Environmental and Therapeutic Approaches to Limit the Consequences of Postnatal Hyperoxia

Abstract

Purpose: : Postnatal hyperoxia in the neonatal rat is characterized by severe and irreversible functional and structural deficits (attenuation of all ERG parameters and complete obliteration of the outer plexiform layer [OPL]) as well as a vasculopathy. We have previously shown that while dark-rearing from P0-14 failed to entirely prevent the functional consequences of OIR, it successfully limited OPL thinning. The purpose of this study was to examine the effect of prolonged dark-rearing (DR) in OIR (P0-30) as well as to explore the angiogenesis-promoting properties of sildenafil citrate in order to limit these sequelae.

Methods: : The hyperoxic-cohort included Sprague Dawley rats exposed to 80% O₂ as previously described from P0-14 (OIR), and was divided into 3 groups: 1) 80% O₂, (12 h at 80 lux, 12h dark; n=12), 2) 80% O₂ + 24h DR up to P14 (n=12), 3) 80% O₂ + 24h DR up to P30 (n=12), 4) 80% O₂ + sildenafil citrate (subcutaneous injection,100mg/kg/day, n=5) and 5) 80% O₂ + 150µl of 10% DMSO (n=5). The normoxic-cohort was divided into 3 groups: 1) 21% O₂, (12h at 80 lux, 12h dark; n=6), 2) 21% O₂ + DR (up to P14; n=15), 3) 21% O₂ + DR (up to P30; n=15) and 4) 21% O₂ + 150 µl of 10% DMSO (n=5). Scotopic (-6.3 to 0.6 log cd.sec.m^-2; 12 hrs dark adaptation) and photopic (0.9 log cd.sec.m^-2; background: 30cd.m^-2) ERGs were recorded at P30, following which retinal histological sections (1 µm) were collected.

Results: : Following hyperoxia, the ERG a-wave, rod Vmax, mixed rod-cone b-wave and photopic b-wave were attenuated to 80%, 25%, 37% and 27% of control, respectively (p<0.05). Hyperoxia + DR (from P0-14) generated waveforms that were attenuated to 66%, 58%, 56%, and 52% of control, while hyperoxia in prolonged DR (from P0-30) further decreased these parameters to 45%, 20%, 28% and 17% of control (p<0.05). Though unidentifiable in OIR rats, both 80% O₂ -DR groups (14- and 30-dyas of DR) reveal a structurally intact OPL that is indistinguishable from the normoxic control (p>0.05), measuring 9 ± .5µm. While sildenafil citrate could not prevent loss of the OPL in OIR rats, their photopic ERGs are better preserved with well-delineated oscillatory potentials.

Conclusions: : While short-term DR limits, albeit not entirely, the functional consequences of OIR, prolonged DR has more of a detrimental effect. The fact that the OPL remains, does not appear to be related to preservation of retinal vasculature during hyperoxia, given that sildenafil citrate was unable to prevent obliteration of the OPL. Photopic oscillatory potential enhancement would suggest that inner retinal vasculature is relatively better preserved following administration of sildenafil citrate. One wonders, therefore, if there is any relationship at all between OPL thinning and retinal vasculature in OIR.

Keywords: retinopathy of prematurity • retina • electroretinography: non-clinical