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

γ-氨基丁酸是中枢神经系统中的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着关键作用。对于抑制性神经传递，GABA A受体必须聚集在突触后膜上，这是一种由α，β和γ亚基组成的五聚体异寡体配基门控氯离子通道。在本研究中，GABA能突触活性和受体介导的传递分子的表达在发育过程中是如何调节和相互关联的，在孤束核(NTS)中进行了研究。GABA能抑制性突触后电流(IPSCs)和受体聚集在突触后膜上所必需的GABA受体γ1亚单位的表达，在出生后早期(P1-6)在NTS尾侧的小神经元中持续可见。出生后1周，这些神经元中的IPSCs和GABA A受体γ1亚单位的表达几乎消失。GABAA受体、γ2-亚基和Gen在NTs中低水平表达。电子显微镜显示GABA能轴体突触数量减少，在发育过程中几乎消失。这些结果表明，GABA能突触活动与γ-1亚单位的表达密切相关，并在特定类型的NTS神经元中受到发育调节，从而建立成人型的MEI局部神经元网络。这种现象可能是在延髓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.