Molecular biology of opioid analgesia.

Opioids provide excellent pain relief in most patients. Yet the responses of patients to individual opioids can vary markedly, even among the mu opioids. Understanding this variability would greatly enhance our ability to treat patients appropriately. Classical pharmacological studies have long implied the existence of multiple subtypes of mu opioid receptors. More recently, a number of variants of the cloned mu opioid receptor have been described. These variants all show the same selectivity for mu opioids, confirming their classification as mu opioid receptors. Yet, they differ in their functional activation by opioids as well as in their localization within cells and regions in the brain. These multiple mu opioid receptors may help explain the range of responses seen clinically among patients for the various opioid drugs.