) that the antigens are not retina-specific, and (3) that Müller cells share epitopes with other glial cells as well as with neurons outside the retina.

Postnatal developmental changes of neurons expressing calcium-binding proteins and GAD mRNA in the pretectal nuclear complex of the cat

The postnatal development of the cat pretectum has been analysed with in situ hybridization and immunohistochemistry with the aim to establish the time course of morphological and neurochemical maturation of parvalbumin (PARV), calbindin-D28k (CALB), and glutamic acid decarboxylase (GAD) expressing neuronal populations. At birth, PARV-ir retinal afferents to the pretectum have already formed distinct termination zones which appear as 3 clusters separated by intercluster regions in coronal sections. The clusters contain two sets of large neurons expressing either PARV or CALB. The two sets of neurons differ in the time at which they grow rapidly. Both sets reach the adult size at P38. PARV-ir retinal fibers contact dendrites of large PARV-negative, and thus presumably CALB-ir neurons. A population of smaller CALB-ir neurons appears within the clusters during the second postnatal week. In intercluster regions, small PARV-ir and CALB-ir neurons are present at birth, but increase in number during development. Only PARV-ir intercluster neurons increase in size between P4 and P38. GAD neurons are present dorsal to the clusters and in intercluster regions from P0 onwards. However, within the clusters GAD neurons do not appear until the second postnatal week. The different onset of marker expression and cellular growth patterns suggest the existence of several populations of CaBP-ir excitatory and inhibitory neurons in the pretectum. The final complement of inhibitory neurons is not present until the second postnatal week. These developmental processes may correlate with the slow maturation of the pretectal motion processing system and the cortico-pretectal projection.

Development of cholinergic and GABAergic neurons in the rat medial septum: different onset of choline acetyltransferase and glutamate decarboxylase mRNA expression

In the present study, we have investigated the developmental expression of the transmitter-synthesizing enzymes choline acetyltransferase (ChAT) and glutamate decarboxylase (GAD) in rat medial septal neurons by using in situ hybridization histochemistry. In addition, we have employed immunostaining for ChAT and the calcium-binding protein parvalbumin, known to be contained in septohippocampal GABAergic neurons. A large number of GAD67 mRNA-expressing neurons were already observed in the septal complex on embryonic day (E) 17, the earliest time point studied. During later developmental stages, there was mainly an increase in the intensity of labeling. Neurons expressing ChAT mRNA were first recognized at E 20, and their number slowly increased during postnatal development of the septal region. The adult pattern of ChAT mRNA-expressing neurons was observed around postnatal day (P) 16. By using a monoclonal ChAT antibody, the first immunoreactive cells were not seen before P 8. Similarly, the first weakly parvalbumin-immunoreactive neurons were seen in the septal complex by the end of the 1st postnatal week. These results indicate that in situ hybridization histochemistry may be an adequate method to monitor the different development of transmitter biosynthesis in cholinergic and GABAergic septal neurons. Moreover, the late onset of ChAT mRNA expression would be compatible with a role of target-derived factors for the differentiation of the cholinergic phenotype.

Development and activity-dependent expression of neuronal marker proteins in organotypic cultures of rat visual cortex

We are interested in activity-dependent mechanisms which govern the structural and functional maturation of neurons in the visual cortex. We have asked whether the expression of neuronal markers microtubule-associated proteins tau, MAP-2, synaptophysin (p38), and the growth-associated protein GAP-43 are dependent on cortical afferents or spontaneous activity. As a model system we have employed organotypic monocultures of rat visual cortex (OTCs, isolated from subcortical structures) in comparison with visual cortex in vivo (innervated by thalamic and other afferents) at different postnatal ages. We know from previous work that the OTCs, like the cortex in vivo, display a high rate of spontaneously generated action potentials. Therefore, as a third objective, we have analysed OTCs grown as monocultures under chronic blockade of spontaneous action potentials. Protein expression was detected by protein blots and/or immunohistochemistry. The proteins examined in this study are expressed in OTCs, even when grown under activity blockade. However, the pattern of expression differs from the cortex in vivo. Tau is expressed much weaker in OTCs than in cortex in vivo. The expression of the major band of about 50 kDa increases over time in vivo and in OTCs. Smaller isoforms of tau are dramatically downregulated, and larger (adult) isoforms do not appear within 35 days in vitro (DIV). Under activity blockade the expression of tau reaches a maximum by 21 DIV and decreases dramatically, so that the protein is hardly detectable by 47 DIV. MAP-2-immunoreactive proteins are localized in somata and dendrites, but also persist in axons. The expression in OTCs of p38 and GAP-43 correlates well with the expression observed in vivo. Synaptophysin (p38) occurs with a similar time course and amount in OTCs as in cortex in vivo. Synaptic boutons appear in all layers, and specialized terminal elements have been observed. Activity blockade slightly affects the p38 expression, although the late postnatal decline in p38 immunoreactivity observed on protein blots from cortex in vivo and in normal OTCs appears more accentuated in activity-blocked OTCs. The GAP-43 expression is prominent from birth onwards in vivo and in OTCs. However, in normal OTCs GAP-43 is not declining as it is in vivo, although it is downregulated in activity-blocked OTCs. As a major finding we report that neuronal markers which are normally expressed in immature neurons and axons during the period of differentiation and structural plasticity are continuously expressed in OTCs, suggesting that a monocultured cortex retains the ability for growth and structural changes longer than the cortex in vivo.

cDNA cloning and expression of rsca1, the rat counterpart of the human spinocerebellar ataxia type 1 gene

Spinocerebellar ataxia type 1 (SCA1) is a neurodegenerative disorder caused by an expanded and unstable (CAG) > 40 repeat within a gene of unknown function. We isolated the complete coding region of the rat SCA1 gene (rsca1), the 5'-untranslated region (UTR) and 1.3 kb of the 3'-UTR. The rat sequence exhibits 90% peptide identity to the human counterpart. In comparison to human, the rat (CAG)n block is reduced to two trinucleotide motifs preceded by three different proline codons not present in man. Furthermore, we investigated the expression of rsca1 in different rat tissues. The rsca1 gene is predominantly expressed in brain throughout all developmental stages. In situ hybridizations reveal high levels of expression in various regions of the adult rat brain, including cerebellum, hippocampus and cortex.

Areal differences of NPY mRNA-expressing neurons are established in the late postnatal rat visual cortex in vivo, but not in organotypic cultures

In order to learn about the factors regulating the postnatal development of neocortical peptidergic neuron populations, we have analysed neurons expressing neuropeptide Y (NPY) by immunohistochemistry and in situ hybridization in developing and adult rat visual cortical areas 17 and 18a in vivo, and in organotypic slice cultures of rat visual cortex. For quantitative analysis, the percentage of NPY mRNA-expressing neurons was determined in supragranular layers I-IV, in infragranular layers V and VI and in the white matter. In vivo, this percentage increased in visual areas 17 and 18a until postnatal day 21 in supra- and infragranular layers. Initially, in both areas the neurons were about equally distributed in supra- and infragranular layers (a ratio of 1:1). During the second postnatal month, the percentage of NPY mRNA-expressing neurons in area 18a declined by approximately 50% in both supra- and infragranular layers, so that the ratio of 1:1 remained constant. In contrast, in area 17 the percentage of neurons in supragranular layers remained fairly constant, but it declined to 50% in infragranular layers, so that by postnatal day 70 the ratio was gradually shifted to 2:1. Throughout development, area 18a contained significantly more NPY mRNA-expressing neurons than area 17. In organotypic slice cultures, a high density of NPY mRNA-expressing neurons had appeared by 10 days in vitro. A much higher percentage of neurons expressed NPY mRNA. The ratio of labelled neurons in supra- versus infragranular layers was 1:1. Both ratio and percentage remained constant from 10-85 days in vitro. The decline in vivo was not caused by an elimination of transient cell types. All cell types persisted into adulthood. Four NPY peptide-immunoreactive neuronal types were classified by axonal morphology in organotypic slice cultures and in vivo; they include (i) cells in layer VI/white matter with horizontal axons and ascending collaterals, (ii) cells in layers V/VI with descending axon and horizontal collaterals, (iii) Martinotti cells in layers V/VI with ascending axons, and (iv) cells in layers III-V with columnar axons. Two further types, bipolar cells with axons descending from dendrites and small basket cells with short horizontal axons, both found in vivo in layers II/III, could not be unequivocally identified in organotypic slice cultures. The NPY-immunoreactive neuron types had already formed a dense innervation of the cultures by 10 days in vitro, which remained stable for up to 85 days in vitro, and resembled the innervation observed in vivo. NPY peptide-immunoreactive neurons in organotypic slice cultures and in vivo were distributed in cortical layers II/III, V and VI and the white matter, but rarely in layers I and IV, which corresponded to the distribution of NPY mRNA-expressing neurons. However, with in situ hybridization more neurons were detectable, especially in layers II/III. A majority of NPY mRNA-expressing neurons co-localized NPY peptide, somatostatin and calbindin. We conclude that intrinsic cues were sufficient to drive the molecular expression of the NPY phenotype, the morphological differentiation and the stabilization of an organotypic NPY innervation in organotypic slice cultures. However, the area- and lamina-specific changes observed in vivo were not observed under monoculture conditions.

Cellular and molecular characterization of a brain-enriched protein tyrosine phosphatase

Regional variations in the expression of a striatal enriched protein tyrosine phosphatase called STEP were studied in the adult rat brain by a combination of immunocytochemistry, lesion studies, Western blotting, and in situ hybridization. Monoclonal antibodies generated against STEP identified multiple polypeptides of M(r) 46, 37, 33 and a doublet of M(r) 64-66 kDa on Western blots. Although the three STEP immunoreactive bands with lower molecular weights were enriched in cytosolic fractions, the 64-66 kDa doublet was enriched in membrane fractions. All of the immunoreactive forms were abundant in the caudate-putamen and were present in lower amounts or were undetectable in other brain regions. In substantia nigra, the M(r) 64-66 kDa doublet was not detected but bands with M(r) 46, 37, and 33 kDa were present. Immunocytochemical and lesion experiments demonstrated that the cytosolic STEP isoforms present in the substantia nigra are in presynaptic axons originating from the projection neurons of the caudate putamen, which innervate this structure. Additional in situ hybridization studies showed that STEP mRNA expression patterns correlate with the patterns of immunocytochemical staining. These findings indicate that there are multiple polypeptide isoforms of STEP enriched in the basal ganglia and related structures which differ in terms of their intracellular locations and functional roles.

Oligodendrocytes differentiate in organotypic cultures of rat visual cortex and myelinate efferent axons

We have investigated the presence and function of glia cells, especially of oligodendrocytes (OL) in organotypic cultures of rat visual cortex grown for 1-6 weeks in vitro. OL identified by strong Galactocerebroside-immunoreactivity (GalC-ir) displayed rather small somata and elaborately ramified processes. They were most concentrated in layers VIa and VIb and the remnant of the white matter. Silver staining revealed long descending or oblique processes in layers V and VI, which were often arranged in patches, and horizontal processes in the white matter. Proximal processes of OL cell bodies were connected to these long processes. DiI-labeling revealed very similar patches of processes, termed OL domains. They were identified as membraneous sheaths formed by processes of single OL around axons passing the OL domain. Confocal microscopy revealed single axons running through the membrane sheaths. We compared the molecular differentiation of glial cells in cultures to the in vivo situation with protein blots and immunohistochemistry for glial cell marker molecules. In homogenates of visual cortex in vivo, protein blots revealed the increase in expression by OL of myelin basic protein (MBP) during the fourth postnatal week. The astrocytic marker glial fibrillary acidic protein (GFAP), blotted as a control, increased over time in vivo, beginning at P14, indicating the differentiation of astrocytes. In homogenates of organotypic cortex cultures, the times course of expression of GFAP was very similar: it increased dramatically during the first 10 DIV, and remained fairly constant in older cultures.(ABSTRACT TRUNCATED AT 250 WORDS)

The SP1 antigen in subplate neurons of the developing cat cortex is an immunoglobulin-like molecule

Monoclonal antibody subplate-1 (mAb SP1) specifically stains somata, dendrites and axons of spiny inverted pyramidal neurons in the subplate zone in the early postnatal kitten neocortex. The SP1 antigen has been previously identified as a cytosolic protein of apparent molecular weight 56 kDa. We have now employed immune-affinity chromatography to further characterize this antigen. An antigen with SP1-like immunoreactivity (ir) is present in various organs, and is particularly enriched in blood plasma. Exsanguination of the organs prior to protein extraction reduces the SP1-ir band dramatically, indicative of a blood-borne molecule. The 56 kDa SP1-ir antigen was purified from plasma by affinity chromatography and subjected to Edman degradation. The first 20 N-terminal amino acids show 80% homology to the N-terminus of immunoglobulin heavy chain of man, the mouse and the dog. If the 56 kDa SP1-ir antigen in plasma is an immunoglobulin, and if an immunoglobulin-like molecule is present in the subplate, then antisera against cat immunoglobulins should stain subplate neurons. A polyclonal antiserum against cat IgG intensely stains the somata and dendrites of subplate neurons. On protein blots, this antiserum recognizes the 56 kDa band, and an additional band of approximately 27 kDa, corresponding in size to immunoglobulin light chains. Preabsorbing mAb SP1 with cat immunoglobulin G abolishes the immunoreactivity in sections of kitten cortex. Further, it dramatically reduces the reactivity on protein blots. The results suggest that the 56 kDa SP1-ir antigen in cortical subplate neurons belongs to the immunoglobulin superfamily.

Inverted pyramidal neurons and interneurons in cat cortical subplate zone are labelled by monoclonal antibody SP1

During development, the subplate zone of the cat neocortex contains neuronal populations with distinct morphological and neurochemical phenotypes. A subset of those are specifically recognized by a mouse monoclonal antibody termed SUBPLATE-1 (SP1), which was generated against tissue homogenates of kitten cortical white matter. SP1 stains cell bodies and proximal dendrites, but rarely distal dendrites, axonal arbors or spines. In order to characterize morphologically the SP1 immunoreactive subplate cell types, we combined SP1 immunohistochemistry with intracellular iontophoretic injections of Lucifer yellow. The majority of double-labelled neurons were inverted pyramids with a single thicker spine-covered dendrite that descended into the white matter and a tuft of thinner spinous dendrites that ascended from the upper somatic pole, but generally remained confined to the white matter. Other double-labelled neurons were multipolar to bitufted, although often equipped with one thicker descending dendrite. In inverted pyramidal cells, the axons originated from the descending dendrite or, more rarely, from the lower portion of the soma, and descended into the white matter. They formed collaterals recurring toward the grey matter. The presence of dendritic spines on double-labelled pyramidal cells and the axonal arborization patterns were two novel features not revealed previously by SP1 immunohistochemistry alone. The inverted pyramidal morphology was typical for double-labelled neurons located in the subplate zone below the apices of the gyri, whereas those located below the flanks or sulci or deep in the white matter often displayed a bitufted or multipolar spinous morphology. A minority of the double-labelled neurons were multipolar with smooth dendrites and locally branching axons. These results suggest that in the cat subplate zone, a majority of the cells expressing the SP1 antigen are spinous, and we termed the spinous subplate cells 'subplate pyramidal neurons'.

The topography of tangential inhibitory connections in the postnatally developing and mature striate cortex of the cat

Clustered intrinsic connections in the striate cortex of kittens originate from an unclustered, diffusely organized pattern prevailing during the first postnatal week. In order to study the participation of inhibitory neurons in this reorganization of the connections, we determined the topography of the inhibitory tangenital connections in the striate cortex of cats ranging in age between 7 and 330 days by combining retrograde transport of fluorescent microspheres with GABA immunohistochemistry. After small intracortical injections of tracer, neurons containing either microspheres only (non-GABAergic neurons) or GABA-like immunoreactivity in addition to microspheres (GABAergic neurons) are labelled at various horizontal distances from the injection. At the end of the first postnatal week, both GABAergic and non-GABAergic neurons are distributed in the horizontal plane in an unclustered fashion. During the second postnatal week, the tangential connections rearrange rapidly to form clusters. The tendency of the cells to form clusters is much weaker, however, in GABAergic than in non-GABAergic neurons. In regions > 500 microns distant from the centre of injection approximately 90% of the non-GABAergic neurons (range 87.5-92.6%) but only 63% (range 57.1-72.3%) of the GABAergic neurons reside within the clusters formed by the non-GABAergic neurons. These proportions do not change systematically with age. In the regions outside the non-GABAergic clusters, GABAergic neurons appear to be evenly distributed and not to aggregate in clusters. From postnatal day 7 forward GABAergic neurons largely retain their overall distribution and density in the horizontal plane. When considering all cortical layers (including the superficial white matter) the lateral spread of the GABAergic neurons is more restricted than that of the non-GABAergic neurons. Systematic changes in the lateral spread of inhibitory connections according to postnatal age were not observed. We conclude that, like the non-GABAergic neurons, the GABAergic neurons have attained an adult-like topography in the horizontal plane by about the end of the second postnatal week. From that time until adulthood they display much weaker clustering, a higher relative occurrence of short axon collaterals and a more restricted lateral distribution than do the excitatory neurons.

Neurotransmitter profile of saccadic omnipause neurons in nucleus raphe interpositus

Saccadic omnipause neurons (OPNs) are essential for the generation of saccadic eye movements. In primates OPNs are located near the midline within the nucleus raphe interpositus (rip). In the present study we used several different neuroanatomical methods to investigate the transmitters associated with OPNs in the monkey. Immunolabeling for the calcium-binding protein parvalbumin was employed to mark OPNs in the monkey and define the homologous cell group in cat and human. The use of antibodies against GABA, glycine (GLY), glutamate (GLU), serotonin (5-HT), and tyrosine hydroxylase revealed that the somata of OPNs are GLY immunoreactive, but they are devoid of GABA and 5-HT immunostaining. In situ hybridization with the GAD67 mRNA probe confirmed the negative GABA immunostaining of OPNs. 3H-GLY was injected into a projection field of OPNs, the rostral interstitial nucleus of the medial longitudinal fascicle (riMLF)--the vertical saccadic burst neuron area. This resulted in selective retrograde labeling of the OPNs in rip, while no labeling was found in the superior colliculus, which sends an excitatory projection to the riMLF. The somata and dendrites of putative burst neurons in the riMLF were contacted by numerous GLY-immunoreactive terminals. The quantitative analysis of immunoreactive terminal-like structures contacting OPNs revealed a strong input from GLY- and GABA-positive terminals on somata and dendrites, whereas GLU-positive puncta were mainly confined to the dendrites. Very few 5-HT and catecholaminergic terminals contacted OPN somata. Our findings suggest that OPNs use GLY as a neurotransmitter, and they receive numerous contacts from GABAergic, glycinergic, and glutaminergic afferents, and significantly fewer from monoaminergic inputs.

LGN-projecting neurons of the cat's pretectum express glutamic acid decarboxylase mRNA

There have been conflicting reports on the chemical nature of the projection of the pretectal nuclei [nucleus of the optic tract and dorsal terminal nucleus of the accessory optic tract (NOT-DTN complex) and posterior pretectal nucleus] to the lateral geniculate nucleus and inferior olive. There is evidence that the pretecto-geniculate pathway is inhibitory. However, most attempts to verify the GABAergic nature of the projection neurons have failed. In order to answer this question, we employed a combination of retrograde transport and in situ hybridization. Rhodamine-labelled latex microspheres were injected into the electrophysiologically identified lateral geniculate nucleus. In addition, fluorescein-labelled latex microspheres were injected into the inferior olive. Retrograde axonal transport labelled large pretectal neurons. We then applied riboprobes specific for glutamic acid decarboxylase mRNA. We were able to demonstrate glutamic acid decarboxylase mRNA expression in up to 70% of lateral geniculate nucleus-projecting NOT-DTN and posterior pretectal nucleus neurons but in none of the pretecto-olivary projection neurons. The results suggest that the pretecto-geniculate projection is GABAergic in nature, which would confirm previous electrophysiological and morphological observations. The pretecto-olivary projection is not GABAergic.

Characterization of neurochemical phenotypes in cultured hypothalamic neurons with immunohistochemistry and in situ hybridization

The expression of neurochemical phenotypes was studied in long-term cultures of dissociated embryonic neurons from rat hypothalamus. With time in culture, these neurons establish a complex dendritic and axonal network, as indicated by staining with antibodies against microtubulin-associated protein (MAP2) and neurofilaments (SMI32 and SMI33) as well as GABA and glutamate decarboxylase mRNA immunoreactivity. Neurons expressing neuropeptide Y (NPY) mRNA and NPY peptide and opioid-like peptides as well as vasopressin were observed. Further, weakly acetylcholinesterase- and NADPH diaphorase (nitric-oxide synthase)-labelled neurons were present. In conclusion, the neurochemical phenotypes reported for hypothalamic neurons in vivo can be observed in these cultures. This indicates that the culture conditions allow morphological and molecular differentiation. These findings support the view that long-term hypothalamic cultures provide a valuable model for studying mechanisms of neurosecretion in hypothalamic networks.

Differential regulation of substance P and somatostatin in Martinotti cells of the developing cat visual cortex

In order to determine their morphological development and ontogenetic fate, Martinotti neurons immunoreactive for substance P and somatostatin have been analysed in the cat visual cortex. Martinotti neurons are located in layers V and VI. They are multipolar to bitufted, and most dendrites remain in layers V and VI. Their typical features is the ascending axon, which emerges from an apical dendrite or from the upper pole of the soma. A number of collaterals branch off in layer V, forming a local terminal plexus. The axon then branches into 2-8 collaterals, which ascend as a bundle to layers III and II, where a second terminal plexus is formed. Some collaterals ascend to layer I where they adopt a horizontal course. Horizontal collaterals in the terminal layers V, III, II, and in layer I may reach up to 400 microns in length. Martinotti neurons begin to differentiate perinatally. The quantitative analysis reveals that the initial time course of differentiation of Martinotti cells is very similar in material stained for substance P and for somatostatin. Double immunofluorescence then confirms that the two peptides are colocalized in Martinotti cells of layers V and VI during the early postnatal period. Further, substance P is colocalized with GABA. Substance P expression in Martinotti cells can be observed only in the immature visual cortex. After postnatal day 15, the Martinotti neuron system becomes less and less detectable by substance P immunoreactivity. It declines to virtually undetectable levels after the third postnatal month. The adult visual cortex is almost devoid of substance P-immunoreactive cell bodies, processes and axon terminals. In situ hybridization confirms this finding, revealing beta-preprotachykinin mRNA-expressing cell bodies in layers V and IV at postnatal day (P)6 and P12, but not in the adult cortex. This suggests a downregulation of the substance P expression at the transcriptional level. In contrast, somatostatin-immunoreactive Martinotti cells, most of which have coexpressed substance P during early postnatal life, can still be observed in the adult cortex. Thus, the Martinotti neurons constitute a persisting cell type, although many individual neurons of this type disappear during the second postnatal month by degeneration and cell death. In summary, while somatostatin is permanently expressed in Martinotti neurons in the cat visual cortex, substance P peptide and mRNA are transiently expressed during an early postnatal period, and apparently are downregulated later in development.

Distribution of neurons expressing substance P receptor messenger RNA in immature and adult cat visual cortex

We have investigated the expression of messenger RNA (mRNA) encoding substance P receptor (SPR) in the visual cortex of adult cats and 17-day-old kittens, using in situ hybridization histochemistry with two digoxigenin-labeled oligodeoxynucleotides complementary to the SPR mRNA. In the adult cortex, a subset of large pyramidal neurons of layer V and layer III is heavily labeled. Other, mainly pyramidal neurons in layers II, III and V are less intensely labeled, but most neurons in these layers appear unlabeled. Neurons in layer IV and VI, and in the white matter do not show hybridization signals above background levels. In the 17-day-old kitten, SPR mRNA-expressing cells are confined to layer V and to the upper white matter (subplate zone), whereas supragranular neurons do not yet contain SPR mRNA. A few neurons in layer VI display moderate labeling. Astrocytes, identified with anti-glial fibrillary acid protein antibodies, did not express detectable levels of SPR mRNA in both adult and kitten visual cortex. These results indicate that SPR mRNA expression is transient in neurons of the white matter, and is developmentally regulated in supragranular layers. In addition, the localization of SPR mRNA in a subset of pyramidal cells suggests that substance P modulates the excitability of certain projection neurons which are the origin of extrinsic connections.

A method of in situ hybridization combined with immunocytochemistry, histochemistry, and tract tracing to characterize the mRNA expressing cell types in heterogeneous neuronal populations

A rapid, sensitive, non-isotopic in situ hybridization histochemistry protocol is presented to study the expression of mRNA at the single cell level in anatomically complex structures of the mammalian central nervous system. The protocol uses digoxigenin-UTP-labeled riboprobes, freefloating sections, and alkaline phosphatase and horseradish peroxidase detection. Modifications have been introduced which preserve the integrity of marker molecules, and as such enable the simultaneous identification and characterization of CNS cell types by tract tracing, histochemical, and immunocytochemical detection of intra- and extracellular markers. All pretreatments that enhance probe penetration have been omitted without substantial loss in sensitivity. The protocol has been successfully extended to vibratome sections with subsequent plastic-embedding and semithin sectioning, which considerably broadens the general applicability of this fast and easy ISHH method.

Morphology of neurons in the white matter of the adult human neocortex

Neurons in the human cerebral cortical white matter below motor, visual, auditory and prefrontal orbital areas have been studied with the Golgi method, immunohistochemistry and diaphorase histochemistry. The majority of white matter neurons are pyramidal cells displaying the typical polarized, spiny dendritic system. The morphological variety includes stellate forms as well as bipolar pyramidal cells, and the expression of a certain morphological phenotype seems to depend on the position of the neuron. Spineless nonpyramidal neurons with multipolar to bitufted dendritic fields constitute less than 10% of the neurons stained for microtubule associated protein (MAP-2). Only 3% of the MAP-2 immunoreactive neurons display nicotine adenine dinucleotide-diaphorase activity. The white matter pyramidal neurons are arranged in radial rows continuous with the columns of layer VI neurons. Neuron density is highest below layer VI, and decreases with increasing distance from the gray matter. White matter neurons are especially abundant below the primary motor cortex, and are least frequent below the visual cortex area 17. In contrast to other mammalian species, the white matter neurons in man are not only present during development, but persist throughout life.

Distribution and morphology of substance P-immunoreactive structures in the olfactory bulb and olfactory peduncle of the common marmoset (Callithrix jacchus), a primate species

The present study describes the morphology and distribution of substance P-immunoreactive (SP-ir) elements in the olfactory bulb (OB) and olfactory peduncle (OP) of the common marmoset (Callithrix jacchus), a primate species. SP-ir neurons are very abundant in the OB and belong to two types. External tufted cells are present in the glomerular layer (GL), whereas granule cells are found in the deeper layers, especially in the granule cell layer (GRL), but also scattered in the OP. SP-ir fibers, putatively of central origin, were identified in the OP. They ascend into the bulbar layers. The SP-chemoarchitecture of the marmoset OB and OP does not differ more from rat, guinea pig and cat, than the SP-chemoarchitecture of these species varies among one another.

The contribution of GABA-ergic neurons to horizontal intrinsic connections in upper layers of the cat's striate cortex

The contribution of neurons containing gamma-aminobutyric acid (GABA) to horizontal intrinsic projections in layers I-III of cat's striate cortex was investigated by combining GABA-immunohistochemistry with axonal tracing. After intracortical injections of Rhodamine-labelled latex microspheres Rhodamine-labelled neurons form patch- or bandlike aggregations (clusters) separated from each other by regions containing fewer, evenly distributed or no labelled neurons. Of the Rhodamine-labelled neurons about 5% display GABA-immunoreactive material (double labelled = DL-neurons). Approximately 70% of the DL-neurons occur at distances of less than 1 mm, and the remaining 30% at distances between 1 mm and 2.5 mm from the injection. About 60% of the DL-neurons reside within clusters and 40% are located in regions between clusters; the respective percentages of the Rhodamine labelled GABA-negative neurons are about 85 and 15. Considering their small number and their spatial distribution inhibitory interneurons seem to make only minor contributions to the clustered pattern of intrinsic connections. Our results demonstrate that the topographical organization of neurons giving origin to lateral inhibitory interactions in upper layers of cat's striate cortex is different from that of neurons mediating excitatory functions.

Substance P- and opioid-immunoreactive structures in olfactory centers of the cat: adult pattern and postnatal development

Substance P (SP)-ir and opioid-ir structures were studied in the cat main olfactory bulb (MOB), accessory olfactory bulb (AOB), and olfactory peduncle. In the MOB, the opioid-ir and the majority of the SP-ir neurons belong to the granule cell type. SP-ir granule cells reside in the deeper granule cell layer, whereas opioid-ir granule cells reside in the superficial granule cell layer, internal plexiform, and mitral cell layer. Many granule cells are observed in the external plexiform and glomerular layer. Other granule cells were found in the bulbar/peduncular white matter, the taenia tecta, and the genu of the corpus callosum. A new substance P-ir cell type was identified in the glomerular layer. This cell type was also identified by using the technique of intracellular injection of Lucifer Yellow. The cell type corresponds neither to the external tufted type nor to the short axon cell types described so far. The AOB resembles the MOB with respect to large numbers of SP-ir and opioid-ir granule cells. In addition, a few opioid-ir neurons, probably superficial mitral cells, were found in the glomerular layer. The AOB is surrounded by islands of immunoreactive granule cells, which connect to the granule cell layer by extremely long processes. Opioid-ir and SP-ir beaded axons pass through the olfactory peduncle terminating on granule cells, and ascend as far as the glomerular layer. All subdivisions of the anterior olfactory nucleus (AON) contain immunoreactive terminal fields. Afferent fibers and terminal plexuses derive from a population of immunoreactive neurons located predominantly in the region of the septo-olfactory junction. They have large somata. Their axons form recurrent collaterals, some of which run rostrally in the peduncular white matter. Others ascend caudally towards the septal region. The fibers seem to remain ipsilaterally, since the olfactory limb of the anterior commissure and the commissure proper are devoid of SP-ir and opioid-ir fibers. During development SP and opioid immunoreactivity were found only in differentiated granule cells. The peptides were not detectable in migrating or immature granule cells, as identified in Golgi-impregnated material. The granule cell population largely develops during postnatal life. The number of opioid-ir granule cells increases slowly and continuously, reaching the adult level not before the sixth postnatal month. Strikingly, SP-ir granule cell number increases fast and reaches a transient peak during the second month. Thereafter it declines (40% decrease) to the adult density, which is similar to that of opioid-ir granule cells.

VIP- and PHI-immunoreactivity in olfactory centers of the adult cat

The purpose of the study was to determine the morphology and distribution of vasoactive intestinal polypeptide- and peptide histidine isoleucine-immunoreactive (VIP- and PHI-ir) neurons and innervation patterns in the main and accessory olfactory bulb, anterior olfactory nucleus, and piriform cortex of the adult cat. In these centers, VIP- and PHI-immunoreactive material are present in the same neuronal types, respectively, therefore summarized as VIP/PHI-ir neurons. In the main olfactory bulb, the majority of VIP/PHI-ir neurons are localized in the external plexiform layer. These neurons give rise to two or more locally branching axons. They form boutons on mitral and external tufted cell bodies. According to the morphology and location, we have classified these neurons as Van Gehuchten cells. Some VIP/PHI-ir neurons are present in the glomerular layer. They have small somata and give rise to dendrites branching exclusively into glomeruli. We have classified these neurons as periglomerular cells. In the granule cell layer, neurons with long apical dendrites and one locally projecting axon are present. In the accessory olfactory bulb, VIP/PHI-ir neurons are localized in the mixed external/mitral/internal plexiform layer. They represent Van Gehuchten cells. In the anterior olfactory nucleus and piriform cortex, VIP/PHI-ir bipolar basket neurons are present. They are localized mainly in layers II/III. These neurons are characterized by a bipolar dendritic pattern and by locally projecting axons forming basket terminals on large immunonegative cell somata. Because of their common morphological features, we summarize them as the retrobulbar VIP/PHI-ir interneuron population. The PHI-ir neurons display the same morphology as the VIP-ir cells. However, they are significantly lower in number with a ratio of VIP-ir to PHI-ir cells about 2:1 in the main and accessory olfactory bulb and in the anterior olfactory nucleus. By contrast, in the piriform cortex the ratio is about 1:1.

Morphology of neuropeptide Y-immunoreactive neurons in the cat olfactory bulb and olfactory peduncle: postnatal development and species comparison

The distribution and morphology of Neuropeptide Y-immunoreactive (NPY-ir) neurons in the olfactory bulb and the olfactory peduncle was studied in the adult cat and rat, and the common marmoset Callithrix jacchus. Significant species differences were not observed. In all three species, the population of NPY-ir neurons is localized in the white matter extending from the main olfactory bulb to the border of the striatum. The neurons are characterized by a conspicuously looping axonal ramification pattern with some major collaterals running toward the olfactory bulb and others running toward the internal olfactory tract. The former, ipsilateral projection terminates in the granule cell layer of the main and accessory olfactory bulb and in layer II/III of the anterior olfactory nucleus. Reconstruction of the latter projection has revealed that the fibers are continuous with the olfactory limb of the anterior commissure and the anterior commissure proper suggesting a commissural contralateral projection. The analysis of the postnatal development of the cat NPY neuron system supports this assumption in a very clear-cut way. In young animals growing fibers are observed to cross the brachium of the commissure. The NPY neuron system develops postnatally. The maximum cell number is reached during the third postnatal week. The appearance of more and more NPY-ir neurons slightly precedes the formation of the terminal fields and of the fiber projection in the internal olfactory tract. The density of this early fiber projection by far exceeds the fiber density observed in the adult. Later in development the fiber density in the olfactory limb and the anterior commissure becomes considerably reduced. In contrast, the plexus density in the anterior olfactory nucleus and the granule cell layer of the main and accessory olfactory bulb undergoes only a slight reduction, and the NPY-ir cell number remains roughly constant. These observations suggest that the ipsilateral NPY-ir projection remains largely unchanged, in contrast to the contralateral projection, which exists to a large extent only for the first four postnatal months. The observation that the NPY neuron system gives rise to a contralateral projection does not support a classification of NPY neurons as short axon cells.

Early postnatal development of vasoactive intestinal polypeptide- and peptide histidine isoleucine-immunoreactive structures in the cat visual cortex

The early postnatal development of neurons containing vasoactive intestinal polypeptide (VIP) and peptide histidine isoleucine (PHI) has been analyzed in visual areas 17 and 18 of cats aged from postnatal day (P) 0 to adulthood. Neuronal types are established mainly by axonal criteria. Both peptides occur in the same neuronal types and display the same postnatal chronology of appearance. Several cell types are transient, which means that they are present in the cortex only for a limited period of development. According to their chronology of appearance the VIP/PHI-immunoreactive (ir) cell types are grouped into three neuronal populations. The first population comprises six cell types which appear early in postnatal life. The pseudohorsetail cells of layer I possess a vertically descending axon which initially gives rise to recurrent collaterals, then forms a bundle passing layers III to V, and finally, horizontal terminal fibers in layer VI. The neurons differentiate at P 4 and disappear by degeneration around P 30. The neurons with columnar dendritic fields of layers IV/V are characterized by a vertical arrangement of long dendrites ascending or descending parallel to each other, thus forming an up to 600 microns long dendritic column. Their axons always descend and terminate in broad fields in layer VI. The neurons appear at P 7 and are present until P 20. The multipolar neurons of layer VI occur in isolated positions and have broad axonal territories. The neurons differentiate at P 7 and persist into adulthood. Bitufted to multipolar neurons of layers II/III have axons descending as a single fiber to layer VI, where they terminate. The neurons appear at P 12 and persist into adulthood. The four cell types described above issue a vertically oriented fiber architecture in layers II-V and a horizontal terminal plexus in layer VI which is dense during the second, third and fourth week. Concurrent with the disappearance of the two transient types the number of descending axonal bundles and the density of the layer VI plexus is reduced, but the latter is maintained during adulthood by the two persisting cell types. Two further cell types belong to the first population: The transient bipolar cells of layers IV, V, and VI have long dendrites which extend through the entire cortical width. Their axons always descend, leave the gray matter, and apparently terminate in the upper white matter. The neurons differentiate concurrently with the pseudohorsetail cells at P 4, are very frequent during the following weeks, and eventually disappear at P 30.(ABSTRACT TRUNCATED AT 400 WORDS)