Synenkephalin is coreleased with Met-enkephalin from neuronal terminals in vitro.

Synenkephalin, the amino-terminal 1-70 residues of proenkephalin, is released intact from bovine globus pallidus and neurohypophysis following potassium-induced depolarization in vitro via a Ca2+-dependent mechanism. The release of synenkephalin accompanies that of Met-enkephalin in a molar ratio of 1/4. In contrast to Met-enkephalin, which is readily destroyed when released, synenkephalin is not destroyed.