Neural detection of gases--carbon dioxide, oxygen--in vertebrates and invertebrates

Modulation of the NO-cGMP pathway has no effect on olfactory responses in the Drosophila antenna

Olfaction is a crucial sensory modality in insects and is underpinned by odor-sensitive sensory neurons expressing odorant receptors that function in the dendrites as odorant-gated ion channels. Along with expression, trafficking, and receptor complexing, the regulation of odorant receptor function is paramount to ensure the extraordinary sensory abilities of insects. However, the full extent of regulation of sensory neuron activity remains to be elucidated. For instance, our understanding of the intracellular effectors that mediate signaling pathways within antennal cells is incomplete within the context of olfaction in vivo. Here, with the use of optical and electrophysiological techniques in live antennal tissue, we investigate whether nitric oxide signaling occurs in the sensory periphery of Drosophila. To answer this, we first query antennal transcriptomic datasets to demonstrate the presence of nitric oxide signaling machinery in antennal tissue. Next, by applying various modulators of the NO-cGMP pathway in open antennal preparations, we show that olfactory responses are unaffected by a wide panel of NO-cGMP pathway inhibitors and activators over short and long timescales. We further examine the action of cAMP and cGMP, cyclic nucleotides previously linked to olfactory processes as intracellular potentiators of receptor functioning, and find that both long-term and short-term applications or microinjections of cGMP have no effect on olfactory responses in vivo as measured by calcium imaging and single sensillum recording. The absence of the effect of cGMP is shown in contrast to cAMP, which elicits increased responses when perfused shortly before olfactory responses in OSNs. Taken together, the apparent absence of nitric oxide signaling in olfactory neurons indicates that this gaseous messenger may play no role as a regulator of olfactory transduction in insects, though may play other physiological roles at the sensory periphery of the antenna.

Euthanasia of laboratory mice: Are isoflurane and sevoflurane real alternatives to carbon dioxide?

In the European Union (EU) millions of laboratory mice are used and killed for experimental and other scientific purposes each year. Although controversially discussed, the use of carbon dioxide (CO₂) is still permitted for killing rodents according to the Directive 2010/63/EU. Within the scope of refinement, our aim was to investigate if isoflurane and sevoflurane are an appropriate alternative killing method to CO₂ in mice. Different concentrations of CO₂ (filling rates of 20%, 60%, 100%; CO₂ 20, 60, 100), isoflurane (Iso 2%, 5%) and sevoflurane (Sevo 4.8%, 8%) were compared in two mouse strains (NMRI, C57Bl/6J) using a broad spectrum of behavioral parameters, including the approach-avoidance test, and analyzing blood for stress parameters (glucose, adrenaline, noradrenaline). We focused in our study on the period from the beginning of the gas inlet to loss of consciousness, as during this period animals are able to perceive pain and distress. Our results show that only higher concentrations of CO₂ (CO₂ 60, 100) and isoflurane (5%) induced surgical tolerance within 300 s in both strains, with CO₂ 100 being the fastest acting inhalant anesthetic. The potency of halogenated ethers depended on the mouse strain, with C57Bl/6J being more susceptible than NMRI mice. Behavioral analysis revealed no specific signs of distress, e. g. stress-induced grooming, and pain, i. e. audible vocalizations, for all inhalant gases. However, adrenaline and noradrenaline plasma concentrations were increased, especially in NMRI mice exposed to CO₂ in high concentrations, whereas we did not observe such increase in animals exposed to isoflurane or sevoflurane. Escape latencies in the approach-avoidance test using C57Bl/6J mice did not differ between the three inhalant gases, however, some animals became recumbent during isoflurane and sevoflurane but not during CO₂ exposure. The rise in catecholamine concentrations suggests that CO₂ exposure might be linked to a higher stress response compared to isoflurane and sevoflurane exposure, although we did not observe a behavioral correlate for that. Follow-up studies investigating other fast-acting stress hormones and central anxiety circuits are needed to confirm our findings.

Neuron type-specific miRNA represses two broadly expressed genes to modulate an avoidance behavior in C. elegans

In this study, Drexel et al. research miRNA-mediated repression of broadly transcribed genes as a strategy for cellular specialization. They show that mir-791, expressed exclusively in the CO₂-sensing neurons in C. elegans, represses two otherwise broadly expressed genes, which are needed for normal neuronal function and behavior of the animals toward CO₂.

Two broad gene classes are distinguished within multicellular organisms: cell type-specific genes, which confer particular cellular properties, and ubiquitous genes that support general cellular functions. However, certain so-called ubiquitous genes show functionally relevant cell type-specific repression. How such repression is achieved is poorly understood. MicroRNAs (miRNAs) are repressors, many of which are expressed with high cell type specificity. Here we show that mir-791, expressed exclusively in the CO₂-sensing neurons in Caenorhabditis elegans, represses two otherwise broadly expressed genes. This repression is necessary for normal neuronal function and behavior of the animals toward CO₂. miRNA-mediated repression of broadly transcribed genes is a previously unappreciated strategy for cellular specialization.

Carbon dioxide sensitivity and its role in multifunctional neurons in the mollusk Onchidium

Intrinsically photoresponsive neurons in the abdominal ganglion of the amphibious mollusk Onchidium named Ip-1 and Ip-2 (Ip-1/2) react to several different stimuli. These neurons respond to light with slow hyperpolarization and to CO2 stimulation with slow depolarization. In this study, increasing the concentration of CO2 in the air caused hyperventilation and enlargement of the pneumostome in the intact animal. In a semi-intact preparation, pouring artificial seawater (ASW) with dissolved CO2 onto the central ganglia caused the previously closed pneumostome to open. In an ASW environment, Ip-1/2 neurons depolarized even under conditions of constant pH (alkaline ASW) and after dissolution of CO2. This depolarization prolonged the firing of action potentials in Ip-1/2 neurons. Adding protons (H+) to ASW caused Ip-1/2 depolarization only when the neurons' membranes were depolarized to a potential above the resting potential. Furthermore, in the presence of the carbonic anhydrase inhibitor acetazolamide (AZ), CO2-induced excitation in Ip-1/2 neurons was increased in both normal and alkaline ASW. These results suggest that when dissolved in ASW, CO2 directly induced the depolarizing response in Ip-1/2 neurons. Since Ip-1/2 neurons participate in pneumostome opening, these results suggest that increased CO2 levels in ASW directly stimulate CO2-sensitive central neurons, promoting ventilation.

Intracellular bicarbonate regulates action potential generation via KCNQ channel modulation

Bicarbonate (HCO3(-)) is an abundant anion that regulates extracellular and intracellular pH. Here, we use patch-clamp techniques to assess regulation of hippocampal CA3 pyramidal cell excitability by HCO3(-) in acute brain slices from C57BL/6 mice. We found that increasing HCO3(-) levels enhances action potential (AP) generation in both the soma and axon initial segment (AIS) by reducing Kv7/KCNQ channel activity, independent of pH (i.e., at a constant pH of 7.3). Conversely, decreasing intracellular HCO3(-) leads to attenuation of AP firing. We show that HCO3(-) interferes with Kv7/KCNQ channel activation by phosphatidylinositol-4,5-biphosphate. Consequently, we propose that, even in the presence of a local depolarizing Cl(-) gradient, HCO3(-) efflux through GABAA receptors may ensure the inhibitory effect of axoaxonic cells at the AIS due to activation of Kv7/KCNQ channels.

TRPs as chemosensors (ROS, RNS, RCS, gasotransmitters)

The transient receptor potential (trp) gene superfamily encodes TRP proteins that act as multimodal sensor cation channels for a wide variety of stimuli from outside and inside the cell. Upon chemical or physical stimulation of cells, TRP channels transduce electrical and/or Ca(2+) signals via their cation channel activities. These functional features of TRP channels allow the body to react and adapt to different forms of environmental changes. Indeed, members of one class of TRP channels have emerged as sensors of reactive oxygen species (ROS), reactive nitrogen species (RNS), reactive carbonyl species (RCS), and gaseous messenger molecules including molecular oxygen (O2), hydrogen sulfide (H2S), and carbon dioxide (CO2). Hydrogen peroxide (H2O2), an ROS, triggers the production of ADP-ribose, which binds and activates TRPM2. In addition to TRPM2, TRPC5, TRPV1, and TRPA1 are also activated by H2O2 via modification of cysteine (Cys) free sulfhydryl groups. Nitric oxide (NO), a vasoactive gaseous molecule, regulates TRP channels directly via Cys S-nitrosylation or indirectly via cyclic GMP (cGMP)/protein kinase G (PKG)-dependent phosphorylation. Anoxia induced by O2-glucose deprivation and severe hypoxia activates TRPM7 and TRPC6, respectively, whereas TRPA1 serves as a sensor of mild hypoxia and hyperoxia in vagal and sensory neurons. TRPA1 also detects other gaseous molecules, such as hydrogen sulfide (H2S) and carbon dioxide (CO2). In this review, we highlight our current knowledge of TRP channels as chemosensors for ROS, RNS, RCS, and gaseous molecules and discuss their functional impacts on physiological and pathological events.

Encoding and representation of intranasal CO2 in the mouse olfactory cortex

Intranasal trigeminal sensory input, often perceived as a burning, tingling, or stinging sensation, is well known to affect odor perception. While both anatomical and functional imaging data suggest that the influence of trigeminal stimuli on odor information processing may occur within the olfactory cortex, direct electrophysiological evidence for the encoding of trigeminal information at this level of processing is unavailable. Here, in agreement with human functional imaging studies, we found that 26% of neurons in the mouse piriform cortex (PCX) display modulation in firing to carbon dioxide (CO2), an odorless stimulant with known trigeminal capacity. Interestingly, CO2 was represented within the PCX by distinct temporal dynamics, differing from those evoked by odor. Experiments with ascending concentrations of isopentyl acetate, an odorant known to elicit both olfactory and trigeminal sensations, resulted in morphing of the temporal dynamics of stimulus-evoked responses. Whereas low concentrations of odorant evoked responses upon stimulus onset, high concentrations of odorant and/or CO2 often evoked responses structured to stimulus offset. These physiological experiments in mice suggest that PCX neurons possess the capacity to encode for stimulus modality (olfactory vs trigeminal) by differential patterns of firing. These data provide mechanistic insights into the influences of trigeminal information on odor processing and place constraints on models of olfactory-trigeminal sensory integration.

O2-sensing neurons control CO2 response in C. elegans

Sensory behaviors are often flexible, allowing animals to generate context-appropriate responses to changing environmental conditions. To investigate the neural basis of behavioral flexibility, we examined the regulation of carbon dioxide (CO2) response in the nematode Caenorhabditis elegans. CO2 is a critical sensory cue for many animals, mediating responses to food, conspecifics, predators, and hosts (Scott, 2011; Buehlmann et al., 2012; Chaisson and Hallem, 2012). In C. elegans, CO2 response is regulated by the polymorphic neuropeptide receptor NPR-1: animals with the N2 allele of npr-1 avoid CO2, whereas animals with the Hawaiian (HW) allele or an npr-1 loss-of-function (lf) mutation appear virtually insensitive to CO2 (Hallem and Sternberg, 2008; McGrath et al., 2009). Here we show that ablating the oxygen (O2)-sensing URX neurons in npr-1(lf) mutants restores CO2 avoidance, suggesting that NPR-1 enables CO2 avoidance by inhibiting URX neurons. URX was previously shown to be activated by increases in ambient O2 (Persson et al., 2009; Zimmer et al., 2009; Busch et al., 2012). We find that, in npr-1(lf) mutants, O2-induced activation of URX inhibits CO2 avoidance. Moreover, both HW and npr-1(lf) animals avoid CO2 under low O2 conditions, when URX is inactive. Our results demonstrate that CO2 response is determined by the activity of O2-sensing neurons and suggest that O2-dependent regulation of CO2 avoidance is likely to be an ecologically relevant mechanism by which nematodes navigate gas gradients.

Sensory cilia in arthropods

In arthropods, the modified primary cilium is a structure common to all peripheral sensory neurons other than photoreceptors. Since its first description in 1958, it has been investigated in great detail in numerous sense organs (sensilla) of many insect species by means of electron microscopy and electrophysiology. The perfection of molecular biological methods has led to an enormous advance in our knowledge about development and function of sensory cilia in the fruitfly since the end of the last century. The cilia show a wealth of adaptations according to their different physiological roles: chemoreception, mechanoreception, hygroreception, and thermoreception. Divergent types of receptors and channels have evolved fulfilling these tasks. The number of olfactory receptor genes can be close to 300 in ants, whereas in crickets slightest mechanical stimuli are detected by the interaction of extremely sophisticated biomechanical devices with mechanosensory cilia. Despite their enormous morphological and physiological divergence, sensilla and sensory cilia develop according to a stereotyped pattern. Intraflagellar transport genes have been found to be decisive for proper development and function.

Natriuretic peptides block synaptic transmission by activating phosphodiesterase 2A and reducing presynaptic PKA activity

The heart peptide hormone atrial natriuretic peptide (ANP) regulates blood pressure by stimulating guanylyl cyclase-A to produce cyclic guanosine monophosphate (cGMP). ANP and guanylyl cyclase-A are also expressed in many brain areas, but their physiological functions and downstream signaling pathways remain enigmatic. Here we investigated the physiological functions of ANP signaling in the neural pathway from the medial habenula (MHb) to the interpeduncular nucleus (IPN). Biochemical assays indicate that ANP increases cGMP accumulation in the IPN of mouse brain slices. Using optogenetic stimulation and electrophysiological recordings, we show that both ANP and brain natriuretic peptide profoundly block glutamate release from MHb neurons. Pharmacological applications reveal that this blockade is mediated by phosphodiesterase 2A (PDE2A) but not by cGMP-stimulated protein kinase-G or cGMP-sensitive cyclic nucleotide-gated channels. In addition, focal infusion of ANP into the IPN enhances stress-induced analgesia, and the enhancement is prevented by PDE2A inhibitors. PDE2A is richly expressed in the axonal terminals of MHb neurons, and its activation by cGMP depletes cyclic adenosine monophosphates. The inhibitory effect of ANP on glutamate release is reversed by selectively activating protein kinase A. These results demonstrate strong presynaptic inhibition by natriuretic peptides in the brain and suggest important physiological and behavioral roles of PDE2A in modulating neurotransmitter release by negative crosstalk between cGMP-signaling and cyclic adenosine monophosphate-signaling pathways.

TRP channels: sensors and transducers of gasotransmitter signals

The transient receptor potential (trp) gene superfamily encodes cation channels that act as multimodal sensors for a wide variety of stimuli from outside and inside the cell. Upon sensing, they transduce electrical and Ca²⁺ signals via their cation channel activities. These functional features of TRP channels allow the body to react and adapt to different forms of environmental changes. Indeed, members of one class of TRP channels have emerged as sensors of gaseous messenger molecules that control various cellular processes. Nitric oxide (NO), a vasoactive gaseous molecule, regulates TRP channels directly via cysteine (Cys) S-nitrosylation or indirectly via cyclic GMP (cGMP)/protein kinase G (PKG)-dependent phosphorylation. Recent studies have revealed that changes in the availability of molecular oxygen (O₂) also control the activation of TRP channels. Anoxia induced by O₂-glucose deprivation and severe hypoxia (1% O₂) activates TRPM7 and TRPC6, respectively, whereas TRPA1 has recently been identified as a novel sensor of hyperoxia and mild hypoxia (15% O₂) in vagal and sensory neurons. TRPA1 also detects other gaseous molecules such as hydrogen sulfide (H₂S) and carbon dioxide (CO₂). In this review, we focus on how signaling by gaseous molecules is sensed and integrated by TRP channels.

Proton production, regulation and pathophysiological roles in the mammalian brain

The recent demonstration of proton signaling in C. elegans muscle contraction suggests a novel mechanism for proton-based intercellular communication and has stimulated enthusiasm for exploring proton signaling in higher organisms. Emerging evidence indicates that protons are produced and regulated in localized space and time. Furthermore, identification of proton regulators and sensors in the brain leads to the speculation that proton production and regulation may be of major importance for both physiological and pathological functions ranging from nociception to learning and memory. Extracellular protons may play a role in signal transmission by not only acting on adjacent cells but also affecting the cell from which they were released. In this review, we summarize the upstream and downstream pathways of proton production and regulation in the mammalian brain, with special emphasis on the proton extruders and sensors that are critical in the homeostatic regulation of pH, and discuss their potential roles in proton signaling under normal and pathophysiological conditions.

The neurobiology of sensing respiratory gases for the control of animal behavior

Aerobic metabolism is fundamental for almost all animal life. Cellular consumption of oxygen (O(2)) and production of carbon dioxide (CO(2)) signal metabolic states and physiological stresses. These respiratory gases are also detected as environmental cues that can signal external food quality and the presence of prey, predators and mates. In both contexts, animal nervous systems are endowed with mechanisms for sensing O(2)/CO(2) to trigger appropriate behaviors and maintain homeostasis of internal O(2)/CO(2). Although different animal species show different behavioral responses to O(2)/CO(2), some underlying molecular mechanisms and pathways that function in the detection of respiratory gases are fundamentally similar and evolutionarily conserved. Studies of Caenorhabditis elegans and Drosophila melanogaster have identified roles for cyclic nucleotide signaling and the hypoxia inducible factor (HIF) transcriptional pathway in mediating behavioral responses to respiratory gases. Understanding how simple invertebrate nervous systems detect respiratory gases to control behavior might reveal general principles common to nematodes, insects and vertebrates that function in the molecular sensing of respiratory gases and the neural control of animal behaviors.

Out of thin air: sensory detection of oxygen and carbon dioxide

Oxygen (O₂) and carbon dioxide (CO₂) levels vary in different environments and locally fluctuate during respiration and photosynthesis. Recent studies in diverse animals have identified sensory neurons that detect these external variations and direct a variety of behaviors. Detection allows animals to stay within a preferred environment as well as identify potential food or dangers. The complexity of sensation is reflected in the fact that neurons compartmentalize detection into increases, decreases, and short-range and long-range cues. Animals also adjust their responses to these prevalent signals in the context of other cues, allowing for flexible behaviors. In general, the molecular mechanisms for detection suggest that sensory neurons adopted ancient strategies for cellular detection and coupled them to brain activity and behavior. This review highlights the multiple strategies that animals use to extract information about their environment from variations in O₂ and CO₂.

Regulation of gene expression by carbon dioxide

Carbon dioxide (CO(2)) is a physiological gas found at low levels in the atmosphere and produced in cells during the process of aerobic respiration. Consequently, the levels of CO(2) within tissues are usually significantly higher than those found externally. Shifts in tissue levels of CO(2) (leading to either hypercapnia or hypocapnia) are associated with a number of pathophysiological conditions in humans and can occur naturally in niche habitats such as those of burrowing animals. Clinical studies have indicated that such altered CO(2) levels can impact upon disease progression. Recent advances in our understanding of the biology of CO(2) has shown that like other physiological gases such as molecular oxygen (O(2)) and nitric oxide (NO), CO(2) levels can be sensed by cells resulting in the initiation of physiological and pathophysiological responses. Acute CO(2) sensing in neurons and peripheral and central chemoreceptors is important in rapidly activated responses including olfactory signalling, taste sensation and cardiorespiratory control. Furthermore, a role for CO(2) in the regulation of gene transcription has recently been identified with exposure of cells and model organisms to high CO(2) leading to suppression of genes involved in the regulation of innate immunity and inflammation. This latter, transcriptional regulatory role for CO(2), has been largely attributed to altered activity of the NF-B family of transcription factors. Here, we review our evolving understanding of how CO(2) impacts upon gene transcription.

Receptor-type guanylate cyclase is required for carbon dioxide sensation by Caenorhabditis elegans

CO(2) is both a critical regulator of animal physiology and an important sensory cue for many animals for host detection, food location, and mate finding. The free-living soil nematode Caenorhabditis elegans shows CO(2) avoidance behavior, which requires a pair of ciliated sensory neurons, the BAG neurons. Using in vivo calcium imaging, we show that CO(2) specifically activates the BAG neurons and that the CO(2)-sensing function of BAG neurons requires TAX-2/TAX-4 cyclic nucleotide-gated ion channels and the receptor-type guanylate cyclase GCY-9. Our results delineate a molecular pathway for CO(2) sensing and suggest that activation of a receptor-type guanylate cyclase is an evolutionarily conserved mechanism by which animals detect environmental CO(2).

TRPA1 is a component of the nociceptive response to CO2

In humans, high concentrations of CO(2), as found in carbonated beverages, evoke a mixture of sensations that include a stinging or pungent quality. The stinging sensation is thought to originate with the activation of nociceptors, which innervate the respiratory, nasal, and oral epithelia. The molecular basis for this sensation is unknown. Here we show that CO(2) specifically activates a subpopulation of trigeminal neurons that express TRPA1, a mustard oil- and cinnamaldehyde-sensitive channel, and that these responses are dependent on a functional TRPA1 gene. TRPA1 is sufficient to mediate responses to CO(2) as TRPA1 channels expressed in HEK-293 cells, but not TRPV1 channels, were activated by bath-applied CO(2). CO(2) can diffuse into cells and produce intracellular acidification, which could gate TRPA1 channels. Consistent with this mechanism, TRPA1 channels in excised patches were activated in a dose-dependent manner by intracellular protons. We conclude that TRPA1, by sensing intracellular acidification, constitutes an important component of the nociceptive response to CO(2).

Integration of CO2 and odorant signals in the mouse olfactory bulb

Carbon dioxide (CO(2)) is an important environmental cue for many animal species. In both vertebrates and invertebrates, CO(2) is detected by a specialized subset of olfactory sensory neurons (OSNs) and mediates several stereotypical behaviors. It remains unknown how CO(2) cues are integrated with other olfactory signals in the mammalian olfactory bulb, the first stage of central olfactory processing. By recording from the mouse olfactory bulb in vivo, we found that CO(2)-activating neurons also respond selectively to odorants, many of which are putative mouse pheromones and natural odorants. In addition, many odorant-responsive bulbar neurons are inhibited by CO(2). For a substantial number of CO(2)-activating neurons, binary mixtures of CO(2) and a specific odorant produce responses that are distinct from those evoked by either CO(2) or the odorant alone. In addition, for a substantial number of CO(2)-inhibiting neurons, CO(2) addition can completely block the action potential firing of the cells to the odorants. These results indicate strong interaction between CO(2) signals and odorant signals in the olfactory bulb, suggesting important roles for the integration of these two signals in CO(2)-mediated behavioral responses.

Ionotropic and metabotropic mechanisms in chemoreception: 'chance or design'?

Chemosensory receptors convert an enormous diversity of chemical signals from the external world into a common language of electrical activity in the brain. Mammals and insects use several families of transmembrane receptor proteins to recognize distinct classes of volatile and non-volatile chemicals that are produced by conspecifics or other environmental sources. A comparison of the signalling mechanisms of mammalian and insect receptors has revealed an unexpected functional distinction: mammals rely almost exclusively on metabotropic ligand-binding receptors, which use second messenger signalling cascades to indirectly activate ion channels, whereas insects use ionotropic receptors, which are gated directly by chemical stimuli, thereby leading to neuronal depolarization. In this review, we consider possible reasons for this dichotomy, taking into account biophysical, cell biological, ecological and evolutionary influences on how information is extracted from chemosensory cues by these animal classes.

Biochemistry. CO2mmon sense

Animals and plants use the same enzyme to detect carbon dioxide.