Knockout of the γ-aminobutyric acid receptor subunit α4 reduces functional δ-containing extrasynaptic receptors in hippocampal pyramidal cells at the onset of puberty

Increased plasmalemmal localization of 4138 GABA(A) receptors (GABARs) occurs in the hippocampal pyramidal cells of female mice at pubertal onset Shen et al., 2010). This increase occurs on both dendritic spines and shafts of CA1 pyramidal cells and is in response to hormone fluctuations that occur at pubertal onset. However, little is known about how the alpha 4 and delta subunits individually mediate the formation of functional, plasmalemmal alpha 4 beta delta GABARs. To determine whether expression of the alpha 4 subunit is necessary for plasmalemmal delta subunit localization at pubertal onset, electron microscopic-immunocytochemistry (EM-ICC) was employed. CA1 pyramidal cells of female alpha 4 knockout (KO) mice were tested for plasmalemmal levels of the delta subunit within dendritic spine and shaft profiles at the onset of puberty. EM-ICC revealed that the alpha 4 and delta subunits localize on dendritic spines and shafts at sites extrasynaptic to GABAergic input at pubertal onset in tissue of wild-type (WT) mice. At pubertal onset, plasmalemmal localization of the delta subunit is reduced 45.9% on dendritic spines, and 56.7% on dendritic shafts with KO of the alpha 4 subunit, as compared to WT tissue, yet levels of intracellular delta immunoreactivity remain unchanged. The decline in plasmalemmal localization is manifested as decreased responsiveness to the GABA agonist gaboxadol at concentrations that are selective for delta-containing GABARs. Additionally, all KO mice have larger dendritic spine and shaft profiles. Our findings demonstrate that alpha 4 subunit expression strongly influences the pubertal increase of delta subunits at the plasma membrane, and that genetic deletion of alpha 4 serves as a functional knock-down of delta-containing GABARs.