METABOTROPIC GLUTAMATE RECEPTOR-MEDIATED SUPPRESSION OF L-TYPE CALCIUM CURRENT IN ACUTELY ISOLATED NEOCORTICAL NEURONS

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-(+/-)-l-aminocyclopentane-1,3-dicarboxylic acid [trans-ACPD (100 muM)] suppressed the peak high-threshold Ca2+ current by 21 +/- 1.7% (mean +/- SE) in 40 of 43 cells from 10- to 2 1 -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-nitroquinoxaline-2,3-dione (CNQX)and DL-2-amino-5-phosphonovaleric acid (APV). EC50s for Ca2+ current suppression were 29 nM for quisqualate, 2.3 muM for glutamate, and 13 muM 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 202-791 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 muM 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 muM inositol- 1,4,5-triphosphate (IP3), or by external application of 1 muM 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. The receptor was identified by its distinctive profile of effective agonists, and the results in cells dialyzed with guanine nucleotides indicated that the response was mediated by linkage to a G protein. The suppression of current was partly Ca2+ dependent, but as yet there is no evidence that the mechanism involved release of Ca2+ from IP3-sensitive intracellular stores.