Preterm birth affects GABA_A receptor subunit mRNA levels during the foetal-to-neonatal transition in guinea pigs

Abstract

Modulation of gamma-aminobutyric acid A (GABAA) receptor signalling by the neurosteroid allopregnanolone has a major role in late gestation neurodevelopment. The objective of this study was to characterize the mRNA levels of GABAA receptor subunits (α4, α5, α6 and δ) that are key to neurosteroid binding in the brain, following preterm birth. Myelination, measured by the myelin basic protein immunostaining, was used to assess maturity of the preterm brains. Foetal guinea pig brains were obtained at 62 days' gestational age (GA, preterm) or at term (69 days). Neonates were delivered by caesarean section, at 62 days GA and term, and maintained until tissue collection at 24 h of age. Subunit mRNA levels were quantified by RT-PCR in the hippocampus and cerebellum of foetal and neonatal brains. Levels of the α6 and δ subunits were markedly lower in the cerebellum of preterm guinea pigs compared with term animals. Importantly, there was an increase in mRNA levels of these subunits during the foetal-to-neonatal transition at term, which was not seen following preterm birth. Myelination was lower in preterm neonatal brains, consistent with marked immaturity. Salivary cortisol concentrations, measured by EIA, were also higher for the preterm neonates, suggesting greater stress. We conclude that there is an adaptive increase in the levels of mRNA of the key GABAA receptor subunits involved in neurosteroid action after term birth, which may compensate for declining allopregnanolone levels. The lower levels of these subunits in preterm neonates may heighten the adverse effect of the premature decline in neurosteroid exposure.

Long-term effects of preterm birth on behavior and neurosteroid sensitivity in the guinea pig

“…Additionally, the results indicate that there are sex differences in the influences of prematurity on these parameters, with males seemingly more affected by early delivery. Furthermore, one of the most important findings was that the deficiencies in neurodevelopment of ex-preterm males seen previously in neonates remained until juvenility, whereas females were able to restore their initial reductions (25,26). Our previous studies in guinea pigs showed markers of mature oligodendrocytes were reduced in preterm compared to term gestational age fetuses, and were markedly affected by preterm delivery in the neonatal hippocampus and subcortical white matter in both sexes (25,26).…”

“…Furthermore, one of the most important findings was that the deficiencies in neurodevelopment of ex-preterm males seen previously in neonates remained until juvenility, whereas females were able to restore their initial reductions (25,26). Our previous studies in guinea pigs showed markers of mature oligodendrocytes were reduced in preterm compared to term gestational age fetuses, and were markedly affected by preterm delivery in the neonatal hippocampus and subcortical white matter in both sexes (25,26). The use of an ex-preterm juvenile model in the present study allowed us to ascertain whether oligodendrocyte expression remains affected until childhood, when many of the school-related disabilities become apparent clinically.…”

“…This reduced allopregnanolone-mediated neuroprotection increases the vulnerability of the neonatal brain to insults such as hypoxia. Furthermore, these preterm animals also show decreased myelination in the CA1 region of the hippocampus and the adjacent subcortical white matter 24 h after delivery, and in the cerebellum at term equivalence (25)(26)(27).…”

“…Previously, we have reported that the expression of these subunits differed between the preterm and term brain at the time of delivery, or allopregnanolone withdrawal, in guinea pigs. Allopregnanolone withdrawal resulted in an adaptive increase in the expression of the δ and α6 subunits in the brain of term guinea pig neonates, and thus represents the feedback impact of allopregnanolone levels on GABA A receptor subunit expression (25). This increase was seemingly absent in the preterm cohort, with expression remaining unchanged between the preterm fetal and neonatal animals 24 h after birth.…”

“…This reduced allopregnanolone-mediated neuroprotection increases the vulnerability of the neonatal brain to insults such as hypoxia. Furthermore, these preterm animals also show decreased myelination in the CA1 region of the hippocampus and the adjacent subcortical white matter 24 h after delivery, and in the cerebellum at term equivalence (25)(26)(27).In this study, we examined the effect of preterm birth on brain development, focusing on the mature myelinating oligodendrocytes and reactive astrocytes that are essential for proper neuronal circuitry and synaptic transmission, to identify effects at adolescence in guinea pigs. We also aimed to ascertain the impact that preterm delivery has on the expression of neurosteroid-sensitive GABA A receptor subunits, in addition to behavior at juvenility, with a particular focus on the hyperactive and anxious behaviors that are common to ex-preterm males and females respectively.…”