ScienceWeek

NEUROBIOLOGY: ULTRAVIOLET VISION IN A BAT

The following points are made by Y. Winter et al (Nature 2003 424:612):

1) Most mammals, with the exception of primates, have dichromatic vision and correspondingly limited color perception(1). Ultraviolet vision was discovered in mammals only a decade ago(2), and in the few rodents and marsupials where it has been found, ultraviolet light is detected by an independent photoreceptor(2,3). Bats orient primarily by echolocation, but they also use vision.

2) All bat species have functional eyes that are used in various contexts(4,5), but their spectral sensitivities and their capacity for spectral discrimination are not known(1). Some bat-pollinated, neotropical plant species have violet blossoms and can even reflect ultraviolet light to a remarkable degree. This raised the question of whether flower bats have dichromatic color vision and whether they perceive the ultraviolet light reflected by some flowers.

3) The authors examined these questions in three behavioral discrimination experiments with neotropical flower bats (four individuals: three female, one male) of the species Glossophaga soricina (Phyllostomidae). First, the detection thresholds for light stimuli at different wavelengths, including the ultraviolet range, were determined. The result was a bimodal spectral-sensitivity function. This led the authors to investigate the ability of G. soricina to discriminate between colors, which as a property of the whole system can be tested only by psychophysical experiments. In a final experiment, the underlying photoreceptor mechanism was examined by using spectral chromatic adaptation. The authors suggest the result proves that these bats are capable of ultraviolet perception through a single receptor mechanism not previously demonstrated in mammals with intact eyes.

4) In summary: The authors demonstrate that a phyllostomid flower bat, Glossophaga soricina, is color-blind but sensitive to ultraviolet light down to a wavelength of 310 nm. behavioral experiments revealed a spectral-sensitivity function with maxima at 510 nm (green) and above 365 nm (ultraviolet). A test for color vision was negative. Chromatic adaptation had the same threshold-elevating effects on ultraviolet and visible test lights, indicating that the same photoreceptor is responsible for both response peaks (ultraviolet and green). Thus, excitation of the beta-band of the visual pigment is the most likely cause of ultraviolet sensitivity. This is a mechanism for ultraviolet vision that has not previously been demonstrated in intact mammalian visual systems.

References (abridged):

1. Jacobs, G. H. The distribution and nature of color vision among the mammals. Biol. Rev. 68, 413-471 (1993)

2. Jacobs, G. H., Neitz, J. & Deegan, J. F. II Retinal receptors in rodents maximally sensitive to ultraviolet light. Nature 353, 655-656 (1991)

3. Jacobs, G. H. & Deegan, J. F. II Sensitivity to ultraviolet light in the gerbil (Meriones unguiculatus): Characteristics and mechanisms. Vision Res. 34, 1433-1441 (1994)

4. Suthers, R., Chase, J. & Braford, B. Visual form discrimination by echolocating bats. Biol. Bull. 137, 535-546 (1969)

5. Chase, J. Differential responses to visual and acoustic cues during escape in the bat Anoura geoffroyi: cue preferences and behavior. Anim. Behav. 31, 526-531 (1983)

Nature http://www.nature.com/nature

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VISUAL TRANSDUCTION IN DROSOPHILA

The following points are made by R.C. Hardie and P. Raghu (Nature 2001 413:186):

1) Phototransduction, the process by which light energy is converted into a photoreceptor's electrical response, has long been at the forefront of studies, not only of sensory transduction, but also cell signaling more generally. Pioneering studies in the 1970s and 80s unraveled the biochemical steps of excitation in vertebrate rods and, together with seminal studies of hormone-stimulated adenylate cyclase, led to the discovery and characterization of G-protein signalling(1). These cascades, whereby heptahelical transmembrane receptors such as rhodopsin catalytically activate heterotrimeric G proteins, are widely found not only in many sensory receptors, but also throughout the body, where they respond to all manner of chemical messengers, such as hormones, neurotransmitters, odorants and tastants.

2) One hallmark of such cascades is their capacity for amplification. Early psychophysical experiments indicating that photoreceptors were capable of responding to single photons(2) were confirmed, first in invertebrates, and later in vertebrate rods, by electrophysiological recordings showing that quantized events (quantum bumps) could be recorded in response to absorption of single photons of light(3,4). Other functional attributes shared by vertebrate and invertebrate photoreceptors include low "dark noise" (spontaneous thermal isomerizations of rhodopsin, which sets the ultimate limit on absolute sensitivity(5)); efficient mechanisms for response termination; the coding of intensity by graded potentials; and the ability to light adapt -- that is, to reduce amplification as background intensity increases.

3) But there are also differences that hint at a dichotomy in the underlying molecular machinery. First, vertebrate photoreceptors hyperpolarize, because the transduction channels close in response to light, whereas in most invertebrates the channels open, leading to depolarization. Second, in rods, the trade-off between amplification and response speed limits human temporal resolution to 10 Hz under dim conditions. But fly photoreceptors possess the fastest known G-protein-signaling pathways, responding around 10 times more quickly than mammalian rods and 100 times faster than toad rods recorded at similar temperatures. Third, rods have only a limited ability to adapt, rapidly saturating as intensity increases; only the less sensitive cones can respond under daylight intensities. By contrast, despite their exquisite sensitivity to single photons, fly photoreceptors successfully light adapt over the entire environmental range, up to approximately 10^(6) effectively absorbed photons per second.

4) In summary: The brain's capacity to analyze and interpret information is limited ultimately by the input it receives. This sets a premium on information capacity of sensory receptors, which can be maximized by optimizing sensitivity, speed and reliability of response. Nowhere is selection pressure for information capacity stronger than in the visual system, where speed and sensitivity can mean the difference between life and death. Phototransduction in flies represents the fastest G-protein-signaling cascade known. Analysis in Drosophila has revealed many of the underlying molecular strategies, leading to the discovery and characterization of signaling molecules of widespread importance.

References (abridged):

1. Hille, B. G protein-coupled mechanisms and nervous signaling. Neuron 9, 187-195 (1992)

2. Hecht, S., Shlaer, S. & Pirenne, M. Energy quanta and vision. J. Gen. Physiol. 25, 819-840 (1942)

3. Yeandle, S. & Spiegler, J. B. Light-evoked and spontaneous discrete waves in the ventral nerve photoreceptor of Limulus. J. Gen. Physiol. 61, 552-571 (1973)

4. Baylor, D. A., Lamb, T. D. & Yau, K.-W. Responses of retinal rods to single photons. J. Physiol. 288, 613-634 (1979)

5. Aho, A. C., Donner, K., Hyden, C., Larsen, L. O. & Reuter, T. Low retinal noise in animals with low body-temperature allows high visual sensitivity. Nature 334, 348-350 (1988)

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