Maturation of GABAergic signaling during brainstem development

Abstract

In the central nervous system, γ-aminobutyric acid (GABA) is known to act as an inhibitory neurotransmitter, via ligand-gated GABAA receptor channels and G protein-coupled GABAB receptors. Recent studies have revealed that the role of the GABAergic system changes developmentally, so that in the immature CNS serves as an excitatory neurotransmitter. In this study, we invastigated factors that influence the developmental changes of the ionotropic GABAA and metabotropic GABAB receptor signaling in mouse brainstem. We examined the signaling modulation of GABAB receptor in the developing mouse brainstem as function of GABAB receptor subtype variants, binding properties of GABAB receptor to different G proteins, and G protein expression variability. We demonstrated that the developmental alteration of the GABAB receptor signaling between neonatal and adult mouse brainstem is not associated with expression difference in the G protein coupling sites of the GABABR2. The investigation of the GABAB signaling pathways in newborn and adult mouse brainstem revealed that GABAB receptors couple to Gαi/o proteins at both ages. Furthermore, we found that GABAB receptors interact also with Gαs proteins, and that this coupling disappears with further maturation. Qualitative and quantitative analysis of the Gα protein subtypes in mouse brainstem revealed diverse expression patterns for the Gα protein subtypes of our interest. The expression levels of the Gαi/o are equal at both developmental stages (newborn and adult), whereas the Gαs variants showed an age-dependent alteration in their expression. The complexity and variability of the Gαs variants as well as their ability to modulate each other, makes it difficult to predict their influence to the GABAB receptor signaling. In addition, we investigated the GABAA receptors in brainstem of MeCP2 knockout mice, a model system which allowed us to examine the Rett syndrome. Brainstem analysis of knockout mice revealed decreased levels of GABAA alpha1 and increased levels of GABAA alpha2 comparing to normal mice of the same age (P7). The two subunits have an exceptional role in the regulation of the GABAA receptor, thus it is proposed that the GABAA alpha1 subunits which are mainly expressed in the mature brain, are involved in synaptic inhibition, whereas the GABAA alpha2 subunits which are mainly expressed in immature brain areas, contribute to the depolarizing GABAA effect.