Synaptic transmission mediated by internal calcium stores in rod photoreceptors

Retinal rod photoreceptors are depolarized in darkness to approximately -40 mV, a state in which they maintain sustained glutamate release despite low levels of calcium channel activation. Blocking voltage-gated calcium channels or ryanodine receptors (RyRs) at the rod presynaptic terminal suppressed synaptic communication to bipolar cells. Spontaneous synaptic events were also inhibited when either of these pathways was blocked. This indicates that both calcium influx and calcium release from internal stores are required for the normal release of transmitter of the rod. RyR-independent release can be evoked by depolarization of a rod to a supraphysiological potential (-20 mV) that activates a large fraction of voltage-gated channels. However, this calcium channel-mediated release depletes rapidly if RyRs are blocked, indicating that RyRs support prolonged glutamate release. Thus, the rod synapse couples a small transmembrane calcium influx with a RyR-dependent amplification mechanism to support continuous vesicle release.

Subcellular, cellular, and circuit mechanisms underlying classical conditioning in Hermissenda crassicornis

A breakthrough for studying the neuronal basis of learning emerged when invertebrates with simple nervous systems, such as the sea slug Hermissenda crassicornis, were shown to exhibit classical conditioning. Hermissenda learns to associate light with turbulence: prior to learning, naive animals move toward light (phototaxis) and contract their foot in response to turbulence; after learning, conditioned animals delay phototaxis in response to light. The photoreceptors of the eye, which receive monosynaptic inputs from statocyst hair cells, are both sensory neurons and the first site of sensory convergence. The memory of light associated with turbulence is stored as changes in intrinsic and synaptic currents in these photoreceptors. The subcellular mechanisms producing these changes include activation of protein kinase C and MAP kinase, which act as coincidence detectors because they are activated by convergent signaling pathways. Pathways of interneurons and motorneurons, where additional changes in excitability and synaptic connections are found, contribute to delayed phototaxis. Bursting activity recorded at several points suggest the existence of small networks that produce complex spatiotemporal firing patterns. Thus, the change in behavior may be produced by a nonlinear transformation of spatiotemporal firing patterns caused by plasticity of synaptic and intrinsic channels. The change in currents and the activation of PKC and MAPK produced by associative learning are similar to those observed in hippocampal and cerebellar neurons after rabbit classical conditioning, suggesting that these represent general mechanisms of memory storage. Thus, the knowledge gained from further study of Hermissenda will continue to illuminate mechanisms of mammalian learning.

Paired-pulse depression at photoreceptor synapses

Synaptic depression produced by repetitive stimulation is likely to be particularly important in shaping responses of second-order retinal neurons at the tonically active photoreceptor synapse. We analyzed the time course and mechanisms of synaptic depression at rod and cone synapses using paired-pulse protocols involving two complementary measurements of exocytosis: (1) paired whole-cell recordings of the postsynaptic current (PSC) in second-order retinal neurons and (2) capacitance measurements of vesicular membrane fusion in rods and cones. PSCs in ON bipolar, OFF bipolar, and horizontal cells evoked by stimulation of either rods or cones recovered from paired-pulse depression (PPD) at rates similar to the recovery of exocytotic capacitance changes in rods and cones. Correlation between presynaptic and postsynaptic measures of recovery from PPD suggests that 80-90% of the depression at these synapses is presynaptic in origin. Consistent with a predominantly presynaptic mechanism, inhibiting desensitization of postsynaptic glutamate receptors had little effect on PPD. The depression of exocytotic capacitance changes exceeded depression of the presynaptic calcium current, suggesting that it is primarily caused by a depletion of synaptic vesicles. In support of this idea, limiting Ca2+ influx by using weaker depolarizing stimuli promoted faster recovery from PPD. Although cones exhibit much faster exocytotic kinetics than rods, exocytotic capacitance changes recovered from PPD at similar rates in both cell types. Thus, depression of release is not likely to contribute to differences in the kinetics of transmission from rods and cones.

Physiological maturation of photoreceptors depends on the voltage-gated sodium channel NaV1.6 (Scn8a)

Voltage-gated sodium channels (VGSCs) ensure the saltatory propagation of action potentials along axons by acting as signal amplifiers at the nodes of Ranvier. In the retina, activity mediated by VGSCs is important for the refinement of the retinotectal map. Here, we conducted a full-field electroretinogram (ERG) study on mice null for the sodium channel NaV1.6. Interestingly, the light-activated hyperpolarization of photoreceptor cells (the a-wave) and the major "downstream" components of the ERG, the b-wave and the oscillatory potentials, are markedly reduced and delayed in these mice. The functional deficit was not associated with any morphological abnormality. We demonstrate that Scn8a is expressed in the ganglion and inner nuclear layers and at low levels in the outer nuclear layer beginning shortly before the observed ERG deficit. Together, our data reveal a previously unappreciated role for VGSCs in the physiological maturation of photoreceptors.

The Phycomyces madA gene encodes a blue-light photoreceptor for phototropism and other light responses

Phycomyces blakesleeanus is a filamentous zygomycete fungus that produces striking elongated single cells that extend up to 10 cm into the air, with each such sporangiophore supporting a sphere containing the spores for dispersal. This organism has served as a model for the detection of environmental signals as diverse as light, chemicals, touch, wind, gravity, and adjacent objects. In particular, sporangiophore growth is regulated by light, and it exhibits phototropism by bending toward near-UV and blue wavelengths and away from far-UV wavelengths in a manner that is physiologically similar to plant phototropic responses. The Phycomyces madA mutants were first isolated more than 40 years ago, and they exhibit reduced sensitivity to light. Here, we identify two (duplicated) homologs in the White Collar 1 family of blue-light photoreceptors in Phycomyces. We describe that the madA mutant strains contain point mutations in one of these genes and that these mutations cosegregate with a defect in phototropism after genetic crosses. Thus, the phototropic responses of fungi through madA and plants through phototropin rely on diverse proteins; however, these proteins share a conserved flavin-binding domain for photon detection.

Cloning and characterization of mr-s, a novel SAM domain protein, predominantly expressed in retinal photoreceptor cells

Background

Sterile alpha motif (SAM) domains are ~70 residues long and have been reported as common protein-protein interaction modules. This domain is found in a large number of proteins, including Polycomb group (PcG) proteins and ETS family transcription factors. In this work, we report the cloning and functional characterization of a novel SAM domain-containing protein, which is predominantly expressed in retinal photoreceptors and the pineal gland and is designated mouse mr-s (major retinal SAM domain protein).

Results

mr-s is evolutionarily conserved from zebrafish through human, organisms through which the mechanism of photoreceptor development is also highly conserved. Phylogenetic analysis suggests that the SAM domain of mr-s is most closely related to a mouse polyhomeotic (ph) ortholog, Mph1/Rae28, which is known as an epigenetic molecule involved in chromatin modifications. These findings provide the possibility that mr-s may play a critical role by regulating gene expression in photoreceptor development. mr-s is preferentially expressed in the photoreceptors at postnatal day 3–6 (P3-6), when photoreceptors undergo terminal differentiation, and in the adult pineal gland. Transcription of mr-s is directly regulated by the cone-rod homeodomain protein Crx. Immunoprecipitation assay showed that the mr-s protein self-associates mainly through the SAM domain-containing region as well as ph. The mr-s protein localizes mainly in the nucleus, when mr-s is overexpressed in HEK293T cells. Moreover, in the luciferase assays, we found that mr-s protein fused to GAL4 DNA-binding domain functions as a transcriptional repressor. We revealed that the repression activity of mr-s is not due to a homophilic interaction through its SAM domain but to the C-terminal region.

Conclusion

We identified a novel gene, mr-s, which is predominantly expressed in retinal photoreceptors and pineal gland. Based on its expression pattern and biochemical analysis, we predict that mr-s may function as a transcriptional repressor in photoreceptor cells and in pinealocytes of the pineal gland.

Photoreceptor adaptation in intrinsically photosensitive retinal ganglion cells

A rare type of mammalian retinal ganglion cell (RGC) expresses the photopigment melanopsin and is a photoreceptor. These intrinsically photosensitive RGCs (ipRGCs) drive circadian-clock resetting, pupillary constriction, and other non-image-forming photic responses. Both the light responses of ipRGCs and the behaviors they drive are remarkably sustained, raising the possibility that, unlike rods and cones, ipRGCs do not adjust their sensitivity according to lighting conditions ("adaptation"). We found, to the contrary, that ipRGC sensitivity is plastic, strongly influenced by lighting history. When exposed to a constant, bright background, the background-evoked response decayed, and responses to superimposed flashes grew in amplitude, indicating light adaptation. After extinction of a light-adapting background, sensitivity recovered progressively in darkness, indicating dark adaptation. Because these adjustments in sensitivity persisted when synapses were blocked, they constitute "photoreceptor adaptation" rather than "network adaptation." Implications for the mechanisms generating various non-image-forming visual responses are discussed.

Sexual dimorphism of short-wavelength photoreceptors in the small white butterfly, Pieris rapae crucivora

The eyes of the female small white butterfly, Pieris rapae crucivora, are furnished with three classes of short-wavelength photoreceptors, with sensitivity peaks in the ultraviolet (UV) (lambda(max) = 360 nm), violet (V) (lambda(max) = 425 nm), and blue (B) (lambda(max) = 453 nm) wavelength range. Analyzing the spectral origin of the photoreceptors, we isolated three novel mRNAs encoding opsins corresponding to short-wavelength-absorbing visual pigments. We localized the opsin mRNAs in the retinal tissue and found that each of the short-wavelength-sensitive photoreceptor classes exclusively expresses one of the opsin mRNAs. We, accordingly, termed the visual pigments PrUV, PrV, and PrB, respectively. The eyes of the male small white butterfly also use three classes of short-wavelength photoreceptors that equally uniquely express PrUV, PrV, and PrB. However, whereas the spectral sensitivities of the male photoreceptors with PrUV and PrB closely correspond to those of the female, the male photoreceptor expressing PrV has a double-peaked blue (dB) spectral sensitivity, strongly deviating from the spectral sensitivity of the female V photoreceptor. The male eyes contain a pigment that distinctly fluoresces under blue-violet as well as UV excitation light. It coexists with the dB photoreceptors and presumably acts as a spectral filter with an absorbance spectrum peaking at 416 nm. The narrow-band spectral sensitivity of the male dB photoreceptors probably evolved to improve the discrimination of the different wing colors of male and female P. rapae crucivora in the short-wavelength region of the spectrum.

A spatiotemporal white noise analysis of photoreceptor responses to UV and green light in the dragonfly median ocellus

Adult dragonflies augment their compound eyes with three simple eyes known as the dorsal ocelli. While the ocellar system is known to mediate stabilizing head reflexes during flight, the ability of the ocellar retina to dynamically resolve the environment is unknown. For the first time, we directly measured the angular sensitivities of the photoreceptors of the dragonfly median (middle) ocellus. We performed a second-order Wiener Kernel analysis of intracellular recordings of light-adapted photoreceptors. These were stimulated with one-dimensional horizontal or vertical patterns of concurrent UV and green light with different contrast levels and at different ambient temperatures. The photoreceptors were found to have anisotropic receptive fields with vertical and horizontal acceptance angles of 15° and 28°, respectively. The first-order (linear) temporal kernels contained significant undershoots whose amplitudes are invariant under changes in the contrast of the stimulus but significantly reduced at higher temperatures. The second-order kernels showed evidence of two distinct nonlinear components: a fast acting self-facilitation, which is dominant in the UV, followed by delayed self- and cross-inhibition of UV and green light responses. No facilitatory interactions between the UV and green light were found, indicating that facilitation of the green and UV responses occurs in isolated compartments. Inhibition between UV and green stimuli was present, indicating that inhibition occurs at a common point in the UV and green response pathways. We present a nonlinear cascade model (NLN) with initial stages consisting of separate UV and green pathways. Each pathway contains a fast facilitating nonlinearity coupled to a linear response. The linear response is described by an extended log-normal model, accounting for the phasic component. The final nonlinearity is composed of self-inhibition in the UV and green pathways and inhibition between these pathways. The model can largely predict the response of the photoreceptors to UV and green light.

Photoreceptor spectral sensitivities in terrestrial animals: adaptations for luminance and colour vision

This review outlines how eyes of terrestrial vertebrates and insects meet the competing requirements of coding both spatial and spectral information. There is no unique solution to this problem. Thus, mammals and honeybees use their long-wavelength receptors for both achromatic (luminance) and colour vision, whereas flies and birds probably use separate sets of photoreceptors for the two purposes. In particular, we look at spectral tuning and diversification among 'long-wavelength' receptors (sensitivity maxima at greater than 500 nm), which play a primary role in luminance vision. Data on spectral sensitivities and phylogeny of visual photopigments can be incorporated into theoretical models to suggest how eyes are adapted to coding natural stimuli. Models indicate, for example, that animal colour vision--involving five or fewer broadly tuned receptors--is well matched to most natural spectra. We can also predict that the particular objects of interest and signal-to-noise ratios will affect the optimal eye design. Nonetheless, it remains difficult to account for the adaptive significance of features such as co-expression of photopigments in single receptors, variation in spectral sensitivities of mammalian L-cone pigments and the diversification of long-wavelength receptors that has occurred in several terrestrial lineages.

Dynamics of a sensory signaling network in a unicellular eukaryote

The processing components and the dynamic signaling network that an individual cell uses to do signal integration and make decisions based on multiple sensory inputs are being identified in a well studied free-swimming unicellular green algal model organism, Chlamydomonas. It has many sensory photoreceptors and measurable behavior associated with its orienting and swimming with respect to light sources in its environment. Study of the dynamics of the beating of its two steering cilia reveals their complex specialization.

Polarization properties of the retinal nerve fiber layer

Recently developed optical techniques provide quantitative structural measurements of the retinal nerve fiber layer (RNFL). A complete interpretation of these measurements requires understanding of the optical properties of the RNFL. This paper gives a review of the polarization properties and relevant anatomy of the ocular tissues, followed by a thorough discussion of the optical properties of the RNFL. The RNFL reflectance arises from light scattering from cylinders. Microtubules are a major component contributing to the reflectance. The RNFL reflectance exhibits weak intrinsic diattenuation and well preserves polarization. RNFL birefringence varies across the retina; the variation suggests that birefringence depends on the ultrastructure of the nerve fiber bundles, which offers hope that measurement of RNFL birefringence may be able to provide early detection of subcellular changes in glaucoma.

Circadian expression of Bmal1 and serotonin-N-acetyltransferase mRNAs in chicken retina cells and pinealocytes in vivo and in vitro

Unlike mammals, rhythmic changes in serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase [AANAT]) transcripts in chicken pineal cells are controlled by an oscillator located in the pinealocytes themselves, which is comprised of clock genes. Asimilar clock-dependent pathway has been postulated to regulate the retinal melatonin rhythm. In chicken retinal photoreceptor cells and pinealocytes, the chicken AANAT gene (cAANAT) is coexpressed with clock genes, including cBmal1 and cClock, which might regulate cAANAT transcription. Here, we have studied the temporal profile of cBmal1, cClock, and cAANAT mRNAexpressions in retinal cells in vivo with chickens housed in a 14/10-h light/dark (LD) cycle for 2 wk and in vitro cultured in a superfusion system for 4 LD cycles. mRNA levels of these genes were analyzed by RT-PCR and compared with their corresponding pineal transcripts. cBmal1 mRNA showed a peak during the light phase between Zeitgeber time (ZT) 8 and 10, preceding the amplitude of the nocturnal increase in cAANAT expression at ZT 16-17. Retinal cBmal1 and cAANAT mRNAs exhibited less robust cycling than their corresponding pineal transcripts in the same animal. cClock mRNAlevels failed to exhibit a well-detectable rhythm. The phase of the rhythms of retinal cBmal1 and cAANAT mRNAs suggests a link between retinal cBmal1 and cAANAT expressions similar to the regulation of pineal cAANAT transcription. Based on the highly conserved nature of the clockwork, it is reasonable to consider that chicken retina and pineal gland might serve as a useful tool for the development of drugs that could influence clock function in man.

Start the clock! Circadian rhythms and development

The contribution of timing cues from the environment to the coordination of early developmental processes is poorly understood. The day-night cycle represents one of the most important, regular environmental changes that animals are exposed to. A key adaptation that allows animals to anticipate daily environmental changes is the circadian clock. In this review, we aim to address when a light-regulated circadian clock first emerges during development and what its functions are at this early stage. In particular, do circadian clocks regulate early developmental processes? We will focus on results obtained with Drosophila and vertebrates, where both circadian clock and developmental control mechanisms have been intensively studied.

Psychophysics of prosthetic vision: III. stochastic rendering, the phosphene image, and perception

For pt.I see ibid., p.Z004024-7. For pt.II see ibid., p.Z004558-61. This paper examines the rendering of luminous spots ("phosphenes") in the visual field, and their stochastic positioning as a means of anti-aliasing the resulting spotty image ("phosphene image"). We derive an equation concerning the correlations of pairs of phosphenes comprising the phosphene image, and show the relationship to the statistics governing the stochastic positioning. We present some examples where stochastic rendering assists the veridical perception of textures, and argue for its superiority as cf. ordered rendering. Our preliminary results suggest that it may be perceptually effective to manufacture disordered arrays of stimulating electrodes for intraocular implantation.

Circadian rhythms of ocular melatonin in the wrasse Halichoeres tenuispinnis, a labrid teleost

Using in vivo and in vitro methods we studied the regulation of ocular melatonin rhythms in the wrasse Halichoeres tenuispinnis, by either light or the circadian clock. Rhythmic changes in ocular melatonin levels under light-dark (LD) cycles were persistent under constant darkness (DD), and had a circadian periodicity of approximately 24h. However, ocular melatonin levels remained low under constant light conditions. When wrasse were exposed to a single 6-h light pulse at three different circadian phases under DD, phase-dependent phase shifts in the circadian rhythms of ocular melatonin were observed. When eyecups were prepared during mid-light periods or at the onset of darkness, and incubated in vitro in either light or dark periods, both time and light conditions affected melatonin release. These results indicate that the melatonin rhythms in the wrasse eye are driven by an ocular circadian clock that is entrained to LD cycles via local photoreceptors.

N-Methyl-D-Aspartate Receptors in the Retina

The vertebrate retina is a "genuine neural center" (Ramón y Cajal), in which glutamate is a major excitatory neurotransmitter. Both N-methyl-D-aspartate (NMDA) and non-NMDA receptors are expressed in the retina. Although non-NMDA receptors and/or metabotropic glutamate receptors are generally thought to be responsible for mediating the transfer of visual signals in the outer retina, there is recent evidence suggesting that NMDA receptors are also expressed in photoreceptors, as well as horizontal and bipolar cells. In the inner retina, NMDA receptors, in addition to other glutamate receptor subtypes, are abundantly expressed to mediate visual signal transmission from bipolar cells to amacrine and ganglion cells, and could be involved in modulation of inhibitory feedback from amacrine cells to bipolar cells. NMDA receptors are extrasynaptically expressed in ganglion cells (and probably amacrine cells) and may play physiological roles in a special mode. Activity of NMDA receptors may be modulated by neuromodulators, such as D-serine and others. This article discusses retinal excitotoxicity mediated by NMDA receptors.

Concepts, facts and artifacts in electron microscopy

This communication illustrates how the electron microscope has contributed to biochemistry by revealing how multienzyme systems in mitochondria are structurally organized to secure high speed ATP synthesis and has extended physiology to the molecular level. Ribonucleoprotein complexes form a gel in the cytoplasm determining the conditions for translation... Photoreceptor stimulation involves two phases, trapping of light by a light reflecting cylinder formed by the outer segment disks and energy transduction by bleaching of photopigment molecules changing the charge of the outer segment disks driving the photoreceptor toward hyperpolarization. Revealing the synaptic connections between retinal neurons extends neurophysiology to the level of information processing by neural circuits, which are designed for high speed processing. Spatial brightness contrast enhancement is eliminated in connection with macular degeneration, which leads to partial blindness, revealing the importance of contrast enhancement for vision.

Stimulus size and intensity alter fundamental receptive-field properties of mouse retinal ganglion cellsin vivo

The receptive field (RF) of most retinal ganglion cells (RGCs) is comprised of an excitatory center and an antagonistic surround. Interactions between these RF elements shape most of the visual responses of RGCs. To begin to investigate center-surround interactions of mouse RGCs quantitatively, we characterized their responses in anin vivopreparation to a variety of spot and full-field stimuli. When RGCs were stimulated with a spot that matched the cell's RF center diameter (optimal spot), all RGCs could be categorized as either ON- or OFF-center. In all RGCs, full-field stimulation significantly reduced both the peak and the mean firing rates evoked with an optimal spot stimulus. Full-field stimulation revealed differences in other response properties between ON- and OFF-center RGCs. With a full-field stimulus, the duration of the OFF-center RGCs response was reduced making them more transient, while the duration of the ON-center RGCs increased making them more sustained. Of most interest, full-field stimulation altered the RF center response sign in approximately half of the OFF-center RGCs, which became either OFF/ON or ON only. In contrast, all ON-center and the other OFF-center cells conserved their RF response sign in the presence of the full-field stimulus. We propose that sign-altering OFF-center RGCs possess an additional RF surround mechanism that underlies this alteration in their response. Of general interest these results suggest that the sole use of full-field stimulation to categorize visual response properties of RGCs does not adequately reflect their RF organization and, therefore, is not an optimal strategy for their classification.

The growth regulators warts/lats and melted interact in a bistable loop to specify opposite fates in Drosophila R8 photoreceptors

Color vision in Drosophila relies on the comparison between two color-sensitive photoreceptors, R7 and R8. Two types of ommatidia in which R7 and R8 contain different rhodopsins are distributed stochastically in the retina and appear to discriminate short (p-subset) or long wavelengths (y-subset). The choice between p and y fates is made in R7, which then instructs R8 to follow the corresponding fate, thus leading to a tight coupling between rhodopsins expressed in R7 and R8. Here, we show that warts, encoding large tumor suppressor (Lats) and melted encoding a PH-domain protein, play opposite roles in defining the yR 8 or pR8 fates. By interacting antagonistically at the transcriptional level, they form a bistable loop that insures a robust commitment of R8 to a single fate, without allowing ambiguity. This represents an unexpected postmitotic role for genes controlling cell proliferation (warts and its partner hippo and salvador) and cell growth (melted).

Guided light and diffraction model of human-eye photoreceptors

The photoreceptors of the living human eye are known to exhibit waveguide-characteristic features. This is evidenced by the Stiles-Crawford effect observed for light incident near the pupil rim, and by the directional component of light reflected off the retina in the related optical Stiles-Crawford effect. We describe a model for the coupling of light to/from photoreceptors on the basis of waveguide theory that includes diffraction between the eye pupil and the photoreceptor apertures, and we show that valuable insight can be gained from a Gaussian approximation to the mode field. We apply this knowledge to a detailed study of the relationship between the Stiles-Crawford effect and its optical counterpart.

Evaluation of retinal function using the Dynamic Focal Cone ERG

The development of effective means of assessing visual function in retinal disease holds the key to improved understanding of pathogenesis, and better monitoring of treatment outcomes. In diseases such as age-related macular degeneration, in which the primary locus of dysfunction is the outer retina, tests which provide a direct measure of the functional integrity of the photoreceptor/retinal pigment epithelium (RPE) complex are of great importance. Recovery of retinal function following adaptation to a bright light requires the healthy function of photoreceptors, RPE, Bruch's membrane and choroidal circulation, making an assessment of this recovery a potentially useful clinical tool. However, current techniques are either subjective in nature, or are influenced by post-retinal processing of visual information. This report describes a novel technique, the 'Dynamic Focal Cone Electro-retinogram (ERG)', which allows direct, objective assessment of the recovery of macular function following photopigment bleach. A series of 41 Hz ERGs was recorded, and ERG amplitude was plotted as a function of time following cessation of the bleach. Normative data was collected from 10 healthy subjects. For all subjects, there was no measurable ERG immediately after the bleach, but the amplitude had returned to a pre-bleach level within 4 min. The amplitude recovery data were adequately described both by an exponential recovery function and by a model based on a rate-limited recovery process. We conclude that this technique provides a clinically applicable, objective measure of outer retinal recovery.

Discriminative responses of squid (Loligo pealeii) photoreceptors to polarized light

Cephalopods behaviorally respond to polarized light. Electrophysiology experiments with the squid, Loligo pealeii, demonstrated that spike responses from individual photoreceptors are a cosine2 function of the e-vector orientation of a polarized stimulus. The discrimination limit to this polarization sensitivity depended upon the difference between the orientation of a polarized stimulus with a preferred e-vector. The limit ranged from 2 degrees to 9.2 degrees with a direct stimulus in the dark or 4.8 degrees -22.1 degrees with non-directed background illumination and the cells were least discriminative at the preferred orientations. This limit can be explained partly by the variability in anatomical alignment of microvilli in the photoreceptors around a dominant axis. A few light-sensitive retinal fibers showed no polarization sensitivity. The coding of polarization information suggests that light intensity is transformed into an average spike rate. This average results from silent periods interspersed between bursts of spikes, each burst possessing a consistent interspike interval. The variations in the length and frequency of silent periods depend upon the difference between the polarization e-vector and a preferred e-vector orientation. The minimal discriminated orientation of a squid photoreceptor agrees well with the minimum behavioral discrimination of polarized light by another cephalopod, the octopus.

Photons, clocks, and consciousness

Light profoundly impacts human consciousness through the stimulation of the visual system and powerfully regulates the human circadian system, which, in turn, has a broad regulatory impact on virtually all tissues in the body. For more than 25 years, the techniques of action spectroscopy have yielded insights into the wavelength sensitivity of circadian input in humans and other mammalian species. The seminal discovery of melanopsin, the photopigment in intrinsically photosensitive retinal ganglion cells, has provided a significant turning point for understanding human circadian phototransduction. Action spectra in humans show that the peak wavelength sensitivity for this newly discovered sensory system is within the blue portion of the spectrum. This is fundamentally different from the three-cone photopic visual system, as well as the individual rod and cone photoreceptor peaks. Studies on rodents, nonhuman primates, and humans indicate that despite having a different wavelength fingerprint, these classic visual photoreceptors still provide an element of input to the circadian system. These findings open the door to innovations in light therapy for circadian and affective disorders, as well as possible architectural light applications.

Contrast gain in the brain

Human sensory systems have the remarkable ability of adjusting sensitivity to the surrounding environment. In this issue of Neuron, Gardner and colleagues used fMRI to show how the visual system shifts its sensitivity to contrast. This process may be helpful for keeping the appearance of contrast constant across a range of spatial frequencies.

Phototransduction in primate cones and blowfly photoreceptors: different mechanisms, different algorithms, similar response

Phototransduction in primate cones is compared with phototransduction in blowfly photoreceptor cells. Phototransduction in the two cell types utilizes not only different molecular mechanisms, but also different signal processing steps, producing range compression, contrast constancy, and an intensity-dependent integration time. The dominant processing step in the primate cone is a strongly compressive nonlinearity due to cGMP hydrolysis by phosphodiesterase. In the blowfly photoreceptor a considerable part of the range compression is performed by the nonlinear membrane of the cell. Despite these differences, both photoreceptor cell types are similarly effective in compressing the wide range of naturally occurring intensities, and in converting intensity variations into contrast variations. A direct comparison of the responses to a natural time series of intensities, simulated in the cone and measured in the blowfly photoreceptor, shows that the responses are quite similar.

Carboxypeptidase E is required for normal synaptic transmission from photoreceptors to the inner retina

Defects in the gene encoding carboxypeptidase E (CPE) in either mouse or human lead to multiple endocrine disorders, including obesity and diabetes. Recent studies on Cpe-/- mice indicated neurological deficits in these animals. As a model system to study the potential role of CPE in neurophysiology, we carried out electroretinography (ERG) and retinal morphological studies on Cpe-/- and Cpe fat/fat mutant mice. Normal retinal morphology was observed by light microscopy in both Cpe-/- and Cpe(fat/fat) mice. However, with increasing age, abnormal retinal function was revealed by ERG. Both Cpe-/- and Cpe fat/fat animals had progressively reduced ERG response sensitivity, decreased b-wave amplitude and delayed implicit time with age, while maintaining a normal a-wave amplitude. Immunohistochemical staining showed specific localization of CPE in photoreceptor synaptic terminals in wild-type (WT) mice, but in both Cpe-/- and Cpe fat/fat mice, CPE was absent in this layer. Bipolar cell morphology and distribution were normal in these mutant mice. Electron microscopy of retinas from Cpe fat/fat mice revealed significantly reduced spherule size, but normal synaptic ribbons and synaptic vesicle density, implicating a reduction in total number of vesicles per synapse in the photoreceptors of these animals. These results suggest that CPE is required for normal-sized photoreceptor synaptic terminal and normal signal transmission to the inner retina.

The Aspergillus nidulans Phytochrome FphA Represses Sexual Development in Red Light

Phytochrome photoreceptors sense red and far-red light through photointerconversion between two stable conformations, a process mediated by a linear tetrapyrrole chromophore. Originally, phytochromes were thought to be confined to photosynthetic organisms including cyanobacteria, but they have been recently discovered in heterotrophic bacteria and fungi, where little is known about their functions. It was shown previously in the ascomycetous fungus Aspergillus nidulans that asexual sporulation is stimulated and sexual development repressed by red light. The effect was reminiscent of a phytochrome response, and indeed phytochrome-like proteins were detected in several fungal genomes. All fungal homologs are more similar to bacterial than plant phytochromes and have multifunctional domains where the phytochrome region and histidine kinase domain are combined in a single protein with a C-terminal response-regulator domain. Here, we show that the A. nidulans phytochrome FphA binds a biliverdin chromophore, acts as a red-light sensor, and represses sexual development under red-light conditions. FphA-GFP is cytoplasmic and excluded from the nuclei, suggesting that red-light photoperception occurs in the cytoplasm. This is the first phytochrome experimentally characterized outside the plant and bacterial kingdoms and the second type of fungal protein identified that functions in photoperception.

Protein Aggregation in Retinal Cells and Approaches to Cell Protection

1. Retinal dystrophies (RD) comprise a group of clinically and genetically heterogeneous retinal disorders, which typically result in the degeneration of photoreceptors followed by the impairment or loss of vision. Although age-related macular degeneration (AMD) and retinitis pigmentosa (RP) are among the most common forms of RD, currently, there is no effective treatment for either disorder. 2. Recently, abnormal protein accumulation and aggregation due to protein misfolding and proteasome inhibition have been implicated in the pathogenesis of RD. In this paper we describe effects of several factors on protein aggregation and survival of photoreceptor cells. 3. Expression of rhodopsin carrying P23H mutation causes its accumulation in intracellular inclusion bodies in a perinuclear area of photoreceptor cells. beta- and gamma-synucleins and heat shock protein Hsp-70, but not alpha-synuclein, protect cultured ocular cells from mutant opsin accumulation. This effect might be explained by their chaperonic activity. 4. Knock-out of alpha- and gamma-synucleins does not affect gross retinal morphology, but induces tyrosine hydroxylase in the inner prexiform layer of the retina. Selegiline-a monoamine oxidase inhibitor used for the treatment of Parkinson's disease, reduces apoptosis and increases viability in cultured retinal pigment epithelium cells (APRE-19). 5. These results suggest that chaperones and selegiline may be considered promising candidates for the protection of ocular cells from the accumulation of misfolded and aggregated proteins.

Lens opacity and photoreceptor degeneration in the zebrafish lens opaque mutant

The zebrafish lens opaque (lop) mutant was identified in a chemical mutagenesis screen. The lop mutant, which develops normally through 4 days postfertilization (dpf), exhibits several signs of lens and retinal degeneration at 7 dpf. Histology revealed disrupted lens fibers and increased numbers of nucleated cells within the mutant lens and anterior chamber. The mutant lens also exhibited aberrant epithelial cell morphologies and lacked a definitive transition zone, which suggests that secondary fiber differentiation was interrupted. In addition, the mutant exhibits severely reduced photoreceptors and a reduction in the number of horizontal cells at 7 dpf. Other retinal cell classes appeared unaffected in the mutant. Transmission electron microscopy and opsin immunohistochemistry showed that the different photoreceptor types were generated at the retinal margin, but the rods and cones failed to mature and disappeared. The mutant lens and retina also displayed increased cell proliferation based on proliferating cell nuclear antigen immunolabeling, suggesting that the lens opacity was due to unregulated cell proliferation and undifferentiated cell accumulation within the mutant lens. The lop mutant phenotype supports recent studies showing the lens has a role in regulating teleost retinal development.

Embryonic stem cell-derived neural progenitors incorporate into degenerating retina and enhance survival of host photoreceptors

Embryonic stem (ES) cells differentiate into all cell types of the body during development, including those of the central nervous system (CNS). After transplantation, stem cells have the potential to replace host cells lost due to injury or disease or to supply host tissues with therapeutic factors and thus provide a functional benefit. In the current study, we assessed whether mouse neuralized ES cells can incorporate into retinal tissue and prevent retinal degeneration in mnd mice. These mice have an inherited lysosomal storage disease characterized by retinal and CNS degeneration. Sixteen weeks after intravitreal transplantation into adult mice, donor cells had incorporated into most layers of the retina, where they resembled retinal neurons in terms of morphology, location in the retina, and expression of cell type-specific marker proteins. Presence of these donor cells was correlated with a reduction in the sizes and numbers of lysosomal storage bodies in host retinal cells. The presence of transplanted donor cells was also accompanied by enhanced survival of host retinal neurons, particularly photoreceptors. These results demonstrate that neuralized ES cells protect host neurons from degeneration and appear to replace at least some types of lost neurons.

Orientation behavior of retinal photoreceptors in alternating electric fields

In alternating electric (AC) fields, particles experience polarizing effects that induce dipoles that orient elongated specimens either parallel or perpendicular to the field lines. In this work we studied the behavior of photoreceptor cells' rod outer segments (ROS) in AC fields of different frequencies. We showed that at low frequencies, ROS orient parallel to the field, whereas at higher frequencies they orient perpendicular to the field lines (in the frequency range from 100 Hz to 10 MHz). We found this behavior to be dependent on the physiological state of cells (due to modifications in their electrical properties). To simulate cell damage, the membrane conductivity was changed by treating the cell with gramicidin A, which resulted in a decrease of cytosol conductivity and, consequently, in a change of the orientation behavior of the treated cells. The change of cell orientation with cytosol conductivity is rather sharp, suggesting the potential of the method for accurate evaluation of the cell physiological status. We modeled the interaction between ROS and AC fields approximating the rod cell by a prolate spheroid with a very long axis. The internal compartment of the ellipsoid was considered to be filled with an inhomogeneous medium consisting of alternating layers of membrane and cytoplasm as media modeling the disks. This theoretical model proved to be in good agreement with the experimental results and enabled the derivation (by fitting with the experimental results) of the membrane and cytosol parameters for normal and damaged cells.

Variation in Stomatopod (Gonodactylus smithii) Color Signal Design Associated with Organismal Condition and Depth

In interactions, many tropical stomatopod species display conspicuous colored body spots that can communicate information about the sender's state (e.g., sex, aggressiveness, etc.). Species inhabiting a variety of depths experience large differences in illumination spectrum and intensity due to filtering of light by water and its constituents. Stomatopod spectral sensitivity is known to vary phenotypically with changes in light environment (associated with depth) that potentially affects the detection of color signals. Animals collected at different depths also have different body coloration. This study examines how spectral differences in colored body spots vary with organismal condition and models the effects of changing body coloration, light environment, and spectral sensitivity on the detection of color signals in a gonodactyloid species, Gonodactylus smithii. Of the seven conspicuous color spots that were measured in G. smithii, three had spectral differences that correlated with sex, aggression, and female reproductive state. A model of color detection in G. smithii indicates that longer-wavelength spectral content was affected most by varying body coloration and light conditions. Most color signals were perceived similarly both by shallow- and by deep-adapted photoreceptor sets over a range of depths (1-13 m). Eye spot ('meral spot') color detection also was invariant over the same depth range in shallow- and deep-adapted, long-wavelength receptors, but deep-adapted receptors continued to maintain a consistent detection of these spots down to 18 meters. These results suggest that meral spot coloration may have evolved as a constant signal when viewed by conspecifics from various depths.

The retinal pigment epithelium in visual function

Located between vessels of the choriocapillaris and light-sensitive outer segments of the photoreceptors, the retinal pigment epithelium (RPE) closely interacts with photoreceptors in the maintenance of visual function. Increasing knowledge of the multiple functions performed by the RPE improved the understanding of many diseases leading to blindness. This review summarizes the current knowledge of RPE functions and describes how failure of these functions causes loss of visual function. Mutations in genes that are expressed in the RPE can lead to photoreceptor degeneration. On the other hand, mutations in genes expressed in photoreceptors can lead to degenerations of the RPE. Thus both tissues can be regarded as a functional unit where both interacting partners depend on each other.

'One receptor' rules in sensory neurons

With the recent explosion in the characterization of different sensory systems, a general rule is emerging: only one type of sensory receptor molecule is expressed per receptor neuron. The visual system is no exception and, in most cases, photoreceptors express only one visual pigment per cell. However, the mechanisms underlying the exclusion of sensory receptors are poorly understood. As expression of a given receptor in a given cell is often stochastic, a decision must first be made to express one of the many receptors of the same family (i.e. one particular rhodopsin) and this expression must correlate with the silencing of the other receptors. Furthermore, the projection center for the receptors in the brain must be informed of the decision in order to process this information. Although cells can choose from up to hundreds of sensory receptors (e.g. in the olfactory system), they make almost no mistakes. Evidence has recently emerged that the exclusion mechanism involves the sensory receptor molecules themselves. Here, we describe the findings from various systems in mammals and Drosophila, and review evidence that in the simple visual system of the fly, rhodopsin molecules play an important role in sensory receptor exclusion.

Regulation of phytochrome B nuclear localization through light-dependent unmasking of nuclear-localization signals

Phytochromes are red and far-red photoreceptors that regulate plant growth and development in response to environmental light cues. Phytochromes exist in two photo-interconvertible conformational states: an inactive Pr form and an active Pfr form. The alteration of phytochromes' subcellular location functions as a major regulatory mechanism of their biological activities. Whereas phytochromes in the Pr form localize in the cytoplasm, phytochromes in the Pfr form accumulate in the nucleus, where they interact with transcription factors to regulate gene expression. The molecular details of the regulation of phytochrome translocation by light are poorly understood. Using Arabidopsis phyB as a model, we demonstrate that the C-terminal PAS-related domain (PRD) is both necessary and sufficient for phyB nuclear import and that the entire C terminus is required for nuclear-body (NB) localization. We also show that phyB's N-terminal bilin lyase domain (BLD) and PHY domain interact directly with the PRD in a light-dependent manner. In vivo localization studies indicate that BLD-PHY is sufficient to regulate phyB's nuclear accumulation. For phyB nuclear localization, our results suggest a molecular mechanism in which the nuclear-localization signal in the PRD is masked by interactions with phyB's chromophore-attachment domains and unmasked by light-dependent conformational changes.

Photoreceptors and visual pigments in the retina of the fully anadromous green sturgeon (Acipenser medirostrus) and the potamodromous pallid sturgeon (Scaphirhynchus albus)

Green sturgeon and pallid sturgeon photoreceptors were studied with scanning electron microscopy (SEM), microspectrophotometry and, in the case of the green sturgeon, retinal whole-mounts. The retinas of both species contain both rods and cones: cones comprise between 23% (whole-mount) and 36% (SEM) of the photoreceptors. The cone population of both species is dominated by large single cones, but a rare small single cone is also present. In both species, most rods have long outer segments of large diameter. A rod with a relatively thin outer segment is present in the pallid sturgeon retina. Mean cone packing density for the entire green sturgeon retina is 4,690+/-891 cones/mm2, with the dorsal retina 14% more dense than the ventral. There is evidence for a horizontal visual streak just above and including the optic disc. Mean rod packing density is 16,006+/-1,668 rods/mm2 for the entire retina, and fairly uniform throughout. Both species have rods with peak absorbance near 540 nm, as well as short-wavelength-sensitive cones (green: 464.5+/-0.7 nm; pallid: 439.7+/-3.5 nm); middle-wavelength-sensitive cones (green: 538.0+/-1.4 nm; pallid: 537.0+/-1.7 nm); and long-wavelength-sensitive cones (green: 613.9+/-3.0 nm; pallid: 617.8+/-7.6 nm).

The retinal G protein-coupled receptor (RGR) enhances isomerohydrolase activity independent of light

Rod and cone visual pigments use 11-cis-retinal, a vitamin A derivative, as their chromophore. Light isomerizes 11-cis- into all-trans-retinal, triggering a conformational transition of the opsin molecule that initiates phototransduction. After bleaching all-trans-retinal leaves the opsin, and light sensitivity must be restored by regeneration of 11-cis-retinal. Under bright light conditions the retinal G protein-coupled receptor (RGR) was reported to support this regeneration by acting as a photoisomerase in a proposed photic visual cycle. We analyzed the contribution of RGR to rhodopsin regeneration under different light regimes and show that regeneration, during light exposure and in darkness, is slowed about 3-fold in Rgr(-/-) mice. These findings are not in line with the proposed function of RGR as a photoisomerase. Instead, RGR, independent of light, accelerates the conversion of retinyl esters to 11-cis-retinal by positively modulating isomerohydrolase activity, a key step in the "classical" visual cycle. Furthermore, we find that light accelerates rhodopsin regeneration, independent of RGR.

Highly effective phosphorylation by G protein-coupled receptor kinase 7 of light-activated visual pigment in cones

Cone photoreceptors show briefer photoresponses than rod photoreceptors. Our previous study showed that visual pigment phosphorylation, a quenching mechanism of light-activated visual pigment, is much more rapid in cones than in rods. Here, we measured the early time course of this rapid phosphorylation with good time resolution and directly compared it with the photoresponse time course in cones. At the time of photoresponse recovery, almost two phosphates were incorporated into a bleached cone pigment molecule, which indicated that the visual pigment phosphorylation coincides with the photoresponse recovery. The rapid phosphorylation in cones is attributed to very high activity of visual pigment kinase [G protein-coupled receptor kinase (GRK) 7] in cones. Because of this high activity, cone pigment is readily phosphorylated at very high bleach levels, which probably explains why cone photoresponses recover quickly even after a very bright light and do not saturate under intense background light. The high GRK7 activity is brought about by high content of a highly potent enzyme. The expression level of GRK7 was 10 times higher than that of rod kinase (GRK1), and the specific activity of a single GRK7 molecule was approximately 10 times higher than that of GRK1. The specific activity of GRK7 is the highest among the GRKs so far known. Our result seems to explain the response characteristics of cone photoreceptors in many aspects, including the nonsaturation of the cone responses during daylight vision.

Structure and function of ribbon synapses

Sensory neurons with short conduction distances can use nonregenerative, graded potentials to modulate transmitter release continuously. This mechanism can transmit information at much higher rates than spiking. Graded signaling requires a synapse to sustain high rates of exocytosis for relatively long periods, and this capacity is the special virtue of ribbon synapses. Vesicles tethered to the ribbon provide a pool for sustained release that is typically fivefold greater than the docked pool available for fast release. The current article, which is part of the TINS Synaptic Connectivity series, reviews recent evidence for this fundamental computational strategy and its underlying cell biology.

Identifying photoreceptors in blind eyes caused by RPE65 mutations: Prerequisite for human gene therapy success

Mutations in RPE65, a gene essential to normal operation of the visual (retinoid) cycle, cause the childhood blindness known as Leber congenital amaurosis (LCA). Retinal gene therapy restores vision to blind canine and murine models of LCA. Gene therapy in blind humans with LCA from RPE65 mutations may also have potential for success but only if the retinal photoreceptor layer is intact, as in the early-disease stage-treated animals. Here, we use high-resolution in vivo microscopy to quantify photoreceptor layer thickness in the human disease to define the relationship of retinal structure to vision and determine the potential for gene therapy success. The normally cone photoreceptor-rich central retina and rod-rich regions were studied. Despite severely reduced cone vision, many RPE65-mutant retinas had near-normal central microstructure. Absent rod vision was associated with a detectable but thinned photoreceptor layer. We asked whether abnormally thinned RPE65-mutant retina with photoreceptor loss would respond to treatment. Gene therapy in Rpe65(-/-) mice at advanced-disease stages, a more faithful mimic of the humans we studied, showed success but only in animals with better-preserved photoreceptor structure. The results indicate that identifying and then targeting retinal locations with retained photoreceptors will be a prerequisite for successful gene therapy in humans with RPE65 mutations and in other retinal degenerative disorders now moving from proof-of-concept studies toward clinical trials.

Photoreceptor evolution: ancient siblings serve different tasks

Photoreceptor cells of vertebrate eyes are fundamentally different from those of invertebrate eyes. New work on the brain of a ragworm now suggests that ancestral bilaterians possessed both types of photoreceptor cell.

K+ channels and their modulation by 5-HT in Drosophila photoreceptors: a modelling study

In order to clarify the role of inactivating and noninactivating K+ conductances in nonspiking neurons, we developed an isopotential model of the Drosophila photoreceptor membrane based on Hodgkin-Huxley-type equations. The model includes voltage dependent potassium conductances, the shaker (gKA) and the delayed rectifier (gKs). The model parameters were derived from published results by Hardie and coworkers and nearly identical model was used also in our previous work (J. E. Niven, M. Vähäsöyrinki, M. Kauranen, R. C. Hardie, M. Juusola, and M. Weckström. The Contribution of shaker K+ channels to the information capacity of Drosophila photoreceptors. Nature. 421:630-634, 2003). The model explains how the two types of channels function together to define the voltage dependent properties of the photoreceptor membrane. Additionally the model enables us to run simulations of conditions which are difficult to achieve in patch clamp, like prolonged membrane depolarizations by light adaptation. Effects of the activation of the delayed rectifier type conductance were found to be in accordance with published experimental work but the inactivation of the shaker channels, in addition to its importance in the determination of the resting potential, produced voltage amplification over equivalent passive membrane under dark adapted conditions. This phenomenon was not present in light adapted conditions. The modulation of the voltage dependence of the conductances as reported by serotonin (5-HT) caused the shaker to act essentially like the delayed rectifier conductance.

The ERG of the Beagle Dog: Evidence Associating a Post b-wave Negativity with the Tapetum Lucidum

As previously reported in the literature, the electroretinogram (ERG) of the Beagle dog includes a large post b-wave negativity, the origin of which is not yet established. In the course of our investigations on the electroretinogram in dogs, we examined two Beagle dogs (2 years apart) who had one eye devoid of a Tapetum Lucidum (TL). Photopic (cone-mediated) and scotopic (rod-mediated) ERGs were obtained according to the guidelines for clinical electroretinography in dog. In both dogs the short-latency ERG components (i.e. a- and b-waves) were found to be within the normal range in amplitude, peak time and morphology O.U. However, the large negative component that, in Beagle dogs, normally follow the b-wave was absent from the photopic as well as the scotopic signals obtained from the TL-free eye. Our results thus suggest a possible contribution of the TL to the ERG of Beagle dogs.