DIFFERENCES BETWEEN PRESYNAPTIC AND POSTSYNAPTIC GABA(B) MECHANISMS IN RAT HIPPOCAMPAL PYRAMIDAL CELLS

1. Whole cell voltage-clamp techniques were used in the CAI region of rat hippocampal slices to study presynaptic and postsynaptic gamma-aminobutyric acid B (GABA(B)) response mechanisms. The effects of the protein kinase C activator phorbol 12,13-diacetate (PDA),barium (Ba2+), and pertussis toxin were compared on the presynaptic and postsynaptic GABA(B) actions of bath-applied baclofen and paired-pulse depression (PPD) of the monosynaptic GABA(A) inhibitory postsynaptic current (IPSC). The magnitude of PPD was dependent on the amplitude of the first response. PPD was predominantly a GABA(B)-mediated effect, as it was very much reduced by the GABA(B) antagonist CGP 35348. 2. PDA enhanced monosynaptic GABA(A) IPSCs through an apparently presynaptic mechanism. Iontophoretic GABA(A) responses were unaffected, and there was no change in E(IPSC). PDA increased the frequency of spontaneous, tetrodotoxin-insensitive IPSCs without significantly affecting their amplitudes. The inactive phorbol ester, 4 alpha-PDA did not alter IPSCs. After PDA application, stimulus intensity was adjusted to produce responses of comparable amplitude to control responses. PDA had a marked and reversible depressant effect on the postsynaptic GABA(B) response and caused a lesser, but still significant, reduction in the baclofen-induced reduction of monosynaptic IPSCs. PDA had no effect on PPD. 3. Ba2+ dramatically reduced postsynaptic GABA(B) responses; it had no effect on PPD. Ba2+ tended to decrease the presynaptic baclofen reduction of IPSCs, although this was not statistically significant. 4. Pertussis toxin, injected 2-3 days earlier into the intact hippocampus, blocked all three GABA(B) responses equally (similar to 70% decrease). 5. We conclude that presynaptic and postsynaptic GABA(B) mechanisms are mediated by G proteins that couple to different mechanisms. Discrepancies with previous work are evidently due to the use of different tissue preparations and different target responses. Even though protein kinase C activation caused a partial reduction in the presynaptic effect of baclofen, its lack of effect on PPD makes a significant role for protein kinase C in modulation of PPD unlikely.