Coordinate release of ATP and GABA at in vitro synapses of lateral hypothalamic neurons

Regulation of GABAergic neurotransmission by purinergic receptors in brain physiology and disease

Purinergic receptors regulate the processing of neural information in the hippocampus and cerebral cortex, structures related to cognitive functions. These receptors are activated when astrocytic and neuronal populations release adenosine triphosphate (ATP) in an autocrine and paracrine manner, following sustained patterns of neuronal activity. The modulation by these receptors of GABAergic transmission has only recently been studied. Through their ramifications, astrocytes and GABAergic interneurons reach large groups of excitatory pyramidal neurons. Their inhibitory effect establishes different synchronization patterns that determine gamma frequency rhythms, which characterize neural activities related to cognitive processes. During early life, GABAergic-mediated synchronization of excitatory signals directs the experience-driven maturation of cognitive development, and dysfunctions concerning this process have been associated with neurological and neuropsychiatric diseases. Purinergic receptors timely modulate GABAergic control over ongoing neural activity and deeply affect neural processing in the hippocampal and neocortical circuitry. Stimulation of A2 receptors increases GABA release from presynaptic terminals, leading to a considerable reduction in neuronal firing of pyramidal neurons. A1 receptors inhibit GABAergic activity but only act in the early postnatal period when GABA produces excitatory signals. P2X and P2Y receptors expressed in pyramidal neurons reduce the inhibitory tone by blocking GABAA receptors. Finally, P2Y receptor activation elicits depolarization of GABAergic neurons and increases GABA release, thus favoring the emergence of gamma oscillations. The present review provides an overall picture of purinergic influence on GABAergic transmission and its consequences on neural processing, extending the discussion to receptor subtypes and their involvement in the onset of brain disorders, including epilepsy and Alzheimer's disease.

Extracellular ATP Is a Homeostatic Messenger That Mediates Cell-Cell Communication in Physiological Processes and Psychiatric Diseases

Neuronal activity is the basis of information encoding and processing in the brain. During neuronal activation, intracellular ATP (adenosine triphosphate) is generated to meet the high-energy demands. Simultaneously, ATP is secreted, increasing the extracellular ATP concentration and acting as a homeostatic messenger that mediates cell-cell communication to prevent aberrant hyperexcitability of the nervous system. In addition to the confined release and fast synaptic signaling of classic neurotransmitters within synaptic clefts, ATP can be released by all brain cells, diffuses widely, and targets different types of purinergic receptors on neurons and glial cells, making it possible to orchestrate brain neuronal activity and participate in various physiological processes, such as sleep and wakefulness, learning and memory, and feeding. Dysregulation of extracellular ATP leads to a destabilizing effect on the neural network, as found in the etiopathology of many psychiatric diseases, including depression, anxiety, schizophrenia, and autism spectrum disorder. In this review, we summarize advances in the understanding of the mechanisms by which extracellular ATP serves as an intercellular signaling molecule to regulate neural activity, with a focus on how it maintains the homeostasis of neural networks. In particular, we also focus on neural activity issues that result from dysregulation of extracellular ATP and propose that aberrant levels of extracellular ATP may play a role in the etiopathology of some psychiatric diseases, highlighting the potential therapeutic targets of ATP signaling in the treatment of these psychiatric diseases. Finally, we suggest potential avenues to further elucidate the role of extracellular ATP in intercellular communication and psychiatric diseases.

Feedback facilitation by adenosine A2A receptors of ATP release from mouse hippocampal nerve terminals

The adenosine modulation system is mostly composed by inhibitory A1 receptors (A1R) and the less abundant facilitatory A2A receptors (A2AR), the latter selectively engaged at high frequency stimulation associated with synaptic plasticity processes in the hippocampus. A2AR are activated by adenosine originated from extracellular ATP through ecto-5'-nucleotidase or CD73-mediated catabolism. Using hippocampal synaptosomes, we now investigated how adenosine receptors modulate the synaptic release of ATP. The A2AR agonist CGS21680 (10-100 nM) enhanced the K+-evoked release of ATP, whereas both SCH58261 and the CD73 inhibitor α,β-methylene ADP (100 μM) decreased ATP release; all these effects were abolished in forebrain A2AR knockout mice. The A1R agonist CPA (10-100 nM) inhibited ATP release, whereas the A1R antagonist DPCPX (100 nM) was devoid of effects. The presence of SCH58261 potentiated CPA-mediated ATP release and uncovered a facilitatory effect of DPCPX. Overall, these findings indicate that ATP release is predominantly controlled by A2AR, which are involved in an apparent feedback loop of A2AR-mediated increased ATP release together with dampening of A1R-mediated inhibition. This study is a tribute to María Teresa Miras-Portugal.

Role of purines in brain development, from neuronal proliferation to synaptic refinement

The purinergic system includes P1 and P2 receptors, which are activated by ATP and its metabolites. They are expressed in adult neuronal and glial cells and are crucial in brain function, including neuromodulation and neuronal signaling. As P1 and P2 receptors are expressed throughout embryogenesis and development, purinergic signaling also has an important role in the development of the peripheral and central nervous system. In this review, we present the expression pattern and activity of purinergic receptors and of their signaling pathways during embryonic and postnatal development of the nervous system. In particular, we review the involvement of the purinergic signaling in all the crucial steps of brain development i.e. in neural stem cell proliferation, neuronal differentiation and migration as well as in astrogliogenesis and oligodendrogenesis. Then, we review data showing a crucial role of the ATP and adenosine signaling pathways in the formation of the peripheral neuromuscular junction and of central GABAergic and glutamatergic synapses. Finally, we examine the consequences of deregulation of the purinergic system during development and discuss the therapeutic potential of targeting it at adult stage in diseases with reactivation of the ATP and adenosine pathway.

ATP-mediated signalling in the central synapses

ATP released from the synaptic terminals and astrocytes can activate neuronal P2 receptors at a variety of locations across the CNS. Although the postsynaptic ATP-mediated signalling does not bring a major contribution into the excitatory transmission, it is instrumental for slow and diffuse modulation of synaptic dynamics and neuronal firing in many CNS areas. Neuronal P2X and P2Y receptors can be activated by ATP released from the synaptic terminals, astrocytes and microglia and thereby can participate in the regulation of synaptic homeostasis and plasticity. There is growing evidence of importance of purinergic regulation of synaptic transmission in different physiological and pathological contexts. Here, we review the main mechanisms underlying the complexity and diversity of purinergic signalling and purinergic modulation in central neurons.

Triggering of Major Brain Disorders by Protons and ATP: The Role of ASICs and P2X Receptors

Adenosine triphosphate (ATP) is well-known as a universal source of energy in living cells. Less known is that this molecule has a variety of important signaling functions: it activates a variety of specific metabotropic (P2Y) and ionotropic (P2X) receptors in neuronal and non-neuronal cell membranes. So, a wide variety of signaling functions well fits the ubiquitous presence of ATP in the tissues. Even more ubiquitous are protons. Apart from the unspecific interaction of protons with any protein, many physiological processes are affected by protons acting on specific ionotropic receptors-acid-sensing ion channels (ASICs). Both protons (acidification) and ATP are locally elevated in various pathological states. Using these fundamentally important molecules as agonists, ASICs and P2X receptors signal a variety of major brain pathologies. Here we briefly outline the physiological roles of ASICs and P2X receptors, focusing on the brain pathologies involving these receptors.

Convergence of adenosine and GABA signaling for synapse stabilization during development

[Figure: see text].

Purinergic Modulation of Activity in the Developing Auditory Pathway

Purinergic P2 receptors, activated by endogenous ATP, are prominently expressed on neuronal and non-neuronal cells during development of the auditory periphery and central auditory neurons. In the mature cochlea, extracellular ATP contributes to ion homeostasis, and has a protective function against noise exposure. Here, we focus on the modulation of activity by extracellular ATP during early postnatal development of the lower auditory pathway. In mammals, spontaneous patterned activity is conveyed along afferent auditory pathways before the onset of acoustically evoked signal processing. During this critical developmental period, inner hair cells fire bursts of action potentials that are believed to provide a developmental code for synaptic maturation and refinement of auditory circuits, thereby establishing a precise tonotopic organization. Endogenous ATP-release triggers such patterned activity by raising the extracellular K⁺ concentration and contributes to firing by increasing the excitability of auditory nerve fibers, spiral ganglion neurons, and specific neuron types within the auditory brainstem, through the activation of diverse P2 receptors. We review recent studies that provide new models on the contribution of purinergic signaling to early development of the afferent auditory pathway. Further, we discuss potential future directions of purinergic research in the auditory system.

Introduction to Purinergic Signaling

Purinergic signaling was proposed in 1972, after it was demonstrated that adenosine 5'-triphosphate (ATP) was a transmitter in nonadrenergic, noncholinergic inhibitory nerves supplying the guinea-pig taenia coli. Later, ATP was identified as an excitatory cotransmitter in sympathetic and parasympathetic nerves, and it is now apparent that ATP acts as a cotransmitter in most, if not all, nerves in both the peripheral nervous system and central nervous system (CNS). ATP acts as a short-term signaling molecule in neurotransmission, neuromodulation, and neurosecretion. It also has potent, long-term (trophic) roles in cell proliferation, differentiation, and death in development and regeneration. Receptors to purines and pyrimidines have been cloned and characterized: P1 adenosine receptors (with four subtypes), P2X ionotropic nucleotide receptors (seven subtypes) and P2Y metabotropic nucleotide receptors (eight subtypes). ATP is released from different cell types by mechanical deformation, and after release, it is rapidly broken down by ectonucleotidases. Purinergic receptors were expressed early in evolution and are widely distributed on many different nonneuronal cell types as well as neurons. Purinergic signaling is involved in embryonic development and in the activities of stem cells. There is a growing understanding about the pathophysiology of purinergic signaling and there are therapeutic developments for a variety of diseases, including stroke and thrombosis, osteoporosis, pain, chronic cough, kidney failure, bladder incontinence, cystic fibrosis, dry eye, cancer, and disorders of the CNS, including Alzheimer's, Parkinson's. and Huntington's disease, multiple sclerosis, epilepsy, migraine, and neuropsychiatric and mood disorders.

Introduction to Purinergic Signalling in the Brain

ATP is a cotransmitter with glutamate, noradrenaline, GABA, acetylcholine and dopamine in the brain. There is a widespread presence of both adenosine (P1) and P2 nucleotide receptors in the brain on both neurons and glial cells. Adenosine receptors play a major role in presynaptic neuromodulation, while P2X ionotropic receptors are involved in fast synaptic transmission and synaptic plasticity. P2Y G protein-coupled receptors are largely involved in presynaptic activities, as well as mediating long-term (trophic) signalling in cell proliferation, differentiation and death during development and regeneration. Both P1 and P2 receptors participate in neuron-glial interactions. Purinergic signalling is involved in control of cerebral vascular tone and remodelling and has been implicated in learning and memory, locomotor and feeding behaviour and sleep. There is increasing interest in the involvement of purinergic signalling in the pathophysiology of the CNS, including trauma, ischaemia, epilepsy, neurodegenerative diseases, neuropsychiatric and mood disorders, and cancer, including gliomas.

Purinergic Signalling in Parkinson's Disease: A Multi-target System to Combat Neurodegeneration

Parkinson's disease (PD) is the second most common neurodegenerative disorder, characterized by progressive loss of dopaminergic neurons that results in characteristic motor and non-motor symptoms. L-3,4 dihydroxyphenylalanine (L-DOPA) is the gold standard therapy for the treatment of PD. However, long-term use of L-DOPA leads to side effects such as dyskinesias and motor fluctuation. Since purines have neurotransmitter and co-transmitter properties, the function of the purinergic system has been thoroughly studied in the nervous system. Adenosine and adenosine 5'-triphosphate (ATP) are modulators of dopaminergic neurotransmission, neuroinflammatory processes, oxidative stress, excitotoxicity and cell death via purinergic receptor subtypes. Aberrant purinergic receptor signalling can be either the cause or the result of numerous pathological conditions, including neurodegenerative disorders. Many data confirm the involvement of purinergic signalling pathways in PD. Modulation of purinergic receptor subtypes, the activity of ectonucleotidases and ATP transporters could be beneficial in the treatment of PD. We give a brief summary of the background of purinergic signalling focusing on its roles in PD. Possible targets for pharmacological treatment are highlighted.

Purinergic modulation of glutamate transmission: An expanding role in stress-linked neuropathology

Chronic stress has been extensively linked to disturbances in glutamatergic signalling. Emerging from this field of research is a considerable number of studies identifying the ability of purines at the pre-, post-, and peri-synaptic levels to tune glutamatergic neurotransmission. While the evidence describing purinergic control of glutamate has continued to grow, there has been relatively little attention given to how chronic stress modulates purinergic functions. The available research on this topic has demonstrated that chronic stress can not only disturb purinergic receptors involved in the regulation of glutamate neurotransmission, but also perturb glial-dependent purinergic signalling. This review will provide a detailed examining of the complex literature relating to glutamatergic-purinergic interactions with a focus on both neuronal and glial contributions. Once these detailed interactions have been described and contextualised, we will integrate recent findings from the field of stress research.

Purines and Carotid Body: New Roles in Pathological Conditions

It is known that adenosine and adenosine-5′-triphosphate (ATP) are excitatory mediators involved in carotid body (CB) hypoxic signaling. The CBs are peripheral chemoreceptors classically defined by O₂, CO₂, and pH sensors. When hypoxia activates the CB, it induces the release of neurotransmitters from chemoreceptor cells leading to an increase in the action potentials frequency at the carotid sinus nerve (CSN). This increase in the firing frequency of the CSN is integrated in the brainstem to induce cardiorespiratory compensatory responses. In the last decade several pathologies, as, hypertension, diabetes, obstructive sleep apnea and heart failure have been associated with CB overactivation. In the first section of the present manuscript we review in a concise manner fundamental aspects of purine metabolism. The second section is devoted to the role of purines on the hypoxic response of the CB, providing the state-of-the art for the presence of adenosine and ATP receptors in the CB; for the role of purines at presynaptic level in CB chemoreceptor cells, as well as, its metabolism and regulation; at postsynaptic level in the CSN activity; and on the ventilatory responses to hypoxia. Recently, we have showed that adenosine is involved in CB hypersensitization during chronic intermittent hypoxia (CIH), which mimics obstructive sleep apnea, since caffeine, a non-selective adenosine receptor antagonist that inhibits A_2A and A_2B adenosine receptors, decreased CSN chemosensory activity in animals subjected to CIH. Apart from this involvement of adenosine in CB sensitization in sleep apnea, it was recently found that P2X3 ATP receptor in the CB contributes to increased chemoreflex hypersensitivity and hypertension in spontaneously hypertension rats. Therefore the last section of this manuscript is devoted to review the recent findings on the role of purines in CB-mediated pathologies as hypertension, diabetes and sleep apnea emphasizing the potential clinical importance of modulating purines levels and action to treat pathologies associated with CB dysfunction.

P2X7 purinergic receptors participate in the expression and extinction processes of contextual fear conditioning memory in mice

The purinergic system consists of two large receptor families - P2X and P2Y. Both are activated by adenosine triphosphate (ATP), although presenting different functions. These receptors are present in several brain regions, including those involved in emotion and stress-related behaviors. Hence, they seem to participate in fear- and anxiety-related responses. However, few studies have investigated the purinergic system in threatening situations, as observed in contextual fear conditioning (CFC). Therefore, this study investigated the involvement of purinergic receptors in the expression and extinction of aversive memories. C57Bl/6 background mice were submitted to the CFC protocol. Wildtype (WT) mice received i.p. injection of either a nonselective P2 receptor (P2R) antagonist, P178 (10 or 30 mg/kg); a selective P2X7 receptor (P2X7R) antagonist, A438079 (10 mg/kg); a selective P2Y1 receptor (P2Y1R) antagonist, MRS2179 (10 mg/kg); or vehicle 10 min prior to or immediately after the extinction session. Additionally, P2X7R KO mice were tested in the CFC protocol. After P2R antagonist treatment, contextual fear recall increased, while acquisition of extinction was impaired. Similar results were observed with the selective P2X7R antagonist, but not with the selective P2Y1R antagonist. Interestingly, P2X7R KO mice showed increased contextual fear recall, associated with impaired acquisition of extinction, in accordance with pharmacologic P2X7R antagonism. Our results suggest that specific pharmacological or genetic blockade of P2X7R promotes anxiogenic-like effects, along with deficits in extinction learning. Thus, these receptors could present an alternative treatment of stress-related psychiatric disorders.

Modulation of Central Synapses by Astrocyte-Released ATP and Postsynaptic P2X Receptors

Communication between neuronal and glial cells is important for neural plasticity. P2X receptors are ATP-gated cation channels widely expressed in the brain where they mediate action of extracellular ATP released by neurons and/or glia. Recent data show that postsynaptic P2X receptors underlie slow neuromodulatory actions rather than fast synaptic transmission at brain synapses. Here, we review these findings with a particular focus on the release of ATP by astrocytes and the diversity of postsynaptic P2X-mediated modulation of synaptic strength and plasticity in the CNS.

Onset of microglial entry into developing quail retina coincides with increased expression of active caspase-3 and is mediated by extracellular ATP and UDP

Microglial cell precursors located in the area of the base of the pecten and the optic nerve head (BP/ONH) start to enter the retina of quail embryos at the 7^th day of incubation (E7), subsequently colonizing the entire retina by central-to-peripheral tangential migration, as previously shown by our group. The present study demonstrates a precise chronological coincidence of the onset of microglial cell entry into the retina with a striking increase in death of retinal cells, as revealed by their active caspase-3 expression and TUNEL staining, in regions dorsal to the BP/ONH area, suggesting that dying retinal cells would contribute to the microglial cell inflow into the retina. However, the molecular mechanisms involved in this inflow are currently unclear. Extracellular nucleotides, such as ATP and UDP, have previously been shown to favor migration of microglia towards brain injuries because they are released by apoptotic cells and stimulate both chemotaxis and chemokinesis in microglial cells via signaling through purinergic receptors. Hence, we tested here the hypothesis that ATP and UDP play a role in the entry and migration of microglial precursors into the developing retina. For this purpose, we used an experimental model system based on organotypic cultures of E6.5 quail embryo retina explants, which mimics the entry and migration of microglial precursors in the in situ developing retina. Inhibition of purinergic signaling by treating retina explants with either apyrase, a nucleotide-hydrolyzing enzyme, or suramin, a broad spectrum antagonist of purinergic receptors, significantly prevents the entry of microglial cells into the retina. In addition, treatment of retina explants with either exogenous ATP or UDP results in significantly increased numbers of microglial cells entering the retina. In light of these findings, we conclude that purinergic signaling by extracellular ATP and UDP is necessary for the entry and migration of microglial cells into the embryonic retina by inducing chemokinesis in these cells.

P2X7 receptors in body temperature, locomotor activity, and brain mRNA and lncRNA responses to sleep deprivation

The ionotropic purine type 2X7 receptor (P2X7R) is a nonspecific cation channel implicated in sleep regulation and brain cytokine release. Many endogenous rhythms covary with sleep, including locomotor activity and core body temperature. Furthermore, brain-hypothalamic cytokines and purines play a role in the regulation of these physiological parameters as well as sleep. We hypothesized that these parameters are also affected by the absence of the P2X7 receptor. Herein, we determine spontaneous expression of body temperature and locomotor activity in wild-type (WT) and P2X7R knockout (KO) mice and how they are affected by sleep deprivation (SD). We also compare hypothalamic, hippocampal, and cortical cytokine- and purine-related receptor and enzyme mRNA expressions before and after SD in WT and P2X7RKO mice. Next, in a hypothesis-generating survey of hypothalamic long noncoding (lnc) RNAs, we compare lncRNA expression levels between strains and after SD. During baseline conditions, P2X7RKO mice had attenuated temperature rhythms compared with WT mice, although locomotor activity patterns were similar in both strains. After 6 h of SD, body temperature and locomotion were enhanced to a greater extent in P2X7RKO mice than in WT mice during the initial 2-3 h after SD. Baseline mRNA levels of cortical TNF-α and P2X4R were higher in the KO mice than WT mice. In response to SD, the KO mice failed to increase hypothalamic adenosine deaminase and P2X4R mRNAs. Further, hypothalamic lncRNA expressions varied by strain, and with SD. Current data are consistent with a role for the P2X7R in thermoregulation and lncRNA involvement in purinergic signaling.

Short- and long-term (trophic) purinergic signalling

There is long-term (trophic) purinergic signalling involving cell proliferation, differentiation, motility and death in the development and regeneration of most systems of the body, in addition to fast purinergic signalling in neurotransmission, neuromodulation and secretion. It is not always easy to distinguish between short- and long-term signalling. For example, adenosine triphosphate (ATP) can sometimes act as a short-term trigger for long-term trophic events that become evident days or even weeks after the original challenge. Examples of short-term purinergic signalling during sympathetic, parasympathetic and enteric neuromuscular transmission and in synaptic transmission in ganglia and in the central nervous system are described, as well as in neuromodulation and secretion. Long-term trophic signalling is described in the immune/defence system, stratified epithelia in visceral organs and skin, embryological development, bone formation and resorption and in cancer. It is likely that the increase in intracellular Ca(2+) in response to both P2X and P2Y purinoceptor activation participates in many short- and long-term physiological effects.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

Functional expression of P2 purinoceptors in a primary neuroglial cell culture of the rat arcuate nucleus

The arcuate nucleus (ARC) plays an important role in the hypothalamic control of energy homeostasis. Expression of various purinoceptor subtypes in the rat ARC and physiological studies suggest a modulatory function of P2 receptors within the neuroglial ARC circuitry. A differentiated mixed neuronal and glial microculture was therefore established from postnatal rat ARC, revealing neuronal expression of ARC-specific transmitters involved in food intake regulation (neuropeptide Y (NPY), proopiomelanocortin (POMC), tyrosine hydroxylase (TH)). Some NPYergic neurons cosynthesized TH, while POMC and TH expression proved to be mutually exclusive. Stimulation with the general purinoceptor agonists 2-methylthioadenosine-5'triphosphate (2-MeSATP) and ATP but not the P2X1/P2X3 receptor subtype agonist α,β-methyleneadenosine-5'triphosphate (α,β-meATP) induced intracellular calcium signals in ARC neurons and astrocytes. Some 5-10% each of 2-MeSATP responsive neurons expressed POMC, NYP or TH. Supporting the calcium imaging data, radioligand binding studies to hypothalamic membranes showed high affinity for 2-MeSATP, ATP but not α,β-meATP to displace [α-(35)S]deoxyadenosine-5'thiotriphosphate ([(35)S]dATPαS) from P2 receptors. Repetitive superfusion with equimolar 2-MeSATP allowed categorization of ARC cells into groups with a high or low (LDD) degree of purinoceptor desensitization, the latter allowing further receptor characterization. Calcium imaging experiments performed at 37°C vs. room temperature showed further reduction of desensitization. Agonist-mediated intracellular calcium signals were suppressed in all LDD neurons but only 25% of astrocytes in the absence of extracellular calcium, suggestive of metabotropic P2Y receptor expression in the majority of ARC astrocytes. The highly P2Y1-selective receptor agonists MRS2365 and 2-methylthioadenosine-5'diphosphate (2-MeSADP) activated 75-85% of all 2-MeSATP-responsive ARC astrocytes. Taking into consideration the high potency to dose-dependently stimulate ARC cells of the LDD group, the high affinity for rat P2X(1-3) and low affinity for rat P2X4, P2X7 and P2Y receptor subtypes except P2Y1 and P2Y13, the agonist 2-MeSATP primarily acted upon P2X2 and P2Y1 purinoceptors to trigger intracellular calcium signaling in ARC neurons and astrocytes.

Differential Modulation of GABAA Receptors Underlies Postsynaptic Depolarization- and Purinoceptor-Mediated Enhancement of Cerebellar Inhibitory Transmission: A Non-Stationary Fluctuation Analysis Study

Cerebellar GABAergic inhibitory transmission between interneurons and Purkinje cells (PCs) undergoes a long-lasting enhancement following different stimulations, such as brief depolarization or activation of purinergic receptors of postsynaptic PCs. The underlying mechanisms, however, are not completely understood. Using a peak-scaled non-stationary fluctuation analysis, we therefore aimed at characterizing changes in the electrophysiological properties of GABA_A receptors in PCs of rat cerebellar cortex during depolarization-induced “rebound potentiation (RP)” and purinoceptor-mediated long-term potentiation (PM-LTP), because both RP and PM-LTP likely depend on postsynaptic mechanisms. Stimulation-evoked inhibitory postsynaptic currents (eIPSCs) were recorded from PCs in neonatal rat cerebellar slices. Our analysis showed that postsynaptic membrane depolarization induced RP of eIPSCs in association with significant increase in the number of synaptic GABA_A receptors without changing the channel conductance. By contrast, bath application of ATP induced PM-LTP of eIPSCs with a significant increase of the channel conductance of GABA_A receptors without affecting the receptor number. Pretreatment with protein kinase A (PKA) inhibitors, H-89 and cAMPS-Rp, completely abolished the PM-LTP. The CaMKII inhibitor KN-62 reported to abolish RP did not alter PM-LTP. These results suggest that the signaling mechanism underlying PM-LTP could involve ATP-induced phosphorylation of synaptic GABA_A receptors, thereby resulting in upregulation of the channel conductance by stimulating adenylyl cyclase-PKA signaling cascade, possibly via activation of P2Y₁₁ purinoceptor. Thus, our findings reveal that postsynaptic GABA_A receptors at the interneuron-PC inhibitory synapses are under the control of two distinct forms of long-term potentiation linked with different second messenger cascades.

Electrophysiological properties of NG2(+) cells: Matching physiological studies with gene expression profiles

NG2(+) glial cells are a dynamic population of non-neuronal cells that give rise to myelinating oligodendrocytes in the central nervous system. These cells express numerous ion channels and neurotransmitter receptors, which endow them with a complex electrophysiological profile that is unique among glial cells. Despite extensive analysis of the electrophysiological properties of these cells, relatively little was known about the molecular identity of the channels and receptors that they express. The generation of new RNA-Seq datasets for NG2(+) cells has provided the means to explore how distinct genes contribute to the physiological properties of these progenitors. In this review, we systematically compare the results obtained through RNA-Seq transcriptional analysis of purified NG2(+) cells to previous physiological and molecular studies of these cells to define the complement of ion channels and neurotransmitter receptors expressed by NG2(+) cells in the mammalian brain and discuss the potential significance of the unique physiological properties of these cells. This article is part of a Special Issue entitled SI:NG2-glia(Invited only).

An introduction to the roles of purinergic signalling in neurodegeneration, neuroprotection and neuroregeneration

Purinergic signalling appears to play important roles in neurodegeneration, neuroprotection and neuroregeneration. Initially there is a brief summary of the background of purinergic signalling, including release of purines and pyrimidines from neural and non-neural cells and their ectoenzymatic degradation, and the current characterisation of P1 (adenosine), and P2X (ion channel) and P2Y (G protein-coupled) nucleotide receptor subtypes. There is also coverage of the localization and roles of purinoceptors in the healthy central nervous system. The focus is then on the roles of purinergic signalling in trauma, ischaemia, stroke and in neurodegenerative diseases, including Alzheimer's, Parkinson's and Huntington's diseases, as well as multiple sclerosis and amyotrophic lateral sclerosis. Neuroprotective mechanisms involving purinergic signalling are considered and its involvement in neuroregeneration, including the role of adult neural stem/progenitor cells. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

ATP release through lysosomal exocytosis from peripheral nerves: the effect of lysosomal exocytosis on peripheral nerve degeneration and regeneration after nerve injury

Studies have shown that lysosomal activation increases in Schwann cells after nerve injury. Lysosomal activation is thought to promote the engulfment of myelin debris or fragments of injured axons in Schwann cells during Wallerian degeneration. However, a recent interpretation of lysosomal activation proposes a different view of the phenomenon. During Wallerian degeneration, lysosomes become secretory vesicles and are activated for lysosomal exocytosis. The lysosomal exocytosis triggers adenosine 5′-triphosphate (ATP) release from peripheral neurons and Schwann cells during Wallerian degeneration. Exocytosis is involved in demyelination and axonal degradation, which facilitate nerve regeneration following nerve degeneration. At this time, released ATP may affect the communication between cells in peripheral nerves. In this review, our description of the relationship between lysosomal exocytosis and Wallerian degeneration has implications for the understanding of peripheral nerve degenerative diseases and peripheral neuropathies, such as Charcot-Marie-Tooth disease or Guillain-Barré syndrome.

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.

Gonadal steroids differentially modulate the actions of orphanin FQ/nociceptin at a physiologically relevant circuit controlling female sexual receptivity

Orphanin FQ/nociceptin (OFQ/N) inhibits the activity of pro-opiomelanocortin (POMC) neurones located in the hypothalamic arcuate nucleus (ARH) that regulate female sexual behaviour and energy balance. We tested the hypothesis that gonadal steroids differentially modulate the ability of OFQ/N to inhibit these cells via presynaptic inhibition of transmitter release and postsynaptic activation of G protein-gated, inwardly-rectifying K(+) (GIRK)-1 channels. Whole-cell patch clamp recordings were performed in hypothalamic slices prepared from ovariectomised rats. OFQ/N (1 μm) decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs), and also caused a robust outward current in the presence of tetrodotoxin, in ARH neurones from vehicle-treated animals. A priming dose of oestradiol benzoate (EB; 2 μg) increased basal mEPSC frequency, markedly diminished both the OFQ/N-induced decrease in mEPSC frequency and the activation of GIRK-1 currents, and potentiated the OFQ/N-induced decrease in mIPSC frequency. Steroid treatment regimens that facilitate sexual receptivity reinstate the basal mEPSC frequency, the OFQ/N-induced decrease in mEPSC frequency and the activation of GIRK-1 currents to levels observed in vehicle-treated controls, and largely abolish the ability of OFQ/N to decrease mIPSC frequency. These effects were observed in an appreciable population of identified POMC neurones, almost one-half of which projected to the medial preoptic nucleus. Taken together, these data reveal that gonadal steroids influence the pleiotropic actions of OFQ/N on ARH neurones, including POMC neurones, in a disparate manner. These temporal changes in OFQ/N responsiveness further implicate this neuropeptide system as a critical mediator of the gonadal steroid regulation of reproductive behaviour.

Neuromodulation by extracellular ATP and P2X receptors in the CNS

Extracellular adenosine 5' triphosphate (ATP) is a widespread cell-to-cell signaling molecule in the brain, where it activates cell surface P2X and P2Y receptors. P2X receptors define a protein family unlike other neurotransmitter-gated ion channels in terms of sequence, subunit topology, assembly, and architecture. Within milliseconds of binding ATP, they catalyze the opening of a cation-selective pore. However, recent data show that P2X receptors often underlie neuromodulatory responses on slower time scales of seconds or longer. Herein, we review these findings at molecular, cellular and systems levels. We propose that, while P2X receptors are fast ligand-gated cation channels, they are most adept at mediating slow neuromodulatory functions that are more widespread and more physiologically utilized than fast ATP synaptic transmission in the CNS.

Interplay between ionotropic receptors modulates inhibitory synaptic strength

The essence of neuronal function is to generate outputs in response to synaptic potentials. Synaptic integration at a synapse determines neuronal outputs in the CNS. In a recent study, we describe that excitatory and inhibitory transmitter-gated channels physically crosstalk each other at the cellular and molecular level. Increased membrane expression of ATP P2X4 receptors by using an interference peptide competing with the intracellular endocytosis motif enhances neuronal excitability, which is further enhanced by reciprocal interaction between post-synaptic ATP- and GABA-gated channels. Molecular interaction is supported by experiments of co-immunoprecipitation and mutagenesis of P2X4 subunit.  Two amino acids in the intracellular carboxyl tail of P2X4 subunit appears to be responsible for this crosstalk. Our recent study provides molecular and electrophysiological evidence for physical interaction between excitatory and inhibitory receptors that appears to be crucial in determining synaptic strength at central synapses.

P2X receptors and their roles in astroglia in the central and peripheral nervous system

Astrocytes are a class of neural cells that control homeostasis at all levels of the central and peripheral nervous system. There is a bidirectional neuron-glia interaction via a number of extracellular signaling molecules, glutamate and ATP being the most widespread. ATP activates ionotropic P2X and metabotropic P2Y receptors, which operate in both neurons and astrocytes. Morphological, biochemical, and functional evidence indicates the expression of astroglial P2X(1/5) heteromeric and P2X(7) homomeric receptors, which mediate physiological and pathophysiological responses. Activation of P2X(1/5) receptors triggers rapid increase of intracellular Na(+) that initiates immediate cellular reactions, such as the depression of the glutamate transporter to keep high glutamate concentrations in the synaptic cleft, the activation of the local lactate shuttle to supply energy substrate to pre- and postsynaptic neuronal structures, and the reversal of the Na(+)/Ca(2+) exchange resulting in additional Ca(2+) entry. The consequences of P2X(7) receptor activation are mostly but not exclusively mediated by the entry of Ca(2+) and result in reorganization of the cytoskeleton, inflammation, apoptosis/necrosis, and proliferation, usually at a prolonged time scale. Thus, astroglia detect by P2X(1/5) and P2X(7) receptors both physiological concentrations of ATP secreted from presynaptic nerve terminals and also much higher concentrations of ATP attained under pathological conditions.

The role of purinergic receptors in retinal function and disease

Extracellular ATP acts as a neurotransmitter in the central and peripheral nervous systems. In this review, the role of purinergic receptors in neuronal signaling and bi-directional glial-neuronal communication in the retina will be considered. There is growing evidence that a range of P2X and P2Y receptors are expressed on most classes of retinal neurons and that activation of P2 receptors modulates retinal function. Furthermore, neuronal control of glial function is achieved through neuronal release of ATP and activation of P2Y receptors expressed by Müller cells. Altered purinergic signaling in Müller cells has been implicated in gliotic changes in the diseased retina and furthermore, elevations in extracellular ATP may lead to apoptosis of retinal neurons.

Purinergic signalling: from normal behaviour to pathological brain function

Purinergic neurotransmission, involving release of ATP as an efferent neurotransmitter was first proposed in 1972. Later, ATP was recognised as a cotransmitter in peripheral nerves and more recently as a cotransmitter with glutamate, noradrenaline, GABA, acetylcholine and dopamine in the CNS. Both ATP, together with some of its enzymatic breakdown products (ADP and adenosine) and uracil nucleotides are now recognised to act via P2X ion channels and P1 and P2Y G protein-coupled receptors, which are widely expressed in the brain. They mediate both fast signalling in neurotransmission and neuromodulation and long-term (trophic) signalling in cell proliferation, differentiation and death. Purinergic signalling is prominent in neurone-glial cell interactions. In this review we discuss first the evidence implicating purinergic signalling in normal behaviour, including learning and memory, sleep and arousal, locomotor activity and exploration, feeding behaviour and mood and motivation. Then we turn to the involvement of P1 and P2 receptors in pathological brain function; firstly in trauma, ischemia and stroke, then in neurodegenerative diseases, including Alzheimer's, Parkinson's and Huntington's, as well as multiple sclerosis and amyotrophic lateral sclerosis. Finally, the role of purinergic signalling in neuropsychiatric diseases (including schizophrenia), epilepsy, migraine, cognitive impairment and neuropathic pain will be considered.

Ionotropic receptors in neuronal-astroglial signalling: what is the role of "excitable" molecules in non-excitable cells

Astroglial cells were long considered to serve merely as the structural and metabolic supporting cast and scenery against which the shining neurones perform their illustrious duties. Relatively recent evidence, however, indicates that astrocytes are intimately involved in many of the brain's functions. Astrocytes possess a diverse assortment of ionotropic transmitter receptors, which enable these glial cells to respond to many of the same signals that act on neurones. Ionotropic receptors mediate neurone-driven signals to astroglial cells in various brain areas including neocortex, hippocampus and cerebellum. Activation of ionotropic receptors trigger rapid signalling events in astroglia; these events, represented by local Ca(2+) or Na(+) signals provide the mechanism for fast neuronal-glial signalling at the synaptic level. Since astrocytes can detect chemical transmitters that are released from neurones and can release their own extracellular signals, gliotransmitters, they are intricately involved in homocellular and heterocellular signalling mechanisms in the nervous system. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Cross-talk between P2X4 and gamma-aminobutyric acid, type A receptors determines synaptic efficacy at a central synapse

The essence of neuronal function is to generate outputs in response to synaptic potentials. Synaptic integration at postsynaptic sites determines neuronal outputs in the CNS. Using immunohistochemical and electrophysiological approaches, we first reveal that steroidogenic factor 1 (SF-1) green fluorescent protein (GFP)-positive neurons in the ventromedial nucleus of the hypothalamus express P2X4 subunits that are activated by exogenous ATP. Increased membrane expression of P2X4 channels by using a peptide competing with P2X4 intracellular endocytosis motif enhances neuronal excitability of SF-1 GFP-positive neurons. This increased excitability is inhibited by a P2X receptor antagonist. Furthermore, increased surface P2X4 receptor expression significantly decreases the frequency and the amplitude of GABAergic postsynaptic currents of SF-1 GFP-positive neurons. Co-immunopurification and pulldown assays reveal that P2X4 receptors complex with aminobutyric acid, type A (GABA(A)) receptors and demonstrate that two amino acids in the carboxyl tail of the P2X4 subunit are crucial for its physical association with GABA(A) receptors. Mutation of these two residues prevents the physical association, thereby blocking cross-inhibition between P2X4 and GABA(A) receptors. Moreover, disruption of the physical coupling using competitive peptides containing the identified motif abolishes current inhibition between P2X4 and GABA(A) receptors in recombinant system and P2X4 receptor-mediated GABAergic depression in SF-1 GFP-positive neurons. Our present work thus provides evidence for cross-talk between excitatory and inhibitory receptors that appears to be crucial in determining GABAergic synaptic strength at a central synapse.

Ionotropic ATP receptors in neuronal-glial communication

In the central nervous system ATP is released from both neurones and astroglial cells acting as a homo- and heterocellular neurotransmitter. Glial cells express numerous purinoceptors of both ionotropic (P2X) and metabotropic (P2Y) varieties. Astroglial P2X receptors can be activated by ongoing synaptic transmission and can mediate fast local signalling through elevation in cytoplasmic Ca(2+) and Na(+) concentrations. These ionic signals can be translated into various physiological messages by numerous pathways, including release of gliotransmitters, metabolic support of neurones and regulation of activity of postsynaptic glutamate and GABA receptors. Ionotropic purinoceptors represent a novel pathway of glia-driven modulation of synaptic signalling that involves the release of ATP from neurones and astrocytes followed by activation of P2X receptors which can regulate synaptic activity by variety of mechanisms expressed in both neuronal and glial compartments.

Resistance of male Sprague-Dawley rats to sucrose-induced obesity: effects of 18-methoxycoronaridine

Evidence suggests that the development of obesity in males and females might be mediated by distinct mechanisms, warranting different treatment approaches. In previous studies from this laboratory, a high sucrose diet induced excessive weight gain in female Sprague-Dawley rats and administration of a selective antagonist of α3β4 nicotinic receptors, 18-methoxycoronaridine (18-MC), prevented this form of obesity. In the present study similar parameters were studied in male rats by using an identical experimental protocol. The effects of repeated administration of 18-MC on body weight gain, deposition of fat, consummatory behavior and biochemical markers of obesity in male rats were also assessed. In contrast to females, males consuming ad libitum quantities of sucrose solution (30%) in combination with normal chow did not become obese; they did not gain excessive weight nor show excessive fat deposition. Repeated administration of 18-MC (20mg/kg, i.p.) attenuated weight gain in both sucrose-consuming and control animals without altering food or fluid intake. The present results indicate that males and females are differentially responsive to high carbohydrate-diet obesity. Such gender disparities could be secondary to sex-specific alterations in cholinergic mechanisms of feeding and body weight regulation.

Ectopic vesicular neurotransmitter release along sensory axons mediates neurovascular coupling via glial calcium signaling

Neurotransmitter release generally is considered to occur at active zones of synapses, and ectopic release of neurotransmitters has been demonstrated in a few instances. However, the mechanism of ectopic neurotransmitter release is poorly understood. We took advantage of the intimate morphological and functional proximity of olfactory receptor axons and specialized glial cells, olfactory ensheathing cells (OECs), to study ectopic neurotransmitter release. Axonal stimulation evoked purinergic and glutamatergic Ca(2+) responses in OECs, indicating ATP and glutamate release. In axons expressing synapto-pHluorin, stimulation evoked an increase in synapto-pHluorin fluorescence, indicative of vesicle fusion. Transmitter release was dependent on Ca(2+) and could be inhibited by bafilomycin A1 and botulinum toxin A. Ca(2+) transients in OECs evoked by ATP, axonal stimulation, and laser photolysis of NP-EGTA resulted in constriction of adjacent blood vessels. Our results indicate that ATP and glutamate are released ectopically by vesicles along axons and mediate neurovascular coupling via glial Ca(2+) signaling.

ATP storage and uptake by isolated pancreatic zymogen granules

ATP is released from pancreatic acini in response to cholinergic and hormonal stimulation. The same stimuli cause exocytosis of ZG (zymogen granules) and release of digestive enzymes. The aim of the present study was to determine whether ZG stored ATP and to characterize the uptake mechanism for ATP transport into the ZG. ZG were isolated and the ATP content was measured using luciferin/luciferase assays and was related to protein in the sample. The estimate of ATP concentration in freshly isolated granules was 40-120 microM. The ATP uptake had an apparent Km value of 4.9+/-2.1 mM when granules were incubated without Mg2+ and a Km value of 0.47+/-0.05 mM in the presence of Mg2+, both in pH 6.0 buffers. The uptake of ATP was significantly higher at pH 7.2 compared with pH 6.0 solutions. The anion transport blockers DIDS (4,4'-di-isothiocyanostilbene-2,2'-disulfonate) and Evans Blue inhibited ATP transport. Western blot analysis on the ZG showed the presence of VNUT (vesicular nucleotide transporter). Together, these findings indicate that VNUT may be responsible for the ATP uptake into ZG. Furthermore, the present study shows the presence of ATP together with digestive enzymes in ZG. This indicates that co-released ATP would regulate P2 receptors in pancreatic ducts and, thus, ductal secretion, and this would aid delivery of enzymes to the duodenum.

Purinergic regulation of hypothalamopituitary functions

The hypothalamus controls the release of hormones by the pituitary and is involved in control of food and water intake, sexual behavior, reproduction and daily cycles in physiological state and behavior, temperature regulation and emotional responses. Adenosine-5'-triphosphate (ATP) and its metabolic products contribute to these functions, acting as agonists for adenosine and P2Y receptors and two-transmembrane domain P2X receptor channels. This review summarizes the recent findings on purinergic receptor expression and their roles in signaling and cellular function in secretory and supporting cells of the hypothalamopituitary system. ATP secretion by these tissues, the enzymes involved in ATP hydrolysis, and the relevance of this pathway for sequential activation of receptors and termination of signaling are also discussed.

Signaling at purinergic P2X receptors

P2X receptors are membrane cation channels gated by extracellular ATP. Seven P2X receptor subunits (P2X(1-7)) are widely distributed in excitable and nonexcitable cells of vertebrates. They play key roles in inter alia afferent signaling (including pain), regulation of renal blood flow, vascular endothelium, and inflammatory responses. We summarize the evidence for these and other roles, emphasizing experimental work with selective receptor antagonists or with knockout mice. The receptors are trimeric membrane proteins: Studies of the biophysical properties of mutated subunits expressed in heterologous cells have indicated parts of the subunits involved in ATP binding, ion permeation (including calcium permeability), and membrane trafficking. We review our current understanding of the molecular properties of P2X receptors, including how this understanding is informed by the identification of distantly related P2X receptors in simple eukaryotes.

Ca2+/calmodulin-dependent kinase II signalling cascade mediates P2X7 receptor-dependent inhibition of neuritogenesis in neuroblastoma cells

ATP, via purinergic P2X receptors, acts as a neurotransmitter and modulator in both the central and peripheral nervous systems, and is also involved in many biological processes, including cell proliferation, differentiation and apoptosis. Previously, we have reported that P2X7 receptor inhibition promotes axonal growth and branching in cultured hippocampal neurons. In this article, we demonstrate that the P2X7 receptor negatively regulates neurite formation in mouse Neuro-2a neuroblastoma cells through a Ca2+/calmodulin-dependent kinase II-related mechanism. Using both molecular and immunocytochemical techniques, we characterized the presence of endogenous P2X1, P2X3, P2X4 and P2X7 subunits in these cells. Of these, the P2X7 receptor was the only functional receptor, as its activation induced intracellular calcium increments similar to those observed in primary neuronal cultures, exhibiting pharmacological properties characteristic of homomeric P2X7 receptors. Patch-clamp experiments were also conducted to fully demonstrate that ionotropic P2X7 receptors mediate nonselective cation currents in this cell line. Pharmacological inhibition of the P2X7 receptor and its knockdown by small hairpin RNA interference resulted in increased neuritogenesis in cells cultured in low serum-containing medium, whereas P2X7 overexpression significantly reduced the formation of neurites. Interestingly, P2X7 receptor inhibition also modified the phosphorylation state of focal adhesion kinase, Akt and glycogen synthase kinase 3, protein kinases that participate in the Ca2+/calmodulin-dependent kinase II signalling cascade and that have been related to neuronal differentiation and axonal growth. Taken together, our results provide the first mechanistic insight into P2X7 receptor-triggered signalling pathways that regulate neurite formation in neuroblastoma cells.

Evolutionary origins of the purinergic signalling system

Purines appear to be the most primitive and widespread chemical messengers in the animal and plant kingdoms. The evidence for purinergic signalling in plants, invertebrates and lower vertebrates is reviewed. Much is based on pharmacological studies, but important recent studies have utilized the techniques of molecular biology and receptors have been cloned and characterized in primitive invertebrates, including the social amoeba Dictyostelium and the platyhelminth Schistosoma, as well as the green algae Ostreococcus, which resemble P2X receptors identified in mammals. This suggests that contrary to earlier speculations, P2X ion channel receptors appeared early in evolution, while G protein-coupled P1 and P2Y receptors were introduced either at the same time or perhaps even later. The absence of gene coding for P2X receptors in some animal groups [e.g. in some insects, roundworms (Caenorhabditis elegans) and the plant Arabidopsis] in contrast to the potent pharmacological actions of nucleotides in the same species, suggests that novel receptors are still to be discovered.

Adenosine in the tuberomammillary nucleus inhibits the histaminergic system via A1 receptors and promotes non-rapid eye movement sleep

Adenosine has been proposed to promote sleep through A(1) receptors (A(1)R's) and/or A(2A) receptors in the brain. We previously reported that A(2A) receptors mediate the sleep-promoting effect of prostaglandin D(2), an endogenous sleep-inducing substance, and that activation of these receptors induces sleep and blockade of them by caffeine results in wakefulness. On the other hand, A(1)R has been suggested to increase sleep by inhibition of the cholinergic region of the basal forebrain. However, the role and target sites of A(1)R in sleep-wake regulation remained controversial. In this study, immunohistochemistry revealed that A(1)R was expressed in histaminergic neurons of the rat tuberomammillary nucleus (TMN). In vivo microdialysis showed that the histamine release in the frontal cortex was decreased by microinjection into the TMN of N(6)-cyclopentyladenosine (CPA), an A(1)R agonist, adenosine or coformycin, an inhibitor of adenosine deaminase, which catabolizes adenosine to inosine. Bilateral injection of CPA into the rat TMN significantly increased the amount and the delta power density of non-rapid eye movement (non-REM; NREM) sleep but did not affect REM sleep. CPA-promoted sleep was observed in WT mice but not in KO mice for A(1)R or histamine H(1) receptor, indicating that the NREM sleep promoted by A(1)R-specific agonist depended on the histaminergic system. Furthermore, the bilateral injection of adenosine or coformycin into the rat TMN increased NREM sleep, which was completely abolished by coadministration of 1,3-dimethyl-8-cyclopenthylxanthine, a selective A(1)R antagonist. These results indicate that endogenous adenosine in the TMN suppresses the histaminergic system via A(1)R to promote NREM sleep.

Pathway-dependent modulation by P2-purinoceptors in the mouse retina

Adenosine trisphosphate (ATP) activates purinoceptors and acts as a neurotransmitter in the nervous system. In the retina, we previously reported that the immunohistochemical distribution of the subset of P2-purinoceptors differs between the ON and OFF pathways. Here, we investigated whether ATP activates P2-purinoceptors and modulates the physiological function of the mouse retina. We also examined if signal processing by P2-purinoceptors is pathway specific. Results showed that ATP activated both ON- and OFF-cholinergic amacrine cells. However, responses in OFF-cholinergic amacrine cells were greater than those in ON-cholinergic amacrine cells. Pharmacological studies in OFF-cholinergic amacrine cells showed that the response of OFF-cholinergic amacrine cells is mediated P2X(2)-purinoceptors. Further, ATP increased gamma-aminobutyric acid (GABA)ergic inhibitory postsynaptic currents (IPSCs) in OFF- but not ON-cholinergic amacrine cells. The increase in GABAergic IPSCs was mediated by P2-purinoceptors. P2-purinoceptor-mediated signals suppressed OFF ganglion cells but activated ON ganglion cells. Our findings indicate that ATP physiologically modulates signal processing of the ON and OFF pathways in a pathway-specific manner through P2-purinoceptors.

P2X receptors and synaptic plasticity

Adenosine triphosphate (ATP) is released in many synapses in the CNS either together with other neurotransmitters, such as glutamate and GABA, or on its own. Postsynaptic action of ATP is mediated through metabotropic P2Y and ionotropic P2X receptors abundantly expressed in neural cells. Activation of P2X receptors induces fast excitatory postsynaptic currents in synapses located in various brain regions, including medial habenula, hippocampus and cortex. P2X receptors display relatively high Ca2+ permeability and can mediate substantial Ca2+ influx at resting membrane potential. P2X receptors can dynamically interact with other neurotransmitter receptors, including N-methyl-D-aspartate (NMDA) receptors, GABA(A) receptors and nicotinic acetylcholine (ACh) receptors. Activation of P2X receptors has multiple modulatory effects on synaptic plasticity, either inhibiting or facilitating the long-term changes of synaptic strength depending on physiological context. At the same time precise mechanisms of P2X-dependent regulation of synaptic plasticity remain elusive. Further understanding of the role of P2X receptors in regulation of synaptic transmission in the CNS requires dissection of P2X-mediated effects on pre-synaptic terminals, postsynaptic membrane and glial cells.