Effect of beta-estradiol on voltage-gated Ca(2+) channels in rat hippocampal neurons: a comparison with dehydroepiandrosterone.

We investigated the effects of beta-estradiol, dehydroepiandrosterone and dehydroepiandrosterone sulfate on intracellular calcium concentration (Ca(2+)) increases induced by gamma-aminobutyric acid (GABA), high K(+) and N-methyl-D-aspartate acid (NMDA) in cultured hippocampal neurons. Acute treatment with beta-estradiol, dehydroepiandrosterone and dehydroepiandrosterone sulfate inhibited the GABA-induced Ca(2+) increases to the similar extent. Tamoxifen, an estrogen receptor antagonist, did not block the inhibitory effects of beta-estradiol. On the other hand, GABA type A (GABA(A)) receptor antagonists, picrotoxin and bicuculline, blocked the GABA-induced Ca(2+) increases. Previously, we demonstrated that GABA- and high K(+)-induced Ca(2+) increases were commonly mediated by voltage-gated calcium channels (VGCCs). Therefore, we examined the effects of these steroids on the high K(+)-induced Ca(2+) increases. The inhibitory effect of beta-estradiol on the high K(+)-induced Ca(2+) increases was much greater than that of dehydroepiandrosterone and dehydroepiandrosterone sulfate. beta-Estradiol inhibited the NMDA-induced Ca(2+) increases with an IC(50) of 51.8 microM and NMDA responses were reduced to half in the presence of 10 micro M nifedipine, indicating that the NMDA-induced Ca(2+) increases also involved VGCCs. Further, we examined the inhibitory effect of beta-estradiol on the high K(+)-induced Ca(2+) increases in the presence of a N-type VGCCs antagonist, 1 microM omega-conotoxin, or a L-type VGCCs antagonist, 10 microM nifedipine. The IC(50) value of beta-estradiol alone (45.5 microM) was similar to that of omega-conotoxin (33.1 microM), while the value combined with nifedipine was reduced to 2.2 microM. beta-Estradiol also abolished the positive modulatory effect of L-type VGCCs agonist, 1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)phenyl]pyridine-3-carboxylic acid methyl ester (Bay K 8644). Our results showed that the inhibitory mechanism of beta-estradiol is different from that of dehydroepiandrosterone and dehydroepiandrosterone sulfate and beta-estradiol may act primarily at L-type VGCCs.