Introduction:
Opioids are a class of drugs that act as agonists at specific opioid receptors in the central nervous system (CNS) and periphery. They mimic the actions of endogenous opioid receptor ligands, such as enkephalins, endorphins, and dynorphins, resulting in the activation of pain-modulating systems.
Opioid Receptor Classification:
Opioid receptors are classified into three main types: mu (μ), kappa (κ), and delta (δ). Each type of receptor has distinct effects and functions in the body. Here’s a classification of opioid receptors and their effects:
| Opioid Receptor | Effects |
|---|---|
| Mu (μ) | – Analgesia (supraspinal, spinal) |
| – Euphoria | |
| – Depression of ventilation | |
| – Low abuse potential | |
| – Physical dependence | |
| – Miosis | |
| – Constipation (marked) | |
| – Bradycardia | |
| – Hypothermia | |
| – Urinary retention | |
| – Diuresis | |
| Kappa (κ) | – Analgesia (supraspinal, spinal) |
| – Dysphoria | |
| – Sedation | |
| – Low abuse potential | |
| – Physical dependence | |
| – Miosis | |
| – Constipation (minimal) | |
| – Urinary retention | |
| Delta (δ) | – Analgesia (supraspinal, spinal) |
Opioid Receptors and Their Locations:
Opioid receptors are found in specific regions of the CNS and periphery. Here are some key locations where opioid receptors are predominantly located:
- In the CNS: Opioid receptors are primarily found in the periaqueductal gray, locus ceruleus, and the rostral ventral medulla in the brain. In the spinal cord, they are present on both interneurons and primary afferent neurons in the dorsal horn.
- In the periphery: Opioid receptors are also found on sensory neurons and immune cells. Immune cells at sites of inflammation can secrete opioid peptides, providing local analgesia.
Conclusion:
Opioids act as agonists at specific opioid receptors in the CNS and periphery, leading to the activation of pain-modulating systems.
