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

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Glossary

Circadian clock – A type of self-sustained molecular oscillator with a period of approximately 24 h. Dopamine – The main retinal catecholamine produced by a type of amacrine or interplexiform cell. Dopamine activates D1 and D2 receptors and plays key roles in light-adaptive processes and in the effects of the retinal clock. Most of the effects of retinal dopamine are through volume transmission; thus, dopamine acts as a neurohormone in the retina.

Electrical conductance – A measure of how easily electrical current flows along a certain path that has a difference in voltage or potential.

Entrainment of a circadian clock – Altering the phase of a circadian oscillator due to the presence of environmental input (e.g., light/dark, temperature) generally in the early or late night.

Ganglion cell – The output neuron of the retina that sends its axon to other parts of the brain.

Gap junction – A type of electrical synapse that is comprised of intercellular channels that directly connect the cytoplasm of two cells and facilitate the cell-to-cell passage of ions and small molecules. Horizontal cell – The second-order interneuron in the outer retina that regulates photoreceptor–bipolar cell synaptic activity.

Melatonin – The neurohormone produced by the retina in the photoreceptor cells and in the pineal gland and whose production is increased at night under the control of circadian clocks.

Mesopic – An adjective that describes the dim ambient light levels between the scotopic and photopic ranges, such as those observed at dawn and dusk, to which both rods and cones can respond. Photopic – An adjective that describes the bright ambient light levels, such as those observed during a sunny day, to which cones, but not rods, can respond.

Rod (cone) pathway – The ensemble of retinal cells (circuit) that sequentially relay the electrical signals through the retina from the rods (cones) to the ganglion cells.

Scotopic – An adjective that describes the very dim ambient light levels, such as those observed during a moonless night, to which rods, but not cones, which have been separated from the retina, can respond.

Synapse – A zone of contact between two cells that allows the transmission of an electrical signal from one cell to the other. Although a synapse may be chemical or electrical (gap junction), when used alone, the term usually refers to a chemical synapse.

Introduction

Most vertebrate retinas contain both rods and cones, photoreceptor cells that detect and transduce visual images into neural signals. These neural signals are then transmitted to bipolar cells, second-order neurons that then signal ganglion cells, the output neurons of the retina (Figure 1). As we live on a planet that rotates, the ambient or background illumination during a bright sunlit day, compared to a moonless night, changes by approximately 20-billion-fold on a daily basis. The retina must be able to function effectively during this dramatic daily environmental change. At least three adaptive mechanisms are thought to have evolved in the vertebrate retina to facilitate its day/night operation. First, because isolated rods and cones, which have been dissociated from the retina, display high and low light sensitivity, respectively, it has been accepted that rods and cones function under different illumination conditions, that is, that rods mediate dim light (scotopic) vision at night and cones mediate bright light (photopic) vision during the day. Second, external environmental factors, such as the level of ambient illumination itself, can modulate the light responses of retinal neurons and alter the operating characteristics of neural networks within the retina. For example, although ganglion cells exhibit a center-surround receptive field under light-adapted conditions, such as occurs in a bright sunlit day, their receptive fields exhibit only center responses if the retina is subsequently maintained in the dark for 30–40 min, such as occurs if an animal moves from a brightly illuminated area to the shade for a prolonged period. Third, intrinsic retinal processes, such as the circadian (24-h) clock or oscillator in the retina, can also modulate the light responses of retinal neurons and alter the operating characteristics of neural networks within the retina. Although it was originally thought that the transition between day vision and night vision is a passive process driven by the intensity of ambient illumination, it is

now known that the retinal clock plays a key role in this mechanism.

A circadian clock is a biological oscillator that has persistent rhythmicity (i.e., a molecular process that rewinds itself) with a period of approximately 24 h under constant environmental conditions (e.g., constant darkness and temperature). Thus, day/night differences in biochemical, morphological, and physiological processes, which are observed under constant environmental conditions, can be attributed to the action of a circadian clock. In the vertebrate retina, many such day/night differences have been observed, including differences in neuronal light responses, dopamine and melatonin content and release, visual sensitivity, retinomotor movements, extracellular pH, photoreceptor disk shedding, and gene expression. A number of these day/night differences have been demonstrated to be under the control of the circadian clock in the retina, and not the circadian clocks located elsewhere in the brain (i.e., suprachiasmatic nucleus of the hypothalamus, pineal gland) or in peripheral organs (i.e., liver).

This article focuses on how the circadian clock in the retina regulates rod and cone pathways, especially concerning circadian regulation of rod–cone coupling.

Our review focuses primarily on findings that have been obtained from the fish retina, because this aspect of circadian function has been most thoroughly studied in the fish. However, the available evidence suggests that the circadian mechanisms, discussed in this article, occur in the vast majority of vertebrate species, including both mammals and nonmammals. These similarities will be noted when evidence from mammalian and other nonfish species is available. Information on how the retinal clock regulates other retinal processes is dealt with elsewhere in this encyclopedia and in other recent reviews and papers, which are listed under the section, titled ‘Further reading’.

Rod and Cone Pathways in the Fish Retina

As shown in Figure 1, rod signals can reach ganglion cells through at least two separate pathways in all vertebrate species that have both rods and cones. First, rod input can reach ganglion cells through cones. In both mammalian and nonmammalian retinas, rods and cones are anatomically connected or coupled by gap junctions, a type of electrical synapse at which rod input can enter the cone

circuit and thereby reach ganglion cells. Although it was thought that rod–cone electrical coupling is relatively weak, recent evidence has demonstrated that rod–cone coupling in both fish and mice is strong at night, but weak during the day due to the action of the retinal clock.

Second, rod input can reach ganglion cells through bipolar cell pathways that do not involve cones. Rods signal bipolar cells at chemical synapses in all vertebrates. In fish, these bipolar cells also receive synaptic contact from cones and, thus, are called mixed rod–cone bipolar cells (Figure 1). In contrast, in mammals, bipolar cells that receive rod input do not receive cone input. Individual ganglion cells in both fish and mammals can be driven by signals from both the rod and cone systems. In fish, mixed rod–cone bipolar cells synapse directly onto ganglion cells, whereas in mammals, rod bipolar cells do not directly synapse onto ganglion cells but instead provide indirect rod input to ganglion cells through AII amacrine cells, which then signal cone bipolar cells.

Day/Night Differences in the Light

Responses of Neurons in the Fish Outer

Retina

The first evidence that supported the idea that the retinal clock regulates rod and cone pathways by modulating the electrical synapses between rods and cones was obtained from electrical recordings of goldfish cone horizontal

cells, second-order cells that receive synaptic contact from cones, but not from rods (Figure 1). Under darkadapted conditions during the day, the hyperpolarizing light responses of these cells are cone driven (Figure 2). However, under dark-adapted conditions at night, the light responses of the cells are dominated by rod input. Specifically, the cells respond to light that is 100 dimmer at night, than in the day. In other words, the threshold stimulus that evokes a response is in the low scotopic range (i.e., intensities to which isolated rods, but not isolated cones, respond) at night, but in the low mesopic range (i.e., the threshold intensity of isolated cones) in the day. In addition, the light responses of dark-adapted cone horizontal cells at night, but not in the day, have a time course that is similar to that of rod horizontal cells; they are slower and of longer duration, especially to brighter light stimuli. Additional support that the cells receive rod input at night, but not in the day, was the demonstration that the L-type (or H1) cone horizontal cells, which make synaptic contact with red (long-wavelength)-sensitive cones, are most sensitive to long-wavelength stimuli during the day, but are most sensitive to middle-wavelength stimuli at night, which is typical of rods. Interestingly, the light responses of rod horizontal cells, which make synaptic contact with rods, but not with cones (Figure 1), are similar in the day and night under dark-adapted conditions (Figure 2).

The day/night differences in the light responses of cone horizontal cells described above are due to the action

of a circadian clock, and not the result of acute darkadaptive or other environmental effects, because similar day/night differences are observed when the fish and/or their in vitro retinas are maintained in constant darkness and temperature for 24–72 h. In addition, prior reversal of the 12-h light/12-h dark cycle in which the fish are maintained, reverses the day/night differences in the light responses of the cone horizontal cells. As circadian clocks can be entrained or phase-shifted by the light/dark cycle, this finding demonstrates that the observed day/ night differences in the light responses of L-type cone horizontal cells are circadian in nature, and not due to day/night differences in environmental factors.

More recently, electrical recordings of the light responses of goldfish cones in the day and night, following 24–72 h of constant darkness, revealed that they are also under circadian control. Specifically, the cone light response threshold is 100 lower at night (i.e., low scotopic) than in the day (Figure 2). In addition, the light responses of dark-adapted cones at night, but not in the day, have a time course that is similar to that of rod horizontal cells; they are slower and of longer duration, especially to brighter light stimuli. Moreover, although recorded cones can be distinguished as either blue (short-wavelength)-sensitive, green (middle-wavelength)-sensitive, or red (long-wavelength)- sensitive during the day, all recorded cones at night are most sensitive to middle-wavelength light, and their sensitivity closely matches that of rods.

The circadian clock regulates the light responses of cones and cone horizontal cells in part through activation of the D2 family of dopamine receptors. Dopamine receptors have been divided into the D1 and D2 receptor families based on their opposite effects on the level of intracellular cyclic adenosine monophosphate (cAMP) and the difference in their affinity for dopamine. D1 receptors are approximately 500 less sensitive to dopamine than D2 receptors, and D1 receptor activation increases cAMP, whereas D2 receptor activation decreases it. Substantial evidence indicates that both the D1 and D2 families of receptors are found in the retina and that rods and cones express D2 receptors, whereas rod and cone horizontal cells express D1 receptors. Moreover, the retinal clock produces a circadian rhythm in dopamine release through melatonin. Specifically, the clock increases melatonin synthesis and release at night, compared to the day. As melatonin inhibits dopamine release, a circadian rhythm in dopamine release is generated, but one in which extracellular dopamine levels are higher in the day, than at night (i.e., the dopamine and melatonin rhythms are in antiphase).

The retinal clock produces a variety of circadian rhythms in the retina, including daily rhythms in retinomotor movements and cyclic guanosine monophosphate (cGMP)-gated cationic channels in cones, by increasing dopamine levels in the day so that D2 receptors are

activated. The clock does not appear to utilize D1 receptors, apparently because it does not increase dopamine levels sufficiently to activate the low-affinity D1 receptors in the retina. Similarly, the retinal clock modulates the strength of rod input to cones and cone horizontal cells by increasing the activation of the D2 receptors on rods and cones during the day. When D2 receptor activation is minimal at night, as evidenced by the lack of effect of D2 receptor antagonists at night, rod input dominates the light responses of cones and cone horizontal cells. In contrast, when the retinal clock activates D2 receptors in the day, rod signals do not reach cones and cone horizontal cells, as evidenced by the finding that application of D2 receptor antagonists during the day increases rod input to cones and cone horizontal cells. Indirect evidence based on data obtained from cone horizontal cells further suggests that the decrease in intracellular cAMP in rods and cones that is evoked by D2 receptor activation in the day eliminates rod input to cones and cone horizontal cells. Although this idea requires further direct testing on cones, a circadian rhythm of cAMP content in retinal photoreceptors, with high levels at night, has been described in several vertebrate species.

The Circadian Clock in the Retina, and Not the Retinal Response to the Ambient Illumination, Controls Rod–Cone Coupling

The day/night differences in the strength of the rod input to cones and cone horizontal cells originate in part from changes in the conductance of rod–cone gap junctions. Following the injection of a membrane-impermeant, gap- junction-permeant, biotinylated tracer (biocytin) during the day, the tracer accumulates in the injected goldfish cone. However, when the experiment is conducted at night, the tracer diffuses to many cones and rods in the vicinity of the injected cell, consistent with an increase in the conductance of the rod–cone gap junctions. The tracer experiments demonstrate that the modulation of the rod–cone gap-junctional conductance is the primary means through which the retinal clock controls the strength of the rod signal that flows into cones and cone horizontal cells (Figure 3).

Consistent with the effects of the clock on the light responses of cones and cone horizontal cells described above, the clock uses dopamine to control the extent of rod–cone tracer coupling. Specifically, the clock-controlled daytime increase in dopamine decreases rod–cone tracer coupling, whereas the nighttime drop in dopamine levels is required to increase the coupling. The day/night difference in rod–cone coupling can be pharmacologically manipulated and the results are consistent with the involvement of D2, and not D1, dopamine receptors. Thus, by increasing dopamine release and D2 receptor activation during the day, the retinal clock decreases rod–cone coupling and

thereby decreases rod input to cones and cone horizontal cells. Conversely, the nighttime decrease in dopamine release enhances rod–cone coupling and increases rod input to cones and cone horizontal cells.

Interestingly, the effects of the clock on rod–cone tracer coupling and on cone light responses are not altered when dim background lights are present. More specifically, similar results are obtained in the day and night, with and without dopamine D2 receptors blocked, when the intensity of the ambient illumination is very dim (i.e., low scotopic), as occurs on a moonless night, or is brighter, but still dim (i.e., mesopic), as occurs on a moonlit night. Most significantly, at night, the extent of rod–cone tracer coupling and the strength of rod input to cones and cone horizontal cells are not decreased even when the intensity of the ambient illumination is in the mesopic range. As the background light level at night normally varies between very dim starlight and dim moonlight conditions, these results indicate that the retinal clock, and not the retinal response to the normal visual environment at night, regulates rod–cone coupling. The clock decreases rod–cone coupling at dawn by increasing D2 receptor activation and increases rod–cone coupling at dusk by reducing D2 receptor activation.

The Circadian Clock in the Mammalian

Retina Controls Rod–Cone Coupling

Although it may be thought that circadian regulation of retinal function occurs in nonmammals, and not in mammals, substantial evidence indicates that the circadian clock in the mammalian retina regulates a variety of cellular phenomena, including melatonin and dopamine production and release, neuronal activity, rod disk shedding, extracellular pH, and rod–cone coupling. Recent findings, using bath application of the gap-junction-permeant, tracer molecule neurobiotin onto mouse retinas that had been cut in

pieces with a razor, indicated that neurobiotin diffused extensively through photoreceptor cell gap junctions under dark-adapted conditions at night and in the day when D2 receptors were blocked, but diffused significantly less under dark-adapted conditions in the day. Thus, it seems likely that the retinal clock controls the strength of rod–cone coupling in most, if not all, mammalian and nonmammalian retinas that have both rods and cones. Moreover, in most vertebrate species, including mammals, the fact that (1) rods and cones are connected by gap junctions, (2) D2 receptors are expressed by rods and cones, but not by horizontal cells, and (3) the retina contains a circadian clock supports this view. Although the clock increases the conductance of rod–cone gap junctions at night, evidence to date does not address whether at night the clock also increases the conductance of cone–cone and/or rod–rod gap junctions, which are also found in vertebrates.

A Circadian Clock Pathway in the Retina

The circadian clock in the fish retina regulates the light responses of cones and cone horizontal cells in part through a melatonin/dopamine pathway that controls rod–cone coupling (Figure 4). Specifically, the clock regulates melatonin synthesis, so that melatonin synthesis and release are kept low during the day and dramatically increased at night. Melatonin inhibits dopamine release from dopaminergic interplexiform cells, and consequently, the extracellular levels of dopamine are the lowest at night. The relief in the inhibition of dopamine release during the day generates an increase in the extracellular levels of dopamine in the day, so that the D2 receptors on photoreceptor cells are activated, which then lowers intracellular cAMP and protein kinase A (PKA) levels in the photoreceptors, decreasing the conductance of rod–cone gap junctions. As a consequence, rod input to cones and cone horizontal cells is decreased. At night, because the clock decreases

dopamine levels below the threshold of D2 receptor activation, the intracellular cAMP level in photoreceptor cells increases, raising the conductance of rod–cone gap junctions and increasing rod input to cones. As a result, rod signals are transmitted to cone horizontal cells, even though these cells do not make direct synaptic contact with rods. Although the circadian pathway illustrated here has been established from data collected in goldfish, accumulative evidence indicates that this pathway is likely conserved among vertebrates, including mammals. However, it is still unresolved as to whether the clock in the mammalian retina increases dopamine release in the day by generating a melatonin rhythm, as occurs in nonmammals, or by direct control of dopamine metabolism in dopaminergic cells.

It is interesting to note that circadian clock pathways in the retina may be different from light-responsive and light/ dark-adaptive pathways. As described above, the retinal clock, and not the retinal response to the level of ambient illumination, uses specific neurotransmitters (i.e., melatonin), neurotransmitter receptors (i.e., melatonin receptors, dopamine D2 receptors), and synaptic processes (i.e., rod–cone gap junctions) to control the strength of rod–cone electrical coupling, These physiological processes, which participate primarily in circadian clock pathways within the retina, may be distinct from other processes that primarily respond to light stimuli, such as those that mediate bright light adaptation (i.e., D1 receptors). In the case of dopamine

receptors, this segregation of pathways may be explained by the difference in the affinity of D1 and D2 receptors for endogenous dopamine. Although the retinal clock increases extracellular dopamine levels sufficiently to activate the high-affinity D2 receptors on rods and cones, the low-affin- ity D1 receptors on horizontal cells are not activated, as evidenced by the absence of a day/night difference in horizontal-cell gap-junctional coupling under dark-adapted conditions. Instead, horizontal cell coupling, which is modulated by D1 receptor activation, is decreased by bright light stimulation during the day, which increases extracellular dopamine levels sufficiently to activate the D1 receptors on horizontal cells. Viewed from this perspective, the D1 and D2 receptor systems in the retina function in a complementary manner; the retinal clock activates D2 receptors at dawn and reduces their activation at dusk, whereas bright lights activate D1 receptors during the day. Retinal function (neurotransmitters, synapses, pathways, adaptation, etc.) may therefore arise from the interplay of both lightresponsive and circadian clock pathways.

Functional Implications of Circadian

Clock Control of Rod–Cone Coupling

As the retinal clock increases the strength of the electrical synapses between rods and cones at night, very dim light

signals from rods can reach cones and then cone horizontal cells at night. Thus, although individual cones that have been separated from the intact retina cannot respond to very dim light (i.e., low scotopic) stimuli, dark-adapted cones in the intact retina at night can do so because the clock opens the electrical synapse between rods and cones at night. Circadian control of rod–cone electrical coupling therefore serves as a synaptic switch for the direct introduction of rod signals to cones and cone pathways at night, but not in the day.

In addition to enabling rod signals to reach cones at night, the circadian increase in rod–cone electrical coupling may also enhance the detection of very dim, large light stimuli by the rod to bipolar cell to ganglion cell circuit. Many rods converge onto each bipolar cell, summing visual signals over a large spatial area at synapses that are highly nonlinear. Although noise in one photoreceptor cell is independent of the noise in nearby photoreceptor cells, dim, large visual objects will produce similar or correlated responses from nearby photoreceptor cells. As a result, an increase in electrical coupling between nearby photoreceptor cells will decrease photoreceptor noise more than it reduces their light responses to dim, large objects. Increased photoreceptor coupling at night will therefore augment the signal-to-noise ratio and the reliability of rod responses to dim, large stimuli before the nonlinear rod to bipolar cell synapse distorts the signal and the noise. Circadian control of rod–cone electrical coupling thus enhances the detection of very dim, large objects at night, and by decreasing rod–cone coupling at dawn, improves the detection of small objects in the day. The absence of rod signals in the cone pathways during the day facilitates the processing of high acuity and color information by the cone pathways during the day. In contrast, the increase in rod–cone coupling at night may maximize nighttime vision and tune the retina to detect large, dim objects. Moreover, circadian control of rod–cone coupling may also mediate in part the circadian rhythm in visual sensitivity that occurs in many vertebrates, including fish and human.

Finally, the nighttime increase in rod–cone coupling may influence photoreceptor survival. Specifically, the metabolic exchange of small signaling molecules and nutrients will likely occur between rods and cones each night because open gap-junctional channels are large enough to allow the diffusion of such small molecules between coupled cells. Healthy rods might improve cone survival by providing coupled cones with nutrients and protective factors at night and/or dying rods might facilitate the death of coupled cones through the diffusion of pro-apoptotic factors each night.

See also: Anatomically Separate Rod and Cone Signaling Pathways; Circadian Metabolism in the Chick Retina; Circadian Regulation of Ion Channels in Photoreceptors; Fish Retinomotor Movements; Limulus Eyes and Their Circadian Regulation; Morphology of Interneurons: Amacrine Cells; Morphology of Interneurons: Bipolar Cells; Morphology of Interneurons: Horizontal Cells; Morphology of Interneurons: Interplexiform Cells; Neurotransmitters and Receptors: Dopamine Receptors; Neurotransmitters and Receptors: Melatonin Receptors; The Physiology of Photoreceptor Synapses and Other Ribbon Synapses.

Further Reading

Barlow, R. B. (2001). Circadian and efferent modulation of visual sensitivity. Progress in Brain Research 131: 487–503.

Bloomfield, S. A. and Dacheux, R. F. (2001). Rod vision: Pathways and processing in the mammalian retina. Progress in Retinal and Eye Research 20: 351–384.

Copenhagen, D. R. (2004). Excitation in the 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, pp. 320–333. Cambridge, MA: MIT Press.

Dowling, J. E. (1987). The Retina, an Approachable Part of the Brain. Cambridge, MA: Harvard University Press.

Green, C. B. and Besharse, J. C. (2004). Retinal circadian clocks and control of retinal physiology. Journal of Biological Rhythms 19: 91–102.

Iuvone, P. M., Tosini, G., Pozdeyev, N., et al. (2005). Circadian clocks, clock networks, arylalkylamine N-acetyltransferase, and melatonin in the retina. Progress in Retinal and Eye Research

24: 433–456.

Raviola, E. and Gilula, N. B. (1973). Gap junctions between photoreceptor cells in the vertebrate retina. Proceedings of

the National Academy of Sciences of the United States of America

70: 1677–1681.

Ribelayga, C. and Mangel, S. C. (2003). Absence of circadian clock regulation of horizontal cell gap junctional coupling reveals two dopamine systems in the goldfish retina. Journal of Comparative Neurology 467: 243–253.

Ribelayga, C., Cao, Y., and Mangel, S. C. (2008). The circadian clock in the retina controls rod–cone coupling. Neuron 59: 790–801.

Ribelayga, C., Wang, Y., and Mangel, S. C. (2002). Dopamine mediates circadian clock regulation of rod and cone input to fish retinal horizontal cells. Journal of Physiology (London) 544: 801–816.

Ribelayga, C., Wang, Y., and Mangel, S. C. (2004). A circadian clock in the fish retina regulates dopamine release via activation of melatonin receptors. Journal of Physiology (London) 554: 467–482.

Tessier-Lavigne, M. and Attwell, D. (1988). The effect of photoreceptor coupling and synapse nonlinearity on signal:noise ratio in early visual processing. Proceedings of the Royal Society of London, Series B 234: 171–197.

Wang, Y. and Mangel, S. C. (1996). A circadian clock regulates rod and cone input to fish retinal cone horizontal cells. Proceedings of the National Academy of Sciences of the United States of America 93: 4655–4660.

Warrant, E. J. (1999). Seeing better at night: Life style, eye design and the optimum strategy of spatial and temporal summation. Vision Research 39: 1611–1630.

Witkovsky, P. (2004). Dopamine and retinal function. Documenta Ophthalmologica 108: 17–40.

Glossary

Circadian rhythm – An endogenously generated biological rhythm with a period of about 24 h. ipRGC – The term stands for intrinsically photosensitive retinal ganglion cell, which is a small subset of retinal ganglion cells that are rendered light sensitive because they express melanopsin, implicated in nonvisual photoresponses. Melanopsin – The opsin-based photopigment of intrinsically photosensitive RGCs.

Nonvisual photoresponses – Physiological or behavioral responses to light that do not require the formation of images. Circadian photoentrainment is an example of a nonvisual photoresponse. Photoentrainment – The synchronization of circadian rhythms to the daily light:dark cycle.

Introduction

Circadian rhythms are biological rhythms that exhibit a period of about 24 h (Latin, circa around þ dies day). They persist in an environment devoid of time cues (Zeitgebers; German, Zeit time þ Geber giver). This persistence of rhythmicity in constant conditions indicates the presence of an internal circadian clock. Circadian clocks are ubiquitous, existing in organisms ranging from cyanobacteria to humans. In metazoans, many tissues are capable of autonomous circadian rhythmicity, however, the phases of such rhythms tend to be orchestrated by a master pacemaker. In mammals, this master circadian pacemaker resides in the hypothalamic suprachiasmatic nuclei (SCN), two bilateral structures that straddle the midline immediately dorsal to the optic chiasm and are separated from each other by the third ventricle. The SCN coordinate the phases of multiple oscillators located in peripheral tissues, including heart, liver, and lung. The intergeniculate leaflet (IGL) of the lateral geniculate complex is also considered a component of the circadian system and receives input from both of the eyes. The IGL has been proposed to function in assessing ambient illumination levels.

While the periods of circadian rhythms are about 24 h, rarely are they exactly 24 h. Similar to a timepiece that runs too slowly or too quickly, the utility of a circadian clock is dependent on occasional resetting of its phase. The primary

resetting agent for most circadian systems is light. Daily fluctuation in ambient irradiance resulting from the Earth’s rotation about its axis is the most predictable diurnally variable feature of the environment. Organisms have evolved photoreceptive mechanisms to communicate this most reliable of Zeitgebers, the day:night cycle, to the circadian machinery so that circadian phase can be synchronized (entrained) with the astronomical day.

The SCN receives retinal input via the retinohypothalamic tract which is comprised of the myelinated axons of retinal ganglion cells (RGCs) (Figure 1). The response of the SCN to phase-shifting stimuli varies throughout the course of the circadian day and is phase-dependent (Figure 2). For photic stimuli, illumination during an animal’s early subjective night results in a phase delay of the circadian clock as measured through the recording of circadian locomotor activity rhythms. By contrast, light pulses administered during the late subjective night cause activity rhythms to be phase advanced. Illumination during the subjective day has a negligible effect on circadian phase. In addition to this phase dependence, the magnitude of the light-induced phase shifts also varies as a function of the duration, intensity, and wavelength composition (color) of the light pulse. Conclusive identification of the photoreceptors that mediate circadian phase shifting has been wrought with problems but significant strides have been made recently.

Nonvisual Photoreception

Nonmammalian vertebrates possess numerous extraocular photoreceptors, some of which are necessary to shift the phase of the circadian system, so it is in proper phase alignment with the day:night cycle. Amphibians, for example, have photoreceptors in the pineal gland, the frontal organ, paraventricular zones of the brain, iris, and skin, in addition to the classical rod and cone photoreceptors of the retina. By contrast, all mammalian photoreception is restricted to the eyes.

One notable exception was a report in humans claiming that blue-light illumination of the popliteal region behind the knee caused shifts in the phase of salivary melatonin and temperature rhythms. This surprising result, which suggested the presence of extraocular photoreception in the mammals, inspired others to investigate the possibility of extraocular photoreception in other animal models. One group found no effect of direct sunlight on bilaterally

Retina

Optic nerve

Retinohypothalamic tract

Optic chiasm

Suprachiasmatic nucleus

Optic tract

Lateral geniculate nucleus

Intergenicluate leaflet

Superior colliculus

Optic radiation

Striate cortex

Retinal projections to the visual system

Retinal projections to the circadian system

Figure 1 Retinal projections to the visual and circadian systems. Projections to the visual system are shown in green and those to the circadian system are shown in blue.

enucleated golden hamsters whose backs were shaved to maximize skin exposure. In these animals, locomotor circadian rhythms and levels of pineal melatonin were unperturbed by sunlight exposure during the animals’ subjective night. Intact control animals, however, showed dramatic phase shifts in activity rhythms and suppression of pineal melatonin levels. Several other groups using human subjects failed to replicate the initial observation in humans indicating that some uncontrolled, nonphotic aspect of the experimental paradigm in the original study caused the observed effects on the circadian system. It is now widely accepted that the anatomical site of mammalian photoreception is exclusively ocular.

Physiological responses to light may be classified as visual or nonvisual. The responses that require the construction of images are considered visual, while those responses that simply require detection of changes in irradiance, such as circadian photoentrainment, may be considered nonvisual. The mammalian retina, the primary focus of this article, subserves both visual and nonvisual photoreception.

Due to the anatomical accessibility of the eye and the profound quality-of-life deficits experienced by the blind, the retina has been one of the most intensely studied tissues. Additionally, the laminar organization of the retina has made it an exquisite model for understanding basic concepts of development and neural communication. Retinal cytoarchitecture has been extensively described for more than 150 years. In fact, the rod and cone cells were postulated to be the light-sensing elements of the retina, as early as the mid-1850s by the German anatomist and physiologist Heinrich Mu¨ller.

In view of this long history of study, it was reasonable to suspect that rods and cones mediated both visual and nonvisual photoreception because no other classes of photoreceptors were known to exist in the mammalian eye. However, several lines of evidence suggested the

existence of a class of ocular photoreceptor that was neither rod nor cone. In 1927, Clyde Keeler recognized that blind mice lacking photoreceptor cells continued to exhibit a pupillary light reflex, although this response was slower and less sensitive than that of fully sighted mice. Seventyfour years later, it was confirmed that the pupillary light reflex does indeed persist in blind mice and is maximally sensitive to blue wavelengths. Phase shifting of the circadian clock in response to light pulses is also maintained in

mice homozygous for the retinal degeneration 1 allele (Pde6brd1/rd1), resulting in the loss of rods and cones. Similar

to the pupillary response, light-induced circadian phase shifting in these visually blind mice exhibits a peak spectral sensitivity in the blue wavelengths. Finally, elevated nighttime levels of serum melatonin can be acutely suppressed by blue light presented to the eyes.

All of these blue-sensitive responses exhibit a peak spectral sensitivity in the 460–480 nm wavelength range, a domain of the electromagnetic spectrum that does not coincide with the peak sensitivity of known rod and cone photopigments of the human or murine retina. Since photoreceptors are defined by their photopigments, which in turn dictate their spectral sensitivity, a common strategy for identifying new photoreceptors is to identify new photopigments which should eventually point to the identification of a new photoreceptor class. The accumulating evidence for novel photoreceptors that mediate nonvisual responses to light led to the discovery of many new opsin-based photopigments. First among these was pinopsin, which was originally identified in the pineal gland of chicken and whose function remains unknown, but most likely plays a role in the photoregulation of melatonin synthesis. Very ancient opsin (VA-opsin), parapinopsin, and parietopsin were subsequently identified in the eye, parapineal, and parietal eye, respectively, of nonmammalian vertebrates. Based on an inspection of completed genomes, it is unlikely that mammalian homologs of these opsins exist.

However, within the last decade new opsins were indeed revealed in the mammals, none of which were localized to rods or cones. Peropsin and retinal G-protein-coupled receptor (RGR) opsin are expressed in the retinal pigment epithelium and are likely to function as accessory photoisomerases which use photic energy to convert all-trans- retinoids to their respective 11-cis isomers. 11-cis-Retinal is the requisite chromophore of all vertebrate signaling opsins. Encephalopsin was originally identified only in the brain and spinal cord of mouse, sites not known to be inherently photoreceptive. A subsequent study claimed the presence of encephalopsin message in the eye, however, the role of encephalopsin in the central nervous system remains unknown. Opn5 is another opsin predicted to exist within the mammalian eye based on the presence of messenger ribonucleic acid (mRNA), although a translated gene product remains to be discovered (Table 1).