Multiple postsynaptic actions of GABA via GABAB receptors on CA1 pyramidal cells of rat hippocampal slices

1. The effects of gamma-aminobutyric acid (GABA) on non-GABAA receptors were investigated with intracellular recordings in CA1 pyramidal cells of rat hippocampal slices in the presence of antagonists of GABAA receptors (50 microM bicuculline and 50 microM picrotoxin), N-methyl-D-aspartate (NMDA) and non-NMDA receptors (100 microM 2-amino-5-phosphonopentanoic acid and 40 microM 6-cyano-7-nitroquinoxaline-2,3-dione, respectively), and of a blocker of GABA uptake (1 mM nipecotic acid). The effects of GABA were compared with those of the selective GABAB agonist (-)baclofen [CGP-11973A; (-)BAC]. 2. In the presence of these antagonists, micropressure application of GABA into stratum radiatum evoked hyperpolarizations with relatively fast peak latency (2 s) and decay (12 s). (-)BAC, in the absence of antagonists, hyperpolarized cells, but with a slower time course (peak latency 8 s, decay 78 s). The mean equilibrium potential (Erev) of responses to GABA (-94 mV; n = 11) was similar to that of (-)BAC (-87 mV; n = 8), suggesting that both responses were mediated by K+ conductances. 3. Bath applications of 1 mM Ba2+ partly antagonized GABA responses in a reversible manner. The mean amplitude of the Ba(2+)-resistant GABA response was 46% of control (n = 16, P < 0.05). In contrast, (-) BAC responses were completely abolished by Ba2+ (n = 15), and the effect was reversible. Thus both GABA and (-)BAC activate a common Ba(2+)-sensitive conductance, but GABA may also activate another Ba(2+)-resistant conductance. 4. The Ba(2+)-resistant GABA response had a similar time course to control GABA responses, but its Erev was more depolarized (-79 mV, n = 8, P < 0.05). 5. During recordings with electrodes containing KCl to reverse the Cl- gradient, although GABA responses were smaller in amplitude, their time course and Erev (-91 mV; n = 10) were similar to those recorded with potassium acetate electrodes. Thus Cl- conductances may not be involved in these non-GABAA responses elicited by GABA. 6. During recordings with electrodes containing CsCl to block outward K+ currents, hyperpolarizing GABA responses were not observed (n = 8). In these conditions, GABA elicited depolarizing responses with a faster time course (peak latency 1 s, decay 5 s) than the hyperpolarizing responses recorded with electrodes containing KCl. Thus GABA may produce hyperpolarizations by activating K+ conductances, but it may also produce an additional depolarzing response via other Cs(+)-insensitive conductances. 7. During recordings with electrodes containing LiCl to interfere with G protein activation, hyperpolarizing GABA responses were blocked and depolarizing responses were unmasked (n = 5). These depolarizing responses were generally similar to those recorded with electrodes containing CsCl. GABA responses were also reduced during recordings with electrodes containing the irreversible G protein activator guanosine-5'-O-(3-thiotriphosphate). Thus hyperpolarizing GABA responses may involve G protein activation, but the depolarizing responses may not. 8. Bath application of the selective GABAB antagonist CGP-35348 (1 mM) did not significantly reduce hyperpolarizing GABA responses (18% reduction in amplitude, n = 6, P > 0.05), but completely suppressed (-)BAC responses (n = 2). The more potent and selective GABAB antagonist CGP-55845A (5 microM) abolished all GABA responses (n = 7). Thus all non-GABAA responses elicited by GABA may be mediated by GABAB receptors. 9. In conclusion, GABA, in the presence of GABAA antagonists, may produce in CA1 pyramidal cells two distinct postsynaptic responses mediated via GABAB receptors and G protein activation: l) GABA [and (-)BAC] may activate a Ba(2+)-sensitive K+ conductance, and 2) GABA [but not (-)BAC] may also generate a Ba(2+)-insensitive K+ conductance. GABA may also generate other ionic changes, via GABAB receptors, resulting in depolarization of pyramidal cells.