1. The effects of metabotropic glutamate receptor (mGluR) stimulation on whole-cell Ca2+ currents were studied in pyramidal neurons isolated from the dorsal frontoparietal neocortex of rat. The selective mGluR agonist cis-(+/-)-1-aminocyclopentane-1,3-dicarboxylic acid [trans-ACPD (100 microM)] suppressed the peak high-threshold Ca2+ current by 21 +/- 1.7% (mean +/- SE) in 40 of 43 cells from 10- to 21-day-old rats. Consistent with previous findings for mGluR, glutamate, quisqualate, and ibotenate [but not alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)] reduced the Ca2+ currents, and the responses were not blocked by the ionotropic glutamate receptor antagonists 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) and DL-2-amino-5-phosphonovaleric acid (APV). EC50S for Ca2+ current suppression were 29 nM for quisqualate, 2.3 microM for glutamate, and 13 microM for trans-ACPD. 2. The low-threshold Ca2+ current was not modulated by trans-ACPD. The component of the high-threshold CA2+ current suppressed by mGluR was determined by pharmacology; the responses were not affected by omega-conotoxin GVIA but were occluded by the dihydropyridine Ca2+ antagonist nifedipine. Ca2+ tail currents prolonged by the dihydropyridine Ca2+ agonist (+)-SDZ 202-79] were suppressed by mGluR stimulation in parallel with the peak current. These findings strongly suggest that L-type Ca2+ channels are modulated by mGluR. 3. In neurons dialyzed with 100 microM guanosine 5'-(gamma-thio)triphosphate (GTP-gamma-S), Ca2+ current suppression was elicited by the first application of trans-ACPD (in 5 of 6 cells), but not by subsequent applications. Responses in neurons dialyzed with 2 mM guanosine 5'-(beta-thio)diphosphate (GDP-beta-S) were significantly smaller than controls. The results are consistent with mGluR acting via linkage to a G protein. 4. The responses to mGluR agonists were smaller when the external Ca2+ was replaced by Ba2+, indicating that some part of the mechanism underlying the current suppression is Ca2+ dependent. Because mGluR stimulates phosphoinositide turnover and release of Ca2+ from intracellular stores in other types of neurons, the possibility of released Ca2+ mediating inactivation of Ca2+ channels was considered. However, the Ca2+ current suppression was not attenuated by strong intracellular Ca2+ buffering [20 mM bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA)], by dialysis with 100 microM inositol-1,4,5-triphosphate (IP3), or by external application of 1 microM thapsigargin. 5. We conclude that in neocortical neurons, one action of mGluR is to suppress the component of high-threshold Ca2+ current conducted by L-type Ca2+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)
Trans-Synaptic Signaling through the Glutamate Receptor Delta-1 Mediates Inhibitory Synapse Formation in Cortical Pyramidal Neurons
