Opioid peptides, opioid receptors and mechanism of down regulation.

Biogenesis of various endogenous opioid peptides, anatomical distribution and the characteristics of multiple receptors with which they interact provides an opportunity for understanding the role of opioid systems and mechanism of opioid tolerance. Cellular and anatomical distribution of opioid receptor and their function is important for identification of neuronal systems and local network involved in initiation of drug action and subsequent development of adaptations resulting from repeated drug use. The details concerning discovery and progress in endogenous opioid peptide research and their distribution in brain have been described in this review. This review also describes opioid receptors, their distribution and mechanism of down regulation, which may be one of the causes for tolerance to opioids. Agonist induced down regulation and recent evidence for involvement of ubiquitin/proteasome system in this process has been discussed.