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 [I-125]Tyr(11)-SRIF-14 in the pr esence of 120 mM NaCl. These sites were characterized by their high af finity for SRIF-14 and SRIF-28, but very low affinity for cyclic pepti des such as octreotide (SMS 201-995) and seglitide (MK 678). The chara cteristics of SS-2 sites are reminiscent of [I-125]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 ra dioligand binding studies performed in rat cortex membranes using [I-1 25]SRIF-14 in the presence of 120 mM NaCl and compared to that of huma n SSTR-1 receptors expressed in human embryonic kidney (HEK 293) cells , using [I-125]SRIF-14. The rank orders of affinity of a variety of SR IF analogues and synthetic peptides for SS-2 binding sites and recombi nant human SSTR-1 receptors were very similar and correlated highly si gnificantly (r = 0.99). However, SS-2 binding correlated also with bin ding to recombinant SSTR-4 receptors (r = 0.91). Autoradiographic stud ies were performed using the radioligand [I-125]CGP 23996 which has be en claimed to label selectively SRIF-2 binding sites and compared with the distribution of SSTR-1 receptor mRNA determined using in situ hyb ridization in rat brain. Although some overlap was observed between th e distribution of SSTR-1 mRNA and [I-125]CGP 23996 binding sites, the latter were clearly more widespread, suggesting this ligand to label S STR-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 syntheti c peptides behaved as agonists at adenylate cyclase and displayed a ra nk order of potency highly similar to that observed for these compound s at SS-2 binding sites. Seglitide acted as an antagonist at SSTR-1 re ceptor mediated inhibition of adenylate cyclase activity with a pK(B) of 4.42. It is concluded that the pharmacological profile of SS-2 bind ing sites resembles most closely that of SSTR-1 receptors (although si milarities with SSTR-4 receptors were observed), that [I-125]CGP 23996 labels presumably several SRIF receptors in rat brain, and that SSTR- 1 receptors are negatively and efficiently coupled to adenylate cyclas e activity.