These are designated by the Greek letters µ (mu), κ (kappa), and δ (delta) All 3 receptor subtypes appear to be involved in antinociceptive and analgesic mechanisms at both spinal and supraspinal levels. The μ-receptor activation plays a major role in the respiratory depressant actions of opioids and together with κ-receptor activation slows gastrointestinal transit; κ-receptor activation also appears to be involved in sedative actions; δ-receptor activation may play a role in the development of tolerance.