Light-transduction in melanopsin-expressing photoreceptors of Amphioxus.

Spatial vision in different organisms is mediated by 2 classes of photoreceptors: microvillar and ciliary. Recently, additional photosensitive cells implicated in nonvisual light-dependent functions have been identified in the mammalian retina. A previously undescribed photopigment, melanopsin, underlies these photoresponses, and it has been proposed that its transduction mechanisms may be akin to the lipid-signaling scheme of invertebrate microvillar receptors, rather than the cyclic-nucleotide cascade of vertebrates. Melanopsin has an ancient origin in deuterostomia, and expresses in 2 morphologically distinct classes of cells in the neural tube of Amphioxus, the most basal extant chordate: pigmented ocelli, and Joseph cells. However, to our knowledge, their physiology and alleged photosensitivity had never been investigated. We dissociated both types of cells, and conclusively demonstrated by patch-electrode recoding that they are primary photoreceptors; their receptor potential is depolarizing, accompanied by an increase in membrane conductance. The action spectrum peaks in the blue region, approximately 470 nm, similar to the absorption of melanopsin in vitro. The light-dependent conductance rectifies inwardly; Na and Ca are differentially implicated in the 2 cell types. Fluorescence Ca imaging reveals that photostimulation rapidly mobilizes calcium from internal stores. Intracellular 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate severely impairs the photoresponse, indicating that light-evoked Ca elevation is an important event in photoexcitation. These observations support the notion that the lineage of microvillar photoreceptors and its associated light-signaling pathway also evolved in the chordates. Thus, Joseph cells and pigmented ocelli of the Amphioxus may represent a link between ancestral rhabdomeric-like light sensors present in prebilaterians and the circadian photoreceptors of higher vertebrates.