Modulation of glutamate release by a nitric oxide/cyclic GMP-dependent pathway

Disturbance of the Glutamate-Glutamine Cycle, Secondary to Hepatic Damage, Compromises Memory Function

Glutamate fulfils many vital functions both at a peripheral level and in the central nervous system (CNS). However, hyperammonemia and hepatic failure induce alterations in glutamatergic neurotransmission, which may be the main cause of hepatic encephalopathy (HE), an imbalance which may explain damage to both learning and memory. Cognitive and motor alterations in hyperammonemia may be caused by a deregulation of the glutamate-glutamine cycle, particularly in astrocytes, due to the blocking of the glutamate excitatory amino-acid transporters 1 and 2 (EAAT1, EAAT2). Excess extracellular glutamate triggers mechanisms involving astrocyte-mediated inflammation, including the release of Ca2+-dependent glutamate from astrocytes, the appearance of excitotoxicity, the formation of reactive oxygen species (ROS), and cell damage. Glutamate re-uptake not only prevents excitotoxicity, but also acts as a vital component in synaptic plasticity and function. The present review outlines the evidence of the relationship between hepatic damage, such as that occurring in HE and hyperammonemia, and changes in glutamine synthetase function, which increase glutamate concentrations in the CNS. These conditions produce dysfunction in neuronal communication. The present review also includes data indicating that hyperammonemia is related to the release of a high level of pro-inflammatory factors, such as interleukin-6, by astrocytes. This neuroinflammatory condition alters the function of the membrane receptors, such as N-methyl-D-aspartate (NMDA), (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) AMPA, and γ-aminobutyric acid (GABA), thus affecting learning and spatial memory. Data indicates that learning and spatial memory, as well as discriminatory or other information acquisition processes in the CNS, are damaged by the appearance of hyperammonemia and, moreover, are associated with a reduction in the production of cyclic guanosine monophosphate (cGMP). Therefore, increased levels of pharmacologically controlled cGMP may be used as a therapeutic tool for improving learning and memory in patients with HE, hyperammonemia, cerebral oedema, or reduced intellectual capacity.

The nitric oxide donor SNAP-induced amino acid neurotransmitter release in cortical neurons. Effects of blockers of voltage-dependent sodium and calcium channels

BACKGROUND

The discovery that nitric oxide (NO) functions as a signalling molecule in the nervous system has radically changed the concept of neuronal communication. NO induces the release of amino acid neurotransmitters but the underlying mechanisms remain to be elucidated.

FINDINGS

The aim of this work was to study the effect of NO on amino acid neurotransmitter release (Asp, Glu, Gly and GABA) in cortical neurons as well as the mechanism underlying the release of these neurotransmitters. Cortical neurons were stimulated with SNAP, a NO donor, and the release of different amino acid neurotransmitters was measured by HPLC. The involvement of voltage dependent Na+ and Ca2+ channels as well as cGMP in its mechanism of action was evaluated.

CONCLUSIONS

Our results indicate that NO induces release of aspartate, glutamate, glycine and GABA in cortical neurons and that this release is inhibited by ODQ, an inhibitor of soluble guanylate cyclase. Thus, the NO effect on amino acid neurotransmission could be mediated by cGMP formation in cortical neurons. Our data also demonstrate that the Na+ and Ca2+ voltage- dependent calcium channels are involved in the NO effects on cortical neurons.

Nitric oxide in the nervous system: biochemical, developmental, and neurobiological aspects

Nitric oxide (NO) is a very reactive molecule, and its short half-life would make it virtually invisible until its discovery. NO activates soluble guanylyl cyclase (sGC), increasing 3',5'-cyclic guanosine monophosphate levels to activate PKGs. Although NO triggers several phosphorylation cascades due to its ability to react with Fe II in heme-containing proteins such as sGC, it also promotes a selective posttranslational modification in cysteine residues by S-nitrosylation, impacting on protein function, stability, and allocation. In the central nervous system (CNS), NO synthesis usually requires a functional coupling of nitric oxide synthase I (NOS I) and proteins such as NMDA receptors or carboxyl-terminal PDZ ligand of NOS (CAPON), which is critical for specificity and triggering of selected pathways. NO also modulates CREB (cAMP-responsive element-binding protein), ERK, AKT, and Src, with important implications for nerve cell survival and differentiation. Differences in the regulation of neuronal death or survival by NO may be explained by several mechanisms involving localization of NOS isoforms, amount of NO being produced or protein sets being modulated. A number of studies show that NO regulates neurotransmitter release and different aspects of synaptic dynamics, such as differentiation of synaptic specializations, microtubule dynamics, architecture of synaptic protein organization, and modulation of synaptic efficacy. NO has also been associated with synaptogenesis or synapse elimination, and it is required for long-term synaptic modifications taking place in axons or dendrites. In spite of tremendous advances in the knowledge of NO biological effects, a full description of its role in the CNS is far from being completely elucidated.

Nitrergic, glutamatergic and gabaergic systems in lithium toxicity

We examined the role of nitrergic, glutamatergic and gamma-aminobutyric acid (GABA)-ergic systems in the mechanism(s) underlying lithium induced acute toxicity. With this aim, lithium (18 mEq/kg, i.p.) intoxicated rats were observed for 3 hr recording their clinical signs and death. Lithium exposure at the dose used produced central nervous system (CNS) depression. Pre-treatment of N(w)-nitro-L-arginine methyl ester (L-NAME) a nonselective nitric oxide synthase inhibitor (10 mg/kg, i.p.), 7-nitroindazole (7-NI) a selective neuronal nitric oxide synthase inhibitor (25 mg/kg, i.p.), nitric oxide precursor L-arginine (1,000 mg/kg, i.p.) and MK-801 a noncompetitive antagonist of N-methyl-D-aspartic acid class of glutamate receptors (0.5 mg/kg, i.p.) all increased CNS depression and mortality in lithium group however, no change was seen in GABA receptor agonist GABA (1,000 mg/kg, i.p.) or D-arginine (1,000 mg/kg, i.p.) a biologically inactive enantiomer of L-arginine pre-treated rats. Glutamic acid decarboxylase (GAD) enzyme activity was measured in hippocampus, cerebral cortex and cerebellum of the different groups of animals. GAD enzyme activity reduced in cerebral cortex but not altered in hippocampus or cerebellum by lithium as compared to the control (saline) group. We conclude that an interaction with nitrergic and glutamatergic systems may have a role in the acute toxicity of lithium in rats.The inhibition of glutamate metabolism may arise from this interaction and the involvement of GABA-ergic system should be further investigated in this toxicity.

Inhibition of excitatory synaptic transmission in the trigeminal motor nucleus by the nitric oxide-cyclic GMP signaling pathway

Nitric oxide (NO) and cyclic GMP (cGMP) suppressed glutamatergic synaptic transmission to trigeminal motoneurons in brain stem slices of neonatal rats. Histological studies showed guanylate cyclase (GC) containing fibers in the trigeminal motor pool. Glutamatergic excitatory postsynaptic currents (EPSCs) were recorded from neonatal trigeminal motoneurons in response to stimulation of the supratrigeminal nucleus (SuV). The NO donors DETA/NONOate (DETA/NO), at a concentration which released 275.1 nM of NO, and Spermine/NONOate (Sper/NO) reduced the amplitude of the EPSC to 52.7±0.6% and 60.1±10.8% of control values, respectively. These actions were not blocked by the GC inhibitors, ODQ or NS-2028. However, in the presence of YC-1 or BAY41-2272, modulators of GC that act as NO sensitizers, lower and otherwise ineffective concentrations of DETA/NO induced a reduction of the EPSC to 60.6±5.2%. Moreover, NO effects were mimicked by 8BrcGMP and by Zaprinast, an inhibitor of Phosphodiesterase 5. Glutamatergic currents evoked by exogenous glutamate were not reduced by DETA/NO nor 8BrcGMP. Paired-pulse facilitation was increased by NO donors. Under "minimal stimulation" conditions NO donors and cGMP increased the failure rate of evoked EPSCs. Protein kinase inhibitors antagonized cGMP effects. The results suggest that NO, through the synthesis of cGMP, presynaptically inhibits glutamatergic synaptic transmission on trigeminal motoneurons. We propose that NO has complex actions on motor pools; specific studies are needed to elucidate their physiological significance in the behaving animal.

Anxiolytic effects induced by inhibition of the nitric oxide-cGMP pathway in the rat dorsal hippocampus

RATIONALE

Conflicting results have been reported regarding the role of the nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) pathway in the hippocampus on anxiety modulation.

OBJECTIVES

To investigate the effects of intrahippocampal injections of drugs that modify the NO-cGMP pathway in rats submitted to two animal models that are sensitive to anxiolytic drugs, the elevated plus-maze and the Vogel punished licking test.

MATERIALS AND METHODS

Male Wistar rats with cannulae aimed at the dentate gyrus of the dorsal hippocampus received microinjections of the NO synthase (NOS) inhibitors N (G)-nitro-L: -arginine methyl ester (LNAME, 15-300 nmol/0.2 microl), N (G)-nitro-L: -arginine (LNOARG, 50-300 nmol/0.2 microl), 7-nitroindazole (7NI, 10-100 nmol/0.2 microl), or the soluble guanylate cyclase inhibitor 1H-oxadiazolo-quinoxalin-1 one (ODQ, 10-100 nmol/0.2 microl), and were submitted to the elevated plus-maze. In a second group, the animals received 7NI, LNAME, or ODQ and were submitted to the Vogel punished licking test. To control for drug-induced changes in locomotor behavior, the animals were submitted to an open arena or to the Rota-rod test.

RESULTS

All drugs increased the exploration of the open arms of the elevated plus-maze. They also increased the number of punished licks in the Vogel test, indicating an anxiolytic effect. The anxiolytic effect of LNAME was prevented by previous treatment with L: -arginine (300 nmol/0.2 microl). Except for the lower dose of LNAME (15 nmol), administration of the NOS inhibitors or ODQ did not change exploratory activity in the open field nor cause any gross locomotor impairment in the Rota-rod test.

CONCLUSION

The results suggest that NO plays an anxiogenic role in the dentate gyrus of the dorsal hippocampus.

Characteristics of GABA Release Induced by Free Radicals in Mouse Hippocampal Slices

The release of the inhibitory neurotransmitter GABA is generally enhanced under potentially cell-damaging conditions. The properties and regulation of preloaded [3H]GABA release from mouse hippocampal slices were now studied in free radical-containing medium in a superfusion system. Free radical production was induced by 0.01% of H2O2 in the medium. H2O2 markedly potentiated GABA release, which was further enhanced about 1.5-fold by K+ stimulation (50 mM). In Ca2+-free media this stimulation was not altered, indicating that the release was mostly Ca2+-independent. Moreover, omission of Na+ increased the release, suggesting that it is mediated by Na+-dependent transporters operating outwards, a conception confirmed by the enhancement with GABA homoexchange. Inhibition of the release with the ion channel inhibitors diisothiocyanostilbene-2,2'-disulphonate and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonate indicates that Cl(-) channels also participate in the process. This release was not modified by the adenosine receptor (A1 and A2a) agonists and ionotropic glutamate receptor agonists kainate, N-methy-D: -aspartate and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, whereas the agonists of metabotropic glutamate receptors of group I [(S)-3,5-dihydroxyphenylglycine] and of group II [(2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate] enhanced it by receptor-mediated mechanisms, the effects being abolished by their respective antagonists. The group III agonist L+-2-amino-4-phosphonobutyrate reduced the evoked GABA release, but this was not affected by the antagonist. Furthermore, the release was reduced by activation of protein kinase C by 4 beta-phorbol 12-myristate 13-acetate and by inhibition of tyrosine kinase by genistein and of phoshoplipase by quinacrine. On the other hand, increasing cGMP levels with the phosphodiesterase inhibitor zaprinast, selective for PDE5, 6 and 9, and NO production with the NO-generating compounds hydroxylamine, sodium nitroprusside and S-nitroso-N-penicillamine enhanced the release. The regulation of GABA release induced by free radical production proved thus to be rather complex. Under potentially cell-damaging conditions, the potentiation of GABA release may be a mechanism to counteract hyperactivity and reduce the effects of excitatory amino acid release. On the other hand, reduction of GABA release could be harmful and contribute to excitotoxic damage and neuronal degeneration.

Modulation of cardiovascular responses and neurotransmission during peripheral nociception following nNOS antagonism within the periaqueductal gray

Nitric oxide (NO) within the dorsal periaqueductal gray matter (dPAG) attenuated cardiovascular responses and changes in the concentrations of glutamate during both mechanical and thermal nociceptive stimulation [Ishide, T., Amer, A., Maher, T.J., Ally, A., 2005. Nitric oxide within periaqueductal gray modulates glutamatergic neurotransmission and cardiovascular responses during mechanical and thermal stimuli. Neurosci. Res. 51, 93-103]. Nitric oxide is synthesized from l-arginine via the enzyme, NO synthase (NOS), which exists in 3 isoforms: endothelial (eNOS), neuronal (nNOS), and inducible (iNOS). In this study, we examined the role of nNOS within the dPAG on cardiovascular responses and extracellular glutamate and GABA concentrations during mechanical and thermal nociception in anesthetized rats. The noxious mechanical stimulus was applied by a bilateral hindpaw pinch for 5 s that increased mean arterial pressure (MAP) and heart rate (HR) by 24+/-4 mm Hg and 41+/-7 bpm, respectively (n=10). Extracellular glutamate levels within the dPAG increased by 10.7+/-1.3 ng/mul while GABA concentrations decreased by 1.9+/-0.5 ng/microl. Bilateral microdialysis of a selective nNOS antagonist, 1-(2-trifluoromethylphenyl)-imidazole (TRIM; 10.0 microM), into the dPAG had no effect on MAP, HR, glutamate and GABA values (P>0.05) during a mechanical stimulation. In a separate set of experiments, a noxious thermal stimulus was generated by immersing the metatarsus of a hindpaw in a water-bath at 52 degrees C for 5 s (n=10). Glutamate, MAP, and HR increased by 14.6+/-2 ng/microl, 45+/-6 mm Hg, and 47+/-7 bpm, while GABA decreased by 2.1+/-0.6 ng/microl. Administration of TRIM into the dPAG significantly enhanced the cardiovascular responses and glutamate increases (P<0.05) but further attenuated GABA changes (P<0.05) during subsequent thermal nociception. These results demonstrate that nNOS within the dPAG plays a differential role in modulating cardiovascular responses and glutamatergic/GABAergic neurotransmission during thermal and mechanical nociception.

Nitric oxide inhibits depolarization-evoked glutamate release from rat cerebellar granule cells

Nitric oxide (NO) modulates the release of various neurotransmitters, some of these are considered to be involved in neuronal plasticity that includes long-term depression in the cerebellum. To date, there have been no reports on the modulation of the exocytotic release of neurotransmitters in the cerebellar granule cells (CGCs) by NO. The aim of this study was to investigate the effects of NO on the exocytotic release of glutamate from rat CGCs. Treatment with NO-related reagents revealed that NO inhibited high-K(+)-evoked glutamate release. Clostridium botulinum type B neurotoxin (BoNT/B) attenuated the enhancement of glutamate release caused by NO synthase (NOS) inhibition; this indicates that NO acts on the high-K(+)-evoked exocytotic pathway. cGMP-related reagents did not affect the high-K(+)-evoked glutamate release. NO-related reagents did not affect Ca(2+) ionophore-induced glutamate release, suggesting that NO inhibits Ca(2+) entry through voltage-dependent Ca(2+) channels (VDCC). Monitoring of intracellular Ca(2+) revealed that NO inhibited high-K(+)-evoked Ca(2+) entry. L-type VDCC blockers inhibited glutamate release and NO did not have an additive effect on the inhibition produced by the L-type VDCC blocker. The inhibition of the high-K(+)-evoked glutamate release by NO was abolished by a reducing reagent; this suggested that NO regulates the high-K(+)-evoked glutamate release from CGCs by redox modulation.

Modulation of GABA release by second messenger substances and NO in mouse brain stem slices under normal and ischemic conditions

GABA is the inhibitory neurotransmitter in most brain stem nuclei. The properties of release of preloaded [(3)H]GABA were now investigated with slices from the mouse brain stem under normal and ischemic (oxygen and glucose deprivation) conditions, using a superfusion system. The ischemic GABA release increased about fourfold in comparison with normal conditions. The tyrosine kinase inhibitor genistein had no effect on GABA release, while the phospholipase inhibitor quinacrine reduced both the basal and K(+)-evoked release in normoxia and ischemia. The activator of protein kinase C (PKC) 4beta-phorbol 12-myristate 13-acetate had no effects on the releases, whereas the PKC inhibitor chelerythrine reduced the basal release in ischemia. When the cyclic guanosine monophosphate (cGMP) levels were increased by superfusion with zaprinast and other phosphodiesterase inhibitors, GABA release was reduced under normal conditions. The NO donors S-nitroso-N-acetylpenicillamine (SNAP) and hydroxylamine (HA) enhanced the basal and K(+)-stimulated release by acting directly on presynaptic terminals. Under ischemic conditions GABA release was enhanced when cGMP levels were increased by zaprinast. This effect was confirmed by inhibition of the release by the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). The NO-producing agents SNAP, HA, and sodium nitroprusside potentiated GABA release in ischemia. These effects were reduced by the NO synthase inhibitor N(G)-nitro-L: -arginine, but not by ODQ. The results show that particularly NO and cGMP regulate both normal and ischemic GABA release in the brain stem. Their effects are however complex.

Mechanism of nitric oxide action on inhibitory GABAergic signaling within the nucleus tractus solitarii

The cellular mechanisms mediating nitric oxide (NO) modulation of the inhibitory transmission in the nucleus tractus solitarii (NTS) remain unclear, even though this could be extremely important for various physiological and pathological processes. Specifically, in the NTS NO-evoked glutamate and gamma-aminobutyric acid (GABA) release might contribute to pathological hypertension. In cultured rat brainstem slices, NTS GABAergic neurons were targeted using an adenoviral vector to express enhanced green fluorescent protein and studied with a combination of patch clamp and confocal microscopy. Low nanomolar concentrations of NO increased intracellular Ca2+ concentration ([Ca2+]i) in somata, dendrites, and putative axons of GABAergic neurons, with axons being the most sensitive compartment. This effect was cGMP mediated and not related to depolarization or indirect presynaptic effects on glutamatergic transmission. Blockade of the cyclic adenosine diphosphate ribose (cADPR)/ryanodine-sensitive stores but not the inositol triphosphate-sensitive stores, inhibited NO effect. Since cADPR/ryanodine-sensitive stores are implicated in the Ca2+-induced Ca2+ release, NO can be expected to potentiate GABA release. In support of this notion, a cADPR antagonist abolished the NO-induced potentiation of GABAergic inhibitory postsynaptic potentials in the NTS. Thus, the NO-cGMP-cADPR-Ca2+ pathway, previously described in sea urchin eggs, also operates in mammalian GABAergic neurons. Potentiation of GABA release by NO may have implications for numerous brain functions.

Cardiovascular responses and neurotransmitter changes during static muscle contraction following blockade of inducible nitric oxide synthase (iNOS) within the ventrolateral medulla

The enzyme nitric oxide synthase (NOS) which is necessary for the production of nitric oxide from L-arginine exists in three isoforms: neuronal NOS (nNOS), endothelial NOS (eNOS), and inducible NOS (iNOS). Our previous studies have demonstrated the roles of nNOS and eNOS within the rostral (RVLM) and caudal ventrolateral medulla (CVLM) in modulating cardiovascular responses during static skeletal muscle contraction via altering localized glutamate and GABA levels (Brain Res. 977 (2003) 80-89; Neuroscience Res. 52 (2005) 21-30). In this study, we investigated the role of iNOS within the RVLM and CVLM on cardiovascular responses and glutamatergic/GABAergic neurotransmission during the exercise pressor reflex. Bilateral microdialysis of a selective iNOS antagonist, aminoguanidine (AGN; 1.0 microM), for 60 min into the RVLM attenuated increases in mean arterial pressure (MAP), heart rate (HR), and extracellular glutamate levels during a static muscle contraction. Levels of GABA within the RVLM were increased. After 120 min of discontinuation of the drug, MAP and HR responses and glutamate/GABA concentrations recovered to baseline values during a subsequent muscle contraction. In contrast, bilateral application of AGN (1.0 microM) into CVLM potentiated cardiovascular responses and glutamate concentration while attenuating levels of GABA during a static muscle contraction. All values recovered after 120 min of discontinuation of the drug. These results demonstrate that iNOS within the ventrolateral medulla plays an important role in modulating cardiovascular responses and glutamatergic/GABAergic neurotransmission that regulates the exercise pressor reflex.

Role of nitric oxide on motor behavior

The present review paper describes results indicating the influence of nitric oxide (NO) on motor control. Our last studies showed that systemic injections of low doses of inhibitors of NO synthase (NOS), the enzyme responsible for NO formation, induce anxiolytic effects in the elevated plus maze whereas higher doses decrease maze exploration. Also, NOS inhibitors decrease locomotion and rearing in an open field arena. These results may involve motor effects of this compounds, since inhibitors of NOS, NG-nitro-L-arginine (L-NOARG), N(G)-nitro-L-arginine methylester (L-NAME), N(G)-monomethyl-L-arginine (L-NMMA), and 7-Nitroindazole (7-NIO), induced catalepsy in mice. This effect was also found in rats after systemic, intracebroventricular or intrastriatal administration. Acute administration of L-NOARG has an additive cataleptic effect with haloperidol, a dopamine D2 antagonist. The catalepsy is also potentiated by WAY 100135 (5-HT1a receptor antagonist), ketanserin (5HT2a and alfal adrenergic receptor antagonist), and ritanserin (5-HT2a and 5HT2c receptor antagonist). Atropine sulfate and biperiden, antimuscarinic drugs, block L-NOARG-induced catalepsy in mice. L-NOARG subchronic administration in mice induces rapid tolerance (3 days) to its cataleptic effects. It also produces cross-tolerance to haloperidol-induced catalepsy. After subchronic L-NOARG treatment there is an increase in the density NADPH-d positive neurons in the dorsal part of nucleus caudate-putamen, nucleus accumbens, and tegmental pedunculupontinus nucleus. In contrast, this treatment decreases NADPH-d neuronal number in the substantia nigra compacta. Considering these results we suggest that (i) NO may modulate motor behavior, probably by interfering with dopaminergic, serotonergic, and cholinergic neurotransmission in the striatum; (ii) Subchronic NO synthesis inhibition induces plastic changes in NO-producing neurons in brain areas related to motor control and causes cross-tolerance to the cataleptic effect of haloperidol, raising the possibility that such treatments could decrease motor side effects associated with antipsychotic medications. Finally, recent studies using experimental Parkinson's disease models suggest an interaction between NO system and neurodegenerative processes in the nigrostriatal pathway. It provides evidence of a protective role of NO. Together, our results indicate that NO may be a key participant on physiological and pathophysiological processes in the nigrostriatal system.

Molecular changes in nNOS protein expression within the ventrolateral medulla following transient focal ischemia affect cardiovascular functions

The majority of human strokes involve an occlusion of the middle cerebral artery and subsequent damage to the brain tissues it perfuses. We have previously reported that reflex cardiovascular changes during a static muscle contraction are attenuated following transient middle cerebral artery occlusion (MCAO) and reperfusion [A. Ally, S.M. Nauli, T.J. Maher, Cardiovascular responses and neurotransmission in the ventrolateral medulla during skeletal muscle contraction following transient middle cerebral artery occlusion and reperfusion, Brain Res. 952 (2002) 176-187]. We hypothesized that the attenuation is a result of altered expression of neuronal nitric oxide synthase (nNOS) within the rostral (RVLM) and caudal ventrolateral medulla (CVLM). In this study, we have compared cardiovascular responses and nNOS protein expression within the four quadrants, i.e., left and right sides of both RVLM and CVLM in sham-operated rats (n = 10) and in rats with a temporary 90-min left-sided MCAO followed by 24 h reperfusion (n = 10). Increases in mean arterial pressure during a static muscle contraction were significantly attenuated in MCAO rats when compared to sham rats. The transient ischemia reduced nNOS expression within the ipsilateral RVLM quadrant compared to the contralateral RVLM or RVLM quadrants of control rats. In contrast, compared to sham rats and the right CVLM quadrant of MCAO rats, nNOS expression was significantly augmented in the ipsilateral CVLM in left-sided MCAO rats. These data suggest that the attenuation of cardiovascular responses during static muscle contraction in MCAO rats is partly due to a reduction in nNOS expression within the ipsilateral RVLM and an overexpression of nNOS abundance within the ipsilateral CVLM. Results demonstrate that nNOS expression within the medulla plays a significant role in mediating cardiovascular responses during static exercise in intact and pathophysiological conditions.

Involvement of the NMDA receptor, NO-cyclic GMP and nuclear factor K-beta in an animal model of repeated trauma

Post-traumatic stress disorder (PTSD) may be associated with shrinkage of the hippocampus, with glutamate release causally related to these events. Recent animal studies strongly implicate activation of the nitric oxide (NO)-cascade in anxiety and stress. Using an animal model of repeated trauma, the effect of stress was investigated on the hippocampal NO-cGMP signalling pathway, specifically the release of nitrogen oxides (NOx) and its modulation by NMDA receptor-, NO-, cGMP- and nuclear factor K-beta (NFK-beta)-selective drugs. Immediately after stress, rats received the glutamate NMDA receptor antagonist, memantine (MEM; 5 mg/kg i.p./d), the NO synthase inhibitor, 7-nitroindazole sodium salt (7-NINA; 20 mg/kg i.p./d), the cGMP-specific PDE inhibitor, sildenafil (SIL; 10 mg/kg i.p./d) or the NFkappa-beta antagonist, pyrollidine dithiocarbamate (PDTC; 70 mg/kg i.p./d), for 7 days. Stress significantly increased hippocampal NOx on day 7 post-stress, which was blocked by either 7-NINA or PDTC, while MEM was without effect. SIL, however, significantly augmented stress-induced NOx accumulation. Increased cGMP therefore acts as a protagonist in driving stress-related events, while both nNOS (neuronal NOS) and iNOS (inducible/immunological NOS) may represent a therapeutic target in preventing the effects of severe stress. The value of NMDA receptor antagonism, however, appears limited in this model.

Nitric oxide within periaqueductal gray modulates glutamatergic neurotransmission and cardiovascular responses during mechanical and thermal stimuli

We have previously reported that nitric oxide (NO) within the rostral ventrolateral medulla (RVLM) attenuates cardiovascular responses and extracellular concentrations of glutamate during thermal, but not during mechanical nociceptive stimulation (Ishide. T., Maher, T.J., Ally, A. 2003. Role of nitric oxide in the ventrolateral medulla on cardiovascular responses and glutamate neurotransmission during mechanical and thermal stimuli. Pharmacol. Res. 47, 59-68). In this study, we examined the role of nitric oxide within the dorsolateral periaqueductal gray matter (PAG), a higher center integrating nociceptive reflexes, on cardiovascular responses and glutamate release during both mechanical and thermal nociception using anesthetized Sprague-Dawley rats. Two types of stimuli were studied, both activating peripheral A(delta) and C fiber polymodal nociceptors. Noxious mechanical stimulus was given by applying a bilateral hindpaw pinch for 5 s. Mechanical stimulation of a hindlimb increased mean arterial pressure (MAP), heart rate (HR), and extracellular fluid glutamate within PAG by 20+/-3 mmHg, 37+/-6 bpm, and 1.7+/-0.3 ng/5 microl, respectively (n=10). Bilateral microdialysis of L-arginine (1.0 microM), a NO precursor, into the PAG significantly attenuated MAP, HR, and glutamate increases during a mechanical stimulation. Subsequent administration of N(G)-methyl-L-arginine (L-NMMA) (1.0 microM), a NO synthase inhibitor, into the PAG blocked the ability of NO within PAG to modulate the cardiovascular responses to mechanical stimulus. The noxious thermal stimulus was generated by immersing the metatarsus of a hindpaw in water-bath at a temperature of 52 degrees C for 5 s. Similar increases were observed following thermal stimulation: 35+/-5 mmHg, 40+/-6 bpm, and 1.14+/-0.4 ng/5 microl (n=10). L-Arginine attenuated both cardiovascular responses and glutamate increase during thermal nociception. These results demonstrate that NO within the dorsolateral PAG plays a role in modulating cardiovascular responses by altering glutamate concentrations during both thermal and mechanical nociception.

Expression of cGMP-dependent protein kinases (I and II) and neuronal nitric oxide synthase in the developing rat cerebellum

The expression of neuronal nitric oxide synthase (nNOS) and the cGMP-dependent protein kinases cGKI and cGKII in rat cerebellum was evaluated at different developmental stages by quantitative RT-PCR and Western blotting. mRNAs coding for these proteins were detected in the cerebella of rats aged 7, 14 and 21 days. Expression levels, nevertheless, varied significantly at each of these developmental stages. While nNOS and cGKI mRNA levels steadily increased during development, cGKII mRNA showed a different behaviour pattern, with similar levels observed on postnatal days 7 and 14 and increased levels noted on postnatal day 21. Moreover, protein expression profiles for nNOS and cGKI showed similar patterns to the mRNAs encoding these proteins. Our results reveal the developmental regulation of the expression of these proteins in the cerebellum, giving rise to higher levels as the cerebellum matures.

Role of nitric oxide in brain regions related to defensive reactions

Nitric oxide synthase (NOS) positive neurons are located in most brain areas related to defensive reactions, including the dorsolateral periaqueductal grey (dlPAG). NOS inhibitors injected into this structure induce anxiolytic-like responses whereas NO donors promote flight reactions. Intra-dlPAG administration of carboxy-PTIO, a NO scavenger, or ODQ, a soluble guanylate cyclase inhibitor, produced anxiolytic-like effects on rats exposed to the elevated plus-maze (EPM). A double-staining experiment using NADPHd histochemistry and c-Fos immunohistochemistry in rats exposed to a cat or to the EPM showed increased activation of NO producing neurons in the dlPAG, paraventricular and lateral nuclei of hypothalamus and dorsal raphe nucleus. Cat exposure also increased activation of NOS neurons in the medial amygdala, dorsal pre-mammillary nucleus and bed nucleus of stria terminalis. Local infusion into the dlPAG of a glutamate NMDA-receptor antagonist (AP7) or a benzodiazepine agonist (midazolam) completely prevented the flight reactions induced by intra-dlPAG administration of SIN-1, a NO donor. The responses were also inhibited by the 5-HT2A/C agonist DOI but not by a 5-HT1A agonist. These results suggest a modulatory role for NO on brain areas related to defensive reactions, probably by interacting with glutamate, serotonin and/or GABA-mediated neurotransmission.

Elements of the nitric oxide/cGMP pathway expressed in cerebellar granule cells: biochemical and functional characterisation

It is known that the nitric oxide (NO)/cGMP pathway affects neuronal development and the expression of the different proteins is developmentally dependent in several brain areas. However, so far there are no data on the expression of the proteins involved in this signalling system during the development of the cerebellar granule cell, one of the most widely used models of neuronal development. This study was accordingly designed to analyse the developmental regulation of neuronal nitric oxide synthase (nNOS), soluble guanylyl cyclase subunits (alpha1, alpha2 and beta1) and cGMP-dependent protein kinases (cGK I and cGK II) in cerebellar granule cells through real time-polymerase chain reaction (RT-PCR) and Western blotting. We were able to detect guanylyl cyclase subunits and cGK I and cGK II in cerebellar granule cells at every stage of development examined (cells freshly isolated from 7-day-old rat pups, and cells cultured for 7 days or 14 days). Expression levels, nevertheless, varied significantly at each stage. nNOS, alpha2 and beta1 and cGK II levels increased during granule cell development, while alpha1 and cGK I showed an opposite behaviour pattern; the levels of these latter proteins diminished as the cells matured. The functionality of this pathway was assessed by stimulating cells kept in culture for 7 days with DEA/NO or with N-methyl-D-aspartate (NMDA). Cells responded by increasing intracellular cGMP and activating cGMP-dependent protein kinase activity, which effectively phosphorylated two well-known substrates of this activity, the vasodilator stimulated phosphoprotein (VASP) and the cAMP response element binding protein (CREB). In summary, through both functional and biochemical tests, this is the first demonstration of a complete NO/cGMP signalling transduction pathway in cerebellar granule cells. Our results also indicate the developmental regulation of the proteins in this system.

Elevation of intracellular cAMP evokes activity-dependent release of adenosine in cultured rat forebrain neurons

Adenosine is an important regulator of neuronal excitability. Zaprinast is a cyclic nucleotide phosphodiesterase inhibitor, and has been shown in the hippocampal slice to suppress excitation. This action can be blocked by an adenosine receptor antagonist, and therefore is presumably due to adenosine release stimulated by exposure to zaprinast. To explore the mechanism of this phenomenon further, we examined the effect of zaprinast on adenosine release itself in cultured rat forebrain neurons. Zaprinast significantly stimulated extracellular adenosine accumulation. The effect of zaprinast on adenosine appeared to be mediated by increasing intracellular cyclic adenosine monophosphate (cAMP) and activation of protein kinase A (PKA): (i) zaprinast stimulated intracellular cAMP accumulation; (ii) a cAMP antagonist (Rp-8-Br-cAMP) significantly reduced the zaprinast effect on adenosine; (iii) an inhibitor of phosphodiesterase (PDE)1 (vinpocetine) and an activator of adenylate cyclase (forskolin) mimicked the effect of zaprinast on adenosine. We also found that zaprinast had no effect on adenosine in astrocyte cultures, and tetrodotoxin completely blocked zaprinast-evoked adenosine accumulation in neuronal cultures, suggesting that neuronal activity was likely to be involved. Consistent with a dependence on neuronal activity, NMDA receptor antagonists (MK-801 and D-APV) and removal of extracellular glutamate by glutamate-pyruvate transaminase blocked the effect of zaprinast. In addition, zaprinast was shown to stimulate glutamate release. Thus, our data suggest that zaprinast-evoked adenosine accumulation is likely to be mediated by stimulation of glutamate release by a cAMP- and PKA-dependent mechanism, most likely by inhibition of PDE1 in neurons. Furthermore, regulation of cAMP, either by inhibiting cAMP-PDE activity or by stimulating adenylate cyclase activity, may play an important role in modulating neuronal excitability. These data suggest the existence of a homeostatic negative feedback loop in which increases in neuronal activity are damped by release of adenosine following activation of glutamate receptors.

Catalepsy induced by intra-striatal administration of nitric oxide synthase inhibitors in rats

Systemic administration of nitric oxide synthase (NOS) inhibitors induces catalepsy in a dose-dependent manner in male Albino-Swiss mice. The objective of the present work was to investigate if similar effects occur in rats and if these effects are centrally mediated. The results showed that systemic administration of N(G)-nitro-L-arginine (L-NOARG, 40-160 mg/kg, i.p.), a non-selective NOS inhibitor, induced catalepsy in rats. Similar effects were found after intracerebroventricular (i.c.v.) injection of L-NOARG (50-200 nmol) or N(G)-nitro-L-arginine methylester (L-NAME, 100-200 nmol). The dose-response curve of the former compound, however, had an inverted U shape. The effect of L-NOARG (100 nmol, i.c.v.) was completely prevented by pre-treatment with L-arginine (300 nmol, i.c.v.) but not by D-arginine (300 nmol, i.c.v.). Intra-striatal injection of N(G)-monomethyl-L-arginine (L-NMMA, 100 nmol), 7-nitroindazole (7-NIO, 100 nmol), L-NOARG (25-100 nmol) or L-NAME (50-200 nmol) also induced catalepsy. Similar to i.c.v. administration, the latter two compounds produced bell-shaped dose-response curves. The cataleptic effect of intra-striatal administration of L-NAME (100 nmol) was reversed by local treatment with L-arginine (100 nmol). These results suggest that interference with the striatal formation of nitric oxide may induce significant motor effects in rats.

Characteristics of GABA release modified by glutamate receptors in mouse hippocampal slices

The major part of hippocampal innervation is glutamatergic, regulated by inhibitory GABA-releasing interneurons. The modulation of [(3)H]GABA release by ionotropic and metabotropic glutamate receptors and by nitric oxide was here characterized in superfused mouse hippocampal slices. The ionotropic glutamate receptor agonists kainate, N-methyl-D-aspartate and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate potentiated the basal GABA release. These effects were blocked by their respective antagonists 6-nitro-7-cyanoquinoxaline-2,3-dione (CNQX), dizocilpine and 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo(f)quinoxaline-7-sulfonamide (NBQX), indicating receptor-mediated mechanisms. The NO-generating compounds S-nitroso-N-acetylpenicillamine (SNAP), sodiumnitroprusside and hydroxylamine enhanced the basal GABA release. Particularly the sodiumnitroprusside-evoked release was attenuated by the NO synthase inhibitor N(G)-nitro-L-arginine (L-NNA) and the inhibitor of soluble guanylyl cyclase 1H-(1,2,4)oxadiazolo(4,3a)quinoxalin-1-one (ODQ), indicating the involvement of the NO/cGMP pathway. This inference is corroborated by the enhancing effect of zaprinast, a phosphodiesterase inhibitor, which is known to increase cGMP levels. The K(+)-stimulated hippocampal GABA release was reduced by the groups I and III agonists of metabotropic glutamate receptors (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylate (t-ACPD) and L-(+)-2-amino-4-phosphonobutyrate (L-AP4), which effects were abolished by their respective antagonists (RS)-1-aminoindan-1,5-dicarboxylate (AIDA) and (RS)-2-cyclopropyl-4-phosphonophenylglycine (CPPG), again indicating modification by receptor-mediated mechanisms.

Stimulatory effects of nitric oxide donors on histamine release in isolated rat gastric mucosal cells

We previously reported stimulatory effects of endogenous and exogenous nitric oxide (NO) on gastric acid secretion. In the present study, we investigated effects of NO donors on release of histamine, which is related to acid secretion, in isolated rat gastric mucosal cells. NO donors such as (+/-)-(E)-4-methyl-2-[(E)-hydroxyimino]-5-nitro-6-methoxy-3-hexanamide (NOR 1) and sodium nitroprusside significantly augmented the histamine release. It was inhibited by 2-(4-carboxyphenyl)-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-amide (carboxy-PTIO), a NO scavenger, and 6-(phenylamino)-5,8-quinolinedione (LY83583), a soluble guanylate cyclase inhibitor. Dibutyryl cyclic GMP also stimulated histamine release. These results suggest that NO donors act on cyclic GMP pathway in isolated gastric mucosal cells, resulting in facilitation of histamine release. NO may stimulate gastric acid secretion through histamine release from the histamine-containing cells, possibly enterochromaffin-like cells.

Hippocampal synaptic plasticity in mice carrying the rd mutation in the gene encoding cGMP phosphodiesterase type 6 (PDE6)

Cyclic GMP (cGMP) has been implicated in the modulation of long-term potentiation (LTP) and depression (LTD) in the hippocampus. Transcripts for subunits of several types of cGMP specific phosphodiesterase are found in the mammalian brain but their relative role in hippocampal function is unclear. The retinal degeneration (rd) mutation in the gene encoding the PDE6B subunit causes a loss of function in PDE6 enzyme and in adult mice homozygous to the mutation it causes blindness. We have used this natural mutation, and the cGMP phosphodiesterase inhibitor zaprinast, in wild-type and rd/rd mouse littermates to investigate whether PDE5 and/or PDE6 regulates excitatory synaptic transmission in the hippocampus. Mice were genotyped using two independent PCR methods. Glutamate-mediated synaptic transmission in the CA1 region or dentate gyrus was unaffected in hippocampal brain slices from mice carrying the rd mutation. Similarly the facilitation of synaptic events by paired-pulse stimuli, and LTP induced by a theta-burst (10 bursts of four events at 100 Hz with a 200-ms inter-burst interval) were normal in rd/rd mice. Inhibition of cGMP-specific PDE activity by zaprinast (10 microM, an inhibitor of PDE5 and PDE6) induced a slowly developing and sustained depression of field synaptic potentials that was quantitatively similar in both wild-type and rd/rd mice. Thus in the CA1 region synaptic plasticity is likely to be regulated by the PDE5 rather than the PDE6 isoform.

Role of nitric oxide in the ventrolateral medulla on cardiovascular responses and glutamate neurotransmission during mechanical and thermal stimuli

We have previously reported that alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-receptor blockade within the rostral ventrolateral medulla (RVLM) attenuates cardiovascular responses and extracellular concentrations of glutamate during mechanical, but not during thermal stimulation [Pharmacol. Res. 43 (2001) 47]. In this study, we examined the role of nitric oxide (NO) within the RVLM on cardiovascular responses and glutamate release during both mechanical and thermal nociception using anesthetized Sprague-Dawley rats. Two types of stimuli were studied, both activating peripheral Adelta and C fiber polymodal nociceptors. Noxious mechanical stimuli were given by applying a bilateral hindpaw pinch for 5s. The noxious thermal stimuli were generated by immersing the metatarsus of both hindpaws in a water bath at a temperature of 52 degrees C for 5s. Mechanical stimulation of both hindlimb extremities increased mean arterial pressure (MAP), heart rate (HR), and extracellular fluid glutamate by 14+/-2 mmHg, 35+/-5 bpm, and 1.4+/-0.3 ng/5 microl, respectively (n=8). Similar responses were observed following thermal stimulation: 40+/-4 mmHg, 44+/-6 bpm, and 0.97+/-0.2 ng/5 microl (n=8). Bilateral microdialysis of L-arginine (1.0 microM), a nitric oxide precursor, into the RVLM had no effects on MAP, HR, and glutamate increases during mechanical stimulation. However, L-arginine attenuated these responses during thermal nociception. Subsequent administration of L-NMMA (1.0 microM), a NOS inhibitor, reversed the attenuations. These results show that nitric oxide most likely plays a role in modulating cardiovascular responses by altering glutamate concentrations within the RVLM during thermal but not mechanical nociception. Overall, the present study delineates the differential central integrative mechanisms that regulate processing of sensory impulses arising from peripheral stimulation.

Nitric oxide formation during cortical spreading depression is critical for rapid subsequent recovery of ionic homeostasis

Cortical spreading depression (CSD) is a temporary disruption of local ionic homeostasis that propagates slowly across the cerebral cortex. Cortical spreading depression promotes lesion progression in experimental stroke, and may contribute to the initiation of migraine attacks. The purpose of this study was to investigate the roles of the marked increase of nitric oxide (NO) formation that occurs with CSD. Microdialysis electrodes were implanted in the cortex of anesthetized rats to perform the following operations within the same region: (1) elicitation of CSD by perfusion of high K+ medium; (2) recording of CSD elicitation; (3) application of the NO synthase inhibitor, NG-nitro-l-arginine methyl ester (l-NAME); and (4) recording of dialysate pH changes. The primary effect of l-NAME (0.3 to 3.0 mmol/L in the perfusion medium) was a marked widening of individual CSD wave, resulting essentially from a delayed initiation of the repolarization phase. This change was due to NO synthase inhibition because it was not observed with the inactive isomer d-NAME, and was reversed by l-arginine. This effect did not appear to be linked to the suppression of a sustained, NO-mediated vascular change associated with the superposition of NO synthase inhibition on high levels of extracellular K+. The delayed initiation of repolarization with local NO synthase inhibition may reflect the suppression of NO-mediated negative feedback mechanisms acting on neuronal or glial processes involved in CSD genesis. However, the possible abrogation of a very brief, NO-mediated vascular change associated with the early phase of CSD cannot be ruled out.

Localization of cGMP-dependent protein kinase type II in rat brain

In brain, signaling pathways initiated by atrial natriuretic peptide, or transmitters which stimulate nitric oxide synthesis, increase cGMP as their second messenger. One important class of target molecules for cGMP is cGMP-dependent protein kinases, and in the present study, biochemical and immunocytochemical analyses demonstrate the widespread distribution of type II cGMP-dependent protein kinase in rat brain, from the cerebral cortex to the brainstem and cerebellum. Also, colocalization of cGMP-dependent protein kinase type II with its activator, cGMP, was found in several brain regions examined after in vitro stimulation of brain slices with sodium nitroprusside. In western blots, cGMP-dependent protein kinase type II was observed in all brain regions examined, although cerebellar cortex and pituitary contained comparatively less of the kinase. Immunocytochemistry revealed cGMP-dependent protein kinase type II in certain neurons, and occasionally in putative oligodendrocytes and astrocytes, however, its most striking and predominant localization was in neuropil. Electron microscopy examination of neuropil in the medial habenula showed localization of the kinase in both axon terminals and dendrites. As a membrane-associated protein, cGMP-dependent protein kinase type II often appeared to be transported to cell processes to a greater extent than being retained in the cell body. Thus, immunocytochemical labeling of cGMP-dependent protein kinase type II often did not coincide with the localization of kinase mRNA previously observed by others using in situ hybridization. We conclude that in contrast to cGMP-dependent protein kinase type I, which has a very restricted localization to cerebellar Purkinje cells and a few other sites, cGMP-dependent protein kinase type II is a very ubiquitous brain protein kinase and thus a more likely candidate for relaying myriad cGMP effects in brain requiring protein phosphorylation.

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

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

Taurine release in the developing and adult mouse hippocampus: involvement of cyclic guanosine monophosphate

The inhibitory neuromodulator taurine is involved in osmoregulation and cell volume adjustments in the central nervous system. In addition, taurine protects neural cells from excitotoxicity and prevents harmful metabolic events evoked by cell-damaging conditions. The release of taurine in nervous cell preparations is greatly enhanced by glutamate receptor agonists and various cell-damaging conditions. NO-generating compounds also increase taurine release in the mouse hippocampus. The further involvement of the NO/cGMP pathway and protein kinases in preloaded [3H]taurine release from hippocampal slices from adult (3-month-old) and developing (7-day-old) mice in normoxia and in ischemia was now studied using a superfusion system. The release was enhanced by 8-Br-cGMP and the phosphodiesterase inhibitor 2-(2-propyloxyphenyl)-8-azapurin-6-one (zaprinast), particularly in the immature hippocampus, indicating that increased cGMP levels induce taurine release. The release was also increased by the inhibitor of soluble guanylyl cyclase, 1H-(1,2,4)oxadiazolo-(4,3a)quinoxalin-1-one (ODQ) and the protein kinase C activator 4beta-phorbol 12-myristate 13-acetate (PMA), but only in the adult hippocampus. The ischemia-induced release was also enhanced by increased cGMP levels in both adult and developing mice, whereas protein kinase inhibitors had no effects in any conditions. The results demonstrate that cGMP is able to modulate hippocampal taurine release in both adult and developing mice, the rise in cGMP levels evoking taurine release in normoxia and in ischemia. This could be part of the neuroprotective properties of taurine, being thus important particularly in cell-damaging conditions and in preventing excitotoxicity.

Effects of excitatory amino acids and nitric oxide on flight behavior elicited from the dorsolateral periaqueductal gray

Microinjection of excitatory amino acids (EAA) into the dorsolateral periaqueductal gray (dlPAG) induces flight reactions while EAA antagonists show anxiolytic effects. Part of the effects mediated by NMDA receptors may involve an increase in nitric oxide (NO) production. We showed that nitric oxide synthase (NOS) inhibitors injected into the dlPAG induced anxiolytic effects. Conversely, SIN-1, a NO donor, produced orientated flight reactions that resemble stimulation of the medial hypothalamus. This compound also produced extensive Fos-like immunoreactivity in this region and in other areas related to defensive reactions such as the medial amygdala and cingulate cortex. Since part of the effects of NO involves increases in guanylate cyclase levels, we found that intra-dlPAG injection of 8-Br-cGMP induced a brief flight reaction followed by increased locomotion. In another experiment, we showed that single or repeated restraint stress produced an increased expression of neuronal NOS in the dlPAG and other areas related to defense, as measured by in situ hybridization, diaphorase histochemistry and immunocytochemistry. Together, these data suggest that NO may participate in the modulation of defensive responses in the dlPAG.

Altered content and modulation of soluble guanylate cyclase in the cerebellum of rats with portacaval anastomosis

It is shown that the glutamate-NO-cGMP pathway is impaired in cerebellum of rats with portacaval anastomosis in vivo as assessed by in vivo brain microdialysis in freely moving rats. NMDA-induced increase in extracellular cGMP in the cerebellum was significantly reduced (by 27%) in rats with portacaval anastomosis. Activation of soluble guanylate cyclase by the NO-generating agent S-nitroso-N-acetyl-penicillamine and by the NO-independent activator YC-1 was also significantly reduced (by 35-40%), indicating that portacaval anastomosis leads to remarkable alterations in the modulation of guanylate cyclase in cerebellum. Moreover, the content of soluble guanylate cyclase was increased ca. two-fold in the cerebellum of rats with portacaval anastomosis. Activation of soluble guanylate cyclase by NO was higher in lymphocytes isolated from rats with portacaval anastomosis (3.3-fold) than in lymphocytes from control rats (2.1-fold). The results reported show that the content and modulation of soluble guanylate cyclase are altered in brain of rats with hepatic failure, resulting in altered function of the glutamate-NO-cGMP pathway in the rat in vivo. This may lead to alterations in cerebral processes such as intercellular communication, circadian rhythms, including the sleep-waking cycle, long-term potentiation, and some forms of learning and memory.

Nitric oxide as modulator of neuronal function

The gas NO is a messenger that modulates neuronal function. The use of NO donors and NO synthase inhibitors as pharmacological tools revealed that this free radical is probably implicated in the regulation of excitability and firing, in long-term potentiation and long-term depression, as well as in memory processes. Moreover, NO modulates neurotransmitter release. In vivo and in vitro studies have shown that, in all brain structures investigated, endogenous NO modulates the release of several neurotransmitters, such as acetylcholine, catecholamines, excitatory and inhibitory amino acids, serotonin, histamine, and adenosine. In most cases, enhanced NO level in the tissue increases the release of neurotransmitters, although decreasing effects have also been observed. Cyclic 3'-5' guanosine monophosphate and glutamate mediate the modulation of transmitter release by NO. Recent observations suggest that the release of some transmitters is dually influenced by NO. Thus, besides modulation by presynaptically located auto- and heteroreceptors, NO released from nitrergic neurons seems to play a universal role in modulating the release of transmitters in the brain.

Presynaptic N-methyl-D-aspartate receptor activation inhibits neurotransmitter release through nitric oxide formation in rat hippocampal nerve terminals

In brain synapses, nitric oxide synthase activation is coupled to N-methyl-D-aspartate-mediated calcium entry at postsynaptic densities through regulatory protein complexes, however a presynaptic equivalent to this signaling mechanism has not yet been identified. Novel evidence indicates that N-methyl-D-aspartate glutamate receptors may play a presynaptic role in synaptic plasticity. Thus, we investigated whether ionotropic glutamate receptor activation in isolated nerve terminals regulates neurotransmitter release, through nitric oxide formation. N-Methyl-D-aspartate dose-dependently inhibited the release of glutamate evoked by 4-aminopyridine (IC(50)=155 microM), and this effect was reversed by the N-methyl-D-aspartate receptor antagonist D-(-)-2-amino-5-phosphopentanoic acid and by the nitric oxide synthase inhibitor, L-nitroarginine, in synaptosomes isolated from whole hippocampus, CA3 and CA1 areas, but not from the dentate gyrus. In contrast, the 4-aminopyridine-evoked release of glutamate was reduced by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid or kainate by a nitric oxide-independent mechanism, since it was not blocked by L-nitroarginine, and N-methyl-D-aspartate, but not alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid or kainate, significantly increased cGMP formation. Presynaptic N-methyl-D-aspartate receptors are probably involved since removing extracellular nitric oxide with the scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide did not block the depression of glutamate release by N-methyl-D-aspartate. The mechanism underlying this depression involves the inhibition of synaptic vesicle exocytosis since N-methyl-D-aspartate/nitric oxide inhibited the release of [3H]glutamate and [14C]GABA evoked by hypertonic sucrose. The results also suggest that presynaptic N-methyl-D-aspartate receptors may function as auto- and heteroreceptors.

Ethanol neurobehavioral teratogenesis and the role of the hippocampal glutamate-N-methyl-D-aspartate receptor-nitric oxide synthase system

The purpose of this review is to evaluate a proposed mechanism for ethanol neurobehavioral teratogenesis in the hippocampus, involving suppression of the glutamate-N-methyl-D-aspartate (NMDA) receptor-nitric oxide synthase (NOS) system. It is postulated that suppression of this signal transduction system in the fetus by chronic maternal consumption of ethanol plays a key role in hippocampal dysmorphology and dysfunction in postnatal life. This mechanism is evaluated critically based on the current literature and our research findings. In view of the apparent time course for loss of CA1 pyramidal cells in the hippocampus produced by chronic prenatal ethanol exposure that manifests in early postnatal life, it is proposed that therapeutic intervention, which targets the glutamate-NMDA receptor-NOS system, may prevent or lessen the magnitude of postnatal hippocampal dysfunction.

Glutamate neurotransmission and nitric oxide interaction within the ventrolateral medulla during cardiovascular responses to muscle contraction

We previously reported that nitric oxide, within the RVLM and CVLM, plays an opposing role in modulating cardiovascular responses during static muscle contraction [B.J. Freda, R.S. Gaitonde, R. Lillaney, A. Ally, Cardiovascular responses to muscle contraction following microdialysis of nitric oxide precursor into ventrolateral medulla, Brain Res. 828 (1999) 60-67]. In this study, we determined whether the effects of administering L-arginine, a precursor for the synthesis of nitric oxide, and N(G)-monomethyl-L-arginine (L-NMMA), a nitric oxide synthase inhibitor, into the rostral (RVLM) and caudal (CVLM) ventrolateral medulla on cardiovascular responses elicited during static muscle contraction were mediated via an alteration of localized glutamate concentrations using microdialysis techniques. In experiments within the RVLM (n=8), muscle contraction increased MAP and HR by 21+/-2 mmHg and 22+/-3 bpm, respectively. Glutamate increased from 1.1+/-0.4 to 4.4+/- 0.6 ng/5 microl measured from bilateral RVLM areas. Microdialysis of L-arginine (1.0 microM) for 30 min attenuated the contraction-evoked increases in MAP, HR, and glutamate levels. After subsequent microdialysis of L-NMMA (1.0 microM) into the RVLM, contraction augmented the pressor and tachycardic responses and glutamate release. In experiments within CVLM (n=8), muscle contraction increased MAP and HR by 22+/-3 mmHg and 20+/-2 bpm, respectively. Glutamate increased from 0.8+/-0. 4 to 3.6+/-0.6 ng/5 microl measured from the CVLM. L-Arginine augmented the cardiovascular responses and glutamate release and L-NMMA attenuated all the effects. Results suggest that nitric oxide within the RVLM and CVLM plays opposing roles in modulating cardiovascular responses during static exercise via decreasing and increasing, respectively, extracellular glutamate levels.

NO-mediated cGMP synthesis in cholinergic neurons in the rat forebrain: effects of lesioning dopaminergic or serotonergic pathways on nNOS and cGMP synthesis

Nitric oxide synthase (NOS) activity and NO-mediated cGMP synthesis were studied in the rat forebrain of control animals and animals which had received a unilateral lesioning of dopaminergic or serotonergic pathways. Lesioning of the dopaminergic innervation using 6-hydroxydopamine resulted in a 50% decrease in NOS activity in the lesioned frontal cortex and caudate putamen. Lesioning of the serotonergic innervation using 5,7-dihydroxytryptamine had no effect on NOS activity. NO-mediated cGMP accumulation in rat forebrain slices was not affected by 6-hydroxydopamine or 5,7, -dihydroxytryptamine lesioning. Using cGMP immunocytochemistry, it was demonstrated that NO-mediated cGMP synthesis was absent from dopaminergic, serotonergic, GABA-ergic and neuronal NOS-containing nerve fibres. A minor colocalization of cGMP immunoreactivity was found in parvalbumin-containing fibres in the cortex. Extensive colocalization between cGMP immunoreactivity and the acetylcholine transporter was found in all cortical areas and in the caudate putamen. There was no effect of the lesions on this colocalization. These results demonstrate NO-mediated cGMP accumulation in cholinergic fibres in the forebrain of the rat and suggest an anterograde signalling function of NO in cholinergic neuronal systems in the cortex and caudate putamen of the rat.

Chemically induced, activity-independent LTD elicited by simultaneous activation of PKG and inhibition of PKA

Although it is widely agreed that cyclic AMP is necessary for the full expression of long-term potentiation of synaptic strength, it is unclear whether cyclic AMP or cyclic AMP-dependent protein kinase (PKA) play roles in the induction of long-term depression (LTD). We show here that two PKA inhibitors, H-89 (10 microM) and KT5720 (1 microM), are unable to block induction of LTD at Schaffer collateral-CA1 synapses in hippocampal slices in vitro. Rather, H-89 enhanced the magnitude of LTD induced by submaximal low-frequency stimulation. Raising [cGMP] with zaprinast (20 microM), a selective type V phosphodiesterase inhibitor, reversibly depressed synaptic potentials. However, coapplication of H-89 plus zaprinast converted this to a robust LTD that depended critically on activation of cyclic GMP-dependent protein kinase (PKG). Chemically induced LTD is activity-independent because it could be induced without stimulation and in tetrodotoxin (0.5 microM). Additionally, chemical LTD did not require activation of N-methyl-D-aspartate or GABA receptors and could be reversed by LTP. Stimulus-induced LTD occluded chemical LTD, suggesting a common expression mechanism. In contrast to bath application, postsynaptic infusion of H-89 into CA1 pyramidal neurons did not enhance LTD, suggesting a presynaptic site of action. Further evidence for a presynaptic locus was supplied by experiments where H-89 applied postsynaptically along with bath application of zaprinast was unable to produce chemical LTD. Thus simultaneous presynaptic generation of cyclic GMP and inhibition of PKA is sufficient to induce LTD of synaptic transmission at Schaffer collateral-CA1 synapses.

Effects of S-nitrosoglutathione on acute vasoconstriction and glutamate release after subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Subarachnoid hemorrhage (SAH) causes acute vasoconstriction that contributes to ischemic brain injury shortly after the initial bleed. It has been theorized that decreased availability of nitric oxide (NO) may contribute to acute vasoconstriction. Therefore we examined the effect of the NO donor N-nitroso glutathione (GSNO) on acute vasoconstriction and early ischemic glutamate release after experimental SAH.

METHODS

SAH was induced by the endovascular suture method in anesthetized rats. GSNO (1 micromol/L/kg, n=31) or saline (n=21) was injected 5 minutes after SAH. Sham-operated rats received GSNO (1 micromol/L/kg, n=5) 5 minutes after sham surgery. Arterial and intracranial pressures, cerebral blood flow (CBF), and extracellular glutamate release were measured serially for 60 minutes after SAH. SAH size was determined, and vascular measurements were made histologically.

RESULTS

GSNO had no effect on resting blood pressure, intracranial pressure, cerebral perfusion pressure, or CBF in sham-operated animals. However, administration of GSNO after SAH was associated with significantly increased CBF (161.6+/-26.6% versus saline 37.1+/-5.5%, 60 minutes after SAH, P<0.05), increased blood vessel diameter (internal carotid artery [ICA] 285.0+/-16.5 microm versus saline 149.2+/-14.1 microm, P<0.01), decreased vessel wall thickness (ICA12.9+/-0.7 microm versus saline 25.1+/-1.6 microm, P<0.01), and decreased extracellular glutamate levels (3315.6+/-1048.3% versus saline469. 7+/-134.3%, P<0.05). Blood pressure decreased transiently, whereas intracranial pressure, cerebral perfusion pressure, and SAH size were not affected.

CONCLUSIONS

These results suggest that GSNO can reverse acute vasoconstriction and prevent ischemic brain injury after SAH. This further implies that acute vasoconstriction contributes significantly to ischemic brain injury after SAH and is mediated in part by decreased availability of NO.

Induction of hippocampal LTD requires nitric-oxide-stimulated PKG activity and Ca2+ release from cyclic ADP-ribose-sensitive stores

Long-term depression (LTD) of synaptic transmission can be induced by several mechanisms, one thought to involve Ca2+-dependent activation of postsynaptic nitric oxide (NO) synthase and subsequent diffusion of NO to the presynaptic terminal. We used the stable NO donor S-nitroso-N-acetylpenicillamine (SNAP) to study the NO-dependent form of LTD at Schaffer collateral-CA1 synapses in vitro. SNAP (100 microM) enhanced the induction of LTD via a cascade that was blocked by the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonopentanoic acid (50 microM), NO guanylyl cyclase inhibitor 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (10 microM), and the PKG inhibitor KT5823 (1 microM). We further show that LTD induced by low-frequency stimulation in the absence of SNAP also is blocked by KT5823 or Rp-8-(4-chlorophenylthio)-guanosine 3',5'-cyclic monophosphorothioate (10 microM), cyclic guanosine 3',5' monophosphate-dependent protein kinase (PKG) inhibitors with different mechanisms of action. Furthermore SNAP-facilitated LTD was blocked when release from intracellular calcium stores was inhibited by ryanodine (10 microM). Finally, two cell-permeant antagonists of the cyclic ADP-ribose binding site on ryanodine receptors also were able to block the induction of LTD. These results support a cascade for induction of homosynaptic, NO-dependent LTD involving activation of guanylyl cyclase, production of guanosine 3',5' cyclic monophosphate and subsequent PKG activation. This process has an additional requirement for release of Ca2+ from ryanodine-sensitive stores, perhaps dependent on the second-messenger cyclic ADP ribose.

Phosphorylation of tyrosine hydroxylase by cGMP-dependent protein kinase in intact bovine chromaffin cells

The phosphorylation of the enzyme tyrosine hydroxylase by the cGMP pathway was investigated in chromaffin cells from the bovine adrenal medulla. The nitric oxide donor, sodium nitroprusside, and the natriuretic peptide, C-type natriuretic peptide, which are able to increase cGMP levels and cGMP-dependent protein kinase activity, produced significant increases in the phosphorylation level of tyrosine hydroxylase in a time- and concentration-dependent manner. The pretreatment of the cells with the soluble guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one blocked the effect of sodium nitroprusside. This result indicates that cGMP production by this enzyme mediated this effect. Experiments performed with a cGMP-dependent protein kinase inhibitor, the Rp-isomer of 8-(4-chlorophenylthio)-cyclic guanosine monophosphorothioate, which blocked the effects of both sodium nitroprusside and C-type natriuretic peptide, demonstrated that the phosphorylation increases evoked by both compounds were mediated by the activation of cGMP-dependent protein kinase. In cells incubated with the adenylyl cyclase activator, forskolin, an increase in the phosphorylation level of the tyrosine hydroxylase was also found. When cells were treated simultaneously with forskolin and sodium nitroprusside or C-type natriuretic peptide, an additive effect on tyrosine hydroxylase phosphorylation was not observed. This suggests that cAMP- and cGMP-dependent protein kinases may phosphorylate the same amino acid residues in the enzyme. Western blot analysis of soluble extracts from chromaffin cells detected specific immunoreactivity for two different commercial antibodies raised against cGMP-dependent protein kinase (both Ialpha and Ibeta isoforms). Electrophoretic mobility correlates with that of purified PKG Ialpha. Because the phosphorylation of the tyrosine hydroxylase correlates with increases in its enzymatic activity and thus with augmentation in the cell capacity to synthesize catecholamines, our results indicate that a cGMP-based second messenger pathway participates in catecholamine biosynthesis regulation in chromaffin cells, a mechanism which may be widespread in other catecholamine-synthesizing cells.

Cardiovascular responses to muscle contraction following microdialysis of nitric oxide precursor into ventrolateral medulla

We determined the effects of administering L-arginine, a precursor for the synthesis of nitric oxide, and L-NMMA (NG-monomethyl-L-arginine), a nitric oxide synthase blocker, into the rostral (RVLM) and caudal (CVLM) ventrolateral medulla on cardiovascular responses elicited during static contraction of the triceps surae muscle. Two microdialysis probes were inserted bilaterally into the RVLM or CVLM of anesthetized Sprague-Dawley rats using stereotaxic guides. For RVLM experiments, static muscle contraction evoked by stimulation of the tibial nerve increased mean arterial pressure (MAP) and heart rate (HR) by 29+/-3 mmHg and 44+/-7 bpm, respectively (n=8). Microdialysis of L-arginine (1.0 microM) for 30 min attenuated the contraction-evoked increases in MAP and HR. After discontinuing L-arginine, L-NMMA (1.0 microM) was microdialyzed into the RVLM for an additional 30 min followed by a muscle contraction. This contraction augmented the pressor response (37+/-4 mmHg) and HR (61+/-11 bpm) with respect to control values. For CVLM experiments, muscle contraction increased MAP and HR by 23+/-3 mmHg and 25+/-5 bpm, respectively (n=9). Microdialysis of L-arginine (1.0 microM) for 30 min potentiated the contraction-evoked increases in MAP and HR. Subsequent administration of L-NMMA (1.0 microM) into the CVLM for an additional 30 min blocked the augmented MAP and HR responses. Developed tensions did not alter during contractions throughout both RVLM and CVLM protocols. These results suggest that nitric oxide, within the RVLM and CVLM, plays an opposing role in modulating cardiovascular responses during static muscle contraction.

Both protein kinase G dependent and independent mechanisms are involved in the modulation of glutamate release by nitric oxide in rat hippocampal nerve terminals

We compared the effects of sodium nitroprusside (SNP), and of 8-bromo guanosine 3',5'-cyclic monophosphate (8-BrcGMP), on the 4-aminopyridine (4-AP)-evoked Ca2+-dependent release of glutamate from hippocampal nerve terminals and further investigated the role of protein kinase G (PKG) in this mechanism. SNP and 8-BrcGMP dose-dependently inhibited glutamate release, however SNP concentrations ([SNP]) > 500 microM abolished the 4-AP evoked release, whereas 8-BrcGMP maximally inhibited the release by about 30%. The inhibition of glutamate release at low concentrations of SNP (< or = 5 microM) was of about 20%, and was reversed by Rp-8(4-chlorophenylthio)guanosine-3',5'-cyclic-monophosphorotioate ) (RpCPTcGMP, 50 nM), but the inhibition at higher concentrations (5 < SNP < or = 50 microM) was insensitive to the PKG inhibitor, but sensitive to [1 H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one] (ODQ), which partially prevented the inhibition. [SNP] > 50 microM strongly inhibited glutamate release, and this was not reversed by either inhibitor. Furthermore, [SNP] < or = 50 microM enhanced cGMP formation, and the observed effects were not related to either decreased Ca2+ entry or ATP/ADP levels. Our results indicate that NO/PKG is the signaling pathway underlying the inhibition of glutamate release at low concentrations of NO, and imply that other NO-dependent, but PKG-independent, mechanisms are activated and have complementary roles at higher NO concentrations.