Characterization and Functional Expression of a Somatostatin Receptor Coupled to Adenylyl Cyclase

Abstract

Receptor ligand binding and functional studies indicate that somatostatin (SRIF) receptors belong to the G protein coupled receptor superfamily. Using the polymerase chain reaction (PCR) and degenerate primers to the third and sixth transmembrane domains of G protein coupled receptors, we have isolated a SRIF receptor cDNA from bovine locus coeruleus (LC). This SRIF receptor (referred to as LCR9) encodes a protein of 368 amino acids and is 94% identical to a human SRIF receptor recently isolated from pancreatic islet cells. Levels of LCR9 mRNA are highest in cerebral cortex, intermediate in thalamus and cerebellum, and lower in pons, LC, dorsal raphe, and substantia nigra. Expression in CHO cells demonstrates that LCR9 binds 125 I-Tyr 1-SRIF with high affinity ( K d ∼ 0.5 n M). SRIF, SRIF28, and MK 678 (a SRIF 1 selective ligand) potently displaced SRIF ligand binding, while a CGP-23996 like compound (compound 1), which displays higher affinity for SRIF2, did not significantly influence ligand binding at concentrations up to 1 μ M. Ligand binding to the expressed receptor was also significantly reduced by GTP. Unlike the receptor isolated from pancreas, expression of LCR9 resulted in SRIF receptor inhibition of adenylyl cyclase. SRIF, SRIF28, and MK 678 all potently inhibited forskolin-stimulated adenylyl cyclase in membranes from LCR9 transfected cells; in contrast the CGP-23996 like compound did not influence adenylyl cyclase activity. SRIF receptor inhibition of adenylyl cyclase was dependent on GTP and was sensitive to pretreatment of membranes with pertussis toxin. The results demonstrate that LCR9 encodes a high-affinity SRIF receptor that is negatively coupled to adenylyl cyclase.