Characterization and distribution of somatostatin SS-1 and SRIF-1 binding sites in rat brain: identitity with SSTR-2 receptors

Abstract

Somatostatin (SRIF) SS-1 binding sites were initially defined in radioligand binding studies performed in rat brain cerebral cortex membranes using [ 125I]204-090 (a radiolabelled Tyr 3 analogue of SMS 201-995, octreotide). SRIF-1 recognition sites were defined in binding studies performed with [ 125I]MK 678 (seglitide). Both SS-1 and SRIF-1 sites were characterized by their high affinity for SRIF-14, SRIF-28 and for cyclic peptides such as octreotide and seglitide, in marked contrast to SS-2 and SRIF-2 sites which have very low affinity for these synthetic SRIF analogues. In the present study, SS-1 and SRIF-1 radioligand binding studies were performed in rat cortex membranes and compared to results obtained in cloned Chinese hamster ovary cells expressing human SSTR-2 receptors using [ 125I]204-090 and/or [ 125I]MK-678. The rank orders of affinity of a variety of SRIF analogues and synthetic peptides for SS-1/SRIF-1 binding sites and recombinant SSTR-2 receptors were very similar and correlated highly significantly ( r = 0.94–0.99); by contrast, correlation between SS-1 and SSTR-5 ( r = 0.44) or SSTR-3 binding ( r = 0.07) was not significant. Autoradiographic studies were performed in rat brain using both radioligands [ 125I]204-090 and [ 125I]MK-678 and compared with the distribution of SSTR-2 receptor mRNA determined using in situ hybridization. A clear overlap was observed between the distribution of SSTR-2 mRNA and binding sites labelled with both radioligands. SSTR-2 receptor-mediated inhibition of forskolin-stimulated adenylate cyclase in Chines hamster ovary cells by a variety of SRIF analogues and short synthetic peptides displayed a rank order of potency highly similar to their rank order of affinity at SS-1/SRIF-1 binding sites. It is concluded that SS-1 and SRIF-1 binding sites respectively labelled with [ 125I]204-090 and [ 125I]MK 678, both display the pharmacological profile of SSTR-2 receptors, that the distribution of [ 125I]204-090 and [ 125I]MK-678 binding sites in rat brain is superimposable and largely comparable to that of SSTR-2 mRNA expression. It is also shown that neither [ 125I]204-090 nor [ 125I]MK-678 label SSTR-3 or SSTR-5 receptors in rat brain. Finally, it is demonstrated that SSTR-2 receptors can very efficiently couple to adenylate cyclase activity in an inhibitory manner.