Development of GABAB subunits and functional GABAB receptors in rat cultured hippocampal neurons

Metabotropic gamma-aminobutyric acid receptors (GABA(B)Rs) play a critical role in inhibitory synaptic transmission in the hippocampus but the ontogeny of their subunit synthesis and synaptic localisation has not been determined. Here we report the distributions and developmental profiles of GABA(B1) and GABA(B2) subunits in cultured rat embryonic hippocampal neurons. Limited levels of GABA(B1) and GABA(B2) immunoreactivity were present at 3 days in vitro (DIV). At 7 DIV, when baclofen-evoked inwardly rectifying K(+) channel-mediated responses first appear in the cells, there was a more widespread expression within the soma and proximal dendrites. Levels of the K(+) channel GIRK 1 were relatively constant at all time points suggesting channel availability does not limit the appearance of functional GABA(B)Rs. At 14 DIV the staining displayed a punctate dendritic distribution and near maximal GABA(B)R-mediated electrophysiological responses were obtained. About half of the puncta for each GABA(B)R subunit in dendrites co-localised with the synaptic marker SV2a suggesting that these subunits are at or very near to synapses. Interestingly, at all ages strong GABA(B)R immunoreactivity was also present in the nuclei of neurons. These results provide an important developmental baseline for future studies aimed at investigating, for example, the trafficking and functional regulation of these receptors.