Characterization and possible function of adenosine 5'-triphosphate receptors in activated rat microglia

P2X7 receptors in Müller glial cells from the human retina

ATP has been shown to be an important extracellular signaling molecule. There are two subgroups of receptors for ATP (and other purines and pyrimidines): the ionotropic P2X and the G-protein-coupled P2Y receptors. Different subtypes of these receptors have been identified by molecular biology, but little is known about their functional properties in the nervous system. Here we present data for the existence of P2 receptors in Müller (glial) cells of the human retina. The cells were studied by immunocytochemistry, electrophysiology, Ca(2+)-microfluorimetry, and molecular biology. They displayed both P2Y and P2X receptors. Freshly enzymatically isolated cells were used throughout the study. Although the [Ca(2+)](i) response to ATP was dominated by release from intracellular stores, there is multiple evidence that the ATP-induced membrane currents were caused by an activation of P2X(7) receptors. Immunocytochemistry and single-cell RT-PCR revealed the expression of P2X(7) receptors by Müller cells. In patch-clamp studies, we found that (1) benzoyl-benzoyl ATP (BzATP) was the most effective agonist to evoke large inward currents and (2) the currents were abolished by P2X antagonists; however, (3) long-lasting application of BzATP did not cause an opening of large pores in addition to the cationic channels. By microfluorimetry it was shown that the P2X receptors mediated a Ca(2+) influx that contributed a small component to the total [Ca(2+)](i) response. Activation of P2X receptors may modulate the uptake of neurotransmitters from the extracellular space by Müller cells in the retina.

Activation of purinergic P2X receptors inhibits P2Y-mediated Ca2+ influx in human microglia

Purinoceptor (P2X and P2Y) mediated Ca2+ signaling in cultured human microglia was studied using Ca2+ sensitive fluorescence microscopy. ATP (at 100 microM) induced a transient increase in [Ca2+]i in both normal and Ca(2+)-free solution suggesting a primary contribution by release from intracellular stores. This conclusion was further supported by the failure of ATP to cause a divalent cationic influx in Mn2+ quenching experiments. However, when fluorescence quenching was repeated after removal of extracellular Na+, ATP induced a large influx of Mn2+, indicating that inward Na+ current through a non-selective P2X-coupled channel may normally suppress divalent cation influx. Inhibition of Mn2+ entry was also found when microglia were depolarized using elevated external K+ in Na(+)-free solutions. The possibility of P2X inhibition of Ca2+ influx was then investigated by minimizing P2X contributions of purinergic responses using either the specific P2Y agonist, ADP-beta-S in the absence of ATP or using ATP combined with PPADS, a specific inhibitor of P2X receptors. In quenching studies both procedures resulted in large increases in Mn2+ influx in contrast to the lack of effect observed with ATP. In addition, perfusion of either ATP plus PPADS or ADP-beta-S alone caused a significantly enhanced duration (about 200%) of the [Ca2+]i response relative to that induced by ATP. These results show that depolarization induced by P2X-mediated Na+ influx inhibits store-operated Ca2+ entry resulting from P2Y activation, thereby modulating purinergic signaling in human microglia.

Acute application of interleukin-1beta induces Ca(2+) responses in human microglia

The effects of the pro-inflammatory cytokine interleukin-1-beta (IL-1beta) on levels of intracellular calcium [Ca(2+)](i) in cultured human microglia have been studied using the fluorescent Ca(2+) indicator fura-2. IL-1beta (2 ng/ml) caused a slow, progressive increase in [Ca(2+)](i) in standard Ca(2+)-containing physiological solution (PSS). A similar effect was observed in separate studies using Ca(2+)-free PSS, however, the mean rate of increase was significantly lower than that measured with PSS. Similar results were obtained in a separate protocol, where cells were exposed to both IL-1beta in Ca(2+)-free PSS and PSS. The slope of the IL-1beta induced increase of [Ca(2+)](i) in Ca(2+)-free PSS was not altered when adenosine triphosphate was added prior to application of the cytokine. These results suggest that IL-1beta-induced responses in human microglia involve both a Ca(2+) entry pathway and a mechanism of intracellular increase other than from IP(3)-sensitive stores.

Activation of mouse microglial cells affects P2 receptor signaling

Microglial cells are the immunocompetent cells of the CNS, which are known to exist in several activation states. Here we investigated the impact of microglial activation on the P2 receptor-mediated intracellular calcium ([Ca(2+)](i)) signaling by means of fluo-3 based Ca(2+)-imaging. Cultured mouse microglial cells were treated with either astrocyte-conditioned medium to induce a ramified morphology or LPS to shift the cells toward the fully activated stage. The extracellular application of ATP (100 microM) induced a [Ca(2+)](i) elevation in 85% of both untreated and ramified microglial cells, whereas only 50% of the LPS-activated cells responded to the stimulus. To characterise the pharmacological profile of microglial P2 receptors we investigated the effects of various P2 agonists on [Ca(2+)](i) in cultured microglial cells. Untreated and ramified microglial cells demonstrated a very similar sensitivity to the different P2 agonists. In contrast, in LPS-activated microglia, a sharp decrease of responses to P2 agonist stimulation was seen. This indicates that microglial activation influences the capability of microglial cells to generate [Ca(2+)](i) signals upon P2 receptor activation.

Platelet-activating factor induced Ca(2+) signaling in human microglia

Increases in intracellular Ca(2+) concentration in human microglial cells in response to platelet-activating factor (PAF) were studied using Ca(2+)-sensitive fluorescence microscopy. In normal physiological solution (PSS), PAF-induced transient increases in [Ca2+](i) which recovered to baseline values within 200 s. Application of PAF in zero-Ca(2+) solution caused the peak response to be decreased to a value near 20% of that recorded in PSS suggesting a primary contribution of Ca(2+) influx for the [Ca2+](i) increase in PSS. To investigate PAF-induced Ca(2+) influx, the contents of intracellular stores were modulated using the SERCA blocker cyclopiazonic acid (CPA). The Ca(2+) signal induced by CPA (10 microM) in zero-Ca(2+) solution showed a peak response about 20% of the amplitude in the presence of external Ca(2+), suggesting the latter response included significant contributions from store-operated Ca(2+) entry. The influx of divalent cations with PAF or CPA was directly measured using Mn(2+) quenching of the fluorescence signal. Although both PAF and CPA induced a similar degree of Mn(2+) influx over time, the PAF effect was very rapid, whereas the CPA action was delayed and only evident about 200 s after application. Overall, the results show that the primary source of the PAF-induced increase of [Ca2+](i) in human microglia was the influx of Ca(2+) from the extracellular space and intracellular Ca(2+)-release contributed only a small part of the total Ca(2+) signal. Nevertheless, Ca(2+)-release induced by PAF (or CPA) serves as an important factor in controlling Ca(2+) entry presumably mediated by activation of store-operated-Ca(2+) channels.

Two different ionotropic receptors are activated by ATP in rat microglia

1. Our aim was to assess whether ATP-induced inward currents in microglia are due to a single or more than one purinergic receptor. The ATP dose-response curve showed two components, whose presence might be due to the activation of high and low affinity receptors. 2. The P2Z/P2X7 specific receptor agonist benzoylbenzoyl-ATP (Bz-ATP) and some P2 receptor agonists were tested. The rank order of potency was Bz-ATP >> ATP = 2-methylthio-ATP (2-MeSATP) > alpha, beta-methylene ATP (alpha,beta-meATP) >= ADP. beta, gamma-MethyleneATP (beta,gamma-meATP), UTP and adenosine were ineffective. 3. The non-specific P2 receptor antagonist suramin antagonized by 92 +/- 2 % the inward current induced by 100 microM ATP, and by 51 +/- 8 and 68 +/- 6 % those induced by 3 mM ATP and 100 microM Bz-ATP, respectively. The P2Z/P2X7 antagonist oxidized ATP (oATP) almost abolished the inward current induced by 3 mM ATP or Bz-ATP, but was ineffective against 100 microM ATP. 4. Inward currents induced by low ATP concentrations (<= 100 microM) were generally followed by an almost complete and irreversible desensitization, while those elicited by ATP >= 1 mM showed only a partial decline. Interestingly, the inward current induced by 100 microM 2-MeSATP showed a large desensitization, while that induced by Bz-ATP did not. 5. In voltage-ramp experiments, the 100 microM ATP-induced current exhibited a slight inward rectification more visible at negative potentials, while the 3 mM ATP-induced current did not. 6. ATP induced a fast and large increase in [Ca2+] that promptly recovered in the continuous presence of low ATP doses, but did not recover in high ATP doses. As with desensitization, the response to Bz-ATP mimicked that of high doses of ATP. 7. When Ca2+ mobilization due to P2Y receptors was blocked by thapsigargin-induced Ca2+ depletion or by pertussis toxin treatment, 10 microM ATP was still able to induce a Ca2+ transient, which represented the contribution of the Ca2+ influx induced by P2X receptors 8. In conclusion, the inward currents and a fraction of the Ca2+ transients induced by ATP in microglia are due to at least two ATP-sensitive receptor channel types, whose different properties and sensitivity to ATP may be associated with different functional roles.

Effects of ATP and elevated K+ on K+ currents and intracellular Ca2+ in human microglia

We have used whole-cell patch-clamp recordings and calcium microfluorescence measurements to study the effects of ATP and elevated external K+ on properties of human microglia. The application of ATP (at 0.1 mM) led to the activation of a transient inward non-selective cationic current at a cell holding potential of -60 mV and a delayed, transient expression of an outward K+ current activated with depolarizing steps applied from holding level. The ATP response included an increase in inward K+ conductance and a depolarizing shift in reversal potential as determined using a voltage ramp waveform applied from -120 to -50 mV. Fura-2 microspectrofluorescence measurements showed intracellular calcium to be increased following the application of ATP. This response was characterized by an initial transient phase, which persisted in Ca2+-free media and was due to release of Ca2+ from intracellular storage sites. The response had a later plateau phase, consistent with Ca2+ influx. In addition, ATP-induced changes in intracellular Ca2+ exhibited prominent desensitization. Elevated external K+ (at 40 mM) increased inward K+ conductance and shifted the reversal potential in the depolarizing direction, with no effect on outward K+ current or the level of internal Ca2+. The results of these experiments show the differential responses of human microglia to ATP and elevated K+, two putative factors associated with neuronal damage in the central nervous system.

Endothelin-induced changes in intracellular calcium in human microglia

Calcium-sensitive spectrofluorometry was used to study the effects of endothelin on levels of intracellular calcium [Ca2+]i in cultured human microglia. Both ET-1 and ET-3 induced transient, non-desensitizing, increases in [Ca2+]i in over 80% of the cells studied. The responses to either ET-1 or ET-3 were significantly diminished in amplitude and duration in Ca2+-free solution suggesting a prominent contribution of Ca2+ influx to the response. ET-1 induced changes in [Ca2+]i were not altered in the presence of the selective ET(A) antagonist BQ610 but were significantly reduced with the selective ET(B) antagonist BQ780. These results confirm the expression of ET(B) receptors on human microglia, these receptors may serve a role in a signaling pathway between microglia and endothelial cells.

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.

Cloning and expression of a P2y purinoceptor from the adult bovine corpus callosum

We have isolated an ATP receptor clone by screening a bovine corpus callosum cDNA library. The clone includes one open reading frame encoding for a protein of 373 amino acid residues (42 kDa) which belongs to the G-protein-coupled receptor superfamily. In Xenopus oocytes, this clone expressed an ATP receptor that triggered an oscillatory current in response to ATP (EC50 approximately 20 microM). This current may have resulted from the activation of phospholipase C, the formation of inositol trisphosphate, and the release of Ca2+, which then opens Cl- channels. The order of potency for ATP receptor agonists was 2-MeSATP approximately ATP >> alpha, beta-MeATP > adenosine, and UTP was ineffective, a pharmacological profile consistent with that of a P2y purinoceptor. Northern blot analysis of mRNAs from various bovine brain tissues showed that the gene is expressed in the cerebellum, medulla, corpus callosum, hippocampus, superior colliculus, frontal cortex, and retina. In situ RT-PCR showed transcripts of the gene in many glial cells and endothelial cells of the corpus callosum. The cloned receptor may play an important role in neuron-glial signaling under normal and pathological conditions.

ATP stimulation of Ca2+ -dependent plasminogen release from cultured microglia

1. ATP (10-100 microM), but not glutamate (100 microM), stimulated the release of plasminogen from microglia in a concentration-dependent manner during a 10 min stimulation. However, neither ATP (100 microM) nor glutamate (100 microM) stimulated the release of NO. A one hour pretreatment with BAPTA-AM (200 microM), which is metabolized in the cytosol to BAPTA (an intracellular Ca2+ chelator), completely inhibited the plasminogen release evoked by ATP (100 microM). The Ca2+ ionophore A23187 induced plasminogen release in a concentration-dependent manner (0.3 microM to 10 microM). 2. ATP induced a transient increase in the intracellular calcium concentration ([Ca2+]i) in a concentration-dependent manner which was very similar to the ATP-evoked plasminogen release, whereas glutamate (100 microM) had no effect on [Ca2+]i (70 out of 70 cells) in microglial cells. A second application of ATP (100 microM) stimulated an increase in [Ca2+]i similar to that of the first application (21 out of 21 cells). 3. The ATP-evoked increase in [Ca2+]i was totally dependent on extracellular Ca2+, 2-Methylthio ATP was active (7 out of 7 cells), but alpha,beta-methylene ATP was inactive (7 out of 7 cells) at inducing an increase in [Ca2+]i. Suramin (100 microM) was shown not to inhibit the ATP-evoked increase in [Ca2+]i (20 out of 20 cells). 2'- and 3'-O-(4-Benzoylbenzoyl)-adenosine 5'-triphosphate (BzATP), a selective agonist of P2X7 receptors, evoked a long-lasting increase in [Ca2+]i even at 1 microM, a concentration at which ATP did not evoke the increase. One hour pretreatment with adenosine 5'-triphosphate-2', 3'-dialdehyde (oxidized ATP, 100 microM), a selective antagonist of P2X7 receptors, blocked the increase in [Ca2+]i induced by ATP (10 and 100 microM). 4. These data suggest that ATP may transit information from neurones to microglia, resulting in an increase in [Ca2+]i via the ionotropic P2X7 receptor which stimulates the release of plasminogen from the microglia.

ATP-mediated cytotoxicity in microglial cells

Microglial cells are known to express purinergic receptors for extracellular ATP of both the P2Y and P2X subtypes. Functional studies have shown that both primary mouse microglial cells and the N9 and N13 microglial cell lines express the pore-forming P2Z/P2X7 receptor. Here we identify the presence of this receptor in N9 and N13 cells with a specific polyclonal Ab and show that microglial cells expressing the P2Z/P2X7 receptor are exquisitively sensitive to ATP-mediated cytotoxicity while clones selected for the lack of this receptor are resistant. Transfection of HEK293 cells with P2X7 (but not P2X2) receptor cDNA confers susceptibility to ATP-mediated cytotoxicity. Morphological and biochemical analysis suggests that ATP-dependent cell death in microglial cells occurs by apoptosis. Finally, microglial cells release ATP via a non-lytic mechanism when activated by bacterial endotoxin, thus suggesting the operation of a purinergic autocrine/paracrine loop.

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

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

ATP release and contraction mediated by different P2-receptor subtypes in guinea-pig ileal smooth muscle

1. The present study was addressed to clarify the subtypes of P2-purinoceptor involved in ATP release and contraction evoked by alpha, beta-methylene ATP (alpha, beta-mATP) and other P2-agonists in guinea-pig ileum. 2. alpha, beta-mATP 100 microM produced a transient and steep contraction followed by ATP release from tissue segments. These maximum responses appeared with different time-courses and their ED50 values were 5 and 25 microM, respectively. The maximum release of ATP by alpha, beta-mATP was markedly reduced by 250 microM suramin, 30 microM pyridoxal-phosphate-6-azophenyl-2',5'-disulphonic acid (PPADS) and 30 microM reactive blue 2 (RB-2), P2-receptor antagonists. However, the contractile response was inhibited by suramin, tetrodotoxin and atropine, but not by PPADS and RB-2. 3. Although the contraction caused by alpha, beta-mATP was strongly diminished by Ca(2+)-removal and nifedipine, and also by tetrodotoxin and atropine at 0.3 microM, the release of ATP was virtually unaffected by these procedures. 4. UTP, beta, gamma-methylene ATP (beta, gamma-mATP) and ADP at 100 microM elicited a moderate release of ATP. The release caused by UTP was virtually unaffected by RB-2. However, these P2-agonists failed to elicit a contraction of the segment. 5. The potency order of all the agonists tested for the release of ATP was alpha, beta-mATP > UTP > beta, gamma-mATP > ADP. 6. In superfusion experiments with cultured smooth muscle cells from the ileum, alpha, beta-mATP (100 microM) enhanced the release of ATP 5 fold above the basal value. This evoked release was inhibited by RB-2. 7. These findings suggest that ATP release and contraction induced by P2-agonists such as alpha, beta-mATP in the guinea-pig ileum result mainly from stimulation of different P2-purinoceptors. P2Y-like purinoceptors on the smooth muscles and, probably, P2X-purinoceptors on cholinergic nerve terminals, respectively. However, the ATP release may also be mediated, in part, by P2U-receptors, because UTP caused RB-2-insensitive ATP release.

Properties of the pore-forming P2X7 purinoceptor in mouse NTW8 microglial cells

1. We have used whole-cell patch clamping methods to study and characterize the cytolytic P2X7 (P2Z) receptor in the NTW8 mouse microglial cell line. 2. At room temperature, in an extracellular solution containing 2 mM Ca2+ and 1 mM Mg2+, 2'- and 3'-O-(4-benzoylbenzoyl)-adenosine-5'-triphosphate (Bz-ATP; 300 microM), or ATP (3 mM), evoked peak whole cell inward currents, at a holding potential of -90 mV, of 549 +/- 191 and 644 +/- 198 pA, respectively. Current-voltage relationships generated with 3 mM ATP reversed at 4.6 mV and did not display strong rectification. 3. In an extracellular solution containing zero Mg2+ and 500 microM Ca2+ (low divalent solution), brief (0.5 s) application of these agonists elicited larger maximal currents (909 +/- 138 and 1818 +/- 218 pA, Bz-ATP and ATP, respectively). Longer application of ATP (1 mM for 30 s) produced larger, slowly developing, currents which reached a plateau after approximately 15-20 s and were reversible on washing. Under these conditions, in the presence of ATP, ethidium bromide uptake could be demonstrated. Further applictions of 1 mM ATP produced rapid currents of the same magnitude as those observed during the 30 s application. Subsequent determination of concentration-effect curves to Bz-ATP, ATP and 2-methylthio-ATP yielded EC50 values of 58.3, 298 and 505 microM, respectively. These affects of ATP were antagonized by pyridoxal-phosphate-6-azophenyl- 2', 4'-disulphonic acid (PPADS; 30 microM) but not suramin (100 microM). 4. In low divalent solution, repeated application of 1 mM ATP for 1 s produced successively larger currents which reached a plateau, after 8 applications, of 466% of the first application current. PPADS (30 microM) prevented this augmentation, while 5-(N,N-hexamethylene)-amiloride (HMA) (100 microM) accelerated it such that maximal augmentation was observed after only one application of ATP in the presence of HMA. At a bath temperature of 32 degrees C, current augmentation also occurred in normal divalent cation containing solution. 5. These data demonstrate that mouse microglial NTW8 cells possess a purinoceptor with pharmacological characteristics resembling the P2X7 receptor. We suggest that the current augmentation phenomenon observed reflects formation of the large cytolytic pore characteristic of this receptor. We have demonstrated that pore formation can occur under normal physiological conditions and can be modulated pharmacologically, both positively and negatively.

Coexistence of purino- and pyrimidinoceptors on activated rat microglial cells

1. Nucleotide-induced currents in untreated (proliferating) and lipopolysaccharide (LPS; 100 ng ml(-1)) treated (non-proliferating) rat microglial cells were recorded by the whole-cell patch-clamp technique. Most experiments were carried out on non-proliferating microglial cells. ATP (100 nM-1 mM), ADP (10 nM-10 mM) and UTP (1 microM-100 mM), but not uridine (100 microM-10 mM) produced a slow outward current at a holding potential of 0 mV. The effect of UTP (1 mM) did not depend on the presence of extracellular Mg2+ (1 mM). The outward current response to UTP (1 mM) was similar in non-proliferating and proliferating microglia. 2. In non-proliferating microglial cells, the ATP (10 microM)-induced outward current was antagonized by suramin (300 microM) or reactive blue 2 (50 microM), whereas 8-(p-sulphophenyl)-theophylline (8-SPT; 100 microM) was inactive. By contrast, the current induced by UTP (1 mM) was increased by suramin (300 microM) and was not altered by reactive blue 2 (50 microM) or 8-SPT (100 microM). 3. The current response to UTP (1 mM) disappeared when K+ was replaced in the pipette solution by an equimolar concentration of Cs+ (150 mM). However, the effect of UTP (1 mM) did not change when most Cl- was replaced with an equimolar concentration of gluconate (145 mM). The application of 4-aminopyridine (1 mM) or Cs+ (1 mM) to the bath solution failed to alter the UTP (1 mM)-induced current. UTP (1 mM) had almost no effect in a nominally Ca2+-free bath medium, or in the presence of charybdotoxin (0.1 microM); the inclusion of U-73122 (5 microM) or heparin (5 mg ml(-1)) into the pipette solution also blocked the responses to UTP (1 mM). By contrast, the effect of ATP (10 microM) persisted under these conditions. 4. I-V relations were determined by delivering fast voltage ramps before and during the application of UTP (1 mM). In the presence of extracellular Cs+ (1 mM) and 4-aminopyridine (1 mM) the UTP-evoked current crossed the zero current level near -75 mV. Omission of Ca2+ from the Cs+ (1 mM)- and 4-aminopyridine (1 mM)-containing bath medium or replacement of K+ by Cs+ (150 mM) in the pipette solution abolished the UTP current. 5. Replacement of GTP (200 microM) by GDP-beta-S (200 microM) in the pipette solution abolished the current evoked by UTP (1 mM). 6. When the pipette solution contained Cs+ (150 mM) instead of K+ and in addition inositol 1,4,5,-trisphosphate (InsP3; 10 microM), an inward current absolutely dependent on extracellular Ca2+ was activated after the establishment of whole-cell recording conditions. This current had a typical delay, a rather slow time course and did not reverse its amplitude up to 100 mV, as measured by fast voltage ramps. 7. A rise of the internal free Ca2+ concentration from 0.01 to 0.5 microM on excised inside-out membrane patches produced single channel activity with a reversal potential of 0 mV in a symmetrical K+ solution. The reversal potential was shifted to negative values, when the extracellular K+ concentration was decreased from 144 to 32 mM. By contrast, a decrease of the extracellular Cl- concentration from 164 to 38 mM did not change the reversal potential. 8. Purine and pyrimidine nucleotides act at separate receptors in rat microglial cells. Pyrimidinoceptors activate via a G protein the enzyme phospholipase C with the subsequent release of InsP3. The depletion of the intracellular Ca2+ pool appears to initiate a capacitative entry of Ca+ from the extracellular space. This Ca2+ then activates a Ca2+-dependent K+ current.

Evidence for P2-purinoceptor-mediated uptake of Ca2+ across a fish (Oreochromis mossambicus) intestinal brush border membrane

We have studied the effect of ATP on Ca2+ uptake in intestinal brush border membrane vesicles (BBMVs) of the teleost tilapia (Oreochromis mossambicus). ATP stimulated Ca2+ uptake 12-fold over the control, with a linear time course. Ionomycin and detergent treatment did not reduce BBMVs' Ca2+ content, indicating the binding of Ca2+ to a membrane component. A rank order of ATP > ADP > AMP was established for the stimulation of Ca2+ uptake. Adenosine, vanadate, adenosine 5'-[alpha, beta-methylene]triphosphate (a P2x purinoceptor agonist) and adenosine 5'-[gamma-thio]triphosphate (a P-type ATPase inhibitor) were without effect. 2-Methylthioadenosine 5'-triphosphate, a P2y purinoceptor agonist, mimicked the stimulation by ATP. As judged from a kinetic comparison, ATP hydrolysis and the stimulation by ATP of Ca2+ uptake were not compatible. The P2 purinoceptor antagonist suramin and the P2y purinoceptor antagonist Reactive Blue-2 inhibited the Ca2+ uptake stimulated by 1 mM ATP (IC50 0.17 mM and 58 microM respectively). We conclude that ATP-stimulated Ca2+ uptake in tilapia intestine is dissociated from ATPase activity and is mediated through a P2 purinoceptor.

ATP-induced membrane currents in ameboid microglia acutely isolated from mouse brain slices

Microglial cells were harvested from the surface of corpus callosum slices acutely isolated from the brain of neonatal (five- to seven-day-old) mice. Transmembrane ionic currents were measured employing a standard whole-cell voltage-clamp technique. The extracellular application of 1 mM ATP triggered the generation of a complex membrane current comprising three components: (i) an initial fast inward current which had a reversal potential at about -20 to -15 mV; (ii) this initial component was followed by a steady-state inward current with reversal potential about -50 to -40 mV; and (iii) a delayed inward current with a reversal potential close to 0 mV. The first two components (fast and steady-state) had an activation threshold at 10 microM ATP, and 100 microM ATP evoked an almost maximal response. In contrast, the third component of ATP-induced inward membrane current could be observed only while 1 mM ATP was applied. The increase in concentration of tetra-anionic form of ATP (ATP4-) by removal of divalent cations from the bath solution substantially lowered the activation threshold for the delayed component of ATP-induced membrane current; conversely, lowering the ATP4- concentration (by replacing Ca2+ with Mg2+) resulted in its disappearance. These results suggest that ATP4- acts as a true agonist for the activation of the delayed ATP-induced membrane current. We conclude that microglial cells express several purinoreceptor subtypes. The activation of these receptors might play a role in intracellular signal transduction in brain microglia.

Properties of K+ and Cl- channels and their involvement in proliferation of rat microglial cells

Essentially pure (>95%) cultures of microglia were established from neopallia of newborn rats and used for whole-cell patch-clamp recording of electrophysiological properties and for proliferation studies. Two types of cultures were examined: 1) "Primary" cultures were grown in culture medium with serum and used within 3 weeks of isolation; 2) and "Colony-stimulating factor (CSF)-1-stimulated" cultures were derived from 3-week-old "primary" cultures by passaging and culturing them for several weeks longer in the presence of conditioned medium enriched in CSF-1. Microglia in the "primary" cultures expressed: 1) an inwardly rectifying K+ current (Kir) that was inhibited by Ba2+; 2) an outwardly rectifying K+ current (Kv) with many similarities to the cloned Kv1.3 channel of lymphocytes, including block by nanomolar concentrations of charybdotoxin (ChTX) and margatoxin (MgTX); and 3) an outwardly rectifying anion current with time- and voltage-independent gating. The anion current is activated reversibly under cell swelling conditions, i.e., after exposure to a hypo-osmotic bathing medium. The anion channels are highly permeable to Cl-, measurably permeable to gluconate (P(gluconate)/ PCl = 0.34), and blocked by flufenamic acid, 4-nitro-2-(3-phenylpropylamino)- benzoic acid (NPPB), and 6, 7-dichloro-2-cyclopentyl-2, 3-dihydro-2-methyl-1-oxo-1H-inden-5-yl (oxy) acetic acid (IAA-94). Microglia in the "CSF-1-stimulated" cultures expressed Kir and Cl- current, but not Kv current. Proliferation in the latter type of cultures could be slowed by omission of the CSF-1 enriched supernatant for 2 days and stimulated by adding back the conditioned medium. This "CSF-1-stimulated" proliferation was inhibited by Ba2+ (Kir blocker), and the Cl(-)-channel blockers flufenamic acid, NPPB, and IAA-94, whereas the Kv blockers ChTX and MgTX had no effect. Thus, Kir and Cl- channels appear to be necessary for "CSF-1-stimulated" proliferation of rat microglia, and there is no evidence that even a transient activation of Kv is necessary.

Trophic actions of extracellular nucleotides and nucleosides on glial and neuronal cells

In addition to their well-established roles as neurotransmitters and neuromodulators, growing evidence suggests that nucleotides and nucleosides might also act as trophic factors in both the central and peripheral nervous systems. Specific extracellular receptor subtypes for these compounds are expressed on neurons, glial and endothelial cells, where they mediate strikingly different effects. These range from induction of cell differentiation and apoptosis, mitogenesis and morphogenetic changes, to stimulation of synthesis or release, or both, of cytokines and neurotrophic factors, both under physiological and pathological conditions. Nucleotides and nucleosides might be involved in the regulation of development and plasticity of the nervous system, and in the pathophysiology of neurodegenerative disorders. Receptors for nucleotides and nucleosides could represent a novel target for the development of therapeutic strategies to treat incurable diseases of the nervous system, including trauma- and ischemia-associated neurodegeneration, demyelinating and aging-associated cognitive disorders.

Molecular mechanisms of microglial activation

Microglial cells are brain macrophages which serve specific functions in the defense of the central nervous system (CNS) against microorganisms, the removal of tissue debris in neurodegenerative diseases or during normal development, and in autoimmune inflammatory disorders of the brain. In cultured microglial cells, several soluble inflammatory mediators such as cytokines and bacterial products like lipopolysaccharide (LPS) were demonstrated to induce a wide range of microglial activities, e.g. increased phagocytosis, chemotaxis, secretion of cytokines, activation of the respiratory burst and induction of nitric oxide synthase. Since heightened microglial activation was shown to play a role in the pathogenesis of experimental inflammatory CNS disorders, understanding the molecular mechanisms of microglial activation may lead to new treatment strategies for neurodegenerative disorders, multiple sclerosis and bacterial or viral infections of the nervous system.

A novel P2-purinoceptor expressed by a subpopulation of astrocytes from the dorsal spinal cord of the rat

1. Astrocytes from the dorsal spinal cord express P2-purinoceptors which, when stimulated, produce a rise in the intracellular level of free Ca2+ ([Ca2+]i). Previously we have found that the P2Y class of receptor is expressed by nearly all astrocytes from the dorsal horn. To determine whether other metabotropic P2-purinoceptor classes are also present, in this study we investigated the effects of UTP. 2. Application of UTP (1-500 microM, 5-20 s) produced a transient rise in [Ca2+]i in a subpopulation of astrocytes. The magnitude of the peak increase in [Ca2+]i was dependent upon UTP concentration and the EC50 was found to be 5.2 +/- 0.2 microM. Ca2+ responses were maximum at 100 microM UTP. 3. The rise in [Ca2+]i in response to UTP was not affected by removal of extracellular Ca2+. On the other hand, application of the sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase inhibitor, thapsigargin, abolished responses to UTP. These findings indicate that UTP stimulates the release of Ca2+ from a thapsigargin-sensitive intracellular pool. 4. The Ca2+ response to UTP was unaffected by treatment with pertussis toxin, suggesting that UTP responses may be mediated via a pertussis toxin-insensitive G protein. 5. While all cells tested (n = 52) responded to the P2Y-purinoceptor agonist, 2-methylthio-ATP, only a subpopulation of astrocytes (n = 67/93) was responsive to UTP. The presence of UTP-sensitive and UTP-insensitive cells requires the existence of two discrete types of receptor. One receptor, expressed by UTP-insensitive cells, appears to be activated selectively by 2-methylthio-ATP. 6. To investigate whether UTP and 2-methylthio-ATP activate a common type of receptor in UTP-responsive cells, a cross-desensitization strategy was used. Desensitization with prolonged exposure to a high concentration of 2-methylthio-ATP failed to affect responses to UTP and vice versa, indicating that receptors activated by UTP are distinct from those activated by 2-methylthio-ATP. 7. The P2-purinoceptor antagonist, suramin (100 microM), blocked Ca2+ responses to UTP and to 2-methylthio-ATP. 8. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), has been reported to block responses mediated by P2X- and P2Y-purinoceptors in other systems and therefore we investigated its effects on responses to 2-methylthio-ATP and to UTP. PPADS was found to block Ca2+ responses to 2-methylthio-ATP in a concentration-dependent manner with an IC50 of 0.92 +/- 0.1 microM. PPADS also blocked UTP-evoked responses and the IC50 was 7.2 +/- 1.9 microM. At a concentration of 10 microM, PPADS produced a rightward shift in the dose-response curve for UTP and did not affect the maximum response. 9. Calcium responses evoked by the muscarinic agonist, carbachol, were unaffected either by suramin (100 microM) or by PPADS (50 microM). 10. The present results indicate the presence of a novel class of metabotropic P2U-purinoceptor in dorsal spinal astrocytes. In contrast to P2Y-purinoceptors, the P2U-purinoceptor is expressed only by a subpopulation of astrocytes and its sensitivity to suramin and PPADS distinguish this receptor from P2U-purinoceptors found in other tissues.

Modulation of potassium currents in cultured murine microglial cells by receptor activation and intracellular pathways

The electrophysiological properties of ameboid microglia from rodent brain are dominated by inwardly rectifying potassium channels and by the lack of outward currents. This channel pattern results in a distinct physiological behavior: depolarizing events, e.g. following adenosine triphosphate receptor activation, can lead to a long lasting membrane depolarization. Here we address the question whether this resting K+ channel activity can be modulated. Intracellular application of guanosine 5'-O-(3-thiotriphosphate) induced an outward current and led to a complete disappearance of the inward current inward rectifier potassium current as measured with the patch clamp technique. Moreover, an elevation in cytosolic calcium concentration (to 1.6 microM) via intracellular perfusion reversibly blocked the inward current. The inhibition of inward currents by guanosine 5'-O-(3-thiotriphosphate) could be enhanced by additional adenosine triphosphate receptor activation. Adenosine triphosphate or tumor necrosis factor receptor activation alone could lead to a transient partial block of the inward rectifier and to the transient appearance of a delayed outward current. We conclude that the activity of the microglia K+ channels and thus the physiological behavior of microglia can be modulated on a time scale of seconds by receptor activation and distinct intracellular pathways.

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.

Signalling via ATP in the nervous system

Strong evidence has been provided that ATP can act as a transmitter not only in smooth muscle but also in peripheral ganglia and in brain. The cloning and molecular identification of two putative ATP receptors supports the previously established pharmacological receptor classifications. This review places into perspective the evidence for ATP as a neural signalling substance by examining sites of storage, release and hydrolysis, as well as potential actions and targets. The action of ATP is related to that of the nucleoside adenosine, and the potential of additional nucleotides to function as neural messenger is examined briefly.

Characterization and transduction mechanisms of purinoceptors in activated rat microglia

1. Purinoceptor agonist-induced currents in untreated (proliferating) and lipopolysaccharide- (LPS; 100 ng ml-1) treated (non-proliferating) rat microglial cells were recorded by the whole-cell patch-clamp technique. 2. In non-proliferating microglia, adenosine (0.01-100 microM), 2-methylthio ATP (3-3000 nM), ATP (0.1-1000 microM), and ATP-gamma-S (1-10 microM), but not alpha,beta-methylene ATP (alpha,beta-MeATP; 100 microM) produced a slow outward current at a holding potential of 0 mV. When K+ was replaced in the pipette solution by an equimolar concentration of Cs+ (150 mM), the 2-methylthio ATP- (300 nM) induced outward current disappeared. The effect of 2-methylthio ATP (300 nM) did not depend on the presence of extracellular Mg2+ (1 mM). The outward current response to 2-methylthio ATP (300 nM) was larger in proliferating than in non-proliferating microglia. 3. ATP (1-1000 microM) evoked a fast inward current at a holding potential of -70 mV in nonproliferating microglia, while adenosine (100-1000 microM) was inactive. When the effects of ATP were compared at 0 and -70 mV, it became evident that ATP is much more potent in evoking the outward current. 4. The 2-methylthio ATP- (300 nM) induced outward current was blocked by suramin (300 microM), but not by 8-(p-sulphophenyl)-theophylline (100 microM), while the adenosine- (1 microM) induced outward current had the reverse sensitivity to these antagonists. 5. The 2-methylthio ATP- (300 nM) induced outward current was inhibited by inclusion of GDP-beta-S(200 microM) into the pipette solution or by preincubation of microglial cells with pertussis toxin(50 ng ml-1) for 12 h. The 2-methylthio ATP- (300 microM) induced inward current was not changed by intracellular GDP-beta-S (200 microM). The outward current response to adenosine (1 microM) was also abolished after pretreatment with pertussis toxin (50 ng ml-1).6. Rat microglia possess both ATP-sensitive P2y- and adenosine-sensitive P1-purinoceptors. The ATP evoked inward current is mediated by P2y-purinoceptors, while the ATP- and adenosine-evoked outward currents are mediated by P2y- and P1-purinoceptors, respectively. The transduction mechanisms of the outward, but not the inward current activation involve a pertussis toxin-sensitive G protein.