Cholecystokinin-like immunoreactivity in frog retina: localization, characterization, and biosynthesis.

We sought to localize and characterize cholecystokinin-like immunoreactivity (CCK-LI) in frog retinal tissues, CCK-LI was present in concentrations of 2.20 +/- 0.19 pmol/g wet weight (mean +/- SE, n = 6) in water extracts of frog retina and 0.23 +/- 0.04 pmol/g in acid extracts. The predominant form of CCK-LI co-eluted with the octapeptide of CCK on Sephadex G50 column chromatography. A minor form which eluted as a molecule larger than the 39 amino acid form of CCK was found only in the acid extracts. By immunohistochemistry, CCK-LI was localized to amacrine cells which give rise to processes ramifying in the distal 2/3 of the inner plexiform layer. Time dependent uptake of 35S-methionine into CCK-LI was observed in cultured frog retinas and this uptake was blocked with 0.2 mM cycloheximide. Our findings suggest that the retina may be a useful system for further characterization of the neural function of CCK-LI.