Inflammatory response of both retinas and superior colliculi to unilateral optic nerve axotomy

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Objective: Unilateral optic nerve crush (ONC) induces a bilateral inflammatory reaction affecting both injured and contralateral un‐injured retinas. However, few works have investigated the spread of inflammation along visual pathway. Our aim was to study the glial response and the underlying molecular changes in both retinas and the superior colliculi (SCi), the main projection areas of retinal ganglion cells in mice. In this species, >90% of RGCs project to the contralateral SC (cSCi).Methods: The left optic nerve of adult pigmented C57Bl/6 male mice were crushed (ONC) at 0.5 mm from the optic disk or underwent surgery without crushing (Sham). One group of intact mice was used as controls. Two studies were carried out:1) At 1, 3, 9 and 30 days post lesion (dpl) both retinas and SCi were freshly dissected and their RNA extracted. Using qPCR we measured the expression levels of TGF‐β1, IL‐1β, TNF‐α, Iba1, GFAP, MHC II and TSPO in retinas and SCi. In SCi extracts we also studied Casp3, CXCR1, Lcn2, IL‐6, CD206, IL‐4, and AQ4. Hptr and Gapdh were used as houskeepings. Intact retinas and SCi were used as controls.2) Three, 9 and 30 days after ONC or sham‐surgery immunodetection of microglia and macroglial cells was carried out in sagittal brain sections. The area occupied by GFAP and MHC II signal as well as the density of Iba1+ and CD68+ cells was quantified in both SCi. Intact brains were used as controls.Results: Sham surgery alone modified gene expression in both retinas and SCi compared to intact animals. Therefore, to isolate the effect of ONC, we compared ONC vs. Sham. After ONC, TNF‐α upregulated early in both retinas and IL‐β remained overexpressed in injured retina at day 30. GFAP overexpression was exclusively observed in injured retina while Iba1 overexpression was presented in both. TSPO and MHCII and Casp3 overexpression was observed only in the cSC.Anatomical microglial activation (rounded somas and CD68 expression) was observed in the cSC from day 3 onwards after ONC. In the ipsilateral SC (iSC) microglial cells did not show signs of activation. Astrocytes hypertrophied in both SCi from day 9 after ONC, but this response was also observed in Sham‐animals.Conclusions: Sham surgery promotes an inflammatory and glial response that must be considered in experimental designs. Unilateral ONC triggers a bilateral response which extends anterogradely and retrogradely along the visual pathway.