Establishment of patterned thalamocortical connections does not require nitric oxide synthase

Neurophysiologic implications of neuronal nitric oxide synthase

The molecular and chemical properties of neuronal nitric oxide synthase (nNOS) have made it a key mediator in many physiological functions and signaling transduction. The NOS monomer is inactive, but the dimer form is active. There are three forms of NOS, which are neuronal (nNOS), inducible (iNOS), and endothelial (eNOS) nitric oxide synthase. nNOS regulates nitric oxide (NO) synthesis which is the mechanism used mostly by neurons to produce NO. nNOS expression and activation is regulated by some important signaling proteins, such as cyclic adenosine monophosphate (cAMP) response element-binding protein (CREB), calmodulin (CaM), heat shock protein 90 (HSP90)/HSP70. nNOS-derived NO has been implicated in modulating many physiological functions, such as synaptic plasticity, learning, memory, neurogenesis, etc. In this review, we have summarized recent studies that have characterized structural features, subcellular localization, and factors that regulate nNOS function. Finally, we have discussed the role of nNOS in the developing brain under a wide range of physiological conditions, especially long-term potentiation and depression.

Ocular dominance columns in V1 are more susceptible than associated callosal patches to imbalance of eye input during precritical and critical periods

In Long Evans rats, ocular dominance columns (ODCs) in V1 overlap with patches of callosal connections. Using anatomical tracers, we found that ODCs and callosal patches are present at postnatal day 10 (P10), several days before eye opening, and about 10 days before the activation of the critical period for ocular dominance plasticity (~P20). In rats monocularly enucleated at P10 and perfused ~P20, ODCs ipsilateral to the remaining eye desegregated, indicating that rat ODCs are highly susceptible to monocular enucleation during a precritical period. Monocular enucleation during the critical period exerted significant, although smaller, effects. Monocular eye lid suture during the critical period led to a significant expansion of the ipsilateral projection from the nondeprived eye, whereas the contralateral projection invaded into, and intermixed with, ipsilateral ODCs innervated by the deprived eye. We propose that this intermixing allows callosal connections to contribute to the effects of monocular deprivation assessed in the hemisphere ipsilateral to the nondeprived eye. The ipsilateral and contralateral projections from the deprived eye did not undergo significant shrinkage. In contrast, we found that callosal patches are less susceptible to imbalance of eye input. In rats monocularly enucleated during either the precritical or critical periods, callosal patches were maintained in the hemisphere ipsilateral to the remaining eye, but desegregated in the hemisphere ipsilateral to the enucleated orbit. Callosal patches were maintained in rats binocularly enucleated at P10 or later. Similarly, monocular deprivation during the critical period had no significant effect on callosal patches in either hemisphere.

Postnatal development of GABAergic interneurons in the neocortical subplate of mice

The subplate (SP) plays important roles in developmental and functional events in the neocortex, such as thalamocortical and corticofugal projection, cortical oscillation generation and corticocortical connectivity. Although accumulated evidence indicates that SP interneurons are crucial for SP function, the molecular composition of SP interneurons as well as their developmental profile and distribution remain largely unclear. In this study, we systematically investigated dynamic development of SP thickness and chemical marker expression in SP interneurons in distinct cortical regions during the first postnatal month. We found that, although the relative area of the SP in the cerebral cortex significantly declined with postnatal development, the absolute thickness did not change markedly. We also found that somatostatin (SOM), the ionotropic serotonin receptor 3A (5HT3AR), and parvalbumin (PV) reliably identify three distinct non-overlapping subpopulations of SP interneurons. The SOM group, which represents ~30% of total SP interneurons, expresses neuronal nitric oxide synthase (nNOS) and calbindin (CB) and colocalizes entirely with neuropeptide Y (NPY). The 5HT3AR group, which accounts for ~60% of the total interneuronal population, expresses calretinin (CR) and GABA-A receptor subunit delta (GABAARδ). The PV group accounts for ~10% of total SP interneurons and coexpressed GABAARδ. Moreover, distinct interneuron subtypes show characteristic temporal and spatial distribution in the SP. nNOS(+) interneurons in the SP increase from the anterior motor cortex to posterior visual cortex, while CR(+) and CB(+) interneurons the opposite. Interestedly, the majority of GABAARδ(+) neurons in SP are non-GABAergic neurons in contrast to other cortical layers. These findings clarify and extend our understanding of SP interneurons in the developing cerebral cortex and will underpin further study of SP function.

Nitric oxide in the nervous system: biochemical, developmental, and neurobiological aspects

Nitric oxide (NO) is a very reactive molecule, and its short half-life would make it virtually invisible until its discovery. NO activates soluble guanylyl cyclase (sGC), increasing 3',5'-cyclic guanosine monophosphate levels to activate PKGs. Although NO triggers several phosphorylation cascades due to its ability to react with Fe II in heme-containing proteins such as sGC, it also promotes a selective posttranslational modification in cysteine residues by S-nitrosylation, impacting on protein function, stability, and allocation. In the central nervous system (CNS), NO synthesis usually requires a functional coupling of nitric oxide synthase I (NOS I) and proteins such as NMDA receptors or carboxyl-terminal PDZ ligand of NOS (CAPON), which is critical for specificity and triggering of selected pathways. NO also modulates CREB (cAMP-responsive element-binding protein), ERK, AKT, and Src, with important implications for nerve cell survival and differentiation. Differences in the regulation of neuronal death or survival by NO may be explained by several mechanisms involving localization of NOS isoforms, amount of NO being produced or protein sets being modulated. A number of studies show that NO regulates neurotransmitter release and different aspects of synaptic dynamics, such as differentiation of synaptic specializations, microtubule dynamics, architecture of synaptic protein organization, and modulation of synaptic efficacy. NO has also been associated with synaptogenesis or synapse elimination, and it is required for long-term synaptic modifications taking place in axons or dendrites. In spite of tremendous advances in the knowledge of NO biological effects, a full description of its role in the CNS is far from being completely elucidated.

Expression of soluble guanylyl cyclase in rat cerebral cortex during postnatal development

Soluble guanylyl cyclase (sGC), the principle "receptor" for nitric oxide (NO), catalyzes the formation of cyclic guanosine monophosphate (cGMP), an intracellular second messenger. Studies in invertebrates have shown that the NO/cGMP pathway is involved in several aspects of neural development, including neuronal migration, dendritic and axonal outgrowth, and synaptogenesis. In vitro studies suggest a developmental role also in mammals. To investigate whether the NO/cGMP pathway might mediate these processes in vivo, we performed immunohistochemistry for sGC on sections from postnatal rat cerebral cortex. Early in postnatal development, migrating neurons in the cortical plate were immunonegative, whereas neurons deeper in the cortex that had completed migration were immunopositive. At the subcellular level, sGC preferentially stained dendrites rather than axons, but, at postnatal day 1 (PND1), sGC was found in a large fraction of axonal growth cones, especially those oriented toward the pial surface. At PND10-20 (the period of maximal synaptogenesis), sGC immunostaining was located mainly in dendritic shafts and was only occasionally associated with spines or axon terminals. These results support a role for the NO/cGMP pathway in dendritic development but argue against a major role in neuronal migration and synaptogenesis.

Distribution of neuronal nitric oxide synthase-immunoreactive neurons in the cerebral cortex and hippocampus during postnatal development

Although many reports have argued a role for nitric oxide (NO) during postnatal development, there has been no combined demonstration in the cerebral cortex and hippocampus. We have investigated the distribution and morphology of neurons and fibers expressing neuronal NO synthase (nNOS) in the cerebral cortex and hippocampal formation of rats during the postnatal development, and correlated these findings with developmental events taking place in these regions. In the cerebral cortex, the nNOS-immunoreactive cells could be divided into two classes : heavily stained neurons and lightly stained neurons. For the lightly stained nNOS-positive neurons, only the cell bodies were observed, whereas for the heavily stained neurons, the cell bodies and their dendrites were visible. During the postnatal days, heavily stained neurons reached their typical morphology in the second week and appeared in all layers except for layer I. In the hippocampus, there was a transient expression of nNOS in the pyramidal cell layer at P3-P7, and this expression disappeared during following days. The adult pattern of staining developed gradually during the postnatal period. This study suggested that these alterations might reflect a region-specific role of NO and a potential developmental role in the postnatal cerebral cortex and hippocampus.

Activity-driven sharpening of the retinotectal projection: the search for retrograde synaptic signaling pathways

Patterned visual activity, acting via NMDA receptors, refines developing retinotectal maps by shaping individual retinal arbors. Because NMDA receptors are postsynaptic but the retinal arbors are presynaptic, there must be retrograde signals generated downstream of Ca(++) entry through NMDA receptors that direct the presynaptic retinal terminals to stabilize and grow or to withdraw. This review defines criteria for retrograde synaptic messengers, and then applies them to the leading candidates: nitric oxide (NO), brain-derived neurotrophic factor (BDNF), and arachidonic acid (AA). NO is not likely to be a general mechanism, as it operates only in selected projections of warm blooded vertebrates to speed up synaptic refinement, but is not essential. BDNF is a neurotrophin with strong growth promoting properties and complex interactions with activity both in its release and receptor signaling, but may modulate rather than mediate the retrograde signaling. AA promotes growth and stabilization of synaptic terminals by tapping into a pre-existing axonal growth-promoting pathway that is utilized by L1, NCAM, N-cadherin, and FGF and acts via PKC, GAP43, and F-actin stabilization, and it shares some overlap with BDNF pathways. The actions of both are consistent with recent demonstrations that activity-driven stabilization includes directed growth of new synaptic contacts. Certain nondiffusible factors (synapse-specific CAMs, ephrins, neurexin/neuroligin, and matrix molecules) may also play a role in activity-driven synapse stabilization. Interactions between these pathways are discussed.

Postnatal development and age-related changes in the distribution of nitric oxide synthase-immunoreactive neurons in the visual system of rats

We have determined postnatal development and age-related changes in the distribution of neuronal nitric oxide synthase (nNOS) in the visual system including superior colliculus (SC), dorsal nucleus of lateral geniculate body (dLGB) and visual cortex of rats. In SC, the number and intensity of immunoreactive cells were increased until postnatal day 21 (P21), and were comparably decreased during the following days. Both neurons and neuropil were intensely stained in dLGB at P21, and the staining intensity was decreased in aged rats. The number of nNOS-immunoreactive cells was increased from P7 to P28 and was decreased in aged rats. The first demonstration of different staining patterns in SC, dLGB and visual cortex during the postnatal days and aging suggests that NO may mediate both postnatal refinement and age-associated visual deficits in some brain regions.

Insights into activity-dependent map formation from the retinotectal system: A middle-of-the-brain perspective

The development of orderly topographic maps in the central nervous system (CNS) results from a collaboration of chemoaffinity cues that establish the coarse organization of the projection and activity-dependent mechanisms that fine-tune the map. Using the retinotectal projection as a model system, we describe evidence that biochemical tags and patterned neural activity work in parallel to produce topographically ordered axonal projections. Finally, we review recent experiments in other CNS projections that support the proposition that cooperation between molecular guidance cues and activity-dependent processes constitutes a general paradigm for CNS map formation.

Development of columnar topography in the excitatory layer 4 to layer 2/3 projection in rat barrel cortex

The excitatory feedforward projection from layer (L) 4 to L2/3 in rat primary somatosensory (S1) cortex exhibits precise, columnar topography that is critical for columnar processing of whisker inputs. Here, we characterize the development of axonal topography in this projection using single-cell reconstructions in S1 slices. In the mature projection [postnatal day (P) 14-26], axons of L4 cells extending into L2/3 were confined almost entirely to the home barrel column, consistent with previous results. At younger ages (P8-11), however, axonal topography was significantly less columnar, with a large proportion of branches innervating neighboring barrel columns representing adjacent whisker rows. Mature topography developed from this initial state by targeted axonal growth within the home column and by growth of barrel columns themselves. Raising rats with all or a subset of whiskers plucked from P8-9, manipulations that induce reorganization of functional whisker maps and synaptic depression at L4 to L2/3 synapses, did not alter normal anatomical development of L4 to L2/3 axons. Thus, development of this projection does not require normal sensory experience after P8, and deprivation-induced reorganization of whisker maps at this age is unlikely to involve physical remodeling of L4 to L2/3 axons.

Nitric oxide synthase inhibition during development: effect on apoptotic death of dopamine neurons

Naturally occurring cell death via apoptosis occurs in the substantia nigra pars compacta (SNc) during rat development, culminating during the perinatal period. We previously showed that lipid peroxidation-mediated oxidative stress is not involved in this cell death process. Nitric oxide (NO) has been proposed to be critical for many developmental processes in brain and has been shown to mediate cell death in neurotoxin models of neurodegenerative disorders. Here, we reported that in vivo pre- and postnatal treatment with the non-specific NO synthase (NOS) inhibitor, L-NAME (60 mg/kg), or with the neuronal NOS inhibitor, 7-NI (30 mg/kg), dramatically decreased the NOS activity as well as the NADPH-diaphorase staining in brain. However, those treatments did not rescue dopamine neurons from developmental death, suggesting that NO is not involved in vivo in developmental death of these neurons or in the overall development of the SNc.

LIS1-no more no less

LIS1 is one of the genes that has a principle role in brain development since hemizygote mutations in LIS1 result in a severe brain malformation known as lissencephaly ('smooth brain'). LIS1 is a WD repeat protein and is known to be involved in several protein complexes that are likely to play a functional role in brain development. We discuss here the brain developmental phenotype observed in mice heterozygote for an N-terminal truncated LIS1 protein in view of known LIS1 protein interactions.

Ocular dominance development revisited

New approaches to the study of ocular dominance development, a model system for the development of neural architecture, indicate that eye-specific columns in primary visual cortex emerge substantially before the onset of the critical period, during which neural connections can be altered by visual experience. The timing, speed and specificity of column emergence implicate molecular patterning mechanisms, along with patterns of neural activity, in the generation of this columnar architecture.

Development of cortical circuits: lessons from ocular dominance columns

The development of ocular dominance columns has served as a Rosetta stone for understanding the mechanisms that guide the construction of cortical circuits. Traditionally, the emergence of ocular dominance columns was thought to be closely tied to the critical period, during which columnar architecture is highly susceptible to alterations in visual input. However, recent findings in cats, monkeys and ferrets indicate that columns develop far earlier, more rapidly and with considerably greater precision than was previously suspected. These observations indicate that the initial establishment of cortical functional architecture, and its subsequent plasticity during the critical period, are distinct developmental phases that might reflect distinct mechanisms.

The receptor tyrosine kinase EphB2 regulates NMDA-dependent synaptic function

Members of the Eph family of receptor tyrosine kinases control many aspects of cellular interactions during development, including axon guidance. Here, we demonstrate that EphB2 also regulates postnatal synaptic function in the mammalian CNS. Mice lacking the EphB2 intracellular kinase domain showed wild-type levels of LTP, whereas mice lacking the entire EphB2 receptor had reduced LTP at hippocampal CA1 and dentate gyrus synapses. Synaptic NMDA-mediated current was reduced in dentate granule neurons in EphB2 null mice, as was synaptically localized NR1 as revealed by immunogold localization. Finally, we show that EphB2 is upregulated in hippocampal pyramidal neurons in vitro and in vivo by stimuli known to induce changes in synaptic structure. Together, these data demonstrate that EphB2 plays an important role in regulating synaptic function.

Targeted mutagenesis of Lis1 disrupts cortical development and LIS1 homodimerization

Lissencephaly is a severe brain malformation in humans. To study the function of the gene mutated in lissencephaly (LIS1), we deleted the first coding exon from the mouse Lis1 gene. The deletion resulted in a shorter protein (sLIS1) that initiates from the second methionine, a unique situation because most LIS1 mutations result in a null allele. This mutation mimics a mutation described in one lissencephaly patient with a milder phenotype. Homozygotes are early lethal, although heterozygotes are viable and fertile. Most strikingly, the morphology of cortical neurons and radial glia is aberrant in the developing cortex, and the neurons migrate more slowly. This is the first demonstration, to our knowledge, of a cellular abnormality in the migrating neurons after Lis1 mutation. Moreover, cortical plate splitting and thalomocortical innervation are also abnormal. Biochemically, the mutant protein is not capable of dimerization, and enzymatic activity is elevated in the embryos, thus a demonstration of the in vivo role of LIS1 as a subunit of PAF-AH. This mutation allows us to determine a hierarchy of functions that are sensitive to LIS1 dosage, thus promoting our understanding of the role of LIS1 in the developing cortex.

Development and plasticity of cortical areas and networks

The development of cortical layers, areas and networks is mediated by a combination of factors that are present in the cortex and are influenced by thalamic input. Electrical activity of thalamocortical afferents has a progressive role in shaping cortex. For early thalamic innervation and patterning, the presence of activity might be sufficient; for features that develop later, such as intracortical networks that mediate emergent responses of cortex, the spatiotemporal pattern of activity often has an instructive role. Experiments that route projections from the retina to the auditory pathway alter the pattern of activity in auditory thalamocortical afferents at a very early stage and reveal the progressive influence of activity on cortical development. Thus, cortical features such as layers and thalamocortical innervation are unaffected, whereas features that develop later, such as intracortical connections, are affected significantly. Surprisingly, the behavioural role of 'rewired' cortex is also influenced profoundly, indicating the importance of patterned activity for this key aspect of cortical function.

Nitric oxide, impulse activity, and neurotrophins in visual system development(1)

Topographic refinement of synaptic connections within the developing visual system involves a variety of molecules which interact with impulse activity in order to produce the precise retinotopic maps found in the adult brain. Nitric oxide (NO) has been implicated in this process, as have various growth factors. Within the subcortical visual system, we have recently shown that nitric oxide contributes to pathway refinement in the superior colliculus (SC). Long-term potentiation (LTP) and long-term depression (LTD) are also expressed in SC during the time that this pathway undergoes refinement. The role of NO has been demonstrated by showing that refinement of ipsilateral fibers in the retinocollicular pathway is significantly delayed in gene knockout mice in which both the endothelial and neuronal isoforms of nitric oxide synthase (NOS) have been disrupted. The effect also depends upon Ca(2+) channels because refinement of both the ipsilateral retinocollicular and retinogeniculate pathways is disrupted in genetic mutants in which the beta3 subunit of the Ca(2+) channel has been deleted. LTD may also be involved in this process, because the time course of its expression correlates with that of pathway refinement and LTD magnitude is depressed by nitrendipine, an L-type Ca(2+) channel blocker. LTP is also expressed during early postnatal development in the LGN and SC and may contribute to synaptic stabilization. The role of neurotrophins in pathway refinement in the visual system is also reviewed.

Early development of ocular dominance columns

The segregation of lateral geniculate nucleus (LGN) axons into ocular dominance columns is believed to involve a prolonged, activity-dependent sorting process. However, visualization of early postnatal ferret LGN axons by direct LGN tracer injections revealed segregated ocular dominance columns <7 days after innervation of layer 4. These early columns were unaffected by experimentally induced imbalances in retinal activity, implying that different mechanisms govern initial column formation and their modification during the subsequent critical period. Instead of activity-dependent plasticity, we propose that ocular dominance column formation relies on the targeting of distinct axonal populations to defined locales in cortical layer 4.

Nitric oxide modulates retinal ganglion cell axon arbor remodeling in vivo

Nitric oxide (NO) has been postulated to act as an activity-dependent retrograde signal that can mediate multiple aspects of synaptic plasticity during development. In the visual system, a role for NO in activity-dependent structural modification of presynaptic arbors has been proposed based on NO's ability to prune inappropriate projections and segregate axon terminals. However, evidence demonstrating that altered NO signaling does not perturb ocular dominance map formation leaves unsettled the role of NO during the in vivo refinement of visual connections. To determine whether NO modulates the structural remodeling of individual presynaptic terminal arbors in vivo we have: 1. Used NADPH-diaphorase histochemistry to determine the onset of NO synthase (NOS) expression in the Xenopus visual system. 2. Used in vivo time-lapse imaging to examine the role of NO during retinal ganglion cell (RGC) axon arborization. We show that NOS expression in the target optic tectum is developmentally regulated and localized to neurons that reside in close proximity to arborizing RGC axons. Moreover, we demonstrate that perturbations in tectal NO levels rapidly and significantly alter the dynamic branching of RGC arbors in vivo. Tectal injection of NO donors increased the addition of new branches, but not their stabilization in the long term. Tectal injection of NOS inhibitors increased the dynamic remodeling of axonal arbors by increasing branch addition and elimination and by lengthening pre-existing branches. Thus, these results indicate that altering NO signaling significantly modifies axon branch dynamics in a manner similar to altering neuronal activity levels (Cohen-Cory, 1999). Consequently, our results support a role for NO during the dynamic remodeling of axon arbors in vivo, and suggest that NO functions as an activity-dependent retrograde signal during the refinement of visual connections.

Normal development of the ipsilateral retinocollicular pathway and its disruption in double endothelial and neuronal nitric oxide synthase gene knockout mice

The development of the ipsilateral retinocollicular pathway involves activity-dependent refinement in which misdirected axons retract to form a precise retinotopic map in adults. This refinement is altered by disruption of genes for the endothelial and neuronal isoforms of nitric oxide synthase (e,nNOS), but the extent of disruption during early development is not known. Therefore, we studied the refinement of this pathway in normal C57/BL6 and e,nNOS double knockouts from P4 to P21 and in adults. Anterograde tracers were injected into one eye to localize the ipsilateral retinal projection (IRP) within the superior colliculus (SC). At P4, the IRP in normal mice was distributed throughout the dorsoventral extent of the superficial gray layer (SGL) across most of the rostrocaudal axis of SC. Between P4 and P9, the pathway retracted to the rostromedial SC, and retracted further between P15 and P21, such that multiple patches of label were seen only in the rostral 200-300 microm. Refinement also began to occur between P4 and P9 in e,nNOS double knockout mice, but labeling was more extensive in P9, P15, and P21 knockout animals. This delay in refinement was confirmed quantitatively at P15 where differences in the area occupied by the pathway were statistically significant. The refinement process is therefore in progress in both normal and e,nNOS knockout mice before eye opening but is significantly delayed in the double knockouts. The IRP in normal mice is also more exuberant at early ages, and the process of refinement more protracted than has been previously reported, suggesting that there is a prolonged critical period of synaptic plasticity.

Plasticity in the development of afferent patterns in the inferior colliculus of the rat after unilateral cochlear ablation

The central nucleus of the inferior colliculus (IC) is the site of convergence for nearly all ascending monaural and binaural projections. Several of these inputs, including inhibitory connections from the dorsal nucleus of the lateral lemniscus (DNLL), are highly ordered and organized into series of afferent bands or patches. Although inputs to the IC from the contralateral DNLL are present in the rat by birth [postnatal day 0 (P0)], the earliest indications of band formation are not evident until P4. Subsequently, the initially diffuse projection segregates into a pattern of bands and interband spaces, and by P12 adult-like, afferent-dense patches are established (Gabriele et al., 2000). To determine the role of the auditory periphery in the development of bands and patches before the onset of hearing (P12/P13), unilateral cochlear ablations were performed at P2 (before any evidence of banding). Rat pups were reared to P12, at which time glass pins coated with 1, 1'-dioctodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate were placed in fixed tissue in the commissure of Probst where DNLL fibers cross the midline. The results indicate that a unilateral cochlear ablation disrupts the normal development of afferent patches in the IC. Although the crossed DNLL projections labeled via commissural dye placement always mirrored each other in P12 controls, ablation cases exhibited a consistent, bilateral asymmetry in pattern formation and relative density of the labeled projections. Possible developmental mechanisms likely to be involved in the establishment of afferent bands and patches before the onset of hearing are discussed.

Acute blockade of nitric oxide synthesis induces disorganization and amplifies lesion-induced plasticity in the rat retinotectal projection

In the rat visual system, the uncrossed retinotectal projection undergoes a topographical refinement within the first two postnatal weeks. We have studied the role of nitric oxide (NO), a retrograde messenger which couples pre- and postsynaptic activation, in the development of the uncrossed retinotectal projection and in the plasticity of this pathway as a result of a restricted retinal lesion in the opposite eye. During development, maximal nitric oxide synthase (NOS) activity was observed in homogenates of tectal tissue at postnatal day 5 (PND 5), followed by a two-step decrease at the end of the topographical fine tuning period (PND 21) and the adult stage (PND 42). We also tested the effects of an acute in vivo blockade of NOS during the development of both animals that had not been operated on, and lesioned animals. Animals ranging from PND 4 to PND 42 were treated either with the NOS inhibitor, L-nitro-arginine (Narg 50 mg/kg ip.) or vehicle (NaCl 0.9%) during 4 days (from PND 4-7 or PND 9-12) or 8 days (from PND 20-27 or PND 34-41). Reduction of NOS activity induced sprouting of the ipsilateral pathway up to the second postnatal week in the animals that had not been operated on. Rats that had been operated on, however, showed an amplification of the lesion-induced plasticity up to the fourth postnatal week under NOS blockade. The data suggest that NO plays a role in the stabilization of retinotectal synapses during the critical period of topographic refinement, and indicate that an acute blockade of retrograde signals enables plastic rearrangements in the visual system within this time window.

Neurotrophins and the dynamic regulation of the neuronal cytoskeleton

The morphology of neuronal axons and dendrites is dependent on the dynamics of the cytoskeleton. An understanding of neurodevelopment and adult neuroplasticity must therefore include a detailed description of the intrinsic and extrinsic mechanisms that regulate the organization and dynamics of actin filaments and microtubules. In this paper we review recent advances in the understanding of the dynamic regulation of neuronal morphology by interactions among cytoskeletal components and the regulation of the cytoskeleton by neurotrophins.

Roles of NMDA receptor activity and nitric oxide production in brain development

The concept that neural activity is important for brain maturation has focused much research interest on the developmental role of the NMDA receptor, a key mediator of experience-dependent synaptic plasticity. However, a mechanism able to link spatial and temporal parameters of synaptic activity during development emerged as a necessary condition to explain how axons segregate into a common brain region and make specific synapses on neuronal sub-populations. To comply with this developmental constraint, it was proposed that nitric oxide (NO), or other substances having similar chemical and biological characteristics, could act as short-lived, activity-dependent spatial signals, able to stabilize active synapses by diffusing through a local volume of tissue. The present article addresses this issue, by reviewing the experimental evidence for a correlated role of the activity of the NMDA receptor and the production of NO in key steps of neural development. Evidence for such a functional coupling emerges not only concerning synaptogenesis and formation of neural maps, for which it was originally proposed, but also for some earlier phases of neurogenesis, such as neural cell proliferation and migration. Regarding synaptogenesis and neural map formation in some cases, there is so far no conclusive experimental evidence for a coupled functional role of NMDA receptor activation and NO production. Some technical problems related to the use of inhibitors of NO formation and of gene knockout animals are discussed. It is also suggested that other substances, known to act as spatial signals in adult synaptic plasticity, could have a role in developmental plasticity. Concerning the crucial developmental phase of neuronal survival or elimination through programmed cell death, the well-documented survival role related to NMDA receptor activation also starts to find evidence for a concomitant requirement of downstream NO production. On the basis of the reviewed literature, some of the major controversial issues are addressed and, in some cases, suggestions for possible future experiments are proposed.

Refinement of the ipsilateral retinocollicular projection is disrupted in double endothelial and neuronal nitric oxide synthase gene knockout mice

Development of retinal connections to the superior colliculus (SC) requires an activity dependent refinement process in which axons gradually become restricted to appropriate retinotopic locations. Nitric oxide has been implicated in this process. We tested this possibility by studying the refinement of the ipsilateral retinocollicular projections (IRP) in normal C57-BL/6 mice and in double knockout mice in which the genes for the edothelial and neuronal isoforms of nitric oxide synthase (e, nNOS) were disrupted. Mice aged between P19 and adulthood were perfused 44-48 h after anterograde injections of WGA-HRP into one eye in order to measure the distribution of the labeled IRP. In normal mice, segregation of the IRP was complete at P21, with the ipsilateral projection restricted to the rostro-medial SC. By contrast, the ipsilateral projection was spread over much more of the SC in double e, nNOS knockouts at P21 with patches of label distributed across the entire medio-lateral axis of the rostral 700 microm. Although the distribution of the ipsilateral projection became more restricted in knockout animals at later ages, it was still more extensive than that of normal mice of the same age at P28 and P42. In the adult, the distribution of axons was similar in both normal and double knockout animals. These results show that refinement of the IRP is delayed when expression of eNOS and nNOS is disrupted, presumably to axons with uncorrelated activity because nitric oxide serves as a repellant molecule during normal development.

Stabilization of growing retinal axons by the combined signaling of nitric oxide and brain-derived neurotrophic factor

The pattern of axonal projections early in the development of the nervous system lacks the precision present in the adult. During a developmental process of refinement, mistargeted projections are eliminated while correct projections are retained. Previous studies suggest that during development nitric oxide (NO) is involved in the elimination of mistargeted retinal axons, whereas brain-derived neurotrophic factor (BDNF) may stabilize retinal axon arbors. It is unclear whether these neuromodulators interact. This study showed that NO induced growth cone collapse and retraction of developing retinal axons. This effect was not attributable to NO-induced neurotoxicity. BDNF protected growth cones and axons from the effects of NO. This effect was specific to BDNF, because neither nerve growth factor (NGF) nor neurotrophin-3 (NT-3) prevented NO-induced growth cone collapse and axon retraction. Exposure to both BDNF and NO, but not either factor alone, stabilized growth cones and axons. Stabilized axons exhibited minimal retraction or extension. This response appears to be a new axon "state" and not simply a partial amelioration of the effect of NO, because lower doses of BDNF or NO allowed axon extension. Furthermore, BDNF/NO-induced growth cone stabilization correlated with the appearance of a cytochalasin D-resistant population of actin filaments. BDNF protection from NO likely was mediated locally at the level of the growth cone, because growth cones or individual filopodia in contact with BDNF-coated beads were protected from NO-induced collapse. These findings suggest a cellular mechanism by which some axonal connections are stabilized and some are eliminated during development.

NOS inhibition during postnatal development leads to increased ipsilateral retinocollicular and retinogeniculate projections in rats

Synthesis of nitric oxide (NO) occurs downstream from activation of N-methyl-D-aspartate (NMDA) receptors; NO reportedly acts as a retrograde messenger, influencing the refinement and stabilization of coactive afferent terminals. Cells and neuropil in the rat superior colliculus (SC) and lateral geniculate body (LGB) show intense, developmentally regulated activity for NO synthase (NOS). To study the role of NO in the development of retinogeniculate and retinotectal axon arbors, we examined primary visual projections of rats that had received intraperitoneal injections of Nomega-nitro-L-arginine (L-NoArg, an NOS inhibitor) on postnatal day 0, and daily thereafter for 4-6 weeks. Treated rats showed significant alterations in ipsilateral retinotectal projections, in the mediolateral and anteroposterior axes; there was an increase in the density of fibres entering the SC, in branch length, and in the numbers of boutons on retinotectal arbors in the treated group. Ipsilaterally projecting retinal axons also showed an increase in density and distribution in the dorsal nucleus of the LGB. If animals were allowed to survive for several months after stopping treatment, similar changes were also noted, but these were much less striking. Our results support the hypothesis that, in the mammalian visual system, NO released from target neurons in the SC and LGB serves as a retrograde signal which feeds back on retinal afferents, influencing their growth. The effects of NOS inhibition are partially reversed after treatment is stopped, indicating that lack of NO synthesis delays the maturation of retinofugal connections, and also that NO plays a constitutive role in their development.

Development of ocular dominance columns in the absence of retinal input

The initial establishment of ocular dominance columns in visual cortex is believed to involve the segregation of overlapping geniculocortical axons into eye-specific patches based on patterns of correlated activity. However, we found that total removal of retinal influence early in visual development did not prevent segregation of geniculocortical axons into alternating stripes with periodicity normal for ocular dominance columns. Because the patterning of geniculocortical afferents resists this dramatic change in the level, source and pattern of spontaneous activity, we propose that formation of ocular dominance columns relies on molecular cues present on thalamic axons, cortical cells or both.

Molecular analysis of developmental plasticity in neocortex

Gene expression studies indicate that during activity-dependent developmental plasticity, N-methyl-D-aspartate receptor activation causes a Ca(2+)-dependent increase in expression of transcription factors and their downstream targets. The products of these plasticity genes then operate collectively to bring about the structural and functional changes that underlie ocular dominance plasticity in visual cortex. Identifying and characterizing plasticity genes provides a tool for molecular dissection of the mechanisms involved. Members of second-messenger pathways identified in adult plasticity paradigms and elements of the transmission machinery are the first candidate plasticity genes tested for their role in activity-dependent developmental plasticity. Knockout mice with deletions of such genes have allowed analyzing their function in the context of different systems and in different paradigms. Studies of mutant mice reveal that activity-dependent plasticity is not necessarily a unified phenomenon. The relative importance of a gene can vary with the context of its expression during different forms of plasticity. Forward genetic screens provide additional new candidates for testing, some with well-defined cellular functions that provide insight into possible plasticity mechanisms.

Electrical activity and gene expression in the development of vertebrate neural circuits

Over the past several decades, anatomical and electrophysiological analyses have demonstrated that the electrical activity of neurons is required for development of the precise patterns of synaptic connectivity found in the adult central nervous system. However, knowledge of the molecular cascades that underlie activity-dependent synaptic development remains rudimentary. As a result, many fundamental issues remain unresolved. Recent advances in differential cloning have begun to provide the tools and insight necessary to bring a molecular level of understanding to principles of activity-dependent synaptic development established via classic systems approaches.

Developmental effects of in vivo and in vitro inhibition of nitric oxide synthase in neurons

The diffusible chemical messenger nitric oxide (NO) is involved in neuronal plasticity and it is, therefore, supposed to play a role in brain development. A shortage of NO during the critical period of brain maturation may theoretically have long-lasting consequences on the organization of the adult brain. We have performed in neonatal rats a chronic inhibition of the enzyme responsible for NO production, nitric oxide synthase (NOS), from postnatal day 3 to postnatal day 23, through administration of the competitive antagonist N-nitro-L-arginine methylester (L-NAME). The calcium-dependent catalytic activity resulted almost completely inhibited throughout the period of treatment and it took more than 4 days after its suspension to get a full recovery. The expression of the neuronal isoform of the enzyme (nNOS), revealed by immunoblotting, was unchanged during the treatment and after it. The histochemical reaction for NADPH diaphorase was reduced at the end of the treatment and recovered in concomitance with the recovery of the catalytic NOS activity. No gross structural alterations were detected in brain morphology. The levels of three neurotransmitter-related and one astrocytic marker were unchanged in the cerebellum, hippocampus and cortex of 60-day-old rats which had been neonatally treated. A similar lack of significant effects on neurochemical brain maturation was also noticed in a parallel series of experiments, in which a short pulse of NOS inhibition was performed at a critical prenatal time of brain development, from gestational day 14 to gestational day 19. In vitro, chronic exposure of cerebellar granule cells to L-NAME (500 microM) resulted in slight decrease of surviving neurons after 8 days in culture and in better resistance to the challenge of stressful culture conditions. The present results suggest that the basic plan of brain organization can be achieved despite an almost complete NOS inhibition during the maturation period. In vitro, NOS inhibition may bring to more pronounced consequences on neuronal viability and function.

The critical period for ocular dominance plasticity in the Ferret's visual cortex

Microelectrode recordings and optical imaging of intrinsic signals were used to define the critical period for susceptibility to monocular deprivation (MD) in the primary visual cortex of the ferret. Ferrets were monocularly deprived for 2, 7 or >14 d, beginning between postnatal day 19 (P19) and P110. The responses of visual cortical neurons to stimulation of the two eyes were used to gauge the onset, peak, and decline of the critical period. MDs ending before P32 produced little or no loss of response to the deprived eye. MDs of 7 d or more beginning around P42 produced the greatest effects. A rapid decline in cortical susceptibility to MD was observed after the seventh week of life, such that MDs beginning between P50 and P65 were approximately half as effective as those beginning on P42; MDs beginning after P100 did not reduce the response to the deprived eye below that to the nondeprived eye. At all ages, 2 d deprivations were 55-85% as effective as 7 d of MD. Maps of intrinsic optical responses from the deprived eye were weaker and less well tuned for orientation than those from the nondeprived eye, with the weakest maps seen in the hemisphere ipsilateral to the deprived eye. Analysis of the effects of 7 d and longer deprivations revealed a second period of plasticity in cortical responses in which MD induced an effect like that of strabismus. After P70, MD caused a marked loss of binocular responses with little or no overall loss of response to the deprived eye. The critical period measured here is compared to other features of development in ferret and cat.

Nitric oxide in the retinotectal system: a signal but not a retrograde messenger during map refinement and segregation

The role of nitric oxide (NO) as a mediator of synaptic plasticity is controversial in both the adult and developing brain. NO generation appears to be necessary for some types of NMDA receptor-dependent synaptic plasticity during development but not for others. Our previous work using several NO donors revealed that Xenopus laevis retinal ganglion cell axons stop growing in response to NO exposure. We demonstrate here that the same response occurs in tectal neuron processes bathed in the NO donor S-nitrosocysteine (SNOC) and in RGC growth cones to which SNOC is very locally applied. We show that NO synthase (NOS) activity is present in the Rana pipiens optic tectum throughout development in a dispersed subpopulation of tectal neurons, although effects of NO on synaptic function in a Rana pipiens tectal slice were varied. We chronically inhibited NOS in doubly innervated Rana tadpole optic tecta using L-N(G)-nitroarginine methyl ester in Elvax. Despite significant NOS inhibition as measured biochemically, eye-specific stripes remained normally segregated. This suggests that NOS activity is not downstream of NMDA receptor activation during retinotectal synaptic competition because NMDA receptor activation is necessary for segregation of retinal afferents into ocular dominance stripes in the doubly innervated tadpole optic tectum. We conclude that NO has some signaling function in the retinotectal pathway, but this function is not critical to the mechanism that refines the projection and causes eye-specific stripes.

Nitrinergic neurons in the developing and adult human telencephalon: transient and permanent patterns of expression in comparison to other mammals

A subpopulation of cerebral cortical neurons constitutively express nitric oxide synthase (NOS) and, upon demand, produce a novel messenger molecule nitric oxide (NO) with a variety of proposed roles in the developing, adult, and diseased brain. With respect to the intensity of their histochemical (NADPH-diaphorase histochemistry) and immunocytochemical (nNOS and eNOS immunocytochemistry) staining, these nitrinergic neurons are generally divided in type I and type II cells. Type I cells are usually large, intensely stained interneurons, scattered throughout all cortical layers; they frequently co-express GABA, neuropeptide Y, and somatostatin, but rarely contain calcium-binding proteins. Type II cells are small and lightly to moderately stained, about 20-fold more numerous than type I cells, located exclusively in supragranular layers, and found almost exclusively in the primate and human brain. In the developing cerebral cortex, nitrinergic neurons are among the earliest differentiating neurons, mostly because the dominant population of prenatal nitrinergic neurons are specific fetal subplate and Cajal-Retzius cells, which are the earliest generated neurons of the cortical anlage. However, at least in the human brain, a subpopulation of principal (pyramidal) cortical neurons transiently express NOS proteins in a regionally specific manner. In fact, transient overexpression of NOS-activity is a well-documented phenomenon in the developing mammalian cerebral cortex, suggesting that nitric oxide plays a significant role in the establishment and refinement of the cortical synaptic circuitry. Nitrinergic neurons are also present in human fetal basal forebrain and basal ganglia from 15 weeks of gestation onwards, thus being among the first chemically differentiated neurons within these brain regions. Finally, a subpopulation of human dorsal pallidal neurons transiently express NADPH-diaphorase activity during midgestation.

Development and organization of ocular dominance bands in primary visual cortex of the sable ferret

Thalamocortical afferents in the visual cortex of the adult sable ferret are segregated into eye-specific ocular dominance bands. The development of ocular dominance bands was studied by transneuronal labeling of the visual cortices of ferret kits between the ages of postnatal day 28 (P28) and P81 after intravitreous injections of either tritiated proline or wheat germ agglutinin-horseradish peroxidase. Laminar specificity was evident in the youngest animals studied and was similar to that in the adult by P50. In P28 and P30 ferret kits, no modulation reminiscent of ocular dominance bands was detectable in the pattern of labeling along layer IV. By P37 a slight fluctuation in the density of labeling in layer IV was evident in serial reconstructions. By P50, the amplitude of modulation had increased considerably but the pattern of ocular dominance bands did not yet appear mature. The pattern and degree of modulation of the ocular dominance bands resembled that in adult animals by P63. Flat mounts of cortex and serial reconstructions of layer IV revealed an unusual arrangement of inputs serving the two eyes in the region rostral to the periodic ocular dominance bands. In this region, inputs serving the contralateral eye were commonly fused along a mediolateral axis, rostral to which were large and sometimes fused patches of ipsilateral input.

Maturation of NADPH-d activity in the rat's barrel-field cortex and its relationship to cytochrome oxidase activity

Histochemical detection of NADPH-d activity in rat barrel-field cortex reveals four types of distributions. (i) A transient, diffuse neuropil staining is visible in the cortical plate and in deeper layers until postnatal day (P) 4. Thereafter, until P15, it is segregated in whisker-specific patches in layer IV, then the pattern gradually disappears, becoming virtually indistinct by P21. This transient patterning of diffuse NADPH-d activity in layer IV disappears after cortical injections of kainic acid and is affected by neonatal damage to the contralateral snout. An intense labeling (ii) of scattered cells and (iii) of a plexus of fibers is present. With maturation, the cells become localized mostly in layers II/III, in the lower part of layer V, and in layer VI. They are sparse in layer I, in upper layer V, and in layer IV where their somata are located primarily in the interbarrel septa. (iv) Light staining of cortical neurons is detected mostly in layers II-IV but occasionally also in layers V-VI. Cytochrome c oxidase (CO)-positive patches associated with barrels are first detected in layer IV around P4-P5; their staining density increases with development, then stays high. In the adult, CO activity is moderate in supragranular layers, highest in the barrels in layer IV, low in upper layer V, medium dense in the deeper half of layer V, and low in lamina VI. Thus, NADPH-d and CO activities are not necessarily colocalized in the rodent barrel-field cortex. The varied (transient and long-lasting) distributions of NADPH-d activity indicate that the enzyme and its associated production of NO serve multiple roles in developing and adult barrel-field cortex.