The suprachiasmatic nucleus (SCN) is the main brain clock in mammals. The SCN synchronizes to the light-dark cycle through the retinohypothalamic tract (RHT). RHT axons release glutamate to activate AMPA-kainate and N-methyl-D-aspartate (NMDA) postsynaptic receptors in ventral SCN neurons. Stimulation of SCN NMDA receptors is necessary for the activation of the signaling cascades that govern the advances and delays of phase. To our knowledge, no research has been performed to analyze the functional synaptic modifications occurring during postnatal development that prepare the circadian system for a proper synchronization to light at adult ages. Here, we studied the pre- and postsynaptic developmental changes between the unmyelinated RHT-SCN connections. Spontaneous NMDA excitatory postsynaptic currents (EPSCs) were greater in amplitude and frequency at postnatal day 34 (P34) than at P8. Similarly, both quantal EPSCs (miniature NMDA and evoked quantal AMPA-kainate) showed a development-dependent increase at analyzed stages, P3-5, P7-9, and P13-18. Moreover, the electrically evoked NMDA and AMPA-kainate components were augmented with age, although the increment was larger for the latter, and the membrane resting potential was more depolarized at early postnatal ages. Finally, the short-term synaptic plasticity was significantly modified during postnatal development as was the estimated number of quanta released and the initial release probability. All of these synaptic modifications in the unmyelinated RHT-SCN synapses suggest that synchronization to light at adult ages requires developmental changes similar to those that occur in myelinated fast communication systems.

中文翻译：

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大鼠产后发育过程中视网膜下丘脑道与视交叉上核之间的功能性突触前和突触后变化。

视交叉上核（SCN）是哺乳动物的主要脑钟。SCN通过视网膜下丘脑束（RHT）同步至明暗循环。RHT轴突释放谷氨酸以激活腹侧SCN神经元中的AMPA海藻酸酯和N-甲基-D-天冬氨酸（NMDA）突触后受体。SCN NMDA受体的刺激是激活信号级联反应的必要条件，而信号级联反应则控制着相的进展和延迟。据我们所知，尚未进行任何研究来分析产后发育过程中发生的功能性突触修饰，从而使昼夜节律系统准备好与成年光同步。在这里，我们研究了无髓的RHT-SCN连接之间的突触前和突触后发展变化。出生后第34天（P34），自发NMDA兴奋性突触后电流（EPSC）的幅度和频率大于P8。同样，定量的EPSC（微型NMDA和诱发的定量AMPA-海藻酸盐）在分析阶段P3-5，P7-9和P13-18均显示出依赖于发育的增加。此外，电诱发的NMDA和AMPA-海藻酸酯组分随年龄增加，尽管后者的增加更大，并且在产后早期膜的静息电位更去极化。最后，短期突触可塑性在出生后的发育过程中得到了显着改变，估计的量子释放数量和初始释放概率也得到了显着改善。