NMDA-R1 subunit of the cerebral cortex co-localizes with neuronal nitric oxide synthase at pre- and postsynaptic sites and in spines

Calcium transients in nNOS neurons underlie distinct phases of the neurovascular response to barrel cortex activation in awake mice

Neuronal nitric oxide (NO) synthase (nNOS), a Ca2+ dependent enzyme, is expressed by distinct populations of neocortical neurons. Although neuronal NO is well known to contribute to the blood flow increase evoked by neural activity, the relationships between nNOS neurons activity and vascular responses in the awake state remain unclear. We imaged the barrel cortex in awake, head-fixed mice through a chronically implanted cranial window. The Ca2+ indicator GCaMP7f was expressed selectively in nNOS neurons using adenoviral gene transfer in nNOScre mice. Air-puffs directed at the contralateral whiskers or spontaneous motion induced Ca2+ transients in 30.2 ± 2.2% or 51.6 ± 3.3% of nNOS neurons, respectively, and evoked local arteriolar dilation. The greatest dilatation (14.8 ± 1.1%) occurred when whisking and motion occurred simultaneously. Ca2+ transients in individual nNOS neurons and local arteriolar dilation showed various degrees of correlation, which was strongest when the activity of whole nNOS neuron ensemble was examined. We also found that some nNOS neurons became active immediately prior to arteriolar dilation, while others were activated gradually after arteriolar dilatation. Discrete nNOS neuron subsets may contribute either to the initiation or to the maintenance of the vascular response, suggesting a previously unappreciated temporal specificity to the role of NO in neurovascular coupling.

Impaired spatial memory in adult vitamin D deficient BALB/c mice is associated with reductions in spine density, nitric oxide, and neural nitric oxide synthase in the hippocampus

Vitamin D deficiency is prevalent in adults and is associated with cognitive impairment. However, the mechanism by which adult vitamin D (AVD) deficiency affects cognitive function remains unclear. We examined spatial memory impairment in AVD-deficient BALB/c mice and its underlying mechanism by measuring spine density, long term potentiation (LTP), nitric oxide (NO), neuronal nitric oxide synthase (nNOS), and endothelial NOS (eNOS) in the hippocampus. Adult male BALB/c mice were fed a control or vitamin D deficient diet for 20 weeks. Spatial memory performance was measured using an active place avoidance (APA) task, where AVD-deficient mice had reduced latency entering the shock zone compared to controls. We characterised hippocampal spine morphology in the CA1 and dentate gyrus (DG) and made electrophysiological recordings in the hippocampus of behaviourally naïve mice to measure LTP. We next measured NO, as well as glutathione, lipid peroxidation and oxidation of protein products and quantified hippocampal immunoreactivity for nNOS and eNOS. Spine morphology analysis revealed a significant reduction in the number of mushroom spines in the CA1 dendrites but not in the DG. There was no effect of diet on LTP. However, hippocampal NO levels were depleted whereas other oxidation markers were unaltered by AVD deficiency. We also showed a reduced nNOS, but not eNOS, immunoreactivity. Finally, vitamin D supplementation for 10 weeks to AVD-deficient mice restored nNOS immunoreactivity to that seen in in control mice. Our results suggest that lower levels of NO and reduced nNOS immunostaining contribute to hippocampal-dependent spatial learning deficits in AVD-deficient mice.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Postsynaptic activity of inhibitory neurons evokes hemodynamic fMRI responses

Functional MRI responses are localized to the synaptic sites of evoked inhibitory neurons, but it is unknown whether, or by what mechanisms, these neurons initiate functional hyperemia. Here, the neuronal origins of these hemodynamic responses were investigated by fMRI or local field potential and blood flow measurements during topical application of pharmacological agents when GABAergic granule cells in the rat olfactory bulb were synaptically targeted. First, to examine if postsynaptic activation of these inhibitory neurons was required for neurovascular coupling, we applied an NMDA receptor antagonist during cerebral blood volume-weighted fMRI acquisition and found that responses below the drug application site (up to ~1.5 mm) significantly decreased within ~30 min. Similarly, large decreases in granule cell postsynaptic activities and blood flow responses were observed when AMPA or NMDA receptor antagonists were applied. Second, inhibition of nitric oxide synthase preferentially decreased the initial, fast component of the blood flow response, while inhibitors of astrocyte-specific glutamate transporters and vasoactive intestinal peptide receptors did not decrease blood flow responses. Third, inhibition of GABA release with a presynaptic GABAB receptor agonist caused less reduction of neuronal and blood flow responses compared to the postsynaptic glutamate receptor antagonists. In conclusion, local hyperemia by synaptically-evoked inhibitory neurons was primarily driven by their postsynaptic activities, possibly through NMDA receptor-dependent calcium signaling that was not wholly dependent on nitric oxide.

Postnatal development of the distribution of nitric oxide-producing neurons in the rat corpus callosum

The postnatal development of nitric oxide (NO)-producing intracallosal neurons was studied in rats by nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry from postnatal day 0 (P0) to P30. NADPH-d-positive neurons (NADPH-d+_Ns) were detected already at P0, mainly in the rostral region of the corpus callosum (cc). Their location and the intensity of staining allowed them to be classified as type I NO-producing neurons. At P0, tufts of intensely labeled fibers, probably corresponding to the callosal septa described in the monkey and human cc, entered the ventral cc region and reached its dorsal portion. From P5, cell bodies and dendrites were often associated to blood vessels. The number of intracallosal NADPH-d+_Ns rose in the first postnatal days to peak at P5, it declined until P10, and then remained almost constant until P30. Their size increased from P0 to P30, dramatically so (>65%) from P0 to P15. From P10 onward their distribution was adult-like, i.e. NADPH-d+_Ns were more numerous in the lateral and intermediate portions of the cc and diminished close to the midline. In conjunction with previous data, these findings indicate that intracallosal NADPH-d+_Ns could have a role in callosal axon guidance, myelination, refinement processes, and callosal blood flow regulation.

NO as a multimodal transmitter in the brain: discovery and current status

NO operates throughout the brain as an intercellular messenger, initiating its varied physiological effects by activating specialized GC-coupled receptors, resulting in the formation of cGMP. In line with the widespread expression of this pathway, NO participates in numerous different brain functions. This review gives an account of the discovery of NO as a signalling molecule in the brain, experiments that originated in the search for a mysterious cGMP-stimulating factor released from central neurones when their NMDA receptors were stimulated, and summarizes the subsequent key steps that helped establish its status as a central transmitter. Currently, various modes of operation are viewed to underlie its diverse behaviour, ranging from very local signalling between synaptic partners (in the orthograde or retrograde directions) to a volume-type transmission whereby NO synthesized by multiple synchronous sources summate spatially and temporally to influence intermingled neuronal or non-neuronal cells, irrespective of anatomical connectivity. LINKED ARTICLES: This article is part of a themed section on Nitric Oxide 20 Years from the 1998 Nobel Prize. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.2/issuetoc.

From synaptically localized to volume transmission by nitric oxide

Nitric oxide (NO) functions widely as a transmitter/diffusible second messenger in the central nervous system, exerting physiological effects in target cells by binding to specialized guanylyl cyclase-coupled receptors, resulting in cGMP generation. Despite having many context-dependent physiological roles and being implicated in numerous disease states, there has been a lack of clarity about the ways that NO operates at the cellular and subcellular levels. Recently, several approaches have been used to try to gain a more concrete, quantitative understanding of this unique signalling pathway. These approaches have included analysing the kinetics of NO receptor function, real-time imaging of cellular NO signal transduction in target cells, and the use of ultrasensitive detector cells to record NO as it is being generated from native sources in brain tissue. The current picture is that, when formed in a synapse, NO is likely to act only very locally, probably mostly within the confines of that synapse, and to exist only in picomolar concentrations. Nevertheless, closely neighbouring synapses may also be within reach, raising the possibility of synaptic crosstalk. By engaging its enzyme-coupled receptors, the low NO concentrations are able to stimulate physiological (submicromolar) increases in cGMP concentration in an activity-dependent manner. When many NO-emitting neurones or synapses are active simultaneously in a tissue region, NO can act more like a volume transmitter to influence, and perhaps coordinate, the behaviour of cells within that region, irrespective of their identity and anatomical connectivity.

Organization, control and function of extrasynaptic NMDA receptors

N-methyl D-aspartate receptors (NMDARs) exist in different forms owing to multiple combinations of subunits that can assemble into a functional receptor. In addition, they are located not only at synapses but also at extrasynaptic sites. There has been intense speculation over the past decade about whether specific NMDAR subtypes and/or locations are responsible for inducing synaptic plasticity and excitotoxicity. Here, we review the latest findings on the organization, subunit composition and endogenous control of NMDARs at extrasynaptic sites and consider their putative functions. Because astrocytes are capable of controlling NMDARs through the release of gliotransmitters, we also discuss the role of the glial environment in regulating the activity of these receptors.

Nitric oxide as a regulatory molecule in the processing of the visual stimulus

Nitric oxide (NO) is a highly reactive gas with considerable diffusion power that is produced pre- and post synaptically in the central nervous system (CNS). In the visual system, it is involved in the processing of the visual information from the retina to superior visual centers. In this review we discuss the main mechanisms through which nitric oxide acts, in physiological levels, on the retina, lateral geniculate nucleus (LGN) and primary visual cortex. In the retina, the cGMP-dependent nitric oxide activity initially amplifies the signal, subsequently increasing the inhibitory activity, suggesting that the signal is "filtered". In the thalamus, on dLGN, neuronal activity is amplified by NO derived from brainstem cholinergic cells, in a cGMP-independent mechanism; the result is the amplification of the signal arriving from retina. Finally, on the visual cortex (V1), NO acts through changes on the cGMP levels, increasing signal detection. These observations suggest that NO works like a filter, modulating the signal along the visual pathways.

The role of nitric oxide in pre-synaptic plasticity and homeostasis

Since the observation that nitric oxide (NO) can act as an intercellular messenger in the brain, the past 25 years have witnessed the steady accumulation of evidence that it acts pre-synaptically at both glutamatergic and GABAergic synapses to alter release-probability in synaptic plasticity. NO does so by acting on the synaptic machinery involved in transmitter release and, in a coordinated fashion, on vesicular recycling mechanisms. In this review, we examine the body of evidence for NO acting as a retrograde factor at synapses, and the evidence from in vivo and in vitro studies that specifically establish NOS1 (neuronal nitric oxide synthase) as the important isoform of NO synthase in this process. The NOS1 isoform is found at two very different locations and at two different spatial scales both in the cortex and hippocampus. On the one hand it is located diffusely in the cytoplasm of a small population of GABAergic neurons and on the other hand the alpha isoform is located discretely at the post-synaptic density (PSD) in spines of pyramidal cells. The present evidence is that the number of NOS1 molecules that exist at the PSD are so low that a spine can only give rise to modest concentrations of NO and therefore only exert a very local action. The NO receptor guanylate cyclase is located both pre- and post-synaptically and this suggests a role for NO in the coordination of local pre- and post-synaptic function during plasticity at individual synapses. Recent evidence shows that NOS1 is also located post-synaptic to GABAergic synapses and plays a pre-synaptic role in GABAergic plasticity as well as glutamatergic plasticity. Studies on the function of NO in plasticity at the cellular level are corroborated by evidence that NO is also involved in experience-dependent plasticity in the cerebral cortex.

Experience-dependent plasticity acts via GluR1 and a novel neuronal nitric oxide synthase-dependent synaptic mechanism in adult cortex

Synaptic plasticity directs development of the nervous system and is thought to underlie memory storage in adult animals. A great deal of our current understanding of the role of AMPA receptors in synaptic plasticity comes from studies on developing cortex and cell cultures. In the present study, we instead focus on plasticity in mature neurons in the neocortex of adult animals. We find that the glutamate receptor 1 (GluR1) subunit of the AMPA receptor is involved in experience-dependent plasticity in adult cortex in vivo and that it acts in addition to neuronal nitric oxide synthase (αNOS1), an enzyme that produces the rapid synaptic signaling molecule nitric oxide (NO). Potentiation of the spared whisker response, following single whisker experience, is ∼33% less in GluR1-null mutants than in wild types. We found that the remaining plasticity depended on αNOS1. Potentiation was reduced by >42% in the single αNOS1-null mutants and completely abolished in GluR1/αNOS1 double-knock-out mice. However, potentiation in GluR1/NOS3 double knock-outs occurred at similar levels to that seen in GluR1 single knock-outs. Synaptic plasticity in the layer IV to II/III pathway in vitro mirrored the results in vivo, in that LTP was present in GluR1/NOS3 double-knock-out mice but not in the GluR1/αNOS1 animals. While basal levels of NO in cortical slices depended on both αNOS1 and NOS3, NMDA receptor-dependent NO release only depended on αNOS1 and not on NOS3. These findings demonstrate that αNOS1 acts in concert with GluR1 to produce experience-dependent plasticity in the neocortex.

Activation of cortical interneurons during sleep: an anatomical link to homeostatic sleep regulation?

Although slow wave activity in the EEG has been linked to homeostatic sleep regulation, the neurobiological substrate of sleep homeostasis is not well understood. Whereas cortical neurons typically exhibit reduced discharge rates during slow wave sleep (SWS), a subpopulation of GABAergic interneurons, which express the enzyme neuronal nitric oxide synthase (nNOS), has recently been found to be activated during SWS. The extent of activation of these nNOS neurons is proportional to homeostatic sleep 'drive'. These cells are an exception among cortical interneurons in that they are projection neurons. We propose that cortical nNOS neurons are positioned to influence neuronal activity across widespread brain areas. They could thus provide a long-sought anatomical link for understanding homeostatic sleep regulation.

Chronic intermittent hypoxia induces NMDA receptor-dependent plasticity and suppresses nitric oxide signaling in the mouse hypothalamic paraventricular nucleus

Chronic intermittent hypoxia (CIH) is a concomitant of sleep apnea that produces a slowly developing chemosensory-dependent blood pressure elevation ascribed in part to NMDA receptor-dependent plasticity and reduced nitric oxide (NO) signaling in the carotid body. The hypothalamic paraventricular nucleus (PVN) is responsive to hypoxic stress and also contains neurons that express NMDA receptors and neuronal nitric oxide synthase (nNOS). We tested the hypothesis that extended (35 d) CIH results in a decrease in the surface/synaptic availability of the essential NMDA NR1 subunit in nNOS-containing neurons and NMDA-induced NO production in the PVN of mice. As compared with controls, the 35 d CIH-exposed mice showed a significant increase in blood pressure and an increased density of NR1 immunogold particles located in the cytoplasm of nNOS-containing dendrites. Neither of these between-group differences was seen after 14 d, even though there was already a reduction in the NR1 plasmalemmal density at this time point. Patch-clamp recording of PVN neurons in slices showed a significant reduction in NMDA currents after either 14 or 35 d exposure to CIH compared with sham controls. In contrast, NO production, as measured by the NO-sensitive fluorescent dye 4-amino-5-methylamino-2',7'-difluorofluorescein, was suppressed only in the 35 d CIH group. We conclude that CIH produces a reduction in the surface/synaptic targeting of NR1 in nNOS neurons and decreases NMDA receptor-mediated currents in the PVN before the emergence of hypertension, the development of which may be enabled by suppression of NO signaling in this brain region.

Presynaptic and postsynaptic NMDA receptors mediate distinct effects of brain-derived neurotrophic factor on synaptic transmission

In addition to its effects on neuronal survival and differentiation, brain-derived neurotrophic factor (BDNF) plays an important role in modulating synaptic transmission and plasticity in many brain areas, most notably the neocortex and hippocampus. These effects may underlie a role for BDNF in learning and memory as well as developmental plasticity. Consistent with localization of the tropomyosin-related kinase B receptor to both sides of the synapse, BDNF appears to have pre- and postsynaptic effects, but the underlying cellular mechanisms are unclear and it is not known whether pre- and postsynaptic modulations by BDNF occur simultaneously. To address these issues, we recorded dual-component (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA] and N-methyl-D-aspartate [NMDA]) miniature excitatory postsynaptic currents (mEPSCs) from cortical and hippocampal pyramidal neurons and dentate gyrus granule cells from acute brain slices. BDNF had no effect on the fast component of mEPSC decay or on the peak amplitude, suggesting that BDNF did not modulate postsynaptic AMPA receptors, although BDNF rapidly modulated NMDA receptors, as seen by an enhancement of the slow component of mEPSC decay that was prevented by blocking postsynaptic NMDA receptors. At the same time, BDNF acted presynaptically to enhance mEPSC frequency. Surprisingly, the effect on frequency was also NMDA receptor dependent, but required activation of presynaptic, not postsynaptic, NMDA receptors. BDNF also enhanced action potential-dependent glutamate release via presynaptic NMDA receptors, an effect that was unmasked when voltage-gated calcium channels were partially inhibited. Our results indicate that BDNF acutely modulates presynaptic release and postsynaptic responsiveness through simultaneous effects on pre- and postsynaptic NMDA receptors.

Morphological evidence for vesicular glutamate release from astrocytes

There is now growing evidence that astrocytes, like neurons, can release transmitters. One transmitter that in a vast number of studies has been shown to be released from astrocytes is glutamate. Although asytrocytic glutamate may be released by several mechanisms, the evidence in favor of exocytosis is most compelling. Astrocytes may respond to neuronal activity by such exocytotic release of glutamate. The astrocyte derived glutamate can in turn activate neuronal glutamate receptors, in particular N-methyl-D-aspartate (NMDA) receptors. Here we review the morphological data supporting that astrocytes possess the machinery for exocytosis of glutamate. We describe the presence of small synaptic-like microvesicles, SNARE proteins and vesicular glutamate transporters in astrocytes, as well as NMDA receptors situated in vicinity of the astrocytic vesicles.

Mechanisms involved in the cerebrovascular dilator effects of N-methyl-d-aspartate in cerebral cortex

Glutamate and its synthetic analogues N-methyl-d-aspartate (NMDA), kainate, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) are potent dilator agents in the cerebral circulation. The close linkage between neural activity-based release and actions of glutamate on neurons and the related decrease in cerebral vascular resistance is a classic example in support of the concept of tight coupling between increased neural activity and cerebral blood flow. However, mechanisms involved in promoting cerebral vasodilator responses to glutamatergic agents are controversial. Here we review the development and current status of this important field of research especially in respect to cerebrovascular responses to NMDA receptor activation.

Adrenomedullin stimulates nitric oxide production from primary rat hypothalamic neurons: roles of calcium and phosphatases

Adrenomedullin (ADM) in the brain plays important roles in the maintenance of homeostasis. Although in vivo evidence has suggested that nitric oxide (NO) mediates ADM's effects in the brain, mechanisms for ADM stimulation of NO production in neurons have not been identified. In the present study, primary hypothalamic neurons were used to characterize ADM-induced NO production and to study the underlying mechanisms. Using Calcium Orange/4-amino-5-methylamino-2',7'-difluorofluorescein fluorescence live cell imaging, we found that ADM (1 or 10 nM, 5 min) significantly elevated [Ca(2+)](i) and NO production in a concentration-dependent manner. Ca(2+) and NO responses induced by 10 nM ADM were abolished by pretreatment with 50 microM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester (BAPTA-AM), an intracellular Ca(2+) chelator, or protein kinase A (PKA) inhibitors 5 microM N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride (H-89) and 50 microM Rp-cAMP. Furthermore, the ADM-induced NO production was significantly attenuated by a protein phosphatase 1/2A inhibitor, okadaic acid (OA; 0.1 microM), or calcineurin inhibitors, tacrolimus (FK506) (1 microM) and cyclosporin A (CsA; 0.1 microM). Using Western blotting, we found that ADM significantly decreased phosphorylation of neuronal nitric-oxide synthase (nNOS) at serine 847. This dephosphorylation was inhibited by 0.1 microM OA, 1 microM FK506, 0.1 microM CsA, or 5 microM H-89, and attenuated by 50 microM BAPTA-AM. These results suggest that, in hypothalamic neurons, ADM elevates [Ca(2+)](i) via PKA-associated mechanisms. The PKA/Ca(2+) cascade leads to protein phosphatase (PP) 1/PP2A- and calcineurin-mediated dephosphorylation of nNOS. We hypothesize that the Ca(2+) increase and nNOS dephosphorylation contribute to activation of nNOS and production of NO in hypothalamic neurons.

Neuronal nitric oxide synthase immunopositive neurons in cat vestibular complex: a light and electron microscopic study

Nitric oxide is a unique neurotransmitter, which participates in many physiological and pathological processes in the organism. Nevertheless, there are little data about the neuronal nitric oxide synthase immunoreactivity (nNOS-ir) in the vestibular complex of a cat. In this respect, the aims of this study were to: (1) demonstrate nNOS-ir in the neurons and fibers, from all major and accessory vestibular nuclei; (2) describe their light microscopic morphology and distribution; (3) investigate and analyze the ultrastructure of the NOS I-immunopositive neurons, fibers, and synaptic boutons. For demonstration of the nNOS-ir, the peroxidase-antiperoxidase-diaminobenzidin method was applied. Immunopositive for nNOS neurons and fibers were present in all major and accessory vestibular nuclei. On the light microscope level, the immunopositive neurons were different in shape and size. According to the latter, they were divided into four groups--small (with diameter less than 15 microm), medium-sized (with diameter from 15 to 30 microm), large type I (with diameter from 30 to 40 microm), and large type II (with diameter greater than 40 microm). On the electron microscope level, the immunoproduct was observed in neurons, dendrites, and terminal boutons. According to the ultrastructural features, the neurons were divided into three groups--small (with diameter less than 15 microm), medium-sized (with diameter from 15 to 30 microm), and large (with diameter greater than 30 microm). At least two types of nNOS-ir synaptic boutons were easily distinguished. As a conclusion, we hope that this study will contribute to a better understanding of the functioning of the vestibular complex in cat and that some of the data presented could be extrapolated to other mammals, including human.

Nicotinic and muscarinic reduction of unitary excitatory postsynaptic potentials in sensory cortex; dual intracellular recording in vitro

We studied the cholinergic modulation of glutamatergic transmission between neighboring layer 5 regular-spiking pyramidal neurons in somatosensory cortical slices from young rats (P10-P26). Brief bath application of 5-10 microM carbachol, a nonspecific cholinergic agonist, decreased the amplitude of evoked unitary excitatory postsynaptic potentials (EPSPs). This effect was blocked by 1 microM atropine, a muscarinic receptor antagonist. Nicotine (10 microM), in contrast to carbachol, reduced EPSPs in nominally magnesium-free solution but not in the presence of 1 mM Mg+2, indicating the involvement of NMDA receptors. Likewise, when the postsynaptic cell was depolarized under voltage clamp to allow NMDA receptor activation in the presence of 1 mM Mg+2, synaptic currents were reduced by nicotine. Nicotinic EPSP reduction was prevented by the NMDA receptor antagonist D-AP5 (50 microM) and by the nicotinic receptor antagonist mecamylamine (10 microM). Both carbachol and nicotine reduced short-term depression of EPSPs evoked by 10 Hz stimulation, indicating that EPSP reduction happens via reduction of presynaptic glutamate release. In the case of nicotine, several possible mechanisms for NMDAR-dependent EPSP reduction are discussed. As a result of NMDA receptor dependence, nicotinic EPSP reduction may serve to reduce the local spread of cortical excitation during heightened sensory activity.

The compartmentalization of prolactin signaling in the mouse mammary gland

In mammary epithelial cells, prolactin (PRL) activates at least two signaling pathways: Jak/Stat and nitric oxide (NO). The former induces differentiation as measured by alpha-lactalbumin accumulation, while experiments with sodium nitroprusside (SNP) show that NO inhibits differentiation. In order to resolve this apparent contradiction, the kinetics, inducibility, and cellular localization of NO production and sensitivity in mammary cells were examined. First, mammary cells remained responsive to PRL throughout the incubation with respect to NO production. Second, although desensitization occurred with continuous PRL exposure, recovery began as quickly as 30 min after PRL withdrawal. Since PRL is secreted in pulses in vivo, complete desensitization was not a likely explanation for the cells' escape from NO inhibition. Finally, the cellular site of transduction was examined using the caveolar disrupting agent, methyl-beta-cyclodextrin (MBCD). MBCD inhibited the accumulation of PRL-induced NO but not alpha-lactalbumin. This finding was confirmed by membrane fractionation studies where the PRL-induced NO production occurred primarily in caveolae and PRL-stimulated tyrosine phosphorylation of Stat5, which transcribes the alpha-lactalbumin gene, occurred predominantly in noncaveolar membranes. Finally, endogenous elevations of NO by arginine did not inhibit differentiation. As such, the inhibition seen with SNP appeared to be an artifact of the ubiquitous generation of NO from SNP. Physiologically, PRL induces NO only in caveolae and this restricted distribution does not interfere with differentiation.

Synaptic and vascular associations of neurons containing cyclooxygenase-2 and nitric oxide synthase in rat somatosensory cortex

Cyclooxygenase-2 (COX-2) is a rate-limiting enzyme for prostanoid synthesis that is present in cortical pyramidal neurons and highly implicated in control of cerebral blood flow during neural activity. We examined the electron microscopic localization of COX-2 and neuronal nitric oxide synthase (nNOS), a functionally related enzyme, in the somatosensory cortex of rat brain to determine the relevant functional sites. COX-2 immunoreactivity was detected in significantly more somatodendritic than axonal profiles, while nNOS was more often seen in axon terminals. The dendritic COX-2 was localized to endomembranes near synaptic inputs from axon terminals, some of which contained nNOS. Conversely, COX-2 terminals formed asymmetric, excitatory-type synapses with dendrites containing nNOS. The dendritic and axonal profiles containing COX-2 as well as those containing nNOS were minimally separated from penetrating arterioles and capillaries by filamentous glial processes. The perivascular COX-2 labeled terminals were among those that also formed axo-dendritic synapses, suggesting that the release of prostanoids and/or excitatory transmitters from a single terminal may simultaneously affect neuronal activity and cerebral blood flow. Thus, COX-2 has a compartmental distribution in somatosensory cortical neurons consistent with the local neuronal synthesis of prostanoids that are involved in neurovascular coupling and whose actions are modulated by nitric oxide.

Changes of [3H]Muscimol, [3H]Flunitrazepam and [3H]MK-801 Binding in Rat Brain by Prolonged Ventricular Infusion of 7-Nitroindazole

In the present study, we have investigated the effects of prolonged inhibition of nitric oxide synthase (NOS) by infusion of neuronal NOS (nNOS) inhibitor, 7-nitroindazole (7-NI), to examine modulation of NMDA and GABAA receptor binding in rat brain. The duration of sleeping time was significantly increased by the pre-treatment with 7-NI (100 mg/kg) 30 min before pentobarbital (40 mg/kg) treatment in rats. However, the duration of pentobarbital-induced sleep was shortened by the prolonged infusion of 7-NI into cerebroventricle for 7 days. We have investigated the effect of NOS inhibitor on NMDA and GABAA receptor binding characteristics in discrete areas of brain regions by using autoradiographic techniques. The GABAA receptors were analyzed by quantitative autoradiography using [3H]muscimol and [3H]flunitrazepam binding, and NMDA receptor binding was analyzed by using [3H]MK-801 binding in rat brain slices. Rats were infused with 7-NI (500 pmol/10 microl/h, i.c.v.) for 7 days, through pre-implanted cannula by osmotic minipumps. The levels of [3H]muscimol were markedly elevated in cortex, caudate putamen, and thalamus while the levels of [3H]flunitrazepam binding were only elevated in cerebellum by NOS inhibitor. However, there was no change in the level of [3H]MK-801 binding except decreasing in the thalamus. These results show that the prolonged inhibition of NOS by 7-NI-infusion highly elevates [3H]muscimol binding in a region-specific manner and decreases the pentobarbital-induced sleep.

Modulation of synaptic function by cGMP and cGMP-gated cation channels

Cyclic nucleotide-gated cation channels have been studied intensively in the primary sensory neurons of the visual and olfactory systems. Using both anatomical and physiological methods we have shown that they have a much more widespread distribution in the nervous system. In many retinal ganglion cells cGMP, but not cAMP, activates a non-selective conductance that has many of the properties of CNG channels. As many neurons also contain cGMP-dependent protein kinases (PKGs), we have used a variety of cGMP analogues to distinguish the actions of cGMP. Sp-8-Br-PET-cGMPS is a potent non-hydrolyzable cGMP analogue that is an agonist of PKG. We found that Sp-8-Br-PET-cGMPS acts as a competitive inhibitor of at least the rod CNG channel. Rp-8-Br-cGMPS has shown the opposite effects, namely as an agonist of the rod CNG channel and an inhibitor of PKG. In dissociated cell cultures and slices of rodent visual cortex cGMP had multiple rapid and reversible effects on transmission at glutamatergic synapses. Extracellular application of 8-Br-cGMP or Sp-8-Br-PET-cGMPS reduced stimulus evoked EPSPs in cortical slices. In cortical cultures both analogs reduced the frequency of spontaneous EPSCs, but not their amplitude. The effects on both EPSPs and EPSCs were presynaptic. The effects on evoked EPSPs may be due, in part, to reduced calcium influx through voltage-gated calcium channels. The effects on spontaneous EPSCs may be due, in part, to modulation of calcium fluxes through internal stores. Similar modulations of synaptic transmission have been found at gabaergic synapses. On postsynaptic cells, PKG activation produced a dramatic enhancement of the responses to applied NMDA. No effects were detected on applied AMPA/kainate or GABA. Together the results suggest that cGMP may use multiple mechanisms to modulate synaptic efficacy and that its actions may include regulating synaptic plasticity and the relative strength of excitatory and inhibitory drive through neural pathways.

The Neonatal Treatment of Rats with Monosodium Glutamate Induces Morphological Changes in the Subfornical Organ

The parenteral administration of monosodium glutamate (MSG) to neonatal rats induces specific lesions in the central nervous system that lead to a well characterized neuroendocrinological dysfunction. Additionally, it has been shown that MSG-treated rats present a blunted blood pressure response to the injection of nitric oxide synthase inhibitors. Recently, a similar cardiovascular alteration has been reported after the electrolytic lesion of the anteroventral region of the third ventricle affecting the connections of the subfornical organ (SFO). We hypothesized that the treatment of neonatal rats with MSG could affect the nitrergic cells of the SFO. In the present work, we have looked for alterations in the NADPH-diaphorase activity (a commonly used marker for nitrergic neurons) in the SFO of MSG-treated rats of either sex and at two different ages. Our results shown that the treatment of neonatal rats with MSG induced a substantial reduction in the volume of the SFO and in the number of its nitrergic cells with regard to control animals. These findings suggest that the SFO could be implicated in some of the cardiovascular alterations observed in MSG-treated rats.

A role for l-glutamate ionotropic receptors in the development of rat neurogenic pulmonary edema

The present study was undertaken to evaluate possible roles of L-glutamate ionotropic receptors in neurogenic pulmonary edema. Perfusion of L-glutamate into the fourth ventricles of rats increased nitric oxide (NO) signals in the efflux solution concentration-dependently, significantly reducing both the occurrence and severity of neurogenic pulmonary edema. This effect was completely reversed by prior intracisternal injection of an NO synthase inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME), or an N-methyl-D-aspartate (NMDA) receptor antagonist, dizocilpine maleate (MK-801), and partially by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a 2-amino-3-hydroxy-5-methyl-4-isoxazol propionic acid (AMPA)/kainic acid receptor antagonist. Administration of MK-801 or CNQX alone, without L-glutamate, almost completely prevented neurogenic pulmonary edema development. These results suggest that endogenous L-glutamate may facilitate underlining disease process, whereas L-glutamate exogenously applied into the fourth ventricle may have an inhibitory action via release of NO, through ionotropic receptors.

Effect of neuronal nitric oxide synthase inhibition on CA2+/calmodulin kinase kinase and CA2+/calmodulin kinase IV activity during hypoxia in cortical nuclei of newborn piglets

The present study tests the hypothesis that cerebral tissue hypoxia results in increased Ca(2+)/calmodulin (CaM) kinase kinase activity and that the administration of nitric oxide synthase inhibitors (N-nitro-l-arginine [NNLA], or 7-nitroindazole sodium [7-NINA]) prior to the onset of hypoxia will prevent the hypoxia-induced increase in the enzyme activity. To test this hypothesis, CaM kinase kinase and CaM kinase IV activities were determined in normoxic, hypoxic, NNLA-treated hypoxic, and 7-NINA-treated hypoxic piglets. Hypoxia was induced (FiO(2)=0.05-0.08x1 h) and confirmed biochemically by tissue levels of ATP and phosphocreatine. CaM kinase kinase activity was determined in a medium containing protein kinase and phosphatase inhibitors, calmodulin, and a specifically designed CaM kinase kinase target peptide. CaM kinase IV activity was determined by (33)P-incorporation into syntide-2 in a buffer containing protein kinase and phosphatase inhibitors. Compared with normoxic animals, ATP and phosphocreatine levels were significantly lower in all hypoxic piglets whether or not pretreated with nitric oxide synthase inhibitors. There was a significant difference among CaM kinase kinase activity (pmol/mg protein/min) in normoxic (76.84+/-14.1), hypoxic (138.86+/-18.2, P<0.05 vs normoxia), NNLA-pretreated hypoxic (91.34+/-19.3; P=NS vs normoxia, P<0.05 vs hypoxia) and 7-NINA-pretreated hypoxic animals (100.12+/-23.3; P=NS vs normoxia, P<0.05 vs hypoxia). There was a significant difference among CaM kinase IV activity (pmol/mg protein/min) in normoxia (1270.80+/-126.1), hypoxia (2680.80+/-136.7; P<0.05 vs normoxia), NNLA-pretreated hypoxia (1666.00+/-154.8; P<0.05 vs normoxia, P<0.05 vs hypoxia), and 7-NINA-pretreated hypoxic (1712.9+/-231.5; P=NS vs normoxia, P<0.05 vs hypoxia). We conclude that the hypoxia-induced increase in CaM kinase kinase and CaM kinase IV activity is mediated by neuronal NOS-derived NO.

GABAergic synapses in hippocampus exocytose aspartate on to NMDA receptors: quantitative immunogold evidence for co-transmission

We previously found evidence for the exocytosis of aspartate from excitatory nerve terminals in hippocampus [J. Neurosci. 18, (1998) 6059]. Here we show, by immunogold electron microscopy in hippocampal slices that aspartate is co-localized and co-exocytosed with GABA from synaptic vesicles in nerve endings assumed to be inhibitory on dentate granule cells and CA1 pyramidal cells. By immunogold double labeling cytochemistry in perfusion fixed hippocampus, we further find that GABA-positive terminals forming symmetric synaptic specializations on perikarya of granule and pyramidal cells express NMDA receptors. In addition, NMDA receptors are present at synapses on granule cell bodies that have asymmetric synaptic specializations and contain high levels of GABA. Glutamate levels are low in the described types of GABA-positive nerve terminals, but high in terminals making asymmetric synapses on dendritic spines, whereas aspartate is localized with high levels in all of these types of terminal. We propose that aspartate is exocytotically released not only from glutamatergic terminals, but also from GABAergic terminals to act on NMDA receptors and may have a role in the regulation of synaptic transmission. Under pathological conditions, release of an excitatory transmitter at an inhibitory synapse could contribute to the development of, for example, epilepsy.

Distinct types of nitric oxide-producing neurons in the developing and adult mouse claustrum

We studied at the light and electron microscopic levels the nitric oxide-producing neurons in the mouse claustrum. Nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry and neuronal nitric oxide synthase (nNOS) immunohistochemical staining were used to reveal putative nitrergic neurons. We also analyzed colocalization of nNOS with the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) as well as the ontogenesis of the nNOS-immunoreactive neurons, providing evidence for different populations of nitrergic neurons in the mouse claustrum. The general staining pattern was similar for the histochemical and the immunohistochemical methods, resulting in neuron and neuropil staining throughout the whole claustrum. We described two populations of nitric oxide-producing neurons in the mouse claustrum on the basis of a different level of nNOS expression. Densely nNOS-stained neurons were mostly GABA immunoreactive, displayed ultrastructural features typically seen in aspiny neurons, and may originate in the subpallium; they were first seen in the claustrum at embryonic stage 17.5 and probably represent local inhibitory interneurons. Densely stained cells were found from rostral to caudal levels throughout the dorsal claustrum and the endopiriform nucleus. Lightly nNOS-stained neurons, on the other hand, were more numerous than densely stained ones, especially in the dorsal claustrum. These claustral lightly stained cells, barely observed in the NADPH-diaphorase reacted sections, were mostly non-GABAergic, and appeared earlier during ontogenesis than densely stained cells (at embryonic stages 15.5-16.5). We suggest that these neurons are probably projection neurons.

Novel treatment of excitotoxicity: targeted disruption of intracellular signalling from glutamate receptors

Glutamate signalling plays key physiological roles in excitatory neurotransmission and CNS plasticity, but also mediates excitotoxicity, the process responsible for triggering neurodegeneration through glutamate receptor overactivation. Excitotoxicity is thought to be a key neurotoxic mechanism in neurological disorders, including brain ischemia, CNS trauma and epilepsy. However, treating excitotoxicity using glutamate receptor antagonists has not proven clinically viable, necessitating more sophisticated approaches. Increasing knowledge of the composition of the postsynaptic density at glutamatergic synapses has allowed us to extend our understanding of the molecular mechanisms of excitotoxicity and to dissect out the distinct signalling pathways responsible for excitotoxic damage. Key molecules in these pathways are physically linked to the cytoplasmic face of glutamate receptors by scaffolding proteins that exhibit binding specificity for some receptors over others. This imparts specificity to physiological and pathological glutamatergic signalling. Recently, we have capitalized on this knowledge and, using targeted peptides to selectively disrupt intracellular interactions linked to glutamate receptors, have blocked excitotoxic signalling in neurones. This therapeutic approach circumvents the negative consequences of blocking glutamate receptors, and may be a practical strategy for treating neurological disorders that involve excitotoxicity.

NMDA induces NOS 1 translocation to the cell membrane in NGF-differentiated PC 12 cells

Glutamatergic-mediated nitric oxide (NO) production occurs via the N-methyl-D-aspartic acid (NMDA) postsynaptic density protein 95 (PSD95)-neuronal nitric oxide synthase (NOS1) ternary complex. To determine whether NOS1 is targeted to the membrane subsequent to NMDA receptor activation, we examined the effect of NMDA on NOS1 subcellular localization in nerve growth factor (NGF) differentiated PC12 cells. No effect on cell viability was observed using a range of NMDA concentrations from 500 to 1000 microM. Within 3 min of stimulation with 750 microM NMDA, increased cytoplasmic NOS1 immunostaining was observed with rapid membrane staining thereafter. This was inhibited by NMDAR inhibition with MK801. This observation was confirmed using subcellular fractionation and immunoblotting. Using 4, 5-diaminofluorescein diacetate (DAF2-DA) staining and a diazotization assay, concurrent NO production was observed. When PC 12 cells were co-treated with either NMDA and N(6)-nitro-L-arginine methyl ester hydrochloride (L-NAME) or (5R, 10S)-(+)-5-methyl-10, 11-dihydro-5H-dibenzo [a, d] cyclohepten-5, 10-imine hydrogen maleate (MK-801), nitric oxide (NO) generation was inhibited. Stimulation in a calcium-free medium did not increase NO levels. Although no evidence of cytotoxicity was observed utilizing either the MTT assay or measures of apoptosis within the maximal interval of NOS1 translocation, cell viability was reduced following 10 h of continuous NMDA exposure. While it has been shown that NMDA triggers NOS1 activation, these results indicate that NMDAR activation also mediates NOS1 targeting to the membrane. Our data validate that NGF-differentiated PC12 cells may be employed as a useful in vitro model to further study the regulation of NOS1 subsequent to NMDAR activation.

Regulation of bone resorption and osteoclast survival by nitric oxide: possible involvement of NMDA-receptor

Nitric oxide has been shown to play an important role in regulation of bone resorption. However, the role of endogenous nitric oxide on osteoclast activity remains still controversial. In this work, using RT-PCR amplification, we demonstrated that rabbit mature osteoclasts express mRNA encoding for neuronal nitric oxide synthase suggesting that this enzyme could be involved in basal nitric oxide production in these cells. Then we assessed the effect of carboxy-PTIO, a nitric oxide scavenger, on in vitro bone resorption and osteoclast survival. Carboxy-PTIO (10-100 microM) inhibited osteoclastic bone resorption in a dose dependent manner and induced osteoclast apoptosis by a mechanism involving caspase 3 activation. These results suggest that basal concentration of endogenous nitric oxide may be essential for normal bone resorption by supporting osteoclast survival. Because osteoclasts express N-methyl-d-aspartate-receptor (NMDA-R), we hypothesized that in osteoclasts NMDA-R may be involved in nitric oxide production as in neuronal cells. We confirmed that blockade of NMDA-R with specific non-competitive antagonists, MK801 and DEP, strongly inhibited bone resorption. As for carboxy-PTIO, we showed that blockade of NMDA-R by both antagonists induced osteoclast apoptosis in a dose dependent manner by a mechanism dependent on caspase 3 activation. Intracellular calcium concentration in osteoclasts decreased within minutes in the presence of both antagonists. Finally, MK801-induced osteoclast apoptosis was partially reversed in the presence of small amount of SNAP (100 nM), a nitric oxide donor, suggesting that the effect of NMDA-R on osteoclast apoptotic cell death could be due to a decrease in nitric oxide production. Taken together, our results are consistent with the hypothesis that NMDA-R on osteoclasts could have a similar function as those in neuronal cells, i.e., to allow a calcium influx, which in turn activates a constitutive neuronal nitric oxide synthase. Nitric oxide generated by this pathway may be essential for osteoclast survival and hence for normal bone resorption.

NMDA receptor blockade in intact adult cortex increases trafficking of NR2A subunits into spines, postsynaptic densities, and axon terminals

Past in vitro studies have used immunofluorescence to show increased clustering of the NR1 subunits of NMDA receptors (NMDAR) following NMDAR blockade, indicating that NMDARs self-regulate trafficking to and from spines. However, since a substantial portion of spinous NMDAR subunits can reside at sites removed from plasma membranes, whether or not these immunofluorescent clusters are synaptic remains to be shown. Also, the NR2A/B subunits undergo activity-dependent switching at synapses, indicating that their subcellular distribution may be regulated differently from the NR1 subunits. We examined the issue of NMDAR autoregulation by determining whether in vivo NMDAR blockade enhances trafficking of the NR2A subunits toward spines and more specifically to postsynaptic densities (PSDs) of already mature synapses. Seven adult rats received unilateral intra-cortical infusion of the NMDAR antagonist, D-AP5 for 1/2-2 h and the inactive enantiomer or the solvent, alone, in the contralateral cortex. Using an electron microscope, approximately 5600 cortical spines originating from the two hemispheres of the seven adult animals were analyzed for the location of NR2A subunits. In six out of the seven cases analyzed, the D-AP5-treated neuropil exhibited increased immunolabeling at PSDs and a concomitantly great increase at non-synaptic sites within spines. NR2A subunits also increased presynaptically within 1/2 h but not after 1 h. These findings indicate that NR2A subunits in intact, adult cortical neurons are prompted to become trafficked into spines and axon terminals by NMDAR inactivity, yielding an increase of a readily available reserve pool and greater localization at both sides of synapses.

Protein inhibitor of nitric oxide synthase (NOS) and the N-methyl-D-aspartate receptor are expressed in the rat and mouse penile nerves and colocalize with penile neuronal NOS

Nitrergic neurotransmission triggering penile erection is mediated by nitric oxide (NO) synthesized in the cavernosal nerves of the penis by penile neuronal NO synthase (PnNOS). In the central nervous system, nNOS is activated by the N-methyl-D-aspartate receptor (NMDAR) and, presumably, is inhibited by the protein inhibitor of NOS (PIN). The PnNOS and NMDAR are expressed in the penis, and PnNOS has been localized in penile nerves. Both proteins colocalize with PIN in the hypothalamus and the spinal cord involved in the control of erection. The present study aimed to elucidate the relationship between PnNOS, PIN, and NMDAR in the penis. It was found that in the rat, PIN was expressed in the pelvic ganglion and the cavernosal nerve, and penile PIN cDNA was cloned, sequenced, and expressed. Immunohistochemistry localized PIN to the cavernosal and dorsal nerve of the penis, whereas NMDAR was not detected in the latter. Dual-fluorescence labeling showed that PnNOS colocalized with PIN in both nerves but with NMDAR only in the cavernosal nerve. Aging did not affect the mRNA levels of PnNOS, nNOS, NMDAR, and PIN. Both PIN and NMDAR were detected in penile nerves of the wild-type and nNOS(-/-) mouse. The PIN protein did not inhibit or bind NOS in penile extracts, and in vivo, PIN cDNA reduced the erectile response to electrical field stimulation. In conclusion, PIN and NMDAR colocalize with PnNOS in penile nerves, but the functional significance of these protein interactions for penile erection remains to be elucidated.

Angiotensin II stimulates basolateral K channels in rat cortical collecting ducts

We used the patch-clamp technique to study the effects of angiotensin II (ANG II) on basolateral K channels in cortical collecting ducts (CCDs). Application of ANG II (100 pM-100 nM) increased the activity of basolateral 18-pS K channels. This effect of ANG II was completely abolished by losartan, which is an antagonist of type 1 angiotensin (AT(1)) receptors. In contrast, inhibition of type 2 angiotensin (AT(2)) receptors did not block the stimulatory effect of ANG II. Also, application of ANG II significantly increased intracellular Ca(2+) concentrations, which were measured with fura 2 dye. To explore the role of Ca(2+)-dependent pathways in the regulation of basolateral K channels, the effects of ANG II on channel activity were examined in the presence of arachidonyltrifluoromethyl ketone to inhibit phospholipase A(2) (PLA(2)), GF-109203X [a protein kinase C (PKC) inhibitor], and N(G)-nitro-l-arginine methyl ester (l-NAME) to inhibit nitric oxide synthase. Inhibition of either PLA(2) or PKC did not block the effect of ANG II on basolateral K-channel activity. However, the stimulatory effect of ANG II was absent in the CCDs treated with l-NAME. Moreover, addition of the membrane-permeant 8-bromo-guanosine 3',5'-cyclic monophosphate (8-bromo-cGMP) not only increased channel activity but also abolished the stimulatory effect of ANG II on channel activity. We conclude that ANG II increases basolateral K-channel activity via the stimulation of AT(1) receptors, and the stimulatory effect of ANG II is mediated by a nitric oxide-dependent cGMP pathway.

L-citrulline immunostaining identifies nitric oxide production sites within neurons

The cellular and subcellular localization of L-citrulline was analyzed in the adult rat brain and compared with that of traditional markers for the presence of nitric oxide synthase. Light, transmission electron, and confocal laser scanning microscopy were used to study tissue sections processed for immunocytochemistry employing a monoclonal antibody against L-citrulline or polyclonal anti-neuronal nitric oxide synthase sera, and double immunofluorescence to detect neuronal nitric oxide synthase and L-citrulline co-localization. The results demonstrate that the same CNS regions and cell types are labeled by neuronal nitric oxide synthase polyclonal antisera and L-citrulline monoclonal antibodies, using both immunocytochemistry and immunofluorescence. Short-term pretreatment with a nitric oxide synthase inhibitor reduces L-citrulline immunostaining, but does not affect neuronal nitric oxide synthase immunoreactivity. In the vestibular brainstem, double immunofluorescence studies show that many, but not all, neuronal nitric oxide synthase-positive cells co-express L-citrulline, and that local intracellular patches of intense L-citrulline accumulation are present in some neurons. Conversely, all L-citrulline-labeled neurons co-express neuronal nitric oxide synthase. Cells expressing neuronal nitric oxide synthase alone are interpreted as neurons with the potential to produce nitric oxide under other stimulus conditions, and the subcellular foci of enhanced L-citrulline staining are viewed as intracellular sites of nitric oxide production. This interpretation is supported by ultrastructural observations of subcellular foci with enhanced L-citrulline and/or neuronal nitric oxide synthase staining that are located primarily at postsynaptic densities and portions of the endoplasmic reticulum. We conclude that nitric oxide is produced and released at focal sites within neurons that are identifiable using L-citrulline as a marker.

Synaptic localization of nitric oxide synthase and soluble guanylyl cyclase in the hippocampus

Functional evidence suggests that nitric oxide released from CA1 pyramidal cells can act as a retrograde messenger to mediate hippocampal long-term potentiation, but the failure to find neuronal nitric oxide synthase (NOS-I) in the dendritic spines of these cells has cast doubt on this suggestion. We hypothesized that NOS-I may be in spines but in a form inaccessible to antibody when using standard histological fixation procedures. Supporting this hypothesis, we found that after a weak fixation protocol shown previously to enhance staining of synaptic proteins, CA1 pyramidal cells exhibit clear immunoreactivity for NOS-I. Confocal microscopy revealed that numerous dendritic spines in the stratum radiatum contained the NR2 subunit of the NMDA receptor and the adaptor protein postsynaptic density-95, and a subset of these spines also contained NOS-I. Quantitative studies showed that only approximately 8% of synaptic puncta (identified by synaptophysin staining) were associated with NOS-I, and approximately 9% contained the beta subunit of soluble guanylyl cyclase (sGC), a major target of NO. However, the majority of NOS-I-positive synaptic puncta was associated with sGC and vice versa. Postembedding immunogold electron microscopy showed that NOS-I concentrates just inside the postsynaptic plasma membrane of asymmetric axospinous synapses in the stratum radiatum of CA1, whereas sGCbeta concentrates just inside the presynaptic membrane. Together, these findings support the possibility that NO may act as a retrograde messenger to help mediate homosynaptic plasticity in a subpopulation of synapses in the stratum radiatum of CA1.

Nitric oxide-mediated expression of Bax protein and DNA fragmentation during hypoxia in neuronal nuclei from newborn piglets

The present study tests the hypothesis that nitric oxide mediates the hypoxia-induced increase in expression of Bax and in DNA fragmentation in the cerebral cortex of newborn piglets, and that administration of N-nitro-L-arginine (NNLA), a nitric oxide synthase inhibitor, will prevent a change in hypoxia-induced expression of apoptotic genes and DNA damage. Piglets were assigned to normoxic, hypoxic, or NNLA-pretreated hypoxic groups. Cerebral tissue hypoxia was documented biochemically by measuring ATP and phosphocreatine (PCr) levels. Cerebral cortical neuronal nuclei were isolated and nuclear proteins were separated electrophoretically and probed with specific antibodies against Bcl-2 or Bax proteins. Neuronal nuclear DNA from normoxic, hypoxic, and NNLA-pretreated hypoxic animals was isolated, separated by electrophoresis on 1% agarose gel and stained with ethidium bromide. Cerebral hypoxia resulted in an increase in nuclear membrane Bax protein levels from 121.33+/-47.7 optical density (OD)xmm(2) in normoxic to 273.67+/-67.3 ODxmm(2) in hypoxic group (P<0.05 vs. normoxic), but levels in NNLA-pretreated hypoxic group were 155.78+/-48.3 ODxmm(2) (P<0.05 vs. hypoxic, P=NS vs. normoxic). Similarly, cerebral hypoxia resulted in the density of DNA fragments increasing from 1530.3+/-309.8 OD/mm(2) in the normoxic group to 5383.3+/-775 OD/mm(2) in the hypoxic group (P<0.05), while levels in NNLA-pretreated hypoxic group were 3574.0+/-952 OD/mm(2) (P<0.05 compared to hypoxic and normoxic groups). The data show that NNLA-pretreatment prevents the hypoxia-induced increase in Bax expression and DNA fragmentation demonstrating that the hypoxia-induced Bax gene expression and the DNA fragmentation are NO-mediated.

Nitration is a mechanism of regulation of the NMDA receptor function during hypoxia

The present study tested the hypothesis that nitration is a mechanism of hypoxia-induced modification of the N-methyl-D-aspartate (NMDA) receptor. To test this hypothesis the effect of hypoxia on the nitration of the NR1, NR2A and NR2B subunits of the NMDA receptor was determined. Furthermore, the effect of administration of a nitric oxide synthase (NOS) inhibitor, N-nitro-L-arginine (NNLA) on the hypoxia-induced nitration of the NMDA receptor subunits as well as the NMDA receptor-mediated Ca2+ influx, an index of NMDA receptor-ion channel function, were determined in cortical synaptosomes. Studies were performed in newborn piglets divided into normoxic, hypoxic and hypoxic-NNLA groups. Hypoxia was induced by decreasing the FiO(2) to 0.07-0.09 for 60 min. Cerebral tissue hypoxia was confirmed by determining the levels of high energy phosphates ATP and phosphocreatine. Nitration of the NMDA receptor subunits was determined by immunoprecipitation using specific antibodies and western blot analysis. NMDA receptor-ion channel-mediated Ca2+ influx was determined using 45Ca2+. There was a significant increase in the nitrated NR1, NR2A and NR2B subunits following hypoxia: 104+/-11 vs. 275+/-18 optical density (OD)xmm(2) for NR1 (P<0.05), 212+/-36 vs. 421+/-16 ODxmm(2) for NR2A (P<0.05) and 246+/-44 vs. 360+/-26 ODxmm(2) for NR2B (P<0.05). This increase in nitrated NR1, NR2A and NR2B subunits of the NMDA receptor was prevented by the administration of NNLA prior to hypoxia (NR1 160+/-19, P=NS, NNLA vs. normoxic; NR2A 304+/-49, P=NS, NNLA vs. normoxic, and NR2B 274+/-19, P=NS, NNLA vs. normoxic). The increase in nitration of the NR1, NR2A and NR2B subunits of the NMDA receptor increased as a function of decreased cerebral high-energy phosphates, ATP and phosphocreatine, during hypoxia. Furthermore, NOS blockade prior to hypoxia resulted in prevention of the hypoxia-induced increase in NMDA receptor-mediated Ca2+ influx. Our results demonstrate that hypoxia results in increased nitration of the NMDA receptor subunits and that administration of an NOS inhibitor prior to hypoxia prevents the hypoxia-induced nitration of the NMDA receptor subunits as well as the hypoxia-induced increase in NMDA receptor-mediated Ca2+ influx. We conclude that nitration is a mechanism of modification of the NMDA receptor function during hypoxia in the newborn piglet brain.

Alpha7 nicotinic acetylcholine receptors occur at postsynaptic densities of AMPA receptor-positive and -negative excitatory synapses in rat sensory cortex

NMDA receptor (NMDAR) activation requires concurrent membrane depolarization, and glutamatergic synapses lacking AMPA receptors (AMPARs) are often considered "silent" in the absence of another source of membrane depolarization. During the second postnatal week, NMDA currents can be enhanced in rat auditory cortex through activation of the alpha7 nicotinic acetylcholine receptor (alpha7nAChR). Electrophysiological results support a mainly presynaptic role for alpha7nAChR at these synapses. However, immunocytochemical evidence that alpha7nAChR is prevalent at postsynaptic sites of glutamatergic synapses in hippocampus and neocortex, along with emerging electrophysiological evidence for postsynaptic nicotinic currents in neocortex and hippocampus, has prompted speculation that alpha7nAChR allows for activation of NMDAR postsynaptically at synapses lacking AMPAR. Here we used dual immunolabeling and electron microscopy to examine the distribution of alpha7nAChR relative to AMPAR (GluR1, GluR2, and GluR3 subunits combined) at excitatory synapses in somatosensory cortex of adult and 1-week-old rats. alpha7nAChR occurred discretely over most of the thick postsynaptic densities in all cortical layers of both age groups. AMPAR immunoreactivity was also detectable at most synapses; its distribution was independent of that of alpha7nAChR. In both age groups, approximately one-quarter of asymmetrical synapses were alpha7nAChR positive and AMPAR negative. The variability of postsynaptic alpha7nAChR labeling density was greater at postnatal day (PD) 7 than in adulthood, and PD 7 neuropil contained a subset of small AMPA receptor-negative synapses with a high density of alpha7nAChR immunoreactivity. These observations support the idea that acetylcholine receptors can aid in activating glutamatergic synapses and work together with AMPA receptors to mediate postsynaptic excitation throughout life.

Involvement of nitric oxide production via kynurenic acid-sensitive glutamate receptors in DOPA-induced depressor responses in the nucleus tractus solitarii of anesthetized rats

We have proposed the hypothesis that L-3,4-dihydroxyphenylalanine (DOPA) plays a role of neurotransmitter of the primary baroreceptor afferents terminating in the nucleus tractus solitarii (NTS). In the present study, we tried to clarify whether glutamate receptors and/or nitric oxide (NO), important modulators for central cardiovascular regulation, are involved in the DOPA-induced cardiovascular responses in the nucleus. Male Wistar rats were anesthetized with urethane and artificially ventilated. Compounds or antisense oligos (17-mer) for neuronal NO synthase were microinjected into depressor sites of the unilateral nucleus. DOPA 30-300 pmol microinjected into the nucleus dose-dependently induced depressor and bradycardic responses. Prior injection of kynurenic acid (600 pmol) suppressed DOPA (300 pmol)-induced responses by approximately 80%. Prior injection of N(G)-monomethyl-L-arginine 100 nmol, a potent NO synthase inhibitor, reversibly attenuated by approximately 90% DOPA-induced responses, while the D-isomer 100 nmol produced no effect. Furthermore, prior injection of neuronal NO synthase antisense oligos (20 pmol) reversibly reduced by approximately 70% responses to DOPA. Sense or scrambled oligos produced no effect. A NO precursor L-arginine (30 nmol) induced depressor and bradycardic responses, but these responses were not affected by kynurenic acid. These results suggest important roles for glutamate receptors and NO in DOPA induced-depressor and bradycardic responses in the NTS.

NMDA receptor and neonatal hypoxic brain injury

The NMDA-type glutamate receptor is a predominant mediator of excitotoxicity in the immature brain due to overexpression of the receptor in the developing brain. Within the development period however, the extent of NMDA receptor mediated processes including hypoxia-induced excitotoxicity may depend on the ontogeny of the NMDA receptor recognition and modulation sites, and subunits leading to altered function of the ion-channel comples. The function of the receptor may be modified by intracellular mechanisms such as phosphorylation/dephosphorylation, nitration, and generation of free radicals including nitric oxide. The susceptibility of the developing brain to hypoxia depends on several factors: the lipid composition of the brain cell membrane; the rate of membrane lipid peroxidation and the status of anti-oxidant defenses; the development and modulation of the NMDA receptor sites; the intracellular Ca(2+) influx mechanisms; expression of apoptotic and antiapoptotic genes such as Bax and Bcl-2; and the activation of initiator caspases and caspase-3, the "executioner" of cell death. The developmental status of these cellular mechanisms and their response to hypoxia determine the fate of the hypoxic cell in the developing brain in the fetus and the newborn.

cGMP-induced presynaptic depression and postsynaptic facilitation at glutamatergic synapses in visual cortex

The mechanisms by which the intracellular messenger cGMP can modulate synaptic efficacy remain poorly understood. Here we report that cGMP, acting through cGMP-dependent protein kinase (PKG), has multiple rapid and reversible effects on synaptic transmission in slices and cultures of rodent visual cortex. Extracellular application of the membrane permeable cGMP analog 8-bromoguanosine-3',5'-cyclic monophosphate (8-Br-cGMP) and the PKG specific activator beta-phenyl-1,N2-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothioate sp-isomer (Sp-8-Br-PET-cGMPS) reduced stimulus-evoked EPSPs in slices. In cortical cultures, both analogs reduced the frequency of spontaneous EPSCs, but not their amplitude. In both slices and cultures, intracellular perfusion of the postsynaptic neurons with a pseudosubstrate inhibitory peptide specific for PKG had no effect on the reduction in EPSPs and EPSCs, indicating that the inhibition occurred at presynaptic sites. Whole-cell calcium currents in cultured cortical neurons were also reduced by both analogs, which may account for the effect on synaptic release. To determine whether cGMP was also acting at postsynaptic sites, we applied exogenous kainate/AMPA and NMDA to the recorded cells directly. cGMP and its analogs showed little effect on the postsynaptic kainate/AMPA responses but produced a dramatic enhancement of NMDA responses. cGMP-induced NMDA potentiation was prevented by the specific PKG inhibitory peptide infused into the postsynaptic cell. In summary, cGMP, acting through PKG, had depressive presynaptic and facilitatory postsynaptic actions at excitatory synapses in the visual cortex. We suggest that these opposing actions may be useful for altering the balance of synaptic inputs to cortical neurons in ways that enhance signals important for synaptic facilitation and neuronal plasticity.

The nNOS/cGMP signal transducing system is involved in the cardiovascular responses induced by activation of NMDA receptors in the rostral ventrolateral medulla of cats

Nitric oxide (NO) is synthesized from L-arginine by NO synthase (NOS). NO stimulates the soluble form of guanylyl cyclase (sGC) and induces accumulation of cyclic guanosine monophosphate (cGMP). The purpose of this study was to examine whether the cardiovascular responses induced by N-methyl-D-aspartate (NMDA) in the rostral ventrolateral medulla (RVLM) depend on the actions of NOS and sGC. In anesthetized cats, the extracellular NO level was measured by in vivo voltammetry using a nafion/porphyrine/o-phenylenediamine-coated carbon-fiber electrode. Microinjection of NMDA into the RVLM produced hypertension and bradycardia associated with NO formation. These NMDA-induced responses were attenuated by prior injections of 7-nitroindazole, a neuronal NO synthase (nNOS) inhibitor, and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, a sGC inhibitor. These findings suggest that NO is involved in the NMDA-induced cardiovascular responses in the RVLM.

Electron microscopic immunocytochemical detection of PSD-95, PSD-93, SAP-102, and SAP-97 at postsynaptic, presynaptic, and nonsynaptic sites of adult and neonatal rat visual cortex

Membrane-associated guanylate kinases (MAGUKs) assemble protein complexes at sites of cell-cell contact. At excitatory synapses in brain, MAGUKs localize to the postsynaptic density (PSD) and interact with N-methyl-D-aspartate (NMDA) glutamate receptors and downstream signaling proteins. However, NMDA receptors are not restricted to the PSDs, as electron microscopic immunocytochemical (EM-ICC) results indicate that NMDA receptors also occur at nonsynaptic portions of dendrites, perhaps functioning as reserves for rapid insertion into synaptic membranes in response to appropriate synaptic activity. NMDA receptors also occur in axons, at least in part to support glutamate-dependent enhancement of transmitter release. In this study, a systematic EM-ICC survey was performed to determine whether the distributions of four neuronal MAGUKs-PSD-95, PSD-93, SAP-102, and SAP-97-resemble that of NMDA receptors. Quantitative analysis revealed that the density of PSD-95 over thick PSDs of asymmetric axo-spinous synaptic junctions is 2-3-fold the level in the immediately adjacent cytoplasm of spines and terminals, while symmetric synapses show no association with PSD-95. Similarly, all four MAGUKs occur over PSDs of spines. However, we also detected MAGUK immunoreactivity, albeit more diffusely, along presynaptic membranes and in the cytoplasm of axons and dendritic shafts. In fact, the overall distribution of PSD-95 within the neuropil is equally prevalent along plasma membranes (including synaptic portions) as in the cytoplasm, away from plasma membranes. These results suggest that MAGUKs have dual roles: to maintain receptors at synapses and to regulate shuttling of receptors between nonsynaptic and synaptic sites.