Learning and memory disabilities in IUGR babies: Functional and molecular analysis in a rat model

Abstract

Introduction: 1Intrauterine growth restriction (IUGR) is the failure of the fetus to achieve its inherent growth potential, and it has frequently been associated with neurodevelopmental problems in childhood. Neurological disorders are mostly associated with IUGR babies with an abnormally high cephalization index (CI) and a brain sparing effect. However, a similar correlation has never been demonstrated in an animal model. The aim of this study was to determine the correlations between CI, functional deficits in learning and memory and alterations in synaptic proteins in a rat model of IUGR.

Methods: 2Utero-placental insufficiency was induced by meso-ovarian vessel cauterization (CMO) in pregnant rats at embryonic day 17 (E17). Learning performance in an aquatic learning test was evaluated 25 days after birth and during 10 days. Some synaptic proteins were analyzed (PSD95, Synaptophysin) by Western blot and immunohistochemistry.

Results: 3Placental insufficiency in CMO pups was associated with spatial memory deficits, which are correlated with a CI above the normal range. CMO pups presented altered levels of synaptic proteins PSD95 and synaptophysin in the hippocampus.

Conclusions: 4The results of this study suggest that learning disabilities may be associated with altered development of excitatory neurotransmission and synaptic plasticity. Although interspecific differences in fetal response to placental insufficiency should be taken into account, the translation of these data to humans suggest that both IUGR babies and babies with a normal birth weight but with intrauterine Doppler alterations and abnormal CI should be closely followed to detect neurodevelopmental alterations during the postnatal period.

Keywords: cephalization index; intrauterine growth restriction; learning; placental insufficiency; spatial memory; synaptic plasticity.

Figure 1

Graphical scheme of the experimental groups and procedures. WMT: water maze test

Figure 2

Learning progression and correlation between escape latency and the cephalization index (CI). (a) Plot representing the average escape latency in four trials performed each day by the controls and the ischemic and Intrauterine growth restriction (IUGR) groups (n = 55, n = 34, N = 22, respectively, with similar representation of both genders) during the 10 days of the study. Learning progression fit to logarithmic curves and ANOVA analysis revealed significant differences in the shape of the curve and the final scores between the controls and the IUGR and ischemic rats (p < .001), with no significant differences between IUGR and ischemic rats. (b) Plot representing the average escape latency for the same animals as in (a), but segregated by gender and distributed into controls and CMO (IUGR + ischemic) experimental groups. Multifactor ANOVA and LSD analysis revealed significant differences in the shape of the learning curves between males and females (p < .01 LSD test), and between their corresponding controls and CMO groups (males p < .05, females p < .01, LSD test). (c) Dispersion plots represent the average escape latency of each individual (control male = 7, control female = 9; ischemic male = 6; ischemic female = 9; IUGR male = 8; IUGR female = 6) with respect to its CI on three representative days of the study. Vertical and horizontal lines on day 10 indicate the 75th percentile of the controls (CI = 1.32; escape latency [EL] = 17.7 s). Error bars in a and b represent the standard error of the mean

Figure 3

Hippocampus histology. Coronal hippocampal sections of control (a, d, g,) ischemic (b, e, h) and Intrauterine growth restriction (IUGR) (c, f, i) rats at P35. (a–c) Nissl stained; d–f) immunostained against NeuN, inset correspond to a higher magnification of the CA1 pyramidal layer; (g–i) GFAP immunostained. CA1, CA2, CA3 hippocampal fields; DG dentate gyrus; H, hilus; SO, stratum oriens; SP, stratum pyramidale; SR, stratum radiatum; SG, stratum granulosum; SLM, stratum lacunosum‐moleculare; SM, stratum moleculare; Scale bar: 150 μm

Figure 4

Distribution of synaptic proteins in the hippocampus. (a) Western blot of PSD95, synaptophysin and SNAP25 proteins in hippocampal tissue from controls, and the ischemic and Intrauterine growth restriction (IUGR) groups of untrained and water maze trained rats. Actin was used as the protein loading control. PSD95 and synaptophysin expression were significantly reduced in the untrained IUGR group, whereas only PSD95 expression was reduced after training. (b) Photomicrographs of coronal sections of the hippocampus of trained control, ischemic and IUGR rats immunostaining of postsynaptic protein PSD95, and presynaptic protein synaptophysin. CA1, CA3, hippocampal subfields; DG, dentate gyrus; H, hilus; SO, stratum oriens; SP, stratum pyramidale; SR, stratum radiatum; SG, stratum granulosum; SLM, stratum lacunosum‐moleculare; SM, stratum moleculare; *p < .05 respect control, ##p < .01 respect ischemic; LSD test. Scale bar: 185 μm