A study on identification of pre-and postsynaptic neural elements of the circadian pacemaker neurons in insects.

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To unravel the wiring of the circadian pacemaker networks underlying rhythmic locomotor activity, we surveyed the pre-and post-synaptic neural elements of the circadian pacemaker neurons, using Drosophila melanogaster and Protophormia terraenovae as model organisms. P. terraenovae has PDF (pigment dispersing factor)-immunopositive neurons (called here Pt-PDF/LNs), corresponding to the Drosophila PDF-immunopositive s-LNvs (small ventral lateral neurons) that function as circadian pacemakers. Therefore, P. terraenovae is also a good model species, paralleling D. melanogaster.In D. melanogaster, the s-LNvs transmit circadian information to the subsets of other clock neurons located in the dorsal protocerebrum of the brain. We examined the synaptic connections of the PDF-positive fibers of s-LNvs in the dorsal protocerebrum, and found that the fibers of s-LNvs contain large dense-cored vesicles (DCVs), as well as numerous small clear vesicles which form divergent output synapses onto unlab … More eled neurites. In addition, three-dimensional reconstruction revealed the presence of input synapses onto the s-LNvs fibers. These results indicate that the s-LNvs receive direct synaptic input in the dorsal protocerebrum, besides exhibiting structural features for supplying the dual outputs from the s-LNvs, either via synaptic outputs or via paracrine release of DCV contents. In P. terraenovae, we demonstrated that the fibers of Pt-PDF/LNs in the accessory medulla receive input synapses from the terminals of extraretinal photoreceptors called 'Pt-eyelets'. Furthermore, in the dorsal protocerebrum, we showed that the fibers of Pt-PDF/LNs form output synapses onto the PL neurons, which locate in the pars lateralis and are indispensable for the induction of reproductive diapause. These results indicate that Pt-PDF/LNs convey the light information detected by the Pt-eyelet to the PL neurons, and that they might play some roles in the time-measurement system and in sending processed information to the PL neurons. Our results provided the first insight into some basic characteristics of the clock's neural circuits. Less