Ординатура / Офтальмология / Английские материалы / The Retina and its Disorders_Besharse, Bok_2011

colors are molecularly similar to eye pigments found in Drosophila, although butterflies may employ a much larger repertoire of pigments than flies. Tantalizing clues from a handful of studies suggest that, aside from diversifying photoreceptor sensitivities, filtering pigments may play a role in the co-evolution of photoreceptors and color-based mating signals. The same genes that mediate both spectral sensitivities in photoreceptors via filtering pigments may also influence the expression of wing pigments expressed in butterfly wings. However, this hypothesis remains to be tested empirically by identifying and molecularly characterizing the genes involved. Filtering pigments and their expression in the butterfly eye offer a fruitful avenue for research into the co-evolution of animal color-based signals and the ability to perceive them.

Evolution of Sexually Dimorphic Eyes:

Opsin Duplications and Sex-Specific

Filtering Pigment Expression

One of the most interesting aspects of butterfly vision is that some species display sexually dimorphic eyes: males and females detect different colors in the same areas of the eye. This feature appears to have evolved independently in two butterfly families, the Lycaenidae and the Pieridae, via two different mechanisms. In the North American butterfly Lycaena rubidus (Lycaenidae), duplication of the B opsin has facilitated the evolution of a sexually dimorphic eye. The eyes of this butterfly express four opsin genes, one UV, two B (B1 and B2), and one LW. In the dorsal part of the eye, L. rubidus males exclusively express the B1 opsin in the R3-R8 photoreceptors, whereas in the female both B1 and LW are expressed. The R1 and R2 cells primarily express UV opsins in this dorsal part of the eye. Therefore, duplication and neofunctionalization of the B1 domain of expression have potentially allowed the evolution of dichromatic and trichromatic color vision in this part of the eye in males and females respectively. The male L. rubidus may use dichromatic color vision in the dorsal part of the eye in behaviors associated with defense of its mating territory. If opsin duplication has facilitated the evolution of sexually dimorphic eyes, we may yet discover these eyes to be more prevalent in the butterflies than previously thought given how frequently such duplications have occurred in this group of insects.

In Pieris rapae subspecies crucivora (Pieridae), sexually dimorphic eyes have also been documented, however in this case mediated by the sexually differentiated expression of filtering pigments in some photoreceptors. The sexual dimorphism occurs in the violet photoreceptors of this species present in some ommatidia. In females, violet photoreceptors match their expected peak spectral sensitivity (lmax ¼ 425 nm). However, in males, the

expression of a violet-absorbing filtering pigment modifies violet photoreceptors into double-peaked blue photoreceptors that have a much narrower spectral sensitivity, with a lmax of 416 nm. This sexual dimorphism may aid males in discriminating the different wing colors of males and females that differ in the short wavelength part of the spectrum, but this hypothesis requires empirical testing through behavioral tests. Not all subspecies of Pieris rapae show sexually dimorphic filtering pigment expression, suggesting that pigment-mediated changes in spectral sensitivities may rapidly facilitate fine-tuning of visual systems according to the ecology of a particular species or subspecies.

Ecological Significance of Butterfly

Visual System Diversity

Why are butterfly eyes so diverse? What are the ecological reasons for such a diversity of color vision systems? To date, however, the ecological and life history factors driving the diversity observed have been far less thoroughly investigated than the molecular basis of this pattern. No clear adaptive matching of the visual pigments to ambient light, as demonstrated in fish, has ever been found in butterflies. For example, the coelacanth lives at a depth of 200 m and only receives light at around 480 nm in wavelength. Its two visual pigment sensitivities peak at around 475 and 485 nm, suggesting adaptive matching to the light environment in this species. In cichlids, visual system spectral sensitivities also seem to match the ambient light environments of these species. In butterflies, the limited evidence available to date suggests that visual pigment diversity may be driven by the social signaling and foraging needs of the animals rather than their particular light environment.

Like many other insects, butterflies utilize vision for a whole range of tasks. They use vision to locate food sources, select suitable oviposition sites, and in intraspecific communication. Papilio butterflies use two of their duplicated LW opsins to see in the green part of the light spectrum when ovipositing and foraging. The lycaenid butterfly Polyommatus icarus uses one of the duplicate B opsins together with the LWopsin-based visual pigment to see green up to 560 nm when foraging. Importantly, opsin gene duplications resulting in the diversification of photoreceptor types have clearly impacted on the foraging behavior of butterflies.

Mate choice in many butterfly species also occurs via color-based signaling. For example, Heliconius butterflies appear to use both colored and polarized light for finding mates. The size of UV-reflecting eye spots on the wings seems to correlate with mating success in Bicyclus anynana. These observations suggest that there might be selective pressure for visual systems to evolve to detect the specific

spectral signals of wings. Correlative evidence suggests that photoreceptor sensitivities have evolved in tandem with wing coloration but empirical evidence is needed.

Conclusions

It is clear that the visual systems enabling butterflies to see in color are functionally very diverse compared to other insects, particularly their moth ancestors. Visual system diversity has been achieved primarily through opsin gene duplication, positive selection at single opsin loci, and heterogeneous expression of filtering pigments in photoreceptors. The evolutionary basis of this pattern of diversification has recently received significant attention, however the exact molecular basis of spectral tuning of butterfly visual pigments remains unknown. Butterfly visual systems offer an excellent opportunity to study the evolutionary and functional genetic links from genotype to phenotype through to behavioral and ecological consequences. To this end, further work is required to explicitly link the pattern of photoreceptor diversification in different butterflies to their ecology, behavior, and life history traits. Only then will we gain a complete understanding of the evolutionary significance of visual system diversification in these insects.

See also: Circadian Rhythms in the Fly’s Visual System; Color Blindness: Acquired; Color Blindness: Inherited; Microvillar and Ciliary Photoreceptors in Molluskan Eyes; The Photoresponse in Squid; Phototransduction: Rhodopsin; Phototransduction: The Visual Cycle.

Further Reading

Arikawa, K., Wakakuwa, M., Qiu, X., Kurasawa, M., and Stavenga, D. G. (2005). Sexual dimorphism of short-wavelength photoreceptors in

the small white butterfly, Pieris rapae crucivora. Journal of Neuroscience 25: 5935–5942.

Bernard, G. D. (1979). Red-absorbing visual pigment of butterflies. Science 203: 1125–1127.

Bernard, G. D. and Remington, C. L. (1991). Color vision in Lycaena butterflies: Spectral tuning of receptor arrays in relation to behavioral ecology. Proceedings of the National Academy of Sciences of the United States of America 88: 2783–2787.

Briscoe, A. D. (2008). Reconstructing the ancestral butterfly eye: Focus on the opsins. Journal of Experimental Biology 211: 1805–1813.

Frentiu, F. D., Bernard, G. D., Sison-Mangus, M. P., Brower, A. V. Z., and Briscoe, A. D. (2007). Gene duplication is an evolutionary mechanism for expanding spectral diversity in the longwavelength photopigments of butterflies. Molecular Biology and Evolution 24: 2016–2028.

Frentiu, F. D., Bernard, G. D., Cuevas, C. I., et al. (2007). Adaptive evolution of color vision as seen through the eyes of butterflies.

Proceedings of the National Academy of Sciences of the United States of America 104: 8634–8640.

Frentiu, F. D. and Briscoe, A. D. (2008). A butterfly eye’s view of birds. BioEssays 30: 1151–1162.

Kelber, A. (1999). Ovipositing butterflies use a red receptor to see green.

Journal of Experimental Biology 202: 2619–2630.

Kelber, A. and Pfaff, M. (1999). True color vision in the orchard butterfly,

Papilio aegeus. Naturwissenschaften 86: 221–224.

Kinoshita, M., Shimada, N., and Arikawa, K. (1998). Colour vision of the foraging swallowtail butterfly Papilio xuthus. Journal of Experimental Biology 202: 95–102.

Sison-Mangus, M. P., Bernard, G. D., Lampel, J., and Briscoe, A. D. (2006). Beauty in the eye of the beholder: The two blue opsins

of lycaenid butterflies and the opsin gene-driven evolution of sexually dimorphic eyes. Journal of Experimental Biology

209: 3079–3090.

Stalleicken, J., Labhart, T., and Mouritsen, H. (2006). Physiological characterization of the compound eye in monarch butterflies with focus on the dorsal rim area. Journal of Comparative Physiology A - Neuroethology Sensory Neural and Behavioral Physiology

192: 321–331.

Stavenga, D. G. and Arikawa, K. (2006). Evolution of color and vision of butterflies. Arthropod Structure and Development

35: 307–318.

Terakita, A. (2005). The opsins. Genome Biology 6: Article No 213. Wahlberg, N., Braby, M. F., Brower, A. V. Z., et al. (2005). Synergistic

effects of combining morphological and molecular data in resolving the phylogeny of butterflies and skippers. Proceedings of the Royal Society B: Biological Sciences 272: 1577–1586.

Wakakuwa, M., Stavenga, D. G., Kurasawa, M., and Arikawa, K. (2004). A unique visual pigment expressed in green, red and deep-red receptors in the eye of the small white butterfly, Pieris rapae crucivora. Journal of Experimental Biology 207: 2803–2810.

Glossary

Cascade – A sequence of biochemical reactions that process a neural signal.

Connexin – A molecule comprising a gap junction that sits in a neuron’s membrane and can join with a connexin from another neuron to create a large nonspecific pore that conducts ions and small molecules.

Ephaptic – Electrical conduction across a synapse between neurons without the mediation of a neurotransmitter.

Henle fiber – The long axon of a foveal cone that extends laterally to terminate outside of the fovea. Invagination – A permanent infolding of a cell’s external membrane, associated in photoreceptors with their synaptic ribbons, and containing fine dendritic processes of bipolar and horizontal cells. Invert – In a postsynaptic cascade, to convert a rising signal into a falling signal.

Low-pass filter – A filter that removes high frequencies and passes low frequencies. Mesopic – The 3-log unit range of background

illuminance in which rod and cone signals temporally sum in cones and the cone bipolar pathway. Microtubule – A small tubule composed of the protein tubulin, often found as arrays in neural axons, involved in transport of cellular components. Pedicle – The cone terminal, with a foot-like flat base, which contains synaptic ribbons.

Ribbon – A presynaptic structure that collects vesicles of neurotransmitter for release. Telodendria – Fine axonal processes extending laterally from the base of the cone terminal, which contact neighboring rod and cone terminals.

Triad – A synapse in the cone terminal that contacts dendritic processes of two horizontal cells and a bipolar cell.

Introduction

The cone photoreceptor is responsible for vision in daylight and provides color vision for most vertebrates. The cone is specialized to compress the large range of environmental illumination into a neural signal that can be

processed by bipolar and ganglion cells in the retina and passed to the brain. The cone soma and synapse support this compression by several mechanisms located in its biophysical properties and at its ribbon synapse. In addition, cones perform spatial filtering because their terminals are electrically coupled to their neighbors and to the surrounding rods through gap junctions.

Structure

Morphology and Topology

The cone is a specialized neuron consisting of an outer segment, inner segment and soma, axon, and axon terminal (Figure 1). The biochemical pathways responsible for transducing light into an electrical signal are contained in the outer segment. The electrical signal passes to the inner segment where it is processed by voltage-gated channels. The inner segment is contiguous with the soma but they lie in distinct retinal layers separated by the external limiting membrane. The cone soma lies in the upper row of the outer nuclear layer. For most mammalian cones, the soma is larger in diameter ( 5 mm) than the outer segment. It contains the nucleus which holds the cell’s DNA, necessary for development and to maintain the cell’s biochemical machinery. Many mammalian species have 20-fold more rods than cones, so the remainder of the outer nuclear layer consists of several layers of rod somas. Cones of most species (and outside the fovea in primate) are spaced semirandomly with a nearestneighbor distance that varies from 5 mm in central retina or the visual streak (e.g., cat, rabbit, and guinea pig) to15 mm in peripheral retina, and are surrounded by rods.

Axon and Terminal

The cone axon carries the electrical signal from the soma to the axon terminal. The axon varies in length depending on the species and eccentricity (distance from central retina) of the photoreceptor. In foveal cones near the center of the primate eye, cone axons extend up to 400–600 mm laterally to allow the outer segments to be packed tightly together in the foveal pit and the axon terminals to be given adequate space outside the fovea to make their synaptic connections with other neurons. These long axons are termed Henle fibers, and the layer containing the horizontally projecting axon fibers is termed Henle’s layer. In other mammals, the cone axon

extends vertically: in cat, the cone axon typically extends50 mm in the outer nuclear layer traversing 6–10 layers of rod somas, and in guinea pig, the axon is shorter, typically 10–25 mm. Typically, cone axons are 1–2 mm in diameter, interspersed between rod somas (35 mm dia.) and rod axons (0.4 mm dia.). The fourfold greater diameter of cone axons suggests a functional interpretation, because space in the outer nuclear layer is limited. One possibility is that the faster cone response might require a larger diameter in a long axon to transmit the cone’s higher frequency response. This was tested with a computational model, and a thin diameter in a long axon was found

sufficient, also consistent with the maintenance of the difference in diameter between cone and rod axons even where they are shorter. Another possibility is related to the axon terminal: the cone terminal has 20-fold more synaptic contacts than the rod, and its cross-sectional area is 16-fold greater. Both rod and cone axons are filled with microtubules, which are essential for transporting synaptic proteins from the nucleus, so the larger diameter of the cone axon may be a consequence of its need to perform more synaptic maintenance. The cone axon terminal is typically 5 mm in diameter, and is flat on its basal surface (toward the ganglion cells); hence, it looks

like a foot and is commonly called a pedicle. The pedicles of adjacent cones align to form a flat plane which lies near the upper edge of the outer plexiform layer.

Synapse

The cone makes chemical synaptic contacts onto 10 types of bipolar cell and two types of horizontal cell (HA and HB in cat and rabbit; H1and H2 in primate) at its axon terminal. The synapse releases glutamate via packets of membrane called synaptic vesicles, which are small ( 30–50 nm) organelles created by endocytosis from the cell’s external membrane. The vesicles diffuse freely around the cytoplasm while they are being filled with glutamate by transporters. The presynaptic machinery in the terminal contains several dense structures known as ribbons because in cross section they are thin and extend vertically away from the cell’s membrane. The active zone, where vesicles are released from the ribbon onto the external membrane, is defined by a trough-shaped band called the arciform density, which contains structural proteins for anchoring the ribbon to the cell membrane and the calcium channels responsible for triggering release. The ribbon allows high continuous release rates, for it collects several rows of vesicles and tethers them for release, to provide a larger readily releasable pool. The mechanism for vesicle release is complex, containing several dozen proteins, and its details are not yet understood, but it is initiated by calcium ions binding to a receptor protein which causes a vesicle to fuse with the external membrane and release its contents into extracellular space. To fuse with the external membrane, the vesicle must be docked near membrane calcium channels at the base of the ribbon. However, there is some evidence for compound fusion, where vesicles fuse with and release into nearby docked vesicles. After release, the vesicle membrane is recycled with a specialized mechanism involving the protein clathrin which coats an invaginating bit of external membrane with a buckyball-type structure and pinches it off as a cytoplasmic vesicle.

Invagination and Triad

The cone’s ribbon synapse is located at the apex of an extension of extracellular space into the bottom surface of the cone terminal, called an invagination, where fine processes of horizontal and bipolar cells extend to form postsynaptic specializations. Each invagination holds two horizontal cell dendritic processes which emanate from different horizontal cells, and one fine dendritic process of an ON bipolar cell. The horizontal cell processes swell up inside the invagination on either side of the bipolar dendrite. Together, the ribbon and postsynaptic processes are called ‘a triad’. The function of the invagination is unknown but it has been suggested to limit diffusion of

the neurotransmitter released by the cone or by horizontal cells for negative feedback. The cone terminal contains 20–50 ribbons, each in a separate invagination, so there are several dozen triads. Off-bipolar cells also contact the cone terminal. Some with alpha-amino-3-hydroxy-5- methyl-4-isoxazolepropionic acid (AMPA) receptors invaginate the cone near the ON bipolar process, and others with kainate receptors contact the base of the pedicle at the so-called flat or basal contacts which lack associated presynaptic vesicles. The flat contacts are located in the space between the invaginations, allowing them to sense the release from several ribbons.

Biophysical Properties

The inner segment and axon terminal of photoreceptors contain several membrane-bound ion channels, including Kv, KCn, BK, and L-type Ca2+. The Kv channels of the inner segment are activated by depolarization, providing an outward current to balance the inward dark current through the light-modulated cyclic guanosine monophosphate (cGMP)-gated channels of the outer segment. These K+ channels provide an adaptational influence, opposing the dark signal, and indirectly opposing the light signal by deactivating with hyperpolarization. The KCn channels, underlying the Ih current, provide a delayed depolarization when activated by hyperpolarization, and calcium-activated potassium channels (BK) help to maintain the resting potential near –40 mV. These are adaptational effects that tend to limit the hyperpolarization from bright light and depolarization in the dark. The cone terminal’s L-type Ca2+ channels (Cav1.4) uniquely include alpha 1D subunits and may have special gating properties. In addition, the terminal contains a calciumactivated chloride current (ICl(Ca)) and calcium-induced calcium release (CICR). Calcium is highly regulated in the terminal, with internal buffering and plasma membrane transporters (protein misfolding cyclic amplifications (PMCAs)), because calcium is the trigger for vesicle release. Calcium is highly compartmentalized at the terminal because calcium in the outer and inner segments and soma performs other cellular functions. In the terminal, ryanodine receptors and IP3 receptors modulate calcium release from internal stores. Voltage-gated sodium channels have been identified in primate cones, possibly to help amplify the cone signal along its axon. The cone terminal contains several transporters with special functions. A proton pump in the vesicles acidifies them so that protons are released along with the glutamate. The protons bind to the local calcium channels to block calcium entry, and thus regulate release. Several transporter proteins carry glutamate, including at least two isoforms that are inserted into the membrane of vesicles to load them with glutamate. A glutamate transporter sitting in the external membrane of the pedicle is important for uptake

of glutamate from the extracellular space. The light response of postsynaptic cells depends on a reduction in glutamate, but when the transporter is blocked pharmacologically the postsynaptic light response is greatly reduced. This implies that the triad synapse is protected to some extent against diffusion by the cone pedicle’s invagination.

Gap Junctions

Cone axon terminals are coupled by gap junctions which carry electrical signals and small molecules directly from each cone to its neighbors. Each gap junction comprises connexin molecules which can form a conductive pore when they are aligned in the membrane of both coupled neurons. The gap junctions are located on telodendria which are basal processes that emanate from the cone terminals to make contact with the neighbors. The cone gap junctions are punctate, that is, they comprise just a few connexin molecules. The connexin has been identified as Cx36. In addition, a cone’s telodendria contact all the surrounding rods with punctate gap junctions. The cone side of the rod-cone gap junction also comprises Cx36, but the identity of the rod-specific connexin molecule is not known. In foveal cones, because their terminals are all located outside the fovea, some of the telodendria extend several hundred micrometers around the perimeter of the fovea to make contact with the neighboring cones, whose axons have extended in the opposite direction from central fovea.

Morphological Implications of Spectral

Sensitivity

The cones of most vertebrates exist in several types that differ in their spectral sensitivity. Until recently, it was difficult to distinguish cone spectral type based on morphology, but antibodies are now available to specifically stain the outer segments by spectral type. Direct imaging is now able to routinely determine the spectral type. The inner segments of birds and some species of reptiles and mammals contain oil droplets which filter out some wavelengths, to narrow the outer segment’s sensitivity and improve the ability for color discrimination. The primate S-cone, sensitive to short wavelengths, is smaller and its terminal makes fewer contacts, so it is distinguishable from middle (M)- and long (L)-wavelength-sensitive cones. In addition, the M- and L-type cones can be distinguished by the number and location of their synaptic connections with bipolar cells. The M- and L-cones are electrically coupled by gap junctions, which blur their spectral sensitivity. The coupling is a reasonable compromise because the coupling increases their contrast sensitivity and the spectral blurring effect is moderate due to the similarity of the M- and L-type spectral sensitivities.

The M- and L-type cones are located randomly in bunches of similar type, which along with the indiscriminate coupling may improve contrast sensitivity without causing much blurring of color. The S-type cone is not coupled strongly to its M- and L-type neighbors, apparently because its spectral sensitivity is farther from the M- and L-type curves.

Function

Adaptation and Predictive Coding

The role of the cone photoreceptor in daylight is crucial because it is the first step in the visual pathway. The cone’s task is to transduce and relay information in the visual environment relevant to an organism’s survival. A major problem faced by the cone is that daylight varies over 5 log units, but a synaptic signal is only able to vary only over 1–2 log units. The problem is one of dynamic range and also of noise because dynamic range limits the maximum signal, and noise limits the minimum discriminable signal. The amount of information available is related to the maximum signal divided by the minimum, that is, the number of distinguishable levels. Since cone transduction is noisy and the ribbon synapse is also noisy, to maximize the amount of information the cone must remove all irrelevant information and selectively transmit the most important signal components with high sensitivity. One way the cone deals with this problem is by removing information about the background level by adaptation. This is accomplished first in the outer segment. In addition, the cone has several more adaptational mechanisms: modulation of the light-evoked signal by voltage-gated channels in the inner segment and terminal, gain reduction at the ribbon synapse, and regulation of its neurotransmitter release by negative feedback. To preserve information about fine temporal detail, the adaptational mechanisms must be slow, and to preserve spatial detail, the adaptational mechanisms must be wide. The theory for such adaptational mechanisms is called predictive coding, in which a temporally slow or spatially wide average signal is subtracted from a photoreceptor. Thus, the cone ribbon synapse carries a signal that is optimally adapted in time and also in space.

Synaptic Transfer Function

Upon depolarization, the ribbon synapse releases glutamate with a nonlinear transfer function, which is controlled by the voltage sensitivity of the L-type calcium channel. The mechanism for release is linearly modulated by calcium because glutamate release is proportional to the terminal’s local calcium concentration. Release is maximal in the dark when the cone is depolarized, and minimal in bright light when the cone is hyperpolarized

(Figure 2). Due to the nonlinear release function, the synaptic gain for small light increments is also maximal in the dark, and minimal in the light. Thus, the synaptic gain is adaptive, that is, it tends to oppose the light-evoked signal and reduce contrast gain in bright light. Negative feedback tends to oppose this effect, that is, it reduces the change in synaptic gain due to a change in background level. Glutamate released by the cone binds to the postsynaptic metabotropic glutamate receptor 6 (mGluR6) receptor to activate a second-messenger cascade which inverts and low-pass filters the signal in the bipolar cell. Glutamate also diffuses out of the invagination to bind to kainate receptors at the tips of off-bipolar cells.

Rate of Vesicle Release

A major source of noise for the cone signal is the random release of vesicles by the ribbon. Although the details of the release mechanism are not known, it is thought to be stochastic (noisy), similar to a modulated Poisson distribution, for which the standard deviation is equal to the square root of the mean. There is also some evidence for more regular release. Protons released along with the vesicle’s packet of glutamate bind to the local calcium channels which may generate a short refractory period, allowing release to be more regular. The cone synapse is thought to release vesicles at 100–200 s 1 per active zone in the dark, but in bright sunlight the rate is reduced to <10 s 1 per active zone. The ability of a ribbon synapse to transmit information about contrast, even with a

Postsynaptic

high release rate, is extremely limited: at a Poisson rate of 200 ves s 1, contrast threshold for a 100 ms response would be 0.2. Therefore to increase sensitivity, several ribbons transmit the same cone signal in parallel. With 20 ribbons, the total release rate is 2000 s 1, for which the contrast threshold would be 0.07. Each bipolar cell contacts several (4–8) ribbons per cone, and bipolar cells (except foveal midget bipolars) normally contact several (4–6) cones, so a bipolar cell integrates signals from one to several dozen cone ribbons.

Electrical Coupling

The gap junctions between cone terminals function as a spatial filter, averaging cone signals to remove noise, and removing high spatial frequencies. The effect is to enlarge the cone receptive field beyond the effect of optical blur. Adjacent L- and M-cones in primate are electrically coupled by gap junctions, which blur to some extent their ability to support color discrimination, implying that their action to reduce noise provides a benefit to luminance contrast discrimination. S-cones are uncoupled to the other types, probably because their spectral sensitivity is so different that it would suffer out of proportion to the potential increase in contrast sensitivity. Cone coupling is paradoxical to some extent because in a linear system which collects cone signals, for example, convergence to a large ganglion cell’s dendritic tree, gap junctions would provide no benefit. However, the cones apparently benefit from the reduction of noise before their signal passes

Presynaptic

200

ON bipolar

OFF bipolar

−70 mV Postsynaptic voltage

through the ribbon synapse which is to some extent nonlinear. The gap junctions coupling a cone to its neighboring rods allow rod signals at mesopic backgrounds, where a rod temporally sums several photon signals, to pass to cones to be transmitted through the cone pathway to ganglion cells. Since cones are 30–100-fold less sensitive than rods, this rod–cone coupling represents a form of adaptation for the cone pathway.

Negative Feedback

Adaptation from spatially pooled horizontal cell feedback at the cone triad is important for retinal signal processing. Glutamate released by the cone ribbon synapse binds to AMPA receptors on the horizontal cell dendrite to depolarize it, and in turn the horizontal cell provides negative feedback to control the release of glutamate by the cone. The negative feedback contributes to an antagonistic surround in bipolar cells and in all other postsynaptic neurons. In some species (e.g., turtles, fish, and putatively in some mammals), horizontal cells receive spectrally specific signals from cones and therefore their negative feedback contributes to cone color opponency. The mechanism for the negative feedback from horizontal cells is not known but several candidates are currently being studied. Horizontal cells synthesize g-aminobutyric acid (GABA) and release it through a transporter working in reverse, and in some species (e.g., turtle, fish, salamander, and frog), GABAergic feedback has been found in the cone terminal. Horizontal cells also generate a feed-forward surround signal in both onand off-bipolar cells, which have GABAA receptors in their dendrites. The effect is to generate a stronger surround signal in the bipolar cell, allowing it to more tightly control its release of neurotransmitter. However, standard GABA-receptor blockers do not affect the surround in most species. Some evidence suggests that feedback may be provided through release of protons in the invagination, because when pH buffers, such as 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) are applied via the perfusion bath in vitro, they block horizontal cell feedback proportionate to their buffering capacity. A third possibility is that the feedback is ephaptic (electrical). The pH and ephaptic theories describe a feedback signal that directly modulates the cone terminal’s calcium current. One benefit of all these putative feedback mechanisms is their freedom from noise that would occur with vesicular release. Feedback is necessary to control neurotransmitter release, but it has one problem, that if too strong it can cause instability from delays in the feedback loop. Oscillations can be evoked by flashed full-field stimuli with a dark mask over the horizontal cell recording site. The dark depolarizes the cone, increasing gain in the feedback loop, which tends to generate oscillations.

Other adaptational mechanisms such as K+ channels can generate similar resonances related to the delay in their response.

Theory of Ephaptic Feedback

The proposed ephaptic feedback modulates the cone pedicle’s calcium channels by controlling the polarization of the external surface of the membrane inside the invagination. When a horizontal cell is hyperpolarized by a remote light (i.e., without modulating local glutamate release), the current that flows from extracellular space into its dendritic tips is increased. The increase in current causes a voltage drop inside the invagination, which shifts the voltage at the external membrane surface more negative. As both surfaces of the membrane have been shifted negative, the calcium channel senses a depolarization (lack of polarization) and thus opens further, which opposes the original light signal. The opposite effect occurs when a cone terminal is hyperpolarized by light but the horizontal cell is not. Light reduces glutamate release, which closes the horizontal cell’s glutamategated (AMPA) channels, decreasing the current flow into the dendritic tip, which shifts the external cone membrane more positive, causing the calcium channel to sense a greater hyperpolarization, which comprises net positive feedback. However, the horizontal cell dendritic tips contain hemichannel gap junctions (connexins) which are not gated by glutamate but shunt the glutamate-gated channels. Their effect is to reduce positive feedback and to enhance negative feedback. One potential problem for the theory is that the hemichannels reduce the light response, and because their conductance is less than the AMPA receptor channels one imagines they should not cause a robust effect. However, the theory is supported by strong evidence, for example, the hemichannels can be blocked by carbenoxolone which reduces the negative feedback, and the same result occurs in knockout animals in which the hemichannels are removed. The relative magnitude of the positive and negative ephaptic feedback effects depend on several factors, including the conductances and the location of the underlying channels. The amount of voltage drop at the external membrane surface depends on the geometry of the current path through extracellular space in the invagination. A robust feedback signal, for example, the typical 5–10 mV shift seen in the calcium current in the cone pedicle, would require a relatively high resistance, and therefore the mechanism is currently controversial. Although the ephaptic theory has not generally been accepted by the field, it can explain some of the phenomena given as evidence for pH feedback. HEPES buffer is known to acidify neurons, and hemichannels are known to be modulated by internal pH, which is consistent with the well-known effect of

HEPES to block the cone surround. An advantage of ephaptic feedback is its short delay, reducing the tendency for instability. A further merit to the ephaptic theory is that it provides a functional role for the invagination, to generate the extracellular voltage shift. Putatively, then, the invagination creates a specialized local environment for the triad synapse, to limit chemical and/or electrical diffusion.

Bandpass Adaptation Filter

The cone terminal comprises a spatiotemporal filter consisting of a low-pass filter from electrical coupling between cones, and a high-pass filter from horizontal cell negative feedback. The signal in horizontal cells is low pass in space because they are large and well coupled: their receptive fields can be 10-fold larger than their dendritic field. The feedback signal in some species has two components, the faster (pH or ephaptic) modulation of the calcium current, and a slower GABAergic component. Therefore, the horizontal cell signal, when it modulates the cone synaptic release, can be characterized as a subtraction of a low-pass filter, effectively making the cone synapse a high-pass filter. The overall effect of electrical coupling and negative feedback is to generate a band-pass filter at the cone terminal. The filter removes spatial and temporal frequencies that transmit less information, improving the performance of the visual pathway which utilizes noisy synapses for transmission.

Conclusion

The cone photoreceptor plays a crucial role in vertebrate vision because it is responsible for transducing fast changes in contrast while ignoring the background light level. To improve its sensitivity over 5 log units of background illumination, the cone contains several mechanisms for adaptation, originating in the transduction cascade, in its biophysical properties, and in the ribbon synapse. The ribbon is part of a complex local circuit called the triad that combines adaptation with spatial filtering. The triad maximizes the amount of information that can be transmitted through noisy synapses over a wide range of environmental light levels.

Acknowledgment

This work was supported by NEI grant EY016607.

See also: Phototransduction: Adaptation in Cones.

Further Reading

Copenhagen, D. R. (2004). Excitation in retina: The flow, filtering, and molecules of visual signaling in the glutamatergic pathways from photoreceptors to ganglion cells. In: Chalupa, L. M. and Werner, J. S. (eds.) The Visual Neurosciences, 2 vols, pp. 234–259. Cambridge, MA: MIT Press.

DeVries, S. H., Li, W., and Saszik, S. (2006). Parallel processing in two transmitter microenvironments at the cone photoreceptor synapse. Neuron 50: 735–748.

Gaal, L., Roska, B., Picaud, S. A., et al. (1998). Postsynaptic response kinetics are controlled by a glutamate transporter at cone photoreceptors. Journal of Neurophysiology 79: 190–196.

Heidelberger, R., Thoreson, W. B., and Witkovsky, P. (2005). Synaptic transmission at retinal ribbon synapses. Progress in Retinal and Eye Research 24: 682–720.

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Glossary

Channels – The groups of visual sensors that are selective for a narrow range of image spatial or temporal structure.

Contrast constancy – At high contrasts, apparent contrast is relatively independent of the parameters that strongly influence contrast-detection threshold.

Contrast-detection threshold – The statistical contrast boundary below which contrast is too low for an image to be detected reliably and above which contrast is high enough for frequent image detection. Often defined as the contrast that produces 75% correct target identifications in forced-choice paradigms.

Contrast sensitivity – The reciprocal of contrastdetection threshold that also represents the transition between visible and invisible images.

Critical flicker frequency – The highest flicker rate of a full contrast image that can be detected reliably.

Forced-choice paradigms – Robust behavioral method used to measure detection or discrimination thresholds. Observers are forced to select between two or more intervals, of which only one contains

a target.

Fourier analysis – Analytical method that calculates the simple sine-wave components whose linear sum forms a given complex image.

Resolution limit – The highest spatial frequency of a full contrast image that can be detected reliably. Spatial frequency – The number of image cycles that fall within a given spatial distance, typically 1 of visual angle.

Temporal frequency – The number of image cycles that fall within 1 s.

Wavelets/gabors – A local filter that is the point-wise product of a two-dimensional (2D) spatial sine wave and a 2D Gaussian envelope.

Most people are familiar with image brightness and contrast from their controls on computer and television displays. The brightness control adjusts the mean luminance of the display uniformly, in order that the intensity of every point in the image increases when brightness is increased or decreases when brightness is reduced. The contrast control adjusts the difference between the lightest and darkest areas of the image. Increasing contrast

makes areas that are below mean luminance darker and areas that are above mean luminance lighter, without changing the mean value. Decreasing contrast draws all values toward the mean, thus making the whole image fainter, similar to viewing the image through fog.

Figure 1 illustrates the effect of changing the contrast of a sine-wave striped pattern (the reasons for using a sine-wave pattern are described below). The top panel shows images of gratings whose contrast increases from 12.5% on the left to 100% on the right. The mean luminance of each image is the same. The traces in the bottom row plot luminance versus position for a horizontal slice through each image.

Contrast-Detection Threshold

A powerful measure of visual sensitivity can be obtained by finding the minimum contrast that is necessary for an image to be detected. This minimum contrast is referred to as contrast-detection threshold (Cthresh) and it is important because it defines the transition at which an image moves from invisible to visible. One method to estimate Cthresh might be to allow a subject to adjust the contrast until an image is just visible. However, this method is highly subjective and large differences in individual criteria for just visible make this measure unreliable.

Psychophysical Assessment of Vision

To overcome these problems, most researchers employ forced-choice procedures that require an observer to identify which of two or more intervals (the more the better) contain the target. An example of a four-alternative forcedchoice (4AFC) detection task is shown in Figure 2(a). In this case, a computer presents a target in one of four positions at random around a central fixation point. The observer’s task is to fixate the central dot and to indicate the location of the target, usually by pressing a computer button. Targets that are below Cthresh (sub-threshold) are rarely detected, whereas targets that are above Cthresh (supra-threshold) are usually detected. Contrast-detection thresholds are therefore probabilistic and are defined as the contrast at which they are correctly detected midway between chance and perfect performance.

It is difficult to cheat on forced-choice methods or to change criteria – the target is either seen, in which case its position is correctly identified, or it is not seen, in which