Organizations and postnatal maturation of local neuronal networks of the nucleus of tractus solitairus

γ-氨基丁酸（GABA）是中枢神经系统中一种主要的抑制性神经递质，被认为在调节兴奋性神经传递中起重要作用。GABAA受体是一种主要由α、β和γ亚基组成的五聚体异聚寡聚体-门控氯离子通道，它必须聚集在突触后膜上才能实现抑制性神经传递。在本研究中，研究人员在孤立束核（NTS）中研究了gaba能突触活性和负责受体介导的传递的分子的表达如何在发育过程中受到调节并相互关联。GABAA能抑制性突触后电流（IPSCs）和GABAA受体γ - 1亚基的表达在出生后早期尾侧NTS的小尺寸神经元中一直很明显（P1-6）。GABAA受体γ - 1亚基是突触后膜受体聚集所必需的。出生后第一周后，这些神经元中IPSCs和GABAA受体γ - 1亚基的表达几乎消失。GABAA受体γ - 2亚基和gephyrin在NTS中一致低水平表达。电镜显示，在发育过程中，gaba能轴体细胞突触数量减少，几乎消失。这些结果表明，gaba能突触活性与γ - 1亚基表达密切相关，并在特定类型的NTS神经元中受到发育调节，以建立成体型的局部神经元网络。这种现象可能是在髓质中建立心血管和呼吸反射回路的基础。

γ-aminobutyric acid (GABA) is a main inhibitory neurotransmitter in the central nervous system and thought to be critically involved in regulating excitatory neurotransmission. For the inhibitory neurotransmission, GABAA receptors, a pentameric hetero-oligomer ligand-gated chloride channel consisting mainly of α,β and γ subunits, must be clustered at the postsynaptic membranes. In the present study, how GABAergic synaptic activity and expression of molecules responsible for the receptor-mediated transmission are regulated and correlated with each other during development,was addressed in the nucleus of the tractus solitarius (NTS). GABAergic inhibitory postsynaptic currents (IPSCs) and expression of GABAA receptqrγ1-subunit, which is essential for the receptor clustering at the postsynaptic membranes, were consistently evident in small sized neurons of the caudal NTS during the early postnatal period (P1-6). After the first postnatal week, both IPSCs and expression of GABAA receptorγ1-subunit virtually disappeared in these neurons. Low level expression of GABAA receptorγ2-subunit and gephyrin were consistently observed in the NTS. Electron microscopy revealed that numbers of GABAergic axosomatic synapses were decreased and virtually disappeared during the development. These results show that GABAergic synaptic activity and γ1 subunit expression were closely associated with each other and that they are developmentally regulated in specific type of NTS neurons for an establishment of mei local neuronal networks of adult type. This phenomenon may underlie an establishment of cardiovascular and respiratory reflex circuits in the medulla.