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

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.

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.

Julius H. Comroe, Jr., distinguished lecture: central chemoreception: then ... and now

The 2010 Julius H. Comroe, Jr., Lecture of the American Physiological Society focuses on evolving ideas in chemoreception for CO₂/pH in terms of what is "sensed," where it is sensed, and how the sensed information is used physiologically. Chemoreception is viewed as involving neurons (and glia) at many sites within the hindbrain, including, but not limited to, the retrotrapezoid nucleus, the medullary raphe, the locus ceruleus, the nucleus tractus solitarius, the lateral hypothalamus (orexin neurons), and the caudal ventrolateral medulla. Central chemoreception also has an important nonadditive interaction with afferent information arising at the carotid body. While ventilation has been viewed as the primary output variable, it appears that airway resistance, arousal, and blood pressure can also be significantly affected. Emphasis is placed on the importance of data derived from studies performed in the absence of anesthesia.

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.

A computational analysis of central CO2 chemosensitivity in Helix aspersa

We created a single-compartment computer model of a CO(2) chemosensory neuron using differential equations adapted from the Hodgkin-Huxley model and measurements of currents in CO(2) chemosensory neurons from Helix aspersa. We incorporated into the model two inward currents, a sodium current and a calcium current, three outward potassium currents, an A-type current (I(KA)), a delayed rectifier current (I(KDR)), a calcium-activated potassium current (I(KCa)), and a proton conductance found in invertebrate cells. All of the potassium channels were inhibited by reduced pH. We also included the pH regulatory process to mimic the effect of the sodium-hydrogen exchanger (NHE) described in these cells during hypercapnic stimulation. The model displayed chemosensory behavior (increased spike frequency during acid stimulation), and all three potassium channels participated in the chemosensory response and shaped the temporal characteristics of the response to acid stimulation. pH-dependent inhibition of I(KA) initiated the response to CO(2), but hypercapnic inhibition of I(KDR) and I(KCa) affected the duration of the excitatory response to hypercapnia. The presence or absence of NHE activity altered the chemosensory response over time and demonstrated the inadvisability of effective intracellular pH (pH(i)) regulation in cells designed to act as chemostats for acid-base regulation. The results of the model indicate that multiple channels contribute to CO(2) chemosensitivity, but the primary sensor is probably I(KA). pH(i) may be a sufficient chemosensory stimulus, but it may not be a necessary stimulus: either pH(i) or extracellular pH can be an effective stimuli if chemosensory neurons express appropriate pH-sensitive channels. The lack of pH(i) regulation is a key feature determining the neuronal activity of chemosensory cells over time, and the balanced lack of pH(i) regulation during hypercapnia probably depends on intracellular activation of pH(i) regulation but extracellular inhibition of pH(i) regulation. These general principles are applicable to all CO(2) chemosensory cells in vertebrate and invertebrate neurons.

Cellular mechanisms involved in CO(2) and acid signaling in chemosensitive neurons

An increase in CO(2)/H(+) is a major stimulus for increased ventilation and is sensed by specialized brain stem neurons called central chemosensitive neurons. These neurons appear to be spread among numerous brain stem regions, and neurons from different regions have different levels of chemosensitivity. Early studies implicated changes of pH as playing a role in chemosensitive signaling, most likely by inhibiting a K(+) channel, depolarizing chemosensitive neurons, and thereby increasing their firing rate. Considerable progress has been made over the past decade in understanding the cellular mechanisms of chemosensitive signaling using reduced preparations. Recent evidence has pointed to an important role of changes of intracellular pH in the response of central chemosensitive neurons to increased CO(2)/H(+) levels. The signaling mechanisms for chemosensitivity may also involve changes of extracellular pH, intracellular Ca(2+), gap junctions, oxidative stress, glial cells, bicarbonate, CO(2), and neurotransmitters. The normal target for these signals is generally believed to be a K(+) channel, although it is likely that many K(+) channels as well as Ca(2+) channels are involved as targets of chemosensitive signals. The results of studies of cellular signaling in central chemosensitive neurons are compared with results in other CO(2)- and/or H(+)-sensitive cells, including peripheral chemoreceptors (carotid body glomus cells), invertebrate central chemoreceptors, avian intrapulmonary chemoreceptors, acid-sensitive taste receptor cells on the tongue, and pain-sensitive nociceptors. A multiple factors model is proposed for central chemosensitive neurons in which multiple signals that affect multiple ion channel targets result in the final neuronal response to changes in CO(2)/H(+).

Heterogeneous patterns of pH regulation in glial cells in the dorsal and ventral medulla

We examined pH regulation in two chemosensitive areas of the brain, the retrotrapezoid nucleus (RTN) and the nucleus tractus solitarius (NTS), to identify the proton transporters involved in regulation of intracellular pH (pHi) in medullary glia. Transverse brain slices from young rats [postnatal day 8 (P8) to P20] were loaded with the pH-sensitive probe 2',7'-bis (2-carboxyethyl)-5,6-carboxyfluorescein after kainic acid treatment removed neurons. Cells were alkalinized when they were depolarized (extracellular K+ increased from 6.24 to 21.24 mM) in the RTN but not in the NTS. This alkaline shift was inhibited by 0.5 mM DIDS. Removal of CO2/HCO3- or Na+ from the perfusate acidified the glial cells, but the acidification after Na+ removal was greater in the RTN than in the NTS. Treatment of the slice with 5-(N-ethyl-N-isopropyl)amiloride (100 microM) in saline containing CO2/HCO3- acidified the cells in both nuclei, but the acidification was greater in the NTS. Restoration of extracellular Cl- after Cl- depletion during the control condition acidified the cells. Immunohistochemical studies of glial fibrillary acid protein demonstrated much denser staining in the RTN compared with the NTS. We conclude that there is evidence of Na+-HCO3- cotransport and Na+/H+ exchange in glia in the RTN and NTS, but the distribution of glia and the distribution of these pH-regulatory functions are not identical in the NTS and RTN. The differential strength of glial pH regulatory function in the RTN and NTS may also alter CO2 chemosensory neuronal function at these two chemosensitive sites in the brain stem.

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.

Focal central chemoreceptor sensitivity in the RTN studied with a CO2 diffusion pipette in vivo

We describe and use a CO2 diffusion pipette to produce a quickly reversible focal acidosis in the retrotrapezoid nucleus region of the rat brain stem. No tissue injection is made. Instead, artificial cerebrospinal fluid (aCSF) equilibrated with CO2 circulates within the micropipette, providing a source for continued CO2 diffusion into the tissue from the pipette tip. Tissue pH electrodes show the acidosis is limited to 500 micron from the tip. In controls (aCSF equilibrated with air), 1-min pipette perfusions increased tissue pH slightly and decreased phrenic nerve amplitude. In moderate- and high-CO2 groups (aCSF equilibrated with 50 or 100% CO2), 1-min perfusions significantly decreased tissue pH and increased phrenic nerve amplitude in a dose-dependent manner. The responses developed and reversed within minutes. Compared with our prior use of medullary acetazolamide injections to produce a focal acidosis, in this approach the acidosis 1) arises and reverses quickly and 2) its intensity can be varied. This allows study of sensitivity and mechanism. We conclude from this initial experiment that retrotrapezoid nucleus region chemoreceptors operate within the normal physiological range of CO2-induced tissue pH changes.

Central chemoreception in the region of the ventral respiratory group in the rat

We injected acetazolamide (AZ; 5 x 10(-6) M, 1 nl) into the region of the ventral respiratory group (VRG) of anesthetized paralyzed ventilated rats. Control injections (mock cerebrospinal fluid, n = 6, or the inactive AZ analogue 2-acetylamino-1,3, 4-thiadiazole-5-sulfon-t-butylamide, n = 6) did not increase the integrated phrenic neurogram [phrenic nerve amplitude (PNA)]. The AZ injections produced a focal region of tissue acidosis with a radius < 300-400 microns and are used as a probe for sites of central chemosensitivity. Injection location is determined by anatomic analysis. Of 22 VRG injections of AZ, 14 increased the amplitude of the PNA over 15-90 min; 8 had no effect. In 17 cases, we measured medullary tissue pH at the injection center and/or at a distant site and reaffirmed the size of the acidotic region produced by such small AZ injections. Of injections with pH electrodes within 300-400 microns of the injection center, all responders showed an acid pH; three nonresponders showed an acid pH, and one an alkaline pH. In a subgroup of five rats, at VRG sites with known respiratory effects identified by prior glutamate injection (10 nl, 100 mM), all subsequent AZ injections produced a PNA response. Simultaneous measurement of PNA and tissue pH responses at the injection center of eight rats did not show a uniform correlation in time; initially, both changed with a similar time course, but PNA recovered more quickly. We conclude that 1) the region of the VRG contains sites of ventilatory chemoreception, 2) ineffective AZ injections do produce a tissue acidosis but at sites with minimal impact on breathing, and 3) tissue pH does not uniquely represent the chemoreceptor stimulus.

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.