Physiological Role Of Nitric Oxide Research Articles

Role of nitric oxide in determination of large intestinal contractility in neonatal rats

Objectives: Nitric oxide (NO) plays a key role in inhibiting the contractility of gut smooth muscles in various species, and NG-nitro-L-arginine methyl ester (L-NAME) is a critical NO synthase inhibitor. Previous research investigating the role of NO in regulating gut motility focused on adult animals. Therefore, more research is needed to determine their status in the gut of newborns. Our study intended to understand how NO impacts the large gut contractility, in vitro, in rats, both neonates and adults, to get a better insight into the physiological role of NO in regulating large gut motility, particularly in neonates. Materials and Methods: In an organ bath preparation, the segments of a large gut (colon and rectum) were subjected to various concentrations of nitroglycerin (NG) (0.01–100 mM), a NO donor, cumulatively. In another group, pre-treatment with L-NAME (100 mM) was done to evaluate the blocking effect of NO on the contractile tension and frequency. Results: NG induced relaxation in the colon and rectum of adult rats in a similar manner. NG caused significantly greater relaxation in neonates’ rectums than in their colons. In neonatal and adult rats, L-NAME pre-application inhibited NG-induced relaxation in contractile tension. Exposure to different concentrations of NG decreased contractile frequency in adult rats’ colons and rectum. However, L-NAME pre-treatment did not affect the decrease in contractile frequency caused by NG. In neonates, NG caused a concentration-dependent reduction in contractile frequency, and a decrease in contractile frequency in the rectum was more than that in the colon. However, L-NAME pre-treatment did not affect the reduction in contractile frequency caused by NG. Conclusion: Nitrergic mechanisms have possibly been present since birth. The intensity of control by NO may be different in the colon and rectum. The differences in NO sensitivity in adults and neonates demonstrated the changes during development.

Nitric Oxide Promoted the Seed Germination of Cynanchum auriculatum under Cadmium Stress

Cynanchum auriculatum, an early food-medicine homologous plant native to Asia, possesses significant nutritional and health benefits. However, the presence of cadmium (Cd) in the soil poses a hazard to the germination and growth of C. auriculatum. As nitric oxide (NO) plays a vital role in plant resistance to heavy metal stress, we used three different concentrations of SNP treatment during the germination phase, aiming to alleviate the inhibitory effects of Cd stress on the seed germination of C. auriculatum. The results indicated that when compared to seeds treated with SNP concentrations of 0.2 mM and 0.8 mM, C. auriculatum seeds treated with 0.4 mM SNP exhibited an improved germination rate and germination index, as well as longer hypocotyl. Furthermore, in comparison to NOS-like, the SNP application stimulated the production of endogenous NO through NR catalysis. Additional investigations showed that the ABA level decreased while the GA level increased under normal conditions, while the SNP application enhanced the accumulation of both ABA and GA in C. auriculatum seeds under Cd stress. Histochemical staining and biochemical indicators demonstrated that SNP treatment enhanced the enzymatic activity of SOD, POD, and CAT, while inhibiting the production of hydrogen peroxide and superoxide anion. Moreover, SNP treatment resulted in increased α-amylase activity, which facilitated starch hydrolysis and the generation of soluble sugar. Ultimately, the seed vitality of C. auriculatum under Cd stress was promoted. Our findings present a theoretical framework for the application of SNP in the seed germination mechanism of C. auriculatum and establish the groundwork for comprehending the physiological role of NO under Cd stress.

Nitric Oxide: Physiological Functions, Delivery, and Biomedical Applications.

Nitric oxide (NO) is a gaseous molecule that has a central role in signaling pathways involved in numerous physiological processes (e.g., vasodilation, neurotransmission, inflammation, apoptosis, and tumor growth). Due to its gaseous form, NO has a short half-life, and its physiology role is concentration dependent, often restricting its function to a target site. Providing NO from an external source is beneficial in promoting cellular functions and treatment of different pathological conditions. Hence, the multifaceted role of NO in physiology and pathology has garnered massive interest in developing strategies to deliver exogenous NO for the treatment of various regenerative and biomedical complexities. NO-releasing platforms or donors capable of delivering NO in a controlled and sustained manner to target tissues or organs have advanced in the past few decades. This review article discusses in detail the generation of NO via the enzymatic functions of NO synthase as well as from NO donors and the multiple biological and pathological processes that NO modulates. The methods for incorporating of NO donors into diverse biomaterials including physical, chemical, or supramolecular techniques are summarized. Then, these NO-releasing platforms are highlighted in terms of advancing treatment strategies for various medical problems.

Elicitation of flavonoid biosynthesis in cell suspension cultures of Haplophyllum virgatum by sodium nitroprusside

Plant cell suspension culture, along with elicitor application, is a suitable technique to obtain large amounts of bioactive compounds. Haplophyllum virgatum is an Iranian endemic plant species. Sodium nitroprusside (SNP) is widely used as a nitric oxide (NO) donor in studies on plants to investigate the physiological roles of NO. Here, the physiological responses were investigated in cell suspension cultures of H. virgatum in the presence of different concentrations of SNP (0, 100, 150, and 200 µM) at various time points (0, 8, 12, 24, 48, 72, and 168 h). The transcript levels of two flavonoid biosynthetic enzymes, including chalcone synthase (CHS) and chalcone isomerase (CHI), and an important transcription factor (R2R3-MYB), were measured in the SNP-treated cells. The content of rutin (the main flavonoid in the Rutaceae family) was also evaluated. Nitric oxide significantly decreased the hydrogen peroxide and malondialdehyde contents by increasing the proline and enzymatic and non-enzymatic antioxidants in the treated cells. Meanwhile, NO significantly elicited flavonoid production, including rutin. These results highlight the effects of NO on flavonoid biosynthesis in H. virgatum suspension cells, probably through gene expression regulation in and upstream of the flavonoid biosynthetic pathway.

A review of the literature: How does prenatal opioid exposure impact placental health and fetal brain development?

In recent years, there has been a sixfold increase in the number of pregnant people with opioid use disorder (OUD). Rates of neonatal opioid withdrawal syndrome (NOWS), previously known as neonatal abstinence syndrome (NAS), have significantly increased in virtually every state and demographic group(HealthcareCostUtilizationProject,HCUP,2010). NOWS is a condition resulting from chronic exposure to either therapeutic opioid use (e.g., medication for OUD, chronic pain conditions) or nonprescribed opioid use. To date, there is no known prenatal treatment to help decrease the risk of infants developing NOWS and subsequent neurodevelopmental outcomes. Given the increasing support for how placental signaling, or placental programming, may play a role in downstream pathology, prospective research investigating how the placenta is affected by chronic opioid exposure morphologically, histologically, and at the cellular level may open up potential treatment opportunities in this field. In this review, we discuss literature exploring the physiological roles of nitric oxide and dopamine not only in the vascular development of the placenta, but also in fetal cerebral blood flow, neurogenesis, neuronal differentiation, and neuronal activity. We also discuss histological preclinical studies that suggest chronic opioid exposure to induce some combination of placental dysfunction and hypoxia in a manner similar to other well-known placental pathologies, as denoted by the compensatory neovascularization and increased utilization of the placenta's supply of trophoblast cells, which play an essential role in placental angiogenesis. Overall, we found that the current literature, while limited, suggests chronic opioid exposure negatively impacts placental function and fetal brain development on a cellular and histopathological level. We conclude that it is worthwhile to consider the placenta as a therapeutic target with the ultimate goal of decreasing the incidence of NOWS and the long-term impacts of prenatal opioid exposure.

Nitric Oxide as a Central Molecule in Hypertension: Focus on the Vasorelaxant Activity of New Nitric Oxide Donors.

Simple SummaryNitric oxide is an important molecule that performs a variety of functions in our bodies, especially in the cardiovascular system. In certain pathological conditions, such as cardiovascular diseases, including hypertension, there is reduced production or bioavailability of nitric oxide. Therefore, compounds that deliver nitric oxide, called nitric oxide donors, are clinically useful. In this review, we discuss the physiological role of nitric oxide, and some of the nitric oxide donors and their clinical uses, focusing on the cardiovascular system. Despite the high number of nitric oxide donors and their known efficacy, it is important to understand the similarities and differences among them and how each of them works, as well as to investigate the development of new molecules that may be better than the NO donors in current use.Cardiovascular diseases include all types of disorders related to the heart or blood vessels. High blood pressure is an important risk factor for cardiac complications and pathological disorders. An increase in circulating angiotensin-II is a potent stimulus for the expression of reactive oxygen species and pro-inflammatory cytokines that activate oxidative stress, perpetuating a deleterious effect in hypertension. Studies demonstrate the capacity of NO to prevent platelet or leukocyte activation and adhesion and inhibition of proliferation, as well as to modulate inflammatory or anti-inflammatory reactions and migration of vascular smooth muscle cells. However, in conditions of low availability of NO, such as during hypertension, these processes are impaired. Currently, there is great interest in the development of compounds capable of releasing NO in a modulated and stable way. Accordingly, compounds containing metal ions coupled to NO are being investigated and are widely recognized as having great relevance in the treatment of different diseases. Therefore, the exogenous administration of NO is an attractive and pharmacological alternative in the study and treatment of hypertension. The present review summarizes the role of nitric oxide in hypertension, focusing on the role of new NO donors, particularly the metal-based drugs and their protagonist activity in vascular function.

Impact of risk factors on serum levels of vasoactive substances in patients with peripheral arterial occlusive disease at different Fontaine’s stages

Introduction: There is a bulk of literature data on the physiological roles of nitric oxide (NO) and endothelin- 1 (ET-1), but many facts remain unknown, especially in certain diseases such as peripheral arterial disease. Material and methods: This is an observational cross-sectional study. The subjects were patients diagnosed with PAD. Serum levels of NO and ET-1 were determined for all patients, and statistical data processing was performed according to the set goals. Results: The study included 64 patients with mean age 60.2 ± 12.7 years, mostly in stage II PAD according to Fontain (46.9%). Statistical analysis failed to determine a significant difference in serum NO or ET-1 values with respect to disease stage, sex, and body mass index (BMI). Certain oscillations were found in the mean values of NO related to smoking and diabetes but without statistical significance. There were also oscillations in the values of ET-1, with higher levels found in women, smokers and non-diabetics in whom this difference reached statistical significance (p = 0.041). Conclusion: Serum levels of NO and ET-1 in this study show some causal relationship with certain risk factors for PAD such as diabetes and smoking, but additional research is needed to fully understand their effects and interactions.

What Lies at the Heart of Photobiomodulation: Light, Cytochrome C Oxidase, and Nitric Oxide-Review of the Evidence.

Objective: The underlying mechanisms of photobiomodulation (PBM) remain elusive. The most attractive hypotheses revolve around the role of cytochrome c oxidase (CCO) and cellular energetics.Background: No reliable demonstration of any PBM-related light-induced mechanistic effect on CCO has been reported. Studies on PBM have proven to be either nonreproducible, of questionable relevance, or involve wavelengths unlikely to be operative in vivo. The literature reveals very few demonstrable mechanistic light effects of any sort on CCO. Nitric oxide (NO) is involved in a number of the reported light effects on CCO. NO inhibits CCO at high reductive pressures by binding to the heme a3 moiety. This complex is white light labile.Methods: The reported photolability of the heme–NO complex seems to be a prime target for PBM studies, as removal of inhibiting NO from the active site of CCO could restore normal activity to inhibited CCO. Another aspect of CCO–NO chemistry has been revealed that shows intriguing possibilities.Results: A novel nitrite reductase activity of solubilized mitochondria has been demonstrated attributable to CCO. NO production was optimal under hypoxic conditions. It was also found that 590 nm irradiation increased NO production by enhancing NO release. The presence of cellular NO has usually been considered metabolically detrimental, but current thinking has expanded the importance and the physiological roles of NO. Evidence shows that NO production is likely to play a role in cardioprotection and defenses against hypoxic damage.Conclusions: Studies combining PBM and hypoxia also point to a connection between light irradiation, hypoxia protection, and NO production. This leads the authors to the possibility that the intrinsic nature of PBM involves the production of NO. The combination of CCO and hemoglobin/myoglobin NO production with photorelease of NO may constitute the heart of PBM.

Nitric Oxide Balance in Health and Diseases: Implications for New Treatment Strategies

Nitric Oxide (NO) is an essential signaling molecule with diverse physiological functions in humans. The steady-state concentration and site of production of nitric oxide determine its effects in biological systems. The human cells are exposed to both beneficial and harmful effects of NO. These dual effects of NO could depend on its local concentration in the cells. Additionally, the rate of synthesis, translocation, direct interaction with other molecules, and signals contribute to the biochemical and physiological effects of NO. In this review, the biochemical and physiological role of NO, particularly in health and disease as touching on cell signaling, oxidative stress, immunity, as well as cardiovascular protection amongst others, is focused on. Therefore, this review objectively discusses the dual functionality of NO in living cells.

Корекція функціонального статусу ендотелію як потенційної мішені у хворих із серцево-судинними захворюваннями та метаболічними розладами

Серцево-судинні (ССЗ) та обмінні захворювання (такі як ожиріння та цукровий діабет) наразі є основними проблемами зі здоров’ям у всьому світі. Одним із найважливіших патофізіологічних зв’язків між цими станами є наявність оксиду азоту (NO), який постійно виробляється з умовно незамінної амінокислоти L-аргініну. NO є важливою паракринною речовиною, яка продукується ендотелієм для регуляції судинного тонусу. Розвиток і прогресування атеросклерозу пов’язано з дисфункцією ендотелію та зниженням біодоступності NO. Встановлено, що ожиріння також пов’язано зі зменшенням продукції NO, викликаної порушенням біодоступності його субстрату — аргініну, а збільшенню вмісту аргініну в ендотелії запобігає гіпертензія, яка часто поєднується з ожирінням. L-аргінін є амінокислотою, необхідною ферменту ендотеліальній NO-синтазі (eNOS) для продукції NO. Мета — бібліографічний огляд наукових публікацій. Результати. У даному огляді розглянуто фізіологічну роль NO у функціонуванні різних системорганізму. Наведено дані щодо метаболізму L-аргініну, його біодоступності та механізмів дії. Проаналізовано літературні дані про ефективність і безпечність застосування різних доз L-аргініну в пацієнтів з ожирінням, цукровим діабетом, інсулінорезистентністю (ІР) та артеріальною гіпертензією. Висновки. Питання дози та тривалості застосування аргініну сьогодні вимагають подальшого вивчення. Результати багатьох досліджень, в яких оцінювали використання аргініну в дорослих з ожирінням, дозволяють припустити, що аргінін може бути безпечним, недорогим і ефективним терапевтичним засобом за ожиріння та позитивно впливає на корекцію метаболічних процесів, а саме на ІР. Ефективність застосування L-аргініну як додаткової терапії до основної схеми лікування отримала підтвердження за артеріальної гіпертензії, цукрового діабету та ІР. Аналіз літератури свідчить, що застосування L-аргініну є безпечним у діапазоні до 30 г на добу, хоча в більшості досліджень використовували дози від 6 до 12 г на добу.

Physiological role of nitric oxide for regulation of arterial stiffness in anesthetized rabbits

We assessed effects of acetylcholine and Nω-Nitro-l-arginine methyl ester hydrochloride (l-NAME) on the cardio-ankle vascular index (CAVI), an indicator of arterial stiffness from origin of aorta to tibial artery, in halothane-anesthetized rabbits. Acetylcholine decreased the blood pressure, femoral vascular resistance and CAVI, whereas l-NAME did not affect the CAVI at a hypertensive dose. The acetylcholine-induced decrement of CAVI was completely suppressed by l-NAME. These results suggest that the arterial stiffness in rabbits may be independent from homeostatic production of nitric oxide, however, it can be decreased by large amounts of nitric oxide that are intrinsically produced by exogenously administered acetylcholine.

Nitric Oxide Signalling in Yeast.

Nitric oxide (NO) is a cellular signalling molecule widely conserved among organisms, including microorganisms such as bacteria, yeasts, and fungi, and higher eukaryotes such as plants and mammals. NO is mainly produced by the activities of NO synthase (NOS) or nitrite reductase (NIR). There are several NO detoxification systems, including NO dioxygenase (NOD) and S-nitrosoglutathione reductase (GSNOR). NO homeostasis, based on the balance between NO synthesis and degradation, is important for regulating its physiological functions, since an excess of NO causes nitrosative stress due to the high reactivity of NO and NO-derived compounds. In yeast, NO may be involved in stress responses, but the role of NO and the mechanism underlying NO signalling are poorly understood due to the lack of mammalian NOS orthologs in the yeast genome. NOS and NIR activities have been observed in yeast cells, but the gene-encoding NOS and the mechanism by which NO production is catalysed by NIR remain unclear. On the other hand, yeast cells employ NOD and GSNOR to maintain intracellular redox balance following endogenous NO production, treatment with exogenous NO, or exposure to environmental stresses. This article reviews NO metabolism (synthesis, degradation) and its regulation in yeast. The physiological roles of NO in yeast, including the oxidative stress response, are also discussed. Such investigations into NO signalling are essential for understanding how NO modulates the genetics and physiology of yeast. In addition to being responsible for the pathology and pharmacology of various degenerative diseases, NO signalling may be a potential target for the construction and engineering of industrial yeast strains.

Nitric oxide signaling in yeast.

As a cellular signaling molecule, nitric oxide (NO) is widely conserved from microorganisms, such as bacteria, yeasts, and fungi, to higher eukaryotes including plants and mammals. NO is mainly produced by NO synthase (NOS) or nitrite reductase (NIR) activity. There are several NO detoxification systems, including NO dioxygenase (NOD) and S-nitrosoglutathione reductase (GSNOR). NO homeostasis based on the balance between NO synthesis and degradation is important for the regulation of its physiological functions because an excess level of NO causes nitrosative stress due to the high reactivity of NO and NO-derived compounds. In yeast, NO may be involved in stress responses, but NO and its signaling have been poorly understood due to the lack of mammalian NOS orthologs in the genome. Even though the activities of NOS and NIR have been observed in yeast cells, the gene encoding NOS and the NO production mechanism catalyzed by NIR remain unclear. On the other hand, yeast cells employ NOD and GSNOR to maintain an intracellular redox balance following endogenous NO production, exogenous NO treatment, or environmental stresses. This article reviews NO metabolism (synthesis, degradation) and its regulation in yeast. The physiological roles of NO in yeast, including the oxidative stress response, are also discussed here. Such investigations into NO signaling are essential for understanding the NO-dependent genetic and physiological modulations. In addition to being responsible for the pathology and pharmacology of various degenerative diseases, NO signaling may be a potential target for the construction and engineering of industrial yeast strains.

Nitric Oxide-Releasing Biomaterials for Biomedical Applications

Nitric oxide (NO), as an essential signaling molecule, participates in various physiological processes such as cardiovascular homeostasis, neuronal transmission, immunomodulation, and tumor growth. The multiple role of NO in physiology and pathophysiology has triggered a massive interest in the strategies of delivering exogenous NO for biomedical applications. Hence, different kinds of NO prodrugs have been developed up to date, including diazeniumdiolates, S-nitrosothiol, metal-nitrosyl, nitrobenzene, and so on. However, the clinical application of these low molecular weight NO donors has been restricted due to the problems of burst release, low payloads, and untargeted delivery. The delivery of NO by biomaterialbased carrier offers a beneficial strategy to realize the controlled and sustained delivery of NO to the targeted tissues or organs. In detail, NO-donor prodrugs have been attached and loaded to diverse biomaterials to fabricate nanoparticles, hydrogels, and coating platforms by means of physical, chemical, or supramolecular techniques. These NO-releasing biomaterials hold promise for a number of biomedical applications ranging from therapy of the ischemic disease and several types of cancer to cardiovascular devices and wound dressing. First, surface coating with NO-releasing biomaterials could mimic the physiological function of vascular endothelium, therefore promoting vascularization and improving the patency of cardiovascular implants. Next, because NO also mediates many important processes that take place after cutaneous injury, NO-releasing biomaterials could serve as ideal wound dressing to accelerate tissue regeneration. Finally, biomaterials enable localized delivery of high dose of NO to tumors in a sustained manner, thus generating potent tumoricidal effect. In this review, we will summarize the progress of different NO-releasing biomaterials, and highlight their biomedical applications with a hope to inspire new perspectives in the area of biomaterial-based NO-delivery systems.

Inducible nitric oxide synthase (NOS-2) in subarachnoid hemorrhage: Regulatory mechanisms and therapeutic implications.

Aneurysmal subarachnoid hemorrhage (SAH) typically carries a poor prognosis. Growing evidence indicates that overabundant production of nitric oxide (NO) may be responsible for a large part of the secondary injury that follows SAH. Although SAH modulates the activity of all three isoforms of nitric oxide synthase (NOS), the inducible isoform, NOS-2, accounts for a majority of NO-mediated secondary injuries after SAH. Here, we review the indispensable physiological roles of NO that must be preserved, even while attempting to downmodulate the pathophysiologic effects of NO that are induced by SAH. We examine the effects of SAH on the function of the various NOS isoforms, with a particular focus on the pathological effects of NOS-2 and on the mechanisms responsible for its transcriptional upregulation. Finally, we review interventions to block NOS-2 upregulation or to counteract its effects, with an emphasis on the potential therapeutic strategies to improve outcomes in patients afflicted with SAH. There is still much to be learned regarding the apparently maladaptive response of NOS-2 and its harmful product NO in SAH. However, the available evidence points to crucial effects that, on balance, are adverse, making the NOS-2/NO/peroxynitrite axis an attractive therapeutic target in SAH.

Production and physiological role of NO in the oral cavity

SummaryNitric oxide (NO) is a free radical which is produced from a wide variety of cells and tissues in the human body. NO is involved in the regulation of many physiological processes, such as vascular relaxation, neurotransmission, immune regulation, and cell death. NO is generated by nitric oxide synthase (NOS), which has three identified isoforms: neuronal type NOS (nNOS), endothelial type NOS (eNOS), and inducible type NOS (iNOS). Different isoforms are expressed depending on the organs, tissues, and cells, and investigation of the types and functions of enzymes expressed in various tissues is underway. The oral cavity is a space in which marked changes have been detected in NO levels, and each tissue is constantly influenced by NO. NO is a component of saliva and is produced by oral bacteria in the oral cavity and released by NOS expressed in oral mucosa. NOS isoforms expressed under normal conditions differ among the oral organs. In addition, the overexpression of NOS was involved in carcinogenesis and tumor growth progression. This review summarized the expression of NOS and functions of NO in oral cavity organs, and their roles in diseases and the influences of treatments.

Endothelial nitric oxide synthase in the microcirculation.

Endothelial nitric oxide synthase (eNOS, NOS3) is responsible for producing nitric oxide (NO)--a key molecule that can directly (or indirectly) act as a vasodilator and anti-inflammatory mediator. In this review, we examine the structural effects of regulation of the eNOS enzyme, including post-translational modifications and subcellular localization. After production, NO diffuses to surrounding cells with a variety of effects. We focus on the physiological role of NO and NO-derived molecules, including microvascular effects on vessel tone and immune response. Regulation of eNOS and NO action is complicated; we address endogenous and exogenous mechanisms of NO regulation with a discussion of pharmacological agents used in clinical and laboratory settings and a proposed role for eNOS in circulating red blood cells.

Implications of glial nitric oxide in neurodegenerative diseases.

Nitric oxide (NO) is a pleiotropic janus-faced molecule synthesized by nitric oxide synthases (NOS) which plays a critical role in a number of physiological and pathological processes in humans. The physiological roles of NO depend on its local concentrations, as well as its availability and the nature of downstream target molecules. Its double-edged sword action has been linked to neurodegenerative disorders. Excessive NO production, as the evoked by inflammatory signals, has been identified as one of the major causative reasons for the pathogenesis of several neurodegenerative diseases. Moreover, excessive NO synthesis under neuroinflammation leads to the formation of reactive nitrogen species and neuronal cell death. There is an intimate relation between microglial activation, NO and neuroinflammation in the human brain. The role of NO in neuroinflammation has been defined in animal models where this neurotransmitter can modulate the inflammatory process acting on key regulatory pathways, such as those associated with excitotoxicity processes induced by glutamate accumulation and microglial activation. Activated glia express inducible NOS and produce NO that triggers calcium mobilization from the endoplasmic reticulum, activating the release of vesicular glutamate from astroglial cells resulting in neuronal death. This change in microglia potentially contributes to the increased age-associated susceptibility and neurodegeneration. In the current review, information is provided about the role of NO, glial activation and age-related processes in the central nervous system (CNS) that may be helpful in the isolation of new therapeutic targets for aging and neurodegenerative diseases.

Nitric oxide augments single Ca2+ channel currents via cGMP-dependent protein kinase in Kenyon cells isolated from the mushroom body of the cricket brain

Behavioral and pharmacological studies in insects have suggested that the nitric oxide (NO)/cyclic GMP (cGMP) signaling pathway is involved in the formation of long-term memory (LTM) associated with olfactory learning. However, the target molecules of NO and the downstream signaling pathway are still not known. In this study, we investigated the action of NO on single voltage-dependent Ca2+ channels in the intrinsic neurons known as Kenyon cells within the mushroom body of the cricket brain, using the cell-attached configuration of the patch-clamp technique. Application of the NO donor S-nitrosoglutathione (GSNO) increased the open probability (NPO) of single Ca2+ channel currents. This GSNO-induced increase was blocked by ODQ, a soluble guanylate cyclase (sGC) inhibitor, suggesting that the NO generated by GSNO acts via sGC to raise cGMP levels. The membrane-permeable cGMP analog 8-Bro-cGMP also increased the NPO of single Ca2+ channel currents. Pretreatment of cells with KT5823, a protein kinase G blocker, abolished the excitatory effect of GSNO. These results suggest that NO augments the activity of single Ca2+ channels via the cGMP/PKG signaling pathway. To gain insight into the physiological role of NO, we examined the effect of GSNO on action potentials of Kenyon cells under current-clamp conditions. Application of GSNO increased the frequency of action potentials elicited by depolarizing current injections, indicating that NO acts as a modulator resulting in a stimulatory signal in Kenyon cells. We discuss the increased Ca2+ influx through these Ca2+ channels via the NO/cGMP signaling cascade in relation to the formation of olfactory LTM.

Cell-specific regulation of neuronal activity by endogenous production of nitric oxide.

Nitric oxide (NO) is a key regulator of neuronal excitability in the nervous system. While most studies have investigated its role as an intercellular messenger/modulator, less is known about potential physiological roles played by NO within NO-producing neurons. We showed previously that intrinsic production of NO within B5 neurons of the pond snail Helisoma trivolvis increased neuronal excitability by acting on three ionic conductances. Here we demonstrate that intrinsically produced NO affected two of the same conductances in another buccal neuron, B19, where it had the opposite, namely inhibitory, effect on neuronal activity. Using single-cell RT-PCR, we show that B19 neurons express NO synthase (NOS) mRNA. The inhibition of intrinsic NO production with NOS inhibitors caused membrane potential depolarization, transient spiking and an increase in input resistance. Inhibition of the main intracellular receptor of NO, soluble guanylyl cyclase, had similar effects on the parameters mentioned above. An investigation of the effects of NO on ion channels revealed that intrinsic NO mediated neuronal hyperpolarization by activating voltage-gated calcium channels that in turn caused the tonic opening of apamin-sensitive calcium-activated potassium channels. The analysis of action potentials in B5 and B19 neurons suggested that the opposite effects on neuronal excitability elicited by intrinsic NO were probably determined by differences in the ionic conductances that shape their action potentials. In summary, we describe a mechanism by which B19 neurons utilise intrinsically produced NO in a cell-type-specific fashion to decrease their neuronal activity, highlighting an important physiological role of NO within NO-producing neurons.