Roles of GABA in the development of the nervous system : analysis of GAD-deficient mice

谷氨酸是以谷氨酸为原料，通过谷氨酸脱羧酶GAD65和GAD67两种亚型合成谷氨酸-氨基丁酸(γ-aminobuyric，GABA)。为了进一步阐明GABA在大脑发育和成熟中的作用，我们建立了GAD基因敲除小鼠并分析了它们的表型。虽然GAD65是在出生后发育起来的，其缺陷会导致癫痫发作和异常的情绪行为，但GAD67的表达与早期胚胎阶段的GABA能神经元的分化同步开始。对GAD67+/-和GFP+/-小鼠上丘内GABA能神经元的发育进行了研究。尽管SC与新皮质一样呈层状结构，但其GABA能神经元与邻近脑室带的其他神经元一起起源。在SC分化出GABA能神经元之前，表面有大量的GABA能纤维。它们可能起源于SC之外，并跨越了中线，表明它们在发育中的作用。与野生型相比，GAD67-/-小鼠的GABA含量不到10%，但它们的大脑结构没有显示出任何严重的缺陷。GAD67基因敲除小鼠的神经功能在出生后因裂板而无法存活，成年动物无法研究其神经功能。从胚胎或新生儿分离的脑干进行电生理研究时，呼吸节律和呼吸神经元抑制性突触电流严重受损。

γ-Aminobutyric acid (GABA) is synthesized from glutamate by two isoforms of glutamic acid decarboxylase (GAD), GAD65 and GAD67. In order to further elucidate the roles of GABA in the developing and mature brain, we have generated GAD gene-knockout mice and analyzed their phenotypes. While GAD65 is developed postnatally and its deficiency results in seizure and abnormal emotional behavior, expression of GAD67 starts simultaneously with differentiation of GABAergic neuron at an early embryonic stage. Development of GABAergic neurons was studied in the superior colliculus (SC) of the GAD67+/- and GFP+/- mice. Although the SC is layered structure as the neocortex, its GABAergic neurons were originated together with other neurons from the nearby ventricular zone. Before GABAergic neurons were differentiated in the SC, abundant GABAergic fibers were transiently present on the surface. They originated probably outside of SC and crossed the midline, suggesting their developmental roles. GAD67-/- mice contained less than 10% of GABA compared with the wild type but did not show any serious defect in their brain structure. Neural function of GAD67 knockout mice did not survive after birth because of cleft plate and their neural function could not studied in adult animals. When the brainstem isolated from the embryo or newborn was studied electrophyiologically, respiratory rhythm and inhibitory synaptic currents in respiratory neurons were greatly impaired.