The opioid receptor subtype mainly involved in mediating analgesic effects is the Mu (µ) receptor.

- Opioid receptors are a group of G-protein-coupled receptors that mediate the effects of endogenous and exogenous opioids.
- There are several subtypes, including Delta (δ), Kappa (κ), Mu (µ), and Nociceptin (ORL-1) receptors. Among these, the Mu (µ) receptor plays the primary role in producing analgesia or pain relief.

- Activation of the Mu receptor leads to potent analgesic effects by inhibiting neurotransmitter release in the pain pathways of the central nervous system, particularly in areas such as the spinal cord, thalamus, and brainstem.
- This receptor is also responsible for many of the classical opioid effects such as euphoria, respiratory depression, and physical dependence.

- While Delta (δ) and Kappa (κ) receptors also contribute to analgesia, their effects are generally less potent or are associated with different side effect profiles.
- The Nociceptin (ORL-1) receptor is structurally related but does not mediate typical opioid analgesic effects and may modulate pain differently.

In summary, the Mu receptor is the primary opioid receptor responsible for mediating strong analgesic responses to opioid drugs like morphine, fentanyl, and oxycodone.

Reference: Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 13th Edition, Chapter 26, Page 528-530