Dynamic and steady-state light adaptation of mouse rod photoreceptors in vivo.

1. Electroretinographic (ERG) methods were used to investigate the effects of background illumination on the responses of mouse rod photoreceptors in vivo. A paired-flash procedure, involving the recording and analysis of the ERG a-wave response to a bright probe flash presented after a brief test flash, was used to derive the rod response to the test flash in steady background light. A related, step-plus-probe procedure was used to derive the step response of the rods to backgrounds of defined strength. 2. Steady background light produced a maintained derived response that was graded with background strength. Determinations of the full time course of the derived weak-flash response in steady background light, and of the effect of background strength on the flash response at fixed post-test-flash times, showed that moderate backgrounds reduce the peak amplitude and duration of the flash response. 3. The response to stepped onset of an approximately half-saturating background (1.2 sc cd m(-2)) exhibited a gradual rise over the first 200-300 ms, and an apparent subsequent relaxation to plateau amplitude within 1 s after background onset. Determinations of normalized amplitudes of the derived response to a test flash presented at 50 or 700 ms after background onset indicated substantial development of background-induced shortening of the test flash response within this 1 s period. These findings indicate a time scale of approximately 1 s or less for the near-completion of light adaptation at this background strength. 4. Properties of the derived response to a stepped background and to test flashes presented in steady background light are in general agreement with photocurrent data obtained from mammalian rods in vitro and suggest that the present results describe, to good approximation, the in vivo desensitization of mouse rods by background light.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina.

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

The relationship between opsin overexpression and photoreceptor degeneration.

PURPOSE: To characterize the process by which overexpression of normal opsin leads to photoreceptor degeneration. METHODS: Three transgenic mouse lines were generated that express different levels of an opsin with three amino acid modifications at the C terminus. These modifications created an epitopic site that can be readily distinguished from the endogenous protein using a bovine opsin-specific antibody. Evidence of degeneration associated with opsin overexpression was provided by anatomic studies and electroretinogram (ERG) recordings. Western blot analysis was used to confirm the production of the transgenic opsin, and an enzyme-linked immunosorbent assay (ELISA) was used to determine the amounts of opsin overexpressed in each line. Immunocytochemistry was used to determine the cellular localization of transgenic opsin. Amounts of 11-cis retinal were determined by extraction and high-performance liquid chromatography (HPLC). RESULTS: Opsin expression levels in the three lines were found to be 123%, 169%, and 222% of the level measured in nontransgenic animals, providing direct correlation between the level of transgene expression and the severity of the degenerative phenotype. In the lower expressing lines, ERG a-wave amplitudes were reduced to less than approximately 30% and 15% of normal values, whereas responses of the highest expressing line were indistinguishable from noise. In the lowest expressor, a 26% elevation in 11-cis retinal was observed, whereas in the medium and the high expressors, 11-cis retinal levels were increased by only 30% to 33%, well below the 69% and 122% increases in opsin levels. CONCLUSIONS: The overexpression of normal opsin induces photoreceptor degeneration that is similar to that seen in many mouse models of retinitis pigmentosa. This degeneration can be induced by opsin levels that exceed by only approximately 23% that of the normal mouse retina. Opsin overexpression has potential implications in retinitis pigmentosa.

Lateral spread of adaptation as measured with the multifocal electroretinogram.

We examined whether lateral spread of adaptation can be observed in the electroretinogram in humans. Specifically, we tested whether the luminance level of a surrounding, nonmodulated annulus affects the multifocal electroretinogram (ERG) response of a modulated central area. Multifocal electroretinograms were recorded in response to an array of 37 unscaled hexagons subtending a retinal area of 38 deg x 35 deg. Responses were recorded in six control subjects. In the first series of experiments, only the center hexagon was modulated, while the surrounding 36 hexagons were held constant at either 0.45, 172, or 340 cd/m2. In a subsequent series of control experiments, modulation depth of the center hexagon was varied and the proximity of the surrounding hexagon systematically altered. For the center-modulated condition, response amplitude and implicit time for the first-order kernel response significantly decreased as a function of increasing surround luminance. Control experiments demonstrated that the effect of the surround illumination was not due to scattered light but was influenced by the proximity of the surrounding annulus. These results demonstrate that lateral adaptation influences can be measured using the multifocal ERG.

Retinoid kinetics in eye tissues of VPP transgenic mice and their normal littermates.

PURPOSE: VPP mice, which possess a mutant transgene for opsin (V20G, P23H, P27L), exhibit a progressive rod degeneration that resembles one form of human autosomal dominant retinitis pigmentosa. In the present study the association of the development of VPP rod degeneration with abnormal operation of the retinoid visual cycle was examined. METHODS: Dark-adapted VPP mice and normal littermates were anesthetized and the pupils dilated. One eye of each animal was illuminated for 2 minutes; the other eye was shielded from the light and served as a control. Each animal was then dark adapted for a defined period (0-300 minutes) and killed. Retinoids contained in the retina, retinal pigment epithelium (RPE), and extracellular medium were recovered by means of formaldehyde-, isopropanol- and ethanol-based extractions and analyzed by high-performance liquid chromatography. RESULTS: Total amounts of retinoid recovered from unilluminated eyes of 2-month-old normal and VPP mice were 425 +/- 90 picomoles per eye and 115 +/- 33 picomoles per eye, respectively (mean +/- SD). Relative distributions of retinoids within normal and VPP eyes were similar. In normal and VPP animals, illumination for 2 minutes produced a similar immediate reduction in the molar percent of total retinoid represented by 11-cis retinal in the retina (average reduction of 34% and 28% in normal and VPP animals, respectively) and a similar transient increase of all-trans retinal in the retina. In both groups the decline of all-trans retinal was accompanied by an increase in total retinyl ester. In normal and VPP animals, a period of approximately 40 minutes or more preceded initiation of the recovery of 11-cis retinal in the retina, and the time course of this recovery was generally similar to that for the decline of retinyl ester. The overall dark-adaptation period required for half-completion of 11-cis retinal recovery was approximately 150 minutes. In neither group did illumination produce a substantial peak of all-trans retinol in the retina. CONCLUSIONS: The evident approximately fourfold reduction of total retinoid in the eyes of 2-month-old VPP mice is consistent with histologic and electroretinographic abnormalities determined in previous studies. Despite this marked abnormality in retinoid content, retinoid cycling in the VPP is remarkably similar to that in normal littermates. The data place constraints on the functional consequences of any abnormality in retinoid processing that may be present at this stage of the VPP rod degeneration.

Sensitivity and kinetics of mouse rod flash responses determined in vivo from paired-flash electroretinograms.

1. Electroretinograms (ERGs) were recorded corneally from C57BL/6J mice using a paired-flash procedure in which a brief test flash at time zero was followed at time tprobe by a bright probe flash of fixed strength, and in which the probe response amplitude was determined at time t = tprobe + 6 ms. Probe responses obtained in a series of paired-flash trials were analysed to derive A(t), a family of amplitudes that putatively represents the massed response of the rod photoreceptors to the test flash. A central aim was to obtain a mathematical description of the normalized derived response A(t)/Amo as a function of Itest, the test flash strength. 2. With fixed tprobe (80 <= tprobe <= 1200 ms), A(t)/Amo was described by the saturating exponential function [1 - exp(-ktItest)], where kt is a time-dependent sensitivity parameter. For t = 86 ms, a time near the peak of A(t), k86 was 7.0 +/- 1.2 (scotopic cd s m-2)-1 (mean +/- s. d.; n = 4). 3. A(t)/Amo data were analysed in relation to the equation below, a time-generalized form of the above exponential function in which (k86Itest) is replaced by the product [k86Itestu(t)], and where u(t) is independent of the test flash strength. The function u(t) was modelled as the product of a scaling factor gamma, an activation term 1 - exp[-alpha(t - td)2]), and a decay term exp(-t/tauomega): A(t)/Amo = 1 - exp[-k86Itestu(t)]; u(t) = gamma(1 - exp[-alpha(t - td)2](exp(-t/tauomega) where td is a brief delay, tauomega is an exponential time constant, and alpha characterizes the acceleration of the activation term. For Itest up to approximately 2.57 scotopic cd s m-2, the overall time course of A(t) was well described by the above equation with gamma = 2.21, td = 3.1 ms, tauomega = 132 ms and alpha = 2.32 x 10-4 ms-2. An approximate halving of alpha improved the fit of the above equation to ERG a-wave and A(t)/Amo data obtained at t about 0-20 ms. 4. Kinetic and sensitivity properties of A(t) suggest that it approximates the in vivo massed photocurrent response of the rods to a test flash, and imply that u(t) in the above equation is the approximate kinetic description of a unit, i.e. single photon, response.

Does rod phototransduction involve the delayed transition of activated rhodopsin to a second, more active catalytic state?

Recovery kinetics of the saturating photocurrent response in amphibian rods suggest regulation of the visual signal by a first-order deactivation reaction with an exponential time constant (tau(c)) of about 2 s. The original hypothesis that tau(c) represents the lifetime of activated rhodopsin (R*) in a single-step deactivation appears at odds with several recent findings, for example, that Ca2+, a known regulator of the enzymatic phosphorylation of R*, does not regulate the value of tau(c). A recently proposed alternative hypothesis, that tau(c) is the lifetime of activated transducin and that the R* lifetime is relatively short (approximately 0.4 s), appears consistent with the Ca2+ data but is difficult to reconcile with a high specific catalytic activity of R*. The present theoretical study proposes a rate-equation model of R* activation and deactivation in amphibian rods that is generally consistent with observed properties of the tau(c)-associated reaction and the action of Ca2+ as well as with the stereotyped nature of the single-photon response. The model is developed by considering the effect of background light on a time-dependent variable, R*eff, defined as the effective total level of R* activity. Central starting assumptions are that Ca2+ reduction mediates the effect of background light on R*eff(t) and that background desensitization of the photocurrent flash response derives from this action of Ca2+. Construction of the model is guided by criteria based on previous experimental findings. Among these are the approximate constancy of background desensitization expressed at near-peak and later times in the flash response, and the large (approximately 10-fold) dynamic range of this desensitization. The proposed model hypothesizes that an event regulated by Ca2+ feedback causes activated rhodopsin to become susceptible to a two-phase, stochastic deactivation process, the second phase of which is characterized by tau(c). A central prediction of the model is the regulated transition of flash-activated R* to "R**", a state exhibiting greatly increased catalytic activity.

Opsin localization and rhodopsin photochemistry in a transgenic mouse model of retinitis pigmentosa.

The VPP mouse is a transgenic strain carrying three mutations (P23H, V20G, P27L) near the N-terminus of opsin, the apoprotein of rhodopsin, the rod photopigment. These animals exhibit a slowly progressive degeneration of the rod photoreceptors, and concomitant changes in retinal function that mimic those seen in humans with autosomal dominant retinitis pigmentosa resulting from a point mutation (P23H) in opsin. In the present study we attempted to determine whether the disease process prevents the translocation of mutant opsin to the rod outer segments of transgenic mice, and whether it affects the photochemical properties of the rhodopsin present within their rod outer segments. Immunocytochemistry with a monoclonal antibody against a region of the C-terminus that recognizes epitopes common to both normal and mutant opsin (monoclonal antibody-1D4), and a polyclonal antibody that reacts preferentially with the mutant opsin (anti-VPP), were used to identify the opsin present in the rods of three-week-old VPP mice and normal littermates. Absorbance spectra, photosensitivity, and regeneration kinetics of rhodopsin in rod outer segment disc membranes were analysed by spectrophotometry. Western blot analysis with anti-VPP antibody indicated the specific binding of this antibody to the mutant opsin. Immunolocalization with monoclonal antibody-1D4 and anti-VPP antibodies suggested a normal translocation of the mutant protein to the outer segments. Aside from a small disparity in the absorbance spectra of rhodopsin obtained from normal and VPP retinas, there were no significant differences in either the ability of opsin to bind 11-cis retinal chromophore, or in the photic sensitivity of rhodopsin. The results indicate that mutant opsin is translated and incorporated into the rod outer segment disc membranes of VPP mice, and that the photochemical properties of rhodopsin in the rods of VPP retinas are similar to those of rhodopsin in normal retinas.

Fourth module of Xenopus interphotoreceptor retinoid-binding protein: activity in retinoid transfer between the retinal pigment epithelium and rod photoreceptors.

PURPOSE: Interphotoreceptor retinoid-binding protein (IRBP), an extracellular protein believed to support the exchange of retinoids between the neural retina and retinal pigment epithelium (RPE) in the vertebrate eye, exhibits a modular, i.e., repeat, structure. The present study was undertaken to determine whether an individual module of IRBP has activity in retinoid transfer between the RPE and rod photoreceptors. METHODS: The retinoid transfer activity of a recombinant protein corresponding to the fourth module of Xenopus laevis IRBP (X4IRBP) was examined in two ways. First, X4IRBP was tested for its ability to support the regeneration of porphyropsin in detached/reattached Xenopus retina/RPE-eyecups. Following illumination and removal of native IRBP, Xenopus eyecups supplemented with 42 microM X4IRBP or (as a control) Ringer's solution were incubated in darkness and then analyzed for regenerated porphyropsin. Second, toad (Bufo marinus) RPE-eyecup preparations were used to evaluate X4IRBP's ability to promote the release of 11-cis retinal from the RPE. RESULTS: The regeneration of porphyropsin in X4IRBP-supplemented Xenopus retina/RPE-eyecups (0.45 +/- 0.04 nmol; mean +/- SEM, n = 11) exceeded that in controls (0.13 +/- 0.02 nmol, n = 11). For promoting the release of 11-cis retinal from the toad RPE, 42 microM X4IRBP was more effective than equimolar bovine serum albumin although considerably less than that of 26 microM native bovine IRBP. CONCLUSIONS: The results indicate a low but significant activity of IRBP's fourth module in reactions relevant to retinoid exchange.

Retinol kinetics in the isolated retina determined by retinoid extraction and HPLC.

Suzuki et al. [Vis. Res. 26, 425-9 (1986); Vis. Res. 28, 1061-70 (1988)] have described a formaldehyde-based (HCHO-based) extraction procedure that efficiently recovers 11-cis retinal initially present as rhodopsin chromophore in photoreceptor membranes. Using the isolated retina of the toad (Bufo marinus), we tested whether this procedure ('HCHO' method), in combination with a formaldehyde-free extraction procedure ('i/h' method) and the analysis of extracted retinoids by high performance liquid chromatography (HPLC), can account quantitatively for light-induced changes in retinoid levels and thus serve as an alternative to spectrophotometry for tracking the formation of all-trans retinol in this intact rod preparation. Initially dark-adapted retinas were incubated in bright light or in darkness and then analysed by homogenization and extraction using the HCHO and i/h methods. Combined data obtained using the two extraction procedures indicated a near-conservation of total retinoid recovered from dark-incubated and illuminated retinas, and thus accounted for light-induced changes in retinoid levels. The HCHO procedure, employing formaldehyde, isopropanol and hexane, was similar to that described by Suzuki et al. and recovered retinaldehydes including chromophoric 11-cis retinal. The i/h procedure utilized isopropanol and hexane and, unlike the HCHO method, efficiently recovered all-trans retinol. Illumination (onset at time zero) that produced an approximately exponential decline of 11-cis retinal (time constant of 24 s) led to an increase and then a gradual decline in all-trans retinal. The normalized peak level of all-trans retinal, representing about 0.54 of the total molar quantity of recovered retinoid, developed with illumination periods of 10-80 s. The normalized level of all-trans retinol reached approximately 0.3 in retinas illuminated for 1 min and, with longer illuminations (up to 30 min), exhibited an approximately exponential further growth to approximately 0.9 with a time constant of 9.2 min. The results indicate the workability of the HCHO and i/h extraction procedures for tracking the in situ conversion of all-trans retinal to all-trans retinol, a reaction thought to be important for both operation of the retinoid visual cycle and shut-off of the phototransduction cascade.

Photoresponses of human rods in vivo derived from paired-flash electroretinograms.

In the human eye, domination of the electroretinogram (ERG) by the b-wave and other postreceptor components ordinarily obscures all but the first few milliseconds of the rod photoreceptor response to a stimulating flash. However, recovery of the rod response after a bright rest flash can be analyzed using a paired-flash paradigm in which the test flash, presented at time zero, is followed at time t by a bright probe flash that rapidly saturates the rods (Birch et al., 1995). In ERG experiments on normal subjects, the hypothesis that a similar method can be used to obtain the full time course of the rod response to test flashes of subsaturating intensity was tested. Rod-only responses to probe flashes presented at varying times t after the test flash were used to derive a family of amplitudes A(t) that represented the putative rod response to the test flash. These rod-only responses to the probe flash were obtained by computational subtraction of the cone-mediated component of each probe flash response. With relatively weak test flashes (11-15 scot-td-s), the time course of the rod response to the test flash derived in this manner was consistent with a four-stage impulse response function of time-to-peak approximately 170 ms. A(170), the amplitude of the derived response at 170 ms, increased with test flash intensity (Itest) to a maximum value Amv and exhibited a dependence on Itest given approximately by the relation, A(170)/Amo = 1 - exp(-kItest), where k = 0.092 (scot-td-s)-1. In steady background light, the falling (i.e. recovery) phase of the derived response began earlier, and the sensitivity parameter k was reduced several-fold from its dark-adapted value. As the sensitivity, sensitivity, kinetics, and light-adaptation properties of the derived response correspond closely with those of photocurrent flash responses previously obtained from isolated rods in vitro, it was concluded that the response derived here from the human ERG approximates the course of the massed in vivo rod response to a test flash.

Rod phototransduction in transgenic mice expressing a mutant opsin gene.

Rod-mediated electroretinograms (ERG's) were recorded from transgenic mice expressing a mouse opsin gene with three point mutations (V20G, P23H, and P27L; termed VPP mice) and from normal littermates. The leading edge of the alpha wave was analyzed in relation to a computational model of rod phototransduction [J. Physiol. 499, 719 (1992)], in which values for the maximum response (RmP3), transduction gain (S), and transduction delay (td) are derived from alpha-wave data. VPP mice exhibited an age-related decrease in RmP3. This decrease was consistent with reductions in the number of rod photoreceptors and in the length of rod outer segments observed in previous histological studies of the VPP retina. Values of S determined for the VPP mice were within the normal range, consistent with a normal amplification of the visual signal in VPP rods. At high stimulus intensities, both normal and VPP mice exhibited a decrease in S, which may reflect depletion of a phototransduction substrate at these stimulus levels. We examined the recovery of the alpha wave after a bright conditioning flash by measuring the rod alpha-wave response to a probe flash presented at varying times after the conditioning stimulus. In both normal and VPP mice a fourfold (0.6-log-unit) increase in conditioning stimulus intensity increased both T50%, the period required for half-maximal recovery, and tau, the exponential time constant describing recovery. However, the increases in T50% and tau were significantly greater in VPP mice, indicating an abnormally slow recovery of the flash response in VPP rods.

Recovery kinetics of human rod phototransduction inferred from the two-branched alpha-wave saturation function.

Electroretinographic data obtained from human subjects show that bright test flashes of increasing intensity induce progressively longer periods of apparent saturation of the rod-mediated electroretinogram (ERG) alpha wave. A prominent feature of the saturation function [the function that relates the saturation period T with the natural logarithm of flash intensity (ln I(f)] is its two-branched character. At relatively low flash intensities (I(f) below approximately 4 x 10(4) scotopic troland second), T increases approximately in proportion to ln I(f) with a slope [delta T/delta (ln I(f)] of approximately 0.3 s. At higher flash intensities, a different linear relation prevails, in which [deltaT/delta(ln I(f) is approximately 2.3 s [Invest. Ophthalmol. Vis. Sci. 36, 1603 (1995)]. Based on a model for photocurrent recovery in isolated single rods [Vis. Neurosci. 8, 9 (1992)], it was suggested that the upper-branch slope of approximately 2.3 s represents tau R*, the lifetime of photoactivated rhodopsin (R*). Here we show that a modified version of this model provides an explanation for the lower branch of the alpha-wave saturation function. In this model, tau E* is the exponential lifetime of an activated species (E*) within the transducin or guanosine 3', 5'-cyclic monophosphate (cGMP) phosphodiesterase stages of rod phototransduction; the generation of E* by a single R* occurs within temporally defined, elemental domains of disk membrane; and Ex, the immediate product of E* deactivation, is converted only slowly (time constant tau Ex) to E, the form susceptible to reactivation by R*. The model predicts that the decay of flash-activated cGMP phosphodiesterase (PDE*) is largely independent of the deactivation kinetics of R* at early postflash times (i.e., at times preceding or comparable with the lifetime tau E*) and that the lower-branch slope (approximately 0.3s) of the a-wave saturation function represent tau E*. The predicted early-stage independence of PDE* decay and R* deactivation furthermore suggests a basis for the relative constancy of the single-photon response observed in studies of isolated rods. Numerical evaluation of the model yields a value of approximately 6.7s for the time constant tau Ex.

Abnormal activation and inactivation mechanisms of rod transduction in patients with autosomal dominant retinitis pigmentosa and the pro-23-his mutation.

PURPOSE: The leading edge of the rod a-wave in normal human subjects can be fit with a computational model of the activation phase of transduction to provide parameters analogous to those obtained from individual photoreceptors. The authors extend this work to the kinetics of recovery after saturating flashes. METHODS: Electroretinograms were recorded from three patients with autosomal dominant retinitis pigmentosa and the pro-23-his rhodopsin mutation, two patients with rod monochromatism, and five normal subjects. Rod-only a-waves were obtained for a series of flashes ranging from 4.4 to 10.1 ln (1.9 to 4.4 log) scot td-sec. One set of parameters describing the activation process was derived from fits to the a-wave model. A double-flash paradigm was used to study inactivation mechanisms. The first flash was achromatic and varied in intensity (I(f)) from 6.1 to 13.9 ln (2.6 to 6.0 log) scot td-sec. The second flash was a short-wavelength probe held constant at 9.3 ln (4.0 log) scot td-sec. Cone components were elicited with a photopically matched long-wavelength stimulus and were computer subtracted. Recovery at each I(f) was followed by measuring the amplitude to the probe flash at various interstimulus intervals (ISI). The critical time (Tc) before the initiation of rod recovery was determined from the function relating relative rod amplitude to ISI. RESULTS: Recovery from activation was similar in normal subjects and in patients with rod monochromatism. Over a large range of I(f) above rod saturation, Tc increased in proportion to ln I(f). The mean slope of the function relating Tc to I(f) was 2.3 s/ln I(f) when I(f) varied between 11 and 13.9 ln scot td-sec. Patients with retinitis pigmentosa and the pro-23-his rhodopsin mutation had a decrease in the gain of activation. They also had significantly slower than normal recovery after high test flash intensities, such that the slope of the function relating Tc to ln I(f) was 12.1 seconds. CONCLUSION: Available data from other species imply that complete, transient activation of transducin (T saturation) occurs within or below the investigated range of flash intensities. Based on the slope of the delay function (delta Tc/ delta ln I(f)) above 11 ln scot td-sec, the authors hypothesize that the lifetime of activated rhodopsin (R) in normal human rods is approximately 2.3 seconds. In patients with the pro-23-his mutation, the gain of the activation mechanism is reduced and the reaction determining the delta Tc/ delta ln I(f) slope is markedly slowed. The activated species that exhibits this prolonged lifetime could be the mutant rhodopsin itself.

Reduced light-dependent phosphorylation of an analog visual pigment containing 9-demethylretinal as its chromophore.

9-Demethyl rhodopsin (9dR), an analog of vertebrate rhodopsin, consists of opsin and a covalently attached chromophore of 11-cis 9-demethylretinal. Electrophysiological evidence that photoactivated 9dR (9dR*) undergoes abnormally slow deactivation in salamander rods (Corson, D. W., Cornwall, M. C., and Pepperberg, D. R. (1994) Visual Neurosci. 11, 91-98) raises the possibility that opsin phosphorylation, a reaction involved in visual pigment deactivation, operates abnormally on 9dR*. This possibility was tested by measuring the light-dependent phosphorylation of 9dR in preparations obtained from bovine rod outer segments. Outer segment membranes containing 9dR or regenerated rhodopsin were flash-illuminated in the presence of [gamma-32P]ATP and rhodopsin kinase, further incubated in darkness, and then analyzed for opsin-bound [32P]Pi. [32P]Pi incorporation by 9dR* increased with both incubation period and bleaching extent but, under all conditions tested, was less than that measured in rhodopsin controls. Results obtained with 30-s incubation periods indicated that the maximal initial rate of incorporation by 9dR* is about 25% of that by photoactivated rhodopsin. The results imply that the low incorporation of Pi by 9dR* results from a reduced rate of phosphorylation by rhodopsin kinase and are consistent with the prolonged lifetime of 9dR* determined electrophysiologically.