UC Santa Cruz

A variety of in vivo imaging techniques have been invented and adopted to study brain function, giving us unprecedented access to dynamics of neuronal circuits from single synapse to large scale network activity. Three-photon microscopy is one of the latest additions, primarily developed for imaging deeper into the brain, and it has shown promises in probing neuronal circuits of an intact animal through highly scattering exoskeleton with minimal or no surgery in mouse, adult zebrafish and fly. Among the model organisms, the fruit fly – Drosophila, has the most comprehensive genetic toolkit, and only the second organism after Caenorhabditis elegans to have a mapped out connectome. The fly brain, with a modest number of neurons (135,000), supports a repertoire of complex behaviors that have been quantitatively studied in laboratories for decades. Reduced complexity, genetic tractability, and the blueprint of neuronal connections make the fly brain is an attractive system to study the neural basis of behavior. The nervous system integrates external stimuli and internal states such as motivation, arousal, drive or emotion to drive adaptive behavior. Hormones and neuromodulators encode internal states and exert influence by modulating or reconfiguring neuronal circuits to shape behavior. Typical in vivo imaging in fly requires removing the cuticle to create an optical window, which perturbs the hormones in the fly's open circulatory system. Here I describe the construction of a three-photon microscope for transcuticle imaging of the fly brain with an emphasis on optimum photon collection. Next, using this non-invasive imaging system, I show how a gut-derived neuropeptide hormone travels through the circulation, modulates a brain circuit, and switches male fly behavior from feeding to mating.