Morphine suppression of neurotransmitter release evoked by sensory stimulation in vivo.

The effects of morphine and naloxone on the release of acetylcholine and amino acid neurotransmitters from sensorimotor cortex were studied employing an in vivo superfusion cannula. Morphine (20 mg/kg) reduced the spontaneous release of acetylcholine but had no detectable effect on the spontaneous release of amino acids. It also suppressed the release of acetylcholine and amino acid neurotransmitters evoked by sensory stimulation of the contralateral sensorimotor cortex via the brachial plexus. Naloxone (5 mg/kg) prevented all of these inhibitory actions of morphine. Naloxone also caused a significant increase in spontaneous acetylcholine release.