Pharmacology
Endogenous opioids include endorphins, enkephalins, dynorphins, and even morphine itself. Morphine appears to mimic endorphins. Endorphins, a contraction of the term endogenous morphines, are responsible for analgesia (reducing pain), causing sleepiness, and feelings of pleasure. They can be released in response to pain, strenuous exercise, orgasm, or excitement.
Morphine is the prototype narcotic drug and is the standard against which all other opioids are tested. It interacts predominantly with the μ-opioid receptor. These μ-binding sites are discretely distributed in the human brain, with high densities in the posterior amygdala, hypothalamus, thalamus, nucleus caudatus, putamen, and certain cortical areas. They are also found on the terminal axons of primary afferents within laminae I and II (substantia gelatinosa) of the spinal cord and in the spinal nucleus of the trigeminal nerve.
Morphine is a phenanthrene opioid receptor agonist – its main effect is binding to and activating the μ-opioid receptors in the central nervous system. In clinical settings, morphine exerts its principal pharmacological effect on the central nervous system and gastrointestinal tract. Its primary actions of therapeutic value are analgesia and sedation. Activation of the μ-opioid receptors is associated with analgesia, sedation, euphoria, physical dependence, and respiratory depression. Morphine is a rapid-acting narcotic, and it is known to bind very strongly to the μ-opioid receptors, and for this reason, it often has a higher incidence of euphoria/dysphoria, respiratory depression, sedation, pruritus, tolerance, and physical and psychological dependence when compared to other opioids at equianalgesic doses. Morphine is also a κ-opioid and δ-opioid receptor agonist, κ-opioid's action is associated with spinal analgesia, miosis (pinpoint pupils) and psychotomimetic effects. δ-opioid is thought to play a role in analgesia. Although morphine does not bind to the σ-receptor, it has been shown that σ-agonists, such as (+)-pentazocine, antagonize morphine analgesia, and σ-antagonists enhance morphine analgesia, suggesting some interaction between morphine and the σ-opioid receptor.
The effects of morphine can be countered with opioid antagonists such as naloxone and naltrexone; the development of tolerance to morphine may be inhibited by NMDA antagonists such as ketamine or dextromethorphan. The rotation of morphine with chemically dissimilar opioids in the long-term treatment of pain will slow down the growth of tolerance in the longer run, particularly agents known to have significantly incomplete cross-tolerance with morphine such as levorphanol, ketobemidone, piritramide, and methadone and its derivatives; all of these drugs also have NMDA antagonist properties. It is believed that the strong opioid with the most incomplete cross-tolerance with morphine is either methadone or dextromoramide.
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