G.B. Arden

Anatomical, electrophysiological and pigmentary aspects of vision in the bush baby: An interpretative study

Abstract An attempt is made to interpret the spectral sensitivity of the dark-adapted bush baby ( Galago crassicaudatus agisymbanus , Coquerel) as a function of the light-absorbing properties of its visual pigment, and the reflectivity and fluorescence of its golden-coloured tapetum. The following observations and measurements have been made: 1. The retina is typically nocturnal. 2. The fine structure of the tapetum is described. 3. The spectral sensitivity curve between 420 and 600 nm has been determined on nine animals by an electrophysiological method. 4. The lens absorption is maximal at 385 nm (density = 0.9), but at 420 nm has a density of 0.1 and a negligible absorption beyond 520 nm. 5. The visual pigment is a substantially homogeneous retinene 1 -based chromoprotein of γ max = 501 nm . The “nominal” retinal density of the pigment is 0.16, and its effective density in the photoreceptors is 0.4. 6. The relative reflectance of the tapetum and the absorptive properties of the tapetal pigment have been measured. The pigment is a derivative of isoalloxazine (probably riboflavin) and exists in situ in the form of crystal plates. Within the framework of certain assumptions it is concluded that tapetal fluorescence does contribute to the visual sensitivity of the bush baby, thus confirming the prediction of Pirie (1959).

New components of the mammalian receptor potential and their relation to visual photochemistry

Abstract The photoreversal potential, associated with the photoregeneration of rhodopsin, resists cooling but disappears when the stimulus flash has brought equilibrium between rhodopsin and its bleached products. In contrast, the positive portion of the ERP (which is the only portion present below 5°C) increases in amplitude if the stimuli photoregenerate rhodopsin. and is present at photochemical equilibrium. Both the positive and negative portions of the ERP are similarly affected by photoregeneration. In the cold eye, where rhodopsin is bleached to metarhodopsin I, the photoreversal is still present. The isolated mammalian pigment epithelium and choroid produces a rapid corneo-negative potential superficially similar to the photoreversal potential, but which is photostable. This potential is much larger in pigmented eyes and probably requires the presence of melanin for its production. It does not resist cooling, and disappears, revealing a positive component. The ERP is shown to originate in an irreversible thermal process later than the primary photoexcitation. It is suggested that the photoreversal potential is the electrical correlate of the formation of rhodopsin from excited photoproducts. Certain differences between the positive ERP in dark- and light-adaptation suggest that this potential contains an additional component, analogous to the photoreversal potential. associated with net bleaching (photolysis).

Effects of hereditary degeneration of the retina on the early receptor potential and the corneo-fundal potential of the rat eye

(1) The base level of the corneo-fundal potential in normal rats matures by 60 days. (2) The light induced changes in potential mature by 30 days. (3) There is no correlation between the potential changes and rhodopsin synthesis. (4) In dystrophic rats the base level of the corneo-fundal potential is normal till 20 days (when retinal degeneration begins), but then stops growing. (5) at 60 days (when the pigment epithelium degenerates) it falls abruptly. (6) The light induced changes are supernormal before 18 days, but decline to zero by 35 days. (7) The early receptor potential (ERP) of normal rats matures by 25 days. (8) In dystrophic rats the ERP is supernormal before the 20th day, but then declines. (9) The amplitude changes indicate that the ERP is only developed by normally organized outer limbs. (10) In dystrophic animals with supernormal ERPs the ERG is already depressed, indicating different sites of origin for ERP, and a- and b-waves.

Mode of generation of the early receptor potential

Abstract The first (positive) phase of the retinal early receptor potential survives fixation in glutaraldehyde. It has a very low temperature coefficient, and has a waveform consistent with the hypothesis that the generating process is a photovoltic effect, attenuated by the tissue time constants. The second (negative) phase is abolished by glutaraldehyde but not by other aldehyde fixatives. The negative component of the pigment epithelium potential is similarly affected. Glutaraldehyde alone prevents extraction of rhodopsin from the rods. The negative component of the e.r.p. is increased by HSO3 ions, and after such treatment, the time course of the potential can be studied to −10°C. No correlation between the potential and the kinetics of rhodopsin breakdown products can be demonstrated. Similarly, agents which affect rhodopsin breakdown products do not alter the negative early receptor potential. It is suggested that the potential is produced by a redox mechanism in the cell membrane, since it is augmented by a reducing agent, and decreased by oxidising agents.

Effects of light-adaptation on the early receptor potential

Abstract In the dark-adapted eyes of albino rats, the amplitude of the early receptor potential (ERP) decreases with successive flashes, but does not disappear. Instead it reaches an equilibrium level, which is higher in dead eyes, and depends on the intensity of the short wave component in the stimulus flash. After equilibrium to white light has been reached, insertion of a filter, which absorbs blue wavelengths but which only slightly reduces the effectiveness of the flash in evoking the ERP, causes a progressive decline to a new equilibrium. When the filter is removed, the responses progressively increase once again. The latter effect is best seen in the dead eye, light-adapted by short exposures to tungsten light. Parallel extractions of dark- and light-adapted retinae show that the ERP amplitude is related to the quantity of rhodopsin in the retinae. The difference spectra of solutions from light-adapted eyes indicates that they contain isorhodopsin. It is concluded that the test flashes used in evoking the ERP cause photoregeneration of pigment, thus preventing complete bleaching and disappearance of the ERP. This in turn implies that in the eye metarhodopsin is relatively stable. The appearance of a new very rapid negative potential is associated with photoregeneration.

Rapid light-induced potentials common to plant and animal tissues.

IRRADIATION of retinal tissue by brief intense flashes of light produces rapid changes in electrical potential. These changes, which occupy only 2–3 msec, are characterized by an initial cornea-positive wave followed by a negative one, and have been referred to collectively as the “early receptor potential” or e.r.p.1. Spectral sensitivity measurements clearly demonstrate that the e.r.p. results from the absorption of light by retinal rhodopsin2, but recent studies (in this and other laboratories) have revealed that an apparently similar potential can be obtained from flash irradiated tissues containing sufficiently high concentrations of melanin (for example, ocular pigment epithelium3,4, iris5 and frog skin5). This communication reports that yet another pigmented tissue, the green leaf, will also produce an “e.r.p. type” potential when irradiated under identical conditions. Important similarities, not only of waveform and time course but also in response to temperature change, suggest that there is a common fundamental mechanism.

The mode of action of diaminophenoxy-alkanes and related compounds on the retina

Abstract The action of two compounds on the activity of the retina has been investigated using the cat ERG and the rabbit d.c. corneo-fundal potential as indices. In small doses the more toxic compound affects the amplitude and waveform of the dark-adapted electroretinogram, but the less toxic compound has only a very transient effect. The less toxic compound does not affect the sensitivity of the eye, as determined by the ERG. Nevertheless, the rate of dark-adaptation is slowed. This occurs in chronic and in acute experiments. The c-wave and corneo-fundal potential (in rabbit) are increased by a dose of these compounds that does not affect the ERG. It is argued that the compounds specifically affect the biochemical reactions which involve visual purple synthesis.

The minimum latency of the a‐wave

1. Intense flashes of light evoke early and late receptor potentials from albino rat eyes.