The dorsomedial telencephalon is essential for active avoidance response in zebrafish

Pradeep Lal:1,2 Mari Hiratani:1 Maximiliano L Suster:1,3 Koichi Kawakami:1,2  1:National Institute of Genetics, Japan 2:The Graduate University for Advanced Studies (SOKENDAI), Japan 

Forebrain structures have been shown to be involved in learning and memory in mammals. However, neural circuits involved in these processes are still largely unexplored. Zebrafish presents tremendous potential in identifying such functional neural circuits common to vertebrate. In this study, we regionalize the adult zebrafish brain and selectively manipulate specific neuronal populations to understand their roles in active avoidance response. First, using the Tol2 transposon-mediated gene trap and enhancer trap methods, we created transgenic fish lines that expressed the Gal4 transactivator in specific tissues. In these lines, Gal4 is visualized with UAS:GFP. We analyzed 349 Gal4 transgenic lines and collected 77 lines that showed strong GFP expression in specific regions of the adult brain. Next, we developed an active avoidance response assay system in which adult zebrafish showed robust learning in associating visual stimuli to electric shock. Finally, to inhibit the function of the Gal4 expressing neurons, we crossed the Gal4 lines with effector fish that carried botulinum neurotoxin gene downstream of UAS. Using the active avoidance response assay, we analyzed 29 double transgenic fish lines and found that 17 lines exhibited abnormalities in avoidance response. From these 17 lines, two transgenic lines had Gal4 expression specific to a subpopulation of neurons in the dorsomedial telencephalon. In mammals, active avoidance response is mediated by amygdala. From this study and previous anatomical studies, we think that dorsomedial telencephalon in zebrafish may be the functional equivalent of amygdala in mammals. Further, we found that the Gal4 expressing neurons in the two lines project their axons to entopeduncular nucleus and hypothalamus in the forebrain. Our present study revealed functional neural circuits involved in active avoidance response in zebrafish and will shed light on understanding of common neuronal basis of fear memories in vertebrate.