The μ opioid receptor: From molecular cloning to functional studies

Opioids have been used and abused by humans for centuries. The μ opioid receptor represents the high affinity binding site for opioid narcotics with high abuse liability such as morphine, codeine and fentanyl. Heroin (diacetylmorphine), a semi-synthetic derivative of morphine, crosses the blood-brain barrier more readily than morphine due to its increased hydrophobicity. Once in the brain heroin is hydrolyzed to morphine, which acts at the μ opioid receptor and results in euphoria, thus conferring the reinforcing properties of heroin. Using molecular biology techniques, the μ opioid receptors from several species have been cloned. This article reviews recent progress in this area, with respect to the two major cellular functions of the μ opioid receptor: reduction of intracellular cAMP concentration by inhibiting adenylyl cyclase activity, and inhibition of neuronal firing by modulating membrane ion channels.