未成熟期脑干听觉核团抑制性与兴奋性突触传递的协同作用决定其对胆红素神经毒性易感

Neonatal auditory system shows selective vulnerability to bilirubin-induced neurotoxicity, namely bilirubin ototoxicity. Auditory brainstem nuclei (ABN) have been extensively speculated as the key targets of the damage, but the beneath mechanisms are still unclear. Mammalian ABN neurons receive of both GABA/glycinergic (using GABA and glycine as neurotransmitter) and glutamatergic (using glutamate as neurotransmitter) synaptic projections, through which coordinating the transmission of nervous signals and the level of cell metabolism. Glutamatergic synaptic transmission always evokes excitatory action in both immature and mature ABN neurons; however, GABA/glycinergic transmission is surprising to present a developmental conversion of the action. In immature mammalian ABN neurons, GABA/glycine are original depolarizing, but gradually change from depolarization to the classic hyperpolarizing-inhibition before hearing onset. Meanwhile, our recent studies have clearly demonstrated the facilitating effects of bilirubin on both GABA/glycinergic and glutamatergic synaptic transmissions in SD rat ABN neurons. Therefore, we speculate reasonably that this kind of facilitating effects may induce strikingly different consequences in terms of bilirubin neurotoxicity. Due to the hyperpolarizing actions of GABA and glycine in mature neurons, their facilitation may serve as great protective roles; however, given the depolarizing actions in immature neurons, the facilitation may result in extensive excitotoxicities in combination with facilitated glutamatergic transmission. This proposition can clearly explain the mechanism beneath the neonatal vulnerability of auditory system to bilirubin ototoxicity. This project is resulted from a well theoretical foundation, together with reasonable technical feasibility. The implementation of the project will be helpful to create a novel theory in regarding to bilirubin neurotoxicity.

未成熟期听觉系统对胆红素神经毒性易感，脑干听觉核团（ABN）是主要受损部位，但机制不明。哺乳动物ABN神经元接受抑制性（GABA/甘氨酸为神经递质）及兴奋性（谷氨酸为神经递质）突触投射，导致神经元胞膜超极化或去极化。无论在听觉发育未成熟或是成熟期，谷氨酸始终产生去极化效应。然而新近发现GABA/甘氨酸不同，从出生至成熟，其经历一个从去极化至超极化效应的逆转过程。我们针对SD大鼠耳蜗核的研究证实了此点。同时发现，胆红素显著激活ABN神经元上述两种突触投射。结合逆转现象我们推测，该激活对未成熟及成熟神经元的病理意义截然相反：因成熟期GABA/甘氨酸的超极化效应，该激活将发挥降低兴奋性的保护作用；而在未成熟期，该激活必将与谷氨酸产生协同去极化效应，诱发神经元过度兴奋的毒性作用。该假说结合了听觉发育与药物毒理研究的新进展，理论依据充分，研究技术成熟，有望破解未成熟期听觉系统对胆红素毒性的易感机制。

未成熟脑干听觉核团对胆红素易感，其机制不明确。既往研究表明胆红素可以促进谷氨酸能突触传递，致神经元产生兴奋毒性，但其同时也可促进γ-氨基丁酸能/甘氨酸能突触传递。由于γ-氨基丁酸能/甘氨酸能突触传递调控的氯离子通道平衡电位在发育过程中会出现极性翻转，这证明γ-氨基丁酸能/甘氨酸能突触传递会增加未成熟神经元的兴奋性。以此为基础我们证实了胆红素通过促进γ-氨基丁酸能/甘氨酸能突触传递导致超早期神经元产生兴奋毒性，这一现象具有钙离子依赖性。我们进一步借助不同亚型电压依赖性钙离子通道的阻滞剂，证明了胆红素对P/Q亚型具有特异性的激活作用，这或许是胞内钙超载的重要原因。最后我们发现NAD+可以通过抑制兴奋性突触传递及细胞内源性兴奋性拮抗与逆转胆红素所致的脑干听觉神经元兴奋毒性，证实其在防治胆红素脑病方面具有良好的应用前景。