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

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.

Distribution of nitric oxide in the rock cavy (Kerodon rupestris) brain II: The brainstem

In a recent paper, we described the distribution of Nitric oxide (NO) in the diencephalon of the rock cavy (Kerodon rupestris). This present paper follows this work, showing the distribution of NO synthesizing neurons in the rock cavy's brainstem. For this, we used immunohistochemistry against the neuronal form of nitric oxide synthase (NOS) and NADPH diaphorase histochemistry. In contrast to the diencephalon in the rock cavy, where the NOS neurons were seen to be limited to some nuclei in the thalamus and hypothalamus, the distribution of NOS in the brainstem is widespread. Neurons immunoreactive to NOS (NOS-ir) were seen as rostral as the precommissural nuclei and as caudal as the caudal and gelatinous parts of the spinal trigeminal nucleus. Places such as the raphe nuclei, trigeminal complex, superior and inferior colliculus, oculomotor complex, periaqueductal grey matter, solitary tract nucleus, laterodorsal tegmental nucleus, pedunculopontine tegmental, and other nuclei of the reticular formation are among the locations with the most NOS-ir neurons. This distribution is similar, but with some differences, to those described for other rodents, indicating that NO also has an important role in rock cavy's physiology.

Effects of nitric oxide on neuromuscular properties of developing zebrafish embryos

Nitric oxide is a bioactive signalling molecule that is known to affect a wide range of neurodevelopmental processes. However, its functional relevance to neuromuscular development is not fully understood. Here we have examined developmental roles of nitric oxide during formation and maturation of neuromuscular contacts in zebrafish. Using histochemical approaches we show that elevating nitric oxide levels reduces the number of neuromuscular synapses within the axial swimming muscles whilst inhibition of nitric oxide biosynthesis has the opposite effect. We further show that nitric oxide signalling does not change synapse density, suggesting that the observed effects are a consequence of previously reported changes in motor axon branch formation. Moreover, we have used in vivo patch clamp electrophysiology to examine the effects of nitric oxide on physiological maturation of zebrafish neuromuscular junctions. We show that developmental exposure to nitric oxide affects the kinetics of spontaneous miniature end plate currents and impacts the neuromuscular drive for locomotion. Taken together, our findings implicate nitrergic signalling in the regulation of zebrafish neuromuscular development and locomotor maturation.

Neuronal nitric oxide synthase expressing neurons: a journey from birth to neuronal circuits

Nitric oxide (NO) is an important signaling molecule crucial for many physiological processes such as synaptic plasticity, vasomotricity, and inflammation. Neuronal nitric oxide synthase (nNOS) is the enzyme responsible for the synthesis of NO by neurons. In the juvenile and mature hippocampus and neocortex nNOS is primarily expressed by subpopulations of GABAergic interneurons. Over the past two decades, many advances have been achieved in the characterization of neocortical and hippocampal nNOS expressing neurons. In this review, we summarize past and present studies that have characterized the electrophysiological, morphological, molecular, and synaptic properties of these neurons. We also discuss recent studies that have shed light on the developmental origins and specification of GABAergic neurons with specific attention to neocortical and hippocampal nNOS expressing GABAergic neurons. Finally, we summarize the roles of NO and nNOS-expressing inhibitory neurons.

Ten-m3 is required for the development of topography in the ipsilateral retinocollicular pathway

Background

The alignment of ipsilaterally and contralaterally projecting retinal axons that view the same part of visual space is fundamental to binocular vision. While much progress has been made regarding the mechanisms which regulate contralateral topography, very little is known of the mechanisms which regulate the mapping of ipsilateral axons such that they align with their contralateral counterparts.

Results

Using the advantageous model provided by the mouse retinocollicular pathway, we have performed anterograde tracing experiments which demonstrate that ipsilateral retinal axons begin to form terminal zones (TZs) in the superior colliculus (SC), within the first few postnatal days. These appear mature by postnatal day 11. Importantly, TZs formed by ipsilaterally-projecting retinal axons are spatially offset from those of contralaterally-projecting axons arising from the same retinotopic location from the outset. This pattern is consistent with that required for adult visuotopy. We further demonstrate that a member of the Ten-m/Odz/Teneurin family of homophilic transmembrane glycoproteins, Ten-m3, is an essential regulator of ipsilateral retinocollicular topography. Ten-m3 mRNA is expressed in a high-medial to low-lateral gradient in the developing SC. This corresponds topographically with its high-ventral to low-dorsal retinal gradient. In Ten-m3 knockout mice, contralateral ventrotemporal axons appropriately target rostromedial SC, whereas ipsilateral axons exhibit dramatic targeting errors along both the mediolateral and rostrocaudal axes of the SC, with a caudal shift of the primary TZ, as well as the formation of secondary, caudolaterally displaced TZs. In addition to these dramatic ipsilateral-specific mapping errors, both contralateral and ipsilateral retinocollicular TZs exhibit more subtle changes in morphology.

Conclusions

We conclude that important aspects of adult visuotopy are established via the differential sensitivity of ipsilateral and contralateral axons to intrinsic guidance cues. Further, we show that Ten-m3 plays a critical role in this process and is particularly important for the mapping of the ipsilateral retinocollicular pathway.

Age-dependent morphology of NADPH diaphorase-positive amacrine cells in the mouse retina

NADPH diaphorase-positive amacrine cells (NAC) were studied in retinal whole mount preparation of mice, ranging from 1 day to 30 months of age. Following a peak in number and size during early development at postnatal day 14, their number and distribution remained well preserved up to senescence. Functional considerations include immunological, vascular and neuro-modulating aspects.

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.

Plasticity of nodose ganglion neurons after capsaicin- and vagotomy-induced nerve damage in adult rats

Previous reports show that vagal afferent innervation of the stomach eventually regenerates from surviving nodose ganglion (NG) neurons after subdiaphragmatic vagotomy. Systemic capsaicin treatment destroys gastric vagal afferent neurons expressing vanilloid receptor 1 (VR1). However, it is not known whether gastric innervation lost after neuronal destruction can be restored. Here, we report that capsaicin-induced damage of NG neurons innervating the stomach in adult rats is followed by restoration of vagal afferent projections. Specifically, we compared measures of neuronal plasticity in NG and vagi after subdiaphragmatic vagotomy or capsaicin treatment. The numbers of VR1-immunoreactive neurons projecting to the stomach were significantly reduced 10 days after either capsaicin treatment or vagotomy. However, the VR1-immunoreactive afferent innervation of the stomach was restored to levels exceeding those of vagotomized rats by 37 days after capsaicin, whereas neither total afferent innervation nor VR1-immunoreactive innervation reached control levels, even by 67 days after vagotomy. Capsaicin treatment significantly increased NG neuronal nitric oxide synthase (nNOS) immunoreactivity at 10 days after capsaicin, and this increase was sustained for the duration of the study, indicating higher nNOS demand in restoration of vagal projections. Vagotomy was associated with a much smaller increase in the number of nNOS-immunoreactive NG neurons, detectable only at 10 days after surgery. The number of nNOS-immunopositive gastric-projecting neurons was dramatically reduced 10 days after either capsaicin treatment or vagotomy but returned to the control level in both groups at 67 days. We found a significantly higher number of growth cones in capsaicin-treated animals compared with controls. Capsaicin significantly increased the number of nNOS-immunopositive and nNOS-immunonegative growth cones in NG at all time points. Vagotomy did not increase the number of nNOS(-) growth cones in NG. We conclude that capsaicin treatment may result in more significant restorative capacities than vagotomy, mainly because of sprouting of capsaicin-insensitive nerve fibers.

The role of nitric oxide on matrix metalloproteinase 2 (MMP2) and MMP9 in placenta and fetus from diabetic rats

Matrix metalloproteinases (MMPs) play an important role in tissue remodeling that accompanies the rapid growth, differentiation, and structural changes of the placenta and several fetal organs. In the present study, we investigated whether the diabetic maternal environment may alter the regulatory homeostasis exerted by nitric oxide (NO) on MMPs activity in the feto-placental unit from rats at midgestation. We found that NADPH-diaphorase activity, which reflects the distribution and activity of NO synthases (NOS), was increased in both placenta and fetuses from diabetic rats when compared with controls. In addition, while a NO donor enhanced MMP2 and MMP9 activities, a NOS inhibitor reduced these activities in the maternal side of the placenta from control rats. This regulatory effect of NO was only observed on MMP9 in the diabetic group. On the other hand, the NO donor did not modify MMP2 and MMP9 activities, while the NOS inhibitor reduced MMP9 activity in the fetal side of both control and diabetic placentas. In the fetuses, MMP2 was enhanced by the NO donor and reduced by the NO inhibitor in both fetuses from control and diabetic rats. Overall, this study demonstrates that NO is able to modulate the activation of MMPs in the feto-placental unit, and provides supportive evidence that increased NOS activity leads to NO overproduction in the feto-placental unit from diabetic rats, an alteration closely related to the observed MMPs dysregulation that may have profound implications in the formation and function of the placenta and the fetal organs.

Embryonic motor neuron dendrite growth is stunted by inhibition of nitric oxide-dependent activation of soluble guanylyl cyclase and protein kinase G

We have examined the participation of a neuronal nitric oxide synthase (nNOS) signaling pathway in the elaboration of motor neuron dendrites during embryonic life. During chick embryogenesis, nNOS is expressed by interneurons that surround the motor neuron pools in the ventral horn. Pseudorabies virus tracing suggests that these cells, while juxtaposed to motor neurons are not synaptically connected to them. The downstream effectors, soluble guanylyl cyclase (sGC) and protein kinase G (PKG), are found in motor neurons as well as several other populations of spinal cord cells. To determine the functional significance of the nNOS/sGC/PKG signaling pathway, pharmacological inhibitors were applied to chick embryos and the effects on motor neuron dendrites monitored. Inhibition of nNOS activity led to a lasting reduction in the overall size and degree of branching of the dendritic tree. These alterations in dendritic architecture were also seen when the activity of sGC or PKG was blocked. Our results suggest that normal motor neuron dendrite elaboration depends, in part, on the activity-dependent generation of NO by ventral horn interneurons, which then activates sGC and PKG in motor neurons.

Combining visual information from the two eyes: the relationship between isthmotectal cells that project to ipsilateral and to contralateral optic tectum using fluorescent retrograde labels in the frog, Rana pipiens

The frog nucleus isthmi (homolog of the mammalian parabigeminal nucleus) is a visually responsive tegmental structure that is reciprocally connected with the ipsilateral optic tectum; cells in nucleus isthmi also project to the contralateral optic tectum. We investigated the location of the isthmotectal cells that project ipsilaterally and contralaterally using three retrograde fluorescent label solutions: Alexa Fluor 488 10,000 mw dextran conjugate; Rhodamine B isothiocyanate; and Nuclear Yellow. Dye solutions were pressure-injected into separate sites in the superficial optic tectum. Following a 6-day survival, brains were fixed, sectioned, and then photographed. Injection of the different labels at separate, discrete locations in the optic tectum result in retrograde filling of singly labeled clusters of cells in both the ipsilateral and contralateral nucleus isthmi. Generally, ipsilaterally projecting cells are dorsal to the contralaterally projecting cells, but there is a slight overlap between the two sets of cells. Nonetheless, when different retrograde labels are injected into opposite tecta, there is no indication that individual cells project to both tecta. The set of cells that project to the ipsilateral tectum and the set of cells that project to the contralateral tectum form a visuotopic map in a roughly vertical, transverse slab. Our results suggest that nucleus isthmi can be separated into two regions with cells in the dorsolateral portion projecting primarily to the ipsilateral optic tectum and cells in the ventrolateral nucleus isthmi projecting primarily to the contralateral optic tectum.

Nitric oxide release from a single cell affects filopodial motility on growth cones of neighboring neurons

Nitric oxide (NO), a gaseous messenger, has been reported to be involved in a variety of functions in the nervous system, ranging from neuronal pathfinding to learning and memory. We have shown previously that the application of NO via NO donors to growth cones of identified Helisoma buccal neurons B5 in vitro induces an increase in filopodial length, a decrease in filopodial number, and a slowing in neurite advance. It is unclear, however, whether NO released from a physiological source would affect growth cone dynamics. Here we used cell bodies of identified neurons known to express the NO synthesizing enzyme nitric oxide synthase (NOS) as a source of constitutive NO production and tested their effect on growth cones of other cells in a sender-receiver paradigm. We showed that B5 cell bodies induced a rapid increase in filopodial length in NO-responsive growth cones, and that this effect was blocked by the NOS inhibitor 7-NI, suggesting that the effect was mediated by NO. Inhibition of soluble guanylyl cyclase (sGC) with ODQ blocked filopodial elongation induced by B5 somata, confirming that NO acted via sGC. We also demonstrate that the effect of NO was reversible and that a cell releasing NO can affect growth cones over a distance of at least 100 microm. Our results suggest that NO released from a physiological source can affect the motility of nearby growth cones and thus should be considered a signaling molecule with the potential to affect the outcome of neuronal pathfinding in vivo.

The roles of nitric oxide in murine cardiovascular development

Nitric oxide (NO) participates in a diverse array of biological functions in mammalian organ systems. Depending on the biochemical environment, the production of NO may result in cytoprotection or cytotoxicity. The paradoxical actions of NO arise from the complexities generated by the redox milieu, NO concentration/bioavailability, and tissue/cell context, which ultimately result in the wide range of regulatory roles observed. Additionally, in physiological versus pathological states, NO often displays diametrically opposing affects in several organ systems. Here, we will discuss the roles of NO during reproduction, organ system development, in particular, the cardiovascular system, and its potential implications in diabetes-induced fetal defects.

Blockade of arachidonic acid pathway induces sprouting in the adult but not in the neonatal uncrossed retinotectal projection

The uncrossed retinotectal projection of rats undergoes extensive axonal elimination and subsequent growth of axonal arbors in topographically appropriate territories within the first two/three postnatal weeks. Nitric oxide has been implicated in development and stabilization of synapses in the retinotectal pathway since blockade of nitric oxide synthesis disrupts the normal pattern of retinal innervation in subcortical nuclei. The present work investigated the role of arachidonic acid pathway in the development and maintenance of ipsilateral retinotectal axons. We also investigated the role of this retrograde messenger in the modulation of plasticity that follows retinal lesions in the opposite eye. Pigmented rats received systemic treatment with quinacrine, a phospholipase A2 inhibitor, indomethacin, a cyclooxygenase inhibitor, nordihydroguaiaretic acid, a 5-lipoxygenase inhibitor or vehicle during 4-8 days at various postnatal ages. Rats given a unilateral temporal retinal lesion were treated with either quinacrine or vehicle during the same period. For anterograde tracing of ipsilateral retinal projections, animals received intraocular injections of horseradish peroxidase. Before the third postnatal week no difference was observed in the laminar or topographic organization of the ipsilateral retinotectal projection between vehicle and treated rats in either normal or lesion conditions. After the third postnatal week, however, systemic blockade of phospholipase A2 or 5-lipoxygenase, but not cyclooxygenase induced sprouting of uncrossed axons throughout the collicular visual layers in unoperated rats. In retinal lesion groups, phospholipase A2 blockade increased the sprouting of uncrossed intact axons to the collicular surface in the same period. The results suggest that arachidonic acid or lipoxygenase metabolites play a role in the maintenance of the retinotectal synapses after the critical period and that the blockade of the arachidonic acid pathway induces reactive sprouting of retinal axons late in development.

Normal eye-specific patterning of retinal inputs to murine subcortical visual nuclei in the absence of brain-derived neurotrophic factor

Brain-derived neurotrophic factor (BDNF) is a preferred ligand for a member of the tropomyosin-related receptor family, trkB. Activation of trkB is implicated in various activity-independent as well as activity-dependent growth processes in many developing and mature neural systems. In the subcortical visual system, where electrical activity has been implicated in normal development, both differential survival, as well as remodeling of axonal arbors, have been suggested to contribute to eye-specific segregation of retinal ganglion cell inputs. Here, we tested whether BDNF is required for eye-specific segregation of visual inputs to the lateral geniculate nucleus and the superior colliculus, and two other major subcortical target fields in mice. We report that eye-specific patterning is normal in two mutants that lack BDNF expression during the segregation period: a germ-line knockout for BDNF, and a conditional mutant in which BDNF expression is absent or greatly reduced in the central nervous system. We conclude that the availability of BDNF is not necessary for eye-specific segregation in subcortical visual nuclei.

Contribution of GABAergic inhibition to synaptic responses and LTD early in postnatal development in the rat superior colliculus

We studied the development of optic tract evoked field potentials (FP) in the rodent superior colliculus (SC) and the effect of GABA antagonists upon their development and upon induction of long-term depression (LTD). Brain slices were cut from Lister Hooded rats. The optic tract was stimulated while recording from the superficial grey layer. GABAergic inhibition was assessed by adding 100 microm picrotoxin and 3 microm CGP55845 antagonists to block GABA A,B,C receptors. LTD was induced with a 50 Hz, 20 s tetanus. At age P2, the FP consisted only of a presynaptic spike. The GABA antagonists had no effect. By P4, the FP consisted of a presynaptic spike, a longer latency population spike, and a field excitatory postsynaptic potential (fEPSP). The fEPSP was slightly prolonged by the GABA antagonists at this age. By P7-P14, a prominent FP with trailing fEPSP was recorded. The GABA antagonists usually had a large effect, with the fEPSP increasing in both amplitude and duration. A mature FP was usually recorded in P15-P23 slices where the GABA antagonist effect remained substantial. LTD could be induced in 17 of 30 control slices from rats aged P4-P26. The average fEPSP amplitude after tetanus was 77.9% of control. Pre-treatment with GABA antagonists produced a short-term potentiation (average 114.0%), rather than LTD, in 14 of 19 cases. This STP was followed by a more prolonged potentiation in 12 of the 14 cases. We conclude that GABAergic inhibitory circuits mature before eye opening and that GABA contributes to induction of LTD in the developing SC.

Expression of neuronal nitric oxide synthase in the developing superficial layers of the rat superior colliculus

We investigated the level of expression of neuronal nitric oxide synthase (nNOS) in the retinorecipient layers of the rat superior colliculus during early postnatal development. Male and female Lister rats ranging in age between the day of birth (P0) and the fourth postnatal week were used in the present study. Two biochemical methods were used, i.e., in vitro measurement of NOS specific activity by the conversion of [3H]-arginine to [3H]-citrulline, and analysis of Western blotting immunoreactive bands from superior colliculus homogenates. As revealed by Western blotting, very weak immunoreactive bands were observed as early as P0-2, and their intensity increased progressively at least until P21. The analysis of specific activity of NOS showed similar results. There was a progressive increase in enzymatic activity until near the end of the second postnatal week, and a nonsignificant tendency to an increase until the end of the third week was also observed. Thus, these results indicated an increase in the amount of nNOS during the first weeks after birth. Our results confirm and extend previous reports using histochemistry for NADPH-diaphorase and immunocytochemistry for nNOS, which showed a progressive increase in the number of stained cells in the superficial layers during the first two postnatal weeks, reaching an adult pattern at the end of the third week. Furthermore, our results suggested that nNOS is present in an active form in the rat superior colliculus during the period of refinement of the retinocollicular pathway.

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.

Nitric oxide acts as a slow-down and search signal in developing neurites

Nitric oxide (NO) has been demonstrated to act as a signaling molecule during neuronal development, but its precise function is unclear. Here we investigate whether NO might function at the neuronal growth cone to affect growth cone motility. We have previously demonstrated that growth cones of identified neurons from the snail Helisoma trivolvis show a rapid and transient increase in filopodial length in response to NO, which was regulated by soluble guanylyl cyclase (sGC) [S. Van Wagenen and V. Rehder (1999) J. Neurobiol., 39, 168-185]. Because in vivo studies have demonstrated that growth cones have longer filopodia and advance more slowly in regions where pathfinding decisions are being made, this study aimed to establish whether NO could function as a combined 'slow-down and search signal' for growth cones by decreasing neurite outgrowth. In the presence of the NO donor NOC-7, neurites of B5 neurons showed a concentration-dependent effect on neurite outgrowth, ranging from slowing at low, stopping at intermediate and collapsing at high concentrations. The effects of the NO donor were mimicked by directly activating sGC with YC-1, or by increasing its product with 8-bromo-cGMP. In addition, blocking sGC in the presence of NO with NS2028 blocked the effect of NO, suggesting that NO affected outgrowth via sGC. Ca2+ imaging of growth cones with Fura-2 indicated that [Ca2+]i increased transiently in the presence of NOC-7. These results support the hypothesis that NO can function as a potent slow/stop signal for developing neurites. When coupled with transient filopodia elongation, this phenomenon emulates growth cone searching behavior.

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.

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.

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.

Properties of LTD and LTP of retinocollicular synaptic transmission in the developing rat superior colliculus

The developing retinocollicular pathway undergoes synaptic refinement in order to form the precise retinotopic pattern seen in adults. To study the mechanisms which underlie refinement, we investigated long-term changes in retinocollicular transmission in rats aged P0-P25. Field potentials (FPs) in the superior colliculus (SC) were evoked by stimulation of optic tract fibers in an in vitro isolated brainstem preparation. High intensity stimulation induced long-term depression (LTD) in the SC after both low (1000 stimuli at 1 Hz) and higher (1000 stimuli at 50 Hz) frequency stimulation. The induction of LTD was independent of activation of NMDA and GABA(A) receptors, because D-APV (100 microM) and bicuculline (10 microM) did not block LTD. Induction of LTD was dependent upon activation of L-type Ca(2+) channels as 10 microM nitrendipine, an L-type Ca(2+) channel blocker, significantly decreased the magnitude of LTD. LTD was down-regulated during development. LTD magnitude was greatest in rats aged P0-P9 and significantly less in rats aged P10-P25. Long-term potentiation (LTP) was induced by low intensity stimulation and only after high frequency tetanus (1000 stimuli at 50 Hz). LTP was NMDA receptor dependent because d-APV (100 microM) completely abolished it. LTP induction was also blocked by the L-type Ca2+ channel blocker nitrendipine. The magnitude of LTP first increased with age, being significantly greater at P7-P13 than at P0-3 and then decreased at P23-25. In summary, both LTD and LTP are present during retinocollicular pathway refinement, but have different transmitter and ionic mechanisms and time courses of expression.

In situ localization of nitric oxide synthase and direct evidence of NO production in rat retinal ganglion cells

The expression of isoforms of nitric oxide synthase (NOS), enzymes responsible for NO production, and the synthesis of nitric oxide (NO) in rat retinal ganglion cells (RGCs) during synaptogenesis for various phases of the pre- and postnatal developmental periods were investigated. The retinas from prenatal, lactating, young, and adult rats were fixed in paraformaldehyde. The cryosections or paraformaldehyde-fixed ganglion cells purified from rat pups were immunostained for constitutive isoforms of NOS (n and eNOS) and observed with a confocal laser scanning microscope. Synthesis of NO in the RGCs was achieved by in vitro stimulation with glutamate. The intracellular NO levels were measured in real time using diaminofluorescein-2 diacetate, a fluorescence indicator of NO. Immunohistochemical analysis revealed nNOS and eNOS expressed in retinal ganglion cells during the first 2 postnatal weeks. Cultured RGCs also expressed nNOS and eNOS in vitro. Intracellular NO levels in cultured RGCs showed spontaneous fluctuation during a 20-min observation. The presence of both a non-specific NOS inhibitor, L-NAME, and a specific nNOS inhibitor, 7-NI, significantly inhibited (P<0.001) the increase of intracellular NO 6 and 8 min after the introduction of L-arginine and glutamate to the medium. This study revealed that all constitutive NOS isoforms are expressed in RGCs and demonstrated that NO is produced by nNOS mainly through stimulation by glutamate in cultured RGCs.

Activity-dependent mapping in the retinotectal projection

It is now 15 years since the discovery that N-methyl-d-aspartate receptor activity is required to maintain the refined topographic organization of retinotectal projections. Recent studies have identified additional components of the signaling pathways required for activity-dependent map formation and maintenance. Nitric oxide and brain-derived neurotrophic factor, candidate retrograde messengers, and serotonin and acetylcholine, modulators of neuronal excitability, all affect mapping. These studies indicate that the mapping process intersects with other processes fundamental to visual system development and function, such as process outgrowth, synaptic turnover and neuromodulation.

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.

Soluble guanylate cyclase is required during development for visual system function in Drosophila

A requirement for nitric oxide (NO) in visual system development has been demonstrated in many model systems, but the role of potential downstream effector molecules has not been established. Developing Drosophila photoreceptors express an NO-sensitive soluble guanylate cyclase (sGC), whereas the optic lobe targets express NO synthase. Both of these molecules are expressed after photoreceptor outgrowth to the optic lobe, when retinal growth cones are actively selecting their postsynaptic partners. We have previously shown that inhibition of the NO-cGMP pathway in vitro leads to overgrowth of retinal axons. Here we examined flies mutant for the alpha subunit gene of the Drosophila sGC (Gcalpha1). This mutation severely reduced but did not abolish GCalpha1 protein levels and NO-stimulated sGC activity in the developing photoreceptors. Although few mutant individuals possessed a disorganized retinal projection pattern, pharmacological NOS inhibition during metamorphosis increased this disorganization in mutants to a greater degree than in the wild type. Adult mutants lacked phototactic behavior, and the off-transient component of electroretinograms was frequently absent or greatly reduced in amplitude. Normal phototaxis and off-transient amplitude were restored by heat shock-mediated Gcalpha1 expression applied during metamorphosis but not in the adult. We propose that diminished sGC activity in the visual system during development causes inappropriate or inadequate formation of first-order retinal synapses, leading to defects in visual system function and visually mediated behavior.

The role of nitric oxide in development of topographic precision in the retinotectal projection of chick

The axonal projection from the retina to the tectum exhibits a precise topographic order in the mature chick such that neighboring ganglion cells send axons to neighboring termination zones in the contralateral tectum. The initial pattern formed during development is much less organized and is refined to the adult pattern during a discrete period of development. Refinement includes elimination of radically aberrant projections, such as those from the temporal side of the retina to posterior regions of the tectum, as well as a more subtle improvement in the topographic precision of the projection. The enzyme that synthesizes nitric oxide is expressed at high levels in the tectum during the developmental period in which the topography improves. Pharmacological blockade of nitric oxide synthesis during this period prevented elimination of topographically inappropriate retinotectal projections in a dose-dependent manner. This effect could not be duplicated by treatment of embryos with a vasoconstrictor, indicating that vascular changes were not a factor. These results show that nitric oxide is involved in refinement of the topography of the retinotectal projection as well as in other aspects of refinement of this projection in developing chick.

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.

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.