Postnatal development and activation of L-type Ca2+ currents in locus ceruleus neurons: implications for a role for Ca2+ in central chemosensitivity

Contribution of membrane-associated oscillators to biological timing at different timescales

Environmental rhythms such as the daily light-dark cycle selected for endogenous clocks. These clocks predict regular environmental changes and provide the basis for well-timed adaptive homeostasis in physiology and behavior of organisms. Endogenous clocks are oscillators that are based on positive feedforward and negative feedback loops. They generate stable rhythms even under constant conditions. Since even weak interactions between oscillators allow for autonomous synchronization, coupling/synchronization of oscillators provides the basis of self-organized physiological timing. Amongst the most thoroughly researched clocks are the endogenous circadian clock neurons in mammals and insects. They comprise nuclear clockworks of transcriptional/translational feedback loops (TTFL) that generate ∼24 h rhythms in clock gene expression entrained to the environmental day-night cycle. It is generally assumed that this TTFL clockwork drives all circadian oscillations within and between clock cells, being the basis of any circadian rhythm in physiology and behavior of organisms. Instead of the current gene-based hierarchical clock model we provide here a systems view of timing. We suggest that a coupled system of autonomous TTFL and posttranslational feedback loop (PTFL) oscillators/clocks that run at multiple timescales governs adaptive, dynamic homeostasis of physiology and behavior. We focus on mammalian and insect neurons as endogenous oscillators at multiple timescales. We suggest that neuronal plasma membrane-associated signalosomes constitute specific autonomous PTFL clocks that generate localized but interlinked oscillations of membrane potential and intracellular messengers with specific endogenous frequencies. In each clock neuron multiscale interactions of TTFL and PTFL oscillators/clocks form a temporally structured oscillatory network with a common complex frequency-band comprising superimposed multiscale oscillations. Coupling between oscillator/clock neurons provides the next level of complexity of an oscillatory network. This systemic dynamic network of molecular and cellular oscillators/clocks is suggested to form the basis of any physiological homeostasis that cycles through dynamic homeostatic setpoints with a characteristic frequency-band as hallmark. We propose that mechanisms of homeostatic plasticity maintain the stability of these dynamic setpoints, whereas Hebbian plasticity enables switching between setpoints via coupling factors, like biogenic amines and/or neuropeptides. They reprogram the network to a new common frequency, a new dynamic setpoint. Our novel hypothesis is up for experimental challenge.

Criteria for central respiratory chemoreceptors: experimental evidence supporting current candidate cell groups

An interoceptive homeostatic system monitors levels of CO2/H+ and provides a proportionate drive to respiratory control networks that adjust lung ventilation to maintain physiologically appropriate levels of CO2 and rapidly regulate tissue acid-base balance. It has long been suspected that the sensory cells responsible for the major CNS contribution to this so-called respiratory CO2/H+ chemoreception are located in the brainstem-but there is still substantial debate in the field as to which specific cells subserve the sensory function. Indeed, at the present time, several cell types have been championed as potential respiratory chemoreceptors, including neurons and astrocytes. In this review, we advance a set of criteria that are necessary and sufficient for definitive acceptance of any cell type as a respiratory chemoreceptor. We examine the extant evidence supporting consideration of the different putative chemoreceptor candidate cell types in the context of these criteria and also note for each where the criteria have not yet been fulfilled. By enumerating these specific criteria we hope to provide a useful heuristic that can be employed both to evaluate the various existing respiratory chemoreceptor candidates, and also to focus effort on specific experimental tests that can satisfy the remaining requirements for definitive acceptance.

Locus Coeruleus Neurons' Firing Pattern Is Regulated by ERG Voltage-Gated K+ Channels

Locus coeruleus (LC) neurons, with their extensive innervations throughout the brain, control a broad range of physiological processes. Several ion channels have been characterized in LC neurons that control intrinsic membrane properties and excitability. However, ERG (ether-à-go-go-related gene) K⁺ channels that are particularly important in setting neuronal firing rhythms and automaticity have not as yet been discovered in the LC. Moreover, the neurophysiological and pathophysiological roles of ERG channels in the brain remain unclear despite their expression in several structures. By performing immunohistochemical investigations, we found that ERG-1A, ERG-1B, ERG-2 and ERG-3 are highly expressed in the LC neurons of mice. To examine the functional role of ERG channels, current-clamp recordings were performed on mouse LC neurons in brain slices under visual control. ERG channel blockade by WAY-123,398, a class III anti-arrhythmic agent, increased the spontaneous firing activity and discharge irregularity of LC neurons. Here, we have shown the presence of distinct ERG channel subunits in the LC which play an imperative role in modulating neuronal discharge patterns. Thus, we propose that ERG channels are important players behind the changes in, and/or maintenance of, LC firing patterns that are implicated in the generation of different behaviors and in several disorders.

Response of TRPM2 Channel to Hypercapnic Acidosis and Role of Zn, Se, and GSH

Hypercapnia can increase the production of reactive oxygen species (ROS) by inducing oxidative stress in cells. Transient receptor potential melastatin 2 (TRPM2) channel activation that is realized by ROS plays a critical role in the cellular mechanism. It was shown that antioxidants such as zinc (Zn), selenium (Se), and glutathione (GSH) can partake in the structures of enzymes and create a protective effect against oxidative stress. This study revealed the relationship between TRPM2 channel and hypercapnia, and the interaction of zinc, selenium, and glutathione. In our study, normoxia, hypercapnia, hypercapnia + Zn, hypercapnia + Se, and hypercapnia + GSH were created, in transfected HEK293 cells. The cells were exposed to normoxia or hypercapnia gasses in two different times (30 min and 60 min), while Zn, Se, and GSH were applied to the cells in the other groups before being exposed to the gas mixtures. The statistical evaluation showed a significant increase in lipid peroxidation (LPO) level and lactate dehydrogenase (LDH)% in the hypercapnia 30 min and 60 min groups, compared to the normoxia 30 min and 60 min groups, and an increase in LPO level and LDH% in the hypercapnia groups that Zn, Se, and GSH were applied. It was determined that in comparison with the normoxia 30 min and 60 min groups, the amount of inward Ca⁺² current across TRPM2 channels and mean current density increased in the groups that were exposed to hypercapnia for 30 min and 60 min, while the same values significantly decreased in the hypercapnia groups that Zn, Se, and GSH were applied. Also, it was shown that oxidative stress rose as the duration of hypercapnia exposure increased. It was concluded that hypercapnia increased oxidative stress and caused cellular membrane damage, while the addition of Zn, Se, and GSH could protect the cell membrane from these damaging effects.

TRPM4 Contributes to Subthreshold Membrane Potential Oscillations in Multiple Mouse Pacemaker Neurons

Select neuronal populations display steady rhythmic neuronal firing that provides tonic excitation to drive downstream networks and behaviors. In noradrenergic neurons of the locus coeruleus (LC), circadian neurons of the suprachiasmatic nucleus (SCN), and CO₂/H⁺-activated neurons of the brainstem retrotrapezoid nucleus (RTN), large subthreshold membrane potential oscillations contribute to the pacemaker-like action potential discharge. The oscillations and firing in LC and SCN involve contributions from leak sodium (NALCN) and L-type calcium channels while recent work from RTN suggested an additional pivotal role for a secondary calcium-activated and voltage-gated cationic current sensitive to TRPM4 channel blockers. Here, we tested whether TRPM4 contributes to subthreshold oscillations in mouse LC and SCN. By RNAscope in situ hybridization, Trpm4 transcripts were detected in both cell groups. In whole-cell recordings from acute slice preparations, prominent voltage-dependent membrane potential oscillations were revealed in LC and SCN after blocking action potentials. These oscillations were inhibited by two chemically-distinct blockers of TRPM4, 9-phenanthrol (9-pt) and 4-chloro-2-[[2-(2-chlorophenoxy)acetyl]amino]benzoic acid (CBA). Under whole-cell voltage clamp, inward currents evoked by oscillation voltage waveforms were inhibited in LC by blocking L-type calcium channels and TRPM4. These data implicate TRPM4 in the large subthreshold membrane potential oscillations that underlie tonic action potential discharge in LC and SCN, providing a voltage-dependent and calcium-dependent cationic current to augment the depolarizing inward Na⁺ and Ca²⁺ currents previously associated with this distinctive electroresponsive property.

Formalin-induced inflammatory pain increases excitability in locus coeruleus neurons

Chronic pain is recognized as an important problem in communities. The locus coeruleus (LC) with extensive ascending and descending projections has a critical role in modulating pain. Some studies indicate how the locus coeruleus-noradrenaline system can remain more active after nociceptive stimulation. In the present study, we examined whether formalin-induced inflammatory pain may affect the electrophysiological properties of LC neurons after 24 h. Inflammatory pain was induced by a subcutaneous injection of 2% formalin (10 μL) into the hind paw of 2-3 week-old male Wistar rats. After 24 h, horizontal slices of brain stem containing the locus coeruleus were prepared and whole-cell patch-clamp recordings were carried out on LC neurons. Findings revealed that LC neurons from formalin injected rats had a significant enhancement in firing rate, half-width and instantaneous frequency of action potentials, but their resting membrane potential, input resistance and afterhyperpolarization amplitude almost remained unchanged. In addition, action potential peak amplitude, maximum rise slope, maximum decay slope, first spike latency and rheobase current significantly decreased in LC neurons obtained from formalin-treated rats. Here, for the first time, we demonstrate that inflammatory pain after 24 h induces hyperexcitability in LC neurons, which in turn may result in changes in noradrenaline release and pain processing.

Calcium, Bioenergetics, and Parkinson's Disease

Degeneration of substantia nigra (SN) dopaminergic (DAergic) neurons is responsible for the core motor deficits of Parkinson’s disease (PD). These neurons are autonomous pacemakers that have large cytosolic Ca²⁺ oscillations that have been linked to basal mitochondrial oxidant stress and turnover. This review explores the origin of Ca²⁺ oscillations and their role in the control of mitochondrial respiration, bioenergetics, and mitochondrial oxidant stress.

Respiratory Control by Phox2b-expressing Neurons in a Locus Coeruleus-preBötzinger Complex Circuit

The locus coeruleus (LC) has been implicated in the control of breathing. Congenital central hypoventilation syndrome results from mutation of the paired-like homeobox 2b (Phox2b) gene that is expressed in LC neurons. The present study was designed to address whether stimulation of Phox2b-expressing LC (Phox2b^LC) neurons affects breathing and to reveal the putative circuit mechanism. A Cre-dependent viral vector encoding a Gq-coupled human M3 muscarinic receptor (hM3Dq) was delivered into the LC of Phox2b-Cre mice. The hM3Dq-transduced neurons were pharmacologically activated while respiratory function was measured by plethysmography. We demonstrated that selective stimulation of Phox2b^LC neurons significantly increased basal ventilation in conscious mice. Genetic ablation of these neurons markedly impaired hypercapnic ventilatory responses. Moreover, stimulation of Phox2b^LC neurons enhanced the activity of preBötzinger complex neurons. Finally, axons of Phox2b^LC neurons projected to the preBötzinger complex. Collectively, Phox2b^LC neurons contribute to the control of breathing most likely via an LC-preBötzinger complex circuit.

Selective neuronal vulnerability in Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease, disabling millions worldwide. Despite the imperative PD poses, at present, there is no cure or means of slowing progression. This gap is attributable to our incomplete understanding of the factors driving pathogenesis. Research over the past several decades suggests that both cell-autonomous and non-cell autonomous processes contribute to the neuronal dysfunction underlying PD symptoms. The thesis of this review is that an intersection of these processes governs the pattern of pathology in PD. Studies of substantia nigra pars compacta (SNc) dopaminergic neurons, whose loss is responsible for the core motor symptoms of PD, suggest that they have a combination of traits-a long, highly branched axon, autonomous activity, and elevated mitochondrial oxidant stress-that predispose them to non-cell autonomous drivers of pathogenesis, like misfolded forms of alpha-synuclein (α-SYN) and inflammation. The literature surrounding these issues will be briefly summarized, and the translational implications of an intersectional hypothesis of PD pathogenesis discussed.

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.

Theoretical perspectives on central chemosensitivity: CO2/H+-sensitive neurons in the locus coeruleus

Central chemoreceptors are highly sensitive neurons that respond to changes in pH and CO₂ levels. An increase in CO₂/H⁺ typically reflects a rise in the firing rate of these neurons, which stimulates an increase in ventilation. Here, we present an ionic current model that reproduces the basic electrophysiological activity of individual CO₂/H⁺-sensitive neurons from the locus coeruleus (LC). We used this model to explore chemoreceptor discharge patterns in response to electrical and chemical stimuli. The modeled neurons showed both stimulus-evoked activity and spontaneous activity under physiological parameters. Neuronal responses to electrical and chemical stimulation showed specific firing patterns of spike frequency adaptation, postinhibitory rebound, and post-stimulation recovery. Conversely, the response to chemical stimulation alone (based on physiological CO₂/H⁺ changes), in the absence of external depolarizing stimulation, showed no signs of postinhibitory rebound or post-stimulation recovery, and no depolarizing sag. A sensitivity analysis for the firing-rate response to the different stimuli revealed that the contribution of an applied stimulus current exceeded that of the chemical signals. The firing-rate response increased indefinitely with injected depolarizing current, but reached saturation with chemical stimuli. Our computational model reproduced the regular pacemaker-like spiking pattern, action potential shape, and most of the membrane properties that characterize CO₂/H⁺-sensitive neurons from the locus coeruleus. This validates the model and highlights its potential as a tool for studying the cellular mechanisms underlying the altered central chemosensitivity present in a variety of disorders such as sudden infant death syndrome, depression, and anxiety. In addition, the model results suggest that small external electrical signals play a greater role in determining the chemosensitive response to changes in CO₂/H⁺ than previously thought. This highlights the importance of considering electrical synaptic transmission in studies of intrinsic chemosensitivity.

The sensory mechanism by which changes in CO₂ and H⁺ levels are detected in the brain is known as central chemoreception. Altered chemoreception is common to a wide variety of clinical conditions, including sleep apnea, sudden infant death syndrome, hyperventilation, depression, anxiety and asthma. In addition, CO₂/H⁺-sensitive neurons are present in some regions of the brain that have been identified as drug targets for the treatment of anxiety and panic disorders. We are interested in understanding the cellular mechanisms that determine and modulate the behavior of these neurons. We previously investigated possible mechanisms underlying their behavior in rats to elucidate whether they respond to changes in intracellular or extracellular pH, CO₂, or a combination of these stimuli. To study the roles that signals and ion channel targets play in individual neurons we develop mathematical models that simulate their electrochemical behavior and their responses to hypercapnic and/or acidotic stimuli. Nowadays, we are focused on using computational tools to explore the firing pattern of such neurons in response to chemical (CO₂/H⁺) and electrical (synaptic) stimulation. Our results reveal significant effects of electrical stimulation on the responses of brainstem neurons and highlight the importance of considering synaptic transmission in experimental studies of chemosensitivity.

Environmentally induced return to juvenile-like chemosensitivity in the respiratory control system of adult bullfrog, Lithobates catesbeianus

KEY POINTS

The degree to which developmental programmes or environmental signals determine physiological phenotypes remains a major question in physiology. Vertebrates change environments during development, confounding interpretation of the degree to which development (i.e. permanent processes) or phenotypic plasticity (i.e. reversible processes) produces phenotypes. Tadpoles mainly breathe water for gas exchange and frogs may breathe water or air depending on their environment and are, therefore, exemplary models to differentiate the degree to which life-stage vs. environmental context drives developmental phenotypes associated with neural control of lung breathing. Using isolated brainstem preparations and patch clamp electrophysiology, we demonstrate that adult bullfrogs acclimatized to water-breathing conditions do not exhibit CO₂ and O₂ chemosensitivity of lung breathing, similar to water-breathing tadpoles. Our results establish that phenotypes associated with developmental stage may arise from plasticity per se and suggest that a developmental trajectory coinciding with environmental change obscures origins of stage-dependent physiological phenotypes by masking plasticity.

ABSTRACT

An unanswered question in developmental physiology is to what extent does the environment vs. a genetic programme produce phenotypes? Developing animals inhabit different environments and switch from one to another. Thus a developmental time course overlapping with environmental change confounds interpretations as to whether development (i.e. permanent processes) or phenotypic plasticity (i.e. reversible processes) generates phenotypes. Tadpoles of the American bullfrog, Lithobates catesbeianus, breathe water at early life-stages and minimally use lungs for gas exchange. As adults, bullfrogs rely on lungs for gas exchange, but spend months per year in ice-covered ponds without lung breathing. Aquatic submergence, therefore, removes environmental pressures requiring lung breathing and enables separation of adulthood from environmental factors associated with adulthood that necessitate control of lung ventilation. To test the hypothesis that postmetamorphic respiratory control phenotypes arise through permanent developmental changes vs. reversible environmental signals, we measured respiratory-related nerve discharge in isolated brainstem preparations and action potential firing from CO₂ -sensitive neurons in bullfrogs acclimatized to semi-terrestrial (air-breathing) and aquatic-overwintering (no air-breathing) habitats. We found that aquatic overwintering significantly reduced neuroventilatory responses to CO₂ and O₂ involved in lung breathing. Strikingly, this gas sensitivity profile reflects that of water-breathing tadpoles. We further demonstrated that aquatic overwintering reduced CO₂ -induced firing responses of chemosensitive neurons. In contrast, respiratory rhythm generating processes remained adult-like after submergence. Our results establish that phenotypes associated with life-stage can arise from phenotypic plasticity per se. This provides evidence that developmental time courses coinciding with environmental changes obscure interpretations regarding origins of stage-dependent physiological phenotypes by masking plasticity.

Strain differences in pH-sensitive K+ channel-expressing cells in chemosensory and nonchemosensory brain stem nuclei

The ventilatory CO2 chemoreflex is inherently low in inbred Brown Norway (BN) rats compared with other strains, including inbred Dahl salt-sensitive (SS) rats. Since the brain stem expression of various pH-sensitive ion channels may be determinants of the CO2 chemoreflex, we tested the hypothesis that there would be fewer pH-sensitive K(+) channel-expressing cells in BN relative to SS rats within brain stem sites associated with respiratory chemoreception, such as the nucleus tractus solitarius (NTS), but not within the pre-Bötzinger complex region, nucleus ambiguus or the hypoglossal motor nucleus. Medullary sections (25 μm) from adult male and female BN and SS rats were stained with primary antibodies targeting TASK-1, Kv1.4, or Kir2.3 K(+) channels, and the total (Nissl-stained) and K(+) channel immunoreactive (-ir) cells counted. For both male and female rats, the numbers of K(+) channel-ir cells within the NTS were reduced in the BN compared with SS rats (P < 0.05), despite equal numbers of total NTS cells. In contrast, we found few differences in the numbers of K(+) channel-ir cells among the strains within the nucleus ambiguus, hypoglossal motor nucleus, or pre-Bötzinger complex regions in both male and female rats. However, there were no predicted functional mutations in each of the K(+) channels studied comparing genomic sequences among these strains. Thus we conclude that the relatively selective reductions in pH-sensitive K(+) channel-expressing cells in the NTS of male and female BN rats may contribute to their severely blunted ventilatory CO2 chemoreflex.

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.

Soluble adenylyl cyclase in the locus coeruleus

Although it has been demonstrated that the CO2-sensitivity in the locus coeruleus (LC) is mediated by changes in pH, the involvement of HCO3(-) in the CO2-detection mechanism in these neurons cannot be excluded. In the present work, we characterized sAC for the first time in the LC and we asked whether this enzyme is important in the detection of changes in HCO3(-)/CO2 levels in these neurons, using an approach that allowed us to isolate CO2 from pH stimulus. sAC mRNA expression and activity were upregulated from 0mM HCO3(-)/0% CO2 to 24 mM HCO3(-)/5% CO2 in the LC but not in the cortex of the brain. Comparing the effects of sAC and tmAC inhibitors in the LC, we observed that both tmAC and sAC contribute to the generation of cAMP during normocapnic conditions but only sAC contributed to the generation of cAMP during isohydric hypercapnia. Furthermore, activation of tmAC induced an increase in sAC expression in LC, but not cortex. sAC may be involved in CO2 sensitivity in the LC, up to its threshold of saturation, with a particular contribution of this enzyme in situations when low HCO3(-) concentrations occur. Its role should be further explored in pathological states to determine whether sAC activation with HCO3(-) alters ventilation.

Mitochondrial oxidant stress in locus coeruleus is regulated by activity and nitric oxide synthase

Loss of noradrenergic locus coeruleus (LC) neurons is a prominent feature of aging-related neurodegenerative diseases, such as Parkinson's disease (PD). The basis of this vulnerability is not understood. To explore possible physiological determinants, we studied LC neurons using electrophysiological and optical approaches in ex vivo mouse brain slices. We found that autonomous activity in LC neurons was accompanied by oscillations in dendritic Ca(2+) concentration that were attributable to the opening of L-type Ca(2+) channels. This oscillation elevated mitochondrial oxidant stress and was attenuated by inhibition of nitric oxide synthase. The relationship between activity and stress was malleable, as arousal and carbon dioxide increased the spike rate but differentially affected mitochondrial oxidant stress. Oxidant stress was also increased in an animal model of PD. Thus, our results point to activity-dependent Ca(2+) entry and a resulting mitochondrial oxidant stress as factors contributing to the vulnerability of LC neurons.

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.