Steroid potentiation and inhibition of N-methyl-D-aspartate receptor-mediated intracellular Ca++ responses: structure-activity studies.

Pregnenolone sulfate and 15 related steroids were investigated for their effects on N-methyl-D-aspartate (NMDA)-induced elevations in intracellular Ca++ ([Ca++]i) in cultured rat hippocampal neurons by microspectrofluorimetry with the Ca(++)-sensitive indicator fura-2. Several pregn-5-ene steroids markedly potentiated NMDA-mediated [Ca++]i responses. Pregnenolone sulfate and its 21-acetoxy derivative and pregnenolone hemisuccinate were the most active. At a concentration of 50 microM, each produced approximately 300% potentiation of 5 microM NMDA responses. In addition, several steroids were identified that inhibited NMDA-induced elevations in [Ca++]i, the most potent of which was 3 alpha-hydroxy-5 beta-pregnan-20-one sulfate (IC50, 37 microM). Concentration-response curves for NMDA in the presence of active steroids revealed noncompetitive interaction(s) of these steroids with the NMDA receptor. Although the mechanism(s) responsible for either steroid-induced augmentation or inhibition of NMDA-receptor responses is unknown, these data suggest the presence of one or more steroid recognition sites with a high degree of structural specificity associated with NMDA receptors. These results further raise the possibility that pregn-5-ene 3-sulfates and pregnane 3-sulfates could be endogenous modulators of NMDA receptor-mediated synaptic events.