Effects of Systemic Morphine on Monkeys and Man: Generalized Suppression of Behavior and Preferential Inhibition of Pain Elicited by Unmyelinated Nociceptors
After years of extensive usage, morphine is still regarded as the most powerful pharmacological tool for control of pain. Despite this long record of success in the clinics, it is clear that morphine 1s not am analgesic, in that pain is not obliterated at systemic dosages that leave respiration intact in man (Javert and Hardy, l951). Clinical patients report that pain can be elicited 1n the presence of morphine. Also, the effects of morphine on psychophysical ratings of phas1cally elicited pain are subtle in comparison with subjective estimates of clinical effectiveness (Beecher, 1957). This disparity of clinical and experimental findings has led to a number of hypotheses concerning the primary actions of morphine: (a) Morphine has been claimed to be an anxfolyt1o, and its, action has been likened to the effects of frontal lobotomy, which is said to attenuate the emotional reactions to pain without disturbing the primary sensations of pain (Freeman and Watts, 1948). This explanation suggests that modifications of pain experiences occur by actions of morphine at telenoephal1o sites rather than via the brain stem — spinal cord circuitry that has been emphasized 1n recent attempts to understand central mechanisms of opiate hypalgesfa (Yaksh, 1981). (b) Morphine has been thought to exert preferential effects on chronic pain, as distinct from phasically elicited pain (Beecher° 1957). Clear definitions of the crucial, distinguishing features of chronic pain or of morphine’s effects on pain are not available, but there are a number of possibilities.
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