Pharmacological identity between somatostatin SS-2 binding sites and SSTR-1 receptors.
Somatostatin (SRIF) SS-2 binding sites were originally defined in rat brain cerebral cortex membranes using [125I]Tyr11-SRIF-14 in the presence of 120 mM NaCl. These sites were characterized by their high affinity for SRIF-14 and SRIF-28, but very low affinity for cyclic peptides such as octreotide (SMS 201-995) and seglitide (MK 678). The characteristics of SS-2 sites are reminiscent of 125I]CGP 23996-labelled sites in rat brain which have been termed SRIF-2 sites. In the present study, the pharmacological profile of SS-2 sites was determined in radioligand binding studies performed in rat cortex membranes using [125I]SRIF-14 in the presence of 120 mM NaCl and compared to that of human SSTR-1 receptors expressed in human embryonic kidney (HEK 293) cells, using [125I]SRIF-14. The rank orders of affinity of a variety of SRIF analogues and synthetic peptides for SS-2 binding sites and recombinant human SSTR-1 receptors were very similar and correlated highly significantly (r = 0.99). However, SS-2 binding correlated also with binding to recombinant SSTR-4 receptors (r = 0.91). Autoradiographic studies were performed using the radioligand [125I]CGP 23996 which has been claimed to label selectively SRIF-2 binding sites and compared with the distribution of SSTR-1 receptor mRNA determined using in situ hybridization in rat brain. Although some overlap was observed between the distribution of SSTR-1 mRNA and [125I]CGP 23996 binding sites, the latter were clearly more widespread, suggesting this ligand to label SSTR-1 and other sites. In addition, inhibition of forskolin-stimulated adenylate cyclase was investigated in HEK 293 cells transfected with human SSTR-1 receptors; a variety of SRIF analogues and short synthetic peptides behaved as agonists at adenylate cyclase and displayed a rank order of potency highly similar to that observed for these compounds at SS-2 binding sites. Seglitide acted as an antagonist at SSTR-1 receptor mediated inhibition of adenylate cyclase activity with a pKB of 4.42. It is concluded that the pharmacological profile of SS-2 binding sites resembles most closely that of SSTR-1 receptors (although similarities with SSTR-4 receptors were observed), that [125I]CGP 23996 labels presumably several SRIF receptors in rat brain, and that SSTR-1 receptors are negatively and efficiently coupled to adenylate cyclase activity.
Hoyer
D
D
SANDOZ Pharma Ltd, Basel, Switzerland.
Pérez
J
J
Schoeffter
P
P
Langenegger
D
D
Schüpbach
E
E
Kaupmann
K
K
Lübbert
H
H
Bruns
C
C
Reubi
J C
JC
D000262:Adenylyl Cyclases / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D001345:Autoradiography; D001665:Binding Sites; D002460:Cell Line; D002540:Cerebral Cortex / Q000378:metabolism; D005576:Colforsin / Q000494:pharmacology; D006801:Humans; D017403:In Situ Hybridization; D066298:In Vitro Techniques; D008969:Molecular Sequence Data; D011869:Radioligand Assay; D051381:Rats; D017208:Rats, Wistar; D017481:Receptors, Somatostatin / Q000145:classification* / Q000187:drug effects / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D013004:Somatostatin / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology
D017481:Receptors, Somatostatin; D011994:Recombinant Proteins; D005576:Colforsin; D013004:Somatostatin; D000262:Adenylyl Cyclases