Vertebrates respond to light with more than just their eyes. In this article, we speculate on the intriguing possibility that a link remains between non-visual opsins and neurohormonal systems that control neuronal circuit formation and activity in mammals. Historically, the retina and pineal gland were considered the only significant light-sensing tissues in vertebrates. However over the last century, evidence has accumulated arguing that extra-ocular tissues in vertebrates influence behavior through non-image-forming photoreception. One such class of extra-ocular light detectors are the long mysterious deep brain photoreceptors. Here, we review recent findings on the cellular identity and the function of deep brain photoreceptors controlling behavior and physiology in zebrafish, and discuss their implications.
Several extra-ocular tissues can detect and respond to photic stimuli. Here, we discuss the identity and behavioral role of deep brain photoreceptors revealed by recent experiments using zebrafish. We hypothesize that an ancient link between non-visual opsins and neurohormonal systems continues to operate in mammals.