How do opiates inhibit neurotransmitter release?

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.