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J. Biol. Chem., Vol. 281, Issue 7, 4274-4284, February 17, 2006
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From the Consiglio Nazionale delle Ricerche Istituto di Neuroscienze and Dipartimento di Scienze Biomediche Sperimentali, Università di Padova, viale G. Colombo 3, 35121 Padova, Italy
Received for publication, September 30, 2005 , and in revised form, November 10, 2005.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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As to the mechanism of glutamate release, it is known that [Ca2+]i elevations in astrocytes can rapidly trigger the fusion with the plasma membrane of glutamate-containing vesicles (12, 13). Although this exocytosis-mediated pathway is most likely involved in astrocyte-to-neuron signaling under physiological conditions, other mechanisms may contribute to this process under both physiological and pathological conditions (14, 15). Recently, a glutamate release mechanism that involves the P2X7R has been proposed (16). This receptor has an established role in cellular toxicity and inflammatory processes (17, 18). Upon sustained activation with ATP, it can form a pore permeable to molecules of relatively large size (<900 Da), thus allowing molecules, such as glutamate, to efflux from, or enter into the cytoplasm according to their concentration gradient. In support of the role of the P2X7R, the release of glutamate from murine-cultured astrocytes triggered by purinergic receptor agonists has been described (16) to be (i) larger upon stimulation with BzATP than with ATP (the latter being a weak P2X7R agonist), (ii) greatly potentiated in divalent ion-free medium, i.e. a condition that favors P2X7R openings, and (iii) blocked by P2X7R antagonists.
Although these results from cultured astrocytes clearly indicate that under selected experimental conditions the P2X7R allows the efflux of glutamate into the extracellular space, no such evidence exists for astrocytes in situ. On the other hand, at physiological concentrations of extracellular Ca2+, ATP is known to trigger a significant release of glutamate from astrocytes (19, 20), which in this case depends on a classic Ca2+-dependent pathway. Indeed, by activating both ionotropic, Ca2+-permeable P2XRs, including the P2X7R and/or metabotropic P2YRs, ATP can evoke an intracellular Ca2+ increase that in turn mediates glutamate release.
The aim of this study was to gain insights into the role of the purinergic receptor P2X7 in the generation of glutamate release from in situ astrocytes. Results obtained in patch clamp recordings from neurons and astrocytes of acute hippocampal slices allow us to conclude that P2X7R activation is not involved in the episodic glutamate release that triggers SICs. In contrast, a P2X7-like receptor, possibly expressed in astrocytes and neurons, contributes significantly to increasing the extracellular concentration of glutamate, in particular when extracellular Ca2+ decreases to very low levels.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Patch Clamp Recordings and Data Analysis—Most of the experimental procedures are similar to that described previously (11). In brief, during recordings slices were perfused with the following saline solution: 120 mM NaCl, 3.2 mM KCl, 1 mM KH2PO4, 26 mM NaHCO3, 2 mM CaCl2, 2.8 mM glucose, and 1 µM glycine at pH 7.4 with 95% O2,5%CO2. Low Ca2+ solution was obtained by replacing CaCl2 with EGTA (0.25 mM). Intracellular pipette solution was 145 mM potassium gluconate, 2 mM MgCl2, 5 mM EGTA, 2 mM Na2ATP, 0.2 mM NaGTP, and 10 mM HEPES to pH 7.2 with KOH. Slices were viewed with an upright Zeiss Axioscope microscope (Carl Zeiss Spa, Milan, IT) equipped with differential interference Nomarski optics, a CCD camera (COHU Inc., San Diego, CA), and a mercury light source for fluorescence excitation. Data were amplified and filtered at 1 KHz with one or two Axopatch-200B amplifiers and sampled at 5 KHz with a Digidata 1200 interface (Axon instruments, Union City, CA). The liquid junction potential at the pipette tip was -15 mV. This value should be added to all voltages to obtain the correct value of the membrane potential in whole-cell configuration. Series resistance (6 < RS < 15 megaohms) was monitored throughout each experiment, and no compensation of RS was applied. All experiments were performed at either room temperature or 35 °C and in the presence of TTX. Astrocytes were identified on the basis of both morphological and electrophysiological criteria. Astrocytes typically have small and round cell soma (diameter 6–10 µm), lack optically apparent large processes, do not fire action potentials upon application of depolarizing current pulses, and have a low input resistance (mean: 15.5 ± 1.9 megaohms, n = 24) and highly negative resting potentials (mean, -77.3 ± 0.7 mV, n = 24). All the astrocytes considered in this study exhibited a linear I-V relationship, typical of passive astrocytes (22). Neurons were voltage-clamped at -60 mV and astrocytes at -75 mV. Clampfit 8.2 (Axon instrument) and Origin 6.0 (Microcal Software, Northampton, MA) software were used for data analysis and fitting.
Transient inward currents with rise time slower than 10 ms and amplitude greater than -20 pA were classified as SICs. SIC frequency, amplitude, and kinetics were measured as described previously (11). In pair recording experiments, the time interval between two SICs was measured as the time interval between the onset of the current in cell 1 and the onset of the current in cell 2. Background noise amplitude was calculated as the peak-to-peak amplitude. Student's t test was performed to determine statistical significance. Data are expressed as mean ± S.E.
Ca2+ Imaging and Dye Uptake—Slice incubation with 10 µM Rhod-2 AM (Molecular Probes, Eugene, OR) and 0.02% pluronic for 50 min at 37 °C under mild stirring resulted in the selective loading of the Ca2+ indicator into astrocytes (23). A confocal laser scanning microscope (TCS SP2 RS, Leica, Mannheim, Germany) was used for monitoring the BzATP-induced [Ca2+]i change in these cells. Slices were continuously perfused at room temperature with the same extracellular solution as used in electrophysiological recording with 0.2 mM sulfinpyrazone. The sampling rate was 2–4 s, and 8 images were averaged for each frame. Rhod-2 fluorescence was excited at 543 nm, and emitted light above 550 nm was collected. The fluorescent signal at a given time point was expressed as 
F/F = (F1 - F0)/F0, where F0 and F1 are the values of the fluorescence in astrocytes at rest and at the given time point, respectively. No background subtraction was applied.
Dye uptake experiments were carried out incubating slices for 5–15 min with 1 µM TTX, 0.5 mg/ml lucifer yellow (LY) with or without 100 µM BzATP in the same solution used in patch clamp recording experiments under mild stirring and continuous bubbling with 95% O2, 5% CO2 to maintain pH 7.4. Incubation was terminated by washing in physiological saline, and fluorescence imaging was performed within 30 min. Fluorescence images were obtained with the same Leica confocal microscope using 458-nm excitation for LY. Cells were visually identified from the differential interference contrast image, and fluorescence intensity was measured from individual cells as the average intensity of fluorescence in a region of interest corresponding to the cell soma. No background subtraction was applied. In the 8-bit scale, a value of fluorescence intensity corresponding to 50 arbitrary units (3-fold the average background noise value: 17.7 ± 0.8 arbitrary units, n = 475) was chosen as the threshold for classifying positive cells. Background noise was not different in control slices and in slices incubated with BzATP or BzATP and low Ca2+. Because of possible damage caused by slice cutting procedures, cells located close to the surface may unspecifically take up the dye. Therefore, cells positively loaded with the dye were considered only when they were located below the first 10–15 µM from the surface.
Drugs—D-AP5, NBQX, 2-methyl-6-(phenylethynyl)pyridine hydrochloride, LY367385 ((S)-(+)- 
-amino-4-carboxy-2-methyl-benzene-acetic acid), TBOA, and TTX were purchased from Tocris Cookson (Buckhurst Hill, UK), and BzATP, ,
-mATP, BBG, OxATP, LY, and carbenoxolone were from Sigma. All chemicals were dissolved in water or Me2SO and then diluted in the recording physiological solution just before use.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Both the transient currents (Fig. 2A) and the tonic current associated with noise increase (Fig. 2, B and C) evoked by BzATP were reversibly blocked by the NMDAR antagonist D-AP5 (50–100 µM) and can, thus, be attributed to the activation of NMDARs. These results clearly indicate that slice perfusion with BzATP triggers the release of glutamate that evokes in CA1 pyramidal neurons both transient and sustained activation of the NMDARs.
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Glutamate Release from Astrocytes Mediates the Transient Neuronal Response—Elevations in the [Ca2+]i evoked in hippocampal CA1 astrocytes by various stimuli, including purinergic receptor agonists, have been recently shown to trigger a pulsatile release of glutamate from these cells (11). Glutamate released from activated astrocytes acts primarily on extra-synaptic NMDARs of CA1 pyramidal neurons to trigger episodic inward currents with characteristic slow kinetics that we have called SICs (11). The transient, slow events evoked by BzATP in CA1 pyramidal neurons described above are mediated exclusively by NMDARs and are insensitive to 1 µM TTX. Therefore, they have the same pharmacological profile of SICs (11) as well as of the slow currents described by Angulo et al. (10). As shown in Fig. 3A, the mean rise time of BzATP-induced transient currents is 70.1 ± 7.9 ms (n = 54), one order of magnitude larger than that of excitatory postsynaptic currents evoked in the same neurons by Schaffer collateral stimulation (rise time, 6.7 ± 0.5 ms, n = 22). The mean decay time was 376.5 ± 24.4 ms (Fig. 3B; n = 54), which is slower than that of the excitatory postsynaptic currents from the same neurons (
1 = 17.9 ± 6.7 ms, 2 = 195.5 ± 14.7 ms, n = 22). The amplitude of these events can reach several hundred pA, with a mean of -115.9 ± 15.6 pA (Fig. 3C; n = 54). These features are also typical of SICs, providing further evidence for the classification of BzATP-evoked transient currents as SICs.
An additional feature of SICs is that they occur with a high degree of synchrony in different pyramidal neurons (10, 11). We investigated this issue by using patch clamp pair recordings from two pyramidal neurons. In two of the four pairs that displayed the transient currents upon BzATP stimulation, synchronous events were observed (Fig. 3D). The inter-event time interval histogram (Fig. 3E) shows that, similar to SICs, 42% of these events (40 of 96) are synchronized in a time window of 200 ms. Taken together, these data suggest that the BzATP-induced transient currents have the same pharmacological and biophysical properties of SICs, i.e. the NMDAR-mediated, slow inward currents that are triggered in CA1 pyramidal neurons by glutamate released from activated astrocytes.
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,-mATP, a purinergic agonist different from BzATP (24, 25), also failed to trigger either a tonic current (Fig. 5, A and B; n = 13) or an increase in background noise (Fig. 5C). Conversely, ,-mATP triggered SICs in 6 of 13 pyramidal neurons (46%) with frequency, amplitude, and kinetics similar to those of SICs evoked by BzATP (Fig. 5D; Table 1).
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To gain further insights into the possible involvement of the P2X7R in the generation of the tonic current, we applied BzATP in the presence of low extracellular Ca2+, a condition that results in a large increase in the P2X7R conductance (29). To reduce the concentration of Ca2+ in the extracellular space, we perfused slices with saline containing 0.25 mM EGTA and no added Ca2+. Noteworthy is that 20 min after the onset of slice perfusion with this saline, a depolarizing stimulus (60 mM K+) still elicited [Ca2+]i elevations in neurons (data not shown). Thus, we confirm previous observations (30) that the reduction of extracellular Ca2+ in slices is a slow process, and after perfusion with a saline containing 0 mM Ca2+ and 0.25 mM EGTA, sufficient Ca2+ remains in the extracellular space to elicit [Ca2+]i elevations through voltage-gated Ca2+ channels.
In 7 neurons in which BzATP triggered either a clear (Fig. 6A, top, n = 3) or a nearly undetectable tonic current (Fig. 6A1, top, n = 4) when a second BzATP challenge was performed in low extracellular Ca2+, the amplitude of the tonic current was drastically increased (Fig. 6, A and A1, bottom, and B, left). The increase in background noise, which is always associated with the tonic current, was similarly enhanced (Fig. 6B, right). In contrast, the kinetic features of SICs evoked by BzATP in low Ca2+ are unchanged with respect to SICs evoked by BzATP in normal Ca2+ (Table 1). Because of the fact that BzATP in low Ca2+ triggered a large increase in the background noise of the trace, SICs of small amplitude were lost in the noise, and thus, only SICs of larger amplitude could be analyzed. As such, the mean amplitude of SICs evoked by BzATP in low Ca2+ results are higher than the mean amplitude of SICs evoked by BzATP in normal Ca2+.
Slice perfusion for 3–4 min with low Ca2+ in the absence of BzATP failed to evoke either a significant tonic current or an increase in noise amplitude (n = 21; Fig. 6, A2 and B). As we showed previously (11), slice perfusion with low Ca2+ induces Ca2+ oscillations in astrocytes and, thus, activates the Ca2+-dependent release of glutamate in these cells that leads to the generation of SICs in CA1 pyramidal neurons. Several SICs can indeed be detected in neurons upon low Ca2+ stimulation (Fig. 6A2). D-AP5 (50–100 µM) reversibly abolished the tonic current evoked by application of BzATP in low Ca2+, demonstrating that it is entirely due to glutamate-mediated activation of NMDA receptors (Fig. 6C). In the presence of the P2X7R antagonist BBG (2 µM), BzATP application in low Ca2+ evoked a negligible tonic current and a small increase in background noise in 6 of 12 neurons (50%; Fig. 6D). Finally, application in low Ca2+ of the weak P2X7R agonist ,-mATP triggered in 4 of 6 neurons (67%) a tonic current of negligible amplitude compared with that triggered by BzATP under the same experimental conditions (not shown).
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BzATP Triggers in Astrocytes a Large Depolarization Mediated by a P2X7-like Receptor—Murine astrocytes in culture have been shown to massively release glutamate through P2X7R openings (16). To test whether functional P2X7Rs are also expressed in in situ astrocytes and if they are involved in the generation of the tonic current, we performed patch clamp recordings from stratum radiatum astrocytes. In each of the cells tested (n = 11), BzATP (100 µM) evoked a slowly developing inward current (mean time to peak, 102 ± 12 s) that was greatly potentiated when BzATP was applied in low Ca2+ (Fig. 8, A and B). The current evoked by BzATP under both normal and low Ca2+ concentrations was almost abolished in the presence of BBG (Fig. 8B).
To directly test the hypothesis that BzATP opens in the astrocytic membrane a channel permeable to large molecules, such as a P2X7-like receptor, we applied voltage steps to whole-cell recorded astrocytes. We found that in all astrocytes tested (n = 5), BzATP application reversibly shifted the reversal potential (Erev) of the whole-cell I-V relationship toward more positive voltages with respect to basal conditions (Fig. 8, C and E). This shift was prevented in astrocytes from slices incubated with BBG (Fig. 8D, E; n = 4). Surprisingly, the overall membrane conductance was found to be unchanged upon BzATP application and to decrease drastically after BzATP washout (Fig. 8, C and F).
Given the low input resistance of the astrocytes and the huge current induced by BzATP, the voltage clamp measurements described above are affected by inadequate clamping of the astrocytic membrane to the imposed voltage. To address this concern we repeated the experiments in current clamp configuration. In all the astrocytes recorded (n = 4), BzATP evoked a slowly increasing depolarization of the astrocytic membrane (Fig. 8, G and H) which, as in the previous voltage-clamp experiments, is greatly potentiated when BzATP is applied in low external Ca2+ and almost completely blocked by BBG (Fig. 8H).
Activation of glutamate receptors by glutamate released from either astrocytes or neurons upon BzATP stimulation is not involved in the BzATP-induced inward current in astrocytes. Indeed, in the presence of 10 µM NBQX, 100 µM LY367385, and 50 µM 2-methyl-6-(phenylethynyl)pyridine hydrochloride, antagonists of the AMPA receptor, metabotropic glutamate receptor (mGluR) 1 and mGluR5, respectively, BzATP triggered in astrocytes an inward current of unchanged amplitude (-590.9 ± 104.3 pA, n = 11, and -552.8 ± 121.2 pA, n = 4, in the absence and in presence of the antagonists, respectively; p > 0.5). The BzATP-induced inward current was also unaffected in the presence of 50 µM D-AP5, antagonist of the NMDA receptor (mean amplitude: -561.7 ± 136.5 pA, n = 3, p > 0.5).
If the large inward current induced by BzATP in the astrocytic membrane is due to a large pore-forming purinergic receptor, it may allow the entrance of molecules of high molecular weight. When BzATP was applied in the virtual absence of external Ca2+, a significant uptake of LY was detected in a subpopulation of astrocytes from the CA1 region (80 of 238, 29 ± 4%; n = 8) (Fig. 9, C (cells 2–5) and E). Electrophysiological characterization of dye-loaded cells demonstrates that these cells display the passive membrane properties typical of astrocytes (n = 3, Fig. 9E).
In patch clamp experiments the BzATP-induced response triggered in low Ca2+ was observed in all astrocytes recorded, whereas BzATP in low Ca2+ induced loading of LY only in a subpopulation of these cells. We suggest that the higher sensitivity of patch clamp with respect to the dye uptake technique may account for this discrepancy. The dye uptake in astrocytes was inhibited in the presence of BBG (Fig. 9E). A BBG-sensitive uptake was also detected in a subpopulation of neurons from the CA3 dye (71 of 168, 40 ± 5%; n = 4, Fig. 9, D and F).
After BzATP incubation, a few pyramidal neurons were also stained with LY in CA1 region. However, since the number of stained CA1 neurons from control slices was similar (Fig. 9G), the dye uptake in these neurons was independent of BzATP action and possibly reflects membrane damage after the slicing procedures.
Taken together, data from electrophysiological and dye uptake experiments support the view that a pore-forming receptor, such as the P2X7R or a P2X7-like receptor, possibly expressed in astrocytes as well as in neurons, can open and either directly or indirectly accounts for the release of glutamate that contributes to the generation of the tonic current in CA1 neurons, in particular under low Ca2+ conditions.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The episodic SIC was previously demonstrated to be due to Ca2+-dependent glutamate release from astrocytes and to represent a form of glutamate-mediated astrocyte-to-neuron signaling that promotes synchronous activity of CA1 pyramidal neurons (10, 11). SICs evoked by BzATP, a powerful although unselective agonist of the P2X7R, were shown here not to be affected by the P2X7R antagonists BBG and OxATP (24, 26, 27). This finding rules out the possibility that BzATP acts on a P2X7R to trigger the release of glutamate-mediating neuronal SICs. Beside the P2X7R, BzATP is known to stimulate most of the other P2X receptors, although with different potencies (24, 25, 36). BzATP can also activate metabotropic P2YRs (25), including the P2Y1R that in mouse stratum radiatum astrocytes mediates Ca2+ elevations triggered by ATP (35). Activation by BzATP of both Ca2+-permeable P2XRs (37) and metabotropic P2YRs can, thus, lead to [Ca2+]i elevations in the astrocytes and triggers the Ca2+-dependent glutamate release pathway that mediates SICs. Our observations that ,-mATP does not effectively activate either P2X7Rs or P2YRs (24, 25) also evoke SIC support for the conclusion that activation of a P2XR other than the P2X7R is sufficient to trigger the release of glutamate from astrocytes that evoke neuronal SICs. The poor specificity of the available pharmacological tools for studying the different purinergic receptors hampers, however, a clear identification of the distinct P2XR type that mediates SICs.
The tonic current, which was always associated with a noise increase in the current trace, was found to be mediated by a purinergic receptor different from the receptor that mediated SICs. The following observations suggest that the P2X7R is a plausible candidate. (i) In contrast to SICs, the tonic current was sensitive to both BBG and OxATP; (ii) ,-mATP, which in contrast to BzATP weakly activated P2X7Rs (25, 37, 38), evoked SICs but failed to trigger the tonic current; (iii) the amplitude of BzATP-induced tonic current was greatly amplified in low extracellular Ca2+, a condition that enhances P2X7R openings (17, 37); (iv) the large amplitude tonic current evoked by BzATP in low extracellular Ca2+ was almost completely abolished in slices incubated with BBG; (v) in low Ca2+, the weak P2X7R agonist ,-mATP triggered a tonic current of negligible amplitude compared with that triggered by BzATP under the same experimental conditions.
It is worth underlying that lowering extracellular Ca2+ results in a 2–3-fold increase in the amplitude of the NMDA receptor-mediated current and reduces inactivation of this receptor (39, 40). Although this change in NMDA receptor intrinsic properties may contribute to increase the amplitude of the tonic current triggered by BzATP-induced release of glutamate in low Ca2+, it is highly unlikely that it can fully account for the observed 25-fold increase in the tonic current amplitude.
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As to the cells that express the P2X7R, astrocytes are good candidates. By reverse transcription-PCR analysis and Western blotting, astrocytes in culture have been shown to express, besides various P2YRs, all the P2XRs except the P2X6R (34). Immunocytochemical studies in rat brain slices also revealed various P2XRs, including the P2X7R, to be expressed in hippocampal astrocytes (42). In murine cultured astrocytes, BzATP has been reported to trigger a massive, slow efflux of glutamate into the extracellular space, suggesting that these cultured cells express functional P2X7Rs (16). The following results from the present study are compatible with a functional expression of P2X7Rs in hippocampal astrocytes in situ. (i) In all astrocytes recorded, in either voltage or current clamp configuration, a large amplitude inward current and a depolarization, respectively, slowly developed upon BzATP application; (ii) both responses were amplified in low Ca2+, and they were almost completely blocked in slices incubated with BBG; (iii) in the presence of specific antagonists of AMPA, NMDA, and type I metabotropic glutamate receptors, BzATP-induced current was unchanged. Therefore, activation of these glutamate receptors in astrocytes by BzATP-induced release of glutamate does not contribute significantly to the BzATP-induced current. These data also argue against the possibility that an increase in the extracellular concentration of K+, which may derive from depolarizing neurons after activation by astrocytic glutamate of AMPA and NMDARs, is involved in BzATP-induced currents in astrocytes; (iv) in a fluorescent dye permeability assay, a BBG-sensitive uptake of lucifer yellow was detected in astrocytes upon stimulation with BzATP in low Ca2+. Dye loading was undetectable in slices incubated with normal Ca2+ as well as in slices incubated with normal Ca2+ and BzATP.
According to these observations, BzATP application should open in the astrocytic membrane a non-selective channel, such as a P2X7R that in the large pore configuration allows cations such as K+,Na+, and Ca2+ and anions such as Cl- and glutamate to flow through the membrane according to their electrochemical gradient, ultimately driving the reversal potential of the whole-cell I-V relationship to more positive values. During BzATP application, the I-V curve was indeed observed to shift significantly along the voltage axis in the depolarizing direction. Although this result is compatible with the opening of a P2X7-like receptor, the overall membrane conductance was surprisingly found to be unchanged upon BzATP application and to decrease drastically after BzATP washout. A possible interpretation of this contradictory result is that the expected increase in membrane conductance due to P2X7-like receptor openings is masked by the overall decrease in membrane conductance that is triggered by BzATP and becomes evident only after BzATP washout.
Taken together, results obtained are compatible with the view that P2X7R openings in astrocytes lead to formation of a large conducting pore that allows the uptake of molecules of relatively large molecular weight. Further experiments are necessary to provide conclusive evidence on this issue.
BzATP applied in normal Ca2+ evoked a tonic current response in 35% of neurons, whereas it evoked an inward current in all astrocytes tested. A plausible hypothesis for this discrepancy is that the variability of the neuronal response reflects the non uniform organization of astrocyte-neuron contacts in the hippocampus (43). Neurons unresponsive to BzATP may be located far away from astrocyte processes and display a tonic current only when the release of glutamate from astrocytes is greatly potentiated, as in the case of BzATP applied in low Ca2+. Indeed, when BzATP was applied in low Ca2+, the tonic current response was observed in all neurons. Noteworthy is that the same neurons that did not display a detectable response to a first BzATP challenge in normal Ca2+ displayed a clear tonic current in response when a second BzATP challenge was applied in low Ca2+.
In the hippocampus, besides astrocytes, CA3 pyramidal neurons as well as neurons from the dentate gyrus have been reported to express the P2X7R (44–46). Although this point was not addressed in detail here, we observed that in slices incubated with low Ca2+ and BzATP, a BBG-sensitive uptake of the dye was detected in a subpopulation of CA3 neurons. Glutamate released through P2X7Rs expressed on CA3 axon terminals of Schaffer collateral pathway may, thus, contribute to the tonic current that we recorded from CA1 neurons.
As to the functional role that glutamate derived from P2X7R activation may have, it has to be taken into account that the endogenous agonist of purinergic receptors, ATP, is a weak P2X7R agonist. It is, thus, unlikely that ATP released under physiological conditions from either axon terminals of Schaffer collaterals (47) and/or astrocytes (35, 48) can reach the high extracellular concentration that is necessary to activate the P2X7R. Most importantly however, such a high concentration may be reached under pathological conditions, such as ischemia and brain trauma (49). The consequent activation of P2X7Rs may result in a massive glutamate release and contributes to increase the extracellular glutamate to the abnormal levels that cause excitotoxic cell death (50). Consistent with this hypothesis, P2X7R inhibition has been recently found to improve the functional recovery and to decrease the death of motoneurons after acute spinal cord injury in rats (51).
It has to be underlined that the expression of the P2X7R in brain cells other than activated microglia (17, 18) is highly debated (52), and conflicting results have been reported. Although previous studies provide indications for the presence of the P2X7R in both astrocytes (42) and neurons from the CA1, CA3, and dentate gyrus region (44, 45, 53, 54), more recent studies cast serious doubts on this conclusion (55, 56).
The results reported in the present study are compatible with the functional expression of the P2X7R in hippocampal cells, either astrocytes and/or neurons. However, also due to the lack of highly specific agonists and antagonists for the different purinergic receptors, we can only suggest that the release of glutamate which triggers the tonic current is due to the activation of the P2X7R- or a P2X7-like receptor. Indeed, other P2XRs, such as the P2X2R and the P2X4R, that have been suggested to result in a progressive formation of a large conducting pore (57, 58) are expressed in stratum radiatum astrocytes (42) and may, thus, be involved in the generation of the tonic current.
In conclusion, the results here reported clearly demonstrate that activation of different purinergic receptors in the hippocampus mediates two types of glutamate release, each evoking a distinct response in CA1 pyramidal neurons. In the first type of release, the P2X7R is not involved. This type of glutamate release from astrocytes evokes in CA1 pyramidal neurons transient NMDAR-mediated responses (SICs) that represent a hallmark of astrocyte-to-neuron communication (11). In the second type of release, the P2X7R or a P2X7-like receptor, may be involved. This release triggers a slowly developing, tonic current and contributes to increase the concentration of extracellular glutamate. This event is potentiated under non-physiological conditions, raising the possibility that it contributes to the excitotoxic action of glutamate in the brain.
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FOOTNOTES |
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1 To whom correspondence should be addressed. Tel.: 39-049-8276075; Fax: 39-049-8276049; E-mail: gcarmi{at}bio.unipd.it.
2 The abbreviations used are: NMDAR, N-methyl-D-aspartate (NMDA) receptor; P2X7R, purinergic P2X7 receptor; [Ca2+]i, intracellular Ca2+ concentration; BzATP, 2- and 3-O-(4-benzoylbenzoyl)ATP; SIC, slow inward current; TTX, tetrodotoxin; LY, lucifer yellow; D-AP5, D-(-)-2-amino-5-phosphonopentanoic acid; NBQX, 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide; TBOA, DL-threo--benzyloxyaspartate; ,-mATP, ,-methylene ATP; BBG, Brilliant Blue G; OxATP, adenosine 5-triphosphate-2,3-dialdehyde; AMPA, -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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an, M., Potokar, M., Pangr
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