Opiates and pain pathways: demonstration of enkephalin synapses on dorsal horn projection neurons.

The participation of the opiate peptide enkephalin in the neural circuitry of the dorsal horn was examined at the light and ultrastructural level through the use of the combined techniques of immunocytochemistry and retrograde transport of horseradish peroxidase. Enkephalin immunoreactive axonal endings made direct synaptic contact with the soma and proximal dendrites of dorsal horn thalamic projection neurons. This observation demonstrates that one major synaptic site of enkephalin modulation of the transfer of nociceptive information in the dorsla horn is on the projection neurons themselves.U