Vertebrate ancient-long opsin: A green-sensitive photoreceptive molecule present in zebrafish deep brain and retinal horizontal cells

Nonretinal/nonpineal photosensitivity has been found in the brain of verteb rates, but the molecular basis for such a "deep brain" photoreception syste m remains unclear. We conducted an extensive search for brain opsin cDNAs o f the zebrafish (Danio rerio), a useful animal model for genetic studies, a nd we have isolated a partial cDNA clone encoding an ortholog of vertebrate ancient (VA) opsin, the function of which is unknown. Subsequent character ization revealed the occurrence of two kinds of mRNAs encoding putative spl icing variants, VA and VA-Long (VAL) opsin, the latter of which is a novel variant of the former. Both opsins shared a common core sequence in the mem brane-spanning domains, but VAL-opsin had a C-terminal tail much longer tha n that of VA-opsin. Functional reconstitution experiments on the recombinan t proteins showed that VAL-opsin with bound 11-cis-retinal is a green-sensi tive pigment (lambda max similar to 500 nm), whereas VA-opsin exhibited no photosensitivity even in the presence of 11-cis-retinal. Immunoreactivity s pecific to this functionally active VAL-opsin was localized at a limited nu mber of cells surrounding the diencephalic ventricle of central thalamus, a nd these cells were distributed over similar to 200 mm along the rostrocaud al axis. Taken together with the previous study on the locus of the teleost brain photosensitivity (von Frisch K, 1911), it is strongly suggested that the VAL-positive cells in the zebrafish brain represent the deep brain pho toreceptors. The VAL-specific immunoreactivity was also detected in a subse t of non-GABAergic horizontal cells in the zebrafish retina. The existence of VAL-opsin, a new member of the rhodopsin superfamily, in these tissues m ay indicate its multiple roles in visual and nonvisual photosensory physiol ogy.