Apoptosis plays a critical role in normal brain development and in a number of neurodegenerative diseases. Recently, opiates have been shown to promote apoptotic death of cells of the immune and nervous systems. In this study, we investigated the effect of morphine on apoptosis of primary human fetal microglial cell, astrocyte and neuronal cell cultures. Exposure of microglia and neurons to 10(-6) M morphine potently induced apoptosis of these brain cells (approximately fourfold increase above untreated control cells). In contrast to microglia and neurons, astrocytes were completely resistant to morphine-induced apoptosis. Concentration-response and time-course studies indicated that neurons were more sensitive than microglia to morphine's effect on apoptosis. Naloxone blocked morphine-induced apoptosis suggesting involvement of an opiate receptor mechanism. Potent inhibition (>70%) of apoptosis by an inhibitor of caspase-3 as well as co-localization of active caspase-3 and DNA fragmentation in microglia or neurons treated with morphine indicated that caspase-3 is involved in the execution phase of morphine-induced apoptosis. The results of these in vitro studies have implications regarding the potential effect of opiates on fetal brain development and on the course of certain neurodegenerative diseases.