Opiates and opioid peptides inhibit the discharge of nerve cells. This action has been considered to result from 'postsynaptic' inhibition of cell firing, or from a 'presynaptic' reduction in the release of excitatory transmitters. In cells of the myenteric plexus and locus coeruleus, opiates directly inhibit cell firing by opening membrane potassium channels and hyperpolarizing the membrane. A hyperpolarization also occurs on nerve cell processes; this can lead to a reduction in transmitter release by blocking action potential propagation and by reducing the entry of calcium during the action potential. Thus both forms of inhibition may result from a single ionic mechanism -an increase in membrane potassium conductance.
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