Metabotropic glutamate receptors coupled to IP3 production mediate inhibition of I-AHP in rat dentate granule neurons

1. Whole cell recordings from dentate granule neurons in the hippocampal slice preparation reveal that (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), a selective agonist at metabotropic glutamate receptors (mGluRs), inhibits a calcium-activated potassium current (I-AHP) 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 I-AHP 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 inhibitory action of ACPD. 2. Inhibition of I-AHP is not secondary to effects on Ca2+ currents, because 10 mu M ACPD, which inhibits I-AHP by 95 +/- 5% (mean +/- SE), reduced the Ca2+ current by only 8 +/- 4%. 3. Activation of mGluRs accelerates the irreversible inhibition of I-AHP that develops when 88 mu M GTP-gamma-S is included in the pipette filling solution, whereas inclusion of 1 mM GDP-beta-S attenuated the inhibitory action of ACPD. These results indicate that the response to mGluR activation is G protein mediated. 4. Group I mGluRs, which includes mGluR1 and mGluR5, are G-protein-coupled receptors that are known to stimulate phospholipase C (PLC)-mediated hydrolysis of phosphoinositides to produce 1,4,5-triphosphate (IP3), which in turn is known to mobilize the release of intracellular Ca2+ The weak but selective mGluR1 agonist (S)-3-hydroxyphenylglycine (100 mu M) completely inhibited I-AHP and the mGluR1 antagonist (S)-4-carboxyphenylglycine (500 mu M) reduced I-AHP inhibition produced by 5 mu M ACPD from 73 +/- 6% to 22 +/- 4%. These results indicate that the mGluR responsible for I-AHP inhibition has a similar pharmacological profile to that of those coupled to IP3 production. 5. The effects of agents known to interfere with IP3 production and action also support IP3 involvement in ACPD action. Neomycin (1 mM in pipette solution), which should reduce IP3 production through inhibition of PLC, reduced the ability of 10 mu M ACPD to inhibit I-AHP from almost 100% to 57 +/- 8% (n = 8). Heparin, an TP, receptor antagonist that reduces Ca2+ mobilization, attenuated the inhibitory action of 10 mu M ACPD from almost 100% to 39 +/- 5% (n = 5). Heparin by itself increased the amplitude and duration of I-AHP, suggesting that resting levels of IP3 are sufficient to suppress of I-AHP partially. 6. In addition to the pool of intracellular Ca2+ that is mobilized by IP3, there is a distinct pool that is responsible for Ca2+-triggered Ca2+ release and is blocked by ryanodine or dantrolene. These drugs caused a small reduction of both I-AHP and the inhibitory action of ACPD. Possibly the Ca2+ signal mobilized by IP3 is partially amplified by Ca2+ released from the ryanodine-sensitive stores. 7. Activation of PLC can also lead to the production of diacyl glycerol and activation of protein kinase C (PKC). However, the inhibitory action of ACPD on I-AHP was not affected by staurosporine at a concentration (1 mu M) that inhibits both protein kinase A (PKA) and PKC and blocks the action of 5-HT to inhibit I-AHP. 8. Activation of PKA by the adenylate cyclase activator forskolin led to inhibition of I-AHP. Although activation of mGluR1 agonists can also stimulate adenylate cyclase and activate PKA, inhibition of PKA and the effect of forskolin on I-AHP with the Walsh peptide did not affect ACPD inhibition of I-AHP 9. All of our results support the hypothesis that mGluR-mediated inhibition of I-AHP is initiated by the production of IP3 and the mobilization of intracellular Ca2+.