Timothy H. Goldsmith

The cone oil droplets of avian retinas

The cone oil droplets of 19 species of birds from 11 families were examined by microspectrophotometry. Individual droplets were expanded with mineral oil, suspended in aqueous glycerol, and absorbance spectra measured between 700 and 320 nm. A classification of oil droplets is proposed, in which objective measurements of their carotenoids are related to the size, position and visual appearance of the droplets under the microscope. Some droplets contain no carotenoid and are transparent at wavelengths longer than 320 nm. Other droplets appear colorless but contain carotenoids absorbing at 385 or 402 nm. The pale droplets that have traditionally been described as greenish contain a mixture of two carotenoids. All of these types are distinct from yellow and red droplets. Red droplets contain astaxanthin esters, and yellow droplets contain a carotenoid with a spectrum similar to zeaxanthin. The 402 nm chromophore is galloxanthin, a C27 apo-carotenoid with 8 double bonds. The in vivo optical densities are 1-4 in the paler droplets, range up to about 8 in the yellow droplets, and can exceed 20 in the red droplets. All droplets that contain carotenoid can exhibit substantial absorption in the near u.v. The frequencies of the several droplet types in the retinas of different species suggests that these organelles respond readily to natural selection and may be involved in more than one function.

THE VISUAL SYSTEM OF INSECTS

Publisher Summary This chapter focuses on the structural organization of compound eyes, which are the principal photoreceptors of adult insects. They are composed of structural units called ommatidia. Compound eyes can be divided into two groups: photopic eyes and scotopic eyes. The former are characteristic of diurnal insects active in bright light, while the latter are found in nocturnal or crepuscular species and have short, fat rhabdoms that are separated from the crystalline cones by a relatively large distance. The chapter further illustrates examples of insect ommatidia. Many insects, particularly males, have eyes divided into two regions that are characterized by markedly different sizes and pigmentation of ommatidia. In some dragonflies, the dorsal facets are nearly twice the diameter of the ventral.

Four spectral classes of cone in the retinas of birds

SummaryThe spectral sensitivity of 15 species of birds has been measured by recording transretinal voltages from opened eyecups. With suitable combinations of colored adapting lights, we find that a variety of passerines have four peaks of photopic sensitivity, with maxima at 370, 450, 480, and 570 nm. Additional sensitivity maxima at 510 nm are found in some species. The spectral sensitivity functions are not altered by bathing the retinas in 50 mM sodium aspartate, suggesting that they reflect the properties of cones and do not result from inhibitory interactions between retinal interneurons.Comparison of the results with a general mathematical model that describes spectral sensitivity functions recorded extracellularly from populations of receptors in different states of adaptation (Goldsmith 1986) shows that the retinal spectral sensitivity functions are consistent with the presence of (at least) four types of cone, but indicate as well that many of the cones that are maximally sensitive in the blue and violet likely contain oil droplets that attenuate the deep violet and near uv.

Comparative studies of crustacean spectral sensitivity

SummarySpectral sensitivity of the lateral eyes of the isopodPorcellio scaber (wood louse) and the decapodsCallinectes sapidus (blue crab),Palaemonetes paludosus (Everglades prawn),Orconectes virilis, andO. immunis (crayfish) have been measured between 300 and 660 nm by determining the reciprocal number of photons required to evoke a constant size retinal action potential. Porcellio is maximally sensitive at 515 nm andCallinectes at 505 nm. Both species have a single pigment system, as spectral sensitivity is unchanged by red light adaptation. Palaemonetes appears to have a dichromatic color vision. Sensitivity of the dark-adapted eye is dominated by a receptor maximally sensitive at 550–555 nm, but red or yellow adaptation discloses a uv pigment with λmax at about 380 nm. Present evidence suggests the 555 and 380 nm pigments are located in different receptor cells. Orconectes has peak sensitivity at 565 nm, but under red light adaptation and close to the electroretinographic threshold a second sensitivity maximum appears at 425 nm. As in the prawn, these peaks seem to indicate the presence of a two-receptor color vision system.The corneas ofOrconectes, Callinectes, andHomarus (lobster) are relatively thick, and microspectrophotometric measurements show near ultraviolet absorption as well as the protein peak at 280 nm. By contrast,Palaemonetes andMusca (housefly), species with near ultraviolet receptors, have thinner corneas which are transparent through the near ultraviolet. The crystalline cone ofPalaemonetes likewise shows no near ultraviolet absorption but a strong protein band at 280 nm.The scarcity of ultraviolet receptors in the compound eyes of crustacea, in contrast to their common occurrence in insects, is thought to be related to the relative absence of ultraviolet wavelengths in most aquatic environments.

Photosensitivity of the Circadian Rhythm and of Visual Receptors in Carotenoid-Depleted Drosophila

Drosophila melanogaster was raised on aseptic diets, with and without β-carotene. The sensitivity of visual receptors in the carotenoid-depleted flies was lowered 3 log units, but the photosensitivity of the circadian rhythm was not affected. This result suggests that the chromophore of the photopigment which mediates light effects on the circadian rhythm is not a carotenoid derivative.

The visual pigment and visual cycle of the lobster,Homarus

SummaryThe visual pigment of the American lobster,Homarus americanus, has been studied in individual isolated rhabdoms by microspectrophotometry. Lobster rhodopsin has λmax at 515 nm and is converted by light to a stable metarhodopsin with λmax at 490 nm. These figures are in good agreement with corresponding values obtained by Wald and Hubbard (1957) in digitonin extracts. Photoregeneration of rhodopsin to metarhodopsin is also observed. The absorbance spectrum of lobster metarhodopsin is invariant with pH in the range 5.4–9, indicating that even after isomerization of the chromophore fromcis totrans, the binding site of the chromophore remains sequestered from the solvent environment. Total axial density of the lobster rhabdom to unpolarized light is about 0.7.As described for several other Crustacea, aldehyde fixation renders the metarhodopsin susceptible to photobleaching, a process that is faster at alkaline than at neutral or acid pH. Small amounts of a photoproduct with λmax at 370 nm are occasionally seen. A slower dark bleaching of lobster rhabdoms (τ1/2−2 h) also occurs, frequently through intermediates with absorption similar to metarhodopsin.The molar extinction coefficient of metarhodopsin is about 1.2 times greater than that of rhodopsin, each measured at their respective λmax. Isomerization of the chromophore fromcis totrans is accompanied by a change in the orientation of the absorption vector of about 3°. The absorption vector of metarhodopsin is either tilted more steeply into the membrane or is less tightly oriented with respect to the microvillar axes.When living lobsters are kept at room temperature, light adaptation does not result in an accumulation of metarhodopsin. At 4 °C, however, the same adapting lights cause a reduction of rhodopsin and an increase in metarhodopsin. There is thus a temperature-sensitive regeneration mechanism that supplements photoregeneration. Following 1 ms, 0.1 joule xenon flashes that convert about 70% of the rhodopsin to metarhodopsin in vivo, dark regeneration occurs in the living eye with half-times of about 25 and 55 min at 22 °C and 15 °C respectively.

Spectral sensitivity of cones in the goldfish, Carassius auratus

The spectral sensitivities of retinal cones isolated from goldfish (Carassius auratus) retinas were measured in the range 277-737 nm by recording membrane photocurrents with suction pipette electrodes (SPE). Cones were identified with lambda max (+/- S.D.) at 623 +/- 6.9 nm, 537 +/- 4.7 nm, 447 +/- 7.7 nm, and about 356 nm (three cells). Two cells (lambda max 572 and 576 nm) possibly represent genetic polymorphism. A single A2 template fits the alpha-band of P447(2), P537(2), and P623(2). HPLC analysis showed 4% retinal:96% 3-dehydroretinal. Sensitivity at 280 nm is nearly half that at the lambda max in the visible. The lambda max of the beta-band (in nm) is a linear function of the lambda max of the alpha-band and follows the same relation as found for A1-based cone pigments of a cyprinid fish.

Sensitivity of Visual Receptors of Carotenoid-Depleted Flies: A Vitamin A Deficiency in an Invertebrate

House flies (Musca domestica) raised under sterile conditions on a diet lacking carotenoids and retinol (vitamin A) have visual receptor sensitivities —as assessed electroretinographically—which average more than 2 log units below normal, both in the near ultraviolet (340 m�) and visible (500 m�) regions of the spectrum. Loss of sensitivity can be prevented by the addition of β-carotene to the larval food. Flies reared for several generations on a carotenoid-free diet, but under conditions where the adults are not kept sterile, do not show a further loss of sensitivity. It is suggested that carotenoid stored in the egg prevents complete blindness in the first generation, and that microorganisms can supply small amounts of carotenoid and thereby prevent complete blindness in the second and successive generations.

Cellular identification of the violet receptor in the crayfish eye

SummaryTen violet receptors in the retinas of crayfish (Procambarus) were injected intracellularly with the fluorescent dye Lucifer Yellow-CH and subsequently identified in histological preparations. All had their cell body located distal to the main rhabdom, in the position of the small, 8th retinular cell. In nine cases it was possible to trace the axon of the violet receptor beyond thelamina ganglionaris, and in four cases, to its termination in themedulla.By contrast, 22 green receptors similarly injected were all found to contribute to the main rhabdom, which is formed by retinular cells 1–7. Their axons synapsed in thelamina ganglionaris.Microspectrophotometry of the 8th cell reveals an absorption peak at 440 nm. As previous microspectrophotometric observations indicated that retinular cells 1–7 all contain a visual pigment with λmax at 530 nm, the microspectrophotometric data confirm that the violet receptor is cell 8.

Microspectrophotometry of the visual pigment of the spider crab Libinia emarginata

SummaryIsolated dark-adapted rhabdoms from the spider crab Libinia emarginata were examined by microspectrophotometry to determine the visual pigments present and their light-sensitive characteristics. The rhabdoms contain a single pigment with λmax=493 nm. Upon one minute irradiation with bright orange light this pigment forms a light-stable photoproduct with nearly the same λmax as the parent pigment but with slightly greater absorption to the long wavelength side of the absorption peak. On exposure to orange or yellow light in the presence of 5% glutaraldehyde, however, the pigment of Libinia rhabdoms bleaches slowly.The photosensitive pigment of properly oriented, transversely illuminated rhabdoms shows isotropic and dichroic regions, corresponding to layers of the rhabdom in which the microvilli are respectively parallel and perpendicular to the direction of propagation of the measuring beam. The maximum dichroic ratio is about 2, with most absorption when the plane of polarization is parallel to the microvillar axes.ZusammenfassungDie isolierten, dunkeladaptierten Rhabdome von Libinia emarginata wurden mikrospektrophotometrisch untersucht, um die Sehfarbstoffe und ihre lichtempfindlichen Eigenschaften zu entdecken. Die Rhabdome enthalten ein einziges Pigment (λmax 493 nm). Nach einer Bestrahlungszeit von 1 min mit hellem orangem Licht entsteht ein lichtfestes Photoprodukt mit beinahe dem gleichen λmax wie das ursprüngliche Pigment, aber mit etwas größerer Absorption gegen die langwellige Seite des Absorptionsmaximums hin. Das Pigment von Libinia-Rhabdomen bleicht langsam aus, wenn man es orangem oder gelbem Licht in Gegenwart von 5% Glutaraldehyd aussetzt.Werden richtig orientierte Rhabdome von der Seite belichtet, so zeigt das lichtempfindliche Pigment teils Isotropismus teils Dichroismus. Im ersten Fall sind die Mikrovilli parallel zur Richtung des Meßstrahles orientiert, im zweiten Fall stehen sie senkrecht dazu. Das größte Dichroismus-Verhältnis liegt bei 2. Man erhält die größte Absorption, wenn die Polarisationsrichtung parallel zur Achse der Mikrovilli steht.