Enlightening the brain: linking deep brain photoreception with behavior and physiology

Abstract

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.

Keywords: behavior; deep brain photoreceptors; melanopsin; neurohormones; zebrafish.

Figure 1

Non-visual opsins are broadly expressed in the zebrafish central nervous system. Schematic representation showing the expression domains of several non-visual opsins in the brain. Expression is based on whole-mount in situ data from opn4a, valopa, tmtopsa, tmtopsb and opn3l opsins (see colored labels). Data from different WT larvae at 3 days post-fertilization were superimposed.

Figure 2

Illustration of dark photokinesis as a non-directional mechanism for light seeking. A: Larvae within an arena are suddenly exposed to complete darkness and are blocked from view of a distant light source. Activity rapidly increases in the dark (dark photokinesis) enabling larvae to eventually escape the arena. B: Once outside the arena, the light source becomes visible and larvae use retinal vision to orient and move directly towards the light source.