Analgesia results when opiates are microinjected into the rostral ventromedial medulla (RVM)1-3. This region, which includes the nucleus raphe magnus and the adjacent reticular formation, is rich in immunoreactive enkephalin-containing neurones and terminals4, and contains neurones that project to the spinal cord dorsal horn where they inhibit identified nociceptive spinothalamic tract neurones5-7. Although opiates have previously been reported either to excite or inhibit RVM cells8, the possibility of an opiate effect being consistent within a physiologically defined subclass has not been examined. Recently we described a class of neurone in the RVM (the off-cell) that abruptly pauses just before a heat-evoked tail-flick reflex9. If off-cells are made to fire continuously by direct electrical stimulation of the RVM, the tail-flick reflex does not occur. We report here that analgesic doses of morphine completely eliminate the pause in firing that precedes the tail-flick reflex. We propose that this disinhibition of off-cells in the RVM is a primary process contributing to opiate inhibition of nociceptor-induced reflexes.
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