Correspondence to: Vladimir J. Kefalov, Department of Neuroscience, Johns Hopkins University School of Medicine, 725 N. Wolfe St., Baltimore, MD 21205. Fax:410-614-3579 E-mail:vkefalov{at}jhmi.edu.
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Abstract |
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We used 11-cis 13-demethylretinal to examine the physiological consequences of retinal's noncovalent interaction with opsin in intact rod and cone photoreceptors during visual pigment regeneration. 11-Cis 13-demethylretinal is an analog of 11-cis retinal in which the 13 position methyl group has been removed. Biochemical experiments have shown that it is capable of binding in the chromophore pocket of opsin, forming a Schiff-base linkage with the protein to produce a pigment, but at a much slower rate than the native 11-cis retinal (Nelson, R., J. Kim deReil, and A. Kropf. 1970. Proc. Nat. Acad. Sci. USA. 66:531538). Experimentally, this slow rate of pigment formation should allow separate physiological examination of the effects of the initial binding of retinal in the pocket and the subsequent formation of the protonated Schiff-base linkage. Currents from solitary rods and cones from the tiger salamander were recorded in darkness before and after bleaching and then after exposure to 11-cis 13-demethylretinal. In bleach-adapted rods, 11-cis 13-demethylretinal caused transient activation of phototransduction, as evidenced by a decrease of the dark current and sensitivity, acceleration of the dim flash responses, and activation of cGMP phosphodiesterase and guanylyl cyclase. The steady state of phototransduction activity was still higher than that of the bleach-adapted rod. In contrast, exposure of bleach-adapted cones to 11-cis 13-demethylretinal resulted in an immediate deactivation of transduction as measured by the same parameters. These results extend the validity of a model for the effects of the noncovalent binding of a retinoid in the chromophore pockets of rod and cone opsins to analogs capable of forming a Schiff-base and imply that the noncovalent binding by itself may play a role for the dark adaptation of photoreceptors.
Key Words: opsin, 11-cis retinal, photoreceptors, pigment regeneration, dark adaptation
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INTRODUCTION |
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Light initiates a cascade of biochemical events in vertebrate photoreceptors that begins with the photoisomerization of the 11-cis retinal and results in the visual response (for reviews, see
The principal biochemical events that occur during dark adaptation are well established (for review, see
Visual pigment formation has been proposed to occur in two steps (
Recent experiments have suggested that these two steps may have distinct physiological consequences and that dark adaptation may be more complicated than previously thought (
The experiments described here were designed to examine and compare the physiological effects of noncovalent and covalent interactions of chromophore and opsin that normally occur during dark adaptation. 11-Cis 13-demethylretinal is a useful tool to address this question. This retinoid has been shown to bind into the chromophore pocket of opsin and to form a Schiff-base linkage with opsin. However, the latter step occurs about nine times slower than with the native 11-cis retinal (
The first part of this article describes experiments that were designed to investigate the separate effects of the noncovalent and covalent interactions between retinal and rod opsin by studying the effect of introducing 11-cis 13-demethylretinal in bleach-adapted rods. The second part deals with the corresponding effects of 11-cis 13-demethylretinal in bleach-adapted cones.
Some of these experiments have been reported previously at meetings of the Biophysical Society (
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MATERIALS AND METHODS |
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All experiments were performed according to procedures approved by the Animal Use and Care Committee of Boston University School of Medicine as being consistent with humane treatment of laboratory animals, and with standards set forth in the Guide for the Care and Use of Laboratory Animals and the Animal Welfare Act.
Rod photoreceptors were mechanically isolated from dark-adapted retinae of larval tiger salamander (Ambystoma tigrinum) by methods that have been described previously (
Measurements of membrane current were made extracellularly by methods that have been described previously (
Light Stimulation
An optical stimulator provided test flashes as well as bleaching lights (
Solutions
The recording chamber was perfused with a saline solution that contained (mM): 110 NaCl, 2.5 KCl, 1.6 MgCl2, 1.0 CaCl2, 10 dextrose, 10 HEPES, pH 7.8, and bovine serum albumin (100 mg liter-1). Entry of solution into the chamber was via a gravity-fed inlet supplied from a reservoir; outflow occurred through a tissue wick located in a trough at one end the chamber.
Test solutions were delivered to the region surrounding the outer segment of the cell by a rapid microperfusion system. The construction and use of this system has been described previously (
The test solution in which the rate constant of guanylyl cyclase was measured (3-isobutyl-1-methylxanthine, IBMX, solution)1 was made by adding IBMX to normal saline solution to a final concentration of 500 µm. The solution in which the rate constant of phosphodiesterase was measured (Li+ solution) was made by replacing NaCl in the saline solution with equimolar LiCl.
11-Cis 13-demethylretinal was synthesized as described previously (-phosphatidylcholine dissolved in a chloroform/methanol solution (Sigma-Aldrich) in a vial and evaporating the solvent under a stream of nitrogen gas. Any residual solvent was removed by lyophilizing the sample for 10 min. 10 ml of saline solution was added to the vial, and the contents were sonicated for 10 min in an ice bath (1 s on/1 s off) at 95 W using a 1.0-cm probe (Vibracell High Intensity Ultrasonic Processor; Sonics and Materials). 1.5 ml of the resulting vesicle solution was added to a vial containing about 300 µg 11-cis 13-demethylretinal and the contents were sonicated for 1.5 min at 30 W. The exact concentration of the retinoid was determined spectrophotometrically by making a 10-fold dilution in ethanol (molar extinction coefficients for 11-cis 13-demethylretinal of 12,600 M-1 cm-1 at 380 nm) and adjusted to 100 µm. 0.5 ml of the retinoid solution was added to the experimental chamber using a calibrated pipetter. No significant photoisomerization of the retinoid in the chamber was expected to occur because of the low intensity and unfavorable spectral composition of the test flashes. All manipulations were done under dim red/infrared light.
Fig 1 shows an average absorbance spectrum of 11-cis 13-demethyl rhodopsin in isolated salamander rod outer segments measured using a photon-counting microspectrophotometer (
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Theory and Analysis of Data
To estimate the change in the rate constants (to be referred to simply as rates) of cGMP phosphodiesterase (PDE) and guanylyl cyclase (GC), we used a method originally devised by
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(1) |
where is the rate of GC, and ß is the rate of PDE (
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(2) |
where i is the photocurrent, im is the maximum possible membrane current, K1/2 is the Michaelis constant for binding of cGMP to the light-sensitive channels, and N is the corresponding Hill coefficient. Sudden block of GC (
0) or of PDE (ß
0) by the corresponding test solution allows derivation from Equation 2 of a formula for the rate of the other enzyme as a function of the current. Thus, assuming that the channel characteristics (K1/2 and N) and im do not change during the step in the test solution, one can obtain the relative rate of PDE compared with that of the dark-adapted cell (see
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(3) |
where D indicates the parameters of the cell in its dark-adapted state and J is the normalized current, i/i D, or the current during the solution change, expressed as a fraction of the steady state membrane current of the dark-adapted cell. After subtracting the junction current, ß was estimated from the slope of the normalized current change plotted on a semilogarithmic graph and then the ratio ß/ßD was taken. Using the same assumptions as in the case of PDE, the relative rate of GC (Equation 4) can also be derived from Equation 2:
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(4) |
where N was taken to be equal to 3 (/
D was calculated.
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RESULTS |
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Bleach-adapted Rods
Photocurrent and light sensitivity.
Current recordings were made from isolated photoreceptors in their dark-adapted state, then after a bleach, and finally after application of 11-cis 13-demethylretinal. The experiment illustrated in Fig 2 shows the effect of 11-cis 13-demethylretinal on the dark current and on the sensitivity of a solitary bleach-adapted rod. The cell was stimulated with a series of test flashes increasing in intensity in half log unit steps to monitor changes in the amplitude of the current and sensitivity. Fig 2 A shows a series of photoresponses recorded from the dark-adapted rod. After bleaching 5% of the pigment, the cell was allowed 20 min to recover to a steady state and a second set of photoresponses was recorded (Fig 2 B). It can be seen that the bleach caused a small reduction in the dark current of ~5 pA. Next, 0.5 ml of saline solution containing vesicles loaded with 11-cis 13-demethylretinal (100 µm in bulk stock solution) was injected into the chamber and allowed to remain there for several minutes before being washed out. When the bleach-adapted rod was exposed to 11-cis 13-demethylretinal (11-cis 13-DM retinal), the current decreased within 20 s to a new lower steady state level (Fig 2 C).
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After the initial decrease in dark current and sensitivity caused by the treatment with 11-cis 13-demethylretinal, partial recovery was observed, even while the cell was still exposed to the retinoid solution. The final steady state dark current (not shown) was greater than the current immediately after exposing the cell to the 11-cis 13-demethylretinal, but it was still less than the current observed in the bleach-adapted state before the retinoid treatment. The kinetics of this partial recovery were studied in detail using repetitive measurements of the rate of guanylyl cyclase and will be discussed below. Unless otherwise specified, all subsequent measurements were made within 5 min of exposure to 11-cis 13-demethylretinal.
Peak photocurrent amplitudes for each set of photoresponses from Fig 2AC, were plotted against flash photon density and fitted with a single hyperbolic saturating function () caused a small decrease in the sensitivity and dark current of the rod from the dark-adapted state (
). Treating the bleached rod with 11-cis 13-demethylretinal produced an additional large decrease in the sensitivity and dark current ().
The effect of 11-cis 13-demethylretinal on the dark current and on the sensitivity was studied in a total of 27 bleach-adapted rods. All rods exhibited a decrease in both the dark current and in the sensitivity in the presence of 11-cis 13-demethylretinal. The bleaches used in these experiments varied from 5 to 20%. The effect of 11-cis 13-demethylretinal on the dark current and the sensitivity was studied in 22 cells in which 5% of the pigment was bleached. Under these conditions, the dark current recorded under steady state was reduced to 92 ± 1% (SEM, n = 22) of its dark-adapted value. Sensitivity was reduced by 0.30 ± 0.14 log units (SEM, n = 18), as measured from changes in the ratio of the response maximum to the semisaturating light intensity of the hyperbolic stimulusresponse functions, S = Rmax/Is (
This recovery of the dark current was accompanied by a slight increase in the sensitivity. Due to the transient nature of the effect, and the fact that several minutes of measurements are required to estimate the sensitivity of the cell, the exact value of this increase could not be measured.
Seven cells were treated a second time with 11-cis 13-demethylretinal. In six of these, the dark current decreased once again to its value measured immediately after the first treatment with 11-cis 13-demethylretinal. In the seventh cell, the second treatment with the retinoid did not induce any change in the dark current.
The effect of 11-cis 13-demethylretinal on the dark current and the sensitivity was also studied in a small number of rods where 10 or 20% of the pigment was bleached. As expected, the decrease in the current compared with the dark-adapted state was higher for the higher bleaches. In rods exposed to a 10% bleach, the dark current was reduced to 76.0 ± 6.4% (SEM, n = 3) of its dark-adapted value. After a 20% bleach, the dark current was reduced to 39.5 ± 2.5% (SEM, n = 2) of that observed before the bleach. However, the further decrease in the current from the bleach-adapted level caused by 11-cis 13-demethylretinal was comparable in the two cases, and was similar to that observed after the 5% bleach: 41.7 ± 15.2% (SEM, n = 3) for the 10% bleach; and 44.5 ± 3.5% (SEM, n = 2) for the 20% bleach. Thus, the percentage decrease of the dark current in the presence of 11-cis 13-demethylretinal was independent of the bleach fraction for the bleaches used in this study. This observation implies that the fraction of opsin bound to 11-cis 13-demethylretinal was the same in each case.
Dim flash kinetics.
Analysis of the time course of responses to dim flashes when photoreceptors are in a different state of adaptation or after a drug treatment can provide information about kinetic changes taking place in the transduction cascade (
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Rate of guanylyl cyclase.
We performed a series of experiments designed to look directly at the effect of 11-cis 13-demethylretinal on the rates of cyclase and phosphodiesterase in bleach-adapted rods. The methods used for measuring the rates of these two enzymes have been described previously (
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After the initial large acceleration, the rate of cyclase gradually decreased over the next 40 min. The steady state rate of cyclase was significantly reduced compared with the rate immediately after the treatment and was only threefold higher than in the bleach-adapted state (Fig 4 D).
Thus, treatment of bleach-adapted rods with 11-cis 13-demethylretinal resulted in an initial large acceleration of the cyclase rate that was followed by a partial recovery. The steady state rate after the treatment was always lower than the initial high rate, but still significantly higher than that of the bleach-adapted state.
The effect of 11-cis 13-demethylretinal on the rate of cyclase was studied in a total of 24 bleach-adapted rods. In all of these, retinal caused acceleration of guanylyl cyclase. The range of pigment bleaches was from 5 to 20%. Higher bleaches were not used because of the difficulty in estimating the significantly higher rates of cyclase and phosphodiesterase when these cells were exposed to 11-cis 13-demethylretinal.
The effect of 100 µm 11-cis 13-demethylretinal in rods after a 5% bleach was studied in detail. In a total of 17 cells under these conditions, the rate of cyclase after the bleach was on average 1.6 ± 0.1 (SEM, n = 17) times higher than in the corresponding dark-adapted state. Treatment initially accelerated cyclase to a rate that was, on average, 4.4 ± 0.4 (SEM, n = 17) times higher than in the bleach-adapted state. The steady state rate of cyclase in the presence of 11-cis 13-demethylretinal was 54 ± 3% (SEM, n = 11) of the rate of cyclase measured immediately after the addition of the retinoid, but still about twofold higher than the pretreatment, bleach-adapted rate of cyclase.
We also studied the effect of 11-cis 13-demethylretinal after 10% bleach in five cells. The acceleration of cyclase caused by the 10% bleach was higher than that of the 5% bleach. The rate of cyclase after the bleach was on average 2.3 ± 0.5 (SEM, n = 5) times higher than in the dark-adapted state. Immediately after adding 11-cis 13-demethylretinal to the bleach-adapted rods, the rate of cyclase was accelerated further to 3.1 ± 0.9 (SEM, n = 5) times that of the bleach-adapted state. The steady state rate of cyclase in the presence of 11-cis 13-demethylretinal was still higher than the rate of the bleach-adapted cell, but it was reduced to 60 ± 10% (SEM, n = 2) of the rate of cyclase immediately after the addition of the retinoid.
In the two cells where the effect of the analog was studied after 20% bleach, the rate of cyclase was accelerated by the bleach on average to 3.5 ± 0.5 (SEM, n = 2) times the rate in the corresponding dark-adapted state. The treatment with 11-cis 13-demethylretinal initially accelerated cyclase 3.1 ± 0.7 (SEM, n = 2) times compared with the bleach-adapted state. The steady state rate of cyclase in the presence of 11-cis 13-demethylretinal was 60 ± 20% (SEM, n = 2) of the initial value, but still about twofold higher than the pretreatment, bleach-adapted steady state value.
The time course of deactivation of cyclase that followed the initial acceleration immediately upon treating the rod was studied in detail in seven cells where 5% of the pigment was bleached. One such experiment is shown on Fig 5. The decrease in the rate of cyclase in each case could be expressed by a single exponential decay function. The time constant for the cell shown in Fig 5 was 22.4 min. The rate of deceleration of cyclase varied significantly from cell to cell and on average it was 17.0 ± 2.2 (SEM, n = 7). This is somewhat faster than the rate of inactivation expected from biochemical experiments (
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The cause for the observed deactivation of the transduction cascade that followed the initial activation by 11-cis 13-demethylretinal was examined next. Because deactivation was observed whether or not excess 11-cis 13-demethylretinal was removed from the chamber, washout of the retinoid could be ruled out as the cause for the slow deactivation of transduction. Another possibility is that the reversal of the initial effect is due to the formation of covalent linkage between the retinoid and free opsin. If that were the case, at least a fraction of the free opsin produced by the bleach would be converted to 11-cis 13-demethyl rhodopsin and, as a result, the amount of free opsin in the rod would be reduced. Therefore, it would be expected that a second treatment with 11-cis 13-demethylretinal would produce a smaller effect or no effect at all, depending on the residual amount of free opsin.
The rate of cyclase was monitored in four bleached rods that were treated multiple times with 11-cis 13-demethylretinal. The retinoid solution was allowed to stay in the chamber for several minutes and then was washed out. Once the rod had reached a steady state after the addition of analog, a second dose of the vesicle solution was introduced into the chamber and its effect on the rate of cyclase was measured. In all experiments, the second treatment induced an additional acceleration of guanylyl cyclase from its steady state rate (Fig 5, right arrow). The effect of the second treatment with 11-cis 13-demethylretinal on the rate of cyclase in three cells was smaller than the corresponding effect after the first treatment (Fig 5). In the fourth cell, each of four repetitive treatments with 11-cis 13-demethylretinal induced approximately the same effect on the rate of cyclase (data not shown). The result of this experiment supports the notion that the reversal of the initial effect is due to the formation of covalent linkage between the retinoid and free opsin. Also in support of this hypothesis is the observation that, similar to the relative rates of pigment regeneration measured in biochemical experiments (
Rate of phosphodiesterase. The effect of 11-cis 13-demethylretinal on the rate of phosphodiesterase in bleach-adapted rods was also studied. The protocol for delivering the retinoid was the same as just described for the case of guanylyl cyclase. Fig 6 shows one example where the effect of 100 µm 11-cis 13-demethylretinal on the rate of phosphodiesterase was studied in a rod after bleaching 5% of the pigment. Fig 6 (left) shows current recordings during the solution change. Fig 6 (right) plots J on a semilogarithmic scale. The slope of this trace, as argued in MATERIALS AND METHODS is proportional to the rate of cGMP hydrolysis by phosphodiesterase in this case. Bleaching 5% of the pigment produced an acceleration of phosphodiesterase by 50% (compare Fig 6A and Fig B). The first measurement of the rate of phosphodiesterase after treating the rod with the analog was performed about 3 min after adding the vesicle solution to the chamber. As a result of the treatment with the retinoid, the rate of phosphodiesterase was increased over threefold compared with the bleach-adapted rate (Fig 6 C). Measurement of phosphodiesterase 50 min later revealed that the rate of phosphodiesterase has decreased and for this cell was about two times higher than the pretreatment, bleach-adapted rate (Fig 6 D).
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The effect of 11-cis 13-demethylretinal on the rate of phosphodiesterase in bleach-adapted rods was studied in a total of 20 cells. The bleach fraction varied from 5 to 20% and the concentration of 11-cis 13-demethylretinal in all experiments was 100 µm. In all 20 bleach-adapted rods tested, 11-cis 13-demethylretinal caused acceleration of phosphodiesterase. In the 16 cells where 5% of the pigment was bleached, the rate of phosphodiesterase in the bleach-adapted state was, on average, 1.8 ± 0.1 (SEM, n = 16) times higher than in the corresponding dark-adapted state. The rate of phosphodiesterase immediately after the addition of the retinoid solution to the bath was, on average, 2.4 ± 0.1 (SEM, n = 16) times higher than in the bleach-adapted state.
An attempt was made to study the time course of the recovery of phosphodiesterase after its initial acceleration after treatment with 11-cis 13-demethylretinal. However, the rate of phosphodiesterase did not change significantly after its initial acceleration in the five rods that were studied. The reason for this lack of effect as compared with the experiments in which the cyclase rate was measured (see Fig 5) is not known.
In the four cells where the bleach fraction was 10 or 20%, the acceleration of phosphodiesterase by both the bleach and the treatment with 11-cis 13-demethylretinal was higher than in the 5% bleach experiments. For the case of 10% bleach, the rate of phosphodiesterase in the bleach-adapted state was 2.0 ± 0.1 (SEM, n = 2) times higher than in the dark-adapted state. The rate of phosphodiesterase in the presence of 11-cis 13-demethylretinal was 2.5 ± 1.3 (SEM, n = 2) times higher than in the bleach-adapted state. For the 20% bleach, the corresponding increase was 3.4 ± 0.4 (SEM, n = 2) for the bleach-adapted to dark-adapted ratio, and 2.7 ± 1.3 (SEM, n = 2) for the acceleration caused by the retinoid.
Phototransduction in dark-adapted rods in the presence of 11-cis 13-demethylretinal.
Control experiments were performed to see if the effect induced by 11-cis 13-demethylretinal in bleach-adapted rods is the result of its binding directly in the chromophore pocket of opsin. The effect of treatment with the retinoid was studied in four dark-adapted rods. In all of them, treatment with 11-cis 13-demethylretinal caused a slight acceleration of the transduction cascade, as evinced by the slight decrease in the dark current and in the sensitivity, as well as by the small acceleration of the dim flash response and of the rates of cyclase and phosphodiesterase (data not shown). However, the amplitude of the changes induced by this treatment was significantly smaller than the corresponding effects after bleaching as little as 5% of the pigment. The rate of cyclase increased on average by 60% and the rate of phosphodiesterase increased by 20%, compared with the severalfold increase in these rates caused by 11-cis 13-demethylretinal in bleach-adapted cells. The slight activation of the phototransduction cascade by 11-cis 13-demethylretinal in dark-adapted rods could be the result of the presence of a small amount of free opsin in rods of freshly dissected animals, as recently argued by
Bleach-adapted Cones
The effect of 11-cis 13-demethylretinal on the activity of phosphodiesterase and cyclase in bleach-adapted red cones was also investigated. Red-sensitive cones were identified by their distinctive morphology and high sensitivity to test flashes at 600 nm. To produce activation of the transduction cascade in the cones comparable with that of 510% bleach-adapted rods, 90% of the cone pigment had to be bleached. The concentration of 11-cis 13-demethylretinal used was 100 µm. A total of five bleach-adapted cones were studied.
Photocurrent and light sensitivity. The experiment illustrated in Fig 7 shows the effect that 11-cis 13-demethylretinal has on the dark current and sensitivity of a bleach-adapted cone. Fig 7 A illustrates a series of superimposed flash responses recorded from the dark-adapted cone. During the exposure to brief bright light that bleached 90% of the pigment, the dark current was completely blocked. Within 10 s after the termination of the bleach, the current recovered to a steady state. The bleach caused a steady decrease in the dark current, as evinced by the decreased amplitude of the saturating flash response (Fig 7 B). On average, bleaching 90% of the pigment caused a decrease in the dark current to 65 ± 5% (SEM, n = 5) of its dark-adapted value.
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When the bleach-adapted cone was exposed to vesicle solution containing 11-cis 13-demethylretinal, the dark current reached a new steady state within 35 s with a value greater than that of the bleach-adapted cone (Fig 7 C). No transient decrease of the current upon adding 11-cis 13-demethylretinal was observed. After the treatment with the retinoid, the amplitude of the current in all of the cones studied was higher than the bleach-adapted state, but still significantly lower than the dark-adapted state of the cell and, on average, was increased to 78 ± 4% (SEM, n = 5) of the dark-adapted value. This effect was not transient and the current remained at this level after flushing the retinoid solution from the chamber. Thus, the treatment with 11-cis 13-demethylretinal caused a recovery of about one third of the dark current lost as a result of the bleach. Considering that, in the case of cones, significant activation of the transduction cascade produces relatively small changes in the dark current (see, for instance,
The sets of photoresponses shown on Fig 7AC, were used to construct the corresponding intensityresponse curves and estimate the effect of 11-cis 13-demethylretinal on the photosensitivity of the bleach-adapted cone (Fig 7 D). Bleaching 90% of the pigment resulted in an ~100-fold (2 log units) decrease in the sensitivity of the cone (Fig 7 D, and
). On average, the bleach caused a loss of sensitivity of 1.8 ± 0.2 (SEM, n = 5) log units. When the bleach-adapted cone was exposed to 11-cis 13-demethylretinal, the sensitivity increased but still remained lower than the sensitivity in the dark-adapted state (Fig 7 D, ). On average, the recovery of sensitivity induced by 11-cis 13-demethylretinal was 0.5 ± 0.1 (SEM, n = 5) log units. Thus, treatment of bleach-adapted cones with the retinoid caused recovery of about one third of the sensitivity lost as a result of the bleach.
Dim flash kinetics.
The observation that 11-cis 13-demethylretinal increases the dark current and sensitivity of bleach-adapted cones suggests that the retinoid down regulates the reactions of the phototransduction cascade. As has been observed after treatment of bleached cones with ß-ionone (
Fig 8 shows normalized dim-flash responses from one cone, elicited in the dark-adapted state, after a 90% bleach, and then after treatment with 11-cis 13-demethylretinal. As a result of the bleach, the photoresponse was accelerated and peaked 40 ms earlier than the dark-adapted response. Treating the bleach-adapted cone with the retinoid partially reversed this acceleration and the response in the treated state peaked only 10 ms earlier than the response in the dark-adapted state. Similar results were obtained from all five tested cones.
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Thus, treatment of bleach-adapted cones with 11-cis 13-demethylretinal caused a partial reversal of the acceleration of the dim flash response caused by the bleach. The effect was not transient and persisted even after washing out the retinoid from the chamber.
Rate of guanylyl cyclase.
We also performed a series of experiments designed to study directly the effect of 11-cis 13-demethylretinal on the level of transduction in cones. Because of the much faster cone response kinetics (
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DISCUSSION |
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Our results demonstrate that treatment of bleach-adapted rods with a vesicle solution containing 11-cis 13-demethylretinal results in: (a) a decrease in the dark current, (b) a decrease in the sensitivity, (c) an acceleration of the dim flash photoresponse, and (d) an increase in the rates of guanylyl cyclase and cGMP phosphodiesterase. Control experiments on dark-adapted rods similarly treated showed that 11-cis 13-demethylretinal produces little or no effect on the level of transduction activity. Taken together, these results suggest that, in bleach-adapted rods, 11-cis 13-demethylretinal combines with opsin and acts as an agonist to activate the phototransduction cascade in the dark in a manner similar to light. This activation is similar to that reported previously to occur in bleached rods exposed to ß-ionone (
These results are consistent with previous biochemical studies in which 11-cis 13-demethylretinal was found to activate opsin (
Fig 10 shows a proposed scheme by which 11-cis 13-demethylretinal may affect the level of transduction in bleach-adapted rods. We speculate that the initial high level of transduction in bleach-adapted rods observed after treatment with 11-cis 13-demethylretinal is the result of the noncovalent binding of the retinoid in the chromophore pocket of free opsin (Fig 10 D). The subsequent partial recovery is due to the covalent attachment of the retinoid to opsin and the formation of 11-cis 13-demethyl rhodopsin. It is likely that Schiff-base formation between opsin and 11-cis 13-demethylretinal does not go to completion (
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The presence of residual amounts of free opsin according to this scheme provides an explanation for our observation that a second treatment with 11-cis 13-demethylretinal produces activation of the transduction cascade, but to a smaller extent than the first treatment. After each treatment, the excess retinoid was flushed from the chamber. The addition of a second dose of 11-cis 13-demethylretinal will result in an increased concentration of free retinoid in the cell. This higher concentration will lead to the increased binding of 11-cis 13-demethylretinal to free opsin, and thus to activation of the transduction cascade. According to this scheme, after the excess of the retinoid is removed, the concentration of free 11-cis 13-demethylretinal in the cell will decrease once again. As a result, the concentration of opsin with noncovalently bound retinoid should decline and the activity of the phototransduction cascade should return to its steady state.
Our studies of the effect of 11-cis 13-demethylretinal on the level of transduction in bleach-adapted cones, on the other hand, failed to demonstrate any activation by the retinoid. Instead, in bleach-adapted cones, we observed an immediate deactivation of transduction upon treatment with 11-cis 13-demethylretinal. Unfortunately, no biochemical studies on the ability of this retinoid to form a pigment with cone opsin have yet been reported. Thus, we cannot rule out the possibility that the observed deactivation of transduction in bleach-adapted cones is the result of the formation of a covalent Schiff-base linkage. However, based on the observation that 11-cis 13-demethyl rhodopsin is inactive in the dark (
Our results on the effect of 11-cis 13-demethylretinal on the level of transduction in bleach-adapted rod and cone photoreceptors complement previous observations of the effects of ß-ionone on phototransduction in bleach-adapted rods and cones. They extend the validity of the models proposed for the effects of the noncovalent binding of a retinoid in the chromophore pockets of rod (
The dark adaptation of bleach-adapted rods and cones most likely occurs in two stages. In the first stage, the noncovalent binding of 11-cis retinal in the chromophore pocket of free rod opsin desensitizes the rod. The analogous reaction in cones results in partial resensitization. The subsequent covalent attachment of retinal to opsin via a protonated Schiff-base in both cell types then completely inactivates the bleached pigment to complete dark adaptation. The opposite effects of the noncovalent binding of retinal to opsin in rods and cones may explain, in part, why dark adaptation of photopic color vision is so much more rapid than scotopic vision mediated by rods.
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Footnotes |
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Dr. Kefalov's present address is Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
1 Abbreviations used in this paper: GC, guanylyl cyclase; IBMX, 3-isobutyl-1-methylxanthine; PDE, phosphodiesterase.
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Acknowledgements |
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We thank Dr. D.A. Cameron for his assistance in acquiring the data for the Fig 1 insert.
This work was supported by National Institutes of Health grants EY07543 (D.W. Corson), EY01157 (M.C. Cornwall), and EY04939 (R.K. Crouch), and by an institutional grant to the Department of Ophthalmology, MUSC, from the Foundation to Prevent Blindness.
Submitted: 19 April 2000
Revised: 15 June 2000
Accepted: 15 June 2000
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References |
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