The role of nitric oxide in development of the patch-cluster system and retinocollicular pathways in the rodent superior colliculus

RNA Sequencing and Pathway Analysis Identify Important Pathways Involved in Hypertrichosis and Intellectual Disability in Patients with Wiedemann-Steiner Syndrome

A growing number of histone modifiers are involved in human neurodevelopmental disorders, suggesting that proper regulation of chromatin state is essential for the development of the central nervous system. Among them, heterozygous de novo variants in KMT2A, a gene coding for histone methyltransferase, have been associated with Wiedemann-Steiner syndrome (WSS), a rare developmental disorder mainly characterized by intellectual disability (ID) and hypertrichosis. As KMT2A is known to regulate the expression of multiple target genes through methylation of lysine 4 of histone 3 (H3K4me), we sought to investigate the transcriptomic consequences of KMT2A variants involved in WSS. Using fibroblasts from four WSS patients harboring loss-of-function KMT2A variants, we performed RNA sequencing and identified a number of genes for which transcription was altered in KMT2A-mutated cells compared to the control ones. Strikingly, analysis of the pathways and biological functions significantly deregulated between patients with WSS and healthy individuals revealed a number of processes predicted to be altered that are relevant for hypertrichosis and intellectual disability, the cardinal signs of this disease.

Sudden decrease in physical activity evokes adipocyte hyperplasia in 70- to 77-day-old rats but not 49- to 56-day-old rats

The cessation of physical activity in rodents and humans initiates obesogenic mechanisms. The overall purpose of the current study was to determine how the cessation of daily physical activity in rats at 49-56 days of age and at 70-77 days of age via wheel lock (WL) affects adipose tissue characteristics. Male Wistar rats began voluntary running at 28 days old and were either killed at 49-56 days old or at 70-77 days old. Two cohorts of rats always had wheel access (RUN), a second two cohorts of rats had wheel access restricted during the last 7 days (7d-WL), and a third two cohorts of rats did not have access to a voluntary running wheel after the first 6 days of (SED). We observed more robust changes with WL in the 70- to 77-day-old rats. Compared with RUN rats, 7d-WL rats exhibited greater rates of gain in fat mass and percent body fat, increased adipocyte number, higher percentage of small adipocytes, and greater cyclin A1 mRNA in epididymal and perirenal adipose tissue. In contrast, 49- to 56-day-old rats had no change in most of the same characteristics. There was no increase in inflammatory mRNA expression in either cohort with WL. These findings suggest that adipose tissue in 70- to 77-day-old rats is more protected from WL than 49- to 56-day-old rats and responds by expansion via hyperplasia.

Nitric oxide synthase regulates morphogenesis of zebrafish spinal cord motoneurons

Nitric oxide (NO) is a signaling molecule that is synthesized in a range of tissues by the NO synthases (NOSs). In the immature nervous system, the neuronal isoform of NOS (NOS1) is often expressed during periods of axon outgrowth and elaboration. However, there is little direct molecular evidence to suggest that NOS1 influences these processes. Here we address the functional role of NOS1 during in vivo zebrafish locomotor circuit development. We show that NOS1 is expressed in a population of interneurons that lie close to nascent motoneurons of the spinal cord. To determine how this protein regulates spinal network assembly, we perturbed NOS1 expression in vivo with antisense morpholino oligonucleotides. This treatment dramatically increased the number of axon collaterals formed by motoneuron axons, an effect mimicked by pharmacological inhibition of the NO/cGMP signaling pathway. In contrast, exogenous elevation of NO/cGMP levels suppressed motor axon branching. These effects were not accompanied by a change in motoneuron number, suggesting that NOS1 does not regulate motoneuron differentiation. Finally we show that perturbation of NO signaling affects the ontogeny of locomotor performance. Our findings provide evidence that NOS1 is a key regulator of motor axon ontogeny in the developing vertebrate spinal cord.

S-nitrosylation of microtubule-associated protein 1B mediates nitric-oxide-induced axon retraction

Treatment of cultured vertebrate neurons with nitric oxide leads to growth-cone collapse, axon retraction and the reconfiguration of axonal microtubules. We show that the light chain of microtubule-associated protein (MAP) 1B is a substrate for S-nitrosylation in vivo, in cultured cells and in vitro. S-nitrosylation occurs at Cys 2457 in the COOH terminus. Nitrosylation of MAP1B leads to enhanced interaction with microtubules and correlates with the inhibition of neuroblastoma cell differentiation. We further show, in dorsal root ganglion neurons, that MAP1B is necessary for neuronal nitric oxide synthase control of growth-cone size, growth-cone collapse and axon retraction. These results reveal an S-nitrosylation-dependent signal-transduction pathway that is involved in regulation of the axonal cytoskeleton and identify MAP1B as a major component of this pathway. We propose that MAP1B acts by inhibiting a microtubule- and dynein-based mechanism that normally prevents axon retraction.

Spike timing-dependent LTP/LTD mediates visual experience-dependent plasticity in a developing retinotectal system

Sensory experience plays an instructive role in the development of the nervous system. Here we showed that visual experience can induce persistent modification of developing retinotectal circuits via spike timing-dependent plasticity (STDP). Pairing light stimuli with spiking of the tectal cell induced persistent enhancement or reduction of light-evoked responses, with a dependence on the relative timing between light stimulus and postsynaptic spiking similar to that for STDP. Using precisely timed sequential three-bar stimulation to mimic a moving bar, we showed that spike timing-dependent LTP/LTD can account for the asymmetric modification of the tectal cell receptive field induced by moving bar. Furthermore, selective inhibition of signaling mediated by brain-derived neurotrophic factor and nitric oxide, which are respectively required for light-induced LTP and LTD, interfered with moving bar-induced temporally specific changes in the tectal cell responses. Together, these findings suggest that STDP can mediate sensory experience-dependent circuit refinement in the developing nervous system.

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.

Nitric oxide synthase inhibition during synaptic maturation decreases synapsin I immunoreactivity in rat brain

During the development of the brain, nitric oxide and synapsins, the latter being phosphoproteins associated to presynaptic membrane vesicles, are abundant in presynaptic terminals and play important and similar roles in neurotransmitter release, morphogenesis, synaptogenesis, and synaptic plasticity. These mechanisms are fundamental for neuronal development and plasticity and constitute important factors for the formation of neuroanatomical structures. Neural nitric oxide synthase (nNOS), synapsin I, and nNOS adapter protein (CAPON) constitute a ternary complex necessary for specific NO and synapsin functions at a presynaptic level. It is not known whether NO absence may affect the presence and/or activity of synapsins during brain development. To understand the role of NO in synaptogenesis, we studied the effects of NOS inhibition on synapsin I expression at a postnatal stage. Rat pups were treated with a competitive NOS antagonist, N-nitro-L-arginine methyl ester, from postnatal days 3 to 23. Control pups received exclusively an equivalent volume of saline solution. Histochemical and immunochemical techniques for NADPH-d and synapsin I, respectively, were carried out. NOS inhibition elicited a significant reduction in synapsin I immunoreactive density and NADPH-d activity in the brain in the analyzed areas-prefrontal cortex, hippocampus, and dorsal thalamus. These data show that the alterations originated by NO and synapsin deficiencies produce a diminution in synaptic density. Thus, functions that depend on the formation of synaptic connections such as learning and memory could be affected by NO deficiency.

Role of nitric oxide in the regulation of neuronal proliferation, survival and differentiation

Nitric oxide (NO), an important cellular messenger, has been linked to both neurodegenerative and neuroprotective actions. In the present review, we focus on recent data establishing a survival and differentiation role for NO in several neural in vitro and in vivo models. Nitric oxide has been found to be essential for survival of neuronal cell lines and primary neurons in culture under various death challenges. Furthermore, its lack may aggravate some neuropathological conditions in experimental animals. Several cellular pathways and signaling systems subserving this neuroprotective role of NO are considered in the review. Survey of recent data related to the developmental role of NO mainly focus on its action as a negative regulator of neuronal precursor cells proliferation and on its role of promotion of neuronal differentiation. Discussion on discrepancies arising from the literature is focused on the Janus-faced properties of the molecule and it is proposed that most controversial results are related to the intrinsic property of NO to compensate among functionally opposed effects. As an example, the increased proliferation of neural cell precursors under conditions of NO shortage may be, later on in the development, compensated by increased elimination through programmed cell death as a consequence of the lack of the survival-promoting action of the molecule. To elucidate these complex, and possibly contrasting, effects of NO is indicated as an important task for future researches.

Control of neuronal nitric oxide synthase and brain-derived neurotrophic factor levels by GABA-A receptors in the developing rat cortex

Gamma-aminobutyric acid (GABA) plays an important morphogenetic role, acting through GABA-A receptors, which are depolarizing in the developing rat brain. Other molecules with major morphogenetic roles are the nitric oxide free radical (NO(.)) and brain-derived neurotrophic factor (BDNF), both of which are involved in the control of synaptic plasticity and apoptosis. In the present work, we investigated the effect of GABA-A receptor activation on neuronal NO(.) synthase (nNOS) and BDNF immunoreactivity in the developing cortex of 5-day-old rats. We also determined the effect of GABA-A receptor activation on phosphorylated cAMP-response element binding protein (pCREB) immunoreactivity in an effort to elucidate the molecular mechanisms involved. Our results show that activation of GABA-A receptors leads to increased numbers of nNOS, BDNF and pCREB, as well as nNOS-pCREB and BDNF-pCREB doubly immunopositive cells. This effect is abolished when L-type Ca(2+) channels are blocked. These results indicate that the following mechanism could be operating: depolarization following GABA-A receptor activation leads to opening of L-type voltage-gated calcium channels, resulting in an increased Ca(2+) influx, which in turn leads to phosphorylation and, thus, activation, of the transcription factor CREB; the phosphorylated CREB can then induce BDNF, as well as nNOS.

Normal retinal development and retinofugal projections in mice lacking the retina-specific variant of actin-binding LIM domain protein

The actin-binding LIM domain protein (abLIM) is the mammalian homologue of UNC-115, a protein mediating axon guidance in C. elegans. AbLIM is widely expressed with three isoforms differing from one another by the length of their amino termini. Experiments utilizing dominant-negative mutants in the chick retina suggested a role for abLIM in axon path finding in retinal ganglion cells (RGCs). To investigate which variant is involved in the regulation of mammalian RGC axon guidance, we analyzed their expression profile in mice. The longest variant, abLIM-L, is highly enriched in the ganglion cell layer. AbLIM-L is up-regulated postnatally which temporally overlaps with the period of RGC axon remodeling. In contrast, the abLIM-M and abLIM-S variants are widespread and remain relatively constant through development. By selective gene targeting, we ablated abLIM-L to explore its functional significance in vivo. AbLIM-L mutant mice exhibit no apparent morphological or functional defects in photoreceptors and inner retinal neurons. Retinofugal projections and synaptic maturation also appear normal. These data suggest that abLIM-M is likely the isoform performing the essential function related to axon guidance.

Brain nitric oxide synthase expression in the developing ferret lateral geniculate nucleus: analysis of time course, localization, and synaptic contacts

Nitric oxide (NO) is a diffusible neurotransmitter that has been implicated in key developmental events, including the refinement of retinogeniculate axons into ON/OFF sublayers in the ferret lateral geniculate nucleus (LGN), and in the formation of eye-specific laminae in other species. To understand the role of NO in the LGN, it is critical to fully characterize the pattern of brain nitric oxide synthase (bNOS) expression within the nucleus, including the phenotype of the neural elements that express it. We have examined the temporal and spatial pattern of bNOS expression in the ferret LGN during the first 6 weeks of postnatal development, and in the adult, by detecting bNOS with a monoclonal antibody as well as beta-nicotinamide adenine dinucleotide phosphate-diaphorase histochemistry. We have found that bNOS is expressed in neurons in the A laminae of the LGN as early as postnatal day 7 (P7), a time coincident with eye-specific segregation of retinal axons. This expression continues through P35, with peak somatodendritic expression at P21. Fluorescent double labeling using antibodies to bNOS and glutamic acid decarboxylase indicate that bNOS is expressed in gamma-aminobutyric acid-ergic interneurons within the A laminae. Electron microscopic examination of bNOS-labeled cells showed synaptic contacts from terminals with two distinct morphologic profiles. Expression of bNOS within interneurons that receive contacts from multiple sources indicates that the synaptic circuitry associated with bNOS activation and the potential targets of NO may be more complex than originally thought and supports a potential new role for interneurons as cellular intermediaries in the refinement of pathways in the LGN. Our findings broaden the window of time that bNOS may be active within the developing LGN, suggesting an expanded role for NO during early postnatal development.

Chemoarchitecture of GABAergic neurons in the ferret superior colliculus

gamma-Aminobutyric acid (GABA)ergic neurons are thought to play a key role both in visual processing and in the complex sensory-motor transformations that take place in the mammalian superior colliculus. To understand the organization of GABAergic neurons in the ferret superior colliculus, we applied antisera to several markers of GABAergic function, including GABA, two isoforms of its synthetic enzyme glutamic acid decarboxylase (GAD-65 and GAD-67), and the GABA transporter, GAT-1. We also applied antisera to several calcium binding proteins (calbindin [CB], calretinin [CR], and parvalbumin [PV]) and neuronal nitric oxide synthase (NOS), chemical markers that colocalize with GABA in some areas of the central nervous system. The distribution of GABAergic neurons in the ferret is similar to that of other mammalian species. GABAergic neurons in the ferret superior colliculus were small, morphologically diverse, and widely distributed throughout all layers of the colliculus. As has been shown in other mammalian species, neurons expressing PV, CB, CR, and NOS were differentially distributed in layers and patches throughout the ferret colliculus. None of these markers, however, showed a distribution that mirrored that of GABAergic neurons. Furthermore, few GABAergic neurons colocalized these neurochemical markers. Only 14% of GABAergic neurons in the superficial layers and 18% of neurons in the deeper layers colocalized PV, 14% of GABAergic neurons in the superficial layers and 10% in the deeper layers colocalized CB, and only 1% of GABAergic neurons in both the superficial and deep layers colocalized nitric oxide synthase. Thus, the arrangement of GABAergic neurons in the ferret superior colliculus is broadly distributed and is distinct from other recognized organizational patterns in the superior colliculus.

Influence of NO downregulation on oscillatory evoked responses in developing rat superior colliculus

Nitric oxide (NO) is involved in neuronal transmission by modulating neurotransmitter release in adults and in stabilizing synaptic connections in developing brains. We investigated the influence of downregulation of NO synthesis on oscillatory components of ON and OFF evoked field potentials in the rat superior colliculus. NO synthesis was decreased by inhibiting nitric oxide synthase (NOS) with an acute microinjection of N(omega)-nitro-L-arginine methyl ester (L-NAME). The study focuses on rhythmic activity by analyzing fast Fourier transform (FFT). Collicular responses were recorded in anesthetized rats, at postnatal days (PND) 13-19 and adults. This time window was chosen because it is centered on eye opening. NO downregulation resulted in a dual effect depending on age and response-type. NO synthesis inhibition decreased the magnitude of oscillations in ON responses in the youngest animals (PND13-14), whereas oscillations of frequencies higher than 20 Hz in OFF responses were increased in all age groups of developing rats. In adults NO downregulation increased oscillations in ON responses and decreased oscillations in OFF responses. L-Arginine was used to increase NOS activity and its injection produced effects opposite to those seen with L-NAME. Slow oscillatory components (7-12 Hz) were unaffected in all experiments. Our data together with results reported in the literature suggest that rhythmic patterns of activity are NO-dependent.

Microtubule reconfiguration during axonal retraction induced by nitric oxide

Axonal retraction is induced by different types of physiological cues and is responsible for the elimination of mistargeted axons. There is broad agreement that alterations in the cytoskeleton underlie axonal retraction. The prevailing view is that axonal retraction involves a wholesale depolymerization of microtubules and microfilaments. However, axons retracting physiologically display a very different morphology than axons induced to retract by experimental depolymerization of microtubules. Experimental depolymerization of microfilaments actually prevents retraction rather than causing it. We have proposed an alternative hypothesis, namely that axonal retraction involves a backward retreat of cytoskeletal elements rather than their wholesale depolymerization. In the present study, we sought to test this hypothesis with regard to microtubules. When a donor of nitric oxide was applied to cultured chick sensory neurons, the majority of axons retracted dramatically within 30-60 min. Retracting axons were characterized by an enlarged distal region, a thin trailing remnant, and sinusoidal bends along the shaft. Quantitative immunofluorescence analyses showed no detectable loss of microtubule mass during retraction, even with regard to the most labile microtubules. Instead, microtubules were reconfigured into coiling and sinusoidal bundles to accommodate the shortening of the axon. Stabilization of microtubules by taxol did not prevent the retraction, even at concentrations of the drug that actually caused microtubule levels to increase. The retractions induced by nitric oxide were remarkably similar to those observed when motor proteins are manipulated, suggesting that these retractions may result from alterations in the activities of the motors that configure microtubules.

Expression of the L-type calcium channel in the developing mouse visual system by use of immunocytochemistry

Developmental refinement of the retinogeniculate and retinocollicular pathways is partially dependent upon Ca(2+) channel function [J. Comp. Neurol. 440 (2001) 177-191]. We have examined the development of the L-type voltage gated Ca(2+) channel to determine if the onset of expression matches this period of refinement. Labeling by an antibody directed against the alpha 1C subunit of this channel was examined in the superior colliculus (SC), lateral geniculate nucleus (LGN), visual cortex (CTX), hippocampus (HC) and cerebellum (CB) in mice aged P3-4, P8-9, P15, P21, P28, and adults. At P3-4, labeled cells within the SC were concentrated within a dense band in the retinorecipient zone of the superficial gray layer. More lightly labeled neurons were seen in other layers. This dense band was still seen at P15, while more labeled neurons were seen in other layers. By P21-P28, labeled neurons were fairly uniformly distributed throughout all layers of SC. Neuronal cell types appeared to be labeled at all ages examined within the LGN. Within CTX, putative layer V-VI pyramidal neurons were well labeled at P4 and later ages, and labeled layer II-III pyramids could be distinguished by P9 and later ages. The dendrites and cell bodies of pyramidal neurons within CA1-CA3 of HC, granule neurons in the dentate gyrus, and Purkinje neurons in CB were labeled at all ages examined. We conclude that the L-type Ca(2+) channel is expressed in many neurons within retinorecipient targets as well as in other brain regions during the developmental period in which pathway refinement and synaptic plasticity occurs.

Inhibition of nitric oxide synthase induces increased production of growth-associated protein 43 in the developing retina of the postnatal rat

We investigated the effects of N(G)-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor, on retinal development in the postnatal rat by immunocytochemistry and immunoblotting using antisera against neuronal nitric oxide synthase (nNOS) or growth-associated protein 43 (GAP-43). An nNOS-immunoreactive band of 155 kDa and a GAP-43-immunoreactive band of 48 kDa were present in the extracts of both control and L-NAME-treated rat retinas. The intensity of the nNOS-immunoreactive band was much weaker in the treated rats, whereas the intensity of the GAP-43-immunoreactive band of 48 kDa was much stronger in the treated rats. Much stronger GAP-43 immunoreactivity was visible in the inner plexiform layer (IPL) of the treated retinas at P10, P14 and P21. Our findings suggest that NO may play an important role in the maturation of the IPL in the developing rat retina.

Development of the visual pathway is disrupted in mice with a targeted disruption of the calcium channel beta(3)-subunit gene

Refinement of the retinal pathways to the superior colliculus (SC) and dorsal lateral geniculate nucleus (dLGN) is mediated by nitric oxide (NO). Long-term depression (LTD) can also be induced in SC and LGN during the time at which these pathways are refined, and this LTD is partially dependent on NO and L-type Ca(2+) channel function. In an effort to determine whether NO-mediated pathway refinement is also mediated by Ca(2+) channel function, we have examined the refinement of the retinocollicular and retinogeniculate pathways in mice which lack the gene for the Ca(2+) channel beta(3) subunit (CCKO) and which have significantly reduced L-type Ca(2+) currents. Injections of the anterograde tracer cholera toxin subunit B/HRP were made into one eye of these knockout animals and in wild-type mice ages postnatal day (P) 13, P19, and P26. After 48 hours, mice were perfused and sections processed by using tetramethylbenzidine histochemistry. Labeling distribution in some animals was analyzed quantitatively. Obvious differences in the distribution of the ipsilateral retinocollicular pathway were observed at P15, with the pathway being more exuberant in CCKO mice. This difference was statistically significant. More subtle differences were seen at P21 and P28. Obvious differences were also seen in the contralateral retinogeniculate pathway which in CCKO mice filled most of the domain normally occupied by ipsilateral eye fibers. This difference was also statistically significant. We conclude that reduction in L-type Ca(2+) currents has an effect on axonal refinement similar to that which occurs in NO knockout mice, which supports the possibility that L-type Ca(2+) channel-dependent LTD mediates NO-dependent axonal refinement.

Structure and projections of white matter neurons in the postnatal rat visual cortex

Transient contributions of subplate neurons to the initial development of the cortex are well-characterized, yet little data are available on a subpopulation of subplate neurons that persist in the white matter (WM) of the cerebral cortex across development. To characterize the WM neurons, differential interference contrast and Nomarski optics were used to visualize individual cells in the WM in slices of rat visual cortex at postnatal ages 9-23. Soma-dendritic morphology and local axonal projection patterns, including probable synaptic innervation sites of their axons, were identified by intracellular filling with biocytin during electrophysiologic recordings. Dendritic branches of all WM neurons, tripartitioned here into upper, middle, and deep divisions, extend throughout the WM and frequently into the overlying cortex. Axonal arborizations from most WM neurons, including apparent boutons, project into adjacent WM with many also innervating overlying cortical layers, whereas some project into the stratum oriens/alveus of the hippocampal formation. Processes of a subset of WM neurons appear to be confined to the WM itself. By using antimicrotubule associated protein (MAP2) immunostaining to quantify the density of WM neurons in rat visual cortex, we find that their overall numbers decrease to approximately 30% of initial levels during postnatal development. During this same developmental period, an increasing percentage of WM neurons contain the synthetic enzyme for nitric oxide, nitric oxide synthase (NOS), as evaluated by immunostaining. Thus, WM neurons that survive the initial perinatal period of cell death are positioned under the laminae of the maturing cortex to potentially modulate the integration of visual signals through either conventional synaptic or nonconventional (diffusible NO signaling) mechanisms.

The development of NADPH-diaphorase and nitric oxide synthase in the visual system of the cichlid fish, Tilapia mariae

The pattern of NADPH-diaphorase expression was studied in the retina and optic tectum of the cichlid fish Tilapia mariae during the first developmental stages. NADPH-diaphorase activity was seen early, at hatching. In the retina a few cell bodies of the retinal inner nuclear layer showed a faint labeling. Scattered labeled cells were found in the stratum periventriculare of the optic tectum, while the optic nerve was unlabeled. Two days after hatching, the number of labeled neurons increased in the inner nuclear layer and a few stained cell bodies were also scattered in the ganglion cell layer. Both the inner and outer plexiform layers showed a diffuse staining and the optic nerve was devoid of labeling. In the optic tectum several positive cells in the periventricular layer, with their dendritic trees extending in the superficial fibrous layer, were found. In 1-month-old Tilapia, NADPH-diaphorase staining and nitric oxide synthase immunoreactivity were found to overlap in both the retina and optic tectum. The density of NADPH-diaphorase labeled neurons in the inner nuclear layer of the retina and in the stratum periventriculare of the optic tectum was largely reduced in comparison with 2 days posthatching embryos. These findings indicated an early and transient production of nitric oxide in the retina and optic tectum of Tilapia, suggesting a functional role for nitric oxide in the development of visual structures in aquatic vertebrates.

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.

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.

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.

Nitric oxide synthase distribution in the cat superior colliculus and co-localization with choline acetyltransferase

Nitric oxide and acetylcholine are important neuromodulators implicated in brain plasticity and disease. We have examined the cellular and fiber localization of nitric oxide in the cat superior colliculus (SC) and its degree of co-localization with ACh using nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry and an antibody to neuronal nitric oxide synthase. ACh was localized using an antibody against choline acetyltransferase. We also made injections of biocytin into the region of the parabrachial brainstem to confirm that this region is a source of nitric oxide containing fibers in SC. NADPHd labeled neurons within the superficial layers of the superior colliculus included pyriform, vertical fusiform, and horizontal morphologies. Labeled neurons in the intermediate gray layer were small to medium in size, and mostly of stellate morphology. Neurons in the deepest layers had mostly vertical or stellate morphologies. NADPHd labeled fibers formed dense patches of terminal boutons within the intermediate gray layer and streams of fibers within the deepest layers of SC. Choline acetyltransferase antibody labeling in adjacent sections indicated that many fibers must contain both labels. Over 94% of neurons in the pedunculopontine tegmental and lateral dorsal tegmental nuclei were also labeled by both NADPHd and choline acetyltransferase. In addition, biocytin labeled fibers from this region were localized in the NADPHd labeled patches. We conclude that nitric oxide is contained in a variety of cell types in SC and that both nitric oxide and ACh likely serve as co-modulators in this midbrain structure.

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.

Failure to disrupt development of cholinergic fiber patches in the superior colliculus in nitric oxide synthase deficient mice

Nitric oxide (NO) has been shown to mediate refinement of glutamatergic axonal pathways during development. In this study, we investigated whether the development of a cholinergic pathway in the intermediate gray layer (IGL) of the mouse superior colliculus (SC) is also mediated by NO. The pathway was labeled using an antibody directed against choline acetyltransferase (ChAT) and its distribution examined in normal C57/BL6 mice and in knockout mice in which the genes for the neuronal isoform of nitric oxide synthase (NOS) or both the endothelial and neuronal isoforms of NOS had been disrupted. We also examined the development of expression of NOS using nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) staining. NADPHd labeled cells were found within the IGL by P8 and formed loose clusters of cells by P12-P15. ChAT and NADPHd labeled fibers were first observed at P12 and gradually established their characteristic two-tiered patchy pattern between P14 and P21. Comparison of the ChAT labeled fiber distribution in normal, single nNOS and double e,nNOS knockout mice revealed no differences between these three groups. We therefore conclude that nitric oxide does not mediate refinement of this cholinergic pathway.

The neuronal form of nitric oxide synthase is required for pattern formation by retinal afferents in the ferret lateral geniculate nucleus

The ferret retinogeniculate projection undergoes activity-dependent refinement of connections that become restricted to eye specific laminae and On/Off sublaminae in the lateral geniculate nucleus (LGN). We have previously shown that the developmental process by which On/Off sublaminae form requires N-methyl-D-aspartate (NMDA) receptors and nitric oxide (NO). In this study, we investigate the role of the neuronal form of NO synthase (nNOS) in sublaminar refinement. This isoform of NOS may be coupled with NMDA receptors at postsynaptic sites. We found that nNOS is present in the developing LGN, and that blocking nNOS during development disrupts the formation of On/Off sublaminae. Endothelial NOS (eNOS) is not expressed in the LGN until after sublaminae have formed. These results suggest that the nNOS isoform is the predominant contributor of NO during development, and support the hypothesis that NO acts downstream of NMDA receptor activation to mediate activity-dependent changes in the patterning of connections in the LGN.

Retinal input induces three firing patterns in neurons of the superficial superior colliculus of neonatal rats

By using an in vitro isolated brain stem preparation, we recorded extracellular responses to electrical stimulation of the optic tract (OT) from 71 neurons in the superficial superior colliculus (SC) of neonatal rats (P1-13). At postnatal day 1 (P1), all tested neurons (n = 10) already received excitatory input from the retina. Sixty-nine (97%) superficial SC neurons of neonatal rats showed three response patterns to OT stimulation, which depended on stimulus intensity. A weak stimulus evoked only one spike that was caused by activation of non-N-methyl-D-aspartate (NMDA) glutamate receptors. A moderate stimulus elicited a short train (<250 ms) of spikes, which was induced by activation of both NMDA and non-NMDA receptors. A strong stimulus gave rise to a long train (>300 ms) of spikes, which was associated with additional activation of L-type high-threshold calcium channels. The long train firing pattern could also be induced either by temporal summation of retinal inputs or by blocking gamma-aminobutyric acid-A receptors. Because retinal ganglion cells show synchronous bursting activity before eye opening at P14, the retinotectal inputs appear to be sufficient to activate L-type calcium channels in the absence of pattern vision. Therefore activation of L-type calcium channels is likely to be an important source for calcium influx into SC neurons in neonatal rats.