Retinoic acid (RA) induced differentiation of SH-SY5Y cells increases the expression of mu opioid receptors (HMOR) and inhibitory G proteins, as well as the efficacy of opioids to inhibit forskolin-induced adenylyl cyclase activity. We examined the time course of the effects of all-trans retinoic acid (RA) on HMOR and c-fos mRNA levels as determined by solution hybridization (using HMOR and rat c-fos riboprobes) in RNA extracts from SH-SY5Y cells. Electrophoretic Mobility Shift Assay (EMSA) and Western blot analysis were used to assess the changes AP-1 DNA binding and the presence of fos-related proteins in nuclear extracts from untreated, vehicle (ethanol) or RA-treated SH-SY5Y cells. Exposure to RA for 0.5 h had no effect on HMOR while after 6-18 h of exposure HMOR in mRNA levels were decreased by 50% and then after 168 h of RA exposure, HMOR mRNA levels were doubled. In contrast, c-fos mRNA levels were unchanged at 0.5 h, but increased by 50% after 18 and 168 h of RA exposure. RA increased AP-1 binding after 18 and 168 h and a pan fos-FRA antibody produced a supershift. Western analysis indicates that RA activates a 45-kDa protein corresponding to the size of the fos B protein. These results identify two signal transduction targets that are regulated by RA during differentiation.