Enkephalins as transmitters in the central nervous system

The distribution of enkephalins and the associated binding sites for opiates strongly suggest that these compounds have a functional role in the brain. The effects of enkephalins administered near cell bodies and nerve terminals are compatible with a transmitter role for these substances. Enkephalins could function as inhibitory transmitters at synapses on cell bodies and could be released as presynaptic transmitters near the terminals of primary afferent fibers. The neurophysiological experiments, however, have not identified a particular synapse at which the enkephalins are released. Reports on the ability of naloxone to modify particular physiological events are not uniform. Collectively, the experimental data do not permit definitive conclusions on the role of enkephalins as transmitters in the brain and spinal cord.