Topical administration of GLP-1 eyedrops improves retinal ganglion cell function by facilitating presynaptic GABA release in early experimental diabetes

Diabetic retinopathy is a prominent cause of blindness in adults, with early retinal ganglion cell (RGC) loss contributing to visual dysfunction or blindness. In the brain, defects in y-aminobutyric acid (GABA) synaptic transmission are associated with pathophysiological and neurodegenerative disorders, whereas glucagon-like peptide-1 (GLP-1) has demonstrated neuroprotective effects. However, it is not yet clear whether diabetes causes alterations in inhibitory input to RGCs and whether and how GLP-1 protects against neurodegeneration in the diabetic retina through regulating inhibitory synaptic transmission to RGCs. In the present study, we used the patch-clamp technique to record GABA subtype A receptor-mediated miniature inhibitory postsynaptic currents (mIPSCs) in RGCs from streptozotocin-induced diabetes model rats. We found that early diabetes (4 weeks of hyperglycemia) decreased the frequency of GABAergic mIPSCs in RGCs without altering their amplitude, suggesting a reduction in the spontaneous release of GABA to RGCs. Topical administration of GLP-1 eyedrops over a period of 2 weeks effectively countered the hyperglycemia-induced downregulation of GABAergic mIPSC frequency, subsequently enhancing the survival of RGCs. Concurrently, the protective effects of GLP-1 on RGCs in diabetic rats were eliminated by topical administration of exendin-9-39, a specific GLP-1 receptor antagonist, or SR95531, a specific antagonist of the GABA subtype A receptor. Furthermore, extracellular perfusion of GLP-1 was found to elevate the frequencies of GABAergic mIPSCs in both ON- and OFF-type RGCs. This elevation was shown to be mediated by activation of the phosphatidylinositol-phospholipase C/inositol 1,4,5-trisphosphate receptor/Ca2+/protein kinase C signaling pathway downstream of GLP-1 receptor activation. Moreover, multielectrode array recordings revealed that GLP-1 functionally augmented the photoresponses of ON-type RGCs. Optomotor response tests demonstrated that diabetic rats exhibited reductions in visual acuity and contrast sensitivity that were significantly ameliorated by topical administration of GLP-1. These results suggest that GLP-1 facilitates the release of GABA onto RGCs through the activation of GLP-1 receptor, leading to the de-excitation of RGC circuits and the inhibition of excitotoxic processes associated with diabetic retinopathy. Collectively, our findings indicate that the GABA system has potential as a therapeutic target for mitigating early-stage diabetic retinopathy. Furthermore, the topical administration of GLP-1 eyedrops represents a non-invasive and effective treatment approach for managing early-stage diabetic retinopathy.