Final answer:
The major subtypes of opioid receptors are mu, delta, and kappa, which are involved in pain modulation and the effects of opioid medications.
Step-by-step explanation:
The major subtypes of opioid receptors include mu (μ), delta (δ), and kappa (κ) opioid receptors. These receptors are part of the endogenous opioid system which plays a role in pain modulation, stress response, and addictive behavior. Mu receptors are primarily responsible for opioid analgesic effects and are the target of many opioid medications. Delta receptors are implicated in modulating pain as well but have a less understood role compared to mu receptors. Kappa receptors are associated with the perception of pain and can produce dysphoria and hallucinations when activated.