ABSTRACT
The activities of the opioid receptors appear to be dependent on the receptor density on the cell surface. Though the general principle of ‘‘spare’’ receptor applies in the opioid receptor regulation of the second-messenger system, the agonist potency, and the effector it regulates are receptor density dependent [1,2]. The importance of opioid receptor density in the pharmacological actions of this drug class is supported by the established relationship between tissue (cell type)-specific expression patterns of a receptor type and the response to opioids, which is ultimately dependent upon the spatial and temporal regulation of gene expression. In addition, opioid receptor density on the cell surface can affect the rate of receptor desensitization [3-5], which is key to the development of drug tolerance and dependence [6]. Thus, the control of the expression of the opioid receptor will determine the opioid agonist activities and may modulate tolerance development.
