The effect of growth restriction and postnatal fluctuating oxygen on the developing brain

Background: Preterm birth is the most common cause of childhood mortality and morbidity in the United Kingdom. Placental insufficiency is significant in the aetiology of preterm delivery and implicit to fetal growth restriction. Preterm growth restricted infants show reduced cerebral cortical volume which correlates with poor neurodevelopmental outcome. Respiratory support in the form of supplemental oxygen or mechanical ventilation is inevitably required by preterm infants with acute and chronic respiratory morbidities which are commonly experienced by growth restricted preterm infants. Postnatal oxygen therapy is also associated with markers of cortical injury in the developing brain. Aims: To define the independent and combined effects of growth restriction and a clinically derived postnatal fluctuating oxygen profile on markers of cortical integrity including (1) laminar thickness (2) cortical transcription factor expression and (3) cerebral growth factor expression, within a rodent model. Methods: Sprague-Dawley rat dams received a normal or protein restricted diet to induce growth restriction. Litters were reared in air or fluctuating hyperoxia for 7 days. Pup brains were weighed, sections through the motor cortex stained with cresyl violet and the thickness of the corpus callosum, superficial (Il-IV) and deep (V-VI) cortical layers were measured. Neuronal subtypes (callosal, superficial, corticospinal and corticothalamic) were immunofluorescently stained using neuronal subtype-specific transcription factors (Satb2, Cuxl, Ctip2, Tbrl respectively). Cerebral expression of trophic factors Insulin-like growth factor-I (IGF-I), Brain derived neurotrophic factor (BDNF), Fibroblast growth factor-2 (FGF-2) and Vascular endothelial growth factor (VEGF) were analysed and circulating serum IGF-I determined. Comparisons were made between study groups. Results: Growth restricted pups had smaller brain weights than normally grown pups but cortical thickness was preserved. Fluctuating hyperoxia did not affect brain weight but cortical thickness was reduced in both normally grown and growth restricted groups. Cortical thinning was observed in the later born superficial cortical layers of both fluctuating oxygen study groups and in the deep layers of pups who were also growth restricted. Cell density and corpus callosum thickness was preserved. Fluctuating oxygen did not affect proportions of neurons expressing Satb2, Ctip2, or Tbrl but was associated with a reduced proportion of superficial neurons expressing Cuxl, intrinsic to cortico-cortical connectivity within the superficial cortex. Circulating IGF-I was reduced in all study groups in comparison to control, most significantly in the growth restricted group exposed to fluctuating oxygen Cerebral expression of IGF-I and FGF-2 were conserved across the groups. VEGF expression was reduced in the dual group. An increased expression of BDNF was observed in the brainstem in all study groups, most significantly the growth restricted group exposed to oxygen. Conclusions: The developing brain is dynamically influenced by the antenatal and postnatal environment. In a rodent model, normal cortical growth is inhibited by in utero growth restriction and postnatal oxygen fluctuation, in part associated with adverse effects on transcription factors and trophic factors instrumental to normal development. In utero growth restriction may permanently alter elements of cerebral development and postnatal oxygen fluctuation,a modifiable factor in neonatal care, may further impair cerebral growth with a preferential disadvantage to later born superficial neurons. Specific effects on neurons intrinsic to cortical circuitry suggest cortical development exposed to preand postnatal growth restriction may be particularly vulnerable to oxygen induced injury.