Cellular origins of cyclic GMP responses to excitatory amino acid receptor agonists in rat cerebellum in vitro

Silva H, P. de Carvalho R, Ventura ALM, Calaza KC, Silveira MS, Kubrusly RCC, de Melo Reis RA. The Healthy and Diseased Retina Seen through Neuron-Glia Interactions

The retina is the sensory tissue responsible for the first stages of visual processing, with a conserved anatomy and functional architecture among vertebrates. To date, retinal eye diseases, such as diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, glaucoma, and others, affect nearly 170 million people worldwide, resulting in vision loss and blindness. To tackle retinal disorders, the developing retina has been explored as a versatile model to study intercellular signaling, as it presents a broad neurochemical repertoire that has been approached in the last decades in terms of signaling and diseases. Retina, dissociated and arranged as typical cultures, as mixed or neuron- and glia-enriched, and/or organized as neurospheres and/or as organoids, are valuable to understand both neuronal and glial compartments, which have contributed to revealing roles and mechanisms between transmitter systems as well as antioxidants, trophic factors, and extracellular matrix proteins. Overall, contributions in understanding neurogenesis, tissue development, differentiation, connectivity, plasticity, and cell death are widely described. A complete access to the genome of several vertebrates, as well as the recent transcriptome at the single cell level at different stages of development, also anticipates future advances in providing cues to target blinding diseases or retinal dysfunctions.

Single-Walled Carbon Nanotube-Based Biosensors for Detection of Bronchial Inflammation

Inflammation is a protective mechanism against invading pathogens and tissue damage. However, the inflammatory process is implicated in a wide range of diseases affecting all organs and body systems. Nitric oxide — a multifunctional signaling molecule that plays a critical role in systemic blood pressure homeostasis, prevention of platelet aggregation, antimicrobial resistance, immunoregulation, tumor suppression and as a neurotransmitter — is used as a surrogate marker for inflammation. However, the most commonly used Griess assay is an indirect and expensive method for the determination of nitric oxide concentration. Hence, single-walled carbon nanotube-based biosensors have been explored as real-time, sensitive, selective and safe methods to determine nitric oxide released during the inflammatory process. In this review, we explore current developments in single-walled carbon nanotube-based biosensors for the detection of nitric oxide in exhaled breath as a direct and noninvasive test for detection of bronchial inflammation.

Historical origins of the discovery of mammalian nitric oxide (nitrogen monoxide) production/physiology/pathophysiology

The award of the 1998 Nobel Prize in Physiology or Medicine to Robert F. Furchgott, Louis J. Ignarro, and Ferid Murad "for their discoveries concerning nitric oxide as a signaling molecule in the cardiovascular system" highlighted the discovery of NO in mammals. This breakthrough also coincided with the discoveries of the role of NO as a cytotoxic effector in the immune system and as an intercellular neurotransmitter in the nervous system. This brief overview describes the chronological development of this trilinear convergence in 1986-1988, including background chemistry and history of human/nitrogen oxide interactions in general.

N10 -carbonyl-substituted phenothiazines inhibiting lipid peroxidation and associated nitric oxide consumption powerfully protect brain tissue against oxidative stress

During some investigations into the mechanism of nitric oxide consumption by brain preparations, several potent inhibitors of this process were identified. Subsequent tests revealed the compounds act by inhibiting lipid peroxidation, a trigger for a form of regulated cell death known as ferroptosis. A quantitative structure–activity study together with XED (eXtended Electron Distributions) field analysis allowed a qualitative understanding of the structure–activity relationships. A representative compound N‐(3,5‐dimethyl‐4H‐1,2,4‐triazol‐4‐yl)‐10H‐phenothiazine‐10‐carboxamide (DT‐PTZ‐C) was able to inhibit completely oxidative damage brought about by two different procedures in organotypic hippocampal slice cultures, displaying a 30‐ to 100‐fold higher potency than the standard vitamin E analogue, Trolox or edaravone. The compounds are novel, small, drug‐like molecules of potential therapeutic use in neurodegenerative disorders and other conditions associated with oxidative stress.

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

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

Nitric oxide as a multimodal brain transmitter

One of the simplest molecules in existence, nitric oxide, burst into all areas of biology some 30 years ago when it was established as a major signalling molecule in the cardiovascular, nervous and immune systems. Most regions of the mammalian brain synthesise nitric oxide and it has many diverse roles both during development and in adulthood. Frequently, nitric oxide synthesis is coupled to the activation of NMDA receptors and its physiological effects are mediated by enzyme-linked receptors that generate cGMP. Generally, nitric oxide appears to operate in two main modes: first, in a near synapse-specific manner acting either retrogradely or anterogradely and, second, when multiple nearby sources are active simultaneously, as a volume transmitter enabling signalling to diverse targets irrespective of anatomical connectivity. The rapid diffusibility of nitric oxide and the efficient capture of fleeting, subnanomolar nitric oxide concentrations by its specialised receptors underlie these modes of operation.

Granular Layer Neurons Control Cerebellar Neurovascular Coupling Through an NMDA Receptor/NO-Dependent System

Neurovascular coupling (NVC) is the process whereby neuronal activity controls blood vessel diameter. In the cerebellum, the molecular layer is regarded as the main NVC determinant. However, the granular layer is a region with variable metabolic demand caused by large activity fluctuations that shows a prominent expression of NMDA receptors (NMDARs) and nitric oxide synthase (NOS) and is therefore much more suitable for effective NVC. Here, we show, in the granular layer of acute rat cerebellar slices, that capillary diameter changes rapidly after mossy fiber stimulation. Vasodilation required neuronal NMDARs and NOS stimulation and subsequent guanylyl cyclase activation that probably occurred in pericytes. Vasoconstriction required metabotropic glutamate receptors and CYP ω-hydroxylase, the enzyme regulating 20-hydroxyeicosatetraenoic acid production. Therefore, granular layer capillaries are controlled by the balance between vasodilating and vasoconstricting systems that could finely tune local blood flow depending on neuronal activity changes at the cerebellar input stage.

SIGNIFICANCE STATEMENT

The neuronal circuitry and the biochemical pathways that control local blood flow supply in the cerebellum are unclear. This is surprising given the emerging role played by this brain structure, not only in motor behavior, but also in cognitive functions. Although previous studies focused on the molecular layer, here, we shift attention onto the mossy fiber granule cell (GrC) relay. We demonstrate that GrC activity causes a robust vasodilation in nearby capillaries via the NMDA receptors-neuronal nitric oxide synthase signaling pathway. At the same time, metabotropic glutamate receptors mediate 20-hydroxyeicosatetraenoic acid-dependent vasoconstriction. These results reveal a complex signaling network that hints for the first time at the granular layer as a major determinant of cerebellar blood-oxygen-level-dependent signals.

Association between plasma cyclic guanosine monophosphate levels and hemodynamic instability during liver transplantation

The activation of cyclic guanosine monophosphate (cGMP) production in patients with end-stage liver disease (ESLD) has been associated with hemodynamic instability during orthotopic liver transplantation (OLT). The aim of this prospective, observational study was to investigate the involvement of cGMP in the mediation of profound hypotension during liver graft reperfusion. An additional objective was to determine whether preoperative cGMP levels are associated with intraoperative hemodynamic instability. Forty-four consecutive patients undergoing OLT were included in the study. Blood samples for cGMP analysis were obtained from (1) the radial artery before the surgical incision; (2) the radial artery, portal vein, and flush blood during the anhepatic phase; and (3) the radial artery 20 minutes after liver graft reperfusion. On the basis of a statistical analysis, the patients were divided into 2 groups: group 1 (preoperative cGMP level ≥ 0.05 μmol/L) and group 2 (preoperative cGMP level < 0.05 μmol/L). We demonstrated a significant correlation between the preoperative levels of cGMP and the amount of catecholamine required to maintain hemodynamic stability during reperfusion (r = 0.52, P < 0.001), the length of the hospital stay (r = 0.38, P = 0.01), and the length of the intensive care unit (ICU) stay (r = 0.44, P = 0.004). We also demonstrated a significantly higher intraoperative catecholamine requirement (P < 0.001) and a prolonged postoperative ICU stay (P = 0.02) in group 1 patients versus group 2 patients. In conclusion, this study demonstrates increased baseline cGMP production in patients with ESLD, which is significantly associated with severe hypotension during OLT. We suggest that preoperative levels of cGMP correlate with hemodynamic instability during liver graft reperfusion.

Nitric oxide modulates sodium vitamin C transporter 2 (SVCT-2) protein expression via protein kinase G (PKG) and nuclear factor-κB (NF-κB)

Ascorbate is an important antioxidant, which also displays important functions in neuronal tissues, including the retina. The retina is responsible for the initial steps of visual processing, which is further refined in cerebral high-order centers. The retina is also a prototypical model for studying physiologic aspects of cells that comprise the nervous system. Of major importance also is the cellular messenger nitric oxide (NO). Previous studies have demonstrated the significance of NO for both survival and proliferation of cultured embryonic retinal cells. Cultured retinal cells express a high-affinity ascorbate transporter, and the release of ascorbate is delicately regulated by ionotropic glutamate receptors. Therefore, we proposed whether there is interplay between the ascorbate transport system and NO signaling pathway in retinal cells. Here we show compelling evidence that ascorbate uptake is tightly controlled by NO and its downstream signaling pathway in culture. NO also modulates the expression of SVCT-2, an effect mediated by cGMP and PKG. Kinetic studies suggest that NO increases the transport capacity for ascorbate, but not the affinity of SVCT-2 for its substrate. Interestingly, NO utilizes the NF-κB pathway, in a PKG-dependent manner, to modulate both SVCT-2 expression and ascorbate uptake. These results demonstrate that NO exerts a fine-tuned control of the availability of ascorbate to cultured retinal cells and strongly reinforces ascorbate as an important bioactive molecule in neuronal tissues.

Picomolar nitric oxide signals from central neurons recorded using ultrasensitive detector cells

Nitric oxide (NO) is a widespread signaling molecule with potentially multifarious actions of relevance to health and disease. A fundamental determinant of how it acts is its concentration, but there remains a lack of coherent information on the patterns of NO release from its sources, such as neurons or endothelial cells, in either normal or pathological conditions. We have used detector cells having the highest recorded NO sensitivity to monitor NO release from brain tissue quantitatively and in real time. Stimulation of NMDA receptors, which are coupled to activation of neuronal NO synthase, routinely generated NO signals from neurons in cerebellar slices. The average computed peak NO concentrations varied across the anatomical layers of the cerebellum, from 12 to 130 pm. The mean value found in the hippocampus was 200 pm. Much variation in the amplitudes recorded by individual detector cells was observed, this being attributable to their location at variable distances from the NO sources. From fits to the data, the NO concentrations at the source surfaces were 120 pm to 1.4 nm, and the underlying rates of NO generation were 36–350 nm/s, depending on area. Our measurements are 4–5 orders of magnitude lower than reported by some electrode recordings in cerebellum or hippocampus. In return, they establish coherence between the NO concentrations able to elicit physiological responses in target cells through guanylyl cyclase-linked NO receptors, the concentrations that neuronal NO synthase is predicted to generate locally, and the concentrations that neurons actually produce.

Activity-dependent regulation of surface glucose transporter-3

Glucose transporter 3 (GLUT3) is the main facilitative glucose transporter in neurons. Glucose provides neurons with a critical energy source for neuronal activity. However, the mechanism by which neuronal activity controls glucose influx via GLUT3 is unknown. We investigated the influence of synaptic stimulation on GLUT3 surface expression and glucose import in primary cultured cortical and hippocampal neurons. Synaptic activity increased surface expression of GLUT3 leading to an elevation of intracellular glucose. The effect was blocked by NMDA receptor (NMDAR) and neuronal nitric oxide synthase (nNOS) inhibition. The Akt inhibitor I (Akt-I) blocked NMDAR-induced GLUT3 surface expression while a nNOS-phosphomimetic mutant (S1412D) enhanced GLUT3 expression at cell surface. These results suggest that NMDAR/Akt-dependent nNOS phosphorylation is coupled to GLUT3 trafficking. We demonstrated that activation of cGMP-dependent protein kinase (cGK) increased the surface expression of GLUT3, which was repressed by Rp-8-pCPT-cGMPS, a potent cell-permeable inhibitor of cGKs. These studies characterize the molecular basis for activity-dependent increases in surface GLUT3 after stimulation of the NMDARs. NMDAR-induced increase in surface GLUT3 represents a novel pathway for control of energy supply during neuronal activity that is critical for maintaining glucose homeostasis during neuronal transmission.

Glutamatergic neurons say NO in the nucleus tractus solitarii

Both glutamate and nitric oxide (NO) may play an important role in cardiovascular reflex and respiratory signal transmission in the nucleus tractus solitarii (NTS). Pharmacological and physiological data have shown that glutamate and NO may be linked in mediating cardiovascular regulation by the NTS. Through tract tracing, multiple-label immunofluorescent staining, confocal microscopic, and electronic microscopic methods, we and other investigators have provided anatomical evidence that supports a role for glutamate and NO as well as an interaction between glutamate and NO in cardiovascular regulation in the NTS. This review article focuses on summarizing and discussing these anatomical findings. We utilized antibodies to markers of glutamatergic neurons and to neuronal NO synthase (nNOS), the enzyme that synthesizes NO in NTS neurons, to study the anatomical relationship between glutamate and NO in rats. Not only were glutamatergic markers and nNOS both found in similar subregions of the NTS and in vagal afferents, they were also frequently colocalized in the same neurons and fibers in the NTS. In addition, glutamatergic markers and nNOS were often present in fibers that were in close apposition to each other. Furthermore, N-methyl-d-aspartate (NMDA) type glutamate receptors and nNOS were often found on the same NTS neurons. Similarly, alpha-amino-3-hydroxy-5-methylisoxozole-proprionic acid (AMPA) type glutamate receptors also frequently colocalized with nNOS in NTS neurons. These findings support the suggestion that the interaction between glutamate and NO may be mediated both through NMDA and AMPA receptors. Finally, by applying tracer to the cut aortic depressor nerve (ADN) to identify nodose ganglion (NG) neurons that transmit cardiovascular signals to the NTS, we observed colocalization of vesicular glutamate transporters (VGluT) and nNOS in the ADN neurons. Thus, taken together, these neuroanatomical data support the hypothesis that glutamate and NO may interact with each other to regulate cardiovascular and likely other visceral functions through the NTS.

Concepts of neural nitric oxide-mediated transmission

As a chemical transmitter in the mammalian central nervous system, nitric oxide (NO) is still thought a bit of an oddity, yet this role extends back to the beginnings of the evolution of the nervous system, predating many of the more familiar neurotransmitters. During the 20 years since it became known, evidence has accumulated for NO subserving an increasing number of functions in the mammalian central nervous system, as anticipated from the wide distribution of its synthetic and signal transduction machinery within it. This review attempts to probe beneath those functions and consider the cellular and molecular mechanisms through which NO evokes short- and long-term modifications in neural performance. With any transmitter, understanding its receptors is vital for decoding the language of communication. The receptor proteins specialised to detect NO are coupled to cGMP formation and provide an astonishing degree of amplification of even brief, low amplitude NO signals. Emphasis is given to the diverse ways in which NO receptor activation initiates changes in neuronal excitability and synaptic strength by acting at pre- and/or postsynaptic locations. Signalling to non-neuronal cells and an unexpected line of communication between endothelial cells and brain cells are also covered. Viewed from a mechanistic perspective, NO conforms to many of the rules governing more conventional neurotransmission, particularly of the metabotropic type, but stands out as being more economical and versatile, attributes that presumably account for its spectacular evolutionary success.

Nitric oxide consumption through lipid peroxidation in brain cell suspensions and homogenates

Mechanisms which inactivate NO (nitric oxide) are probably important in governing the physiological and pathological effects of this ubiquitous signalling molecule. Cells isolated from the cerebellum, a brain region rich in the NO signalling pathway, consume NO avidly. This property was preserved in brain homogenates and required both particulate and supernatant fractions. A purified fraction of the particulate component was rich in phospholipids, and NO consumption was inhibited by procedures that inhibited lipid peroxidation, namely a transition metal chelator, the vitamin E analogue Trolox and ascorbate oxidase. The requirement for the supernatant was accounted for by its content of ascorbate which catalyses metal-dependent lipid peroxidation. The NO-degrading activity of the homogenate was mimicked by a representative mixture of brain lipids together with ascorbate and, under these conditions, the lipids underwent peroxidation. In a suspension of cerebellar cells, there was a continuous low level of lipid peroxidation, and consumption of NO by the cells was decreased by approx. 50% by lipid-peroxidation inhibitors. Lipid peroxidation was also abolished when NO was supplied at a continuously low rate (approximately 100 nM/min), which explains why NO consumption by this process is saturable. Part of the activity remaining after the inhibition of lipid peroxidation was accounted for by contaminating red blood cells, but there was also another component whose activity was greatly enhanced when the cells were maintained under air-equilibrated conditions. A similar NO-consuming process was present in cerebellar glial cells grown in tissue culture but not in blood platelets or leucocytes, suggesting a specialized mechanism.

Distribution and expression pattern of the nitrergic system in the cerebellum of the sheep

The nitrergic system produces nitric oxide as an atypical neurotransmitter in the nervous system. Nitric oxide is produced from l-arginine through specific enzymes known as nitric oxide synthases. Of these, the more abundant form in neurons is the constitutive neuronal nitric oxide synthase, although the inducible isoform can be expressed as well, especially following stress or other injuries. The excessive formation of nitric oxide results in protein nitration, particularly at tyrosine residues, thus the presence of nitrotyrosine can be used as a marker of nitric oxide production. In previous studies we have shown the distribution of the components of the nitrergic system in the cerebellum of rodents, where neuronal nitric oxide synthase immunoreactivity was present in stellate and basket cells, and occasionally in granule cells. Here, we present evidence that in the sheep, as a model of larger mammals, most cerebellar neurons display an intense immunostaining for neuronal nitric oxide synthase, including unipolar brush cells, and Lugaro and Golgi neurons, which are not immunoreactive in rodents. In addition, weak immunoreactivity for inducible nitric oxide synthase and nitrotyrosine was found in particular cell types, indicating a basal expression for these markers. Our results suggest a larger dependence on the nitrergic system for the cerebella of larger mammals. Since this increase happens in both activating and inhibitory neurons of the cerebellar circuitry, we propose that in these animals there is a higher steady-state regulation of the cerebellum based on nitric oxide.

30 some years of heme oxygenase: from a "molecular wrecking ball" to a "mesmerizing" trigger of cellular events

In the beginning, the microsomal HO system was presumed to be made of one isozymes, now known as HO-1, which was cytP450-dependent; and, was thought to be of physiological significance solely in the context of catalysis of hemoglobin heme to bile pigments and CO. A succession of discoveries including characterization of the system as an independent mono-oxygenase, identification of a second form, called HO-2, free radical quenching activity of bile pigments, analogous function of CO in cell signaling to NO, and characterization of the system as HSP32 cognates has led to such an impressive expansion in the number of reports dealing with the HO system that surpass anyone's expectation. This review is a compilation of certain older findings and recent events that together ensure placement of the HO system in the mainstream research for decades to come.

Hindbrain chemical mediators of reflex-induced inhibition of gastric tone produced by esophageal distension and intravenous nicotine

The purpose of this study was to activate a vagovagal reflex by using esophageal distension and nicotine and test whether hindbrain nitric oxide and norepinephrine are involved in this reflex function. We used double-labeling immunocytochemical methods to determine whether esophageal distension (and nicotine) activates c-Fos expression in nitrergic and noradrenergic neurons in the nucleus tractus solitarii (NTS). We also studied c-Fos expression in the dorsal motor nucleus of the vagus (DMV) neurons projecting to the periphery. Esophageal distension caused 19.7 +/- 2.3% of the noradrenergic NTS neurons located 0.60 mm rostral to the calamus scriptorius (CS) to be activated but had little effect on c-Fos in DMV neurons. Intravenous administration of nicotine caused 19.7 +/- 4.2% of the noradrenergic NTS neurons 0.90 mm rostral to CS to be activated and, as reported previously, had no effect on c-Fos expression in DMV neurons. To determine whether norepinephrine and nitric oxide were central mediators of esophageal distension-induced decrease in intragastric pressure (balloon recording), N(G)-nitro-L-arginine methyl ester microinjected into the NTS (n = 5), but not into the DMV, blocked the vagovagal reflex. Conversely, alpha2-adrenergic blockers microinjected into the DMV (n = 7), but not into the NTS, blocked the vagovagal reflex. These data, in combination with our earlier pharmacological microinjection data with nicotine, indicate that both esophageal distension and nicotine produce nitric oxide in the NTS, which then activates noradrenergic neurons that terminate on and inhibit DMV neurons.

Mitochondrial nitric-oxide synthase: enzyme expression, characterization, and regulation

Nitric oxide is generated in vivo by nitric-oxide synthase (NOS) during the conversion of L-Arg to citrulline. Using a variety of biological systems and approaches emerging evidence has been accumulated for the occurrence of a mitochondrial NOS (mtNOS), identified as the alpha isoform of neuronal or NOS-1. Under physiological conditions, the production of nitric oxide by mitochondria has an important implication for the maintenance of the cellular metabolism, i.e. modulates the oxygen consumption of the organelles through the competitive (with oxygen) and reversible inhibition of cytochrome c oxidase. The transient inhibition suits the continuously changing energy and oxygen requirements of the tissue; it is a short-term regulation with profound pathophysiological consequences. This review describes the identification of mtNOS and the role of posttranslational modifications on mtNOS' activity and regulation.

Inhibition of nitric oxide release in vivo by ethanol

OBJECTIVE

Previous studies indicate that the nitric oxide (NO(.)) pathway is involved in the acute or chronic effects of ethanol on the central nervous system. However, direct evidence for the effect of ethanol on NO(.) production in vivo is lacking, and it is not clear whether it is inhibition or stimulation of the NO(.) pathway that contributes to the behavioral effects of ethanol. Herein the release of NO(.) in the rat striatum in vivo in response to NMDA receptor activation--the dominant mechanism controlling NO(.) formation-has been investigated after systemic or local injections of ethanol.

METHODS

NMDA-induced release of authentic NO(.) was measured directly in the striatum of urethane-anesthetized (1.2 g/kg intraperitoneally) male Sprague-Dawley rats by using a direct-current amperometric method coupled to an electrically modified carbon microelectrode. An injector cannula was implanted in the proximity of the electrode (250 microm apart) for focal drugs application.

RESULTS

Local application of NMDA (1 microl, 100 microM) produced a sharp and transient NO(.) signal. Systemic ethanol, 1 or 2.5 g/kg intraperitoneally, caused a long-lasting, dose-dependent inhibition of NMDA-induced NO(.) release to 12.2 +/- 5.9 and 6.4 +/- 3.7% of control, respectively, 60 min after ethanol administration. Dizocilpine (0.5 mg/kg intraperitoneally) mimicked the ethanol effect, inhibiting NO release to 1.6 +/- 0.66% of control. Local application of ethanol (1 microl, 2.5% v/v) in the striatum reduced the NMDA-induced response to 28.6 +/- 3.8% of control. Focal application of the competitive NMDA receptor antagonist D-(-)-2-amino-5-phosphonopentanoic acid (100 microM) or the nonselective NO synthase inhibitor L-N(G)-nitro-arginine methyl esther (100 microM) also caused inhibition of NMDA-induced NO(.) release to 2.4 +/- 0.7 and 4.3 +/- 0.9% of control, respectively.

CONCLUSIONS

Ethanol, at pharmacologically significant doses, strongly inhibits striatal NO(.) production and release apparently through inhibition of NMDA receptor function. Inhibition of NMDA receptor-mediated activation of the NO(.) pathway could be a primary neurobiological mechanism contributing to the effects of ethanol.

Dual effect of diazepam on cGMP levels in rat brain slices

The effect of diazepam on NO-mediated cGMP synthesis was studied in rat brain slices. It was found that diazepam dose-dependently decreased cGMP synthesis in cerebellar slices, with an inhibition of 90% at 1 mM diazepam. cGMP levels in the presence of diazepam were not restored to control levels by the addition of 0.1 mM sodium nitroprusside, whereas the decrease in cerebellar cGMP levels induced by 0.1 mM L-NAME was restored by the simultaneous application of NO-donors. In addition to the decrease of cGMP levels in neuronal structures induced by 1 mM diazepam, we observed increased cGMP immunoreactivity in glial cells in the cerebellum, the hippocampus, and the cerebral cortex. The significance of this observation is discussed.

Pulse of nitric oxide release in response to activation of N-methyl-D-aspartate receptors in the rat striatum: rapid desensitization, inhibition by receptor antagonists, and potentiation by glycine

Increased activity of glutamate N-methyl-d-aspartate (NMDA) receptors is the dominant mechanism by which nitric oxide (NO.) is generated. By using a selective direct-current amperometry method, we characterized real time NO* release in vivo in response to chemical stimulation of NMDA receptors in the rat striatum. The application of NMDA caused the appearance of a sharp and transient oxidation signal. Concentration-response studies (10-500 microM) indicated an EC(50) of 48 microM. The NMDA-induced amperometric signal was suppressed by focal application of the nitric-oxide synthase inhibitor L-nitro-arginine methyl ester (L-NAME, 100 microM) or D-(-)-2-amino-5-phosphonopentanoic acid (AP-5, 100 microM) or by systemic injection of dizocilpine (1 mg/kg i.p.), drugs that, when given alone, had no effect on baseline oxidation current. Repeated injections of NMDA at short intervals (approximately 80 s) resulted in a progressive reduction of the amperometric signal with a decay half-life of 1.36 min. Sixty min after the last NMDA application the amperometric response was restored to initial levels. Finally, the coapplication of glycine (50 or 100 microM), which, when given alone had no effect, potentiated the NMDA-induced response. Thus, NMDA receptor-mediated activation of striatal NO* system shuts off quickly and undergoes rapid desensitization, suggesting a feedback inhibition of NMDA receptor function. To the extent of NO* release can represent a correlate of NMDA receptor activity, its amperometric detection could be useful to assess in vivo the state of excitatory transmission under physiological, pharmacological, or pathological conditions.

Transmission of Arterial Baroreflex Signals Depends on Neuronal Nitric Oxide Synthase

Because inhibition of neuronal nitric oxide synthase in the nucleus tractus solitarii blocks cardiovascular responses to activation of local glutamate receptors, and because glutamate is a neurotransmitter of baroreceptor afferent nerves, we sought to test the hypothesis that neuronal nitric oxide synthase inhibition would block baroreflex transmission and cause hypertension. We determined reflex heart rate responses to intravenous phenylephrine and sodium nitroprusside in 5 anesthetized rats before and after bilateral microinjection (100 nL) of the neuronal nitric oxide synthase inhibitor AR-R 17477 (7.5 nmol) into the nucleus tractus solitarii. The inhibitor significantly increased mean arterial pressure without affecting heart rate, and it significantly reduced the gain of the baroreflex. After administration of the inhibitor, reflex responses of heart rate to changes in mean arterial pressure were always less than those responses to the same, or less, change in mean arterial pressure in the same animal without administration of the inhibitor. Microinjection of saline (100 nL) bilaterally into the nucleus tractus solitarii did not lead to hypertension or change baroreflex responses. These data support the hypothesis and suggest that neuronal nitric oxide synthase is critical to transmission of baroreflex signals through the nucleus tractus solitarii.

Astrocyte glycogen metabolism is required for neural activity during aglycemia or intense stimulation in mouse white matter

We tested the hypothesis that inhibiting glycogen degradation accelerates compound action potential (CAP) failure in mouse optic nerve (MON) during aglycemia or high-intensity stimulation. Axon function was assessed as the evoked CAP, and glycogen content was measured biochemically. Isofagomine, a novel inhibitor of central nervous system (CNS) glycogen phosphorylase, significantly increased glycogen content under normoglycemic conditions. When MONs were bathed in artificial cerebrospinal fluid (aCSF) containing 10 mM glucose, the CAP failed 16 min after exposure to glucose-free aCSF. MONs bathed in aCSF plus isofagomine displayed accelerated CAP failure on glucose removal. Similar results were obtained in MONs bathed in 30 mM glucose, which increased baseline glycogen concentration. The ability of isofagomine to increase glycogen content thus was not translated into delayed CAP failure. This is likely due to the inability of the tissue to metabolize glycogen in the presence of isofagomine, highlighting the importance of glycogen in sustaining neural function during aglycemia. The hypothesis that glycogen breakdown supports intense neural activity was tested by blocking glycogen breakdown during periods of high-frequency stimulation. The CAP area declined more rapidly when glycogen metabolism was inhibited by isofagomine, explicitly showing an important physiological role for glycogen metabolism during neural activity.

Kinetics of nitric oxide-cyclic GMP signalling in CNS cells and its possible regulation by cyclic GMP

Physiologically, nitric oxide (NO) signal transduction occurs through soluble guanylyl cyclase (sGC), which catalyses cyclic GMP (cGMP) formation. Knowledge of the kinetics of NO-evoked cGMP signals is therefore critical for understanding how NO signals are decoded. Studies on cerebellar astrocytes showed that sGC undergoes a desensitizing profile of activity, which, in league with phosphodiesterases (PDEs), was hypothesized to diversify cGMP responses in different cells. The hypothesis was tested by examining the kinetics of cGMP in rat striatal cells, in which cGMP accumulated in neurones in response to NO. Based on the effects of selective PDE inhibitors, cGMP hydrolysis following exposure to NO was attributed to a cGMP-stimulated PDE (PDE 2). Analysis of NO-induced cGMP accumulation in the presence of a PDE inhibitor indicated that sGC underwent marked desensitization. However, the desensitization kinetics determined under these conditions described poorly the cGMP profile observed in the absence of the PDE inhibitor. An explanation shown plausible theoretically was that cGMP determines the level of sGC desensitization. In support, tests in cerebellar astrocytes indicated an inverse relationship between cGMP level and recovery of sGC from its desensitized state. We suggest that the degree of sGC desensitization is related to the cGMP concentration and that this effect is not mediated by (de)phosphorylation.

Immunocytochemistry of cGMP in the Cerebellum of the Immature, Adult, and Aged Rat: the Involvement of Nitric Oxide. A Micropharmacological Study

In this study we describe the localization of formaldehyde-fixed cGMP-immunoreactivity (cGMP-IR) in rat cerebellar tissue slices incubated in vitro. In the absence of phosphodiesterase inhibition, cGMP-immunofluorescence was of low intensity in tissue slices prepared from immature cerebella. Addition of isobutylmethylxanthine (IBMX) to the incubation medium resulted in the appearance of cGMP-IR in clusters of astrocytes in the internal granular layer. Addition of N-methyl-d-aspartate (NMDA), kainic acid, atrial natriuretic factor (ANF), or sodium nitroprusside (SNP) gave an intense cGMP-IR in Bergmann fibres, Bergmann cell bodies, and astrocytes in the internal granular layer. Astrocytes in the white matter showed cGMP-IR after incubation of the slice in the presence of ANF or nitroprusside, but not after NMDA or kainic acid. In addition, after SNP stimulation of cGMP production, cGMP-IR was found in fibres which were not positive for glial fibrillary acidic protein (GFAP). In the adult cerebellar slice, intense basal cGMP-immunostaining was observed in Bergmann fibres, Bergmann cell bodies, and astrocytes in the granular layer. No cGMP-IR was observed in Purkinje cells. Stimulation of the cGMP-content in the glial structures by NMDA, ANF, or SNP, was suggested by the immunocytochemical results. However, when measured biochemically, only the effect of SNP was statistically significant, and immunocytochemistry showed that SNP clearly stimulated cGMP synthesis in neuronal cell structures. In the cerebellum of the aged rat a reduced cGMP-IR was found compared to the adult, in the same structures which showed cGMP-IR in the adult. Basal cGMP-immunostaining was reduced in the presence of haemoglobin, methylene blue, by inhibiting nitric oxide synthesis with NG-monomethyl-l-arginine (NGMAr), or by depletion of external Ca2+. Also the stimulatory effect of NMDA and of ANF (partly) on the cGMP-IR was inhibited by these compounds. cGMP-IR after stimulation of guanylate cyclase by SNP was reduced by the concomitant presence of haemoglobin or methylene blue, but not by NGMAr, or by omission of Ca2+. Our results point to an important role for cGMP in the functioning of glial tissue in the cerebellum and also suggest a role for nitric oxide as an intercellular mediator in the functioning of glutamate and ANF in the cerebellum.

Pharmacology of the nitric oxide receptor, soluble guanylyl cyclase, in cerebellar cells

The nitric oxide (NO) receptor, soluble guanylyl cyclase (sGC), is commonly manipulated pharmacologically in two ways. Inhibition of activity is achieved using 1-H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-l-one (ODQ) which oxidizes the haem prosthetic group to which NO binds, while the compound 3-(5-hydroxymethyl-2-furyl)-1-benzylindazole (YC-1) is considered an 'allosteric' activator. Knowledge of how these agents function and interact in a normal cellular environment is limited. These issues were addressed using rat cerebellar cells. Inhibition by ODQ was not simply competitive with NO. The rate of onset was ODQ concentration-dependent and developed in two kinetic phases. Recovery from inhibition occurred with a half-time of approximately 5 min. YC-1 slowed the rate at which sGC deactivated on removal of NO by 45 fold, consistent with YC-1 increasing the potency of NO for sGC. YC-1 also enhanced the maximal response to NO by 2 fold. Furthermore, when added to cells in which sGC was 90% desensitized, YC-1 abruptly enhanced sGC activity to a degree that indicated partial reversal of desensitization. After pre-exposure to YC-1, sGC became resistant to inhibition by ODQ. In addition, YC-1 rapidly reversed inhibition by ODQ in cells and for purified sGC, suggesting that YC-1 either increases the NO affinity of the oxidized sGC haem or reverses haem oxidation. It is concluded that the actions of ODQ and YC-1 on sGC are broadly similar in cells and purified preparations. Additionally, YC-1 transiently reverses sGC desensitization in cells. It is hypothesized that YC-1 has multiple actions on sGC, and thereby both modifies the NO binding site and enhances agonist efficacy.

Nitric oxide inactivation in brain by a novel O2-dependent mechanism resulting in the formation of nitrate ions

In order for nitric oxide (NO) to function as a biological messenger it has to be inactivated, but little is known of how this is achieved. In cells from the brain, we have recently shown the existence of a powerful NO sink that 'shapes' NO signals for targeting its receptor, soluble guanylate cyclase, whilst simultaneously preventing NO rising to toxic concentrations [Griffiths and Garthwaite (2001) J. Physiol. (Cambridge, U.K.) 536, 855-862]. In the present study, the properties of this sink were investigated further. Inactivation of NO was preserved in rat brain homogenates. In both cerebellar cell suspensions and brain homogenates, NO inactivation required O(2) and, from measurements in homogenates, the principal end-product was NO(-)(3), which is also the main product of endogenously formed NO in vivo. Direct chemical reaction with O(2), superoxide anions or haemoglobin was not responsible. Consumption of NO was, however, inhibited by heat or protease treatment. Pharmacological tests were negative for several candidate enzymes, namely cytochrome c oxidase, H(2)O(2)-dependent haem peroxidases, prostaglandin H synthase, 12/15-lipoxygenase and a flavohaemoglobin-like NO dioxygenase. The capacity of the NO sink in cells was limited because regeneration of the activity was slow (2 h). It is concluded that NO is consumed in the brain through a novel protein, ultimately forming NO(-)(3), and that the slow regeneration of the activity provides a scenario for NO to become toxic.

The shaping of nitric oxide signals by a cellular sink

1. The functioning of nitric oxide (NO) as a biological messenger necessitates that there be an inactivation mechanism. Cell suspensions from a rat brain region rich in the NO signalling pathway (cerebellum) were used to investigate the existence of such a mechanism and to determine its properties. 2. The cells consumed NO in a manner that could not be explained by reaction with O(2), superoxide ions or contaminating red blood cells. Functionally, the mechanism was able to convert constant rates of NO formation into low steady-state NO concentrations. For example, with NO produced at 90 nM min(-1), the cells (20 x 10(6) ml(-1)) held NO at 20 nM. Various other cell types behaved similarly. 3. The influence of NO inactivation on the ability of NO to access its receptor, soluble guanylyl cyclase, was explored by measuring cGMP accumulation in response to the clamped NO concentrations. The extrapolated steady-state EC(50) for NO was 2 nM, a concentration readily achieved by low NO release rates, despite inactivation. 4. When confronted by higher NO release rates for several minutes, the clamping mechanism failed, resulting in a progressive rise in NO concentration. While the clamp was maintained, cellular respiration was unaffected but, as it failed, respiration became inhibited by NO. The IC(50) was measured to be 120 nM (at 100-140 microM O(2)). 5. It is concluded that cerebellar (and other) cells possess a powerful NO inactivation mechanism that, extrapolated to the whole tissue, would impose on NO a half-life of around 100 ms. This and other properties of the device appear ideal for shaping low-level NO signals for activating its receptor, soluble guanylyl cyclase, whilst avoiding adverse effects on mitochondrial function. The exhaustibility of the mechanism provides a scenario for NO to become toxic.

Nitroxidergic influences on cardiovascular control by NTS: a link with glutamate

Glutamate (GLU) receptor activation, which is important in cardiovascular reflex transmission through the nucleus tractus solitarii (NTS), leads to release of nitric oxide (NO.) from central nitroxidergic neurons. Therefore, we hypothesized that GLU and NO. are linked in cardiovascular control by NTS. We first sought to determine if NO. released into NTS led to cardiovascular changes like those produced by GLU and found that the nitrosothiol S-nitrosocysteine, but not NO. itself or other NO. donors, elicited such responses in anesthetized rats. The responses were dependent on activation of soluble guanylate cyclase but, not being affected by a scavenger of NO., likely did not depend on release of NO. into the extracellular space. Responses to ionotropic GLU agonists in NTS, like those to S-nitrosocysteine, were inhibited by inhibition of soluble guanylate cyclase. Inhibition of neuronal NO. synthase (nNOS) also inhibited responses to ionotropic GLU agonists. The apparent physiologic link between GLU and NO. mechanisms in NTS was further supported by anatomical studies that demonstrated frequent association between GLU-containing nerve terminals and neurons containing nNOS. Furthermore, GLU receptors were often found on NTS neurons that were immunoreactive for nNOS. The anatomical relationships between GLU and nNOS and GLU receptors and nNOS were more pronounced in some subnuclei of NTS than in others. While seen in subnuclei that are known to receive cardiovascular afferents, the association was even more prominent in subnuclei that receive gastrointestinal afferents. These studies support a role for nitroxidergic neurons in mediating cardiovascular and other visceral reflex responses that result from release of GLU into the NTS.

Subunits of the nitric oxide receptor, soluble guanylyl cyclase, expressed in rat brain

Despite the widespread use of nitric oxide as a signalling molecule in the central nervous system, the molecular makeup of its receptor, soluble guanylyl cyclase (sGC), therein is poorly understood. Accordingly, RT-PCR and in situ hybridization were used to identify sGC subunits expressed in rat brain. In addition to the expected mRNA for alpha 1 and beta1 subunits, message for the beta 2 subunit was detected in the cerebellum at all developmental stages investigated (1--150 days postnatum). The use of degenerate primers allowed the identification of mRNA coding for the rat alpha 2 subunit, which was also expressed at every age studied. All but beta 2 were detected by in situ hybridization in the brains of both 8-day-old and adult rats. The distribution patterns indicated that in some areas, e.g. caudate-putamen and nucleus accumbens, sGC probably exists mainly as the alpha 1 beta 1 heterodimer. In others, e.g. hippocampus and olfactory bulb, alpha 2 beta 1 is likely to be dominant. In the cerebellum, alpha 1 and beta 1 message was strong in the Purkinje cell layer but was not confined to Purkinje cells: smaller cells, presumed to be the Bergmann glia, were also labelled. In contrast, alpha 2 mRNA was concentrated in cerebellar granule cells. Western blotting indicated an excess of alpha 1 over beta 1 protein in the cerebellum, the reverse of what was found in the lung. It is concluded that, in molecular terms, sGC is likely to be more complex and exhibit more regional variation in the brain than previously thought. The functional consequences of this heterogeneity require investigation.

Chronic exposure to aluminium decreases NADPH-diaphorase positive neurons in the rat cerebral cortex

Aluminium (Al) exposure is neurotoxic and is considered a possible etiological factor for many neurodegenerative disorders. Since it is known that Al impairs the glutamate-nitric oxide-cGMP pathway in neurons, this study was carried out to monitor the expression of NADPH-d in some central nervous system areas of rats after chronic administration of Al in drinking water. We tested three different nervous areas known to contain NADPH-diaphorase positive neurons: two cortical area (somatosensory cerebral cortex and cerebral cortex), a deep brain area (dorsolateral periaqueductal gray matter) and a spinal area (lumbar enlargement of the spinal cord). Our data showed that Al significantly decreased NADPH-d positive neurons in the cerebral cortex and the NADPH-d staining of many granular neurons in the cerebellum. We also found that Al did not cause neuron loss or apoptosis in the cerebral cortex. These findings suggest that the cortical nitroxidergic neurons and granule cells were a specific target of Al neurotoxicity.

Subtle alterations in NMDA-stimulated cyclic GMP levels following lateral fluid percussion brain injury

This study examined whether NMDA-stimulated cyclic GMP levels were altered at two different time points following lateral fluid percussion injury. At 60 min and 15 days postinjury, the left and right hippocampi were dissected and chopped into mini-prisms. Each hippocampus was divided into five equal parts and incubated with either the phosphodiesterase inhibitor IBMX (3-isobutyl-1-methylxanthine, 500 microM) alone, IBMX and N-methyl-D-aspartic acid (NMDA) OR IBMX, NMDA, and glycine (10 MM). Two concentrations of NMDA were used: 500 or 1,000 microM. Tissues were then assayed for levels of cyclic GMP. Results indicated that there were no changes in basal levels of cyclic GMP at either postinjury time point. At 60 min postinjury, there were no significant main effects for injury or drug concentration. There was a significant injury x side interaction effect with increased levels of NMDA-stimulated cyclic GMP in the hippocampus ipsilateral to the injury impact and decreased cyclic GMP levels in the contralateral hippocampus. There were no significant alterations in NMDA-stimulated cyclic GMP levels at 15 days postinjury. The data from this study indicated that NMDA-stimulated cyclic GMP accumulation is differentially altered in the hippocampus ipsilateral and contralateral to the site of the injury at 1 h after injury, but is normalized by 15 days postinjury. These findings implicate NMDA-mediated intracellular signaling processes in the acute excitotoxic response to injury.

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.

Apposition of neuronal elements containing nitric oxide synthase and glutamate in the nucleus tractus solitarii of rat: a confocal microscopic analysis

The distribution of glutamate and neuronal nitric oxide synthase in the rat nucleus tractus solitarii was investigated by double fluorescent immunohistochemistry combined with confocal laser scanning microscopy. Cells and fibers that exhibited neuronal nitric oxide synthase immunoreactivity alone, glutamate immunoreactivity alone or both immunolabels were present in all subnuclei of the nucleus tractus solitarii, but staining intensities differed between the subnuclei. The percentages of double-labeled glutamate-immunoreactive cells also differed between the subnuclei. The central subnucleus contained the highest percentage of double-labeled glutamate-immunoreactive cells and the medial subnucleus contained the lowest. The percentages of double-labeled neuronal nitric oxide synthase-immunoreactive neurons likewise differed between the subnuclei. The central subnucleus contained the highest percentage of double-labeled neuronal nitric oxide synthase-immunoreactive neurons and the commissural subnucleus contained the lowest. Because of our interest in cardiovascular regulation, the anatomical relationship between glutamate-immunoreactive and neuronal nitric oxide synthase-immunoreactive fibers in the dorsolateral and commissural subnuclei was further examined at higher magnification. Close appositions were observed between neuronal nitric oxide synthase-immunoreactive and glutamate-immunoreactive fibers, between double-labeled and glutamate-immunoreactive fibers, and between neuronal nitric oxide synthase-immunoreactive and double-labeled fibers. We recognized that a single visual perspective might cause labeled fibers that pass in close proximity to appear to make contact. Therefore, we constructed three-dimensional images from serial optical sections obtained from the dorsolateral and commissural subnuclei by means of a confocal scanning microscope. Rotation of the three-dimensional images caused some fibers that had seemed to be in close apposition to other structures to separate from those structures. In contrast, some glutamate-immunoreactive and some neuronal nitric oxide synthase-immunoreactive fibers remained in close apposition regardless of the angle at which they were viewed. This study supports there being an anatomical link between glutamatergic and nitroxidergic systems in the nucleus tractus solitarii. Recognized physiological interactions between the two systems could occur through such a link.

Rapid desensitization of the nitric oxide receptor, soluble guanylyl cyclase, underlies diversity of cellular cGMP responses

A major receptor for nitric oxide (NO) is the cGMP-synthesizing enzyme, soluble guanylyl cyclase (sGC), but it is not known how this enzyme behaves in cells. In cerebellar cells, NO (from diethylamine NONOate) increased astrocytic cGMP with a potency (EC(50) </= 20 nM) higher than that reported for purified sGC. Deactivation of NO-stimulated sGC activity, studied by trapping free NO with hemoglobin, took place within seconds (or less) rather than the minute time scale reported for the purified enzyme. Measurement of the rates of accumulation and degradation of cGMP were used to follow the activity of sGC over time. The peak activity, occurring within seconds of adding NO, was swiftly followed by desensitization to a steady-state level 8-fold lower. The same desensitizing profile was observed when the net sGC activity was increased or decreased or when cGMP breakdown was inhibited. Recovery from desensitization was relatively slow (half-time = 1.5 min). When the cells were lysed, sGC desensitization was lost. Analysis of the transient cGMP response to NO in human platelets showed that sGC underwent a similar desensitization. The results indicate that, in its natural environment, sGC behaves much more like a neurotransmitter receptor than had been expected from previous enzymological studies, and that hitherto unknown sGC regulatory factors exist. Rapid sGC desensitization, in concert with variations in the rate of cGMP breakdown, provides a fundamental mechanism for shaping cellular cGMP responses and is likely to be important in decoding NO signals under physiological and pathophysiological conditions.