GCG and diabetes mellitus: A study utilizing STZ-diabetic rats demonstrated that hyperglycemia downregulates GLP-1R expression in the retina [83], although Hernandez et al. found no significant differences in the GLP-1R expression levels in retinas derived from diabetic patients and db/db mice versus those from non-diabetic controls despite GLP-1 levels being lower in the retinas of those with diabetes [47].