G. Höglund

Trichromatic visual system in an insect and its sensitivity control by blue light

SummaryThe spectral sensitivity of the visual cells in the compound eye of the mothDeilephila elpenor was determined by electrophysiological mass recordings during exposure to monochromatic adapting light. Three types of receptors were identified. The receptors are maximally sensitive at about 350 nm (ultraviolet), 450 nm (violet), and 525 nm (green). The spectral sensitivity of the green receptors is identical to a nomogram for a rhodopsin with λmax at 525 nm. The spectral sensitivity of the other two receptors rather well agrees with nomograms for corresponding rhodopsins. The recordings indicate that the green receptors occur in larger number than the other receptors. The ultra-violet and violet receptors probably occur in about equal number.The sensitivity after monochromatic adapting illumination varies with the wavelength of the adapting light, but is not proportional to the spectral sensitivity of the receptors. The sensitivity is proportional to the concentration of visual pigment at photoequilibrium. The equilibrium is determined by the absorbance coefficients of the visual pigment and its photoproduct at each wavelength. The concentration of the visual pigment, and thereby the sensitivity, is maximal at about 450 nm, and minimal at wavelengths exceeding about 570 nm.The light from a clear sky keeps the relative concentration of visual pigment in the green receptors, and the relative sensitivity, at about 0.62. The pigment concentration in the ultra-violet receptors is about 0.8 to 0.9, and that in the violet receptors probably about 0.6. At low ambient light intensities a chemical regeneration of the visual pigments may cause an increase in sensitivity. At higher intensities the concentrations of the visual pigments remain constant. Due to the constant pigment concentrations the input signals from the receptors to the central nervous system contain unequivocal information about variations in intensity and spectral distribution of the stimulating light.

Pigment migration and spectral sensitivity in the compound eye of moths

SummaryThe spectral sensitivity of dark adapted eyes of the moth Manduca sexta (Lepidoptera: Sphingidae) was measured between 350 and 650 nm by determining the relative number of photons necessary to elicit a retinal potential of 50 μV. The spectral sensitivity was determined in eyes with the screening pigment: a) in the extreme distal position, b) in a proximal position, and c) with the pigment removed. Sensitivity maxima were found at about 350 to 370, 450 and 530 nm, irrespective of pigment position.Pigment movement is associated with a change in light attenuation of 2 to 3 log units at all wavelengths between 350 and 650 nm. Only small variations, about 0.6 log units, in screening effect were observed throughout the measured spectrum, and therefore the effect of pigment movement on spectral sensitivity is small.There is a slightly larger decline in sensitivity of eyes with the pigment removed compared to eyes with the pigment in the distal position at wavelengths above 530 nm. This result suggests that the distally located pigments have a slight screening effect.

The Photopigments in an Insect Retina

Colour vision is not an exclusive property of vertebrates. Also insects can discriminate wavelengths. The best known example is the honeybee, as shown by training experiments (1) and electrophysiological recordings (2,3). The peripheral wavelength discrimination is accomplished by at least three receptor types. The spectral sensitivity of the receptors fairly well agrees with resonance spectra for rhodopsins (3), and bee heads contain retinol and retinal (4). These results suggest that the visual pigments in insects are rhodopsins, i. e. they consist of retinal bound to a protein.

Spectral absorption by screening pigment granules in the compound eyes of a moth and a wasp

SummaryThe spectral absorption by single ommin containing pigment granules or clusters of granules from compound eyes was measured spectrophotometrically between 300 and 700 nm. The measurements were made on fresh and fixed slices from compound eyes of Celerio euphorbiae and Vespa spec. In the visible part of the spectrum there is an absorption maximum between 540 and 550 nm, situated nearly 30 nm more towards the red than that of pure ommin in solution. A frequently found side maximum of variable height at about 450 nm is probably caused by oxidized xanthommatin occurring additionally within the granules. The absorption increases from 350 nm towards shorter wavelengths, and gradually declines between 550 and 700 nm.

Light induced retinal screening pigment migration independent of visual cell activity

Summary1.The migration of the granules in the retinal screening pigment cells was studied in the compound eye of the mothDeilephila elpenor. Pigment position was determined by observing the size of the ‘glow’ caused by reflection of incident light.2.The spectral efficiency of the pigment migration was tested by wavelengths between 352 and 608 nm. Maximal pigment movement was seen after exposure to wavelengths between 352 and 443 nm. The movement decreased with wavelength between 443 and 530 nm. (Maximal spectral sensitivity, and efficiency, of the visual cell layer is to 525 nm).3.Exposure to white light caused pigment expansioninspiteofclampingthe visual cell receptor potential.4.Blue (λ = 458 nm) light elicited pigment expansion also after the screening pigment cell layer had been isolated from the visual cells.5.In animals dying in darkness the screening pigment remained in the energy consuming contracted position while the body had already become completely stiff.The results suggest that:a)photosensitive system, maximally sensitive to UV and blue wavelengths, is located within the screening pigment cells, and that absorption of photons by this system directly interferes with the energy consuming mechanism which keeps the pigment contracted. Alternatively, pigment expansion is triggered by the UV (λmax 350 nm) and violet (λmax = 450 nm), but not by the green (λmax = 525 nm), sensitive visual cells.7.moths are trapped by UV wavelengths because these wavelengths elicit pigment expansion. Dim ambient light is then absorbed by the screening pigment, and the animal flies towards the trap which is the only light visible to the animal. During sunset the pigment migrates to the dark-adapted position as soon as the intensity of the short wavelength light is sufficiently low, whereby discrimination of variations in ambient light intensity is enhanced.

Ultra-violet and blue induced migration of screening pigment in the retina of the mothDeilephila elpenor

Summary1.The present study concerns the migration of the screening pigment in the compound eye of the mothDeilephila elpenor and its relation to the activity of the visual cells. Movements of the screening pigment were determined by microphotometric measurement, and the electroretinogram (ERG) was used as an index of retinal activity.2.A light stimulus caused a reflectance change that proceeded in three phases: an initial increase in reflectance, followed by a decrease and a final slow increase. The amplitude of the decrease, i.e. of the pigment dispersion, was found to be determined by the total number of light quanta incident on the eye. The reflectance decrease could be approximated by a first order photochemical reaction with a maximal summation time of at least 80 s.3.The spectral efficiency of the reflectance change between 338 and 530 nm differed from the spectral efficiency of the dark adapted retina recorded electrophysiologically, but was similar to the efficiency recorded during selective light adaptation of the green sensitive visual cells. Highest sensitivity was recorded in response to UV stimuli. Correspondingly, there was no distinct correlation between the amplitude of the ERG and the amplitude of the simultaneously recorded decrease in reflectance after UV, blue-violet or green light. The green stimulus elicited the largest ERG response but the smallest pigment dispersion. These results indicate that the contribution of the green sensitive visual cells to the pigment dispersion is small.4.The screening pigment was observed by transmission microscopy in preparations consisting of only the screening pigment cells, dioptric structures, and small visual cell rests kept in a depolarizing, high extracellular potassium concentration. UV and blue-violet light stimuli induced pigment dispersion similar to that observed in intact eyes. The result suggests that photosensitive pigment(s) are located in some element outside the visual cells, but does not preclude that the visual cells participate in the normal regulation of pigment dispersion.

Auditory degeneration after exposure to toluene in two genotypes of mice.

Two inbred strains of mice, CBA/Ca (with a moderate hearing loss starting late in life) and C57BL/6J (with an early onset of spontaneous auditory degeneration), were exposed to toluene by inhalation (1000 ppm, 12 h/day, 7 days) at either 1 or 6 months of age. Thresholds of auditory brainstem response (ABR) were measured 3–5 days after exposure and assessed repeatedly up to the age of 16 months (C57) or 23 months (CBA). Both strains of mice exposed to toluene at 1 month of age showed a mild loss of sensitivity at a high frequency (31.5 kHz) shortly after exposure. With increasing age, toluene exposure had little effect on the aging process of the auditory system in CBA mice but accelerated age-related hearing loss in C57 mice. The results indicate that toluene exposure can aggravate auditory deterioration only in mice with a strong genetic predisposition to spontaneously precocious age-related hearing loss.

Localization of the pupil trigger in insect superposition eyes

SummaryIn the superposition eyes of the sphingid moth Deilephila and the neuropteran Ascalaphus, adjustment to different intensities is subserved by longitudinal migrations of screening pigment in specialized pigment cells. Using ophthalmoscopic techniques we have localized the light-sensitive trigger that controls pigment position.In both species, local illumination of a small spot anywhere within the eye glow of a dark-adapted eye evokes local light adaptation in the ommatidia whose facets receive the light. Details of the response pattern demonstrate that a distal light-sensitive trigger is located axially in the ommatidium, just beneath the crystalline cone, and extends with less sensitivity deep into the clear zone. The distal trigger in Deilephila was shown to be predominantly UV sensitive, and a UV-absorbing structure, presumably the distal trigger, was observed near the proximal tip of the crystalline cone.In Ascalaphus we also found another trigger located more proximally, which causes local pigment reaction in the ommatidia whose rhabdoms are illuminated (the centre of the eye glow). The light-sensitive trigger for this response appears to be the rhabdom itself.

RECEPTOR SENSITIVITY AND PIGMENT POSITION IN THE COMPOUND EYE OF NOCTURNAL LEPIDOPTERA.

Abstract Recent investigations on nocturnal moths have shown the relation between the position of the secondary pigment and the sensitivity of the eye (1,2,3,4,5, 6). The present investigation was undertaken to obtain information concerning the mechanism of this action.

Photoreconvertible fluorophore systems in rhabdomeres, Semper cells and corneal lenses in the compound eye of the blowfly.

Summary1.The primary aim of the experiments described in this article was to localize the origin of the complex fluorescence in the compound eye of flies. The eye tissue was dissected and the fluorescence from cells and cell organelles was recorded by microspectrofluorometry. Using this technique, fluorophore systems were detected in the rhabdomeres, Semper cells and corneal lenses. The fluorophore systems are photoreconvertible by UV and blue light.2.The fluorophore systems in the rhabdomeres and Semper cells are similar. The intensity of the fluorescence from the microvilli is enhanced up to 29 × by adaptation to UV light. The enhancement is inversely related to the rhodopsin content in the microvilli, indicating that the chromophoric group of the fluorophore is not a vitamin A derivative.3.The enhancement of the fluorescence by UV light strongly depends on pH, suggesting that the photoreconvertible fluorophore systems in the microvilli and Semper cells are photosensitive redox pigments. These redox systems are probably located in the membranes of the microvilli in the photoreceptors, and in the endoplasmic reticulum of the Semper cells, or they are coupled to filaments in the cytoskeleton of both cell types.4.Preliminary reaction schemes for the photoreactions based on the recorded excitation and emission spectra and photokinetics were developed. A primary pigment in the microvillous structure,AR, or in organelles in the Semper cells,AS, is converted by UV light into an excited stateAR* orAS*, which either relaxes to the primary pigment by photon emission, or converts into an intermediate X, which by proton uptake changes into stable products,BRorBS.Blue illumination convertsBRandBSinto the excited statesBR*andBS*, which either relax by photon emission toBRorBS, or convert into an intermediate Y, which after deprotonation reconverts into the primary pigmentARorAS.5.Estimation of the molecular density showed that the concentration of the fluorophore in the microvilli presumably is almost equal to maximal rhodopsin concentration. The high density suggests that the fluorophores have a specific function in transduction or adaptation of the visual process.