Molecular and functional mechanisms of long-term visual habituation in larval zebrafish

Long-term habituation allows animals to learn to ignore persistent but unimportant stimuli. Despite being the most basic form of learning, a consensus model on the underlying mechanisms has yet to emerge. We have exploited a visual habituation paradigm in larval zebrafish, where larvae will learn to reduce their reactions to abrupt global dimming (a dark flash). Using a drug-screening approach, we first identified pathways that potently altered habituation learning, including GABAergic inhibition, Melatonin and Estrogen signaling. To determine how habituation manifests at the circuit-level, we used whole-brain pERK and 2-photon Ca2+ imaging. These analyses identified 12 classes of neurons that differ in their stimulus response profile, rate of adaptation during learning, anatomical location, and GABAergic identity. By analyzing how GABA and Melatonin alter population activity, we propose a model for dark flash habituation in which the suppression of activity begins early in the visual pathway but downstream of the retina. This suppression is mediated by GABAergic inhibitory motifs resulting in heterogeneous inhibition of distinct neuronal types in the dark flash circuit. Our results have identified multiple molecular pathways acting in functional cell types underlying a form of long-term plasticity in a vertebrate brain, and allow us to propose the first iteration of a model for how and where this learning process is encoded in individual neurons to shape learned behaviour. ### Competing Interest Statement The authors have declared no competing interest.