The word narcotics (or opiates) refers to drugs that act on specific receptors in the central nervous system (CNS) to reduce perception of pain. In general, they do not eliminate pain, but the patient is not as bothered by the pain. They act on three major classes of receptors in the CNS, called opioid receptors and designated mu (μ), kappa (κ), and delta (δ). Most of the actions of the narcotic analgesics are mediated by the μ receptor. Some actions are mediated through the κ and δ receptors.
Divide the narcotics into four groups:
Agonists; use morphine as the prototype
Weak agonist/reuptake inhibitors
A partial listing of drugs in this class is included in the following table:
|Agonists ||Weak Agonists/Reuptake Inhibitors ||Mixed Agonist-Antagonists ||Antagonists |
| ||nalbuphine ||methylnaltrexone |
The most important drug names in this class are in capital letters in the table. Notice that there are many more agonists than antagonists and that there is only one important mixed agonist-antagonist.
Remember the names of the antagonists (NALOXONE and NALTREXONE) and the most important mixed agonist-antagonist (PENTAZOCINE). Everything else is an agonist.
Of course, that statement was somewhat simplified. Use morphine as the prototype drug in this class. The other agonists have the same general properties. They vary in things like potency and duration of action.
ACTIONS OF MORPHINE AND THE OTHER AGONISTS
Spasm of smooth muscle of the gastrointestinal (GI) and genitourinary (GU) tracts, including the biliary tract
Morphine has actions in many organ systems. These actions are sometimes used for therapeutic purposes and sometimes are considered side effects. Therefore, learning the important actions means that you have learned both therapeutic uses and adverse effects at one time.
CNS: In most people, morphine produces drowsiness and sedation in addition to the reduction in awareness of pain. Initial doses of morphine often cause nausea, more often in ambulatory patients than in those who are bedridden. This effect is due to direct stimulation of the chemoreceptor trigger zone in the medulla oblongata and to an increase in vestibular sensitivity.
Morphine is an effective cough suppressant because it has a direct effect in the medulla. Morphine is not used for this purpose, but codeine (another agonist) is frequently prescribed for its cough suppressive action.
EYE: Morphine produces pupillary constriction by a direct action in the brain nucleus of the oculomotor nerve (Edinger–Westphal). This is the classic pinpoint pupil that you will hear mentioned in the emergency room.