PHYSIOLOGY

Physiology

Physiology

Question 1

Morphine is the principal alkaloid of opium, and is used for the control of moderate to severe pain. The word opium itself is derived from the Greek name for juice, the drug being obtained from the juice of the poppy, Papaver somniferum. Opium contains more than 20 distinct alkaloids. In 1806, Setürner reported the isolation of an opium alkaloid that he named morphine after Morpheus, the Greek god of dreams.

Morphine and other morphinelike drugs (heroin, codeine, or methadone) produce analgesia primarily through their interaction with opioid receptors located in the central nervous system and periphery. The existence of multiple opioid receptors was proposed in 1977 and confirmed by various studies. Soon after the demonstration of these opioid receptors, three classes of endogenous opioid peptides were isolated and identified: the enkephalins, the endorphins, and the dynorphins (Gutstein & Akil, 2001).

Although there are now many compounds with pharmacological properties similar to those produced by morphine, this “old” drug remains the most useful in clinical settings. However, in spite of its efficacy, morphine treatment has some associated problems. Side effects such as nausea, vomiting, constipation, drowsiness, confusion, and the variability in analgesic response between patients are common clinical problems during morphine therapy, with respiratory depression being a less frequent but more serious side effect (Martindale, 2002). Although the development of dependence and/or tolerance is not generally a problem when morphine is used in patients with opioid-sensitive pain, the possibility of tolerance, dependence, and addiction with long-term use may complicate its clinical use and can create barriers to its adequate prescription (Schafer et al., 2001). In fact, the development of tolerance, physical dependence, and addiction with repeated use is a characteristic feature of all the opioid drugs. Tolerance can be defined as a loss of potency of a drug after its repeated administration, so that doses have to be increased to achieve the same effect. Drug addiction of the morphine type is a state arising from repeated administration of morphine or morphinelike drugs (heroin, pethidine, etc.); it is characterized by an overwhelming need to continue taking the drug or one with similar properties. Abrupt withdrawal of morphine or morphinelike drugs from persons physically dependent on them precipitates a withdrawal syndrome (Martindale, 2002). However, when morphine and morphinelike drugs are used correctly in appropriate doses to treat morphine-sensitive pain, tolerance, dependence, and severe side effects are not a clinical problem (McQuay, 1989). What happens when morphine or opioids are given to someone in pain is different from what happens when they are given to someone not in pain (McQuay, 1999). In patients, tolerance may be due to a disease progression or increased nociception, and they may develop a drug-seeking behavior that represents an attempt to get appropriate pain relief (Schug, Merry, & Acland, 1991).

During morphine treatment, common side effects such as nausea, constipation, and drowsiness are usually controlled by appropriate measures. What complicates morphine use in the clinical setting is the variability between patients in their pharmacological ...