The developmental expression of neuronal nitric oxide synthase in cerebellar granule cells is sensitive to GABA and neurotrophins

Neuronal Nitric Oxide Synthase Critically Regulates the Endocannabinoid Pathway in the Murine Cerebellum During Development

The cerebellum is a major site of endocannabinoid (eCB) production and signaling. The predominant eCB within the cerebellum, 2-arachidonoylglycerol (2-AG), is produced by a metabotropic glutamate receptor type 1 (mGluR1)-initiated signaling cascade within Purkinje neurons (PNs). 2-AG retrogradely stimulates cannabinoid 1 receptors (CB1Rs) located on presynaptic terminals. The activated CB1R decreases neurotransmitter release and leads to the production of nitric oxide (NO), a gaseous molecule. Recently, our group discovered that during development in mice lacking neuronal nitric oxide synthase (nNOS-/-), PNs display an excitotoxic phenotype associated with overactivated mGluR1. Considering the importance of mGluR1 in 2-AG synthesis, the present study explored the role of nNOS-derived NO in regulating the eCB pathway within the cerebella of wildtype (WT) and nNOS-/- mice at postnatal day 7 (PD7), 2 weeks (2 W), and 7 weeks (7 W). Our analysis showed that diacylglycerol lipase α, the enzyme that catalyzes 2-AG production, was elevated at early postnatal ages, and followed by elevated levels of 2-AG in nNOS-/- cerebella compared to WT. CB1R expression in nNOS-/- cerebella was upregulated at PD7 but decreased at 2 W and 7 W when compared to age-matched WT mice cerebella. Importantly, treating organotypic nNOS-/- cerebellar slice cultures with an NO-donor-attenuated CB1R levels after 7 days in vitro. In addition, expression of the eCB hydrolases fatty acid amide hydrolase and monoacylglycerol lipase were significantly downregulated in nNOS-/- cerebella compared to WT cerebella at 7 W. Together, these results reveal a novel role for nNOS/NO in regulating eCB signaling in the cerebellum.

Distribution of neuronal nitric oxide synthase immunoreactivity in adult male Sprague-Dawley rat brain

Neuronal NOS (nNOS) accounts for most of the NO production in the nervous system that modulates synaptic transmission and neuroplasticity. Although previous studies have selectively described the localisation of nNOS in specific brain regions, a comprehensive distribution profile of nNOS in the brain is lacking. Here we provided a detailed morphological characterization on the rostro-caudal distribution of neurons and fibres exhibiting positive nNOS-immunoreactivity in adult Sprague-Dawley rat brain. Our results demonstrated that neurons and fibres in the brain regions that exhibited high nNOS immunoreactivity include the olfactory-related areas, intermediate endopiriform nucleus, Islands of Calleja, subfornical organ, ventral lateral geniculate nucleus, parafascicular thalamic nucleus, superior colliculus, lateral terminal nucleus, pedunculopontine tegmental nucleus, periaqueductal gray, dorsal raphe nucleus, supragenual nucleus, nucleus of the trapezoid body, and the cerebellum. Moderate nNOS immunoreactivity was detected in the cerebral cortex, caudate putamen, hippocampus, thalamus, hypothalamus, amygdala, and the spinal cord. Finally, low NOS immunoreactivity were found in the corpus callosum, fornix, globus pallidus, anterior commissure, and the dorsal hippocampal commissure. In conclusion, this study provides a comprehensive view of the morphology and localisation of nNOS immunoreactivity in the brain that would contribute to a better understanding of the role played by nNOS in the brain.

The neuronal nitric oxide synthase (nNOS) gene and neuroprotection against alcohol toxicity

When a mother abuses alcohol during pregnancy, the offspring can suffer a myriad of abnormalities, collectively known as fetal alcohol spectrum disorder (FASD). Foremost among these abnormalities is central nervous system dysfunction, which commonly manifests itself as mental retardation, clumsiness, hyperactivity, and poor attention span. These behavior problems are due, in large part, to alcohol-induced neuronal losses in the developing fetal brain. However, not all fetuses are equally affected by maternal alcohol consumption during pregnancy. While some fetuses are severely affected and develop hallmarks of FASD later in life, others exhibit no evident neuropathology or behavioral abnormalities. This variation is likely due, at least in part, to differences in fetal genetics. This review focuses on one particular gene, neuronal nitric oxide synthase, whose mutation worsens alcohol-induced neuronal death, both in vitro and in vivo. In addition, ectopic expression of the neuronal nitric oxide synthase gene protects neurons against alcohol toxicity. The gene encodes an enzyme that produces nitric oxide (NO), which facilitates the protective effects of neuronal growth factors and which underlies the ability of neurons to resist alcohol toxicity as they mature. Nitric oxide exerts its protective effects against alcohol via a specific signaling pathway, the NO-cGMP-PKG pathway. Pharmacologic manipulation of this pathway could be of therapeutic use in preventing or ameliorating FASD.

Efficient and graded gene expression in glia and neurons of primary cerebellar cultures transduced by lentiviral vectors

Lentiviral vectors are valuable tools to express genes of interest in living animals and stem cell cultures. The use of promoters in lentiviral constructs has been successfully used to drive gene expression in particular cell types including neurons and glia of the central nervous system in vivo. However, their suitability in cell culture is less well documented. In this paper, we describe lentiviral vectors containing neuronal promoters of the murine stem cell virus, of the synapsin 1 gene, the tubulin alpha 1 gene, and the calmodulin kinase II gene, and the glial promoter of the glial fibrillary acidic protein gene to drive reporter gene expression in primary dissociated cerebellar cell cultures and in slice cultures. While the glial promoter was highly specific for glia, the neuronal promoters were active in neurons and glia of dissociated cultures to a comparable extent. In slice cultures, neuronal and glial promoters demonstrated higher, but not absolute selectivity for particular cell types. In addition, the promoters allowed for an efficient and graded expression of genes in dissociated cultures. By using selected combinations of vectors, it was also possible to drive the expression of two genes in one cell type with high efficiency. A gene of interest in combination with a reporter gene can thus be expressed in a graded manner to reveal gene function in a rather short time and in a complex cellular environment.

Altered arginine metabolism in the central nervous system (CNS) of the Cln3-/- mouse model of juvenile Batten disease

BACKGROUND

Juvenile neuronal ceroid lipofuscinoses (JNCL) or juvenile Batten disease is a recessively inherited childhood neurodegenerative disorder resulting from a mutation in CLN3, which encodes a putative lysosomal protein of unknown function.

AIM

Recent evidence suggests that a disruption in CLN3 function results in altered regulation of arginine transport into lysosomes, and may influence intracellular arginine levels. We sought to investigate the possible consequences of arginine dysregulation in the brain of the Cln3(-/-) mouse model of JNCL.

METHODS

Using a combination of enzyme assays, metabolite profiling, quantitative reverse-transcription polymerase chain reaction and Western blotting, we analysed the activities and expression of enzymes involved in arginine metabolism in the cerebral cortex and cerebellum of Cln3(-/-) mice over several developmental time points.

RESULTS

We report subtle, but significant changes in the activities of enzymes involved in the citrulline-NO recycling pathway, and altered regulation of neuronal nitric oxide synthase in the cortex and cerebellum of Cln3(-/-) mice. In addition, a significant decrease in arginine transport into cerebellar granule cells was observed, despite an apparent upregulation of the cationic amino acid transporter-1 transporter at the cell surface. Our results provide further evidence that CLN3 function and arginine homeostasis are intricately related, and that cellular mechanisms may act to compensate for the loss of this protein.

CONCLUSIONS

This and other studies indicate that CLN3 dysfunction in JNCL may result in multiple disturbances in metabolism that together contribute to the pathophysiological processes underlying this disease.

The protective effect of neuronal nitric oxide synthase (nNOS) against alcohol toxicity depends upon the NO-cGMP-PKG pathway and NF-kappaB

Fetal alcohol syndrome (FAS) stems from maternal alcohol abuse during pregnancy and is an important cause of mental retardation and hyperactivity in children. In the developing brain, alcohol can kill neurons, leading to microencephaly. However, due to their genetic makeup, some individuals are less vulnerable than others to alcohol's neurotoxic effects. Animal studies have demonstrated that one particular gene, neuronal nitric oxide synthase (nNOS), protects developing neurons in vivo against alcohol-induced death. We utilized pharmacologic techniques to demonstrate that nNOS protects neurons against alcohol toxicity by activating the NO-cGMP-PKG signaling pathway. Cerebellar granule cell cultures derived from mice carrying a null mutation for nNOS (nNOS-/- mice) were substantially more vulnerable than cultures from wild-type mice to alcohol-induced cell death. However, activation of the pathway at sites downstream of nNOS protected the cultures against alcohol toxicity. Conversely, blockade of the pathway rendered wild-type cultures vulnerable to alcohol-induced death. We further identified NF-kappaB as the downstream effector through which nNOS and the NO-cGMP-PKG pathway signal their neuroprotective effects. Tumor necrosis factor-alpha (TNF-alpha), which activates NF-kappaB, ameliorated alcohol-induced cell death in nNOS-/- and wild-type cultures, while an NF-kappaB inhibitor (NFi) blocked the protective effects of TNF-alpha and worsened alcohol-induced cell death. Furthermore, NFi blocked the protective effects of NO-cGMP-PKG pathway activators, demonstrating that NF-kappaB is downstream of the NO-cGMP-PKG pathway. As wild-type neurons matured in culture, they became resistant to alcohol toxicity. However, this maturation-dependent alcohol resistance did not occur in nNOS-/- mice and could be reversed in wild-type mice with NFi, demonstrating that nitric oxide and NF-kappaB are crucial for the development of alcohol resistance with age. Thus, nNOS protects developing neurons against alcohol toxicity by activating the NO-cGMP-PKG-NF-kappaB pathway and is crucial for the acquisition of maturation-dependent alcohol resistance.

Sex differences in the control of neuronal nitric oxide synthase by GABA-A receptors in the developing rat diencephalon

The nitric oxide free radical (NO(*)), which is synthesized by neuronal nitric oxide synthase (nNOS), is known to play an important morphogenetic role in the developing rat brain. In the cortex, the levels of nNOS are regulated by phosphorylated cAMP response element binding protein (pCREB) downstream of GABA-A receptor activation. During early stages of neonatal development, binding of GABA to its type A receptors leads to depolarization of the neuronal membrane. One of the developmental processes mediated through GABA-A receptors is the sexual differentiation of the brain. In the present work, we investigated the effect of GABA-A receptor activation on nNOS and pCREB immunoreactivity in the developing diencephalon of 5-day-old male and female rats. Our results showed that in the bed nucleus of the stria terminalis activation of GABA-A receptors leads to increased numbers of nNOS, and pCREB as well as nNOS-pCREB doubly immunopositive cells only in the males while in the posterior hypothalamus this effect is observed in both sexes. The GABA-A receptor-mediated increase in nNOS and pCREB is abolished when L-type voltage-gated Ca(2+) channels are blocked. These results indicate that the following mechanism could be operating in a gonadal hormone-dependent and brain area-specific manner during neonatal rat brain development: 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 nNOS.

Proliferation of cerebellar precursor cells is negatively regulated by nitric oxide in newborn rat

The diffusible messenger, nitric oxide plays multiple roles in neuroprotection, neurodegeneration and brain plasticity. Its involvement in neurogenesis has been disputed, on the basis of results on models in vivo and in culture. We report here that pharmacological blockade of nitric oxide production in rat pups resulted, during a restricted time window of the first three postnatal days, in increased cerebellar proliferation rate, as assessed through tritiated thymidine or BrdU incorporation into DNA. This was accompanied by increased expression of Myc, a transcription factor essential for cerebellar development, and of the cell cycle regulating gene, cyclin D1. These effects were mediated downstream by the nitric oxide-dependent second messenger, cGMP. Schedules of pharmacological NO deprivation targeted to later developmental stages (from postnatal day 3 to 7), no longer increased proliferation, probably because of partial escape of the cGMP level from nitric oxide control. Though limited to a brief temporal window, the proliferative effect of neonatal nitric oxide deprivation could be traced into adulthood. Indeed, the number of BrdU-labeled surviving cells, most of which were of neuronal phenotype, was larger in the cerebellum of 60-day-old rats that had been subjected to NO deprivation during the first three postnatal days than in control rats. Experiments on cell cultures from neonatal cerebellum confirmed that nitric oxide deprivation stimulated proliferation of cerebellar precursor cells and that this effect was not additive with the proliferative action of sonic hedgehog peptide. The finding that nitric oxide deprivation during early cerebellar neurogenesis, stimulates a brief increase in cell proliferation may contribute to a better understanding of the controversial role of nitric oxide in brain development.

Antagonism by NOS inhibition of the behavioral effects of benzodiazepine and GABAA receptor agonists in the mouse elevated plus-maze

Earlier we implicated nitric oxide (NO) in mediation of the behavioral effects of benzodiazepines. Since benzodiazepines work through facilitation of GABAergic inhibitory neurotransmission, this study was designed to determine whether the direct-acting gamma-aminobutyric acidA (GABAA) receptor agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) evokes behavioral effects similar to those of benzodiazepines and whether behavioral effects of THIP are also NO dependent. When challenged with either chlordiazepoxide or THIP in an elevated plus-maze paradigm, male NIH Swiss mice exhibited a dose-related increase in open-arm activity. The chlordiazepoxide-induced effects were sensitive to antagonism by a benzodiazepine antagonist, and the effects of THIP were blocked by a GABAA receptor antagonist. Pretreatment with the NO synthase (NOS) inhibitor L-NG-nitro arginine antagonized the effects of both chlordiazepoxide and THIP; similar pretreatment with the D-isomer, D-NG-nitro arginine, which is inactive as an NOS inhibitor, was without effect on chlordiazepoxide and THIP. These findings indicate that chlordiazepoxide and THIP evoke similar behavioral effects in mice in the elevated plus-maze through actions on different parts of the GABAA receptor, and that NO appears to play a key role in mediation of the behavioral effects of both chlordiazepoxide and THIP.

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.

The role of NADPH oxidase and neuronal nitric oxide synthase in zinc-induced poly(ADP-ribose) polymerase activation and cell death in cortical culture

In the present study, we examined the role and the mechanism of poly(ADP-ribose) polymerase (PARP) and poly(ADP-ribose) glycohydrolase (PARG) activation in zinc-induced cell death in cortical culture. After brief exposure to 400 microM zinc, cortical cells exhibited DNA fragmentation, increased poly(ADP-ribosyl)ation, and decreased levels of nicotinamide adenine dinucleotide (NAD) and ATP and subsequently underwent cell death. Inhibitors of PARP/PARG attenuated both zinc-induced NAD/ATP depletion and cell death, thereby implicating the PARP/PARG cascade in these processes. The zinc-inducible enzymes NADPH oxidase and neuronal nitric oxide synthase (nNOS) contributed to PARP activation as their inhibitors attenuated zinc-induced poly(ADP-ribosyl)ation. Levels of nitric oxide and nitrites increased following zinc exposure, consistent with NOS activation. In addition, Western blots and RT-PCR analysis revealed that protein and mRNA levels of nNOS specifically increased following zinc exposure in a manner similar to that of NADPH oxidase. The present study demonstrates that induction of NADPH oxidase and nNOS actively contributes to PARP/PARG-mediated NAD/ATP depletion and cell death induced by zinc in cortical culture.

Brain-derived neurotrophic factor modulates dendritic morphology of cerebellar basket and stellate cells: an in vitro study

The dendrites of cerebellar basket/stellate cells show a highly stereotyped orientation relative to granule cell axons (parallel fibers) and Purkinje cell dendrites. This specific morphology is acquired during the early postnatal phase of cerebellar development, when basket/stellate cells become synaptically integrated with Purkinje neurons and granule cells. In the present study, we used primary cerebellar cultures to test how the spatial arrangement of granule cell axons affects basket/stellate cell dendritic morphology. In addition, we sought to determine whether active signals as might be provided by granule cells, i.e. synaptic input and the neurotrophin, brain-derived neurotrophic factor, affect basket/stellate cell development. Our results confirm the critical role of parallel fiber orientation for basket/stellate dendritic morphogenesis. Moreover, we found that both electrical activity and brain-derived neurotrophic factor increased basket and stellate cell dendritic arborization. Together with previously published findings, our data led to the conclusion that both structural cues and active interneuronal signaling collaborate to bring about the precise morphogenesis of cerebellar basket/stellate cells. The distinct responses of various cerebellar phenotypes towards the morphogenetic effects of brain-derived neurotrophic factor suggest that this neurotrophin, within the developing cerebellum, enhances synaptic connectivity by concerting the formation of appropriate pre- and postsynaptic structures.

Nitric oxide (NO.) stabilizes whereas nitrosonium (NO+) enhances filopodial outgrowth by rat retinal ganglion cells in vitro

Recent observations suggest that nitric oxide (NO(.)) can increase or decrease growth cone motility. Here, these apparently paradoxical results are explained by distinct actions of different NO-related species. Filopodial morphology of 223 rat retinal ganglion cells was monitored under computer-enhanced video microscopy in the presence of NO synthase (NOS) substrates or inhibitors, donors of specific NO-related species, and membrane-permeant cyclic nucleotide analogs. Physiological NOS activity induced filopodial outgrowth, whereas inhibition of NOS stabilized filopodia. Similar to NOS, nitrosonium (NO(+) transfer) and peroxynitrite (ONOO(-)), which can regulate the activity of growth-associated proteins by S-nitrosylation and oxidation, respectively, induced filopodial outgrowth. In contrast, NO(.), which stimulates guanylate cyclase to increase cGMP, stabilized filopodial activity. Thus disparate NO-related species may offer a dynamic process of filopodial growth regulation.

Partial cloning of constitutive and inducible nitric oxide synthases and detailed neuronal expression of NOS mRNA in the cerebellum and optic tectum of adult Atlantic salmon (Salmo salar)

Studies of different species have implicated nitric oxide (NO) synthase (NOS) in various physiological and pathological events. Three major NOS isoforms are present in the brain of mammals; endothelial NOS (eNOS), neuronal NOS (nNOS) and inducible NOS (iNOS). Little is known about the significance of the presence of these proteins in the brain. We report the first investigation into the presence of nNOS and iNOS isoforms in a teleost, adult Atlantic salmon (Salmo salar). Complementary DNA was synthesized from cerebellum and thymus mRNA using RT-PCR techniques with primers against conserved regions of NOS. Cloning and sequencing revealed a partial gene sequence of 560 bp corresponding to mammalian nNOS from cerebellum cDNA. The predicted protein sequence of identified salmon nNOS possessed 85% identity to that of mammalian nNOS. Northern blot analysis of different tissues revealed expression in brain and heart, and indicated expression of three different nNOS mRNAs in the brain. In addition, a 389 bp sequence corresponding to iNOS was identified in thymus cDNA. Salmon iNOS is almost identical to rainbow trout iNOS (95%), but shows much less amino acid identity to goldfish (65%) and mammalian (52%) iNOS. Phylogenetically, all vertebrate nNOS and iNOS homologues are clustered separately. Expression studies by means of in situ hybridization revealed abundant nNOS mRNA transcripts in distinct neuronal populations throughout the Purkinje cell layer of the corpus cerebellum and the periventricular layer of the optic tectum. Our data show that adult Atlantic salmon possess a gene encoding an nNOS isoform and putative alternatively spliced forms, which are expressed in distinct neuronal populations in the cerebellum and optic tectum, and in yet unidentified cell types in the heart. The data suggest that the arising of different vertebrate NOS isoforms is an evolutionary old event. The well conserved sequences present in salmon and mammalian nNOS may reflect their importance in protein function, whereas interspecies distributional differences in cellular expression of nNOS and sequence differences of iNOS may reflect variations and specializations in routes of NO action in the vertebrate phylogeny.

Effects of brain-derived neurotrophic factor on the development of NADPH-diaphorase/nitric oxide synthase-positive amacrine cells in the rodent retina

Amacrine neurons expressing nitric oxide synthase (NOS) contain brain-derived neurotrophic factor (BDNF) receptors and respond to exogenous BDNF [Klöcker, N., Cellerino, A. & Bähr, M. (1998) J. Neurosci., 18, 1038-1046]. We analysed the effects of BDNF on the development of neurons which express NOS in the mouse and rat retina. Rat pups received a total of three intraocular injections of BDNF at intervals of 48 h, starting at postnatal day 16 (P16), and were killed at P22. The retinas were stained for NADPH-diaphorase, a histological marker of NOS. NOS-expressing neurons were found in both the inner nuclear layer (INL) and the ganglion cell layer (GCL). Two classes of NOS-expressing neurons, type I and type II, had already been distinguished in the INL [Koistinaho, J. & Sagar, S.M. (1995) In Osborne, N.N. & Chader, G.J. (eds), Progress in Retinal and Eye Research, Vol. 15. Oxford University Press, pp. 69-87] and a third one in the GCL. Up-regulation of NADPH-diaphorase activity was observed after BDNF treatment. The number of type I neurons remained stable, whereas the number of type II neurons and NOS-positive neurons in the GCL increased significantly (P < 0.001). Type I and type II neurons were significantly larger in BDNF-treated retinas. Double-labelling experiments revealed that BDNF induces NADPH-diaphorase in dopaminergic neurons and amacrine cells displaced to the GCL, but not in retinal ganglion cells. In mice homozygous for a null mutation of the bdnf gene, the intensity of NADPH-diaphorase labelling in both somata and processes was reduced, but the number of labelled neurons was not dramatically reduced. These findings indicate that BDNF regulates the neurotransmitter phenotype of NOS-expressing amacrine neurons under physiological conditions, but is not required for their survival.

The differentiation of cerebellar interneurons is independent of their mitotic history

A narrow time window centered around the terminal mitosis of their precursors has been recognized to be critical for the determination and/or realization of the developmental fate of a variety of neuronal phenotypes. In contrast, individual cell lineages in the cerebellum get separated early during embryonic development, and at least precursors for granule neurons have been found to be specified while still proliferating. We utilized primary dissociated cultures to address the issue of whether the faithful development of cerebellar granule cells and basket/stellate cells is dependent on their mitotic history and on the completion of a fixed number of cell cycles. Neuroblasts derived from embryonic cerebellar anlagen and transferred into primary dissociated cultures stopped proliferating as assessed by a loss of expression of the cell proliferation marker, Ki-67, and a failure to incorporate 5-bromo-2'-deoxyuridine. Although these cells had been forced to leave the proliferating cell pool prematurely, they developed into granule neurons or basket/stellate cells as judged by their distinct pattern of expression of specific molecular markers and the acquisition of a typical morphology. This included the cell intrinsic capacity of granule neurons to position their afferent synapses specifically to their dendrites. Thus, the competence of cerebellar interneurons to differentiate appropriately is independent of the precise timing of their final mitosis; however, their sensitivity towards extrinsic developmental signals appears to vary in a cell cycle-dependent manner, as suggested by the failure to survive of those cells that were in S-phase at the time of cultivation.

NMDA induces BDNF expression in the albino rat retina in vivo

The effect of an intravitreal injection of NMDA on the expression of brain-derived neurotrophic factor (BDNF) in retinal ganglion cells was investigated in rats. Forty-eight hours after intravitreal injection of NMDA retinal ganglion cell BDNF immunoreactivity was practically obliterated, as was the choline acetyltransferase (ChAT) immunoreactivity associated with a subset of amacrine cells. However, 2h following treatment with NMDA the BDNF immunoreactivity and BDNF mRNA associated with the ganglion cells was enhanced while the amacrine cell ChAT immunoreactivity was clearly reduced and the levels of mRNA coding for rhodopsin and Thy-1 did not change. However, 4h after NMDA injection the increase in BDNF mRNA was now no longer apparent. The results show that synthesis of BDNF is increased in the ganglion cells immediately following an insult by NMDA. It is suggested that this is a natural protective mechanism of rat retinal ganglion cells.

Neuronal-type NO synthase: transcript diversity and expressional regulation

Of the three established isoforms of NO synthase, the gene for the neuronal-type enzyme (NOS I) is by far the largest and most complicated one. The genomic locus of the human NOS I gene is located on chromosome 12 and distributed over a region greater than 200 kb. The nucleotide sequence corresponding to the major neuronal mRNA transcript is encoded by 29 exons. The full-length open reading frame codes for a protein of 1434 amino acids with a predicted molecular weight of 160.8 kDa. However, both in rodents and in humans, multiple, tissue-specific or developmentally regulated NOS I mRNA transcripts have been reported. They arise from the initiation by different transcriptional units containing alternative promoters (at least eight in the human gene), cassette exon deletions or insertions, and/or the usage of alternate polyadenylation signals. Depending on the insertions and deletions, translation results in functional or nonfunctional proteins. The use of alternative promoters can influence gene expression by various means. Indeed, NOS I is not a static, constitutively expressed enzyme, but subject to expressional regulation by various compounds and conditions. The molecular mechanisms underlying these regulations are currently being studied in several laboratories including our own.

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

Nitric oxide (NO) has been implicated as a retrograde signal in the process of refining axonal pathways during brain development. To determine some of the factors involved in this process, we have used two model pathway systems in the rat and mouse superior colliculus (SC). The first, the patch-cluster system, consists of clusters of neurons in the intermediate gray layer (igl) which transiently express NO during development and which receive input from a cholinergic pathway from the parabrachial brainstem as well as from other pathways containing different transmitters. The second system, the retinocollicular pathway, consists of glutamatergic fibers that project to the superficial gray layer. We have used both nitric oxide synthase inhibition (nw-nitro-L-arginine, NoArg) and single (nNOS) and double (nNOS and eNOS) gene knockout mice to examine the effect that reduction in NOS has upon the development of these two systems. The onset of NOS expression in rat, as revealed by nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) labeling, occurred in igl cells as early as postnatal day P5, with clusters being well-established by P14. Cholinergic fibers were first visible at P10 and formed obvious patches and tiers by P14. Intraperitoneal injections of NoArg from P1-P22 had no effect upon the development of these cholinergic patches. The pathway also developed normally in both single and double-knockout mice. In contrast, the ipsilateral retinocollicular pathway was altered in the double, but not in the single knockout mouse. This pathway is exuberant during the first week of life, being distributed across much of the mediolateral axis of the rostral SC. By P8-P15, this pathway has retracted to the most mediorostral SC. This refinement was delayed substantially in the double NOS gene knockout mouse. Ipsilateral fibers were found within 3-5 separate medio-lateral patches within the rostral 600 microns of SC at P15, and patches of abnormal size and extent were also seen at P18. We conclude from these results that NO plays a role in pathway development in the rodent SC, but only in glutamatergic pathways and only when both endothelial and neuronal forms of NOS have been deleted. The mechanism of this effect must involve pathway elimination in situations where there is non-correlated electrical activity. It is likely that NO promotes fiber retraction rather than fiber stabilization in these developing nerve fibers.

Nitric oxide as a signaling molecule in visual system development

The lateral geniculate nucleus (LGN) of the ferret is characterized by the readily discernible anatomical patterning of afferent terminations from the retina into both eye-specific layers and On/Off sublaminae. The eye-specific layers form during the first post-natal week, and On/Off sublaminae become apparent during the third to fourth post-natal weeks. The post-natal appearance of these patterns thus provides an advantageous model for the study of the mechanisms of activity-dependent development. The second phase of pattern formation, the appearance of On/Off sublaminae, involves the elaboration of appropriately placed axonal terminals and the restriction (or retraction) of inappropriately placed terminals. Previous work has demonstrated that this process is dependent on the activation of NMDA-receptors. Other studies have provided strong evidence that nitric oxide, a diffusible gas which is produced downstream of NMDA-receptor activation, acts as a retrograde messenger molecule to induce changes in pre-synaptic structures. In this article we review the evidence that nitric oxide plays a role in activity-dependent synaptic plasticity in the developing retinogeniculate pathway. The role of nitric oxide in other aspects of visual system development is also discussed.

Analysis of NO synthase expression in neuronal, astroglial and fibroblast-like derivatives differentiating from PCC7-Mz1 embryonic carcinoma cells

We studied the expression of the NO synthase isoforms in an in vitro model of neural development using RT-PCR, Western blot and immunohistochemistry. Murine PCC7-Mz1 cells (Jostock et al., Eur. J. Cell Biol. 76, 63-76, 1998) differentiate in the presence of all-trans retinoic acid and dibutyryl cAMP along the neural pathway into neuron-like, fibroblast-like and astroglia-like cells. Undifferentiated cells showed immunofluorescent staining for neuronal-type NOS I and endothelial-type NOS III. This expression pattern was retained in those cells differentiating into neurofilament- and tau protein-positive neuronal cells. Thymocyte alloantigen (Thy1.2/CD 90.2)-positive fibroblasts, appearing around day 3, and glial fibrillary acidic protein (GFAP)-positive astroglial cells, appearing after day 6 of differentiation, stained negative for any NOS isoform. Starting at day 6 of differentiation, expression of inducible-type NOS II could be stimulated with cytokines in a subset of cells, which may represent activated astrocytes. NOS II was always undetectable in non-induced cultures. These data indicate that the ability of stem cells to express NOS I and NOS III is only retained when the cells differentiate along the neuronal lineage, while a small subpopulation of cells acquires the ability to express NOS II in response to cytokines.