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 hippocamp al slice preparation reveal that (1S,3R)-1-aminocyclopentane-1,3-dicar boxylic acid (ACPD), a selective agonist at metabotropic glutamate rec eptors (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 intracel lular Ca2+ may be involved in this inhibitory action of ACPD. 2. Inhib ition of I-AHP is not secondary to effects on Ca2+ currents, because 1 0 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-g amma-S is included in the pipette filling solution, whereas inclusion of 1 mM GDP-beta-S attenuated the inhibitory action of ACPD. These res ults indicate that the response to mGluR activation is G protein media ted. 4. Group I mGluRs, which includes mGluR1 and mGluR5, are G-protei n-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 intracellula r Ca2+ The weak but selective mGluR1 agonist (S)-3-hydroxyphenylglycin e (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 pharmacologic al profile to that of those coupled to IP3 production. 5. The effects of agents known to interfere with IP3 production and action also suppo rt IP3 involvement in ACPD action. Neomycin (1 mM in pipette solution) , which should reduce IP3 production through inhibition of PLC, reduce d 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 Ca2mobilization, attenuated the inhibitory action of 10 mu M ACPD from al most 100% to 39 +/- 5% (n = 5). Heparin by itself increased the amplit ude and duration of I-AHP, suggesting that resting levels of IP3 are s ufficient to suppress of I-AHP partially. 6. In addition to the pool o f intracellular Ca2+ that is mobilized by IP3, there is a distinct poo l that is responsible for Ca2+-triggered Ca2+ release and is blocked b y ryanodine or dantrolene. These drugs caused a small reduction of bot h I-AHP and the inhibitory action of ACPD. Possibly the Ca2+ signal mo bilized by IP3 is partially amplified by Ca2+ released from the ryanod ine-sensitive stores. 7. Activation of PLC can also lead to the produc tion of diacyl glycerol and activation of protein kinase C (PKC). Howe ver, the inhibitory action of ACPD on I-AHP was not affected by stauro sporine 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. Act ivation of PKA by the adenylate cyclase activator forskolin led to inh ibition of I-AHP. Although activation of mGluR1 agonists can also stim ulate adenylate cyclase and activate PKA, inhibition of PKA and the ef fect 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 I P3 and the mobilization of intracellular Ca2+.