ROLE OF OPIOIDS IN PAIN MANAGEMENT
Opioids have been used to control pain for centuries. They are extracted from the opium poppy plant, Papaver somniferum. From the substance extracted, roughly 9% to 14% is morphine and 0.8% to 2.5% is codeine.7 Opioids are used to treat many symptoms and ailments, including diarrhea, moderate to severe pain, and persistent cough.
Opioids work through receptors in the CNS, including mu, kappa, and delta opioid receptors and the opioid-like receptor nociceptin.7 The principal receptors associated with pain physiology and inhibition are mu and kappa. (Morphine, the gold standard for treating severe pain, is an opioid agonist that binds to mu and kappa receptors.)
Most opioids that are used clinically bind to mu receptors; these drugs provide analgesia but also present the risk for adverse effects, such as decreased respiratory drive, miosis, and decreased motor function of the gastrointestinal (GI) tract, which can lead to constipation.8 Because mu receptors are located mainly in the brain and spinal cord (as well as the GI tract), opioids also produce a feeling of euphoria that can lead to dependence.
Kappa receptors, in contrast, are located mainly in the limbic system, diencephalic area, and spinal cord. When these receptors are activated, they can produce spinal analgesia, dyspnea, dependence, and dysphoria.
Delta receptors also play a role in pain management and are associated with emotional and affective components of the experience of pain.7 They are largely located in the brain; when activated, they can lead to spinal and supraspinal anesthesia, as well as decreased gastric motility.8 Delta receptors have not been studied as much as mu and kappa receptors, but it has been suggested that they play a role in psychologic dependency.
Depending on the effect that a drug has on these receptors, it can be considered a full (or pure) opioid agonist, a partial agonist, or a mixed agonist–antagonist. By binding to opioid receptors, opioid agonists provide pain relief. Health care providers often prescribe a full agonist, such as morphine, hydrocodone, codeine, or oxycodone, to treat pain. Partial agonists, such as buprenorphine and butorphanol, often decrease activity at mu receptor sites. Mixed agonist–antagonists either block or bind opioids at receptor sites.9 Medications that block mu and kappa receptors are considered opioid antagonists, which are used not to treat pain but rather to reverse the effect of opioids (eg, naloxone).6
Table 1 lists commonly used opioids, their analgesic duration, and the standard approved dosages.10
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