A psychophysical analysis of morphine analgesia

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Abstract
I.v. administration of 0.04-0.08 mg/kg morphine sulfate reduced both sensory intensity and unpleasantness visual analog scale (VAS) responses to graded 5 s nociceptive temperature stimuli (45-51.degree. C) in a dose-dependent manner. The lower doses of morphine (0.04 and 0.06 mg/kg) resulted in statistically reliable reductions in affective but not sensory intensity VAS responses, possibly reflecting supraspinal effects on brain regions involved in affect and motivation. The highest dose of morphine tested (0.08 mg/kg) reduced both sensory and affective VAS responses to graded nociceptive stimuli as well as VAS sensory responses to 1st and 2nd pain evoked by brief heat pulses. Morphine also had an especially potent inhibitory effect on temporal summation of 2nd pain that occurs when intense nociceptive stimuli occur at rates > 0.3/s. Current hypotheses about neural mechanisms of narcotic analgesia are supported. The relative effects of morphine on sensory and affective dimensions of experimental pain are clarified. The derived morphine dose-analgesic response functions also provide a reference standard for quantitatively comparing magnitudes of different CNS-mediated forms of analgesia.