Ultrastructural organization of slice cultures from rat visual cortex

Development of membrane properties of rat neocortical neurons studied in organotypic cocultures

Development of membrane properties of layer IV neocortical neurons, which receive thalamic inputs, and other layer neurons, which receive cortical inputs, were studied in organotypic cocultures of a rat visual cortical slice with either a block of the lateral geniculate nucleus or another cortical slice. In all types of cells, the membrane time constant, input resistance and spike width decreased markedly with days in culture whereas maximum rate of rise of the spike increased. Except for layer V/VI neurons, spike height and resting potentials also tended to increase during development. These changes were more prominent in superficial neurons regardless of afferent connectivity. The results suggest that the membrane properties of cortical neurons develop primarily based on laminar specificity.

Glial expression of the proenkephalin gene in slice cultures of the subventricular zone

The proenkephalin (PEnk) gene is expressed in rats in the neocortical subventricular zone (nSVZ) of the lateral ventricle during the first postnatal week, when precursors of astro- and oligoglial cells of the rat neocortex proliferate in this area. To study the expression of the gene in the glial precursors, slices containing the nSVZ were prepared from the brains of newborn and 7-day-old rats. After 1-5 d of cultivation, numerous cells that expressed PEnk mRNA were found in the nSVZ with in situ hybridization. Some of these cells coexpressed the glial fibrillary acidic protein (GFAP), indicating that they were of astroglial origin. Activation of protein kinase A with 8Br.cAMP strongly enhanced the number of cells that expressed the PEnk gene in slices prepared from the brains of newborn or 7-d-old rats. Also pituitary adenylate cyclase activating polypeptide (PACAP) proved to be effective. After stimulation with 8Br.cAMP or PACAP-38, PEnk mRNA-containing cells were found in the subventricular zone as well as in the adjacent area through which glial cells migrate on their way to the neocortex. It has therefore been concluded that protein kinase A may regulate the expression of the PEnk gene expression in glial precursors in the nSVZ.

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.

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.

Activity-dependent regulation of dendritic spine density on cortical pyramidal neurons in organotypic slice cultures

In order to examine the effects of activity on spine production and/or maintenance in the cerebral cortex, we have compared the number of dendritic spines on pyramidal neurons in slices of P0 mouse somatosensory cortex maintained in organotypic slice cultures under conditions that altered basal levels of spontaneous electrical activity. Cultures chronically exposed to 100 microM picrotoxin (PTX) for 14 days exhibited significantly elevated levels of electrical activity when compared to neurons in control cultures. Pyramidal neurons raised in the presence of PTX showed significantly higher densities of dendritic spines on primary apical, secondary apical, and secondary basal dendrites when compared to control cultures. The PTX-induced increase in spine density was dose dependent and appeared to saturate at 100 microM. Cultures exhibiting little or no spontaneous activity, as a result of growth in a combination of PTX and tetrodotoxin (TTX), showed significantly fewer dendritic spines compared to cultures maintained in PTX alone. These results demonstrate that the density of spines on layers V and VI pyramidal neurons can be modulated by growth conditions that alter the levels of spontaneous electrical activity.

Astrocytes alter their polarity in organotypic slice cultures of rat visual cortex

The ultrastructure of astrocytes in an organotypic slice culture of the rat visual cortex was investigated using ultrathin sections and freeze-fracture replicas. After a culture period of 9-15 days, a glial scaffold formed that separated the bulk of the slice neuropil from the medium and the underlying plasma clot. However, the glial cells and processes did not build a dense barrier but allowed the outgrowth of neurites. A basal lamina covering the medium-oriented surface of the astrocytes was not found. In freeze-fracture replicas, orthogonal arrays of particles (OAP) were characteristic components of astrocytic membranes. The OAP density in membranes bordering the medium was 35 +/- 13 OAP/microns 2, corresponding to 2.5% of this membrane area; the OAP density in membranes within the slice neuropil was 22 +/- 12 OAP/microns 2, corresponding to 1.4% of this membrane area. Although the difference was significant, it was greatly reduced when comparing OAP densities in endfoot and non-endfoot membranes in vivo. Another node of polarity was recognized in astrocytes of the organotypic slice culture. In membranes of astrocytes bordering upon the medium, the density of non-OAP intramembranous particles (IMP) was clearly higher (1130 +/- 136 IMP/microns 2) than in membranes of astrocytes in the center of the slice (700 +/- 172 IMP/microns 2). This pronounced IMP-related polarity was observed neither in vivo nor in cultured astrocytes. The present study suggests, together with data from the literature, that the distribution of astrocytic OAP across the cell surface is influenced by the existence of a basal lamina and neuronal activity, and that astrocytes possess a more remarkable plasticity of membrane structure than previously suspected.

Cortical circuitry in a dish

In the mammalian neocortex, local connections as well as projections to and from the cortex are organized in cortical layers. Recent studies have demonstrated that the formation of the patterns of afferent and efferent cortical connections in organotypic co-cultures are similar to those observed in vivo. These findings provide some insights into the cellular strategies that operate during the development of layer-specific cortical connections.

Growth-promoting interactions between the murine neocortex and thalamus in organotypic co-cultures

The aim of this study was to assess whether developing cerebral cortex produces diffusible factors that can affect the growth of thalamic cells and, if so, what the role of these factors might be during the formation of thalamocortical connections. We studied interactions between cultured organotypic explants from mice maintained in defined serum-free medium. First, we cultured explants of embryonic dorsolateral thalamus in isolation from any other tissue; after culture, these explants were viewed intact and then sectioned. We estimated the numbers of healthy and pyknotic cells before and after culture, and the rates of mitosis in the explants during culture (using bromodeoxyuridine). Based on these data, we concluded that the majority of cells in the thalamic explants survived, although significant numbers of pyknotic cells did accumulate. Thalamic explants extended either very few or no neurites when cultured alone. We then cultured explants of embryonic thalamus near to explants from other tissues. A gap was always maintained between the explants, and we measured the length and density of neurite outgrowth from each thalamic explant. Slices of embryonic cortex promoted a small but significant increase in the amount of growth from thalamic explants. Postnatal cortex stimulated much more profuse neurite outgrowth; postnatal cerebellum had less of an effect, and postnatal medulla or liver had none. We showed that there was significantly more outgrowth from thalamic explants cultured in medium that had been preconditioned with cortical slices than from thalamic explants cultured in control medium, confirming that diffusible factors were produced by the cortex. The survival and mitotic rates of thalamic cells were unaffected by co-culture with the cortex. We conclude that the developing cortex releases diffusible factors that stimulate the growth of thalamic neurites and that other regions of the brain may also release the same substance(s). The lack of a specific source of thalamic growth promoting factor(s) argues against a role for these factors in guiding thalamic axons to specific targets; indeed, we were unable to demonstrate any chemotropic guidance of thalamic axons towards cortical explants in collagen gels. Since postnatal cortex has a more potent stimulatory effect than prenatal cortex, it seems possible that, in vivo, the cortical-derived factors act mainly on thalamocortical axons that have located their targets and are in the process of arborizing and refining their connections.

Aspects of early postnatal development of cortical neurons that proceed independently of normally present extrinsic influences

To examine the contribution of local versus extrinsic influences on postnatal development of cortical neurons, we compared the maturation of deep (infragranular) layer neurons in isolated slices of neocortex grown in organotypic culture to a similar population of neurons developing in vivo. All slice cultures were prepared from sensorimotor cortices of newborn mice (P0) and neurons in these cultures were examined at daily intervals during the first 9 days in vitro (DIV). The maturational state of neurons developing in vivo over this same time period was assessed in acute slices prepared from animals of equivalent postnatal age, P1-P9. Electrophysiological recordings were obtained from neurons in both cultured and acute slices, using Lucifer yellow filled whole-cell recording electrodes, enabling subsequent morphometric analysis of the labeled cells. We report significant changes in both cellular morphology and electrical membrane properties of these deep layer cortical neurons during the first week in culture. Morphological maturation over this time period was characterized by a two- to three-fold increase in cell body size and total process length, and an increase in dendritic complexity. In this same population of cells a three-fold decrease in input resistance and changes in the action potential waveform, including a two-fold decrease in the AP duration, also occur. The degree of morphological and electrophysiological differentiation of individual neurons was highly correlated across developmental ages, suggesting that the maturational state of a cell is reflected in both cellular morphology and intrinsic membrane properties. A remarkably similar pattern of neuronal maturation was observed in neurons in layers V, VI/SP examined in acute slices prepared from animals between P1-P9. Because our culture system preserves many aspects of the local cortical environment while eliminating normal extrinsic influences (including thalamic, brainstem, and callosal connections), our findings argue that this early phase of neuronal differentiation, including the rate and extent of dendritic growth and development of AP waveform, results from instructive and/or permissive local influences, and appears to proceed independently of the many normally present extrinsic factors.

Formation of synapses between basal forebrain afferents and cerebral cortex neurons: an electron microscopic study in organotypic slice cultures

Co-cultures of rat basal forebrain and cerebral cortex were maintained from 1 to 5 weeks in vitro with serum-free defined medium. The formation of synaptic connections between basal forebrain afferent fibres and cortical neurons was studied by specific labelling with three staining techniques, including (i) neuronal tract tracing with the fluorescent dye 1,1'-dioctodecyl-3,3,3'3'- tetramethylindocarbocyanine perchlorate, (ii) acetylcholinesterase histochemistry, and (iii) choline acetyltransferase immunocytochemistry. Both basal forebrain and cerebral cortex tissue displayed organotypic characteristics in culture. Cerebral cortex revealed a dense innervation by axonal projections from the basal forebrain. All three labelling techniques produced similar results at the light microscopic level, with densest innervation located in the marginal zone. At the fine structural level, the 1,1'-dioctodecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate-, acetylcholinesterase- and choline acetyltransferase-stained basal forebrain afferents all revealed a number of synaptic contacts with cortical neurons. The contacts displayed consistent synaptic features, including presynaptic accumulation of small round vesicles, cleft widening, and postsynaptic densities forming symmetric synapses. These morphological characteristics of connections formed in vitro are similar to basal forebrain cholinergic projections to cerebral cortex in normal brain. Based on these results, this tissue culture model appears to be an useful tool for investigations of the development of cholinergic innervation of cerebral cortex.

Reconstructing cortical connections in a dish

During development of the cortex, efferent projection neurons located in distinct cortical layers send their axons to different targets, and afferent fibers establish connections with cortical target cells of a particular layer. Recent studies have shown that layer- and cell-specific afferent and efferent cortical connections established in culture are similar to those observed in vivo. The results of these experiments provide evidence for the existence of diffusible and membrane-bound guidance factors for specific sets of axons. Furthermore, they suggest the use of different molecules to navigate axons towards their target, regulate target innervation and mediate target cell recognition.

Formation of specific efferent connections in organotypic slice cultures from rat visual cortex cocultured with lateral geniculate nucleus and superior colliculus

Cells in the cerebral cortex project to many distant regions in the brain. Each cortical target receives input from a specific population of cells which have a characteristic morphology and which are located in a distinct cortical layer. In an attempt to learn about the mechanisms by which this stereotypic output pattern is generated during development, we have studied the formation of cortical projections in an in vitro system. Slices from developing rat visual cortex were cocultured with slices from the superior colliculus, the major target of cells in layer 5, and the lateral geniculate nucleus, the major target of cells in layer 6. Cortical neurons which established connections with tectal and thalamic explants were retrogradely labelled with fluorescent dyes. It was found that, in vitro, different populations of neurons project to these two targets, and that the laminar position and cellular morphology of the projecting cells were similar to their in vivo counterparts. These specific connections were established when the target explants were placed either next to the white matter or next to the pial side of cortical slice cultures. The axons of cells projecting to ectopic positioned explants reoriented their trajectories and grew through the cortical grey matter directly towards their targets. Thus subcortical targets exert an orienting effect specifically on their innervating cells and attract growing axons of the appropriate cells at a distance. These results suggest that different targets release different molecules that act selectively on specific populations of neurons. Therefore, chemotropic guidance is likely to play a significant role in the development of specific connections between cortical neurons and their target areas.

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.