Somatostatin receptor binding in rat cerebral cortex. Characterization using a nonreducible somatostatin analog.

tissue was much more difficult to detect, and only recently have reports appeared which demonstrate the presence of somatostatin receptors in preparations of rat brain (5-71, adrenal (8), and pituitary (9-11). One major problem was that the somatostatin analogs used as radioligands were readily degraded by proteolytic enzymes present in the tissue preparations. In addition to aminopeptidase and endopeptidase activities previously shown to inactivate somatostatin (12), we also considered the possibility that enzymatic reduction of the disulfide bond of somatostatin analogs such as N-'2'I-tyrosinyl-somatostatin and ['2'I-Tyr]somatostatin would also hinder their use as radioligands in binding studies. The conversion of these peptides to the linear, reduced form would lower their receptor affinity, based on the decreased biological activity of several noncyclizable somatostatin analogs (2). Nonreducible, cyclic analogs of somatostatin, in which methylene groups replace the sulfur atoms, were reported to retain high biological activity (13). In an effort to circumvent the possible problem of disulfide reduction, we have used in the present studies a tyrosylated dicarba analog of somatostatin, CGP 23996.' We demonstrated that 'I-labeled CGP 23996 was highly resistant to degradation, even at 37 C, and proved to be suitable for the identification and characterization of somatostatin receptors on synaptosomal membranes from rat cerebral cortex. EXPERIMENTAL PROCEDURES