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Evidence that disinhibition of brain stem neurones contributes to morphine analgesia

Abstract

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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Fields, H., Vanegas, H., Hentall, I. et al. Evidence that disinhibition of brain stem neurones contributes to morphine analgesia. Nature 306, 684–686 (1983). https://doi.org/10.1038/306684a0

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