Cinnamon Attenuated Long-Term IGT-Induced Retinal Abnormalities via Regulation of Glucose Homeostasis in Neonatal Streptozotocin Induced Rat Model

Abstract

Diabetic retinopathy (DR) is one of the major causes of blindness all over the world. According to the previous studies, impaired glucose tolerance (IGT) has been linked to retinal dysfunction/vascular damage. Decreased retinal function is an initial event of early DR. Although the biochemical and molecular events are not fully understood, glial activation, angiogenesis and oxidative stress are some of the pathways associated with early retinal abnormalities. Since IGT is associated with development of retinal dysfunction/vascular damage; as a preventive strategy, we have studied beneficial effect of Cinnamon as a hypoglycaemic agent on long-term IGT induced retinal abnormalities using neonatal streptozotocin (nSTZ) rat model. Control, IGT rats were maintained on AIN-93M diet alone and another set of IGT rats were maintained on AIN-93M diet with 3% Cinnamon for 8 months. At the end of the study, untreated IGT rats developed retinal functional abnormalities as assessed by electroretinogram (ERG) and the retinal structure did not alter as assessed by H&E staining. Further, increase in expressions of GFAP, VEGF and decreased expression of rhodopsin in untreated IGT rat retinas. 4-HNE, a marker of oxidative stress was also elevated in IGT state. Supplementation of Cinnamon to IGT rats had lowered fasting and postprandial glucose levels and also prevented retinal functional abnormalities. Further, Cinnamon protected photoreceptor cell damage, suppressed glial activation, angiogenesis and oxidative stress as there was an improved rhodopsin expression, inhibited elevated expressions of GFAP, VEGF and 4-HNE respectively. In conclusion, Cinnamon attenuated IGT induced retinal abnormalities probably through its hypoglycemic property.

Keywords: Cinnamon; Impaired glucose tolerance; Neonatal streptozotocin; Retinal abnormalities.

Fig. 1

Schematic representation of the experiment

Fig. 2

Glucose and insulin response during OGTT at second month (a, b) and tenth month (c, d) of the experimental period. Values are mean ± SE n = 9–12 animals. The asterisk (*) denotes that data are significantly different from control group and the hash mark (^#) denotes that data are significantly different from IGT group

Fig. 3

Fasting (a) and postprandial blood glucose levels (b) during the experimental period assessed using glucometer (one touch horizon). All values are mean ± SE, n = 9–12 animals. The asterisk (*) denotes that data are significantly different from control group and the hash mark (^#) denotes that data are significantly different from IGT group

Fig. 4

Electroretinogram data. Scotopic b-wave amplitudes (a), Scotopic a-wave amplitudes (b), Representative wave forms of scotopic oscillatory potentials (c) and representative wave forms of scotopic oscillatory potentials (d) of control, IGT and IGT + Cin groups. Values are mean ± SE. The asterisk (*) denotes that data are significantly different from control group

Fig. 5

Morphology of retinas control, IGT and IGT + Cin group stained with H&E (a). Expression of VEGF, Rho, GFAP and 4-HNE in control, IGT and IGT rats fed with cinnamon (IGT + Cin) group rat retinas by IHC (b). White arrow indicated at the site of expression of VEGF, GFAP, rhodopsin and 4-HNE. GCL ganglion cell layer, INL inner nuclear layer, ONL outer nuclear layer

Fig. 6

Expression of VEGF, GFAP, Rhodopsin and 4-HNE at protein level by western blot (a) and densitometry analysis (b) in control, IGT and IGT rats fed with cinnamon (IGT + Cin) group rat retinas. Results are mean ± SE for three independent protein preparations for each group. The asterisk (*) denotes that data are significantly different from control group

Fig. 7

Expression of VEGF, GFAP and Rhodopsin at mRNA levels by qRT-PCR in control, IGT and IGT rats fed with cinnamon (IGT + Cin) group rat retinas. Gene expression results for these genes were normalized to beta actin. Results are mean ± SE for three independent mRNA preparations for each group. The asterisk (*) denotes that data are significantly different from control group and the hash mark (^#) denotes that data are significantly different from IGT group