The opiates have a wide range of actions. The opiates remain the most widely used drugs for the relief of moderate to severe pain (Fig. 1). Their actions on pain perception are unique. Rather than blocking the transmission of pain impulses into the central nervous system, like local anesthetics, or working on the sensitization of pain fibers, like the antiinflammatory drugs, opiates relieve the ''suffering component'' of pain. Patients often report that the pain is still there, but it just does not hurt anymore. This ability to relieve the pain without interfering with other aspects of sensation provides a major advantage. Furthermore, opiates do not display a ''ceiling effect,'' implying that even very severe pain can be successfully relieved at sufficiently high drug doses. Unfortunately, side effects become increasingly troublesome as the dose is increased and the ability of the patient to tolerate these unwanted actions may interfere with the ability to administer adequate doses of the drug.
The actions of the opiates are not limited to pain. The additional actions most often encountered when treating patients are constipation, sedation, and respiratory depression. All these actions are mediated through opioid receptors and can be reversed by opioid antagonists, although evidence is mounting that they may be produced by different opioid receptor subtypes. Opiates influence gastrointestinal transit both centrally and peripherally. Although this action can be troublesome in pain management, it is valuable in the treatment of conditions associated with increased gastrointestinal motility, such as diarrhea.
Respiratory depression is another important opioid action. Opiates such as morphine depress respiratory
Methadone Figure 1
Structures of selected opiates.
Methadone Figure 1
depression in a dose-dependent manner and at sufficiently high doses breathing stops. The potential of respiratory arrest is a concern with very high doses of the drugs in naive patients, but typically it is not an issue with patients chronically on the drugs due to the development of tolerance to the action.
Sedation is also a common effect of morphine and related drugs. Indeed, morphine was initially named for Morpheus, the god of sleep. Like other opioid actions, tolerance will develop to sedation, but it may develop more slowly and to a lesser degree than tolerance to the analgesic actions of the drug. Opioids have many other actions. Their effects on the endocrine system are extensive and recent studies have also implicated them in immune function.
Morphine and related opiates produce their actions by activating receptors in the brain, mimicking a family of peptides with similar actions that are naturally occurring within the nervous system, termed the endorphins (Table I). The concept of peptide neurotransmitters has expanded dramatically, the endorphins are now only one family of many active endogenous peptides. This article discusses the four major groupings of opioid and opioid-like peptides and their receptors and also their physiological and pharmacological importance.
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