Deep-brain photoreception links luminance detection to motor output in Xenopus frog tadpoles

Abstract

Non-visual photoreceptors are widely distributed in the retina and brain but their roles in animal behaviour remain poorly understood. Here we document a novel form of deep-brain photoreception in Xenopus laevis frog tadpoles. The isolated nervous system retains sensitivity to light even when devoid of input from classical eye and pineal photoreceptors. These preparations produce regular bouts of rhythmic swimming activity in ambient light but fall silent in the dark. This sensitivity is tuned to short wavelength UV light; illumination at 400nm initiates motor activity over a broad range of intensities while longer wavelengths do not cause a response. The photosensitive tissue is located in a small region of caudal diencephalon - this region is necessary to retainresponses to illumination while its focal illumination is sufficient to drive them. We present evidence for photoreception via the lightsensitive proteins OPN5 and/or cryptochrome 1, since populations of OPN5-positive and cryptochrome-positive cells reside within the caudal diencephalon. This represents a hitherto undescribed vertebrate pathway that links luminance detection to motor output. The pathway provides a simple mechanism for light avoidance and/or it may reinforce classical circadian systems.