Hipóxia-isquemia transitória pré-natal e desenvolvimento da retina de ratos: alterações morfológicas e funcionais

Abstract

Prenatal hypoxia-ischemia (HI) is one of the main causes of neurodevelopmental impairment in the newborn and is associated with cerebral palsy, attention problems, hyperactivity, epilepsy and sensory alterations, including visual processing problems. Thus, the investigation of the effects of prenatal HI on retinal development offers great potential to elucidate mechanisms related to the effects of HI during pregnancy. The objective of this study was to evaluate the effects of prenatal systemic HI on retinal ganglion cells (RGCs) and visual function of Wistar rats during postnatal development. Specifically, we evaluated the impact of prenatal HI on the number of RGCs and on the functional properties of the imaging and non-forming pathways. At the eighteenth day of gestation (E18), pregnant rats had their uterine horns exposed and during 45 minutes and the uterine and ovarian blood flow obstructed by artery clamping (HI group). The animals of the control group were obtained from pregnant females submitted to the same surgical procedure, except for the clamping of the arteries (SH group). The histological (hematoxylin and eosin, HE) and immunohistochemical (Brn3a) processing of HI and SH animal retinas were done on the second, ninth, twenty-third and thirtieth postnatal days. The evaluation of the functional properties of vision was made only on the 30th postnatal day through analysis of the pupillary reflex (non-image forming path) and the visual cliff (image forming pathway) test. Finally, a qualitative comparison of the RGCs projections between HI and SH animals was made by anterograde axonal fluorescence marking. The animals of the HI group presented a lower evolution in the gain of body mass compared to the animals of the SH group and, interestingly, the eye opening of the majority of the HI animals was precocious in relation to the SH. Regarding the evaluation of the number of RGCs, although no difference was observed in the number of cells in this layer between the HI and SH groups for HE stained retinas, quantification of the number of RGCs by Brn3a-labeled evidenced a significant reduction in the number of RGCs from HI animals at most of the ages analyzed. This result may explain the reduction of the optic nerve thickness and the lower density of the number of retinal projections in the dorsal geniculate nucleus of the HI animals in relation to the SH animals observed on the 30th postnatal day. The evaluation of the pupillary reflex revealed that the HI animals could not sustain the pupillary constriction under continuous illumination. In addition, no differences were observed between the experimental groups regarding pharmacologically induced ciliary muscle constriction. In the visual cliff test HI rats showed a preference for the deep side (depth perception deficit), contrary to SH, which descended mostly to the shallow side (perception of normal depth). Taken together, our results demonstrate that prenatal transient HI causes loss in the number of CGRs and suggest that such loss may be related to changes in the visual function of the animals.