DISTINCT GABA(B) ACTIONS VIA SYNAPTIC AND EXTRASYNAPTIC RECEPTORS IN RAT HIPPOCAMPUS IN-VITRO

Intracellular recordings were obtained from pyramidal cells to examine gamma-aminobutyric acid-B (GABA(B))-mediated synaptic mechanisms in t he CA1 region of rat hippocampal slices. To investigate if heterogeneo us ionic mechanisms linked to GABA(B) receptors originate from distinc t sets of inhibitory fibers, GABA(B)-mediated monosynaptic late inhibi tory postsynaptic potentials (IPSPs) were elicited in the presence of antagonists of ionotropic glutamate and GABA(A) receptors and of an in hibitor of GABA uptake and were compared after direct stimulation of i nhibitory fibers in three different CA1 layers: stratum oriens, radiat um, and lacunosum-moleculare. No significant differences were found in mean amplitude, rise time, or time to decay to half-amplitude of IPSP s evoked from the three layers. Mean equilibrium potential(E-rev) of l ate IPSPs was similar for all groups and close to the equilibrium pote ntial of K+. Bath application of the GABA(B) antagonist CGP55845A bloc ked all monosynaptic late IPSPs. During recordings with micropipettes containing guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S), the mean amplitude of all GABA(B) IPSPs gradually was reduced. Bath applicatio n of Ba2+ completely eliminated monosynaptic late IPSPs evoked from an y of the stimulation sites. Late IPSPs were blocked completely during Ba2+ applications that reduced the GABA(B)-mediated hyperpolarizations elicited by local application of exogenous GABA only by similar to 50 %. These results indicate that heterogenous K+ conductances activated by GABA(B) receptors do not originate from separate sets of inhibitory fibers in these layers. To examine if synchronous release of GABA fro m a larger number of inhibitory fibers could activate heterogeneous GA BA(B) mechanisms, giant GABA(B) IPSPs were induced by 4-aminopyridine (4-AP) in the presence of antagonists of ionotropic glutamate and GABA (A) receptors. The amplitude and time course 4-AP-induced late IPSPs w ere approximately double that of evoked monosynaptic late IPSPs, but t heir voltage sensitivity, E-rev, and antagonism by the GABA(B) antagon ist CGP55845A and intracellular GTP gamma S were similar. Ba2+ complet ely abolished 4-AP-induced late IPSPs, whereas responses elicited by e xogenous GABA were only reduced by similar to 50% in the same cells. T hese results indicate that synchronous activation of large numbers of inhibitory fibers, as induced by 4-AP, may not activate heterogenous G ABA(B)-mediated conductances. Similarly, Ba2+ almost completely blocke d late inhibitory postsynaptic currents evoked by stimulus trains. Ove rall, our results show that exogenous GABA can activate heterogenous K + conductances via GABA, receptors, but that GABA released synapticall y, either by electrical stimulation or 4-AP application, can only acti vate K+ conductances homogeneously sensitive to Ba2+. Thus GABA(B) rec eptors located at synaptic and extrasynaptic sites on hippocampal pyra midal cells may be linked to distinct K+ conductances.