Independent maturation of the GABAB receptor subunits GABAB1 and GABAB2 during postnatal development in rodent brain

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Abstract
GABAB receptors mediate slow inhibitory GABAergic neurotransmission. They are encoded by two distinct subunits, GABAB1 (GBR1) and GABAB2 (GBR2), with two major isoforms of GBR1, GBR1a and GBR1b, arising from differential promoter usage. Heterodimerization of GBR1 and GBR2 is essential for GABAB receptor function, as shown in recombinant expression systems and in GBR1−/− mice. GABAB receptors are highly expressed during ontogeny, prior to synaptogenesis, but their developmental function remains elusive. Here we investigated the postnatal development of GABAB receptors in rodent brain, focusing on potential differences in the spatial and temporal expression pattern of GBR1 and GBR2. Immunohistochemistry with subunit‐specific antibodies revealed a widespread staining for GBR1a and GBR2 in neonatal rodent brain. During the first 2 weeks, these two subunits exhibited largely overlapping regional distribution, but with profound distinctions in cellular and subcellular localization. The adult‐like pattern was established during the third week, with a prominent up‐regulation of GBR1b, extensively codistributed with GBR2. Several unexpected features were noted at early stages, notably, a selective GBR2 staining of axonal tracts, such as the corticothalamic projection, and a prominent GBR1 expression in astrocytes. The specificity of the antibody labeling was verified in GBR1‐ and GBR2‐knockout mice. In addition, the analysis of these mutants revealed a partial preservation of GBR2 staining in GBR1−/− mice and vice versa. Altogether, the results suggest a functional role for GBR1 and GBR2 proteins in immature brain in addition to their contribution to dimeric GABAB receptor complexes. J. Comp. Neurol. 477:235–252, 2004.