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Biomedical subjects

E F MacNichol

Publications and source records attributed to E F MacNichol.

At least 19 recordsLinked to original sources
PubMed1994

Relief of opsin desensitization and prolonged excitation of rod photoreceptors by 9-desmethylretinal.

The 9-methyl group of 11-cis-retinal plays a crucial role in photoexcitation of the visual pigment rhodopsin. A hydrogen-substituted analogue, 11-cis-9-desmethylretinal, combines with opsin to form a pigment that produces abnormal photoproducts and diminished activation of the GTP-binding protein transducin in vitro. We have measured the formation of this analogue pigment in bleached salamander rods and determined the size and shape of its quantal response. In addition, we have characterized the influence of opsin and newly formed analogue pigment on the quantal response to native porphyropsin. We find that, as 11-cis-9-desmethylretinal combines with opsin in bleached rods, the amplitude of the quantal response from residual native pigment is elevated by approximately 7.5-fold to 0.15 +/- 0.09 pA, a value close to the amplitude of the quantal response before bleach (0.31 +/- 0.10 pA). When activated by light, the new analogue pigment produces a quantal response that is approximately 30-fold smaller and decays approximately 5 times more slowly than that of native pigment in unbleached cells. We conclude that the 9-methyl group of retinal is not critical for conversion of opsin to its nondesensitizing state but that it is critical for the normal processes of activation and deactivation of metarhodopsin that give rise to the quantal response.

Action Potentials↗
PubMed1993

Noncovalent occupancy of the retinal-binding pocket of opsin diminishes bleaching adaptation of retinal cones.

Bright light bleaches visual pigment and leads to a persistent desensitization of isolated rod and cone photoreceptors called bleaching adaptation. Bleaching adaptation results from the combined effects of pigment depletion and adaptational modulation of certain cellular reactions in the visual transduction cascade. Here, we present evidence that in solitary cone photoreceptors isolated from the salamander retina, the latter effect is due to the presence of free opsin in the outer segment. Also, we demonstrate that this "opsin adaptation" can be reversed by treating the cells with synthetic retinoids similar to 11-cis retinal but having polyene chains too short to form protonated Schiff base attachments to opsin.

Adaptation, Physiological↗
PubMed1993

Visual pigment bleaching in isolated salamander retinal cones. Microspectrophotometry and light adaptation.

Visual pigment bleaching desensitizes rod photoreceptors greatly in excess of that due to loss of quantum catch. Whether this phenomenon also occurs in cone photoreceptors was investigated for isolated salamander red-sensitive cones. In parallel experiments, (a) visual pigment depletion by steps of bleaching light was measured by microspectrophotometry, and (b) flash sensitivity was measured by recording light-sensitive membrane current. In isolated cones, visual pigment bleaching permanently reduced flash sensitivity significantly below that due to the reduction in quantum catch, and there was little spontaneous recovery of visual pigment. The "extra" desensitization due to bleaching was most prominent up to bleaches of approximately 80% visual pigment and reached a level approximately 1 log unit beyond that due to loss of quantum catch. At higher bleaches, the effect of loss of quantum catch became more important. Bleaching did not greatly reduce the maximum light-suppressible membrane current. A 99% reduction of the visual pigment permanently reduced the circulating current by only 30%. Visual pigment bleaching speeded up the kinetics of dim flash responses. All electrical effects of bleaching were reversed on exposure to 11-cis retinal, which probably caused visual pigment regeneration. Light adaptation in photopic vision is known to involve significant visual pigment depletion. The present results indicate that cones operate with a maintained circulating current even after a large pigment depletion. It is shown how Weber/Fechner behavior may still be observed in photopic vision when the contributions of bleaching to adaptation are included.

Absorptiometry, Photon↗
PubMed1990

Transduction noise induced by 4-hydroxy retinals in rod photoreceptors.

New visual pigments were formed with 4-hydroxy retinals in isolated vertebrate rod photoreceptors by exposing bleached rods from the tiger salamander, Ambystoma tigrinum, to lipid vesicles containing the analogues. Formation of physiologically active pigment was demonstrated by the restoration of sensitivity and by a shift of approximately 50 nm in the peak of both the visual pigment absorptance spectrum and rod spectral sensitivity spectrum from approximately 520 to approximately 470 nm for 11-cis 4-hydroxy retinal. Membrane current recordings from the inner segments of isolated rods revealed excess fluctuations in membrane current after formation of the new pigment in bleached cells or after exposure of unbleached cells to flashes in the presence of the analogue. The excess current fluctuations are similar to the fluctuations elicited by steady light producing a few discrete responses per second, a rate approximately 100 times greater than the normal rate of spontaneous events in darkness. These results suggest that analogues of retinal can produce alterations in the frequency of production of discrete responses in darkness in rod photoreceptors.

Ambystoma↗
PubMed1990

Sensitization of bleached rod photoreceptors by 11-cis-locked analogues of retinal.

Photoactivation of rhodopsin initiates both excitation and adaptation in vertebrate rod photoreceptors. Bleaching of rhodopsin to free opsin and all-trans-retinal in isolated rods produces a stable desensitization (bleaching adaptation) that is much larger than expected from pigment depletion alone. In our experiments, a 93% bleach produced a 500-fold increase in the light intensity required for saturation of the light response. This component of adaptation was 32-fold larger than the 16-fold increase expected from pigment depletion alone. 11-cis-Retinal, when delivered to isolated rods from liposomes, combines with free opsin to form a bleachable photopigment that fully restores sensitivity. 11-cis-Locked analogues of retinal combine with opsin to form unbleachable pigments in isolated bleached rods from the tiger salamander. They restore sensitivity to a substantial (16- to 25-fold) but incomplete extent. The analogues apparently relieve a stable component of adaptation when they interact with opsin. Because these analogues do not detectably excite rods, the structural requirements of both retinal and opsin for the relief of adaptation are different from those of excitation. The biochemical basis of light adaptation resulting from pigment bleaching and the minimum structural requirements of retinal for its relief remain to be determined.

Ambystoma↗
PubMed1990

Cellular mechanisms that underlie bleaching and background adaptation.

Experiments were performed on rod photoreceptors isolated from the eye of the larval tiger salamander to determine if the same or different mechanisms underlie the desensitization produced by dim background light (background adaptation) and that which persists in the steady state in darkness after a significant fraction of the photopigment is bleached (bleaching adaptation). We have examined adaptational effects after light that bleached between approximately 50% and 95% of the photopigment under conditions which preclude pigment regeneration. The steady-state desensitization, far greater than that predicted by quantum-catch loss, is relieved upon regeneration of the visual pigment with 11-cis retinal. We measured the spread of desensitization along the long axis of the rod after a local bright bleach at one end by comparing responses to dim local test flashes elicited in different regions of the outer segment, before and after bleaching. The space constant for this spread was less than 2.5 microns. We have previously measured the space constant for the longitudinal spread of desensitization during a local dim background in Ambystoma rods to be 7 microns. This is similar to a space constant of 6 microns measured under similar conditions in Bufo rods by Lamb et al. (1981. J. Physiol. 319:463-496). If calcium carries the signal for background desensitization, this difference in space constant for background and bleaching adaptation precludes it as the messenger for the steady component of bleaching adaptation. Experiments with isobutylmethyl xanthine (IBMX) also indicate that Ca2+ as well as c-GMP are unlikely regulators of bleaching desensitization, since elevation of cytosolic levels of both of these internal messengers by IBMX has little effect on sensitivity in bleach-adapted cells. All of our findings are consistent with the notion that bleaching adaptation is not mediated by a freely diffusible cytoplasmic messenger.

1-Methyl-3-isobutylxanthine↗
PubMed1988

Increased preferential absorption in human atherosclerotic plaque with oral beta carotene. Implications for laser endarterectomy.

Patients undergoing carotid endarterectomy were pretreated with low-dose, oral beta carotene to determine whether the carotenoid content of plaque could be increased in vivo. Beta carotene-treated patients had a 50-fold increase in their plaque beta carotene level from 0.066 to 3.3 micrograms beta carotene/g plaque. Microscopy and microspectrophotometry demonstrated that plaque from beta carotene-treated patients had higher carotenoid levels and higher absorption (450-500 nm) compared with control specimens, but normal media was unaffected. This demonstration of increased preferential absorption by plaque suggests that selective ablation of atherosclerotic plaque may be enhanced by pretreating patients with oral beta carotene.

Absorption↗
PubMed1984

Blue-sensitive cones in the primate retina: microspectrophotometry of the visual pigment.

Direct absorbance and bleaching absorbance-difference spectra were obtained using a photon-counting microspectrophotometer from the outer segments of ten blue-sensitive cones of macaque monkeys. The peak wavelength (lambda max) of the direct measurements was 426 +/- 3.4 nm, whereas the lambda max of the bleaching difference was 434 +/- 6.6 nm. We consider these values to be upper and lower bounds since both measurements may be shifted in opposite directions by wavelength-dependent effects. Therefore, the true peak sensitivity must be close to 430 nm.

Animals↗
PubMed1984

Absorptance and spectral sensitivity measurements of rod photoreceptors of the tiger salamander, Ambystoma tigrinum.

The spectral sensitivity of the extracellularly-recorded photoresponse of isolated rods of the tiger salamander, Ambystoma tigrinum, was compared to the absorptance spectrum. Both measurements were made with the same optical system on the same portion of each cell to avoid errors that could occur when the two kinds of measurement were made under different conditions. The relative spectral sensitivity and absorptance spectrum were found to be in excellent agreement between 450 and 700 nm.

Ambystoma↗
PubMed1983

Cellular mechanisms for color-coding in holostean retinas and the evolution of color vision.

Electrophysiological recording and microspectrophotometry were used to analyze retinal function in representatives of the two surviving genera of holostean grade fish--the bowfin (Amia calva) and gars (Lepisosteus sp.). The properties of the cone photopigments, horizontal cells and ganglion cells show that these holostean retinas have cellular mechanisms for color vision which are fundamentally similar to those previously described for teleosts, turtle and mammals. These findings suggest that trichromatic receptor systems and opponent color-coding mechanisms may have evolved in primitive Neopterygii or more ancient fish, before the advent of teleosts. In conjunction with other recent data on living representatives of primitive fishes, these findings also add renewed plausibility for the view that vertebrate color vision could have taken a common origin some 400 million years ago from an ancestral aquatic jawed vertebrate.

Action Potentials↗
PubMed1983

Spatial localization of bleaching adaptation in isolated vertebrate rod photoreceptors.

Bleaching of a large fraction of the rhodopsin in isolated rod outer segments results in an irreversible desensitization of the rod. This desensitization is referred to as bleaching adaptation. The logarithm of the sensitivity of the rod during bleaching adaptation has been found by a number of workers to be linearly related to the concentration of unbleached rhodopsin. We have measured the desensitization due to bleaching adaptation produced by a spatially confined stimulus and found that its effects are highly local. The space constant for the spread of desensitization was less than 4 microns. The small apparent spread of desensitization beyond the bleached regions probably can be accounted for by defocusing and light scatter. Thus, the involvement of a freely diffusible transmitter in bleaching adaptation does not appear to be required.

Adaptation, Physiological↗
PubMed1980

Electrical responses and photopigments of twin cones in the retina of the walleye.

1. The properties of twin and single cones in the retina of the walleye (Stizostedion vitreum vitreum) were studied by intracellular recording, dye injection and microspectrophotometry. 2. Twin cones generate hyperpolarizing responses to central illumination, can receive depolarizing influences (feed-back) from the receptive field surround, and show no detectable dye coupling when injected with Procion yellow. In seventeen of eighteen dye-injected cones, fluorescence was intense in the inner segment and undetectable or weak in the cone pedicle. 3. Both members of the twin cone contain the same photopigment in their outer segments. It absorbs maximally at about 605 nm. 4. A 533 nm green-sensitive photopigment was found in single cones. No blue-sensitive cones have been found. 5. With the exception of a modest discrepancy in the violet, the absorptance spectrum of the 605 nm photopigment of twin cones agrees closely with the action spectrum measured by intracellular recording. 6. The spectral properties established by the twin cone's photopigment are not detectably altered by the hyperpolarizing influences arising from nearby cones or by the depolarizing influences arising from the receptive field surround. 7. The twin cones of the walleye retina are thus "identical twins', both photochemically and physiologically, and seem designed to function as long-wave, spectrally univariant receptor units for colour vision. 8. The available evidence suggests that identical twin cones differ functionally from double cones and non-identical twin cones. 9. Although they outnumber single cones by about three to one in adults, identifiable twin cones were rarely observed in the cone population of retinas examined 3-5 days after birth. If walleye twin cones develop by fusion of single cones this process apparently occurs only for cones containing the 605 nm photopigment.

Animals↗
PubMed1979

Intraretinal distribution of cone pigments in certain teleost fishes.

Microspectrophotometric investigations of visual pigments in the teleost family Cichlidae determined that morphological "twin cones" need not be "pigment twins" as well. In each species there were two pigments that could be found in these cells; a "longwave" and a "shortwave" type whose precise spectral location varies for each species, making the terms red and green inadequate to describe them. Studies of the receptor mosaic with the nitro-blue tetrazolium chloride reduction technique permitted the sampling of larger receptor populations and confirmed that twin cones in several cichlid species could be either longwave-longwave, longwave-shortwave, or shortwave-shortwave pairs, and that the relative proportions of these twin cone types vary in different parts of the retinas. Nonuniform distribution of pigment types was also evident in the eyes of several other species from a variety of piscine taxa.

Animals↗
PubMed1979

Triple cones found in retinas of 3 fish species.

Triple cones were found in the retinas of 3 species of fishes indigenous to the Woods Hole area. The function of these triple cones can not be deduced from the behavior patterns of these fishes.

Animals↗