title: Light-adapting migration of the screening-pigment in crayfish photoreceptors is a two-stage movement comprising an all-or-nothing initial phase.
abstract: The light-adapting response of the screening-pigment in crayfish retinal photoreceptors, previously described as a monophasic movement, was found to consist of two stages with different properties: (1) a rapid initial expansion that once started proceeds for at least half of the full distance, and (2) a slower and more variable continuation of the movement. The two components were resolved in isolated eyes stimulated under conditions expected to restrict Na+ influx into the photoreceptors. Only the second stage of the response to light was inhibited when Na+ was substituted with choline, or if the normal saline contained amiloride, a diuretic that hinders Na+ entry across many cell membranes. Amiloride in a medium without Na+ delayed, but did not curb, the first stage, whereas the rest of the movement was markedly restrained. Partial replacement of Na+ with Li+ blocked the second stage without affecting the rapid initial shift triggered by light. Exposure of dark-adapted eyes to high Na+ levels or to ouabain in the presence of Na+ in the dark also elicited a two-staged pigment dispersion to the light-adapted position. Low Na+ concentrations or amiloride affected the latency, but not the rate or extent, of the first stage of migration in ouabain-treated eyes. Consistent though less significant results were obtained with cyanide and the Na+ ionophore monensin. These observations suggest a differential control of pigment position over two defined domains along the photoreceptors, probably to integrate a double mechanism of light-adaptation: an all-or-nothing partial shift of the pigment screen as a safety factor against overexposure, followed by a regulated adjustment according to stimulation intensity.
authors.affiliation: Departamento de Biología Celular, Centro de Investigación y de Estudios Avanzados del IPN, México, DF.
authors.forename: E
authors.initials: E
authors.lastname: Frixione
authors.forename: O
authors.initials: O
authors.lastname: Pérez-Olvera
mesh_terms: D000221:Adaptation, Ocular / Q000502:physiology*; D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D003400:Astacoidea / Q000502:physiology*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002794:Choline / Q000378:metabolism; D008027:Light; D008094:Lithium / Q000494:pharmacology; D008985:Monensin / Q000494:pharmacology; D010042:Ouabain / Q000494:pharmacology; D010786:Photoreceptor Cells / Q000378:metabolism / Q000502:physiology*; D010860:Pigments, Biological / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism
chemical_list: D010860:Pigments, Biological; D010042:Ouabain; D000584:Amiloride; D008985:Monensin; D008094:Lithium; D012964:Sodium; D002794:Choline