A primary sensory cortical interareal feedforward inhibitory circuit for tacto-visual integration

Tactile sensation and vision are often both utilized for the exploration of objects that are within reach though it is not known whether or how these two distinct sensory systems combine such information. Here in mice, we used a combination of stereo photogrammetry for 3D reconstruction of the whisker array, brain-wide anatomical tracing and functional connectivity analysis to explore the possibility of tacto-visual convergence in sensory space and within the circuitry of the primary visual cortex (VISp). Strikingly, we find that stimulation of the contralateral whisker array suppresses visually evoked activity in a tacto-visual sub-region of VISp whose visual space representation closely overlaps with the whisker search space. This suppression is mediated by local fast-spiking interneurons that receive a direct cortico-cortical input predominantly from layer 6 neurons located in the posterior primary somatosensory barrel cortex (SSp-bfd). These data demonstrate functional convergence within and between two primary sensory cortical areas for multisensory object detection and recognition.

Visual deprivation independent shift of ocular dominance induced by cross-modal plasticity

There is convincing evidence that the deprivation of one sense can lead to adaptive neuronal changes in spared primary sensory cortices. However, the repercussions of late-onset sensory deprivations on functionality of the remaining sensory cortices are poorly understood. Using repeated intrinsic signal imaging we investigated the effects of whisker or auditory deprivation (WD or AD, respectively) on responsiveness of the binocular primary visual cortex (V1) in fully adult mice. The binocular zone of mice is innervated by both eyes, with the contralateral eye always dominating V1 input over ipsilateral eye input, the normal ocular dominance (OD) ratio. Strikingly, we found that 3 days of WD or AD induced a transient shift of OD, which was mediated by a potentiation of V1 input through the ipsilateral eye. This cross-modal effect was accompanied by strengthening of layer 4 synapses in V1, required visual experience through the ipsilateral eye and was mediated by an increase of the excitation/inhibition ratio in V1. Finally, we demonstrate that both WD and AD induced a long-lasting improvement of visual performance. Our data provide evidence that the deprivation of a non-visual sensory modality cross-modally induces experience dependent V1 plasticity and improves visual behavior, even in adult mice.

Cross-modal restoration of ocular dominance plasticity in adult mice

The temporal closure of one eye in juvenile and young adult mice induces a shift of the ocular dominance (OD) of neurons in the binocular visual cortex. However, OD plasticity typically declines with age and is completely absent in matured mice beyond postnatal day (PD) 110. As it has been shown that the deprivation of one sensory input can induce neuronal alterations in non-deprived sensory cortices, we here investigated whether cross-modal interactions have the potential to reinstall OD plasticity in matured mice. Strikingly, using intrinsic signal imaging we could demonstrate that both whisker deprivation and auditory deprivation for only one week reinstated OD plasticity in fully adult mice. These OD shifts were always mediated by an increase of V1 responsiveness to visual stimulation of the open eye, a characteristic feature of OD plasticity normally only found in young adult mice. Moreover, systemic administration of the competitive NMDA receptor antagonist CPP completely abolished cross-modally induced OD plasticity. Taken together, we demonstrate here for the first time that the deprivation of non-visual senses has the potential to rejuvenate the adult visual cortex.

Cross-modal refinement of visual performance after brief somatosensory deprivation in adult mice

It is well established that the congenital lack of one sensory modality enhances functionality in the spared senses. However, whether a late onset deprivation of one sense leads to such alterations is largely unknown. Here, we investigated whether a somatosensory deprivation induced by bilateral whisker removal affects visual acuity and contrast sensitivity in fully adult mice. Using the visual cortex-dependent visual water task, we found that a brief somatosensory deprivation markedly improved behavioral visual acuity and contrast sensitivity by about 40%. Determining these attributes of vision using periodic optical imaging of intrinsic signals in the same mice revealed that visual cortex responses elicited by weak visual stimuli were massively increased after somatosensory deprivation. Strikingly, comparison of visual acuity and contrast sensitivity values determined by the visual water task and intrinsic signal imaging revealed that these measurements were almost identical, even at the level of individual animals. In summary, our results suggest that a brief manipulation of somatosensory experience profoundly boosts visual cortex-dependent vision in adults.

Homeostatic plasticity and synaptic scaling in the adult mouse auditory cortex

It has been demonstrated that sensory deprivation results in homeostatic adjustments recovering neuronal activity of the deprived cortex. For example, deprived vision multiplicatively scales up mEPSC amplitudes in the primary visual cortex, commonly referred to as synaptic scaling. However, whether synaptic scaling also occurs in auditory cortex after auditory deprivation remains elusive. Using periodic intrinsic optical imaging in adult mice, we show that conductive hearing loss (CHL), initially led to a reduction of primary auditory cortex (A1) responsiveness to sounds. However, this was followed by a complete recovery of A1 activity evoked sounds above the threshold for bone conduction, 3 days after CHL. Over the same time course patch-clamp experiments in slices revealed that mEPSC amplitudes in A1 layers 2/3 pyramids scaled up multiplicatively in CHL mice. No recovery of sensory evoked A1 activation was evident in TNFα KO animals, which lack synaptic scaling. Additionally, we could show that the suppressive effect of sounds on visually evoked visual cortex activity completely recovered along with TNFα dependent A1 homeostasis in WT animals. This is the first demonstration of homeostatic multiplicative synaptic scaling in the adult A1. These findings suggest that mild hearing loss massively affects auditory processing in adult A1.

Data on the effect of conductive hearing loss on auditory and visual cortex activity revealed by intrinsic signal imaging

This data article provides additional data related to the research article entitled “Simultaneous intrinsic signal imaging of auditory and visual cortex reveals profound effects of acute hearing loss on visual processing” (Teichert and Bolz, 2017) [1]. The primary auditory and visual cortex (A1 and V1) of adult male C57BL/6J mice (P120-P240) were mapped simultaneously using intrinsic signal imaging (Kalatsky and Stryker, 2003) [2]. A1 and V1 activity evoked by combined auditory and visual stimulation were measured before and after conductive hearing loss (CHL) induced by bilateral malleus removal. We provide data showing that A1 responsiveness evoked by sounds of different sound pressure levels (SPL) decreased after CHL whereas visually evoked V1 activity increased after this intervention. In addition, we also provide imaging data on percentage of V1 activity increases after CHL compared to pre-CHL.

Thalamic afferents influence cortical progenitors via ephrin A5-EphA4 interactions

The phenotype of excitatory cerebral cortex neurons is specified at the progenitor level, orchestrated by various intrinsic and extrinsic factors. Here, we provide evidence for a subcortical contribution to cortical progenitor regulation by thalamic axons via ephrin A5-EphA4 interactions. Ephrin A5 is expressed by thalamic axons and represents a high-affinity ligand for EphA4 receptors detected in cortical precursors. Recombinant ephrin A5-Fc protein, as well as ephrin A ligand-expressing, thalamic axons affect the output of cortical progenitor division in vitro. Ephrin A5-deficient mice show an altered division mode of radial glial cells (RGCs) accompanied by increased numbers of intermediate progenitor cells (IPCs) and an elevated neuronal production for the deep cortical layers at E13.5. In turn, at E16.5 the pool of IPCs is diminished, accompanied by reduced rates of generated neurons destined for the upper cortical layers. This correlates with extended infragranular layers at the expense of superficial cortical layers in adult ephrin A5-deficient and EphA4-deficient mice. We suggest that ephrin A5 ligands imported by invading thalamic axons interact with EphA4-expressing RGCs, thereby contributing to the fine-tuning of IPC generation and thus the proper neuronal output for cortical layers.

A dual role of EphB1/ephrin-B3 reverse signaling on migrating striatal and cortical neurons originating in the preoptic area: should I stay or go away?

During embryonic development the preoptic area (POA) gives rise to two populations of neurons which are generated at the same time, cortical interneurons and striatal cells. POA-derived cortical interneurons take a superficial path and avoid the developing striatum (Str) when they migrate to their target region. We found that EphB1, which is expressed in the striatal anlage, prevents cortical interneurons from entering the Str via ephrin-B3 reverse signaling. In contrast, for striatal neurons which also express ephrin-B3, EphB1 acts as a stop signal. This dual role of EphB1 is due to differences in ephrin-B3 reverse signaling cascades. For striatal neurons, binding of EphB1 to ephrin-B3 reduces endogenously high levels of pSrc and pFAK, which then causes the cells to stop migration. In contrast, in cortical interneurons EphB1-ephrin-B3 reverse signaling leads to phosphorylation of Src and focal adhesion kinase (FAK) which then mediates repulsion. Consistent with these in vitro findings, in an ephrin-B3 knockout mouse line, we discovered misrouted cortical interneurons in the Str and an over-migration of striatal neurons in their target region. Thus, EphB1/ephrin-B3 reverse signaling has a different impact on two sets of neurons which are generated at the same time and place: it can act as a repulsive cue for migrating neurons or it can terminate neuronal migration, a novel role of the Eph/ephrin system.

DISC1 knockdown impairs the tangential migration of cortical interneurons by affecting the actin cytoskeleton

Disrupted-in-Schizophrenia 1 (DISC1) is a risk gene for a spectrum of major mental disorders. It has been shown to regulate radial migration as well as dendritic arborization during neurodevelopment and corticogenesis. In a previous study we demonstrated through in vitro experiments that DISC1 also controls the tangential migration of cortical interneurons originating from the medial ganglionic eminence (MGE). Here we first show that DISC1 is necessary for the proper tangential migration of cortical interneurons in the intact brain. Expression of EGFP under the Lhx6 promotor allowed us to analyze exclusively interneurons transfected in the MGE after in utero electroporation. After 3 days in utero, DISC1 deficient interneurons displayed prolonged leading processes and, compared to control, fewer neurons reached the cortex. Time-lapse video microscopy of cortical feeder-layers revealed a decreased migration velocity due to a reduction of soma translocations. Immunostainings indicated that DISC1 is co-localized with F-actin in the growth cone-like structure of the leading process. DISC1 knockdown reduced F-actin levels whereas the overall actin level was not altered. Moreover, DISC1 knockdown also decreased levels of phosphorylated Girdin, which cross-links F-actin, as well as the Girdin-activator pAkt. In contrast, using time-lapse video microscopy of fluorescence-tagged tubulin and EB3 in fibroblasts, we found no effects on microtubule polymerization when DISC1 was reduced. However, DISC1 affected the acetylation of microtubules in the leading processes of MGE-derived cortical interneurons. Together, our results provide a mechanism how DISC1 might contribute to interneuron migration thereby explaining the reduced number of specific classes of cortical interneurons in some DISC1 mouse models.

EphA/ephrin A reverse signaling promotes the migration of cortical interneurons from the medial ganglionic eminence

Inhibitory interneurons control the flow of information and synchronization in the cerebral cortex at the circuit level. During embryonic development, multiple subtypes of cortical interneurons are generated in different regions of the ventral telencephalon, such as the medial and caudal ganglionic eminence (MGE and CGE), as well as the preoptic area (POA). These neurons then migrate over long distances towards their cortical target areas. Diverse families of diffusible and cell-bound signaling molecules, including the Eph/ephrin system, regulate and orchestrate interneuron migration. Ephrin A3 and A5, for instance, are expressed at the borders of the pathway of MGE-derived interneurons and prevent these cells from entering inappropriate regions via EphA4 forward signaling. We found that MGE-derived interneurons, in addition to EphA4, also express ephrin A and B ligands, suggesting Eph/ephrin forward and reverse signaling in the same cell. In vitro and in vivo approaches showed that EphA4-induced reverse signaling in MGE-derived interneurons promotes their migration and that this effect is mediated by ephrin A2 ligands. In EphA4 mutant mice, as well as after ephrin A2 knockdown using in utero electroporation, we found delayed interneuron migration at embryonic stages. Thus, besides functions in guiding MGE-derived interneurons to the cortex through forward signaling, here we describe a novel role of the ephrins in driving these neurons to their target via reverse signaling.

Integration of opposing semaphorin guidance cues in cortical axons

Previous work demonstrated that members of the semaphorin family, Sema3A and Sema3C, act as repulsive and attractive guidance signals, respectively, for cortical axons. During the development of corticofugal projections, these semaphorins are expressed in adjacent cortical zones, but there is a considerable overlap between Sema3A and Sema3C expression in the subventricular zone. We used different in vitro assays to examine the response of cortical axons exposed to defined mixtures of these opposing guidance cues. Results showed that even at very low concentrations, Sema3A overrides the effects of Sema3C. Moreover, experiments with function-blocking antibodies directed against neuropilin provided insights into how cortical axons integrate disparate guidance signals at the receptor level. These in vitro data suggest that the pathway of corticofugal axons is defined by an attractive cue in the intermediate zone, where Sema3C is expressed alone. To directly test this hypothesis in vivo, we performed axon-tracing experiments in Sema3C-deficient mice. Compared with wild-type animals, corticofugal axons take a more superficial route in Sema3C(-/-) mice, and the corticofugal pathway is more compacted. This phenotype is expected when an attractive cue for cortical axons, Sema3C, is eliminated and a repulsive cue, Sema3A, becomes predominant.

Ephrins guide migrating cortical interneurons in the basal telencephalon

Cortical interneurons are born in the proliferative zones of the ganglionic eminences in the subpallium and migrate to the developing cortex along well-defined tangential routes. The mechanisms regulating interneuron migration are not completely understood. Here we examine the role of class-A members of the Eph/ephrin system in directing the migration of interneurons. In situ hybridizations demonstrated that ephrin-A3 is expressed in the developing striatum, an area that is strictly avoided by migrating cortical interneurons in vivo, which express the EphA4 receptor. We then examined interneuron migration in grafting experiments, where explants of the medial ganglionic eminence (MGE) from enhanced green fluorescent protein-expressing transgenic mice were homotopically grafted into host slices from wildtype littermate embryos. After blocking ephrin-A ligands, many interneurons invaded the striatal anlage. Moreover, stripe assay experiments revealed that ephrin-A3 acts as a repellent cue for neurons from the medial ganglionic eminence. Downregulation of the EphA4 receptor via siRNA transfection reduced the repulsive effect of ephrin-A3, indicating that EphA4 mediates at least in part the repulsive effect of ephrin-A3 on these cells. Together, these results suggest that ephrin-A3 acts as a repulsive cue that restricts cortical interneurons from entering inappropriate regions and thus contributes to define the migratory route of cortical interneurons.

Structural alterations of spiny stellate cells in the somatosensory cortex in ephrin-A5-deficient mice

Previous studies demonstrated that in ephrin-A5-deficient mice corticothalamic arbors are reduced by more than 50% in layer 4 of the somatosensory cortex (S1), where ephrin-A5 is normally expressed. Here we examined possible consequences of the reduced thalamic input on spiny stellate cells, the target neurons of thalamocortical afferents. Using ballistic delivery of particles coated with lipophilic dyes in fixed slices and confocal laser-microscopy, we could quantitatively analyze the morphology of these neurons. Cells were examined in S1 at postnatal day 8 (P8), when thalamic afferents establish synaptic contacts and the dendrites of their target cells are covered with filopodia, and at P23, after synapse formation and replacement of filopodia by spines. Our results indicate that at P8 the dendrites of cells in mutant animals exhibit more filopodia and are more branched than dendrites of wildtype cells. In contrast, there is no difference in the extent of the dendritic tree between knockout and control animals. At P23, dendrites of neurons in ephrin-A5-deficient mice are still more branched, but possess fewer spines than wildtype cells. Thus, at early stages layer 4 neurons appear to compensate the reduced thalamic input by increasing dendritic branching and the density of filopodia. However, while at later stages the dendrites of layer 4 neurons in mutants are still more branched, their spine density is now lower than in wildtype cells. Taken together, these data demonstrate that the structure of spiny stellate cells is shaped by thalamic input and Eph receptor signaling.

Chondroitin sulfate acts in concert with semaphorin 3A to guide tangential migration of cortical interneurons in the ventral telencephalon

Chondroitin sulfate (CS) carrying proteoglycans (PGs) are widely expressed in the nervous system, and there is increasing evidence that they regulate developmental mechanisms like neurite outgrowth, axonal guidance and neuronal migration. Moreover, they can also act indirectly by organizing and/or modulating growth factors and guidance molecules. We found that chondroitin-4-sulfate is coexpressed with semaphorin 3A (Sema 3A) in the striatal mantle zone (SMZ), a nontarget region of neuropilin (Nrp)-1-expressing cortical interneurons flanking their migratory route in the subpallium. Using in vitro assays, we showed that CS PGs exert a repulsive effect on cortical interneurons, independently of Sema 3A, due to the CS side chains. We further showed that extracellular Sema 3A binds to CS. Disrupting Sema 3A-Nrp-1 signaling led migrating medial ganglionic eminence neurons to inappropriately invade the SMZ and even more so after removal of the CS side chains. Moreover, we found that soluble Sema 3A enhances the CS-induced repulsion in vitro. We concluded that CS acts as a repellent for cortical interneurons and that, in addition, CS restricts secreted Sema 3A within SMZ. Thus, both molecules act in concert to repel cortical interneurons from the SMZ during tangential migration toward the cerebral cortex.

Ephrin-A5 acts as a repulsive cue for migrating cortical interneurons

Cortical interneurons are born in the germinative zones of the ganglionic eminences in the subpallium, and migrate tangentially in spatially and temporally well-defined corridors into the neocortex. Because ephrin-A5 is expressed in the ventricular zone (VZ) of the ganglionic eminences at these developmental stages, we examined the possible effects of this molecule on interneuron migration. Double-immunocytochemistry of dissociated neurons from the medial ganglionic eminences (MGE) revealed that calbindin-positive cells express the EphA4-receptor. In situ, EphA4 is strongly expressed in the subventricular zone of the ganglionic eminences. Using different in vitro assays, we found that ephrin-A5 acts as a repellent cue for MGE neurons. We then examined interneuron migration in slice overlay experiments, where MGE-derived explants from enhanced green fluorescent protein-expressing transgenic mice were homotopically grafted into host slices from wild-type littermate embryos. In these in vitro preparations, interneurons recapitulated in vivo cell migration in several respects. However, interneurons in brain slices also migrated in the VZ of the ganglionic eminences, a region that is strictly avoided in vivo. In situ hybridizations revealed that ephrin-A5 became downregulated in the VZ in vitro. When recombinant ephrin-A5-Fc was added to the slices, it preferentially bound to the VZ, and migrating MGE neurons avoided the VZ as in vivo. The restoration of the normal migration pathway in slices required ephrin-A5 clustering and signalling of Src family kinases. Together, these experiments suggest that ephrin-A5 acts as an inhibitory flank that contributes to define the pathway of migrating interneurons.

Acetylcholine influences growth cone motility and morphology of developing thalamic axons

The neurotransmitter acetylcholine (ACh) is expressed in the developing telencephalon at the time when thalamic axons project to the cortex, long before synapses are being formed. Since previous studies demonstrated an influence of ACh on neurite extension we used different in vitro assays to examine possible effects of ACh on the growth of thalamic axons. In explant cultures, application of ACh reduced the length of thalamic axons in a dose dependent manner, an effect that could also be evoked by selective muscarinic and nicotinic agonists. Time-lapse imaging of thalamic axons exposed to microscopic gradients of ACh revealed that growth cones no longer advanced, but maintained high filopodial activity. This growth cone pausing was not accompanied by axon retraction or growth cone collapse. It could at least partially be blocked by muscarinic and nicotinic antagonists, indicating that both types of ACh receptors contribute to mediate these effects on thalamic axons. Finally, we also found that ACh changed the morphology of growth cones; they became larger and extended more filopodia. Since such changes in the structure and motility of growth cones are observed at decision regions along the path of many fiber populations including thalamic axons, we suggest that ACh plays a role during the elaboration of thalamocortical projections.

The specificity of interactions between the cortex and the thalamus

The functioning of the adult mammalian cerebral cortex depends critically upon precise interconnections between specific thalamic nuclei and distinct cortical regions. Therefore, one central issue in understanding cortical development is determining the cellular and molecular strategies underlying the specification of thalamocortical projections. We address the role of axon-axon interactions and membrane-bound guidance molecules in the establishment of the development of layer-specific patterns of afferent and efferent cortical connections does not depend upon neuronal activity. We present evidence that activity conveyed by thalamic afferents is required for the elaboration of the columnar specificity of cortical circuits.

Axons of callosal neurons bifurcate transiently at the white matter before consolidating an interhemispheric projection

The main alternative output routes of adult cortical axons are the internal capsule and the corpus callosum. How do callosal axons choose their trajectories? We hypothesized that bifurcation followed by elimination of one branch is a developmental strategy for accomplishing this aim. Using embryonic and postnatal mice, we labelled cortical projecting neurons and quantified their axonal bifurcations in correlation with the mediolateral position of their somata. Bifurcating axons were numerous in the younger brains but declined during further development. Most bifurcating axons pertained to neurons located in the dorsolateral cortex. Moreover, callosal neurons bifurcate more often than subcortically projecting cells. We then quantified bifurcations formed by dissociated green fluorescent cells plated onto cortical slices. Cells grown over dorsolateral cortex bifurcated more often than those grown over medial cortex, irrespective of their positional origin in the donor. Removal of intermediate targets from the slices prevented bifurcation. We concluded that transient bifurcation and elimination of the lateral branch is a strategy employed by developing callosal axons in search of their targets. As cell body position and intermediate targets determine axon behaviour, we suggest that bifurcations are regulated by cues expressed in the environment.

Multiple effects of ephrin-A5 on cortical neurons are mediated by SRC family kinases

The Eph receptor tyrosine kinases and their membrane-bound ligands, the ephrins, are involved in a variety of developmental processes such as axonal guidance, cell migration, cell adhesion, proliferation, and differentiation. In addition to repulsive effects, ephrins can also induce attractive responses. Up to now, little was known about the underlying signaling mechanisms that regulate attractive versus repulsive effects. In this study, we show that ephrin-A5 enhances the motility of cortical neurons that is dependent on the activity of Src-family kinases (SFKs). Ephrin-A5 further changes the adhesive properties of neurons by inducing the formation of cell aggregates. Using the stripe assay, we found that the motogenic effect of ephrin-A5 is the result of repulsive ephrin-A interactions. Blocking SFK function leads to a conversion of repulsion into adhesion, suggesting that SFKs can act as a biological switch for the response of EphA receptors. Finally, we discovered a ligand-induced release of membrane particles containing EphA receptors, suggesting membrane ripping as a novel mechanism to overcome the "ephrin paradox" of repulsion after high-affinity receptor-ligand binding.

Connecting thalamus and cortex: The role of ephrins

The complex task of wiring up the brain during embryonic development is achieved by a multitude of guidance signals acting in complex combinations to drive growing axons to their proper targets. The somatosensory system provides an extensively studied model system featuring many universal mechanisms of neural development. In rodents, it constitutes an important model to study how precise topographic connections are achieved. Recent evidence suggests that the Eph/ephrin family of guidance molecules is of pivotal importance for the development of the somatosensory system. Members of Eph/ephrin family are thought to be involved in the global presorting of thalamic axons projecting to the cortex, in labeling specific cortical areas for innervation, in providing topographic cues within the target area, and in distinguishing cortical layers for intracortical wiring. The Eph/ephrin system also seems to contribute to the formation of specific corticothalamic feedback projections. So far, the functions of only a few members of the Eph/ephrin family have been examined, but expression analysis indicates complex combinatorial effects of these signaling molecules. Understanding the Eph/ephrin wiring code is expected to yield new insights into the development and plasticity of brain circuits involved in higher functions.

Neurogenesis in the adult dentate gyrus after cortical infarcts: Effects of infarct location, N-methyl-d-aspartate receptor blockade and anti-inflammatory treatment

Stimulation of cell proliferation and neurogenesis in the adult dentate gyrus has been observed after focal and global brain ischemia but only little is known about the underlying mechanisms. We here analyzed neurogenesis in the dentate gyrus after small cortical infarcts leaving the hippocampal formation and subcortical regions intact. Using the photothrombosis model in adult rats, focal ischemic infarcts were induced in different cortical areas (sensorimotor forelimb and hindlimb cortex) and proliferating cells were labeled at days 3-14 after infarct induction with bromodeoxyuridine. At 2, 4, and 10 weeks after ischemia, immunocytochemistry was performed with immature neuronal (doublecortin), mature neuronal (neuronal nuclei antigen) and glial (calcium-binding protein beta S100beta) markers. When compared with sham-operated controls, animals with infarcts in the forelimb as well as hindlimb cortex revealed an increase in survival of newborn progenitor cells at four and 10 weeks after the insult with predominance at the ipsilateral side. Triple immunofluorescence and confocal laser scanning microscopy revealed an increase in neurogenesis in all groups that was more pronounced 10 weeks after the infarct. Application of the N-methyl-D-aspartate (NMDA)-receptor antagonist MK-801 during lesion induction significantly enhanced neurogenesis in the dentate gyrus. An even stronger increase in newborn neurons was observed after anti-inflammatory treatment with indomethacine during the first 16 days of the experiment. The present study demonstrates that small cortical infarcts leaving subcortical structures intact increase neurogenesis in the dentate gyrus and that these processes can be stimulated by N-methyl-D-aspartate receptor blockade and anti-inflammatory treatment.

Multiple roles of ephrins during the formation of thalamocortical projections: Maps and more

The functional architecture of the cerebral cortex is based on intrinsic connections that precisely link neurons from distinct cortical laminae as well as layer-specific afferent and efferent projections. Experimental strategies using in vitro assays originally developed by Friedrich Bonhoeffer have suggested that positional cues confined to individual layers regulate the assembly of local cortical circuits and the formation of thalamocortical projections. One of these wiring molecules is ephrinA5, a ligand for Eph receptor tyrosine kinases. EphrinA5 and Eph receptors exhibit highly dynamic expression patterns in distinct regions of the cortex and thalamus during early and late stages of thalamocortical and cortical circuit formation. In vitro assays suggest that ephrinA5 is a multifunctional wiring molecule for different populations of cortical and thalamic axons. Additionally, the expression patterns of ephrinA5 during cortical development are consistent with this molecule regulating, in alternative ways, specific components of thalamic and cortical connectivity. To test this directly, the organization of thalamocortical projections was examined in mice lacking ephrinA5 gene expression. The anatomical studies in ephrinA5 knockout animals revealed a miswiring of limbic thalamic projections and changes in neocortical circuits that were predicted from the expression pattern and the in vitro analysis of ephrinA5 function.

BK channel openers inhibit migration of human glioma cells

Large-conductance Ca(2+)-activated K(+) channels (BK channels) are highly expressed in human glioma cells. However, less is known about their biological function in these cells. We used the patch-clamp technique to investigate activation properties of BK channels and time-lapse microscopy to evaluate the role of BK channel activation in migration of 1321N1 human glioma cells. In whole cells, internal perfusion with a solution containing 500 nM free Ca(2+) and external application of the BK channel opener phloretin (100 micro M) shifted the activation threshold of BK channel currents toward more negative voltages of about -30 mV, which is close to the resting potential of the cells. The concentration of intracellular Ca(2+) in fura-2-loaded 1321N1 cells was measured to be 235+/-19 nM and was increased to 472+/-25 nM after treatment with phloretin. Phloretin and another BK channel opener NS1619 (100 micro M) reduced the migration velocity by about 50%. A similar reduction was observed following muscarinic stimulation of glioma cells with acetylcholine (100 micro M). The effects of phloretin, NS1619 and acetylcholine on cell migration were completely abolished by co-application of the specific BK channel blockers paxilline (5 micro M) and iberiotoxin (100 nM). The phloretin-induced increase in intracellular Ca(2+) was unaffected by the removal of extracellular Ca(2+) and co-application of paxilline. These findings indicate that glioma cell migration was inhibited through BK channel activation, independent of intracellular Ca(2+).

Differential regulation of thalamic and cortical axonal growth by hepatocyte growth factor/scatter factor

The initial axonal projections between the cerebral cortex and thalamus are established during embryogenesis. Chemoattractants and repellents are thought to provide specific guidance cues for directional growth of these pathways. Hepatocyte growth factor/scatter factor (HGF/SF) serves as an attractant for developing motor neurons, and its distribution in embryonic pallidum, pallium and thalamus suggests a similar role in forebrain development. We examined the effectiveness of HGF/SF in regulating thalamic and cortical neuronal growth using in vitro assays. HGF/SF increased neurite outgrowth of thalamic, but not cortical neurons, grown in dissociated cultures or as explants. HGF/SF also exhibited a chemoattractant property for thalamic axons, promoting the extension of neurites towards an HGF/SF source. These experiments demonstrate HGF/SF has the capacity to selectively direct thalamocortical projections into an intermediate target, the pallidum, and eventually to their final cortical destination.

Miswiring of limbic thalamocortical projections in the absence of ephrin-A5

Axon guidance cues of the ephrin ligand family have been hypothesized to regulate the formation of thalamocortical connections, but in vivo evidence for such a role has not been examined directly. To test whether ephrin-mediated repulsive cues participate in sorting the projections originating from distinct thalamic nuclei, we analyzed the organization of somatosensory and anterior cingulate afferents postnatally in mice lacking ephrin-A5 gene expression. Projections from ventrobasal and laterodorsal nuclei to their respective sensory and limbic cortical areas developed normally. However, a portion of limbic thalamic neurons from the laterodorsal nucleus also formed additional projections to somatosensory cortical territories, thus maintaining inappropriate dual projections to multiple cortical regions. These results suggest that ephrin-A5 is not required for the formation of normal cortical projections from the appropriate thalamic nuclei, but rather acts as a guidance cue that restricts limbic thalamic axons from inappropriate neocortical regions.

Ephrins regulate the formation of terminal axonal arbors during the development of thalamocortical projections

The development of connections between thalamic afferents and their cortical target cells occurs in a highly precise manner. Thalamic axons enter the cortex through deep cortical layers, then stop their growth in layer 4 and elaborate terminal arbors specifically within this layer. The mechanisms that underlie target layer recognition for thalamocortical projections are not known. We compared the growth pattern of thalamic explants cultured on membrane substrates purified from cortical layer 4, the main recipient layer for thalamic axons, and cortical layer 5, a non-target layer. Thalamic axons exhibited a reduced growth rate and an increased branching density on their appropriate target membranes compared with non-target substrate. When confronted with alternating stripes of both membrane substrates, thalamic axons grew preferentially on their target membrane stripes. Enzymatic treatment of cortical membranes revealed that growth, branching and guidance of thalamic axons are independently regulated by attractive and repulsive cues differentially expressed in distinct cortical layers. These results indicate that multiple membrane-associated molecules collectively contribute to the laminar targeting of thalamic afferents. Furthermore, we found that interfering with the function of Eph tyrosine kinase receptors and their ligands, ephrins, abolished the preferential branching of thalamic axons on their target membranes, and that recombinant ephrin-A5 ligand elicited a branch-promoting activity on thalamic axons. We conclude that interactions between Eph receptors and ephrins mediate branch formation of thalamic axons and thereby may play a role in the establishment of layer-specific thalamocortical connections.

Semaphorin 3A-vascular endothelial growth factor-165 balance mediates migration and apoptosis of neural progenitor cells by the recruitment of shared receptor

The dynamic and coordinated interaction between cells and their microenvironment controls cell migration, proliferation, and apoptosis, mediated by different cell surface molecules. We have studied the response of a neuroectodermal progenitor cell line, Dev, to a guidance molecule, semaphorin 3A (Sema3A), described previously as a repellent-collapsing signal for axons, and we have shown that Sema3A acts as a repellent guidance cue for migrating progenitor cells and, on prolonged application, induces apoptosis. Both repulsion and induction of cell death are mediated by neuropilin-1, the ligand-binding component of the Sema3A receptor. The vascular endothelial growth factor, VEGF165, antagonizes Sema3A-induced apoptosis and promotes cell survival, migration, and proliferation. Surprisingly, repulsion by Sema3A also depends on expression of VEGFR1, a VEGF165 receptor, expressed in Dev cells. Moreover, we found that these repulsive effects of Sema3A require tyrosine kinase activity, which can be attributed to VEGFR1. These results indicate that the balance between guidance molecules and angiogenic factors can modulate the migration, apoptosis (or survival), and proliferation of neural progenitor cells through shared receptors.

Axonal surface molecules act in combination with semaphorin 3a during the establishment of corticothalamic projections

Interactions between growing axons are considered to play important roles for the establishment of precise neuronal connections during the development of the nervous system. Here we used time-lapse imaging techniques to examine the behavior of neocortical and thalamic axons when they encounter each other in vitro. Results indicate that axonal growth cones are able to respond to specific cues expressed on the surface of fibers. Thalamic growth cones often extended along the surface of other thalamic axons and, likewise, cortical growth cones formed fascicles with cortical axons. In contrast, after contacts between cortical and thalamic fibers, in most cases growth cones collapsed and retracted from the axons. Collapse assays using membrane preparations from cortical or thalamic explants demonstrated the existence of cell-type specific collapsing factors whose activity was enhanced by a member of the semaphorin protein family, Sema3A (expressed in the thalamocortical pathway), as it increased the rate of homotypic fasciculations and at the same time amplified the segregation between cortical and thalamic axons. The interaction between axonal surface molecules and environmental cues might mediate the segregation of afferent and efferent fiber tracts in the neocortical white matter.

The polysialic acid moiety of the neural cell adhesion molecule is involved in intraretinal guidance of retinal ganglion cell axons

We have characterized the antigen recognized by mab10, a monoclonal antibody that has been shown to modify outgrowth of thalamic and cortical axons in vitro, and investigated the influence of this antibody on axonal growth in the chicken retina in vivo. Immunopurification, peptide sequencing, and biochemical characterization proved the epitope recognized by mab10 to be polysialic acid (PSA), associated with the neural cell adhesion molecule (NCAM). Intravitreal injections of antibody-secreting hybridoma cells were combined with whole-mount studies using the fluorescent tracer 1,1'-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate (DiI). Pathfinding at the optic fissure was affected, resulting in a failure of axons to exit into the nerve. Misprojections also occurred in more peripheral areas of the retina; however, axons eventually oriented toward the center. Similar projection errors were observed after enzymatic removal of PSA by injecting endoneuraminidase N (endo N). Quantitative measurements of the optic nerve diameter as well as the width of the optic fiber layer confirmed that many axons failed to leave the retina and grew back in the optic fiber layer of the retina. Our findings suggest that NCAM-linked PSA is involved in guiding ganglion cell axons in the retina and at the optic fissure.

Spatial distributions of guidance molecules regulate chemorepulsion and chemoattraction of growth cones

It is generally assumed that gradients of chemotropic molecules are instrumental to the wiring of the nervous system. Recently, two members of the secreted class III semaphorin protein family have been implicated as repulsive (Sema3A) and attractive (Sema3C) guidance molecules for cortical axons (). Here, we show that stabilized gradients of increasing semaphorin concentrations elicit stereotyped responses from cortical growth cones, independent of the absolute concentration and the slope of these gradients. In contrast, neither repulsive effects of Sema3A nor attractive effects of Sema3C were observed when axons were growing toward decreasing semaphorin concentrations. Thus, growth cone guidance by gradients of chemotropic molecules is robust and reproducible, because it is primarily independent of the exact dimensions of the gradients.

Opposing roles for neurotrophin-3 in targeting and collateral formation of distinct sets of developing cortical neurons

Neurotrophin-3 and its receptor TrkC are expressed during the development of the mammalian cerebral cortex. To examine whether neurotrophin-3 might play a role in the elaboration of layer-specific cortical circuits, slices of layer 6 and layers 2/3 neurons were cultured in the presence of exogenously applied neurotrophin-3. Results indicate that neurotrophin-3 promotes axonal branching of layer 6 axons, which target neurotrophin-3-expressing layers in vivo, and that it inhibits branching of layers 2/3 axons, which avoid neurotrophin-3-expressing layers. Such opposing effects of neurotrophin-3 on axonal branching were also observed with embryonic cortical neurons, indicating that the response to neurotrophin-3 is specified at early developmental stages, prior to cell migration. In addition to its effects on fiber branching, axonal guidance assays also indicate that neurotrophin-3 is an attractive signal for layer 6 axons and a repellent guidance cue for layers 2/3 axons. Experiments with specific antibodies to neutralize neurotrophin-3 in cortical membranes revealed that endogenous levels of neurotrophin-3 are sufficient to regulate branching and targeting of cortical axons. These opposing effects of neurotrophin-3 on specific populations of axons demonstrate that it could serve as one of the signals for the elaboration of local cortical circuits.

Semaphorins act as attractive and repulsive guidance signals during the development of cortical projections

Members of the semaphorin family have been implicated in mediating axonal guidance in the nervous system by their ability to collapse growth cones and to function as chemorepellents. The present findings show that recombinant Semaphorin D has similar effects on cortical axons and, in addition, inhibits axonal branching. In contrast, semaphorin E acts as an attractive guidance signal for cortical axons. Attractive effects were only observed when growth cones encountered increasing concentrations or a patterned distribution of Semaphorin E, but not when they are exposed to uniform concentrations of this molecule. Specific binding sites for Semaphorin D and Semaphorin E were present on cortical fibers both in vitro and in vivo at the time when corticofugal projections are established. In situ hybridization analysis revealed that the population of cortical neurons used in our experiments express neuropilin-1 and neuropilin-2, which are essential components of receptors for the class III semaphorins. Moreover, semD mRNA was detected in the ventricular zone of the neocortex whereas semE mRNA was restricted to the subventricular zone. Taken together, these results indicate that semaphorins are bifunctional molecules whose effects depend on their spatial distribution. The coordinated expression of different semaphorins, together with their specific activities on cortical axons, suggests that multiple guidance signals contribute to the formation of precise corticofugal pathways.

Membrane-associated molecules guide limbic and nonlimbic thalamocortical projections

Membrane-associated signals expressed in restricted domains of the developing cerebral cortex may mediate axon target recognition during the establishment of thalamocortical projections, which form in a highly precise manner during development. To test this hypothesis, we first analyzed the outgrowth of thalamic explants from limbic and nonlimbic nuclei on membrane substrates prepared from limbic cortex and neocortex. The results show that different thalamic fiber populations are able to discriminate between membrane substrates prepared from target and nontarget cortical regions. A candidate molecule that could mediate selective choice in the thalamocortical system is the limbic system-associated membrane protein (LAMP), which is an early marker of cortical and subcortical limbic regions (Pimenta et al.,1995) that can promote outgrowth of limbic axons. Limbic thalamic and cortical axons showed preferences for recombinant LAMP (rLAMP) in a stripe assay. Incubation of cortical membranes with an antibody against LAMP prevented the ability of limbic thalamic fibers to distinguish between membranes from limbic cortex and neocortex. Strikingly, nonlimbic thalamic fibers also responded to LAMP, but in contrast to limbic thalamic fibers, rLAMP inhibited branch formation and acted as a repulsive axonal guidance signal for nonlimbic thalamic axons. The present studies indicate that LAMP fulfills a role as a selective guidance cue in the developing thalamocortical system.

Dual action of a ligand for Eph receptor tyrosine kinases on specific populations of axons during the development of cortical circuits

The structural basis of cortical columns are radially oriented axon collaterals that form precise connections between distinct cortical layers. During development, these connections are highly specified from the initial outgrowth of collateral branches. Our previous work provided evidence for positional cues confined to individual layers that induce and/or prevent the formation of axon collaterals in specific populations of cortical neurons. Here we demonstrated with in situ hybridization techniques that mRNA of the Eph receptor tyrosine kinase EphA5 and one of its ligands, ephrin-A5, are present in distinct cortical layers, at a time when intrinsic connections are being formed in the cortex. Axonal guidance assays indicate that ephrin-A5 is a repellent signal for a populations of axons that in vivo avoid the cortical layer expressing ephrin-A5. In contrast to its established role as a repulsive axonal guidance signal, ephrin-A5 specifically mediates sprouting of those cortical axons that target the ephrin-A5-expressing layer in vivo. These results identify a novel function of ephrin-A5 on axonal arbor formation. The laminar distribution and the dual action on specific populations of axons suggest that ephrin-A5 plays a role in the assembly of local cortical circuits.

How do wiring molecules specify cortical connections?

The laminar and columnar organization of the cortex is reflected in the projections to and from the cortex and in the intracortical connections. During development of the cerebral cortex, growing axons are able to distinguish between the different cortical layers, and cortical axons originating from different laminae respond to different layer-specific signals. Here we consider some experimental systems for identifying mechanisms that contribute to the elaboration of layer-specific cortical connections.

Area-specific regulation of gamma-aminobutyric acid type A receptor subtypes by thalamic afferents in developing rat neocortex

Targeting and innervation of the cerebral cortex by thalamic afferents is a key event in the specification of cortical areas. The molecular targets of thalamic regulation, however, have remained elusive. We now demonstrate that thalamic afferents regulate the expression of gamma-aminobutyric acid type A (GABAA) receptors in developing rat neocortex, leading to the area-specific expression of receptor subtypes in the primary visual (V1) and somatosensory (S1) areas. Most strikingly, the alpha1- and alpha5-GABAA receptors exhibited a reciprocal expression pattern, which precisely reflected the distribution of thalamocortical afferents at postnatal day 7. Following unilateral lesions at the birth of the thalamic nuclei innervating V1 and S1 (lateral geniculate nucleus and ventrobasal complex, respectively), profound changes in subunit expression were detected 1 week later in the deprived cortical territories (layers III-IV of V1 and S1). The expression of the alpha1 subunit was strongly down-regulated in these layers to a level comparable to that in neighboring areas. Conversely, the alpha5 subunit was up-regulated and areal boundaries were no longer discernible in the lesioned hemisphere. Changes similar to the alpha5 subunit were also seen for the alpha2 and alpha3 subunits. These results indicate that the differential expression of GABAA receptor subtypes in developing neocortex is dependent on thalamic innervation, contributing to the emergence of functionally distinct areas.

Membrane-associated molecules regulate the formation of layer-specific cortical circuits

The columnar organization of the mammalian neocortex is based on radially oriented axon collaterals which precisely link cells from distinct cortical layers. During development, these interlaminar connections are specific from their initial outgrowth: collaterals form only in the target layers and there are no transient axonal collaterals in the nontarget layers. To examine whether positional cues within individual cortical layers regulate the laminar specificity of collateral formation, explants of cells destined for different cortical layers were cultured on membranes prepared from target and nontarget layers. Axonal growth and branching were examined on homogeneous membrane substrates and on alternating stripes of membranes from different layers. Results show that axons branch preferentially on membrane substrates from those layers that they would target in vivo. In addition, when cortical axons were given a choice to grow on membranes from either their target or their nontarget layer, they exhibited a clear preference for the target layers. This indicates that membrane-associated cues confined to individual layers regulate the formation of collaterals of cortical axons and restrict their growth to their target layers. Heat inactivation of membranes from target layers resulted in reduced axonal branching. The same manipulation of membranes from nontarget layers increased axonal branching for one population of cortical neurons. Taken together, these results suggest that membrane-associated molecules confined to individual layers induce and prevent the formation of axon collaterals in distinct populations of cortical neurons. Thus, the expression of layer-specific cues provides important constraints for the remodeling of local circuits during cortical development.

Tenascin-C synthesis and influence on axonal growth during rat cortical development

Several putative guidance molecules are restricted to the marginal and subplate zones, the major fibre tracts in the developing cortex. It is presently unknown how their distribution is achieved and how these molecules affect neurite extension. Tenascin-C is of particular interest in this context, because it may either promote or deflect growing axons depending on its mode of presentation. Therefore, the cellular origin of tenascin-C in the developing rat cortex and its effects on the extension of cortical afferents and efferents were examined. Tenascin-C protein is first restricted to the marginal and subplate zones and spreads later into the developing grey matter, in close correlation with afferent innervation. In situ hybridization showed that tenascin-C mRNA is first confined to the ventricular zone, at some distance from the location of the protein, while at later stages tenascin-C-synthesizing cells become scattered throughout the cortical thickness, concomitant with the spread of the protein. In order to assess its function, monoclonal antibodies directed against different domains of tenascin-C were used in a quantitative axonal outgrowth assay. These perturbation experiments suggested that distinct tenascin-C fibronectin type III repeats sustain the growth of thalamic and cortical axons on cortical membrane carpets, whereas the EGF-type repeats are not involved. The combination of different antibodies revealed that separate fibronectin-type III repeats exert cooperative effects. These results suggest that ventricular zone cells regulate the establishment of thalamic and cortical axonal projections through locally restricted deposition of tenascin-C.

Developmental strategies underlying the elaboration of cortical circuits

The mammalian cerebral cortex is organized in layers and columns, which are reflected in the local intrinsic connections and in the projections to and from the cortex. It is well established that the development of the columnar architecture is under the influence of neuronal activity, but little is known about the mechanisms that control the laminar specificity of cortical circuits. Here we review some recent studies which show that diffusible and membrane-associated molecules provide sufficient information to reconstruct layer-specific intrinsic and extrinsic cortical circuits under in vitro conditions.

Dual action of a carbohydrate epitope on afferent and efferent axons in cortical development

During development of the mammalian cerebral cortex, ingrowing afferents from the thalamus take a path that is different from that of axons leaving the cortical plate. Thalamic axons arrive at the cortex at the time before their target cells of layer 4 are generated in the ventricular zone, but they invade the cortex only shortly before these cells have migrated to their final position in the cortex. Growth-promoting molecules are up-regulated in the developing cortical plate during this period. To identify such molecules, we have generated monoclonal antibodies against membrane preparations from rat postnatal cortex. In Western blots, one antibody (mAb 10) recognized a carbohydrate epitope of a glycoprotein with an apparent molecular weight extending from 180 to 370 kDa. Immunohistochemical staining revealed that the staining pattern of mAb 10 at embryonic stages delineates the pathway of thalamocortical axons, with only very faint labeling of the corticofugal pathway. In vitro assays in combination with time-lapse imaging indicated that mAb 10 has opposite effects on the growth of thalamic and cortical axons. The growth speed and axonal elongation of thalamic fibers on postnatal cortical membranes preincubated with mAb 10 was reduced compared with untreated cortical membranes. In contrast, cortical axons grew faster and stopped their growth less frequently after addition of mAb 10 to a cortical membrane substrate. Taken together, these results suggest that a carbohydrate moiety of a membrane-associated glycoprotein plays a role in the segregation of afferent and efferent cortical axons in the white matter. Moreover, the epitope recognized by mAb 10 might also contribute to regulation of the timing of the thalamocortical innervation at later developmental stages.

Specification of layer-specific connections in the developing cortex

One of the basic tasks of neurobiology is to understand how the precision and specificity of neuronal connections is achieved during development. In this paper we reviewed some recent in vitro studies on the developing mammalian cerebral cortex that have been made towards this end. The results of these experiments provided evidence that membrane-associated molecules are instrumental for the formation of specific afferent and efferent cortical projections. Substrate-bound molecules guide growing axons towards their target, regulate the timing of thalamocortical innervation and mediate target cell recognition. Moreover, a newly described glycoprotein, defined by a monoclonal antibody, revealed a molecular heterogeneity in the developing white matter. Since this molecule has opposite effects on thalamic and cortical axons, it might play a role in the segregation of axons running to and from the cortex. Substrate-bound cues are important during the formation of local cortical circuits. In vitro assays demonstrated that molecular components confined to individual cortical layers control the laminar specificity of cortical axon branching. This suggests that similar developmental strategies contribute to the laminar specification of extrinsic and intrinsic cortical circuits. Thus substrate-bound molecules might provide the framework for subsequent activity-dependent mechanisms that control the elaboration of precise connections between the cortical columns. A major challenge ahead is to identify the factors that mediate these processes and to determine their mode of action. Recently, two families of proteins, the netrins and the semaphorins/collapsins, have been identified as growth cone signals in the developing spinal cord (reviewed in Goodman, 1994; Colamarino and Tessier-Lavigne, 1995a; Dodd and Schuchardt, 1995; Kennedy and Tessier-Lavigne, 1995). Semaphorins/collapsins appear to regulate axonal guidance by repelling growth cones and by inhibiting axonal branching and synapse formation. Originally, netrins have been purified as diffusible chemoattractants for commissural axons of the dorsal spinal cord, but it is now well established that they can also function as chemorepellent factors for other classes of neurons. Since netrins are related to extracellular matrix components and since they can bind to the cell surface, they might also act as local guidance cues. A possible role of netrins and semaphorins/collapsins in the development of cortical connections is likely to be resolved in the near future. The identification of the factors that regulate specific branching patterns of cortical neurons might provide a better understanding of cortical development, but it might also be relevant to some aspects of plasticity and repair in the adult cortex.

Guidance of thalamocortical axons by growth-promoting molecules in developing rat cerebral cortex

Substrate-bound guidance cues play an important role during the development of thalamocortical projections. We used time-lapse video microscopy to study the growth behaviour of thalamic axons on different substrates. On embryonic cortical membranes and on a pure laminin substrate, thalamic fibres advanced relatively slowly (approximately 15 microns/h) and on average their growth cones retracted transiently every approximately 5 h. In contrast, on membranes prepared from early postnatal cortex, thalamic fibres grew twice as fast and spontaneous growth cone collapse occurred approximately 8 times less often. Experiments in which we used the sugar-binding lectin peanut agglutinin or heat inactivation to change the membrane properties indicated that these differences are due to growth-supporting molecules on postnatal cortical membranes. When offered a choice between embryonic and postnatal cortical membranes, thalamic axons preferred the postnatal membrane substrate. Time-lapse imaging revealed that borders between these two substrates effectively guided thalamic fibres, and in most cases axons changed their direction without collapse of the growth cone. Our results suggest that thalamic axons can be guided by the spatial distribution of growth-promoting molecules in the developing cortex.