A P2 purinoceptor activated by ADP in rat medullar neurons

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Comparative hydrolysis of extracellular adenine nucleotides and adenosine in synaptic membranes from porcine brain cortex, hippocampus, cerebellum and medulla oblongata

We have investigated the metabolism of extracellular adenine nucleotides and adenosine in porcine brain. The cortex synaptic plasma membranes hydrolyzed ATP to ADP, AMP and adenosine. We also observed a slow hydrolysis of adenosine with the concomitant accumulation of inosine. These results indicate that NTPDase1, NTPDase2, ecto-5'-nucleotidase, and adenosine deaminase are present in cortex synaptic membranes from porcine brain. We further showed that all these enzymes are also abundant in synaptic membranes from hippocampus, cerebellum, and medulla oblongata and compared their specific activities. Brain cortex and hippocampus exhibited higher activities of NTPDase1 and NTPDase2 than cerebellum and medulla oblongata. It was consistent with the high level of the expression of NTPDases in the two first structures. Adenosine deaminase activity was found in all brain structures analyzed; however, it was lower than the activity of ecto-nucleotidases. Taken together, our data suggest that investigated enzymes have a ubiquitous abundance in porcine brain, and observed differences in their activities in cortex, hippocampus, cerebellum, and medulla oblongata may correlate with the pattern of P2 receptor expression in these brain areas. In addition, low activity of adenosine deaminase may indicate that nonenzymatic mechanism(s) are responsible for the termination of P1 receptor signaling in porcine brain.

Molecular physiology of P2 receptors in the central nervous system

Neurons of the central nervous system (CNS) are endowed with ATP-sensitive receptors belonging to the P2X (ligand-gated cationic channels) and P2Y (G protein-coupled receptors) types. Whereas a number of P2X receptors mediate fast synaptic responses to the transmitter ATP, P2Y receptors mediate either slow changes of the membrane potential in response to non-synaptically released ATP or the interaction with receptors for other transmitters. To date seven P2X and seven P2Y receptors of human origin have been molecularly identified and functionally characterized. P2X subunits may occur as homooligomers or as heterooligomeric assemblies of more than one subunit. P2X(7) subunits do not form heterooligomeric assemblies and are unique in mediating apoptosis and necrosis of glial cells and possibly also of neurons. The P2X(2), P2X(4), P2X(4)/P2X(6) and P2Y(1) receptors appear to be the predominant neuronal types. The localisation of these receptors may be at the somato-dendritic region (postsynaptic) or at the nerve terminals (presynaptic). Postsynaptic P2 receptors appear to be mostly excitatory, while presynaptic P2 receptors may be either excitatory (P2X) or inhibitory (P2Y). Since in the CNS the stimulation of a single neuron may activate multiple networks, a concomitant stimulation of facilitatory and inhibitory circuits as a result of ATP release is also possible. Finally, the enzymatic degradation of ATP may lead to the local generation of adenosine which can modulate via A(1) or A(2A) receptor-activation the ATP effect.

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.

Diverse signal transduction pathways mediated by endogenous P2 receptors in cultured rat cerebral cortical neurons

The present study was conducted to assess the intracellular signaling pathways mediated by receptors for ATP, uridine triphosphate (UTP), and 2-methylthio ATP (2-MeSATP), by monitoring patch-clamp currents and intracellular calcium mobilization in cultured rat cortical cerebral neurons. All three agonists evoked potassium currents and increased the intracellular free Ca2+ concentration ([Ca2+]i), and these effects were inhibited by the broad G-protein inhibitor guanosine-5'-O-(2-thiodiphosphate) (GDPbetaS) but not by the Gi/o-protein inhibitor pertussis toxin (PTX). UTP-evoked currents were inhibited by either the phospholipase C inhibitor neomycin or the selective protein kinase C (PKC) inhibitor GF109203X, and the rise in cytosolic Ca2+ was inhibited by either neomycin or the inositol 1,4,5-trisphosphate (IP3) receptor antagonist heparin, indicating that the UTP receptor involved phospholipase C-mediated phosphatidylinositol signaling. In contrast, 2-MeSATP-induced currents and rise in cytosolic Ca2+ were not inhibited by either neomycin, or GF109203X, or heparin. 2-MeSATP elicited single-channel currents in the cell-attached patch-clamp configuration and also in excised patches. The G-protein activator GTP gamma S induced single-channel currents in a fashion that mimicked the effect of 2-MeSATP. These data suggest that 2 MeSATP activated potassium channels by a direct action of G-protein beta gamma subunits and increased [Ca2+]i by a mechanism independent of phospholipase C stimulation and IP3 production. ATP-evoked currents were partially inhibited by either neomycin or GF109203X, although the rise in cytosolic Ca2+ was not affected by these inhibitors. ATP produced single-channel currents with two major classes of the slope conductance (86 and 95 pS) in cell-attached patches, each of which is consistent with that achieved by 2-MeSATP (85 pS) or UTP (96 pS); the currents with the lower conductance were observed in the outside-out patch-clamp configuration. These results indicate that P2 receptors for UTP and 2-MeSATP are linked to a PTX-insensitive G-protein involving different signal transduction pathways and that ATP responses are mediated by both of these P2 receptors.

Distribution of [35S]dATP alpha S binding sites in the adult rat neuraxis

Highly abundant, saturable and specific binding sites for [35S]2'-deoxyadenosine 5'-O-(1-thio) triphosphate ([35S]dATP alpha S, Kd: 9 +/- 2 nM; Bmax: 39 +/- 8 pmol/mg protein) are present in adult rat brain membranes and have characteristics consistent with those expected for a P2Y1 receptor. The anatomical distribution of these binding sites in the brain and spinal cord was examined using in vitro autoradiography. The [35S]dATP alpha S binding sites showed a widespread distribution throughout the brain and spinal cord. They could be displaced by a large excess (100 microM) of 2-methylthioATP (2MeS-ATP) but not by uridine-5'-triphosphate (UTP) or alpha,beta-methyleneATP (alpha,beta-meATP). Within the cortical regions labelling was of equal medium density. However, discrete structures and nuclei within the olfactory bulb, subcortical telencephalon, hippocampal complex, thalamic regions and mesencephalon displayed a variety of densities. Within the spinal cord, gray matter was labelled at a greater density than the funiculi. The present study clarifies the anatomical distribution of P2Y1 and closely related receptors within the central nervous system of rat and extends the evidence that those receptors are abundant and widely distributed within the neuraxis.

Electrophysiological effects of ATP on brain neurones

1. The electrophysiological effects of ATP on brain neurones are either due to the direct activation of P2 purinoceptors by the unmetabolized nucleotide or to the indirect activation of P1. purinoceptors by the degradation product adenosine. 2. Two subtypes of P2 purinoceptors are involved, a ligand-activated ion channel (P2X) and a G protein-coupled receptor (P2Y). Hence, the stimulation of P2X purinoceptors leads to a cationic conductance increase, while the stimulation of P2Y purinoceptors leads to a G protein-mediated opening or closure of potassium channels. 3. ATP may induce a calcium-dependent potassium current by increasing the intracellular Ca2+ concentration. This is due either to the entry of Ca2+ via P2X purinoceptors or to the activation of metabotropic P2Y purinoceptors followed by signaling via the G protein/phospholipase C/inositol 1,4,5-trisphosphate (IP3) cascade. Eventually, IP3 releases Ca2+ from its intracellular pools. 4. There is no convincing evidence for the presence of P2U purinoceptors sensitive to both ATP and UTP, or pyrimidinoceptors sensitive to UTP only, in the central nervous system (CNS). 5. ATP-sensitive P2X and P2Y purinoceptors show a wide distribution in the CNS and appear to regulate important neuronal functions.

Adenosine activates the K+ channel and enhances cytosolic Ca2+ release via a P2Y purinoceptor in hippocampal neurons

The effects of adenosine on hippocampal neurons were examined by patch-clamp recording and Ca2+ imaging using fura-2 fluorescence. In the whole-cell patch-clamp configuration, adenosine evoked outwardly rectifying K+ currents in a dose-dependent manner. These currents were not inhibited by a nonselective P1 purinoceptor antagonist or selective adenosine A1, A2A receptor antagonists and moreover, selective adenosine A1, A2A receptor agonists evoked no current. In contrast, P2 purinoceptor agonists produced similar outward currents with the order of potency: ADP > or = 2-methylthio ATP > ATP > adenosine >> AMP. No response was obtained to UTP, alpha, beta-methylene ATP or beta, gamma-methylene ATP. The intracellular perfusion of a broad G-protein inactivator, guanosine-5'-O-(2-thiodiphosphate) (GDP beta S), abolished adenosine-evoked currents, whereas a Gi/Go-protein inhibitor, pertussis toxin, had no effect. Furthermore, the currents were blocked by a phospholipase C inhibitor, neomycin, or specific protein kinase C inhibitors, GF109203X (bisindolyl maleimide, C25H24N4O2) and protein kinase C inhibitor peptide. In the cell-attached patch-clamp configuration, adenosine elicited single-channel currents with two major kinds of slope conductances. Likewise, application of adenosine outside the patch electrode again produced single-channel currents with same conductances. A potent protein kinase C activator, 12-O-tetradecanoylphorbol-13-acetate (TPA), induced single-channel currents in a fashion that mimics the effect of adenosine. The evoked currents were blocked by GF109203X. In addition, adenosine enhanced intracellular free Ca2+ concentration ([Ca2+]i). This [Ca2+]i increase was inhibited by GDP beta S or neomycin, but was not affected by pertussis toxin. These results, thus, suggest that adenosine activates the K+ channel and enhances cytosolic Ca2+ release via a P2Y purinoceptor linked to a pertussis toxin-insensitive G-protein, which is involved in a phospholipase C-mediated phospholipid-signaling pathway.

ATP-regulated K+ channel and cytosolic Ca2+ mobilization in cultured rat spinal neurons

ATP activated the K+ channel responsible for outwardly rectifying currents via a P2Y purinoceptor linked to a pertussis toxin-insensitive G-protein in cultured rat spinal neurons. The evoked currents were inhibited by a selective protein kinase C inhibitor, GF109203X, whereas a phospholipase C inhibitor, neomycin had no effect. These indicate that the currents are regulated by phospholipase C-independent protein kinase C activation. In addition, ATP enhanced intracellular free Ca2+ concentration. The increase in intracellular free Ca2+ concentration was inhibited by a broad G-protein inhibitor, GDP beta S, but not affected by neomycin or an inositol 1,4,5-triphosphate receptor antagonist, heparin, suggesting that the cytosolic Ca2+ mobilization is regulated by a mechanism independent of a phospholipase C-mediated phosphatidylinositol signaling. These results, thus, demonstrate that ATP has dual actions on the coupled K+ channel and cytosolic Ca2+ release.

Repetitive applications of ATP potentiate potassium current by Ca2+/calmodulin kinase in cultured rat hippocampal neurons

ATP evoked whole-cell potassium currents in hippocampal neurons. The second application of ATP to the same cell potentiated the current amplitude to around 140% and the current potentiation was maintained by further applications. A calmodulin inhibitor, W-7, or a selective Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitor, KN-62, inhibited the current potentiation, although a selective protein kinase C inhibitor, GF109203X, or a selective cAMP-dependent protein kinase inhibitor, H-89, had no effect. In addition, ATP enhanced intracellular free Ca2+ concentration, which may activate CaMKII, but this enhancement was blocked by repetitive applications. These results provide an indication that CaMKII may be involved in the current potentiation by repetitive applications of ATP.

Adenosine evokes potassium currents by protein kinase C activated via a novel signaling pathway in superior colliculus neurons

Adenosine evoked whole-cell potassium currents and enhanced intracellular free Ca2+ concentration ([Ca2+]i) in superior colliculus neurons through a P2Y purinoceptor linked to a pertussis toxin-insensitive G-protein, possibly Gq-protein, which is involved in a protein kinase C (PKC) activation pathway. The [Ca2+]i increase was inhibited by a phospholipase C (PLC) inhibitor, whereas the evoked currents were not affected by a PLC inhibitor or a phospholipase A2 (PLA2) inhibitor. Adenosine elicited single channel currents via PKC activation in cell-attached patches and furthermore, those currents with conductances of the same slope were induced even in excised patches, suggesting that PKC can be activated only by cell membrane factors without intracellular components. These results thus indicate that the P2Y purinoceptor-coupled potassium channel is regulated via a novel PKC activation pathway independent of PLC or PLA2.