Postnatal Hyperoxia Triggers Neurotrophic Factor Upregulation With Mixed Success

Abstract

Purpose:: We've shown that the developing retina is unequally susceptible to hyperoxia: functional and histological anomalies being worse with exposure during the second week of life compared to the first. We aimed at identifying the cellular and molecular mechanisms at the origin of these differences.

Methods:: Sprague Dawley (SD) and Long Evans (LE) rats (n=60) exposed to 80% O₂ from P0-6 or P6-14 were compared to rats raised in room air (n=20). Immunohistochemistry and western-blots were performed on retinas collected upon removal from hyperoxia (P6 and P14) or at P60. Antibodies were used against neurotrophic factors and retinal cellular structures.

Results:: Western-blots at P6 and P14 revealed significant (p<.05) upregulation of CNTF (LE and SD) and BDNF (SD only) following hyperoxia and co-immunostaining with GFAP at P60 suggests that Muller cells substantiate this increase. Immunohistochemistry confirmed localization of both factors in the INL and GCL of exposed rats. In contrast, FGF-2 levels were unchanged. At P60, minimal changes followed P0-6 exposure, while that of P6-14 caused a central and peripheral loss of synapses between bipolar and amacrine cells (anti-PKC and parvalbumin), photoreceptor and bipolar cells (anti-bassoon and mGluR6) and horizontal cells (anti-calbindin), findings that were more pronounced in LE rats. Finally, in LE rats an increase in TUNEL-positive cells was observed in the INL and ONL [33.2% and 116.7% more than control, respectively (p<.05)] after P6-14 exposure while fewer were seen in the INL of LE and SD rats [67.5% and 55.6% less than control, respectively (p<.05)] following the P0-6 exposure.

Conclusions:: Studies of ours suggested that structural and functional consequences of postnatal hyperoxia depend on retinal maturity at time of exposure. Our results propose that neurotrophic factors might play a role in this differential susceptibility in that they would succesfully protect the retina from hyperoxic injury during the first but not the second week of life. This is further corroborated by the retina’s inability to curtail cell death following exposure from P6-14 as it seems to for early exposures. Our findings propose a previously undescribed involvement of neurotrophic factors, increased cell death and synaptic retraction that are likely involved in generating the functional consequences of OIR.

Keywords: retinopathy of prematurity • oxidation/oxidative or free radical damage • immunohistochemistry