VA Opsin-Based Photoreceptors in the Hypothalamus of Birds

Abstract

Studies in the 1930s demonstrated that birds possess photoreceptors that are located within the hypothalamus and regulate photoperiodic responses to day length. Most recently, photoperiod has been shown to alter the activity of the pars tuberalis to release thyrotrophin, which ultimately drives a reproductive response. Despite these significant findings, the cellular and molecular identity of the hypothalamic photoreceptors has remained a mystery. Action spectra implicated an opsin-based photopigment system, but further identification based on rod- or cone-opsin probes failed, suggesting the utilization of a novel opsin. The vertebrate ancient (VA) opsin photopigments were isolated in 1997 but were thought to have a restricted taxonomic distribution, confined to the agnatha and teleost fish. Here, we report the isolation of VA opsin from chicken and show that the two isoforms spliced from this gene (cVAL and cVA) are capable of forming functional photopigments. Further, we show that VA opsin is expressed within a population of hypothalamic neurons with extensive projections to the median eminence. These results provide the most complete cellular and molecular description of a deep brain photoreceptor in any vertebrate and strongly implicate VA opsin in mediating the avian photoperiodic response.

A mammalian neural tissue opsin (Opsin 5) is a deep brain photoreceptor in birds

“…Foster et al, Foster and Follett, and Halford et al reported an action spectrum for the photoperiodic response in quail with a λ max of ≈483-492 nm (9,10,16). Careful analysis of their results, however, revealed that light with a wavelength of 410 nm also caused a photoperiodic response.…”

“…Expression of melanopsin has been demonstrated in the lateral septal region as well as the retina and pineal gland in chicks (15). Recently, vertebrate ancient (VA) opsin-like immunoreactivity was detected in the avian anterior hypothalamus (16). Although spectral sensitivities of avian melanopsin and VA opsin remain to be determined, both mammalian melanopsin and teleost VA opsin have a λ max of ∼480 nm (44,45).…”

“…Surprisingly, in several studies, rhodopsin-like immunoreactivity has not been successfully detected in the quail (13,14). Recently, mRNA expression of melanopsin (Opn4) in the chick septal region and VA opsin-like immunoreactivity in the chicken and quail anterior hypothalamus have been reported, and these opsins are suggested to be candidate deep brain photoreceptors (15,16). However, the functional significance of these candidate photopigments remains unknown, along with the identity of the deep brain photoreceptors.…”

Shedding light on photoperiodism

“…Therefore, it remains a possibility that the UV and blue wavelengths tested by Nakane et al (2) stimulated testis growth through photopigments other than Opn5. As previously mentioned, VA-opsin has indeed been identified in the developing (19) and adult avian brain (11); it has also been suggested (albeit not tested) that a TSH-mediated photoperiodic response, virtually identical to the one presented by Nakane et al (2), is initiated by VA-opsin activation (11).…”

“…Rhodopsin-like immunoreactivity was initially observed in the tuberal hypothalamus and septal forebrain regions of the pigeon (8,9). Later studies indicated that melanopsin and vertebrate ancient opsin (VA-opsin) were found in the avian brain (10,11).…”