||Evidences showed that increased enkephalin binding site and delta opioid receptor mRNA expression were found in a cloned rat pheochromocytoma PC12h cell after NGF treatment. Since PC12 cells will undergo a phenotypic transition from chromaffin-like cell to neurite-bearing neuronal cell after NGF treatment, the cell line is thought to be a good in vitro model for studying the NGF-induced neuronal differentiation. We therefore set-up experiments to study the role of opioid receptor in NGF-induced neuronal differentiation. Our data indicated that the mRNA of delta, but not mu or kappa opioid receptor was elevated by NGF treatment in a dose-response and time-dependent manner. The mRNAs of delta opioid receptor reached to the highest level on day 5 post NGF treatment. In addition, a protein kinase C inhibitor, H7, could partially block the elevated mRNA levels of delta opioid receptor after NGF treatment. This result indicated a protein kinase C might involve in the pathway which transduce the NGF signal to target gene, the delta opioid receptor gene. To investigate the role of opioid receptor in neuronal differentiation, we analyzed the effect of opioid agonist (morphine) or antagonist (naloxone) on NGF induced PC12 cell differentiation. Our results showed that naloxone combined with NGF could increase the ratio of NGF-induced differentiated cell (P<0.05) after 10 days treatment. And this effect could be reversed by the addition of morphine, indicating that this effect may be through the opioid receptor, and possibly the delta-type receptor. However, naloxone combined with NGF did not affect the NGF-induced neurite outgrowth and total cell number after 10 days treatment. Furthermore, our data also showed that the addition of morphine or naloxone combined with NGF could decrease PC12 apoptosis after 10 days in virto culture.