CHARACTERIZATION AND DISTRIBUTION OF SOMATOSTATIN SS-1 AND SRIF-1 BINDING-SITES IN RAT-BRAIN - IDENTITY WITH SSTR-2 RECEPTORS

Somatostatin (SRIF) SS-1 binding sites were initially defined in radio ligand binding studies performed in rat brain cerebral cortex membrane s using [I-125]204-090 (a radiolabelled Tyr(3) analogue of SMS 201-995 , octreotide). SRIF-1 recognition sites were defined in binding studie s performed with [I-125]MK 678 (seglitide). Both SS-1 and SRIF-1 sites were characterized by their high affinity for SRIF-14, SRIF-28 and fo r cyclic peptides such as octreotide and seglitide, in marked contrast to SS-2 and SRIF-2 sites which have very low affinity for these synth etic SRIF analogues. In the present study, SS-1 and SRIF-1 radioligand binding studies were performed in rat cortex membranes and compared t o results obtained in cloned Chinese hamster ovary cells expressing hu man SSTR-2 receptors using [I-l25]204-090 and/or [I-125]MK-678. The ra nk orders of affinity of a variety of SRIF analogues and synthetic pep tides for SS-1/SRIF-1 binding sites and recombinant SSTR-2 receptors w ere 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 [I-125]204-090 and [I-1 25]MK-678 and compared with the distribution of SSTR-2 receptor mRNA d etermined using in situ hybridization. A clear overlap was observed be tween the distribution of SSTR-2 mRNA and binding sites labelled with both radioligands. SSTR-2 receptor-mediated inhibition of forskolin-st imulated 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 r espectively labelled with [I-125]204-090 and [I-125]MK 678, both displ ay the pharmacological profile of SSTR-2 receptors, that the distribut ion of [I-125]204-090 and [I-125]MK-678 binding sites in rat brain is superimposable and largely comparable to that of SSTR-2 mRNA expressio n. It is also shown that neither [I-125]204-090 nor [I-125]MK-678 labe l SSTR-3 or SSTR-5 receptors in rat brain. Finally, it is demonstrated that SSTR-2 receptors can very efficiently couple to adenylate cyclas e activity in an inhibitory manner.