Opioid drugs act primarily on which receptors of the central nervous system?

1. Opioids include heroin, morphine and codeine. 2. They bind specific opioid receptors present principally in the CNS and gastrointestinal tract. 3. Activation gives analgesia (pain relief) and can produce sedation and euphoria. 4. Chronic use leads to tolerance and severe dependence. _Source: syllabus Biology, Ch 7 "Human Health and Disease", §7.5_