Opiate antagonistic properties of an octapeptide somatostatin analog

The somatostatin analog D-Phe-Cys-Phe-D-Trp-Lys-Thr-Cys-NH-CH(CH2OH)CHOHCH3 (SMS 201-995) displaces [3h[naloxone from its binding sites (IC50, 38 +/- 60 nM), being more than 200 times more potent than somatostatin. As measured by the difference between [3H]dihydromorphine, [3H][D-Ala2,D-Leu5]enkephalin, and (-)-[3H]bremazocine binding, SMS 201-995 appears to be highly specific for the opiate mu binding site. Electrophysiological data from hippocampal cultures and results from animal studies (tail flick, mydriasis) demonstrate the opiate antagonistic properties of SMS 201-995. SMS 201-995 is an opiate mu antagonist with a peptide structure. That this property is displayed by a somatostatin analog is somewhat unexpected.