PRESYNAPTIC ACTIONS OF GABA AND BACLOFEN IN CA1 REGION OF THE GUINEA-PIG HIPPOCAMPUS IN-VITRO

The presynaptic actions of GABA and (+/-)baclofen on the stratum radia tum in the CA1 region of guinea-pig hippocampal slices were investigat ed using a modified grease-gap recording technique. D.c. potential shi fts were recorded in response to varying concentrations of GABA and (/-)baclofen. In Ca2+-free media containing tetrodotoxin, bath applicat ions of GABA (2.5 mu M to 20 mM) produced depolarizations which were c oncentration-dependent. Maximum depolarization was attained with 1O mM GABA. Superfusion of(+/-)baclofen (0.125-500 mu M) produced a concent ration-dependent hyperpolarization which peaked at a concentration of 250 mu M. The GABA-induced depolarization but not the (+/-)baclofen-in duced hyperpolarization was depressed by the GABA(A) antagonists bicuc ulline and picrotoxinin. The(+/-)baclofen-induced hyperpolarization bu t not the GABA-induced depolarization was suppressed by CGP 35 348, a GABA(B) antagonist. In the presence of bicuculline, GABA (0.5-5.0 mM) occasionally caused a hyperpolarization which could be blocked by CGP 35 348. These results indicate that the primary presynaptic action of GABA on the d.c. potential in the CA1 region of the hippocampus is to produce a GABA(A) receptor-mediated depolarization, while (+/-)baclofe n induces a GABA(B) receptor-mediated hyperpolarization. The grease-ga p d.c. potential recording technique, described in this paper, is expe cted to be useful in examining changes in the membrane potentials of p resynaptic terminals.