Abstract
In lower vertebrates, brain photoreceptor cells express vertebrate-specific non-visual opsins. We previously revealed that a pineal-related organ-specific opsin, parapinopsin, is UV-sensitive and allows pineal wavelength discrimination in lampreys and teleost. The Australian pouched lamprey was recently reported as having two parapinopsin-related genes. We demonstrate that a parapinopsin-like opsin from the Japanese river lamprey exhibits different molecular properties and distribution than parapinopsin. This opsin activates Gi-type G protein in a mammalian cell culture assay in a light-dependent manner. Heterologous action spectroscopy revealed that the opsin forms a violet to blue-sensitive pigment. Interestingly, the opsin is co-localised with green-sensitive P-opsin in the cells of the M5 nucleus of Schober (M5NS) in the mesencephalon of the river and brook lamprey. Some opsins-containing cells of the river lamprey have cilia and others an axon projecting to the retina. The opsins of the brook lamprey are co-localised in the cilia of centrifugal neurons projecting to the retina, suggesting that cells expressing the parapinopsin-like opsin and P-opsin are sensitive to violet to green light. Moreover, we found neural connections between M5NS cells expressing the opsins and the retina. These findings suggest that the retinal activity might be modulated by brain photoreception.
Highlights
In lower vertebrates, brain photoreceptor cells express vertebrate-specific non-visual opsins
We found that parapinopsin, which is first found in catfish pineal and parapineal organs[5], is a UV-sensitive opsin and is involved in the pineal wavelength discrimination of the river lamprey[14]
We investigated immunohistochemically if in a second species, the brook lamprey (L. reissneri), centrifugal retinal neurons in M5 nucleus of Schober (M5NS) express bPPL and P-opsin (Fig. 6)
Summary
Brain photoreceptor cells express vertebrate-specific non-visual opsins. Four types of vertebrate-specific non-visual opsins, which are close to but clearly distinguished from vertebrate visual opsins, are found in non-mammalian vertebrates, including lampreys, teleost, frogs, lizards and birds[2] These opsins are categorised into vertebrate non-visual opsin group: pinopsin[3,4], parapinopsin[5] and parietopsin[6] are characterised as opsins of pineal and related organs, and vertebrate ancient opsin (VA/VAL opsin)[7,8] are distributed to the brain tissues, including the diencephalon and mesencephalon, in addition to the retina[7,8,9,10]. These vertebrate non-visual opsins are thought to be involved in the photoreceptive functions of the pineal and related organs and of the deep brain photoreceptors in non-mammalian vertebrates[11]. Investigation of the functional differences among multiple parapinopsin-related opsins in the river lamprey is of interest
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