Rod Bipolar Cell Dysfunction During Diabetes is Prevented by Dietary Supplementation With Fish Oil

Abstract

Purpose:: Neuronal, glial and vascular changes within the retina during diabetes are most likely linked. Increases in protein kinase C ß (PKC) activity during diabetes has been associated with vascular complications within the retina. PKC is also highly expressed within rod bipolar cells (RBC). Therefore, the aim of this project was to examine whether rod bipolar cell function, especially PKC activity and expression is altered during diabetes and whether this could be prevented by dietary supplementation with omega 3 polyunsaturated fatty acid (fish oil).

Methods:: Sprague-Dawley rats were fed either a diet supplemented with or without fish oil. The diets were commenced at birth through to adulthood, where animals where then assigned to either control or diabetic groups (n=12 for each group). Diabetes was induced by injection with streptozotocin (STZ; 50mg/kg) and followed for 13 weeks. A standard electroretinogram (ERG) protocol was used to measure retinal function and the kinetics of the b-wave used to assess RBC function. Membranous and cytoplasmic PKC activities were measured from homogenised retinal tissue. Phosphorylated PKC immunoreactivity within RBC axon terminals was examined on vertical cryostat sections taken from rat retinae that had been fixed in 4% paraformaldehyde for 30 mins. This was quantitatively assessed by determining the membranous and cytoplasmic pPKC intensities and this was expressed as a ratio.

Results:: RBC function was assessed by measuring the kinetics of the descending limb of the b-wave. Diabetes caused a significant increase in the width of the b-wave when compared to controls. A diet supplemented with fish oil was found to prevent this change. Diabetes induced an increase in membranous PKC activity in whole retinae that was reduced in diabetic animals fed fish oil. However, the increase in the membranous-cytoplasmic ratio of phosphorylated-PKC within RBC terminals in diabetic animals was not prevented by fish oil supplementation.

Conclusions:: The results of this study indicate that rod bipolar cell function is altered during diabetes and can be prevented by dietary supplementation with fish oils. However, the improvement in rod bipolar cell function is not explained by a change in expression of PKC within RBC axon terminals.

Keywords: retina • diabetes • bipolar cells