Neuromodulation: purinergic signaling in respiratory control

Purinergic signaling mediates neuroglial interactions to modulate sighs

Sighs prevent the collapse of alveoli in the lungs, initiate arousal under hypoxic conditions, and are an expression of sadness and relief. Sighs are periodically superimposed on normal breaths, known as eupnea. Implicated in the generation of these rhythmic behaviors is the preBötzinger complex (preBötC). Our experimental evidence suggests that purinergic signaling is necessary to generate spontaneous and hypoxia-induced sighs in a mouse model. Our results demonstrate that driving calcium increases in astrocytes through pharmacological methods robustly increases sigh, but not eupnea, frequency. Calcium imaging of preBötC slices corroborates this finding with an increase in astrocytic calcium upon application of sigh modulators, increasing intracellular calcium through g-protein signaling. Moreover, photo-activation of preBötC astrocytes is sufficient to elicit sigh activity, and this response is blocked with purinergic antagonists. We conclude that sighs are modulated through neuron-glia coupling in the preBötC network, where the distinct modulatory responses of neurons and glia allow for both rhythms to be independently regulated.

Brain H+ /CO2 sensing and control by glial cells

Maintenance of constant brain pH is critically important to support the activity of individual neurons, effective communication within the neuronal circuits, and, thus, efficient processing of information by the brain. This review article focuses on how glial cells detect and respond to changes in brain tissue pH and concentration of CO2 , and then trigger systemic and local adaptive mechanisms that ensure a stable milieu for the operation of brain circuits. We give a detailed account of the cellular and molecular mechanisms underlying sensitivity of glial cells to H+ and CO2 and discuss the role of glial chemosensitivity and signaling in operation of three key mechanisms that work in concert to keep the brain pH constant. We discuss evidence suggesting that astrocytes and marginal glial cells of the brainstem are critically important for central respiratory CO2 chemoreception-a fundamental physiological mechanism that regulates breathing in accord with changes in blood and brain pH and partial pressure of CO2 in order to maintain systemic pH homeostasis. We review evidence suggesting that astrocytes are also responsible for the maintenance of local brain tissue extracellular pH in conditions of variable acid loads associated with changes in the neuronal activity and metabolism, and discuss potential role of these glial cells in mediating the effects of CO2 on cerebral vasculature.

Regulation of blood vessels by ATP in the ventral medullary surface in a rat model of Parkinson's disease

Parkinson's disease (PD) patients often experience impairment of autonomic and respiratory functions. These include conditions such as orthostatic hypotension and sleep apnea, which are highly correlated with dysfunctional central chemoreception. Blood flow is a fundamental determinant of tissue CO₂/H⁺, yet the extent to which blood flow regulation within chemoreceptor regions contributes to respiratory behavior during neurological disease remains unknown. Here, we tested the hypothesis that 6-hydroxydopamine injection to inducing a known model of PD results in dysfunctional vascular homeostasis, biochemical dysregulation, and glial morphology of the ventral medullary surface (VMS). We show that hypercapnia (FiCO₂ = 10%) induced elevated VMS pial vessel constriction in PD animals through a P2-receptor dependent mechanism. Similarly, we found a greater CO₂-induced vascular constriction after ARL67156 (an ectonucleotidase inhibitor) in control and PD-induced animals. In addition, we also report that weighted gene correlational network analysis of the proteomic data showed a protein expression module differentially represented between both groups. This module showed that gene ontology enrichment for components of the ATP machinery were reduced in our PD-model compared to control animals. Altogether, our data indicate that dysfunction in purinergic signaling, potentially through altered ATP bioavailability in the VMS region, may compromise the RTN neuroglial vascular unit in a PD animal model.

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.

The lamprey respiratory network: Some evolutionary aspects

Breathing is a complex behaviour that involves rhythm generating networks. In this review, we examine the main characteristics of respiratory rhythm generation in vertebrates and, in particular, we describe the main results of our studies on the role of neural mechanisms involved in the neuromodulation of the lamprey respiration. The lamprey respiratory rhythm generator is located in the paratrigeminal respiratory group (pTRG) and shows similarities with the mammalian preBötzinger complex. In fact, within the pTRG a major role is played by glutamate, but also GABA and glycine display important contributions. In addition, neuromodulatory influences are exerted by opioids, substance P, acetylcholine and serotonin. Both structures respond to exogenous ATP with a biphasic response and astrocytes there located strongly contribute to the modulation of the respiratory pattern. The results emphasize that some important characteristics of the respiratory rhythm generating network are, to a great extent, maintained throughout evolution.

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.

Molecular characterization of frog vocal neurons using constellation pharmacology

Identification and characterization of neuronal cell classes in motor circuits are essential for understanding the neural basis of behavior. It is a challenging task, especially in a non-genetic-model organism, to identify cell-specific expression of functional macromolecules. Here, we performed constellation pharmacology, calcium imaging of dissociated neurons to pharmacologically identify functional receptors expressed by vocal neurons in adult male and female African clawed frogs, Xenopus laevis. Previously we identified a population of vocal neurons called fast trill neurons (FTNs) in the amphibian parabrachial nucleus (PB) that express N-methyl-d-aspartate (NMDA) receptors and GABA and/or glycine receptors. Using constellation pharmacology, we identified four cell classes of putative fast trill neurons (pFTNs, responsive to both NMDA and GABA/glycine applications). We discovered that some pFTNs responded to the application of substance P (SP), acetylcholine (ACh), or both. Electrophysiological recordings obtained from FTNs using an ex vivo preparation verified that SP and/or ACh depolarize FTNs. Bilateral injection of ACh, SP, or their antagonists into PBs showed that ACh receptors are not sufficient but necessary for vocal production, and SP receptors play a role in shaping the morphology of vocalizations. Additionally, we discovered that the PB of adult female X. laevis also contains all the subclasses of neurons at a similar frequency as in males, despite their sexually distinct vocalizations. These results reveal novel neuromodulators that regulate X. laevis vocal production and demonstrate the power of constellation pharmacology in identifying the neuronal subtypes marked by functional expression of cell-specific receptors in non-genetic-model organisms.NEW & NOTEWORTHY Molecular profiles of neurons are critical for understanding the neuronal functions, but their identification is challenging especially in non-genetic-model organisms. Here, we characterized the functional expression of membrane macromolecules in vocal neurons of African clawed frogs, Xenopus laevis, using a technique called constellation pharmacology. We discovered that receptors for acetylcholine and/or substance P are expressed by some classes of vocal neurons, and their activation plays a role in the production of normal vocalizations.

The Purinome and the preBötzinger Complex - A Ménage of Unexplored Mechanisms That May Modulate/Shape the Hypoxic Ventilatory Response

Exploration of purinergic signaling in brainstem homeostatic control processes is challenging the traditional view that the biphasic hypoxic ventilatory response, which comprises a rapid initial increase in breathing followed by a slower secondary depression, reflects the interaction between peripheral chemoreceptor-mediated excitation and central inhibition. While controversial, accumulating evidence supports that in addition to peripheral excitation, interactions between central excitatory and inhibitory purinergic mechanisms shape this key homeostatic reflex. The objective of this review is to present our working model of how purinergic signaling modulates the glutamatergic inspiratory synapse in the preBötzinger Complex (key site of inspiratory rhythm generation) to shape the hypoxic ventilatory response. It is based on the perspective that has emerged from decades of analysis of glutamatergic synapses in the hippocampus, where the actions of extracellular ATP are determined by a complex signaling system, the purinome. The purinome involves not only the actions of ATP and adenosine at P2 and P1 receptors, respectively, but diverse families of enzymes and transporters that collectively determine the rate of ATP degradation, adenosine accumulation and adenosine clearance. We summarize current knowledge of the roles played by these different purinergic elements in the hypoxic ventilatory response, often drawing on examples from other brain regions, and look ahead to many unanswered questions and remaining challenges.

Adenosine Signaling through A1 Receptors Inhibits Chemosensitive Neurons in the Retrotrapezoid Nucleus

A subset of neurons in the retrotrapezoid nucleus (RTN) function as respiratory chemoreceptors by regulating depth and frequency of breathing in response to changes in tissue CO₂/H⁺. The activity of chemosensitive RTN neurons is also subject to modulation by CO₂/H⁺-dependent purinergic signaling. However, mechanisms contributing to purinergic regulation of RTN chemoreceptors are not entirely clear. Recent evidence suggests adenosine inhibits RTN chemoreception in vivo by activation of A1 receptors. The goal of this study was to characterize effects of adenosine on chemosensitive RTN neurons and identify intrinsic and synaptic mechanisms underlying this response. Cell-attached recordings from RTN chemoreceptors in slices from rat or wild-type mouse pups (mixed sex) show that exposure to adenosine (1 µM) inhibits chemoreceptor activity by an A1 receptor-dependent mechanism. However, exposure to a selective A1 receptor antagonist (8-cyclopentyl-1,3-dipropylxanthine, DPCPX; 30 nM) alone did not potentiate CO₂/H⁺-stimulated activity, suggesting activation of A1 receptors does not limit chemoreceptor activity under these reduced conditions. Whole-cell voltage-clamp from chemosensitive RTN neurons shows that exposure to adenosine activated an inward rectifying K⁺ conductance, and at the network level, adenosine preferentially decreased frequency of EPSCs but not IPSCs. These results show that adenosine activation of A1 receptors inhibits chemosensitive RTN neurons by direct activation of a G-protein-regulated inward-rectifier K⁺ (GIRK)-like conductance, and presynaptically, by suppression of excitatory synaptic input to chemoreceptors.

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.

Advances in cellular and integrative control of oxygen homeostasis within the central nervous system

Mammals must continuously regulate the levels of O2 and CO2 , which is particularly important for the brain. Failure to maintain adequate O2 /CO2 homeostasis has been associated with numerous disorders including sleep apnoea, Rett syndrome and sudden infant death syndrome. But, O2 /CO2 homeostasis poses major regulatory challenges, even in the healthy brain. Neuronal activities change in a differentiated, spatially and temporally complex manner, which is reflected in equally complex changes in O2 demand. This raises important questions: is oxygen sensing an emergent property, locally generated within all active neuronal networks, and/or the property of specialized O2 -sensitive CNS regions? Increasing evidence suggests that the regulation of the brain's redox state involves properties that are intrinsic to many networks, but that specialized regions in the brainstem orchestrate the integrated control of respiratory and cardiovascular functions. Although the levels of O2 in arterial blood and the CNS are very different, neuro-glial interactions and purinergic signalling are critical for both peripheral and CNS chemosensation. Indeed, the specificity of neuroglial interactions seems to determine the differential responses to O2 , CO2 and the changes in pH.

Inhibition of the hypercapnic ventilatory response by adenosine in the retrotrapezoid nucleus in awake rats

The brain regulates breathing in response to changes in tissue CO₂/H⁺ via a process called central chemoreception. Neurons and astrocytes in the retrotrapezoid nucleus (RTN) function as respiratory chemoreceptors. The role of astrocytes in this process appears to involve CO₂/H⁺-dependent release of ATP to enhance activity of chemosensitive RTN neurons. Considering that in most brain regions extracellular ATP is rapidly broken down to adenosine by ectonucleotidase activity and since adenosine is a potent neuromodulator, we wondered whether adenosine signaling contributes to RTN chemoreceptor function. To explore this possibility, we pharmacologically manipulated activity of adenosine receptors in the RTN under control conditions and during inhalation of 7-10% CO₂ (hypercapnia). In urethane-anesthetized or unrestrained conscious rats, bilateral injections of adenosine into the RTN blunted the hypercapnia ventilatory response. The inhibitory effect of adenosine on breathing was blunted by prior RTN injection of a broad spectrum adenosine receptor blocker (8-PT) or a selective A1-receptor blocker (DPCPX). Although RTN injections of 8PT, DPCPX or the ectonucleotidase inhibitor ARL67156 did not affected baseline breathing in either anesthetized or awake rats. We did find that RTN application of DPCPX or ARL67156 potentiated the respiratory frequency response to CO₂, suggesting a portion of ATP released in the RTN during high CO₂/H⁺ is converted to adenosine and serves to limit chemoreceptor function. These results identify adenosine as a novel purinergic regulator of RTN chemoreceptor function during hypercapnia.

Respiratory rhythm irregularity after carotid body denervation in rats

Respiratory activity is controlled by inputs from the peripheral and central chemoreceptors. Since overactivity of the carotid bodies, the main peripheral chemoreceptors, is linked to the pathophysiology of disparate metabolic and cardiovascular diseases, carotid body denervation (CBD) has been proposed as a potential treatment. However, long-term effects of CBD on the respiratory rhythm and regularity of breathing remain unknown. Here, we show that five weeks after bilateral CBD in rats, the respiratory rhythm was slower and less regular. Ten weeks after bilateral CBD, the respiratory frequency was not different from the sham-operated group, but the regularity of the respiratory rhythm was still reduced. Increased frequency of randomly occurring apneas is likely to be responsible for the irregular breathing pattern after CBD. These results should be taken into consideration since any treatment that reduces the stability of the respiratory rhythm might exacerbate the cardio-respiratory instability and worsen the cardiovascular outcomes.

ATP and astrocytes play a prominent role in the control of the respiratory pattern generator in the lamprey

KEY POINTS

The paratrigeminal respiratory group (pTRG) is responsible for the respiratory pattern generation in the lamprey. The role of ATP and astrocytes, known to control respiratory activity in mammals, was investigated in the lamprey respiratory network. ATP microinjected into the pTRG induces a biphasic response consisting of marked increases in respiratory frequency mediated by P2X receptors followed by a decrease in the respiratory motor output due to the ATP metabolite adenosine. We provide evidence that astrocytes are involved in the genesis of the normal respiratory pattern, ATP-induced responses and acidification-induced increases of the respiratory activity. The function of astrocytes in rhythmic networks appears to be phylogenetically conserved.

ABSTRACT

The role of ATP and astrocytes in respiratory rhythm modulation has been recently investigated in neonatal rodents. However, no information on the role of ATP and astrocytes within the respiratory network of the lamprey is available, particularly within the paratrigeminal respiratory group (pTRG), the proposed respiratory central pattern generator. To address these issues, the present study was carried out on isolated brainstems of the adult lamprey. Bath application of ATP caused marked increases in respiratory frequency followed by decreases in the respiratory motor output, mediated by the ATP metabolite adenosine at the level of the pTRG. Bath applications and microinjections of agonists and antagonists of purinergic receptors showed that ATP increased respiratory activity through an action on pTRG P2X receptors. To disclose the respiratory role of astrocytes, we used bath application of the gliotoxin aminoadipic acid, which dramatically depressed the respiratory motor output that, however, promptly recovered following glutamine application. Furthermore, the excitatory responses to ATP-γ-S (a non-hydrolysable ATP analogue), but not to substance P, microinjected into the pTRG, were abolished. Finally, we also demonstrated that acidification-induced increases in respiratory activity were ATP-independent, but mediated by the astrocytes' glutamate-glutamine cycle. The results show for the first time that ATP and especially astrocytes strongly contribute to the modulation of the lamprey respiratory pattern. Their role in the modulation or maintenance of rhythmic neuronal activities appears to be phylogenetically conserved.

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.

Purinergic signalling mediates bidirectional crosstalk between chemoreceptor type I and glial-like type II cells of the rat carotid body

KEY POINTS

Carotid body chemoreceptors are organized in clusters containing receptor type I and contiguous glial-like type II cells. While type I cells depolarize and release ATP during chemostimulation, the role of type II cells which express purinergic P2Y2 receptors (P2Y2Rs) and ATP-permeable pannexin-1 (Panx-1) channels, is unclear. Here, we show that in isolated rat chemoreceptor clusters, type I cell depolarization induced by hypoxia, hypercapnia, or high K(+) caused delayed intracellular Ca(2+) elevations (Δ[Ca(2+)]i) in nearby type II cells that were inhibited by the P2Y2R blocker suramin, or by the nucleoside hydrolase apyrase. Likewise, stimulation of P2Y2Rs on type II cells caused a delayed, secondary Δ[Ca(2+)]i in nearby type I cells that was inhibited by blockers of Panx-1 channels, adenosine A2A receptors and 5'-ectonucleotidase. We propose that reciprocal crosstalk between type I and type II cells contributes to sensory processing in the carotid body via purinergic signalling pathways.

ABSTRACT

The mammalian carotid body (CB) is excited by blood-borne stimuli including hypoxia and acid hypercapnia, leading to respiratory and cardiovascular reflex responses. This chemosensory organ consists of innervated clusters of receptor type I cells, ensheathed by processes of adjacent glial-like type II cells. ATP is a major excitatory neurotransmitter released from type I cells and type II cells express purinergic P2Y2 receptors (P2Y2Rs), the activation of which leads to the opening of ATP-permeable, pannexin-1 (Panx-1) channels. While these properties support crosstalk between type I and type II cells during chemotransduction, direct evidence is lacking. To address this, we first exposed isolated rat chemoreceptor clusters to acute hypoxia, isohydric hypercapnia, or the depolarizing stimulus high K(+), and monitored intracellular [Ca(2+)] using Fura-2. As expected, these stimuli induced intracellular [Ca(2+)] elevations (Δ[Ca(2+)]i) in type I cells. Interestingly, however, there was often a delayed, secondary Δ[Ca(2+)]i in nearby type II cells that was reversibly inhibited by the P2Y2R antagonist suramin, or by the nucleoside hydrolase apyrase. By contrast, type II cell stimulation with the P2Y2R agonist uridine-5'-triphosphate (100 μm) often led to a delayed, secondary Δ[Ca(2+)]i response in nearby type I cells that was reversibly inhibited by the Panx-1 blocker carbenoxolone (5 μm). This Δ[Ca(2+)]i response was also strongly inhibited by blockers of either the adenosine A2A receptor (SCH 58261) or of the 5'-ectonucleotidase (AOPCP), suggesting it was due to adenosine arising from breakdown of ATP released through Panx-1 channels. Collectively, these data strongly suggest that purinergic signalling mechanisms mediate crosstalk between CB chemoreceptor and glial cells during chemotransduction.

Purine nucleosides in neuroregeneration and neuroprotection

In the present review, we stress the importance of the purine nucleosides, adenosine and guanosine, in protecting the nervous system, both centrally and peripherally, via activation of their receptors and intracellular signalling mechanisms. A most novel part of the review focus on the mechanisms of neuronal regeneration that are targeted by nucleosides, including a recently identified action of adenosine on axonal growth and microtubule dynamics. Discussion on the role of the purine nucleosides transversally with the most established neurotrophic factors, e.g. brain derived neurotrophic factor (BDNF), glial derived neurotrophic factor (GDNF), is also focused considering the intimate relationship between some adenosine receptors, as is the case of the A2A receptors, and receptors for neurotrophins. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Impact of ectonucleotidases in autonomic nervous functions

Adenine and uracil nucleotides play key functions in the autonomic nervous system (ANS). For instance, ATP acts as a neurotransmitter, co-transmitter and neuromodulator in the ANS. The purinergic system encompasses (1) receptors that respond to extracellular purines, which are designated as P1 and P2 purinoceptors, (2) purine release and uptake, and (3) a cascade of enzymes that regulate the concentration of purines near the cell surface. Ectonucleotidases and adenosine deaminase (ADA) are enzymes responsible for the hydrolysis of ATP (and other nucleotides such as ADP, UTP, UDP, AMP) and adenosine, respectively. Accordingly, these enzymes are expected to play an important role in the control of neuro-effector transmission in tissues innervated by both the sympathetic and parasympathetic divisions of the ANS. Indeed, ectonucleotidases have the ability to either terminate P2 receptor responses initiated by nucleoside triphosphates (ATP and UTP), and/or to favor the activation of ADP (e.g. P2Y1,12,13) and UDP (e.g. P2Y6) and/or adenosine (P1) specific receptors. In addition, ectonucleotidases can also importantly protect some P2 receptors from desensitization (e.g. P2X1, P2Y1). In this review, we present the (putative) roles of ectonucleotidases and ADA in the ANS with a focus on their regulatory activity at neuro-effector junctions in the following tissues: heart, vas deferens, urinary bladder, salivary glands, blood vessels and the intestine. We also present their implication in nociceptive transmission.

Neuroglia and their roles in central respiratory control; an overview

While once viewed as mere housekeepers, providing structural and metabolic support for neurons, it is now clear that neuroglia do much more. Phylogenetically, they have undergone enormous proliferation and diversification as central nervous systems grew in their complexity. In addition, they: i) are morphologically and functionally diverse; ii) play numerous, vital roles in maintaining CNS homeostasis; iii) are key players in brain development and responses to injury; and, iv) via gliotransmission, are likely participants in information processing. In this review, we discuss the diverse roles of neuroglia in maintaining homeostasis in the CNS, their evolutionary origins, the different types of neuroglia and their functional significance for respiratory control, and finally consider evidence that they contribute to the processing of chemosensory information in the respiratory network and the homeostatic control of blood gases.

Independent purinergic mechanisms of central and peripheral chemoreception in the rostral ventrolateral medulla

The rostral ventrolateral medulla oblongata (RVLM) contains two functionally distinct types of neurons that control and orchestrate cardiovascular and respiratory responses to hypoxia and hypercapnia. One group is composed of the central chemoreceptor neurons of the retrotrapezoid nucleus, which provides a CO₂/H(+) -dependent drive to breathe and serves as an integration centre and a point of convergence of chemosensory information from other central and peripheral sites, including the carotid bodies. The second cluster of RVLM cells forms a population of neurons belonging to the C1 catecholaminergic group that controls sympathetic vasomotor tone in resting conditions and in conditions of hypoxia and hypercapnia. Recent evidence suggests that ATP-mediated purinergic signalling at the level of the RVLM co-ordinates cardiovascular and respiratory responses triggered by hypoxia and hypercapnia by activating retrotrapezoid nucleus and C1 neurons, respectively. The role of ATP-mediated signalling in the RVLM mechanisms of cardiovascular and respiratory activities is the main subject of this short review.

Differentiating connexin hemichannels and pannexin channels in cellular ATP release

Adenosine triphosphate (ATP) plays a fundamental role in cellular communication, with its extracellular accumulation triggering purinergic signaling cascades in a diversity of cell types. While the roles for purinergic signaling in health and disease have been well established, identification and differentiation of the specific mechanisms controlling cellular ATP release is less well understood. Multiple mechanisms have been proposed to regulate ATP release with connexin (Cx) hemichannels and pannexin (Panx) channels receiving major focus. However, segregating the specific roles of Panxs and Cxs in ATP release in a plethora of physiological and pathological contexts has remained enigmatic. This multifaceted problem has arisen from the selectivity of pharmacological inhibitors for Panxs and Cxs, methodological differences in assessing Panx and Cx function and the potential compensation by other isoforms in gene silencing and genetic knockout models. Consequently, there remains a void in the current understanding of specific contributions of Panxs and Cxs in releasing ATP during homeostasis and disease. Differentiating the distinct signaling pathways that regulate these two channels will advance our current knowledge of cellular communication and aid in the development of novel rationally-designed drugs for modulation of Panx and Cx activity, respectively.