Hypotheses in light detection by vertebrate ancient opsin in the bird brain

Extra-retinal photoreception is common across fish and avian species. In birds, the hypothalamus contains non-visual photoreceptors that detect light and regulate multiple endocrine systems. To date, light-dependent control of seasonal reproduction is one of the most well-studied systems that require deep brain photoreception. However, the precise photoreceptor(s) that detect light and the neuroendocrine connection between opsin-expressing cells and the gonadotropin-releasing hormone-1 (GnRH1) system remain poorly defined. In the past couple of decades, two opsin molecules have been proposed to link light detection with seasonal reproduction in birds: neuropsin (Opn5) and vertebrate ancient opsin (VA opsin). Only VA opsin is expressed in GnRH1 cells and has an absorption spectrum that matches the action spectrum of the avian photoperiodic reproductive response. This perspective describes how the annual change in daylength, referred to as photoperiod, regulates the neuroendocrine control of seasonal reproduction. The opsin genes are then outlined, and the cellular phototransduction cascade is described, highlighting the common feature of hyperpolarization in response to light stimulation. We then discuss the latest evidence using short-hairpin RNA to temporarily knock down VA opsin and Opn5 on transcripts involved in the neuroendocrine regulation of reproduction. Based on emerging data, we outline three theoretical scenarios in which VA opsin might regulate GnRH1 synthesis and release in birds. The models proposed provide a series of testable hypotheses that can be used to improve our understanding of avian light detection by VA opsin or other opsin-expressing cells in the brain.