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-ami nobutyric acid B (GABA(B)) response mechanisms. The effects of the pro tein kinase C activator phorbol 12,13-diacetate (PDA),barium (Ba2+), a nd pertussis toxin were compared on the presynaptic and postsynaptic G ABA(B) actions of bath-applied baclofen and paired-pulse depression (P PD) of the monosynaptic GABA(A) inhibitory postsynaptic current (IPSC) . The magnitude of PPD was dependent on the amplitude of the first res ponse. PPD was predominantly a GABA(B)-mediated effect, as it was very much reduced by the GABA(B) antagonist CGP 35348. 2. PDA enhanced mon osynaptic GABA(A) IPSCs through an apparently presynaptic mechanism. I ontophoretic GABA(A) responses were unaffected, and there was no chang e in E(IPSC). PDA increased the frequency of spontaneous, tetrodotoxin -insensitive IPSCs without significantly affecting their amplitudes. T he inactive phorbol ester, 4 alpha-PDA did not alter IPSCs. After PDA application, stimulus intensity was adjusted to produce responses of c omparable amplitude to control responses. PDA had a marked and reversi ble depressant effect on the postsynaptic GABA(B) response and caused a lesser, but still significant, reduction in the baclofen-induced red uction of monosynaptic IPSCs. PDA had no effect on PPD. 3. Ba2+ dramat ically 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 thr ee 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 pre vious work are evidently due to the use of different tissue preparatio ns and different target responses. Even though protein kinase C activa tion 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.