The hypothalamic photoreceptors regulating seasonal reproduction in birds: a prime role for VA opsin.

José M García-Fernández,Stuart N Peirson,Rafael Cernuda-Cernuda,Wayne I.L Davies,Michael Turton,Stephanie Halford,Jessica Rodgers,Russell G Foster,Brian K Follett,Steven Hughes,Mark W Hankins

doi:10.1016/j.yfrne.2014.11.001

Abstract

Extraretinal photoreceptors located within the medio-basal hypothalamus regulate the photoperiodic control of seasonal reproduction in birds. An action spectrum for this response describes an opsin photopigment with a λmax of ∼ 492 nm. Beyond this however, the specific identity of the photopigment remains unresolved. Several candidates have emerged including rod-opsin; melanopsin (OPN4); neuropsin (OPN5); and vertebrate ancient (VA) opsin. These contenders are evaluated against key criteria used routinely in photobiology to link orphan photopigments to specific biological responses. To date, only VA opsin can easily satisfy all criteria and we propose that this photopigment represents the prime candidate for encoding daylength and driving seasonal breeding in birds. We also show that VA opsin is co-expressed with both gonadotropin-releasing hormone (GnRH) and arginine-vasotocin (AVT) neurons. These new data suggest that GnRH and AVT neurosecretory pathways are endogenously photosensitive and that our current understanding of how these systems are regulated will require substantial revision.

Highlights

Reproduction and other seasonal phenomena in temperate birds are profoundly influenced by changes in photoperiod
Whole-cell patch clamp recordings from transfected fluorescent cells, expressing either chicken VA (cVA) or cVAL, demonstrated light dependent inward currents in the presence, but not the absence, of chromophore (Fig. 1). These results demonstrate that both cVA and cVAL can elicit a retinal-dependent light response in vitro, consistent with the activity of a fully functional opsin photopigment that can couple to a ubiquitous G-protein cascade in Neuro-2A cells
There are two associated themes that are considered in this paper: The first reflects upon the candidacy of vertebrate ancient (VA) opsin as the photopigment of a deep brain photoreceptor regulating one or more avian photoperiodic responses; and the second theme based upon entirely new data considers the unexpected finding that VA opsin may confer endogenous photosensitivity to two populations of neurosecretory cells within the avian brain

Summary

Introduction

Reproduction and other seasonal phenomena in temperate birds are profoundly influenced by changes in photoperiod. This approach yielded protein, which when reconstituted with excess 11-cis retinal and subjected to UV–Vis spectrophotometry, and produced a characteristic opsin/vitamin A difference spectrum with a kmax value of 490 nm (R2 = 0.7) (Fig. 2C) These in vivo and in vitro approaches yielded absorbance maxima at $490 nm, which is 2 nm away from the reported 492 nm peak of the action spectrum for the avian photoperiodic response (Hankins et al, 2008; Young, 1962). A long-wavelength-shifted photopigment in the 40 °C temperature environment of the avian brain would have a greatly diminished capacity to detect light-induced isomerisation from thermally induced isomerisation of the chromophore As a result, these data fully support the criteria listed, that the absorbance characteristics of VA opsin photopigment should be appropriately matched to the spectral qualities of the available light within the hypothalamus. As a result we provide below full methodological details for these findings

Antibodies

Immunostaining

Co-localization results

Discussion

Conclusions