CXCR4, a chemokine receptor constitutively expressed in the brain, binds both ligands, the chemokine SDF-1alpha and the HIV envelope glycoprotein gp120(IIIB). There seem to be intracellular differences between the neuronal apoptosis induced by SDF-1alpha and that induced by gp120(IIIB), but the apoptotic pathways involved have not been compared in human neuronal cells. In this study, we characterized the apoptotic intracellular pathways activated by neurotoxic concentrations of SDF-1alpha and gp120(IIIB) in human neuroblastoma cells SK-N-SH. SDF-1alpha (10 nM) and gp120(IIIB) (2 nM) induced similar levels of apoptosis after 24 h of incubation (49 +/- 4% and 48 +/- 3%, respectively, of the neurons were apoptotic). SDF1alpha-induced apoptosis was completely abolished by the inhibition of Src phosphorylation by PP2. Exposure to SDF-1alpha (10 nM) triggered an increase in Src phosphorylation, with a maximum after 20 min of incubation (1.80 +/- 0.24 times higher than control, P = 0.01). NMDA calcium flux was enhanced only if cells were incubated with SDF-1alpha for 20 min before applying NMDA. By contrast, gp120(IIIB)-induced apoptosis was not affected by the inhibition of Src phosphorylation. Moreover, gp120(IIIB) enhanced NMDA calcium flux immediately, without modifying Src phosphorylation status. Finally, levels of phospho-JNK increased following exposure to gp120(IIIB) (by a factor of 1.46 +/- 0.4 at 120 min, P = 0.03), but not after exposure to SDF-1alpha. Thus, SDF-1alpha and gp120(IIIB) induced a similar level of neuronal apoptosis, but by activating different intracellular pathways. SDF-1alpha enhanced NMDA activity indirectly via Src phosphorylation, whereas gp120(IIIB) probably activated the NMDA receptor directly and phosphorylated JNK.