Impaired light adaptation of ON-sustained ganglion cells in early diabetes is attributable to diminished dopamine D4 receptor sensitivity

Purpose: It has been known for some time that normal retinal signaling is disrupted early on in diabetes, before the onset of the vascular pathologies associated with diabetic retinopathy. There is growing evidence that levels of retinal dopamine, a neuromodulator that mediates light adaptation, may also be reduced in early diabetes. Previously, we have shown that after six weeks of diabetes in a mouse model, light adaptation is impaired at the level of ON-sustained (ON-s) ganglion cells. The purpose of this study was to determine whether changes in dopamine receptor sensitivity contribute to this dysfunction. Here we used single cell retinal patch-clamp recordings from the mouse retina to determine how activating dopamine type D4 receptors (D4Rs) changes the light-evoked and spontaneous excitatory inputs to ON-s ganglion cells, in both control and diabetic animals. We also used in-situ fluorescent hybridization to assess whether D4R expression was impacted by diabetes. We found that D4R activation had a smaller impact on light-evoked excitatory inputs to ON-s ganglion cells in diabetic retinas compared to controls. This impaired D4R signaling is not attributable to a decline in D4R expression, as we found increased D4R mRNA density in the outer plexiform layer in diabetic retinas. This suggests that the cellular machinery of dopaminergic signaling is itself disrupted in early diabetes and may be amenable to chronic dopamine supplementation therapy.