# Light intensity and opsin sensitivity shape the morphology of cone photoreceptor outer segments

## TL;DR

This study shows that the shape of light-sensitive parts in zebrafish cone cells is influenced by the type of light they detect and the intensity of light exposure.

## Contribution

The study reveals a conserved mechanism linking opsin sensitivity and light intensity to cone outer segment morphology through long-term neural activity.

## Key findings

- Cone outer segment morphology correlates with the wavelength sensitivity of expressed opsins.
- Long-term neural activity, not transient changes, drives morphological plasticity in cone outer segments.
- Light intensity, including changes from lipid droplets or environmental factors, modulates outer segment shape.

## Abstract

Regulation of neural cell morphology remains a fundamental question in neuroscience. Photoreceptor cells, a specialized class of neurons capable of initiating the phototransduction cascade, exhibit distinct structural and morphological characteristics. While the structural and morphological differences between rod and cone photoreceptors have been extensively studied, the variability in the morphology of cone outer segments (OS) remains largely unexplored. Zebrafish possess four distinct cone types, each displaying unique OS morphologies. By modulating opsin expression across cone types, we reveal that the morphology of the cone OS correlates directly with the wavelength sensitivity of the expressed opsins, with cones expressing longer wavelength-sensitive opsins exhibiting elongated OS. This regulatory mechanism is conserved across various vertebrates. Furthermore, we show that alterations in light intensity—induced by ectopic lipid droplet formation in the light path or by changing the environment light intensity—can also modulate OS morphology. Notably, this morphological plasticity is not transient, but rather dependent on long-term neural activity. Based on these findings, we propose a model for the regulation of cone OS length. Our data suggest that both opsin sensitivity and light intensity shape cone OS morphology through long-term neural activity, providing critical insights into neural plasticity in these light-sensitive neurons.

Photoreceptor cells exhibit variability in the morphology of the light-absorbing outer segment, the basis of which is unclear. This study in zebrafish shows that the wavelength sensitivity of the expressed opsin and the intensity of light exposure determine outer segment morphology, with implications for our understanding of neuronal plasticity.