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.

Central control of autonomic function.

In this review, current understanding of the control of autonomic function is outlined and its development over the last 50 years highlighted. Using the control of the cardiovascular system as the primary tool, the importance of the patterning of autonomic outflows is shown to be crucial in both homeostasis and behaviour. Technical advances have made it possible to obtain a clearer idea of how the central nervous system evolves patterns of autonomic discharge that optimise autonomic changes to support motor and behavioural responses. The specific roles of sympathetic and parasympathetic preganglionic neurones and premotor neurones are surveyed and the importance of their roles in integrating afferent inputs that result from peripheral sensory inputs and drive from multiple levels of the neuraxis is outlined. The autonomic control of the viscera, including the urinogenital organs and other organs is discussed briefly. The current ability to use animal models to monitor and modulate autonomic neural discharge and simultaneously co-relate this with end-organ activity is shown to have translational potential. There is every prospect that these studies will lead to the identification of new therapies for pathophysiological conditions.

Cardiac vagal preganglionic neurones: An update.

The autonomic nervous system controls the heart by dynamic recruitment and withdrawal of cardiac parasympathetic and sympathetic activities. These activities are generated by groups of sympathoexcitatory and vagal preganglionic neurones residing in a close proximity to each other within well-defined structures of the brainstem. This short essay provides a general overview and an update on the latest developments in our understanding of the central nervous origins and functional significance of cardiac vagal tone. Significant experimental evidence suggests that distinct groups of cardiac vagal preganglionic neurones with different patterns of activity control nodal tissue (controlling the heart rate and atrioventricular conductance) and the ventricular myocardium (modulating its contractility and excitability).

Cardioprotection evoked by remote ischaemic preconditioning is critically dependent on the activity of vagal pre-ganglionic neurones.

AIMS: Innate mechanisms of inter-organ protection underlie the phenomenon of remote ischaemic preconditioning (RPc) in which episode(s) of ischaemia and reperfusion in tissues remote from the heart reduce myocardial ischaemia/reperfusion injury. The uncertainty surrounding the mechanism(s) underlying RPc centres on whether humoral factor(s) produced during ischaemia/reperfusion of remote tissue and released into the systemic circulation mediate RPc, or whether a neural signal is required. While these two hypotheses may not be incompatible, one approach to clarify the potential role of a neural pathway requires targeted disruption or activation of discrete central nervous substrate(s). METHODS AND RESULTS: Using a rat model of myocardial ischaemia/reperfusion injury in combination with viral gene transfer, pharmaco-, and optogenetics, we tested the hypothesis that RPc cardioprotection depends on the activity of vagal pre-ganglionic neurones and consequently an intact parasympathetic drive. For cell-specific silencing or activation, neurones of the brainstem dorsal motor nucleus of the vagus nerve (DVMN) were targeted using viral vectors to express a Drosophila allatostatin receptor (AlstR) or light-sensitive fast channelrhodopsin variant (ChIEF), respectively. RPc cardioprotection, elicited by ischaemia/reperfusion of the limbs, was abolished when DVMN neurones transduced to express AlstR were silenced by selective ligand allatostatin or in conditions of systemic muscarinic receptor blockade with atropine. In the absence of remote ischaemia/reperfusion, optogenetic activation of DVMN neurones transduced to express ChIEF reduced infarct size, mimicking the effect of RPc. CONCLUSION: These data indicate a crucial dependence of RPc cardioprotection against ischaemia/reperfusion injury upon the activity of a distinct population of vagal pre-ganglionic neurones.

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.

Astrocytes control breathing through pH-dependent release of ATP.

Astrocytes provide structural and metabolic support for neuronal networks, but direct evidence demonstrating their active role in complex behaviors is limited. Central respiratory chemosensitivity is an essential mechanism that, via regulation of breathing, maintains constant levels of blood and brain pH and partial pressure of CO2. We found that astrocytes of the brainstem chemoreceptor areas are highly chemosensitive. They responded to physiological decreases in pH with vigorous elevations in intracellular Ca2+ and release of adenosine triphosphate (ATP). ATP propagated astrocytic Ca2+ excitation, activated chemoreceptor neurons, and induced adaptive increases in breathing. Mimicking pH-evoked Ca2+ responses by means of optogenetic stimulation of astrocytes expressing channelrhodopsin-2 activated chemoreceptor neurons via an ATP-dependent mechanism and triggered robust respiratory responses in vivo. This demonstrates a potentially crucial role for brain glial cells in mediating a fundamental physiological reflex.

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.

To breathe or not to breathe? That is the question.

Our understanding of the role of the brain in respiratory rhythm generation and regulation began the early nineteenth century. Over the next 150 years the neuronal groups in the medulla oblongata and pons that were involved in eupnoea and in gasping were identified by techniques involving the lesioning of areas of the lower brainstem, several transections across the brainstem and focal electrical stimulation. An incomplete picture emerged that stressed the importance of the ventral medulla. Subsequent electrophysiological studies in in vivo, in situ and in vitro preparations have revealed the importance of restricted groups of neurones in this area, within the Bötzinger and pre-Bötzinger nuclei, that are the essential kernel for rhythm generation. The outputs to the spinal motoneurones responsible for the patterning of inspiratory and expiratory discharge are shaped by inputs from these neurones and others within the respiratory complex that determine the activity of respiratory bulbospinal neurones. It is clear that the developmental stage of the preparation is often critical for the pattern of respiratory activity that is generated and that these patterns have important physiological consequences. The models that are currently considered to explain rhythmogenesis are critically evaluated. The respiratory network is subject to regulation from peripheral and central chemoreceptors, amongst other afferent inputs, which act to ensure respiratory homeostasis. The roles of peripheral chemoreceptors as primarily O(2) sensors are considered, and the evolution of ideas surrounding their roles is described. New insights into the transduction mechanisms of chemoreception in the carotid body and chemosensitive areas of the ventral medullary surface, specifically in monitoring CO(2) levels, are reviewed. As new experimental tools, both genetic and cellular, are emerging, it can be expected that the detailed network architecture and synaptic interactions that pattern respiratory activity in relation to behavioural activity will be revealed over the next years.

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.

Beneficial effect of the central nervous system beta-adrenoceptor blockade on the failing heart.

Heart failure patients are routinely given beta-adrenoceptor antagonists (beta-blockers), although the mechanism(s) underlying their beneficial effects is not fully resolved. It is not entirely clear how long-term application of negative inotropic compounds improves cardiac performance, slows remodeling processes, and decreases mortality. All beta-blockers, which produce a beneficial effect in heart failure, have in common a high degree of lipophilicity and, therefore, have the ability to cross the blood-brain barrier. Here, we show that blockade of beta-adrenoceptors directly in the brain (chronic intracerebroventricular administration of metoprolol) attenuates the progression of left ventricular remodeling in a rat model of myocardial infarction-induced heart failure. These results provide the first direct evidence that the action of certain beta-blockers in the brain could contribute to their beneficial effect on the failing heart.

Release of ATP in the central nervous system during systemic inflammation: real-time measurement in the hypothalamus of conscious rabbits.

Receptors for extracellular ATP (both ionotropic and metabotropic) are widely expressed in the CNS both in neurones and glia. ATP can modulate neuronal activity in many parts of the brain and contributes to the central nervous control of several physiological functions. Here we show that during the systemic inflammatory response the extracellular concentrations of ATP increase in the anterior hypothalamus and this has a profound effect on the development of the thermoregulatory febrile response. In conscious rabbits we measured ATP release in real time with novel amperometric biosensors and monitored a marked increase in the concentration of ATP (4.0 +/- 0.7 microm) in the anterior hypothalamus in response to intravenous injection of bacterial endotoxin - lipopolysaccharide (LPS). No ATP release was observed in the posterior hypothalamus. The release of ATP coincided with the development of the initial phase of the febrile response, starting 18 +/- 2 min and reaching its peak 45 +/- 2 min after LPS injection. Application of the ATP receptor antagonists pyridoxal-5'-phosphate-6-azophenyl-2',4'-disulphonic acid, Brilliant Blue G or periodate oxidized ATP dialdehyde to the site of ATP release in the anterior hypothalamus markedly augmented and prolonged the febrile response. These data indicate that during the development of the systemic inflammation, ATP is released in the anterior hypothalamus to limit the magnitude and duration of fever. This release may also have a profound effect on the hypothalamic control of other physiological functions in which ATP and related purines have been implicated to play modulatory roles, such as food intake, hormone secretion, cardiovascular activity and sleep.

ATP is a mediator of chemosensory transduction in the central nervous system.

Extracellular signalling by the purine nucleotide ATP has long been associated with sensory function. In the periphery, ATP mediates nociception, mechanosensitivity, thermal sensitivity and O2 chemosensitivity. These processes share a common mechanism that involves the release of ATP to excite afferent fibres via activation of ionotropic P2X and/or metabotropic P2Y receptors. Chemosensors located in the brainstem are crucial for the maintenance of physiological levels of blood gases through the regulation of breathing. Here we show that an increase in pCO2 in the arterial blood triggers the immediate release of ATP from three chemosensitive regions located on the ventral surface of the medulla oblongata. Blockade of ATP receptors at these sites diminishes the chemosensory control of breathing, suggesting that ATP release constitutes a key step in central chemosensory transduction. These new data suggest that ATP, a phylogenetically ancient, unique and simple molecule, has been widely used in the evolution of afferent systems to mediate distinct forms of sensory transduction not only in the periphery but also within the central nervous system.

P2 receptor blockade attenuates fever and cytokine responses induced by lipopolysaccharide in rats.

Adenosine 5'-triphosphate (ATP) has been shown to induce release of cytokines implicated in fever, including interleukin(IL)-1beta, IL-6, and tumour necrosis factor-alpha (TNF-alpha). The role of ATP-mediated purinergic signalling in fever and cytokine release during systemic inflammation was investigated by studying the effects of P2 receptor antagonists suramin, pyridoxal-5'-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), and Brilliant Blue G (BBG) on changes in body temperature and the increases in plasma levels of IL-1beta, IL-6, and TNFalpha induced by bacterial lipopolysaccharide (LPS) in rats. LPS (Escherichia coli; 50 microg kg(-1))-induced febrile response was attenuated by suramin (25 mg kg(-1) and 100 mg kg(-1)), PPADS (25 mg kg(-1)), and a more selective P2X(7) receptor antagonist BBG (100 mg kg(-1)) injected intraperitoneally before the induction of fever. The increase in plasma concentrations of IL-1beta and IL-6, measured 1 h after LPS treatment, was reduced by PPADS (25 mg kg(-1)) and BBG (100 mg kg(-1)). LPS-induced increase in plasma TNF-alpha concentration was also markedly attenuated by BBG (100 mg kg(-1)), but not by PPADS (25 mg kg(-1)). These data indicate that purinergic signalling plays an important role in the mechanisms responsible for the LPS-induced febrile response and increases in the levels of circulating cytokines. We suggest that ATP acting via P2X(7) receptors induces release of pyrogenic cytokines to mediate fever during systemic inflammation.

Release of ATP in the ventral medulla during hypoxia in rats: role in hypoxic ventilatory response.

P2X2 receptor subunits of the ATP-gated ion channels are expressed by physiologically identified respiratory neurons in the ventral respiratory column, implicating ATP in the control of respiratory activity. We now show that, during hypoxia, release of ATP in the ventrolateral medulla (VLM) plays an important role in the hypoxic ventilatory response in rats. By measuring ATP release in real time at the ventral surface of the medulla with novel amperometric biosensors, we found that hypoxia (10% O2; 5 min) induced a marked increase in the concentration of ATP (approximately 3 microm). This ATP release occurred after the initiation of enhanced respiratory activity but coincided with the later hypoxia-induced slowing of the respiratory rhythm. ATP was also released at the ventral surface of the medulla during hypoxia in peripherally chemodenervated animals (vagi, aortic, and carotid sinus nerve sectioned). By using horizontal slices of the rat medulla, we found that, during hypoxia, ATP is produced throughout the VLM in the locations corresponding to the ventral respiratory column. Blockade of ATP receptors in the VLM (microinjection of P2 receptor antagonist pyridoxal-5'-phosphate-6-azophenyl-2',4'-disulphonic acid; 100 mum) augmented the hypoxia-induced secondary slowing of the respiratory rhythm. Our findings suggest that ATP released within the ventral respiratory column is involved in maintenance of the respiratory activity in conditions when hypoxia-induced slowing of respiration occurs. These data illustrate a new functional role for ATP-mediated purinergic signaling in the medullary mechanisms controlling respiratory activity.

Effect of stimulation of anterior hypothalamic area on urinary bladder function of the anesthetized rat.

The hypothalamus is a key area for the integration of the autonomic features of affective behavior. Hypothalamic defence area (HDA) stimulation evokes major cardiorespiratory changes as well as modifications of general autonomic activity both in the anesthetized and conscious animal. Micturition is due to an increase in pelvic parasympathetic activity and, in the cat, the anterior hypothalamus has been implicated in urinary bladder control with the demonstration of a dorsolateral vesicoconstrictor pathway and a ventromedial inhibitory pathway. In this study we have investigated the effect of electrical and chemical stimulation of the HDA on bladder pressure and contractions in rat. Female rats (n = 15) were anesthetized, paralyzed and ventilated artificially. Arterial blood pressure, heart rate, urinary bladder pressure and pelvic nerve activity were recorded. HDA was electrically (1 ms, 100 Hz, 5-10 s train at intensities up to 150 micro A) and chemically (sodium glutamate, 50 nl, 2mM) stimulated. For statistical analysis the t-test was used, data were expressed as mean +/- SEM. Values of t were taken as significant when p < 0.05.HDA stimulation at 100-150 micro A evoked changes of both mean blood pressure (mBP) and bladder pressure (BlP). However, stimulation at < 30 micro A allowed a distinction within HDA of two different regions, at the same antero-posterior and lateral level, but separated 100-150 micro m in depth, which evoked differential effects on blood pressure and urinary bladder pressure. Results show that low intensity stimulation at ventral sites evoked a significant increase of mBP (from 102 +/- 5.9 to 127 +/- 8.6 mmHg, n = 10, p < 0.0001) with little changes of BlP (from 12 +/- 2.2 to 16 +/- 2.9 cmH(2)O, n = 10, p < 0.0005), whilst at more dorsal sites significant increases of BlP were elicited (from 12 +/- 8.3 to 38 +/- 4.6 cmH(2)O,n = 10, p < 0.0001) with only a small rise of mBP (from 102 +/- 6.2 to 111 +/- 9.8 mmHg, n = 10, p < 0.005). Glutamate injections at dorsal sites evoked a rise of BlP (from 11 +/- 2.2 to 30 +/- 3.0 cmH(2)O (n = 5; p < 0.0001) with small changes in BP, whilst at ventral sites (n = 4) glutamate microinjections evoked changes in BP but not of BlP. In conclusion stimulation at different sites within HDA can elicit separate changes in BP and BlP.

ATP is a key mediator of central and peripheral chemosensory transduction.

Recent evidence suggests that ATP is a mediator of central (within the ventral surface of the medulla) and peripheral (within the carotid body) chemosensory transduction. This short review discusses the data obtained in experiments in vivo and in vitro supporting this hypothesis. P2 receptors for ATP are expressed within the ventrolateral medulla as well as by the peripheral chemosensory afferent neurones. Blockade of P2 receptors in the ventrolateral medulla attenuates the CO2-induced increase in respiration while blockade of purinergic signalling impairs carotid body function and diminishes the ventilatory response to hypoxia. Furthermore, ATP is released from the ventral surface of the medulla during hypercapnia and from the carotid body during hypoxia. Finally, exogenous ATP applied on the ventral surface of the medulla evokes rapid increase in phrenic nerve activity, while ATP applied to the carotid body evokes marked excitation of the carotid sinus nerve afferents. We suggest that in the ventrolateral medulla ATP is produced following CO2/H(+)-induced activation of central chemosensory elements (neuronal and/or glial) and acts within the respiratory network to produce physiologically relevant changes in ventilation. In the carotid body, ATP contributes in a significant manner to the transmission of the sensitivity of the carotid body to changes in arterial PO2 and may be considered as a key transmitter released by chemoreceptor cells to activate endings of the sinus nerve afferent fibres.

Fever in systemic inflammation: roles of purines.

Extracellular purine nucleotide and nucleoside signalling molecules, such as ATP and adenosine, acting through specific receptors (P2 and P1, respectively) play significant roles in the mechanisms underlying the febrile response. A variety of P2 and P1 receptor subunits have been identified in the hypothalamus, the area of the brain that orchestrates the febrile response. Importantly, both ATP and adenosine have been shown to modulate release and/or action of cytokines that are implicated in fever, as well as to be involved in the central mechanisms of cardiovascular and respiratory control. Our data indicate that at the level of the anterior hypothalamus extracellular ATP is involved in the control of the development of fever. A population of warm-sensitive neurones in the anterior hypothalamus is likely to be the site of action of ATP on body temperature. ATP-induced cytokine release does not appear to play a significant role in the hypothalamic mechanisms leading to the development of the febrile response. However, the blockade of fever by P2 receptor antagonists given systemically suggests that ATP-mediated signalling may play a role in the release of pyrogenic cytokines in the periphery. At the level of the anterior hypothalamus adenosine appears to be released tonically, and acts to maintain body temperature under afebrile conditions. There is also evidence that adenosine-mediated signalling may play a role in the hypothalamic mechanisms controlling the degree of body temperature increase during fever. Our investigations have identified possible mechanisms by which purines modulate the febrile response. The actions of purines on body temperature during fever are most likely "site specific" (brain vs. periphery), may or may not involve their effect on cytokine release and/or action, and are likely to involve P2 and P1 receptors of different subtypes. Further extensive studies are needed to elucidate these mechanisms in greater detail and may lead to the development of new approaches for modifying febrile, cytokine and acute-phase responses to infection.

Pivotal role of nucleotide P2X2 receptor subunit of the ATP-gated ion channel mediating ventilatory responses to hypoxia.

In mammals, the ventilatory response to decreased oxygen tension in the arterial blood is initiated by excitation of specialized O2-sensitive chemoreceptor cells in the carotid body that release neurotransmitters to activate endings of the sinus nerve afferent fibers. We investigated the role of ATP acting via ionotropic P2X receptors in the carotid body function and ventilatory response to hypoxia in mice. Mice deficient in P2X2 receptor subunit showed a markedly attenuated ventilatory response to hypoxia, whereas the response to hypoxia in P2X3-deficient mice was comparable with that seen in wild-type controls. P2X2 and P2X3 receptor subunit deficiency did not affect the ventilatory responses to hypercapnia. P2X2 subunit deficiency resulted in a dramatic reduction in the responses of the carotid sinus nerve to hypoxia in the in vitro carotid body-sinus nerve preparation. ATP and its stable analog alpha,beta-methyleneATP both evoked rapid excitation of sinus nerve afferents, and the P2 receptor antagonist PPADS (pyridoxal-5'-phosphate-6-azophenyl-2',4'-disulphonic acid) (100 microm) blocked hypoxia-induced increase in sinus nerve discharge. Immunoreactivities for P2X2 and P2X3 subunits were both detected on afferent terminals surrounding clusters of glomus cells in the wild-type animals but were absent in mice deficient in P2X2 and P2X3 receptor subunits. These observations provide the first definitive evidence that, in the carotid body, ATP is a key transmitter released by chemoreceptor cells to activate endings of the sinus nerve afferent fibers. We conclude that P2X receptors containing the P2X2 subunit play a pivotal role in carotid body function and in mediating ventilatory responses to hypoxia.