Brain effects of fetal growth restriction and their prevention in an animal model

Abstract

BACKGROUND: Chronic hypoxia due to placental insufficiency and prenatal undernutrition are probably the two major causes worldwide of an adverse intrauterine environment having an impact on neurodevelopment. Clinically, both situations manifest as an intrauterine growth restriction (IUGR), a situation defined as a significant reduction in fetal growth resulting in a birth weight below the 10th percentile. This situation is a well-recognized cause of neurobehavioral and cognitive impairments extending beyond childhood and early adulthood period. Although all these evidence, the structural ground of these functional impairments and the pathophysiological mechanisms are not fully characterized. An improvement in these two aspects would allow us to propose different therapeutic strategies aiming to ameliorate and even revert the long-lasting consequences of the IUGR.

HYPOTHESIS: We hypothesized that IUGR produces subtle structural brain changes that underlie the long-term neurobehavioral and cognitive impairments. The severity of the neurodevelopmental consequences might be related to the severity of the prenatal insult (reduction in nutrients with or without a reduction in oxygen). High-resolution brain imaging along with specific histological techniques focused on neuronal connectivity could evidence these structural brain changes. Additionally, we hypothesized that an early postnatal stimulation might ameliorate the structural and functional impairments that persist at the long-term period after IUGR.

METHODS: Two animal models of IUGR were used in this thesis: i. A cohort of pregnant rabbits was randomized to reproduce an undernutrition model based on maternal food reduction intake, ii. Another cohort of pregnant rabbits was randomized to the placental insufficiency model based on the surgical ligation of 40-50% of the uteroplacental vessels that irrigate each gestational sac. After the delivery in both models, IUGR and controls animals were followed up to the 70th postnatal days. At the 30th postnatal days, a subgroup of IUGR animals was randomized to an environmental enrichment strategy. In all the groups at the neonatal period, general motor skills, reflexes, and olfactory sensitivity were evaluated. Similarly, at the 70th postnatal days, anxiety, memory, and learning were evaluated. Afterward, animals were sacrificed and brains were fixed and diffusion MRI was then performed. In a subset of animals, changes at the microstructural level and differences in the number of fibers in two specific brain circuits (anxiety and memory circuits) were performed by using a Voxel-Based approach

(VBA) and Tractography analysis, respectively. Moreover, brain networks were obtained and evaluated by means of a Connectomics. Finally, a subgroup of animals was also histologically evaluated by means of dendritic spine and perineural nets evaluation in the Hippocampus.

RESULTS: IUGR animals showed poorer functional performance in both moments, especially in the model of placental insufficiency. At the long-term period, IUGR animals presented an altered brain network architecture, being again these differences more pronounced in the placental insufficiency model. Moreover, VBA analysis and Tractography analysis evidenced microstructural brain changes mostly affecting gray matter and a decreased in the number of fibers involved in the anxiety and memory circuits in the IUGR animals in comparison to controls. At the cellular level, IUGR animals presented abnormal neuronal connectivity with changes in the dendritic spine density and in the perineural nets. In contrast, stimulated IUGR animals presented a functional and structural improvement in comparison to non-stimulated IUGR animals over the long-term period.

CONCLUSIONS: This thesis adds to the previous evidence new insights regarding the pathophysiological mechanisms underlying IUGR and gives strong evidence linking IUGR with altered brain connectivity as the basis for the neurological sequelae associated with IUGR. Additionally, it gives preliminary evidence suggesting that a strategy based on physical, sensory, cognitive as well as social stimulation applied during early postnatal life, where brain plasticity is higher, might ameliorate the neurodevelopmental consequences of IUGR.