ATP and glutamate are released from separate neurones in the rat medial habenula nucleus: frequency dependence and adenosine-mediated inhibition of release

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.

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.

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.

Functional Principles of Posterior Septal Inputs to the Medial Habenula

The medial habenula (MHb) is an epithalamic hub contributing to expression and extinction of aversive states by bridging forebrain areas and midbrain monoaminergic centers. Although contradictory information exists regarding their synaptic properties, the physiology of the excitatory inputs to the MHb from the posterior septum remains elusive. Here, combining optogenetics-based mapping with ex vivo and in vivo physiology, we examine the synaptic properties of posterior septal afferents to the MHb and how they influence behavior. We demonstrate that MHb cells receive sparse inputs producing purely glutamatergic responses via calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), heterotrimeric GluN2A-GluN2B-GluN1 N-methyl-D-aspartate (NMDA) receptors, and inhibitory group II metabotropic glutamate receptors. We describe the complex integration dynamics of these components by MHb cells. Finally, we combine ex vivo data with realistic afferent firing patterns recorded in vivo to demonstrate that efficient optogenetic septal stimulation in the MHb induces anxiolysis and promotes locomotion, contributing long-awaited evidence in favor of the importance of this septo-habenular pathway.

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Medial habenular (MHb) neurons receive sparse inputs from the posterior septum (PS)

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PS afferents to the MHb function in a purely glutamatergic mode

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Excitatory ionotropic and inhibitory metabotropic receptors convey PS inputs in the MHb

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PS activation in the MHb increases locomotion and induces anxiolysis

The dorsal diencephalic conduction system in reward processing: Spotlight on the anatomy and functions of the habenular complex

The dorsal diencephalic conduction system (DDC) is a highly conserved pathway in vertebrates that provides a route for the neural information to flow from forebrain to midbrain structures. It contains the bilaterally paired habenular nuclei along with two fiber tracts, the stria medullaris and the fasciculus retroflexus. The habenula is the principal player in mediating the dialogue between forebrain and midbrain regions, and functional abnormalities in this structure have often been attributed to pathologies like mood disorders and substance use disorder. Following Matsumoto and Hikosaka seminal work on the lateral habenula as a source of negative reward signals, the last decade has witnessed a great surge of interest in the role of the DDC in reward-related processes. However, despite significant progress in research, much work remains to unfold the behavioral functions of this intriguing, yet complex, pathway. This review describes the current state of knowledge on the DDC with respect to its anatomy, connectivity, and functions in reward and aversion processes.

The habenulo-interpeduncular pathway in nicotine aversion and withdrawal

Progress has been made over the last decade in our understanding of the brain areas and circuits involved in nicotine reward and withdrawal, leading to models of addiction that assign different addictive behaviors to distinct, yet overlapping, neural circuits (Koob and Volkow, 2010; Lobo and Nestler, 2011; Tuesta et al., 2011; Volkow et al., 2011). Recently the habenulo-interpeduncular (Hb-IPN) midbrain pathway has re-emerged as a new critical crossroad that influences the brain response to nicotine. This brain area is particularly enriched in nicotinic acetylcholine receptor (nAChR) subunits α5, α3 and β4 encoded by the CHRNA5-A3-B4 gene cluster, which has been associated with vulnerability to tobacco dependence in human genetics studies. This finding, together with studies in mice involving deletion and replacement of nAChR subunits, and investigations of the circuitry, cell types and electrophysiological properties, have begun to identify the molecular mechanisms that take place in the MHb-IPN which underlie critical aspects of nicotine dependence. In the current review we describe the anatomical and functional connections of the MHb-IPN system, as well as the contribution of specific nAChRs subtypes in nicotine-mediated behaviors. Finally, we discuss the specific electrophysiological properties of MHb-IPN neuronal populations and how nicotine exposure alters their cellular physiology, highlighting the unique role of the MHb-IPN in the context of nicotine aversion and withdrawal. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

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.

The medial habenula contains a specific nonstellate subtype of astrocyte expressing the ectonucleotidase NTPDase2

ATP-mediated synaptic transmission represents the only transmitter-gated Ca(2+)-entry pathway in neurons of the rodent medial habenula. In addition to direct purinergic receptor-mediated synaptic inputs, the medial habenula contains purinergic systems that modulate synaptic transmission. Purinergic signaling is modulated or terminated by ectonucleotidase, nucleotide-hydrolyzing enzymes of the cell surface. Here we identify the major ectonucleotidase responsible for the hydrolysis of extracellular ATP in the mouse medial habenula as ectonucleoside triphosphate diphosphohydrolase 2 (NTPDase2), using immunostaining and enzyme histochemistry. Double labeling experiments reveal that the enzyme is expressed by astrocytes enwrapping the densely packed neurons and also the myelinated fiber bundles of the stria medullaris. NTPDase2 immunoreactivity is absent from the lateral habenula. The analysis of mice expressing enhanced green fluorescent protein under the promoter of glial fibrillary acidic protein revealed that the medial habenula harbors a highly polar type of astrocytes with very long laminar cellular processes, untypical for grey matter astrocytes. Its morphology strongly differs from that of the stellate astrocytes in the adjacent lateral habenula. Our results suggest that the mouse medial habenula contains a specific perineuronal nonstellate subtype of astrocyte that expresses the ectonucleotidase NTPDase2 and is in a strategic position to modulate purinergic transmission in this subnucleus.

NT2 derived neuronal and astrocytic network signalling

A major focus of stem cell research is the generation of neurons that may then be implanted to treat neurodegenerative diseases. However, a picture is emerging where astrocytes are partners to neurons in sustaining and modulating brain function. We therefore investigated the functional properties of NT2 derived astrocytes and neurons using electrophysiological and calcium imaging approaches. NT2 neurons (NT2Ns) expressed sodium dependent action potentials, as well as responses to depolarisation and the neurotransmitter glutamate. NT2Ns exhibited spontaneous and coordinated calcium elevations in clusters and in extended processes, indicating local and long distance signalling. Tetrodotoxin sensitive network activity could also be evoked by electrical stimulation. Similarly, NT2 astrocytes (NT2As) exhibited morphology and functional properties consistent with this glial cell type. NT2As responded to neuronal activity and to exogenously applied neurotransmitters with calcium elevations, and in contrast to neurons, also exhibited spontaneous rhythmic calcium oscillations. NT2As also generated propagating calcium waves that were gap junction and purinergic signalling dependent. Our results show that NT2 derived astrocytes exhibit appropriate functionality and that NT2N networks interact with NT2A networks in co-culture. These findings underline the utility of such cultures to investigate human brain cell type signalling under controlled conditions. Furthermore, since stem cell derived neuron function and survival is of great importance therapeutically, our findings suggest that the presence of complementary astrocytes may be valuable in supporting stem cell derived neuronal networks. Indeed, this also supports the intriguing possibility of selective therapeutic replacement of astrocytes in diseases where these cells are either lost or lose functionality.

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.

Neurochemical phenotypes of the afferent and efferent projections of the mouse medial habenula

The medial habenula (MHb) is a key bridge between limbic forebrain and midbrain monoaminergic centers. Although its exact behavioral function remains enigmatic, it is implicated in regulating many behaviors such as stress responses and circadian rhythm. Fundamental information such as the neurotransmitters in the afferent and efferent projections of the MHb remains unclear. By combining retrograde tract tracing and genetic labeling of GABAergic neurons in mice, we find that the medial septum (MS) and nucleus of diagonal band (NDB) provide GABA-ergic input to the MHb. By anterograde tracing and immunostaining against a marker of glutamatergic synapses, we find that the projection from the triangular septal nucleus (TS) to the MHb has the capacity to release glutamate. This suggests that in addition to ATP, glutamate is another neurotransmitter for the TS-->MHb projection. Finally, by combining anterograde tracing and immunostaining, we find that the MHb neurons projecting to the interpeduncular nucleus (IPN) have the capacity to release glutamate. This suggests that, in addition to acetylcholine and substance P, glutamate is another neurotransmitter used by MHb projection neurons. Our findings reveal the organization of two key neurotransmitters for the MHb afferent and efferent projections, and lay framework for future functional studies of this pathway in the brain.

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.

Purinergic signalling in the nervous system: an overview

Purinergic receptors, represented by several families, are arguably the most abundant receptors in living organisms and appeared early in evolution. After slow acceptance, purinergic signalling in both peripheral and central nervous systems is a rapidly expanding field. Here, we emphasize purinergic co-transmission, mechanisms of release and breakdown of ATP, ion channel and G-protein-coupled-receptor subtypes for purines and pyrimidines, the role of purines and pyrimidines in neuron-glial communication and interactions of this system with other transmitter systems. We also highlight recent data involving purinergic signalling in pathological conditions, including pain, trauma, ischaemia, epilepsy, migraine, psychiatric disorders and drug addiction, which we expect will lead to the development of therapeutic strategies for these disorders with novel mechanisms of action.

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.

Nicotinic receptor activation occludes purinergic control of central cardiorespiratory network responses to hypoxia/hypercapnia

Prenatal nicotine exposure alters the cardiorespiratory network responses to hypoxia/hypercapnia; however the mechanism(s) responsible for these cardiorespiratory network responses and their alteration by prenatal nicotine exposure are unknown. We used an in vitro medullary slice that allows simultaneous examination of rhythmic respiratory-related activity and excitatory synaptic neurotransmission to cardioinhibitory vagal neurons (CVNs). Respiratory related increases in glutamatergic neurotransmission only occurred on recovery from hypoxia/hypercapnia in unexposed animals. These responses were not altered by nicotinic antagonists but were mediated in part by activation of P2 purinergic receptors. Prenatal nicotine exposure transformed central cardiorespiratory responses to hypoxia/hypercapnia; CVNs received a respiratory related glutamatergic neurotransmission during periods of hypoxia and hypercapnia, whereas increases in glutamatergic neurotransmission during recovery were absent. The excitatory neurotransmission to CVNs during hypoxia/hypercapnia in prenatal nicotine-exposed animals were wholly dependent on nicotinic receptor activation. In the presence of nicotinic antagonists, the responses in prenatal nicotine animals reverted to the pattern of responses in unexposed animals in which an increase in glutamatergic neurotransmission occurred not during but only on recovery from hypoxia/hypercapnia, and this recruited excitatory pathway was blocked by P2 receptor antagonists. These data identify a new functional role for purinergic receptors in the cardiorespiratory responses to hypoxia/hypercapnia and their role in occluding nicotinic receptor activation with prenatal nicotine exposure.

Absence of GABA type A signaling in adult medial habenular neurons

Neural inhibition in the brain is mainly mediated by ionotropic GABA type A receptors. Apart from the GABA type A receptors, both K(+)-Cl(-) cotransporter isoform 2 and the GABA-synthesizing enzyme, glutamic acid decarboxylase, are essential determinants for GABA type A receptor-mediated inhibition. By using immunofluorescent staining, we observed that K(+)-Cl(-) cotransporter isoform 2, GABA type A receptor beta2/3 subunits and a presynaptically localized glutamic acid decarboxylase isoform, glutamic acid decarboxylase 65, were all absent in adult Sprague-Dawley rat medial habenular nucleus, while immunopositive staining for glutamic acid decarboxylase 67, GABA and GABA type B receptor type 2 subunit were present in the medial habenular nucleus. Consistent with the lack of GABA type A signaling as detected by immunohistochemistry, GABA (100 muM) evoked no measurable currents in the medial habenular nucleus but induced bicuculline-sensitive currents in the lateral habenular nucleus and in the CA1 area of hippocampus. We also failed to record miniature inhibitory postsynaptic currents in medial habenular nucleus neurons. These results support the idea that GABAergic transmission in medial habenular nucleus is probably not mediated by any of the most common GABA type A receptor subtypes. Our data suggest that GABA type B receptor-mediated inhibition may play a role in balancing neuronal excitation in this special region. Further exploration for factors determining medial habenular nucleus neural inhibition will lead to a more complete understanding of control of synaptic balance in the CNS.

Purinergic transmission in the central nervous system

The adenosine 5'-triphosphate (ATP), discovered in 1929 by Karl Lohman, Cyrus Hartwell Fiske, and Yellagaprada SubbaRow, acts as an important extracellular signaling molecule. In the CNS, ATP can be released from synaptic terminals, either on its own or together with other neurotransmitters. After the release from the presynaptic terminals, ATP binds to a plethora of ionotropic and metabotropic receptors, which mediate its action as an excitatory neurotransmitter. Furthermore, ATP also acts as an important mediator in neuronal-glial communications because glial cells are endowed with numerous ATP receptors, which trigger Ca(2+) signaling events and membrane currents in both macro and microglia. In addition, ATP can be released from astroglial cells, thereby acting as a mediator of glial-glial and glial-neuronal signaling.

Vesicular release of ATP at central synapses

Adenosine triphosphate (ATP) acts as a fast excitatory transmitter in several regions of the central nervous system (CNS) including the medial habenula, dorsal horn, locus coeruleus, hippocampus, and somatosensory cortex. Postsynaptic actions of ATP are mediated through an extended family of P2X receptors, widely expressed throughout the CNS. ATP is released via several pathways, including exocytosis from presynaptic terminals and diffusion through large transmembrane pores (e.g., hemichannels, P2X(7) receptors, or volume-sensitive chloride channels) expressed in astroglial membranes. In presynaptic terminals, ATP is accumulated and stored in the synaptic vesicles. In different presynaptic terminals, these vesicles may contain ATP only or ATP and another neurotransmitter [e.g., gamma-amino-butyric acid (GABA) or glutamate]; in the latter case, two transmitters can be coreleased. Here, we discuss the mechanisms of vesicular release of ATP in the CNS and present our own data, which indicate that in central neuronal terminals, ATP is primarily stored and released from distinct pool of vesicles; the release of ATP is not synchronized either with GABA or with glutamate.

Increased resistance of glioma cell lines to extracellular ATP cytotoxicity

Glioblastomas are the most common form of primary tumors of the central nervous system (CNS) and despite treatment, patients with these tumors have a very poor prognosis. ATP and other nucleotides and nucleosides are very important signaling molecule in physiological and pathological conditions in the CNS. ATP is degraded very slowly by gliomas when compared to astrocytes, potentially resulting in the accumulation of extracellular ATP around gliomas. Cell lysis caused by excitotoxic death or by tumor resection may liberate intracellular ATP, a known mitotic factor for glioma cells. The aim of this study is to examine the effects on cytotoxicity induced by extracellular ATP in U138-MG human glioma cell line and C6 rat glioma cell line compared to hippocampal organotypic cell cultures. The cytotoxicity of ATP (0.1, 0.5, 5 mM) was measured using propidium iodide and LDH assays. Caspases assay was performed to identify apoptotic cell death. Results showed that the glioma cells present resistance to death induced by ATP when compared with a normal tissue. High ATP concentrations (5 mM) induced cell death after 24 h in organotypic cell cultures but not in glioma cell lines. Our data indicate that ATP released in these situations can induce cell death of the normal tissue surrounding the tumor, potentially opening space to the fast growth and invasion of the tumor.

ATP- and adenosine-mediated signaling in the central nervous system: synaptic purinoceptors: the stage for ATP to play its "dual-role"

The studies aiming to understand the function of purinoceptors in the central nervous system (CNS), which has been explored mostly in isolated and cultured cell systems, are now at the stage of identifying their physiological and pathophysiological significance in the native organs, tissues, and whole animals. The results of our recent studies made in brain slice preparations are not in full accordance with what have been demonstrated in isolated cells, mostly due to strong interplay between ATP receptors, adenosine receptors, and ecto-nucleotidases. This suggests that these proteins form coordinated regulation systems in the native tissue, controlling the local network behaviors through regulating the balance between the effects of ATP and adenosine on synaptic transmissions. We propose that this tripartite regulation system by extracellular purines may be an important target of CNS drugs.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Anti-addictive actions of an iboga alkaloid congener: a novel mechanism for a novel treatment

18-Methoxycoronaridine (18-MC), a novel iboga alkaloid congener that decreases drug self-administration in several animal models, may be a potential treatment for multiple forms of drug abuse. In animal models, 18-MC reduced intravenous morphine, cocaine, methamphetamine and nicotine self-administration, oral alcohol and nicotine intake, and attenuated signs of opioid withdrawal, but had no effect on responding for a nondrug reinforcer (water) and produced no apparent toxicity [Brain Res. 719 (1996) 29; NeuroReport 11 (2000) 2013; Pharmacol. Biochem. Behav. 58 (1997) 615; Psychopharmacology (Berl.) 139 (1998) 274; NeuroReport 9 (1998) 1283; Ann. N. Y. Acad. Sci. 914 (2000) 369]. Consistent with a relationship among drug sensitization, mesolimbic dopamine, and drug-seeking behavior, 18-MC also blocked the sensitized dopamine responses to morphine and cocaine in the nucleus accumbens. An extensive series of receptor studies showed that 18-MC was most potent and somewhat selective as an antagonist at alpha3beta4 nicotinic receptors. Low-dose combinations of 18-MC with other drugs known to have this same action (e.g., mecamylamine, dextromethorphan, bupropion) decreased morphine, methamphetamine, and nicotine self-administration in rats at doses that were ineffective if administered alone. Together, the data support the hypothesis that diencephalic pathways having high densities of alpha3beta4 nicotinic receptors modulate mesocorticolimbic pathways more directly involved in drug reinforcement. Antagonists of alpha3beta4 nicotinic receptors may represent a totally novel approach to treating multiple addictive disorders, and 18-MC might be the first of a new class of synthetic agents acting via this novel mechanism and having a broad spectrum of activity.

Long-term potentiation of glutamatergic synaptic transmission induced by activation of presynaptic P2Y receptors in the rat medial habenula nucleus

A novel form of long-term potentiation of glutamatergic synaptic transmission is described in the rat medial habenula nucleus. It occurs when uridine 5'-triphosphate is bath applied at low micromolar concentrations and is prevented by Reactive Blue 2, suggesting that it is mediated by P2Y4 receptors. Uridine 5'-diphosphate can also cause such a Reactive Blue 2-sensitive potentiation, but at higher concentrations (200 microm), suggesting that this might also be an effect on the relatively uridine 5'-diphosphate-insensitive P2Y4 receptor. The potentiation is due to an increase in presynaptic release probability. It requires neither depolarization nor calcium influx postsynaptically and is thus probably non-Hebbian. When potentiation due to low concentrations of uridine 5'-triphosphate is inhibited in the presence of Reactive Blue 2, uridine 5'-triphosphate causes instead a significant inhibition of glutamate release. We suggest that this inhibition may be mediated by a Reactive Blue 2-insensitive P2Y2-like receptor. At higher concentrations of uridine 5'-triphosphate (200 micro m), the inhibitory effect dominates such that even in the absence of Reactive Blue 2 no potentiation is seen.

Transducing system operated by adenosine A(2A) receptors to facilitate acetylcholine release in the rat hippocampus

Although molecular biology studies indicate the presence of adenosine A(2A) receptors in the rat hippocampus, the pharmacological characterization of adenosine A(2A) receptor binding and of its putative facilitatory effects has revealed features essentially different from these found for adenosine A(2A) receptors in most preparations. We now confirmed that activation of adenosine A(2A) receptors with 2-[4-(2-p-carboxyethyl)phenylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680, 1-30 nM) or 2-hexynyl-5'-N-ethylcarboxamidoadenosine (HENECA, 3-100 nM) facilitated the veratridine-evoked [3H]acetylcholine release from hippocampal synaptosomes with a maximal effect of 14+/-2% and 16+/-2%, respectively. These effects were prevented by the adenosine A(2A) receptor antagonists, 4-(2-[7-amino-2-[2-furyl][1,2,4]-triazolo[2,3a][1,3,5]triazin-5-yl-amino]ethyl)phenol (ZM 241385, 20 nM) and 5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH 58261, 20 nM), but not by the adenosine A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 20 nM). Adenosine A(2A) receptors may activate adenylate cyclase and protein kinase A since CGS 21680 (10 nM) facilitation of [3H]acetylcholine release was occluded by 8-bromo-cAMP (0.5 mM) and forskolin (10 microM) and prevented by H-89 (1 microM), but unaffected by phorbol-12,13-didecanoate (250 nM) or bisindolylmaleimide I (1 microM). The existence of adenosine A(2A) receptors in hippocampal nerve terminals was further confirmed by a Western blot immunoreactivity qualitatively identical to that found in the striatum. This constitutes the first pharmacological identification of canonical adenosine A(2A) receptors coupling to the expected cAMP/protein kinase A pathway in the hippocampus with the expected immunoreactivity.

Molecular physiology of P2X receptors

P2X receptors are membrane ion channels that open in response to the binding of extracellular ATP. Seven genes in vertebrates encode P2X receptor subunits, which are 40-50% identical in amino acid sequence. Each subunit has two transmembrane domains, separated by an extracellular domain (approximately 280 amino acids). Channels form as multimers of several subunits. Homomeric P2X1, P2X2, P2X3, P2X4, P2X5, and P2X7 channels and heteromeric P2X2/3 and P2X1/5 channels have been most fully characterized following heterologous expression. Some agonists (e.g., alphabeta-methylene ATP) and antagonists [e.g., 2',3'-O-(2,4,6-trinitrophenyl)-ATP] are strongly selective for receptors containing P2X1 and P2X3 subunits. All P2X receptors are permeable to small monovalent cations; some have significant calcium or anion permeability. In many cells, activation of homomeric P2X7 receptors induces a permeability increase to larger organic cations including some fluorescent dyes and also signals to the cytoskeleton; these changes probably involve additional interacting proteins. P2X receptors are abundantly distributed, and functional responses are seen in neurons, glia, epithelia, endothelia, bone, muscle, and hemopoietic tissues. The molecular composition of native receptors is becoming understood, and some cells express more than one type of P2X receptor. On smooth muscles, P2X receptors respond to ATP released from sympathetic motor nerves (e.g., in ejaculation). On sensory nerves, they are involved in the initiation of afferent signals in several viscera (e.g., bladder, intestine) and play a key role in sensing tissue-damaging and inflammatory stimuli. Paracrine roles for ATP signaling through P2X receptors are likely in neurohypophysis, ducted glands, airway epithelia, kidney, bone, and hemopoietic tissues. In the last case, P2X7 receptor activation stimulates cytokine release by engaging intracellular signaling pathways.

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

Autonomic and limbic information is integrated within the lateral hypothalamus (LH), and excitability of LH neurons is important in the control of feeding and behavioral arousal. Despite the prominent expression of P2X-type ATP receptors throughout the hypothalamus, the role of ATP in LH excitability is not known. Perforated-patch-clamp recordings of synaptically coupled neurons from both embryonic chick and postnatal mouse lateral hypothalamus in vitro reveal robust stimulus-evoked purinergic synaptic transmission. Suprathreshold activation elicits reliable and concurrent release of ATP with GABA. Tetrodotoxin-resistant P2X receptor-mediated events are readily observed at LH synapses from the embryonic chick, whereas GABA miniature postsynaptic currents (mPSCs) are recorded in innervated LH neurons from either embryonic chicks or postnatal mice. Two distinct mPSCs are recorded at ATP-GABA cosynapses; one has a monoexponential decay phase and is modulated by flunitrazepam, and the other has a decay phase that is best fit by a sum of two exponential functions (tau(fast) and tau(slow)), and only the tau(slow) component is affected by flunitrazepam. Bicuculline does not completely inhibit all mPSCs. The remaining bicuculline-resistant mPSCs are blocked by suramin, and their decay phase is briefer than that of GABAergic mPSCs. Furthermore, at a holding potential intermediate for the reversal potentials of GABA(A) and P2X receptors, little or no current is observed, consistent with concomitant release (and detection) of GABA and ATP. Together, our data suggest that a subset of spontaneous and evoked PSCs arise from the concurrent activation of both GABA(A) and P2X receptors.

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

Autonomic and limbic information is integrated within the lateral hypothalamus (LH), and excitability of LH neurons is important in the control of feeding and behavioral arousal. Despite the prominent expression of P2X-type ATP receptors throughout the hypothalamus, the role of ATP in LH excitability is not known. Perforated-patch-clamp recordings of synaptically coupled neurons from both embryonic chick and postnatal mouse lateral hypothalamus in vitro reveal robust stimulus-evoked purinergic synaptic transmission. Suprathreshold activation elicits reliable and concurrent release of ATP with GABA. Tetrodotoxin-resistant P2X receptor-mediated events are readily observed at LH synapses from the embryonic chick, whereas GABA miniature postsynaptic currents (mPSCs) are recorded in innervated LH neurons from either embryonic chicks or postnatal mice. Two distinct mPSCs are recorded at ATP-GABA cosynapses; one has a monoexponential decay phase and is modulated by flunitrazepam, and the other has a decay phase that is best fit by a sum of two exponential functions (tau(fast) and tau(slow)), and only the tau(slow) component is affected by flunitrazepam. Bicuculline does not completely inhibit all mPSCs. The remaining bicuculline-resistant mPSCs are blocked by suramin, and their decay phase is briefer than that of GABAergic mPSCs. Furthermore, at a holding potential intermediate for the reversal potentials of GABA(A) and P2X receptors, little or no current is observed, consistent with concomitant release (and detection) of GABA and ATP. Together, our data suggest that a subset of spontaneous and evoked PSCs arise from the concurrent activation of both GABA(A) and P2X receptors.

Interaction between P2Y and NMDA receptors in layer V pyramidal neurons of the rat prefrontal cortex

In the first part of this study, monosynaptic excitatory postsynaptic potentials (EPSPs) in layer V of the rat prefrontal cortex were evoked by electrical stimulation of layer I. Recordings by intracellular sharp microelectrodes showed that EPSPs were concentration-dependently facilitated by the P2 receptor antagonistic ATP analogue 2-methylthio ATP (2-MeSATP), while ATP itself depressed the synaptic potentials. The inhibitory effect of ATP turned into facilitation in the presence of the adenosine A(1) receptor antagonist DPCPX. The 2-MeSATP-induced potentiation of EPSP amplitudes were prevented by the P2 receptor antagonists PPADS and Suramin. The EPSP was almost abolished by coapplication of the NMDA receptor antagonist AP-5 and the AMPA/kainate receptor antagonist CNQX. After blockade of the NMDA receptor-mediated part of the EPSP by AP-5, the stimulatory effect of 2-MeSATP disappeared. When NMDA or AMPA were pressure-applied onto pyramidal cells, only the NMDA-induced depolarization was potentiated by 2-MeSATP. In the second part of the study, NMDA-induced currents were measured by whole-cell patch-clamp pipettes. ATP, 2-MeSATP, UDP and UTP potentiated the response to NMDA, while ADP-beta-S was inactive. PPADS antagonized the effect of ATP. Synaptic isolation of pyramidal neurons by a Ca(2+)-free medium or tetrodotoxin did not alter the effect of ATP which, however, was markedly depressed when GTP in the micropipette was replaced by GDP-beta-S. These observations suggest that in layer V pyramidal neurons of the prefrontal cortex postsynaptically localized P2Y receptors interact with NMDA receptor-channels.

P2 receptor modulation and cytotoxic function in cultured CNS neurons

In this study we investigate the presence, modulation and biological function of P2 receptors and extracellular ATP in cultured cerebellar granule neurons. As we demonstrate by RT-PCR and western blotting, both P2X and P2Y receptor subtypes are expressed and furthermore regulated as a function of neuronal maturation. In early primary cultures, mRNA for most of the P2 receptor subtypes, except P2X(6), are found, while in older cultures only P2X(3), P2Y(1) and P2Y(6) mRNA persist. In contrast, P2 receptor proteins are more prominent in mature neurons, with the exception of P2Y(1). We also report that extracellular ATP acts as a cell death mediator for fully differentiated and mature granule neurons, for dissociated striatal primary cells and hippocampal organotypic cultures, inducing both apoptotic and necrotic features of degeneration. ATP causes cell death with EC(50) in the 20-50 microM range within few minutes of exposure and with a time lapse of at most two hours. Additional agonists for P2 receptors induce toxic effects, whereas selected antagonists are protective. Cellular swelling, lactic dehydrogenase release and nuclei fragmentation are among the features of ATP-evoked cell death, which also include direct P2 receptor modulation. Comparably to P2 receptor antagonists previously shown preventing glutamate-toxicity, here we report that competitive and non-competitive NMDA receptor antagonists inhibit the detrimental consequences of extracellular ATP. Due to the massive extracellular release of purine nucleotides and nucleosides often occurring during a toxic insult, our data indicate that extracellular ATP can now be included among the potential causes of CNS neurodegenerative events.

Opioid receptor regulation of muscarinic acetylcholine receptor-mediated synaptic responses in the hippocampus

A common feature of many synapses is their regulation by neurotransmitters other than those released from the presynaptic terminal. This aspect of synaptic transmission is often mediated by activation of G protein coupled receptors (GPCRs) and has been most extensively studied at amino acid-mediated synapses where ligand gated receptors mediate the postsynaptic signal. Here we have investigated how opioid receptors modulate synaptic transmission mediated by muscarinic acetylcholine receptors (mAChRs) in hippocampal CA1 pyramidal neurones. Using a cocktail of glutamate and gamma-amino-butyric acid (GABA) receptor antagonists a slow pirenzepine-sensitive excitatory postsynaptic potential (EPSP(M)) that was associated with a small increase in cell input resistance could be evoked in isolation. This response was enhanced by the acetylcholine (ACh) esterase inhibitor physostigmine (1 microM) and depressed by the vesicular ACh transport inhibitor vesamicol (50 microM). The mu-opioid receptor agonists DAMGO (1-5 microM) and etonitazene (100 nM), but not the delta- and kappa-opioid receptor selective agonists DTLET (1 microM) and U-50488 (1 microM), potentiated this EPSP(M) (up to 327%) without affecting cell membrane potential or input resistance; an effect that was totally reversed by naloxone (5 microM). In contrast, postsynaptic depolarizations and increases in cell input resistance evoked by carbachol (3 microM) were unaffected by DAMGO (1-5 microM) but were abolished by atropine (1 microM). Taken together these data provide good evidence for a mu-opioid receptor-mediated presynaptic enhancement of mAChR-mediated EPSPs in hippocampal CA1 pyramidal neurones.

Fast synaptic transmission mediated by P2X receptors in CA3 pyramidal cells of rat hippocampal slice cultures

1. A fast ATP-mediated synaptic current was identified in CA3 pyramidal cells in organotypic hippocampal slice cultures. In the presence of inhibitors for ionotropic glutamate and GABA receptors, extracellular stimulation in the pyramidal cell layer evoked fast synaptic currents that reversed near 0 mV, reflecting an increase in a non-selective cationic conductance. This response was mimicked by focal application of ATP. Antagonists of ionotropic P2X receptors reduced both synaptic and ATP-induced currents. 2. Using a pharmacological approach, the source of synaptically released ATP was determined. Synaptic ATP responses were insensitive to presynaptic blockade of GABAergic transmission between interneurons and CA3 pyramidal cells with the mu-opioid receptor agonist D-Ala(2),MePhe(4),Met(O)(5)-ol-enkephalin (FK33-824), but were blocked by adenosine, which inhibits glutamate release from synaptic terminals in the hippocampus. However, selective inhibition of mossy fibre glutamatergic transmission with the metabotropic glutamate receptor group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) did not affect the response. This result points to the associational fibres as the source of the ATP-mediated synaptic response. 3. These results suggest that ATP, coreleased with glutamate, induces a synaptic response in CA3 pyramidal cells that is observed mainly under conditions of synchronous discharge from multiple presynaptic inputs.

Distinct Localization of P2X receptors at excitatory postsynaptic specializations

ATP mediates fast excitatory synaptic transmission in some regions of the central nervous system through activation of P2X receptors. Nonetheless, the functional significance of ATP-mediated neurotransmission is not yet understood. Using postembedding immunocytochemistry, we describe the distribution of P2X(2), P2X(4), and P2X(6) subunits in cerebellum and in the CA1 region of the hippocampus. Dendritic spines of cerebellar Purkinje cells showed immunogold labeling for all three subunits when apposed to parallel fiber (PF) terminals. In contrast, no immunogold labeling was observed on dendritic spines or cell bodies receiving inputs from climbing fibers and basket cells, respectively. In CA1 pyramidal cells, postsynaptic membranes apposed to terminals of Schaffer collaterals were immunogold-labeled for P2X(2), P2X(4), and P2X(6) subunits. Immunolabeling was also observed perisynaptically and intracellularly in relation to membranes of the endoplasmic reticulum. The analysis of the tangential distribution of gold particles showed that they were preferentially located at the peripheral portion of the postsynaptic specialization of both parallel fiber and Schaffer collateral synapses. By double imunogold labeling using antibodies for P2X receptor subunits and GluR2/3 subunits of the AMPA glutamate receptors, we show that synapses expressing P2X receptors are also glutamatergic. The present study shows for the first time qualitatively and quantitatively the precise localization of P2X receptors in brain. Moreover, our data indicate that P2X receptors may play a significant role at glutamatergic synapses.

Molecular physiology of P2X receptors and ATP signalling at synapses

ATP is found in every cell, where it is a major source of energy. But in the nervous system, ATP also has additional actions, which include its role in fast synaptic transmission and modulation. Here I discuss the 'fast' actions of ATP at synapses, the properties of the receptors that are activated by ATP and the physiology of ATP signalling, with emphasis on its role in pain processing.

Intracellular pathways regulating ciliary beating of rat brain ependymal cells

1. The mammalian brain ventricles are lined with ciliated ependymal cells. As yet little is known about the mechanisms by which neurotransmitters regulate cilia beat frequency (CBF). 2. Application of 5-HT to ependymal cells in cultured rat brainstem slices caused CBF to increase. 5-HT had an EC50 of 30 microM and at 100 microM attained a near-maximal CBF increase of 52.7 +/- 4.1 % (mean +/- s.d.) (n = 8). 3. Bathing slices in Ca2+-free solution markedly reduced the 5-HT-mediated increase in CBF. Fluorescence measurements revealed that 5-HT caused a marked transient elevation in cytosolic Ca2+ ([Ca2+]c) that then slowly decreased to a plateau level. Analysis showed that the [Ca2+]c transient was due to release of Ca2+ from inositol 1,4,5-trisphosphate (IP3)-sensitive stores; the plateau was probably due to extracellular Ca2+ influx through Ca2+ release-activated Ca2+ (CRAC) channels. 4. Application of ATP caused a sustained decrease in CBF. ATP had an EC50 of about 50 microM and 100 microM ATP resulted in a maximal 57.5 +/- 6.5 % (n = 12) decrease in CBF. The ATP-induced decrease in CBF was unaffected by lowering extracellular [Ca2+], and no changes in [Ca2+]c were observed. Exposure of ependymal cells to forskolin caused a decrease in CBF. Ciliated ependymal cells loaded with caged cAMP exhibited a 54.3 +/- 7.5 % (n = 9) decrease in CBF following uncaging. These results suggest that ATP reduces CBF by a Ca2+-independent cAMP-mediated pathway. 5. Application of 5-HT and adenosine-5'-O-3-thiotriphosphate (ATP-gamma-S) to acutely isolated ciliated ependymal cells resulted in CBF responses similar to those of ependymal cells in cultured slices suggesting that these neurotransmitters act directly on these cells. 6. The opposite response of ciliated ependymal cells to 5-HT and ATP provides a novel mechanism for their active involvement in central nervous system signalling.

Adenosine as a neuromodulator and as a homeostatic regulator in the nervous system: different roles, different sources and different receptors

Adenosine exerts two parallel modulatory roles in the CNS, acting as a homeostatic modulator and also as a neuromodulator at the synaptic level. We will present evidence to suggest that these two different modulatory roles are fulfilled by extracellular adenosine originated from different metabolic sources, and involve receptors with different sub-cellular localisation. It is widely accepted that adenosine is an inhibitory modulator in the CNS, a notion that stems from the preponderant role of inhibitory adenosine A(1) receptors in defining the homeostatic modulatory role of adenosine. However, we will review recent data that suggests that the synaptically localised neuromodulatory role of adenosine depend on a balanced activation of inhibitory A(1) receptors and mostly facilitatory A(2A) receptors. This balanced activation of A(1) and A(2A) adenosine receptors depends not only on the transient levels of extracellular adenosine, but also on the direct interaction between A(1) and A(2A) receptors, which control each other's action.

Distinct modulation of evoked and spontaneous EPSCs by purinoceptors in the nucleus tractus solitarii of the rat

Whole-cell transmembrane currents of second-order neurones in the caudal part of the nucleus tractus solitarii (cNTS) of brainstem slices of the rat were recorded to analyse the effects of adenosine 5'-triphosphate (ATP) on: (1) EPSCs evoked by the solitary tract stimulation (eEPSCs) and (2) spontaneous EPSCs (sEPSCs). ATP (10-6 to 10-4 m) significantly reduced the amplitude of eEPSCs to 46.6 +/- 7.4 % and increased the frequency of sEPSCs to 268.0 +/- 71.5 % of the control without significant changes in sEPSC amplitude. These opposite effects of ATP on eEPSCs and sEPSCs were concurrently observed in about 80 % of cNTS neurones recorded. The reduction of eEPSC amplitude by ATP was similarly observed with the addition of an equimolar solution of adenosine but not with alpha,beta-methylene ATP and was suppressed by 8-cyclopentyltheophylline (CPT) and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). Addition of pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) did not affect the reduction of eEPSC amplitude by ATP. The increase in sEPSC frequency by ATP remained under tetrodotoxin addition but was abolished in the presence of PPADS. It is suggested that ATP activates: (1) presynaptic adenosine A1 receptors, after being hydrolysed to adenosine, reducing evoked release of glutamate from the primary afferent terminals and (2) presynaptic P2X receptors on the axon terminals of intrinsic excitatory cNTS neurones facilitating spontaneous release of glutamate. This is the first evidence that ATP modulates excitatory synaptic inputs arising from distinct origins and converging on a single postsynaptic neurone in diametrically opposite directions through activation of distinct presynaptic purinoceptors.

Distinct Localization of P2X receptors at excitatory postsynaptic specializations

ATP mediates fast excitatory synaptic transmission in some regions of the central nervous system through activation of P2X receptors. Nonetheless, the functional significance of ATP-mediated neurotransmission is not yet understood. Using postembedding immunocytochemistry, we describe the distribution of P2X(2), P2X(4), and P2X(6) subunits in cerebellum and in the CA1 region of the hippocampus. Dendritic spines of cerebellar Purkinje cells showed immunogold labeling for all three subunits when apposed to parallel fiber (PF) terminals. In contrast, no immunogold labeling was observed on dendritic spines or cell bodies receiving inputs from climbing fibers and basket cells, respectively. In CA1 pyramidal cells, postsynaptic membranes apposed to terminals of Schaffer collaterals were immunogold-labeled for P2X(2), P2X(4), and P2X(6) subunits. Immunolabeling was also observed perisynaptically and intracellularly in relation to membranes of the endoplasmic reticulum. The analysis of the tangential distribution of gold particles showed that they were preferentially located at the peripheral portion of the postsynaptic specialization of both parallel fiber and Schaffer collateral synapses. By double imunogold labeling using antibodies for P2X receptor subunits and GluR2/3 subunits of the AMPA glutamate receptors, we show that synapses expressing P2X receptors are also glutamatergic. The present study shows for the first time qualitatively and quantitatively the precise localization of P2X receptors in brain. Moreover, our data indicate that P2X receptors may play a significant role at glutamatergic synapses.

A role for adenosine in modulating cardio-respiratory responses: a mini-review

Evidence is provided that purine nucleotides play a major role in modulating cardio-respiratory control through actions at specific sites in the central nervous system (CNS). The accumulation of adenosine in the extracellular space of the nucleus tractus solitarius (NTS) on hypothalamic defence area stimulation is described, and is shown to result from the extracellular breakdown of neurally released adenosine triphosphate (ATP). The actions of adenosine in modulating synaptic actions in the NTS are discussed in relation to the potential presynaptic action of adenosine in modulating transmitter release. Actions of adenosine and ATP at other sites in the medulla concerned with cardio-respiratory control are discussed. The inter-relationships between ATP and adenosine are mentioned.

Extracellular ATP reduces optically monitored electrical signals in hippocampal slices through metabolism to adenosine

Electrical signals in rat hippocampal slices were optically monitored using a voltage-sensitive dye to determine whether extracellular ATP exhibits direct effects through its own receptors or indirect effects after its hydrolysis to adenosine. The dentate gyrus was stimulated and electrical signals in the CA1 and the CA3 region were analyzed. The signals were divided into two components: a transient component peaking within 10 ms (fast component) and a subsequent sustained component (slow component). ATP (10 to 100 microM) inhibited both the fast and the slow components in the CA1 region by about 30% and 70%, respectively. ADP, AMP and adenosine also inhibited the fast and the slow components. The inhibition by ATP was antagonized by aminophylline and other adenosine receptor antagonists, and by alpha,beta-methylene ADP, an inhibitor of 5'-nucleotidases. These results suggest that extracellular ATP inhibits neuronal electrical signals in hippocampal slices after its metabolism to adenosine.

Adenosine A2A receptors regulate the extracellular accumulation of excitatory amino acids upon metabolic dysfunction in chick cultured retinal cells

The role of endogenous extracellular adenosine as a tonic modulator of the extracellular accumulation of excitatory amino acids (glutamate and aspartate) caused by metabolic inhibition was investigated in cultured retinal cells. The selective adenosine A2A receptor antagonist, 4-[2-[7-amino-2-(2-furyl)(1,2,4)-triazin-5-ylamino]-ethyl]ph enol (ZM241385) (50 nM), increased the release of glutamate (three- to four-fold) and of aspartate (nearly two-fold) upon iodoacetic acid-induced glycolysis inhibition, in the presence or in the absence of Ca2+. Blockade of tonic activation of A2A receptors by ZM241385 also increased (nearly two-fold) the ischemia-induced release of glutamate and aspartate. Furthermore, another selective A2A receptor antagonist, 5-amino-7-(2-phenylethyl)-2-(2-furyl)pyrazolo[4,3-e]-1,2,4-triazolo[1,5- c] pyrimidine (SCH58261), also increased the release of aspartate and glutamate by about two-fold in cells submitted to glycolysis inhibition. In contrast, the selective adenosine A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) (100 nM), did not significantly modify the extracellular accumulation of either glutamate or aspartate caused by inducers of chemical ischemia or glycolytic inhibitors. Inhibition of glycolysis also increased (about three-fold) the extracellular accumulation of GABA, which was virtually unchanged by ZM241385. Furthermore, the GABAA receptor antagonist, bicuculline (10 microM), only increased (nearly two-fold) the iodoacetic acid-induced Ca(2+)-dependent release of glutamate, whereas the GABAB receptor antagonist, 3-aminopropyl(diethoxymethyl) phosphinic acid, CGP35348 (100 microM), was devoid of effects on the extracellular accumulation of glutamate and aspartate. These results show that endogenous extracellular adenosine, which rises under conditions of inhibited glycolysis, tonically inhibits the extracellular accumulation of excitatory amino acid through the activation of A2A, but not A1, adenosine receptors, and this effect is independent of GABAA and GABAB functions in the cultured retinal cells.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Ca2+ permeability and kinetics of glutamate receptors in rat medial habenula neurones: implications for purinergic transmission in this nucleus

1. We have previously investigated P2X receptor-mediated synaptic currents in medial habenula neurones and shown that they can be calcium permeable. We now investigate the receptor properties of glutamate, the other, more abundant excitatory transmitter, to determine its receptor subtypes and their relative calcium permeability. This may have implications for the physiological role of the P2X receptors which mediate synaptic currents. 2. Using fast application of ATP, L-glutamate or kainate to nucleated patches, glutamate receptors were determined to be of the AMPA subtype but no functional P2X receptors were detected. 3. The deactivation and desensitization rates of the AMPA channel were determined to have time constants of 1.77 +/- 0.21 ms (n = 10) and 4.01 +/- 0.85 ms (n = 9) at -60 mV, respectively. AMPA receptors recovered from desensitization with two exponential components with time constants of 21.08 +/- 2.95 and 233.60 +/- 51.1 ms (n = 3). None of the deactivation or desensitization properties of the GluR channels depended on membrane potential. 4. The current-voltage relationship under different ionic conditions revealed that the GluR channel was equally permeable to Cs+ and Na+ but relatively impermeable to Ca2+ (PCa/PCs = 0.13, n = 6). 5. For both synaptic currents and somatic currents activated by fast application of L-glutamate to nucleated patches, decay time constants were similar at +/-60 mV in the presence of Mg2+ ions. Thus GluR channels appear to be of the AMPA subtype and not the NMDA subtype. 6. Thus, under the conditions of this study, neurones of the medial habenula lack functional NMDA receptors and possess AMPA receptors that have low permeability to Ca2+. We conclude that the P2X receptor-mediated synaptic currents are the only calcium-permeable fast-transmitter gated currents in these neurones which may be important for their physiological function.