In studies on interactions between the AAS and the MOP receptors, using SH-SY5Y neuroblastoma cells, nandrolone treatment was seen to induce a downregulation of this opioid receptor ( Guarino and Spampinato, 2008 ). The observed decline in the MOP receptor was found to be both time- and concentration dependent. In contrast to the nandrolone effect on the DOP receptor, this effect was abolished in the presence of an androgen receptor antagonist. The nandrolone -induced downregulation of the MOP receptor was confirmed by binding assays indicating a significant decrease in the MOP receptor protein in nandrolone -treated cells. Further treatment of the cells including a transcription inhibitor and transfection studies with a MOP receptor luciferase promoter/reporter construct suggests that nandrolone may regulate the expression of the MOP receptor through posttranscriptional mechanisms that requires the androgen receptor ( Guarino and Spampinato, 2008 ).