Somatostatin analogs with affinity for opiate receptors in rat brain binding assay

The somatostatin analogs D-Phe-Cys-D-Trp-Lys-Thr-Cys-Thr and the corresponding penicillamine compounds have been prepared and tested for their ability to displace [3H]naloxone and [3H] [D-Ala2, D-Leu5]enkephalin from rat brain receptors. While somatostatin and the cystine containing peptide displayed little or no preference for either receptor system, the substitution of penicillamine at position two or seven resulted in analogs that displayed opposite receptor selectivity. The substitution of tyrosine for phenylalanine at position three resulted in a large increase in opiate receptor affinity which may be related to the known requirement for a phenolic hydroxyl moiety in the rigid opiate and enkephalin systems. Conformational properties of these analogs were also examined and related to their affinity for opiate and somatostatin receptors in the rat brain.