Existence of nitric oxide synthase in rat hippocampal pyramidal cells

Regulation of myelin basic protein phosphorylation by mitogen-activated protein kinase during increased action potential firing in the hippocampus

Myelin basic protein (MBP) phosphorylation is a complex regulatory process that modulates the contribution of MBP to the stability of the myelin sheath. Recent research has demonstrated the modulation of MBP phosphorylation by mitogen-activated protein kinase (MAPK) during myelinogenesis and in the demyelinating disease multiple sclerosis. Here we investigated the physiological regulation of MBP phosphorylation by MAPK during neuronal activity in the alveus, the myelinated output fibers of the hippocampus. Using a phosphospecific antibody that recognizes the predominant MAPK phosphorylation site in MBP, Thr95, we found that MBP phosphorylation is regulated by high-frequency stimulation but not low-frequency stimulation of the alveus. This change was blocked by application of tetrodotoxin, indicating that action potential propagation in axons is required. It is interesting that the change in MBP phosphorylation was attenuated by the reactive oxygen species scavengers superoxide dismutase and catalase and the nitric oxide synthase inhibitor N-nitro-L-arginine. Removal of extracellular calcium also blocked the changes in MBP phosphorylation. Thus, we propose that during periods of increased neuronal activity, calcium activates axonal nitric oxide synthase, which generates the intercellular messengers nitric oxide and superoxide and regulates the phosphorylation state of MBP by MAPK.

7-Nitroindazole, a selective inhibitor of nNOS, increases hippocampal extracellular glutamate concentration in status epilepticus induced by kainic acid in rats

The glutamate extracellular concentration is controlled by metabolic and neuronal pathways via release and uptake mechanisms. Stimulation of glutamate receptors induces neuronal nitric oxide (NO) release, which in turn modulates glutamate transmission. In this study, the influence of neuronally derived NO on hippocampal glutamate extracellular concentration was investigated in conditions of intense metabolic activation, i.e., during status epilepticus induced by systemic kainic acid (KA). Glutamate, arginine and citrulline concentrations were measured by microdialysis coupled to HPLC. Experiments were performed in conscious rats implanted with a microdialysis probe within the hippocampal CA3 area. Three groups were used: (1) rats treated with KA i.p. (12 mg/kg) and vehicle locally, via the microdialysis probe (n = 9); (2) rats given KA i.p. and a selective inhibitor of neuronal NO synthase, 7-nitroindazole (7-NI, 1.25 mM) locally (n = 13); (3) rats treated with saline i.p. and 7-NI locally (n = 7). Infusion of 7-NI or vehicle was performed throughout the second hour of status epilepticus. In groups 1 and 3, no significant modifications of extracellular glutamate, arginine and citrulline concentrations were measured. In group 2, the local application of 7-NI in the hippocampus during status epilepticus significantly increased extracellular glutamate and arginine concentrations, whereas citrulline concentration remained constant. The concomitant increases of extracellular glutamate and arginine concentrations under local 7-NI perfusion in seizure conditions, suggest that glutamate and arginine are linked in a common metabolic pathway and/or that glutamate is involved in the cross-talk between glia and neurons. A cerebrovascular effect of 7-NI which triggers glutamate release may also occur.

Nitrinergic neurons in the developing and adult human telencephalon: transient and permanent patterns of expression in comparison to other mammals

A subpopulation of cerebral cortical neurons constitutively express nitric oxide synthase (NOS) and, upon demand, produce a novel messenger molecule nitric oxide (NO) with a variety of proposed roles in the developing, adult, and diseased brain. With respect to the intensity of their histochemical (NADPH-diaphorase histochemistry) and immunocytochemical (nNOS and eNOS immunocytochemistry) staining, these nitrinergic neurons are generally divided in type I and type II cells. Type I cells are usually large, intensely stained interneurons, scattered throughout all cortical layers; they frequently co-express GABA, neuropeptide Y, and somatostatin, but rarely contain calcium-binding proteins. Type II cells are small and lightly to moderately stained, about 20-fold more numerous than type I cells, located exclusively in supragranular layers, and found almost exclusively in the primate and human brain. In the developing cerebral cortex, nitrinergic neurons are among the earliest differentiating neurons, mostly because the dominant population of prenatal nitrinergic neurons are specific fetal subplate and Cajal-Retzius cells, which are the earliest generated neurons of the cortical anlage. However, at least in the human brain, a subpopulation of principal (pyramidal) cortical neurons transiently express NOS proteins in a regionally specific manner. In fact, transient overexpression of NOS-activity is a well-documented phenomenon in the developing mammalian cerebral cortex, suggesting that nitric oxide plays a significant role in the establishment and refinement of the cortical synaptic circuitry. Nitrinergic neurons are also present in human fetal basal forebrain and basal ganglia from 15 weeks of gestation onwards, thus being among the first chemically differentiated neurons within these brain regions. Finally, a subpopulation of human dorsal pallidal neurons transiently express NADPH-diaphorase activity during midgestation.

Mice deficient in endothelial nitric oxide synthase exhibit a selective deficit in hippocampal long-term potentiation

Long-term potentiation, a persistent increase in synaptic efficacy, may require a retrograde signal originating in the postsynaptic cell that induces an increase in presynaptic neurotransmitter release. We have constructed a mouse homozygous for a targeted null mutation in the endothelial isoform of nitric oxide synthase and report that long-term potentiation in the CA1 region of these mice is entirely absent under weak stimulation conditions. Application of a membrane-permeant guanosine-3',5'-cyclic monophosphate analogue during tetanus fails to compensate for this deficit, suggesting that nitric oxide produced by endothelial nitric oxide synthase may affect long-term potentiation through a cascade that does not include guanylyl cyclase. We also report that strong tetanic stimulation can induce robust long-term potentiation in these mice which is not blocked by pharmacological inhibitors of nitric oxide synthase. Furthermore, mice lacking endothelial nitric oxide synthase show no shift in the frequency-response curve for the induction of long-term potentiation. Basal synaptic transmission, paired-pulse facilitation and the electrical properties of CA1 cells in these mice were similar to controls. These results support a selective role for endothelial nitric oxide synthase in long-term potentiation, but also demonstrate that nitric oxide synthase is not involved in this process under all conditions.

Corticosterone and phenytoin reduce neuronal nitric oxide synthase messenger RNA expression in rat hippocampus

The production and release of the corticosteroids, namely the glucocorticoids and the mineralocorticoids, are regulated by various stimuli, including stress. Previous studies from our laboratory have shown that chronic exposure to stress or to stress levels of glucocorticoids produces atrophy of the apical dendrites of CA3 pyramidal neurons in the hippocampus. This stress-induced dendritic remodeling is blocked by the anti-epileptic drug phenytoin, which suppresses glutamate release, and also by N-methyl-D-aspartate receptor antagonists. These results suggest an interaction between glucocorticoids and excitatory amino acids in the development of stress-induced atrophy of CA3 pyramidal neurons. Since nitric oxide is proposed to play an important role in mediating both the physiological and pathophysiological actions of excitatory amino acids, we examined the regulation of neuronal nitric oxide synthase messenger RNA expression by corticosterone and phenytoin in the rat hippocampus. The expression of neuronal nitric oxide synthase messenger RNA in hippocampal pyramidal neurons and granule neurons of the dentate gyrus was unaffected by 21-day administration of corticosterone (40 mg/kg), phenytoin (40 mg/kg) or the combination of corticosterone and phenytoin. However, in hippocampal interneurons, corticosterone/ phenytoin co-administration led to a significant reduction in neuronal nitric oxide synthase messenger RNA levels when compared with vehicle controls. These results suggest that, during exposure to stress levels of corticosterone, phenytoin inhibits glucocorticoid-induced atrophy of CA3 pyramidal neurons by reducing neuronal nitric oxide synthase expression in hippocampal interneurons. Moreover, these results may provide another example of synaptic plasticity in the hippocampus mediated by nitric oxide synthase.

In vivo studies of the cerebral glutamate receptor/NO/cGMP pathway

Overwhelming evidence indicates that the glutamate/nitric oxide (NO) synthase/soluble guanylyl cyclase system is of primary importance in a variety of physiological and pathological processes of the brain. Most of our knowledge on this neurochemical pathway derives from in vitro and ex vivo studies but the recent improvement of microdialysis techniques combined with extremely sensitive measurements of the amplified end-product cyclic GMP (cGMP) has given new impulses to the investigation of this cascade of events, its modulation by neurotransmitters and its functional relevance, in a living brain. The first reports, appeared in the early 90's, have demonstrated that microdialysis monitoring of cGMP in the extracellular environment of the cerebellum and hippocampus exactly reflects what is expected to occur at the intracellular level; thus, in vivo extracellular cGMP is sensitive to NO-synthase and soluble guanylyl cyclase inhibitors, can be increased by NO-donors or phosphodiesterase blockers and is modulated by glutamate receptor stimulation in a NO-dependent fashion. Since then, other microdialysis studies have been reported showing that the brain NO synthase/guanylyl cyclase pathway is mainly controlled by NMDA, AMPA and metabotropic glutamate receptors but can be also influenced by other transmitters (GABA, acetylcholine, neuropeptides) through polysynaptic circuits interacting with the glutamatergic system. The available data indicate that this technique, applied to freely-moving animals and combined with behavioural tests, could be useful to get a better insight into the functional roles played by NO and cGMP in physiological and pathological situations such as learning, memory formation, epilepsy, cerebral ischemia and neurodegenerative diseases.

Nitric oxide production in the CA1 field of the gerbil hippocampus after transient forebrain ischemia : effects of 7-nitroindazole and NG-nitro-L-arginine methyl ester

BACKGROUND AND PURPOSE

The present study was designed to examine the time course of nitric oxide (NO) production and the source of NO in the CA1 field of the gerbil hippocampus after transient forebrain ischemia.

METHODS

The production of NO in the CA1 field of the hippocampus after transient ischemia was monitored consecutively by measuring total NO metabolites (NOx-, NO2- plus NO3-) with the use of brain microdialysis. 7-Nitroindazole (7-NI) and NG-nitro-L-arginine methyl ester were used to dissect the relative contributions of neuronal NO synthase and endothelial NO synthase to the NO production. The histological outcomes of 7-NI in 5- and 10-minute global ischemia were also evaluated.

RESULTS

The production of NO in the CA1 field of the hippocampus after ischemia was dependent on the severity of ischemia. Ischemia for 2 or 5 minutes did not induce a significant increase in NOx- levels in the CA1 field of the hippocampus after reperfusion, whereas the 10- and 15-minute ischemias produced significant and persistent increases in NOx- levels. 7-NI did not inhibit the basal NOx- levels and showed no effects on NOx- levels after 5 minutes of ischemia. However, it completely inhibited the increased NOx- levels after 10 or 15 minutes of ischemia. 7-NI provided minor neuroprotection in 5 minutes but not in 10 minutes of global ischemia.

CONCLUSIONS

The increased NO level in the CA1 field of the hippocampus after ischemia is produced mostly by neuronal NO synthase, whereas the basal NO level mainly originates from endothelial NO synthase. The observed neuroprotective effect of 7-NI in 5-minute global ischemia in gerbils may not be due to neuronal NO synthase inhibition by this drug.

Blockade of tetrahydrobiopterin synthesis protects neurons after transient forebrain ischemia in rat: a novel role for the cofactor

The generation of nitric oxide (NO) aggravates neuronal injury. (6R)-5,6,7,8-Tetrahydro-L-biopterin (BH4) is an essential cofactor in the synthesis of NO by nitric oxide synthase (NOS). We attempted to attenuate neuron degeneration by blocking the synthesis of the cofactor BH4 using N-acetyl-3-O-methyldopamine (NAMDA). In vitro data demonstrate that NAMDA inhibited GTP cyclohydrolase I, the rate-limiting enzyme for BH4 biosynthesis, and reduced nitrite accumulation, an oxidative metabolite of NO, without directly inhibiting NOS activity. Animals exposed to transient forebrain ischemia and treated with NAMDA demonstrated marked reductions in ischemia-induced BH4 levels, NADPH-diaphorase activity, and caspase-3 gene expression in the CA1 hippocampus. Moreover, delayed neuronal injury in the CA1 hippocampal region was significantly attenuated by NAMDA. For the first time, these data demonstrate that a cofactor, BH4, plays a significant role in the generation of ischemic neuronal death, and that blockade of BH4 biosynthesis may provide novel strategies for neuroprotection.

Purification and characterization of rat hippocampal CA3-dendritic spines associated with mossy fiber terminals

We report a revised and improved isolation procedure for CA3-dendritic spines, most of them still in association with mossy fiber terminals resulting in a 7.5-fold enrichment over nuclei and a 29-fold enrichment over myelin. Additionally, red blood cells, medullated fibers, mitochondria and small synaptosomes were significantly depleted. We show by high resolution electron microscopy that this subcellular fraction contains numerous dendritic spines with a rich ultrastructure, e.g. an intact spine apparatus, membranous organelles, free and membrane-bound polyribosomes, endocytic structures and mitochondria. This improved experimental system will allow us to study aspects of post-synaptic functions at the biochemical and molecular level.

Citron, a Rho-target, interacts with PSD-95/SAP-90 at glutamatergic synapses in the thalamus

Proteins of the membrane-associated guanylate kinase family play an important role in the anchoring and clustering of neurotransmitter receptors in the postsynaptic density (PSD) at many central synapses. However, relatively little is known about how these multifunctional scaffold proteins might provide a privileged site for activity- and cell type-dependent specification of the postsynaptic signaling machinery. Rho signaling pathway has classically been implicated in mechanisms of axonal outgrowth, dendrogenesis, and cell migration during neural development, but its contribution remains unclear at the synapses in the mature CNS. Here, we present evidence that Citron, a Rho-effector in the brain, is enriched in the PSD fraction and interacts with PSD-95/synapse-associated protein (SAP)-90 both in vivo and in vitro. Citron colocalization with PSD-95 occurred, not exclusively but certainly, at glutamatergic synapses in a limited set of neurons, such as the thalamic excitatory neurons; Citron expression, however, could not be detected in the principal neurons of the hippocampus and the cerebellum in the adult mouse brain. In a heterologous system, Citron was shown to form a heteromeric complex not only with PSD-95 but also with NMDA receptors. Thus, Citron-PSD-95/SAP-90 interaction may provide a region- and cell type-specific link between the Rho signaling cascade and the synaptic NMDA receptor complex.

Nitric oxide as a retrograde messenger during long-term potentiation in hippocampus

Nitric oxide (NO) is widespread in the nervous system and is thought to play a role in a variety of different neuronal functions, including learning and memory (see other chapters, this volume). A number of behavioral studies have indicated that NO is involved in several types of learning such as motor learning (Yanagihara and Kondo, 1996), avoidance learning (Barati and Kopf, 1996; Myslivecek et al., 1996), olfactory learning (Okere et. al., 1996; Kendrick et al., 1997), and spatial learning (Holscher et al., 1995; Yamada et al., 1996) (for review of earlier papers see Hawkins, 1996). Moreover, NO is thought to be involved in neuronal plasticity contributing to these different types of learning in different brain areas including the cerebellum (chapter by R. Tsien, this volume) and hippocampus. In this chapter we review evidence on the role of NO in long-term potentiation (LTP), a type of synaptic plasticity in hippocampus that is believed to contribute to declarative forms of learning such as spatial learning.

Genetic analysis of NOS isoforms using nNOS and eNOS knockout animals

All three major isoforms of nitric oxide synthase (NOS) are expressed in the brain. Because of complex and overlapping expression patterns (Marletta, 1994; Nathan and Xie, 1994), the particular NOS isoform involved in many processes is not clear. In fact, NO generated by separate isoforms may have different roles and potentially opposing effects (Iadecola et al., 1994). We have taken a genetic approach, to disrupt or knockout the genes for NOS isoforms to circumvent some of the limitations of pharmacologic agents. This approach allows the study of each individual NOS isoform in physiologic processes in the context of intact animals. It gives insights into possible developmental roles for NO and parallel processes that may compensate for the absence of each NOS isoform. We have made nNOS and eNOS knockout mice, as well as double knockout mice that lack both nNOS and eNOS isoforms (Huang et al., 1993; Huang et al., 1995; Son et al., 1996). In this chapter, we review some of the physiologic roles for NO that have been elucidated making use of these mice, including regulation of cerebral blood flow, response to cerebral ischemia, regulation of neurotransmitter release in the brain, and development of synaptic plasticity. Other chapters will discuss results using NOS knockout animals in studies of long term potentiation (see Hawkins, this volume), neuronal development (see Mize, this volume), and potential mechanisms for protection in nNOS knockout mice (Moskowitz, M.A.; Dawson, V.L, this volume).

NOS- and non-NOS NADPH diaphorases in the insular cortex of the Syrian golden hamster

We had previously shown NADPH diaphorase activity in fixed tissue slices of the insular cortex of the Syrian golden hamster (Mesocricetus auratus). The objective of this work was to determine the chemical identity of agents responsible for the observed NADPH diaphorase activities. Three different enzymatic NADPH diaphorase activities were distinguished in the insular cortex. (a) The activity seen in endothelial cells was not characterized histochemically, but it co-localized with eNOS-like immunoreactivity. (b) The neuronal Type I activity showed little sensitivity to 10(-5) M dicoumarol, could use either alpha- or beta-NADPH with almost equal facility, and co-localized with nNOS-like immunoreactivity. This activity was primarily attributable to nNOS. (c) The neuronal Type II activity was greatly attenuated by 10(-5) M dicoumarol, had a strong preference for beta-NADPH (rather than alpha-NADPH), and did not co-localize with any NOS-like immunoreactivity. These characteristics also apply to the NADPH diaphorase activity observed in the diffuse blue band in Layers II and III of agranular and dysgranular insular cortex and in the meshwork of cortical fibers. This staining was due primarily to a dicoumarol-sensitive dehydrogenase(s), either an isozyme of DT diaphorase (EC 1.6.99.2), or NADPH dehydrogenase (quinone) (EC 1.6. 99.6), or to a novel dicoumarol-sensitive NADPH dehydrogenase.

Blockade of tetrahydrobiopterin synthesis protects neurons after transient forebrain ischemia in rat: a novel role for the cofactor

The generation of nitric oxide (NO) aggravates neuronal injury. (6R)-5,6,7,8-Tetrahydro-L-biopterin (BH4) is an essential cofactor in the synthesis of NO by nitric oxide synthase (NOS). We attempted to attenuate neuron degeneration by blocking the synthesis of the cofactor BH4 using N-acetyl-3-O-methyldopamine (NAMDA). In vitro data demonstrate that NAMDA inhibited GTP cyclohydrolase I, the rate-limiting enzyme for BH4 biosynthesis, and reduced nitrite accumulation, an oxidative metabolite of NO, without directly inhibiting NOS activity. Animals exposed to transient forebrain ischemia and treated with NAMDA demonstrated marked reductions in ischemia-induced BH4 levels, NADPH-diaphorase activity, and caspase-3 gene expression in the CA1 hippocampus. Moreover, delayed neuronal injury in the CA1 hippocampal region was significantly attenuated by NAMDA. For the first time, these data demonstrate that a cofactor, BH4, plays a significant role in the generation of ischemic neuronal death, and that blockade of BH4 biosynthesis may provide novel strategies for neuroprotection.

Citron, a Rho-target, interacts with PSD-95/SAP-90 at glutamatergic synapses in the thalamus

Proteins of the membrane-associated guanylate kinase family play an important role in the anchoring and clustering of neurotransmitter receptors in the postsynaptic density (PSD) at many central synapses. However, relatively little is known about how these multifunctional scaffold proteins might provide a privileged site for activity- and cell type-dependent specification of the postsynaptic signaling machinery. Rho signaling pathway has classically been implicated in mechanisms of axonal outgrowth, dendrogenesis, and cell migration during neural development, but its contribution remains unclear at the synapses in the mature CNS. Here, we present evidence that Citron, a Rho-effector in the brain, is enriched in the PSD fraction and interacts with PSD-95/synapse-associated protein (SAP)-90 both in vivo and in vitro. Citron colocalization with PSD-95 occurred, not exclusively but certainly, at glutamatergic synapses in a limited set of neurons, such as the thalamic excitatory neurons; Citron expression, however, could not be detected in the principal neurons of the hippocampus and the cerebellum in the adult mouse brain. In a heterologous system, Citron was shown to form a heteromeric complex not only with PSD-95 but also with NMDA receptors. Thus, Citron-PSD-95/SAP-90 interaction may provide a region- and cell type-specific link between the Rho signaling cascade and the synaptic NMDA receptor complex.

Regulation of nitric oxide synthase messenger RNA expression in the rat hippocampus by glucocorticoids

Nitric oxide and glucocorticoids have been implicated in learning and memory, as well as in regulation of the stress response. By use of the in situ hybridization technique, we examined the role of glucocorticoids in the regulation of nitric oxide synthase messenger RNA in the hippocampus. In control animals, nitric oxide synthase subtype I (neuronal) messenger RNA was expressed in the CA1, CA3 and dentate gyrus of the hippocampus. Nitric oxide synthase subtype I expression was almost absent in CA2 pyramidal neurons. Neither subtype II (immunological) nor subtype III (endothelial) nitric oxide synthase messenger RNAs were observed in neurons of the hippocampal subfields. Bilateral removal of the adrenal glands resulted in a significant increase in nitric oxide synthase subtype I messenger RNA expression in the CA1 and CA3 pyramidal neurons and in granular cells of the dentate gyrus. To a lesser degree, the nitric oxide synthase subtype I messenger RNA signal was increased in CA2 pyramidal neurons. Daily administration of glucocorticoids for one week attenuated the adrenalectomy-induced increased level of expression of the messenger RNA encoding nitric oxide synthase subtype I in all areas studied. Because adrenalectomy, which suppresses the production of glucocorticoids, increases nitric oxide synthase expression, and replacement of adrenalectomized animals with glucocorticoids restores the basal levels of nitric oxide synthase subtype I expression, our results demonstrate an up-regulation of nitric oxide synthase subtype I messenger RNA in the absence of glucocorticoids in the hippocampus. The present findings suggest an involvement of the stress axis in the regulation of the synaptic plasticity process mediated by nitric oxide in the hippocampus.

The selective inhibitor of neuronal nitric oxide synthase, 7-nitroindazole, reduces the delayed neuronal damage due to forebrain ischemia in rats

BACKGROUND AND PURPOSE

The present study was designed to investigate whether neuronally derived nitric oxide (NO) plays a toxic role in the cascade of cellular events triggered by global cerebral ischemia in rats.

METHODS

7-Nitroindazole (7-NI) was used as a selective inhibitor of neuronal NO synthase. Global ischemia was induced for 20 minutes in anesthetized rats following the four-vessel occlusion model. Electroencephalogram and brain and body temperatures were continuously monitored. All rats were thermoregulated for the entire duration of anesthesia. 7-NI (25 mg/kg) or its vehicle was given intraperitoneally just after the carotid clamping and again 1 hour later. Rats were randomly divided into four groups: (1) vehicle (n = 7); (2) 7-NI (n = 7); (3) L-arginine (300 mg/kg IP) +7-NI (n = 7); and (4) 7-NI associated with warming to 37 degrees C for 7 hours after disruption of anesthesia to compensate for the decrease in temperature induced by 7-NI (n = 9). Seven days after ischemia, hippocampal CA1 damage was evaluated by classic histology. The lesion was scored with the use of a point scale, and the surviving neurons were counted.

RESULTS

Lesion scores were significantly lower and neuron counts higher in the two (warmed and unwarmed) groups of rats in which 7-NI was given alone than in vehicle- and L-arginine +7-NI-treated rats.

CONCLUSIONS

The results indicate that 7-NI was neuroprotective in 20-minute global ischemia in rats and that the neuroprotective effect of 7-NI was mostly due to the blockade of NO synthesis, suggesting that NO released from neurons in ischemic conditions has a deleterious influence on hippocampal pyramidal neurons.

Inhibition of neuronal (type 1) nitric oxide synthase prevents hyperaemia and hippocampal lesions resulting from kainate-induced seizures

The possible roles for nitric oxide produced by neurons in epileptic conditions have been investigated from two different aspects: microcirculation and delayed damage. Our aim was to determine whether the selective inhibition of neuronal (type 1) nitric oxide synthase by 7-nitroindazole, during seizures induced by systemic kainate, modifies hippocampal blood flow and oxygen supply and influences the subsequent hippocampal damage. Experiments were performed in conscious Wistar rats whose electroencephalogram was recorded. 7-Nitroindazole (25 mg/kg, i.p.) or its vehicle was injected 30 min before kainate administration (10 mg/kg, i.p.) and then twice at 1-h intervals. Kainate triggered typical limbic seizures evolving into status epilepticus, identified by uninterrupted electroencephalographic spike activity. The seizures were stopped by diazepam (5 mg/kg, i.p.) after 1 h of status epilepticus. Three types of experiments were performed in vehicle- and 7-nitroindazole-treated rats. (1) Hippocampal nitric oxide synthase activity was measured under basal conditions, at 1 h after the onset of the status epilepticus and at 24 h after its termination (n = 4-6 per group). (2) Hippocampal blood flow and tissue partial pressure of oxygen were measured simultaneously by mass spectrometry for the whole duration of the experiment, while systemic variables and body temperature were monitored (n = 6 per group). (3) Hippocampal damage was revealed by Cresyl Violet staining and evaluated with a lesion score seven days after status epilepticus (n = 12 per group). Hippocampal nitric oxide synthase activity was not significantly modified during status epilepticus or the following day in vehicle-treated rats. In contrast, it was inhibited by 57% in 7-nitroindazole-treated rats, both in basal conditions and after 1 h of status epilepticus, but was not different from its basal level 24 h later. 7-Nitroindazole significantly decreased basal hippocampal blood flow and tissue partial pressure in oxygen by 30% and 35%, respectively without affecting any systemic or thermal variable. During status epilepticus, 7-nitroindazole significantly reduced the increase in hippocampal blood flow by 70% and prevented any increase in the tissue partial pressure of oxygen. Seven days later, the hippocampal damage in the CA1 and CA3 layers was significantly less in 7-nitroindazole-treated rats than in vehicle-treated rats. These results indicate that the inhibition of neuronal nitric oxide synthase by 7-nitroindazole protects neurons from seizure-induced toxicity despite reducing blood flow and oxygen supply to the hippocampus.

Light and electron microscopic study of neuronal nitric oxide synthase-immunoreactive neurons in the rat subiculum

Neurons in the rat subiculum that are capable of producing nitric oxide were studied by using an antibody to the neuronal isoform of nitric oxide synthase (nNOS). In the light microscope, the staining pattern with the nNOS antibody closely resembled that seen following histochemical processing with nicotinamide adenine dinucleotide phosphate diaphorase. Immunostained neurons were found in all layers, and, in addition, large dendrites in the apical dendrite layer were also immunopositive. Although a few immunolabelled cells had the typical morphology of interneurons, most were found to have the characteristics of pyramidal neurons. In the subiculum, these immunoreactive pyramidal neurons were concentrated mainly in the most superficial cell layers and closest to the CA1 region, but pyramidal neurons in the CA1 layer of the hippocampus were consistently immunonegative. Immunopositive profiles in the subiculum were studied in the electron microscope and compared with unlabelled structures. Ultrastructural criteria suggest that both pyramidal and nonpyramidal subicular neurons are immunopositive for nNOS. Large, spiny dendrites and smaller, varicose dendrites were found to be immunoreactive for nNOS. Vesicle-containing profiles were probably presynaptic axons, and immunopositive boutons were seen to make symmetrical and asymmetrical synaptic contacts.

Specific pattern of nitric oxide synthase expression in glial cells after hippocampal injury

In the central nervous system (CNS), nitric oxide (NO) is thought to be involved in a variety of functions including synaptic plasticity, long term potentiation, and neurotoxicity. The aim of the present study was to investigate the expression of nitric oxide synthase (NOS) in the mouse CNS, following surgical injury to the hippocampus. NOS expression was assessed by histochemical detection of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-diaphorase) activity and immunohistochemistry of the inducible NOS (iNOS). Two days after injury to the CA1 hippocampal field, NADPH-diaphorase activity was detected in pyramidal and granular neurons and also in glial cells in the hippocampus, in contrast to the non-injured one where NADPH-diaphorase staining was observed only in a few interneurons. NADPH-diaphorase histochemistry combined with immunolabelling for GFAP and F4/80 demonstrated that these glial cells were astrocytes and microglia. This pattern of NOS expression is induced specifically after a hippocampal injury since lesion to the prefrontal or cerebellar cortex leads to NOS activity only in monocytes/macrophages like cells. Despite the large expression of NOS detected by NADPH-diaphorase histochemistry after lesioning the hippocampus, immunostaining for iNOS was confined to microglia. The fact that induction of high levels of NOS activity are detected in glial cells after a lesion to the hippocampus could be accounted for by the sensitivity of this structure to a high release of glutamate.

NMDAR1 glutamate receptor subunit isoforms in neostriatal, neocortical, and hippocampal nitric oxide synthase neurons

Nitric oxide (NO), an unconventional and diffusible neurotransmitter, is synthesized by nitric oxide synthase (NOS). NMDA glutamate receptors are potent regulators of NO synthesis. We have used dual-label immunofluorescence and confocal microscopy to examine forebrain neurons in the rat that contain high levels of neuronal NOS (nNOS) for the presence of the NMDAR1 receptor subunit protein and regions of this protein encoded by three alternative spliced segments of the NMDAR1 mRNA: N1, C1, and C2. In the neostriatum, neocortex, and hippocampus, nNOS-labeled neurons exhibit strong NMDAR1 immunoreactivity (-ir). In all three of these regions, nNOS-positive neurons are characterized by the absence of immunoreactivity for the C1 segment of NMDAR1, whereas C1-ir is abundant in most nNOS-negative neurons. In addition, nNOS-ir neurons exhibit selective staining for the alternative C2' terminus of NMDAR1 that is produced when the C2 segment is absent. These results demonstrate directly that neurons with abundant nNOS-ir contain NMDAR1 receptor subunit proteins and that the NMDAR1 isoforms present in these cells differ from those of most other neurons in these regions. The distinct NMDA receptor phenotype of these nNOS-positive neurons is likely to contribute to both the physiological regulation of NO release by glutamate as well as to NO-mediated excitotoxic injury.

NMDAR1 glutamate receptor subunit isoforms in neostriatal, neocortical, and hippocampal nitric oxide synthase neurons

Nitric oxide (NO), an unconventional and diffusible neurotransmitter, is synthesized by nitric oxide synthase (NOS). NMDA glutamate receptors are potent regulators of NO synthesis. We have used dual-label immunofluorescence and confocal microscopy to examine forebrain neurons in the rat that contain high levels of neuronal NOS (nNOS) for the presence of the NMDAR1 receptor subunit protein and regions of this protein encoded by three alternative spliced segments of the NMDAR1 mRNA: N1, C1, and C2. In the neostriatum, neocortex, and hippocampus, nNOS-labeled neurons exhibit strong NMDAR1 immunoreactivity (-ir). In all three of these regions, nNOS-positive neurons are characterized by the absence of immunoreactivity for the C1 segment of NMDAR1, whereas C1-ir is abundant in most nNOS-negative neurons. In addition, nNOS-ir neurons exhibit selective staining for the alternative C2' terminus of NMDAR1 that is produced when the C2 segment is absent. These results demonstrate directly that neurons with abundant nNOS-ir contain NMDAR1 receptor subunit proteins and that the NMDAR1 isoforms present in these cells differ from those of most other neurons in these regions. The distinct NMDA receptor phenotype of these nNOS-positive neurons is likely to contribute to both the physiological regulation of NO release by glutamate as well as to NO-mediated excitotoxic injury.

Precise distribution of neuronal nitric oxide synthase mRNA in the rat brain revealed by non-radioisotopic in situ hybridization

Regional distribution of neurons expressing neuronal nitric oxide synthase mRNA in the rat brain was examined by non-radioisotopic in situ hybridization, using digoxigenin-labeled complementary RNA probes. Clustering of intensely positive neurons was observed in discrete areas including the main and accessory olfactory bulbs, the islands of Calleja, the amygdala, the paraventricular nucleus of the thalamus, several hypothalamic nuclei, the lateral geniculate nucleus, the magnocellular nucleus of the posterior commissure, the superior and inferior colliculi, the laterodorsal and pedunculopontine tegmental nuclei, the nucleus of the trapezoid body, the nucleus of the solitary tract and the cerebellum. Strongly-stained isolated neurons were scattered mainly in the cerebral cortex, the basal ganglia and the brain stem, especially the medulla reticular formation. In the hippocampus, an almost uniform distribution of moderately stained neurons was observed in the granular cell layer of the dentate gyrus and in the pyramidal cell layer of the Ammon's horn, while more intensely stained isolated neurons were scattered over the entire hippocampal region. These observations can serve as a good basis for studies on function and gene regulation of neuronal nitric oxide synthase.

Potassium ion- and nitric oxide-induced exocytosis from populations of hippocampal synapses during synaptic maturation in vitro

The development of mechanisms of neurotransmitter release is an important component in the formation of functional synaptic connections. Synaptic neurotransmitter release can be modulated by nitric oxide, a compound shown to have a variety of physiologic functions in the nervous system. The goal of this study was to determine whether, during synaptic maturation, nitric oxide is capable of affecting exocytosis of synaptic vesicles, and to compare its effects with those elicited by strongly depolarizing stimuli. To address these questions we examined vesicle release from large numbers of individual synapses of hippocampal neurons between five and 13 days in culture. Synaptic vesicles were labelled by uptake of the styrylpyridinium dye N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide (FM1-43) and their release was monitored by fluorescence imaging. Across populations of developing synapses, there was a good correspondence between FM1-43 staining and synapsin immunocytochemistry. A marked heterogeneity was observed in the ability to release vesicles both after potassium and nitric oxide stimulation. In less mature populations of synapses, the rate of potassium- and nitric oxide-induced exocytosis gradually increased, while at later stages nitric oxide-induced responses levelled off and potassium-induced responses continued to rise. Application of nitric oxide donors did not trigger any detectable changes in intracellular calcium. Combined immunocytochemical analysis of cultured hippocampal neurons for neuronal nitric oxide synthase and synapsin revealed that nitric oxide synthase was present within neurites of cultured hippocampal neurons, largely distributed in a bead-like pattern which partially overlapped presynaptic sites. Stimulation of the N-methyl-D-aspartate receptor while blocking propagation of action potentials with tetrodotoxin resulted in exocytosis from numerous individually resolved sites. Preincubation of neurons with an nitric oxide synthase inhibitor or addition of an nitric oxide scavenger eliminated these responses indicating a role for nitric oxide in N-methyl-D-aspartate-stimulated exocytosis. Using fluorescence imaging of individually resolved synaptic sites, we provide direct evidence for an effect of nitric oxide on vesicular neurotransmitter release in intact neurons. Nitric oxide is capable to produce this effect at all stages of synaptic development and acts independently of calcium influx. We show that nitric oxide synthase is present at synaptic sites and endogenously produced nitric oxide is sufficient to cause exocytosis. Taken together, these experiments suggest a possible role for nitric oxide in calcium-independent transmitter release in populations of synapses at all stages of maturation.

Differential expression of NADPH-diaphorase between electrophysiologically-defined classes of pyramidal neurons in rat ventral subiculum, in vitro

The subiculum is the major output region of the hippocampal formation. We have studied pyramidal neurons in slices of rat ventral subiculum to determine if there is a correlation between nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) activity and electrophysiological phenotype. The majority of NADPH-d-positive pyramidal neurons were found in the superficial cell layer (i.e. nearest to the hippocampal fissure) of the subiculum and appreciable NADPH-d activity was absent from pyramidal neurons in area CA1. This distribution of NADPH-d activity was mimicked by that of immunoreactivity for the neuronal isoform of nitric oxide synthase. Subicular pyramidal neurons were classified, electrophysiologically, as intrinsically burst-firing or regular spiking. After electrophysiological characterization, neurons were filled with Neurobiotin and revealed using fluorescence immunocytochemistry. The slices containing these neurons were also processed for NADPH-d. NADPH-d activity was found in six out of eight regular spiking neurons but was not found in any of 13 intrinsically burst-firing neurons (P=0.0008, Fisher's Exact Test). We conclude that in rat ventral subiculum, NADPH-d activity is present in a proportion of pyramidal neurons and indicates the presence of the neuronal isoform of nitric oxide synthase. Furthermore, amongst pyramidal neurons, NADPH-d activity is distributed preferentially to those with the regular spiking phenotype. The distribution of regular spiking neurons suggests that they may not be present to the same extent in all subicular output pathways. Thus, the actions of nitric oxide may be relatively specific to particular hippocampal connections.

Nitric oxide, the enigmatic neuronal messenger: its role in synaptic plasticity

The discovery of the intercellular messenger nitric oxide (NO) stimulated new concepts of how synaptic plasticity could be induced in the nervous system. While initial reports found evidence that NO is of importance for the formation of long-term potentiation of synaptic transmission (LTP) and spatial learning in rats, later reports failed to confirm these results. Novel approaches such as deletion of the gene that encodes NO synthase in mice showed that the neuronal and the endothelial isoforms are expressed in neurones. Deletion of both isoforms reduced the inducibility of LTP. Furthermore, novel selective inhibitors of NO synthase impaired spatial learning. These results support the hypothesis that NO plays an important role in synaptic transmission and explain some but not all previously contradictory results.

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

The majority of nitric oxide's (NO) physiologic and pathologic actions in the brain has been linked to NMDA receptor activation. In order to determine how the NO-synthesizing enzyme within brain, neuronal NO synthase (nNOS), and NMDA receptors are functionally linked, previous studies have used in situ hybridization techniques in combination with light microscopic immunocytochemistry to show that the two are expressed within single neurons. However, this light microscopic finding does not guarantee that NMDA receptors are distributed sufficiently close to nNOS within single neurons to allow direct interaction of the two. Thus, in this study, dual immuno-electron microscopy was performed to determine whether nNOS and NMDA receptors co-exist within fine neuronal processes. We show that nNOS and the obligatory subunit of functional NMDA receptors, i.e. the NMDA-R1, co-exist within dendritic shafts, spines and terminals of the adult rat visual cortex. Axon terminals form asymmetric synaptic junctions with the dually labeled dendrites, suggesting that the presynaptic terminals release glutamate. Axons and dendrites expressing one without the other also are detected. These results indicate that it is possible for the generation of NO to be temporally coordinated with glutamatergic synaptic transmission at axo-dendritic and axo-axonic junctions and that NO may be generated independently of glutamatergic synaptic transmission. Together, our observations point to a greater complexity than previously recognized for glutamatergic neurotransmission, based on the joint versus independent actions of NO relative to NMDA receptors at pre- versus postsynaptic sites.

Synaptic plasticity: a role for nitric oxide in LTP

Nitric oxide is back in the spotlight with a new series of studies showing that it plays an important role in long-term potentiation, the best-studied type of synaptic plasticity in the central nervous system thought likely to play an important role in learning and memory.

Transient synaptic potentiation in the visual cortex. I. Cellular mechanisms

The cellular mechanisms that underlie transient synaptic potentiation were studied in visual cortical slices of adult guinea pigs (> or = age 5 wk postnatal). Postsynaptic potentials (PSPs) elicited by stimulation of the white matter/layer VI border were recorded with conventional intracellular techniques from layer II/III neurons. Transient potentiation (average duration 23 +/- 3 min, mean +/- SE) was evoked by 60 low-frequency (0.1 Hz) pairings of weak afferent stimulation with coincident intracellular depolarizing pulses (80 ms) of the postsynaptic cell. Fifty-one percent (47 of 92) of the pairing protocols led to significant enhancement (+26 +/- 3%) of the PSP peak amplitude. Blockade of action potential output from the recorded neuron during pairing with Lidocaine, N-ethyl bromide quaternary salt in the recording micropipette did not reduce the likelihood of potentiation (7 of 14 protocols = 50%). Thus transient synaptic potentiation does not require action potential output from the paired cell or recurrent synaptic activation in the local cortical circuit. Rather, the modification occurs at synaptic sites that directly impinge onto the activated neuron. Intracellular postsynaptic blockade of inhibitory PSPs only onto the paired cell with the chloride channel blocker 4,4'-dinitro-stilbene-2,2'-disulfonic acid and the potassium channel blocker cesium in he micropipette also did not reduce the likelihood of induction of potentiation (6 of 9 protocols = 67%). These results suggest that the potentiation is due to a true upregulation of excitatory synaptic transmission and that it does not require a reduction of inhibitory components of the compound PSP for induction. Chelation of postsynaptic intracellular calcium with 1,2-bis-2-aminophenoxy ethane-N,N,N',N'-tetraacetic acid (BAPTA) in all cases effectively blocked the induction of potentiation (no change in the PSP, 9 of 13 protocols; induction of synaptic depression, 4 of 13 protocols), suggesting that a rise in the intracellular postsynaptic calcium level is critical for the pairing-induced synaptic potentiation to occur. Bath application of the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (APV) reversibly blocked potentiation of the PSP peak amplitude in most cells (14 of 16) that were capable of significant potentiation of control solution. Blockade of nitric oxide production with bath application of the competitive inhibitor of nitric oxide synthase, L-nitro-arginine (LNA), did not significantly affect the likelihood of synaptic potentiation (11 of 20 cells). It did, however, block subsequent enhancement for several cells (2 of 4) that had previously had their inputs potentiated. Moreover, LNA increased the overall average magnitude of synaptic potentiation (with an additional +28%) when induction was successful. These results suggest that endogenous cortical nitric oxide production can both positively and negatively modulate this NMDA receptor-mediated type of synaptic plasticity.

Up-regulation of nitric oxide synthase messenger RNA in an integrated forebrain circuit involved in oxytocin secretion

The hypothalamo-neurohypophysial system contains high levels of neuronal nitric oxide synthase and this increases further during times of neurohormone demand, such as that following osmotic stimulation. Using double in situ hybridization, we demonstrate here an increase in the expression of nitric oxide synthase messenger RNA by oxytocin neurons, but not vasopressin neurons, of the supraoptic nucleus at the time of lactation, when oxytocin is in demand due to another neuroendocrine stimulus, the milk-ejection reflex. In addition, using immunocytochemical retrograde tracing, we show that neurons of the subfornical organ, median preoptic nucleus and organum vasculosum of the lamina terminalis, which project to the supraoptic nucleus, contain nitric oxide synthase. These three structures of the lamina terminalis, together with the hypothalamo-neurohypophysial system, make up the forebrain osmoresponsive circuit that controls osmotically-stimulated release of oxytocin in the rat. The expression of nitric oxide synthase messenger RNA in the lamina terminalis was also shown to increase during lactation. The increases in nitric oxide synthase messenger RNA were not apparent during pregnancy. These results provide evidence for an integrated nitric oxide synthase-containing neural network involved in the regulation of the hypothalamo-neurohypophysial axis. The expression of nitric oxide synthase messenger RNA increases in this circuit during lactation and correlates with a reduction in the sensitivity of the circuit to osmotic stimuli also present in lactation but not pregnancy. As nitric oxide is believed to attenuate neurohormone release, it seems that the increased nitric oxide synthase messenger RNA expression detected here during lactation at a time of high oxytocin demand may be involved in reducing the sensitivity of the whole forebrain circuit to osmotic stimuli.

Up-regulation of nitric oxide synthase and its mRNA in vagal motor nuclei following axotomy in rat

Effects of vagotomy on nitric oxide synthase (NOS) protein and mRNA levels in the dorsal motor nucleus of vagus (DMV) and nucleus ambiguus (NA) of rats were examined by nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) staining, brain NOS (bNOS) immunostaining and in situ hybridization. NADPH-d staining and bNOS immunoreactivity increased in neurons of the ipsilateral DMV and NA 5, 10, and 20 days after vagotomy. These changes were not observed in unoperated or sham-operated rats. In situ hybridization showed that bNOS mRNA levels were also elevated in neurons of DMV and NA on the operated side. Our results suggest that transection of vagal efferents up-regulates bNOS and its mRNA expression in the DMV and NA.

Long-term potentiation is reduced in mice that are doubly mutant in endothelial and neuronal nitric oxide synthase

Nitric oxide (NO) has been implicated in hippocampal long-term potentiation (LTP), but LTP is normal in mice with a targeted mutation in the neuronal form of NO synthase (nNOS-). LTP was also normal in mice with a targeted mutation in endothelial NOS (eNOS-), but LTP in stratum radiatum of CA1 was significantly reduced in doubly mutant mice (nNOS-/eNOS-). By contrast, LTP in stratum oriens was normal in the doubly mutant mice. These results provide the first genetic evidence that NOS is involved in LTP in stratum radiatum and suggest that the neuronal and endothelial forms can compensate for each other in mice with a single mutation. They further suggest that there is also a NOS-independent component of LTP in stratum radiatum and that LTP in stratum oriens is largely NOS independent.

Characteristics of nitric oxide synthase type I of rat cerebellar astrocytes

We have previously reported that stimulation of astrocyte cultures by particular agonists and calcium ionophores induces cyclic GMP formation through activation of a constitutive nitric oxide synthase (NOS) and that astrocytes from cerebellum show the largest response. In the present work we have used rat cerebellar astrocyteenriched primary cultures to identify and characterise the isoform of NOS expressed in these cells. The specific NOS activity in astrocyte homogenates, determined by conversion of [3H]arginine to [3H]citrulline, was ten times lower than in homogenates from cerebellar granule neurons. Upon centrifugation at 100,000 g, the astroglial activity was recovered in the supernatant, whereas in neurons around 30% of the activity remained particulate. The cytosolic NOS activities of both astrocytes and granule neurons displayed the same Km for L-arginine, dependency of calcium, and sensitivity to NOS inhibitors. Expression of NOS-I in astrocyte cytosolic fractions was revealed by Western blot with a specific polyclonal antiserum against recombinant NOS-I. Double immunofluorescence labelling using anti-glial fibrillary acidic protein (GFAP) and anti-NOS-I antibodies revealed that a minor population of the GFAP-positive cells, usually in clusters, presented a strong NOS-I immunostaining that was predominantly located around the nuclei and had a granular appearance, indicating association with the endoplasmic reticulum-Golgi system. Astrocytes of stellate morphology also showed immunoreactivity in the processes. Similar staining was observed with the avidin-biotin-peroxidase complex using different anti-NOS-I antisera. With this method the majority of cells showed a weak NOS-I immunoreactivity around the nuclei and cytosol. A similar pattern was observed with the NADPH-diaphorase reaction. These results demonstrate that the NOS-I expressed in astrocytes presents the same biochemical characteristics as the predominant neuronal isoform but may differ in intracellular location.

Basal and apical synapses of CA1 pyramidal cells employ different LTP induction mechanisms

Nitric oxide (NO) production has been widely reported to be required for the induction of long-term potentiation (LTP) in hippocampal CA1 cells. Of the two constitutive isoforms of NO synthase, the endothelial form (eNOS) has been implicated in the induction of LTP in these cells. The distribution of eNOS within CA1 cells is not uniform, however, being present in the cell bodies and apical dendrites but absent from the basal dendrites. Using extracellular and intracellular recording techniques, we demonstrate that LTP induction in stratum radiatum synapses (onto apical dendrites) is dependent on NO production, being attenuated by pretreatment with a NOS inhibitor. LTP induced in stratum oriens synapses (onto basal dendrites) is, however, resistant to NOS inhibitors. Both forms of LTP require the activation of N-methyl-D-aspartate (NMDA) receptors because induction of LTP in both stratum radiatum and stratum oriens is blocked by AP5. Thus, it appears that synapses onto apical and basal dendrites of CA1 cells use different cellular mechanisms of LTP induction.

Localization of NADPH diaphorase/nitric oxide synthase in the optic nerve of the normal guinea pig: a light and electron microscopic study

The aim of this study is to determine the presence and subcellular distribution of NADPH diaphorase (NADPH-d)/nitric oxide synthase (NOS) in the optic nerve of the normal guinea pig. Optic nerve specimens were stained by NADPH-d histochemistry, and double labeled by combining NADPH-d histochemistry with immunostaining for (a) anti-glial fibrillary acidic protein (GFAP) antibody for recognition of astrocytes, (b) griffonia simplicifilia B4-isolectin (GSA-IB4) horse radish peroxidase (HRP)-conjugate for identification of microglia, or (c) oligodendrocyte-associated antibodies to carbonic anhydrase isoenzyme II (CA-II) or to galactocerebroside (GalC) for visualization of oligodendrocytes. In addition, constitutive NOS (cNOS) and inducible NOS (iNOS) immunostaining were used for colocalization with NADPH-d histochemistry. Light microscopy revealed NADPH-d reaction product in the blood vessels and neuroglia of the unmyelinated optic nerve head and myelinated retrobulbar optic nerve. Double labeling with GFAP immunoperoxidase combined with NADPH-d histochemistry revealed both activities in astrocytes. Microglia were labeled with GSA-IB4 isolectin HRP-conjugate, but they did not have NADPH-d activity. Oligodendroglia were immunolabeled with anti CA-II or anti GalC antibodies, but they did not have NADPH-d activity. Both iNOS and cNOS immunoperoxidase labeled astrocytes, but not microglia or oligodendroglia. Under transmission electron microscopy, NADPH-d reaction product appeared as electron-dense particles. These particles were seen in the cytoplasm of endothelial cells, perivascular smooth muscle cells and fibrous astrocytes. Axons and myelin were devoid of NADPH-d activity. This study demonstrates the existence and cellular distribution of NADPH-d/NOS activity in endothelial cells, perivascular smooth muscle cells and fibrous astrocytes of the optic nerve of the normal guinea pig. The presence of these non-neuronal sources of NOS in the optic nerve provides the foundation for future comparative studies of the functional role of reactive oxygen induced toxicity in disorders affecting the optic nerve.

Astrocytes and Bergmann glia as an important site of nitric oxide synthase I

In the central nervous system nitric oxide appears to be critically involved in a number of physiological and pathological processes. Although there is convincing evidence for expression of nitric oxide synthase in cultured glial cells, demonstration of this enzyme in glial cells in situ remained largely unsatisfactory. In the present study we applied immunostaining to freeze-dried sections of snap-frozen hippocampi and cerebella of rats and to sections of freeze-dried brain tissue in order to minimize diffusion artefacts and thus to obtain more precise information about the true in situ localization of nitric oxide synthase. Here we show that astrocytes and Bergmann glia react strongly with antibodies raised against cerebellar nitric oxide synthase and against a type I nitric oxide synthase-specific C-terminal peptide, respectively. This finding was further substantiated by histochemical localization of NADPH-diaphorase activity in astrocytes and Bergmann glia as well as by immunoreactivity of both types of glia cells with antibodies to the NADPH-delivering enzyme glucose-6-phosphate dehydrogenase. We conclude, that astrocytes are important sites of nitric oxide synthase I in brain, suggesting that these cells might use nitric oxide as gaseous messenger molecule for various aspects of glia-neuron signalling.

Nitric oxide synthase in cerebral ischemia. Possible contribution of nitric oxide synthase activation in brain microvessels to cerebral ischemic injury

The results of our continuing studies on the role of nitric oxide (NO) in cellular mechanisms of ischemic brain damage as well as related reports from other laboratories are summarized in this paper. Repetitive ip administration of NG-nitro-L-arginine (L-NNA), a NO synthase (NOS) inhibitor, protected against neuronal necrosis in the gerbil hippocampal CA1 field after transient forebrain ischemia with a bell-shaped response curve, the optimal dose being 3 mg/kg. Repeated ip administration of L-NNA also mitigated rat brain edema or infarction following permanent and transient middle cerebral artery (MCA) occlusion with a U-shaped response. The significantly ameliorative dose-range and optimal dose were 0.01-1 mg/kg and 0.03 mg/kg, respectively. Studies using a NO-sensitive microelectrode revealed that NO concentration in the affected hemisphere was remarkably increased by 15-45 min and subsequently by 1.5-4 h after MCA occlusion. Restoration of blood flow after 2 h-MCA occlusion resulted in enhanced NO production by 1-2 h after reperfusion. Administration of L-NNA (1 mg/kg, ip) diminished the increments in NO production during ischemia and reperfusion, leading to a remarkable reduction in infarct volume. In brain microvessels obtained from the affected hemisphere, Ca(2+)-dependent constitutive NOS (cNOS) was activated significantly at 15 min, and Ca(2+)-independent inducible NOS (iNOS) was activated invariably at 4 h and 24 h after MCA occlusion. Two hour reperfusion following 2 h-MCA occlusion caused more than fivefold increases in cNOS activity with no apparent alterations in iNOS activity. Thus, we report here based on available evidence that there is good reason to think that NOS activation in brain microvessels may play a role in the cellular mechanisms underlying ischemic brain injury.

N-methyl-D-aspartate-evoked release of cyclo-oxygenase products in rabbit hippocampus: an in vivo microdialysis study

In vivo microdialysis of the rabbit hippocampus was used to study the effects of N-methyl-D-aspartate (NMDA) receptor stimulation on dialysate concentrations of thromboxane B2 (Tx B2)- and 6-keto prostaglandin F1 alpha (6-keto PGF1 alpha)-immunoreactive materials that are stable metabolites of biologically active thromboxane A2 and prostacyclin. All pharmacological substances were applied in the dialysis medium. The application of 1 mM NMDA for 20 min resulted in five- and eightfold increases in Tx B2 and 6-keto PGF1 alpha concentrations, respectively. An increase in NMDA concentration to 2.5 mM did not potentiate a peak eicosanoid release, but significantly prolonged this effect. Either 10 microM MK-801 or the extrusion of Ca2+ from the dialysis medium inhibited the release by about 50%. Quinacrine, a phospholipase A2 inhibitor (250 microM), decreased the NMDA-evoked eicosanoid release by 30%, whereas 10 microM indomethacin, a cyclo-oxygenase inhibitor, completely suppressed the release. One hundred micromolar furegrelate, an inhibitor of thromboxane synthase, reduced by 75% Tx B2 release with concomitant 100% increase in 6-keto PGF1 alpha formation. Thus, stimulation of NMDA receptors induces calcium-dependent formation of thromboxane A2 and prostacyclin in the hippocampus, which may have pathophysiological implications. The neuronal site of their formation seems probable, although a transcellular mechanism of their synthesis should be also considered.

Ganglioside GM1 prevents N-methyl-D-aspartate neurotoxicity in rabbit hippocampus in vivo. Effects on calcium homeostasis

Microdialysis was used to apply 1 mM N-methyl-D-aspartate (NMDA) for 20 min to the hippocampus of rabbits, control and pre-treated with GM1 ganglioside (im injections of 30 mg/kg for 3 d, twice a day). Concentrations of ionized Ca2+ and 6-keto prostaglandin F1 alpha (6-keto PGF1 alpha)-immunoreactive material in the dialyzates and 45Ca and [14C]sucrose efflux from the prelabeled hippocampus were determined. After 24 h, the morphology of the hippocampal neurons was examined. In control animals, the application of NMDA resulted in 25% decrease in Ca2+ concentration and in 1000% increase in 6-keto PGF 1 alpha concentration in the dialyzates. A 30% decrease in 45Ca efflux was accompanied by 20% increase in [14C]sucrose efflux, reflecting a corresponding reduction of the extracellular space volume. A degeneration of CA1 pyramidal neurons in the vicinity of a microdialysis probe was observed. In GM1-treated rabbits the NMDA-induced decrease in Ca2+ concentrations in the dialyzates was not reduced significantly, whereas a 70% stimulation of 45Ca efflux was noted, with a concomitant 40% reduction of 6-keto-PG F1 alpha release. NMDA-evoked increase in [14C]sucrose efflux did not differ from the control. In these animals CA1 neurons were well preserved. These results indicate that the pretreatment with GM1 results in activation of calcium extrusion from the NMDA-stimulated rabbit hippocampal neurons that alleviates destabilization of calcium homeostasis and reduces NMDA-evoked neuronal injury.

Resetting the biological clock: mediation of nocturnal circadian shifts by glutamate and NO

Circadian rhythms of mammals are timed by an endogenous clock with a period of about 24 hours located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Light synchronizes this clock to the external environment by daily adjustments in the phase of the circadian oscillation. The mechanism has been thought to involve the release of excitatory amino acids from retinal afferents to the SCN. Brief treatment of rat SCN in vitro with glutamate (Glu), N-methyl-D-aspartate (NMDA), or nitric oxide (NO) generators produced lightlike phase shifts of circadian rhythms. The SCN exhibited calcium-dependent nitric oxide synthase (NOS) activity. Antagonists of NMDA or NOS pathways blocked Glu effects in vitro, and intracerebroventricular injection of a NOS inhibitor in vivo blocked the light-induced resetting of behavioral rhythms. Together, these data indicate that Glu release, NMDA receptor activation, NOS stimulation, and NO production link light activation of the retina to cellular changes within the SCN mediating the phase resetting of the biological clock.