Analogues of somatostatin bind selectively to brain somatostatin receptor subtypes

Somatostatin (SRIF) is a neurotransmitter that produces its multiple effects in the CNS through interactions with membrane-bound receptors. Subtypes of SRIF receptors are found in the CNS that are distinguished by their sensitivities to the cyclic hexapeptide MK-678, such that SRIF1 receptors are sensitive to MK-678 and SRIF2 receptors are insensitive to MK-678. In the present study, we further examined the selectivities of a series of structurally diverse SRIF analogues for SRIF receptor subtypes. SRIF receptors were labeled by 125I-Tyr11-SRIF, which has indistinguishable affinities for SRIF receptor subtypes. The inhibition by MK-678 was incomplete, indicating this peptide is highly selective for a subtype of SRIF receptor that we have termed the SRIF1 receptor. The binding of 125I-MK-678 to SRIF1 receptors was monophasically inhibited by SRIF, the octapeptides (such as SMS-201-995), and the hexapeptides (such as MK-678), consistent with the highly selective labeling of a subtype of SRIF receptor. In contrast, the smaller CGP-23996-like analogues did not inhibit 125I-MK-678 binding to SRIF1 receptors. The binding of 125I-CGP-23996 to SRIF receptors was inhibited by SRIF and the octapeptides with Hill coefficients of less than 1, indicating that 125I-CGP-23996 labels multiple SRIF receptor subtypes. The hexapeptides and CGP-23996-like compounds produced only partial inhibitions of 125I-CGP-23996 binding, which were additive, indicating selective interactions of these compounds with the different receptor subpopulations labeled by 125I-CGP-23996. 125I-Tyr11-SRIF binding and 125I-CGP-23996 binding to SRIF receptors were likewise only partially affected by 100 microM guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), a concentration that completely abolishes specific 125I-MK-678 binding to SRIF1 receptors. The component of 125I-CGP-23996 labeling that was sensitive to GTP gamma S was also MK-678 sensitive. Thus, two subpopulations of SRIF receptors exist in the CNS. The SRIF1 receptor is sensitive to cyclic hexapeptides such as MK-678 and to GTP gamma S but insensitive to smaller CGP-23996-like compounds. The SRIF2 receptor is sensitive to the CGP-23996-like compounds and can be selectively labeled by 125I-CGP-23996 in the presence of high concentrations of the hexapeptides or GTP gamma S because, unlike the SRIF1 receptor, the SRIF2 receptor is insensitive to these agents. The SRIF receptor subtype-selective peptide analogues will be useful in the future characterization of the functions mediated by SRIF receptor subtypes in the CNS.