Somatostatin in the retina

One single gene codes for the peptide somatostatin in mammals. Among species the propeptide shows a high degree of homology as the porcine and human propeptides are identical and only differs in two positions compared with the rat propeptide. Somatostatin and prosomatostatin are therefore very suitable for a comparative investigation of the posttranslatoric processing. Antibodies directed against four putative parts of the processing products of the prosomatostatin molecule were used in order to investigate the comparative biosynthetic processing of the precursor for the neuropeptide somatostatin, in the retina from frog, rat, gerbil, mink and monkey. The antibodies were employed to quantify, characterize, and localize somatostatin and prosomatostatin by biochemical and histochemical techniques. Further, somatostatin producing cells were demonstrated by in-situ hybridization using an antisense probe for somatostatin. Somatostatin and prosomatostatin were demonstrated in retinal extracts from all five species investigated. Retinal posttranslational processing of the prosomatostatin molecule varies between the species. In rat and monkey only one form of the bioactive somatostatin (SS-14) is present. In frog, gerbil and mink two known bioactive forms of somatostatin (SS-14 and SS-28) were present. In all five species prosomatostatin-64 (proSS-64) was recognized. In addition, the fragment somatostatin 1-28(1-12) was detected in rat and monkey retinae. In the rat somatostatin and prosomatostatin were demonstrated in subpopulations of amacrine cells located in the inner part of the retina. Cell bodies were labelled at the proximal boarder of the inner nuclear layer and in the ganglion cell layer. From these cells processes were limited to the plexiform layers of the retina and were never observed leaving the retina. By in-situ hybridization autoradiographic signals were demonstrated according to the cell bodies represented by immunohistochemistry. No seasonal variation was observed in the processing pattern of the peptide, investigated in the frog retina. However, the concentration measured during wintertime was almost two fold the concentration measured during summertime. In conclusion, the findings indicate that somatostatin and prosomatostatin are well conserved peptides in vertebrate retinae. Species differences in posttranslational processing excist. The physiological implication of this fact needs to be solved.