Somatostatin and ACTH are peptides with partial antagonist-like selectivity for opiate receptors.

The peptides, Met-enkephalin and somatostatin were analysed for competitive affinity to opiate agonist receptor sites in rat brain. Met-enkephalin, like morphinomimetic compounds, being equal to marked selectivity for agonist sites while somatostatin and ACTH showed much less selectivity, being equal to partial antagonists like nalorphine. These results are compared with those observed for these peptides in vivo. It is suggested that somatostatin and acth act as partial agonist--antagonist on opiate receptors in the cns.