Purpose: Genetic factors appear to modulate the incidence and severity of retinopathy of prematurity (ROP). Different rat strains may be analogous to genetic differences across human ethnic groups. We investigated the incidence and severity of neovascularization (NV) in Brown Norway (BN) and Sprague Dawley (SD) rats in oxygen-induced retinopathy (OIR) and acidosis-induced retinopathy (AIR) models for ROP. We also studied whether there was a difference in retinal vascular endothelial growth factor (VEGF) mRNA levels in OIR animals.
Methods: Newborn SD and BN rats (110 in both groups) were raised in standardized litters of ten (four OIR, twelve AIR, six non-gavaged room air controls). Beginning on day 1 of life, OIR litters were exposed to seven cycles of hyperoxia (80% O2, 20.5 h) and hypoxia (10% O2, 0.5 h) with a gradual return to 80% O2 over 3 h. This was followed by room air recovery for five days. OIR and OIR control rats were sacrificed on day 13 of life. AIR rats were gavaged twice daily with NH4Cl (10 mmol/kg) from day 2 to day 7 of life, or 15 mmol/kg twice daily on day 2 and then 10 mmol/kg twice daily from day 3 to day 7. AIR and AIR control rats were sacrificed on day 10 or day 13. Retinas from left eyes were dissected, ADPase stained and flatmounted. Presence and severity of NV (clock hours, 0 to 12) was scored by a masked observer. Right retinas from OIR and room air controls were processed for VEGF mRNA analysis.
Results: In OIR rats, the incidence of NV was higher in BN than SD rats (100% compared to 5%, p<0.0001). NV was more severe in BN rats (1 to 10 clock hours, median 7 clock hours compared to 0 to 1 clock hours, p=0.0001). In contrast, the incidence of NV in AIR rats was similar in BN and SD rats (4% compared to 0%, p=1.0) in the 10 mmol/kg study, and 18% compared to 0%, (p= 0.15) in the 15 mmol/kg study. Increased levels of retinal VEGF mRNA were found in BN OIR animals when compared to BN room air controls (1.4 fold increase) whereas retinal VEGF mRNA levels were similar between SD OIR and SD room air control animals.
Conclusions: BN rats differ from SD rats in incidence and severity of NV in OIR. The findings in AIR were limited by the low incidence of NV and intolerance to higher multiple doses of NH4Cl. In OIR, the higher severity of NV was associated with higher retinal VEGF mRNA in BN OIR rats. Studies are warranted to investigate the genetic differences between Brown Norway and Sprague Dawley rats in models of ROP. These genetic studies may yield further clues into the pathogenesis of clinical ROP.