The Dopaminergic System in the Ventral Tegmental Area Contributes to Morphine Analgesia and Tolerance.

Morphine has a strong analgesic effect and is suitable for various types of pain, so it is widely used. But long-term usage of morphine can lead to drug tolerance, which limits its clinical application. The complex mechanisms underlying the development of morphine analgesia into tolerance involve multiple nuclei in the brain. Recent studies reveal the signaling at the cellular and molecular levels as well as neural circuits contributing to morphine analgesia and tolerance in the ventral tegmental area (VTA), which is traditionally considered a critical center of opioid reward and addiction. Existing studies show that dopamine receptors and μ-opioid receptors participate in morphine tolerance through the altered activities of dopaminergic and/or non-dopaminergic neurons in the VTA. Several neural circuits related to the VTA are also involved in the regulation of morphine analgesia and the development of drug tolerance. Reviewing specific cellular and molecular targets and related neural circuits may provide novel precautionary strategies for morphine tolerance.