Metabotropic ATP receptor in hippocampal and thalamic neurones: pharmacology and modulation of Ca2+ mobilizing mechanisms

Implications of purinergic receptor-mediated intracellular calcium transients in neural differentiation

Purinergic receptors participate, in almost every cell type, in controlling metabolic activities and many physiological functions including signal transmission, proliferation and differentiation. While most of P2Y receptors induce transient elevations of intracellular calcium concentration by activation of intracellular calcium pools and forward these signals as waves which can also be transmitted into neighboring cells, P2X receptors produce calcium spikes which also include activation of voltage-operating calcium channels. P2Y and P2X receptors induce calcium transients that activate transcription factors responsible for the progress of differentiation through mediators including calmodulin and calcineurin. Expression of P2X2 as well as of P2X7 receptors increases in differentiating neurons and glial cells, respectively. Gene expression silencing assays indicate that these receptors are important for the progress of differentiation and neuronal or glial fate determination. Metabotropic receptors, mostly P2Y1 and P2Y2 subtypes, act on embryonic cells or cells at the neural progenitor stage by inducing proliferation as well as by regulation of neural differentiation through NFAT translocation. The scope of this review is to discuss the roles of purinergic receptor-induced calcium spike and wave activity and its codification in neurodevelopmental and neurodifferentiation processes.

Stimulation of bursting in pre-Bötzinger neurons by Epac through calcium release and modulation of TRPM4 and K-ATP channels

The exchange factor directly activated by cAMP (Epac) can couple cAMP production to the activation of particular membrane and cytoplasmic targets. Using patch-clamp recordings and calcium imaging in organotypic brainstem slices, we examined the role of Epac in pre-Bötzinger complex, an essential part of the respiratory network. The selective agonist 8-(4-chlorophenylthio)-2'-O-methyl-cAMP (8-pCPT) sensitized calcium mobilisation from inositol-1,4,5-trisphosphate-sensitive internal stores that stimulated TRPM4 (transient receptor potential cation channel, subfamily M, Melastatin) channels and potentiated the bursts of action potentials. 8-pCPT actions were abolished after inhibition of phospholipase C with U73122 and depletion of calcium stores with thapsigargin. Caffeine-sensitive release channels were not modulated by 8-pCPT. Epac inhibited ATP-sensitive K(+) channels that also led to the enhancement of bursting by 8-pCPT. Bursting activity, spontaneous calcium transients and activity of TRPM4 and ATP-sensitive K(+) channels were potentiated after brief exposures to bradykinin and incubation with wortmannin produced opposite effects that can be explained by changes in phosphatidylinositol 4,5-bisphosphate levels. 8-pCPT stimulated the respiratory motor output in functionally intact preparations and the effects of bradykinin and wortmannin were identical to those observed in organotypic slices. The data thus indicate a novel pathway of controlling bursting activity in pre-Bötzinger complex neurons through Epac that can involved in reinforcement of the respiratory activity by cAMP.

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.

Increase of intracellular Ca2+ by P2Y but not P2X receptors in cultured cortical multipolar neurons of the rat

The expression and functionality of P2X/P2Y receptor subtypes in multipolar nonpyramidal neurons of mixed cortical cell cultures were investigated by means of immunocytochemistry and fura-2 microfluorimetry. The morphological studies revealed that most of the neurons are immunoreactive for GABA and express a range of P2X/P2Y receptors, predominantly of the P2X(2,4,6) and P2Y(1,2) subtypes. P2X(1) and P2X(7) receptor immunoreactivity (IR) was found on thin axon-like processes and presynaptic structures, respectively. Application of ATP caused a small concentration-dependent increase in intracellular Ca2+ concentration ([Ca2+]i) in most investigated neurons, whereas only about the half of these cells responded to 2',3'-O-(benzoyl-4-benzoyl)-ATP (BzATP), ADPbetaS, 2MeSADP, or 2MeSATP and even fewer cells to UTP. In contrast, alpha,beta-meATP, UDP, and UDP-glucose failed to produce any [Ca2+]i signaling. The response to ATP itself was inhibited by pyridoxal-5'-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), Reactive Blue 2, 2'-deoxy-N(6)-methyl adenosine 3',5'-diphosphate (MRS2179), and suramin (300 microM) as well as by a cyclopiazonic acid-induced depletion of intracellular Ca2+ stores. A Ca2+-free external medium tended to decrease the ATP-induced [Ca2+]i transients, although this action did not reach statistical significance. Various blockers of voltage-sensitive Ca2+ channels and the gap junction inhibitor carbenoxolone did not interfere with the effect of ATP, whereas a combination of the ionotropic glutamate receptor antagonists D(-)-2-amino-5-phosphonopentanoic acid (AP5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) decreased it. Cross-desensitization experiments between ADPbetaS or UTP and ATP suggested that ATP acts on the one hand via P2Y(1,2) receptors and on the other hand by additional signaling mechanisms. These mechanisms may involve the release of glutamate (which in consequence activates ionotropic glutamate receptors) and the entry of Ca2+ via store-operated Ca2+ channels. Evidence for the presence of functional P2X receptors, in particular P2X(7), remains elusive.

Respiratory circuits: function, mechanisms, topology, and pathology

Neuroscientists have long sought to understand how circuits in the nervous system are organized to generate the precise neural outputs that underlie particular behaviors. Recent studies deepened our understanding of the mechanisms responsible for the generation of the rhythmic output for breathing. Here, the author focuses on issues that are pertinent for the respiratory network and considers its organization and how it derives the functional output. The author discusses pacemaker and network mechanisms of rhythm generation, which are now combined into a novel concept of emergent network activity due to coherent excitation of pacemaker groups. He discusses subcellular basis of this hypothesis and possible mechanisms of synchronization within respiratory network. These new findings in respiratory neuroscience are further applied to explain modifications in breathing during hypoxia and possible origins of respiratory disorders that may be acquired during neural development and aging.

Imaging cytoplasmic cAMP in mouse brainstem neurons

BACKGROUND

cAMP is an ubiquitous second messenger mediating various neuronal functions, often as a consequence of increased intracellular Ca2+ levels. While imaging of calcium is commonly used in neuroscience applications, probing for cAMP levels has not yet been performed in living vertebrate neuronal tissue before.

RESULTS

Using a strictly neuron-restricted promoter we virally transduced neurons in the organotypic brainstem slices which contained pre-Bötzinger complex, constituting the rhythm-generating part of the respiratory network. Fluorescent cAMP sensor Epac1-camps was expressed both in neuronal cell bodies and neurites, allowing us to measure intracellular distribution of cAMP, its absolute levels and time-dependent changes in response to physiological stimuli. We recorded [cAMP]i changes in the micromolar range after modulation of adenylate cyclase, inhibition of phosphodiesterase and activation of G-protein-coupled metabotropic receptors. [cAMP]i levels increased after membrane depolarisation and release of Ca2+ from internal stores. The effects developed slowly and reached their maximum after transient [Ca2+]i elevations subsided. Ca2+-dependent [cAMP]i transients were suppressed after blockade of adenylate cyclase with 0.1 mM adenylate cyclase inhibitor 2'5'-dideoxyadenosine and potentiated after inhibiting phosphodiesterase with isobutylmethylxanthine and rolipram. During paired stimulations, the second depolarisation and Ca2+ release evoked bigger cAMP responses. These effects were abolished after inhibition of protein kinase A with H-89 pointing to the important role of phosphorylation of calcium channels in the potentiation of [cAMP]i transients.

CONCLUSION

We constructed and characterized a neuron-specific cAMP probe based on Epac1-camps. Using viral gene transfer we showed its efficient expression in organotypic brainstem preparations. Strong fluorescence, resistance to photobleaching and possibility of direct estimation of [cAMP] levels using dual wavelength measurements make the probe useful in studies of neurons and the mechanisms of their plasticity. Epac1-camps was applied to examine the crosstalk between Ca2+ and cAMP signalling and revealed a synergism of actions of these two second messengers.

New insights into purinergic receptor signaling in neuronal differentiation, neuroprotection, and brain disorders

Ionotropic P2X and metabotropic P2Y purinergic receptors are expressed in the central nervous system and participate in the synaptic process particularly associated with acetylcholine, GABA, and glutamate neurotransmission. As a result of activation, the P2 receptors promote the elevation of free intracellular calcium concentration as the main signaling pathway. Purinergic signaling is present in early stages of embryogenesis and is involved in processes of cell proliferation, migration, and differentiation. The use of new techniques such as knockout animals, in vitro models of neuronal differentiation, antisense oligonucleotides to induce downregulation of purinergic receptor gene expression, and the development of selective inhibitors for purinergic receptor subtypes contribute to the comprehension of the role of purinergic signaling during neurogenesis. In this review, we shall discuss the participation of purinergic receptors in developmental processes and in brain physiology, including neuron-glia interactions and pathophysiology.

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.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

Novel vistas of calcium-mediated signalling in the thalamus

Traditionally, the role of calcium ions (Ca(2+)) in thalamic neurons has been viewed as that of electrical charge carriers. Recent experimental findings in thalamic cells have only begun to unravel a highly complex Ca(2+) signalling network that exploits extra- and intracellular Ca(2+) sources. In thalamocortical relay neurons, interactions between T-type Ca(2+) channel activation, Ca(2+)-dependent regulation of adenylyl cyclase activity and the hyperpolarization-activated cation current ( I(h)) regulate oscillatory burst firing during periods of sleep and generalized epilepsy, while a functional triad between Ca(2+) influx through high-voltage-activated (most likely L-type) Ca(2+) channels, Ca(2+)-induced Ca(2+) release via ryanodine receptors (RyRs) and a repolarizing mechanism (possibly via K(+) channels of the BK(Ca) type) supports tonic spike firing as required during wakefulness. The mechanisms seem to be located mostly at dendritic and somatic sites, respectively. One functional compartment involving local GABAergic interneurons in certain thalamic relay nuclei is the glomerulus, in which the dendritic release of GABA is regulated by Ca(2+) influx via canonical transient receptor potential channels (TRPC), thereby presumably enabling transmitters of extrathalamic input systems that are coupled to phospholipase C (PLC)-activating receptors to control feed-forward inhibition in the thalamus. Functional interplay between T-type Ca(2+) channels in dendrites and the A-type K(+) current controls burst firing, contributing to the range of oscillatory activity observed in these interneurons. GABAergic neurons in the reticular thalamic (RT) nucleus recruit a specific set of Ca(2+)-dependent mechanisms for the generation of rhythmic burst firing, of which a particular T-type Ca(2+) channel in the dendritic membrane, the Ca(2+)-dependent activation of non-specific cation channels ( I(CAN)) and of K(+) channels (SK(Ca) type) are key players. Glial Ca(2+) signalling in the thalamus appears to be a basic mechanism of the dynamic and integrated exchange of information between glial cells and neurons. The conclusion from these observations is that a localized calcium signalling network exists in all neuronal and probably also glial cell types in the thalamus and that this network is dedicated to the precise regulation of the functional mode of the thalamus during various behavioural states.

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.

The endoplasmic reticulum and neuronal calcium signalling

The endoplasmic reticulum (ER) is a multifunctional signalling organelle regulating a wide range of neuronal functional responses. The ER is intimately involved in intracellular Ca(2+) signalling, producing local or global cytosolic calcium fluctuations via Ca(2+)-induced Ca(2+) release (CICR) or inositol-1,4,5-trisphosphate-induced Ca(2+) release (IICR). The CICR and IICR are controlled by two subsets of Ca(2+) release channels residing in the ER membrane, the Ca(2+)-gated Ca(2+) release channels, generally known as ryanodine receptors (RyRs) and InsP(3)-gated Ca(2+) release channels, referred to as InsP(3)-receptors (InsP(3)Rs). Both types of Ca(2+) release channels are expressed abundantly in nerve cells and their activation triggers cytoplasmic Ca(2+) signals important for synaptic transmission and plasticity. The RyRs and InsP(3)Rs show heterogeneous localisation in distinct cellular sub-compartments, conferring thus specificity in local Ca(2+) signals. At the same time, the ER Ca(2+) store emerges as a single interconnected pool fenced by the endomembrane. The continuity of the ER Ca(2+) store could play an important role in various aspects of neuronal signalling. For example, Ca(2+) ions may diffuse within the ER lumen with comparative ease, endowing this organelle with the capacity for "Ca(2+) tunnelling". Thus, continuous intra-ER Ca(2+) highways may be very important for the rapid replenishment of parts of the pool subjected to excessive stimulation (e.g. in small compartments within dendritic spines), the facilitated removal of localised Ca(2+) loads, and finally in conveying Ca(2+) signals from the site of entry towards the cell interior and nucleus.

Metabotropic purinoreceptors in rat dorsal horn neurones: predominant dendritic location

Elevations of cytosolic free Ca2+ concentration ([Ca2+]i) induced by addition of ATP have been compared in rat dorsal horn neurones in slices and after their isolation. ATP application induced in neurones in situ a rise of [Ca2+]i by 201 +/- 12 nM. In Ca2+-free external solution the rise was 156 +/- 14 nM (n = 45 of 76), indicating the presence of active purinergic metabotropic receptors in about 59% of neurones. [Ca2+]i transients induced by 2MeSATP in Ca2+-free external solution were completely abolished by 10 microM PPADS, indicating that some of the corresponding receptors are of the P2Y1 type. In acutely isolated neurones which lost their dendrites, there were no metabotropic response. The results confirm the presence of metabotropic postsynaptic purinoreceptors located in the dendritic tree of dorsal horn neurones.

Purinergic and muscarinic receptor activation activates a common calcium entry pathway in rat neocortical neurons and glial cells

The nature of metabotropic purinergic and muscarinic receptor-mediated increases in intracellular calcium in primary rat neocortical neurons and glial cells has been investigated using fluorescence imaging techniques. Bath-application of ATP and muscarine (10 microM) elicited a characteristic increase in intracellular calcium in both neurons and glial cells. The profile of this response consisted of an initial transient increase followed by a sustained elevation (the plateau phase) which was dependent on extracellular calcium. Examination of the pharmacological basis of the purinergic receptor-mediated calcium response using 10 microM 2-methyl-thio ATP (MeS-ATP) and UTP revealed that P(2Y) receptor activation underlies this response. The calcium influx pathway responsible for the sustained calcium response was inhibited by metal ions. In both cell types La(3+) and Zn(2+) (100 microM) effectively inhibited the plateau phase of the response, whilst 100 microM Ni(2+) had little or no effect. In conclusion, P(2Y) purinergic and muscarinic receptor activation evoke a sustained increase in intracellular calcium in neocortical neurons and glial cells. This response has similar characteristics to that we have previously described following mGlu(5) activation. We propose that in these cell types stimulation of metabotropic receptors coupled to phosphoinositide turnover activates a common calcium entry pathway that is distinct from voltage-gated calcium channels and resembles store-operated calcium entry.

Ca2+-induced Ca2+ release supports the relay mode of activity in thalamocortical cells

Ca2+ ions play an important role during rhythmic bursting of thalamocortical neurons within sleep. The function of Ca2+ during the tonic relay mode of these neurons during wakefulness is less clear. Here, we report that tonic activity in thalamocortical cells results in an increase in the intracellular Ca2+ concentration and subsequent release of Ca2+ from intracellular stores mediated via ryanodine receptors (RyRs). Blockade of Ca2+ release shifted the regular firing of single action potentials toward the generation of spike clusters. Regular spike firing and intracellular Ca2+ release thus appear to be functionally coupled in a positive feedback manner, thereby supporting the relay mode of thalamocortical cells during wakefulness. Regulatory influences may be coupled to this system via the cyclic ADP ribose pathway.

Regulation of transmembrane signaling by ganglioside GM1: interaction of anti-GM1 with Neuro2a cells

Interaction of antibodies to ganglioside GM1 with Neuro2a cells was studied to investigate the role of GM1 in cell signaling. Binding of anti-GM1 to Neuro2a cells induced the formation of 3H-inositol phosphates (3H-IPs) and elevated the intracellular Ca2+ concentration [Ca2+]i. The rise in [Ca2+]i was due to the influx of Ca2+ from the extracellular medium and release from intracellular Ca2+ pools. The Ca2+ influx pathway did not allow the permeation of Na+ or K+. The influx was inhibited by amiloride, a specific blocker of T-type Ca2+ channels, whereas nifedipine and diltiazem, blockers of L-type Ca2+ channels, did not have any effect. Thus, anti-GM1 appears to activate a T-type Ca2+ channel in Neuro2a cells. The intracellular Ca2+ release was inhibited by pretreatment of cells with neomycin sulfate, phorbol dibutyrate, and pertussis toxin (PTx), which also inhibited the 3H-IP formation in Neuro2a cells. Addition of caffeine neither elevated the [Ca2+]i nor affected the anti-GM1-induced [Ca2+]i rise. The data reveal that the binding of anti-GM1 to Neuro2a cells activates phospholipase C via a PTx-sensitive G protein, which leads to formation of IPs and release of Ca2+ from inositol trisphosphate-sensitive pool of endoplasmic reticulum. Anti-GM1 also arrested the differentiation of Neuro2a cells in culture and significantly stimulated their proliferation. This stimulatory effect of anti-GM1 on cell proliferation was blocked by amiloride but not by PTx, suggesting that the influx of Ca2+ was essentially required for cell proliferation. Our data suggest a role for GM1 in the regulation of transmembrane signaling events and cell growth.

Iono- and metabotropically induced purinergic calcium signalling in rat neocortical neurons

ATP receptor-mediated Ca2+ concentration changes were recorded from neocortical neurones in brain slices from 2 week-old rats. To measure the cytoplasmic concentration of Ca2+ ([Ca2+]i) slices were incubated with fura-2/AM, and the microfluorimetry system was focused on an individual cell. During transients the intracellular level of [Ca2+]i in the majority of neocortical neurones (98 of 102) varied in the concentration range of ATP 5-2000 microM between 41. 3+/-5 and 163+/-7 nM. The rank order of efficacy for purinoreceptor agonists in concentration 100 microM was: ATPgammaS>ATP>ADP>>AMP approximately Adenosine approximately alpha,beta-methylene ATP>UTP. 10 microM PPADS, a P2-purinoreceptor antagonist, reduced the ATP-induced [Ca2+]i response by 26%+/-4%. After elimination of calcium from extracellular solution the first ATP-induced [Ca2+]i transient decreased to 65+/-8%, suggesting the participation of metabotropic P2y triggered Ca-release in the generation of the transient. Elevation of cytosolic Ca2+ by activation of plasmalemmal Ca2+ channels failed to potentiate such release indicating the absence of effective reloading of the corresponding stores. No Ca2+-induced Ca2+-release has been observed in the investigated neurons.

Nucleotide receptors in the nervous system. An abundant component using diverse transduction mechanisms

Extracellular nucleotides achieve their role as cell-to-cell communicators by acting at cell surface transmembrane receptors-the P2 receptors. Before molecular cloning led to the isolation of any P2-receptor sequence, a small number of receptor types had been proposed on the basis of pharmacological evidence. The application of molecular biology to this field of receptor research has indicated that a great underestimation of the number of receptor subtypes and of their abundance had occurred. There are now known to be seven characterized P2Y (G protein linked) receptors and the same number again of P2X receptors of the transmitter-gated ion channel type. In this review, we discuss the properties of these cloned receptors, their distribution within the nervous system, and their methods of signal transduction.

Early expression of a novel nucleotide receptor in the neural plate of Xenopus embryos

Extracellular ATP functions as a neurotransmitter and neuromodulator in the adult nervous system, and a signaling molecule in non-neural tissue, acting either via ligand-gated ion channels (P2X) or G-protein-coupled receptors (P2Y). ATP can cause an increase in intracellular Ca2+ (Ca2+i) in embryonic cells and so regulate cell proliferation, migration, and differentiation. We have isolated a Xenopus cDNA encoding a novel P2Y receptor, XlP2Y, which is expressed abundantly in developing embryos. Recombinant XlP2Y responds equally to all five naturally occurring nucleoside triphosphates (ATP, UTP, CTP, GTP, and ITP), which elicit a biphasic Ca2+-dependent Cl- current (ICl,Ca) where the second phase persists for up to 60 min. XlP2Y also causes a continuous release of Ca2+i and a low level persistent activation of ICl,Ca in Xenopus oocytes through the spontaneous efflux of ATP. mRNAs for XlP2Y are expressed transiently in the neural plate and tailbud during Xenopus development, coincident with neurogenesis. This restricted pattern of expression and novel pharmacological features confer unique properties to XlP2Y, which may play a key role in the early development of neural tissue.

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.

ATP receptor activation potentiates a voltage-dependent Ca channel in hippocampal neurons

Activation of a purinergic P2 receptor by adenosine 5'-triphosphate (ATP) has previously been shown to open a non-selective cation channel with a reversal potential of approximately 0 mV. We examined the effect of P2 receptor activation on voltage-gated ionic currents in acutely isolated CA3 pyramidal neurons from guinea pig hippocampus using the whole-cell-patch technique. Under conditions designed to isolate current through voltage-dependent Ca channels (ICa), ATP (50 microM) potentiated ICa by 36%. This increase in ICa desensitized back to control levels within 4 min. In contrast to the non-selective cation channel, ICa elicited from a holding potential (HP) of -100 mV showed significant potentiation in response to ATP when depolarized to a test potential (TP) of -10 mV but showed no effect on ICa when the same neuron was alternately depolarized to TP = -70 mV. No change in holding current at HP = -100 mV occurred. Tail currents were unaffected by ATP exposure suggesting that ICa potentiation was not due to modulation of L-type Ca channels. This potentiation was also observed either with ATP-gamma-s, the slowly hydrolyzable ATP analog, or with ATP in the presence of alpha, beta-methylene-ADP, an ectonucleotidase inhibitor, indicating that the effects observed were not due to activation of an adenosine receptor that required ATP hydrolysis. The potentiation of ICa was not observed with the P2X agonist, beta, gamma-methylene-ATP. These results suggest that ATP receptors can modulate voltage- as well as ligand-gated channels permeable to calcium and may play an important role in the dynamics of intracellular Ca2+ in these neurons.

Mechanisms of intracellular calcium regulation in adult astrocytes

Microfluorimetric techniques were used to measure changes in intracellular calcium in astrocytes cultured from the forebrain of the adult rat. Application of ATP consistently raised intracellular calcium. The response persisted in the absence of extracellular calcium, but then quickly declined upon repeated agonist application. Thapsigargin abolished responses to nucleotides following depletion of the endoplasmic reticular calcium stores. Calcium release was inhibited by caffeine, but was dramatically increased through inositol phosphate receptor sensitization by the sulphydryl reagent thimerosal. Responses to repeated nucleotide applications resulted in a gradual decline of peak calcium concentrations, suggesting a (post)receptor-mediated desensitization or gradual depletion of the internal calcium stores. Subsequent application of ionomycin suggested intracellular calcium depletion as the relevant mechanism. Depletion of the internal calcium stores with ATP, ionomycin or thapsigargin failed to reveal a calcium influx pathway. These results suggest that the capacitative mechanism of calcium entry does not operate in response to nucleotide receptor activation in these cells, and that the immediate refilling of the internal calcium stores is primarily determined by re-uptake of cytosolic calcium into the endoplasmic reticulum. A complete refilling of this calcium store by extracellular calcium may be a much slower process. Control of these signal transduction pathways is crucial to the maintenance of the calcium/energy homeostasis of the adult astrocyte in the central nervous system.

Ethanol actions on the mechanisms of Ca2+ mobilization in rat hippocampal cells are mediated by protein kinase C

The effects of ethanol on intracellular free Ca(2+) concentration, [Ca](i), were studied in cultured rat hippocampal neurons using fluo-3 and confocal microscopy. Ethanol application transiently elevAted [Ca](i) due to Ca(2+)-induced Ca(2+) release from internal stores since the effect was observed also in solutions containing zero Ca(2+) or 0.3 mM La(3+) and restoration of external Ca(2+) content led to secondary response in presence of ethanol. The sites of highest [Ca]i increases correlated well with those obtained after Ca(2+) release from caffeine-and IP3-sensitive internal stores. After single ethanol exposure the caffeine-evoked [Ca](i) transients were potentiated whereas Ca(2+) release induced by IP(3)-mobilizing agonists was suppressed. Similar effects were observed by activation of protein kinase C (PKC) by phorbol esters which also occluded ethanol actions. Ethanol increased fluorescence of Rim-1, a PKC indicator dye. The data obtained are consistent with ethanol activation of PKC whereby Ca(2+) release via ryanodine receptors is potentiated and IP(3) receptors are down-modulated. Since the effects of both ethanol and phorbol esters were mimicked by cytochalasins B and D, PKC-induced cytoskeleton phosphorylation and its subsequent rearrangements can be responsible for observed effects.

Plasmalemmal and intracellular Ca2+ pumps as main determinants of slow Ca2+ buffering in rat hippocampal neurones

Using the Ca(2+)-sensitive fluorescent indicator dye fura-2, the mechanisms by which cytoplasmic free Ca2+ concentration, [Ca]i, decays to resting levels were studied in neurones cultured from the rat hippocampus. The time-course of [Ca]i restoration after transient elevations due to CaCl2 injections or brief exposures to 50 mM K Cl were biexponential. Application of specific inhibitors of systems participating in Ca2+ removal from cytoplasm changed both basal [Ca]i and the slow phase of recovery, but the fast phase was unaltered by any treatment. Inhibition of the plasmalemmal Ca2+ pump by external alkalinization or intracellular acidification was reversible, whereas calmodulin inhibitors (calmidazolium and triftazine, W-13) acted irreversibly. The net effects of blockers of the intracellular Ca2+ pump, thapsigargin (Tg) and t-BuHQ, were similar. Suppression of mitochondrial Ca2+ uptake or Ca2+ extrusion due to Na+/Ca2+ exchange, reversibly increased [Ca]i but the time-course of [Ca]i clearance was marginally changed. After glutamate application [Ca]i restoration was prolonged which was mediated by concomitant intracellular acidification causing inhibition of plasmalemmal Ca2+ ATPase. It is concluded that Ca2+ homeostasis in rat hippocampal neurones is mainly determined by Ca2+ pumps in both the surface membrane and internal stores, whereas Na+/Ca2+ exchange and mitochondria play a minor role.

Effect of P2-purinoceptor antagonists on glutamatergic transmission in the rat hippocampus

1. A study has been made of the effects of P2-purinoceptor antagonists on the evoked excitatory postsynaptic currents (e.p.s.cs) generated in CA1 pyramidal cells on stimulation of Schaffer collaterals and in CA3 pyramidal cells on stimulation of mossy fibres. The effects of these antagonists on currents generated in the cells on application of glutamate has also been determined. 2. Suramin blocked the evoked e.p.s.cs with an 50% inhibition (ID50) of 62 +/- 8 microM (mean +/- s.e.mean, n = 17), spontaneous miniature e.p.s.cs and the currents induced by application of 100 microM glutamate with an ID50 = 121 +/- 36 microM (n = 15) in all the cells studied. 3. Reactive Blue 2 (RB-2) in a concentration of 200 microM decreased the e.p.s.cs by 80 +/- 10% (n = 6) and the glutamate-activated currents by 83 +/- 3% (n = 6). 4. Pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) in the concentration-range of 40-500 microM decreased the amplitude of the e.p.s.cs in 12 out of 13 cells studied. PPADS at 200 microM reduced the amplitude of the e.p.s.cs by 60 +/- 10% (n = 3). PPADS did not affect the glutamate-induced currents in 4 cells and produced potentiation of the current amplitude by 60 +/- 10% in 4 other cells. 5. These results suggest that both presynaptic and postsynaptic P2-purinoceptors in the hippocampus can modulate the release and action of endogenous glutamate.

Purinoceptors in the nervous system

The purine nucleoside adenosine and the purine nucleotide ATP play different roles in the nervous system. Adenosine acts on a family of G protein coupled receptors, collectively called adenosine receptors or P1 purinoceptors. Four members of this family have been cloned and pharmacologically characterized: A1, A2A, A2B and A3. Their distribution, pharmacology and biological roles are briefly discussed. In particular, the evidence that adenosine acting at A1 receptors regulates the release of several neurotransmitters and that adenosine acting at A2A receptors modulates dopaminergic transmission is summarized. ATP acts on receptors called P2 purinoceptors, which appear to fall into at least two main families--G protein coupled receptors and intrinsic ion channels. Their subclassification is becoming clearer as receptors are cloned and new selective agonists and/or antagonists are becoming available. There is an interesting potential for development of drugs targeted at purines or their receptors.

Nucleotides as extracellular signalling molecules

There is now wide acceptance that ATP and other nucleotides are ubiquitous extracellular chemical messengers. ATP and diadenosine polyphosphates can be released from synaptosomes. They act on a large and diverse family of P2 purinoceptors, four of which have been cloned. This receptor family can be divided into two distinct classes: ligand-gated ion channels for P2X receptors and G protein-coupled receptors for P2Y, P2U, P2T and P2D receptors. The P2Y, P2U and P2D receptors have a fairly wide tissue distribution, while the P2X receptor is mainly found in neurons and muscles and the P2T and P2Z receptors confined to platelets and immune cells, respectively. Inositol phosphate and calcium signalling appear to be the predominant mechanisms for transducing the G-protein linked P2 receptor signals. Multiple P2 receptors are expressed by neurons and glia in the CNS and also in neuroendocrine cells. ATP and other nucleotides may therefore have important roles not only as a neurotransmitter but also as a neuroendocrine regulatory messenger.