Differential regulation by pregnenolone sulfate of intracellular Ca2 increase by amino acids in primary cultured rat cortical neurons

We investigated the effects of pregnenolone sulfate (PS) on the [Ca 2+ ]; increase induced by y-aminobutyric acid (GABA) and N-methyl-D-aspartate (NMDA) using fluorescence imaging. PS inhibited the 50μM GABA-induced increase in [Ca 2+ ], in a dose-dependent manner with an IC50 of 30 μM. The inhibitory effect of PS was apparent within 5min and was in a non-competitive manner, suggesting that PS may act directly to the membrane level but indirectly to the GABA binding sites. Our previous study has already shown that the GABA-induced Ca 2+ increase involves GABA A receptors and the similar pathway to a high K-induced Ca 2+ response (Takebayashi et al., 1996). Because 50 μM of PS could not inhibit a 25 mM K -induced Ca 2+ increase, it seems likely that the site of the inhibitory action of PS on the GABA-induced Ca 2 increase may be independent of the pathway of the high K + -induced Ca 2+ response, but rather at GABA A receptor complex. In contrast, PS potentiated the 50 μM NMDA-induced increase in [Ca 2+ ], in a dose-dependent manner. The magnitude of the NMDA response was approximately doubled in the presence of 100 PM of PS. However, PS did not affect the acetylcholine(Ach)-induced increase in [Ca 2+ ],. Furthermore, corticosterone had little effect on the GABA- and NMDA-induced Ca 2+ increases, indicating that the alteration of the Ca 2+ response is specific for PS. In conclusion, it is suggested that PS modulates differentially [Ca 2+ ] i increase induced by GABA and NMDA.