CO2 chemoreception in the pulmonate snail, Helix aspersa

Welfare of invertebrates: a pilot study on a new land snail stunning technique

The almost complete absence of regulations to protect invertebrates is a common condition in legal systems, including the European one, especially when it comes to invertebrates intended for human consumption. Thus, in the vast majority of cases, edible invertebrates do not receive even the most basic protection at slaughter. Despite recent research indicating that invertebrates are capable of feeling pain and stress, the humane step of stunning is not used on them. This is also the case for land snails, which are gastropod invertebrates whose consumption has now reached significant levels, already involving tonnes and that is expected to increase significantly as edible snail farming becomes more popular as a relatively low-cost, easy-to-perform, and sustainable alternative animal husbandry, thereby making land snails an increasingly economically important species. This paper presents and investigates a proposed stunning method based on the immersion of mollusks in CO2-supplemented and refrigerated water that could be used in the snail meat production chain to reduce the slaughter suffering of millions of these invertebrates. To this end, body condition descriptors (hemolymph parameters) in snails were determined before and after CO2 treatment in cold water, while generating useful data for defining a preliminary set of reference intervals for basal values.

Two central pattern generators from the crab, Cancer borealis, respond robustly and differentially to extreme extracellular pH

The activity of neuronal circuits depends on the properties of the constituent neurons and their underlying synaptic and intrinsic currents. We describe the effects of extreme changes in extracellular pH – from pH 5.5 to 10.4 – on two central pattern generating networks, the stomatogastric and cardiac ganglia of the crab, Cancer borealis. Given that the physiological properties of ion channels are known to be sensitive to pH within the range tested, it is surprising that these rhythms generally remained robust from pH 6.1 to pH 8.8. The pH sensitivity of these rhythms was highly variable between animals and, unexpectedly, between ganglia. Animal-to-animal variability was likely a consequence of similar network performance arising from variable sets of underlying conductances. Together, these results illustrate the potential difficulty in generalizing the effects of environmental perturbation across circuits, even within the same animal.

Life in a Near-Future Atmosphere: Carbon Dioxide Enrichment Increases Plant Growth and Alters the Behavior of a Terrestrial Snail but not a Terrestrial Beetle

Abstract In addition to effects on climate and water acidification, anthropogenic atmospheric releases of carbon dioxide may also directly impact terrestrial organisms that use CO2 as a chemical cue. We wondered how common organisms would respond to near-future levels of CO2 – levels that may occur by 2025. We chose two common but taxonomically and ecologically dissimilar organisms (Theba pisana helicid snails and Adesmia dilatata tenebrionid beetles) to examine the behavioral effects of a slight rise (~10 ppm) of CO2 on animal abundance and plant growth in the Negev Desert of Israel. We found that plots with supplementary CO2 exhibited greater plant growth than control plots over a 50-day experiment, but increased growth did not alter beetle or snail numbers.

In laboratory experiments with higher levels of augmented CO2 paired with food rewards, we found that snails did not change their climbing behavior when presented with CO2 alone, but they avoided food and climbed away when CO2 was paired with food. Beetles in the laboratory were attracted to food regardless of CO2 levels although high levels of CO2 (1200–1300 ppm) reduced movement.

The direct effects of near-future CO2 levels may augment plant growth but have only minor influence on terrestrial snails and beetles. However, the effects of CO2 on climate change in desert habitats like the Negev may be more severe due to a predicted rise in temperature and a decline in precipitation.

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.

Control of breathing in invertebrate model systems

The invertebrates have adopted a myriad of breathing strategies to facilitate the extraction of adequate quantities of oxygen from their surrounding environments. Their respiratory structures can take a wide variety of forms, including integumentary surfaces, lungs, gills, tracheal systems, and even parallel combinations of these same gas exchange structures. Like their vertebrate counterparts, the invertebrates have evolved elaborate control strategies to regulate their breathing activity. Our goal in this article is to present the reader with a description of what is known regarding the control of breathing in some of the specific invertebrate species that have been used as model systems to study different mechanistic aspects of the control of breathing. We will examine how several species have been used to study fundamental principles of respiratory rhythm generation, central and peripheral chemosensory modulation of breathing, and plasticity in the control of breathing. We will also present the reader with an overview of some of the behavioral and neuronal adaptability that has been extensively documented in these animals. By presenting explicit invertebrate species as model organisms, we will illustrate mechanistic principles that form the neuronal foundation of respiratory control, and moreover appear likely to be conserved across not only invertebrates, but vertebrate species as well.

Ionic mechanisms of central CO(2) chemosensitivity

A comparative analysis of chemosensory systems in invertebrates and vertebrates reveals that different animals use similar strategies when sensing CO(2) to control respiration. A variety of animals possess neurons that respond to changes in pH. These respiratory chemoreceptor neurons seem to rely largely on pH-dependent inhibition of potassium channels, but the channels do not appear to be uniquely adapted to detect pH. The 'chemosensory' potassium channels identified thus far are widely distributed, common potassium channels. The pH-sensitivity is a common feature of the channels whether the channels are in chemosensory neurons or not. Thus, the pattern of synaptic connectivity and the mix of potassium channels expressed seem to determine whether a neuron is chemosensory or not, rather than any special adaptation of a channel for pH-sensitivity. Moreover, there are often multiple pH-sensitive channels in each chemosensory neuron. These ionic mechanisms may, however, be only part of the chemosensory process, and pH-dependent modulation of synaptic activity seems to contribute to central chemosensitivity as well. In addition, the exploration of the mechanisms of pH-dependent modulation of ion channel activity in chemosensory cells is incomplete: additional mechanisms of pH modulation of channel activity may be found, and addition conductances, other than potassium channels, may participate in the chemosensory process.

CO2chemosensitivity inHelix aspersa: three potassium currents mediate pH-sensitive neuronal spike timing

Elevated levels of carbon dioxide increase lung ventilation in Helix aspersa. The hypercapnic response originates from a discrete respiratory chemosensory region in the dorsal subesophageal ganglia that contains CO2-sensitive neurons. We tested the hypothesis that pH-dependent inhibition of potassium channels in neurons in this region mediated the chemosensory response to CO2. Cells isolated from the dorsal subesophageal ganglia retained CO2chemosensitivity and exhibited membrane depolarization and/or an increase in input resistance during an acid challenge. Isolated somata expressed two voltage-dependent potassium channels, an A-type and a delayed-rectifier-type channel ( IKAand IKDR). Both conductances were inhibited during hypercapnia. The pattern of voltage dependence indicated that IKAwas affected by extracellular or intracellular pH, but the activity of IKDRwas modulated by extracellular pH only. Application of inhibitors of either channel mimicked many of the effects of acidification in isolated cells and neurons in situ. We also detected evidence of a pH-sensitive calcium-activated potassium channel ( IKCa) in neurons in situ. The results of these studies support the hypothesis that IKAinitiates the chemosensory response, and IKDRand IKCaprolong the period of activation of CO2-sensitive neurons. Thus multiple potassium channels are inhibited by acidosis, and the combined effect of pH-dependent inhibition of these channels enhances neuronal excitability and mediates CO2chemosensory responses in H. aspersa. We did not find a single “chemosensory channel,” and the chemosensitive channels that we did find were not unique in any way that we could detect. The protein “machinery” of CO2chemosensitivity is probably widespread among neurons, and the selection process whereby a neuron acts or does not act as a respiratory CO2chemosensor probably depends on the resting membrane potential and synaptic connectivity.

The Potential Role of CO2in Initiation and Maintenance of Estivation in the Land SnailHelix lucorum

Elevated CO(2) levels are hypothesized to play a role in the initiation and maintenance of estivation in snails through disturbances of acid-base status. The aim of our study was to identify the ambient CO(2) threshold that induces disturbances in acid-base status in the air-breathing land snail Helix lucorum. Acid-base parameters were determined in the hemolymph of snails acclimated to 0.5%, 1%, 2%, 4%, and 8% CO(2) in air for 20 d. In addition, we evaluated the effects of long-term acclimation on metabolic rate and on levels of D-lactate dehydrogenase activity (D-LDH) and of D-lactate in snails after 20 d of exposure to increased CO(2) levels. Helix lucorum proved to be unable to compensate for a decrease in extracellular pH (pH(e)) when acclimated to levels higher than 1% CO(2) in air. The rate of oxygen consumption started to decrease when snails were acclimated to 0.5% CO(2) in air. However, there was no correlation between the drops in pH(e) and in metabolic rate. Long-term acclimation to elevated CO(2) levels induced an increase in the activity of D-LDH with a concomitant accumulation of D-lactate in tissues. This indicates that long-term acclimation to elevated ambient CO(2) levels could reduce the aerobic capacity of land snails and trigger expression of anaerobic pathways of ATP turnover. The threshold levels of ambient CO(2) that induce changes in acid-base status and elicit metabolic depression in adult land snails H. lucorum are higher than the future atmospheric levels that are expected to result from human use of fossil energy resources.

Ontogeny of central CO2 chemoreception: chemosensitivity in the ventral medulla of developing bullfrogs

Sites of central CO2 chemosensitivity were investigated in isolated brain stems from Rana catesbeiana tadpoles and frogs. Respiratory neurograms were made from cranial nerve (CN) 7 and spinal nerve 2. Superfusion of the brain stem with hypercapnic artificial cerebrospinal fluid elicited increased fictive lung ventilation. The effect of focal perfusion of hypercapnic artificial cerebrospinal fluid on discrete areas of the ventral medulla was assessed. Sites of chemosensitivity, which are active continuously throughout development, were identified adjacent to CN 5 and CN 10 on the ventral surface of the medulla. In early- and middle-stage tadpoles and frogs, unilateral stimulation within either site was sufficient to elicit the hypercapnic response, but simultaneous stimulation within both sites was required in late-stage tadpoles. The chemosensitive sites were individually disrupted by unilateral application of 1 mg/ml protease, and the sensitivity to bath application or focal perfusion of hypercapnia was reassessed. Protease lesions at CN 10 abolished the entire hypercapnic response, but lesions at CN 5 affected only the hypercapnic response originating from the CN 5 site. Neurons within the chemosensitive sites were also destroyed by unilateral application of 1 mM kainic acid, and the sensitivity to bath or focal application of hypercapnia was reassessed. Kainic acid lesions within either site abolished the hypercapnic response. Using a vital dye, we determined that kainic acid destroyed neurons by only within 100 microm of the ventral medullary surface. Thus, regardless of developmental stage, neurons necessary for CO2 sensitivity are located in the ventral medulla adjacent to CN 5 and 10.

Hypoxia-induced respiratory patterned activity in Lymnaea originates at the periphery

Respiration in Lymnaea is a hypoxia-driven rhythmic behavior, which is controlled by an identified network of central pattern generating (CPG) neurons. However, the precise site(s) (i.e., central or peripheral) at which hypoxia acts and the cellular mechanisms by which the respiratory chemosensory drive is conveyed to the CPG were previously unknown. Using semi-intact and isolated ganglionic preparations, we provide the first direct evidence that the hypoxia-induced respiratory drive originates at the periphery (not within the central ring ganglia) and that it is conveyed to the CPG neurons via the right pedal dorsal neuron 1 (RPeD1). The respiratory discharge frequency increased when the periphery, but not the CNS, was made hypoxic. We found that in the semi-intact preparations, the frequency of spontaneously occurring respiratory bursts was significantly lower than in isolated ganglionic preparations. Thus the periphery exerts a suppressive regulatory control on respiratory discharges in the intact animal. Moreover, both anoxia (0% O(2)) and hypercapnia (10% CO(2)) produce a reduction in respiratory discharges in semi-intact, but not isolated preparations. However, the effects of CO(2) may be mediated through pH changes of the perfusate. Finally, we demonstrate that chronic exposure of the animals to hypoxia (90% N(2)), prior to intracellular recordings, significantly enhanced the rate of spontaneously occurring respiratory discharges in semi-intact preparations, even if they were maintained in normoxic saline for several hours. Moreover, we demonstrate that the peripherally originated hypoxia signal is likely conveyed to the CPG neurons via RPeD1. In summary, the data presented in this study demonstrate the important role played by the periphery and the RPeD1 neuron in regulating respiration in response to hypoxia in Lymnaea.

Intracellular pH regulation in neurons from chemosensitive and nonchemosensitive regions of Helix aspersa

We used 2',7'-bis(carboxyethyl)-5(6)-carboxyflourescein (BCECF), a pH-sensitive fluorescent dye, to study intracellular pH (pH(i)) regulation in neurons in CO(2) chemoreceptor and nonchemoreceptor regions in the pulmonate, terrestrial snail, Helix aspersa. We studied pH(i) during hypercapnic acidosis, after ammonia prepulse, and during isohydric hypercapnia. In all treatment conditions, pH(i) fell to similar levels in chemoreceptor and nonchemoreceptor regions. However, pH(i) recovery was consistently slower in chemoreceptor regions compared with nonchemoreceptor regions, and pH(i) recovery was slower in all regions when extracellular pH (pH(e)) was also reduced. We also studied the effect of amiloride and DIDS on pH(i) regulation during isohydric hypercapnia. An amiloride-sensitive mechanism was the dominant pH(i) regulatory process during acidosis. We conclude that pH(e) modulates and slows pH(i) regulation in chemoreceptor regions to a greater extent than in nonchemoreceptor regions by inhibiting an amiloride-sensitive Na(+)/H(+) exchanger. Although the phylogenetic distance between vertebrates and invertebrates is large, similar results have been reported in CO(2)-sensitive regions within the rat brain stem.

CO2, brainstem chemoreceptors and breathing

The regulation of breathing relies upon chemical feedback concerning the levels of CO2 and O2. The carotid bodies, which detect O2, provide tonic excitation to brainstem respiratory neurons under normal conditions and dramatic excitation if O2 levels fall. Feedback for CO2 involves the carotid body and receptors in the brainstem, central chemoreceptors. Small increases in CO2 produce large increases in breathing. Decreases in CO2 below normal can, in sleep and anesthesia, decrease breathing, even to apnea. Central chemoreceptors, once thought localized to the surface of the ventral medulla, are likely distributed more widely with sites presently identified in the: (1) ventrolateral medulla; (2) nucleus of the solitary tract; (3) ventral respiratory group; (4) locus ceruleus; (5) caudal medullary raphé; and (6) fastigial nucleus of the cerebellum. Why so many chemoreceptor sites? Hypotheses, some with supporting data, include the following. Geographical specificity; all regions of the brainstem with respiratory neurons contain chemoreceptors. Stimulus intensity; some sites operate in the physiological range of CO2 values, others only with more extreme changes. Stimulus specificity; CO2 or pH may be sensed by multiple mechanisms. Temporal specificity; some sites respond more quickly to changes on blood or brain CO2 or pH. Syncytium; chemosensitive neurons may be connected via low resistance, gap junctions. Arousal state: sites may vary in effectiveness and importance dependent on state of arousal. Overall, as judged by experiments of nature, and in the laboratory, central chemoreceptors are critical for adequate breathing in sleep, but other aspects of the control system can maintain breathing in wakefulness.

Diethyl pyrocarbonate (DEPC) inhibits CO2 chemosensitivity in Helix aspersa

Central CO2 chemoreceptors in poikilothermic vertebrates may not regulate ventilation at a particular pH setpoint; central chemoreceptor responses may more accurately reflect the relative charge state (alpha) of the imidazole of histidine. We have tested the alphastat hypothesis in the terrestrial, air breathing, pulmonate snail, Helix aspersa, by chemically modifying histidine residues in the central CO2 chemoreceptor area of this animal using diethyl pyrocarbonate (DEPC). After focal application of 20 mM DEPC to the central CO2 chemoreceptor region, the pneumostome, a respiratory, CO2 responsive organ in the snail, no longer responded to hypercapnic, acidotic stimulation of the central chemoreceptor area. However, pneumostomal responses to hypoxic stimulation of the pneumostome and to focal stimulation of the central chemoreceptor area with sodium nitroprusside, a respiratory stimulant in H. aspersa, remained intact after DEPC treatment. Furthermore, DEPC treatment of the central chemoreceptor area blocked pneumostomal responses to ammonia pre-pulse treatment, which changes intracellular pH, while extracellular pH is held constant. These results resemble mammalian responses to DEPC treatment and indicate that central chemoreceptor responses in H. aspersa may originate from changes in the alpha of intracellular histidine residues.

Comparative aspects of central CO2 chemoreception

We compare and contrast the putative mechanisms underlying CO2 chemoreceptor function in air breathing vertebrates and terrestrial pulmonate snails. We discuss the role of intracellular pH (pHi) in central respiratory responses to CO2 and describe a variety of patterns of pHi regulation in chemosensory areas. One pattern, in which pHi retains a fixed relationship to the CO2 stimulus over time, seems well suited to chemoreceptor cells. Alphastat regulation of ventilation is apparent in both air breathing vertebrates and terrestrial pulmonate snails. Diethyl pyrocarbonate inhibits respiratory responses to hypercapnia in both groups of animals. The neuronal basis of chemosensitivity is similar, in that putative chemoreceptor cells depolarize during hypercapnic stimulation, but the ionic basis of excitability appears to be a potassium conductance in the vertebrates studied to date and a calcium conductance in the snails. Despite divergent evolutionary histories, chemosensory responses and mechanisms are remarkably similar in air breathing vertebrates and terrestrial pulmonate snails.

Carbonic anhydrase and CO2 chemoreception in the pulmonate snail Helix aspersa

We have studied the effects of carbonic anhydrase inhibition on the hypercapnic ventilatory response of the pulmonate snail, Helix aspersa, in an isolated brain-pneumostome preparation. We found that the cell permeant carbonic anhydrase inhibitor, acetazolamide (ACTZ), increased pneumostomal opening and ventilation during normocapnia (2-3% CO2) and decreased the rate of pneumostomal response to step changes in CO2 (4.5%), but did not change the steady-state ventilatory response to elevated CO2 (4.5%) compared to the inactive ACTZ analogue, N2-substituted 2-acetylamino-1,3,4-thiadiazole (Cl 13850). In contrast, the cell impermeant carbonic anhydrase inhibitor, quartenary ammonium sulfonilamide (QAS), had no effect on the pneumostomal response to CO2 compared to Cl 13850. Using Hansson's histochemical technique to stain for carbonic anhydrase activity, we identified a small number of neurons in the subesophageal ganglia that exhibited carbonic anhydrase activity. Some of these cells were in the region of CO2-sensitivity. In conclusion, carbonic anhydrase inhibition slows the ventilatory response to rapid changes in CO2, but does not affect the intrinsic ability of H. aspersa to respond to CO2. The ventilatory effects of carbonic anhydrase inhibition may be attributed to the intracellular actions of the carbonic anhydrase enzyme.

Central chemoreceptor stimulus in the terrestrial, pulmonate snail, Helix aspersa

We studied the effect of hypercapnic and fixed acid central chemoreceptor stimulation on the pneumostome in the pulmonate snail, Helix aspersa. We found that focal stimulation of the central chemoreceptor area of the pulmonate snail brain with hypercapnic solutions more effectively increased the pneumostomal area than did fixed acid stimulation at the same extracellular pH. Disrupting intracellular pH regulation by inhibiting Cl- transport, either pharmacologically (DIDS) or by ion substitution (Cl(-)-free perfusate), enhanced pneumostomal responses to CO2. While maintaining a constant perfusate pH, addition of NH4Cl to the perfusate resulted in pneumostomal closure; whereas removal of NH4Cl from the bath resulted in pneumostomal opening. In conclusion, the ventilatory response to CO2 in H. aspersa does not require Cl- transport or conductance. Furthermore, changing pHi alone is an adequate stimulus for the central chemoreceptors in the snail.