A mitogen-activated protein kinase pathway is required for mu-opioid receptor desensitization.

The mu-opioid receptor mediates not only the beneficial painkilling effects of opiates like morphine but also the detrimental effects of chronic exposure such as tolerance and dependence. Different studies have linked tolerance to opioid receptor desensitization. Agonist activation of the mu-opioid receptor stimulates a mitogen-activated protein kinase (MAPK) activity, but the functional significance of this pathway remains unclear. We have focused on the MAPK signaling cascade to study mu-opioid receptor desensitization. We report that inhibition of the MAPK pathway blocks desensitization of mu-opioid receptor signaling as well as the loss of receptor density due to internalization. Our results suggest that a feedback signal emanating from the MAPK cascade is required for mu-opioid receptor desensitization.