Mechanisms of presynaptic inhibition by metabotropic glutamate receptor (mGluR) agonists were investigated in neonatal rat hippocampal CA1 region using the optical recording technique recently developed. Following selective loading of presynaptic terminals with a fluorescent Ca2+ indicator dye rhod-2 AM, changes in Ca2+ signals and the corresponding field excitatory postsynaptic potentials (EPSPs) induced by single electrical stimuli to the Schaffer collateral-commissural (SCC) pathway were recorded simultaneously. Application of a mGluR agonist, 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD; 100 microM) or (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD; 100 microM), reversibly reduced both the field EPSP and the presynaptic Ca2+ transient, and the quantitative relationship between them was quite similar to that observed during application of Cd2+, a non-selective Ca2+ channel blocker, or in a Ca(2+)-free solution. Application of 4-aminopyridine (4-AP; 1 mM), a blocker of certain subtypes of voltage-dependent K+ channels, significantly inhibited the 1S,3R-ACPD effect. Application of DCG-IV, a novel mGluR2/mGluR3-selective agonist, suppressed field EPSPs only slightly even at a high dose (3 microM). These results suggest that activation of presynaptic mGluR different from mGluR2/mGluR3 suppresses the action potential-triggered Ca2+ influx, probably via 4-AP-sensitive mechanisms, and thereby reduces glutamate release in neonatal rat hippocampal CA1 region.