Cerebellar GABAergic inhibitory transmission is under heterosynaptic control mediated by diverse chemical messengers. Here, we investigated roles of metabotropic P2Y purinoceptors (P2YRs) on GABAergic synapses between cerebellar interneurons and Purkinje cells (PCs). Activation of P2Y purinoceptors by two selective agonists, ADP and 2-methylthio-ADP (2MeSADP), elicited two distinct forms of synaptic plasticity of GABAergic transmission in the cerebellar cortex. First, the two agonists induced long-lasting enhancement of stimulation-evoked GABAergic IPSCs as well as GABAA receptor currents in PCs. This effect was completely abolished by intracellular infusion of the Ca2+-chelating agent BAPTA. Measurements of intracellular Ca2+ ([Ca2+]i) dynamics showed that puff application of 2MeSADP produced an increase in [Ca2+]i of PCs and that this increase persisted in an external Ca2+-deficient medium. These results suggest that P2Y activation postsynaptically elicits long-term enhancement of GABAA receptor sensitivity of PCs through a Gq-mediated increase in [Ca2+]i. The other action of P2YR agonists on cerebellar GABAergic synapses was that they produced a short-term increase in the frequency and the amplitude of spontaneous GABAA receptor-mediated IPSCs in PCs in a manner sensitive to a P2Y1R antagonist, N6-methyl 2′-deoxyadenosine 3′,5′-bisphosphate. This action appeared to be attributable to an excitability increase in presynaptic GABAergic interneurons, because ADP excited all Lugaro cells examined and some of interneurons in the molecular layer. These results suggest that activation of cerebellar P2Y purinoceptors leads to modulation of GABAergic transmission in different spatial and temporal domains, namely short-term and long-term plasticity through presynaptic and postsynaptic mechanisms at interneuron→PC inhibitory synapses in the rat cerebellar cortex.
Neurotransmission at central synapses is under the control of heterosynaptic interactions that use a variety of chemical messengers, including neurotransmitters themselves and other modulators. Among them, ATP and its metabolites have been reported to take part in mediating various forms of chemical signaling through activation of purinoceptors expressed in peripheral tissues and CNS synapses. Purinoceptors have been classified into three main subgroups, adenosine/P1, ionotropic P2X, and metabotropic P2Y receptors (P2XR and P2YR), which differ in their pharmacological profiles, transduction mechanisms, and molecular structures (Ralevic and Burnstock, 1998). These purinoceptor subtypes distribute differentially to neurons and glial cells throughout the mammalian CNS, including the cerebellum and other brain areas (Evans et al., 1992; Kirischuk et al., 1995; Salter and Hicks, 1995; North and Barnard, 1997; Mateo et al., 1998; North, 2002). Previous studies have shown that activation of adenosine/P1 receptors elicits presynaptic and postsynaptic inhibitory actions on central and peripheral synapses. Furthermore, P2XR and P2YR have been implicated in modulation of neurotransmission at central excitatory glutamatergic synapses (Mendoza-Fernandez et al., 2000; Gomez-Villafuertes et al., 2001; Wirkner et al., 2002). However, relatively little is known about the role played by P2YRs in inhibitory synapses, although their wide distribution in the CNS has been shown by radioligand binding (Simon et al., 1995) and immunohistochemical studies (Moore et al., 2000a,b).
Therefore, this study aimed at examining roles of P2Y purinoceptor activation at inhibitory GABAergic synapses in the cerebellar cortex. Consequently, we find that P2YR agonists elicited short-term and long-lasting enhancement of spontaneous and stimulation-evoked GABAergic transmission between cerebellar interneurons and Purkinje cells (PCs). Furthermore, our findings suggest that the short-term modulation of GABAergic synapses is induced by a presynaptic mechanism of recruiting a population of interneurons, at least in part Lugaro cells, after the P2YR activation and that the P2YR-mediated long-term enhancement is caused by a postsynaptic mechanism of increasing the GABAA receptor sensitivity in PCs through G-protein-coupled intracellular Ca2+ ([Ca2+]i) elevation.
Part of this study has been published previously in abstract form (Saitow et al., 2003).
Materials and Methods
Preparation. Experiments were performed using thin slices cut from the cerebellum of 12- to 18-d-old Wistar rats in accordance with the guidelines of Nippon Medical School (Tokyo, Japan), as described previously (Saitow et al., 2000). In brief, animals of either sex were deeply anesthetized with halothane, and their brains were rapidly removed. Parasagittal slices (250 μm thick) were cut using a vibratome (VT1000S; Leica, Nussloch, Germany) at ∼4°C in an Na+-deficient saline that contained (in mm): 299.2 sucrose, 3.4 KCl, 0.3 CaCl2, 3.0 MgCl2, 10 HEPES, 0.6 NaH2PO4, and 10 glucose. The slices were kept for at least 1 h in a humidified interface on a glass-filter chamber supplied with oxygenated artificial CSF (ACSF) that contained (in mm): 138.6 NaCl, 3.4 KCl, 2.5 CaCl2, 1.0 MaCl2, 21.0 NaHCO3, 0.6 NaH2PO4, and 10 glucose. The pH of ACSF was maintained at 7.4 by bubbling with 95% O2-5% CO2 gas.
Patch-clamp recording. Individual slices were transferred to a recording chamber attached on the stage of a microscope (BX50WI; Olympus, Tokyo, Japan) and continuously superfused with the oxygenated ACSF at a flow rate of 1.5 ml/min and temperature of 25-27°C, and 10 μm 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) was added in the ACSF to isolate GABAA inhibitory synaptic currents from contaminating ionotropic glutamate receptor-mediated excitatory synaptic responses. Patch electrodes used for whole-cell voltage-clamp recordings from PCs were filled with an internal solution that contained (in mm): 75 CsCl, 75 Cs methanesulfonate, 5.0 KCl, 0.1 K-EGTA, 5.0 Na-HEPES, 3.0 Mg-ATP, and 0.4 Na-GTP, pH 7.4, having resistances of 2-4 MΩ. Whole-cell recordings were also obtained from cerebellar interneurons with electrodes filled with a different internal solution containing (in mm): 150.0 K methanesulfonate, 5.0 KCl, 0.1 K-EGTA, 5.0 Na-HEPES, 3.0 Mg-ATP, and 0.4 Na-GTP, pH 7.4, having resistances of 5-7 MΩ. Neurons were visually identified under Nomarski optics with a water-immersion objective (63×; numerical aperture, 0.90; Olympus). Membrane currents and potentials were recorded with a patch-clamp amplifier (EPC-9; HEKA Elektronik, Lambrecht, Germany), and data were analyzed using PulseFit software (HEKA Elektronik), Kyplot software (Kyence, Tokyo, Japan), and Mini analysis software, version 5 (Synaptosoft, Decatur, GA). Data were also continuously stored during experiments on a videotape recorder after digitizing through a pulse code modulation data recorder for off-line analysis (NF Electronic Instruments, Tokyo, Japan). All signals were filtered at 1 kHz and sampled at 2 kHz, and spontaneous and miniature synaptic currents were analyzed with a threshold of -10 to -40 pA.
Ca2+ imaging. Digital fluorescence imaging was performed with a Fluoview confocal microscope system (Olympus) mounted on the microscope. Fluorescence was excited using the 488 nm wavelength of an argon laser, and emissions were detected through a 515 nm barrier filter. For experiments using a cell-permeable Ca2+ indicator, Fluo-3 AM (Molecular Probes, Eugene, OR), slices were stained with ACSF containing 5 μm Fluo-3 AM and 0.02% (w/v) pluronic acid F-127 (Sigma, St. Louis, MO) for 30-40 min at room temperature. Before onset of experiments, slices loaded with the indicator were left in normal ACSF for 30 min to allow for hydrolysis of the AM ester. Fluorescence time courses were then recorded in frame scan mode, and images were sampled at 5 s intervals. For experiments using a cell-impermeable Ca2+ indicator, Oregon Green BAPTA-1 (OGB-1; Molecular Probes), the indicator was infused into PCs via the recording patch electrode filled with the Cs-base internal solution supplemented with OGB-1 (200 μm). Fluorescence time courses were then recorded in frame scan mode. Images were acquired with a sampling rate of 150 ms per frame while the PC was held at -60 mV. [Ca2+]i changes are expressed as the ratio of increases in the fluorescence intensity over resting levels (ΔF/F0). Under the present experimental conditions, there was no significant attenuation of fluorescence level (F0) during the course of imaging unless the cell was stimulated with depolarization pulses.
Chemicals. Research chemicals were obtained from the following sources: ADP and 2-methylthio-ADP (2MeSADP) from Sigma; CNQX, N6-methyl 2′-deoxyadenosine 3′,5′-bisphosphate (MRS2179), 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), and 3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl-P-benzyl-phosphinic acid from Tocris Cookson (Bristol, UK); and tetrodotoxin (TTX) from Sankyo (Tokyo, Japan).
Statistics. Numerical data are given as means ± SEM, where n represents the number of independent experiments. Difference between experimental groups was evaluated using Student's paired or unpaired t test. For the test of differences in cumulative curves, Kolmogorov-Smirnov (K-S) test was used.
Effects of P2YR agonists on cerebellar GABAergic transmission
Using two P2YR agonists, ADP and 2MeSADP, we examined the effect of P2YR activation on cerebellar GABAergic synapses. Application of the P2YR agonists produced two distinguishable actions on GABAA receptor-mediated transmission onto PCs, as illustrated in Figures 1 and 3. First, ADP transiently increased the amplitude and the frequency of spontaneous IPSCs (sIPSCs) in PCs (Fig. 1A,C,D): amplitude increase was 148.4 ± 13.5% of control, and frequency increase was 145.6 ± 12.4% of control (n = 8). 2MeSADP also enhanced sIPSCs (see Fig. 3A,B): amplitude increase was 165.6 ± 22.1% of baseline, and frequency increase was 155.6 ± 20.0% of baseline (n = 6) (Fig. 1C,D). Enhancement of sIPSCs by the P2YR agonists showed a fast time course, rapidly attaining a peak effect and reversibly recovering after washout. A conspicuous feature of this enhancement was that ADP recruited a population of sIPSCs with much larger amplitudes (Fig. 1B), which suggests that the agonist excites presynaptic GABAergic interneurons and elicits action potential-dependent GABA release. In fact, a transient increase in the sIPSC amplitude after ADP application was completely abolished by 1 μm TTX (Fig. 1C,D). The ADP-induced transient enhancement of sIPSCs in PCs was markedly suppressed by treatment with the P2Y1R antagonist MRS2179 (Nandanan et al., 2000) (Fig. 2A): in the presence of 100 μm MRS2179, the ADP-induced increases in the amplitude and frequency of sIPSCs were only 103 ± 3.9 and 111.2 ± 5.6% of baselines, respectively (n = 4) (Fig. 2B,C). The blocking effect of MRS2179 recovered partially 10 min after washing out of the antagonist: the ADP-induced increases in sIPSC amplitude and frequency were 130 ± 3.5 and 133.1 ± 5.7%, respectively (p < 0.05 vs the MRS2179 treatment; n = 4). These pharmacological data indicate that the ADP-induced short-term enhancement of GABA release is mediated through the activation of P2Y purinoceptors.
Second, the P2YR agonist ADP and 2MeSADP also enhanced stimulation-evoked IPSCs (eIPSCs) in PCs (Fig. 3). However, in contrast to the effects on sIPSCs, the onset of enhancement of eIPSCs was gradual, reaching to a peak effect in 20 min and shifting to long-term potentiation (LTP). The extent of eIPSC amplitude increase by ADP and 2MeSADP (determined at 20 min after application) was 148 ± 31.7 and 134.3 ± 12.0%, respectively (n = 8 for each agonist) (Fig. 3C,D). Although sIPSCs also exhibited a transient increase during the agonist application and a gradual increase in the mean amplitude after the application, the extent of gradual increase in the sIPSCs (open circles) was less than that of eIPSCs (filled circles), as exemplified in Figure 3B, which suggests that spontaneous and evoked GABA release is differentially affected by the purinoceptor activation as a result of heterogeneous localizations of the synaptic sites and/or GABAA receptor subtypes responsible for sIPSCs and eIPSCs in PCs. ATP also mimicked the ADP-induced short-term enhancement of sIPSCs and LTP of eIPSCs (data not shown).
Postsynaptic mechanisms contribute to P2Y1R-mediated LTP of GABAergic transmission
To determine whether this P2YR agonist-induced LTP of eIPSCs is caused by a presynaptic or postsynaptic mechanism, we compared paired-pulse ratios (PPRs) of eIPSCs (evoked with a 50 ms interval) before and 20 min after the agonist application. The PPR did not show any significant changes (Fig. 3E,F): 0.82 ± 0.07 before and 0.80 ± 0.07 after application of ADP (n = 8; p > 0.3) and 0.80 ± 0.04 before and 0.80 ± 0.05 after application of 2MeSADP (n = 8; p > 0.8). It is therefore likely that postsynaptic mechanisms contribute to this LTP. The above results thus indicate that activation of P2Y purinoceptors in the cerebellar cortex can lead to short-term enhancement of sIPSCs with a presynaptic mechanism and LTP of evoked GABAergic transmission with a postsynaptic form at interneuron→ PC inhibitory synapses.
To further examine the mechanisms underlying P2YR-mediated modulation of cerebellar GABAergic transmission, we then examined the effect of P2YR agonists on the GABAA receptor sensitivity of PCs. Both ADP and the P2Y1R-selective agonist 2MeSADP increased the amplitude of GABAA receptor currents produced in response to brief puff application of exogenous GABA from the micropipette placed in the vicinity of PC primary dendrites in the presence of 1 μm TTX (Fig. 4A,B). The extent of GABA current increase by ADP and 2MeSADP (determined 3 min after agonist applications) was 152.5 ± 26.5% (n = 10) and 132.0 ± 5.6% (n = 12), respectively. These effects were also slowly developing and long lasting, reaching peak levels in 20-30 min: 208 ± 24.5% for ADP (n = 10) and 180 ± 13.6% for 2Me-SADP (n = 12), and there was no significant difference in the magnitudes of enhancement induced by the two agonists (p > 0.7). The time course of the P2YR agonist-induced increase in GABA currents was almost consistent with that of eIPSC enhancement (Fig. 3C,D). Furthermore, the two agonists increased the mean amplitude of miniature IPSCs (mIPSCs) in PCs (Fig. 4C). The P2YR agonist-induced increase in mIPSC amplitude was also gradual and long lasting (Fig. 4Cc), being consistent with the changes in eIPSCs and exogenous GABA-induced currents. These results suggest that P2YR activation elicits slow-onset LTP of GABAergic transmission at interneuron→ PC synapses through a postsynaptic mechanism.
Intracellular Ca2+ increase triggers P2Y1R-mediated LTP at cerebellar GABAergic synapses
It has been shown previously that purinoceptor agonists, such as ATP and ADP, increase the level of cytoplasmic Ca2+ in PCs (Kirischuk et al., 1996) and that the increase in the GABAA receptor sensitivity of PCs after stimulation of climbing fiber inputs [referred to as “rebound potentiation” (RP)] is associated with a depolarization-induced increase in intracellular Ca2+ level (Kano et al., 1992). Therefore, in the experiments illustrated in Figure 5, we sought to determine whether the P2YR-mediated LTP of GABAergic transmission is triggered by intracellular Ca2+ elevation in PCs. To monitor [Ca2+]i dynamics in PCs, we used Fluo-3 AM, a cell-permeable calcium indicator (loaded extracellularly), and Oregon green BAPTA-1, a cell-impermeable indicator (loaded intracellularly via the recording patch pipette). Bath application of 50 μm ADP for 2 min transiently increased [Ca2+]i in the PC layer when measured with Fluo-3 AM (Fig. 5A,B); the average increase was 28 ± 2.5% at 10-20 s after application (n = 7). This increase was abolished by pretreatment with a potent P2Y1R antagonist, MRS2179: the average change was 0.2 ± 0.3% at 10-20 s after ADP application (p < 0.001; n = 7). After washing out of MRS2179, the ADP-induced [Ca2+]i elevation recovered (Fig. 5C); the average increase was 14.1 ± 0.3% (p < 0.005; n = 6). The results indicate that activation of P2Y1R is responsible for the ADP-induced [Ca2+]i elevation in PCs. It is also likely that the P2Y1 purinoceptor subtype mediates not only the ADP-induced short-term enhancement of GABAergic transmission at cerebellar interneuron→ PC inhibitory synapses but also purinoceptor-mediated LTP of postsynaptic GABAA receptor currents in PCs via the ADP-induced [Ca2+]i elevation.
P2Y1R has been classified as a member of G-protein-coupled metabotropic receptors linking to IP3-sensitive Ca2+ stores in a variety of cells (Marrelli, 2001; Junankar et al., 2002; Gallagher and Salter, 2003; Hu et al., 2003; Ju et al., 2003). Thus, using combined techniques of electrophysiology and Ca2+ imaging, we examined whether purinoceptor activation can increase Ca2+ influx or stimulate internal Ca2+ stores. To address this issue, we tested the effects of a Ca2+-deficient medium on [Ca2+]i elevation triggered sequentially by P2YR activation and depolarization in PCs loaded with Oregon Green BAPTA-1 via the recording patch pipette. A depolarizing voltage step (from -60 to 0 mV for 20 ms) induced a sharp increase in [Ca2+]i in PCs (Fig. 5D, Depol.): the average increase was 277 ± 33.8% at the peak in the control solution (n = 5). This response was markedly suppressed in Ca2+-deficient solution: the average increase was only 6.3 ± 1.0% of the control (p < 0.001; n = 6) (Fig. 5G). Focal application of 2MeSADP (200 μm, 5000 ms) to dendritic shafts of PCs increased [Ca2+]i in the dendritic fields (Fig. 5D,E): the average increase was 52 ± 0.7% at the peak in the control solution (n = 8). However, the P2Y1R agonist-induced Ca2+ increase was not significantly altered by the Ca2+-deficient solution: the average increase was 91 ± 14% of the control (p > 0.8; n = 8) (Fig. 5F). The effects of the Ca2+-deficient medium were invariably observed when the sequence of application of the agonist and depolarization were alternated (data not shown). The results strongly suggest that intracellular Ca2+ mobilization, but not influx, contributes to the P2Y1R-mediated Ca2+ elevation, thereby leading to LTP of GABAergic transmission at interneuron→ PC inhibitory synapses. Additional support for this idea was provided in the experiment shown in Figure 6, in which we examined the effects of intracellular infusion of a fast Ca2+ chelator, BAPTA (20 mm), on the purinoceptor-mediated enhancement of GABAA receptor currents. BAPTA injection into PCs via the recording pipette virtually abolished LTP of GABAA receptor-mediated responses at 25 min after ADP application (208 ± 24.5% under control condition and 108.0 ± 12.2% with BAPTA injection; p < 0.001; n = 7).
Identification of presynaptic neurons responsible for P2Y1R-mediated short-term modulation
Because the purinoceptor-mediated short-term enhancement of GABA release is prominent, we sought to determine which GABAergic neurons are involved in this short-term synaptic plasticity. Possible candidates might be interneurons in the molecular layer (ML), such as basket cells (BCs) and stellate cells (SCs), and Lugaro cells in the granule cell layer (GCL) (Dieudonne and Dumoulin, 2000). To examine the excitability changes after P2Y1R agonist application, current-clamp recordings were made from interneurons in the ML and GCL, to which a constant depolarizing current pulse (+60 pA for 500 ms) was injected, and the number of spikes during the depolarizing pulse was counted before, during, and after 50 μm ADP application (Fig. 7A). Interneurons in the ML (presumably BCs) exhibited different responsiveness to ADP. As shown in Figure 7B, in 4 of 13 ML interneurons, ADP increased the number of spikes during the depolarizing current pulse. However, there was no significant difference in changes in the average number of spikes before and after ADP application (before, 7.2 ± 0.9; during ADP application, 7.3 ± 1.0; after washout, 6.8 ± 1.2; p > 0.7; n = 13). In 3 of 13 cells, the excitability was decreased after ADP application (control, 8.1 ± 1.3; ADP, 5 ± 1.8; n = 3). This inhibitory action of ADP on ML interneurons persisted in the presence of an A1 receptor antagonist, DPCPX (1 μm), which makes it unlikely that a metabolite of ADP, namely adenosine, inhibits these ML interneurons. In contrast, the excitability of Lugaro cells in the GCL was significantly increased by ADP (before, 3.1 ± 0.5; during ADP application, 5.4 ± 0.3; p < 0.05; n = 6) (Fig. 7C). As reported previously (Dieudonne and Dumoulin, 2000), Lugaro cells could be identified by the shape of fusiform soma, extending processes from both poles in the sagittal plane (Fig. 7D) and almost silent at the resting membrane potential (mean, -60.1 ± 2.9 mV; n = 13). Moreover, the number of spikes induced by current injection in Lugaro cells was lower than that in ML interneurons (Fig. 7C).
Because Lugaro cells have been reported to make GABAergic synaptic contacts with PCs (Dean et al., 2003), we attempted to perform double recordings from Lugaro cells and PCs. However, we could not detect synaptic connections between them (n = 13). Consequently, we then compared the effects of ADP and serotonin (5-HT) on GABAergic synapses on PCs, because 5-HT has been shown to activate Lugaro cells (Dieudonne, 2001), thereby triggering GABAergic synaptic responses in PCs. As shown in Figure 8A, both ADP (50 μm) and 5-HT (10 μm), when sequentially applied to the same PC (at intervals of 5-10 min), elicited GABAergic IPSCs with large amplitudes and increased the frequency of sIPSCs. On average, ADP was more potent than 5-HT in increasing the amplitude and frequency of sIPSCs (Fig. 8B,C): amplitude increases by ADP and 5-HT were 140.9 ± 10.3 and 105.5 ± 3.4% of baseline, respectively (p < 0.001; ADP vs 5-HT; n = 9), and frequency increases by ADP and 5-HT were 138.0 ± 7.9 and 111.3 ± 3.2%, respectively (p < 0.001; ADP vs 5-HT; n = 9). In addition, the regression analysis showed that there are no significant correlations between the effects of ADP and 5-HT on sIPSC frequency and amplitude: r = -0.3 (n = 9) for the frequency, and r = -0.16 for the amplitude (n = 9). Together, the results suggest that P2Y1 purinoceptor-mediated short-term enhancement of GABAergic synaptic activity in PCs is caused by activation of cerebellar interneurons in the ML and GCL and that sIPSCs with large amplitudes induced by P2YR agonists are, at least in part, caused by recruitment of Lugaro cell activation in the GCL. This notion was further supported by the experiments illustrated in Figure 9, in which we examined whether ADP can indeed excite cerebellar interneurons. Both ADP and 5-HT profoundly excited Lugaro cells (Fig. 9A-C). The effects of both agonists on interneurons in the ML were complex: the action-potential firing of some ML interneurons was increased by ADP (Fig. 9D), but these effects were variable among individual ML interneurons (Fig. 9E): 5 of 17 ML interneurons examined (29.4%) were excited by ADP (also see Fig. 7).
The finding of this study demonstrates that activation of P2Y1 purinoceptors in the cerebellar cortex elicits two different forms of short-term and long-term plasticity at inhibitory GABAergic synapses between interneurons and PCs. The short-term enhancement of GABAergic synaptic activity in PCs appears to involve a presynaptic mechanism of recruiting a population of Lugaro cells in the GCL and GABAergic interneurons in the ML excited after activation of purinoceptors expressed in these interneurons. In contrast, the P2YR-mediated LTP of stimulationevoked GABAergic transmission onto PCs appears to be induced by a postsynaptic mechanism of increasing the GABAA receptor sensitivity resulting from intracellular Ca2+ elevation caused by G-protein-coupled stimulation of Ca2+ stores in PCs. It is therefore strongly suggested that one subtype of prinoceptors, P2Y1R, which is expressed in the cerebellar cortex, plays a distinct role in upregulation of the gain control at cerebellar inhibitory GABAergic synapses in different temporal and spatial domains, thereby inhibiting the excitability level of the cerebellar output PCs.
Presynaptic enhancement of GABA release at cerebellar inhibitory synapses
The metabotropic P2YR-mediated short-term enhancement of inhibitory GABAergic synapses found in the cerebellar cortex is unique, because extracellular nucleotides, such as ATP, have been shown previously to modulate neurotransmitter release at central and peripheral synapses via acting on ionotropic P2X-type purinoceptors (Boehm, 1999; Nakatsuka and Gu, 2001; Khakh et al., 2003; Nakatsuka et al., 2003). The observation in this study is that the P2Y1R activation increased both frequency and amplitude of sIPSCs in an action potential-dependent manner (Fig. 1), which indicates that P2Y1R agonists excite presynaptic GABAergic neurons, leading to increased GABAergic synaptic activity in PCs. One possible ionic mechanism that may underlie the P2YR-induced excitation of presynaptic neurons is suppression of potassium channels, because activation of P2Y1-like receptors has been shown recently to inhibit a voltage-dependent K+ current in Xenopus spinal neurons (Brown and Dale, 2002).
Searching for presynaptic neurons responsible for the P2YR-mediated short-term enhancement in PCs revealed that GABAergic Lugaro cells in the GCL and some of the interneurons in the ML are responsive to P2YR agonists, resulting in increases in the excitability (Fig. 7) and direct excitation of these neurons (Fig. 9). The excitability increase in Lugaro cells induced by P2YR agonists was more potent than that induced by 5-HT, which has been shown to elicit GABAergic synaptic activity in PCs through activation of Lugaro cells (Dieudonne, 2001), suggesting that P2Y-type purinoceptors in Lugaro cells also play a pivotal role in modulation of the excitability level of PCs by regulating the activity of GABAergic inputs to the cerebellar output neuron (Dean et al., 2003). Approximately 30% of interneurons in the ML responded to the P2YR agonists (Figs. 7, 9), indicating that they are heterogeneous in terms of the P2Y purinoceptor sensitivity. Because a single PC receives multiple converging GABAergic inputs from 8 to 10 interneurons in the ML (Korbo et al., 1993; Saitow et al., 2000), it seems that individual PCs receive GABAergic inputs from two to three purinoceptor agonist-sensitive interneurons in the ML.
P2Y1R-mediated LTP of GABAergic synapses via a postsynaptic mechanism
Purinoceptor stimulation in the cerebellar cortex was found to induce a distinct form of synaptic plasticity, namely LTP of GABAergic transmission on PCs, through a postsynaptic mechanism. The P2Y1R agonists elicited a long-lasting increase in stimulation-evoked IPSCs as well as GABAA receptor sensitivity in PCs (Figs. 3, 4). This P2Y1R-mediated LTP is reminiscent of the phenomenon named rebound potentiation, exhibiting a long-term increase in GABAA currents in PCs after postsynaptic depolarization (Llano et al., 1991; Kano et al., 1992). Both RP and P2Y1R LTP were associated with intracellular Ca2+ elevation and abolished by intracellular BAPTA injection (Llano et al., 1991; Kano et al., 1992) (Fig. 6). However, there are distinct features between them: (1) the time course of P2Y1R LTP was much slower than that of RP, and (2) RP depends on Ca2+ influx via voltage-gated Ca2+ channels (Llano et al., 1991), whereas P2Y1R LTP persists in Ca2+-deficient medium (Fig. 5). Although both phenomena appear to rely on intracellular Ca2+ elevation and subsequent Ca2+-induced Ca2+ release and IP3-induced Ca2+ release from intracellular stores (Hashimoto et al., 1996), it is possible that different signaling pathways might be coupled to upregulation of the GABAA receptor sensitivity in PCs. Although protein phosphorylation of GABAA receptors mediated by protein kinase A and calmodulin kinase II is implicated in the RP (Kano et al., 1996; Kawaguchi and Hirano, 2000), it has been reported that GABAA receptors are phosphorylated by other kinases, such as protein kinase C (Fancsik et al., 2000) and tyrosine kinase (Wan et al., 1997; Dunne et al., 1998). Additional study is therefore needed to determine whether the P2YR-mediated modulation of GABAA receptor sensitivity also involves protein kinases. It should be noted that the RP in PCs was still able to be induced after depolarization of PCs in the presence of the P2Y1 antagonist MRS2179 (our unpublished observation).
Furthermore, it has been reported recently that activation of metabotropic glutamate receptor 1 in PCs elicits a slow excitatory synaptic response through Gq11-coupled activation of a transient receptor potential channel 1 (TRPC1) cation channel (Kim et al., 2003) and that metabotropic purinoceptors couple to TRPC7 (Clapham, 2003). However, the P2Y1R activation in PCs did not alter the electrical properties of PCs, such as membrane current and conductance (data not shown), which makes it unlikely that TRPC channels are primary targets of P2Y1 purinoceptors in PCs for the induction of LTP at GABAergic synapses on PCs.
The purinoceptor-mediated short-term and long-term upregulation of cerebellar inhibitory GABAergic synapses between interneurons and PCs appears to play a critical role in information flow within the cerebellar cortex. Endogenous nucleotides, such as ATP and/or ADP, are potential candidates that mediate the P2YR-activated two forms of synaptic plasticity. ATP has been implicated as an extracellular mediator liberated by astrocytes for chemical communications between glia and neurons (Bezzi and Volterra, 2001; Haydon, 2001). In fact, astroglia display profound Ca2+ waves in response to chemical stimuli and thereby elicit massive waves of extracellular ATP (Guthrie et al., 1999). It has been also shown that glutamatergic activity-dependent release of ATP from astrocytes can modulate the strength of neurotransmission in the hippocampus (Zhang et al., 2003). In the cerebellum, Bergmann glia have been shown to receive glutamatergic inputs derived from parallel fibers and climbing fibers (Matsui and Jahr, 2003) and express neurotransmitter receptors, including Ca2+-permeable glutamate receptors and several types of P2Y receptors (Kirischuk et al., 1995; Iino et al., 2001). Therefore, it seems that the Bergmann glial cell is a most likely candidate for the source of nucleotides that take part in the P2YR-mediated short-term and long-term plasticity at cerebellar GABAergic synapses found in this study. This possibility awaits additional examinations.
This work was supported by the Core Research for Evolutional Science and Technology, Japan Science and Technology Agency.
Correspondence should be addressed to Shiro Konishi, Waseda-Olympus Bioscience Research Institute, Helios, #05-01/02, 11 Biopolis, Singapore 138667. E-mail:.
T. Murakoshi's present address: Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Tokyo 153-8902, Japan.
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