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Biomedical subjects

D R Pepperberg

Publications and source records attributed to D R Pepperberg.

54 records · Page 3Linked to original sources
PubMed1984

Rhodopsin photoproducts and the visual response of vertebrate rods.

Flash responses of the photoreceptors in the isolated, all-rod retina of the skate were recorded extracellularly during exposure of the retina to relatively weak background light of fixed intensity. Under conditions expected to yield approximately constant levels of (previously) bleached visual pigment and transient bleaching intermediates, (quasi-)steady-state, increment receptor thresholds were examined in relation to the prevailing extent of rhodopsin bleaching. The results indicate that, at least for moderate bleaches, the change in log threshold associated with prior bleaching becomes smaller with increasing value of background intensity. This behavior of the increment threshold data is discussed in the context of a recent model (Pepperberg, 1984; Vision Res. 24, 357-366) linking values of photoreceptor threshold during light and dark adaptation with states of the visual pigment molecule.

Adaptation, Ocular↗
PubMed1984

Cycloheptatrienylidene analog of 11-cis retinal. Formation of pigment in photoreceptor membranes.

In biochemical and electrophysiologic studies employing the bullfrog (Rana catesbeiana) and the rat, the authors examined the interaction of opsin and an 11-cis-locked analog of retinal. In previously bleached preparations of bullfrog receptor outer segments (ROS) and isolated retinas, incubation with the aldehyde form (I) of the analog leads to the appearance of a pigment that is degraded slowly by hydroxylamine but is relatively resistant to photolysis. In the ROS preparation, the analog pigment (lambda max of difference spectrum congruent to 497 nm) also forms on incubation with NADP+ and the alcohol form (II) of the analog. In vitamin A deprived rats possessing only approximately 45% of the normal complement of rhodopsin, intraperitoneal injection of II leads within 1 day to the appearance of the analog pigment in the photoreceptors, at levels representing a major fraction of the opsin initially available for pigment formation. Formation of the analog pigment appears to have no significant effect on the sensitivity of electroretinographic b-wave responses recorded from the rat eye; furthermore, administration of II appears to suppress the sensitizing activity of all-trans retinol injected 1 day later. The data are discussed in relation to other studies examining chromophore-opsin interactions and electrophysiologic changes associated with the formation of rhodopsin in situ.

Animals↗
PubMed1982

Light-dependent effects of a hydrolysis-resistant analog of GTP on rod photoresponses in the toad retina.

Responses to 100-ms flashes were recorded intracellularly from dark- and light-adapted rod photoreceptors in the isolated retina of the toad, Bufo marinus. Properties of photoresponses were analyzed under each condition of adaptation when retinas were superfused with 1.0 mM guanosine 5'-[beta, gamma-methylene]triphosphate (p[CH2]ppG), a hydrolysis-resistant analog od GTP. When applied to retinas that previously had been subjected to intense light (approximately 30% bleach), p[CH2]ppG increased both the amplitude and duration of photoresponses. By contrast, treatment of dark-adapted retinas with p[CH2]ppG did not alter these response parameters. When similarly applied to either dark- or light-adapted retinas, GTP had no effects on amplitude or duration of photoresponses. These results are discussed in terms of GTP-dependent mechanisms for rod adaptation.

Animals↗
PubMed1982

Desensitization of skate photoreceptors by bleaching and background light.

Through extracellular measurements of photoreceptor responses to flashed stimuli, we examined how the bleaching of rhodopsin affects increment receptor threshold in the isolated retina of the skate (Raja oscellata and R. erinacea). Both initially unbleached and previously bleached photoreceptors, when exposed to full-field luminous backgrounds of fixed intensity, attain approximately stable levels of increment threshold that vary with the intensity of the background light. Values of stabilized increment thresholds measured after various extents of bleaching (less than approximately 50%), when plotted against background intensity in log-log coordinates, tend to converge with increasing intensity of the background; this relationship of the increment threshold functions resembles that which Blakemore and Rushton (1965b) found to describe the transient effect of bleaching on psychophysical increment threshold for the human rod mechanism. Our data are consistent with the possibility that related photochemical processes govern the stabilized levels of receptor sensitivity exhibited by the isolated retina (a) during steady illumination and (b) long after substantial bleaching.

Animals↗
PubMed1982

Utilization of retinoids in the bullfrog retina.

The capacity to generate 11-cis retinal from retinoids arising naturally in the eye was examined in the retina of the bullfrog, Rana catesbeiana. Retinoids, co-suspended with phosphatidylcholine, were applied topically to the photoreceptor surface of the isolated retina after substantial bleaching of the native visual pigment. The increase in photoreceptor sensitivity associated with the formation of rhodopsin, used as an assay for the appearance of 11-cis retinal in the receptors, was analyzed by extracellular measurement of the photoreceptor potential; in separate experiments using the isolated retina or receptor outer segment preparations, the formation of rhodopsin was measured spectrophotometrically. Treatments with the 11-cis isomers of retinal and retinol induced significant increases in both the rhodopsin content and photic sensitivity of previously bleached receptors. The all-trans isomers of retinyl palmitate, retinol, and retinal, as well as the 11-cis isomer of retinyl palmitate, were inactive by both the electrophysiological and spectrophotometric criteria for the generation of rhodopsin. Treatment with any one of the "inactive" retinoids did not abolish the capacity of subsequently applied 11-cis retinal or 11-cis retinol to promote the formation of rhodopsin. The data are discussed in relation to the interconversions of retinoids ("visual cycle of vitamin A") thought to mediate the regeneration of rhodopsin in vivo after extensive bleaching.

Animals↗
PubMed1978

Visual pigment and photoreceptor sensitivity in the isolated skate retina.

Photoreceptor potentials were recorded extracellularly from the aspartate-treated, isolated retina of the skate (Raja oscellata and R. erinacea), and the effects of externally applied retinal were studied both electrophysiologically and spectrophotometrically. In the absence of applied retinal, strong light adaptation leads to an irreversible depletion of rhodopsin and a sustained elevation of receptor threshold. For example, after the bleaching of 60% of the rhodopsin initially present in dark-adapted receptors, the threshold of the receptor response stabilizes at a level about 3 log units above the dark-adapted value. The application of 11-cis retinal to strongly light-adapted photoreceptors induces both a rapid, substantial lowering of receptor threshold and a shift of the entire intensity-response curve toward greater sensitivity. Exogenous 11-cis retinal also promotes the formation of rhodopsin in bleached photoreceptors with a time-course similar to that of the sensitization measured electrophysiologically. All-trans and 13-cis retinal, when applied to strongly light-adapted receptors, fail to promote either an increase in receptor sensitivity or the formation of significant amounts of light-sensitive pigment within the receptors. However, 9-cis retinal isin. These findings provide strong evidence that the regeneration of visual pigment in the photoreceptors directly regulates the process of photochemical dark adaptation.

Adaptation, Physiological↗
PubMed1978

Sensitizing activity of 9,13-dicis retinal in bleached photoreceptors of the skate.

9,13-dicis Retinal was externally applied to photoreceptors of isolated skate retina that previously had been desensitized by bleaching irradiation. This treatment led to a significant lowering of photoreceptor threshold and to the intracellular formation of isorhodopsin II, an artificial visual pigment containing 9,13-dicis retinal as its chromophore. These results suggest that isorhodopsin II can function in situ to promote an increase in the visual sensitivity of skate photoreceptors.

Animals↗
PubMed1976

Visual adaptation: effects of externally applied retinal on the light-adapted, isolated skate retina.

Incubation with externally applied 11-cis retinal induces a marked increase of visual sensitivity within partially bleached skate photoreceptors. This activity of 11-cis retinal is duplicated by 9-cis retinal, but not by all-trans retinal. The sensitization of photoreceptors promoted by 11-cis and 9-cis retinal is accompanied by the formation of rhodopsin and isorhodopsin, respectively.

Adaptation, Ocular↗
PubMedDate not supplied

Versatile software for collection, analysis and plotting of biochemical data using the IBM PC.

A software package, OMNILAB, has been written for the IBM PC, XT and AT computers to be a general purpose system for data collection, analysis, and display. The program supports collection of dta from a variety of absorbance detectors, from pH meters and from other instruments that output time-varying analog voltages in the ranges of millivolts or volts. The program includes capabilities for averaging of data, baseline subtraction, integration of curves, and versatile formatting for both video and hardcopy display of data.

Data Collection↗
PubMedDate not supplied

Effect of hydroxylamine on the subcellular distribution of arrestin (S-antigen) in rod photoreceptors.

The immunocytochemical labeling of arrestin (S-antigen) in photoreceptors of the ovine retina was examined following incubation of the retina with hydroxylamine (NH2OH), an agent known to inhibit the phosphorylation of photoactivated rhodopsin. Intact, isolated retinas bathed in medium containing 20 mM NH2OH, or in control medium lacking NH2OH, were maintained in darkness or exposed to bright light for 3 min (dark-adapted and light-adapted conditions, respectively); further incubated in darkness for 10 min; and then fixed and prepared for cryosectioning. Cryosections were incubated with anti-S-antigen monoclonal antibody MAb A2G5; with secondary antibodies that were conjugated with horseradish peroxidase; and with either 3-amino-9-ethyl carbazole or diaminobenzidine as chromogen. Anti-arrestin labeling in cryosections was then analyzed densitometrically using a light-microscopic image processing system. In dark-adapted control retinas, labeling density of the photoreceptor outer segment (OS) layer (0.061 +/- 0.004; average +/- S.E.M.) was less than that of the inner segment (IS) layer (0.138 +/- 0.011). In light-adapted control retinas, OS labeling density (0.139 +/- 0.007) exceeded IS labeling density (0.095 +/- 0.005). Incubation with NH2OH eliminated this light-dependent increase in labeling of the OS relative to that of the IS, i.e. eliminated the increase in relative OS/IS labeling. Densities of labeling were 0.110 +/- 0.006 (OS) and 0.183 +/- 0.006 (IS) in NH2OH-treated dark-adapted retinas vs. 0.078 +/- 0.004 (OS) and 0.182 +/- 0.008 (IS) in NH2OH-treated light-adapted retinas. Anti-arrestin labeling was also examined in retinas that were exposed to 3 min or 13 min of bright light and then immediately fixed.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation, Ocular↗
PubMedDate not supplied

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.

Adaptation, Ocular↗
PubMedDate not supplied

Modulation of transduction gain in light adaptation of retinal rods.

The effect of light adaptation on the period of photocurrent saturation induced by a bright stimulating flash was examined in rod photoreceptors of the larval-stage tiger salamander (Ambystoma tigrinum). Using suction electrodes, photocurrent responses to brief flashes were recorded from single, isolated rods in the presence and absence of steady background illumination. Background light decreased the saturation period (T) measured at fixed flash intensity (fixed If) and in this respect light-adapted the saturating response. Effects of the background on responses to weak (i.e. subsaturating) and bright flashes were compared with changes in a parameter, phi = e-delta T/TR*, where delta T is the decrease in saturation period, and where TR* is the slope of the line that relates T and ln If in a given state of adaptation. Dark- and light-adapted responses to flash intensities IDf and ILf, respectively, exhibited similar absolute peak photocurrent and falling-phase kinetics when IDf and ILf satisfied the relation, IDf = phi (ILf + IbTR*), where Ib is the background intensity. It is argued that phi approximates the relative PDE*/R* gain of transduction, i.e. the relative peak level of activated cGMP phosphodiesterase (PDE*) produced by a given, small amount of photoactivated visual pigment (R*). Interpreted on this view, the results imply that light adaptation derives largely from a decrease in PDE*/R gain, rather than from the stimulation of guanylate cyclase activity. The data are consistent with the possibility that modulation of the lifetime of PDE* underlies the background dependence of phi.

Adaptation, Ocular↗
PubMedDate not supplied

Evidence for the prolonged photoactivated lifetime of an analogue visual pigment containing 11-cis 9-desmethylretinal.

Following bright flashes, rod photoreceptors exhibit a period of photocurrent saturation that increases linearly with the logarithm of flash intensity. In a recent report, Pepperberg et al. (1992) presented evidence that the slope of the function relating the saturation period (T) to the natural logarithm of flash intensity (ln If) represents the exponential lifetime (tau) of photoactivated visual pigment: tau = delta T/delta [ln If]. In salamander rods, 11-cis 9-desmethylretinal combines with opsin to form 9-desmethyl rhodopsin. Dim flash responses mediated by this analogue visual pigment exhibited slow recovery kinetics relative to those of native pigment (Corson et al., 1991). This observation raises the hypothesis that the physiological lifetime of photoactivated 9-desmethyl rhodopsin is substantially longer than that of native visual pigment. To test this hypothesis, we have examined the relation between the period of photocurrent saturation and flash intensity in salamander rods containing a mixture of the two pigments. Brief stimuli at two widely separated wavelengths (440 and 640 nm) elicited saturating photocurrent responses that were preferentially mediated by 9-desmethyl rhodopsin or residual native pigment, respectively. Plots of T vs. ln If revealed a linear increase in the period of response saturation over a large range of saturating intensities at both wavelengths. However, the slope of the relation between T and ln If with 440-nm flashes was more than twice as large (4.1 +/- 0.5 s, n = 5) as that measured with 640-nm flashes (1.7 +/- 0.4 s). For rods subjected only to bleaching of the native pigment, or to bleaching and resensitization with 11-cis retinal, the slope of the relation between T and ln If remained independent of wavelength and indistinguishable from that of native pigment in unbleached cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMedDate not supplied

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.

Adaptation, Ocular↗
PubMedDate not supplied

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.

Adaptation, Physiological↗