Existence of nitric oxide synthase in rat hippocampal pyramidal cells

The regulatory role of nitric oxide in morphine-induced analgesia in the descending path of pain from the dorsal hippocampus to the dorsolateral periaqueductal gray

BACKGROUND

Nitric oxide (NO) levels in brain nuclei, such as the hippocampus and brainstem, are involved in morphine analgesia, but the relationship between the dorsal hippocampus (dH) and the dorsolateral periaqueductal gray matter (dlPAG) needs to be clarified, which is our goal.

METHODS

Wistar rats were simultaneously equipped with a stereotaxic device with unilateral guide cannula at dH and dlPAG. After recovery, they were divided into control and experimental groups. Formalin (50 μL of 2.5%) was inoculated into the left hind paw of rat. Morphine (6 mg/kg) was administered intraperitoneally (i.p.) 10 min before formalin injection. L-Arginine (0.25, 0.5, 1 and 2 μg/rat), and L-NAME (0.25, 0.5, 1 and 2 μg/rat), unrelatedly or with the respect in the order of injection were used in the nuclei before morphine injection (i.p.). Activation of the neuronal NO synthase (nNOS) in the brains of all animals was measured using NADPH-diaphorase, a selective biochemical marker of nNOS.

RESULTS

Morphine reduced inflammatory pain in the early and late stages of the rat formalin test. The morphine response was attenuated by before injection of single L-arginine but not L-NAME in the two target areas. However, the acute phase result was stopped due to L-NAME pretreatment. When L-NAME was injected into dlPAG before injecting L-arginine at dH, the morphine response did not decrease at all, indicating a modulatory role of NO in dlPAG, which was confirmed by NADPH-d staining.

CONCLUSIONS

High levels of NO in dlPAG may regulate pain process in downward synaptic interactions.

Effect of enriched environment and predictable chronic stress on spatial memory in adolescent rats: Predominant expression of BDNF, nNOS, and interestingly malondialdehyde in the right hippocampus

Little is known about the mechanisms that promote divergence of function between left and right in the hippocampus, which is most affected by external factors and critical for spatial memory. We investigated the levels of memory-related mediators in the left and right hippocampus and spatial memory in rats exposed to predictable chronic stress (PCS) and an enriched environment (EE) during adolescence. Twenty-eight-day-old Sprague-Dawley rats were divided into control (standard cages), PCS (15 min/day immobilization stress for four weeks), and EE (one hour/day environmentally enriched cages for four weeks) groups. After the applications, spatial memory was tested with the Morris water maze, and the serum levels of corticosterone were evaluated. The levels of brain-derived neurotrophic factor (BDNF) and neuronal nitric oxide synthase (nNOS), which are critical for synaptic plasticity; malondialdehyde (MDA; lipid-peroxidation indicator); protein carbonyl (protein-oxidation indicator); and superoxide dismutase (antioxidant enzyme) were evaluated in the left and right hippocampus. Corticosterone levels in both the PCS and EE groups did not change compared with control. In both the PCS and EE groups, spatial memory improved and BDNF was increased in both halves of the hippocampus, still there was an asymmetry. nNOS levels were increased in the dentate gyrus and CA1 regions of the right hippocampus in both PCS and EE groups. MDA levels were increased but PCO levels were decreased in the right hippocampus in both the PCS and EE groups, but SOD did not change in either half of the hippocampus. Our results suggest that both PCS and EE improved spatial memory by increasing BDNF and nNOS in the right hippocampus and that, interestingly; MDA could be the physiological signal molecule in the right hippocampus for spatial memory process.

Neuronal nitric oxide synthase and affective disorders

Affective disorders including major depressive disorder (MDD), bipolar disorder (BPD), and general anxiety affect more than 10% of population in the world. Notably, neuronal nitric oxide synthase (nNOS), a downstream signal molecule of N-methyl-D-aspartate receptors (NMDARs) activation, is abundant in many regions of the brain such as the prefrontal cortex (PFC), hippocampus, amygdala, dorsal raphe nucleus (DRN), locus coeruleus (LC), and hypothalamus, which are closely associated with the pathophysiology of affective disorders. Decreased levels of the neurotransmitters including 5-hydroxytryptamine or serotonin (5-HT), noradrenalin (NA), and dopamine (DA) as well as hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis are common pathological changes of MDD, BPD, and anxiety. Increasing data suggests that nNOS in the hippocampus play a crucial role in the etiology of MDD whereas nNOS-related dysregulation of the nitrergic system in the LC is closely associated with the pathogenesis of BPD. Moreover, hippocampal nNOS is implicated in the role of serotonin receptor 1 A (5-HTR1 A) in modulating anxiety behaviors. Augment of nNOS and its carboxy-terminal PDZ ligand (CAPON) complex mediate stress-induced anxiety and disrupting the nNOS-CAPON interaction by small molecular drug generates anxiolytic effect. To date, however, the function of nNOS in affective disorders is not well reviewed. Here, we summarize works about nNOS and its signal mechanisms implicated in the pathophysiology of affective disorders. On the basis of this review, it is suggested that future research should more fully focus on the role of nNOS in the pathomechanism and treatment of affective disorders.

Endothelial Nitric Oxide Synthase Is Present in Dendritic Spines of Neurons in Primary Cultures

Nitric oxide exerts important regulatory functions in various brain processes. Its synthesis in neurons has been most commonly ascribed to the neuronal nitric oxide synthase (nNOS) isoform. However, the endothelial isoform (eNOS), which is significantly associated with caveolae in different cell types, has been implicated in synaptic plasticity and is enriched in the dendrites of CA1 hippocampal neurons. Using high resolution microscopy and co-distribution analysis of eNOS with synaptic and raft proteins, we now show for the first time in primary cortical and hippocampal neuronal cultures, virtually devoid of endothelial cells, that eNOS is present in neurons and is localized in dendritic spines. Moreover, eNOS is present in a postsynaptic density-enriched biochemical fraction isolated from these neuronal cultures. In addition, qPCR analysis reveals that both the nNOS as well as the eNOS transcripts are present in neuronal cultures. Moreover, eNOS inhibition in cortical cells has a negative impact on cell survival after excitotoxic stimulation with N-methyl-D-aspartate (NMDA). Consistent with previous results that indicated nitric oxide production in response to the neurotrophin BDNF, we could detect eNOS in immunoprecipitates of the BDNF receptor TrkB while nNOS could not be detected. Taken together, our results show that eNOS is located at excitatory synapses where it could represent a source for NO production and thus, the contribution of eNOS-derived nitric oxide to the regulation of neuronal survival and function deserves further investigations.

■ REVIEW : Glial NO: Normal and Pathological Roles

All nervous system cell types can be induced with cytokines or bacterial products to make nitric oxide, at least in culture. The signaling pathways invoked by inducers that result in transcriptional activation of the nitric oxide synthase gene are becoming clear, and modulators of this induction have been discovered. Much suggestive and, recently, more definitive evidence has accumulated for induction of nitric oxide synthase in glial cells in vivo associated with viral infection, as well as in animal models of trauma, ischemia, and autoimmunity. Whether nitric oxide from this source contributes to or limits the attendant conditions is not yet clear. The Neuroscientist 2:90-99, 1996

Potential pharmacological strategies for the improved treatment of organophosphate-induced neurotoxicity

Organophosphates (OP) are highly toxic compounds that cause cholinergic neuronal excitotoxicity and dysfunction by irreversible inhibition of acetylcholinesterase, resulting in delayed brain damage. This delayed secondary neuronal destruction, which arises primarily in the cholinergic areas of the brain that contain dense accumulations of cholinergic neurons and the majority of cholinergic projection, could be largely responsible for persistent profound neuropsychiatric and neurological impairments such as memory, cognitive, mental, emotional, motor, and sensory deficits in the victims of OP poisoning. The therapeutic strategies for reducing neuronal brain damage must adopt a multifunctional approach to the various steps of brain deterioration: (i) standard treatment with atropine and related anticholinergic compounds; (ii) anti-excitotoxic therapies to prevent cerebral edema, blockage of calcium influx, inhibition of apoptosis, and allow for the control of seizure; (iii) neuroprotection by aid of antioxidants and N-methyl-d-aspartate (NMDA) antagonists (multifunctional drug therapy), to inhibit/limit the secondary neuronal damage; and (iv) therapies targeting chronic neuropsychiatric and neurological symptoms. These neuroprotective strategies may prevent secondary neuronal damage in both early and late stages of OP poisoning, and thus may be a beneficial approach to treating the neuropsychological and neuronal impairments resulting from OP toxicity.

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.

Neurovascular signaling in the brain and the pathological consequences of hypertension

The execution and maintenance of all brain functions are dependent on a continuous flow of blood to meet the metabolic needs of the tissue. To ensure the delivery of resources required for neural processing and the maintenance of neural homeostasis, the cerebral vasculature is elaborately and extensively regulated by signaling from neurons, glia, interneurons, and perivascular nerves. Hypertension is associated with impaired neurovascular regulation of the cerebral circulation and culminates in neurodegeneration and cognitive dysfunction. Here, we review the physiological processes of neurovascular signaling in the brain and discuss mechanisms of hypertensive neurovascular dysfunction.

The nNOS-p38MAPK pathway is mediated by NOS1AP during neuronal death

Neuronal nitric oxide synthase (nNOS) and p38MAPK are strongly implicated in excitotoxicity, a mechanism common to many neurodegenerative conditions, but the intermediary mechanism is unclear. NOS1AP is encoded by a gene recently associated with sudden cardiac death, diabetes-associated complications, and schizophrenia (Arking et al., 2006; Becker et al., 2008; Brzustowicz, 2008; Lehtinen et al., 2008). Here we find it interacts with p38MAPK-activating kinase MKK3. Excitotoxic stimulus induces recruitment of NOS1AP to nNOS in rat cortical neuron culture. Excitotoxic activation of p38MAPK and subsequent neuronal death are reduced by competing with the nNOS:NOS1AP interaction and by knockdown with NOS1AP-targeting siRNAs. We designed a cell-permeable peptide that competes for the unique PDZ domain of nNOS that interacts with NOS1AP. This peptide inhibits NMDA-induced recruitment of NOS1AP to nNOS and in vivo in rat, doubles surviving tissue in a severe model of neonatal hypoxia-ischemia, a major cause of neonatal death and pediatric disability. The highly unusual sequence specificity of the nNOS:NOS1AP interaction and involvement in excitotoxic signaling may provide future opportunities for generation of neuroprotectants with high specificity.

A critical role of NO/cGMP/PKG dependent pathway in hippocampal post-ischemic LTP: modulation by zonisamide

Nitric oxide (NO) is an intercellular retrograde messenger involved in several physiological processes such as synaptic plasticity, hippocampal long-term potentiation (LTP), and learning and memory. Moreover NO signaling is implicated in the pathophysiology of brain ischemia. In this study, we have characterized the role of NO/cGMP signaling cascade in the induction and maintenance of post-ischemic LTP (iLTP) in rat brain slices. Moreover, we have investigated the possible inhibitory action of zonisamide (ZNS) on this pathological form of synaptic plasticity as well as the effects of this antiepileptic drug (AED) on physiological activity-dependent LTP. Finally, we have characterized the possible interaction between ZNS and the NO/cGMP/PKG-dependent pathway involved in iLTP. Here, we provided the first evidence that an oxygen and glucose deprivation episode can induce, in CA1 hippocampal slices, iLTP by modulation of the NO/cGMP/PKG pathway. Additionally, we found that while ZNS application did not affect short-term synaptic plasticity and LTP induced by high-frequency stimulation, it significantly reduced iLTP. This reduction was mimicked by bath application of NO synthase inhibitors and a soluble guanyl cyclase inhibitor. The effect of ZNS was prevented by either the application of a NO donor or drugs increasing intracellular levels of cGMP and activating PKG. These findings are in line with the possible use of AEDs, such as ZNS, as a possible neuroprotective strategy in brain ischemia. Moreover, these findings strongly suggest that NO/cGMP/PKG intracellular cascade might represent a physiological target for neuroprotection in pathological forms of synaptic plasticity such as hippocampal iLTP.

NMDA receptor activation enhances inhibitory GABAergic transmission onto hippocampal pyramidal neurons via presynaptic and postsynaptic mechanisms

N-methyl-d-aspartate (NMDA) receptors (NMDARs) are implicated in synaptic plasticity and modulation of glutamatergic excitatory transmission. Effect of NMDAR activation on inhibitory GABAergic transmission remains largely unknown. Here, we report that a brief application of NMDA could induce two distinct actions in CA1 pyramidal neurons in mouse hippocampal slices: 1) an inward current attributed to activation of postsynaptic NMDARs; and 2) fast phasic synaptic currents, namely spontaneous inhibitory postsynaptic currents (sIPSCs), mediated by GABAA receptors in pyramidal neurons. The mean amplitude of sIPSCs was also increased by NMDA. This profound increase in the sIPSC frequency and amplitude was markedly suppressed by the sodium channel blocker TTX, whereas the frequency and mean amplitude of miniature IPSCs were not significantly affected by NMDA, suggesting that NMDA elicits repetitive firing in GABAergic interneurons, thereby leading to GABA release from multiple synaptic sites of single GABAergic axons. We found that the NMDAR open-channel blocker MK-801 injected into recorded pyramidal neurons suppressed the NMDA-induced increase of sIPSCs, which raises the possibility that the firing of interneurons may not be the sole factor and certain retrograde messengers may also be involved in the NMDA-mediated enhancement of GABAergic transmission. Our results from pharmacological tests suggest that the nitric oxide signaling pathway is mobilized by NMDAR activation in CA1 pyramidal neurons, which in turn retrogradely facilitates GABA release from the presynaptic terminals. Thus NMDARs at glutamatergic synapses on both CA1 pyramidal neurons and interneurons appear to exert feedback and feedforward inhibition for determining the spike timing of the hippocampal microcircuit.

Role of ventral hippocampal nitric oxide/cGMP pathway in anxiety-related behaviors in rats submitted to the elevated T-maze

The L-arginine/nitric oxide (NO)/cGMP pathways have been implicated in the control of a variety of physiological mechanisms and are believed to participate in the modulation of anxiety in the CNS. The aim of this study was to investigate the effects of N(G)-nitro-L-arginine-methyl-ester (L-NAME), a non-selective inhibitor of NO synthase (NOS); 7-nitroindazole (7-NI), a preferential inhibitor of neuronal NOS; and sodium nitroprusside (SNP), an NO donor, administered into the ventral hippocampus (VH) of rats submitted to the elevated T-maze (ETM). The ETM, an animal model derived from the elevated plus-maze, allows the measurement of two defensive behavioral responses in the same rat: inhibitory avoidance and escape. Results showed that L-NAME and 7-NI impaired the acquisition of inhibitory avoidance and prolonged escape latency in the ETM, suggesting an anxiolytic-like and panicolytic-like effect, respectively. SNP facilitated the acquisition of inhibitory avoidance without interfering with escape performance, suggesting an anxiogenic-like effect. Treatment with methylene blue did not alter per se any of the behavioral responses measured in the ETM, but blocked the effect promoted by SNP. Thus, altogether these results suggest that NO in the VH is critically involved in the modulation of defensive behavior of rats exposed to the ETM.

Involvement of nitrergic system in the anticonvulsant effect of the cannabinoid CB(1) agonist ACEA in the pentylenetetrazole-induced seizure in mice

Cannabinoid system plays a pivotal role in the seizure threshold modulation which is mainly mediated through activation of the cannabinoid CB(1) receptor. There is also several evidence of interaction between cannabinoid system and other neurotransmitters including nitric oxide (NO) system. Using model of clonic seizure induced by pentylenetetrazole (PTZ) in male NMRI mice, we investigated whether NO is involved in the effects of cannabinoids on the seizure threshold. Injection of the selective cannabinoid CB(1) agonist ACEA (2mg/kg, i.p.) significantly (P<0.01) increased the seizure threshold which was prevented (P<0.001) by pretreatment with the selective CB(1) antagonist AM251 (1mg/kg, i.p.). The NO precursor l-arginine (50 and 100mg/kg, i.p.) potentiated the anticonvulsant effects of the sub-effective dose of ACEA (1mg/kg, i.p.). Pretreatment with non-effective doses of the non-specific NOS inhibitor l-NAME (15 and 30mg/kg, i.p.) and the specific neuronal NOS inhibitor 7-NI (40 and 80mg/kg, i.p.) but not the inducible NOS inhibitor aminoguanidine (10, 50 and 100mg/kg, i.p.) prevented the anticonvulsant effect of ACEA (2mg/kg, i.p.). Co-administration of non-effective dose of AM251 (0.5mg/kg) with both low and per se non-effective doses of l-NAME (1mg/kg, i.p.) and 7-NI (10mg/kg, i.p.) had significant (P<0.01) effect in preventing the anticonvulsant effect of ACEA (2mg/kg, i.p.). Our findings demonstrated that central NO system could be involved in the anticonvulsant properties of the specific cannabinoid CB(1) agonist ACEA, emphasizing on the interaction between two systems in the seizure modulation.

Pharmacologic suppression of oxidative damage and dendritic degeneration following kainic acid-induced excitotoxicity in mouse cerebrum

Intense seizure activity associated with status epilepticus and excitatory amino acid (EAA) imbalance initiates oxidative damage and neuronal injury in CA1 of the ventral hippocampus. We tested the hypothesis that dendritic degeneration of pyramidal neurons in the CA1 hippocampal area resulting from seizure-induced neurotoxicity is modulated by cerebral oxidative damage. Kainic acid (KA, 1 nmol/5 microl) was injected intracerebroventricularly to C57Bl/6 mice. F2-isoprostanes (F2-IsoPs) and F4-neuroprostanes (F4-NeuroPs) were used as surrogate measures of in vivo oxidative stress and biomarkers of lipid peroxidation. Nitric oxide synthase (NOS) activity was quantified by evaluating citrulline level and pyramidal neuron dendrites and spines were evaluated using rapid Golgi stains and a Neurolucida system. KA produced severe seizures in mice immediately after its administration and a significant (p<0.001) increase in F2-IsoPs, F4-NeuroPs and citrulline levels were seen 30 min following treatment. At the same time, hippocampal pyramidal neurons showed significant (p<0.001) reduction in dendritic length and spine density. In contrast, no significant change in neuronal dendrite and spine density or F2-IsoP, F4-NeuroPs and citrulline levels were found in mice pretreated with vitamin E (alpha-tocopherol, 100mg/kg, i.p.) for 3 days, or with N-tert-butyl-alpha-phenylnitrone (PBN, 200mg/kg, i.p.) or ibuprofen (inhibitors of cyclooxygenase, COX, 14 microg/ml of drinking water) for 2 weeks prior to KA treatment. These findings indicate novel interactions among free radical-induced generation of F2-IsoPs and F4-NeuroPs, nitric oxide and dendritic degeneration, closely associate oxidative damage to neuronal membranes with degeneration of the dendritic system, and point to possible interventions to limit severe damage in acute neurological disorders.

Layer-specific differences in reactive oxygen species levels after oxygen-glucose deprivation in acute hippocampal slices

The major role of reactive oxygen species (ROS) in the pathomechanism of ischemia have been widely recognized. Still, measurements of the precise time course and regional distribution of ischemia-induced ROS level changes in acute brain slices have been missing. By using acute hippocampal slices and the fluorescent dye CM-H2DCFDA, we showed that reoxygenation after in vitro ischemia (oxygen-glucose deprivation; OGD) increased ROS levels in the hippocampal CA1 layers vulnerable to ischemia but did not have significant effects in the resistant stratum granulosum in the dentate gyrus (DG). Production of ROS started during OGD, but, contrary to reoxygenation, it manifested as a ROS level increase exclusively in the presence of catalase and glutathione peroxidase inhibition. The mechanism of ROS production involves the activation of NMDA receptors and nitric oxide synthases. The inhibition of ROS response by either AP-5 or L-NAME together with the ROS sensitivity profile of the dye suggest that peroxynitrite, the reaction product of superoxide and nitric oxide, plays a role in the response. Direct visualization of layer-specific effects of ROS production and its scavenging, shown for the first time in acute hippocampal slices, suggests that distinct ROS homeostasis may underlie the different ischemic vulnerability of CA1 and DG.

Involvement of nitric oxide in depolarization-induced suppression of inhibition in hippocampal pyramidal cells during activation of cholinergic receptors

Several types of neurons are able to regulate their synaptic inputs via releasing retrograde signal molecules, such as endocannabinoids or nitric oxide (NO). Here we show that, during activation of cholinergic receptors, retrograde signaling by NO controls CB1 cannabinoid receptor (CB1R)-dependent depolarization-induced suppression of inhibition (DSI). Spontaneously occurring IPSCs were recorded in CA1 pyramidal neurons in the presence of carbachol, and DSI was induced by a 1-s-long depolarization step. We found that, in addition to the inhibition of CB1Rs, blocking the NO signaling pathway at various points also disrupted DSI. Inhibitors of NO synthase (NOS) or NO-sensitive guanylyl cyclase (NO-sGC) diminished DSI, whereas a cGMP analog or an NO donor inhibited IPSCs and partially occluded DSI in a CB1R-dependent manner. Furthermore, an NO scavenger applied extracellularly or postsynaptically also decreased DSI, whereas L-arginine, the precursor for NO, prolonged it. DSI of electrically evoked IPSCs was also blocked by an inhibitor of NOS in the presence, but not in the absence, of carbachol. In line with our electrophysiological data, double immunohistochemical staining revealed an NO-donor-induced cGMP accumulation in CB1R-positive axon terminals. Using electron microscopy, we demonstrated the postsynaptic localization of neuronal NOS at symmetrical synapses formed by CB1R-positive axon terminals on pyramidal cell bodies, whereas NO-sGC was found in the presynaptic terminals. These electrophysiological and anatomical results in the hippocampus suggest that NO is involved in depolarization-induced CB1R-mediated suppression of IPSCs as a retrograde signal molecule and that operation of this cascade is conditional on cholinergic receptor activation.

Hippocampal GABAergic synapses possess the molecular machinery for retrograde nitric oxide signaling

Nitric oxide (NO) plays an important role in synaptic plasticity as a retrograde messenger at glutamatergic synapses. Here we describe that, in hippocampal pyramidal cells, neuronal nitric oxide synthase (nNOS) is also associated with the postsynaptic active zones of GABAergic symmetrical synapses terminating on their somata, dendrites, and axon initial segments in both mice and rats. The NO receptor nitric oxide-sensitive guanylyl cyclase (NOsGC) is present in the brain in two functional subunit compositions: alpha1beta1 and alpha2beta1. The beta1 subunit is expressed in both pyramidal cells and interneurons in the hippocampus. Using immunohistochemistry and in situ hybridization methods, we describe that the alpha1 subunit is detectable only in interneurons, which are always positive for beta1 subunit as well; however, pyramidal cells are labeled only for beta1 and alpha2 subunits. With double-immunofluorescent staining, we also found that most cholecystokinin- and parvalbumin-positive and smaller proportion of the somatostatin- and nNOS-positive interneurons are alpha1 subunit positive. We also found that the alpha1 subunit is present in parvalbumin- and cholecystokinin-positive interneuron terminals that establish synapses on somata, dendrites, or axon initial segments. Our results demonstrate that NOsGC, composed of alpha1beta1 subunits, is selectively expressed in different types of interneurons and is present in their presynaptic GABAergic terminals, in which it may serve as a receptor for NO produced postsynaptically by nNOS in the very same synapse.

Splice-isoform specific immunolocalization of neuronal nitric oxide synthase in mouse and rat brain reveals that the PDZ-complex-building nNOSalpha beta-finger is largely exposed to antibodies

Knock out mice deficient for the splice-isoform alphaalpha of neuronal nitric oxide synthase (nNOSalphaalpha) display residual nitric oxide synthase activity and immunosignal. To attribute this signal to the two minor neuronal nitric oxide synthase splice variants, betabeta and gammagamma, we generated isoform-specific anti-peptide antibodies against the nNOSalphaalpha specific betabeta-finger motif involved in PDZ domain scaffolding and the nNOSbetabeta specific N-terminus. The nNOSalphaalpha betabeta-finger-specific antibody clearly recognized the 160-kDa band of recombinant nNOSalphaalpha on Western blots. Using immunocytochemistry, this antibody displayed, in rats and wild-type mice, a labeling pattern similar to but not identical with that obtained using a commercial pan-nNOS antibody. This similarity indicates that the majority of immunocytochemically detectable nNOS is not likely to be complexed with PDZ-domain proteins via the betabeta-finger motif. This conclusion was confirmed by the inhibition of PSD-95/nNOS interaction by the nNOSalphaalpha betabeta-finger antibody in pull-down assays. By contrast, nNOSalphaalpha betabeta-finger labeling was clearly reduced in hippocampal and cortical neuropil areas enriched in NMDA receptor complex containing spine synapses. In nNOSalphaalpha knock out mice, nNOSalphaalpha was not detectable, whereas the pan-nNOS antibody showed a distinct labeling of cell bodies throughout the brain, most likely reflecting betabeta/gammagamma-isoforms in these cells. The nNOSbetabeta antibody clearly detected bacterial expressed nNOSbetabeta fusion protein and nNOSbetabeta in overexpressing HEK cells by Western blotting. Immunocytochemically, individual cell bodies in striatum, cerebral cortex, and in some brain stem nuclei were labeled in knock out but not in wild-type mice, indicating an upregulation of nNOSbetabeta in nNOSalphaalpha deficient animals.

Inhibition of nitric oxide synthase increases synaptophysin mRNA expression in the hippocampal formation of rats

Synaptophysin is a protein involved in the biogenesis of synaptic vesicles and budding. It has been used as an important tool to investigate plastic effects on synaptic transmission. Nitric oxide (NO) can influence plastic changes in specific brain regions related to cognition and emotion. Experimental evidence suggests that NO and synaptophysin are co-localized in several brain regions and that NO may change synaptophysin expression. Therefore, the aim of the present work was to investigate if inhibition of NO formation would change synaptophysin mRNA expression in the hippocampal formation. Male Wistar rats received single or repeated (once a day for 4 days) i.p. injections of saline or l-nitro-arginine (l-NOARG, 40mg/kg), a non-selective inhibitor of nitric oxide synthase (NOS). Twenty-four hours after the last injection the animals were sacrificed and their brains removed for 'in situ' hybridization study using (35)S-labeled oligonucleotide probe complementary to synaptophysin mRNA. The results were analyzed by computerized densitometry. Acute administration of l-NOARG induced a significant (p<0.05, ANOVA) increase in synaptophysin mRNA expression in the dentate gyrus, CA1 and CA3. The effect disappeared after repeated drug administration. No change was found in the striatum, cingulated cortex, substantia nigra or nucleus accumbens. These results reinforce the proposal that nitric oxide is involved in plastic events in the hippocampus.

Concentration dynamics of nitric oxide in rat hippocampal subregions evoked by stimulation of the NMDA glutamate receptor

Nitric oxide (*NO) production in response to stimulation of the NMDA glutamate receptor is implicated not only in the synaptic plasticity in hippocampus but may also participate in excitotoxic cell death. Using *NO-selective microssensors inserted into the diffusional field of *NO in acute hippocampal slices, we describe the *NO concentration dynamics evoked by NMDA receptor activation and report profound differences along the trisynaptic loop of the hippocampus. We measured the oxygen gradient across the slice thickness and conclude that *NO measurements were performed at cell layers experiencing physiological oxygen tensions. Recordings performed at increasing distances from the point of NMDA receptor stimulation resulted in a progressive decrease of *NO signals, reaching undetectable levels for distances >400 microm, supporting the notion of a wide diffusional spread of endogenously generated *NO in the hippocampus. Neither a picoinjection nor a continuous perfusion of NMDA resulted in high steady-state *NO levels; rather all signals were transient, suggesting that cells are able to efficiently respond to high *NO concentrations (typically 200-400 nM) bringing it to very low nM levels; the claimed high micromolar *NO range achieved by excessive stimulation of NMDA receptor may have to be reevaluated. The distinct responses to NMDA receptor stimulation along the trysynaptic loop suggest a differential *NO activity and/or regulation among the hippocampal subregions. These findings may be relevant for the understanding of the role of *NO in physiologic mechanisms in the hippocampus and the differential sensitivity of the hippocampal subregions to NMDA receptor-dependent neurodegeneration.

Memory consolidation of Pavlovian fear conditioning requires nitric oxide signaling in the lateral amygdala

Nitric oxide (NO) has been widely implicated in synaptic plasticity and memory formation. In studies of long-term potentiation (LTP), NO is thought to serve as a 'retrograde messenger' that contributes to presynaptic aspects of LTP expression. In this study, we examined the role of NO signaling in Pavlovian fear conditioning. We first show that neuronal nitric oxide synthase is localized in the lateral nucleus of the amygdala (LA), a critical site of plasticity in fear conditioning. We next show that NO signaling is required for LTP at thalamic inputs to the LA and for the long-term consolidation of auditory fear conditioning. Collectively, the findings suggest that NO signaling is an important component of memory formation of auditory fear conditioning, possibly as a retrograde signal that participates in presynaptic aspects of plasticity in the LA.

Nitric oxide donors induce neurotrophin-like survival signaling and protect neurons against apoptosis

Our previous results showed that inhibition of protein tyrosine phosphatases (PTP) by orthovanadate is an appropriate strategy to mimic nerve growth factor (NGF) effects in neurons, including enhanced phosphorylation of TrkA, stimulation of downstream survival signaling pathways, and protection against apoptotic stress. In this study, we wanted to trigger such NGF-like survival signaling in primary hippocampal neurons with the more specific PTP inhibitors ethyl-3,4-dephostatin (DPN), 4-O-methyl-ethyl-3,4-dephostatin (Me-DPN), and methoxime-3,4-dephostatin. It was striking that only the nitric oxide (NO)-releasing dephostatin analogs DPN and Me-DPN, but not the nitrosamine-free methoxime derivative (which did not release NO), enhanced TrkA phosphorylation and protected the neurons against staurosporine (STS)-induced apoptosis. The established NO donor S-nitroso-N-acetylpenicillamine (SNAP) also enhanced TrkA phosphorylation and prevented apoptosis similarly to DPN and Me-DPN. Analysis of the major signaling pathways downstream of TrkA revealed that both SNAP and DPN enhanced phosphorylation of Akt and the mitogen-activated kinases (MAPK) Erk1/2. Blocking of these signaling pathways by the PI3-K inhibitor wortmannin or the MAPK kinase inhibitor U0126 [1,4-diamino-2,3-dicyano-1,4-bis(2-aminophynyltio)butadiene] equally abolished the neuroprotective effect of the NO donors. It was striking that inhibition of the soluble guanylyl cyclase (sGC) by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) or protein kinase G (PKG) inhibition by (9S,10R,12R)-2,3,9,10,11,12-hexahydro-10-methoxy-2,9-dimethyl-1-oxo-9,12-epoxy-1H-diindolo-[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid methyl ester (KT5823) also blocked the neuroprotective effect of the NO donors, and ODQ clearly attenuated SNAP-induced phosphorylation of TrkA, Akt, and MAPK. In conclusion, NO release by the dephostatin derivatives and subsequent stimulation of sGC and PKG is essential for their neuroprotective effects. In primary neurons, such NO-activated survival signaling involves NGF-like effects, including enhanced phosphorylation of TrkA and activation of PI3-K/Akt and MAPK pathways.

Presynaptic and postsynaptic roles of NO, cGK, and RhoA in long-lasting potentiation and aggregation of synaptic proteins

Recent results suggest that long-lasting potentiation at hippocampal synapses involves the rapid formation of clusters or puncta of presynaptic as well as postsynaptic proteins, both of which are blocked by antagonists of NMDA receptors and an inhibitor of actin polymerization. We have investigated whether the increase in puncta involves retrograde signaling through the NO-cGMP-cGK pathway and also examined the possible roles of two classes of molecules that regulate the actin cytoskeleton: Ena/VASP proteins and Rho GTPases. Our results suggest that NO, cGMP, cGK, actin, and Rho GTPases including RhoA play important roles in the potentiation and act directly in both the presynaptic and postsynaptic neurons, where they contribute to the increase in puncta of synaptic proteins. cGK phosphorylates synaptic VASP during the potentiation, whereas Rho GTPases act both in parallel and upstream of cGMP, in part by maintaining the synaptic localization of soluble guanylyl cyclase.

The NOS/sGC pathway in the rat central nervous system: a microdialysis overview

It is now well established that nitric oxide is involved in a variety of physiopathological processes in the central nervous system, which mainly result from the interaction of this gaseous molecule with the heme group of soluble guanylyl cyclase and the elevation of intracellular cGMP in target neurons. During the last decade, several studies have monitored extracellular cGMP, by means of intracerebral microdialysis, to investigate in vivo the functioning and modulation of this neurochemical pathway under different experimental conditions and in various brain regions. In this review, we summarise some of the most relevant results obtained in this research field.

Nitric oxide synthase in rat visual cortex: an immunohistochemical study

The aim of the present study was to identify the distribution of two isoforms of the nitric oxide synthase (NOS), the neuronal (nNOS) and the endothelial (eNOS) form, in rat visual cortex. Immunohistochemical localisation of each NOS isoform was studied with three tissue-processing protocols. In the first one, immunohistochemical reactions were made on 30-microm-thick sections with membrane detergents, Triton or Saponin, used to increase the permeability of the tissue for the antibodies. In the second protocol, we excluded these detergents from all solutions to avoid a destruction of the cellular membrane. In the third protocol, we used thin paraffin sections (5 microm thick) to assure delivery of the antibodies to intracellular structures. Our data demonstrate, that both neuronal and endothelial isoforms of the NOS are present in the visual cortex. Among the neurones labelled by the antibodies against eNOS or nNOS, some excitatory cells were definitely present. nNOS immunopositive were neurones and a dense network of fibres, presumably axons. Some of the neurones were heavily labelled in a Golgi-like manner, while others showed only weak labelling. eNOS immunopositivity was found in the blood vessels and in neurones. eNOS positive neurones were much more numerous than nNOS-containing cells, and represent about 60% of the cortical cells. However, with antibodies against eNOS, we never observed neurone-specific cell features. The NOS-containing cells found in our present study represent a possible morphological substrate for production of nitric oxide (NO).

Nitric oxide in brain: diffusion, targets and concentration dynamics in hippocampal subregions

Nitric oxide (NO(*)) is a diffusible regulatory molecule involved in a wide range of physiological and pathological events. At the tissue level, a local and temporary increase in NO(*) concentration is translated into a cellular signal. From our current knowledge of biological synthesis and decay, the kinetics and mechanisms that determine NO(*) concentration dynamics in tissues are poorly understood. Generally, NO(*) mediates its effects by stimulating (e.g., guanylate cyclase) or inhibiting (e.g., cytochrome oxidase) transition metal-containing proteins and by post-translational modification of proteins (e.g., formation of nitrosothiol adducts). The borderline between the physiological and pathological activities of NO(*) is a matter of controversy, but tissue redox environment, supramolecular organization and compartmentalisation of NO(*) targets are important features in determining NO(*) actions. In brain, NO(*) synthesis in the dependency of glutamate NMDA receptor is a paradigmatic example; the NMDA-subtype glutamate receptor triggers intracellular signalling pathways that govern neuronal plasticity, development, senescence and disease, suggesting a role for NO(*) in these processes. Measurements of NO(*) in the different subregions of hippocampus, in a glutamate NMDA receptor-dependent fashion, by means of electrochemical selective microsensors illustrate the concentration dynamics of NO(*) in the sub-regions of this brain area. The analysis of NO(*) concentration-time profiles in the hippocampus requires consideration of at least two interrelated issues, also addressed in this review. NO(*) diffusion in a biological medium and regulation of NO(*) activity.

Acute and delayed restraint stress-induced changes in nitric oxide producing neurons in limbic regions

RATIONALE

Microinjection into the dentate gyrus of the hippocampus of N(omega)-nitro-l-arginine methyl ester hydrochloride (l-NAME), a nitric oxide synthase (NOS) inhibitor, induces antinociceptive effect 5 days after a single restraint episode. The mechanisms of this stress-antinociceptive modulatory effect have not been investigated but may involve plastic changes in the hippocampal formation (HF).

OBJECTIVE

The objective of the present study was to investigate possible mechanisms of the stress-modulating effect on antinociception induced by NOS inhibition in the hippocampus. We analyzed the effects of restraint stress on neuronal NOS (nNOS) expression and nicotinamide adenine dinucleotide phosphate-diaphorase histochemical activity (NADPH-d) in the HF and related brain regions.

METHODS

Male Wistar rats (n=6-11/group) were submitted to a single (acute stress) or repeated (5 days) episodes of 2-h restraint. Control animals remained in their home cages being all animals daily handled during this period. In the fifth day, animals received unilateral microinjection of l-NAME (150 nmol/0.2 microl) or saline (control) into the dentate gyrus of the dorsal hippocampus (DG). Immediately before and after drug microinjection tail-flick reflex latency or hotplate licking reaction was measured. Animals were killed i. immediately; ii. 5 days after acute stress; or iii. after repeated stress. NADPH-d and nNOS expression were quantified in the HF, caudate-putamen, secondary somatosensorial, entorhinal and piriform cortices and amygdaloid complex.

RESULTS

Five days after one or five restraint episodes l-NAME microinjection into the DG elicited antinociceptive effect (analysis of variance [ANOVA], P<0.05). Acute restraint stress induced a significant increase in the density of neurons expressing NADPH-d and nNOS in the amygdaloid nuclei. nNOS expression increased also in the DG and piriform cortex. Five days after a single or repeated restraint stress there was an additional increase in NADPH-d- and nNOS-positive neurons in CA1, CA3, and entorhinal cortex. No changes were seen in non-limbic regions such as the caudate-putamen and secondary somatosensorial cortex.

CONCLUSION

The results confirm that the dorsal hippocampus participates in the modulation of stress consequences. They also show that a single stress episode causes acute changes in nitric oxide system in the amygdala complex and delayed modifications in the HF. The delayed (5 days) antinociceptive effect of NOS inhibition in the HF after a single restraint episode suggests that those latter modifications may have functional consequences. It remains to be tested if the acute amygdala and delayed hippocampal changes are causally related.

Species, strain and developmental variations in hippocampal neuronal and endothelial nitric oxide synthase clarify discrepancies in nitric oxide-dependent synaptic plasticity

Nitric oxide (NO) has been implicated in long-term potentiation (LTP) in pyramidal neurons in cellular area 1 (CA1) of the hippocampus. However, considerable confusion exists about the exact role of NO, and the contribution of the endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS) isoforms of NO synthase to NO-dependent LTP (NO-LTP), with results often varying, depending on the organism and experimental paradigm used. Using immunohistochemistry and in situ hybridization, we contrast NO synthase expression and activity in rat, mouse, and human hippocampus. nNOS is prominently expressed in all CA1 pyramidal cells of C57B6 mice and humans, while in rats and SV129 mice, its levels are much lower and restricted to the caudal hippocampus. By contrast, eNOS is restricted to endothelial cells. We observe N-methyl-D-aspartate-dependent citrulline production in pyramidal cells of mouse hippocampus, which is absent in nNOS(Delta/Delta) animals. Finally, we observe robust nNOS expression in human CA1 pyramidal cells.The considerable axial, developmental, strain and species-dependent variations in nNOS expression in CA1 pyramidal neurons can explain much of the variation observed in reports of NO-dependent LTP. Moreover, our data suggest that NO produced by eNOS in endothelial cells may play a paracrine role in modulating LTP.

Retrograde regulation of synaptic vesicle endocytosis and recycling

Sustained release of neurotransmitter depends upon the recycling of synaptic vesicles. Until now, it has been assumed that vesicle recycling is regulated by signals from the presynaptic bouton alone, but results from rat hippocampal neurons reported here indicate that this need not be the case. Fluorescence imaging and pharmacological analysis show that a nitric oxide (NO) signal generated postsynaptically can regulate endocytosis and at least one later step in synaptic vesicle recycling. The proposed retrograde pathway involves an NMDA receptor (NMDAR)-dependent postsynaptic production of NO, diffusion of NO to a presynaptic site, and a cGMP-dependent increase in presynaptic phosphatidylinositol 4,5-biphosphate (PIP2). These results indicate that the regulation of synaptic vesicle recycling may integrate a much broader range of neural activity signals than previously recognized, including postsynaptic depolarization and the activation of NMDARs at both immediate and nearby postsynaptic active zones.

Protective role of melatonin in domoic acid-induced neuronal damage in the hippocampus of adult rats

Domoic acid (DA), a kainite-receptor agonist and potent inducer of neurotoxicity, has been administered intravenously in adult rats in the present study (0.75 mg/kg body weight) to demonstrate neuronal degeneration followed by glial activation and their involvement with inducible nitric oxide synthase (iNOS) in the hippocampus. An equal volume of normal saline was administered in control rats. The pineal hormone melatonin, which protects the neurons efficiently against excitotoxicity mediated by sensitive glutamate receptor, was administered intraperitoneally (10 mg/kg body weight), 20 min before, immediately after, and 1 h and 2 h after the DA administration, to demonstrate its role in therapeutic strategy. Histopathological analysis (Nissl staining) demonstrated extensive neuronal damage in the pyramidal neurons of CA1, CA3 subfields and hilus of the dentate gyrus (DG) in the hippocampus at 5 days after DA administration. Sparsely distributed glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes were observed in the hippocampus at 4-24 h after DA administration and in the control rats. Astrogliosis was evidenced by increased GFAP immunoreactivity in the areas of severe neuronal degeneration at 5 days after DA administration. Along with this, microglial cells exhibited an intense immunoreaction with OX-42, indicating upregulation of complement type 3 receptors (CR3). Ultrastructural study revealed swollen or shrunken degenerating neurons in the CA1, CA3 subfields and hilus of the DG and hypertrophied astrocytes showing accumulation of intermediate filament bundles in the cytoplasm were observed after administration of DA. Although no significant change could be observed in the mRNA level of iNOS expression between the DA-treated rats and controls at 4-24 h and at 5-day time intervals, double immunofluorescense revealed co-expression of induced iNOS with GFAP immunoreactive astrocytes, but not in the microglial cells, and iNOS expression in the neurons of the hippocampal subfields at 5 days after DA administration. Expression of iNOS was not observed in the hippocampus of control rats. DA-induced neuronal death, glial activation, and iNOS protein expression were attenuated significantly by melatonin treatment and were comparable to the control groups. The results of the present study suggest that melatonin holds potential for the treatment of pathologies associated with DA-induced brain damage. It is speculated that astrogliosis and induction of iNOS protein expression in the neurons and astrocytes of the hippocampus may be in response to DA-induced neuronal degeneration.

Enhancement of dendritic branching in cultured hippocampal neurons by 17beta-estradiol is mediated by nitric oxide

Both 17beta-estradiol (E2) and nitric oxide (NO) are important in neuronal development, learning and memory, and age-related memory changes. There is growing evidence that a number of estrogen receptor-mediated effects of estradiol utilize nitric oxide as an intermediary. The role of estradiol in hippocampal neuronal differentiation and function has particular implications for learning and memory. Low levels of estradiol (10nM) significantly increase dendritic branching in cultured embryonic rat hippocampal neurons (158% of control). This study investigates the hypothesis that the estrogen-stimulated increase in dendritic branching is mediated by nitric oxide. We found that nitric oxide donors also produce significantly increased dendritic branching S-nitroso-N-acetylpenicillamine (SNAP: 119%; 2,2'-(hydroxynitrosohydrazino)bis-ethanamine (NOC-18): 128% of control). We then determined that the increases in dendritic branching stimulated by estradiol or by a nitric oxide donor were both blocked by an inhibitor of guanylyl cyclase. Dendritic branching was also stimulated by a cell permeable analog of cyclic guanosine monophosphate (dibutyryl-cGMP: 173% of control). Estradiol-stimulated dendritic branching was reversed by the nitric oxide scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl imidazoline-1-oxyl 3-oxide (carboxy-PTIO). This study provides evidence that estradiol influences the development of embryonic hippocampal neurons in culture by increasing the production of nitric oxide or by increasing the sensitivity of the neurons to nitric oxide. Nitric oxide in turn stimulates dendritic branching via activation of guanylyl cyclase.

Upregulation of neuronal nitric oxide synthase in in vitro stellate astrocytes and in vivo reactive astrocytes after electrically induced status epilepticus

Neuronal nitric oxide synthase (nNOS) is a constitutively expressed and calcium-dependent enzyme. Despite predominantly expressed in neurons, nNOS has been also found in astrocytes, although at lower expression levels. We have studied the regulation of nNOS expression in cultured rat astrocytes from cortex and spinal cord by Western blotting and immunocytochemistry. nNOS was not detectable in cultured astrocytes grown in serum-containing medium (SCM), but was highly expressed after serum deprivation. Accordingly, calcium-dependent NOS activity and both intracellular nitrite levels and nitrotyrosine immunoreactivity after glutamate stimulation were higher in serum-deprived astrocytes than in cells grown in SCM. Serum deprivation induced a modification of astrocytes morphology, from flat to stellate. nNOS up-regulation was also observed in reactive astrocytes of rat hippocampi after electrically induced status epilepticus, as demonstrated by double-labeling experiments. Thus, nNOS upregulation occurs in both in vitro stellate and in vivo reactive astrocytes, suggesting a possible involvement of glial nNOS in neurological diseases characterized by reactive gliosis.

Physiologic pH changes modulate calcium ion dependence of brain nitric oxide synthase in Carassius auratus

Species of the fish genus Carassius survive prolonged anoxia. Nitric oxide (NO) regulates cerebral blood flow in these fish during normoxic conditions whereas adenosine is the main vasoregulating molecule during anoxia. We investigated the calcium ion dependence of Carassius auratus brain NO synthase (NOS) as a function of pH. The physiological pH decrease from 7.2 to 6.8, which takes place during anoxia, greatly decreases NOS activity. This strong pH dependence is mainly due to variation of the calcium sensitivity of the enzyme. The EC(50) is 0.15 microM at pH 7.2 and 2.1 microM at pH 6.8 for the soluble enzyme. The particulate enzyme is also dependent on pH variations. The reduced sensitivity to calcium ions at acidic pH decreases both NO and H(2)O(2) production, saving the cells by suppression of the formation of potentially toxic nitrogen and oxygen species. Modulation of NOS activity by variation of its calcium affinity within the range of physiological pH constitutes an important and rapid mechanism to control the formation of NO and H(2)O(2) during normoxia-anoxia and anoxia-normoxia transitions.

Delayed stress-induced antinociceptive effect of nitric oxide synthase inhibition in the dentate gyrus of rats

Stimulation of the hippocampal formation can modulate nociceptive mechanisms, whereas painful stimuli can activate this structure. Stress exposure can produce plastic changes in the hippocampus. Nitric oxide (NO) is an important neuroregulatory agent present in the hippocampus. The objective of the present study was to investigate the effects of intrahippocampal administration of N(omega)-nitro-L-arginine methyl ester hydrochloride (L-NAME), an inhibitor of NO synthase (NOS), on nociceptive processes in stressed and nonstressed rats. Male Wistar rats (n=6-11/group) received unilateral microinjection of L-NAME (50-300 nmol/0.2 microl) into the dentate gyrus (DG) of the dorsal hippocampus. Immediately after the injection tail-flick reflex latency was measured. Stressed animals were submitted to 2 h of restraint and tested immediately or 1, 2, 5 or 10 days later. L-NAME failed to modify nociception in nonstressed rats. However, 5 days after, restraint L-NAME, at all doses tested, produced an antinociceptive effect (ANOVA, P<.05). The dose-response curve had an inverted U shape. L-NAME antinociceptive effect was antagonized by previous treatment with L-arginine (150 nmol/0.2 microl, P<.05). The results suggest that the modulation of nociceptive processes by NO in the dorsal hippocampus is dependent on previous stress exposure and on poststress interval.

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.

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.

Phosphorylation of neuronal nitric oxide synthase at Ser847 by CaM-KII in the hippocampus of rat brain after transient forebrain ischemia

The authors previously demonstrated that Ca2+/calmodulin (CaM)-dependent protein kinase IIalpha (CaM-KIIalpha) can phosphorylate neuronal nitric oxide synthase (nNOS) at Ser847 and attenuate NOS activity in neuronal cells. In the present study, they established that forebrain ischemia causes an increase in the phosphorylation of nNOS at Ser847 in the hippocampus. This nNOS phosphorylation appeared to be catalyzed by CaM-KII: (1) it correlated with the autophosphorylation of CaM-KIIalpha; (2) it was blocked by the CaM-KII inhibitor, KN-93; and (3) nNOS and CaM-KIIalpha were found to coexist in the hippocampus. Examination of the spatial relation between nNOS and CaM-KIIalpha in the brain revealed coexistence in the hippocampus but not in the cortex during reperfusion, with a concomitant increase in autophosphorylation of CaM-KIIalpha. The phosphorylation of nNOS at Ser847 probably takes place in nonpyramidal hippocampal neurons, which increased after 30 minutes of reperfusion in the hippocampus, whereas no significant increase was detected in the cortex. An intraventricular injection of KN-93 significantly decreased the phosphorylation of nNOS in the hippocampus. These results point to CaM-KII as a protein kinase, which by its colocalization may attenuate the activity of nNOS through its Ser847 phosphorylation, and may thus contribute to promotion of tolerance to postischemic damage in hippocampal neurons.

Nitrergic neurons make synapses on dual-input dendritic spines of neurons in the cerebral cortex and the striatum of the rat: implication for a postsynaptic action of nitric oxide

Pre-embedding electron microscopic immunocytochemistry was used to examine the ultrastructure of neurons containing nitric oxide synthase and to evaluate their synaptic relationships with target neurons in the striatum and sensorimotor cerebral cortex. Intense nitric oxide synthase immunoreactivity was found by light and electron microscopy in a type of aspiny neuron scattered in these two regions. The intensity of the labeling was uniform in the soma, dendrites and axon terminals of these neurons. In both forebrain regions, nitric oxide synthase-immunoreactive neurons received synaptic contacts from unlabeled terminals, which were mostly apposed to small-caliber dendrites. The unlabeled symmetric contacts were generally about four times as abundant as the unlabeled asymmetric contacts on the nitric oxide synthase-immunoreactive neurons. Terminals labeled for nitric oxide synthase were filled with synaptic vesicles and were observed to contact unlabeled neurons. Only 54% (in the cerebral cortex) and 44.3% (in the striatum) of the nitric oxide synthase-immunoreactive terminals making apposition with the target structures were observed to form synaptic membrane specializations within the plane of the randomly sampled sections. The most common targets of nitric oxide synthase-immunoreactive terminals were thin dendritic shafts (54% of the immunoreactive terminals in the cortex and 75.7% of the immunoreactive terminals in the striatum), while dendritic spines were a common secondary target (42% of the immunoreactive terminals in the cortex and 20.6% of the immunoreactive terminals in the striatum). The spines contacted by nitric oxide synthase-immunoreactive terminals typically also received an asymmetric synaptic contact from an unlabeled axon terminal. These findings suggest that: (i) nitric oxide synthase-immunoreactive neurons in the cortex and striatum preponderantly receive inhibitory input; (ii) nitric oxide synthase-containing terminals commonly make synaptic contact with target structures in the cortex and striatum; (iii) spines targeted by nitric oxide synthase-containing terminals in the cortex and striatum commonly receive an asymmetric contact as well, which may provide a basis for a synaptic interaction of nitric oxide with excitatory input to individual spines.

nNOS expression in reactive astrocytes correlates with increased cell death related DNA damage in the hippocampus and entorhinal cortex in Alzheimer's disease

The immunocytochemical distribution of the neuronal form of nitric oxide synthase (nNOS) was compared with neuropathological changes and with cell death related DNA damage (as revealed by in situ end labeling, ISEL) in the hippocampal formation and entorhinal cortex of 12 age-matched control subjects and 12 Alzheimer's disease (AD) patients. Unlike controls, numerous nNOS-positive reactive astrocytes were found in AD patients around beta-amyloid plaques in CA1 and subiculum and at the places of clear and overt neuron loss, particularly in the entorhinal cortex layer II and CA4. This is the first evidence of nNOS-like immunoreactivity in reactive astrocytes in AD. In contrast to controls, in all but one AD subject, large numbers of ISEL-positive neuronal nuclei and microglial cells were found in the CA1 and CA4 regions and subiculum. Semiquantitative analysis showed that neuronal DNA fragmentation in AD match with the distribution of nNOS-expressing reactive astroglial cells in CA1 (r = 0.74, P < 0.01) and CA4 (r = 0.58, P < 0.05). A portion of the nNOS-positive CA2/CA3 pyramidal neurons was found to be spared even in the most affected hippocampi. A significant inverse correlation between nNOS expression and immunoreactivity to abnormally phosphorylated tau proteins (as revealed by AT8 monoclonal antibody) in perikarya of these CA2/3 neurons (r = -0.85, P < 0.01) suggests that nNOS expression may provide selective resistance to neuronal degeneration in AD. In conclusion, our results imply that an upregulated production of NO by reactive astrocytes may play a key role in the pathogenesis of AD.

The distribution of neuronal nitric oxide synthase in the nucleus tractus solitarii of the squirrel monkey

The distribution of neuronal nitric oxide synthase (nNOS) containing neurons and fibers in subnuclei of the nucleus tractus solitarii (NTS) in the squirrel monkey, Saimuri sciureus, was investigated by nNOS immunohistochemistry and nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry. Generally, the staining pattern of nNOS and NADPH-diaphorase in the NTS was similar. A high density of neurons and fibers exhibiting both nNOS immunoreactivity and NADPH-diaphorase reactivity was present in the central, medial, intermediate, and dorsolateral subnuclei of the NTS. A moderate density of neurons and fibers that stained for both nNOS and NADPH-diaphorase was noted in the interstitial and ventromedial subnuclei. The gelatinosus and commissural subnuclei contained a low density of neurons and fibers exhibiting nNOS immunoreactivity and NADPH-diaphorase staining. The dorsal motor nucleus of vagus contained a high density of nNOS immunopositive and NADPH-diaphorase containing neurons and fibers at the rostral level, but contained a moderate density of positive fibers and very few positive neurons at the intermediate, subpostremal and commissural NTS levels. Incongruence was noted, however, between nNOS immunostaining and NADPH-diaphorase staining in blood vessels in the brainstem. Capillaries and small vessels exhibited strong staining for NADPH-diaphorase but no nNOS immunoreactivity. In summary, this work substantiates the presence of nNOS in subnuclei of the monkey NTS and is consistent with a role for NO(.) in neurotransmission in primate NTS.

Bio-imaging of nitric oxide-producing neurones in slices of rat brain using 4,5-diaminofluorescein

4,5-Diaminofluorescein (DAF-2) was used to identify individual nitric oxide (NO)-producing neurones in brain slices in vitro. Coronal slices of midbrain or hippocampus, 300 microm thick from young adult rats, were incubated for 30 min in 1 microM DAF-2 diacetate (DAF-2 DA) and maintained in ACSF at 33 degrees C. Illumination at 450-490 nm revealed punctate fluorescence in neurones in the lateral tegmental nucleus, dorsal raphe nucleus, dorsolateral periaqueductal grey matter, deep collicular layers and cortical areas. Neurones in the hippocampal pyramidal cell layer, molecular layer of the dentate gyrus and the hilus fluoresced also. The fluorescence was abolished by pre-incubation of slices with L-NAME (100 microM-1 mM), the inhibitor of constitutive nitric oxide synthase (NOS), but not by D-NAME (100 microM) or L-NIL (5-50 microM), an inhibitor of inducible NOS. In some superficially located arterioles, there were small regions of bright fluorescence close to the outer smooth muscle wall and diffuse fluorescence within the adjacent smooth muscle cells. A diffuse fluorescence was also seen in some superficially located capillaries. Basal production of NO was not seen within deeper blood vessels. DAF-2 DA offers a sensitive indicator for visualising basal production of NO with high spatial resolution and could provide a means of identifying NOS-containing neurones in brain slices in vitro prior to neurophysiological study.

Reactive oxygen species mediate activity-dependent neuron-glia signaling in output fibers of the hippocampus

Nonsynaptic signaling is becoming increasingly appreciated in studies of activity-dependent changes in the nervous system. We investigated the types of neuronal activity that elicit nonsynaptic communication between neurons and glial cells in hippocampal output fibers. High-frequency, but not low-frequency, action potential firing in myelinated CA1 axons of the hippocampus resulted in increased phosphorylation of the oligodendrocyte-specific protein myelin basic protein (MBP). This change was blocked by tetrodotoxin, indicating that axonally generated action potentials were necessary to regulate the phosphorylation state of MBP. Furthermore, scavengers of the reactive oxygen species superoxide and hydrogen peroxide and nitric oxide synthase inhibitors prevented activation of this neuron-glia signaling pathway. These results indicate that, during periods of increased neuronal activity in area CA1 of the hippocampus, reactive oxygen and nitrogen species are generated, which diffuse to neighboring oligodendrocytes and result in post-translational modifications of MBP, a key structural protein in myelin. Thus, in addition to their well-known capacity for activity-dependent neuron-neuron signaling, hippocampal pyramidal neurons possess a mechanism for activity-dependent neuron-glia signaling.

Reactive oxygen species mediate activity-dependent neuron-glia signaling in output fibers of the hippocampus

Nonsynaptic signaling is becoming increasingly appreciated in studies of activity-dependent changes in the nervous system. We investigated the types of neuronal activity that elicit nonsynaptic communication between neurons and glial cells in hippocampal output fibers. High-frequency, but not low-frequency, action potential firing in myelinated CA1 axons of the hippocampus resulted in increased phosphorylation of the oligodendrocyte-specific protein myelin basic protein (MBP). This change was blocked by tetrodotoxin, indicating that axonally generated action potentials were necessary to regulate the phosphorylation state of MBP. Furthermore, scavengers of the reactive oxygen species superoxide and hydrogen peroxide and nitric oxide synthase inhibitors prevented activation of this neuron-glia signaling pathway. These results indicate that, during periods of increased neuronal activity in area CA1 of the hippocampus, reactive oxygen and nitrogen species are generated, which diffuse to neighboring oligodendrocytes and result in post-translational modifications of MBP, a key structural protein in myelin. Thus, in addition to their well-known capacity for activity-dependent neuron-neuron signaling, hippocampal pyramidal neurons possess a mechanism for activity-dependent neuron-glia signaling.