Derivatives of the opium poppy have been used to relieve severe
pain for hundreds (possibly thousands) of years. Morphine, the prototypic opioid agonist,
does so with remarkable efficacy. This alkaloid (named after Morpheus,
the Greek god of dreams) is extracted from crude opium, which is
obtained from the opium poppy seed pod. Morphine remains the standard
against which all drugs that have strong analgesic action are compared
in terms of efficacy and potency. These drugs are collectively known
as opioid analgesics and include not only the natural opium alkaloids
and semisynthetic alkaloid derivatives from opium, but also synthetic
surrogates (opioid-like analgesic drugs whose actions are blocked
by the nonselective antagonist naloxone),
and endogenous peptides that interact with several opioid receptor
subtypes. Opioid analgesics are characterized by their ability to
relieve moderate to severe pain. Many opioids also have useful antitussive
or antidiarrheal effects.
Analgesic drug therapy and many physical therapy interventions
are aimed at the same outcome: pain relief. For this reason, analgesic
drugs are among the most frequently prescribed medications for patients
receiving rehabilitative therapy. It is essential for physical therapists
to be familiar with the intended uses and common adverse effects
of opioid analgesics to effectively monitor patient outcomes. The
opioid medications are classified as controlled substances because
of their potential for abuse; therefore, an understanding of the
clinical signs of physical dependence and tolerance is also needed.
The major subclasses of opioid agonist and antagonist drugs are
outlined in Figure 20–1.
Major subclasses of opioid agonists and antagonists.
The other major group of analgesic drugs comprises aspirin and
the other nonsteroidal anti-inflammatory drugs (NSAIDs). The NSAIDs
have a significantly lower maximal efficacy than the opioids and
have no addiction liability. Therefore, they are not considered
narcotics and are available with an ordinary prescription or even
over-the-counter. The NSAIDs are discussed in Chapter 34.
Although they are not peptides, opioid alkaloids (e.g., morphine)
produce analgesia through actions in regions of the brain that contain
endogenous peptides which have opioid-like pharmacologic properties.
The general term currently used for these substances is endogenous
opioid peptides, which replaces the previous term endorphins. Three families of endogenous
opioid peptides have been described: enkephalins, dynorphans, and endorphins. These peptides are synthesized
in the soma of neurons and transported to the nerve endings where
they accumulate in synaptic vesicles and are released from nerve
terminals. Although chemically quite different from the opioid alkaloids,
these endogenous peptides bind to opioid receptors and modulate
transmission in the brain and spinal cord, and at the primary afferents.
It remains unclear whether these peptides function as classic neurotransmitters
or as modulator neuropeptides. Opioid peptides are also found in
the adrenal medulla and the neural plexus (enteric nervous system)
of the gut.
The opioid analgesics and related drugs ...