Molecular biology of somatostatin receptors

The diverse physiological effects of somatostatin are mediated by a family of cell surface receptors that bind somatostatin selectively and with high affinity. The somatostatin receptors are members of the seven transmembrane segment receptor superfamily and molecular cloning studies have identified five types, designated sstr1-5. The human somatostatin receptors vary in size from 364 (sstr5) to 418 (sstr3) amino acids with 46-61% amino acid identity between receptors, and 105 amino acids are invariant. The sequences of the seven putative alpha-helical membrane-spanning domains are more highly conserved than those of the extracellular N- and intracellular C-terminal domains. Two forms of sstr2 have been identified in the mouse, sstr2A and sstr2B, which differ in size and sequence of the intracellular C-terminal domain. These two forms of sstr2 are products of a common gene and are generated by alternative splicing with sstr2A and sstr2B being the products of the unspliced and spliced forms, respectively, of sstr2 mRNA. Thus, functional diversity within the somatostatin receptor family may result from the expression of multiple types as well as from alternative splicing. The five somatostatin receptors have distinct patterns of expression in the central nervous system and peripheral tissues. They have also been expressed in vitro and shown to have different pharmacological properties. Somatostatin analogues selective for sstr2, sstr3 and sstr5 have been identified which will facilitate in vivo studies of the functions of these somatostatin receptors. Such studies to date suggest that sstr2 mediates inhibition of growth hormone secretion and sstr5 mediates inhibition of insulin secretion. The molecular cloning and functional characterization of the somatostatin receptor family is a first step in elucidating the diverse effects of somatostatin on cellular functions.