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1. Whole cell recordings from dentate granule neurons in the hippocampal slice preparation reveal that (1 S, 3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), a selective agonist at metabotropic glutamate receptors (mGluRs), inhibits a calcium-activated potassium current (IAHP) responsible for the postspike after-hyperpolarization. Inclusion of 1 mM of the Ca2+ chelator ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid in the patch pipette reduced the inhibitory action of ACPD on IAHP while having no effect on a similar action of serotonin (5-HT). Thus the known action of ACPD of mobilizing intracellular Ca2+ may be involved in this inhibitor action of ACPD. 2. Inhibition of IAHP is not secondary to effects on Ca2+ currents, because 10 microM ACPD, which inhibits IAHP by 95 +/- 5% (mean +/- SE), reduced the Ca2+ current by only 8 +/- 4%. 3. Activation of mGluRs accelerates the …