Mu-opioid receptors are a type of protein found in the brain and other parts of the body. They play a crucial role in the body's response to pain and are involved in the regulation of mood, stress, and reward. When activated by natural substances like endorphins or synthetic drugs such as morphine, these receptors can produce feelings of pain relief and euphoria. These receptors are part of the larger opioid receptor family, which also includes delta and kappa receptors. Activation of mu-opioid receptors can lead to various effects, including sedation, respiratory depression, and potential for addiction, making them significant in both pain management and substance use disorders.