G-Protein-Coupled Modulation of Presynaptic Calcium ... - CiteSeerX

The Journal of Neuroscience, May 1, 1998, 18(9):3138–3146

G-Protein-Coupled Modulation of Presynaptic Calcium Currents and Transmitter Release by a GABAB Receptor Tomoyuki Takahashi, Yoshinao Kajikawa, and Tetsuhiro Tsujimoto Department of Neurophysiology, University of Tokyo Faculty of Medicine, Tokyo 113, Japan

Presynaptic GABAB receptors play a regulatory role in central synaptic transmission. To elucidate their underlying mechanism of action, we have made whole-cell recordings of calcium and potassium currents from a giant presynaptic terminal, the calyx of Held, and EPSCs from its postsynaptic target in the medial nucleus of the trapezoid body of rat brainstem slices. The GABAB receptor agonist baclofen suppressed EPSCs and presynaptic calcium currents but had no effect on voltagedependent potassium currents. The calcium current–EPSC relationship measured during baclofen application was similar to that observed on reducing [Ca 21]o , suggesting that the presynaptic inhibition generated by baclofen is caused largely by the suppression of presynaptic calcium influx. Presynaptic loading of the GDP analog guanosine-59-O-(2-thiodiphosphate)

(GDPbS) abolished the effect of baclofen on both presynaptic calcium currents and EPSCs. The nonhydrolyzable GTP analog guanosine 59-O-(3-thiotriphosphate) (GTPgS) suppressed presynaptic calcium currents and occluded the effect of baclofen on presynaptic calcium currents and EPSCs. Photoactivation of GTPgS induced an inward rectifying potassium current at the calyx of Held, whereas baclofen had no such effect. We conclude that presynaptic GABAB receptors suppress transmitter release through G-protein-coupled inhibition of calcium currents.

GABAB receptors are widely distributed in the presynaptic and postsynaptic membranes of vertebrate central neurons, and they modulate synaptic transmission by either suppressing transmitter release or hyperpolarizing postsynaptic cells (Thompson et al., 1993; Kaupmann et al., 1997). At neuronal somata, GABAB receptors are known to activate G-proteins, thereby enhancing inwardly rectif ying potassium channels (Andrade et al., 1986; Sodickson and Bean, 1996) or suppressing calcium channels (Dolphin and Scott, 1987; Scholtz and Miller, 1991; Mintz and Bean, 1993). Compared with the wealth of information on the postsynaptic mechanism of GABAB receptors, much less is known about their presynaptic mechanism of action. In particular, it is not known whether the effector of presynaptic GABAB receptors is a potassium channel (Saint et al., 1990; Thompson and Gahwiler, 1992), a calcium channel (Scholtz and Miller, 1991; Pfrieger et al., 1994; Wu and Saggau, 1995; Dittman and Regehr, 1996, 1997), or exocytotic machinery downstream of calcium influx (Scanziani et al., 1992; Dittman and Regehr, 1996; Rohrbacher et al., 1997). Also, an involvement of G-proteins in GABAB receptor-mediated presynaptic inhibition remains to be directly demonstrated (Thompson et al., 1993). The calyx of Held is an ideal preparation for directly testing these issues using patch-clamp techniques (Forsythe, 1994; Borst et al., 1995; Takahashi et al., 1996). The presynaptic terminal possesses

GABAB receptors as well as metabotropic glutamate receptors and adenosine receptors (Barnes-Davies and Forsythe, 1995). Here we demonstrate that the G-protein-coupled inhibition of calcium channels underlies the GABAB receptor-mediated presynaptic inhibition.

Received Nov. 26, 1997; revised Feb. 11, 1998; accepted Feb. 12, 1998. This work was supported by the “Research for the Future” Program by The Japan Society for the Promotion of Sciences. We thank M. Farrant, I. D. Forsythe, T. Manabe, and K . Kobayashi for critically reading this manuscript. We are also gratef ul to R. Y. Tsien and V. Lev-Ram for their technical advice on the caged compound photolysis system and to Novartis Pharma (Basel, Switzerland) for the generous gift of CGP35348. Correspondence should be addressed to Tomoyuki Takahashi, Department of Neurophysiology, University of Tokyo Faculty of Medicine, Tokyo 113, Japan. Copyright © 1998 Society for Neuroscience 0270-6474/98/183138-09$05.00/0

Key words: GABAB receptor; presynaptic inhibition; Gprotein; calcium currents; inwardly rectifying potassium currents; the calyx of Held; presynaptic recording

MATERIALS AND METHODS Preparation and solutions. Transverse slices of the superior olivary complex were prepared from 14- to 19-d-old Wistar rats killed by decapitation under halothane anesthesia. The medial nucleus of trapezoid body (M N TB) neurons and calyces were viewed with a 403, 633 (Z eiss), or 603 (Olympus Optical, Tokyo, Japan) water immersion lens attached to an upright microscope (Axioskop, Z eiss). Each slice was perf used with artificial C SF (aC SF) containing 120 mM NaC l, 2.5 mM KC l, 26 mM NaHC O3 , 1.25 mM NaH2PO4 , 2 mM C aC l2 , 1 mM MgC l2 , 10 mM glucose, 0.5 mM myo-inositol, 2 mM sodium pyruvate, 0.5 mM ascorbic acid, and 4 mM lactic acid, pH 7.4, with 5% C O2 and 95% O2. To isolate C a 21 currents, 10 mM tetraethylammonium (TEA) chloride and 1 mM tetrodotoxin (TTX) were included in the aC SF. The postsynaptic patch pipette was filled with a solution (A) containing 97.5 mM potassium gluconate, 32.5 mM KC l, 10 mM H EPES, 5 mM EGTA, and 1 mM MgC l2 , pH adjusted to 7.4 with KOH. N-(2,6-diethylphenylcarbamoylmethyl)triethyl-ammonium bromide (QX314, 5 mM) was included in the postsynaptic pipette solution to suppress action potential generation when aC SF did not contain TTX. For recording EPSC s, the aC SF routinely contained bicuculline (10 mM) and strychnine (0.5 mM) to block spontaneous inhibitory synaptic currents. For recording presynaptic calcium currents (IpC a), the presynaptic pipette was filled with a solution (B) containing 110 mM C sC l, 40 mM H EPES, 0.5 mM EGTA, 1 mM MgC l2 , 2 mM ATP, 0.5 mM GTP, 12 mM Na2 phosphocreatinine, and 10 mM TEA, pH adjusted to 7.4 with C sOH. Presynaptic potassium currents were recorded with solution A. The presynaptic pipette solutions routinely contained 2 mM ATP (ATP-Mg salt), 12 mM phosphocreatinine, and 0.5 mM GTP, unless noted otherwise. For paired recordings 10 mM potassium glutamate or cesium glutamate (equimolar replacement of KC l or C sC l) was also included in the presynaptic pipette solution (Borst et al., 1995; Takahashi et al., 1996). Recording and data anal ysis. Whole-cell patch-clamp recordings were made from M N TB neurons, presynaptic calyces, or simultaneously from

Takahashi et al. • Presynaptic Ca Current Modulation by a GABAB Receptor

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Figure 1. Inhibitory effects of GABAB receptor agonists on EPSC s. EPSC s were evoked by extracellular stimulation. A (top row), Reversible inhibition of EPSCs by baclofen (2 mM) and attenuation of the baclofen effect by CGP35348 (100 mM) in an M N TB neuron. Bottom row, Inhibitory effect of GABA (20 mM) and its attenuation by CGP35348 in another M N TB neuron. The magnitude of inhibition by CGP35348 on the effect of baclofen and GABA was 82.8 6 2.3% (n 5 3) and 60.7 6 11% (n 5 3), respectively. B, Dose-dependent suppression of EPSC s by baclofen and GABA. Cumulative dose-dependent effects of baclofen (top) and GABA (bottom) on the amplitude of EPSC s recorded from M N TB neurons. Sample records from individual MNTB neurons are shown in the inset. C alibration: 2 nA, 10 msec. The curves fitted to data points derived from the following equation: magnitude of inhibition (%) 5 maximal inhibition (%)/[1 1 (IC50 /agonist concentration)n]. For baclofen and GABA, maximal inhibition was 82.0 and 90.8%, IC50 was 0.77 and 9.97 mM, and Hill coefficient ( n) was 0.90 and 1.21, respectively. Magnitude of EPSC suppression by 20 mM baclofen was 80.1 6 2.6% at the cumulative-dose application (n 5 4), which was not significantly different ( p 5 0.13; Student’s t test) from that at the single-dose application (72.5 6 4.3%; n 5 8).

both structures (Takahashi et al., 1996). EPSC s were evoked at 0.1 Hz by extracellular stimulation of presynaptic axons near the midline of a slice with a bipolar platinum electrode (Barnes-Davies and Forsythe, 1995) in a relatively thick slice (250 mm) or by presynaptic action potentials or C a 21 currents elicited by a whole-cell pipette in thin slice (150 mm). The electrode resistances were 4 –7 MV for the postsynaptic pipette and 6 –10 MV for the presynaptic pipette. The series resistance of presynaptic

recording was typically 10 –20 MV and was compensated by 60 –90%. Current or potential recordings were made with a patch-clamp amplifier (Axopatch 200B, Axon Instruments, Foster C ity, CA). Unless noted otherwise, records were low-pass-filtered at 2.5–20 kHz and digitized at 5–50 kHz by a CED 1401 interface (C ambridge Electronic Design). Leak currents were subtracted for presynaptic currents by a scaled pulse divided by n (P/ N) protocol. The liquid junctional potential between the

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Figure 2. Baclofen-induced suppression of EPSC s is blocked by GDPbS. Simultaneous presynaptic and postsynaptic recordings at the caly x–M N TB synapse. EPSCs were evoked by action potentials elicited by a depolarizing current pulse (2–10 msec) applied to a calyx through a whole-cell patch pipette. The postsynaptic holding potential was 270 mV. A, Reversible suppression of EPSCs by baclofen (20 mM). B, Blocking effect of GDPbS (tri-lithium salt, 3 mM) in the pipette on baclofen-induced suppression of EPSC s (a, b). A lower concentration of GDPbS (0.2 mM) did not prevent the effect of baclofen (data not shown). After the pipettes were retracted, a second paired recording was made from the same structures with a presynaptic pipette containing GTP (0.5 mM) instead of GDPbS. Baclofen clearly suppressed the EPSC s (c, d), which gradually recovered after washout ( e). Complete recovery of EPSCs took 5–10 min (Fig. 7). When LiC l (9 mM) was included in the presynaptic pipette the baclofen effect was not attenuated (not shown). The amplitudes of EPSCs were normalized against the mean of the first seven (with GDPbS) or six (with GTP) data points before baclofen application in each experiment; the data point represents means and the error bars represent SEMs derived from paired recording experiments at three different synapses. Vertical calibration scales indicate 80 mV for presynaptic membrane potentials (A and B) and 1.25 nA ( A) or 0.6 nA ( B) for EPSCs. Scale bars, 10 msec. pipette solution and aC SF was 17.5 mV for solution A and 13.3 mV for solution B. The value of reversal potentials (see Fig. 6C) was corrected for these junction potentials. Drug application. GABAB receptor agonists were bath-applied by switching superf usates by solenoid valves. C aged GTPgS [S-(DM N PEcaged) GTPgS; Molecular Probes, Eugene, OR] was applied at 38 mM into calyces by dialysis from whole-cell pipettes. C are was taken to protect the compound from short wavelength light during this procedure. A flash of light was given from a mercury lamp light source (50 W) through a filter (360 6 20 nm) by opening a shutter for a given period (2– 4 sec). Application of the light flash without loading caged compound had no effect on the synaptic transmission or IpC a under normal experimental conditions, although an excessive illumination sometimes induced a transient potentiation of IpC a or an increase in the frequency of spontaneous synaptic currents. E xperiments were carried at room temperature (22–26°C).

RESULTS Presynaptic inhibition mediated by GABAB receptors at the calyx of Held A single extracellular stimulation evoked a large and rapidly decaying EPSC in a principal cell of the medial nucleus of trapezoid body (M N TB) under whole-cell voltage clamp. As

reported previously (Barnes-Davies and Forsythe, 1995), bathapplication of the GABAB receptor agonist baclofen suppressed EPSCs in a reversible manner (Fig. 1 A). This baclofen effect was detectable at 0.2 mM, increased dose-dependently, and reached a maximal at ;20 mM (Fig. 1 B). Similarly, the inhibitory transmitter GABA suppressed EPSCs (Fig. 1). The 50% inhibitory concentration (IC50 ) of baclofen was estimated from the dose– response curve to be 0.8 mM, whereas that for GABA was 10 mM (Fig. 1 B). Thus baclofen was about 10 times more potent than GABA in inhibiting EPSCs at this synapse. The inhibitory effects of both baclofen and GABA were largely attenuated by the GABAB receptor antagonist CGP35348 (100 mM) (Fig. 1 A), indicating that the effects of baclofen and GABA were indeed mediated by the GABAB receptor.

Involvement of G-proteins in GABAB receptormediated presynaptic inhibition An EPSC was evoked by a presynaptic action potential in a simultaneous whole-cell recording from the calyx of Held and a target MNTB cell (Fig. 2 A). Baclofen suppressed the EPSC


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are involved in the GABAB receptor-mediated presynaptic inhibition.

Inhibition of presynaptic calcium currents by baclofen

Figure 3. Suppression of presynaptic C a 21 currents by baclofen. The calyx was voltage-clamped at 280 mV, and IpC a was evoked by a 10 msec depolarizing pulse. In this experiment, [C a 21]o was reduced to 1 mM to allow better voltage-clamp performance. A, IpC a induced in a caly x by a depolarizing voltage step to 210 mV in the absence and presence of baclofen (20 mM, superimposed). B, The tail currents are normalized at the peak and superimposed. C, Current–voltage relationships of IpC a before (open circles) and after ( filled circles) baclofen application. Mean values 6 SEMs obtained from six calyces are shown.

without affecting the presynaptic action potential. The magnitude of suppression of EPSC s by baclofen (20 mM) was 78.5 6 0.71% (mean 6 SEM; n 5 4 cells), which was comparable with that for the extracellularly evoked EPSC s (Fig. 1 and legend). The presynaptic action potential had a peak amplitude of 90.5 6 12 mV and a half-width of 0.76 6 0.19 msec (n 5 4 calyces), which remained at 99.0 6 2.4% and 113 6 25%, respectively, during baclofen application. Baclofen had no effect on the presynaptic membrane potential or conductance (see below). To directly address an involvement of presynaptic G-proteins in the action of baclofen, the GDP analog guanosine-59-O-(2thiodiphosphate) (GDPbS, 3 mM) was included in the presynaptic whole-cell pipette. In this condition, baclofen no longer suppressed EPSC s (102 6 2.6%; n 5 4 pairs) (Fig. 2 B). After the whole-cell pipette containing GDPbS was retracted, another paired recording was made again at the same synapse, this time with a presynaptic pipette solution containing GTP. Baclofen clearly suppressed EPSC s by 67 6 14% (n 5 3 pairs after GDPbS washout) (Fig. 2 B). Thus presynaptic GDPbS blocked the effect of baclofen in a reversible manner. Similarly, when the nonhydrolyzable GTP analog guanosine 59-O-(3-thiotriphosphate) (GTPgS, 200 mM) was included in the presynaptic whole-cell pipette, baclofen had no effect on EPSC s (105 6 7.6%; n 5 3 pairs; data not shown). These results indicate that the effect of baclofen on EPSC s is indeed presynaptic and that G-proteins

To identify an effector of the presynaptic GABAB receptor, we first examined whether presynaptic calcium currents (IpCa) could be modulated by baclofen. As illustrated in Figure 3A, baclofen slowed activation kinetics of IpCa and reduced its amplitude. When measured at the peak of the control current (1.3 msec from onset) at 210 mV, the magnitude of IpCa suppression was 38.0 6 3.8% (n 5 6). The baclofen-induced suppression of IpCa was not associated with a shift in the current–voltage (I–V ) relationship (Fig. 3C). As shown in Figure 3 A,B, after a 10 msec depolarizing pulse (to 210 mV) IpCa deactivated exponentially with a fast time constant (0.14 6 0.03 msec; n 5 8). Baclofen had no effect on this deactivation time constant (0.14 6 0.05 msec after baclofen). This suggests that baclofen has little effect on the presynaptic Ca 21 channel open time. These characteristics of the baclofeninduced inhibition of IpCa are similar to those reported for somatic Ca 21 currents (Dolphin and Scott, 1987; Scholtz and Miller, 1991; Mintz and Bean, 1993; Lambert and Wilson, 1996). To study further the involvement of G-proteins in the baclofeninduced suppression of IpCa, caged GTPgS (38 mM) was loaded into a calyx through a whole-cell patch pipette (Fig. 4 A). After it was confirmed that baclofen reversibly suppressed IpCa (a–c), a flash of ultraviolet light (UV, 340 –380 nm) was applied for 2– 4 sec (arrow) to induce a photo-release of the caged GTPgS compound. After the flash, IpCa gradually diminished in amplitude and slowed in its rising phase (c, d). After IpCa amplitude reached a steady level, a second application of baclofen no longer attenuated IpCa (d, e). In agreement with this result, when GTPgS (200 mM) was included in the presynaptic whole-cell pipette, IpCa exhibited a similarly slow rise, and baclofen had no significant effect on the current amplitude (99.8 6 1.6%; n 5 4) (Fig. 4 B). When GDPbS (3 mM) was included in the pipette, IpCa had a normal rise time, but baclofen was again ineffective on IpCa (96.9 6 1.2%; n 5 5) (Fig. 4C). These results indicate that the inhibitory effect of baclofen on the presynaptic calcium current is mediated by G-proteins.

Lack of baclofen effect on presynaptic potassium currents We next examined whether baclofen might modulate potassium currents. Voltage-dependent outward potassium currents were evoked by depolarizing a presynaptic terminal in the presence of TTX (1 mM) (Forsythe, 1994). As illustrated in Figure 5, the potassium current before and after baclofen application was nearly identical at all voltages examined. Thus, GABAB receptors do not seem to be coupled with voltage-gated potassium channels at the presynaptic terminal. In neuronal somata, baclofen enhances inwardly rectifying potassium currents by activating G-proteins (Andrade et al., 1986; Sodickson and Bean, 1996). We examined whether baclofen might similarly enhance the inward rectifying potassium current at the presynaptic terminal. As illustrated in Figure 6 A, baclofen applied at 270 mV holding potential had no effect on the holding current or the membrane conductance (98.5 6 1.8%; n 5 9) measured by a ramp command voltage pulse (Fig. 6C). The inwardly rectifying potassium current is known to be blocked by a low concentration of Ba 21 (Hagiwara et al., 1978). Bathapplication of Ba 21 (100 mM) caused a small inward current accompanied by a slight decrease in membrane conductance (to

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Figure 4. Block of baclofen-induced IpC a suppression by GTPgS or GDPbS. A, Occlusion of baclofen effect by GTPgS. IpC a was evoked in a calyx by a 20 msec depolarizing step from 270 mV to 213 mV. Baclofen (20 mM) suppressed IpCa, which recovered partially (a–c, superimposed). After a light flash given at an arrow for 2 sec, IpCa diminished gradually (c, d). A second application of baclofen after the flash had no effect on IpCa (d, e). Essentially the same result was obtained in two other calyces. B, C, Little effect of baclofen on IpC a (evoked by a 20 msec depolarizing pulse from 280 mV to 210 mV) was observed in the presence of GTPgS (200 mM, B) or GDPbS (3 mM, C) in the presynaptic pipette. B and C are from different calyces. A similar result was obtained in another caly x for each case.

Figure 5. Lack of baclofen effect on voltage-gated potassium currents. Inset, Outward potassium currents evoked by 20 mV depolarizing steps from the holding potential of 280 mV to 120 mV in the presence of TTX before (left) and after (middle) baclofen (20 mM) application. The superimposed traces before and after baclofen application overlapped almost completely (right). The amplitude of the potassium current was normalized against the value at 0 mV and mean 6 SEMs of five calyces before (open circles) and after ( filled triangles) baclofen application are plotted against membrane potential.

84.5 6 4.6%; n 5 6) (Fig. 6 A), suggesting that the inwardly rectifying channels might weakly contribute to the resting conductance of the presynaptic terminal. After a calyx was loaded with caged GTPgS, photo-release of GTPgS by a flash (Fig. 6B, arrow) induced a prominent outward current accompanied by an increase in membrane conductance. After the outward current reached a steady level, subsequent application of Ba 21 (100 mM) largely abolished this current. When Ba 21 was washed out, the outward current gradually recovered, with an increase in membrane conductance (not shown). The Ba 21-sensitive current induced by GTPgS was extracted as a difference current before and after the Ba 21 application (Fig. 6 B, a and b). This current rectified inwardly and reversed at 292 6 1.1 mV (n 5 4) close to the theoretical potassium equilibrium potential (99.5 mV; Fig. 6C, arrow), indicating that it is a G-protein-activated inwardly rectifying potassium current (GIRK) (Kubo et al., 1993). Thus GIRK is present in the presynaptic terminal but cannot be activated by GABAB receptors. Similar to Ba 21, extracellular Cs 1 blocks inwardly rectifying potassium currents (Hagiwara et al., 1976; Sodickson and Bean, 1996) as well as the inwardly rectifying cationic currents Ih (Halliwell and Adams, 1982; Takahashi, 1990). Bath-application of Ba 21 or Cs 1 (both at 1 mM) had no effect on EPSCs evoked extracellularly (Fig. 7). Baclofen applied in the presence of Ba 21 or Cs 1 suppressed EPSCs to a similar extent as in control: 72.5 6


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Figure 6. Effects of baclofen and GTPgS on presynaptic holding current and membrane conductance. A calyx was voltage-clamped at the holding potential of 270 mV, and a ramp command voltage from 250 to 2130 mV (C, top left) was applied every 20 sec. A, Baclofen (20 mM) had no effect on the holding current or input conductance. Ba 21 (100 mM) caused a slight inward current associated with a decrease in conductance in the same caly x. B, In another caly x, photo-release of GTPgS by a UV flash (arrow) induced an outward current accompanied by an increase in input conductance. This current was suppressed by Ba 21 (100 mM, b). Application of the light flash without loading caged compound had no effect on the holding current or membrane conductance. The outward current was not observed after GTPgS photolysis with the C s1-based internal solution for IpC a recordings (Fig. 4 A). C, Currents (a, b, bottom) corresponding to a command voltage (top) after photolysis of caged GTPgS compound before ( a) and after ( b) application of Ba 21. Right, Ba 21-sensitive current extracted as a difference current (a–b). Arrow indicates theoretical equilibrium potential for potassium ions calculated from the internal and external potassium activities. The difference current between before and after photolysis had a similar reversal potential, but inward rectification was less prominent (data not shown). Membrane potential was corrected for the liquid junction potential between the external and internal solution (17.5 mV) for this current– voltage relationship. The data in this figure were low-passfiltered at 100 Hz and sampled at 1 kHz.

4.3% in control (n 5 8), 72.6 6 2.0% in Ba 21 (n 5 4), and 74.8 6 1.5% in C s 1 (n 5 4), respectively. These results suggest further that neither GIRK nor Ih is involved in the GABAB receptormediated presynaptic inhibition at the caly x–M N TB synapse.

Lack of contribution of exocytotic machinery to GABAB receptor-mediated presynaptic inhibition 21

To examine whether the exocytotic process downstream of Ca influx is involved in GABAB receptor-mediated presynaptic inhibition, we made simultaneous pre- and postsynaptic recordings and compared the IpC a–EPSC relationship between two conditions: first after baclofen application and then after reduction of [Ca 21]o (Takahashi et al., 1996). When baclofen was applied, EPSC s diminished concomitantly with IpC a (Fig. 8 A, i, ii). Similarly, when [C a 21]o was reduced by replacement with [Mg 21]o , both EPSC s and IpC a were diminished in parallel (Fig. 8 A, iii, iv). When the IpC a–EPSC relations were plotted for data obtained after baclofen application and after [C a 21]o reduction, the two relationships largely overlapped with each other (Fig. 8 B). At the seven synapses examined, the slope in the regression lines after baclofen application was similar to that after [Ca 21]o reduction (Fig. 8 B, inset) (no significant difference in paired t test).

The result was essentially the same when the charge instead of the peak amplitude for IpCa and EPSCs was compared. Thus, the baclofen-induced suppression of EPSCs can be explained mostly by a reduction of IpCa.

DISCUSSION Inhibition of calcium currents and transmission by GABAB receptor through G-protein In this study, using paired whole-cell recordings from the brain stem giant presynaptic terminal and postsynaptic cell, we have demonstrated that presynaptic GABAB receptors are linked through G-proteins to Ca 21 channels, thereby suppressing transmitter release. The IpCa at the calyx–MNTB synapse is almost exclusively P-type at the age range examined (Forsythe et al., 1998). It is possible that bg complex of heterotrimeric G-protein may interact with the a1A subunit, thereby suppressing P-type Ca 21 channel activity (De Waard et al., 1997). Such a membranedelimited mechanism is consistent with our finding that the magnitude of the baclofen-induced suppression of EPSCs evoked via a presynaptic whole-cell pipette was similar to that of EPSCs evoked via an extracellular pipette. Thus diffusible intracellular

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Figure 7. Effect of external Ba 21 and C s 1 on GABAB receptormediated presynaptic inhibition. EPSC s were evoked in M N TB principal cells by extracellular stimulation. A, Baclofen-induced suppression of EPSCs was similar before and after Ba 21 application (1 mM). EPSC s before, during baclofen application (20 mM), and after washout are superimposed on top in the absence (lef t) and presence (right) of Ba 21. Note the small polysynaptic EPSC component observed at the decay of monosynaptic EPSC. B, Baclofen suppressed EPSC s similarly in the absence and presence of external C s 1 (1 mM). EPSC s before and after baclofen application are superimposed on top in the absence (lef t) and presence (right) of Cs 1. A and B are from different cells.

messengers, likely to be washed out during whole-cell recording, may not be essentially involved in the baclofen-induced suppression of EPSC s. IpC a is also suppressed by a metabotropic glutamate receptor (mGluR) agonist (Takahashi et al., 1996). It remains to be seen whether a common G-protein mediates the presynaptic inhibition by mGluRs and GABAB receptors.

Presynaptic potassium channels are not coupled with GABAB receptor Presynaptic potassium conductances are thought to be important in the regulation of transmitter release (Augustine, 1990). The receptor-mediated inhibition of a potassium conductance is known to enhance synaptic efficacy in invertebrate nervous systems (Kandel and Schwartz, 1982). At mammalian neuronal somata, GABAB receptors potentiate transient potassium currents (Saint et al., 1990) or activate inwardly rectif ying potassium currents through G-protein activation (Andrade et al., 1986; Sodickson and Bean, 1996). It has been postulated that an enhancement of presynaptic potassium currents may underlie GABAB receptor-mediated presynaptic inhibition (Saint et al., 1990; Thompson and Gahwiler, 1992). However, direct record-


ings from the calyx presynaptic terminals revealed that baclofen had no effect on the voltage-gated potassium currents or inwardly rectifying potassium currents. Furthermore, the inward rectifier channel blockers Ba 21 or Cs 1 had no effect on the baclofeninduced inhibition of EPSCs. These results indicate that potassium conductances are not significantly involved in the GABAB receptor-mediated presynaptic inhibition at this fast excitatory synapse. In our present study, Ba 21 had no effect on GABAB receptormediated presynaptic inhibition as reported at other central synapses (Allerton et al., 1989; Lambert at al., 1991; Thompson and Gahwiler, 1992; Hirata et al., 1995). Although Ba 21 was reported to inhibit the effect of baclofen on monosynaptic IPSCs in hippocampal CA3 cells (Thompson and Gahwiler, 1992), this was not confirmed in a study using another blocking agent of inward rectifying potassium channels (Lambert and Wilson, 1993). Furthermore, transgenic mice lacking a GIRK gene exhibited a normal magnitude of GABAB receptor-mediated presynaptic inhibition at hippocampal synapses (Luscher et al., 1997). Thus, so far there is no direct evidence to indicate an involvement of potassium conductances in the receptor-mediated presynaptic inhibition at mammalian central synapses. At the calyx of Held, an inwardly rectifying potassium current could be activated by intracellular application of GTPgS but not by baclofen. This result may imply that the GABAB receptors and G-proteins coupled with GIRK are distinct from those coupled with voltage-gated calcium channels, as proposed previously on the basis of pharmacological differences between the presynaptic and postsynaptic effect of baclofen (Dutar and Nicoll, 1988). However, it is also possible that GIRK is localized outside of the functional domain of G-proteins coupled with GABAB receptors at the presynaptic nerve terminal. Baclofen had no effect on presynaptic spike waveform being consistent with the lack of involvement of potassium conductance (also see Dittman and Regehr, 1996). Although baclofen suppressed the presynaptic calcium conductance, this was not apparent in the action potential waveform. This might be attributable to the large potassium conductance masking the calcium conductance. In fact, even after synaptic transmission was abolished by reducing [Ca 21]o , the presynaptic action potential waveform remained similar at this (our unpublished observation) and other synapses (Sabatini and Regehr, 1997).

The exocytotic machinery for evoked transmitter release is not affected by GABAB receptors The IpCa–EPSC relationship during baclofen application was similar to that during reduction of [Ca 21]o. This suggests that the exocytotic machinery downstream of Ca 21 entry is not involved in GABAB receptor-mediated presynaptic inhibition at the calyx– MNTB synapse, as is the case for mGluR-mediated presynaptic inhibition (Takahashi et al., 1996). In the case of baclofen, similar conclusions were made from studies using Ca 21 indicators at hippocampal synapses (Wu and Saggau, 1995; Dittman and Regehr, 1997) (but see Dittman and Regehr, 1996). The direct involvement of the exocytotic machinery in receptor-mediated presynaptic inhibition has been postulated from the observation that baclofen suppressed the frequency of spontaneous miniature synaptic currents in a [Ca 21]o-independent manner (Scanziani et al., 1992; Rohrbacher et al., 1997). However, differential modulations of the frequency of miniature events and the amplitude of evoked synaptic responses by various manipulations are well known (Fu and Poo, 1991; Geppert et al., 1994; Cummings et al.,


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Figure 8. Comparison of IpC a–EPSC relationships during baclofen application and [C a 21]o reduction. Paired recording from a caly x and its target cell. A, Effects of baclofen (20 mM; i, ii) and [C a 21]o reduction (iii, iv) on IpC a (Pre) and EPSC s (Post). IpC a was evoked by 1 msec depolarizing command pulse from 270 mV to 210 mV. Records before and after baclofen application or [C a 21]o reduction are superimposed on top row. B, Double logarithmic plot of IpCa–EPSC relation during baclofen application ( filled circles with a dotted regression line) and [C a 21]o reduction (open circles with a solid regression line). Data points above 90% in EPSC amplitude were excluded from these plots to minimize constrainment. The slope value was 2.29 for baclofen and 2.33 for [Ca 21]o reduction, respectively. E xcluding the minimal point from each relationship had no significant effect on the slope values (2.15 and 2.09, respectively, for baclofen and [C a 21]o reduction). Inset graph, The slope value of regression lines compared between [C a 21]o reduction and baclofen application at seven synapses. No significant difference with p 5 0.24 in paired t test. The mean slope value was 1.73 6 0.17 for baclofen and 1.77 6 0.17 for [Ca 21]o reduction, respectively.

1996; Hori et al., 1996). A distinct mechanism may operate in the modulation of synchronous and asynchronous transmitter release. In conclusion, this study has demonstrated G-protein-coupled modulation of presynaptic C a 21 channels on activation of GABAB receptor-mediated presynaptic inhibition; neither potassium channels nor modulation of the exocytotic machinery downstream of C a 21 influx plays a significant role. On binding a ligand, presynaptic GABAB receptors activate G-proteins and suppress Ca 21 currents, thereby reducing transmitter release. Given the wide distribution of presynaptic GABAB receptors at synapses throughout the nervous system, this mechanism would be of general application.

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