The mechanisms of the induction of long-term depression (LTD) of field excitatory postsynaptic potentials (EPSPs) and whole cell patch-clamped excitatory postsynaptic currents (EPSCs) were studied in the dentate gyrus of the rat hippocampus. LTD of field EPSPs measuring 40% of control at 30 min poststimulation was induced by low-frequency stimulation consisting of 900 pulses at 1 Hz. LTD of EPSCs measuring 37% of control was induced by a pairing procedure consisting of 60 pulses at 1 Hz applied under voltage clamp at a holding potential of -40 mV. The induction of LTD of field EPSPs was dependent on an influx of extracellular calcium, being reduced in a low-Ca2+ (0.8 mM) medium. However, substantial LTD (26%) was still induced in such a medium, demonstrating the relatively low sensitivity of LTD induction to the level of extracellular Ca2+. A high concentration of the N-methyl-D-aspartate receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP5) (100 microM) did not significantly inhibit the induction of LTD of EPSCs evoked by the intracellular pairing procedure. D-AP5 partially reduced the magnitude of LTD of field EPSPs, but substantial LTD was still induced in the presence of AP5. The induction of LTD was strongly inhibited by Ni2+ (50 microM) but not by nifedipine (10 microM), indicating that Ca2+ influx via T-type, but not L-type, Ca2+ channels is required for the induction of LTD. The induction of LTD was strongly inhibited by thapsigargin, an agent known to deplete intracellular Ca2+ stores. The induction of LTD, but not long-term potentiation (LTP), was also strongly inhibited by ruthenium red, an agent known to block the ryanodine receptors located on intracellular Ca2+ stores. These results demonstrate that Ca2+ release from intracellular Ca2+ stores is required for the induction of LTD, but not LTP. The results of the present experiments suggest that the induction of LTD involves the entry of Ca2+ via low-voltage-activated voltage-gated Ca2+ channels followed by release of Ca2+ from intracellular ryanodine-receptor-sensitive Ca2+ stores.