THE M1 ANTAGONIST PIRENZEPINE SUPPRESSES HETEROSYNAPTIC STD IN THE CA1 REGION

THE effect of the muscarinic M1 and M2 antagonists (pirenzepine and ga llamine) on heterosynaptic short-term depression (HSTD) of the CA1 pop ulation spike was studied in rat hippocampal slices. HSTD was evoked b y a train of 30-50 pulses at 5 Hz (theta-like stimulation; TLS) delive red to the stratum oriens. Bath-applied pirenzepine (10(-6)-3 x 10(-6) M) significantly suppressed the magnitude of HSTD in the tested radia tum input (56 +/- 26% of control, p < 0.05). In contrast, gallamine (1 0(-6)-10(-5) M) did not suppress HSTD (102 +/- 7%). An enhancement of HSTD during application of eserine was also antagonized by pirenzepine (71 +/- 18%, p < 0.01), but not by gallamine (152 +/- 85%). These dat a suggest that acetylcholine synaptically released during TLS of the s tratum oriens significantly contributes to HSTD in the CA1 region by a ctivation of the M1 muscarinic receptor subtype.