β²/β³-di- and α/β³-tetrapeptide derivatives as potent agonists at somatostatin sst₄ receptors

Four linear β²/β³-di- and α/β³-tetrapeptides (1-4) were investigated as somatostatin sst₄ receptor agonists on recombinant human and mouse somatostatin receptors. Human somatostatin receptor subtypes 1-5 (sst_1-5), and mouse somatostatin receptor subtypes 1,3,4 and 5, were characterised using the agonist radioligands [¹²⁵I]LTT-SRIF-28, [¹²⁵I][Tyr¹⁰]CST₁₄ and [¹²⁵I]CGP 23996 in stably transfected Chinese hamster lung fibroblast (CCL39) cells. The peptides bound selectively to sst₄ receptors with nanomolar affinity (pK_d=5.4-7.8). The peptides were investigated on second messenger systems both as agonists, and as antagonists to SRIF-14-mediated effects in CCL39 cells expressing mouse sst₄ receptors, via measurement of inhibition of forskolin-stimulated adenylate cyclase activity, and stimulation of luciferase expression. The peptides showed full agonism or pronounced partial agonism (40 to 100% relative intrinsic activity) in both inhibition of forskolin-stimulated adenylate cyclase activity (pEC₅₀=5.5-6.8), and luciferase expression (pEC₅₀=5.5-6.5). The agonist potential was confirmed since antagonism was very difficult to establish. The data show that β²/β³-di- and α/β³-tetrapeptide derivatives have agonist potential at recombinant somatostatin sst₄ receptors. Therefore, they may be used to elucidate physiological and biochemical effects mediated by sst₄, and may also have potential as therapeutic agents.