Abstract

Opioid receptors, including mu (MOR for mu-opioid receptor), delta (DOR for delta-opioid receptor) and kappa (KOR for kappaopioid receptor), belong to the super family of G-protein coupled receptors (GPCRs). Those receptors, and particularly the MOR, are involved in pain control and are the targets of numerous drugs including morphine and its derivatives. The different opioid receptors are widely and differentially distributed throughout the human central nervous system and peripheral tissues [1]. Molecular studies revealed that MOR and DOR are highly expressed in brain regions with subtle differences; DOR are found at higher levels compared to MOR in cerebral cortex, putamen, nucleus accumbens, caudate nucleus, temporal lobe, and hippocampus. The cerebral distribution of DOR is in agreement with their involvement in motor as well as in cognitive functions. In contrast, higher levels of MOR are measured in cerebellum, the spinal cord and dorsal root ganglia suggesting that they may play an important role in the control of nociception. KOR are expressed in different brain regions although at moderate amounts and are implicated