Beta subunits of the olfactory cyclic nucleotide-gated channel form a nitric oxide activated Ca2+ channel

Nitric oxide in the insular cortex modulates baroreflex responses in a cGMP-independent pathway

Insular cortex is a brain structure involved in the modulation of autonomic activity and cardiovascular function. The nitric oxide/cyclic guanosine-3',5'-monophosphate pathway is a prominent signaling mechanism in the central nervous system, controlling behavioral and physiological responses. Nevertheless, despite evidence regarding the presence of nitric oxide-synthesizing neurons in the insular cortex, its role in the control of autonomic and cardiovascular function has never been reported. Thus, the present study aimed to investigate the involvement of nitric oxide/cyclic guanosine-3',5'-monophosphate pathway mediated by neuronal nitric oxide synthase (nNOS) activation within the insular cortex in the modulation of baroreflex responses in unanesthetized rats. For this, we evaluated the effect of bilateral microinjection of either the nitric oxide scavenger carboxy-PTIO, the selective neuronal nitric oxide synthase inhibitor N^ω-Propyl-l-arginine or the soluble guanylate cyclase inhibitor ODQ into the insular cortex on the bradycardia evoked by blood pressure increases in response to intravenous infusion of phenylephrine, and the tachycardia caused by blood pressure decreases evoked by intravenous infusion of sodium nitroprusside. Bilateral microinjection of either NPLA or carboxy-PTIO into the insular cortex increased the reflex bradycardic response, whereas the reflex tachycardia was decreased by these treatments. Bilateral microinjection of the soluble guanylate cyclase inhibitor into the insular cortex did not affect any parameter of baroreflex function evaluated. Overall, our findings provide evidence that insular cortex nitrergic signaling, acting via neuronal nitric oxide synthase, plays a prominent role in control of baroreflex function. However, control of reflex responses seems to be independent of soluble guanylate cyclase activation.

Thioredoxin shapes the C. elegans sensory response to Pseudomonas produced nitric oxide

Nitric oxide (NO) is released into the air by NO-producing organisms; however, it is unclear if animals utilize NO as a sensory cue. We show that C. elegans avoids Pseudomonas aeruginosa (PA14) in part by detecting PA14-produced NO. PA14 mutants deficient for NO production fail to elicit avoidance and NO donors repel worms. PA14 and NO avoidance are mediated by a chemosensory neuron (ASJ) and these responses require receptor guanylate cyclases and cyclic nucleotide gated ion channels. ASJ exhibits calcium increases at both the onset and removal of NO. These NO-evoked ON and OFF calcium transients are affected by a redox sensing protein, TRX-1/thioredoxin. TRX-1’s trans-nitrosylation activity inhibits the ON transient whereas TRX-1’s de-nitrosylation activity promotes the OFF transient. Thus, C. elegans exploits bacterially produced NO as a cue to mediate avoidance and TRX-1 endows ASJ with a bi-phasic response to NO exposure.

Nitric oxide is a colorless gas that contains one nitrogen atom and one oxygen atom. Found at very low levels in the air, this gas is produced by the intense heat of lightning strikes and by combustion engines. Almost all living organisms also produce nitric oxide. In animals, for example, nitric oxide regulates blood pressure and signaling between neurons. However, it was not known if animals could detect nitric oxide in their environment and respond to it.

Caenorhabditis elegans is a worm that has been intensively studied in many fields of biology. Unlike most animals, it cannot make nitric oxide. Yet, living in the soil, C. elegans does come into contact with many microbes that can, including the bacterium Pseudomonas aeruginosa. These bacteria can infect and kill C. elegans, and so the worm typically avoids them. Hao, Yang et al. asked whether C. elegans does so by detecting the nitric oxide that these harmful bacteria release into their environment.

First, worms were added to a petri dish where a small patch of P. aeruginosa was growing. Consistent with previous results, the worms had all moved away from the bacteria after a few hours. The experiments were then repeated with mutant bacteria that cannot produce nitric oxide. The worms were less likely to avoid these mutant bacteria, suggesting that C. elegans does indeed avoid infection by detecting bacterially produced nitric oxide.

Next, using a range of techniques, Hao, Yang et al. showed that C. elegans avoids nitric oxide released into its environment by detecting the gas via a pair of sensory neurons. These neurons require several specific proteins to be able to detect nitric oxide and respond to it. In particular, a protein called Thioredoxin was found to determine the beginning and end of the worm’s sensory response to nitric oxide.

All of these proteins are also found in many other animals, and so it is possible that these findings may be relevant to other species too. Further studies are now needed to confirm whether other organisms can sense nitric oxide from their environment and, if so, how their nervous systems equip them to do this.

The role of nitric oxide in pre-synaptic plasticity and homeostasis

Since the observation that nitric oxide (NO) can act as an intercellular messenger in the brain, the past 25 years have witnessed the steady accumulation of evidence that it acts pre-synaptically at both glutamatergic and GABAergic synapses to alter release-probability in synaptic plasticity. NO does so by acting on the synaptic machinery involved in transmitter release and, in a coordinated fashion, on vesicular recycling mechanisms. In this review, we examine the body of evidence for NO acting as a retrograde factor at synapses, and the evidence from in vivo and in vitro studies that specifically establish NOS1 (neuronal nitric oxide synthase) as the important isoform of NO synthase in this process. The NOS1 isoform is found at two very different locations and at two different spatial scales both in the cortex and hippocampus. On the one hand it is located diffusely in the cytoplasm of a small population of GABAergic neurons and on the other hand the alpha isoform is located discretely at the post-synaptic density (PSD) in spines of pyramidal cells. The present evidence is that the number of NOS1 molecules that exist at the PSD are so low that a spine can only give rise to modest concentrations of NO and therefore only exert a very local action. The NO receptor guanylate cyclase is located both pre- and post-synaptically and this suggests a role for NO in the coordination of local pre- and post-synaptic function during plasticity at individual synapses. Recent evidence shows that NOS1 is also located post-synaptic to GABAergic synapses and plays a pre-synaptic role in GABAergic plasticity as well as glutamatergic plasticity. Studies on the function of NO in plasticity at the cellular level are corroborated by evidence that NO is also involved in experience-dependent plasticity in the cerebral cortex.

Differential regulation by cyclic nucleotides of the CNGA4 and CNGB1b subunits in olfactory cyclic nucleotide-gated channels

Olfactory cyclic nucleotide-gated (CNG) ion channels are essential contributors to signal transduction of olfactory sensory neurons. The activity of the channels is controlled by the cyclic nucleotides guanosine 3',5'-monophosphate (cGMP) and adenosine 3',5'-monophosphate (cAMP). The olfactory CNG channels are composed of two CNGA2 subunits, one CNGA4 and one CNGB1b subunit, each containing a cyclic nucleotide-binding domain. Using patch-clamp fluorometry, we measured ligand binding and channel activation simultaneously and showed that cGMP activated olfactory CNG channels not only by binding to the two CNGA2 subunits but also by binding to the CNGA4 subunit. In a channel in which the CNGA2 subunits were compromised for ligand binding, cGMP binding to CNGA4 was sufficient to partly activate the channel. In contrast, in heterotetrameric channels, the CNGB1b subunit did not bind cGMP, but channels with this subunit showed activation by cAMP. Thus, the modulatory subunits participate actively in translating ligand binding to activation of heterotetrameric olfactory CNG channels and enable the channels to differentiate between cyclic nucleotides.

Thioredoxin 1-mediated post-translational modifications: reduction, transnitrosylation, denitrosylation, and related proteomics methodologies

Despite the significance of redox post-translational modifications (PTMs) in regulating diverse signal transduction pathways, the enzymatic systems that catalyze reversible and specific oxidative or reductive modifications have yet to be firmly established. Thioredoxin 1 (Trx1) is a conserved antioxidant protein that is well known for its disulfide reductase activity. Interestingly, Trx1 is also able to transnitrosylate or denitrosylate (defined as processes to transfer or remove a nitric oxide entity to/from substrates) specific proteins. An intricate redox regulatory mechanism has recently been uncovered that accounts for the ability of Trx1 to catalyze these different redox PTMs. In this review, we will summarize the available evidence in support of Trx1 as a specific disulfide reductase, and denitrosylation and transnitrosylation agent, as well as the biological significance of the diverse array of Trx1-regulated pathways and processes under different physiological contexts. The dramatic progress in redox proteomics techniques has enabled the identification of an increasing number of proteins, including peroxiredoxin 1, whose disulfide bond formation and nitrosylation status are regulated by Trx1. This review will also summarize the advancements of redox proteomics techniques for the identification of the protein targets of Trx1-mediated PTMs. Collectively, these studies have shed light on the mechanisms that regulate Trx1-mediated reduction, transnitrosylation, and denitrosylation of specific target proteins, solidifying the role of Trx1 as a master regulator of redox signal transduction.

Pheromone transduction in moths

Calling female moths attract their mates late at night with intermittent release of a species-specific sex-pheromone blend. Mean frequency of pheromone filaments encodes distance to the calling female. In their zig-zagging upwind search male moths encounter turbulent pheromone blend filaments at highly variable concentrations and frequencies. The male moth antennae are delicately designed to detect and distinguish even traces of these sex pheromones amongst the abundance of other odors. Its olfactory receptor neurons sense even single pheromone molecules and track intermittent pheromone filaments of highly variable frequencies up to about 30 Hz over a wide concentration range. In the hawkmoth Manduca sexta brief, weak pheromone stimuli as encountered during flight are detected via a metabotropic PLCβ-dependent signal transduction cascade which leads to transient changes in intracellular Ca²⁺ concentrations. Strong or long pheromone stimuli, which are possibly perceived in direct contact with the female, activate receptor-guanylyl cyclases causing long-term adaptation. In addition, depending on endogenous rhythms of the moth's physiological state, hormones such as the stress hormone octopamine modulate second messenger levels in sensory neurons. High octopamine levels during the activity phase maximize temporal resolution cAMP-dependently as a prerequisite to mate location. Thus, I suggest that sliding adjustment of odor response threshold and kinetics is based upon relative concentration ratios of intracellular Ca²⁺ and cyclic nucleotide levels which gate different ion channels synergistically. In addition, I propose a new hypothesis for the cyclic nucleotide-dependent ion channel formed by insect olfactory receptor/coreceptor complexes. Instead of being employed for an ionotropic mechanism of odor detection it is proposed to control subthreshold membrane potential oscillation of sensory neurons, as a basis for temporal encoding of odors.

Localization of eNOS in the olfactory epithelium of the rat

Nitric oxide (NO) is a free radical and produced from L-arginine by nitric oxide synthase (NOS). Since NO is recently suggested to be involved in olfactory perception, the expression of eNOS, an isoform of NOS, was examined in the rat olfactory epithelium. The activity of NADPH-diaphorase was also examined as a marker of NOS. In the dorsomedial region of the nasal cavity, intensely positive reactions for NADPH-diaphorase were observed in the entire cytoplasm of sensory cells (olfactory cells). By immunohistochemistry, intensely positive reactions for eNOS were also found in the dorsomedial region of the nasal cavity. These reactions were observed on the free border of the olfactory epithelium. By immunoelectron microscopy, positive reactions for eNOS were found in the cilia of olfactory cells. In addition, in situ hybridization analysis of the olfactory epithelium revealed the expression of eNOS mRNA in the olfactory cells. These results indicate the presence of eNOS in the olfactory cells of the rat, and differential expression of eNOS in the olfactory epithelium depending on the regions of the nasal cavity. In addition, NO produced by eNOS may be involved in olfactory perception in the cilia of olfactory cells.

NADPH-diaphorase activity and NO synthase expression in the olfactory epithelium of the bovine

NADPH-diaphorase (NADPH-d) staining of the bovine olfactory epithelium was compared with the immunohistochemical localization of nitric oxide synthase (NOS), soluble guanylyl cyclase, and cGMP (cyclic guanosine 3',5'-monophosphate). Out of the three isoforms, only the inducible NOS (NOS-II) was found at the epithelial surface correlating with the strong labelling for NADPH-d. In contrast, light diaphorase staining associated with deeper epithelial regions did not coincide with any NOS immunoreactivity. As there is overlapping expression of NOS-II, soluble guanylyl cyclase and cGMP at the luminal surface morphologically occupied by dendritic knobs of olfactory receptor neurons and microvillar endings of supporting cells, the nitric oxide (NO)/cGMP pathway is likely to be involved in modulating the odour signals during olfactory transduction.

A novel cyclic nucleotide-gated ion channel enriched in synaptic terminals of isotocin neurons in zebrafish brain and pituitary

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels opened by binding of intracellular cyclic GMP or cyclic AMP. CNG channels mediate sensory transduction in the rods and cones of the retina and in olfactory sensory neurons, but in addition, CNG channels are also expressed elsewhere in the CNS, where their physiological roles have not yet been well defined. Besides the CNG channel subtypes that mediate vision and olfaction, zebrafish has an additional subtype, CNGA5, which is expressed almost exclusively in the brain. We have generated CNGA5-specific monoclonal antibodies, which we use here to show that immunoreactivity for CNGA5 channels is highly enriched in synaptic terminals of a discrete set of neurons that project to a subregion of the pituitary, as well as diffusely in the brain and spinal cord. Double labeling with a variety of antibodies against pituitary hormones revealed that CNGA5 is located in the terminals of neuroendocrine cells that secrete the nonapeptide hormone/transmitter isotocin in the neurohypophysis, brain, and spinal cord. Furthermore, we show that CNGA5 channels expressed in Xenopus oocytes are highly permeable to Ca(2+), which suggests that the channels are capable of modulating isotocin release in the zebrafish brain and pituitary. Isotocin is the teleost homolog of the mammalian hormone oxytocin, and like oxytocin, it regulates reproductive and social behavior. Therefore, the high calcium permeability of CNGA5 channels and their strategic location in isotocin-secreting synaptic terminals suggest that activation of CNGA5 channels in response to cyclic nucleotide signaling may have wide-ranging neuroendocrine and behavioral effects.

Odorant-dependent generation of nitric oxide in Mammalian olfactory sensory neurons

The gaseous signalling molecule nitric oxide (NO) is involved in various physiological processes including regulation of blood pressure, immunocytotoxicity and neurotransmission. In the mammalian olfactory bulb (OB), NO plays a role in the formation of olfactory memory evoked by pheromones as well as conventional odorants. While NO generated by the neuronal isoform of NO synthase (nNOS) regulates neurogenesis in the olfactory epithelium, NO has not been implicated in olfactory signal transduction. We now show the expression and function of the endothelial isoform of NO synthase (eNOS) in mature olfactory sensory neurons (OSNs) of adult mice. Using NO-sensitive micro electrodes, we show that stimulation liberates NO from isolated wild-type OSNs, but not from OSNs of eNOS deficient mice. Integrated electrophysiological recordings (electro-olfactograms or EOGs) from the olfactory epithelium of these mice show that NO plays a significant role in modulating adaptation. Evidence for the presence of eNOS in mature mammalian OSNs and its involvement in odorant adaptation implicates NO as an important new element involved in olfactory signal transduction. As a diffusible messenger, NO could also have additional functions related to cross adaptation, regeneration, and maintenance of MOE homeostasis.

NO/cGMP-dependent modulation of synaptic transmission

Nitric oxide (NO) is a multifunctional messenger in the CNS that can signal both in antero- and retrograde directions across synapses. Many effects of NO are mediated through its canonical receptor, the soluble guanylyl cyclase, and the second messenger cyclic guanosine-3',5'-monophosphate (cGMP). An increase of cGMP can also arise independently of NO via activation of membrane-bound particulate guanylyl cyclases by natriuretic peptides. The classical targets of cGMP are cGMP-dependent protein kinases (cGKs), cyclic nucleotide hydrolysing phosphodiesterases, and cyclic nucleotide-gated (CNG) cation channels. The NO/cGMP/cGK signalling cascade has been linked to the modulation of transmitter release and synaptic plasticity by numerous pharmacological and genetic studies. This review focuses on the role of NO as a retrograde messenger in long-term potentiation of transmitter release in the hippocampus. Presynaptic mechanisms of NO/cGMP/cGK signalling will be discussed with recently identified potential downstream components such as CaMKII, the vasodilator-stimulated phosphoprotein, and regulators of G protein signalling. NO has further been suggested to increase transmitter release through presynaptic clustering of a-synuclein. Alternative modes of NO/cGMP signalling resulting in inhibition of transmitter release and long-term depression of synaptic activity will also be addressed, as well as anterograde NO signalling in the cerebellum. Finally, emerging evidence for cGMP signalling through CNG channels and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels will be discussed.

New perspectives on the mechanisms through which nitric oxide may affect learning and memory processes

Nitric oxide (NO) has been well established as a molecule necessary for memory consolidation. Interestingly, the majority of research has focused on only a single mechanism through which NO acts, namely the up-regulation of guanylate cyclase (GC). However, since NO and NO-derived reactive nitrogen species are capable of interacting with a broad array of enzymes, ion channels and receptors, a singular focus on GC appears short-sighted. Although NO inhibits the action of a number of molecules there are four, in addition to GC, which are up-regulated by the direct presence of NO, or NO-derived radicals, and implicated in memory processing. They are: cyclic nucleotide-gated channels; large conductance calcium-activated potassium channels; ryanodine receptor calcium release (RyR) channels; and the enzyme mono(ADP-ribosyl) transferase. This review presents evidence that not only are these four molecules worthy of investigation as GC-independent mechanisms through which NO may act, but that behavioural evidence already exists suggesting a relationship between NO and the RyR channel.

Characterization of a novel cyclic nucleotide-gated channel from zebrafish brain

Cyclic nucleotide-gated (CNG) channels have been well characterized in the sensory receptors of vision and olfaction, but their characteristics in other tissues remain largely unknown. Here, we report characterization of a novel brain-specific CNG channel from zebrafish. Unique among CNG channels, the transcript is expressed mainly in the brain. When expressed in Xenopus oocytes, the channel's electrophysiological properties are distinct compared to CNG channels from either rods (CNGA1), olfactory receptors (CNGA2), or cones (CNGA3). The channel is less sensitive to cAMP than cGMP (K(1/2) of 280 and 7 microM, respectively), with a maximum cAMP efficacy at least 80% of that with saturating levels of cGMP. The single-channel conductance of 58pS is larger than most other CNG channels. Like other CNG channels the channel is relatively nonselective among monovalent cations. However, unlike other CNG channels, there was rundown of the macroscopic current within 30-100 min after patch excision.

Morituri te salutant? Olfactory signal transduction and the role of phosphoinositides

During the past 150 years, researchers have investigated the cellular, physiological, and molecular mechanisms underlying the sense of smell. Based on these efforts, a conclusive model of olfactory signal transduction in the vertebrate's nose is now available, spanning from G-protein-mediated odorant receptors to ion channels, which are linked by a cyclic adenosine 3',5'-monophosphate-mediated signal transduction cascade. Here we review some historical milestones in the chronology of olfactory research, particularly emphasising the role of cyclic nucleotides and inositol trisphosphate as alternative second messengers in olfactory cells. We will describe the functional anatomy of the nose, outline the cellular composition of the olfactory epithelium, and describe the discovery of the molecular backbone of the olfactory signal transduction cascade. We then summarize our current model, in which cyclic adenosine monophosphate is the sole excitatory second messenger in olfactory sensory neurons. Finally, a possible significance of microvillous olfactory epithelial cells and inositol trisphosphate in olfaction will be discussed.

Intracellular Thiol-mediated Modulation of Epithelial Sodium Channel Activity

The epithelial sodium channel ENaC is physiologically important in the kidney for the regulation of the extracellular fluid volume, and in the lungs for the maintenance of the appropriate airway surface liquid volume that lines the pulmonary epithelium. Besides the regulation of ENaC by hormones, intracellular factors such as Na(+) ions, pH, or Ca(2+) are responsible for fast adaptive responses of ENaC activity to changes in the intracellular milieu. In this study, we show that ENaC is rapidly and reversibly inhibited by internal sulfhydryl-reactive molecules such as methanethiosulfonate derivatives of different sizes, the metal cations Cd(2+) and Zn(2+), or copper(II) phenanthroline, a mild oxidizing agent that promotes the formation of disulfide bonds. At the single channel level, these agents applied intracellularly induce the appearance of long channel closures, suggesting an effect on ENaC gating. The intracellular reducing agent dithiothreitol fully reverses the rundown of ENaC activity in inside-out patches. Our observations suggest that changes in intracellular redox potential modulate ENaC activity and may regulate ENaC-mediated Na(+) transport in epithelia. Finally, substitution experiments reveal that multiple cysteine residues in the amino and carboxyl termini of ENaC subunits are responsible for this thiol-mediated inhibition of ENaC.

A conserved tripeptide in CNG and HCN channels regulates ligand gating by controlling C-terminal oligomerization

Cyclic nucleotides directly enhance the opening of the tetrameric CNG and HCN channels, although the mechanism remains unclear. We examined why HCN and certain CNG subunits form functional homomeric channels, whereas other CNG subunits only function in heteromeric channels. The "defect" in the CNGA4 subunit that prevents its homomeric expression was localized to its C-linker, which connects the transmembrane domain to the binding domain and contains a tripeptide that decreases the efficacy of ligand gating. Remarkably, replacement of the homologous HCN tripeptide with the CNGA4 sequence transformed cAMP into an inverse agonist that inhibits HCN channel opening. Using analytical ultracentrifugation, we identified the structural basis for this gating switch: whereas cAMP normally enhances the assembly of HCN C-terminal domains into a tetrameric gating ring, inclusion of the CNGA4 tripeptide reversed this action so that cAMP now causes gating ring disassembly. Thus, ligand gating depends on the dynamic oligomerization of C-terminal binding domains.

Arachidonic acid regulates two Ca2+ entry pathways via nitric oxide

Several regulated Ca2+ entry pathways have been identified, with capacitative Ca2+ entry (CCE) being the most characterized. In the present study, we examined Ca2+ entry pathways regulated by arachidonic acid (AA) in mouse parotid acini. AA induced Ca2+ release from intracellular stores, and increased Ca2+ entry. AA inhibited thapsigargin (Tg)-induced CCE, whereas AA activated Ca2+ entry when CCE was blocked by gadolinium (Gd3+). AA-induced Ca2+ entry was associated with depletion of calcium from ryanodine-sensitive stores; both AA-induced Ca2+ release and Ca2+ entry were inhibited by tetracaine and the nitric oxide synthase (NOS) inhibitor, 7-nitroindazole (7-NI). The nitric oxide (NO) donor, 1,2,3,4-ox-triazolium,5-amino-3-(3,4-dichlorophenyl)-chloride (GEA 3162), but not 8-bromo-cGMP, mimicked the effects of AA in inhibiting CCE. Results suggest that AA acts via nitric acid to inhibit the CCE pathway that is selective for Ca2+, and to activate a second Ca2+ entry pathway that is dependent on depletion of Ca2+ from ryanodine-sensitive stores.

Rat cerebellar granule cells are protected from glutamate-induced excitotoxicity by S-nitrosoglutathione but not glutathione

In cultured rat cerebellar granule cells, glutamate or N-methyl-d-aspartate (NMDA) activation of the NMDA receptor caused a sustained increase in cytosolic Ca(2+) levels ([Ca(2+)](i)), reactive oxygen species (ROS) generation, and cell death (respective EC(50) values for glutamate were 12, 30, and 38 microM) but no increase in caspase-3 activity. Removal of extracellular Ca(2+) blocked all three glutamate-induced effects, whereas pretreatment with an ROS scavenger inhibited glutamate-induced cell death but had no effect on the [Ca(2+)](i) increase. This indicates that glutamate-induced cell death is attributable to [Ca(2+)](i) increase and ROS generation, and the [Ca(2+)](i) increase precedes ROS generation. Apoptotic cell death was not seen until 24 h after exposure of cells to glutamate. S-nitrosoglutathione abolished glutamate-induced ROS generation and cell death, and only a transient [Ca(2+)](i) increase was seen; similar results were observed with another nitric oxide (NO) donor, S-nitroso-N-acetylpenicillamine, but not with glutathione, which suggests that the effects were caused by NO. The transient [Ca(2+)](i) increase and the abolishment of ROS generation induced by glutamate and S-nitrosoglutathione were still seen in the presence of an ROS scavenger. Glial cells, which were present in the cultures used, showed no [Ca(2+)](i) increase in the presence of glutamate, and glutamate-induced granule cell death was independent of the percentage of glial cells. In conclusion, NO donors protect cultured cerebellar granule cells from glutamate-induced cell death, which is mediated by ROS generated by a sustained [Ca(2+)](i) increase, and glial cells provide negligible protection against glutamate-induced excitotoxicity.

Atypical relaxation by scorpion venom in the lamb urethral smooth muscle involves both NO-dependent and -independent responses

The sustained depolarisation induced by alpha-toxins from scorpion venom (20 microg/ml(-1)) was used to test the hypothesis that an endogenous, photo-sensitive, nitrocompound could act as a stable nitrergic transmitter in the sheep (lamb) urethra. Scorpion venom-treatment effectively abolished neurogenic responses to electrical field stimulation, but it did not modify the spontaneous urethral photorelaxation. On the other hand, scorpion venom induced an atypical relaxation in noradrenaline-contracted preparations, which could be reverted, but not prevented, by tetrodotoxin (TTX, 1 microM). However, after TTX-pretreatment, relaxations elicited by scorpion venom were significantly delayed and slowed down, and similar responses were obtained in the presence of ouabain (10 microM), low sodium medium, or after the inhibition of the NO-cGMP pathway. Although the involvement of K(+) and Cl(-) channels can be ruled out since both charybdotoxin (300 nM) and chlorotoxin (50 nM) did not elicit any urethral relaxation nor modified the scorpion venom-induced one. However, a slow Ca(2+) channel seems to be involved. GVIA omega-conotoxin (1 microM), but not MVIIC omega-conotoxin (1 microM), significantly inhibited both EFS- and scorpion venom-induced relaxations and almost abolished the partial relaxation that was resistant to NO synthase inhibition. On the other hand, the presence of L-cis-diltiazem (0.3 mM), a selective inhibitor of cyclic nucleotide gated channels (CNGCs), also delayed and slowed down relaxation induced by scorpion venom, as well as abolish its reversal by TTX. L-cis-diltiazem pre-treatment induced a progressive decay in urethral relaxation brought about by electrical field stimulation only when repetitive, long duration stimulation protocols were used. Taken together, our results do not support the hypothesis of the endogenous, photo-sensitive, urethral nitrocompound as reflecting a stable nitrergic transmitter instead of NO. However, they suggest the involvement of both a NO-cGMP-dependent and TTX-sensitive component and a NO-independent response, mediated by GVIA omega-conotoxin-sensitive Ca(2+) channels, in the neurogenic relaxation of the urethral muscle. In addition, the likely involvement of CNGCs as an additional component of the cGMP signalling mechanism is suggested.

NO activates the olfactory cyclic nucleotide-gated conductance independent from cGMP in isolated rat olfactory receptor neurons

Nitric oxide (NO) is a putative neuronal messenger in the olfactory epithelium. We analysed the electrical responses of rat olfactory receptor neurones to NO stimulation in the whole-cell patch clamp mode and found that NO activates the cyclic nucleotide-gated conductance, independent from cyclic GMP. As NO may also open calcium-dependent potassium channels and raise cyclic GMP levels, it appears to act as a complex modulator of olfactory receptor neuron physiology in vivo.

Nitric oxide inhibits prolactin secretion in pituitary cells downstream of voltage-gated calcium influx

The coupling between nitric oxide (NO)-cGMP signaling pathway and prolactin (PRL) release in pituitary lactotrophs has been established previously. However, the messenger that mediates the action of this signaling pathway on hormone secretion and the secretory mechanism affected, calcium dependent or independent, have not been identified. In cultured pituitary cells, basal PRL release was controlled by spontaneous voltage-gated calcium influx and was further enhanced by depolarization of cells and stimulation with TRH. Inhibition of constitutively expressed neuronal NO synthase decreased NO and cGMP levels and increased basal PRL release. The addition of a slowly releasable NO donor increased cGMP levels and inhibited basal PRL release in a time-dependent manner. Expression of inducible NO synthase also increased NO and cGMP levels and inhibited basal, depolarization-induced, and TRH-induced PRL release, whereas inhibition of this enzyme decreased NO and cGMP production and recovered PRL release. None of these treatments affected spontaneous and stimulated voltage-gated calcium influx. At basal NO levels, the addition of permeable cGMP analogs did not inhibit PRL secretion. At elevated NO levels, inhibition of cGMP production and facilitation of its degradation did not reverse inhibited PRL secretion. These experiments indicate that NO inhibits calcium-dependent PRL secretion in a cGMP-independent manner and downstream of voltage-gated calcium influx.

GEA3162 stimulates Ca2+ entry in neutrophils

We showed that 5-amino-3-(3,4-dichlorophenyl)1,2,3,4-oxatriazolium (GEA3162), a lipophilic nitric oxide (NO)-releasing agent, induced Ca(2+) entry into rat neutrophils in a concentration-dependent manner, whereas the guanylyl cyclase inhibitors, 6-anilino-5,8-quinolinequinone (LY83583) and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), had no effect on GEA3162-induced response. The GEA3162-induced Ca(2+) entry was not observed in a Ca(2+)-free medium. GEA3162 did not potentiate but reduced the store-emptying activated Ca(2+) entry caused by cyclopiazonic acid. Stimulation of cells with GEA3162 in the absence of extracellular Ca(2+) followed by addition of cations showed that only Ca(2+) but not Ba(2+) and Sr(2+) entry occurs. Store-operated Ca(2+) entry was sensitive to La(3+) and Ni(2+) inhibition, whereas the GEA3162-induced Ca(2+) entry was sensitive to La(3+) but resistant to Ni(2+). cis-N-(2-Phenylcyclopentyl)azacyclotridec-1-en-2-amine (MDL-12,330A) and calyculin A diminished the Ca(2+) entry activated by cyclopiazonic acid as well as by GEA3162. In contrast, 2-aminoethyldiphenyl borate (2-APB) diminished cyclopiazonic acid-but enhanced GEA3162-induced [Ca(2+)](i) change. Genistein effectively attenuated the cyclopiazonic acid-but slightly inhibited GEA3162-induced [Ca(2+)](i) change. Application of neomycin and high extracellular Ca(2+) concentration did not induce [Ca(2+)](i) rise. These data suggest that GEA3162 induced Ca(2+) entry and regulated Ca(2+) signal, through direct protein thiol oxidation. The action of GEA3162 demonstrates characteristics that distinguish it from the store-operated mechanism in neutrophils and therefore is likely to represent an entirely distinct pathway. Extracellular Ca(2+)-sensing receptor is not existing in neutrophils.

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels first identified in retinal photoreceptors and olfactory sensory neurons (OSNs). They are opened by the direct binding of cyclic nucleotides, cAMP and cGMP. Although their activity shows very little voltage dependence, CNG channels belong to the superfamily of voltage-gated ion channels. Like their cousins the voltage-gated K+ channels, CNG channels form heterotetrameric complexes consisting of two or three different types of subunits. Six different genes encoding CNG channels, four A subunits (A1 to A4) and two B subunits (B1 and B3), give rise to three different channels in rod and cone photoreceptors and in OSNs. Important functional features of these channels, i.e., ligand sensitivity and selectivity, ion permeation, and gating, are determined by the subunit composition of the respective channel complex. The function of CNG channels has been firmly established in retinal photoreceptors and in OSNs. Studies on their presence in other sensory and nonsensory cells have produced mixed results, and their purported roles in neuronal pathfinding or synaptic plasticity are not as well understood as their role in sensory neurons. Similarly, the function of invertebrate homologs found in Caenorhabditis elegans, Drosophila, and Limulus is largely unknown, except for two subunits of C. elegans that play a role in chemosensation. CNG channels are nonselective cation channels that do not discriminate well between alkali ions and even pass divalent cations, in particular Ca2+. Ca2+ entry through CNG channels is important for both excitation and adaptation of sensory cells. CNG channel activity is modulated by Ca2+/calmodulin and by phosphorylation. Other factors may also be involved in channel regulation. Mutations in CNG channel genes give rise to retinal degeneration and color blindness. In particular, mutations in the A and B subunits of the CNG channel expressed in human cones cause various forms of complete and incomplete achromatopsia.

Expression of subunits for the cAMP-sensitive 'olfactory' cyclic nucleotide-gated ion channel in the cochlea: implications for signal transduction

Cyclic nucleotide-gated (CNG) ion channels have been implicated as functioning in sensory transduction and in second-messenger modulation of synaptic neurotransmitter release. The olfactory, cAMP-sensitive CNG ion channel in vivo is considered to comprise the pore-forming CNG2 subunit together with CNG5 and CNG4.3 modulatory subunits. The expression of these 'olfactory' CNG subunit transcripts in microdissected subfractions of the rat cochlea and hair cell libraries has been investigated with RT-PCR. Unmodified transcripts of CNG2 were detected in the organ of Corti, lateral wall and spiral ganglion subfractions. CNG5 message was found in both the sensory organ of Corti and the non-sensory lateral wall subfractions but not in the spiral ganglion subfraction. The CNG5 sequence obtained for the organ of Corti fraction encompassed 78% of the olfactory CNG5 cDNA sequence. CNG5 message has also been detected in an inner hair cell cDNA library. In the lateral wall, unmodified CNG5 sequence was observed as well as truncated versions of CNG5 transcripts, one of which was also found in the rat brain. The truncated versions were characterized by deletions that resulted in a shift in reading frame and the premature appearance of a stop codon. The 'olfactory' CNG4.3 cDNA was amplified from all three subfractions. Within the cochlea, CNG2 immunoreactivity was selectively distributed in a pattern similar to that of adenylyl cyclase type I. Immunoreactivity to CNG2 has been localized to stereocilia of inner hair cells. CNG5 immunoreactivity was associated with stereocilia and lateral plasma membranes of outer hair cells. We conclude that transcripts necessary for a functional cAMP-sensitive CNG ion channel are present in the cochlea resulting from combinations of CNG2 with CNG5 and CNG4.3. Further, the localization of CNG2 and CNG5 immunoreactivity to hair cell stereocilia suggests a role for cAMP-sensitive CNG channels in hair cell signal transduction.

The nitric oxide/cyclic GMP pathway in the olfactory processing system of the terrestrial slug Limax marginatus

To examine the distribution of nitric oxide (NO)-generative cells and NO-responsive cells in the tentacles and procerebral lobes (olfactory processing center) of terrestrial slugs, we applied NADPH diaphorase (NADPH-d) histochemistry and NO-induced cyclic GMP (cGMP)-like immunohistochemistry. We found that NADPH-d reactive cells/fibers and cGMP-like immunoreactive cells/fibers were different, but they were localized adjacent to each other, in both the tentacles and the procerebral lobes. Then, we measured the concentration of NO that was generated around the procerebral lobes using an NO sensitive electrode, when the olfactory nerve was electrically stimulated as a replacement for an odorant stimulus. Stimulation of the olfactory nerve evoked an increase in NO concentration at nanomolar levels, suggesting that binding of nanomolar concentrations of NO to the prosthetic heme group activates soluble guanylyl cyclase. Taken together with previously reported physiological data, our results, therefore, showed that the NO/cGMP pathways are involved in slug olfactory processing.

Central role of the CNGA4 channel subunit in Ca2+-calmodulin-dependent odor adaptation

Heteromultimeric cyclic nucleotide-gated (CNG) channels play a central role in the transduction of odorant signals and subsequent adaptation. The contributions of individual subunits to native channel function in olfactory receptor neurons remain unclear. Here, we show that the targeted deletion of the mouse CNGA4 gene, which encodes a modulatory CNG subunit, results in a defect in odorant-dependent adaptation. Channels in excised membrane patches from the CNGA4 null mouse exhibited slower Ca2+-calmodulin-mediated channel desensitization. Thus, the CNGA4 subunit accelerates the Ca2+-mediated negative feedback in olfactory signaling and allows rapid adaptation in this sensory system.

cGMP-mediated facilitation in nerve terminals by enhancement of the spike afterhyperpolarization

cGMP has long been suspected to play a role in synaptic plasticity, but the inaccessibility of nerve terminals to electrical recording has impeded tests of this hypothesis. In posterior pituitary nerve terminals, nitric oxide enhanced Ca(2+)-activated K+ channel activity by activating guanylate cyclase and PKG. This enhancement occurred only at depolarized potentials, so the spike threshold remained unaltered but the afterhyperpolarization became larger. During spike trains, the enhanced afterhyperpolarization promoted Na+ channel recovery from inactivation, thus reducing action potential failures and allowing more Ca(2+) to enter. Activating guanylate cyclase, either with applied nitric oxide, or with physiological stimulation to activate nitric oxide synthase, increased action potential firing. Thus, the cGMP/nitric oxide cascade generates a short-term, use-dependent enhancement of release.

Thiol-reactive agents biphasically regulate inositol 1,4,5-trisphosphate binding and Ca(2+) release activities in bovine adrenal cortex microsomes

Within all endocrine cells, the inositol 1,4,5-trisphosphate (InsP(3)) receptor plays an important role in regulation of the intracellular Ca(2+) concentration. In the present study we showed that a single short-term treatment with either N-ethylmaleimide (known to decrease InsP(3) receptor activity) or thimerosal (known to increase InsP(3) receptor activity) caused time-dependent biphasic effects on the InsP(3) binding activity of bovine adrenal cortex microsomes. The early potentiating effect of thiol-reactive agents translated into a 2-fold increase in binding affinity and Ca(2+) release efficiency. The late dampening effect of thiol-reactive agents translated into a continuous reduction of the maximal binding capacity of the microsomes with a concomitant decrease in Ca(2+) release efficiency. Under these conditions, Western blot analyses demonstrated that the level of InsP(3) receptor protein was not modified. Sequential treatments with thimerosal and the reducing agent dithiothreitol showed that the InsP(3) receptor can readily oscillate between high and low affinity states that are related to its alkylation state. Our results suggest a common mode of action of thiol-reactive agents on the InsP(3) receptor. These results also support the contention that cellular mechanisms of thiol group modification could play important roles in regulation of the intracellular Ca(2+) concentration.

X inactivation of the OCNC1 channel gene reveals a role for activity-dependent competition in the olfactory system

The organization of neuronal systems is often dependent on activity and competition between cells. In olfaction, the X-linked OCNC1 channel subunit is subject to random inactivation and is essential for odorant-evoked activity. Reporter-tagged OCNC1 mutant mice permit the visualization of OCNC1-deficient olfactory neurons and their projections. In heterozygous females, X inactivation creates a mosaic with two populations of genetically distinct neurons. OCNC1-deficient neurons are slowly and specifically depleted from the olfactory epithelium and display unusual patterns of projection to the olfactory bulb. Remarkably, this depletion is dependent on odorant exposure and is reversed by odorant deprivation. This suggests that odorants and the activity they evoke are critical for neuronal survival in a competitive environment and implicate evoked activity in the organization and maintenance of the olfactory system.

Heme oxygenase-2 immunoreactivity in developing and mature bovine olfactory epithelium

In the present study the localization of heme oxygenase-2 (HO-2) in developing and mature olfactory epithelium of the bovine is investigated using immunohistochemistry and post embedding immunogold labelling. HO-2 immunoreactivity is first seen in epithelial cells localized along the luminal surface of the olfactory pit. Up to midgestation the number of HO-2 immunoreactive cells increases throughout all layers of the developing olfactory epithelium. From midgestation through adulthood immunostaining is restricted to perinuclear cytoplasm and axons of mature olfactory receptor neurons localized in intermediate epithelial regions. The temporal and spatial expression patterns of HO-2 immunohistochemistry support the notion that CO plays a role in neuronal differentiation while its presence in mature neurons might be functionally related to olfactory transduction.

Novel activation of non-selective cationic channels by dinitrosyl iron-thiosulfate in PC12 cells

Low molecular mass dinitrosyl iron complexes (DNICs) are nitrosating agents and it is known that the dinitrosyl iron moiety can be transferred to proteins. The aim of the present study was to determine if the formation of protein-bound dinitrosyl iron can modulate ionic channel activity. In PC12 cells, dinitrosyl iron-thiosulfate (50 microM) caused irreversible activation of a depolarizing inward current (IDNIC). IDNIC was partially inhibited by the metal chelator diethyldithiocarbamate (DETC, 1 mM), but not by the reducing/denitrosylating agent dithiothreitol (DTT, 5 mM). The activation of IDNIC was not reproduced by application of nitric oxide (NO., 100 microM), S-nitrocysteine (200 microM) or ferrous iron-thiosulfate (50 microM), and was not prevented by the irreversible guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4, 3-a]quinoxalin-1-one (ODQ, 1 microM). Similarly, intracellular perfusion of dinitrosyl iron-thiosulfate (100 microM) did not result in activation of IDNIC. Ion replacement experiments show that the DETC-sensitive component of IDNIC is a non-selective cationic current. In accordance, IDNIC was blocked by antagonists of receptor-operated calcium entry, gadolinium (25 microM) and SK&F 96365 (25 microM). Single-channel measurements from outside-out patches reveal that the DETC-sensitive component of IDNIC is an inward current carried by a cationic channel having a conductance of 50 pS. The present observations suggest that the formation of ion channel-bound dinitrosyl iron represents another mechanism of regulation of ion channel activity by NO.-related species, which may be particularly important in pathophysiological processes where NO. is overproduced.

Cyclic nucleotide-gated channels mediate membrane depolarization following activation of store-operated calcium entry in endothelial cells

Calcium agonists induce membrane depolarization in endothelial cells through an unknown mechanism. Present studies tested the hypothesis that pulmonary artery endothelial cells express a cyclic nucleotide-gated (CNG) cation channel activated by store-operated calcium entry to produce membrane depolarization. In the whole-cell configuration, voltage-clamped cells revealed a large non-inactivating, outwardly rectifying cationic current in the absence of extra- or intracellular Ca(2+) that was reduced upon replenishment of Ca(2+). The inward current was non-selective for K(+), Na(+), Cs(+), and Rb(+) and was not inhibited by high tetraethylammonium concentrations. cAMP and cGMP stimulated the current and changed the cation permeability to favor Na(+). Moreover, 8-bromo-cAMP stimulated the current in voltage-clamped cells in the perforated patch mode. The cationic current was inhibited by the CNG channel blocker LY83,583, and reverse transcriptase-polymerase chain reaction cloning identified expression of a CNG channel resembling that seen in olfactory neurons. Activation of store-operated calcium entry using thapsigargin increased a current through the CNG channel. Stimulation of the current paralleled pulmonary artery endothelial cell membrane depolarization, and both the current and membrane depolarization were abolished using LY83,583. Taken together, these data demonstrate activation of store-operated calcium entry stimulates a CNG channel producing membrane depolarization. Such membrane depolarization may contribute to slow feedback inhibition of store-operated calcium entry.

Heme oxygenase-2 and nitric oxide synthase immunoreactivity of bovine olfactory receptor neurons and a comparison with the distribution of NADPH-diaphorase staining

It has recently been suggested that, in addition to nitric oxide (NO), carbon monoxide (CO) is an important gaseous messenger which might be involved in vertebrate olfactory transduction because its effects include activation of guanylyl cyclase and the formation of cGMP. As there is no information regarding the presence of heme oxygenase-2 -- the constitutive isoform of the heme oxygenase system -- in olfactory neurons of non-rodent species, we have investigated the distribution pattern of heme oxygenase-2 in the olfactory epithelium of the bovine, a representative of macrosmatics. Localization of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) activity of the olfactory epithelium was compared with heme oxygenase-2 and NO synthase (NOS) immunoreactivities in order to obtain possible hints at functional significance. NADPH-d activity was particularly intense in apical dendrites of receptor neurons. It was also found in Bowman glands and intraepithelial duct cells. Less intense, discrete NADPH-d activity was present also at intermediate and basal levels of the olfactory epithelium, corresponding to the layer of receptor neuron somata and basal cells. While heme oxygenase-2 activity mainly occurred in neuronal perikarya, a very intense NOS immunoreactivity, exclusively for the inducible isoform, was detected in the apical dendrites. Ultrastructurally, NADPH-d histochemistry showed distinct labelling of membranes, in particular of endoplasmic reticulum, mitochondria and nucleus. The coincident localization of the moderate NADPH-d activity and heme oxygenase-2 immunoreactivity in receptor cell perikarya suggest a functional association between NADPH-cytochrome P450 reductase and heme oxygenase-2. In contrast, dendritic localization of NADPH-d activity is topically and possibly functionally related to the presence of the inducible isoform of NOS. The results suggest that both CO and NO may be generated in bovine receptor neurons and thus involved in odorant stimulation. Based on immunocytochemical localization of synthesizing enzymes, NO might be regarded as a direct regulator of transduction related processes while CO might act as a modulator of the initial signal.

A single intracellular cysteine residue is responsible for the activation of the olfactory cyclic nucleotide-gated channel by NO

The activation of cyclic nucleotide-gated (CNG) channels is the final step in olfactory and visual transduction. Previously we have shown that, in addition to their activation by cyclic nucleotides, nitric oxide (NO)-generating compounds can directly open olfactory CNG channels through a redox reaction that results in the S-nitrosylation of a free SH group on a cysteine residue. To identify the target site(s) of NO, we have now mutated the four candidate intracellular cysteine residues Cys-460, Cys-484, Cys-520, and Cys-552 of the rat olfactory rCNG2 (alpha) channel into serine residues. All mutant channels continue to be activated by cyclic nucleotides, but only one of them, the C460S mutant channel, exhibited a total loss of NO sensitivity. This result was further supported by a similar lack of NO sensitivity that we found for a natural mutant of this precise cysteine residue, the Drosophila melanogaster CNG channel. Cys-460 is located in the C-linker region of the channel known to be important in channel gating. Kinetic analyses suggested that at least two of these Cys-460 residues on different channel subunits were involved in the activation by NO. Our results show that one single cysteine residue is responsible for NO sensitivity but that several channel subunits need to be activated for channel opening by NO.

Cyclic GMP evoked calcium transients in olfactory receptor cell growth cones

Nitric oxide-induced calcium transients in growth cones are believed to be mediated by cyclic nucleotides. Because nitric oxide is thought to influence the development of olfactory receptor cells (ORCs), we have begun to explore the effect of cyclic nucleotides on ORC growth cones. Cultured ORCs were loaded with fluo-3 AM and confocal imaging was employed to monitor calcium transients following cyclic nucleotide-gated channel activation. Application of 8-bromo-cGMP at the growth cone caused transient increases in fluorescence which were restricted to the growth cone and lasted tens of seconds. The signal was abolished by LY83583, an inhibitor of cyclic nucleotide-gated channels. 8-Bromo-cGMP also inhibited further extension of growth cones. The data indicate that ORC growth cones exhibit cGMP-dependent calcium transients that are consistent with those generated by cyclic nucleotide-gated channels.

Cyclic nucleotide-gated channels in non-sensory organs

Cyclic nucleotide-gated channels represent a class of ion channels activated directly by the binding of either cyclic-GMP or cyclic-AMP. They carry both mono and divalent cations, but select calcium over sodium. In the majority of the cases studied, binding of cyclic nucleotides to the channel results in the opening of the channel and the influx of calcium. As a consequence, cytosolic free calcium levels increase leading to the modifications of calcium-dependent processes. This represents and important link in the chain of events leading to the physiological response. Cyclic nucleotide-gated channels were discovered in sensory cell types, in the retina, and in olfactory cells, and were extensively studied in those cells. However, it is becoming increasingly evident that such channels are present not only in sensory systems, but in most, if not all, cell types where cyclic nucleotides play a role in signal transduction. A hypothesis is presented here which attributes physiological importance to these channels in non-sensory organs. Four examples of such channels in non-sensory cells are discussed in detail: those in the liver, in the heart, in the brain, and in the testis with the emphasis on the possible physiological roles that these channels might have in these organs.

Nitric oxide and carbon monoxide modulate oscillations of olfactory interneurons in a terrestrial mollusk

Spontaneous or odor-induced oscillations in local field potential are a general feature of olfactory processing centers in a large number of vertebrate and invertebrate species. The ubiquity of such oscillations in the olfactory bulb of vertebrates and analogous structures in arthropods and mollusks suggests that oscillations are fundamental to the computations performed during processing of odor stimuli. Diffusible intercellular messengers such as nitric oxide (NO) and carbon monoxide (CO) also are associated with central olfactory structures in a wide array of species. We use the procerebral (PC) lobe of the terrestrial mollusk Limax maximus to demonstrate a role for NO and CO in the oscillatory dynamics of the PC lobe: synthesizing enzymes for NO and CO are associated with the PC lobes of Limax, application of NO to the Limax PC lobe increases the local field potential oscillation frequency, whereas block of NO synthesis slows or stops the oscillation, the bursting cells of the PC lobe that drive the field potential oscillation are driven to higher burst frequency by application of NO, the nonbursting cells of the PC lobe receive trains of inhibitory postsynaptic potentials, presumably from bursting cells, due to application of NO, and application of CO to the PC lobe by photolysis of caged CO results in an increase in oscillation frequency proportional to CO dosage.

Ca(2+) entry activated by S-nitrosylation. Relationship to store-operated ca(2+) entry

The coupling between Ca(2+) pools and store-operated Ca(2+) entry channels (SOCs) remains an unresolved question. Recently, we revealed that Ca(2+) entry could be activated in response to S-nitrosylation and that this process was stimulated by Ca(2+) pool emptying (Favre, C. J., Ufret-Vincenty, C. A., Stone, M. R., Ma, H-T. , and Gill, D. L. (1998) J. Biol. Chem. 273, 30855-30858). In DDT(1)MF-2 smooth muscle cells and DC-3F fibroblasts, Ca(2+) entry activated by the lipophilic NO donor, GEA3162 (5-amino-3-(3, 4-dichlorophenyl)1,2,3,4-oxatriazolium), or the alkylator, N-ethylmaleimide, was observed to be strongly activated by transient external Ca(2+) removal, closely resembling activation of SOC activity in the same cells. The nonadditivity of SOC and NO donor-activated Ca(2+) entry suggested a single entry mechanism. Calyculin A-induced reorganization of the actin cytoskeleton prevented SOC but had no effect on GEA3162-induced Ca(2+) entry. However, a single entry mechanism could account for both SOC and NO donor-activated entry if the latter reflected direct modification of the entry channel by S-nitrosylation, bypassing the normal coupling process between channels and pools. Small differences between SOC and GEA3162-activated Ba(2+) entry and sensitivity to blockade by La(3+) were observed, and in HEK293 cells SOC activity was observed without a response to thiol modification. It is concluded that in some cells, S-nitrosylation modifies an entry mechanism closely related to SOC and/or part of the regulatory machinery for SOC-mediated Ca(2+) entry.

Direct actions of nitric oxide on rat neurohypophysial K+ channels

1. Nitric oxide (NO) has been shown to modulate neuropeptide secretion from the posterior pituitary. Here we show that NO activates large-conductance Ca2+-activated K+ (BK) channels in posterior pituitary nerve terminals. 2. NO, generated either by the photolysis of caged-NO or with chemical donors, irreversibly enhanced the component of whole-terminal K+ current due to BK channels and increased the activity of BK channels in excised patches. NO also inhibited the transient A-current. The time courses of these effects on K+ current were very different; activation of BK channels developed slowly over several minutes whereas inhibition of A-current immediately followed NO uncaging. 3. Activation of BK channels by NO occurred in the presence of guanylyl cyclase inhibitors and after removal of ATP or GTP from the pipette solution, suggesting a cGMP-independent signalling pathway. 4. The sulfhydryl alkylating agent N-ethyl maleimide (NEM) increased BK channel activity. Pretreatment with NEM occluded NO activation. 5. NO activation of BK channels occurred independently of voltage and cytoplasmic Ca2+ concentration. In addition, NO removed the strict Ca2+ requirement for channel activation, rendering channels highly active even at nanomolar Ca2+ levels. 6. These results suggest that NO, or a reactive nitrogen byproduct, chemically modifies nerve terminal BK channels or a closely associated protein and thereby produces an increase in channel activity. Such activation is likely to inhibit impulse activity in posterior pituitary nerve terminals and this may explain the inhibitory action of NO on secretion.

Nitric oxide transforms serotonin into an inactive form and this affects neuromodulation

Nitric oxide is a highly reactive molecule, diffusible and therefore ubiquitous in the central nervous system. Consequently, nitric oxide or nitric oxide-derived nitrogen oxides must enter into contact with neuromodulators and they can modify these molecules, especially monoamines, and thus change their regulatory action on synaptic transmission. We tested this possibility on a well-known, identified cholinergic synapse of Aplysia buccal ganglion, in which we have found that evoked acetylcholine release was decreased by extracellularly applied serotonin. We show that this modulatory effect of serotonin was largely reduced not only in the presence of 3-morpholinosydnonimine, a nitric oxide donor, but also when endogenous nitric oxide synthase was activated. We have shown that this decrease in the serotonin effect is due to the formation of chemical derivatives of serotonin, mainly a symmetric serotonin dimer, 4-nitroso-serotonin and 4-nitro-serotonin, which are ineffective in reproducing the modulatory effect of serotonin. Serotonin is involved in the regulation of several central functions, such as sleep-wake activity or mood. The consequences of chemical modifications of serotonin by nitric oxide must be taken into account in physiological as well as pathological situations. In addition, our results highlight the importance of the physiological implications of interactions between free radicals and neuromediators in the nervous system.

Nitric oxide activates a potassium current in olfactory receptor neurons from Caudiverbera caudiverbera and Xenopus laevis

The putative role of nitric oxide (NO) in the physiology of olfactory receptor neurons (ORNs) is controversial. Here we report that pulses of NO caused an outward current in voltage-clamped isolated olfactory neurons. The I-V relation of this effect, its sensitivity to charybdotoxin and its dependence on external potassium suggest that NO activates a K(+)-conductance. As blockers of soluble guanylyl cyclases failed to affect the current, we conclude that NO opens K(+)-channels in a cGMP-independent manner.

Oscillatory dynamics and information processing in olfactory systems

Oscillatory dynamics is a universal design feature of olfactory information-processing systems. Recent results in honeybees and terrestrial slugs suggest that oscillations underlie temporal patterns of olfactory interneuron responses critical for odor discrimination. Additional general design features in olfactory information-processing systems include (1) the use of central processing areas receiving direct olfactory input for odor memory storage and (2) modulation of circuit dynamics and olfactory memory function by nitric oxide. Recent results in the procerebral lobe of the terrestrial slug Limax maximus, an olfactory analyzer with oscillatory dynamics and propagating activity waves, suggest that Lucifer Yellow can be used to reveal a band-shaped group of procerebral neurons involved in the storage of an odor memory. A model has been constructed to relate wave propagation and odor memory bands in the procerebral lobe of L. maximus and to relate these findings to glomerular odor representations in arthropods and vertebrates.

The native rat olfactory cyclic nucleotide-gated channel is composed of three distinct subunits

Cyclic nucleotide-gated (CNG) channels play central roles in visual and olfactory signal transduction. In the retina, rod photoreceptors express the subunits CNCalpha1 and CNCbeta1a. In cone photoreceptors, only CNCalpha2 expression has been demonstrated so far. Rat olfactory sensory neurons (OSNs) express two homologous subunits, here designated CNCalpha3 and CNCalpha4. This paper describes the characterization of CNCbeta1b, a third subunit expressed in OSNs and establishes it as a component of the native channel. CNCbeta1b is an alternate splice form of the rod photoreceptor CNCbeta1a subunit. Analysis of mRNA and protein expression together suggest co-expression of all three subunits in sensory cilia of OSNs. From single-channel analyses of native rat olfactory channels and of channels expressed heterologously from all possible combinations of the CNCalpha3, -alpha4, and -beta1b subunits, we conclude that the native CNG channel in OSNs is composed of all three subunits. Thus, CNG channels in both rod photoreceptors and olfactory sensory neurons result from coassembly of specific alpha subunits with various forms of an alternatively spliced beta subunit.

Cyclic nucleotide-gated channels. Molecular mechanisms of activation

Activation of cyclic nucleotide-gated (CNG) channels represents the final step in the transduction pathways in both vision and olfaction. Over the past several years, CNG channels have been found in a variety of other cell types where they might fulfill various physiological functions. The olfactory and photoreceptor CNG channels rely on the binding of at least two molecules of cAMP or cGMP at intracellular sites on the channel protein to open a nonspecific cation conductance with a significant permeability to Ca ions. A series of elegant experiments with cloned channels and chimeric constructs has revealed significant information regarding the binding and gating reactions that lead to CNG channel activation. These recent studies have identified several regions as well as specific amino acid residues distributed on the retinal or the olfactory CNG channel subunits that play a key role in channel regulation. In this review, we will focus on these specific molecular sites of activation and modulation of CNG channels.

Controversial issues in vertebrate olfactory transduction

A number of controversial issues in olfactory transduction are discussed including the matter of multiple transduction pathways, with a new experiment proposed. Evidence is reviewed concerning the fact that cyclic AMP is the only pathway mediating olfactory transduction. Two knockout mice have been produced: a knockout for a cyclic nucleotide-gated channel and a G(olf) knockout. The results obtained with both mice are consistent with cyclic AMP being the only second messenger. The evidence for gaseous second channel messengers is also reviewed. Slow gating kinetics of the cyclic nucleotide-gated channel and the detection of single-odorant molecules are reviewed. A new phenomenon in which odorants can block odorant responses is discussed.