Developmental regulation of GABAB receptors and downstream molecules in the mouse brain

Metabotropic GABA (GABAB) receptors have modulatory functions on neuronal excitability and neurotransmitter release. To fulfil these functions, GABAB receptors form macromolecular signaling complexes with G proteins, effectors, and other associated proteins. Here we investigated the postnatal development of GABAB receptors (GABAB1 and GABAB2 subunits) in mouse brain, focusing on potential similarities in the spatial and temporal expression pattern of their associated proteins CaV2.1, Gαo, Gβ5, and RGS7, using histoblots, immunofluorescence, and immunoelectron microscopic techniques. At all ages analyzed, histoblot showed that the six proteins were widely expressed in the brain, with mostly an overlapping pattern throughout postnatal development. In the hippocampus, immunoelectron microscopy and quantitative analysis of immunoparticles for GABAB1, GABAB2, Gαo, Gβ5, and RGS7 revealed their progressive enrichment around excitatory synapses on dendritic spines of CA1 pyramidal cells toward P15. At presynaptic sites, GABAB receptors colocalize with CaV2.1, Gαo, Gβ5, and RGS7 in the active zone and extrasynaptic membranes of axon terminals, establishing synapses on dendritic spines of CA1 pyramidal cells. In the cerebellum, double immunofluorescence at P7 and P10 revealed the colocalization of GABAB1 and CaV2.1 in the whole dendritic tree of developing Purkinje cells. Immunoelectron microscopy at P15 showed that GABAB1, GABAB2, CaV2.1, Gαo, Gβ5, and RGS7 are distributed along the dendritic surface of Purkinje cells, enriched close to excitatory synapses in spines. Altogether, these data suggest that macromolecular complexes composed of GABAB1 /GABAB2/CaV2.1/ Gαo/Gβ5/RGS7 are pre-assembled during key stages of postnatal development in hippocampal and cerebellar neurons.