Effects of glycosylation inhibitors on the frog retina

Effects of the intraocular injection of three inhibitors of glycosylation (tunicamycin, castanospermine, and swainsonine) on the rhodopsin content and the integrity of disc membranes in frog retina were studied. The administration of 10 or 100 micrograms of tunicamycin resulted in a 78% loss of rhodopsin in the frog retina which also exhibited a significant reduction in the length of photoreceptor outer segments (as examined under light microscope). This suggests that the synthesis and/or insertion of rhodopsin into the disc membrane is inhibited by tunicamycin. In contrast, injections of up to 250 micrograms of castanospermine and swainsonine resulted in neither a decrease in rhodopsin content nor a change in the length of photoreceptor outer segments. Examination of retinal tissue homogenates for alpha Mannosidase and alpha-Glucosidase activities revealed homogenates for alpha Mannosidase and alpha-Glucosidase activities revealed these hydrolase activities to be significantly decreased (70%). We suggest the possibility that specific oligosaccharide processing reactions may not be required for the insertion of rhodopsin and subsequent assembly of disc membranes in frog photoreceptors.