Regional distribution of the Mr 15,000 somatostatin precursor, somatostatin-28 and somatostatin-14 in the rat brain suggests a differential intracellular processing of the high molecular weight species.

Three different forms of immunoreactive somatostatin (Mr 15,000, 3,000 and 1,600) were immunologically detected in extracts made from six neural structures of the rat brain. The largest represents the proform, while the smaller were identified by high pressure liquid chromatography with bovine somatostatin-28 (S-28) and -14 (S-14) respectively. In each of the brain structures studied highly variable proportions of precursor, S-28 and S-14 were found. These observations provide suggestive evidence that the intracellular processing of the 15,000 Mr proform may occur differently in the various somatostatinergic pathways of the brain. They argue in favor of a biological role for the precursor in providing distinct relative proportions of S-14 and S-28 in specific rat brain regions.