Department of Physiology and Biophysics, School of Medicine, State University of New York, Buffalo, New York 14214
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ABSTRACT |
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Zhang, Jian, Ning Tian, and Malcolm M. Slaughter. Neuronal discriminator formed by metabotropic -aminobutyric acid receptors. J. Neurophysiol. 80: 3365-3368, 1998. Neurotransmitters function in one of two modes, promoting either inhibition or excitation. However, the metabotropic
-aminobutyric acid receptor (GABABR) system can switch between these modes. In the presence of a small excitatory stimulus, the GABABR mediates a shunting inhibition that suppresses excitation. However, in the presence of a strong excitatory stimulus, the GABABR potentiates the response. This bipartite action is accomplished by linking the GABABR to two electrogenic mechanisms; one activates an outward current and another reduces an outward current. As a consequence, the GABABR serves as a discriminator that reduces the influence of weak signals while augmenting responses to strong signals. In retinal ganglion cells, this mechanism acts to promote the communication of phasic information.
Whole cell recordings were obtained from neurons in the ganglion cell layer of the tiger salamander (Ambystoma tigrinum) retinal slice as previously described (Werblin 1978 The biophysical properties that generate a neuronal discriminator are illustrated in Fig. 1. Baclofen was applied to selectively activate GABABRs (Bowery et al. 1980
INTRODUCTION
Abstract
Introduction
Methods
Results
References
-Aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the brain, activates both ionotropic and metabotropic receptors (Dunlap 1981
). The former gates a chloride conductance that provides a powerful shunting inhibition. However, the metabotropic GABA receptor (GABABR) initiates subtler and more varied responses. Generally, GABABRs either increase a potassium conductance (Newberry and Nicoll 1984
) or reduce a voltage-dependent calcium conductance (Dolphin and Scott 1986
). Frequently, these two mechanisms are partitioned across the synapse. For example, in hippocampus postsynaptic GABABRs activate a potassium conductance responsible for a slow inhibitory postsynaptic potential (Dutar and Nicoll 1988
). Presynaptically, GABABRs suppress calcium current and reduce transmitter release. Thus these two complementary responses combine to inhibit synaptic transmission. However, in the retina both GABABR responses coexist on the ganglion cell. As a consequence, they produce a unique switching mechanism that filters synaptic signals based on the strength of the input.
METHODS
Abstract
Introduction
Methods
Results
References
; Wu 1987
). Experiments employed an Axoclamp 2A amplifier and PCLAMP software. Recording electrodes (~5 M
) contained (in mM) 106 K-gluconate, 5 NaCl, 2 MgCl2, 5 ethylene glycol-bis(
-aminoethyl ether)-N,N,N',N'-tetraacetic acid, and 5 N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES, pH 7.4). Voltages were corrected for tip potential. The Ringer solution contained (in mM) 111 NaCl, 2.5 KCl, 1.8 CaCl2, 1 MgCl2, 10 dextrose, and 5 HEPES (pH 7.8). Drugs were applied by superfusion. In calcium channel current experiments, the pipette solution also contained 4 mM ATP, 20 mM phosphocreatine, and 50 units/ml creatine phosphokinase, and the Ringer solution contained 10 mM barium and 40 mM tetraethylammonium (TEA; equimolar replacement of all calcium and some sodium).
RESULTS
Abstract
Introduction
Methods
Results
References
). This agonist affected every cell tested in the ganglion cell layer. Two distinct GABABR actions were observed. At voltages around the resting membrane potential (
100 to
50 mV), baclofen application produced a small current that reversed near
80 mV, close to the calculated potassium reversal potential (Fig. 1A). The GABABR current shown in this figure is the difference current generated by voltage ramps in the presence and absence of 100 µM baclofen. The conductance produced by baclofen was 423 ± 95 pS (mean ± SE, n = 6). The resting conductance of these cells was 592 ± 101 pS. Thus the GABABR shunting conductance represented a large fraction of the resting conductance, which indicates that it can elicit significant inhibition.
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FIG. 1.
-Aminobutyric acid receptors (GABABRs) regulate 2 opposing conductances. A: difference current (baclofen
control) illustrates a baclofen-elicited, small current that reversed near
80 mV. This difference current was obtained from voltage-clamped ganglion cells that were ramped from
100 to
50 mV. B: voltage-clamped ganglion cell was ramped between
90 and +20 mV. A large outward current was evoked (1), which was partially suppressed by 100 µM baclofen (2). B, inset: inward barium current (
) observed in a voltage-clamped ganglion cell that was stepped from
70 to 0 mV. This current was suppressed by baclofen (
). C: by using a protocol similar to that in B, an outward current was evoked (1), this was partially suppressed by 100 µM cadmium (2), and cadmium occluded the effect of 100 µM baclofen (3). The inset, following the paradigm in A, plots the difference current between traces 2 and 3 and illustrates that cadmium did not block the small, baclofen-elicited current observed at negative voltages. D: at less negative voltages, the net effect of baclofen was to produce a large inward current. This trace is the difference current from B (trace 2
trace 1).
40 mV. Baclofen blocked only a portion of this outward current (Fig. 1B, trace 2). Potassium channel blockers inhibited this outward current (not shown). Calcium channel blockers also suppressed a portion of this outward current (Fig. 1C, trace 2) and occluded the action of baclofen (Fig. 1C, trace 3). This suggests that GABABRs suppressed calcium-dependent outward current. The inset in Fig. 1B demonstrates that baclofen reduced a voltage-activated calcium current in these ganglion cells (Bindokas and Ishida 1991
; Zhang et al. 1997
). To measure calcium channel current, the slice was pretreated with 40 mM TEA and 10 mM barium. Under these conditions, a step from
70 mV to +10 mV evoked an inward barium current (
) that was partially suppressed by 100 µM baclofen (
; mean reduction of 40 ± 3%, n = 22). In combination, these results indicate that GABABRs reduced a voltage-dependent calcium current, which in turn reduced a large, calcium-dependent potassium conductance.
90 and
50 mV (trace 3 minus trace 2 of Fig. 1C).
10 pA) was suppressed by baclofen. However, baclofen augmented the voltage response to stronger depolarizing currents (+30 and +45 pA, Fig. 2A). Baclofen also produced a 20% average increase in spiking during a +45-pA current step (control = 22.6/s spikes, 50 µM baclofen = 27.1/s, n = 9, Wilcoxin's paired signed-ranks test P < 0.05). This differential effect can be explained by the discrete actions of the two GABABR conductances. A small depolarization was shunted by the baclofen-activated, calcium-independent potassium conductance. However, a larger amplitude depolarization moved the cell voltage to a region where baclofen produced a potentiating net inward current. Cadmium application replicated the latter effect of baclofen (Fig. 2B). This supports the hypothesis that enhancement was due to reduction of calcium-dependent potassium currents. Cadmium did not suppress the response to small currents (+15 pA) (data not shown), consistent with the calcium independence of this component of GABABR action.
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FIG. 2.
GABABR stimulation produces opposing effects on responses to weak and strong inward currents. A: in current-clamp recordings from ganglion cells, baclofen reduced the response to small current injections (+15 and 10 pA) but enhanced the response to stronger currents (+30 and +45 pA). B: Both baclofen and cadmium enhanced responses to the stronger currents (+45 pA).
; Lukasiewicz and Werblin 1988
). Correspondingly, ganglion cell EPSCs correlate with spike frequency but not with EPSPs (Diamond and Copenhagen 1995
).
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FIG. 3.
A: when the retinal slice was stimulated by light (diffuse, yellow LED), the ganglion cell's excitatory postsynaptic current (right) was more transient than the excitatory postsynaptic potential (left). B: in current-clamp records from another ganglion cell, baclofen caused an enhancement of the early phase of the light-evoked voltage response (a') and suppression of the late phase (b').
; Müller et al. 1992
; Slaughter and Bai 1989
). Although part of this effect may be due to presynaptic actions of baclofen, at least part is due to the direct, dual effects of baclofen on the ganglion cell.
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ACKNOWLEDGEMENTS |
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This work was supported by grant EY-05725 from the National Eye Institute.
Present addresses: J. Zhang, Dept. of Ophthalmology and Visual Science, University of Texas Medical School, 6431 Fannin, Suite 7.024, Houston, TX 77030; N. Tian, Dept. of Ophthalmology, Box 0730, University of California, San Francisco, CA 94143.
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FOOTNOTES |
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Address for reprint requests: M. Slaughter, SUNY, Dept. of Physiology, 124 Sherman Hall, Buffalo, NY 14214.
Received 22 June 1998; accepted in final form 11 August 1998.
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REFERENCES |
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