UCSF

Expression of halorhodopsin (NpHR), a light-driven microbial chloride pump, allows for optical control of membrane potential and reversible silencing of transfected neurons and muscle cells ("optogenetics"). Zebrafish are ideal model organisms for experiments involving localized light delivery, since they are optically clear, genetically tractable and display many behaviors within the first two weeks of life.

Here, the optogenetic approach was adapted to zebrafish by creating transgenic zebrafish lines coding for light-activatable pumps (NpHR) and channels (channelrhodopsin-2, ChR2) and by implementing optical methods for the precise application of light. We studied the neural circuits underlying the generation of fast eye movements (saccades) during the optokinetic response (OKR). The OKR is a visual behavior, which minimizes retinal slip during rotating motion. Applying light through thin optic fibers positioned above the animal's head enabled us to target small groups of cells and to simultaneously test the effect of their silencing on the generation of saccades during the OKR. We identified a cell population in rhombomere 5 that is required and sufficient for the generation of saccades in zebrafish larvae. The discovered similarities between zebrafish and mammalian oculomotor circuitry are in line with a possible homology of the saccade generator in fish and mammals.

Furthermore, we studied the cardiac conduction system throughout development using these tools. By automated sequential illumination of small areas of the developing heart with a digital micromirror device (DMD), while simultaneously monitoring heart contractions, we identified the regions responsible for initiating and relaying cardiac conduction. By directly targeting the pacemaker areas with well-defined light pulses, we were able to induce disease-like states (tachycardia, bradycardia, AV blocks).

Together, our studies have introduced a versatile optogenetic toolkit for precise loss-of-function and gain-of-function analysis of neural circuits, behavior, and physiology.