BACKGROUND: In the present study, we examined the effect of morphine on NO- and peroxynitrite-induced cell death using a human neuroblastoma SH-SY5Y cell line which abundantly expresses micro, delta and K-opioid receptors. METHODS: The cultured cells were pretreated with morphine (100 micrometer) and exposed to 3-morpholinosydnonimine (SIN-1, 1mM). Agarose gel electrophoresis of DNA was done with the extracts from SH-SY5Y cells. The cells were treated with selective ligands for opioid receptor subtypes and with PI3-kinase inhibitors. Cell damage was assessed by using an MTT assay. Spectrophotometric absorption spectra were measured from the mixture of morphine (100 micrometer) plus peroxynitrite (1 mM) at room temperature. RESULTS: SIN-1 treated cells showed the occurrence of a specific form of chromosomal DNA fragmentation which pretreatment with morphine inhibited. The selective ligands for opioid receptor subtypes, [D-Ala2, N-Me-Phe4, Gly-ol5]enkephalin (DAMGO, micro-opioid receptor agonist), [D-Pen2,5] enkephalin (DPDPE, delta-opioid receptor agonist) and U-69593 (K-opioid receptor agonist) at a concentration of 10 micrometer did not prevent the cell death induced by SIN-1. Naloxone (20 micrometer) hardly antagonized the effect of morphine in SIN-1-induced cell death. The PI3-kinase inhibitors Wortmannin and LY294002 did not inhibit the action of morphine on apoptotic cell death. In the measurements of spectrophotometric absorption spectra, the peak of the absorbance of the mixture of morphine plus peroxynitrite at 295 300 nm disappeared three minutes after mixing. CONCLUSIONS: The present study showed that morphine protected the human neuroblastoma cell line,SH-SY5Y, from peroxynitrite-induced apoptotic cell death. However, it is suggested that the protective action of morphine is not via the activation of opioid receptors and/or the PI3-kinase pathway but possibly via direct chemical reaction.