Long-Term Functional Outcomes and Correlation with Regional Brain Connectivity by MRI Diffusion Tractography Metrics in a Near-Term Rabbit Model of Intrauterine Growth Restriction

Abstract

BackgroundIntrauterine growth restriction (IUGR) affects 5–10% of all newborns and is associated with increased risk of memory, attention and anxiety problems in late childhood and adolescence. The neurostructural correlates of long-term abnormal neurodevelopment associated with IUGR are unknown. Thus, the aim of this study was to provide a comprehensive description of the long-term functional and neurostructural correlates of abnormal neurodevelopment associated with IUGR in a near-term rabbit model (delivered at 30 days of gestation) and evaluate the development of quantitative imaging biomarkers of abnormal neurodevelopment based on diffusion magnetic resonance imaging (MRI) parameters and connectivity.MethodologyAt +70 postnatal days, 10 cases and 11 controls were functionally evaluated with the Open Field Behavioral Test which evaluates anxiety and attention and the Object Recognition Task that evaluates short-term memory and attention. Subsequently, brains were collected, fixed and a high resolution MRI was performed. Differences in diffusion parameters were analyzed by means of voxel-based and connectivity analysis measuring the number of fibers reconstructed within anxiety, attention and short-term memory networks over the total fibers.Principal FindingsThe results of the neurobehavioral and cognitive assessment showed a significant higher degree of anxiety, attention and memory problems in cases compared to controls in most of the variables explored. Voxel-based analysis (VBA) revealed significant differences between groups in multiple brain regions mainly in grey matter structures, whereas connectivity analysis demonstrated lower ratios of fibers within the networks in cases, reaching the statistical significance only in the left hemisphere for both networks. Finally, VBA and connectivity results were also correlated with functional outcome.ConclusionsThe rabbit model used reproduced long-term functional impairments and their neurostructural correlates of abnormal neurodevelopment associated with IUGR. The description of the pattern of microstructural changes underlying functional defects may help to develop biomarkers based in diffusion MRI and connectivity analysis.

Programming the brain: Common outcomes and gaps in knowledge from animal studies of IUGR

“…Corpus callosum ↓ [115] ↓width [162] NEURONAL DENSITY Cerebrum = [79] ↓insular, temporal and occipital cortex, indirect evidence [117] Hippocampus = [85] ↑ neuronal proliferation, adolescent females [131] = [113,115] ↑degenerating neurons, CA3 [113] ↓indirect evidence [117] Dentate gyrus = [101] Cerebellum = [79] ↓ indirect evidence via MRI [117] Fornix ↑ degenerating neurons [113] Entorhinal cortex ↓ [113,115] ↑ degenerating neurons [113] Cingulate cortex = [113] External capsule ↑ degenerating neurons [113] Prefrontal cortex = [115] ↓ indirect evidence [117] GABAergic interneurons ↑ prefrontal cortex [115] …”

“…Axonal density ↓ left hemispheric anxiety and memory pathways [117] Axonal degeneration + cingulate and somatosensory cortices, internal capsule, pontocerebellar tract [115] Microstructural reorganisation + [163] MYELINATION Cerebrum = [162] ↓ [117] Corpus callosum = [222] ↓ d60 [40] Cingulum ↓ d60 [112] Internal and external capsule = d60 [112] Astrogliosis Hippocampus ↑CA1 [113,115] Dentate gyrus ↑ [113,115] Entorhinal cortex ↑ [113,115] Cingulum ↑ [113,115] = [94] Fornix ↑ [113] Motor cortex = [115] Somatosensory cortex ↑ [115] Neonatal neurobehaviour = reflexes [79] ↓ righting reflex, d3-4 males, d3 females [87] ↓cliff avoidance, d7 females, d8, both sexes [87] ↓negative geotaxis, d7-8 males [87] = reflexes d10-21 [165] ↓surface righting, d2-9 ↓ negative geotaxis d 4-15 [129] ↓righting reflexes, locomotion, head turning and smell test scores as d1 neonates [116] Neuromotor ↓ grip strength adult males [165] ↓ motor learning, males [129] ↓motor learning, adults [222] Spatial learning = adult males [95] = adults [97,165] = adult males [167] ↓initial simple maze tests (lambs) [122] = extended simple maze testing, obstacle course tasks, tmaze tasks (lambs) [122] ↓initial simple maze tests (male lambs and young adults) [150] Revers...…”

“…Behavioural anxiety = male pups [79] =/↑ adult males [93,94] ↓ adults [98,216] ↑ reactivity to physical restraint and surprise, adults [84] ↑ adolescent females = adolescent males [131] ↑ adults [117] ↑ low birth weight female lambs [150] Spontaneous ambulation = adult males [94,95] ↑ females [165] ↑ in isolation tasks, adults [84] ↑ adults [113][114][115] ↓ adult males [160] ↓ Hyperactivity ↑ adult females [165] ↑adults [113][114][115] Exploratory behaviour ↑adult females [216] ↑adults [113][114][115] ↓ adults [117,163] Response to novelty ↓novelty seeking, adults [84] = adults [113] …”

“…low birth weight in humans [30,39,42,148], existing studies do suggest long-term structural 283 damage following experimental IUGR. These include damage which occurs during exposure to 284 restriction and persist from fetal life, such as decreased neuronal density [117], which can be 285 contributed by grey matter loss in utero resulting in decreased neuron numbers in later life. This also includes further changes that develop after birth, including decreased myelination [112,117,287 149].…”

“…These include damage which occurs during exposure to 284 restriction and persist from fetal life, such as decreased neuronal density [117], which can be 285 contributed by grey matter loss in utero resulting in decreased neuron numbers in later life. This also includes further changes that develop after birth, including decreased myelination [112,117,287 149]. Studies in adolescent and adult animals ( Table 5) learning or memory impairments ( Table 6).…”