Brain Imaging with Improved GCaMPs in Zebrafish

Akira Muto1, Junichi Nakai2, Koichi Kawakami1

1)Molecular Developmental Biology, National Institute of Genetics, 2)Saitama University Brain Science Institute, Saitama, Japan

 Calcium imaging with DNA-coded calcium indicators, such as GCaMPs, is a powerful means for monitoring the activity of genetically identified neurons. We generated UAS:GCaMP transgenic zebrafish and tested their capability to probe neuronal activities in various types of neurons in combination with GAL4 enhancer/gene trap lines. When GCaMP was expressed in the spinal motoneurons, we could detect calcium signals that were correlated to spontaneous muscle contractions. Next we tested if GCaMP can report neural activity in the sensory system. Anatomical study has shown that the map of spatial visual field is represented in the retina, and further projected to the optic tectum. To functionally confirm this retinotopy, we expressed GCaMP in the retinal ganglion cells (RGCs), or in the optic tectum. Axonal terminals of the RGCs in the neuropil of the tectum showed calcium signals in response to visual stimuli to the retina. Tectal cells also showed calcium signals in the neuropil where the tectal cell dendrites make synapses with RGC axons. In both cases, visual stimulus on the temporal side in the visual field elicited calcium signals in the posterior neuropil. In contrast, anterior neuropil showed calcium signals when the stimulus was on the nasal side. Thus, our imaging with GCaMP functionally demonstrated the retinotopic map that was shown previously by anatomical studies. Finally, we expressed GCaMP in the forebrain. We discovered that subpopulation of forebrain cells were active without any obvious sensory stimuli. The neuronal ensemble showed synchronized activity to some extent. The origin and role of these activities remain to be elucidated. We conclude that the GCaMP technology, in combination with the Gal4-UAS system, provides an excellent tool to study functional neural circuits in zebrafish in vivo.