Purpose: To characterise the role of microglia in the neurovascular unit and vascular regulation within the inner retina and explore changes during early steptozotocin (STZ)-induced diabetes.

Methods: Immunohistochemistry was used to characterise the extent of microglial (anti-Iba1) contact with retinal synapses (anti-Vglut1) and the inner retinal vasculature (IB4). Transmission electron microscopy was used to detail the microglial-vascular association using the Cx3cr1 GFP/+ transgenic animal where one copy of the microglial receptor, Cx3cr1 was replaced with enhanced green fluorescent protein. Capillary responses were quantified in retinal explants (Cx3cr1 GFP/+ mice and dark agouti rats) incubated in Ames and perfused with fractalkine (200ng/ml), the sole ligand for Cx3cr1. Diabetes was induced in dark agouti rats with a single intraperitoneal injection of STZ (55mg/kg) and animals taken after 4 weeks of hyperglycaemia.

Results: The majority of inner retinal microglia contact both neuronal synapses and capillaries (73±8%), with half of these associated with the smallest vessels (< 8µm). At the ultrastructural level, microglial processes directly oppose pericytes and endothelial cells, wrapping around vessel lumen. Using fractalkine, the sole ligand to Cx3cr1, specific capillary constriction was observed in areas of microglial contact (-31±8% of vehicle response, n= 6, p< 0.01), while no change in vascular diameter was evident in areas devoid of microglial contact. During early stages of STZ-induced diabetes there was increased microglial-vascular interaction (20±8%, n> 10, p< 0.05) and capillary constriction (control, 9.5±0.3 μm; STZ, 8.7±0.3 μm, n= 12, p< 0.05). Ex vivo addition of fractalkine to diabetic retinae did not result in further capillary constriction (+ 4.0±3%, n= 5, p> 0.05).

Conclusions: Retinal microglia form part of the neurovascular unit within the retina and are capable of vasoregulation. Changes in microglial regulation of the retinal vasculature early in diabetes may contribute to the progression of retinal pathology.