Post-transcriptional regulation of opioid receptors in the nervous system

Three types of opioid receptors exist in the animals, each is encoded by a single gene, i.e., the mu opioid receptor gene, the delta opioid receptor gene, and the kappa opioid receptor gene. However, each opioid receptor gene produces multiple mRNA variants as a result of alternative promoter usages, splicing and/or polyadenylation. As such, a large reservoir of regulatory events has evolved for the control of the production of mRNA variants or differentially modified proteins from each opioid receptor gene. This review focuses on post-transcriptional events for the regulation of opioid receptor expression or activities, including alternative splicing, mRNA stability, translation, RNA polyadenylation, RNA transport, and covalent modification of the receptors. Variation at the mRNA level contributes, primarily, to the control of spatial and temporal expression of these receptors in different parts of neurons; whereas modification at the receptors is the key for controlling the duration and amplitude of signals generated from these receptors.