Long-term outer retinal damage in rats exposed to neonatal hypoxia-ischemia

Abstract

Purpose : It has previously been demonstrated that neonatal hypoxia-ischemia (HI) induces inner retinal damage in rats, while sparing the outer retina up to 2 months of age. Given the disruption of both the retinal and choroidal blood flow in this animal model, we hypothesized that the outer retina may eventually be affected. The purpose of this study was to determine the long-term effects of neonatal HI on the outer retina.

Methods : Neonatal HI was induced in postnatal day 10 (P10) male Long-Evans rat pups by a left common carotid artery ligation followed by a 2-hour hypoxia (8% oxygen). Sham operated rats served as control. Flash electroretinograms (ERGs) were recorded from the left eyes at P14, 60 and 120 to assess retinal function. Following the ERGs at each time-point, the eyes were collected for RPE/choroid/sclera flatmounts, which were stained with phalloidin and isolectin B4 to label RPE and microglia, respectively. The number of microglia was counted on confocal images obtained from the centre, mid-periphery and periphery.

Results : The ERG a-wave amplitude (photoreceptor function) did not differ significantly between control and HI rats at P14 and P60, but was significantly attenuated at P120 in HI rats compared to control (65% decrease, p<0.01). The ERG b-wave amplitude (inner retina function) was significantly attenuated in HI rats compared to control at P60 (75% decrease, p<0.01) and P120 (86% decrease, p<0.001). RPE/choroid/sclera flatmounts revealed infiltration of microglia at the level of the RPE in the peripheral retina of HI rats as early as P14, which showed a 3-fold and 7-fold increase in number at P60 and P120, respectively. At P120, microglia were found in the centre and mid-periphery as well. The RPE cells showed signs of disruption at P60 and P120, but not at P14.

Conclusions : Our data suggest that anomalies of the outer retina (photoreceptor and RPE) take longer to develop than those of the inner retina. An increasing number of microglia with disease progression suggests that the damage may be mediated by inflammation.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.