The role of nitric oxide and NMDA receptors in the development of motor neuron dendrites

Extracellular molecular signals shaping dendrite architecture during brain development

Proper growth and branching of dendrites are crucial for adequate central nervous system (CNS) functioning. The neuronal dendritic geometry determines the mode and quality of information processing. Any defects in dendrite development will disrupt neuronal circuit formation, affecting brain function. Besides cell-intrinsic programmes, extrinsic factors regulate various aspects of dendritic development. Among these extrinsic factors are extracellular molecular signals which can shape the dendrite architecture during early development. This review will focus on extrinsic factors regulating dendritic growth during early neuronal development, including neurotransmitters, neurotrophins, extracellular matrix proteins, contact-mediated ligands, and secreted and diffusible cues. How these extracellular molecular signals contribute to dendritic growth has been investigated in developing nervous systems using different species, different areas within the CNS, and different neuronal types. The response of the dendritic tree to these extracellular molecular signals can result in growth-promoting or growth-limiting effects, and it depends on the receptor subtype, receptor quantity, receptor efficiency, the animal model used, the developmental time windows, and finally, the targeted signal cascade. This article reviews our current understanding of the role of various extracellular signals in the establishment of the architecture of the dendrites.

The contribution of an imbalanced redox signalling to neurological and neurodegenerative conditions

Nitric oxide and other redox active molecules such as oxygen free radicals provide essential signalling in diverse neuronal functions, but their excess production and insufficient scavenging induces cytotoxic redox stress which is associated with numerous neurodegenerative and neurological conditions. A further component of redox signalling is mediated by a homeostatic regulation of divalent metal ions, the imbalance of which contributes to neuronal dysfunction. Additional antioxidant molecules such as glutathione and enzymes such as super oxide dismutase are involved in maintaining a physiological redox status within neurons. When cellular processes are perturbed and generation of free radicals overwhelms the antioxidants capacity of the neurons, a resulting redox damage leads to neuronal dysfunction and cell death. Cellular sources for production of redox-active molecules may include NADPH oxidases, mitochondria, cytochrome P450 and nitric oxide (NO)-generating enzymes, such as endothelial, neuronal and inducible NO synthases. Several neurodegenerative and developmental neurological conditions are associated with an imbalanced redox state as a result of neuroinflammatory processes leading to nitrosative and oxidative stress. Ongoing research aims at understanding the causes and consequences of such imbalanced redox homeostasis and its role in neuronal dysfunction.

Postnatal development of inner lamina II interneurons of the rat medullary dorsal horn

ABSTRACT

Pain processing in young mammals is immature. Despite the central role of the medullary dorsal horn (MDH) in processing orofacial sensory information, the maturation of the neurons within the MDH has been largely overlooked. Combining in vitro electrophysiological recordings and 3D morphological analysis over the first postnatal month in rats, we investigated the age-dependent development of the neurons within the inner lamina II (IIi) of the MDH. We show the lamina IIi neuronal population transition into a more hyperpolarized state, with modification of the action potential waveform, and a shift from single spiking, at early postnatal ages, to tonic firing and initial bursting at later stages. These physiological changes are associated with a strong structural remodelling of the neuronal morphology with most of the modifications occurring after the third postnatal week. Among the lamina IIi neuronal population, the subpopulation of interneurons expressing the γ isoform of the protein kinase C (PKCγ+) are key elements for the circuits underlying facial mechanical allodynia. How do they develop from the rest of the lamina IIi constitute an important question that remained to be addressed. Here, we show that PKCγ+ interneurons display electrophysiological changes over time comparable with the PKCγ- population. However, they show a distinctive increase of the soma volume and primary branches length, as opposed to the PKCγ- population. Together, our data demonstrate a novel pattern of late postnatal maturation of lamina IIi interneurons, with a spotlight on PKCγ+ interneurons, that may be relevant for the development of orofacial sensitivity.

Postsynaptic and Presynaptic NMDARs Have Distinct Roles in Visual Circuit Development

To study contributions of N-methyl-D-aspartate receptors (NMDARs) in presynaptic and postsynaptic neurons of the developing visual system, we microinject antisense Morpholino oligonucleotide (MO) against GluN1 into one cell of two-cell-stage Xenopus laevis embryos. The resulting bilateral segregation of MO induces postsynaptic NMDAR (postNMDAR) knockdown in tectal neurons on one side and presynaptic NMDAR (preNMDAR) knockdown in ganglion cells projecting to the other side. PostNMDAR knockdown reduces evoked NMDAR- and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated retinotectal currents. Although the frequency of spontaneous synaptic events is increased, the probability of evoked release is reduced. PreNMDAR knockdown results in larger evoked and unitary synaptic responses. Structurally, postNMDAR and preNMDAR knockdown produce complementary effects. Axonal arbor complexity is reduced by preNMDAR-MO and increased by postNMDAR-MO, whereas tectal dendritic arbors exhibit the inverse. The current study illustrates distinct roles for pre- and postNMDARs in circuit development and reveals extensive transsynaptic regulation of form and function.

Nitric oxide and sex differences in dendritic branching and arborization

Nitric oxide (NO) is an important signaling molecule with many functions in the nervous system. Derived from the enzymatic conversion of arginine by several nitric oxide synthases (NOS), NO plays significant roles in neuronal developmental events such as the establishment of dendritic branching or arbors. A brief summary of the discovery, molecular biology, and chemistry of NO, and a description of important NO-mediated signal transduction pathways with emphasis on the role for NO in the development of dendritic branching during neurodevelopment are presented. Important sex differences in neuronal nitric oxide synthase expression during neuronal development are considered. Finally, a survey of endogenous and exogenous substances that disrupt dendritic patterning is presented with particular emphasis on how these molecules may drive NO-mediated sex differences in dendritic branching.

The role of nitric oxide in brain disorders: Autism spectrum disorder and other psychiatric, neurological, and neurodegenerative disorders

Nitric oxide (NO) is a multifunctional signalling molecule and a neurotransmitter that plays an important role in physiological and pathophysiological processes. In physiological conditions, NO regulates cell survival, differentiation and proliferation of neurons. It also regulates synaptic activity, plasticity and vesicle trafficking. NO affects cellular signalling through protein S-nitrosylation, the NO-mediated posttranslational modification of cysteine thiols (SNO). SNO can affect protein activity, protein-protein interaction and protein localization. Numerous studies have shown that excessive NO and SNO can lead to nitrosative stress in the nervous system, contributing to neuropathology. In this review, we summarize the role of NO and SNO in the progression of neurodevelopmental, psychiatric and neurodegenerative disorders, with special attention to autism spectrum disorder (ASD). We provide mechanistic insights into the contribution of NO in diverse brain disorders. Finally, we suggest that pharmacological agents that can inhibit or augment the production of NO as well as new approaches to modulate the formation of SNO-proteins can serve as a promising approach for the treatment of diverse brain disorders.

Oxycodone Decreases Dendritic Complexity in Female but not Male Rat Striatal Neurons In Vitro

The use of oxycodone in the past two decades has dramatically risen, yet the amount of research regarding how it impacts neuronal health is lacking. As prescription use and misuse in women of reproductive age increases there has been a corresponding increase in the number of infants who have been exposed to oxycodone in utero. Given the critical role of the striatum in motor control and reward regulation, the aim of the current study was to examine the effects of oxycodone on developing rat striatal neurons. Sex-specific effects of oxycodone on neuronal cytoarchitecture were examined in cultured rat striatal neurons with a primary focus on dendritic arborization. Neurons were extracted from either male or female embryonic day 18 rat striata and cultured and exposed to varying concentrations of oxycodone over a ten-day period. Dendritic complexity of the neurons was measured using Sholl analysis. Results indicate that oxycodone inhibits dendritic complexity in a dose-dependent manner in female but not male striatal neurons. Additional analysis indicated the number of non-primary dendrites in female striatal neurons significantly decreased with increasing concentrations of oxycodone, while the number of primary dendrites as well as the length of primary and non-primary dendrites was unaffected by oxycodone treatment in both sexes. These in vitro findings demonstrate sex-specific effects of oxycodone on the development of striatal dendritic architecture which may be important for understanding the effects of oxycodone exposure in utero.

Glutamate Activity Regulates and Dendritic Development of J-RGCs

Retinal ganglion cells (RGCs) have a wide variety of dendritic architectures, which are critical for the formation of their function-specific synaptic circuitry. The developmental regulation of the dendrites of RGCs is thought to be subtype dependent. The purpose of this study is to characterize the dendritic development of a genetically identified RGC subtype, JamB RGCs (J-RGCs), and the roles of glutamate receptor activity on the dendritic development of these cells. We show that the dendrites of J-RGCs are strictly ramified in the outer portion of the inner plexiform layer (IPL) of the retina at the age of postnatal day 8 (P8), mimicking the ramification pattern of adults. However, several other important features of dendrites undergo substantial developmental refinement after P8. From P8 to P13, the dendritic development of J-RGCs is characterized by a dramatic increase of dendritic length and the size of the dendritic field. After eye opening, the dendritic development of J-RGCs is characterized by a tremendous decrease of the number of dendritic protrusions (spine-like structures) and a consolidation of the size of the dendritic field. To determine whether the dendritic development of J-RGCs is regulated by glutamatergic activity, we conditionally knocked out the expression of an obligatory subunit of N-methyl-D-aspartate receptors (NMDARs), NR1 (Grin1), in J-RGCs. We found that J-RGCs with the NMDAR mutation have decreased dendrite outgrowth and dendritic field expansion but increased number of dendritic protrusions before eye opening. To determine if visual experience regulates the development of J-RGC dendrites, we raised the mice in complete darkness after birth. Light deprivation prevented the decrease in the number of dendritic protrusions and the consolidation of the dendritic field of wild type (WT) mice after eye opening. However, light deprivation has no additional effect on the number of dendritic protrusions or the size of the dendritic field of J-RGCs with NMDAR mutation. Together, these results revealed the roles of light stimulation and NMDAR activity on the dendritic development of J-RGCs.

Developmental Alcohol Exposure Impairs Activity-Dependent S-Nitrosylation of NDEL1 for Neuronal Maturation

Neuronal nitric oxide synthase is involved in diverse signaling cascades that regulate neuronal development and functions via S-Nitrosylation-mediated mechanism or the soluble guanylate cyclase (sGC)/cyclic guanosine monophosphate (cGMP) pathway activated by nitric oxide. Although it has been studied extensively in vitro and in invertebrate animals, effects on mammalian brain development and underlying mechanisms remain poorly understood. Here we report that genetic deletion of "Nos1" disrupts dendritic development, whereas pharmacological inhibition of the sGC/cGMP pathway does not alter dendritic growth during cerebral cortex development. Instead, nuclear distribution element-like (NDEL1), a protein that regulates dendritic development, is specifically S-nitrosylated at cysteine 203, thereby accelerating dendritic arborization. This post-translational modification is enhanced by N-methyl-D-aspartate receptor-mediated neuronal activity, the main regulator of dendritic formation. Notably, we found that disruption of S-Nitrosylation of NDEL1 mediates impaired dendritic maturation caused by developmental alcohol exposure, a model of developmental brain abnormalities resulting from maternal alcohol use. These results highlight S-Nitrosylation as a key activity-dependent mechanism underlying neonatal brain maturation and suggest that reduction of S-Nitrosylation of NDEL1 acts as a pathological factor mediating neurodevelopmental abnormalities caused by maternal alcohol exposure.

Emerging Roles of Filopodia and Dendritic Spines in Motoneuron Plasticity during Development and Disease

Motoneurons develop extensive dendritic trees for receiving excitatory and inhibitory synaptic inputs to perform a variety of complex motor tasks. At birth, the somatodendritic domains of mouse hypoglossal and lumbar motoneurons have dense filopodia and spines. Consistent with Vaughn's synaptotropic hypothesis, we propose a developmental unified-hybrid model implicating filopodia in motoneuron spinogenesis/synaptogenesis and dendritic growth and branching critical for circuit formation and synaptic plasticity at embryonic/prenatal/neonatal period. Filopodia density decreases and spine density initially increases until postnatal day 15 (P15) and then decreases by P30. Spine distribution shifts towards the distal dendrites, and spines become shorter (stubby), coinciding with decreases in frequency and increases in amplitude of excitatory postsynaptic currents with maturation. In transgenic mice, either overexpressing the mutated human Cu/Zn-superoxide dismutase (hSOD1^G93A) gene or deficient in GABAergic/glycinergic synaptic transmission (gephyrin, GAD-67, or VGAT gene knockout), hypoglossal motoneurons develop excitatory glutamatergic synaptic hyperactivity. Functional synaptic hyperactivity is associated with increased dendritic growth, branching, and increased spine and filopodia density, involving actin-based cytoskeletal and structural remodelling. Energy-dependent ionic pumps that maintain intracellular sodium/calcium homeostasis are chronically challenged by activity and selectively overwhelmed by hyperactivity which eventually causes sustained membrane depolarization leading to excitotoxicity, activating microglia to phagocytose degenerating neurons under neuropathological conditions.

S-Nitrosylation in neurogenesis and neuronal development

BACKGROUND

Nitric oxide (NO) is a pleiotropic messenger molecule. The multidimensional actions of NO species are, in part, mediated by their redox nature. Oxidative posttranslational modification of cysteine residues to regulate protein function, termed S-nitrosylation, constitutes a major form of redox-based signaling by NO.

SCOPE OF REVIEW

S-Nitrosylation directly modifies a number of cytoplasmic and nuclear proteins in neurons. S-Nitrosylation modulates neuronal development by reaction with specific proteins, including the transcription factor MEF2. This review focuses on the impact of S-nitrosylation on neurogenesis and neuronal development.

MAJOR CONCLUSIONS

Functional characterization of S-nitrosylated proteins that regulate neuronal development represents a rapidly emerging field. Recent studies reveal that S-nitrosylation-mediated redox signaling plays an important role in several biological processes essential for neuronal differentiation and maturation.

GENERAL SIGNIFICANCE

Investigation of S-nitrosylation in the nervous system has elucidated new molecular and cellular mechanisms for neuronal development. S-Nitrosylated proteins in signaling networks modulate key events in brain development. Dysregulation of this redox-signaling pathway may contribute to neurodevelopmental disabilities such as autism spectrum disorder (ASD). Thus, further elucidation of the involvement of S-nitrosylation in brain development may offer potential therapeutic avenues for neurodevelopmental disorders. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

Modulation of glycinergic synaptic transmission in the trigeminal and hypoglossal motor nuclei by the nitric oxide-cyclicGMP signaling pathway

In a previous work we found that nitric oxide (NO) and cyclicGMP (cGMP) inhibit glutamatergic synaptic transmission in trigeminal motoneurons (MnV). Here we study the actions of the NO/cGMP signaling pathway on glycinergic synaptic transmission in trigeminal and hypoglossal motoneurons (MnXII) in brain stem slices of neonatal rats. Glycinergic inhibitory postsynaptic currents (IPSCs) were recorded in MnV by stimulation of the supratrigeminal nucleus (SuV) and in MnXII by stimulation of the nucleus of Roller. The NO donor DETA/NONOate (DETA/NO) reduced the amplitude of the IPSC to 58.1±4.2% of control values in MnV. In the presence of YC-1, a modulator of guanylate cyclase that acts as a NO sensitizer, lower and otherwise ineffective concentrations of DETA/NO induced a reduction of the IPSC to 47.2±15.6%. NO effects were mimicked by 8 bromo cyclicGMP (8BrcGMP). They were accompanied by an increase in the paired pulse facilitation (PPF) and in the failure rate of evoked IPSCs. 8BrcGMP did not modify the glycinergic currents elicited by exogenous glycine. In MnXII the IPSCs were also reduced by NO donors and 8BrcGMP to 52.9±6.3% and 45.9±4% of control values, respectively. In these neurons, but not in MnV, we also observed excitatory postsynaptic actions of NO donors. We propose that the differences between the two motor pools may be due to a differential development of the nitrergic system in the two nuclei. Our data show that NO, through its second messenger cGMP, reduces inhibitory glycinergic synaptic transmission in both MnV and MnXII. For MnV, evidence in favor of presynaptic inhibition of glycine release is presented. Given our previous data together with the current results, we propose that the NO/cGMP signaling pathway participates pre- and postsynaptically in the combined regulation of MnV and MnXII activities in motor acts in which they participate.

Hippocampus and nitric oxide

Since it was first identified to play an important role in relaxation of blood vessels, nitric oxide has been demonstrated to regulate many biological processes, especially in the central nervous system. Of the three types of enzymes that produce nitric oxide in humans and rodents, neuronal type is found almost exclusively in the nervous system. This gaseous molecule is a nonclassical neurotransmitter, which maintains the activities of neural cells and regulates the normal functions of brain. It appears to play a role in promoting the transfer of nerve signals from one neuron to another, maintaining the synaptic strength. Meanwhile, nitric oxide is a unique regulator on neurogenesis and synaptogenesis, producing the positive or negative effects upon different signal pathways or cellular origins and locations. Based on its significant roles in neural plasticity, nitric oxide is involved in a number of central nervous diseases, such as ischemia, depression, anxiety, and Alzheimer's disease. Clarifying the profiles of nitric oxide in the brain tissues and its participation in pathophysiological processes opens a new avenue for development of new therapeutic strategies. Thus, this chapter specifies the effects of nitric oxide in the hippocampus, a key structure implicated in the modulation of mood and memories, exhibiting the trend of future research on nitric oxide.

NOS1AP is associated with increased severity of PTSD and depression in untreated combat veterans

BACKGROUND

Posttraumatic stress disorder (PTSD) and depressive disorder are over represented in combat veterans. Veterans with both disorders have an increased risk of suicide. The nitric oxide synthase 1 adaptor protein (NOS1AP) gene, which modulates stress-evoked N-methyl-d-aspartate (NMDA) activity, was investigated in combat veterans.

METHODS

A comprehensive genetic analysis of NOS1AP and its association with PTSD was investigated in Vietnam combat veterans with PTSD (n=121) and a group of healthy control individuals (n=237). PTSD patients were assessed for symptom severity and level of depression using the Mississippi Scale for Combat-Related PTSD and the Beck Depression Inventory-II (BDI).

RESULTS

The G allele of NOS1AP SNP rs386231 was significantly associated with PTSD (p=0.002). Analysis of variance revealed significant differences in BDI-II and Mississippi scores between genotypes for rs386231 with the GG genotype associated with increased severity of depression (p=0.002 F=6.839) and higher Mississippi Scale for Combat-Related PTSD scores (p=0.033). Haplotype analysis revealed that the C/G haplotype (rs451275/rs386231) was significantly associated with PTSD (p=0.001).

LIMITATIONS

The sample sizes in our study were not sufficient to detect SNP associations with very small effects. In addition the study was limited by its cross sectional design.

CONCLUSIONS

This is the first study reporting that a variant of the NOS1AP gene is associated with PTSD. Our data also suggest that a genetic variant in NOS1AP may increase the susceptibility to severe depression in patients with PTSD and increased risk for suicide.

The molecular basis of experience-dependent motor system development

Neurons in the vertebrate nervous system acquire their mature features over an extended period in pre-natal and early post-natal life. The interaction of the organism with its environment (“experience”) has been shown to profoundly influence sensory neuron development. Over the past ~2 decades, it has become increasingly clear that motor system development is also experience-dependent. Glutamate receptors of the N-methyl-D-aspartate (NMDA) subtype have been implicated in both sensory and motor system experience-dependent development. An additional molecular mechanism involves the GluA1 subunit of the 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid (AMPA) subtype glutamate receptors. GluA1-dependent development operates in an NMDA-R independent manner and uses a distinct set of signaling molecules. The synapse associated protein of 97 kDa molecular weight (SAP97) is key. A deeper understanding of how experiences guides motor system development may lead to new ways to improve function after central nervous system insult.

Spinal cord transection modifies neuronal nitric oxide synthase expression in medullar reticular nuclei and in the spinal cord and increases parvalbumin immunopositivity in motoneurons below the site of injury in experimental rabbits

Using immunohistochemistry, we detected the expression of neuronal nitric oxide synthase (nNOS) in ventral medullary gigantocellular reticular nuclei and in the lumbosacral spinal cord 10 days after thoracic transection in experimental rabbits. We tried to determine whether neurons located below the site of injury are protected by the calcium binding protein parvalbumin (PV). Changes of nNOS immunoreactivity (IR) in spinal cord were correlated with the level of nNOS protein in dorsal and ventral horns. Ten days after transection, nNOS was upregulated predominantly in lateral gigantocellular nuclei. In the spinal cord, we revealed a significant increase of nNOS protein in the dorsal horn. This is consistent with a higher density of punctate and fiber-like immunostaining for nNOS in laminae III-IV and the up-regulation of nNOS-IR in neurons of the deep dorsal horn. After surgery, the perikarya of motoneurons remained nNOS immunonegative. Contrary to nNOS, the PV-IR was upregulated in α-motoneurons and small-sized neurons of the ventral horn. However, its expression was considerably reduced in neurons of the deep dorsal horn. The findings indicate that spinal transection affects nNOS and PV in different neuronal circuits.

Correlation between hippocampal levels of neural, epithelial and inducible NOS and spatial learning skills in rats

In the present study, to better understand the role of different nitric oxide synthase (NOS) isoforms in hippocampus-dependent forms of learning, we examined the expression of neural, endothelial, and inducible NOS in the hippocampus of young-adult rats classified as "poor" and "good" learners on the basis of their performance in the partially baited 12-arm radial maze. Taking into consideration strain-dependent differences in learning skills and NOS expression, experiments were performed on two different lines of laboratory rats: the inbred Wistar (W) and the outcrossed Wistar/Spraque-Dawley (W/S) line. The hippocampal levels of NOS proteins were assessed by Western Blotting. In the present study, genetically more homogenous W rats showed a slower rate of learning compared to the genetically less homogenous outcrossed W/S rats. The deficient performance in the W rat group compared to outcrossed W/S rats, and in "poor" learners of both groups compared to "good" learners was due to a higher percentage of reference memory errors. The overall NOS levels were significantly higher in W group compared to outcrossed W/S rats. In both rat lines, the rate of learning positively correlated with hippocampal levels of nNOS and negatively correlated with iNOS levels. Hippocampal eNOS levels correlated negatively with animals' performance but only in the W rats. These results suggest that all 3 NOS isoforms are implemented but play different roles in neural signaling.

Interactions between the NO-citrulline cycle and brain-derived neurotrophic factor in differentiation of neural stem cells

The diffusible messenger NO plays multiple roles in neuroprotection, neurodegeneration, and brain plasticity. Argininosuccinate synthase (AS) is a ubiquitous enzyme in mammals and the key enzyme of the NO-citrulline cycle, because it provides the substrate L-arginine for subsequent NO synthesis by inducible, endothelial, and neuronal NO synthase (NOS). Here, we provide evidence for the participation of AS and of the NO-citrulline cycle in the progress of differentiation of neural stem cells (NSC) into neurons, astrocytes, and oligodendrocytes. AS expression and activity and neuronal NOS expression, as well as l-arginine and NO(x) production, increased along neural differentiation, whereas endothelial NOS expression was augmented in conditions of chronic NOS inhibition during differentiation, indicating that this NOS isoform is amenable to modulation by extracellular cues. AS and NOS inhibition caused a delay in the progress of neural differentiation, as suggested by the decreased percentage of terminally differentiated cells. On the other hand, BDNF reversed the delay of neural differentiation of NSC caused by inhibition of NO(x) production. A likely cause is the lack of NO, which up-regulated p75 neurotrophin receptor expression, a receptor required for BDNF-induced differentiation of NSC. We conclude that the NO-citrulline cycle acts together with BDNF for maintaining the progress of neural differentiation.

Electrical activity as a developmental regulator in the formation of spinal cord circuits

Spinal cord development is a complex process involving generation of the appropriate number of cells, acquisition of distinctive phenotypes and establishment of functional connections that enable execution of critical functions such as sensation and locomotion. Here we review the basic cellular events occurring during spinal cord development, highlighting studies that demonstrate the roles of electrical activity in this process. We conclude that the participation of different forms of electrical activity is evident from the beginning of spinal cord development and intermingles with other developmental cues and programs to implement dynamic and integrated control of spinal cord function.

Cell class-specific regulation of neocortical dendrite and spine growth by AMPA receptor splice and editing variants

Glutamatergic transmission converging on calcium signaling plays a key role in dendritic differentiation. In early development, AMPA receptor (AMPAR) transcripts are extensively spliced and edited to generate subunits that differ in their biophysical properties. Whether these subunits have specific roles in the context of structural differentiation is unclear. We have investigated the role of nine GluA variants and revealed a correlation between the expression of flip variants and the period of major dendritic growth. In interneurons, only GluA1(Q)-flip increased dendritic length and branching. In pyramidal cells, GluA2(Q)-flop, GluA2(Q)-flip, GluA3(Q)-flip and calcium-impermeable GluA2(R)-flip promoted dendritic growth, suggesting that flip variants with slower desensitization kinetics are more important than receptors with elevated calcium permeability. Imaging revealed significantly higher calcium signals in pyramidal cells transfected with GluA2(R)-flip as compared with GluA2(R)-flop, suggesting a contribution of voltage-activated calcium channels. Indeed, dendritic growth induced by GluA2(R)-flip in pyramidal cells was prevented by blocking NMDA receptors (NMDARs) or voltage-gated calcium channels (VGCCs), suggesting that they act downstream of AMPARs. Intriguingly, the action of GluA1(Q)-flip in interneurons was also dependent on NMDARs and VGCCs. Cell class-specific effects were not observed for spine formation, as GluA2(Q)-flip and GluA2(Q)-flop increased spine density in pyramidal cells as well as in interneurons. The results suggest that AMPAR variants expressed early in development are important determinants for activity-dependent dendritic growth in a cell type-specific and cell compartment-specific manner.

Nitrergic ventro-medial medullary neurons activated during cholinergically induced active (rapid eye movement) sleep in the cat

The rostral ventro-medial medullary reticular formation is a complex structure that is involved with a variety of motor functions. It contains glycinergic neurons that are activated during active (rapid eye movement (REM)) sleep (AS); these neurons appear to be responsible for the postsynaptic inhibition of motoneurons that occurs during this state. We have reported that neurons in this same region contain nitric oxide (NO) synthase and that they innervate brainstem motor pools. In the present study we examined the c-fos expression of these neurons after carbachol-induced active sleep (C-AS). Three control and four experimental cats were employed to identify c-fos expressing nitrergic neurons using immunocytochemical techniques to detect the Fos protein together with neuronal nitric oxide synthase (nNOS) or nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase activity. The classical neurotransmitter content of the nitrergic cells in this region was examined through the combination of immunocytochemical techniques for the detection of glutamate, glycine, choline acetyltransferase (ChAT), tyrosine hydroxilase (TH) or GABA together with nNOS. During C-AS, there was a 1074% increase in the number of nitrergic neurons that expressed c-fos. These neurons did not contain glycine, ChAT, TH or GABA, but a subpopulation (15%) of them displayed glutamate-like immunoreactivity. Therefore, some of these neurons contain both an excitatory neurotransmitter (glutamate) and an excitatory neuromodulator (NO); the neurotransmitter content of the rest of them remains to be determined. Because some of the nitrergic neurons innervate brainstem motoneurons it is possible that they participate in the generation of tonic and excitatory phasic motor events that occur during AS. We also suggest that these nitrergic neurons may be involved in autonomic regulation during this state. In addition, because NO has trophic effects on target neurons, the present findings represent the first, albeit indirect, evidence for a possible trophic function of this nature during AS.

Differential roles of NMDA Receptor Subtypes NR2A and NR2B in dendritic branch development and requirement of RasGRF1

N-methyl-D-aspartate receptors (NMDARs) are known to regulate axonal refinement and dendritic branching. However, because NMDARs are abundantly present as tri-heteromers (e.g., NR1/NR2A/NR2B) during development, the precise role of the individual subunits NR2A and NR2B in these processes has not been elucidated. Ventral spinal cord neurons (VSCNs) provide a unique opportunity to address this problem, because the expression of both NR2A and NR2B (but not NR1) is downregulated in culture. Exogenous NR2A or NR2B were introduced into these naturally NR2-null neurons at 4 DIV, and electrophysiological recordings at 11 DIV confirmed that synaptic NR1NR2A receptors and NR1NR2B receptors were formed, respectively. Analysis of the dendritic architecture showed that introduction of NR2B, but not NR2A, dramatically increased the number of secondary and tertiary dendritic branches of VSCNs. Whole cell patch-clamp recordings further indicated that the newly formed branches in NR2B-expressing neurons were able to establish functional synapses because the frequency of miniature AMPA-receptor synaptic currents was increased. Using previously described mutants, we also found that disruption of the interaction between NR2B and RasGRF1 dramatically impaired dendritic branch formation in VSCNs. The differential role of the NR2A and NR2B subunits and the requirement for RasGRF1 in regulating branch formation was corroborated in hippocampal cultures. We conclude that the association between NR1NR2B-receptors and RasGRF1 is needed for dendritic branch formation in VSCNs and hippocampal neurons in vitro. The dominated NR2A expression and the limited interactions of this subunit with the signaling protein RasGRF1 may contribute to the restricted dendritic arbor development in the adult CNS.

Spatiotemporal patterns of dexamethasone-induced Ras protein 1 expression in the central nervous system of rats with experimental autoimmune encephalomyelitis

Dexamethasone-induced Ras protein 1 (Dexras 1), a brain-enriched member of Ras subfamily of guanosine triphosphatases, as a novel physiologic nitric oxide (NO) effector, anchor neuronal nitric oxide synthase (nNOS) that could form a ternary complex with carboxy-terminal PDZ ligand of nNOS (CAPON) and nNOS, to specific targets to enhance NO signaling. The present study was to explore the expression pattern of Dexras 1 in the development of experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis. Western blot and immunochemistry analysis showed that the gene and protein expression of Dexras 1 in the central nervous system (CNS) of rats increased significantly during the process of EAE compared with control groups (p < 0.01) and maintain a high level in the remission period. The protein expressions of nNOS and CAPON in hippocampus were approximately paralleled Dexras 1. Immunofluorescence revealed that both neurons and glial cells expressed the Dexras 1 in EAE CNS. Importantly, the damaged CNS in EAE-affected rats showed the codistribution between Dexras 1 and caspase 3, indicating the role of Dexras 1 played in the apoptotic process in EAE. Furthermore, colocalizations of Dexras 1 were observed in neurons and glial cells in CNS with nNOS or CAPON, supporting the ternary complex in this model. Thus, these findings suggest the postulation that Dexras 1 might participate into CNS neuronal cell death and demyelination in the whole process of EAE through regulating the NO signaling by binding to nNOS and CAPON.

Nitrosothiol reactivity profiling identifies S-nitrosylated proteins with unexpected stability

Nitric oxide (NO) regulates protein function by S-nitrosylation of cysteine to form nitrosothiols. Nitrosothiols are highly susceptible to nonenzymatic degradation by cytosolic reducing agents. Here we show that although most protein nitrosothiols are rapidly degraded by cytosolic reductants, a small subset form unusually stable S-nitrosylated proteins. Our findings suggest that stable S-nitrosylation reflects a protein conformation change that shields the nitrosothiol. To identify stable protein nitrosothiols, we developed a proteomic method for profiling S-nitrosylation. We examined the stability of over 100 S-nitrosylated proteins, and identified 10 stable nitrosothiols. These proteins remained S-nitrosylated in cells after NO synthesis was inhibited, unlike most S-nitrosylated proteins. Taken together, our data identify a class of NO targets that form stable nitrosothiols in the cell and are likely to mediate the persistent cellular effects of NO.

Focal degeneration of astrocytes in amyotrophic lateral sclerosis

Astrocytes emerge as key players in motor neuron degeneration in Amyotrophic Lateral Sclerosis (ALS). Whether astrocytes cause direct damage by releasing toxic factors or contribute indirectly through the loss of physiological functions is unclear. Here we identify in the hSOD1(G93A) transgenic mouse model of ALS a degenerative process of the astrocytes, restricted to those directly surrounding spinal motor neurons. This phenomenon manifests with an early onset and becomes significant concomitant with the loss of motor cells and the appearance of clinical symptoms. Contrary to wild-type astrocytes, mutant hSOD1-expressing astrocytes are highly vulnerable to glutamate and undergo cell death mediated by the metabotropic type-5 receptor (mGluR5). Blocking mGluR5 in vivo slows down astrocytic degeneration, delays the onset of the disease and slightly extends survival in hSOD1(G93A) transgenic mice. We propose that excitotoxicity in ALS affects both motor neurons and astrocytes, favouring their local interactive degeneration. This new mechanistic hypothesis has implications for therapeutic interventions.

Modest alterations in patterns of motor neuron dendrite morphology in the Fmr1 knockout mouse model for fragile X

Fragile X, an inheritable form of mental retardation, is caused by the inactivation of a gene on the X chromosome, FMR1 which codes for an RNA binding protein, fragile X mental retardation protein. Loss of this protein is associated with reduced complexities of neuronal dendrites and alterations in spine morphology in a number of cortical brain regions, and these deficits may underlie the cognitive impairment observed in fragile X patients. Among the many symptoms of fragile X are altered motor functions, although the neuronal basis for these remains unclear. In this study we investigated whether knockout of Fmr1 in the mouse model of fragile X altered dendrite morphology in developing spinal cord motor neurons. We find that Fmr1 knockout leads to modest alterations in the distribution of dendritic arbor across the span of the motor neuron dendritic tree in 2- and 4-week-old mice, compared to wild-type controls, consistent with slower rates of extension and abnormal pruning of intermediate dendritic segments. These studies suggest that some motor deficits in fragile X patients may be due to abnormal maturation of dendritic patterning within spinal motor neurons, and suggest that strategies aimed at preventing motor impairment in fragile X patients may be targeted at motor functions during early development.

Expression of the N-methyl-D-aspartate receptor subunit NR3B regulates dendrite morphogenesis in spinal motor neurons

During postnatal development, the dendrites of spinal motor neurons are refined in an activity-dependent manner that can be influenced by blocking activation of N-methyl-D-aspartate (NMDA) receptors. In late postnatal life, dendritic refinement ceases, and dendrite architecture is unaffected by NMDA antagonists; however the molecular substrate for limiting dendritic plasticity is not understood. During late postnatal development, expression of the NR3B NMDA receptor subunit, a putative dominant-negative subunit that reduces glutamate-induced ionic currents, is upregulated within motor neurons. To investigate whether increasing NR3B expression may contribute to the loss in late development of activity-dependent dendritic reorganization in the spinal cord, we over-expressed NR3B in cultured rat spinal motor neurons, and compared its effects on dendrite morphology with the effects of pharmacological blockade of NMDA receptors. We found that over-expression of the NR3B receptor subunit increased the length and complexity of dendritic arbor, and increased numbers of dendritic filopodia, suggesting that NR3B promotes the addition of branch segments in developing motor neurons. In contrast, blockade of NMDA receptor activity by the NMDA antagonist DL-2-amino-5-phosphonovalerate (AP5) had little effect on the overall length or complexity of dendritic arbor. Instead, treatment with AP5 resulted in significant reorganization of dendritic arbor in a manner that favored addition of dendritic segments of high branch orders, at the expense of those closer to the cell body. These results suggest that expression of the NR3B subunit may participate in activity-dependent reorganization of dendritic architecture, but via a mechanism that may be inconsistent with loss of NMDA receptor activity.

Differential regulation of dendrite complexity by AMPA receptor subunits GluR1 and GluR2 in motor neurons

Activity-dependent developmental mechanisms in many regions of the central nervous system are thought to be responsible for shaping dendritic architecture and connectivity, although the molecular mechanisms underlying these events remain obscure. Since AMPA glutamate receptors are developmentally regulated in spinal motor neurons, we have investigated the role of activation of AMPA receptors in dendritic outgrowth of spinal motor neurons by overexpression of two subunits, GluR1 and GluR2, and find that dendrite outgrowth is differentially controlled by expression of these subunits. Overexpression of GluR1 was associated with greater numbers of filopodia, and an increase in the length and complexity of dendritic arbor. In contrast, GluR2 expression did not alter dendritic complexity, but was associated with a moderate increase in length of arbor, and decreased numbers of filopodia. Neither GluR1 nor GluR2 had any effect on the motility of filopodia. In addition, GluR1 but not GluR2 expression increased the density of dendritic puncta incorporating a GFP-labeled PSD95, suggesting that GluR1 may mediate its effect in part by augmenting the number of excitatory synapses within motor neuron dendrites. Together these results suggest that in spinal motor neurons, AMPA receptors composed of GluR1 subunits may facilitate neurotrophic mechanisms in these neurons, permitting sustained dendrite outgrowth and synaptogenesis, whereas expression of AMPA receptors containing GluR2 acts to preserve existing dendritic arbor. Thus, the observed downregulation of GluR1 in motor neurons during postnatal development may limit the formation of new dendrite segments and synapses, promoting stabilized synaptic connectivity.

Neuromodulation and developmental plasticity in the locomotor system of anuran amphibians during metamorphosis

Metamorphosis in frogs has long fascinated laymen and scientists alike. This remarkable developmental transformation involves the simultaneous remodelling of almost every organ in the body, including the gut, associated with a switch in diet from filter feeder to predator, and the visual system, from laterally-directed monocular to forward-directed binocular vision. In the context of locomotion there is the complete loss of the tail, the main structure involved in generating thrust during swimming in larvae, and the gain of the limbs which produce rhythmic extension-flexion kicks during swimming and jumping. Here we review recent evidence from experiments utilizing novel in vitro isolated preparations of the Xenopus laevis spinal cord and brainstem which remain viable for several days and can generate motor rhythms similar to those that would normally drive locomotion in vivo. The results indicate that the developing limb circuitry is born from within the existing axial-based network, which acts like a functional scaffold. Initially the limb activity shares the same left-right alternation coordination and relatively high frequency as the tail swimming network. Only later, once the limbs are fully functional, does the limb network break free to produce left-right synchrony of limb motoneuron bursting and with a different, slower cadence than the tail-based system. During the initial formation of the limb networks nitric oxide-producing neurons appear in the spinal cord, but occupy regions other than those in which the new limb circuitry is developing. Now exogenous nitric oxide facilitates locomotor activity, in contrast to its inhibitory effects on swimming at earlier larval stages of development.

Changes in mRNA for CAPON and Dexras1 in adult rat following sciatic nerve transection

Peripheral nerve transection has been implicated to cause a production of neuronal nitric oxide synthase (nNOS), which may influence a range of post-axotomy processes necessary for neuronal survival and nerve regeneration. Carboxy-terminal post synaptic density protein/Drosophila disc large tumor suppressor/zonula occuldens-1 protein (PDZ) ligand of neuronal nitric oxide synthase (CAPON), as an adaptor, interacts with nNOS via the PDZ domain helping regulate nNOS activity at postsynaptic sites in neurons. And Dexras1, a small G protein mediating multiple signal transductions, has been reported to form a complex with CAPON and nNOS. A role for the physiologic linkage by CAPON of nNOS to Dexras1 has suggested that NO-mediated activation of Dexras1 is markedly enhanced by CAPON. We investigated the changes in mRNA for CAPON, Dexras1 and nNOS in the sciatic nerve, dorsal root ganglia and lumbar spinal cord of adult rat following sciatic axotomy by TaqMan quantitative real-time PCR and in situ hybridization combined with immunofluorescence. Signals of mRNA for CAPON and Dexras1 were initially expressed in these neural tissues mentioned, transiently increased at certain time periods after sciatic axotomy and finally recovered to the basal level. It was also found that nNOS mRNA underwent a similar change pattern during this process. These results suggest that CAPON as well as Dexras1 may be involved in the different pathological conditions including nerve regeneration, neuron loss or survival and even pain process, possibly via regulating the nNOS activity or through the downstream targets of Dexras1.

Developmental regulation of PSD-95 and nNOS expression in lumbar spinal cord of rats

Postsynaptic density (PSD)-95 is originally isolated from glutamatergic synapse where it serves as a physical tether to allow neuronal nitric oxide synthase (nNOS) signaling by N-methyl-D-aspartate receptor (NMDAR) activity. Considering the physiological importance of glutamate receptor and nitric oxide (NO) during development, we examined the spatiotemporal expression of PSD-95 and nNOS in the lumbar spinal cord at a postnatal stage. Temporally, both gene and protein levels of them gradually increased with age after birth, peaked at the postnatal day 14 (P14), and then decreased to an adult level. In addition, the enhanced coimmunoprecipitations between PSD-95 and nNOS were detected in developing spinal cord. Spatially, PSD-95 staining codistributed with nNOS in NeuN-positive motor neurons and sensory neurons at P14. These findings indicate that PSD-95 and nNOS might collectively participate in spinal cord development.

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.

Development and neuromodulation of spinal locomotor networks in the metamorphosing frog

Metamorphosis in the anuran frog, Xenopus laevis, involves profound structural and functional transformations in most of the organism's physiological systems as it encounters a complete alteration in body plan, habitat, mode of respiration and diet. The metamorphic process also involves a transition in locomotory strategy from axial-based undulatory swimming using alternating contractions of left and right trunk muscles, to bilaterally-synchronous kicking of the newly developed hindlimbs in the young adult. At critical stages during this behavioural switch, functional larval and adult locomotor systems co-exist in the same animal, implying a progressive and dynamic reconfiguration of underlying spinal circuitry and neuronal properties as limbs are added and the tail regresses. To elucidate the neurobiological basis of this developmental process, we use electrophysiological, pharmacological and neuroanatomical approaches to study isolated in vitro brain stem/spinal cord preparations at different metamorphic stages. Our data show that the emergence of secondary limb motor circuitry, as it supersedes the primary larval network, spans a developmental period when limb circuitry is present but not functional, functional but co-opted into the axial network, functionally separable from the axial network, and ultimately alone after axial circuitry disappears with tail resorption. Furthermore, recent experiments on spontaneously active in vitro preparations from intermediate metamorphic stage animals have revealed that the biogenic amines serotonin (5-HT) and noradrenaline (NA) exert short-term adaptive control over circuit activity and inter-network coordination: whereas bath-applied 5-HT couples axial and appendicular rhythms into a single unified pattern, NA has an opposite decoupling effect. Moreover, the progressive and region-specific appearance of spinal cord neurons that contain another neuromodulator, nitric oxide (NO), suggests it plays a role in the maturation of limb locomotor circuitry. In summary, during Xenopus metamorphosis the network responsible for limb movements is progressively segregated from an axial precursor, and supra- and intra-spinal modulatory inputs are likely to play crucial roles in both its functional flexibility and maturation.

Postnatal switching of NMDA receptor subunits from NR2B to NR2A in rat facial motor neurons

The subunit composition of N-methyl-D-aspartate (NMDA) receptors affects their function under normal and pathological conditions. Functional NMDA receptors are expressed in lower motor neurons, but their subunit composition has not been defined. Here, we employed electrophysiology, quantitative PCR, and immunohistochemistry to investigate the subunit composition of NMDA receptors in postnatal motor neurons of the Wistar rat facial nucleus (FN). Whole-cell patch clamp recordings of acutely dissociated motor neurons from postnatal days 3 and 4 (P3-P4) showed that ifenprodil, a specific antagonist of the NMDA receptor 2B (NR2B) subunit, inhibited 91.62%+/-2.02% of NMDA-induced current, whereas NVP-AAM007, a specific antagonist of the NMDA receptor 2A (NR2A) subunit, inhibited much less of the current (16.69%+/-3.28%). Starting from P5, the inhibitory effects of ifenprodil and NVP-AAM007 gradually decreased and increased, respectively, such that the effect of NVP-AAM007 exceeded that of ifenprodil by P10. At P14, most of the NMDA-induced current was inhibited by NVP-AAM007 (84.59%+/-3.35%). Consistent with this, NR2B mRNA and protein were expressed highly at P3 and then gradually decreased by more than 75% by P14 in FN motor neurons, while NR1 was expressed stably over the same ages. However, NR2A mRNA and protein showed relatively constant levels between P3-P10 and decreased to 45% and 75% of the P3 level, respectively, by P14. Thus, analysis of functional NMDA receptors is critical to revealing subunit switching, which may be an important step in postnatal development of FN motor neurons.

Developmental and regional expression of NADPH-diaphorase/nitric oxide synthase in spinal cord neurons correlates with the emergence of limb motor networks in metamorphosing Xenopus laevis

Metamorphosis in anuran amphibians requires a complete transformation in locomotor strategy from undulatory tadpole swimming to adult quadrupedal propulsion. The underlying reconfiguration of spinal networks may be influenced by various neuromodulators including nitric oxide, which is known to play an important role in CNS development and plasticity in diverse species, including metamorphosis of amphibians. Using NADPH-diaphorase (NADPH-d) staining and neuronal nitric oxide synthase (nNOS) immunofluorescence labelling, the expression and developmental distribution of NOS-containing neurons in the spinal cord and brainstem were analysed in all metamorphic stages of Xenopus laevis. Wholemount preparations of the spinal cord from early stages of metamorphosis (coincident with emergence of the fore- and hindlimb buds) revealed two clusters of NOS-positive neurons interspersed with areas devoid of stained somata. These cells were distributed in three topographic subgroups, the most ventral of which had axonal projections that crossed the ventral commissure. Motoneurons innervating the fore- and hindlimb buds were retrogradely labelled with horseradish peroxidase (HRP) to determine their position in relation to the two NOS-expressing cord regions. Limb motoneurons and NOS-positive cells did not overlap, indicating that during early stages of metamorphosis nitrergic neurons are excluded from regions where spinal limb circuits are forming. As metamorphosis progresses, NOS expression became distributed along the length of the spinal cord together with an increase in the number and intensity of labelled cells and fibers. NOS expression reached a peak as the forelimbs emerge then declined. These findings are consistent with a role for nitric oxide (NO) in the developmental transition from undulatory swimming to quadrupedal locomotion.

Bi-directional control of motor neuron dendrite remodeling by the calcium permeability of AMPA receptors

Motor neurons express particularly high levels of the AMPA receptor subunit GluR1(Q)flip (GluR1(Q)i) during the period in early postnatal life when their dendritic tree grows and becomes more branched. To investigate how GluR1-containing AMPA receptors contribute to dendrite morphogenesis, we characterized a mutant form of GluR1 (containing a histidine in the Q/R editing site) with unique electrophysiological properties. Most notably, AMPA receptors assembled from GluR1(H)i display less calcium permeability than AMPA receptors assembled from GluR1(Q)i. Expression of GluR1(Q)i in vivo or in vitro led to an increase in dendrite branching with no net change in the overall tree size while GluR1(H)i led to a loss of branches and a net reduction in overall tree size. GluR1(H)i-dependent dendrite atrophy is mediated by protein phosphatase 2B. The results suggest that the electrophysiological properties of cell surface AMPA receptors, specifically their permeability to calcium, can be a central determinant of whether the dendrites undergo activity-dependent branching or atrophy.

The role of phosphodiesterase isoforms 2, 5, and 9 in the regulation of NO-dependent and NO-independent cGMP production in the rat cervical spinal cord

NO-responsive, cGMP-producing structures are abundantly present in the cervical spinal cord. NO-mediated cGMP synthesis has been implicated in nociceptive signaling and it has been demonstrated that cGMP has a role establishing synaptic connections in the spinal cord during development. As cGMP levels are controlled by the activity of soluble guanylyl cyclase (synthesis) and the phosphodiesterase (PDE) activity (breakdown), we studied the influence of PDE activity on NO-stimulated cGMP levels in the rat cervical spinal cord. cGMP-immunoreactivity (cGMP-IR) was localized in sections prepared from slices incubated in vitro. A number of reported PDE isoform-selective PDE inhibitors was studied in combination with diethylamineNONOate (DEANO) as a NO-donor including isobutyl-methylxanthine (IBMX) as a non-selective PDE inhibitor. We studied 8-methoxy-IBMX as a selective PDE1 inhibitor, erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) and BAY 60-7550 as selective PDE2 inhibitors, sildenafil as a selective PDE5 inhibitor, dipyridamole as a mixed type PDE5 and PDE10 inhibitor, rolipram as a PDE4 inhibitor, and SCH 81566 as a selective PDE9 inhibitor. cGMP-IR structures (nerve fibers, axons, and terminals) were characterized using the following neurochemical markers: vesicular transporter molecules for acetylcholine, GABA, and glutamate (type 1 and type 2), parvalbumin, glutamate transporter molecule EAAT3, synaptophysin, substance P, calcitonin gene-related peptide, and isolectin B4. Most intense cGMP-IR was observed in the dorsal lamina. Ventral motor neurons were devoid of cGMP-IR. cGMP-IR was observed in GABAergic, and glutamatergic terminals in all gray matter laminae. cGMP-IR was abundantly colocalized with anti-vesicular glutamate transporter 2 (vGLUT2), however not with the anti-vesicular glutamate transporter 1 (vGLUT1), suggesting a functional difference between structures expressing vGLUT1 or vGLUT2. cGMP-IR did not colocalize with substance P- or calcitonin-gene related peptide-IR structures, however did partially colocalize with isolectin B4 in the dorsal horn. cGMP-IR in cholinergic structures was observed in dorsal root fibers entering the spinal cord, occasionally in laminae 1-3, in laminae 8 and 9 in isolated boutons and in the C-type terminals, and in small cells and varicosities in lamina 10. This latter observation suggests that the proprioceptive interneurons arising in lamina 10 are also NO-responsive. No region-specific nor a constant co-expression of cGMP-IR with various neuronal markers was observed after incubation of the slices with one of the selected PDE inhibitors. Expression of the mRNA of PDE2, 5, and 9 was observed in all lamina. The ventral motor neurons and the ependymal cells lining the central canal expressed all three PDE isoforms. Incubation of the slices in the presence of IBMX, DEANO in combination with BAY 41-2272, a NO-independent activator of soluble guanylyl cyclase, provided evidence for endogenous NO synthesis in the slice preparations and enhanced cGMP-IR in all lamina. Under these conditions cGMP-IR colocalized with substance P in a subpopulation of substance P-IR fibers. It is concluded that NO functions as a retrograde neurotransmitter in the spinal cord but that also postsynaptic structures are NO-responsive by producing cGMP. cGMP-IR in a subpopulation of isolectin B4 positive fibers and boutons is indicative for a role of NO-cGMP signaling in nociceptive processing. cGMP levels in the spinal cord are controlled by the concerted action of a number of PDE isoforms, which can be present in the same cell.

Function of cGMP-Dependent Protein Kinases as Revealed by Gene Deletion

Over the past few years, a wealth of biochemical and functional data have been gathered on mammalian cGMP-dependent protein kinases (cGKs). In mammals, three different kinases are encoded by two genes. Mutant and chimeric cGK proteins generated by molecular biology techniques yielded important biochemical knowledge, such as the function of the NH2-terminal domains of cGKI and cGKII, the identity of the cGMP-binding sites of cGKI, and the substrate specificity of the enzymes. Genetic approaches have proven especially useful for the analysis of the biological functions of cGKs. Recently, some of the in vivo targets and mechanisms leading to changes in neuronal adaptation, smooth muscle relaxation and growth, intestinal water secretion, bone growth, renin secretion, and other important functions have been identified. These data show that cGKs are signaling molecules involved in many biological functions.

NMDA receptor binding declines differentially in three spinal motor nuclei during postnatal development

The NMDA subtype of glutamate receptors mediates a variety of neuronal processes involved in the development of dendritic morphology. For example, NMDA receptor antagonism during the early postnatal period attenuates dendritic growth in spinal motoneurons. NMDA receptors are present in high levels in the spinal cord early in the postnatal period and decline during development, a period of extensive dendritic plasticity in the spinal cord. Previous studies have suggested that an adult pattern of distribution of NMDA receptors is established as early as postnatal day (P)21 (day of birth = P1). However, given that dendritic growth in spinal motoneurons is not complete by this age and that NMDA receptor activation is necessary for dendritic growth, we assessed NMDA receptor binding in specific spinal motor nuclei during normal development. NMDA receptors were labeled with [3H]MK-801 at P7, P14, P28, P49, and in adult male rats. Receptor binding in the spinal nucleus of the bulbocavernosus (SNB), dorsolateral nucleus (DLN) and retrodorsolateral nucleus (RDLN) was measured using in vitro quantitative autoradiography. NMDA receptor binding over the SNB, DLN and RDLN in intact males was initially high, and declined to adult levels. However, the time course of the decline differed across nuclei. The local decline in NMDA receptor binding observed in the SNB and DLN is coincident with the periods of dendritic growth in these nuclei, further supporting a role for NMDA receptors in the development of motoneuron dendritic morphology.

Differential environmental exposure alters NMDA but not AMPA receptor subunit expression in nucleus accumbens core and shell

Environmentally enriched (EE) rats show neurobehavioral differences relative to less stimulated, socially isolated (SI) littermates. Although experience-dependent cortical changes are presumed to underlie learning differences in these differentially housed animals, EE rats show reduced reward-seeking behavior and altered cytoarchitecture and dopaminergic function in the nucleus accumbens (NAcc), a brain area involved in adaptive, goal-directed activity. Given that glutamate and its interaction with dopamine regulate motivational and associative processing in this brain region, we assessed expression of the NR1 subunit of the N-methyl-D-aspartate (NMDA) receptor and the GluR1 subunit of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor in the NAcc core and shell of EE and SI rats. Our results indicate fewer intensely stained NR1 immunopositive neurons in both core and shell of EE relative to SI rats. No such differences were observed in GluR1 staining. These results suggest that environmental experience alters NMDA but not AMPA receptor expression in NAcc. Increased expression of the NR1 subunit in the NAcc of SI rats may augment impulsivity and reward-seeking behavior relative to EE rats.

Developmental changes in spinal motoneuron dendrites in neonatal mice

We examined the age-dependent morphological changes of lumbar spinal motoneurons (MNs) in neonatal Swiss-Webster mice during the first 2 weeks of postnatal life. Neurons labeled by intracellular injection of biocytin in hemisected lumbosacral spinal cords in vitro were reconstructed from serial sections. Digitized data were compared for young (P3; postnatal days 2-4; n = 9) and older animals (P11; postnatal days 10-13; n = 8). As expected, measures of dendritic size (e.g., stem branch diameter, total surface area, maximum distance to tips, and lateral tree spread) were all significantly greater for P11 than for P3 mice. In contrast, the number of dendrites per MN and parameters related to tree topology (e.g., terminations per tree and maximum branch order), although slightly greater for P11 animals, were not significantly different between the two ages. Dendrite growth appeared to be proportional throughout the tree because the ratios between average terminal and internal branch lengths were similar for the two groups. Furthermore, this elongation was proportional to enlargement of overall spinal cord dimensions. A variety of other morphometric measures showed no significant difference between age groups. The relative constancy of MN dendritic topology up to P13 was surprising, given the striking maturation in motor function during this time period.

Neuronal nitric oxide synthase knock-out mice show impaired cognitive performance

Nitric oxide (NO) plays a role in a series of neurobiological functions, underlying behavior and memory. The functional role of nNOS derived NO in cognitive functions, however, is elusive. We decided to study cognitive functions in the Morris water maze (MWM) and the multiple T-maze (MTM) in 3-month-old male nNOS-knock-out mice (nNOS KO). To study the influence of neurology and behavior, we performed tests in an observational battery, the rota-rod, the elevated plus maze (EPM), the open field (OF), and a social interaction test. In the memory and relearning task of the MWM, most nNOS KO failed whereas performing better in the MTM. nNOS KO displayed significantly increased frequency of grooming, center crossings, and entries into the center in the OF. The observational battery revealed significantly increased scores for touch-escape reaction, body position, locomotion, and pelvic- and tail-elevation together with reduced vocalization. In the EPM, the time spent in the closed arm and the grooming frequency were significantly increased whereas urination was absent. We conclude that nNOS KO show impaired spatial performance in the MWM and herewith confirm the role of nNOS in cognitive functions such as processing, maintenance, and recall of memory. It must be taken into account that the major behavioral findings of increased grooming and anxiety-related behaviors may have led to impaired function in the MWM. The fact that nNOS KO performed well in the MTM, reflecting a low stress situation points to the interpretation that nNOS inhibition affects cognitive functions under stressful conditions (MWM) only.

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.

Effects of limb exercise after spinal cord injury on motor neuron dendrite structure

An integration center subserving locomotor leg movements resides in the upper lumbar spinal cord. If this neuronal network is preserved after a spinal cord injury, it is possible to stimulate this circuitry to initiate and promote walking. The several effective approaches (electrical stimulation, pharmacologic agents, physical therapy training programs) may all share a common modus operandi of altering synaptic activity within segmental spinal cord. To understand the neural substrate for the use-dependent behavioral improvement, we studied the dendritic architecture of spinal motor neurons. In the first experiment, we compared three groups of animals: animals with an intact spinal cord, animals that had a complete spinal cord transection (SCT) and animals with SCT who engaged in a daily exercise program of actively moving paralyzed hindlimbs through the motions of walking. When compared with animals with an intact spinal cord, the motor neurons from animals with SCT displayed marked atrophy, with loss of dendritic membrane and elimination of branching throughout the visible tree within transverse tissue slices. None of these regressive changes were found in the motor neurons from SCT animals that underwent exercise. In a second experiment, we inquired whether exercise of animals with an intact spinal cord influenced dendrite structure. Increased exercise had very modest effects on dendrite morphology, indicating an upper limit of use-dependent dendrite growth. Our findings suggest that the dendritic tree of motor neurons deprived of descending influences is rapidly pruned, and this finding is not observed in motor neurons after SCT if hindlimbs are exercised. The functional benefits of exercise after SCT injury may be subserved, in part, by stabilizing or remodeling the dendritic tree of motor neurons below the injury site.

7-Nitroindazole upregulates phosphorylated cAMP response element binding protein, polysialylated-neural cell adhesion molecule and tryptophan hydroxylase expression in the adult rat hippocampus

We examined the effect of nitric oxide (NO) on the expression of phosphorylated cAMP response element binding (pCREB) protein and on the polysialylated form of the neural cell adhesion molecule (PSA-NCAM), in the hippocampus of the adult rat, by administering 7-nitroindazole (7-NI) for 5 days, to block neuronal nitric oxide synthase (nNOS) activity. It was found that PSA-NCAM and pCREB immunoreactive cells were increased two-fold in the dentate gyrus by this treatment. In addition, the immunoreactivities of serotonin (5-HT, 5-hydroxytryptamine), known to be involved in neural plasticity, and of tryptophan hydroxylase immunoreactive-fibers in the dentate gyrus were both upregulated by 7-NI. Our results indicate that nNOS inhibition upregulates neural plasticity and increases 5-HT in the rat hippocampus, and suggest the existence of regulatory relationships between NO, neural plasticity and 5-HT.

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.

The inhibition of nitric oxide synthase enhances PSA-NCAM expression and CREB phosphorylation in the rat hippocampus

It is well known that nitric oxide (NO) acts downstream of NMDA receptor activation, which regulates the neural plasticity in the brain. In the present study, the effect of L-NAME, a non-selective nitric oxide synthase (NOS) inhibitor, on neural plasticity in the hippocampus was investigated. L-NAME increased the expression of PSA-NCAM and pCREB in the adult rat hippocampus. The co-localization of PSA-NCAM and pCREB indicates a possible relationship between the two in the granule cell layer in the dentate gyrus. Our results demonstrate that NO, as a subsignal of NMDA receptors, could be involved in the structural plasticity of the granule cell layer in the dentate gyrus by regulating the expression of PSA-NCAM and pCREB in the hippocampus.

Development of a sexually dimorphic neuromuscular system in male rats after spinal transection: morphologic changes and implications for estrogen sites of action

The lumbar spinal cord of rats contains the sexually dimorphic, steroid-sensitive spinal nucleus of the bulbocavernosus (SNB). In male rats, SNB motoneurons exhibit a biphasic pattern of dendritic growth, having an initial period of exuberant growth followed by a period of retraction to mature lengths by 7 weeks of age. This growth is steroid dependent: dendrites fail to grow after castration, but growth is supported in castrates treated with estradiol. In this experiment, we examined whether supraspinal afferent input by means of descending spinal tracts to the SNB was involved in the normal postnatal development of SNB motoneurons, and whether the effect of estradiol on SNB dendritic growth could be explained by an indirect action of estradiol on supraspinal afferents. Motoneuron morphology was assessed in normal males, early- or late-postnatally transected males, castrated males left untreated or treated with estradiol, and transected castrates treated with estradiol. SNB motoneurons were retrogradely labeled with cholera toxin-horseradish peroxidase during both the growth and retraction phases of dendritic development and reconstructed in three dimensions. The removal of supraspinal afferents resulted in extremely local effects within the developing SNB arbor, as well as transient alterations in somal growth. Furthermore, spinal transection did not block the trophic effect of estradiol on supporting SNB dendritic growth, indicating that estrogens do not act by means of supraspinal input to support SNB motoneuron development.