Suppression of 5-HT₂ receptor signaling impairs normal eye development in zebrafish

Serotonin (5-HT) signaling plays essential roles in vertebrate development beyond neurotransmission, including in ocular morphogenesis. Here, we investigated the effects of Ritanserin, a non-selective 5-HT₂ receptor antagonist, on zebrafish embryonic development with a focus on eye formation. While Ritanserin exposure (0.1-10  μM) did not affect mortality or hatching rates up to 96 h post-fertilization (hpf), higher concentrations (5-10  μM) induced developmental abnormalities, such as microphthalmia, pericardial edema, blood congestion, and craniofacial malformations by 5 days post-fertilization (dpf). Ritanserin significantly reduced the expression of cyp19a1b, encoding brain aromatase (AroB) involved in local estrogen synthesis within the eye, and pax6a, a key regulator of retinal neurogenesis, without altering cyp19a1a levels. Co-treatment with exogenous 5-HT or estradiol (E₂) partially rescued eye size, optic nerve morphology, pax6a expression, and reduced retinal apoptosis. Moreover, visual deficits induced by Ritanserin, as shown by impaired visual background adaptation (VBA) and optomotor responses (OMR), were similarly reversed by 5-HT or E₂. These findings demonstrate that 5-HT₂ signaling is critical for proper eye development in zebrafish, likely through regulation of estrogen synthesis and neuroretinal gene expression, and suggest that serotonergic disruption during early development can lead to structural and functional visual deficits.