An alternative approach to the identification of respiratory central chemoreceptors in the brainstem

The Modulation by Anesthetics and Analgesics of Respiratory Rhythm in the Nervous System

Rhythmic eupneic breathing in mammals depends on the coordinated activities of the neural system that sends cranial and spinal motor outputs to respiratory muscles. These outputs modulate lung ventilation and adjust respiratory airflow, which depends on the upper airway patency and ventilatory musculature. Anesthetics are widely used in clinical practice worldwide. In addition to clinically necessary pharmacological effects, respiratory depression is a critical side effect induced by most general anesthetics. Therefore, understanding how general anesthetics modulate the respiratory system is important for the development of safer general anesthetics. Currently used volatile anesthetics and most intravenous anesthetics induce inhibitory effects on respiratory outputs. Various general anesthetics produce differential effects on respiratory characteristics, including the respiratory rate, tidal volume, airway resistance, and ventilatory response. At the cellular and molecular levels, the mechanisms underlying anesthetic-induced breathing depression mainly include modulation of synaptic transmission of ligand-gated ionotropic receptors (e.g., γ-aminobutyric acid, N-methyl-D-aspartate, and nicotinic acetylcholine receptors) and ion channels (e.g., voltage-gated sodium, calcium, and potassium channels, two-pore domain potassium channels, and sodium leak channels), which affect neuronal firing in brainstem respiratory and peripheral chemoreceptor areas. The present review comprehensively summarizes the modulation of the respiratory system by clinically used general anesthetics, including the effects at the molecular, cellular, anatomic, and behavioral levels. Specifically, analgesics, such as opioids, which cause respiratory depression and the "opioid crisis", are discussed. Finally, underlying strategies of respiratory stimulation that target general anesthetics and/or analgesics are summarized.

Current Drug Treatment Strategies for Atrial Fibrillation and TASK-1 Inhibition as an Emerging Novel Therapy Option

Atrial fibrillation (AF) is the most common sustained arrhythmia with a prevalence of up to 4% and an upwards trend due to demographic changes. It is associated with an increase in mortality and stroke incidences. While stroke risk can be significantly reduced through anticoagulant therapy, adequate treatment of other AF related symptoms remains an unmet medical need in many cases. Two main treatment strategies are available: rate control that modulates ventricular heart rate and prevents tachymyopathy as well as rhythm control that aims to restore and sustain sinus rhythm. Rate control can be achieved through drugs or ablation of the atrioventricular node, rendering the patient pacemaker-dependent. For rhythm control electrical cardioversion and pharmacological cardioversion can be used. While electrical cardioversion requires fasting and sedation of the patient, antiarrhythmic drugs have other limitations. Most antiarrhythmic drugs carry a risk for pro-arrhythmic effects and are contraindicated in patients with structural heart diseases. Furthermore, catheter ablation of pulmonary veins can be performed with its risk of intraprocedural complications and varying success. In recent years TASK-1 has been introduced as a new target for AF therapy. Upregulation of TASK-1 in AF patients contributes to prolongation of the action potential duration. In a porcine model of AF, TASK-1 inhibition by gene therapy or pharmacological compounds induced cardioversion to sinus rhythm. The DOxapram Conversion TO Sinus rhythm (DOCTOS)-Trial will reveal whether doxapram, a potent TASK-1 inhibitor, can be used for acute cardioversion of persistent and paroxysmal AF in patients, potentially leading to a new treatment option for AF.

Integration of Central and Peripheral Respiratory Chemoreflexes

A debate has raged since the discovery of central and peripheral respiratory chemoreceptors as to whether the reflexes they mediate combine in an additive (i.e., no interaction), hypoadditive or hyperadditive manner. Here we critically review pertinent literature related to O2 and CO2 sensing from the perspective of system integration and summarize many of the studies on which these seemingly opposing views are based. Despite the intensity and quality of this debate, we have yet to reach consensus, either within or between species. In reviewing this literature, we are struck by the merits of the approaches and preparations that have been brought to bear on this question. This suggests that either the nature of combination is not important to system responses, contrary to what has long been supposed, or that the nature of the combination is more malleable than previously assumed, changing depending on physiological state and/or respiratory requirement.

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.

The cellular building blocks of breathing

Respiratory brainstem neurons fulfill critical roles in controlling breathing: they generate the activity patterns for breathing and contribute to various sensory responses including changes in O2 and CO2. These complex sensorimotor tasks depend on the dynamic interplay between numerous cellular building blocks that consist of voltage-, calcium-, and ATP-dependent ionic conductances, various ionotropic and metabotropic synaptic mechanisms, as well as neuromodulators acting on G-protein coupled receptors and second messenger systems. As described in this review, the sensorimotor responses of the respiratory network emerge through the state-dependent integration of all these building blocks. There is no known respiratory function that involves only a small number of intrinsic, synaptic, or modulatory properties. Because of the complex integration of numerous intrinsic, synaptic, and modulatory mechanisms, the respiratory network is capable of continuously adapting to changes in the external and internal environment, which makes breathing one of the most integrated behaviors. Not surprisingly, inspiration is critical not only in the control of ventilation, but also in the context of "inspiring behaviors" such as arousal of the mind and even creativity. Far-reaching implications apply also to the underlying network mechanisms, as lessons learned from the respiratory network apply to network functions in general.

Neuronal expression of bitter taste receptors and downstream signaling molecules in the rat brainstem

Previous studies have shown that molecules of the taste transduction pathway may serve as biochemical markers for chemoreceptive cells in respiratory and gastrointestinal tracts. In this study, we tested the hypothesis that brainstem neurons contain signaling molecules similar to those in taste buds which may sense the chemical composition of brain extracellular fluids. We used the reverse transcription polymerase chain reaction (RT-PCR), Western blot and immunohistochemical techniques to evaluate presence of different bitter-responsive type 2 taste receptors (T2Rs), their associated G-protein α-gustducin, the downstream signaling molecules phospholipase C isoform β2 (PLC-β2) and transient receptor potential melastatin 5 (TRPM5) in the brainstem of rats. RT-PCR confirmed the mRNA coding for α-gustducin, PLC-β2, TRPM5 and rT2R1 but not that of rT2R16, rT2R26 and rT2R38 in the medulla oblongata. Western blotting confirmed the presence of α-gustducin at the protein level in rat brainstem. Immunohistochemistry identified cells expressing α-gustducin and PLC-β2 at multiple cardiorespiratory and CO(2)/H(+) chemosensory sites, including rostral ventral medulla, facial, parapyramidal, solitary tract, hypoglossal and raphe nuclei. In the medullary raphe, α-gustducin and PLC-β2 were colocalized with a subpopulation of tryptophan hydroxylase (TPH)-immunoreactive serotonergic neurons, a subset of which has respiratory CO(2)/H(+) chemosensitivity. Presence of the T2R1 gene and other genes and proteins of the bitter taste transduction pathway in the brainstem implies additional functions for taste receptors and their effector molecules apart from their gustatory function.

Respiratory responses to hypercapnia and hypoxia in mice with genetic ablation of Kir5.1 (Kcnj16)

Inward rectifier (Kir) potassium channels contribute to the control of electrical activity in excitable tissues and their activity is modulated by many biochemical factors, including protons. Heteromeric Kir4.1-Kir5.1 channels are highly pH sensitive within the physiological range of pH changes and are strongly expressed by the peripheral chemosensors as well as in the brainstem pH-sensitive areas which mediate respiratory responses to changes in blood and brain levels of P(CO(2))/[H(+)]. In the present study, Kir5.1 knockout mice (Kir5.1(-/-)) were used to determine the role of these channels in the chemosensory control of breathing. We found that Kir5.1(-/-) mice presented with persistent metabolic acidosis and a clear respiratory phenotype. Despite metabolic acidosis, ventilation at rest and in hyperoxic hypercapnia were similar in wild-type and Kir5.1(-/-) mice. Ventilatory responses to hypoxia and normoxic hypercapnia were significantly reduced in Kir5.1(-/-) mice; however, carotid body chemoafferent responses to hypoxia and CO(2) were not affected. In the in situ brainstem-spinal cord preparations with denervated peripheral chemoreceptors, resting phrenic nerve activity and phrenic nerve responses to respiratory acidosis or isohydric hypercapnia were also similar in Kir5.1(-/-) and wild-type mice. In in situ preparations of Kir5.1(-/-) mice with intact peripheral chemoreceptors, application of CN(-) resulted in a significantly reduced phrenic nerve response, suggesting that the relay of peripheral chemosensory information to the CNS is compromised. We suggest that this compensatory modulation of the peripheral chemosensory inputs develops in Kir5.1(-/-) mice in order to counteract the effect of continuing metabolic acidosis on the activity of the peripheral chemoreceptors. These results therefore suggest that despite their intrinsic pH sensitivity, Kir4.1-Kir5.1 channels are dispensable for functional central and peripheral respiratory 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.

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.

Inward rectifier channel, ROMK, is localized to the apical tips of glial-like cells in mouse taste buds

Cells in taste buds are closely packed, with little extracellular space. Tight junctions and other barriers further limit permeability and may result in buildup of extracellular K(+) following action potentials. In many tissues, inwardly rectifying K channels such as the renal outer medullary K (ROMK) channel (also called Kir1.1 and derived from the Kcnj1 gene) help to redistribute K(+). Using reverse-transcription polymerase chain reaction (RT-PCR), we defined ROMK splice variants in mouse kidney and report here the expression of a single one of these, ROMK2, in a subset of mouse taste cells. With quantitative (q)RT-PCR, we show the abundance of ROMK mRNA in taste buds is vallate > foliate > > palate > > fungiform. ROMK protein follows the same pattern of prevalence as mRNA, and is essentially undetectable by immunohistochemistry in fungiform taste buds. ROMK protein is localized to the apical tips of a subset of taste cells. Using tissues from PLCbeta2-GFP and GAD1-GFP transgenic mice, we show that ROMK is not found in PLCbeta2-expressing type II/receptor cells or in GAD1-expressing type III/presynaptic cells. Instead, ROMK is found, by single-cell RT-PCR and immunofluorescence, in most cells that are positive for the taste glial cell marker, Ectonucleotidase2. ROMK is precisely localized to the apical tips of these cells, at and above apical tight junctions. We propose that in taste buds, ROMK in type I/glial-like cells may serve a homeostatic function, excreting excess K(+) through the apical pore, and allowing excitable taste cells to maintain a hyperpolarized resting membrane potential.

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.

Retrotrapezoid nucleus and central chemoreception

The 'distributed chemoreception theory' attributes the central chemoreflex (the stimulation of breathing by CNS acidification) to the cumulative effects of pH on multiple classes of respiratory neurons as well as on their tonic sources of drive. Opinions differ as to how many classes of pH-sensitive neurons contribute to the central chemoreflex but the number of candidates is high and growing fast. The 'specialized chemoreceptor theory', endorsed here, attributes the chemoreflex to a limited number of specialized neurons. These neurons (the central chemoreceptors) would drive a respiratory pattern generator that is not or minimally activated by acidification. In this review we first describe the properties of the retrotrapezoid nucleus (RTN) and argue that this nucleus may contain the most important central chemoreceptors. Next, we subject the assumptions that underlie the distributed chemoreception theory to a critical analysis. We propose several explanations for the apparent contradiction between the two competing theories of central chemoreception. We attribute much of the current controversy to premature extrapolations of the effects of acidification on neurons recorded in vitro (chemosensitivity) and to a semantic confusion between chemosensitivity and chemoreception (the mechanism by which CO(2) or pH activates breathing in vivo).

Hypercapnia modulates synaptic interaction of cultured brainstem neurons

CO(2) is an important metabolic product whose concentrations are constantly monitored by CO(2) chemoreceptors. However, the high systemic CO(2) sensitivity may not be achieved by the CO(2) chemoreceptors without neuronal network processes. To show modulation of network properties during hypercapnia, we studied brainstem neurons dissociated from embryonic rats (P17-19) in multielectrode arrays (MEA) after initial period (3 weeks) of culture. Spike trains of 33,622 pairs of units were analyzed using peri-event histograms (PEH). The amplitude of peri-central peaks between two CO(2)-stimulated units increased and the peak latency decreased during hypercapnia. Similar enhancement of synaptic strength was observed in those sharing a common input. These phenomena were not seen in CO(2)-unresponsive neurons. The amplitude of peri-central peaks between two CO(2) inhibited units also increased without changing latency. Over 60% CO(2)-stimulated neurons studied received mono-/oligosynaptic inputs from other CO(2)-stimulated cells, whereas only approximately 10% CO(2)-unresponsive neurons had such synaptic inputs. A small number of brainstem neurons showed electrical couplings. The coupling efficiency of CO(2)-stimulated but not CO(2)-unresponsive units was suppressed by approximately 50% with high PCO(2). Inhibitory synaptic projections were also found, which was barely affected by hypercapnia. Consistent with the strengthening of excitatory synaptic connections, CO(2) sensitivity of post-synaptic neurons was significantly higher than presynaptic neurons. The difference was eliminated with blockade of presynaptic input. Based on these indirect assessments of synaptic interaction, our PEH analysis suggests that hypercapnia appears to modulate excitatory synaptic transmissions, especially those between CO(2)-stimulated neurons.

Effects of leucine-enkephalin on potassium currents in neurons in the rat respiratory center in vitro

Experiments to identify the neuronal mechanisms underlying the respiratory activity of the opioid peptide leucine-enkephalin were performed on transverse slices of the rat brainstem in voltage-clamped conditions; studies addressed the effects of this peptide (10 nM-1 microM) on the potassium A current and the inward potassium current of neurons in two areas of the respiratory center: the ventrolateral area of the solitary tract nucleus and the pre-Bötzinger complex. The parameters of the A current assessed in all respiratory center neurons studied showed no change in the presence of leucine-enkephalin. At the same time, leucine-enkephalin produced reversible increases in the amplitude of the inward potassium current. These results provide evidence that the inhibitory effect of leucine-enkephalin at the level of respiratory center neurons is at least in part explained by its stimulatory action on the inward potassium current but is not associated with modulation of the potassium A current.

Protein kinase C dependent inhibition of the heteromeric Kir4.1-Kir5.1 channel

Heteromultimerization of Kir4.1 and Kir5.1 leads to a channel with distinct functional properties. The heteromeric Kir4.1-Kir5.1 channel is expressed in the eye, kidney and brainstem and has CO(2)/pH sensitivity in the physiological range, suggesting a candidate molecule for the regulation of K(+) homeostasis and central CO(2) chemoreception. It is known that K(+) transport in renal epithelium and brainstem CO(2) chemosensitivity are subject to modulation by hormones and neurotransmitters that activate distinct intracellular signaling pathways. If the Kir4.1-Kir5.1 channel is involved in pH-dependent regulation of cellular functions, it may also be regulated by some of the intracellular signaling systems. Therefore, we undertook studies to determine whether PKC modulates the heteromeric Kir4.1-Kir5.1 channel. The channel expressed using a Kir4.1-Kir5.1 tandem dimer construct was inhibited by the PKC activator PMA in a dose-dependent manner. The channel inhibition was produced via reduction of the P(open). The effect of PMA was abolished by specific PKC inhibitors. In contrast, exposure of oocytes to forskolin (a PKA activator) had no significant effect on Kir4.1-Kir5.1 currents. The channel inhibition appeared to be independent of PIP(2) depletion and PKC-dependent internalization. Several consensus sequences of potential PKC phosphorylation sites were identified in the Kir4.1 and Kir5.1 subunits by sequence scan. Although the C-terminal peptides of both Kir4.1 and Kir5.1 were phosphorylated in vitro, site-directed mutagenesis of individual residues failed to reveal the PKC phosphorylation sites suggesting that the channel may have multiple phosphorylation sites. Taken together, these results suggest that the Kir4.1-Kir5.1 but not the homomeric Kir4.1 channel is strongly inhibited by PKC activation.

Purinergic P2 receptors modulate excitability but do not mediate pH sensitivity of RTN respiratory chemoreceptors

The cellular mechanism(s) by which the brain senses changes in pH to regulate breathing (i.e., central chemoreception) have remained incompletely understood, in large part because the central respiratory chemoreceptors have themselves eluded detection. Here, we recorded from a newly identified population of central chemoreceptors located in the retrotrapezoid nucleus (RTN) on the ventral surface of the brainstem to test a recently proposed role for purinergic P2 receptor signaling in central respiratory chemoreception (Gourine et al., 2005). Using loose-patch current-clamp recordings in brainstem slices from rat pups (postnatal day 7-12), we indeed show purinergic modulation of pH-sensitive RTN neurons: activation of P2X receptors indirectly inhibited RTN firing by increasing inhibitory input, whereas P2Y receptor stimulation caused direct excitation of RTN chemoreceptors. However, after blocking P2 receptors with the broad-spectrum antagonists PPADS (pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate) or RB2 (reactive blue 2), the pH sensitivity of RTN neurons remained intact. Therefore, we conclude that purinergic signaling can modulate RTN neuron activity but does not mediate the pH sensing intrinsic to these central respiratory chemoreceptors.

Lack of the Kir4.1 channel subunit abolishes K+ buffering properties of astrocytes in the ventral respiratory group: impact on extracellular K+ regulation

Ongoing rhythmic neuronal activity in the ventral respiratory group (VRG) of the brain stem results in periodic changes of extracellular K+. To estimate the involvement of the weakly inwardly rectifying K+ channel Kir4.1 (KCNJ10) in extracellular K+ clearance, we examined its functional expression in astrocytes of the respiratory network. Kir4.1 was expressed in astroglial cells of the VRG, predominantly in fine astrocytic processes surrounding capillaries and in close proximity to VRG neurons. Kir4.1 expression was up-regulated during early postnatal development. The physiological role of astrocytic Kir4.1 was studied using mice with a null mutation in the Kir4.1 channel gene that were interbred with transgenic mice expressing the enhanced green fluorescent protein in their astrocytes. The membrane potential was depolarized in astrocytes of Kir4.1-/- mice, and Ba2+-sensitive inward K+ currents were diminished. Brain slices from Kir4.1-/- mice, containing the pre-Bötzinger complex, which generates a respiratory rhythm, did not show any obvious differences in rhythmic bursting activity compared with wild-type controls, indicating that the lack of Kir4.1 channels alone does not impair respiratory network activity. Extracellular K+ measurements revealed that Kir4.1 channels contribute to extracellular K+ regulation. Kir4.1 channels reduce baseline K+ levels, and they compensate for the K+ undershoot. Our data indicate that Kir4.1 channels 1) are expressed in perineuronal processes of astrocytes, 2) constitute the major part of the astrocytic Kir conductance, and 3) contribute to regulation of extracellular K+ in the respiratory network.

CO2 central chemosensitivity: why are there so many sensing molecules?

CO2 central chemoreceptors (CCRs) play a critical role in respiratory and cardiovascular controls. Although the primary sensory cells and their neuronal networks remain elusive, recent studies have begun to shed insight into the molecular mechanisms of several pH sensitive proteins. These putative CO2/pH-sensing molecules are expressed in the brainstem, detect P(CO2) at physiological levels, and couple the P(CO2) to membrane excitability. Functional analysis suggests that multiple CO2/pH-sensing molecules are needed to achieve high sensitivity and broad bandwidth of the CCRs. In contrast to the diversity of pH sensitive molecules, molecular mechanisms for CO2 sensing are rather general. The sensing molecules detect pH changes rather than molecular CO2. One or a few titratable amino acid residues in these proteins are usually involved. Protonation of these residues may lead to a change in protein conformation that is coupled to a change in channel activity. Depending on the location of the protonation sites, a membrane protein can detect extra- and/or intracellular pH.

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.

Kir channels in the CNS: emerging new roles and implications for neurological diseases

Inwardly rectifying potassium (Kir) channels have long been regarded as transmembrane proteins that regulate the membrane potential of neurons and that are responsible for [K(+)] siphoning in glial cells. The subunit diversity within the Kir channel family is growing rapidly and this is reflected in the multitude of roles that Kir channels play in the central nervous system (CNS). Kir channels are known to control cell differentiation, modify CNS hormone secretion, modulate neurotransmitter release in the nigrostriatal system, may act as hypoxia-sensors and regulate cerebral artery dilatation. The increasing availability of genetic mouse models that express inactive Kir channel subunits has opened new insights into their role in developing and adult mammalian tissues and during the course of CNS disorders. New aspects with respect to the role of Kir channels during CNS cell differentiation and neurogenesis are also emerging. Dysfunction of Kir channels in animal models can lead to severe phenotypes ranging from early postnatal death to an increased susceptibility to develop epileptic seizures. In this review, we summarize the in vivo data that demonstrate the role of Kir channels in regulating morphogenetic events, such as the proliferation, differentiation and survival of neurons and glial cells. We describe the way in which the gating of Kir channel subunits plays an important role in polygenic CNS diseases, such as white matter disease, epilepsy and Parkinson's disease.

Breathing: rhythmicity, plasticity, chemosensitivity

Breathing is a vital behavior that is particularly amenable to experimental investigation. We review recent progress on three problems of broad interest. (i) Where and how is respiratory rhythm generated? The preBötzinger Complex is a critical site, whereas pacemaker neurons may not be essential. The possibility that coupled oscillators are involved is considered. (ii) What are the mechanisms that underlie the plasticity necessary for adaptive changes in breathing? Serotonin-dependent long-term facilitation following intermittent hypoxia is an important example of such plasticity, and a model that can account for this adaptive behavior is discussed. (iii) Where and how are the regulated variables CO2 and pH sensed? These sensors are essential if breathing is to be appropriate for metabolism. Neurons with appropriate chemosensitivity are spread throughout the brainstem; their individual properties and collective role are just beginning to be understood.

Maturation of respiratory control in the behaving mammal

Whereas in vitro techniques have contributed greatly to our understanding of detailed neuronal mechanisms of respiratory control, the integrated function of respiratory behavior requires studying conscious, unsedated subjects. Noninvasive approaches, meticulous chronic instrumentation for the recording of multiple respiratory indices, and correlations with brain studies performed after physiological manipulations in vivo can all be employed to get to some understanding of the maturation of respiratory control in the mammal. This article is a selective and critical overview of recent literature on methodologies that can be used in behaving subjects, the relationship of respiration to sleep-wake states, respiratory patterns during normoxia, and on respiratory responsiveness to hypercarbia and hypoxia, all emphasizing processes during development. It is hoped that this review will encourage new investigators interested in the regulation of breathing to resort to experimental approaches that will reveal the mysteries of respiratory behavior in the integrated organism.

Central chemosensitivity of respiration: a brief overview

In this introductory article we make use of the work reviewed in detail by a number of contributors to this Special Issue (Respir. Physiol., 2001) to provide an outline of current approaches to identifying brainstem CO(2)/pH-chemosensitive neurones. The section headings which we have adopted are intended to reflect particular issues rather than experimental techniques, though some of these issues arise out of the choice of preparation and the advantages and limitations which follow from such a choice. We have also considered whether, in spite of the diversity in the kinds of neurones usually considered to be chemosensitive, there are any indications for shared or uniform features. Again, this is based on the material published together in this volume. Finally, and more speculatively, we suggest that the dendritic organization of chemosensitive neurones may play an important role in chemoreception, not simply as a means of sampling the stimulus but also as a way of compartmentalizing the effects of pH in relation to other aspects of a neurone's activity.

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.