Purinergic signalling in the medullary mechanisms of respiratory control in the rat: respiratory neurones express the P2X2 receptor subunit

Immediate and sustained increases in the activity of vagal preganglionic neurons during exercise and after exercise training

AIMS

The brain controls the heart by dynamic recruitment and withdrawal of cardiac parasympathetic (vagal) and sympathetic activity. Autonomic control is essential for the development of cardiovascular responses during exercise, however, the patterns of changes in the activity of the two autonomic limbs, and their functional interactions in orchestrating physiological responses during exercise, are not fully understood. The aim of this study was to characterize changes in vagal parasympathetic drive in response to exercise and exercise training by directly recording the electrical activity of vagal preganglionic neurons in experimental animals (rats).

METHODS AND RESULTS

Single unit recordings were made using carbon-fibre microelectrodes from the populations of vagal preganglionic neurons of the nucleus ambiguus (NA) and the dorsal vagal motor nucleus of the brainstem. It was found that (i) vagal preganglionic neurons of the NA and the dorsal vagal motor nucleus are strongly activated during bouts of acute exercise, and (ii) exercise training markedly increases the resting activity of both populations of vagal preganglionic neurons and augments the excitatory responses of NA neurons during exercise.

CONCLUSIONS

These data show that central vagal drive increases during exercise and provide the first direct neurophysiological evidence that exercise training increases vagal tone. The data argue against the notion of exercise-induced central vagal withdrawal during exercise. We propose that robust increases in the activity of vagal preganglionic neurons during bouts of exercise underlie activity-dependent plasticity, leading to higher resting vagal tone that confers multiple health benefits associated with regular exercise.

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.

Neurosteroids as positive and negative allosteric modulators of ligand-gated ion channels: P2X receptor perspective

Neurosteroids are steroids synthesized de novo in the brain from cholesterol in an independent manner from peripheral steroid sources. The term "neuroactive steroid" includes all steroids independent of their origin, and newly synthesized analogs of neurosteroids that modify neuronal activities. In vivo application of neuroactive steroids induces potent anxiolytic, antidepressant, anticonvulsant, sedative, analgesic and amnesic effects, mainly through interaction with the γ-aminobutyric acid type-A receptor (GABA_AR). However, neuroactive steroids also act as positive or negative allosteric regulators on several ligand-gated channels including N-methyl-d-aspartate receptors (NMDARs), nicotinic acetylcholine receptors (nAChRs) and ATP-gated purinergic P2X receptors. Seven different P2X subunits (P2X1-7) can assemble to form homotrimeric or heterotrimeric ion channels permeable for monovalent cations and calcium. Among them, P2X2, P2X4, and P2X7 are the most abundant within the brain and can be regulated by neurosteroids. Transmembrane domains are necessary for neurosteroid binding, however, no generic motif of amino acids can accurately predict the neurosteroid binding site for any of the ligand-gated ion channels including P2X. Here, we will review what is currently known about the modulation of rat and human P2X by neuroactive steroids and the possible structural determinants underlying neurosteroid-induced potentiation and inhibition of the P2X2 and P2X4 receptors.

Geoff Burnstock, purinergic signalling, and chemosensory control of breathing

This article is the authors' contribution to the tribute issue in honour of Geoffrey Burnstock, the founder of this journal and the field of purinergic signalling. We give a brief account of the results of experimental studies which at the beginning received valuable input from Geoff, who both directly and indirectly influenced our research undertaken over the last two decades. Research into the mechanisms controlling breathing identified ATP as the common mediator of the central and peripheral chemosensory transduction. Studies of the sources and mechanisms of chemosensory ATP release in the CNS suggested that this signalling pathway is universally engaged in conditions of increased metabolic demand by brain glial cells - astrocytes. Astrocytes appear to function as versatile CNS metabolic sensors that detect changes in brain tissue pH, CO2, oxygen, and cerebral perfusion pressure. Experimental studies on various aspects of astrocyte biology generated data indicating that the function of these omnipresent glial cells and communication between astrocytes and neurons are governed by purinergic signalling, - first discovered by Geoff Burnstock in the 70's and researched through his entire scientific career.

A common role for astrocytes in rhythmic behaviours?

Astrocytes are a functionally diverse form of glial cell involved in various aspects of nervous system infrastructure, from the metabolic and structural support of neurons to direct neuromodulation of synaptic activity. Investigating how astrocytes behave in functionally related circuits may help us understand whether there is any conserved logic to the role of astrocytes within neuronal networks. Astrocytes are implicated as key neuromodulatory cells within neural circuits that control a number of rhythmic behaviours such as breathing, locomotion and circadian sleep-wake cycles. In this review, we examine the evidence that astrocytes are directly involved in the regulation of the neural circuits underlying six different rhythmic behaviours: locomotion, breathing, chewing, gastrointestinal motility, circadian sleep-wake cycles and oscillatory feeding behaviour. We discuss how astrocytes are integrated into the neuronal networks that regulate these behaviours, and identify the potential gliotransmission signalling mechanisms involved. From reviewing the evidence of astrocytic involvement in a range of rhythmic behaviours, we reveal a heterogenous array of gliotransmission mechanisms, which help to regulate neuronal networks. However, we also observe an intriguing thread of commonality, in the form of purinergic gliotransmission, which is frequently utilised to facilitate feedback inhibition within rhythmic networks to constrain a given behaviour within its operational range.

The retrotrapezoid nucleus and the neuromodulation of breathing

Breathing is regulated by a host of arousal and sleep-wake state-dependent neuromodulators to maintain respiratory homeostasis. Modulators such as acetylcholine, norepinephrine, histamine, serotonin (5-HT), adenosine triphosphate (ATP), substance P, somatostatin, bombesin, orexin, and leptin can serve complementary or off-setting functions depending on the target cell type and signaling mechanisms engaged. Abnormalities in any of these modulatory mechanisms can destabilize breathing, suggesting that modulatory mechanisms are not overly redundant but rather work in concert to maintain stable respiratory output. The present review focuses on the modulation of a specific cluster of neurons located in the ventral medullary surface, named retrotrapezoid nucleus, that are activated by changes in tissue CO₂/H⁺ and regulate several aspects of breathing, including inspiration and active expiration.

Development of a Mouse Reporter Strain for the Purinergic P2X2 Receptor

The ATP-sensitive P2X₂ ionotropic receptor plays a critical role in a number of signal processes including taste and hearing, carotid body detection of hypoxia, the exercise pressor reflex and sensory transduction of mechanical stimuli in the airways and bladder. Elucidation of the role of P2X₂ has been hindered by the lack of selective tools. In particular, detection of P2X₂ using established pharmacological and biochemical techniques yields dramatically different expression patterns, particularly in the peripheral and central nervous systems. Here, we have developed a knock-in P2X₂-cre mouse, which we crossed with a cre-sensitive tdTomato reporter mouse to determine P2X₂ expression. P2X₂ was found in more than 80% of nodose vagal afferent neurons, but not in jugular vagal afferent neurons. Reporter expression correlated in vagal neurons with sensitivity to α,β methylene ATP (αβmATP). P2X₂ was expressed in 75% of petrosal afferents, but only 12% and 4% of dorsal root ganglia (DRG) and trigeminal afferents, respectively. P2X₂ expression was limited to very few cell types systemically. Together with the central terminals of P2X₂-expressing afferents, reporter expression in the CNS was mainly found in brainstem neurons projecting mossy fibers to the cerebellum, with little expression in the hippocampus or cortex. The structure of peripheral terminals of P2X₂-expressing afferents was demonstrated in the tongue (taste buds), carotid body, trachea and esophagus. P2X₂ was observed in hair cells and support cells in the cochlear, but not in spiral afferent neurons. This mouse strain provides a novel approach to the identification and manipulation of P2X₂-expressing cell types.

O2-Dependent Protein Internalization Underlies Astrocytic Sensing of Acute Hypoxia by Restricting Multimodal TRPA1 Channel Responses

Hypoxia sensors are essential for regulating local oxygen (O₂) homeostasis within the body. This is especially pertinent within the CNS, which is particularly vulnerable to O₂ deprivation due to high energetic demand. Here, we reveal hypoxia-monitoring function exerted by astrocytes through an O₂-regulated protein trafficking mechanism within the CNS. Strikingly, cultured mouse astrocytes isolated from the parafacial respiratory group (pFRG) and retrotrapezoid nucleus (RTN) region are capable of rapidly responding to moderate hypoxia via the sensor cation channel transient receptor potential (TRP) A1 but, unlike multimodal sensory neurons, are inert to hyperoxia and other TRPA1 activators (carbon dioxide, electrophiles, and oxidants) in normoxia. Mechanistically, O₂ suppresses TRPA1 channel activity by protein internalization via O₂-dependent proline hydroxylation and subsequent ubiquitination by an E3 ubiquitin ligase, NEDD4-1 (neural precursor cell-expressed developmentally down-regulated protein 4). Hypoxia inhibits this process and instantly accumulates TRPA1 proteins at the plasma membrane, inducing TRPA1-mediated Ca²⁺ influx that triggers ATP release from pFRG/RTN astrocytes, potentiating respiratory center activity. Furthermore, astrocyte-specific Trpa1 disruption in a mouse brainstem-spinal cord preparation impedes the amplitude augmentation of the central autonomic respiratory output during hypoxia. Thus, reversible coupling of the TRPA1 channels with O₂-dependent protein translocation allows astrocytes to act as acute hypoxia sensors in the medullary respiratory center.

Neurokinin-1 receptor activation is sufficient to restore the hypercapnic ventilatory response in the Substance P-deficient naked mole-rat

Naked mole-rats (NMRs) live in large colonies within densely populated underground burrows. Their collective respiration generates significant metabolic carbon dioxide (CO₂) that diffuses slowly out of the burrow network, creating a hypercapnic environment. Currently, the physiological mechanisms that underlie the ability of NMRs to tolerate environmental hypercapnia are largely unknown. To address this, we used whole-body plethysmography and respirometry to elucidate the hypercapnic ventilatory and metabolic responses of awake, freely behaving NMRs to 0%-10% CO₂. We found that NMRs have a blunted hypercapnic ventilatory response (HCVR): ventilation increased only in 10% CO₂. Conversely, metabolism was unaffected by hypercapnia. NMRs are insensitive to cutaneous acid-based pain caused by modified substance P (SP)-mediated peripheral neurotransmission, and SP is also an important neuromodulator of ventilation. Therefore, we re-evaluated physiological responses to hypercapnia in NMRs after an intraperitoneal injection of exogenous substance P (2 mg/kg) or a long-lived isoform of substance P {[pGlu5-MePhe8-MeGly9]SP(5-11), DiMe-C7; 40-400 μg/kg}. We found that both drugs restored hypercapnia sensitivity and unmasked an HCVR in animals breathing 2%-10% CO₂. Taken together, our findings indicate that NMRs are remarkably tolerant of hypercapnic environments and have a blunted HCVR; however, the signaling network architecture required for a "normal" HCVR is retained but endogenously inactive. This muting of chemosensitivity likely suits the ecophysiology of this species, which presumably experiences hypercapnia regularly in their underground niche.

Release of ATP by pre-Bötzinger complex astrocytes contributes to the hypoxic ventilatory response via a Ca2+ -dependent P2Y1 receptor mechanism

KEY POINTS

The ventilatory response to reduced oxygen (hypoxia) is biphasic, comprising an initial increase in ventilation followed by a secondary depression. Our findings indicate that, during hypoxia, astrocytes in the pre-Bötzinger complex (preBötC), a critical site of inspiratory rhythm generation, release a gliotransmitter that acts via P2Y1 receptors to stimulate ventilation and reduce the secondary depression. In vitro analyses reveal that ATP excitation of the preBötC involves P2Y1 receptor-mediated release of Ca2+ from intracellular stores. By identifying a role for gliotransmission and the sites, P2 receptor subtype, and signalling mechanisms via which ATP modulates breathing during hypoxia, these data advance our understanding of the mechanisms underlying the hypoxic ventilatory response and highlight the significance of purinergic signalling and gliotransmission in homeostatic control. Clinically, these findings are relevant to conditions in which hypoxia and respiratory depression are implicated, including apnoea of prematurity, sleep disordered breathing and congestive heart failure.

ABSTRACT

The hypoxic ventilatory response (HVR) is biphasic, consisting of a phase I increase in ventilation followed by a secondary depression (to a steady-state phase II) that can be life-threatening in premature infants who suffer from frequent apnoeas and respiratory depression. ATP released in the ventrolateral medulla oblongata during hypoxia attenuates the secondary depression. We explored a working hypothesis that vesicular release of ATP by astrocytes in the pre-Bötzinger Complex (preBötC) inspiratory rhythm-generating network acts via P2Y1 receptors to mediate this effect. Blockade of vesicular exocytosis in preBötC astrocytes bilaterally (using an adenoviral vector to specifically express tetanus toxin light chain in astrocytes) reduced the HVR in anaesthetized rats, indicating that exocytotic release of a gliotransmitter within the preBötC contributes to the hypoxia-induced increases in ventilation. Unilateral blockade of P2Y1 receptors in the preBötC via local antagonist injection enhanced the secondary respiratory depression, suggesting that a significant component of the phase II increase in ventilation is mediated by ATP acting at P2Y1 receptors. In vitro responses of the preBötC inspiratory network, preBötC inspiratory neurons and cultured preBötC glia to purinergic agents demonstrated that the P2Y1 receptor-mediated increase in fictive inspiratory frequency involves Ca2+ recruitment from intracellular stores leading to increases in intracellular Ca2+ ([Ca2+ ]i ) in inspiratory neurons and glia. These data suggest that ATP is released by preBötC astrocytes during hypoxia and acts via P2Y1 receptors on inspiratory neurons (and/or glia) to evoke Ca2+ release from intracellular stores and an increase in ventilation that counteracts the hypoxic respiratory depression.

Brain metabolic sensing and metabolic signaling at the level of an astrocyte

Astrocytes support neuronal function by providing essential structural and nutritional support, neurotransmitter trafficking and recycling and may also contribute to brain information processing. In this article we review published results and report new data suggesting that astrocytes function as versatile metabolic sensors of central nervous system (CNS) milieu and play an important role in the maintenance of brain metabolic homeostasis. We discuss anatomical and functional features of astrocytes that allow them to detect and respond to changes in the brain parenchymal levels of metabolic substrates (oxygen and glucose), and metabolic waste products (carbon dioxide). We report data suggesting that astrocytes are also sensitive to circulating endocrine signals-hormones like ghrelin, glucagon-like peptide-1 and leptin, that have a major impact on the CNS mechanisms controlling food intake and energy balance. We discuss signaling mechanisms that mediate communication between astrocytes and neurons and consider how these mechanisms are recruited by astrocytes activated in response to various metabolic challenges. We review experimental data suggesting that astrocytes modulate the activities of the respiratory and autonomic neuronal networks that ensure adaptive changes in breathing and sympathetic drive in order to support the physiological and behavioral demands of the organism in ever-changing environmental conditions. Finally, we discuss evidence suggesting that altered astroglial function may contribute to the pathogenesis of disparate neurological, respiratory and cardiovascular disorders such as Rett syndrome and systemic arterial hypertension.

On the existence of a central respiratory oxygen sensor

A commonly held view that dominates both the scientific and educational literature is that in terrestrial mammals the central nervous system lacks a physiological hypoxia sensor capable of triggering increases in lung ventilation in response to decreases in Po₂ of the brain parenchyma. Indeed, a normocapnic hypoxic ventilatory response has never been observed in humans following bilateral resection of the carotid bodies. In contrast, almost complete or partial recovery of the hypoxic ventilatory response after denervation/removal of the peripheral respiratory oxygen chemoreceptors has been demonstrated in many experimental animals when assessed in an awake state. In this essay we review the experimental evidence obtained using in vitro and in vivo animal models, results of human studies, and discuss potential mechanisms underlying the effects of CNS hypoxia on breathing. We consider experimental limitations and discuss potential reasons why the recovery of the hypoxic ventilatory response has not been observed in humans. We review recent experimental evidence suggesting that the lower brain stem contains functional oxygen sensitive elements capable of stimulating respiratory activity independently of peripheral chemoreceptor input.

CO2-Induced ATP-Dependent Release of Acetylcholine on the Ventral Surface of the Medulla Oblongata

Complex mechanisms that detect changes in brainstem parenchymal PCO2/[H+] and trigger adaptive changes in lung ventilation are responsible for central respiratory CO2 chemosensitivity. Previous studies of chemosensory signalling pathways suggest that at the level of the ventral surface of the medulla oblongata (VMS), CO2-induced changes in ventilation are (at least in part) mediated by the release and actions of ATP and/or acetylcholine (ACh). Here we performed simultaneous real-time biosensor recordings of CO2-induced ATP and ACh release from the VMS in vivo and in vitro, to test the hypothesis that central respiratory CO2 chemosensory transduction involves simultaneous recruitment of purinergic and cholinergic signalling pathways. In anaesthetised and artificially ventilated rats, an increase in inspired CO2 triggered ACh release on the VMS with a peak amplitude of ~5 μM. Release of ACh was only detected after the onset of CO2-induced activation of the respiratory activity and was markedly reduced (by ~70%) by ATP receptor blockade. In horizontal slices of the VMS, CO2-induced release of ATP was reliably detected, whereas CO2 or bath application of ATP (100 μM) failed to trigger release of ACh. These results suggest that during hypercapnia locally produced ATP induces or potentiates the release of ACh (likely from the medullary projections of distal groups of cholinergic neurones), which may also contribute to the development and/or maintenance of the ventilatory response to CO2.

Time Domains of the Hypoxic Ventilatory Response and Their Molecular Basis

Ventilatory responses to hypoxia vary widely depending on the pattern and length of hypoxic exposure. Acute, prolonged, or intermittent hypoxic episodes can increase or decrease breathing for seconds to years, both during the hypoxic stimulus, and also after its removal. These myriad effects are the result of a complicated web of molecular interactions that underlie plasticity in the respiratory control reflex circuits and ultimately control the physiology of breathing in hypoxia. Since the time domains of the physiological hypoxic ventilatory response (HVR) were identified, considerable research effort has gone toward elucidating the underlying molecular mechanisms that mediate these varied responses. This research has begun to describe complicated and plastic interactions in the relay circuits between the peripheral chemoreceptors and the ventilatory control circuits within the central nervous system. Intriguingly, many of these molecular pathways seem to share key components between the different time domains, suggesting that varied physiological HVRs are the result of specific modifications to overlapping pathways. This review highlights what has been discovered regarding the cell and molecular level control of the time domains of the HVR, and highlights key areas where further research is required. Understanding the molecular control of ventilation in hypoxia has important implications for basic physiology and is emerging as an important component of several clinical fields. © 2016 American Physiological Society. Compr Physiol 6:1345-1385, 2016.

Purinergic receptor blockade in the retrotrapezoid nucleus attenuates the respiratory chemoreflexes in awake rats

AIM

Recent evidence suggests that adenosine triphosfate (ATP)-mediated purinergic signalling at the level of the rostral ventrolateral medulla contributes to both central and peripheral chemoreceptor control of breathing and blood pressure: neurones in the retrotrapezoid nucleus (RTN) function as central chemoreceptors in part by responding to CO2 -evoked ATP release by activation of yet unknown P2 receptors, and nearby catecholaminergic C1 neurones regulate blood pressure responses to peripheral chemoreceptor activation by a P2Y1 receptor-dependent mechanism. However, potential contributions of purinergic signalling in the RTN to cardiorespiratory function in conscious animals have not been tested.

METHODS

Cardiorespiratory activity of unrestrained awake rats was measured in response to RTN injections of ATP, and during exposure to hypercapnia (7% CO2 ) or hypoxia (8% O2 ) under control conditions and after bilateral RTN injections of P2 receptor blockers (PPADS or MRS2179).

RESULTS

Unilateral injection of ATP into the RTN increased cardiorespiratory output by a P2-receptor-dependent mechanism. We also show that bilateral RTN injections of a non-specific P2 receptor blocker (pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS) reduced the ventilatory response to hypercapnia (7% CO2 ) and hypoxia (8% O2 ) in unanesthetized rats. Conversely, bilateral injections of a specific P2Y1 receptor blocker (MRS2179) into the RTN had no measurable effect on ventilatory responses elicited by hypercapnia or hypoxia.

CONCLUSION

These data exclude P2Y1 receptor involvement in the chemosensory control of breathing at the level of the RTN and show that ATP-mediated purinergic signalling contributes to central and peripheral chemoreflex control of breathing and blood pressure in awake rats.

Neurochemical and electrical modulation of the locus coeruleus: contribution to CO2drive to breathe

The locus coeruleus (LC) is a dorsal pontine region, situated bilaterally on the floor of the fourth ventricle. It is considered to be the major source of noradrenergic innervation in the brain. These neurons are highly sensitive to CO₂/pH, and chemical lesions of LC neurons largely attenuate the hypercapnic ventilatory response in unanesthetized adult rats. Developmental dysfunctions in these neurons are linked to pathological conditions such as Rett and sudden infant death syndromes, which can impair the control of the cardio-respiratory system. LC is densely innervated by fibers that contain glutamate, serotonin, and adenosine triphosphate, and these neurotransmitters strongly affect LC activity, including central chemoreflexes. Aside from neurochemical modulation, LC neurons are also strongly electrically coupled, specifically through gap junctions, which play a role in the CO₂ ventilatory response. This article reviews the available data on the role of chemical and electrical neuromodulation of the LC in the control of ventilation.

P2Y1 receptor-mediated potentiation of inspiratory motor output in neonatal rat in vitro

PreBötzinger complex inspiratory rhythm-generating networks are excited by metabotropic purinergic receptor subtype 1 (P2Y1R) activation. Despite this, and the fact that inspiratory MNs express P2Y1Rs, the role of P2Y1Rs in modulating motor output is not known for any MN pool. We used rhythmically active brainstem-spinal cord and medullary slice preparations from neonatal rats to investigate the effects of P2Y1R signalling on inspiratory output of phrenic and XII MNs that innervate diaphragm and airway muscles, respectively. MRS2365 (P2Y1R agonist, 0.1 mm) potentiated XII inspiratory burst amplitude by 60 ± 9%; 10-fold higher concentrations potentiated C4 burst amplitude by 25 ± 7%. In whole-cell voltage-clamped XII MNs, MRS2365 evoked small inward currents and potentiated spontaneous EPSCs and inspiratory synaptic currents, but these effects were absent in TTX at resting membrane potential. Voltage ramps revealed a persistent inward current (PIC) that was attenuated by: flufenamic acid (FFA), a blocker of the Ca(2+)-dependent non-selective cation current ICAN; high intracellular concentrations of BAPTA, which buffers Ca(2+) increases necessary for activation of ICAN; and 9-phenanthrol, a selective blocker of TRPM4 channels (candidate for ICAN). Real-time PCR analysis of mRNA extracted from XII punches and laser-microdissected XII MNs revealed the transcript for TRPM4. MRS2365 potentiated the PIC and this potentiation was blocked by FFA, which also blocked the MRS2365 potentiation of glutamate currents. These data suggest that XII MNs are more sensitive to P2Y1R modulation than phrenic MNs and that the P2Y1R potentiation of inspiratory output occurs in part via potentiation of TRPM4-mediated ICAN, which amplifies inspiratory inputs.

Cxs and Panx- hemichannels in peripheral and central chemosensing in mammals

Connexins (Cxs) and Pannexins (Panx) form hemichannels at the plasma membrane of animals. Despite their low open probability under physiological conditions, these hemichannels release signaling molecules (i.e., ATP, Glutamate, PGE₂) to the extracellular space, thus subserving several important physiological processes. Oxygen and CO₂ sensing are fundamental to the normal functioning of vertebrate organisms. Fluctuations in blood PO₂, PCO₂ and pH are sensed at the carotid bifurcations of adult mammals by glomus cells of the carotid bodies. Likewise, changes in pH and/or PCO₂ of cerebrospinal fluid are sensed by central chemoreceptors, a group of specialized neurones distributed in the ventrolateral medulla (VLM), raphe nuclei, and some other brainstem areas. After many years of research, the molecular mechanisms involved in chemosensing process are not completely understood. This manuscript will review data regarding relationships between chemosensitive cells and the expression of channels formed by Cxs and Panx, with special emphasis on hemichannels.

Introduction and perspective, historical note

P2 nucleotide receptors were proposed to consist of two subfamilies based on pharmacology in 1985, named P2X and P2Y receptors. Later, this was confirmed following cloning of the receptors for nucleotides and studies of transduction mechanisms in the early 1990s. P2X receptors are ion channels and seven subtypes are recognized that form trimeric homomultimers or heteromultimers. P2X receptors are involved in neuromuscular and synaptic neurotransmission and neuromodulation. They are also expressed on many types of non-neuronal cells to mediate smooth muscle contraction, secretion, and immune modulation. The emphasis in this review will be on the pathophysiology of P2X receptors and therapeutic potential of P2X receptor agonists and antagonists for neurodegenerative and inflammatory disorders, visceral and neuropathic pain, irritable bowel syndrome, diabetes, kidney failure, bladder incontinence and cancer, as well as disorders if the special senses, airways, skin, cardiovascular, and musculoskeletal systems.

Neuromodulation: purinergic signaling in respiratory control

The main functions of the respiratory neural network are to produce a coordinated, efficient, rhythmic motor behavior and maintain homeostatic control over blood oxygen and CO2/pH levels. Purinergic (ATP) signaling features prominently in these homeostatic reflexes. The signaling actions of ATP are produced through its binding to a diversity of ionotropic P2X and metabotropic P2Y receptors. However, its net effect on neuronal and network excitability is determined by the interaction between the three limbs of a complex system comprising the signaling actions of ATP at P2Rs, the distribution of multiple ectonucleotidases that differentially metabolize ATP into ADP, AMP, and adenosine (ADO), and the signaling actions of ATP metabolites, especially ADP at P2YRs and ADO at P1Rs. Understanding the significance of purinergic signaling is further complicated by the fact that neurons, glia, and the vasculature differentially express P2 and P1Rs, and that both neurons and glia release ATP. This article reviews at cellular, synaptic, and network levels, current understanding and emerging concepts about the diverse roles played by this three-part signaling system in: mediating the chemosensitivity of respiratory networks to hypoxia and CO2/pH; modulating the activity of rhythm generating networks and inspiratory motoneurons, and; controlling blood flow through the cerebral vasculature.

ATP in the locus coeruleus as a modulator of cardiorespiratory control in unanaesthetized male rats

Locus coeruleus (LC) noradrenergic neurons are chemosensitive to CO2 and pH in mammals and amphibians and are involved in the CO2-related drive to breathe. Purinergic neuromodulation in the LC is of particular interest because ATP acts as a neuromodulator in brainstem regions involved in cardiovascular and respiratory regulation, such as the LC. ATP acting on LC P2 receptors influences the release of noradrenaline. Thus, the goal of the present study was to investigate the role of LC purinergic neuromodulation of ventilatory and cardiovascular responses in normocapnic and hypercapnic conditions in unanaesthetized male Wistar rats. We assessed the purinergic modulation of cardiorespiratory systems by microinjecting an ATP P2X receptor agonist [α,β-methylene ATP (α,β-meATP), 0.5 or 1 nmol in 40 nl] and two non-selective P2 receptor antagonists [pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), 0.5 or 1 nmol in 40 nl; and suramin, 1 nmol in 40 nl] into the LC. Pulmonary ventilation (measured by plethysmography), mean arterial pressure (MAP) and heart rate (HR) were determined before and after unilateral microinjection (40 nl) of α,β-meATP, PPADS, suramin or 0.9% saline (vehicle) into the LC. These measurements were made during a 60 min exposure to normocapnic conditions or a 30 min exposure to 7% CO2. Subsequently, animals undergoing pharmacological treatment were subjected to a 30 min exposure to normocapnic conditions as a recovery period. In normocapnic conditions, α,β-meATP did not affect any parameter, whereas PPADS decreased respiratory frequency and increased MAP and HR. Suramin increased MAP and HR but did not change ventilation. Moreover, hypercapnic conditions induced an increase in ventilation and a decrease in HR in all groups. In hypercapnic conditions, α,β-meATP increased ventilation but did not change cardiovascular parameters, whereas PPADS increased MAP but did not alter ventilation, and suramin increased both ventilation and MAP. Thus, our data suggest that purinergic signalling, specifically through P2 receptors, in the LC plays an important role in cardiorespiratory control in normocapnic and hypercapnic conditions in unanaesthetized rats.

Purinergic modulation of hypoxic regulation via the rostral ventral lateral medulla in rats

Anatomical studies have demonstrated the existence of purinergic receptors in the rostral ventral lateral medulla (RVLM), a site containing some respiratory-related neurons. However, little is known about the functional role of these receptors in acute hypoxia. In the present study, we found that both the amplitude and frequency of phrenic nerve discharges were increased during hypoxia. Microinjection of adenosine 5'-triphosphate (ATP) (0.2M, 10-70nl) into the RVLM increased the hypoxic respiratory response and showed significant dose-dependency. An identical microinjection protocol of pyridoxalphosphate-6-azophenyl-2',4'-disulfonate (PPADS), a broad-spectrum P2 receptor antagonist, into the RVLM markedly attenuated the respiratory effects evoked by hypoxic ventilation. Immunohistochemical analysis showed that the P2X(2) receptor was present in the postsynaptic membrane of the RVLM neuronal cell bodies and levels of this receptor were significantly increased after acute hypoxic challenge. These results suggest that RVLM purinergic P2 receptors may contribute to respiratory control by regulating the acute hypoxic ventilatory response.

Purinergic signaling in the airways

Evidence for a significant role and impact of purinergic signaling in normal and diseased airways is now beyond dispute. The present review intends to provide the current state of knowledge of the involvement of purinergic pathways in the upper and lower airways and lungs, thereby differentiating the involvement of different tissues, such as the epithelial lining, immune cells, airway smooth muscle, vasculature, peripheral and central innervation, and neuroendocrine system. In addition to the vast number of well illustrated functions for purinergic signaling in the healthy respiratory tract, increasing data pointing to enhanced levels of ATP and/or adenosine in airway secretions of patients with airway damage and respiratory diseases corroborates the emerging view that purines act as clinically important mediators resulting in either proinflammatory or protective responses. Purinergic signaling has been implicated in lung injury and in the pathogenesis of a wide range of respiratory disorders and diseases, including asthma, chronic obstructive pulmonary disease, inflammation, cystic fibrosis, lung cancer, and pulmonary hypertension. These ostensibly enigmatic actions are based on widely different mechanisms, which are influenced by the cellular microenvironment, but especially the subtypes of purine receptors involved and the activity of distinct members of the ectonucleotidase family, the latter being potential protein targets for therapeutic implementation.

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.

Purinergic modulation of preBötzinger complex inspiratory rhythm in rodents: the interaction between ATP and adenosine

ATP signalling in the CNS is mediated by a three-part system comprising the actions of ATP (and ADP) at P2 receptors (P2Rs), adenosine (ADO) at P1 receptors (P1Rs), and ectonucleotidases that degrade ATP into ADO. ATP excites preBötzinger complex (preBötC) inspiratory rhythm-generating networks where its release attenuates the hypoxic depression of breathing. Its metabolite, ADO, inhibits breathing through unknown mechanisms that may involve the preBötC. Our objective is to understand the dynamics of this signalling system and its influence on preBötC networks. We show that the preBötC of mouse and rat is sensitive to P2Y(1) purinoceptor (P2Y(1)R) activation, responding with a >2-fold increase in frequency. Remarkably, the mouse preBötC is insensitive to ATP. Only after block of A(1) ADORs is the ATP-evoked, P2Y(1)R-mediated frequency increase observed. This demonstrates that ATP is rapidly degraded to ADO, which activates inhibitory A(1)Rs, counteracting the P2Y(1)R-mediated excitation. ADO sensitivity of mouse preBötC was confirmed by a frequency decrease that was absent in rat. Differential ectonucleotidase activities are likely to contribute to the negligible ATP sensitivity of mouse preBötC. Real-time PCR analysis of ectonucleotidase isoforms in preBötC punches revealed TNAP (degrades ATP to ADO) or ENTPDase2 (favours production of excitatory ADP) as the primary constituent in mouse and rat, respectively. These data further establish the sensitivity of this vital network to P2Y(1)R-mediated excitation, emphasizing that individual components of the three-part signalling system dramatically alter network responses to ATP. Data also suggest therapeutic potential may derive from methods that alter the ATP-ADO balance to favour the excitatory actions of ATP.

Astrocytes as brain interoceptors

Astrocytes form a vascular-neuronal interface and provide CNS neural networks with essential structural and metabolic support. They embrace all penetrating arterioles and capillaries, enwrap multiple neuronal somata, thousands of individual synapses, and upon activation release gliotransmitters (ATP, glutamate and D-serine) capable of modulating neuronal activity. The aim of this brief report is to review recent data implicating astrocytes in the brain mechanisms responsible for the detection of different sensory modalities and transmitting sensory information to the relevant neural networks controlling vital behaviours. The concept of astrocytes as brain interoceptors is strongly supported by our recent data obtained from studies of the central nervous mechanisms underlying the chemosensory control of breathing. At the level of the medulla oblongata, astrocytes indeed act as functional central respiratory chemoreceptors, sensing changes in the arterial blood and brain levels of /pH and then imparting these changes on the activity of the respiratory network to induce adaptive changes in lung ventilation.

P2X receptors in health and disease

Seven P2X receptor subunits have been cloned which form functional homo- and heterotrimers. These are cation-selective channels, equally permeable to Na(+) and K(+) and with significant Ca(2+) permeability. The three-dimensional structure of the P2X receptor is described. The channel pore is formed by the α-helical transmembrane spanning region 2 of each subunit. When ATP binds to a P2X receptor, the pore opens within milliseconds, allowing the cations to flow. P2X receptors are expressed on both central and peripheral neurons, where they are involved in neuromuscular and synaptic neurotransmission and neuromodulation. They are also expressed in most types of nonneuronal cells and mediate a wide range of actions, such as contraction of smooth muscle, secretion, and immunomodulation. Changes in the expression of P2X receptors have been characterized in many pathological conditions of the cardiovascular, gastrointestinal, respiratory, and urinogenital systems and in the brain and special senses. The therapeutic potential of P2X receptor agonists and antagonists is currently being investigated in a range of disorders, including chronic neuropathic and inflammatory pain, depression, cystic fibrosis, dry eye, irritable bowel syndrome, interstitial cystitis, dysfunctional urinary bladder, and cancer.

Purinergic mechanisms of lateral parabrachial nucleus facilitate sodium depletion-induced NaCl intake

Purinergic receptors are present in the lateral parabrachial nucleus (LPBN), a pontine structure involved in the control of sodium intake. In the present study, we investigated the effects of α,β-methyleneadenosine 5'-triphosphate (α,β-methylene ATP, selective P2X purinergic agonist) alone or combined with pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, P2X purinergic antagonist) or suramin (non-selective P2 purinergic antagonist) injected into the LPBN on sodium depletion-induced 1.8% NaCl intake. Male Holtzman rats with stainless steel cannulas implanted into the LPBN were used. Sodium depletion was induced by treating rats with the diuretic furosemide (20mg/kg of body weight) followed by 24h of sodium-deficient diet. Bilateral injections of α,β-methylene ATP (2.0 and 4.0nmol/0.2μl) into the LPBN increased sodium depletion-induced 1.8% NaCl intake (25.3±0.8 and 26.5±0.9ml/120min, respectively, vs. saline: 15.2±1.3ml/120min). PPADS (4nmol/0.2μl) alone into the LPBN did not change 1.8% NaCl intake, however, pretreatment with PPADS into the LPBN abolished the effects of α,β-methylene ATP on 1.8% NaCl intake (16.9±0.9ml/120min). Suramin (2.0nmol/0.2μl) alone into the LPBN reduced sodium depletion-induced 1.8% NaCl intake (5.7±1.9ml/120min, vs. saline: 15.5±1.1ml/120min), without changing 2% sucrose intake or 24h water deprivation-induced water intake. The combination of suramin and α,β-methylene ATP into the LPBN produced no change of 1.8% NaCl intake (15.2±1.2ml/120min). The results suggest that purinergic P2 receptor activation in the LPBN facilitates NaCl intake, probably by restraining LPBN mechanisms that inhibit sodium intake.

Ventilatory and carotid body chemoreceptor responses to purinergic P2X receptor antagonists in newborn rats

Adenosine triphosphate, acting through purinergic P2X receptors, has been shown to stimulate ventilation and increase carotid body chemoreceptor activity in adult rats. However, its role during postnatal development of the ventilatory response to hypoxia is yet unknown. Using whole body plethysmography, we measured ventilation in normoxia and in moderate hypoxia (12% fraction of inspired O₂, 20 min) before and after intraperitoneal injection of suramin (P2X₂ and P2X₃ receptor antagonist, 40 mg/kg) in 4-, 7-, 12-, and 21-day-old rats. Suramin reduced baseline breathing (∼20%) and the response to hypoxia (∼30%) in all rats, with a relatively constant effect across ages. We then tested the effect of the specific P2X₃ antagonist, A-317491 (150 mg/kg), in rats aged 4, 7, and 21 days. As with suramin, A-317491 reduced baseline ventilation (∼55%) and the hypoxic response (∼40%) at all ages studied. Single-unit carotid body chemoreceptor activity was recorded in vitro in 4-, 7-, and 21-day-old rats. Suramin (100 μM) and A-317491 (10 μM) significantly depressed the sinus nerve chemosensory discharge rate (∼80%) in normoxia (Po₂ ∼150 Torr) and hypoxia (Po₂ ∼60 Torr), and this decrease was constant across ages. We conclude that, in newborn rats, P2X purinergic receptors are involved in the regulation of breathing under basal and hypoxic condition, and P2X₃-containing receptors play a major role in carotid body function. However, these effects are not age dependent within the age range studied.

ATP, glia and central respiratory control

An increase in PCO(2) in the arterial blood triggers immediate release of ATP from the ventral chemosensory site(s) on the surface of the medulla oblongata. Systemic hypoxia in anesthetized rats was also associated with increased ATP release on the ventral medullary surface. During both hypoxia and hypercapnia, ATP and possibly other gliotransmitters released in the ventral medulla seemed to enhance cardiorespiratory responses to these stressors, and some of this ATP was proposed to be derived from astrocytes. Astrocytes also play a vital role controlling local blood flow. Astrocytes are activated by neurotransmitter release - especially glutamate and ATP. The astrocytic activation is manifest as a rise in intracellular Ca(2+) that is closely coupled to the metabolic activity of neurons in the active area. The activation of astrocytes spreads as a wave from astrocyte to astrocyte and causes release of ATP, adenosine, and other gliotransmitters that may alter neuronal function in the region of astrocytic activation. In addition, ATP, adenosine and other vasoactive substances, when released at the endfeet of astrocytes, interact with vascular receptors that may either dilate or constrict the vessels in the region closely adjacent to the site of neuronal activity. Thus, astrocytes seem to integrate neuronal metabolic needs by responding to the level of neuronal activity to regulate local blood flow and cardiorespiratory responses to hypoxia and hypercapnia to match substrate need (oxygen and glucose) with substrate availability and with the removal of CO(2). In so doing, astrocytes assume a larger role in information processing and in the regulation of neuronal activity and homeostasis of the entire organism than has been ascribed to them in the past.

Connexin hemichannel-mediated CO2-dependent release of ATP in the medulla oblongata contributes to central respiratory chemosensitivity

Arterial PCO2, a major determinant of breathing, is detected by chemosensors located in the brainstem. These are important for maintaining physiological levels of PCO2 in the blood and brain, yet the mechanisms by which the brain senses CO(2) remain controversial. As ATP release at the ventral surface of the brainstem has been causally linked to the adaptive changes in ventilation in response to hypercapnia, we have studied the mechanisms of CO(2)-dependent ATP release in slices containing the ventral surface of the medulla oblongata. We found that CO(2)-dependent ATP release occurs in the absence of extracellular acidification and correlates directly with the level of PCO2. ATP release is independent of extracellular Ca(2+) and may occur via the opening of a gap junction hemichannel. As agents that act on connexin channels block this release, but compounds selective for pannexin-1 have no effect, we conclude that a connexin hemichannel is involved in CO(2)-dependent ATP release. We have used molecular, genetic and immunocytochemical techniques to demonstrate that in the medulla oblongata connexin 26 (Cx26) is preferentially expressed near the ventral surface. The leptomeninges, subpial astrocytes and astrocytes ensheathing penetrating blood vessels at the ventral surface of the medulla can be loaded with dye in a CO(2)-dependent manner, suggesting that gating of a hemichannel is involved in ATP release. This distribution of CO(2)-dependent dye loading closely mirrors that of Cx26 expression and colocalizes to glial fibrillary acidic protein (GFAP)-positive cells. In vivo, blockers with selectivity for Cx26 reduce hypercapnia-evoked ATP release and the consequent adaptive enhancement of breathing. We therefore propose that Cx26-mediated release of ATP in response to changes in PCO2 is an important mechanism contributing to central respiratory chemosensitivity.

ATP overflow in skeletal muscle 1A arterioles

The purpose of this study was to investigate the sources of ATP in the 1A arteriole, and to investigate age-related changes in ATP overflow. Arterioles (1A) from the red portion of the gastrocnemius muscle were isolated, cannulated and pressurized in a microvessel chamber with field stimulation electrodes. ATP overflow was determined using probes specific for ATP and null probes that were constructed similar to the ATP probes, but did not contain the enzyme coating. ATP concentrations were determined using a normal curve (0.78 to 25 micromol l(-1) ATP). ATP overflow occurred in two phases. Phase one began in the first 20 s following stimulation and phase two started 35 s after field stimulation. Tetrodotoxin, a potent neurotoxin that blocks action potential generation in nerves, abolished both phases of ATP overflow. alpha1-Receptor blockade resulted in a small decrease in ATP overflow in phase two, but endothelial removal resulted in an increase in ATP overflow. ATP overflow was lowest in 6-month-old rats and highest in 12- and 2-month-old rats (P<0.05). ATP overflow measured via biosensors was of neural origin with a small contribution from the vascular smooth muscle. The endothelium seems to play an important role in attenuating ATP overflow in 1A arterioles.

Glia contribute to the purinergic modulation of inspiratory rhythm-generating networks

Glia modulate neuronal activity by releasing transmitters in a process called gliotransmission. The role of this process in controlling the activity of neuronal networks underlying motor behavior is unknown. ATP features prominently in gliotransmission; it also contributes to the homeostatic ventilatory response evoked by low oxygen through mechanisms that likely include excitation of preBötzinger complex (preBötC) neural networks, brainstem centers critical for breathing. We therefore inhibited glial function in rhythmically active inspiratory networks in vitro to determine whether glia contribute to preBötC ATP sensitivity. Glial toxins markedly reduced preBötC responses to ATP, but not other modulators. Furthermore, since preBötC glia responded to ATP with increased intracellular Ca(2+) and glutamate release, we conclude that glia contribute to the ATP sensitivity of preBötC networks, and possibly the hypoxic ventilatory response. Data reveal a role for glia in signal processing within brainstem motor networks that may be relevant to similar networks throughout the neuraxis.

Current ideas on central chemoreception by neurons and glial cells in the retrotrapezoid nucleus

Central chemoreception is the mechanism by which CO2/pH-sensitive neurons (i.e., chemoreceptors) regulate breathing in response to changes in tissue pH. A region of the brain stem called the retrotrapezoid nucleus (RTN) is thought to be an important site of chemoreception (23), and recent evidence suggests that RTN chemoreception involves two interrelated mechanisms: H+-mediated activation of pH-sensitive neurons (38) and purinergic signaling (19), possibly from pH-sensitive glial cells. A third, potentially important, aspect of RTN chemoreception is the regulation of blood flow, which is an important determinate of tissue pH and consequently chemoreceptor activity. It is well established in vivo that changes in cerebral blood flow can profoundly affect the chemoreflex (2); e.g., limiting blood flow by vasoconstriction acidifies tissue pH and increases the ventilatory response to CO2, whereas vasodilation can wash out metabolically produced CO2 from tissue to increase tissue pH and decrease the stimulus at chemoreceptors. In this review, we will summarize the defining characteristics of pH-sensitive neurons and discuss potential contributions of pH-sensitive glial cells as both a source of purinergic drive to pH-sensitive neurons and a modulator of vasculature tone.

Tripartite purinergic modulation of central respiratory networks during perinatal development: the influence of ATP, ectonucleotidases, and ATP metabolites

ATP released during hypoxia from the ventrolateral medulla activates purinergic receptors (P2Rs) to attenuate the secondary hypoxic depression of breathing by a mechanism that likely involves a P2Y(1)R-mediated excitation of preBötzinger complex (preBötC) inspiratory rhythm-generating networks. In this study, we used rhythmically active in vitro preparations from embryonic and postnatal rats and ATP microinjection into the rostral ventral respiratory group (rVRG)/preBötC to reveal that these networks are sensitive to ATP when rhythm emerges at embryonic day 17 (E17). The peak frequency elicited by ATP at E19 and postnatally was the same ( approximately 45 bursts/min), but relative sensitivity was threefold greater at E19, reflecting a lower baseline frequency (5.6 +/- 0.9 vs 19.0 +/- 1.3 bursts/min). Combining microinjection techniques with ATP biosensors revealed that ATP concentration in the rVRG/preBötC falls rapidly as a result of active processes and closely correlates with inspiratory frequency. A phosphate assay established that preBötC-containing tissue punches degrade ATP at rates that increase perinatally. Thus, the agonist profile [ATP/ADP/adenosine (ADO)] produced after ATP release in the rVRG/preBötC will change perinatally. Electrophysiology further established that the ATP metabolite ADP is excitatory and that, in fetal but not postnatal animals, ADO at A(1) receptors exerts a tonic depressive action on rhythm, whereas A(1) antagonists extend the excitatory action of ATP on inspiratory rhythm. These data demonstrate that ATP is a potent excitatory modulator of the rVRG/preBötC inspiratory network from the time it becomes active and that ATP actions are determined by a dynamic interaction between the actions of ATP at P2 receptors, ectonucleotidases that degrade ATP, and ATP metabolites on P2Y and P1 receptors.

Purines and sensory nerves

P2X and P2Y nucleotide receptors are described on sensory neurons and their peripheral and central terminals in dorsal root, nodose, trigeminal, petrosal, retinal and enteric ganglia. Peripheral terminals are activated by ATP released from local cells by mechanical deformation, hypoxia or various local agents in the carotid body, lung, gut, bladder, inner ear, eye, nasal organ, taste buds, skin, muscle and joints mediating reflex responses and nociception. Purinergic receptors on fibres in the dorsal spinal cord and brain stem are involved in reflex control of visceral and cardiovascular activity, as well as relaying nociceptive impulses to pain centres. Purinergic mechanisms are enhanced in inflammatory conditions and may be involved in migraine, pain, diseases of the special senses, bladder and gut, and the possibility that they are also implicated in arthritis, respiratory disorders and some central nervous system disorders is discussed. Finally, the development and evolution of purinergic sensory mechanisms are considered.

Antidipsogenic effects of central adenosine-5'-triphosphate

Besides other physiological functions, adenosine-5'-triphosphate (ATP) is also a neurotransmitter that acts on purinergic receptors. In spite of the presence of purinergic receptors in forebrain areas involved with fluid-electrolyte balance, the effect of ATP on water intake has not been investigated. Therefore, we studied the effects of intracerebroventricular (icv) injections of ATP (100, 200 and 300 nmol/microL) alone or combined with DPCPX or PPADS (P1 and P2 purinergic antagonists, respectively, 25 nmol/microL) on water intake induced by water deprivation. In addition, the effect of icv ATP was also tested on water intake induced by intragastric load of 12% NaCl (2 mL/rat), acute treatment with the diuretic/natriuretic furosemide (20 mg/kg), icv angiotensin II (50 ng/microL) or icv carbachol (a cholinergic agonist, 4 nmol/microL), on sodium depletion-induced 1.8% NaCl intake, and on food intake induced by food deprivation. Male Holtzman rats (280-320 g, N = 7-11) had cannulas implanted into the lateral ventricle. Icv ATP (300 nmol/microL) reduced water intake induced by water deprivation (13.1 +/- 1.9 vs saline: 19.0 +/- 1.4 mL/2 h; P < 0.05), an effect blocked by pre-treatment with PPADS, but not DPCPX. Icv ATP also reduced water intake induced by NaCl intragastric load (5.6 +/- 0.9 vs saline: 10.3 +/- 1.4 mL/2 h; P < 0.05), acute furosemide treatment (0.5 +/- 0.2 vs saline: 2.3 +/- 0.6 mL/15 min; P < 0.05), and icv angiotensin II (2.2 +/- 0.8 vs saline: 10.4 +/- 2.0 mL/2 h; P < 0.05), without changing icv carbachol-induced water intake, sodium depletion-induced 1.8% NaCl intake and food deprivation-induced food intake. These data suggest that central ATP, acting on purinergic P2 receptors, reduces water intake induced by intracellular and extracellular dehydration.

Chemosensory pathways in the brainstem controlling cardiorespiratory activity

Cardiorespiratory activity is controlled by a network of neurons located within the lower brainstem. The basic rhythm of breathing is generated by neuronal circuits within the medullary pre-Bötzinger complex, modulated by pontine and other inputs from cell groups within the medulla oblongata and then transmitted to bulbospinal pre-motor neurons that relay the respiratory pattern to cranial and spinal motor neurons controlling respiratory muscles. Cardiovascular sympathetic and vagal activities have characteristic discharges that are patterned by respiratory activity. This patterning ensures ventilation-perfusion matching for optimal respiratory gas exchange within the lungs. Peripheral arterial chemoreceptors and central respiratory chemoreceptors are crucial for the maintenance of cardiorespiratory homeostasis. Inputs from these receptors ensure adaptive changes in the respiratory and cardiovascular motor outputs in various environmental and physiological conditions. Many of the connections in the reflex pathway that mediates the peripheral arterial chemoreceptor input have been established. The nucleus tractus solitarii, the ventral respiratory network, pre-sympathetic circuitry and vagal pre-ganglionic neurons at the level of the medulla oblongata are integral components, although supramedullary structures also play a role in patterning autonomic outflows according to behavioural requirements. These medullary structures mediate cardiorespiratory reflexes that are initiated by the carotid and aortic bodies in response to acute changes in PO(2), PCO(2) and pH in the arterial blood. The level of arterial PCO(2) is the primary factor in determining respiratory drive and although there is a significant role of the arterial chemoreceptors, the principal sensor is located either at or in close proximity to the ventral surface of the medulla. The cellular and molecular mechanisms of central chemosensitivity as well as the neural basis for the integration of central and peripheral chemosensory inputs within the medulla remain challenging issues, but ones that have some emerging answers.

ATP in central respiratory control: a three-part signaling system

The landmark demonstrations in 2005 that ATP released centrally during hypoxia and hypercapnia contributes to the respective ventilatory responses validated a decade-old hypothesis and ignited interest in the potential significance of P2 receptor signaling in central respiratory control. Our objective in this review is to provide a non-specialist overview of ATP signaling from the perspective that it is a three-part system where the net effects are determined by an interaction between the signaling actions of ATP and adenosine at P2 and P1 receptors, respectively, and a family of enzymes (ectonucleotidases) that breakdown ATP into adenosine. We review the rationale for the original interest in P2 signaling in respiratory control, the evolution of this hypothesis, and the mechanisms by which ATP might affect respiratory behaviour. The potential significance of P2 receptor, P1 receptor and ectonucleotidase diversity for the different compartments of the respiratory control system is also considered. We conclude with a look to future questions and technical challenges.

Purinergic signalling in autonomic control

Intercellular purinergic signalling, which utilizes ATP as a transmitter, is fundamental for the operation of the autonomic nervous system. ATP is released together with 'classical' transmitters from sympathetic and parasympathetic nerves supplying various peripheral targets, modulates neurotransmission in autonomic ganglia, has an important role in local enteric neural control and coordination of intestinal secretion and motility, and acts as a common mediator for several distinct sensory modalities. Recently, the role of ATP-mediated signalling in the central nervous control of autonomic function has been addressed. Emerging data demonstrate that in the brain ATP is involved in the operation of several key cardiorespiratory reflexes, contributes to central processing of viscerosensory information, mediates central CO(2) chemosensory transduction and triggers adaptive changes in breathing, and modulates the activities of the brainstem vagal preganglionic, presympathetic and respiratory neural networks.

Release of ATP and glutamate in the nucleus tractus solitarii mediate pulmonary stretch receptor (Breuer-Hering) reflex pathway

The Breuer-Hering inflation reflex is initiated by activation of the slowly adapting pulmonary stretch receptor afferents (SARs), which monosynaptically activate second-order relay neurones in the dorsal medullary nucleus of the solitary tract (NTS). Here we demonstrate that during lung inflation SARs release both ATP and glutamate from their central terminals to activate these NTS neurones. In anaesthetized and artificially ventilated rats, ATP- and glutamate-selective microelectrode biosensors placed in the NTS detected rhythmic release of both transmitters phase-locked to lung inflation. This release of ATP and glutamate was independent of the centrally generated respiratory rhythm and could be reversibly abolished during the blockade of the afferent transmission in the vagus nerve by topical application of local anaesthetic. Microionophoretic application of ATP increased the activity of all tested NTS second-order relay neurones which receive monosynaptic inputs from the SARs. Unilateral microinjection of ATP into the NTS site where pulmonary stretch receptor afferents terminate produced central apnoea, mimicking the effect of lung inflation. Application of P2 and glutamate receptor antagonists (pyridoxal-5'-phosphate-6-azophenyl-2',4'-disulphonic acid, suramin and kynurenic acid) significantly decreased baseline lung inflation-induced firing of the second-order relay neurones. These data demonstrate that ATP and glutamate are released in the NTS from the central terminals of the lung stretch receptor afferents, activate the second-order relay neurones and hence mediate the key respiratory reflex - the Breuer-Hering inflation reflex.

ATP-gated P2X cation-channels

P2X receptors are ATP-gated cation channels with important roles in diverse pathophysiological processes. Substantial progress has been made in the last few years with the discovery of both subunit selective antagonists and modulators. The purpose of this brief review is to summarize the advances in the pharmacology of P2X receptors, with key properties presented in an easy to access format. Ligand-gated ion channels consist of three families in mammals; the ionotropic glutamate receptors, the Cys-loop receptors (for GABA, ACh, glycine and serotonin) and the P2X receptors for ATP. The first two of these are considered in articles accompanying this Special Issue. Here we consider the pharmacological properties of P2X receptors. We do not present a detailed discussion of P2X receptor physiological roles or structure-function studies. Moreover, the pharmacological basis for discriminating between the main subtypes of P2X receptor and their nomenclature has been published by the Nomenclature Committee of the International Union of Pharmacology (NC-IUPHAR) P2X Receptor Subcommittee, and so these aspects are not revisited here. Instead in this brief article we seek to present a summary of the pharmacology of recombinant homomeric and heteromeric P2X receptors, with particular emphasis on new antagonists. In this article we have tried to present as much information as possible in two tables in the hope this will be useful as a day-to-day resource, and also because an excellent and detailed review has recently been published.

ATP sensitivity of preBötzinger complex neurones in neonatal rat in vitro: mechanism underlying a P2 receptor-mediated increase in inspiratory frequency

P2 receptor (R) signalling plays an important role in the central ventilatory response to hypoxia. The frequency increase that results from activation of P2Y(1)Rs in the preBötzinger complex (preBötC; putative site of inspiratory rhythm generation) may contribute, but neither the cellular nor ionic mechanism(s) underlying these effects are known. We applied whole-cell recording to rhythmically-active medullary slices from neonatal rat to define, in preBötC neurones, the candidate cellular and ionic mechanisms through which ATP influences rhythm, and tested the hypothesis that putative rhythmogenic preBötC neurones are uniquely sensitive to ATP. ATP (1 mm) evoked inward currents in all non-respiratory neurones and the majority of respiratory neurons, which included inspiratory, expiratory and putative rhythmogenic inspiratory neurones identified by sensitivity to substance P (1 microM) and DAMGO (50 microM) or by voltage-dependent pacemaker-like activity. ATP current densities were similar in all classes of preBötC respiratory neurone. Reversal potentials and input resistance changes for ATP currents in respiratory neurones suggested they resulted from either inhibition of a K(+) channel or activation of a mixed cationic conductance. The P2YR agonist 2MeSADP (1 mm) evoked only the latter type of current in inspiratory and pacemaker-like neurones. In summary, putative rhythmogenic preBötC neurones were sensitive to ATP. However, this sensitivity was not unique; ATP evoked similar currents in all types of preBötC respiratory neurone. The P2Y(1)R-mediated frequency increase is therefore more likely to reflect activation of a mixed cationic conductance in multiple types of preBötC neurone than excitation of one, highly sensitive group.

Involvement of L-glutamate and ATP in the neurotransmission of the sympathoexcitatory component of the chemoreflex in the commissural nucleus tractus solitarii of awake rats and in the working heart-brainstem preparation

Peripheral chemoreflex activation with potassium cyanide (KCN) in awake rats or in the working heart-brainstem preparation (WHBP) produces: (a) a sympathoexcitatory/pressor response; (b) bradycardia; and (c) an increase in the frequency of breathing. Our main aim was to evaluate neurotransmitters involved in mediating the sympathoexcitatory component of the chemoreflex within the nucleus tractus solitarii (NTS). In previous studies in conscious rats, the reflex bradycardia, but not the pressor response, was reduced by antagonism of either ionotropic glutamate or purinergic P2 receptors within the NTS. In the present study we evaluated a possible dual role of both P2 and NMDA receptors in the NTS for processing the sympathoexcitatory component (pressor response) of the chemoreflex in awake rats as well as in the WHBP. Simultaneous blockade of ionotropic glutamate receptors and P2 receptors by sequential microinjections of kynurenic acid (KYN, 2 nmol (50 nl)(-1)) and pyridoxalphosphate-6-azophenyl-2',4'-disulphonate (PPADS, 0.25 nmol (50 nl)(-1)) into the commissural NTS in awake rats produced a significant reduction in both the pressor (+38+/-3 versus +8+/-3 mmHg) and bradycardic responses (-172+/-18 versus -16+/-13 beats min(-1); n=13), but no significant changes in the tachypnoea measured using plethysmography (270+/-30 versus 240+/-21 cycles min(-1), n=7) following chemoreflex activation in awake rats. Control microinjections of saline produced no significant changes in these reflex responses. In WHBP, microinjection of KYN (2 nmol (20 nl)(-1)) and PPADS (1.6 nmol (20 nl)(-1)) into the commissural NTS attenuated significantly both the increase in thoracic sympathetic activity (+52+/-2% versus +17+/-1%) and the bradycardic response (-151+/-17 versus -21+/-3 beats min(-1)) but produced no significant changes in the increase of the frequency of phrenic nerve discharge (+0.24+/-0.02 versus +0.20+/-0.02 Hz). The data indicate that combined microinjections of PPADS and KYN into the commissural NTS in both awake rats and the WHBP are required to produce a significant reduction in the sympathoexcitatory response (pressor response) to peripheral chemoreflex activation. We conclude that glutamatergic and purinergic mechanisms are part of the complex neurotransmission system of the sympathoexcitatory component of the chemoreflex at the level of the commissural NTS.