Opioid Tolerance in Adult and Neonatal Rats

One of the main actions of natural and synthetic opioids is powerful analgesia. For many decades, opioid-based drugs, such as morphine, remain to be the most effective analgesics and are widely used in current clinical management of various pain conditions. They are particularly required to chronically treat severe and persistent pain in patients suffering from chronic diseases, such as cancer. Repeated use of opioid drugs, however, induces the development of opioid tolerance that reduces their analgesic potency so that increasing doses are required to maintain the desired level of analgesia. As opioids also have undesirable and even dangerous side effects at higher doses, such as respiratory depression, opioid tolerance is, therefore, a major problem in clinical management of pain.