Extracellular recording from the guinea-pig myenteric plexus and the action of morphine

Extracellular recordings were made with glass suction electrodes from neurons in isolated ganglia of the myenteric plexus of the guinea-pig ileum. The activity recorded was considered to arise in neuronal somata as a consequence of the mechanical contact with the recording electrode. The spike activity was depressed by normorphine (10 nM-1 micrometer) and this action was prevented by low concentrations of naloxone. Morphine remained effective in solutions which contained 1 or 12 mM K+. The findings support other evidence that morphine acts by hyperpolarizing the soma membrane of myenteric neurons, but do not elucidate the ionic mechanism of the effect.