Regulation of gene expression is one of the mechanisms by which neuronal activity elicits long-term changes in neuronal phenotype and function. Although activity-dependent induction of immediate-early genes has been extensively studied, much less is known about the late-response genes. We have investigated the activity-dependent regulation of delta-opioid receptor (DOR) mRNA levels in NG108-15 cells. Transsynaptic activation was mimicked by depolarization with 55 mM KCl or veratridine. Both treatments lead to a time-dependent increase of DOR mRNA levels. Ca2+ entry through L-type voltage-dependent Ca2+ channels activated by depolarization appears to be involved, because L-type channel blockers reduced the induction of DOR expression. Ca2+ binding to calmodulin is the next step in the signal transduction pathway, because a calmodulin antagonist, W7, reduced the effect of veratridine. A selective inhibitor of calmodulin kinases (KN-62) and cyclosporin, an inhibitor of calcineurin, also antagonized the depolarization-induced increase in DOR mRNA levels, which indicates that both calcium/calmodulin-dependent enzymes are involved in the activity-dependent induction of DOR gene expression. Induction of DOR gene expression by an activity-dependent increase in intracellular Ca2+ concentration may serve as a feedback regulatory mechanism because activation of DOR leads to hyperpolarization and lower excitability of neurons.