Effects of light-adaptation on the early receptor potential

Rapid charge movements and photosensitivity of visual pigments in salamander rods and cones

1. Photosensitivities of visual pigments were determined by measuring early receptor currents (ERCs) in voltage-clamped photoreceptors from larval salamanders. 2. As expected from previous work of others, the ERC elicited by a brief flash consisted of a rapid inward component followed by a larger and slower outward component. The magnitude of the outward component corresponded to the movement of about 0.18 electronic charge across the membrane per photoisomerization. 3. The time course of the ERC was independent of the flash intensity, the flash wavelength and the magnitude of the response. The outward component of the cone ERC declined about twice as rapidly as the outward component of the rod ERC.. 4. The amplitude of the ERC decreased as successive flashes bleached the cell's pigment. Using the proportional relation between the size of the ERC and the number of pigment molecules photoisomerized, photosensitivities of the native A2 pigments in rods, red-sensitive cones, blue-sensitive cones and UV-sensitive cones were determined. Calculated solution photosensitivities for rhodopsin, red-sensitive and blue-sensitive cone pigments were not significantly different and the average value for all three pigments at their respective absorption maxima was (7.3 +/- 1.6) x 10(-9) micron 2 molecule-1. A value of 44.0 x 10(-9) micron 2 molecule-1 was obtained in a single UV-sensitive cone. 5. Substitution of the native dehydroretinal chromophore in the red-sensitive cone pigment with 11-cis-retinal increased the solution photosensitivity to (9.6 +/- 0.62) x 10(-9) micron 2 molecule-1. 6. We conclude that cone pigments have large molecular absorption cross-sections and high quantum efficiencies of photoisomerization. These properties seem well suited for the receptive molecules of a highly sensitive, miniaturized transducer.

Early receptor potential recordings for clinical routine

An apparatus and an electrode for early receptor potential recordings in clinical routine are introduced. One-hundred and eighty normal eyes were compared with cases of diffuse and localized retinal diseases. In retinitis pigmentosa patients a decrease in R1 and R2 amplitudes and in some cases a much faster time course was found. Eyes with circumscribed retinal affectations produced a wide variety of amplitudes, overlapping with the range of normals. In cases of monochromatism, the ERP recording established the diagnosis because of an absent R1 wave.

Flash bleaching of rhodopsin in the human retina

1. Measurements were made of the photo-sensitivity of rhodopsin in the living human retina following bleaching with (a) a flash less than 1 msec in duration, (b) a continuous exposure lasting 30 sec.2. The apparatus consisted of a fundus reflectometer coupled to a computer for on-line processing. Three subjects were studied.3. The relations between the quantity of pigment bleached and the incident energy are so similar for the flash and continuous exposure tests that the occurrence of photo-reversal (such as has been reported in similar circumstances for visual pigment in solution) seems to be ruled out.4. Nonetheless, the two experimental conditions lead qualitatively to two different photo-chemical states in as much as the density difference spectrum for the flash exposure is displaced toward longer wave-lengths in comparison with the data obtained with the continuous exposure.5. The results are discussed with reference to the sequence of events following the bleaching of visual pigments solutions as reported by other workers.

The minimum latency of the a-wave

1. Intense flashes of light evoke early and late receptor potentials from albino rat eyes.2. When the temperature is lowered the amplitude of the early receptor potential (e.r.p.) is found to decrease much more than that of the a-wave. Below 5 degrees the a-wave can be seen without a preceding negative e.r.p.3. The a-wave, but not the e.r.p., is suppressed by an intense flash, so that subsequent stimuli only evoke e.r.p.s.4. The responses of such light-adapted eyes have been used to determine the contribution of the a-wave to the response of dark-adapted eyes.5. These calculated a-waves begin during the e.r.p. with an initial portion which is biphasic.6. The initial portion has a temperature dependence similar to that of the a-wave. It is not related to the photoreversal of rhodopsin caused by the stimulating flashes.7. The minimum latency of the a-wave, extrapolated to body temperature, is approximately 300 musec.8. The site and mode of generation of the a-wave is discussed.

New evidence supporting the linkage to extracellular space of outer segment saccules of frog cones but not rods

Previous electron microscopic examinations of outer segments of photoreceptors suggest that many flattened saccules of cones are continuous with the cell membrane and that their lumina connect with the extracellular compartment but that most saccules in rods appear to lack these connections. The saccules probably contain photolabile pigment, and certain potentials appear to result from dipole formation during pigment bleaching. The detection of dipoles from rod saccules may require that the lumina of rod saccules connect with extracellular space, and questions have been raised whether the interpretation of micrographs is correct or the isolation of rod saccules is the result of artifact. Accordingly, lanthanum and barium precipitates were produced near fixed and unfixed frog photoreceptors. Lanthanum precipitates appeared to infiltrate the saccules of fixed cones and the few surviving cones exposed prior to fixation, but no rod saccules were infiltrated except occasional, most basal saccules or saccules within narrow zones of probable damage. Barium precipitates did not infiltrate saccules of either variety of unfixed photoreceptor, but they did occasionally infiltrate around the saccules at points of damage in rod outer segments. The results thus support the view of the patency of saccules of frog cones and are consistent with, but do not prove, the isolation of saccules of frog rods.

Early receptor potential: photoreversible charge displacement in rhodopsin

When the eye is illuminated by an intense flash, the visual pigment rhodopsin begins to pass rapidly through a series of intermediate states, eventually becoming bleached. If a second flash is delivered during the lifetimes of these intermediates the rhodopsin can be photoregenerated. A fast electrical response of the visual receptors, the early receptor potential, is elicited by the first flash. A similar response is elicited by the second flash, but the polarity of this response is reversed. Moreover, this response can be separated into three components, each arising from the action of light on a different intermediate. It is likely that all these fast responses, including the early receptor potential, arise from charge displacements in the visual-pigment molecule.