Light adaptation and photopigment bleaching in cone photoreceptors in situ in the retina of the turtle

Abstract

Light adaptation and photopigment bleaching in cone photoreceptors were studied in the intact, superfused retina of the turtle (Pseudemys scripta elegans). A new method for measuring changes in the photopigment of cones is described. Action spectrum measurements indicate that the signals arise from the red-sensitive cones. Measurements of steady-state bleaching are well described by the monomolecular bleaching equation with a half-bleaching constant of about 5.5 log photons sec-1 microns-2. Quantitative data on light adaptation were obtained by intracellular recording from 15 red- sensitive cones over nearly 8 decades of background illumination obtained from a helium-neon laser (632.8 nm). The steady-state membrane potential, Rs, and the rate of photoisomerization of the photopigment, Pi, rose in parallel with background illumination and then stabilized over the upper 4 decades of illumination. These results are described by the relation Rs = k Pi0.27, and suggest that about 5 x 10(6) photoisomerizations sec-1 lead to the closure of half the cone's light- sensitive channels in the steady state. A full range of decremental and incremental flashes was used to investigate stimulus-response relations. Cones tended to generate responses of approximately constant amplitude to flashes of constant contrast over a substantial range of contrast (< or = 3 x) and background illumination (approximately 3–4 decades). This suggests that a substantial component of contrast constancy in vertebrate vision may originate in cones. Over nearly 7 decades, the small-signal step sensitivity was found to conform closely to Weber's law (sensitivity is inversely proportional to background illumination). Thus, Weber's law extends into the ultra-high-intensity realm, some 3 decades higher than previously known for vertebrate cones. Over the upper 3–4 decades of illumination, Weber's law behavior can be explained by the depletion of photopigment (reduced probability of the photon catch). There remains a substantial low-intensity domain for which light adaptation and Weber's law behavior are presumably mediated by other mechanisms within the cone. These might be the calcium- and/or cGMP-dependent mechanisms recently suggested by others.

Aging of human short-wave cone pathways

“…This is consistent with similar responses from S cones to increments and decrements. We are not aware of direct measurements of isolated S-cone responses to increments and decrements; however, nonmammalian M and L cones undergo voltage excursions of approximately equal magnitude for intensity increments and decrements (28,29), and this is likely to be the case for primate S cones, given their comparable temporal dynamics (27). Note that there are substantial individual differences in IRF amplitude; however, in each case, the ON and OFF amplitudes are correlated.…”

Why rods and cones?

“…7 For the scotopic system, the sensitivity also follows Weber's Law over a narrow range, though at the lowest intensities it instead follows an approximately square-root law (reviewed in Barlow 8 ).…”

Highly effective phosphorylation by G protein-coupled receptor kinase 7 of light-activated visual pigment in cones

“…When an increment stimulus is given at this point, both rods and cones are hyperpolarized. When a momentary decrease in background illumination is given instead, rods do not respond (3,4), but cones depolarize quickly (4,5). This difference in the response to a decrement stimulus is another notable difference between rods and cones and is attributable to the difference in the speed of the recovery of a photoresponse.…”

“…Rods are saturated with bright background light and do not respond to more intense light (3). In contrast, cones are not saturated and respond to very bright light (4,5). The molecular mechanisms underlying in the differences of these response characteristics are not yet known.…”