ABSTRACT
The habenula (Hb) is an evolutionarily conserved diencephalic structure in vertebrates. It is considered as an emotion center and plays critical roles in regulating diverse types of emotion-related behaviors, including anxiety, fear, reward, depression, and nicotine withdrawal. On the one hand, action selection- and emotion-relevant inputs are transferred to the Hb through the basal ganglia and limbic system, respectively. At the same time, sensory inputs of multiple modalities also converge on the Hb. Among them, the visual input of the Hb from the retina ganglion cells ‒ thalamus pathway has been found to play a critical role in light-preference behavior of zebrafish. On the other hand, the Hb projects to two main neuromodulatory systems, the dopaminergic system and the serotoninergic system. As the Hb receives both internal emotion inputs and external sensory inputs and regulates the function of neuromodulatory systems, its functions are quite diverse and complex. In this review, we summarize the progress in both the structure and connection of the Hb and propose future study direction.