Developmental expression of GABA(A) receptor subunit mRNAs in individual hippocampal neurons in vitro and in vivo.

The GABA(A) receptor is a heterooligomeric protein complex composed of multiple receptor subunits. Developmental changes in the pattern of expression of 11 GABA(A) receptor subunits in individual rat embryonic hippocampal neurons on days 1-21 in culture and acutely dissociated hippocampal neurons from postnatal day (PND) 5 rat pups were investigated using the technique of single-cell mRNA amplification. We demonstrate that multiple GABA(A) receptor subunits are expressed within individual hippocampal neurons, with most cells simultaneously expressing alpha1, alpha2, alpha5, beta1, and gamma2 mRNAs. Further, relative expression of several GABA(A) receptor subunit mRNAs changes significantly in embryonic hippocampal neurons during in vitro development, with the relative abundance (compared with beta-actin) of alpha1, alpha5, and gamma2 mRNAs increasing 2.3-, 2.7-, and 3.8-fold, respectively, from days 1 to 14, and beta1 increasing 5-fold from days 1 to 21. In situ hybridization with antisense digoxigenin-labeled alpha1, beta1, and gamma2 RNA probes demonstrates a similar increase in expression of subunit mRNAs as embryonic hippocampal neurons mature in vitro. Relative abundances of alpha1, beta1, and gamma2 subunit mRNAs in acutely dissociated PND 5 hippocampal neurons are also significantly greater than in embryonic day 17 neurons on day 1 in vitro and exceed the peak values seen in cultured neurons on days 14-21, suggesting that GABA(A) receptor subunit mRNA expression within individual hippocampal neurons follows a similar, if somewhat delayed, developmental pattern in vitro compared with in vivo. These findings suggest that embryonic hippocampal neuronal culture provides a useful model in which to study the developmental regulation of GABA(A) receptor expression and that developmental changes in GABA(A) receptor subunit expression may underlie some of the differences in functional properties of GABA(A) receptors in neonatal and mature hippocampal neurons.