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

S-Glutathionylation underscores the modulation of the heteromeric Kir4.1-Kir5.1 channel in oxidative stress

The Kir4.1 channel is expressed in the brainstem, retina and kidney where it acts on K(+) transportation and pH-dependent membrane potential regulation. Its heteromerization with Kir5.1 leads to K(+) currents with distinct properties such as single-channel conductance, rectification, pH sensitivity and phosphorylation modulation. Here we show that Kir5.1 also enables S-glutathionylation to the heteromeric channel. Expressed in HEK cells, an exposure to the oxidant H(2)O(2) or diamide produced concentration-dependent inhibitions of the whole-cell Kir4.1-Kir5.1 currents. In inside-out patches, currents were inhibited strongly by a combination of diamide/GSH or H(2)O(2)/GSH but not by either alone. The currents were also suppressed by GSSG and the thiol oxidants pyridine disulfides (PDSs), suggesting S-glutathionylation. In contrast, none of the exposures had significant effects on the homomeric Kir4.1 channel. Cys158 in the TM2 helix of Kir5.1 was critical for the S-glutathionylation, which was accessible to intracellular but not extracellular oxidants. Site-directed mutagenesis of this residue (C158A or C158T) abolished the Kir4.1-Kir5.1 current modulation by oxidants, and eliminated almost completely the biochemical interaction of Kir5.1 with GSH. In tandem Kir4.1-Kir5.1 constructs, the channel with a single Cys158 was inhibited to the same degree as the wild-type channel, suggesting that one glutathione moiety is sufficient to block the channel. Consistent with the location of Cys158, GSSG inhibited the channel only when the channel was open, indicating that the channel inhibition was state dependent. The finding that the heteromeric Kir4.1-Kir5.1 channel but not the homomeric Kir4.1 is subject to the S-glutathionylation thus suggests a novel Kir4.1-Kir5.1 channel modulation mechanism that is likely to occur in oxidative stress.

Preinspiratory calcium rise in putative pre-Botzinger complex astrocytes

The neural inspiratory activity originates from a ventrolateral medullary region called the pre-Bötzinger complex (preBötC), yet the mechanism underlying respiratory rhythmogenesis is not completely understood. Recently, the role of not only neurons but astrocytes in the central respiratory control has attracted considerable attention. Here we report our discovery that an intracellular calcium rise in a subset of putative astrocytes precedes inspiratory neuronal firing in rhythmically active slices. Functional calcium imaging from hundreds of preBötC cells revealed that a subset of putative astrocytes exhibited rhythmic calcium elevations preceding inspiratory neuronal activity with a time lag of approximately 2 s. These preinspiratory putative astrocytes maintained their rhythmic activities even during the blockade of neuronal activity with tetrodotoxin, whereas the rhythm frequency was lowered and the intercellular phases of these rhythms were decoupled. In addition, optogenetic stimulation of preBötC putative astrocytes induced firing of inspiratory neurons. These findings raise the possibility that astrocytes in the preBötC are actively involved in respiratory rhythm generation in rhythmically active slices.

Regulation of ventral surface CO2/H+-sensitive neurons by purinergic signalling

Central chemoreception is the mechanism by which the brain regulates breathing in response to changes in tissue CO(2)H+. A brainstem region called the retrotrapezoid nucleus (RTN) contains a population of CO2/H+-sensitive neurons that appears to function as an important chemoreceptor. Evidence also indicates that CO2-evoked ATP release from RTN astrocytes modulates activity of CO2/H+-sensitive neurons; however, the extent to which purinergic signalling contributes to chemoreception by RTN neurons is not clear and the mechanism(s) underlying CO2/H+-evoked ATP release is not fully elucidated. The goals of this study are to determine the extent to which ATP contributes to RTN chemoreception both in vivo and in vitro, and whether purinergic drive to chemoreceptors relies on extracellular Ca(2+) or gap junction hemichannels. We also examine the possible contribution of P2Y1 receptors expressed in the RTN to the purinergic drive to breathe.We show that purinergic signalling contributes, in part, to the CO(2)/H+ sensitivity of RTN neurons. In vivo, phrenic nerve recordings of respiratory activity in adult rats show that bilateral injections of pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS, a P2 receptor blocker) decreased the ventilatory response to CO2 by 30%. In vitro, loose-patch recordings from RTN neurons show that P2 receptor blockers decreased responsiveness to both 10% and 15% CO2 also by 30%. In the slice, the contribution of purinergic signalling to RTN chemoreception did not increase with temperature (22–35◦C) and was retained in low extracellular Ca2+ medium. Conversely, the gap junction blockers carbenoxolone and cobalt decreased neuronal CO2/H+ sensitivity by an amount similar to P2 receptor antagonists. Inhibition of the P2Y1 receptor in the RTN had no effect on CO2 responsivness in vitro or in vivo; thus, the identity of P2 receptors underlying the purinergic component of RTN chemoreception remains unknown. These results support the possibility that CO2/H+-evoked ATP release is mediated by a mechanism involving gap junction hemichannels.

Postsynaptic mechanisms of CO(2) responses in parafacial respiratory neurons of newborn rats

The parafacial respiratory group (pFRG) in the rostral ventrolateral medulla of the newborn rat is predominantly composed of pre-inspiratory (Pre-I) neurons and is involved in respiratory rhythm generation. The subgroup located close to the ventral surface (at least partially overlapping the retrotrapezoid nucleus, RTN) expresses the Phox2b transcription factor and responds to hypercapnic stimulation with strong depolarization, which suggests it has a role in central chemoreception. Although a CO(2) response of pFRG/RTN neurons has been confirmed in the presence of tetrodotoxin (TTX), it is unknown whether the depolarization involved in this response is induced by a direct postsynaptic response of pFRG/RTN neurons or by any presynaptic components mediated by Ca(2+)-dependent mechanisms. In this study, we examined the effects of ATP or substance P receptor antagonists on hypercapnic responses of rostral pFRG/RTN neurons. We tested effects of Cd(2+) and low Ca(2+)-high Mg(2+) in the presence of TTX. The experiments were performed in in vitro brainstem–spinal cord preparations from newborn rats in which Pre-I neurons reflect the discharge pattern of the pFRG. We found that ATP receptor and substance P receptor antagonists do not block membrane potential responses to hypercapnic stimulation (2%→8%) of pFRG/RTN neurons in the rostral parafacial region.Moreover, rostral pFRG/RTN neurons were depolarized by hypercapnia under conditions where the contribution of presynaptic components was inhibited in the presence of TTX and Cd(2+) or in a low Ca(2+)-high Mg(2+) solution containing TTX and Cd(2+). All cases (except some cases in a low Ca(2+)-high Mg(2+) solution) of membrane depolarization by hypercapnic stimulation were accompanied with an increase in input resistance. These neurons were predominantly Phox2b immunoreactive. Our findings suggest that the response of pFRG/RTN neurons to hypercapnia is induced by direct action on the postsynaptic membrane via closing of K(+) channels.

The retrotrapezoid nucleus and breathing

The retrotrapezoid nucleus (RTN) is located in the rostral medulla oblongata close to the ventral surface and consists of a bilateral cluster of glutamatergic neurons that are non-aminergic and express homeodomain transcription factor Phox2b throughout life. These neurons respond vigorously to increases in local pCO(2) via cell-autonomous and paracrine (glial) mechanisms and receive additional chemosensory information from the carotid bodies. RTN neurons exclusively innervate the regions of the brainstem that contain the respiratory pattern generator (RPG). Lesion or inhibition of RTN neurons largely attenuates the respiratory chemoreflex of adult rats whereas their activation increases respiratory rate, inspiratory amplitude and active expiration. Phox2b mutations that cause congenital central hypoventilation syndrome in humans prevent the development of RTN neurons in mice. Selective deletion of the RTN Phox2b-VGLUT2 neurons by genetic means in mice eliminates the respiratory chemoreflex in neonates.In short, RTN Phox2b-VGLUT2 neurons are a major nodal point of the CNS network that regulates pCO(2) via breathing and these cells are probable central chemoreceptors.

Measurement of purine release with microelectrode biosensors

Purinergic signalling departs from traditional paradigms of neurotransmission in the variety of release mechanisms and routes of production of extracellular ATP and adenosine. Direct real-time measurements of these purinergic agents have been of great value in understanding the functional roles of this signalling system in a number of diverse contexts. Here, we review the methods for measuring purine release, introduce the concept of microelectrode biosensors for ATP and adenosine and explain how these have been used to provide new mechanistic insight in respiratory chemoreception, synaptic physiology, eye development and purine salvage. We finish by considering the association of purine release with pathological conditions and examine the possibilities that biosensors for purines may one day be a standard part of the clinical diagnostic tool chest.

Breathing without CO(2) chemosensitivity in conditional Phox2b mutants

Breathing is a spontaneous, rhythmic motor behavior critical for maintaining O(2), CO(2), and pH homeostasis. In mammals, it is generated by a neuronal network in the lower brainstem, the respiratory rhythm generator (Feldman et al., 2003). A century-old tenet in respiratory physiology posits that the respiratory chemoreflex, the stimulation of breathing by an increase in partial pressure of CO(2) in the blood, is indispensable for rhythmic breathing. Here we have revisited this postulate with the help of mouse genetics. We have engineered a conditional mouse mutant in which the toxic PHOX2B(27Ala) mutation that causes congenital central hypoventilation syndrome in man is targeted to the retrotrapezoid nucleus, a site essential for central chemosensitivity. The mutants lack a retrotrapezoid nucleus and their breathing is not stimulated by elevated CO(2) at least up to postnatal day 9 and they barely respond as juveniles, but nevertheless survive, breathe normally beyond the first days after birth, and maintain blood PCO(2) within the normal range. Input from peripheral chemoreceptors that sense PO(2) in the blood appears to compensate for the missing CO(2) response since silencing them by high O(2) abolishes rhythmic breathing. CO(2) chemosensitivity partially recovered in adulthood. Hence, during the early life of rodents, the excitatory input normally afforded by elevated CO(2) is dispensable for life-sustaining breathing and maintaining CO(2) homeostasis in the blood.

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.

The disruption of central CO2 chemosensitivity in a mouse model of Rett syndrome

People with Rett syndrome (RTT) have breathing instability in addition to other neuropathological manifestations. The breathing disturbances contribute to the high incidence of unexplained death and abnormal brain development. However, the cellular mechanisms underlying the breathing abnormalities remain unclear. To test the hypothesis that the central CO(2) chemoreception in these people is disrupted, we studied the CO(2) chemosensitivity in a mouse model of RTT. The Mecp2-null mice showed a selective loss of their respiratory response to 1-3% CO(2) (mild hypercapnia), whereas they displayed more regular breathing in response to 6-9% CO(2) (severe hypercapnia). The defect was alleviated with the NE uptake blocker desipramine (10 mg·kg(-1)·day(-1) ip, for 5-7 days). Consistent with the in vivo observations, in vitro studies in brain slices indicated that CO(2) chemosensitivity of locus coeruleus (LC) neurons was impaired in Mecp2-null mice. Two major neuronal pH-sensitive Kir currents that resembled homomeric Kir4.1 and heteromeric Ki4.1/Kir5.1 channels were identified in the LC neurons. The screening of Kir channels with real-time PCR indicated the overexpression of Kir4.1 in the LC region of Mecp2-null mice. In a heterologous expression system, an overexpression of Kir4.1 resulted in a reduction in the pH sensitivity of the heteromeric Kir4.1-Kir5.1 channels. Given that Kir4.1 and Kir5.1 subunits are also expressed in brain stem respiration-related areas, the Kir4.1 overexpression may not allow CO(2) to be detected until hypercapnia becomes severe, leading to periodical hyper- and hypoventilation in Mecp2-null mice and, perhaps, in people with RTT as well.

Impaired respiratory and body temperature control upon acute serotonergic neuron inhibition

Physiological homeostasis is essential for organism survival. Highly responsive neuronal networks are involved, but their constituent neurons are just beginning to be resolved. To query brain serotonergic neurons in homeostasis, we used a neuronal silencing tool, mouse RC::FPDi (based on the synthetic G protein-coupled receptor Di), designed for cell type-specific, ligand-inducible, and reversible suppression of action potential firing. In mice harboring Di-expressing serotonergic neurons, administration of the ligand clozapine-N-oxide (CNO) by systemic injection attenuated the chemoreflex that normally increases respiration in response to tissue carbon dioxide (CO(2)) elevation and acidosis. At the cellular level, CNO suppressed firing rate increases evoked by CO(2) acidosis. Body thermoregulation at room temperature was also disrupted after CNO triggering of Di; core temperatures plummeted, then recovered. This work establishes that serotonergic neurons regulate life-sustaining respiratory and thermoregulatory networks, and demonstrates a noninvasive tool for mapping neuron 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.

ATP: a ubiquitous gliotransmitter integrating neuron-glial networks

Astrocytes are ideally situated to integrate glial and neuronal functions and neurovascular coupling by way of their multiple contacts with neurons, glia and blood vessels. There is a high degree of specialisation of astroglial membranes at the different sites of contact, including the expression of neurotransmitter receptors, ion channels, transporters and gap junctional proteins. An apparently universal property of astrocytes throughout the CNS is their responsiveness to ATP acting via metabotropic P2Y receptors, with a prominent role for the P2Y1 receptor subtype. Activation of astroglial P2Y receptors triggers a rise in intracellular calcium, which is the substrate for astroglial excitability and intercellular communication. In addition, astrocytes have a number of mechanisms for the release of ATP, which can be considered a 'gliotransmitter'. Astrocytes may be the most widespread source of ATP release in the CNS, and astroglial ATP and its metabolite adenosine activate purine receptors on neurons, microglia, oligodendrocytes and blood vessels. There is compelling evidence that astroglial ATP and adenosine regulate neuronal synaptic strength, although the physiological significance of this astrocyte-to-neuron signalling is questioned. A less appreciated aspect of astrocyte signalling is that they also release neurotransmitters onto other glia. Notably, both ATP and adenosine control microglial behaviour and regulate oligodendrocyte differentiation and myelination. P2 receptors also mediate injury responses in all glial cell types, with a prominent role for the P2X7 receptor subtype. In addition, ATP is a potent vasoconstrictor and astrocytes provide a route for coupling blood flow to neuronal activity by way of their synaptic and perivascular connections. Thus, astrocytes are the fulcrum of neuron-glial-vascular networks and purinergic signalling is the primary mechanism by which astrocytes can integrate the functions of these diverse elements.

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.