Increased neurosteroid sensitivity of hippocampal gaba a receptors during postnatal development

Abstract

To investigate developmental changes in neurosteroid modulation of GABA A receptors, whole-cell currents were elicited by applying GABA with allopregnanolone or pregnenolone sulfate (PS) to dentate granule cells (DGCs), acutely isolated from 7–14-day-old and adult rats. GABA evoked larger currents from dentate granule cells acutely isolated from adult rats (adult DGCs) than from neonatal DGCs, due to increased efficacy (1662±267 pA in adult DGCs versus 1094±198 pA in neonatal DGCs, P=0.004), and current density (0.072±0.01 pA/μm 2 in neonatal rat DGCs to 0.178±0.02 pA/μm 2 in adult DGCs), but unchanged potency (EC 50 was 18.5±2 μM in adult DGCs, and 26.6±7.9 μM in neonatal DGCs, P=0.21). Allopregnanolone sensitivity of GABA A receptor currents increased during development due to an increased potency (21.1±4.7 nM in adult DGCs versus 94.6±9 nM in neonatal DGCs, P=0.0002). The potency and efficacy of PS inhibition of GABA A receptor currents were remained unchanged during development (13±6 μM and 13.2±5.9 μM, P=0.71 and 85.5%±3.5% and 83.6%±0.8%, P=0.29, respectively). To investigate possible mechanism of developmental changes in GABA A receptor properties, in situ hybridization for α1, α4 and γ2 subunit mRNAs was performed in dentate gyrus of the two age groups. Qualitatively, α1 subunit mRNA was expressed at low levels in neonatal rats while it was well expressed in adult rats. The α4 and γ2 subunits were well expressed in the dentate gyrus of adult and neonatal rats. Immunohistochemical staining for α1 subunit in hippocampal slices from neonatal and adult rats was examined under confocal laser scanning microscope. This demonstrated that cell bodies and dendrites of granule cells are moderately positive for the α1 staining in adult rats but weakly so in neonatal rats. Higher-magnification images demonstrate large number of clusters of α1-subunit in the cell bodies of dentate granule cells of adult rat but rare clusters in granule cells of neonatal rats. Maturation of GABA A receptors in DGCs is characterized by increased number of GABA A receptors that are more sensitive to endogenous neurosteroid allopregnanolone, which might be related to increased expression of α1 subunit.