Biosynthesis of Somatostatin-Like Immunoreactivity by Frog Retinas In Vitro

Abstract: Somatostatin-like immunoreactivity has been localized to a wide variety of central nervous system neurons, including the retina. We utilized the unique advantages the retina provides for in vitro studies of nerves to examine the biosynthesis of somatostatin. Extracts of frog retinas pulse-labeled with [³⁵S]cysteine for various time periods revealed uptake of radioactivity into material adsorbable by anti-somatostatin antibody linked to affinity beads. This uptake increased in a curvilinear fashion for 4 h and was inhibited by cycloheximide (0.2 m M ) or by boiling the retinas prior to labeling. Pulse-chase experiments revealed that affinity-adsorbable radioactivity from retinal extracts decreased with time of incubation in chase medium; 89% of this decrease could be accounted for by increases in the affinity-adsorbable radioactivity of the chase medium. Chromatography of the retinal extracts on Sephadex G50 (superfine) revealed four elution peaks, whereas only one peak, co-eluted with somatostatin-14, could be identified in the medium. Chromatographic elution patterns of affinity-adsorbable radioactivity from extracts of pulse-labeled retinas incubated in chase medium for various times showed a gradual shift of radioactivity from the earlier-eluting peaks to the later ones. These studies indicate that biosynthesis of somatostatin occurs in frog retinas in vitro. The retina may be a useful model for further study of peptidergic neurons.