Molecular cloning, localization and circadian expression of chicken melanopsin (Opn4): differential regulation of expression in pineal and retinal cell types

Abstract

The avian retina and pineal gland contain autonomous circadian oscillators and photo-entrainment pathways, but the photopigment(s) that mediate entrainment have not been definitively identified. Melanopsin (Opn4) is a novel opsin involved in entrainment of circadian rhythms in mammals. Here, we report the cDNA cloning of chicken melanopsin and show its expression in retina, brain and pineal gland. Like the melanopsins identified in amphibians and mammals, chicken melanopsin is more similar to the invertebrate retinaldehydebased photopigments than the retinaldehyde-based photopigments typically found in vertebrates. In retina, melanopsin mRNA is expressed in cells of all retinal layers. In pineal gland, expression was strong throughout the parenchyma of the gland. In brain, expression was observed in a few discrete nuclei, including the lateral septal area and medial preoptic nucleus. The retina and pineal gland showed distinct diurnal expression patterns. In pineal gland, melanopsin mRNA levels were highest at night at Zeitgeber time (ZT) 16. In contrast, transcript levels in the whole retina reached their highest levels in the early morning (ZT 0-4). Further analysis of melanopsin mRNA expression in retinal layers isolated by laser capture microdissection revealed different patterns in different layers. There was diurnal expression in all retinal layers except the ganglion cell layer, where heavy expression was localized to a small number of cells. Expression of melanopsin mRNA peaked during the daytime in the retinal pigment epithelium and inner nuclear layer but, like in the pineal, at night in the photoreceptors. Localization and regulation of melanopsin mRNA in the retina and pineal gland is consistent with the hypothesis that this novel photopigment plays a role in photic regulation of circadian function in these tissues.

A non‐canonical photopigment, melanopsin, is expressed in the differentiating ganglion, horizontal, and bipolar cells of the chicken retina

“…The nucleotide sequence of cOpn4-a was identical to the one deposited in GenBank (accession number, AY036061; Chaurasia et al, 2005), encoding a 543-amino-acid protein (Fig. 1B).…”

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

“…Although rhodopsin-like immunoreactivity has been observed in the pigeon septal region (11,12), multiple independent studies failed to detect similar signals in the quail brain (13,14). 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).…”

“…However, we cannot exclude the possibility that some of the chicken probes did not work in quail. In addition, it is reported that expression of some opsin genes is rhythmically regulated (15,41). Because we collected brains in the middle of the day, the expression of some genes may have been too low for detection at that time point.…”

“…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.…”

Novel functions for Period 3 and Exo-rhodopsin in rhythmic transcription and melatonin biosynthesis within the zebrafish pineal organ

“…Many other opsin genes are also expressed with a strong daily rhythm [6][7][8]13,17,[23][24][25]29,31,33,35,42,49,50,55,59]. However, the function of this rhythmic expression is not well understood.…”