Contribution of Ca2+-activated K+ channels to central chemosensitivity in cultivated neurons of fetal rat medulla

The Role of Ca2+ and BK Channels of Locus Coeruleus (LC) Neurons as a Brake to the CO2 Chemosensitivity Response of Rats

The cellular mechanisms by which LC neurons respond to hypercapnia are usually attributed to an "accelerator" whereby hypercapnic acidosis causes an inhibition of K⁺ channels or activation of Na⁺ and Ca⁺² channels to depolarize CO₂-sensitive neurons. Nevertheless, it is still unknown if this "accelerator" mechanism could be controlled by a brake phenomenon. Whole-cell patch clamping, fluorescence imaging microscopy and plethysmography were used to study the chemosensitive response of the LC neurons. Hypercapnic acidosis activates L-type Ca²⁺ channels and large conductance Ca-activated K⁺ (BK) channels, which function as a "brake" on the chemosensitive response of LC neurons. Our findings indicate that both Ca²⁺ and BK currents develop over the first 2 weeks of postnatal life in rat LC slices and that this brake pathway may cause the developmental decrease in the chemosensitive firing rate response of LC neurons to hypercapnic acidosis. Inhibition of this brake by paxilline (BK channel inhibitor) returns the magnitude of the chemosensitive firing rate response from LC neurons in rats older than P10 to high values similar to those in LC neurons from younger rats. Inhibition of BK channels in LC neurons by bilateral injections of paxilline into the LC results in a significant increase in the hypercapnic ventilatory response of adult rats. Our findings indicate that a BK channel-based braking system helps to determine the chemosensitive respiratory drive of LC neurons and contributes to the hypercapnic ventilatory response. Perhaps, abnormalities of this braking system could result in hypercapnia-induced respiratory disorders and panic responses.

CO2 carbamylation of proteins as a mechanism in physiology

Carbamate bonds occur following the nucleophilic attack of CO2 on to an amine. In proteins, this can occur at lysine side chains or at the N-terminus. For CO2 binding to occur an amine must be present in the NH2 form and consequently carbamates represent a site-specific post-translational modification, occurring only in environments of reduced hydration. Due to the specific nature of these interactions, coupled with the inability of these bonds to survive protein preparation methods, carbamate reactions appear rare. However, more biologically important examples continue to emerge that use carbamates as key parts of their mechanisms. In this review, we discuss specific examples of carbamate bond formation and their biological consequences with an aim to highlight this important, and often forgotten, biochemical group.

Environmental CO2 inhibits Caenorhabditis elegans egg-laying by modulating olfactory neurons and evokes widespread changes in neural activity

Carbon dioxide (CO2) gradients are ubiquitous and provide animals with information about their environment, such as the potential presence of prey or predators. The nematode Caenorhabditis elegans avoids elevated CO2, and previous work identified three neuron pairs called "BAG," "AFD," and "ASE" that respond to CO2 stimuli. Using in vivo Ca(2+) imaging and behavioral analysis, we show that C. elegans can detect CO2 independently of these sensory pathways. Many of the C. elegans sensory neurons we examined, including the AWC olfactory neurons, the ASJ and ASK gustatory neurons, and the ASH and ADL nociceptors, respond to a rise in CO2 with a rise in Ca(2+). In contrast, glial sheath cells harboring the sensory endings of C. elegans' major chemosensory neurons exhibit strong and sustained decreases in Ca(2+) in response to high CO2. Some of these CO2 responses appear to be cell intrinsic. Worms therefore may couple detection of CO2 to that of other cues at the earliest stages of sensory processing. We show that C. elegans persistently suppresses oviposition at high CO2. Hermaphrodite-specific neurons (HSNs), the executive neurons driving egg-laying, are tonically inhibited when CO2 is elevated. CO2 modulates the egg-laying system partly through the AWC olfactory neurons: High CO2 tonically activates AWC by a cGMP-dependent mechanism, and AWC output inhibits the HSNs. Our work shows that CO2 is a more complex sensory cue for C. elegans than previously thought, both in terms of behavior and neural circuitry.

A HCO(3)(-)-dependent mechanism involving soluble adenylyl cyclase for the activation of Ca²⁺ currents in locus coeruleus neurons

Hypercapnic acidosis activates Ca²⁺ channels and increases intracellular Ca²⁺ levels in neurons of the locus coeruleus, a known chemosensitive region involved in respiratory control. We have also shown that large conductance Ca²⁺-activated K⁺ channels, in conjunction with this pathway, limits the hypercapnic-induced increase in firing rate in locus coeruleus neurons. Here, we present evidence that the Ca²⁺ current is activated by a HCO(3)(-)-sensitive pathway. The increase in HCO(3)(-) associated with hypercapnia activates HCO(3)(-)-sensitive adenylyl cyclase (soluble adenylyl cyclase). This results in an increase in cyclic adenosine monophosphate levels and activation of Ca²⁺ channels via cyclic adenosine monophosphate-activated protein kinase A. We also show the presence of soluble adenylyl cyclase in the cytoplasm of locus coeruleus neurons, and that the cyclic adenosine monophosphate analogue db-cyclic adenosine monophosphate increases Ca²⁺i. Disrupting this pathway by decreasing HCO(3)(-) levels during acidification or inhibiting either soluble adenylyl cyclase or protein kinase A, but not transmembrane adenylyl cyclase, can increase the magnitude of the firing rate response to hypercapnia in locus coeruleus neurons from older neonates to the same extent as inhibition of K⁺ channels. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

Transient outwardly rectifying A currents are involved in the firing rate response to altered CO2 in chemosensitive locus coeruleus neurons from neonatal rats

The effect of hypercapnia on outwardly rectifying currents was examined in locus coeruleus (LC) neurons in slices from neonatal rats [postnatal day 3 (P3)-P15]. Two outwardly rectifying currents [4-aminopyridine (4-AP)-sensitive transient current and tetraethyl ammonium (TEA)-sensitive sustained current] were found in LC neurons. 4-AP induced a membrane depolarization of 3.6 ± 0.6 mV (n = 4), while TEA induced a smaller membrane depolarization of 1.2 ± 0.3 mV (n = 4). Hypercapnic acidosis (HA) inhibited both currents. The maximal amplitude of the TEA-sensitive current was reduced by 52.1 ± 4.5% (n = 5) in 15% CO2 [extracellular pH (pHo) 7.00, intracellular pH (pHi) 6.96]. The maximal amplitude of the 4-AP-sensitive current was reduced by 34.5 ± 3.0% (n = 6) in 15% CO2 (pHo 7.00, pHi 6.96), by 29.4 ± 6.8% (n = 6) in 10% CO2 (pHo 7.15, pHi 7.14), and increased by 29.0 ± 6.4% (n = 6) in 2.5% CO2 (pHo 7.75, pHi 7.35). 4-AP completely blocked hypercapnia-induced increased firing rate, but TEA did not affect it. When LC neurons were exposed to HA with either pHo or pHi constant, the 4-AP-sensitive current was inhibited. The data show that the 4-AP-sensitive current (likely an A current) is inhibited by decreases in either pHo or pHi. The change of the A current by various levels of CO2 is correlated with the change in firing rate induced by CO2, implicating the 4-AP-sensitive current in chemosensitive signaling in LC neurons.

Redefining the components of central CO2 chemosensitivity--towards a better understanding of mechanism

Abstract

The field of CO₂ chemosensitivity has developed considerably in recent years. There has been a mounting number of competing nuclei proposed as chemosensitive along with an ever increasing list of potential chemosensory transducing molecules. Is it really possible that all of these areas and candidate molecules are involved in the detection of chemosensory stimuli? How do we discriminate rigorously between molecules that are chemosensory transducers at the head of a physiological reflexversusthose that just happen to display sensitivity to a chemosensory stimulus? Equally, how do we differentiate between nuclei that have a primary chemosensory function, versusthose that are relays in the pathway? We have approached these questions by proposing rigorous definitions for the different components of the chemosensory reflex, going from the salient molecules and ions, through the components of transduction to the identity of chemosensitive cells and chemosensitive nuclei. Our definitions include practical and rigorous experimental tests that can be used to establish the identity of these components. We begin by describing the need for central CO₂ chemosensitivity and the problems that the field has faced. By comparing chemosensory mechanisms to those in the visual system we suggest stricter definitions for the components of the chemosensory pathway. We then, considering these definitions, re-evaluate current knowledge of chemosensory transduction, and propose the ‘multiple salient signal hypothesis’ as a framework for understanding the multiplicity of transduction mechanisms and brain areas seemingly involved in chemosensitivity.

CO2-dependent opening of an inwardly rectifying K+ channel

CO₂ chemosensing is a vital function for the maintenance of life that helps to control acid–base balance. Most studies have reported that CO₂ is measured via its proxy, pH. Here we report an inwardly rectifying channel, in outside-out excised patches from HeLa cells that was sensitive to modest changes in PCO₂ under conditions of constant extracellular pH. As PCO₂ increased, the open probability of the channel increased. The single-channel currents had a conductance of 6.7 pS and a reversal potential of –70 mV, which lay between the K⁺ and Cl^– equilibrium potentials. This reversal potential was shifted by +61 mV following a tenfold increase in extracellular [K⁺] but was insensitive to variations of extracellular [Cl^–]. The single-channel conductance increased with extracellular [K⁺]. We propose that this channel is a member of the Kir family. In addition to this K⁺ channel, we found that many of the excised patches also contained a conductance carried via a Cl^–-selective channel. This CO₂-sensitive Kir channel may hyperpolarize excitable cells and provides a potential mechanism for CO₂-dependent inhibition during hypercapnia.

Astrocyte chemoreceptors: mechanisms of H+ sensing by astrocytes in the retrotrapezoid nucleus and their possible contribution to respiratory drive

Central chemoreception is the mechanism by which CO(2)/pH-sensitive neurons (i.e. chemoreceptors) regulate breathing, presumably in response to changes in tissue pH. A region of the brainstem called the retrotrapezoid nucleus (RTN) is thought to be an important site of chemoreception; select neurons (i.e. chemoreceptors) in this region sense changes in CO(2)/H(+) and send excitatory glutamatergic drive to respiratory centres to modulate the depth and frequency of breathing. Purinergic signalling may also contribute to chemoreception; for instance, it was shown in vivo that CO(2)/H(+) facilitates ATP release within the RTN to stimulate breathing, and recent evidence suggests that CO(2)/H(+)-sensitive RTN astrocytes are the source of this purinergic drive to breathe. In this review, we summarize evidence that RTN astrocytes sense changes in CO(2)/H(+), identify mechanisms that are likely to confer CO(2)/H(+) sensitivity to RTN astrocytes, including inhibition of heteromeric Kir4.1-Kir5.1 channels and activation of a depolarizing inward current generated by the sodium bicarbonate cotransporter, and discuss the extent to which astrocytes contribute to respiratory drive.

Astrocytes in the retrotrapezoid nucleus sense H+ by inhibition of a Kir4.1-Kir5.1-like current and may contribute to chemoreception by a purinergic mechanism

Central chemoreception is the mechanism by which CO(2)/pH sensors regulate breathing in response to tissue pH changes. There is compelling evidence that pH-sensitive neurons in the retrotrapezoid nucleus (RTN) are important chemoreceptors. Evidence also indicates that CO(2)/H(+)-evoked adenosine 5'-triphosphate (ATP) release in the RTN, from pH-sensitive astrocytes, contributes to chemoreception. However, mechanism(s) by which RTN astrocytes sense pH is unknown and their contribution to chemoreception remains controversial. Here, we use the brain slice preparation and a combination of patch-clamp electrophysiology and immunohistochemistry to confirm that RTN astrocytes are pH sensitive and to determine mechanisms by which they sense pH. We show that pH-sensitive RTN glia are immunoreactive for aldehyde dehydrogenase 1L1, a marker of astrocytes. In HEPES buffer the pH-sensitive current expressed by RTN astrocytes reversed near E(K(+)) (the equilibrium potential for K(+)) and was inhibited by Ba(2+) and desipramine (blocker of Kir4.1-containing channels), characteristics most consistent with heteromeric Kir4.1-Kir5.1 channels. In bicarbonate buffer, the sodium/bicarbonate cotransporter also contributed to the CO(2)/H(+)-sensitive current in RTN astrocytes. To test the hypothesis that RTN astrocytes contribute to chemoreception by a purinergic mechanism, we used fluorocitrate to selectively depolarize astrocytes while measuring neuronal activity. We found that fluorocitrate increased baseline activity and pH sensitivity of RTN neurons by a P2-receptor-dependent mechanism, suggesting that astrocytes may release ATP to activate RTN chemoreceptors. We also found in bicarbonate but not HEPES buffer that P2-receptor antagonists decreased CO(2) sensitivity of RTN neurons. We conclude that RTN astrocytes sense CO(2)/H(+) in part by inhibition of a Kir4.1-Kir5.1-like current and may provide an excitatory purinergic drive to pH-sensitive neurons.

Physiological carbon dioxide, bicarbonate, and pH sensing

In biological systems, carbon dioxide exists in equilibrium with bicarbonate and protons. The individual components of this equilibrium (i.e., CO₂, HCO₃⁻, and H(+)), which must be sensed to be able to maintain cellular and organismal pH, also function as signals to modulate multiple physiological functions. Yet, the molecular sensors for CO₂/HCO₃⁻/pH remained unknown until recently. Here, we review recent progress in delineating molecular and cellular mechanisms for sensing CO₂, HCO₃⁻, and pH.

Retrotrapezoid nucleus, respiratory chemosensitivity and breathing automaticity

Breathing automaticity and CO(2) regulation are inseparable neural processes. The retrotrapezoid nucleus (RTN), a group of glutamatergic neurons that express the transcription factor Phox2b, may be a crucial nodal point through which breathing automaticity is regulated to maintain CO(2) constant. This review updates the analysis presented in prior publications. Additional evidence that RTN neurons have central respiratory chemoreceptor properties is presented, but this is only one of many factors that determine their activity. The RTN is also regulated by powerful inputs from the carotid bodies and, at least in the adult, by many other synaptic inputs. We also analyze how RTN neurons may control the activity of the downstream central respiratory pattern generator. Specifically, we review the evidence which suggests that RTN neurons (a) innervate the entire ventral respiratory column and (b) control both inspiration and expiration. Finally, we argue that the RTN neurons are the adult form of the parafacial respiratory group in neonate rats.

Immunohistochemical distribution of inwardly rectifying K+ channels in the medulla oblongata of the rat

The inwardly rectifying K+ channels, Kir1.1, Kir2.3 and Kir4.1-Kir5.1, are the candidate chemosensory molecules for CO2/H+. We determined the mRNA expression and immunohistochemical localization of these channels in the medulla oblongata of the rat. RT-PCR analysis revealed mRNAs of Kir1.1, Kir2.3, Kir4.1 and Kir5.1 were detected in the medulla. The immunoreactivities for Kir1.1, Kir2.3, Kir4.1, and Kir5.1 were observed in the medulla, and immunolabeling pattern was varied by the subunit. Immunoreactivities for Kir1.1 and Kir2.3 were observed in the nerve cell bodies and glial cells both in the chemosensory areas [nucleus tractus solitarius (NTS), nucleus raphe obscurus (RO), pre-Bötzinger complex (PreBötC)] and non-chemosensory area [hypoglossal nucleus (XII), inferior olive nucleus (IO)]. Kir4.1 immunoreactivity was observed in the glial cells and neuropil, especially in XII and IO. Kir5.1 immunoreactivity was observed in the nerve cell bodies in the XII, RO, and PreBötC, but not in the NTS or IO. In the NTS, a dense network of varicose nerve fibers showed immunoreactivity for Kir5.1. Our findings suggest that Kir channels may not act specific to the central chemoreception, but regulate the ionic properties of cellular membranes in various neurons and glial cells.

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.

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.

Calcium and avian intrapulmonary chemoreceptor response to CO2

Intrapulmonary chemoreceptors (IPC) are highly responsive respiratory chemoreceptors that innervate the lungs of birds and diapsid reptiles. IPC are stimulated by low levels of lung Pco(2), inhibited by high levels of lung Pco(2), and their vagal afferents serve as a sensory limb for reflex adjustments of breathing depth and rate. Most IPC exhibit both phasic and tonic sensitivity to CO(2), and spike frequency adaptation (SFA) contributes to their phasic CO(2) responsiveness. To test whether CO(2) responsiveness and SFA in IPC is modulated by a Ca(2+)-linked mechanism, we quantified the role of transmembrane Ca(2+) fluxes and Ca(2+)-related channels on single-unit IPC function in response to phasic changes in inspired Pco(2). We found that 1) broad-spectrum blockade of Ca(2+) channels using cadmium or cobalt and blockade of L-type Ca(2+) channels using nifedipine increased IPC discharge; 2) activation of L-type Ca(2+) channels using BAY K 8644 reduced IPC discharge; 3) blockade of Ca(2+)-activated potassium channels using charybdotoxin (antagonist of large-conductance Ca(2+)-dependent K(+) channel) increased IPC discharge, but neither charybdotoxin nor apamin affected SFA; and 4) blockade of chloride channels, including Ca(2+)-activated chloride channels, with niflumic acid decreased IPC discharge at low Pco(2) and increased IPC discharge at high Pco(2), resulting in a net attenuation of the IPC CO(2) response. We conclude that Ca(2+) influx through L-type Ca(2+) channels has an inhibitory effect on IPC afferent discharge and CO(2) sensitivity, that spike frequency adaptation is not due to apamin- or charybdotoxin-sensitive Ca(2+)-activated K(+) channels in IPC, and that chloride channels blocked by niflumic acid help modulate IPC CO(2) responses.

Serotonergic and glutamatergic neurons at the ventral medullary surface of the human infant: Observations relevant to central chemosensitivity in early human life

Central chemoreception is the mechanism by which the brain detects the level of carbon dioxide (CO(2)) in the arterial blood and alters breathing accordingly in order to maintain it within physiological levels. The ventral surface of the medulla oblongata (VMS) of animals has long been recognized as a site of chemosensitivity, culminating in the recent identification of chemosensitive serotonergic (5-HT) and glutamatergic (Glut) neurons in this region. In this study, we analyzed the distribution of 5-HT and Glut neurons and their receptors in the arcuate nucleus (Arc) at the VMS of the human infant, using single-and double-label immunohistochemistry with specific antibodies. We also examined the expression of astrocytes, as experimental evidence suggests that astrocytes mediate, at least in part, central chemosensitivity via 5-HT and/or Glut receptors. We identified a small number of 5-HT neurons (approximately 5% of Arc neurons), distributed over the entire extent of the VMS, a large number of Glut neurons (approximately 95% of Arc neurons) that localized almost exclusively to the medial Arc, and a large number of astrocytes distributed across the entire extent of the VMS. The Arc also contained 5-HT(1A), kainate (GluR5), and 5-HT(2A) receptors, which localized predominantly to 5-HT neurons, glutamate neurons and astrocytes, respectively. Astrocytes also expressed the vesicular glutamate transporter 2 and low levels of 5-HT(1A) and kainate (GluR5) receptors, indicating that astrocytes may store and release glutamate, possibly in response to stimulation by 5-HT and/or Glut. These observations suggest that important functional interactions exist between 5-HT, glutamate, and astrocytes in the Arc. They also support the idea that the Arc is homologous to chemosensitive zones at the VMS in experimental animals. These data are important towards delineating the role of the human Arc in modulation of homeostasis, and its dysfunction in brainstem-associated pathologies such as the sudden infant death syndrome.

Response of membrane potential and intracellular pH to hypercapnia in neurons and astrocytes from rat retrotrapezoid nucleus

We compared the response to hypercapnia (10%) in neurons and astrocytes among a distinct area of the retrotrapezoid nucleus (RTN), the mediocaudal RTN (mcRTN), and more intermediate and rostral RTN areas (irRTN) in medullary brain slices from neonatal rats. Hypercapnic acidosis (HA) caused pH(o) to decline from 7.45 to 7.15 and a maintained intracellular acidification of 0.15 +/- 0.02 pH unit in 90% of neurons from both areas (n = 16). HA excited 44% of mcRTN (7/16) and 38% of irRTN neurons (6/16), increasing firing rate by 167 +/- 75% (chemosensitivity index, CI, 256 +/- 72%) and 310 +/- 93% (CI 292 +/- 50%), respectively. These responses did not vary throughout neonatal development. We compared the responses of mcRTN neurons to HA (decreased pH(i) and pH(o)) and isohydric hypercapnia (IH; decreased pH(i) with constant pH(o)). Neurons excited by HA (firing rate increased 156 +/- 46%; n = 5) were similarly excited by IH (firing rate increased 167 +/- 38%; n = 5). In astrocytes from both RTN areas, HA caused a maintained intracellular acidification of 0.17 +/- 0.02 pH unit (n = 6) and a depolarization of 5 +/- 1 mV (n = 12). In summary, many neurons (42%) from the RTN are highly responsive (CI 248%) to HA; this may reflect both synaptically driven and intrinsic mechanisms of CO(2) sensitivity. Changes of pH(i) are more significant than changes of pH(o) in chemosensory signaling in RTN neurons. Finally, the lack of pH(i) regulation in response to HA suggests that astrocytes do not enhance extracellular acidification during hypercapnia in the RTN.

Sodium/Proton Exchanger 3 in the Medulla Oblongata and Set Point of Breathing Control

RATIONALE

In vivo inhibition of the sodium/proton exchanger 3 (NHE3) in chemosensitive neurons of the ventrolateral brainstem augments central respiratory drive in anesthetized rabbits.

OBJECTIVES

To further explore the possible role of this exchanger for the control of breathing, we examined the individual relationship between brainstem NHE3 abundance and ventilation in rabbits during wakefulness.

METHODS

In 32 adult male rabbits on standard nutritional alkali load, alveolar ventilation, metabolic CO2 production, and blood gases were determined, together with arterial and urinary acid-base status and renal base control functions. Expression of NHE3 in brainstem tissue from the obex region was determined by quantitative real-time reverse-transcription polymerase chain reaction analysis.

MEASUREMENTS AND MAIN RESULTS

Regarding the distribution above and below the median, we classified high and low brainstem NHE3 animals, expressing a mean (+/- SEM) NHE3 mRNA of 2.08 +/- 0.28 and 0.72 +/- 0.06 fg cDNA/mg RNA, respectively. Alveolar ventilation of high brainstem NHE3 animals was lower than that of low brainstem NHE3 animals (715 +/- 36 vs. 919 +/- 41 ml . minute(-1); p < 0.01), a finding also reflected by a marked difference in Pa(CO2) (5.24 +/- 0.16 vs. 4.44 +/- 0.15 kPa; p < 0.01). Among possible secondary factors, CO2 production, systemic base excess, and fractional renal base reabsorption were not found to be different.

CONCLUSIONS

We conclude that the level of brainstem NHE3 expression-most likely via intracellular pH modulation-contributes to the individual control of breathing and Pa(CO2) in conscious rabbits by adjusting the set point and the loop gain of the system.

Imidazole binding reagent diethyl pyrocarbonate (DEPC) inhibits avian intrapulmonary chemoreceptor discharge in vivo

Data indicate that avian intrapulmonary chemoreceptors (IPC) transduce CO2 stimuli by sensing the products of CO2 hydration, [H+] and [HCO3-]. The alphastat regulation hypothesis of physiological pH sensitivity suggests that proteins sense [H+] through changes in the ionization state of imidazole groups (alphaIm). To test whether imidazole is involved with IPC CO2 sensitivity, we administered diethyl pyrocarbonate (DEPC) intravenously while recording from IPC exposed to varying levels of inspired CO2. At physiological pH, DEPC converts pH sensitive imidazole groups to pH-insensitive N-carbethoxyhistidyl residues. Single cell extracellular neural recordings were made from vagal filaments in anesthetized, unidirectionally ventilated Anas platyrhynchos. Without DEPC, IPC discharge rate was inversely proportional to inspired CO2 with characteristic dynamic responses to rapid CO2 alterations (n = 10). After DEPC treatment (> or = 15 mM), mean sensitivity of IPC discharge to static inspired CO2 levels was decreased 75% (P < 0.05), and mean peak dynamic IPC discharge rate was decreased 80% (P < 0.05). Additionally, we tested whether DEPC might alter IPC discharge by binding imidazole groups in the enzyme carbonic anhydrase (CA), but we found no effect on CA catalytic rate. We conclude that DEPC inhibits IPC CO2 signal transduction by modifying imidazole groups on acid-sensitive proteins other than CA, possibly membrane acid-base exchangers or ion channels. These data support the alphastat regulation hypothesis in IPC CO2 respiratory chemoreception and suggests a more direct link between H+ and membrane excitability.

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(+).

Respiratory control by ventral surface chemoreceptor neurons in rats

A long-standing theory posits that central chemoreception, the CNS mechanism for CO(2) detection and regulation of breathing, involves neurons located at the ventral surface of the medulla oblongata (VMS). Using in vivo and in vitro electrophysiological recordings, we identify VMS neurons within the rat retrotrapezoid nucleus (RTN) that have characteristics befitting these elusive chemoreceptors. These glutamatergic neurons are vigorously activated by CO(2) in vivo, whereas serotonergic neurons are not. Their CO(2) sensitivity is unaffected by pharmacological blockade of the respiratory pattern generator and persists without carotid body input. RTN CO(2)-sensitive neurons have extensive dendrites along the VMS and they innervate key pontomedullary respiratory centers. In brainstem slices, a subset of RTN neurons with markedly similar morphology is robustly activated by acidification and CO(2). Their pH sensitivity is intrinsic and involves a background K(+) current. In short, the CO(2)-sensitive neurons of the RTN are good candidates for the long sought-after VMS chemoreceptors.

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.

Highly H+-sensitive neurons in the caudal ventrolateral medulla of the rat

The ventral surface of the caudal ventrolateral medulla (cVLM) has been shown to generate intense respiratory responses after surface acid-base stimulation. With respect to their chemosensitive characteristics, cVLM neurons have been less studied than other rostral-most regions of the brainstem. The purpose of these experiments was to determine the bioelectric responses of cVLM neurons to acidic stimuli and to determine their chemosensitive properties. Using extracellular and microiontophoretic techniques, we recorded electrical activities from 117 neurons in an area close to the ventral surface of the cVLM in anaesthetised rats. All neurons were tested for their sensitivity to H+. The fluorescent probe BCECF was used to measure extracellular pH changes produced by the microiontophoretic injection of H+ in brainstem slices. This procedure provided an estimation of the local changes in pH produced by microiontophoretic H+ application in the anaesthetised rat. Neurons coupled to the respiratory cycle, R (n = 51), were not responsive to direct stimulation with H+. Sixty-six neurons that did respond to H+ stimulation were uncoupled from respiration, and identified as NR neurons. These neurons presented distinct ranges of H+ sensitivity. The neuronal sensitivity to H+ was mainly assessed by the slope of the stimulus-response curve, where the steeper the slope, the higher the H+ sensitivity. On this basis, NR neurons were classed as being either weakly or highly sensitive to H+. NR neurons with a high H+ sensitivity (n = 12) showed an average value of 34.17 +/- 7.44 spikes s-1 (100 nC)-1 (mean +/- S.D.) for maximal slope and an EC50 of 126.76 +/- 33 nC. Suprathreshold H+ stimulation of highly sensitive NR neurons elicited bursting pattern responses coupled to the respiratory cycle. The bursting responses, which were synchronised with the inspiratory phase and the early expiratory phase of the respiratory cycle, lasted for several seconds before returning to the steady state firing pattern characteristic of the pre-stimulus condition. These NR neurons, which possess the capacity to detect distinct H+ concentrations in the extracellular microenvironment, are excellent candidates to serve in a chemoreceptor capacity in the caudal medulla.

Central sympathetic chemosensitivity and Kir1 potassium channels in the cat

The possible involvement of potassium channels in central chemosensitivity, with special reference to the Kir1.1 potassium channel, was investigated by studying the CO(2) response of presympathetic neurons in the rostroventrolateral medulla (RVLM) in the absence or presence of various K(+) channel inhibitors. Synaptic input to RVLM neurons was blocked by local injection of omega-agatoxin and omega-conotoxin. Activity of RVLM neurons was measured by recording the electrical activity in preganglionic (WR-T(3)) or postganglionic (renal) sympathetic nerves after perfusion of the lower brainstem via the left vertebral artery with CO(2)-enriched saline solution. Unspecific K(+) channel blockade by BaCl(2) reduced the excitatory response of sympathetic activity after CO(2)-perfusion to 56% of control. A quantitatively similar inhibition of the central CO(2) response was obtained after administration of 9-fluorenylmethylchloroformate (FMOC-Cl) which eliminates pH sensitivity of Kir1 and Kir4.1. Furthermore, two structurally different Kir1 inhibiting toxins, tertiapin and Lq2, also reduced the central CO(2) response to approximately 50% of control. In contrast, charybdotoxin (CTX) had no effect on the CO(2) response. Using RT-PCR the expression of mRNA homologous to rat Kir1 mRNA was identified in the cat medulla oblongata. These data suggest that a modulation of potassium channel activity possibly via Kir1 may contribute to central chemosensitivity.

Multiple targets of chemosensitive signaling in locus coeruleus neurons: role of K+ and Ca2+ channels

We studied chemosensitive signaling in locus coeruleus (LC) neurons using both perforated and whole cell patch techniques. Upon inhibition of fast Na(+) spikes by tetrodotoxin (TTX), hypercapnic acidosis [HA; 15% CO(2), extracellular pH (pH(o)) 6.8] induced small, slow spikes. These spikes were inhibited by Co(2+) or nifedipine and were attributed to activation of L-type Ca(2+) channels by HA. Upon inhibition of both Na(+) and Ca(2+) spikes, HA resulted in a membrane depolarization of 3.52 +/- 0.61 mV (n = 17) that was reduced by tetraethylammonium (TEA) (1.49 +/- 0.70 mV, n = 7; P < 0.05) and absent (-0.97 +/- 0.73 mV, n = 7; P < 0.001) upon exposure to isohydric hypercapnia (IH; 15% CO(2), 77 mM HCO(3)(-), pH(o) 7.45). Either HA or IH, but not 50 mM Na-propionate, activated Ca(2+) channels. Inhibition of L-type Ca(2+) channels by nifedipine reduced HA-induced increased firing rate and eliminated IH-induced increased firing rate. We conclude that chemosensitive signals (e.g., HA or IH) have multiple targets in LC neurons, including TEA-sensitive K(+) channels and TWIK-related acid-sensitive K(+) (TASK) channels. Furthermore, HA and IH activate L-type Ca(2+) channels, and this activation is part of chemosensitive signaling in LC neurons.

The role of calcium-activated potassium channels in respiratory control

The introduction of pharmacological and genetic tools has helped to understand the functional role of specific ion channel subtypes in the neural control of breathing. This review outlines the role one family of potassium channels, those, which are regulated by intracellular calcium. Calcium-activated potassium channels (K(+)ca) play important roles in determining the duration of single action potentials and in the frequency of action potentials in excitable cells. They have been classified based on their unitary conductances combined with their inhibition by a variety of pharmacologic substances. In this review the role of K(+)ca channels in respiratory control is outlined. The predictions of their contributions from modeling the respiratory system and the results of experiments in which they are individually blocked are discussed. Recently mice have been generated in which the expression of a single member of the K(+)ca channel family can be conditionally regulated. The repression of this small-conductance K(+)ca channel (SK3) as well as its overexpression affect baseline respiratory rhythm and the ventilatory response to hypoxia.

Role of intracellular and extracellular pH in the chemosensitive response of rat locus coeruleus neurones

The chemosensitive response of locus coeruleus (LC) neurones to changes in intracellular pH (pH(i)), extracellular pH (pH(o)) and molecular CO(2) were investigated using neonatal rat brainstem slices. A new technique was developed that involves the use of perforated patch recordings in combination with fluorescence imaging microscopy to simultaneously measure pH(i) and membrane potential (V(m)). Hypercapnic acidosis (15 % CO(2), pH(o) 6.8) resulted in a maintained fall in pH(i) of 0.31 pH units and a 93 % increase in the firing rate of LC neurones. On the other hand, isohydric hypercapnia (15 % CO(2), 77 mM HCO(3)(-), pH(o) 7.45) resulted in a smaller and transient fall in pH(i) of about 0.17 pH units and an increase in firing rate of 76 %. Acidified Hepes (N-2-hydroxyethylpiperazine-N'-2- ethanesulfonic acid)-buffered medium (pH(o) 6.8) resulted in a progressive fall in pH(i) of over 0.43 pH units and an increase in firing rate of 126 %. Isosmotic addition of 50 mM propionate to the standard HCO(3)(-)-buffered medium (5 % CO(2), 26 mM HCO(3)(-), pH(o) 7.45) resulted in a transient fall in pH(i) of 0.18 pH units but little increase in firing rate. Isocapnic acidosis (5 % CO(2), 7 mM HCO(3)(-), pH(o) 6.8) resulted in a slow intracellular acidification to a maximum fall of about 0.26 pH units and a 72 % increase in firing rate. For all treatments, the changes in pH(i) preceded or occurred simultaneously with the changes in firing rate and were considerably slower than the changes in pH(o). In conclusion, an increased firing rate of LC neurones in response to acid challenges was best correlated with the magnitude and the rate of fall in pH(i), indicating that a decrease in pH(i) is a major part of the intracellular signalling pathway that transduces an acid challenge into an increased firing rate in LC neurones.

TASK-1 is a highly modulated pH-sensitive 'leak' K(+) channel expressed in brainstem respiratory neurons

Central respiratory chemoreceptors adjust respiratory drive in a homeostatic response to alterations in brain pH and/or P(CO(2)). Multiple brainstem sites are proposed as neural substrates for central chemoreception, but molecular substrates that underlie chemosensitivity in respiratory neurons have not been identified. In rat brainstem neurons expressing transcripts for TASK-1, a two-pore domain K(+) channel, we characterized K(+) currents with kinetic and voltage-dependent properties identical to cloned rat TASK-1 currents. Native currents were sensitive to acid and alkaline shifts in the same physiological pH range as TASK-1 (pK approximately 7.4), and native and cloned pH-sensitive currents were modulated similarly by neurotransmitters and inhalational anesthetics. This pH-sensitive TASK-1 channel is an attractive candidate to mediate chemoreception because it is functionally expressed in respiratory-related neurons, including airway motoneurons and putative chemoreceptor neurons of locus coeruleus (LC). Inhibition of TASK-1 channels by extracellular acidosis can depolarize and increase excitability in those cells, thereby contributing to chemoreceptor function in LC neurons and directly enhancing respiratory motoneuronal output.

Ventrolateral neurons of medullary organotypic cultures: intracellular pH regulation and bioelectric activity

The hypothesized role of the intracellular pH (pH(i)) as a proximate stimulus for central chemosensitive neurons is reviewed on the basis of data obtained from organotypic cultures of the medulla oblongata (obex level) of new born rats (OMC). Within OMC a subset of neurons responds to hypercapnia as do neurons in the same (or similar) brain areas in vivo. Maneuvers altering intra- and/or extracellular pH (pH(o)) such as hypercapnia, bicarbonate-withdrawal, or ammonium pre-pulses, evoked well defined changes of the neuronal pH(i). During hypercapnia (pH(o) 7.0) or bicarbonate-withdrawal (pH(o) 7.4) most ventrolateral neurons adopted a pH(i) which was < or = 0.2 pH units below the steady state pH(i), while signs of pH(i)-regulation occurred only in a small fraction of neurons. During all treatments leading to intracellular acidosis, bioelectric activity of chemosensitive neurons increased and was often indistinguishable from the response to hypercapnia, regardless of whether pH(o) was unchanged, decreased or increased during the treatment. These data strongly suggest that the pH(i) acts as proximate stimulus. The mode of acid extrusion of chemosensitive neurons is, therefore, of major importance for the control of central chemosensitivity. Immunocytochemical data, pH(i) measurements and neuropharmacological studies with novel drugs pointed to the Na(+)/H(+) exchanger subtype 3 (NHE3) as a main acid extruder in ventrolateral chemosensitive neurons. Possible functions and neuropharmacological strategies arising from this very local NHE3 expression are discussed.

Intracellular pH regulation of neurons in chemosensitive and nonchemosensitive areas of brain slices

The role of changes of intracellular pH (pH(i)) as the proximal signal in central chemosensitive neurons has been studied. pH(i) recovery from acidification is mediated by Na(+)/H(+) exchange in all medullary neurons and pH(i) recovery from alkalinization is mediated by Cl(-)/HCO(3)(-) exchange in most medullary neurons. These exchangers are more sensitive to inhibition by changes in extracellular pH (pH(o)) in neurons from chemosensitive regions compared to those from nonchemosensitive regions. Thus, neurons from chemosensitive regions exhibit a maintained intracellular acidification in response to hypercapnic acidosis but they show pH(i) recovery in response to isohydric hypercapnia. A similar pattern of pH(i) response is seen in other CO(2)/H(+)-responsive cells, including glomus cells, sour taste receptor cells, and chemosensitive neurons from snails, suggesting that a maintained fall of pH(i) is a common feature of the proximal signal in all CO(2)/H(+)-sensitive cells. To further evaluate the potential role of pH(i) changes as proximal signals for chemosensitive neurons, studies must be done to: determine why a lack of pH(i) recovery from hypercapnic acidosis is seen in some nonchemosensitive neurons; establish a correlation between hypercapnia-induced changes of pH(i) and membrane potential (V(m)); compare the hypercapnia-induced pH(i) changes seen in neuronal cell bodies with those in dendritic processes; understand why the V(m) response to hypercapnia of many chemosensitive neurons is washed out when using whole cell patch pipettes; and employ knock out mice to investigate the role of certain proteins in the CO(2)/H(+) response of chemosensitive neurons.

Avian intrapulmonary chemoreceptor discharge rate is increased by anion exchange blocker 'DIDS'

Avian intrapulmonary chemoreceptors (IPC) are neurons that sense lung P(CO(2)) and provide phasic feedback for the control of breathing in birds. To try to understand mechanisms of CO(2) transduction and intracellular pH regulation in IPC, the anion exchange inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) was used to block transmembrane Cl(-)/HCO(3)(-) transport. Single-unit IPC discharge rates were measured at steady intrapulmonary CO(2) levels and during step changes in CO(2) in 15 anesthetized, unidirectionally ventilated adult mallard ducks (Anas platyrhynchos). Measurements were repeated after giving 50, 100 and 200 micromol/kg cumulative i.v. dosages of DIDS. Mean IPC discharge rates at steady (tonic) P(CO(2)) levels were significantly increased by 100 and 200 micromol/kg DIDS, but not by 50 micromol/kg DIDS. Mean dynamic (phasic) IPC responses to CO(2) steps were not significantly affected by DIDS. Results indicate that the DIDS-sensitive Cl(-)/HCO(3)(-) membrane exchanger is involved with tonic CO(2) signal transduction in IPC. However, because some individual IPC were unaffected by DIDS, yet still altered their discharge rate with CO(2), additional mechanisms besides the Cl(-)/HCO(3)(-) exchange are probably required for CO(2) chemotransduction in IPC.

Modulation of kir4.1 and kir5.1 by hypercapnia and intracellular acidosis

CO2 chemoreception may be mediated by the modulation of certain ion channels in neurons. Kir4.1 and Kir5.1, two members of the inward rectifier K+ channel family, are expressed in several brain regions including the brainstem. To test the hypothesis that Kir4.1 and Kir5. 1 are modulated by CO2 and pH, we carried out experiments by expressing Kir4.1 and coexpressing Kir4.1 with Kir5.1 (Kir4.1-Kir5. 1) in Xenopus oocytes. K+ currents were then studied using two-electrode voltage clamp and excised patches. Exposure of the oocytes to CO2 (5, 10 and 15 %) produced a concentration-dependent inhibition of the whole-cell K+ currents. This inhibition was fast and reversible. Exposure to 15 % CO2 suppressed Kir4.1 currents by approximately 20 % and Kir4.1-Kir5.1 currents by approximately 60 %. The effect of CO2 was likely to be mediated by intracellular acidification, because selective intracellular, but not extracellular, acidification to the measured hypercapnic pH levels lowered the currents as effectively as hypercapnia. In excised inside-out patches, exposure of the cytosolic side of membranes to solutions with various pH levels brought about a dose-dependent inhibition of the macroscopic K+ currents. The pK value (-log of dissociation constant) for the inhibition was 6.03 in the Kir4.1 channels, while it was 7.45 in Kir4.1-Kir5.1 channels, an increase in pH sensitivity of 1.4 pH units. Hypercapnia without changing pH did not inhibit the Kir4.1 and Kir4.1-Kir5.1 currents, suggesting that these channels are inhibited by protons rather than molecular CO2. A lysine residue in the N terminus of Kir4.1 is critical. Mutation of this lysine at position 67 to methionine (K67M) completely eliminated the CO2 sensitivity of both the homomeric Kir4. 1 and heteromeric Kir4.1-Kir5.1. These results therefore indicate that the Kir4.1 channel is inhibited during hypercapnia by a decrease in intracellular pH, and the coexpression of Kir4.1 with Kir5.1 greatly enhances channel sensitivity to CO2/pH and may enable cells to detect both increases and decreases in PCO2 and intracellular pH at physiological levels.

CO(2) inhibits specific inward rectifier K(+) channels by decreases in intra- and extracellular pH

Hypercapnia has been shown to affect cellular excitability by modulating K(+) channels. To understand the mechanisms for this modulation, four cloned K(+) channels were studied by expressing them in Xenopus oocytes. Exposures of the oocytes to CO(2) for 4-6 min produced reversible and concentration-dependent inhibitions of Kir1.1 and Kir2.3 currents, but had no effect on Kir2.1 and Kir6.1 currents. Intra- and extracellular pH (pH(i), pH(o)) dropped during CO(2) exposures. The inhibition of Kir2.3 currents was mediated by reductions in both intra- and extracellular pH, whereas the suppression of Kir1.1 resulted from intracellular acidification. In cell-free excised inside-out patches with cytosolic-soluble factors washed out, a decrease in pH(i) produced a fast and reversible inhibition of macroscopic Kir2.3 currents. The degree of this inhibition was similar to that produced by hypercapnia when compared at the same pH(i) level. Exposure of cytosolic surface of patch membranes to a perfusate bubbled with 15% CO(2) without changing pH failed to inhibit the Kir2.3 currents. These results therefore indicate that (1) hypercapnia inhibits specific K(+) channels, (2) these inhibitions are caused by intra- and extracellular protons rather than molecular CO(2), and (3) these effects are independent of cytosol-soluble factors.

Chemosensitivity of non-respiratory rat CNS neurons in tissue culture

Neurons from many brainstem nuclei involved in respiratory control increase their firing rate in response to acidosis in vitro, suggesting that they are central chemoreceptors. This property has been considered to be either unique to neurons involved in respiratory control, or at least very unusual for non-respiratory neurons. However, recordings of intrinsic pH responses of neurons have not been made from enough non-respiratory regions of the CNS to be certain this assumption is true. Here, we have quantified changes in firing rate of neurons cultured from the hippocampus (n=43), neocortex (n=33), and cerebellum (n=29) in response to changes in CO(2) between 3% and 9% (pH approximately 7.6-7.2) after blockade of glutamatergic and GABAergic transmission. The responses of neurons from these three regions were similar, with a subset of neurons (12% of the total 105) inhibited by acidosis, decreasing their firing rate to a mean of 70% of control in response to a decrease in pH of 0.2. Some neurons (5% of total) were stimulated by acidosis, with an increase in firing rate to a mean of 175% of control in response to a decrease in pH of 0.2. We previously quantified chemosensitivity of neurons from the medullary raphe using the same methods [W. Wang, J.H. Pizzonia, G.B. Richerson, Chemosensitivity of rat medullary raphe neurones in primary tissue culture, J. Physiol., 511 (1998) 433-450]. Compared to these non-respiratory neurons, more raphe neurons were stimulated by acidosis (22%), and the average response was greater (to 300% of control) in response to the same stimulus. Thus, over a physiologically relevant pH range, stimulation by acidosis occurs in a significant percentage of neurons not involved in respiratory chemoreception. However, the degree of chemosensitivity of these neurons was less than medullary raphe neurons under the same conditions. Chemosensitivity is not an all-or-none neuronal property, and the degree of chemosensitivity may be relevant to the role neurons play in sensing pH in vivo.

Identification of a critical motif responsible for gating of Kir2.3 channel by intracellular protons

Protons are involved in gating Kir2.3. To identify the molecular motif in the Kir2.3 channel protein that is responsible for this process, experiments were performed using wild-type and mutated Kir2. 3 and Kir2.1. CO2 and low pHi strongly inhibited wild-type Kir2.3 but not Kir2.1 in whole cell voltage clamp and excised inside-out patches. This CO2/pH sensitivity was completely eliminated in a mutant Kir2.3 in which the N terminus was substituted with that in Kir2.1, whereas a similar replacement of its C terminus had no effect. Site-specific mutations of all titratable residues in the N terminus, however, did not change the CO2/pH sensitivity. Using several chimeras generated systematically in the N terminus, a 10-residue motif near the M1 region was identified in which only three amino acids are different between Kir2.3 and Kir2.1. Mutations of these residues, especially Thr53, dramatically reduced the pH sensitivity of Kir2.3. Introducing these residues or even a single threonine to the corresponding positions of Kir2.1 made the mutant channel pH-sensitive. Thus, a critical motif responsible for gating Kir2.3 by protons was identified in the N terminus, which contained about 10 residues centered by Thr53.

Hyperpolarization-activated inward currents contribute to spontaneous electrical activity and CO2/H+ sensitivity of cultivated neurons of fetal rat medulla

Neurons growing out from cultivated fetal medullary slices that exhibited spontaneous electrical activity after blockade of synaptic transmission were investigated by the patch-clamp technique for their response to decreases in the extracellular pH. Increases in the [H+], induced by increases in pCO2, resulted in a decrease in spike frequency associated with a decrease in the rate of depolarization preceding each action potential. The type of ion channel, contributing to interspike depolarization, and which may therefore be the site of CO2/H+ action, was identified by application of agents that inhibited the hyperpolarization-activated cation, IH, channel (Cs+ and ZD7288). Application of Cs+ and ZD7288 slightly hyperpolarized the cell membrane, decreased the interspike slope and inhibited CO2/H+-induced modulations of spike frequency in one group of CO2-inhibited medullary neurons, suggesting that IH contributes to spontaneous neuronal activity and to CO2/H+-sensitivity. CO2/H+ effects on IH were further confirmed in voltage-clamp experiments. Increasing the bath CO2 from 2% to 9% reduced the IH amplitude, shifted the mean EH from -54 to -60 mV, lengthened the voltage-dependent delay of current activation and increased the time-constants of activation at all potentials studied. It is concluded that depolarizing inward currents through IH channels participate in the gradual ramp-like change in membrane potential which depolarizes the cell up to the threshold of Na+ spike generation. CO2/H+-induced inhibition of IH reduces the contribution of this ion current to the interspike depolarization and accounts for the CO2/H+-induced decrease in spike frequency in one type of CO2/H+-inhibited medullary cells.