Chapter 180

The term opioids refers broadly to all compounds related to opium that possess analgesic and sedative properties. The term narcotic refers to a broader group of agents and is predominantly used by law enforcement to designate a variety of controlled substances with abuse or addictive potential. Because narcotic is not descriptive of a unique drug class, use of this term in the practice of medicine is discouraged.

Opioid abuse is a significant public health issue in the U.S. Between 2004 and 2006, ED visits associated with opioid abuse or misuse increased 43%.1 The opioids most commonly reported to be involved are, in order of frequency, heroin, oxycodone, hydrocodone, and methadone.1 Between 1999 and 2002, there was a 91% increase in the number of deaths attributed to poisoning from prescription opioids.2 The majority of prescription opioid overdose deaths were associated with diversion, doctor shopping, and nonmedical use of these drugs.3 During 2008, the American Association of Poison Control Centers received reports of 126,456 exposures to opioids; 84,861 were multidrug exposures that included opioids and 41,595 were single-drug exposures to an opioid. There were 401 fatalities that mentioned an opioid and 66 of these fatalities associated with only an opioid.4

Opioids modulate nociception in the terminals of afferent nerves in the central nervous system, peripheral nervous system, and GI tract. Opioids are agonists at the three primary opioid receptors: μ (mu), κ (kappa), and δ (delta). Opioid receptors are similar to other G protein–coupled receptors in that they are transmembrane proteins that undergo conformational change when activated by external molecules, and this change then alters some aspect of intracellular function. Opioid receptors vary widely in morphology and distribution. Also, the specificity and affinity of an opioid for a particular receptor is variable. For example, tramadol possesses 1/6000 the affinity of morphine at the μ-receptor site.

Stimulation of the μ-receptors results in analgesia, sedation, miosis, respiratory depression, cough suppression, euphoria, and decreased GI motility. Stimulation of κ-receptors results in weaker analgesia, sedation, miosis, decreased intestinal motility, dysphoria, and hallucinations. Stimulation of the δ-receptors results in analgesia, although less than does stimulation of the μ-receptors, and produces some antidepressant effect, but the clinical role of δ-receptor stimulation is largely unknown. All currently available opioid agonists have activity at the μ-receptor and result in some degree of respiratory depression.

There is interplay between opioid receptors and other transmembrane receptors found in the nervous system. One example is that opioid binding to μ-receptors in the nucleus accumbens results in the localized release of dopamine (the “dopamine pleasure pathway”). A second example is that the analgesic effect of morphine is enhanced in the presence of N-methyl-d-aspartate receptor blockers such as amantadine. A third example is the induction of mast cell histamine release by morphine and meperidine.

Opioids can be categorized as naturally occurring compounds (termed opiates), chemical modifications of natural compounds ...

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