Defects in dendrite and spine maturation and synaptogenesis associated with an anxious-depressive-like phenotype of GABA_A receptor-deficient mice

Mice that were rendered heterozygous for the γ2 subunit of GABA_A receptors (γ2^+/- mice) have been characterized extensively as a model for major depressive disorder. The phenotype of these mice includes behavior indicative of heightened anxiety, despair, and anhedonia, as well as defects in hippocampus-dependent pattern separation, HPA axis hyperactivity and increased responsiveness to antidepressant drugs. The γ2^+/- model thereby provides strong support for the GABAergic deficit hypothesis of major depressive disorder. Here we show that γ2^+/- mice additionally exhibit specific defects in late stage survival of adult-born hippocampal granule cells, including reduced complexity of dendritic arbors and impaired maturation of synaptic spines. Moreover, cortical γ2^+/- neurons cultured in vitro show marked deficits in GABAergic innervation selectively when grown under competitive conditions that may mimic the environment of adult-born hippocampal granule cells. Finally, brain extracts of γ2^+/- mice show a numerical but insignificant trend (p = 0.06) for transiently reduced expression of brain derived neurotrophic factor (BDNF) at three weeks of age, which might contribute to the previously reported developmental origin of the behavioral phenotype of γ2^+/- mice. The data indicate increasing congruence of the GABAergic, glutamatergic, stress-based and neurotrophic deficit hypotheses of major depressive disorder. This article is part of the Special Issue entitled 'GABAergic Signaling in Health and Disease'.