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

Histamine and histidine decarboxylase in the olfactory system and brain of the common cuttlefish Sepia officinalis (Linnaeus, 1758)

Cephalopods are radically different from any other invertebrate. Their molluscan heritage, innovative nervous system, and specialized behaviors create a unique blend of characteristics that are sometimes reminiscent of vertebrate features. For example, despite differences in the organization and development of their nervous systems, both vertebrates and cephalopods use many of the same neurotransmitters. One neurotransmitter, histamine (HA), has been well studied in both vertebrates and invertebrates, including molluscs. While HA was previously suggested to be present in the cephalopod central nervous system (CNS), Scaros, Croll, and Baratte only recently described the localization of HA in the olfactory system of the cuttlefish Sepia officinalis. Here, we describe the location of HA using an anti-HA antibody and a probe for histidine decarboxylase (HDC), a synthetic enzyme for HA. We extended previous descriptions of HA in the olfactory organ, nerve, and lobe, and describe HDC staining in the same regions. We found HDC-positive cell populations throughout the CNS, including the optic gland and the peduncle, optic, dorso-lateral, basal, subvertical, frontal, magnocellular, and buccal lobes. The distribution of HA in the olfactory system of S. officinalis is similar to the presence of HA in the chemosensory organs of gastropods but is different than the sensory systems in vertebrates or arthropods. However, HA's widespread abundance throughout the rest of the CNS of Sepia is a similarity shared with gastropods, vertebrates, and arthropods. Its widespread use with differing functions across Animalia provokes questions regarding the evolutionary history and adaptability of HA as a transmitter.

A review of the actions of Nitric Oxide in development and neuronal function in major invertebrate model systems

Ever since the late-eighties when endothelium-derived relaxing factor was found to be the gas nitric oxide, endogenous nitric oxide production has been observed in virtually all animal groups tested and additionally in plants, diatoms, slime molds and bacteria. The fact that this new messenger was actually a gas and therefore didn't obey the established rules of neurotransmission made it even more intriguing. In just 30 years there is now too much information for useful comprehensive reviews even if limited to animals alone. Therefore this review attempts to survey the actions of nitric oxide on development and neuronal function in selected major invertebrate models only so allowing some detailed discussion but still covering most of the primary references. Invertebrate model systems have some very useful advantages over more expensive and demanding animal models such as large, easily identifiable neurons and simple circuits in tissues that are typically far easier to keep viable. A table summarizing this information along with the major relevant references has been included for convenience.

Do terrestrial gastropods use olfactory cues to locate and select food actively?

Having been investigated for over 40 years, some aspects of the biology of terrestrial gastropod's olfactory system have been challenging and highly contentious, while others still remain unresolved. For example, a number of terrestrial gastropod species can track the odor of food, while others have no strong preferences toward food odor; rather they find it by random encounter. Here, while assessing the most recent findings and comparing them with earlier studies, the aspects of the food selection based on olfactory cues are examined critically to highlight the speculations and controversies that have arisen. We analyzed and compared the potential role of airborne odors in the feeding behavior of several terrestrial gastropod species. The available results indicate that in the foraging of most of the terrestrial gastropod species odor cues contribute substantially to food finding and selection. The results also suggest, however, that what they will actually consume largely depends on where they live and the species of gastropod that they are. Due to the voluminous literature relevant to this object, this review is not intended to be exhaustive. Instead, I selected what I consider to be the most important or critical in studies regarding the role of the olfaction in feeding of terrestrial gastropods.

Phase-Dependent Modulation of Oscillatory Phase and Synchrony by Long-Lasting Depolarizing Inputs in Central Neurons

Oscillatory neural activities have been implicated in various types of information processing in the CNS. The procerebral (PC) lobe of the land mollusk Limax valentianus shows an ongoing oscillatory local field potential (LFP). Olfactory input increases both the frequency and spatial synchrony of the LFP oscillation by a nitric oxide (NO)-mediated mechanism, but how NO modulates the activity in a specific manner has been unclear. In the present study, we used electrical stimulation and NO uncaging to systematically analyze the response of the LFP oscillation and found phase-dependent effects on phase shifting and synchrony. The neurons that presumably release NO in the PC lobe preferentially fired at phases in which NO has a synchronizing effect, suggesting that the timing of NO release is regulated to induce a stereotyped response to natural sensory stimuli. The phase-response curve (PRC) describes the timing dependence of responses of an oscillatory system to external input. PRCs are usually constructed by recording the temporal shifts of the neural activity in response to brief electrical pulses. However, NO evokes a long-lasting depolarization persisting for several cycles of oscillation. The phase-response relationship obtained by NO stimulation was approximately the integral of the PRC. A similar relationship was also shown for regular firing of mouse cerebellar Purkinje cells receiving step depolarization, suggesting the generality of the results to oscillatory neural systems with highly distinct properties. These results indicate novel dynamic effects of long-lasting inputs on network oscillation and synchrony, which are based on simple and ubiquitous mechanisms.

Nitric Oxide-Mediated Modulation of Central Network Dynamics during Olfactory Perception

Nitric oxide (NO) modulates the dynamics of central olfactory networks and has been implicated in olfactory processing including learning. Land mollusks have a specialized olfactory lobe in the brain called the procerebral (PC) lobe. The PC lobe produces ongoing local field potential (LFP) oscillation, which is modulated by olfactory stimulation. We hypothesized that NO should be released in the PC lobe in response to olfactory stimulation, and to prove this, we applied an NO electrode to the PC lobe of the land slug Limax in an isolated tentacle-brain preparation. Olfactory stimulation applied to the olfactory epithelium transiently increased the NO concentration in the PC lobe, and this was blocked by the NO synthase inhibitor L-NAME at 3.7 mM. L-NAME at this concentration did not block the ongoing LFP oscillation, but did block the frequency increase during olfactory stimulation. Olfactory stimulation also enhanced spatial synchronicity of activity, and this response was also blocked by L-NAME. Single electrical stimulation of the superior tentacle nerve (STN) mimicked the effects of olfactory stimulation on LFP frequency and synchronicity, and both of these effects were blocked by L-NAME. L-NAME did not block synaptic transmission from the STN to the nonbursting (NB)-type PC lobe neurons, which presumably produce NO in an activity-dependent manner. Previous behavioral experiments have revealed impairment of olfactory discrimination after L-NAME injection. The recording conditions in the present work likely reproduce the in vivo brain state in those behavioral experiments. We speculate that the dynamical effects of NO released during olfactory perception underlie precise odor representation and memory formation in the brain, presumably through regulation of NB neuron activity.

Mitral and tufted cells are potential cellular targets of nitration in the olfactory bulb of aged mice

Olfactory sensory function declines with age; though, the underlying molecular changes that occur in the olfactory bulb (OB) are relatively unknown. An important cellular signaling molecule involved in the processing, modulation, and formation of olfactory memories is nitric oxide (NO). However, excess NO can result in the production of peroxynitrite to cause oxidative and nitrosative stress. In this study, we assessed whether changes in the expression of 3-nitrotyrosine (3-NT), a neurochemical marker of peroxynitrite and thus oxidative damage, exists in the OB of young, adult, middle-aged, and aged mice. Our results demonstrate that OB 3-NT levels increase with age in normal C57BL/6 mice. Moreover, in aged mice, 3-NT immunoreactivity was found in some blood vessels and microglia throughout the OB. Notably, large and strongly immunoreactive puncta were found in mitral and tufted cells, and these were identified as lipofuscin granules. Additionally, we found many small-labeled puncta within the glomeruli of the glomerular layer and in the external plexiform layer, and these were localized to mitochondria and discrete segments of mitral and tufted dendritic plasma membranes. These results suggest that mitral and tufted cells are potential cellular targets of nitration, along with microglia and blood vessels, in the OB during aging.

Structure and regulation of soluble guanylate cyclase

Nitric oxide (NO) is an essential signaling molecule in biological systems. In mammals, the diatomic gas is critical to the cyclic guanosine monophosphate (cGMP) pathway as it functions as the primary activator of soluble guanylate cyclase (sGC). NO is synthesized from l-arginine and oxygen (O(2)) by the enzyme nitric oxide synthase (NOS). Once produced, NO rapidly diffuses across cell membranes and binds to the heme cofactor of sGC. sGC forms a stable complex with NO and carbon monoxide (CO), but not with O(2). The binding of NO to sGC leads to significant increases in cGMP levels. The second messenger then directly modulates phosphodiesterases (PDEs), ion-gated channels, or cGMP-dependent protein kinases to regulate physiological functions, including vasodilation, platelet aggregation, and neurotransmission. Many studies are focused on elucidating the molecular mechanism of sGC activation and deactivation with a goal of therapeutic intervention in diseases involving the NO/cGMP-signaling pathway. This review summarizes the current understanding of sGC structure and regulation as well as recent developments in NO signaling.

Two pairs of tentacles and a pair of procerebra: optimized functions and redundant structures in the sensory and central organs involved in olfactory learning of terrestrial pulmonates

Terrestrial pulmonates can learn olfactory-aversion tasks and retain them in their long-term memory. To elucidate the cellular mechanisms underlying learning and memory, researchers have focused on both the peripheral and central components of olfaction: two pairs of tentacles (the superior and inferior tentacles) and a pair of procerebra, respectively. Data from tentacle-amputation experiments showed that either pair of tentacles is sufficient for olfactory learning. Results of procerebrum lesion experiments showed that the procerebra are necessary for olfactory learning but that either one of the two procerebra, rather than both, is used for each olfactory learning event. Together, these data suggest that there is a redundancy in the structures of terrestrial pulmonates necessary for olfactory learning. In our commentary we exemplify and discuss functional optimization and structural redundancy in the sensory and central organs involved in olfactory learning and memory in terrestrial pulmonates.

Effects of tentacle amputation and regeneration on the morphology and activity of the olfactory center of the terrestrial slug Limax valentianus

The tentacles of pulmonates regenerate spontaneously following amputation. The regenerated tentacle is equipped with all the elements necessary for normal olfactory functioning, and the slugs can behave as well as they did before the tentacle amputation. However, it is not known what changes occur to the olfactory center procerebrum in the brain at the morphological and physiological levels. Here, we investigated the innervation of tentacular nerves into the procerebrum by examining the size of the terminal mass (input layer from tentacular nerves) of the procerebrum and also by staining afferent nerves immunohistochemically at 15, 58 and 75 days following unilateral amputation of the superior and inferior tentacles. The size of the terminal mass was significantly decreased, and the Phe-Met-Arg-Phe-NH2ergic (FMRFamidergic) afferent nerves disappeared by 15 days following the tentacle amputation. However, the size of the terminal mass had recovered substantially by 58 days, as the tentacle regenerated. The FMRFamidergic innervation into the cerebral ganglion was also restored by this time. An extended recovery (75 days), however, did not result in any further increase in the size of the terminal mass. We also recorded the local field potential (LFP) oscillation in the procerebrum. We found that the oscillatory frequency of the LFP had decreased at 15 days following the tentacle amputation but had recovered at 58 and 75 days. These results suggest that the amputation and regrowth of the tentacle are accompanied by the respective degeneration and re-innervation of olfactory nerves, and these changes in the innervation status affect the basal state of LFP oscillation.

Effects of cadmium exposure and intermittent anoxia on nitric oxide metabolism in eastern oysters, Crassostrea virginica

Nitric oxide (NO) is an intracellular signaling molecule synthesized by a group of enzymes called nitric oxide synthases (NOS) and involved in regulation of many cellular functions including mitochondrial metabolism and bioenergetics. In invertebrates, the involvement of NO in bioenergetics and metabolic responses to environmental stress is poorly understood. We determined sensitivity of mitochondrial and cellular respiration to NO and the effects of cadmium (Cd) and intermittent anoxia on NO metabolism in eastern oysters, Crassostrea virginica. NOS activity was strongly suppressed by exposure to 50 μg l–1 Cd for 30 days (4.76 vs 1.19 pmol NO min–1 mg–1 protein in control and Cd-exposed oysters, respectively) and further decreased during anoxic exposure in Cd-exposed oysters but not in their control counterparts. Nitrate/nitrite content (indicative of NO levels) decreased during anoxic exposure to less than 10% of the normoxic values and recovered within 1 h of re-oxygenation in control oysters. In Cd-exposed oysters, the recovery of the normoxic NO levels lagged behind, reflecting their lower NOS activity. Oyster mitochondrial respiration was inhibited by exogenous NO, with sensitivity on a par with that of mammalian mitochondria, and ADP-stimulated mitochondrial respiration was significantly more sensitive to NO than resting respiration. In isolated gill cells, manipulations of endogenous NOS activity either with a specific NOS inhibitor (aminoguanidine) or a NOS substrate (l-arginine) had no effect on respiration, likely due to the fact that mitochondria in the resting state are relatively NO insensitive. Likewise, Cd-induced stimulation of cellular respiration did not correlate with decreased NOS activity in isolated gill cells. High sensitivity of phosphorylating (ADP-stimulated) oyster mitochondria to NO suggests that regulation of bioenergetics is an evolutionarily conserved function of NO and that NO-dependent regulation of metabolism may be most prominent under the conditions of high metabolic flux when the ADP-to-ATP ratio is high.

Investigation of changes in NO content during long-term sensitization in edible snail using EPR-spectroscopy: effects of antibodies to calcium-binding protein S-100

EPR-spectroscopy experiments (electron paramagnetic resonance) demonstrated a decrease in NO production in the nervous system and heart of edible snail Helix lucorum after formation of long-term sensitization, a neurobiological model of anxiety and depression. The protective effect of antibodies to Ca(2+)-binding protein S-100 in dilution of 10(-12) on the formation of long-term sensitization was accompanied by partial recovery of NO synthesis in the nervous system and heart. These findings indicate that the imbalance in Ca(2+)-binding protein S-100 can lead to inhibition or modulation of some processes during plastic reorganization in the body and especially during pathological processes.

Regulation of enteric neuron migration by the gaseous messenger molecules CO and NO

The enteric nervous system (ENS) of insects is a useful model to study cell motility. Using small-molecule compounds to activate or inactivate biosynthetic enzymes, we demonstrate that the gaseous messenger molecules carbon monoxide (CO) and nitric oxide (NO) regulate neuron migration in the locust ENS. CO is produced by heme oxygenase (HO) enzymes and has the potential to signal via the sGC/cGMP pathway. While migrating on the midgut, the enteric neurons express immunoreactivity for HO. Here, we show that inhibition of HO by metalloporphyrins promotes enteric neuron migration in intact locust embryos. Thus, the blocking of enzyme activity results in a gain of function. The suppression of migratory behavior by activation of HO or application of a CO donor strongly implicates the release of CO as an inhibitory signal for neuron migration in vivo. Conversely, inhibition of nitric oxide synthase or application of the extracellular gaseous molecule scavenger hemoglobin reduces cell migration. The cellular distribution of NO and CO biosynthetic enzymes, together with the results of the chemical manipulations in whole embryo culture suggest CO as a modulator of transcellular NO signals during neuronal migration. Thus, we provide the first evidence that CO regulates embryonic nervous system development in a rather simple invertebrate model.

Caged molecules: principles and practical considerations

A caged molecule is an inert but photosensitive molecule that is transformed by photolysis into a biologically active molecule at high speed (typically 1 msec). The process is referred to as photorelease. The spatial resolution of photorelease is limited by the properties of light; submicrometer resolution is potentially achievable. Therefore, focal photorelease of caged molecules enables one to control biological processes with high spatio-temporal precision. The principles underlying caged molecules as well as practical considerations for their use are discussed in this unit.

Tonic and stimulus-evoked nitric oxide production in the mouse olfactory bulb

Nitric oxide (NO) has been long assumed to play a key role in mammalian olfaction. This was based largely on circumstantial evidence, i.e. prominent staining for nitric oxide synthase (NOS) and cyclic guanosine 3',5'-cyclic monophosphate (cGMP) or soluble guanylyl cyclase, an effector enzyme activated by NO, in local interneurons of the olfactory bulb. Here we employ innovative custom-fabricated NO micro-sensors to obtain the first direct, time-resolved measurements of NO signaling in the olfactory bulb. In 400 microm thick mouse olfactory bulb slices, we detected a steady average basal level of 87 nM NO in the extracellular space of mitral or granule cell layers. This NO 'tone' was sensitive to NOS substrate manipulation (200 microM L-arginine, 2 mM N(G)-nitro-L-arginine methyl ester) and Mg(2+) modulation of N-methyl-D-aspartate (NMDA) receptor conductance. Electrical stimulation of olfactory nerve fibers evoked transient (peak at 10 s) increments in NO levels 90-100 nM above baseline. In the anesthetized mouse, NO micro-sensors inserted into the granule cell layer detected NO transients averaging 55 nM in amplitude and peaking at 3.4 s after onset of a 5 s odorant stimulation. These findings suggest dual roles for NO signaling in the olfactory bulb: tonic inhibitory control of principal neurons, and regulation of circuit dynamics during odor information processing.

Nitric oxide and respiratory rhythm in mammals: a new modulator of phase transition?

NO (nitric oxide) modulates several central pattern generators, but its role in respiratory rhythmogenesis and its mode of action on medullary respiratory neurons during normoxia are unknown. We analysed the actions of NO on the mammalian respiratory network at the system and cellular levels. Given systemically, the NO donor diethylamine NONOate increased post-inspiratory duration in vagus, phrenic and hypoglossal nerves, whereas blockade of NO generation with L-NAME (N(G)-nitro-L-arginine methyl ester) produced the opposite response. At the cellular level, we pressure-ejected the NO donor on to respiratory neurons. NO had both inhibitory and excitatory effects on all types of respiratory neurons. Inhibitory effects involved soluble guanylate cyclase, as they were blocked with ODQ (1H-[1,2,4]oxadiazolo[4,3a]quinoxalin-1-one), whereas excitations were antagonized by uric acid and possibly mediated via peroxynitrite. Importantly, NO facilitated both GABA (gamma-aminobutyric acid)- and NMDA (N-methyl-D-aspartate)-induced neuronal responses, but this was restricted to post-inspiratory and pre-inspiratory neurons; other neuron types showed additive effects only. Our results support NO as modulator of centrally generated respiratory activity and specifically of ligand-mediated responses in respiratory neuron types involved in respiratory phase transition.

Chemotopic odorant coding in a mammalian olfactory system

Systematic mapping studies involving 365 odorant chemicals have shown that glomerular responses in the rat olfactory bulb are organized spatially in patterns that are related to the chemistry of the odorant stimuli. This organization involves the spatial clustering of principal responses to numerous odorants that share key aspects of chemistry such as functional groups, hydrocarbon structural elements, and/or overall molecular properties related to water solubility. In several of the clusters, responses shift progressively in position according to odorant carbon chain length. These response domains appear to be constructed from orderly projections of sensory neurons in the olfactory epithelium and may also involve chromatography across the nasal mucosa. The spatial clustering of glomerular responses may serve to "tune" the principal responses of bulbar projection neurons by way of inhibitory interneuronal networks, allowing the projection neurons to respond to a narrower range of stimuli than their associated sensory neurons. When glomerular activity patterns are viewed relative to the overall level of glomerular activation, the patterns accurately predict the perception of odor quality, thereby supporting the notion that spatial patterns of activity are the key factors underlying that aspect of the olfactory code. A critical analysis suggests that alternative coding mechanisms for odor quality, such as those based on temporal patterns of responses, enjoy little experimental support.

Inhibition of nitric oxide and soluble guanylyl cyclase signaling affects olfactory neuron activity in the moth, Manduca sexta

Nitric oxide is emerging as an important modulator of many physiological processes including olfaction, yet the function of this gas in the processing of olfactory information remains poorly understood. In the antennal lobe of the moth, Manduca sexta, nitric oxide is produced in response to odor stimulation, and many interneurons express soluble guanylyl cyclase, a well-characterized nitric oxide target. We used intracellular recording and staining coupled with pharmacological manipulation of nitric oxide and soluble guanylyl cyclase to test the hypothesis that nitric oxide modulates odor responsiveness in olfactory interneurons through soluble guanylyl cyclase-dependent pathways. Nitric oxide synthase inhibition resulted in pronounced effects on the resting level of firing and the responses to odor stimulation in most interneurons. Effects ranged from bursting to strong attenuation of activity and were often accompanied by membrane depolarization coupled with a change in input resistance. Blocking nitric oxide activation of soluble guanylyl cyclase signaling mimicked the effects of nitric oxide synthase inhibitors in a subset of olfactory neurons, while other cells were differentially affected by this treatment. Together, these results suggest that nitric oxide is required for proper olfactory function, and likely acts through soluble guanylyl cyclase-dependent and -independent mechanisms in different subsets of neurons.

Gaseous neuromodulator-related genes expressed in the brain of honeybee Apis mellifera

Nitric oxide (NO), hydrogen sulfide (H2S), and carbon monoxide (CO) are thought to act as gaseous neuromodulators in the brain across species. For example, in the brain of honeybee Apis mellifera, NO plays important roles in olfactory learning and discrimination, but the existence of H2S- and CO-mediated signaling pathways remains unknown. In the present study, we identified the genes of nitric oxide synthase (NOS), soluble guanylyl cyclase (sGC), cystathionine beta-synthase (CBS), and heme oxygenase (HO) from the honeybee brain. The honeybee brain contains at least one gene for each of NOS, CBS, and HO. The deduced proteins for NOS, CBS, and HO are thought to contain domains to generate NO, H2S, and CO, respectively, and to contain putative Ca2+/calmodulin-binding domains. On the other hand, the honeybee brain contains three subunits of sGC: sGCalpha1, sGCbeta1, and sGCbeta3. Phylogenetic analysis of sGC revealed that Apis sGCalpha1 and sGCbeta1 are closely related to NO- and CO-sensitive sGC subunits, whereas Apis sGCbeta3 is closely related to insect O2-sensitive sGC subunits. In addition, we performed in situ hybridization for Apis NOS mRNA and NADPH-diaphorase histochemistry in the honeybee brain. The NOS gene was strongly expressed in the optic lobes and in the Kenyon cells of the mushroom bodies. NOS activity was detected in the optic lobes, the mushroom bodies, the central body complex, the lateral protocerebral lobes, and the antennal lobes. These findings suggest that NO is involved in various brain functions and that H2S and CO can be endogenously produced in the honeybee brain.

Functions of the subesophageal ganglion in the medicinal leech revealed by ablation of neuromeres in embryos

Two general trends in the evolution of the nervous system have been toward centralization of neuronal somata and cephalization of the central nervous system (CNS). These organizational trends are apparent in the nervous system of annelid worms, including leeches. To determine if the anterior brain of the leech serves functions similar to those of the brains of more complex organisms, including vertebrates, we ablated one of the two major regions of the cephalic brain--the subesophageal ganglion (SubEG). For anatomical reasons, ablations were performed in embryos, rather than in adults. At the end of embryonic development, we observed the leeches' spontaneous behaviour and their responses to moderate touch. We observed that, although the midbody ganglia of the leech CNS display a high degree of local autonomy, the cephalic brain provides generalized excitation to the rest of the CNS, is a source of selective inhibition that modulates behaviour, integrates sensory information from the head with signals from the rest of the body, and plays an important role in organizing at least some complicated whole-body behaviours. These roles of the leech cephalic brain are common features of brain function in many organisms, and our results are consistent with the hypothesis that they arose early in evolution and have been conserved in complex nervous systems.

Carbon Monoxide: Endogenous Production, Physiological Functions, and Pharmacological Applications

Over the last decade, studies have unraveled many aspects of endogenous production and physiological functions of carbon monoxide (CO). The majority of endogenous CO is produced in a reaction catalyzed by the enzyme heme oxygenase (HO). Inducible HO (HO-1) and constitutive HO (HO-2) are mostly recognized for their roles in the oxidation of heme and production of CO and biliverdin, whereas the biological function of the third HO isoform, HO-3, is still unclear. The tissue type-specific distribution of these HO isoforms is largely linked to the specific biological actions of CO on different systems. CO functions as a signaling molecule in the neuronal system, involving the regulation of neurotransmitters and neuropeptide release, learning and memory, and odor response adaptation and many other neuronal activities. The vasorelaxant property and cardiac protection effect of CO have been documented. A plethora of studies have also shown the importance of the roles of CO in the immune, respiratory, reproductive, gastrointestinal, kidney, and liver systems. Our understanding of the cellular and molecular mechanisms that regulate the production and mediate the physiological actions of CO has greatly advanced. Many diseases, including neurodegenerations, hypertension, heart failure, and inflammation, have been linked to the abnormality in CO metabolism and function. Enhancement of endogenous CO production and direct delivery of exogenous CO have found their applications in many health research fields and clinical settings. Future studies will further clarify the gasotransmitter role of CO, provide insight into the pathogenic mechanisms of many CO abnormality-related diseases, and pave the way for innovative preventive and therapeutic strategies based on the physiologic effects of CO.

Nitric oxide modulates peristaltic muscle activity associated with fluid circulation in the sea pansy Renilla koellikeri

Nitric oxide (NO) is a well-known regulator of vascular activities in vertebrates and it has also been implicated as a vasodilatatory agent in a cephalopod. In the sea pansy Renilla koellikeri, an octocorallian representative of the most basal animals with a nervous system, we investigated the role of NO in peristalsis, an activity that moves body fluids through the coelenteron (gastrovascular cavity) of the polyps across the colony. NO donors increased the amplitude of peristaltic contractions and increased tonic contractions in relaxed preparations, but caused a relaxation of basal tension in contracted preparations. The NO synthase (NOS) inhibitors L-NAME (N(ω)-nitro-L-arginine methyl ester) and 7-nitroindazole reduced the amplitude of peristaltic contractions and lowered basal tension. In contrast, aminoguanidine, a specific inhibitor of inducible NOS, increased the amplitude but reduced the rate of peristalsis. Zaprinast, a cGMP-specific phosphodiesterase inhibitor, decreased the amplitude of peristaltic contractions, a decrease that was amplified by dibutyryl cGMP. In contrast,the inhibitor of soluble guanylyl cyclase ODQ(1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one) enhanced peristalsis. Putative NOS-containing neurons, revealed by NADPH-diaphorase activity and citrulline immunohistochemistry, were observed in the basiectoderm at the base of the autozooid polyp tentacles and in a nerve-net around the oral disc. Their neurites ran up the tentacles and down to the polyp body wall, crossing from the ectoderm through the mesoglea and into the endoderm musculature where musculo-epithelial cells were also reactive. These data suggest that two distinct nitrergic pathways, one of which is mediated by cGMP, regulate peristalsis and muscle tone in the sea pansy and that these pathways may involve NOS-containing ectodermal neurons and musculo-epithelial cells.

Odorant-evoked nitric oxide signals in the antennal lobe of Manduca sexta

The gaseous signaling molecule nitric oxide (NO) can affect the activities of neurons and neural networks in many different systems. The strong expression of NO synthase (NOS) in the primary synaptic neuropil (the antennal lobe in insects and the olfactory bulb in vertebrates) of the olfactory system of most organisms, and the unique spheroidal geometry of olfactory glomeruli in those neuropils, have led to suggestions that NO signaling is important for processing olfactory information. No direct evidence exists, however, that NO signals are produced in olfactory glomeruli. We investigated the production of NO in the antennal lobe of the moth, Manduca sexta, by using immunocytochemistry and real-time optical imaging with a NO-sensitive fluorescent marker, diaminofluorescein diacetate. We confirmed that NOS was expressed in the axons of olfactory receptor neurons projecting to all glomeruli. Soluble guanylyl cyclase, the best characterized target of NO, was found in a subset of postsynaptic antennal lobe neurons that included projection neurons, a small number of GABA-immunoreactive neurons, and a serotonin-immunoreactive neuron. We found that odorant stimulation evoked NO signals that were reproducible and spatially focused. Different odorants evoked spatially distinct patterns of NO production. Increased concentrations of pheromone and plant odorants caused increases in peak signal intensity. Increased concentrations of plant odorants also evoked a dramatic increase in signal area. The results of these experiments show clearly that odorant stimulation can evoke NO production in the olfactory system. The NO signals produced are likely to play an important role in processing olfactory information.

Impairment of olfactory discrimination by blockade of nitric oxide activity in the terrestrial slug Limax valentianus

The terrestrial slug Limax readily associates an innately preferred food odor with the aversive taste of quinidine. We investigated slugs' olfactory discrimination capability among structurally similar alcohols and the effects of inhibition of nitric oxide (NO) synthesis to the olfactory discrimination behavior, using an olfactory discriminatory learning task. Limax could discriminate among the odor of 1-octanol (OT), 3-methylcyclohexanol (MC) and 1-hexanol (HX). OT was perceptually more similar to HX than was MC for them. When NO synthesis was inhibited by injecting N-nitro-L-arginine methyl ester (l-NAME) shortly before the discrimination test, slugs could not discriminate between OT and HX whereas the retrieval of olfactory memory and the discrimination between OT and MC remained intact. These results indicate that the NO cascade plays a crucial role for fine olfactory discrimination in Limax.

Heme oxygenase inhibition reduces neuronal activation evoked by bicuculline in newborn pigs

Carbon monoxide (CO) is a product of heme degradation by heme oxygenase (HO) that is highly expressed in the brain. The present study addresses the hypothesis that CO can be involved in brain neuronal function. The effects of the HO inhibitor, tin protoporphyrin (SnPP), on brain electrical activity and pial arteriolar diameter were examined using quantitative electroencephalography (EEG) and cranial window techniques in the bicuculline model of sustained generalized seizures in newborn pigs. SnPP (3 mg/kg i.v.) inhibits brain HO as indicated by blocking cerebral vasodilation to heme and decreasing CO concentration in cortical periarachnoid cerebrospinal fluid. The quantitative spectral analysis of digitalized scalp EEG recordings was performed to determine the EEG amplitude and spectral power within a 1-15-Hz frequency range. SnPP did not affect basal brain EEG parameters. Bicuculline (3 mg/kg i.v.) immediately (in <1 min) evoked bursts of brain electrical activity characterized by four- to seven-fold increases in EEG amplitude and power in all analyzed frequency bands that occurred simultaneously with cerebral vasodilation. Increased EEG activity and cerebral vasodilation were sustained for a 2h period. SnPP inhibited cerebral vasodilation but did not affect the EEG amplitude evoked by bicuculline. However, 20-40% reductions of the power in 7.5 Hz (theta), 10 and 12.5 Hz (alpha), and a 15-Hz (beta) bands, the major evoked EEG spectral components, were observed for the duration of seizures in SnPP-treated animals. These findings suggest that endogenous CO can have proconvulsant action and affect neuronal activation during seizures.

Nitric oxide synthase and soluble guanylyl cyclase underlying the modulation of electrical oscillations in a central olfactory organ

We have isolated and characterized the cDNAs for nitric oxide synthase (NOS) and soluble guanylyl cyclase (sGC) from the terrestrial slug Limax marginatus, and examined the presence and distribution of their mRNAs in the central nervous system using histological techniques and a reverse transcription-polymerase chain reaction method. Our results showed that both bursting and nonbursting neurons in the procerebral lobes contain the mRNAs for both NOS and sGC. We further found that the oscillation frequency of electrical activity in the procerebral lobes increases with increasing intracellular concentrations of cyclic GMP (cGMP). Taken together with previous data on the NO-induced cGMP-like immunoreactivity and on the anatomical distribution of neurites and the localization of synapses of bursting and nonbursting neurons, our present results suggest that NO-induced changes in cGMP concentration modulate the oscillation frequency in the procerebral lobes by acting on the olfactory input pathways, but possibly not on the output pathways, in slugs. .

Immunocytochemical evidence for nitric oxide- and carbon monoxide-producing neurons in the stomatogastric nervous system of the crayfishCherax quadricarinatus

Nitric oxide (NO) and carbon monoxide (CO) have been shown to serve neuromodulatory roles in both vertebrates and invertebrates. Here, we use antibodies to their respective biosynthetic enzymes, nitric oxide synthase (NOS) and heme oxygenase 2 (HO-2), to map the distribution of putative gas-producing neurons in the stomatogastric nervous system (STNS) of the crayfish Cherax quadricarinatus. In this species, NOS immunolabeling is found in the neuropil of the stomatogastric ganglion (STG). This staining originates from two immunopositive axons that project to the STG through the superior oesophageal and stomatogastric nerves, presumably from cell bodies located in the commissural ganglia (CoGs). HO-2 immunoreactivity is present in small diameter fibers and varicosities in the periphery of nerves located in the anterior portion of the STNS. This labeling originates from approximately 12 somata in each CoG. Transmission electron microscopy done on the nerves of the anterior STNS shows they contain a neuroendocrine plexus. Collectively, our results indicate that NO- and CO-producing neurons are likely to exist in the crayfish STNS. Moreover, these gases appear to be produced by distinct subsets of the neurons present there. The localization of NO to the STG neuropil suggests that it serves as a locally released modulator or is involved in the local release of other substances within this ganglion. The presence of CO in the neurohemal plexus of the anterior STNS suggests that it serves as a circulating hormone or is involved in the control of neuroendocrine release from this plexus.

Carbon monoxide neurotransmission activated by CK2 phosphorylation of heme oxygenase-2

Carbon monoxide (CO) is a putative gaseous neurotransmitter that lacks vesicular storage and must be synthesized rapidly following neuronal depolarization. We show that the biosynthetic enzyme for CO, heme oxygenase-2 (HO2), is activated during neuronal stimulation by phosphorylation by CK2 (formerly casein kinase 2). Phorbol ester treatment of hippocampal cultures results in the phosphorylation and activation of HO2 by CK2, implicating protein kinase C (PKC) in CK2 stimulation. Odorant treatment of olfactory receptor neurons augments HO2 phosphorylation and activity as well as cyclic guanosine monophosphate (cGMP) levels, with all of these effects selectively blocked by CK2 inhibitors. Likewise, CO-mediated nonadrenergic, noncholinergic (NANC) relaxation of the internal anal sphincter requires CK2 activity. Our findings provide a molecular mechanism for the rapid neuronal activation of CO biosynthesis, as required for a gaseous neurotransmitter.

NOVELNEURALMODULATORS

The discovery that nitric oxide (NO) is produced by neurons and regulates synaptic activity has challenged the definition of a neurotransmitter. NO is not stored in synaptic vesicles and does not act at conventional receptors on the surface of adjacent neurons. The toxic gases carbon monoxide (CO) and hydrogen sulfide (H2S) are also produced by neurons and modulate synaptic activity. D-serine synthesis and release by astrocytes as an endogenous ligand for the "glycine" site of N-methyl D-aspartate (NMDA) receptors defy the concept that a neurotransmitter must be synthesized by neurons. We review the properties of these "atypical" neural modulators.

Slime-trail tracking in the predatory snail, Euglandina rosea

Euglandina rosea, a predatory land snail, tracks prey and mates by following slime trails. Euglandina follow slime trails more than 80% of the time, following trails of their own species, but not those of prey snails, in the direction that they were laid. The attractive elements of prey slime are small, water-soluble compounds detected by specialized lip extensions. Although olfaction plays no role in trail following, strong odors disrupt tracking. Inhibition of nitric oxide synthase also disrupts slime trail following, suggesting a role for nitric oxide in neural processing of slime trail stimuli. Euglandina can be conditioned to follow novel trails of glutamate or arginine paired with feeding on prey snails. These experiments demonstrate that slime-trail tracking in Euglandina is a robust, easily measured behavior that makes a good model system for studying sensory processing and learning in a novel modality.

Influence of NO downregulation on oscillatory evoked responses in developing rat superior colliculus

Nitric oxide (NO) is involved in neuronal transmission by modulating neurotransmitter release in adults and in stabilizing synaptic connections in developing brains. We investigated the influence of downregulation of NO synthesis on oscillatory components of ON and OFF evoked field potentials in the rat superior colliculus. NO synthesis was decreased by inhibiting nitric oxide synthase (NOS) with an acute microinjection of N(omega)-nitro-L-arginine methyl ester (L-NAME). The study focuses on rhythmic activity by analyzing fast Fourier transform (FFT). Collicular responses were recorded in anesthetized rats, at postnatal days (PND) 13-19 and adults. This time window was chosen because it is centered on eye opening. NO downregulation resulted in a dual effect depending on age and response-type. NO synthesis inhibition decreased the magnitude of oscillations in ON responses in the youngest animals (PND13-14), whereas oscillations of frequencies higher than 20 Hz in OFF responses were increased in all age groups of developing rats. In adults NO downregulation increased oscillations in ON responses and decreased oscillations in OFF responses. L-Arginine was used to increase NOS activity and its injection produced effects opposite to those seen with L-NAME. Slow oscillatory components (7-12 Hz) were unaffected in all experiments. Our data together with results reported in the literature suggest that rhythmic patterns of activity are NO-dependent.

Regulation of early embryonic behavior by nitric oxide in the pond snail Helisoma trivolvis

Helisoma trivolvis embryos display a cilia-driven rotational behavior that is regulated by a pair of serotonergic neurons named ENC1s. As these cilio-excitatory motor neurons contain an apical dendrite ending in a chemosensory dendritic knob at the embryonic surface, they probably function as sensorimotor neurons. Given that nitric oxide (NO) is often associated with sensory neurons in invertebrates, and has also been implicated in the control of ciliary activity, we examined the expression of NO synthase (NOS) activity and possible function of NO in regulating the rotational behavior in H. trivolvis embryos. NADPH diaphorase histochemistry on stage E25-E30 embryos revealed NOS expression in the protonephridia, buccal mass,dorsolateral ciliary cells and the sensory dendritic knobs of ENC1. At stages E35-40, the pedal ciliary cells and ENC1's soma, apical dendrite and proximal descending axon were also stained. In stage E25 embryos, optimal doses of the NO donors SNAP and SNP increased the rate of embryonic rotation by twofold, in contrast to the fourfold increase caused by 100 μmol l-1serotonin. The NOS inhibitors L-NAME (10 mmol l-1) and 7-NI (100μmol l-1) decreased the rotation rate by approximately 50%,whereas co-addition of L-NAME and SNAP caused a twofold increase. In an analysis of the surge and inter-surge subcomponents of the rotational behavior, the NO donors increased the inter-surge rotation rate and the surge amplitude. In contrast, the NO inhibitors decreased the inter-surge rotation rate and the frequency of surges. These data suggest that the embryonic rotational behavior depends in part on the constitutive excitatory actions of NO on ENC1 and ciliary cells.

Rhythmic activity from transverse brainstem slice of neonatal rat is modulated by nitric oxide

We investigated the role of nitric oxide (NO) in the modulation of respiratory-like activity recorded from hypoglossal rootlets in brainstem slices of neonatal rats (P0-P8). Sodium nitroprusside (SNP), S-Nitroso-N-acetyl-D,L-penicillamine (SNAP) and diethylamine-NO (DEA-NO), three NO-donors, reversibly increased hypoglossal burst amplitude with inconsistent effects on burst frequency. Similar effects were also obtained with the endogenous substrate of nitric oxide synthase (NOS), L-arginine, whereas the inactive enantiomer D-arginine had no effect. The NO-trap agent methylene blue significantly depressed both the amplitude and frequency of hypoglossal activity while hemoglobin depressed only the amplitude. Furthermore, the addition of NO-trap agents significantly attenuated the excitatory response to SNP. Inhibiting NOS with either N(omega)-Nitro-L-Arginine (L-NNA) or 7-Nitroindazole (7-NI), decreased the amplitude of hypoglossal activity with no effects on frequency. Histochemical analysis of NADPH-diaphorase activity, a marker for NOS, was performed on slices not treated pharmacologically and in brainstem sections of newborn rats, perfused in situ. Comparison between in vitro and in vivo conditions indicated that NOS activity was maintained in slice preparations. Neurons in the ambiguus and hypoglossal nuclei (dorsal division) exhibited a granular staining, suggesting the presence of NADPHd-positive terminals. Neurons with cytoplasmic staining were identified in regions connected to the hypoglossal nucleus (nucleus tractus solitarius, paramedian and gigantocellular reticular nuclei). These neurons might be involved in nitrergic control of hypoglossal activity. Both pharmacological and histochemical data suggest that endogenous NO may reinforce the output activity of the medullary respiratory network.

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.

Histochemical and immunohistochemical localization of neuronal nitric oxide synthase in the olfactory epithelium of the axolotl, Ambystoma mexicanum

The aim of this study was to describe the anatomic distribution of neuronal nitric oxide synthase immunoreactivity (nNOS-IR) and nicotinamide-adenine dinucleotide phosphate-diaphorase (NADPH-d) staining in the olfactory epithelium of the axolotl, juvenile, and neotenic adult, Ambystoma mexicanum. Nitric oxide (NO, nitrogen monoxide) is a widespread molecule that has been identified both as a neuromodulator and as an intracellular messenger. In the olfactory system, NO has been proposed to play a role in olfactory transduction. Nitric oxide synthase (NOS) can be detected by histochemical (NADPH-d) and immunohistochemical techniques. NADPH-d staining has been described in olfactory receptor neurons (ORN) of several species; however, nNOS-IR has not always been found at ORN. Present results show intense NADPH-d staining and nNOS-IR in the dendrites and cell bodies of ORN in both the nasal cavity and the vomeronasal organ of axolotls. Unilateral olfactory axotomy was conducted to confirm that labels were at ORN. Two weeks after this procedure an important decrease in NADPH-d staining and nNOS-IR was observed. The remaining labels were mostly in basal cells. By 5 weeks postaxotomy both labels were almost totally absent. Thus, both NADPH-d staining and nNOS-IR were mainly localized in ORN. NADPH-d staining and nNOS-IR were also found in nerve fibers surrounding arterioles, as well as in secretory and duct cells of the Bowman's glands. This last anatomical localization suggests that in the A. mexicanum NO might be involved in functions other than only olfactory transduction, such as regulation of local blood flow, glandular secretion, and ORN development.

Serotonin and NO complementarily regulate generation of oscillatory activity in the olfactory CNS of a terrestrial mollusk

Synchronous oscillation of membrane potentials, generated by assemblies of neurons, is a prominent feature in the olfactory systems of many vertebrate and invertebrate species. However, its generation mechanism is still controversial. Biogenic amines play important roles for mammalian olfactory learning and are also implicated in molluscan olfactory learning. Here, we investigated the role of serotonin, a biogenic amine, in the oscillatory dynamics in the procerebrum (PC), the molluscan olfactory center. Serotonin receptor blockers inhibited the spontaneous synchronous oscillatory activity of low frequency (approximately 0.5 Hz) in the PC. This was due to diminishing the periodic slow oscillation of membrane potential in bursting (B) neurons, which are essential neuronal elements for the synchronous oscillation in the PC. On the other hand, serotonin enhanced the amplitude of the slow oscillation in B neurons and subsequently increased the number of spikes in each oscillatory cycle. These results show that the extracellular serotonin level regulates the oscillation amplitude in B neurons and thus serotonin may be called an oscillation generator in the PC. Although nitric oxide (NO) is known to also be a crucial factor for generating the PC oscillatory activity and setting the PC oscillation frequency, the present study showed that NO only regulates the oscillation frequency in B neurons but could not increase the spikes in each oscillatory cycle. These results suggest complementary regulation of the PC oscillatory activity: NO determines the probability of occurrence of slow potentials in B neurons, whereas serotonin regulates the amplitude in each cycle of the oscillatory activity in B neurons.

Neural network partitioning by NO and cGMP

The stomatogastric ganglion (STG) of the crab Cancer productus contains approximately 30 neurons arrayed into two different networks (gastric mill and pyloric), each of which produces a distinct motor pattern in vitro. Here we show that the functional division of the STG into these two networks requires intact NO-cGMP signaling. Multiple nitric oxide synthase (NOS)-like proteins are expressed in the stomatogastric nervous system, and NO appears to be released as an orthograde transmitter from descending inputs to the STG. The receptor of NO, a soluble guanylate cyclase (sGC), is expressed in a subset of neurons in both motor networks. When NO diffusion or sGC activation are blocked within the ganglion, the two networks combine into a single conjoint circuit. The gastric mill motor rhythm breaks down, and several gastric neurons pattern switch and begin firing in pyloric time. The functional reorganization of the STG is both rapid and reversible, and the gastric mill motor rhythm is restored when the ganglion is returned to normal saline. Finally, pharmacological manipulations of the NO-cGMP pathway are ineffective when descending modulatory inputs to the STG are blocked. This suggests that the NO-cGMP pathway may interact with other biochemical cascades to partition rhythmic motor output from the ganglion.

Model for olfactory discrimination and learning in Limax procerebrum incorporating oscillatory dynamics and wave propagation

We extend our model of the procerebral (PC) lobe of Limax, which is comprised of a layer of coupled oscillators and a layer of memory neurons, each layer 4 rows by 20 columns, corresponding to the cell body layer (burster cells) and neuropil layer (nonburster cells) of the PC lobe. A gradient of connections in the layer of model burster cells induces periodic wave propagation, as measured in the PC lobe. We study odor representations in the biological PC lobe using the technique of Kimura and coworkers. Lucifer yellow injection into intact Limax after appetitive or aversive odor learning results in a band or patch of labeled cells in the PC lobe with the band long axis normal to the axis of wave propagation. Learning two odors yields two parallel bands of labeled PC cells. We introduce olfactory input to our model PC lobe such that each odor maximally activates a unique row of four cells which produces a short-term memory trace of odor stimulation. A winner-take-all synaptic competition enabled by collapse of the phase gradient during odor presentation produces a single short-term memory band for each odor. The short-term memory is converted to long-term memory if odor stimulation is followed by activation of an input pathway for the unconditioned stimulus (US) which presumably results in release of one or more neuromodulatory amines or peptides in the PC lobe.

Modulation of oscillatory neural activities by cholinergic activation of interneurons in the olfactory center of a terrestrial slug

The neurons in the procerebrum (PC) of the terrestrial slug Limax marginatus show regular oscillation of their membrane potential, and the oscillation has been implicated in olfactory processing. The neural mechanisms for the generation and modulation of the oscillation have been poorly understood. In the present work, we examined the ionic conductances evoked by acetylcholine (ACh) in the PC neurons and the effects of ACh application on the population activities of intrinsic and extrinsic neurons. The PC neurons are categorized into bursting neurons, which are putative local inhibitory neurons, and nonbursting neurons, which likely mediate the input and output of information in the PC. Bath application of ACh augmented the local field potential oscillation in the PC. Perforated patch recording from single PC neurons revealed that ACh has direct excitatory effects on bursting neurons, while it suppresses the activity of nonbursting neurons, possibly via augmented inhibitory synaptic input from bursting neurons. The correlation between the membrane potential of bursting neurons and the frequency of oscillation suggests that bursting neurons are the main determinant of the oscillation frequency. Application of ACh also resulted in a reduction of the oscillation amplitude in the olfactory nerve, suggesting that the frequency modulation in the oscillatory network could change the activities in the follower neurons.

The effects of carbon monoxide as a neurotransmitter

Neural tissues generate carbon monoxide. Although neuronal carbon monoxide does not appear to be released in a directed manner, heme-derived carbon monoxide affects neuronally mediated activities. This rather suggests that endogenously formed carbon monoxide is an important neuromodulator. In addition, it appears that carbon monoxide may contribute to various neuropathological conditions.

A colorimetric sensor array for odour visualization

Array-based vapour-sensing devices are used to detect and differentiate between chemically diverse analytes. These systems--based on cross-responsive sensor elements--aim to mimic the mammalian olfactory system by producing composite responses unique to each odorant. Previous work has concentrated on a variety of non-specific chemical interactions to detect non-coordinating organic vapours. But the most odiferous, toxic compounds often bind readily to metal ions. Here we report a simple optical chemical sensing method that utilizes the colour change induced in an array of metalloporphyrin dyes upon ligand binding while minimizing the need for extensive signal transduction hardware. The chemoselective response of a library of immobilized vapour-sensing metalloporphyrin dyes permits the visual identification of a wide range of ligating (alcohols, amines, ethers, phosphines, phosphites, thioethers and thiols) and even weakly ligating (arenes, halocarbons and ketones) vapours. Water vapour does not affect the performance of the device, which shows a good linear response to single analytes, and interpretable responses to analyte mixtures. Unique colour fingerprints can be obtained at analyte concentrations below 2 parts per million, and responses to below 100 parts per billion have been observed. We expect that this type of sensing array will be of practical importance for general-purpose vapour dosimeters and analyte-specific detectors (for insecticides, drugs or neurotoxins, for example).