How does methadone primarily exert its effects?

Methadone primarily exerts its effects by acting on mu-opioid receptors, which are part of the body's natural pain management system. These receptors, when activated by opioids like methadone, lead to an increase in pain relief and can also produce feelings of euphoria. This action not only helps manage chronic pain but also plays a crucial role in treating opioid use disorder. Methadone's effective binding to mu-opioid receptors allows it to help minimize withdrawal symptoms and cravings in those recovering from addiction.

While other options mention different mechanisms, they do not specifically describe the primary action of methadone. For instance, competitive antagonists would block receptor activity rather than promote it, and effects on GABA pathways or serotonin reuptake are not the main mechanisms through which methadone operates. Therefore, the key mechanism is through its action on mu-opioid receptors, making this the correct answer.