Evidence for P2-purinoceptor-mediated inhibition of noradrenaline release in rat brain cortex

Involvement of P2Y1, P2Y6, A1 and A2A Receptors in the Purinergic Inhibition of NMDA-Evoked Noradrenaline Release in the Rat Brain Cortex

In the cerebral cortex, glutamate activates NMDA receptors (NMDARs), localized in noradrenergic neurons, inducing noradrenaline release that may have a permissive effect on glutamatergic transmission, and therefore, on the modulation of long-term plasticity. ATP is co-released with noradrenaline, and with its metabolites (ADP and adenosine) is involved in the purinergic modulation of electrically-evoked noradrenaline release. However, it is not known if noradrenaline release evoked by activation of NMDARs is also under purinergic modulation. The present study aimed to investigate and to characterize the purinergic modulation of noradrenaline release evoked by NMDARs. Stimulation of rat cortical slices with 30 µM NMDA increased noradrenaline release, which was inhibited by ATP upon metabolization into ADP and adenosine and by the selective agonists of A1 and A2A receptors, CPA and CGS2680, respectively. It was also inhibited by UTP and UDP, which are mainly released under pathophysiological situations. Characterization of the effects mediated by these compounds indicated the involvement of P2Y1, P2Y6, A1 and A2A receptors. It is concluded that, in the rat brain cortex, NMDA-evoked noradrenaline release is modulated by several purinergic receptors that may represent a relevant mechanism to regulate the permissive effect of noradrenaline on NMDA-induced neuroplasticity.

Inhibitors of ectonucleotidases have paradoxical effects on synaptic transmission in the mouse cortex

Extracellular adenosine plays prominent roles in the brain in both physiological and pathological conditions. Adenosine can be generated following the degradation of extracellular nucleotides by various types of ectonucleotidases. Several ectonucleotidases are present in the brain parenchyma: ecto-nucleotide triphosphate diphosphohydrolases 1 and 3 (NTPDase 1 and 3), ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP 1), ecto-5'-nucleotidase (eN), and tissue non-specific alkaline phosphatase (TNAP, whose function in the brain has received little attention). Here we examined, in a living brain preparation, the role of these ectonucleotidases in generating extracellular adenosine. We recorded local field potentials evoked by electrical stimulation of the lateral olfactory tract in the mouse piriform cortex in vitro. Variations in adenosine level were evaluated by measuring changes in presynaptic inhibition generated by adenosine A1 receptors (A1Rs) activation. A1R-mediated presynaptic inhibition was present endogenously and was enhanced by bath-applied AMP and ATP. We hypothesized that inhibiting ectonucleotidases would reduce extracellular adenosine concentration, which would result in a weakening of presynaptic inhibition. However, inhibiting TNAP had no effect in controlling endogenous adenosine action and no effect on presynaptic inhibition induced by bath-applied AMP. Furthermore, contrary to our expectation, inhibiting TNAP reinforced, rather than reduced, presynaptic inhibition induced by bath-applied ATP. Similarly, inhibition of NTPDase 1 and 3, NPP1 and eN induced stronger, rather than weaker, presynaptic inhibition, both in endogenous condition and with bath-applied ATP and AMP. Consequently, attempts to suppress the functions of extracellular adenosine by blocking its extracellular synthesis in living brain tissue could have functional impacts opposite to those anticipated.

Activation of basal forebrain purinergic P2 receptors promotes wakefulness in mice

The functions of purinergic P2 receptors (P2Rs) for extracellular adenosine triphosphate (ATP) are poorly understood. Here, for the first time, we show that activation of P2Rs in an important arousal region, the basal forebrain (BF), promotes wakefulness, whereas inhibition of P2Rs promotes sleep. Infusion of a non-hydrolysable P2R agonist, ATP-γ-S, into mouse BF increased wakefulness following sleep deprivation. ATP-γ-S depolarized BF cholinergic and cortically-projecting GABAergic neurons in vitro, an effect blocked by antagonists of ionotropic P2Rs (P2XRs) or glutamate receptors. In vivo, ATP-γ-S infusion increased BF glutamate release. Thus, activation of BF P2XRs promotes glutamate release and excitation of wake-active neurons. Conversely, pharmacological antagonism of BF P2XRs decreased spontaneous wakefulness during the dark (active) period. Together with previous findings, our results suggest sleep-wake regulation by BF extracellular ATP involves a balance between excitatory, wakefulness-promoting effects mediated by direct activation of P2XRs and inhibitory, sleep-promoting effects mediated by degradation to adenosine.

An Update on P2Y13 Receptor Signalling and Function

The distribution of nucleotide P2Y receptors across different tissues suggests that they fulfil key roles in a number of physiological and pathological conditions. P2Y₁₃ is one of the latest P2Y receptors identified, a novel member of the Gi-coupled P2Y receptor subfamily that responds to ADP, together with P2Y₁₂ and P2Y₁₄. Pharmacological studies drew attention to this new ADP receptor, with a pharmacology that overlaps that of P2Y₁₂ receptors but with unique features and roles. The P2RY12-14 genes all reside on human chromosome 3 at 3q25.1 and their strong sequence homology supports their evolutionary origin through gene duplication. Polymorphisms of P2Y₁₃ receptors have been reported in different human populations, yet their consequences remain unknown. The P2Y₁₃ receptor is versatile in its signalling, extending beyond the canonical signalling of a Gi-coupled receptor. Not only can it couple to different G proteins (Gs/Gq) but the P2Y₁₃ receptor can also trigger several intracellular pathways related to the activation of MAPKs (mitogen-activated protein kinases) and the phosphatidylinositol 3-kinase/Akt/glycogen synthase kinase 3 axis. Moreover, the availability of P2Y₁₃ receptor knockout mice has highlighted the specific functions in which it is involved, mainly in the regulation of cholesterol and glucose metabolism, bone homeostasis and aspects of central nervous system function like pain transmission and neuroprotection. This review summarizes our current understanding of this elusive receptor, not only at the pharmacological and molecular level but also, in terms of its signalling properties and specific functions, helping to clarify the involvement of P2Y₁₃ receptors in pathological situations.

P2Y receptors in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia, affecting more than 10% of people over the age of 65. Age is the greatest risk factor for AD, although a combination of genetic, lifestyle and environmental factors also contribute to disease development. Common features of AD are the formation of plaques composed of beta-amyloid peptides (Aβ) and neuronal death in brain regions involved in learning and memory. Although Aβ is neurotoxic, the primary mechanisms by which Aβ affects AD development remain uncertain and controversial. Mouse models overexpressing amyloid precursor protein and Aβ have revealed that Aβ has potent effects on neuroinflammation and cerebral blood flow that contribute to AD progression. Therefore, it is important to consider how endogenous signalling in the brain responds to Aβ and contributes to AD pathology. In recent years, Aβ has been shown to affect ATP release from brain and blood cells and alter the expression of G protein-coupled P2Y receptors that respond to ATP and other nucleotides. Accumulating evidence reveals a prominent role for P2Y receptors in AD pathology, including Aβ production and elimination, neuroinflammation, neuronal function and cerebral blood flow.

Neurochemical and electrical modulation of the locus coeruleus: contribution to CO2drive to breathe

The locus coeruleus (LC) is a dorsal pontine region, situated bilaterally on the floor of the fourth ventricle. It is considered to be the major source of noradrenergic innervation in the brain. These neurons are highly sensitive to CO₂/pH, and chemical lesions of LC neurons largely attenuate the hypercapnic ventilatory response in unanesthetized adult rats. Developmental dysfunctions in these neurons are linked to pathological conditions such as Rett and sudden infant death syndromes, which can impair the control of the cardio-respiratory system. LC is densely innervated by fibers that contain glutamate, serotonin, and adenosine triphosphate, and these neurotransmitters strongly affect LC activity, including central chemoreflexes. Aside from neurochemical modulation, LC neurons are also strongly electrically coupled, specifically through gap junctions, which play a role in the CO₂ ventilatory response. This article reviews the available data on the role of chemical and electrical neuromodulation of the LC in the control of ventilation.

Purinergic modulation of norepinephrine release and uptake in rat brain cortex: contribution of glial cells

The pathogenesis of psychiatric and neurodegenerative diseases is often associated with a deregulation of noradrenergic transmission. Considering the potential involvement of purinergic signaling in the modulation of noradrenergic transmission in the brain cortex, this study aimed to identify the P2Y receptor subtypes involved in the modulation of neuronal release and neuronal/glial uptake of norepinephrine. Electrical stimulation (100 pulses at 5 Hz) of rat cortical slices induced norepinephrine release that was inhibited by ATP and ADP (0.01–1 mM), adenosine 5′- O-(2-thiodiphosphate) (ADPβS, 0.03–0.3 mM), and UDP (0.1–1 mM). The effect of ADPβS was mediated by P2Y1receptors and possibly by A1/P2Y1heterodimers since it was attenuated by the A1receptor antagonist DPCPX and by the P2Y1receptor antagonist MRS 2500 but was resistant to the effect of adenosine deaminase (ADA). UDP inhibited norepinephrine release through activation of P2Y6receptors, an effect that was abolished by the P2Y6receptor antagonist MRS 2578 and by DPCPX, indicating that it depends on the formation and/or release of adenosine and activation of A1receptors. Supporting this hypothesis, the inhibitory effect of UDP was also prevented by inhibition of ectonucleotidases, by ADA and was attenuated by the inhibitor of nucleoside transporter 6-[(4-nitrobenzyl)thio]-9-β-d-ribofuranosylpurine (NBTI). Additionally, the inhibitory effect of UDP was attenuated when norepinephrine uptake 1 or 2 was inhibited. In astroglial cultures, ADPβS and UDP increased norepinephrine uptake mainly by activation of P2Y1and P2Y6receptors, respectively. The results indicate that neuronal and glial P2Y1and P2Y6receptors may represent new targets of intervention to regulate noradrenergic transmission in CNS diseases.

ATP in the locus coeruleus as a modulator of cardiorespiratory control in unanaesthetized male rats

Locus coeruleus (LC) noradrenergic neurons are chemosensitive to CO2 and pH in mammals and amphibians and are involved in the CO2-related drive to breathe. Purinergic neuromodulation in the LC is of particular interest because ATP acts as a neuromodulator in brainstem regions involved in cardiovascular and respiratory regulation, such as the LC. ATP acting on LC P2 receptors influences the release of noradrenaline. Thus, the goal of the present study was to investigate the role of LC purinergic neuromodulation of ventilatory and cardiovascular responses in normocapnic and hypercapnic conditions in unanaesthetized male Wistar rats. We assessed the purinergic modulation of cardiorespiratory systems by microinjecting an ATP P2X receptor agonist [α,β-methylene ATP (α,β-meATP), 0.5 or 1 nmol in 40 nl] and two non-selective P2 receptor antagonists [pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), 0.5 or 1 nmol in 40 nl; and suramin, 1 nmol in 40 nl] into the LC. Pulmonary ventilation (measured by plethysmography), mean arterial pressure (MAP) and heart rate (HR) were determined before and after unilateral microinjection (40 nl) of α,β-meATP, PPADS, suramin or 0.9% saline (vehicle) into the LC. These measurements were made during a 60 min exposure to normocapnic conditions or a 30 min exposure to 7% CO2. Subsequently, animals undergoing pharmacological treatment were subjected to a 30 min exposure to normocapnic conditions as a recovery period. In normocapnic conditions, α,β-meATP did not affect any parameter, whereas PPADS decreased respiratory frequency and increased MAP and HR. Suramin increased MAP and HR but did not change ventilation. Moreover, hypercapnic conditions induced an increase in ventilation and a decrease in HR in all groups. In hypercapnic conditions, α,β-meATP increased ventilation but did not change cardiovascular parameters, whereas PPADS increased MAP but did not alter ventilation, and suramin increased both ventilation and MAP. Thus, our data suggest that purinergic signalling, specifically through P2 receptors, in the LC plays an important role in cardiorespiratory control in normocapnic and hypercapnic conditions in unanaesthetized rats.

P2 receptor signaling in neurons and glial cells of the central nervous system

Purine and pyrimidine nucleotides are extracellular signaling molecules in the central nervous system (CNS) leaving the intracellular space of various CNS cell types via nonexocytotic mechanisms. In addition, ATP is a neuro-and gliotransmitter released by exocytosis from neurons and neuroglia. These nucleotides activate P2 receptors of the P2X (ligand-gated cationic channels) and P2Y (G protein-coupled receptors) types. In mammalians, seven P2X and eight P2Y receptor subunits occur; three P2X subtypes form homomeric or heteromeric P2X receptors. P2Y subtypes may also hetero-oligomerize with each other as well as with other G protein-coupled receptors. P2X receptors are able to physically associate with various types of ligand-gated ion channels and thereby to interact with them. The P2 receptor homomers or heteromers exhibit specific sensitivities against pharmacological ligands and have preferential functional roles. They may be situated at both presynaptic (nerve terminals) and postsynaptic (somatodendritic) sites of neurons, where they modulate either transmitter release or the postsynaptic sensitivity to neurotransmitters. P2 receptors exist at neuroglia (e.g., astrocytes, oligodendrocytes) and microglia in the CNS. The neuroglial P2 receptors subserve the neuron-glia cross talk especially via their end-feets projecting to neighboring synapses. In addition, glial networks are able to communicate through coordinated oscillations of their intracellular Ca(2+) over considerable distances. P2 receptors are involved in the physiological regulation of CNS functions as well as in its pathophysiological dysregulation. Normal (motivation, reward, embryonic and postnatal development, neuroregeneration) and abnormal regulatory mechanisms (pain, neuroinflammation, neurodegeneration, epilepsy) are important examples for the significance of P2 receptor-mediated/modulated processes.

P2Y receptor mediated inhibitory modulation of noradrenaline release in response to electrical field stimulation and ischemic conditions in superfused rat hippocampus slices

In this study, the inhibitory regulation of the release of noradrenaline (NA) by P2 receptors was investigated in hippocampus slices pre-incubated with [(3)H]NA. Electrical field stimulation (EFS; 2 Hz, 240 shocks, and 1 ms) released NA in an outside [Ca(2+)]-dependent manner, and agonists of P2Y receptors inhibited the EFS-evoked [(3)H]NA release with pharmacological profile similar to that of the P2Y(1) and P2Y(13) receptor subtypes. This inhibitory modulation was counteracted by bicuculline and 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline + 2-amino-5-phosphonovalerate and 2-amino-4-phosphonobutyrate. In contrast, the excess release in response to 30 min combined oxygen and glucose deprivation was outside [Ca(2+)] independent, but still sensitive to the inhibition of both facilitatory P2X(1) and inhibitory P2Y(1) receptors. Whereas mRNA encoding P2Y(12) and P2Y(13) receptor subunits were expressed in the brainstem, P2Y(1) receptor immunoreactivity was localized to neuronal somata and dendrites innervated by the mossy fiber terminals in the CA3 region of the hippocampus, as well as somata of granule cells and interneurons in the dentate gyrus. In summary, in addition to the known facilitatory modulation via P2X receptors, EFS-evoked [(3)H]NA outflow in the hippocampus is subject to inhibitory modulation by P2Y(1)/P2Y(13) receptors. Furthermore, endogenous activation of both facilitatory and inhibitory P2 receptors may participate in the modulation of pathological NA release under ischemic-like conditions.

Presynaptic adenosine and P2Y receptors

Adenine-based purines, such as adenosine and ATP, are ubiquitous molecules that, in addition to their roles in metabolism, act as modulators of neurotransmitter release through activation of presynaptic P1 purinoceptors or adenosine receptors (activated by adenosine) and P2 receptors (activated by nucleotides). Of the latter, the P2Y receptors are G protein-coupled, whereas the P2X receptors are ligand-gated ion channels and not covered in this review.

Modulation of neurotransmitter release by P2X and P2Y receptors in the rat spinal cord

In this study, the P2 receptor-mediated modulation of [3H]glutamate and [3H]noradrenaline release were examined in rat spinal cord slices. Adenosine 5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP), and 2-methylthioadenosine 5'-diphosphate (2-MeSADP) decreased the electrical stimulation-evoked [3H]glutamate efflux with the following order of potency: ADP>2-MeSADP>ATP. The effect of ATP was antagonized by suramin (300microM), the P2Y12,13 receptor antagonist 2-methylthioadenosine 5'-monophosphate (2-MeSAMP, 10microM), and partly by 4-[[4-Formyl-5-hydroxy-6-methyl-3-[(phosphonooxy)methyl]-2-pyridinyl]azo]-1,3-benzenedisulfonic acid (PPADS, 30microM) and the P2Y1 receptor antagonist 2'-deoxy-N6-methyladenosine 3',5'-diphosphate (MRS 2179, 10muM). ATP, ADP and 2-MeSADP also decreased evoked [3H]noradrenaline outflow; the order of agonist potency was ADP> or =2-MeSADP>ATP. The effect of ATP was reversed by 2-MeSAMP (10microM), and partly by MRS 2179 (10microM). By contrast, 2-methylthioadenosine-5'-triphosphate (2-MeSATP, 10-300microM) increased resting and electrically evoked [3H]glutamate and [3H]noradrenaline efflux, and this effect was prevented by the P2X1 receptor selective antagonist 4,4',4'',4'''-[carbonylbis[imino-5,1,3-benzenetriyl bis (carbonyl-imino)]] tetrakis (benzene-1,3-disulfonic acid) octasodium salt (NF449, 100nM). Reverse transcriptase polymerase chain reaction (RT-PCR) analysis revealed that mRNAs encoding P2Y12 and P2Y13 receptors are expressed in the brainstem, whereas P2Y13 but not P2Y12 receptor mRNA is present in the dorsal root ganglion and spinal cord. P2Y1 receptor expression in the spinal cord is also demonstrated at the protein level. In conclusion, inhibitory P2Y and facilitatory P2X1-like receptors, involved in the regulation of glutamate (P2Y13 and/or P2Y1) and noradrenaline (P2Y13 and/or P2Y1, P2Y12) release have been identified, which provide novel target sites for analgesics acting at the spinal cord level.

P2 receptor-mediated modulation of neurotransmitter release-an update

Presynaptic nerve terminals are equipped with a number of presynaptic auto- and heteroreceptors, including ionotropic P2X and metabotropic P2Y receptors. P2 receptors serve as modulation sites of transmitter release by ATP and other nucleotides released by neuronal activity and pathological signals. A wide variety of P2X and P2Y receptors expressed at pre- and postsynaptic sites as well as in glial cells are involved directly or indirectly in the modulation of neurotransmitter release. Nucleotides are released from synaptic and nonsynaptic sites throughout the nervous system and might reach concentrations high enough to activate these receptors. By providing a fine-tuning mechanism these receptors also offer attractive sites for pharmacotherapy in nervous system diseases. Here we review the rapidly emerging data on the modulation of transmitter release by facilitatory and inhibitory P2 receptors and the receptor subtypes involved in these interactions.

ATP release and its prejunctional modulation

We studied some properties of the release of noradrenaline and ATP in isolated sympathetically innervated tissues. Release was elicited by electric stimulation and assessed as overflow of tritiated compounds (after labelling with [3H]noradrenaline) and enzymically measured ATP, respectively. Evans blue, which inhibits ectonucleotidases, greatly increased the evoked overflow of ATP, indicating that a major part of the ATP was metabolized after release. Much of the ATP was postjunctional in origin. The neural fraction was isolated when postjunctional release was suppressed by prazosin (alpha 1-adrenoceptor antagonist) and suramin (P2 purinoceptor antagonist). Comparison of neural ATP and [3H]-noradrenaline release showed that prostaglandin E2 reduced the release of both co-transmitters to a similar extent. Activation of prejunctional alpha 2-adrenoceptors, however, preferentially reduced the release of [3H]noradrenaline, and activation of prejunctional A1 purinoceptors reduced preferentially the release of ATP. Nucleotides such as ATP depressed the release of [3H]noradrenaline through two receptors: the well-known prejunctional A1 receptors and a separate group of prejunctional P2 purinoceptors. P2 antagonists increased the release of [3H]-noradrenaline. Overall, the results indicate differential storage, release and modulation of release of the two sympathetic co-transmitters. They also indicate that postganglionic sympathetic axons possess receptors for both co-transmitters: alpha 2 and P2 autoreceptors.

Immunohistological Determination of Ecto-nucleoside Triphosphate Diphosphohydrolase1 (NTPDase1) and 5′-nucleotidase in Rat Hippocampus Reveals Overlapping Distribution

Distribution of two enzymes involved in the ectonucleotidase enzyme chain, ecto-nucleoside triphosphate diphosphohydrolase1 (NTPDase1) and ecto-5'-nucleotidase, was assessed by immunohistochemistry in the rat hippocampus. Obtained results have shown co-expression of the enzymes in the hippocampal region, as well as wide and strikingly similar cellular distribution. Both enzymes were expressed at the surface of pyramidal neurons in the CA1 and CA2 sections, while cells in the CA3 section were faintly stained. The granule cell layer of the dentate gyrus was moderately stained for NTPDase1, as well as for ecto-5'-nucleotidase. Glial association for ecto-5'-nucleotidase was also observed, and fiber tracts were intensively stained for both enzymes. This is the first comparative study of NTPDase1 and ecto-5'-nucleotidase distribution in the rat hippocampus. Obtained results suggest that the broad overlapping distribution of these enzymes in neurons and glial cells reflects the functional importance of ectonucleotidase actions in the nervous system.

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.

Functions of neuronal P2Y receptors

Within the last 15 years, at least eight different G protein-coupled nucleotide receptors, i.e., P2Y receptors, have been characterized by molecular means. While ionotropic P2X receptors are mainly involved in fast synaptic neurotransmission, P2Y receptors rather mediate slower neuromodulatory effects. This P2Y receptor-dependent neuromodulation relies on changes in synaptic transmission via either pre- or postsynaptic sites of action. At both sites, the regulation of voltage-gated or transmitter-gated ion channels via G protein-linked signaling cascades has been identified as the predominant underlying mechanisms. In addition, neuronal P2Y receptors have been found to be involved in neurotoxic and neurotrophic effects of extracellular adenosine 5-triphosphate. This review provides an overview of the most prominent actions mediated by neuronal P2Y receptors and describes the signaling cascades involved.

Purinergic modulation of cardiovascular function in the rat locus coeruleus

1 The purpose of the present study was to determine whether purines exerted a physiological role in central cardiovascular modulation at the level of the locus coeruleus (LC). 2 In pentobarbitone-anaesthetised Wistar-Kyoto rats, unilateral microinjection of ATP or alpha,beta-methyleneATP into the LC elicited dose-related decreases in blood pressure and heart rate. Unilateral microinjection of the P2 purinoceptor antagonists suramin and PPADS, caused pressor and tachycardic responses. Administration of the selective P2X(1) receptor antagonist NF-279 had no effect. While both ATP and L-glutamate (L-GLU) resulted in depressor responses after intra-LC microinjection, following intra-LC microinjection of P2 purinoceptor antagonists into the LC, the effects of subsequent administration of either ATP or L-GLU were functionally reversed, such that a pressor response ensued. 3 Microinjection of noradrenaline into the LC caused an increase in blood pressure and heart rate; however, the alpha(2)-adrenoceptor antagonist idazoxan had no cardiovascular effects, but did prevent the pressor response to PPADS or suramin. In addition, coinjection of idazoxan with either suramin or PPADS abolished the ATP and L-GLU mediated pressor responses observed following either suramin or PPADS administration. 4 The present data suggest that firstly, purines are capable of acting within the LC to ultimately modulate the cardiovascular system and secondly, that there is apparently a functional interaction between tonically active purinergic and noradrenergic systems within the LC of the rat.

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.

Calcium signaling mediated by P2Y receptors in mouse taste cells

Evidence implicates a number of neuroactive substances and their receptors in mediating complex cell-to-cell communications in the taste bud. Recently, we found that ATP, a ubiquitous neurotransmitter/neuromodulator, mobilizes intracellular Ca2+ in taste cells by activating P2Y receptors. Here, P2Y receptor-cellular response coupling was characterized in detail using single cell ratio photometry and the inhibitory analysis. The sequence of underlying events was shown to include ATP-dependent activation of PLC, IP3 production, and IP3 receptor-mediated Ca2+ release followed by Ca2+ influx. Data obtained favor SOC channels rather than receptor-operated channels as a pathway for Ca2+ influx that accompanies Ca2+ release. Intracellular Ca2+ mobilized by ATP is apparently extruded by the plasma membrane Ca2+-ATPase, while a contribution of the Na+/Ca2+ exchange and other mechanisms of Ca2+ clearance is negligible. Cyclic AMP-dependent phosphorylation is likely to control a gain of the phosphoinositide cascade involved in ATP transduction. ATP-responsive taste cells are abundant in circumvallate, foliate, and fungiform papillae. Taken together, our observations point to a putative role for ATP as a neurotransmitter operative in the taste bud.

P2Y receptor regulation of cultured rat cerebral cortical cells: calcium responses and mRNA expression in neurons and glia

1 We have investigated increases in cytosolic Ca(2+) in response to nucleotides in mixed rat cerebrocortical cultures (neurons and glia in similar numbers) and in essentially neuron-free glial cultures. 2 In both cultures, the agonist-response profile was 2-methylthioADP(2MeSADP)>2-methylthioATP(2MeSATP)>ADP>ATP>adenosine 5'-O-(3-thiotriphosphate), consistent with a P2Y(1) receptor. The maximal responses to 2MeSADP, 2MeSATP and ADP were identical, but that to ATP was higher. 3 Suramin, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid, reactive blue 2 (RB2), and adenosine biphosphate (A3P5P) were antagonists with apparent pA(2) values of 5.5 for suramin, 6.4 for RB2, and 4.7 for A3P5P. 4 Single cell imaging divided the cells from the mixed neuronal-glial cultures into two populations: responsive (neurons) and unresponsive (glial cells) to high [K(+)]. The response of cells to nucleotides was almost exclusively limited to those not responsive to high K(+). 5 In the presence of extracellular Mn(2+), the response of the mixed cultures to 30 mM K(+) and 20 micro M Bay K 8644 was attenuated. However, when 2MeSADP was added there was no reduction in response in cultures previously loaded with Mn(2+). This further indicated that the 2MeSADP response was not in the neurons. 6 Reverse transcriptase-polymerase chain reaction studies detected transcripts for P2Y(1), P2Y(4) and P2Y(6) in RNA preparations from embryonic rat cortex, and from both mixed and glial cultures. P2Y(2) transcripts were not detected in the embryonic cortex. 7 Based on this and previous work, it is proposed that the principal P2Y influences in the brain are on cytosolic Ca(2+) in glial cells and presynaptic sites on neurons.

An interaction between ATP and high K+: mutual impairment of ATP- and high K(+)-evoked [Ca2+]i increase in NG 108-15 cells

The interaction between ATP- and high K(+)-evoked increase in intracellular free calcium concentration ([Ca2+]i) was investigated to gain an insight into the mechanism of interaction of ATP with voltage-sensitive calcium channels. [Ca2+]i was measured in the neuronal model, neuroblastoma x glioma hybrid cells (NG 108-15), using the fluorescence indicator fura-2. In the presence of 1.8 mM extracellular Ca2+, ATP induced a rapid, concentration-dependent increase in [Ca2+]i. High K+ (50 mM) evoked a [Ca2+]i rise from 109 +/- 11 nM to 387 +/- 81 nM (n = 16). The application of either of these two [Ca2+]i-increase provoking agents in sequence with the other caused impairment of the latter effect. The mutual desensitization of the responses to ATP and high K+ strongly suggests that both agents rely at least in part on the same source of Ca2+ for elevation of [Ca2+]i in NG 108-15 cells.

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.

Antinociceptive effects of intracerebroventricularly administered P2 purinoceptor agonists in the rat

We examined the effects of adenosine 5'-triphosphate (ATP) and its analogues administered intracerebroventricularly on nociceptive thresholds in rats. Intracerebroventricular (i.c.v.) administration of ATP (10 and 100 nmol/rat), alpha,beta-methylene-ATP (1-30 nmol/rat) and 2', 3'-O-(4-benzoylbenzoyl)-ATP (1-30 nmol/rat) dose-dependently elevated the mechanical nociceptive threshold in the paw pressure test. These antinociceptive effects were rapid and short-lasting, peaking at 5 min and disappearing by 20 min after the administration. However, i.c.v. administration of beta,gamma-methylene-ATP (1-30 nmol/rat) and UTP (10 and 100 nmol/rat) had no significant effects on the mechanical nociceptive threshold. In other tests, i.c.v. administration of alpha,beta-methylene-ATP (10 and 30 nmol/rat) prolonged the thermal nociceptive latency in the hot plate test, but only a higher dose (30 nmol/rat) of alpha,beta-methylene-ATP prolonged the latency in the tail flick test. alpha,beta-Methylene-ATP produced no motor deficit in the inclined plane test. These results suggest that P2X purinoceptors play an inhibitory role in nociception at the supraspinal level.

Modulation of GABA-augmented norepinephrine release in female rat brain slices by opioids and adenosine

GABAA receptor activation augments electrically-stimulated release of norepinephrine (NE) from rat brain slices. Because this effect is not observed in synaptoneurosomes, GABA probably acts on inhibitory interneurons to disinhibit NE release. To determine whether opioids or adenosine influence GABA-augmented NE release, hypothalamic and cortical slices from female rats were superfused with GABA or vehicle in the presence and absence of 10 microM morphine or 100 microM adenosine. GABA augments [3H]NE release in the cortex and hypothalamus. Morphine alone has no effect on [3H]NE release, but attenuates GABA augmentation of [3H]NE release in both brain regions. Adenosine alone modestly inhibits [3H]NE release in the cortex, but not in the hypothalamus. Adenosine inhibits GABA-augmented [3H]NE release in both brain regions. The general protein kinase inhibitor H-7, augments [3H]NE release in both brain regions and may have additive effects with GABA in cortical slices. These results implicate opioid and adenosine interneurons and possibly protein kinases in regulating GABAergic influences on NE transmission.

Co-transmitter function of ATP in central catecholaminergic neurons of the rat

Intracellular recordings were made in a mid-pontine slice preparation of the rat brain containing the nucleus locus coeruleus. Focal electrical stimulation evoked biphasic synaptic potentials consisting of early depolarizing (d.p.s.p.) and late hyperpolarizing (i.p.s.p.) components. The alpha(2)-adrenoceptor antagonist idazoxan inhibited the i.p.s.p. without altering the d.p.s.p. All of the following experiments were carried out in the presence of kynurenic acid and picrotoxin to block the glutamatergic and GABAergic fractions of the d.p.s.p., respectively. Guanethidine, which is known to inhibit noradrenaline and ATP release from nerve terminals of postganglionic sympathetic nerves, depressed both the d.p.s.p. and the i.p.s.p. in a concentration-dependent manner. Damage of catecholaminergic nerve terminals by 6-hydroxydopamine also decreased both the d.p.s.p. and the i.p.s.p. The P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) depressed the d.p.s.p., whereas the i.p.s.p. remained unaffected. The further application of PPADS did not increase the depression of the d.p.s.p. by guanethidine. Superfusion with the mixed alpha-adrenoceptor agonist noradrenaline or the selective P2 receptor agonist adenosine 5'-O-(2-thiodiphosphate) inhibited both the d.p.s.p. and the i.p.s.p. The inhibitory effects of these agonists were prevented by the respective antagonists idazoxan or suramin. In the presence of suramin noradrenaline failed to inhibit the residual d.p.s.p. Superfused noradrenaline potentiated rather than inhibited responses to pressure-applied alpha,beta-methylene-ATP; superfused adenosine 5'-O-(2-thiodiphosphate) did not interact with pressure-applied noradrenaline. In conclusion, we present electrophysiological evidence for the co-release of ATP and catecholamines in the CNS. At the cell somata of neurons in the locus coeruleus, noradrenaline and ATP activate inhibitory alpha(2)-adrenoceptors and excitatory P2 receptors, respectively. In addition, inhibitory presynaptic autoreceptors of the alpha(2) and P2 types appear to regulate release of the two co-transmitters.

The effect of nucleotides and adenosine on stimulus-evoked glutamate release from rat brain cortical slices

Evidence has previously been presented that P1 receptors for adenosine, and P2 receptors for nucleotides such as ATP, regulate stimulus-evoked release of biogenic amines from nerve terminals in the brain. Here we investigated whether adenosine and nucleotides exert presynaptic control over depolarisation-elicited glutamate release. Slices of rat brain cortex were perfused and stimulated with pulses of 46 mM K(+) in the presence of the glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (0.2 mM). High K(+) substantially increased efflux of glutamate from the slices. Basal glutamate release was unchanged by the presence of nucleotides or adenosine at concentrations of 300 microM. Adenosine, ATP, ADP and adenosine 5'-O-(3-thiotriphoshate) at 300 microM attenuated depolarisation-evoked release of glutamate. However UTP, 2-methylthio ATP, 2-methylthio ADP, and alpha,beta-methylene ATP at 300 microM had no effect on stimulated glutamate efflux. Adenosine deaminase blocked the effect of adenosine, but left the response to ATP unchanged. The A(1) antagonist 8-cyclopentyl-1, 3-dipropylxanthine antagonised the inhibitory effect of both adenosine and ATP. Cibacron blue 3GA inhibited stimulus-evoked glutamate release when applied alone. When cibacron blue 3GA was present with ATP, stimulus-evoked glutamate release was almost eliminated. However, this P2 antagonist had no effect on the inhibition by adenosine. These results show that the release of glutamate from depolarised nerve terminals of the rat cerebral cortex is inhibited by adenosine and ATP. ATP appears to act directly and not through conversion to adenosine.

Presynaptic P2 receptors?

Although the emphasis in ATP research has been on postjunctional receptors, there is also evidence for presynaptic receptors regulating transmitter release in the autonomic nervous system. Recent work has attempted to identify similar mechanisms in the central nervous system. Some of the existing results can be explained by the metabolism of nucleotides to adenosine or adenosine 5'-monophosphate (AMP). However, studies of presynaptic effects using sensitive electrophysiological tests such as paired-pulse interactions indicate that nucleotides can act at presynaptic sites, but that their effects may be mediated by a release of adenosine. Results are also described which indicate that, under some conditions, nucleotides can mediate phenomena such as long-term potentiation, which probably involves a significant presynaptic element. In part these effects may involve a nucleotide-induced release of adenosine and the simultaneous activation of P1 and P2 receptors.

P2 receptor-mediated inhibition of dopamine release in rat neostriatum

Axon terminal nucleotide P2 receptors mediating an inhibition of transmitter release have, so far, been detected in various sympathetically innervated tissues,(8,27) and on central noradrenergic,(14,26) glutamatergic(15) and serotonergic neurons. (28) We have now investigated the effect of ATP and related nucleotides on the release of endogenous dopamine from slices of rat neostriatum using fast cyclic voltammetry. Mutual interactions between the two neurotransmitters have been observed previously: ATP and related nucleotides induce a release of dopamine in PC12 pheochromocytoma cells, a frequently used model for sympathetic neurons;(10,22) they also increase the dopamine concentration in rat brain measured by in vivo microdialysis(16,32) and stimulate the uptake of dopamine by rat striatal synaptosomes.(3) Dopamine, in contrast, facilitates activation of ligand-gated cation channels (i. e. P2X(2) receptors) by ATP.(11,20) Here, we show that ATP and two of its analogues decrease the electrically evoked release of endogenous dopamine in rat neostriatum. The inhibitory effect of ATP is blocked by the P2 receptor antagonists suramin, reactive blue 2 and cibacron blue 3GA. Suramin, in addition, partly prevents the attenuation of dopamine release evoked by a single stimulus that follows a brief train of high-frequency pulses.These findings suggest the existence of release-inhibiting P2 receptors on dopaminergic nerve terminals and indicate that dopaminergic transmission in rat neostriatum might be modulated by an endogenous P2 receptor ligand, presumably ATP.

Suramin-sensitive suppression of paired-pulse inhibition by adenine nucleotides in rat hippocampal slices

In order to assess the possible presence of presynaptic P2 receptors for nucleotides in the hippocampus, adenosine triphosphate and betagamma-methyleneATP have been examined on paired-pulse inhibition in rat hippocampal slices. Both compounds reproduced the effects of adenosine and reduced the amount of paired-pulse inhibition at an interpulse interval of 10 ms and increased the amount of facilitation at intervals of 20 and 50 ms. These effects were prevented by 8-phenyltheophylline and adenosine deaminase, indicating their mediation by adenosine. The effects were also reduced by suramin at 50 microM, suggesting the possible activation of P2 receptors. It is suggested that a population of P2 receptors may exist which promote the release of endogenous adenosine in the hippocampus.

Characterization and tissue location of the neural adenosine receptor in the rat ileum

1. The aim of the present investigation was to characterize and determine the tissue location of the adenosine receptors present in the rat ileum using a method that detects drug action on the cholinergic nerves innervating the longitudinal and circular muscles. 2. The non-selective adenosine agonist, NECA (10 and 100 nM) caused significant concentration-related reductions in the circular muscle responses to transmural stimulation over the frequency range of 2.5-40 Hz, but did not affect the responses of the longitudinal muscle, nor did it reduce the muscle responses of the guinea-pig ileum. 3. The affinity order of antagonists at inhibiting the effect of NECA on the circular muscle was: CPDPX>8-PT>DMPX with apparent pA2 values of 9.31, 7.54 and 5.63 respectively. CPDPX (10-100 nM) caused parallel displacements of the concentration-effect curves to CPA with a pKb value of 9.15 and Schild slope of 1.03. 4. The agonists previously tested in the rat jejunum peristaltic reflex preparation were also shown to inhibit responses of the rat ileum in the following decreasing order of potency: CPA>NECA>2-CADO>R-PIA>S-PIA>>PAA. In addition, CHA and CCPA were also potent agonists. NECA (100 nM) and CPA (32 nM) did not inhibit carbachol (1 microM)-induced tone of tissues pre-treated with TTX (1 microM). 5. In conclusion, the rat ileum contains inhibitory A1 adenosine receptors situated on cholinergic nerve endings innervating the circular muscle.

Only pyrimidinoceptors are functionally expressed in mouse neuroblastoma cell lines

The ability of UTP, UDP, ATP, and ADP to influence inositol phospholipid hydrolysis in neuroblastoma origin cell lines was assessed. The mouse neuroblastoma lines N1E 115, Neuro 2a, and NB4 1A3 and the rat glioma/mouse neuroblastoma hybrid line NG108-15 gave robust responses to both UTP and UDP, which were essentially equipotent. Thus a range of cell lines of mouse neuroblastoma origin express a pyrimidine-selective P2Y receptor. The NG108-15 cells were the only cell type tested at which ATP and ADP displayed activity with EC50 values of greater than 100 microM, compared with values of 0.58 and 1.25 microM for UTP and UDP, respectively. In contrast to the cell lines derived from mouse neuroblastoma, the human neuroblastoma lines SH-SY5Y and SK-N-SH did not respond to any nucleotides, although both responded well to carbachol.

Characterization and tissue location of the neural adenosine receptor in the rat ileum

1. The aim of the present investigation was to characterize and determine the tissue location of the adenosine receptors present in the rat ileum using a method that detects drug action on the cholinergic nerves innervating the longitudinal and circular muscles. 2. The non-selective adenosine agonist, NECA (10 and 100 nM) caused significant concentration-related reductions in the circular muscle responses to transmural stimulation over the frequency range of 2.5-40 Hz, but did not affect the responses of the longitudinal muscle, nor did it reduce the muscle responses of the guinea-pig ileum. 3. The affinity order of antagonists at inhibiting the effect of NECA on the circular muscle was: CPDPX>8-PT>DMPX with apparent pA2 values of 9.31, 7.54 and 5.63 respectively. CPDPX (10-100 nM) caused parallel displacements of the concentration-effect curves to CPA with a pKb value of 9.15 and Schild slope of 1.03. 4. The agonists previously tested in the rat jejunum peristaltic reflex preparation were also shown to inhibit responses of the rat ileum in the following decreasing order of potency: CPA>NECA>2-CADO>R-PIA>S-PIA>>PAA. In addition, CHA and CCPA were also potent agonists. NECA (100 nM) and CPA (32 nM) did not inhibit carbachol (1 microM)-induced tone of tissues pre-treated with TTX (1 microM). 5. In conclusion, the rat ileum contains inhibitory A1 adenosine receptors situated on cholinergic nerve endings innervating the circular muscle.

Dual coupling of heterologously-expressed rat P2Y6 nucleotide receptors to N-type Ca2+ and M-type K+ currents in rat sympathetic neurones

1. The P2Y6 receptor is a uridine nucleotide-specific G protein-linked receptor previously reported to stimulate the phosphoinositide (PI) pathway. We have investigated its effect in neurones, by micro-injecting its cRNA into dissociated rat sympathetic neurones and recording responses of N-type Ca2+ (I(Ca(N))) and M-type K+ (I(K(M))) currents. 2. In P2Y6 cRNA-injected neurones, UDP or UTP produced a voltage-dependent inhibition of I(Ca(N)) by approximately 53% in whole-cell (disrupted-patch) mode and by 73% in perforated-patch mode; no inhibition occurred in control cells. Mean IC50 values (whole-cell) were: UDP, 5.9+/-0.3 nM; UTP, 20+/-1 nM. ATP and ADP (1 microM) had no significant effect. Pertussis toxin (PTX) substantially (approximately 60%) reduced UTP-mediated inhibition in disrupted patch mode but not in perforated-patch mode. 3. Uridine nucleotides also inhibited I(K(M)) in P2Y6 cRNA-injected cells (by up to 71% at 10 microM UTP; perforated-patch). Mean IC50 values were: UDP, 30+/-3 nM; UTP, 115+/-12 nM. ATP (10 microM) again had no effect. No significant inhibition occurred in control cells. Inhibition was PTX-resistant. 4. Thus, the P2Y6 receptor, like the P2Y2 subtype studied in this system, couples to both of these two neuronal ion channels through at least two different G proteins. However, the P2Y6 receptor displays a much higher sensitivity to its agonists than the P2Y2 receptor in this expression system and higher than previously reported using other expression methods. The very high sensitivity to both UDP and UTP suggests that it might be preferentially activated by any locally released uridine nucleotides.

ATP receptors for the protection of hippocampal functions

The inhibitory effects of extracellular adenosine 5'-triphosphate (ATP) are reviewed in the present paper. ATP inhibits the release of the excitatory transmitter glutamate and stimulates the release of the inhibitory transmitter GABA from hippocampal neurons. Also, ATP activates potassium conductance directly through G protein, resulting in hyperpolarization of membrane potential. ATP activates microglia to secrete plasminogen that promotes the development of mesencephalic dopaminergic neurons and enhances neurite outgrowth from explants of neocortical tissue. Moreover, ATP may protect hippocampal neurons from excitotoxic cell death by preserving mitochondrial function. Thus, ATP may have a role in the protection of the function of hippocampus from over-stimulation by glutamate.

P2Y2 nucleotide receptors expressed heterologously in sympathetic neurons inhibit both N-type Ca2+ and M-type K+ currents

The P2Y2 receptor is a uridine/adenosine triphosphate (UTP/ATP)-sensitive G-protein-linked nucleotide receptor that previously has been reported to stimulate the phosphoinositide signaling pathway. Messenger RNA for this receptor has been detected in brain tissue. We have investigated the coupling of the molecularly defined rat P2Y2 receptor to neuronal N-type Ca2+ channels and to M-type K+ channels by heterologous expression in rat superior cervical sympathetic (SCG) neurons. After the injection of P2Y2 cRNA, UTP inhibited the currents carried by both types of ion channel. As previously reported [Filippov AK, Webb TE, Barnard EA, Brown DA (1997) Inhibition by heterologously expressed P2Y2 nuerones. Br J Pharmacol 121:849-851], UTP inhibited the Ca2+ current (ICa(N)) by up to 64%, with an IC50 of approximately 0.5 microM. We now find that UTP also inhibited the K+M current (IK(M)) by up to 61%, with an IC50 of approximately 1.5 microM. UTP had no effect on either current in neurons not injected with P2Y2 cRNA. Structure-activity relations for the inhibition of ICa(N) and IK(M) in P2Y2 cRNA-injected neurons were similar, with UTP >/= ATP > ITP >> GTP,UDP. However, coupling to these two channels involved different G-proteins: pretreatment with Pertussis toxin (PTX) did not affect UTP-induced inhibition of IK(M) but reduced inhibition of ICa(N) by approximately 60% and abolished the voltage-dependent component of this inhibition. In unclamped neurons, UTP greatly facilitated depolarization-induced action potential discharges. Thus, the single P2Y2 receptor can couple to at least two G-proteins to inhibit both Ca2+N and K+M channels with near-equal facility. This implies that the P2Y2 receptor may induce a broad range of effector responses in the nervous system.

P2Y2 nucleotide receptors expressed heterologously in sympathetic neurons inhibit both N-type Ca2+ and M-type K+ currents

The P2Y2 receptor is a uridine/adenosine triphosphate (UTP/ATP)-sensitive G-protein-linked nucleotide receptor that previously has been reported to stimulate the phosphoinositide signaling pathway. Messenger RNA for this receptor has been detected in brain tissue. We have investigated the coupling of the molecularly defined rat P2Y2 receptor to neuronal N-type Ca2+ channels and to M-type K+ channels by heterologous expression in rat superior cervical sympathetic (SCG) neurons. After the injection of P2Y2 cRNA, UTP inhibited the currents carried by both types of ion channel. As previously reported [Filippov AK, Webb TE, Barnard EA, Brown DA (1997) Inhibition by heterologously expressed P2Y2 nuerones. Br J Pharmacol 121:849-851], UTP inhibited the Ca2+ current (ICa(N)) by up to 64%, with an IC50 of approximately 0.5 microM. We now find that UTP also inhibited the K+M current (IK(M)) by up to 61%, with an IC50 of approximately 1.5 microM. UTP had no effect on either current in neurons not injected with P2Y2 cRNA. Structure-activity relations for the inhibition of ICa(N) and IK(M) in P2Y2 cRNA-injected neurons were similar, with UTP >/= ATP > ITP >> GTP,UDP. However, coupling to these two channels involved different G-proteins: pretreatment with Pertussis toxin (PTX) did not affect UTP-induced inhibition of IK(M) but reduced inhibition of ICa(N) by approximately 60% and abolished the voltage-dependent component of this inhibition. In unclamped neurons, UTP greatly facilitated depolarization-induced action potential discharges. Thus, the single P2Y2 receptor can couple to at least two G-proteins to inhibit both Ca2+N and K+M channels with near-equal facility. This implies that the P2Y2 receptor may induce a broad range of effector responses in the nervous system.

Adenosine monophosphate as a mediator of ATP effects at P1 purinoceptors

1. When perfused with a medium containing no added magnesium and 4-aminopyridine (4AP) (50 microM) hippocampal slices generated epileptiform bursts of an interictal nature. We have shown in a previous study that adenosine 5'-triphosphate (ATP) depressed epileptiform activity and that this effect was blocked by the adenosine A1 receptor antagonist cyclopentyltheophylline but was not affected by adenosine deaminase. This implied that ATP might act indirectly at P1 receptors or at a xanthine-sensitive P2 receptor. The aim of the present study was to investigate further the action of ATP on epileptiform activity. 2. ATP can be metabolized by ecto-nucleotidases to adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP) and adenosine, respectively. Each of these metabolites can activate receptors in its own right: P2 receptors for ADP and P1 receptors for AMP and adenosine. 3. We now show that both AMP and ATP (50 microM) significantly decrease epileptiform discharge rate in a rapid and reversible manner. 5'Adenylic acid deaminase (AMP deaminase, AMPase) (0.2 u ml(-1)), when perfused alone did not significantly alter the discharge rate over the 10 min superfusion period used for drug application. When perfused concurrently with AMP (50 microM), AMP deaminase prevented the depressant effect of AMP on discharge rate. 4. AMP deaminase, at a concentration of 0.2 u ml(-1) which annulled the effect of AMP (50 microM), prevented the inhibitory activity of ATP (50 microM). A higher concentration of ATP (200 microM) depressed the frequency of spontaneous bursts to approximately 30% control and this response was also prevented by AMP deaminase. 5. Superfusion of the slices with 5'-nucleotidase also prevented the inhibitory activity of ATP on epileptiform discharges. 6. The results suggest that AMP mediates the inhibitory effects of ATP on epileptiform activity, a conclusion which can explain the earlier finding that cyclopentyltheophylline but not adenosine deaminase inhibited the effect of ATP. A corollary to this is that, when examining the pharmacology of ATP, care must be taken to inactivate AMP with AMP deaminase, as well as adenosine with adenosine deaminase, before a direct action of ATP on P1 receptors can be postulated. Failure to do so may have led to erroneous conclusions in some previous studies of nucleotide activity on nucleotide receptors.

Developmental modulation of glutamatergic inspiratory drive to hypoglossal motoneurons

Proper function of hypoglossal motoneurons (XII MNs) innervating tongue muscles is critical for respiratory control of the airway. Morphological and electrophysiological properties of XII MNs change during postnatal development, as do modulatory systems. Despite these changes, the system producing respiratory movements must remain fully functional throughout life. Modulatory systems have therefore received considerable attention since coordination of their development with a developing neuromuscular system may be critical for maintenance of continuous, efficient breathing. Developmental modulation of XII inspiratory activity by three transmitter systems is examined. Thyrotropin-releasing hormone (TRH) mediates an increase in MN input resistance (RN) in juvenile but not neonate MNs, and this likely underlies the developmental increase in TRH potentiation of inspiratory activity. Norepinephrine (NE) potentiation of inspiratory activity, which in the neonate is produced in part by an alpha 1-mediated increase in RN, also increases postnatally. Effects of purinergic transmission on XII inspiratory activity remain constant during the first 2 weeks of postnatal development. Adenosine-triphosphate (ATP) produces tonic excitation and inspiratory potentiation that likely result from activation of postsynaptic P2 receptors. A secondary inhibitory effect likely results from hydrolysis of ATP to adenosine and activation of presynaptic A1 adenosine receptors. The functional relevance of these postnatal changes is discussed.

Diadenosine polyphosphates evoke Ca2+ transients in guinea-pig brain via receptors distinct from those for ATP

1. The ability of diadenosine polyphosphates, namely P1,P2-di(adenosine) pyrophosphate (Ap2A), P1,P3-di(adenosine) triphosphate (Ap3A), P1,P4-di(adenosine) tetraphosphate (Ap4A), P1,P5-di(adenosine) pentaphosphate (Ap5A) and P1,P6-di(adenosine) hexaphosphate (Ap6A) to evoke Ca2+ signals in synaptosomes prepared from three different regions of the guinea-pig brain was examined. 2. In synaptosomal preparations from the paleocortex (cortex), diencephalon/brainstem (midbrain) and cerebellum all the dinucleotides evoked Ca2+ signals that were concentration dependent over the range 1-300 microM. ATP and its synthetic analogues, alpha,beta-methylene ATP, 2-methylthio ATP and adenosine 5'-O-(2-thio)diphosphate (all 100 microM) also evoked Ca2+ signals in these preparations. 3. In the midbrain and cerebellum preparations, responses to ATP and its analogues were attenuated or abolished by the P2 receptor antagonist suramin (100 microM) but responses to the dinucleotides were not. Also, desensitization by a dinucleotide blocked responses to dinucleotides but not mononucleotides, and desensitization by a mononucleotide blocked responses to mononucleotides but not dinucleotides. 4. In cortical preparations, suramin (100 microM) blocked responses to both classes of nucleotides. Furthermore, there was mutual cross-desensitization between the mono- and dinucleotides. 5. The adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine, did not affect responses evoked by the dinucleotides, nor did the pyrimidine UTP. 6. It is concluded that there are specific dinucleotide receptors, activated by diadenosine polyphosphates, but not ATP or UTP, on synaptic terminals in guinea-pig diencephalon/ brainstem and cerebellum. These receptors bear a similarity to the dinucleotide receptor (P4 receptor) in rat brain. In guinea-pig cerebral cortex synaptosomes, diadenosine polyphosphates appear to act via the same receptor as ATP.