A New Method for Imaging Neuropeptide Release in the Brain

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Neuropeptides modulate synapses and circuits to control mood and a wide variety of behaviors including appetite, pain perception and circadian rhythms. Despite the importance of neuropeptides, it is not possible to detect synaptic neuropeptide release in the intact living brain with current methods. However, we recently developed a new optical approach for imaging exocytosis of neuropeptide-containing dense-core vesicles (DCVs) at intact living Drosophila synapses. This approach is based on inserting a fluorogen activating protein (FAP), which confers fluorescence on the normally nonfluorescent dye malachite green (MG), into a proneuropeptide, thus targeting the FAP to the DCV lumen. Following extracellular application of membrane impermeant MG derivatives that are small enough to pass through fusion pores, activity-evoked fusions of individual DCVs can readily be resolved at the Drosophila neuromuscular junction. Furthermore, we detected a novel mode of DCV spontaneous exocytosis that is distinguished by its sensitivity to perturbations of the secretory apparatus (e.g. resistance to tetanus toxin). Finally, preliminary studies show that the neuropeptide-FAP approach is applicable to studying circadian peptidergic neurons in the intact adult Drosophila brain. Therefore, to demonstrate the utility of FAP imaging, this approach will be used to answer fundamental questions regarding the function of the circadian circuit in the Drosophila. For example, we will determine the timing of neuropeptide release by multiple neurons and address whether a newly discovered mode of spontaneous release accounts for tetanus toxin resistant behavioral effects of a fly neuropeptide. Furthermore, we will use new dyes, a second spectrally distinct FAP variant and genomic engineering to enable simultaneous imaging of synaptic release of two neuropeptides under native transcriptional control. In addition to testing specific hypotheses about peptidergic transmission in the circadian circuit of the adult fly brain, these studies will serve as proof of principle examples for applying real time neuropeptide-FAP imaging to other systems including the mammalian central nervous system.