MULTIPLE POSTSYNAPTIC ACTIONS OF GABA VIA GABA(B) RECEPTORS ON CA1 PYRAMIDAL CELLS OF RAT HIPPOCAMPAL SLICES

1. The effects of gamma-aminobutyric acid (GABA) on non-GABA(A) recept ors were investigated with intracellular recordings in CA1 pyramidal c ells of rat hippocampal slices in the presence of antagonists of GABA( A) receptors (50 mu M bicuculline and 50 mu M picrotoxin), N-methyl-D- aspartate (NMDA) and non-NMDA receptors (100 mu M 2-amino-5-phosphonop entanoic acid and 40 mu M 6-cyano-7-nitroquinoxaline-2,3-dione, respec tively), and of a blocker of GABA uptake (1 mM nipecotic acid). The ef fects of GABA were compared with those of the selective GABA(B) agonis t (-)baclofen [CGP-11973A; (-)BAC]. 2. In the presence of these antago nists, 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, bu t with a slower time course (peak latency 8 s, decay 78 s). The mean e quilibrium potential (E(rev)) of responses to GABA (-94 mV; n = 11) wa s similar to that of (-)BAC (-87 mV; n = 8), suggesting that both resp onses were mediated by K+ conductances. 3. Bath applications of 1 mM B a2+ partly antagonized GABA responses in a reversible manner. The mean amplitude of the Ba2+-resistant GABA response was 46% of control (n = 16, P < 0.05). In contrast, (-)BAC responses were completely abolishe d by Ba2+ (n = 15), and the effect was reversible. Thus both GABA and (-)BAC activate a common Ba2+-sensitive conductance, but GABA may also activate another Ba2+-resistant conductance. 4. The Ba2+-resistant GA BA response had a similar time course to control GABA responses, but i ts E(rev) was more depolarized (-79 mV, n = 8, P < 0.05). 5. During re cordings with electrodes containing KCl to reverse the Cl- gradient, a lthough GABA responses were smaller in amplitude, their time course an d E(rev) (-91 mV; n = 10) were similar to those recorded with potassiu m acetate electrodes. Thus Cl- conductances may not be involved in the se non-GABA(A) responses elicited by GABA. 6. During recordings with e lectrodes containing CsCl to block outward K+ currents, hyperpolarizin g GABA responses were not observed (n = 8). In these conditions, GABA elicited depolarizing responses with a faster time course (peak latenc y 1 s, decay 5 s) than the hyperpolarizing responses recorded with ele ctrodes containing KCl. Thus GABA may produce hyperpolarizations by ac tivating K+ conductances, but it may also produce an additional depola rizing response via other Cs+-insensitive conductances. 7. During reco rdings with electrodes containing LiCl to interfere with G protein act ivation, hyperpolarizing GABA responses were blocked and depolarizing responses were unmasked (n = 5). These depolarizing responses were gen erally similar to those recorded with electrodes containing CsCl. GABA responses were also reduced during recordings with electrodes contain ing the irreversible G protein activator guanosine-5'-O-(3-thiotriphos phate). Thus hyperpolarizing GABA responses may involve G protein acti vation, but the depolarizing responses may not. 8. Bath application of the selective GABA(B) antagonist CGP-35348 (1 mM) did not significant ly 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 GABA(B) antagonist CGP-55845A (5 mu M) a bolished all GABA responses (n = 7). Thus all non-GABA(A) responses el icited by GABA may be mediated by GABA(B) receptors. 9. In conclusion, GABA, in the presence of GABA(A) antagonists, may produce in CA1 pyra midal cells two distinct postsynaptic responses mediated via GABA(B) r eceptors and G protein activation: 1) GABA [and (-)BAC] may activate a Ba2+-sensitive K+ conductance, and 2) GABA [but not (-)BAC] may also generate a Ba2+-insensitive K+ conductance. GABA may also generate oth er ionic changes, via GABA(B) receptors, resulting in depolarization o f pyramidal cells.