Abstract
Abstract: :
Purpose:Diabetic retinopathy is a common complication of diabetes that causes significant visual impairment. Recently, oxidative stress in diabetes has been shown to disturb the GABA synthesis and degradation pathways in the retina; however, the effect of these changes on GABA signaling has not been studied. In this study, we examine the GABA response from isolated bipolar cells from normal vs diabetic rat retinae using electrophysiological techniques. Methods: Rats received 60 mg/kg streptozotocin IV dissolved in 0.9% Na citrate buffer (pH 4.5). Diabetes was established by demonstrating a BGL >350 mg/dl at sacrifice (2, 4, or 6 wks). Individual bipolar cells were isolated from the retina and maintained in culture for up to 8 hrs. The whole cell version of the patch clamp technique was used to monitor the membrane current changes elicited by the application of GABA and other pharmacological agents. Results: GABA elicited chloride currents from rat retinal bipolar cells exhibit both transient and sustained components, which are likely mediated by GABAa and GABAc receptors respectively. The majority of the response to 100 uM GABA for bipolar cells from both normal and diabetic retinae is contributed by GABAc receptors and is resistant to bicuculline blockade. The amplitude of the GABA response from bipolar cells isolated from diabetic retinae was enhanced compared to control (peak response of –537 vs –472 pA). The sustained response was also elevated (–323 vs –261 pA, p<0.05), indicating an upregulation of GABAc receptor activity in the diabetic retina. THIP, a GABAa receptor agonist and GABAc receptor antagonist, was used to investigate GABAa receptor activity. Bipolar cells from diabetic retinae had a larger response to 500 uM THIP compared to control (–105 vs –73 pA, p<0.05). An alteration in the redox environment occurs in the diabetic retina and could effect GABAa receptors which are sensitive to redox modulation. Whereas the application of the reducing agent DTT lead to a 50% enhancement of the THIP response for control bipolar cells (p<0.01), no significant change was observed for diabetic cells. Furthermore, DTT treatment eliminated the difference between the control and diabetic responses to THIP, suggesting that GABAa receptors are already in a reduced state in the diabetic retina. Conclusions: Our results indicate that bipolar cells in the diabetic retina are more sensitive to GABA, with enhanced activity of both GABAa and GABAc receptors. The altered redox status of the diabetic retina may be one of the underlying mechanisms for the change in the activity of GABA receptors.
Keywords: diabetes • inhibitory neurotransmitters • bipolar cells