Darvaux-Hubert, I. | bioRxiv Science

bioRxiv2026

Melatonin modulates habituation learning via the convergent action of two MT1-type receptors.

Melatonin is a hormone produced by the pineal gland and retina, with roles in sleep and circadian rhythms as well as other neuronal processes, including the regulation of learning and memory. Using larval zebrafish, we previously identified Melatonin as a potent modulator of habituation learning: the progressive suppression of responses to a repeated stimulus. Here we have extended these analyses and have found that Melatonin has strong effects on separate aspects of habituation learning: it potentiates habituation of response probability and latency, while simultaneously inhibiting habituation of movement amplitude and duration. We performed a systematic CRISPR mutagenesis of all six zebrafish Melatonin receptors, and found that only two MT1-type receptors (Mtnr1aa and Mtnr1al) are required for Melatonin's effects on habituation. These receptors showed a cooperative interaction, with each individual mutant showing reduced sensitivity to Melatonin, and mtnr1aa;mtnr1al double mutants showing complete insensitivity. To map where these receptors act, we used whole-brain activity mapping. Despite distinct footprints, both perturbed a shared set of brain areas, including the torus longitudinalis, cerebellum, and preoptic area, with the double mutant producing the most extensive phenotype by combining and expanding the effects seen in each single mutant. Thus, we propose that two MT1-type receptors act at overlapping circuit nodes to reinforce Melatonin's bidirectional control of learning.

neuroscience↗

bioRxiv2025

Estradiol promotes habituation learning via an unidentified target, bypassing the suppressive effects of established Estrogen Receptors

Habituating to the constant stimuli in the environment is a critical learning process conserved across species. We use a larval zebrafish visual response to sudden darkness as a model for studying habituation learning, where zebrafish reduce their responses to repeated stimulations. In this paradigm, treatment with estradiol strongly increases learning rate, resulting in more strongly suppressed responses. We used mutant lines for the Estrogen Receptors (esr1, esr2a, esr2b, gper1) in an attempt to identify the receptor(s) mediating these effects. These experiments failed to identify a necessary receptor (or combination of receptors). Surprisingly, esr1, esr2a, and gper1 mutants showed weak but consistent increases in habituation, indicating these receptors suppress habituation learning. These experiments demonstrate that estradiol is a complex modulator of learning in our model, where the learning-promoting effects are mediated by an unidentified estradiol target, and the classical Estrogen Receptors act in competition to subtly suppress learning.

neuroscience↗