Table1_The Neuronal Circuit of the Dorsal Circadian Clock Neurons in Drosophila melanogaster.DOCX

Drosophila’s dorsal clock neurons (DNs) consist of four clusters (DN1as, DN1ps, DN2s, and DN3s) that largely differ in size. While the DN1as and the DN2s encompass only two neurons, the DN1ps consist of ∼15 neurons, and the DN3s comprise ∼40 neurons per brain hemisphere. In comparison to the well-characterized lateral clock neurons (LNs), the neuroanatomy and function of the DNs are still not clear. Over the past decade, numerous studies have addressed their role in the fly’s circadian system, leading to several sometimes divergent results. Nonetheless, these studies agreed that the DNs are important to fine-tune activity under light and temperature cycles and play essential roles in linking the output from the LNs to downstream neurons that control sleep and metabolism. Here, we used the Flybow system, specific split-GAL4 lines, trans-Tango, and the recently published fly connectome (called hemibrain) to describe the morphology of the DNs in greater detail, including their synaptic connections to other clock and non-clock neurons. We show that some DN groups are largely heterogenous. While certain DNs are strongly connected with the LNs, others are mainly output neurons that signal to circuits downstream of the clock. Among the latter are mushroom body neurons, central complex neurons, tubercle bulb neurons, neurosecretory cells in the pars intercerebralis, and other still unidentified partners. This heterogeneity of the DNs may explain some of the conflicting results previously found about their functionality. Most importantly, we identify two putative novel communication centers of the clock network: one fiber bundle in the superior lateral protocerebrum running toward the anterior optic tubercle and one fiber hub in the posterior lateral protocerebrum. Both are invaded by several DNs and LNs and might play an instrumental role in the clock network.

果蝇背部时钟神经元（DNs）由四个簇（DN1as、DN1ps、DN2s和DN3s）组成，这些簇在大小上存在显著差异。其中，DN1as和DN2s仅包含两个神经元，DN1ps由约15个神经元构成，而DN3s每个脑半球大约包含40个神经元。与已充分表征的侧时钟神经元（LNs）相比，DNs的神经解剖学和功能尚不明确。在过去十年中，众多研究探讨了它们在果蝇昼夜节律系统中的作用，导致了一些有时相互矛盾的结果。然而，这些研究一致认为，DNs对于精细调节光照和温度周期下的活动至关重要，并在将LNs的输出与控制睡眠和代谢的下级神经元连接中扮演着至关重要的角色。在本研究中，我们利用Flybow系统、特定的分裂GAL4线、trans-Tango以及最近发表的果蝇连接组（称为半脑）来更详细地描述DNs的形态，包括它们与其他时钟和非时钟神经元的突触连接。我们发现某些DN组在很大程度上是异质的。虽然某些DN与LNs有强烈的连接，但其他DN主要作为输出神经元，向时钟下游的回路发出信号。后者包括蘑菇体神经元、中枢复合体神经元、丘脑球神经元、间脑部分中的神经分泌细胞以及其他尚未确定的合作伙伴。DNs的这种异质性可能解释了之前关于其功能性的某些矛盾结果。最重要的是，我们确定了时钟网络中两个假定的新型通信中心：一个在顶侧原脑中向眼前视泡延伸的纤维束，以及一个在后侧原脑中的纤维枢纽。两者都被多个DNs和LNs所侵犯，可能在时钟网络中发挥关键作用。