PREGNENOLONE SULFATE POTENTIATION OF N-METHYL-D-ASPARTATE RECEPTOR CHANNELS IN HIPPOCAMPAL-NEURONS

Many actions of the classical gonadal and adrenal steroid hormones are at the level of transcriptional regulation. Recent studies have shown , however, that endogenous brain metabolites of steroids exert importa nt nongenomic modulatory effects on neuronal mechanisms. Potentiation of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor by th e neurosteroid pregnenolone sulfate (PS) was studied using cultured hi ppocampal neurons and patch-clamp techniques The magnitude of NMDA-act ivated whole-cell currents was approximately doubled in the presence o f 100 mum PS. The dose-response curve of PS action showed significant potentiation above 250 nm and a half-maximal effect at approximately 2 9 mum. Maximum potentiation was reached within 25 sec, and the potenti ation was completely reversed with 60 sec of washout. The enhancement of the NMDA current is probably not due to activation of a new ionic c onductance, because the reversal potential of the I-V curve did not sh ift in the presence of PS. Potentiation is specific for the NMDA subty pe of glutamate receptor; non-NMDA currents showed only a slight inhib ition (approximately 6%) in the presence of 50 mum PS. Potentiation of the NMDA current by PS occurred in the presence of saturating concent rations of NMDA and glycine, indicating that at saturating concentrati ons of the coagonists PS does not change the affinity between the coag onists and the NMDA receptor. The dose-response relations for NMDA and glycine were shifted slightly to the left, and the percent potentiati on was significantly higher for lower concentrations of coagonists, su ggesting that at low concentrations of the coagonists PS may slightly increase their affinity for the NMDA receptor. The fractional open tim e (nP(o)) of single NMDA-activated channels was potentiated by PS in p atch-clamp recordings using both the outside-out and cell-attached con figurations. The potentiation of nP(o) resulted from increases in the frequency of opening and in the mean channel open time. No effect was seen on single-channel conductances.