Experience-dependent choice of fear responses is regulated by the habenula in zebrafish

Masakazu Agetsuma1, Hidenori Aizawa1, Tazu Aoki1,Mikako Takahoko1, Ryoko Nakayama1,Toshiyuki Shiraki1, Midori Goto1, Koichi Kawakami2, Shin-ichi Higashijima3, Hitoshi Okamoto1

1)RIKEN Brain Science Institute, Wako, Japan, 2)National Institute of Genetics, Mishima, Japan, 3)Okazaki Institute for Integrative Bioscience, Okazaki, Japan

 In the last annual meeting, we reported that habenula is an evolutionarily highly conserved diencephalic structure from fish to mammals, and the chronic silencing of zebrafish habenula with genetic methods enhanced freezing responses against the conditioned stimuli after the cued fear conditioning. Here we show more detailed analyses of the mechanisms regulated by the habenula. In zebrafish, the lateral subnuclei of the dorsal habenula (dHbL) are asymmetrically connected with the dorsal and intermediate parts of the interpeduncular nucleus (d/iIPN), which is shown to further project to the periaqueductal gray (PAG). The specific and chronic silencing of this pathway by tetanus toxin rendered animals extraordinarily prone to freeze against conditioned fear stimuli, while, in the same conditioning paradigm, we found that the control fish learnt to show opponent flight behaviors. We also found that there was no difference in the basic locomotor activity and the US sensitivity during the fear conditioning tasks. The modification of fear behaviors by this pathway is experience-dependent. We also used the nitroreductase-based inducible cell death system, and the acute inactivation of this pathway also induced significantly enhanced immobility. These results suggest that habenula might contribute more directly to the information processing for the behavioral choice. Under threatening environments, choice of a suitable response is critical for survival. Our results demonstrate an unprecedented role of the dHbL-d/iIPN-PAG pathway in the choice of fear responses, which might be crucial for the animal survival in the wild.