Effect of hyperglycemia and diabetes on the expression of AMPA and kainate receptor subunits in rat retina

Abstract

Abstract: : Purpose: Several evidences indicate that glutamate may be involved in neurodegeneration in diabetic retinopathy (DR). We previously showed that glutamate uptake and release are altered under hyperglycemic conditions in retinal neurons. We investigated the effect of hyperglycemia and diabetes on the expression of ionotropic glutamate receptor subunits in rat retinal cell cultures and in diabetic retinas, and how hyperglycemia can affect the Ca²⁺ permeability of AMPA receptors in cell cultures. Methods: Rat retinal cells, cultured in MEM with 10% FCS, were exposed either to 25 mM D–glucose (final concentration 30 mM) or to 25 mM D–mannitol for 7 days. Diabetes was induced in 2–month old male Wistar rats with an injection of streptozotocin (65 mg/kg). Changes in total protein expression of GluR1, GluR2, GluR6/7 and KA2 subunits were assessed by western blot analysis of cell and retinal extracts (1 week after the onset of diabetes). Neurons with Ca²⁺–permeable AMPA receptors were identified with the Co²⁺ staining technique. Results: We found that KA2 subunit protein expression was increased both in cell cultures exposed to high glucose (204.3±42.3% of the control) and in diabetic retinas (143.9±12.8% of age–matched controls). The GluR6/7 subunits expression decreased in cells exposed to hyperglycemia (62.4±3.3% of the control) while in the case of diabetic rats there were no significant changes. The expression of GluR1 subunit decreased in glucose–treated cells (67.3±4.6% of the control) and in diabetic retinas (75.0±7.7% of the control). GluR2 subunit immunoreactivity increased (225.5±49.1% of the control) in cells exposed to high glucose, and a decrease in the number of neurons stained by the Co²⁺ technique was also observed. The presence of mannitol did not cause significant changes. Conclusions: Hyperglycemia and diabetes may cause changes on the expression of AMPA and KA receptor subunits in retina, which may have implications for neurodegeneration in DR. Particularly the upregulation of GluR2 subunit, may be a defensive mechanism against Ca²⁺ overload.

Keywords: diabetic retinopathy • excitatory neurotransmitters • retinal degenerations: cell biology