In this study we investigated the effects of sigma receptor ligands and neurosteroids on bradykinin-induced intracellular calcium concentration ([Ca2+](i)) changes in SH-SY5Y neuroblastoma cells. [Ca2+](i) levels in cells loaded with fura-2 were monitored with dual-wavelength ratiometric fluorescence measurement. Submicromolar concentrations of bradykinin elicited [Ca2+](i) responses with a fast rise followed by a slow decline in these cells. Preincubation of low micromolar concentrations of the neurosteroids pregnenolone, dehydroepiandrosterone (DHEA), or the prototypic sigma (sigma) receptor agonist (+)pentazocine potentiated bradykinin-induced [Ca2+](i) changes in SH-SY5Y cells. The sigma receptor antagonist haloperidol blocked the enhancing effects on [Ca2+](i) by (+)pentazocine or pregnenolone. Progesterone did not significantly affect the basal [Ca2+](i) level or bradykinin-induced [Ca2+](i) changes in these cells. However, coincubation of progesterone with (+)pentazocine, pregnenolone, or DHEA reversed their potentiating effects. The antagonistic effects of haloperidol and progesterone on the potentiating effects of (+)pentazocine and pregnenolone suggested that these ligands might act through a common mechanism. We further showed that progesterone, pregnenolone, and DHEA competed for [H-3](+)pentazocine binding in SH-SY5Y cells with K-i values of 0.13 +/- 0.03 muM, 0.98 +/- 0.34 muM, and 5.2 +/- 1.4 muM, respectively. Thus, the modulation of bradykinin-induced [Ca2+](i) changes by neurosteroids in these cells is likely due to their actions on sigma receptors.
