1. Gamma-aminobutyric acid-B (GABAB) autoreceptor-dependent and -independent components of paired-pulse depression (PPD) at inhibitory synapses in area CA3 of the rat hippocampus were studied using whole-cell recording techniques. Inhibitory fibers were activated directly in the presence of the ionotropic glutamate receptor antagonists 6,7-dinitroquinoxaline-2,3,dione (20 microM) and D-2-amino-5-phosphonovalerate (20 microM). 2. When pairs of monosynaptic inhibitory postsynaptic currents (eIPSCs) were evoked with an interstimulus interval of 200 ms, the amplitude of the second response (eIPSC2) was depressed when compared with the first (eIPSC1). The GABAB receptor agonist baclofen (10 microM) depressed both responses, but eIPSC1 was depressed more than eIPSC2, resulting in PPD that was comparatively smaller. Addition of the GABAB receptor antagonist CGP 55845A (1 microM) completely reversed depression of eIPSC1 by baclofen and increased the amplitude of eIPSC2 above the control value, such that PPD in the combination of baclofen and CGP 55845A was equivalent to that in baclofen alone. The ratio eIPSC2/eIPSC1 was 0.64 under control conditions, 0.77 in the presence of baclofen, and 0.79 in the presence of baclofen and CGP 55845A. These results demonstrate the existence of two components of PPD at inhibitory synapses, one that depends on activation of GABAB autoreceptors (GABAB receptor-dependent PPD) and one that does not (GABAB receptor-independent PPD). 3. When the number of inhibitory fibers activated was lowered by decreasing the stimulus intensity, eIPSC2/eIPSC1 was 0.76 under control conditions, 0.75 in the presence of baclofen, and 0.76 in the presence of baclofen and CGP 55845A. These results indicate that GABAB receptor-dependent PPD requires activation of several presynaptic inhibitory neurons, whereas GABAB receptor-independent PPD does not. 4. The time-courses of the GABAB-dependent and -independent components of PPD were compared by varying the interstimulus interval in the absence and presence of CGP 55845A. GABAB-dependent PPD was maximal at an interstimulus interval of 100 ms and was undetectable at 1 s. In contrast, GABAB-independent PPD was maximal at 5 ms and 1 s, was slightly less pronounced at intermediate intervals (50-200 ms), and was present at intervals as long as 5 s. 5. GABAB-independent PPD was not blocked by antagonists at opioid receptors (10 microM naloxone) or muscarinic acetylcholine receptors (10 microM atropine). GABAB-independent PPD could not be accounted for by a decrease in driving force because of Cl- redistribution.(ABSTRACT TRUNCATED AT 400 WORDS)