Fig. 1.

GABA_A receptor subunits are differentially expressed during neuronal lineage progression. Using quantitative flow cytometry of TnTx/A2B5 surface-labeling patterns and intensities in conjunction with microdissection, we identified the E19 neocortical cells as neural precursors (DN), neuroglial (SP-A2B5) or neuronal (SP-TnTx^lo or DP-TnTx^lo) progenitors from the VZ/SVZ, or postmitotic/postmigratory differentiating neurons (SP-TnTx^hi or DP-TnTx^hi) from the CP/SP regions (Maric et al., 2000b). After surface phenotyping, the cells were fixed and processed for the expression of 13 GABA_A receptor and 2 GABA_B receptor subunits (see Materials and Methods).Left, As an example, the log–log dot density plots inpseudocolor show the TnTx/A2B5 labeling pattern characteristic of E19 neocortical cells and the distributions of the γ3 GABA_A receptor subunit in the context of either surface label. Boundaries (crosshairs) between labeled and unlabeled cells and between cells expressing low and high levels of TnTx labeling (TnTx^lo and TnTx^hi) have been drawn empirically to quantify the percentages of cells in each subpopulation. Middle, Right, The bar graphs summarize the percentage (means ± SEM) of immunopositive cells in each subpopulation from three independent experiments. Approximately 35–65% of DN neural precursor cells in the VZ/SVZ express α4, β1, γ1, γ3, or δ GABA_A receptor subunits (black bars) and both GABA_B R1 and R2 receptor subunits (white bars), whereas SP-A2B5 neuroglial progenitors are virtually devoid of GABA_A and GABA_Breceptors. Most SP-TnTx^lo neuronal progenitors (75–90%) from VZ/SVZ express α4, β1, or γ1, whereas ∼50% exhibit γ3 or δ, and ∼30% are α3⁺, β2⁺, or γ2⁺. Many of these cells (60–75%) also express both GABA_B receptor subunits. Some DP-TnTx^lo neuronal progenitors (20–40%) express α4, β1, or γ1 subunits, and ∼15% express both GABA_B subunits. Almost all SP-TnTx^hineurons (85–95%) from the CP/SP are α3⁺, β3⁺, γ2⁺, or γ3⁺, as are many DP-TnTx^hineurons (55–75%). Approximately half of the SP-TnTx^hi neurons exhibit GABA_B receptor subunits, whereas only few DP-TnTx^hi neurons (∼10%) express GABA_B receptors. None of the neocortical populations express α1 and α6 GABA_A receptor subunits at E19.