The screening effect of the pigment epithelium on the retinal rods in the frog

Microtubule Motor Transport of Organelles in a Specialized Epithelium: The RPE

The retinal pigment epithelium (RPE) is a uniquely polarized epithelium that lies adjacent to the photoreceptor cells in the retina, and is essential for photoreceptor function and viability. Two major motile organelles present in the RPE are the melanosomes, which are important for absorbing stray light, and phagosomes that result from the phagocytosis of the distal tips of the photoreceptor cilium, known as the photoreceptor outer segment (POS). These organelles are transported along microtubules, aligned with the apical-basal axis of the RPE. Although they undergo a directional migration, the organelles exhibit bidirectional movements, indicating both kinesin and dynein motor function in their transport. Apical melanosome localization requires dynein; it has been suggested that kinesin contribution might be complex with the involvement of more than one type of kinesin. POS phagosomes undergo bidirectional movements; roles of both plus- and minus-end directed motors appear to be important in the efficient degradation of phagosomes. This function is directly related to retinal health, with defects in motor proteins, or in the association of the phagosomes with the motors, resulting in retinal degenerative pathologies.

Protein kinase A regulation of pigment granule motility in retinal pigment epithelial cells from fish, Lepomis spp

Retinomotor movements include elongation and contraction of rod and cone photoreceptors, and mass migration of melanin-containing pigment granules (melanosomes) of the retinal pigment epithelium (RPE) within the eyes of fish, frogs, and other lower vertebrates. Eyes of these animals do not contain dilatable pupils; therefore the repositioning of the rods and cones and a moveable curtain of pigment granules serve to modulate light intensity within the eye. RPE from sunfish (Lepomis spp.) can be isolated from the eye and dissociated into single cells, allowing in vitro studies of the cytoskeletal and regulatory mechanisms of organelle movement. Pigment granule aggregation from distal tips of apical projections into the cell body can be triggered by the application of underivatized cAMP, and dispersion is effected by cAMP washout in the presence of dopamine. While the phenomenon of cAMP-dependent pigment granule aggregation in isolated RPE was described many years ago, whether cAMP acts through the canonical cAMP-PKA pathway to stimulate motility has never been demonstrated. Here, we show that pharmacological inhibition of PKA blocks pigment granule aggregation, and microinjection of protein kinase A catalytic subunit triggers pigment granule aggregation. Treatment with a cAMP agonist that activates the Rap GEF, Epac (Effector protein activated by cAMP), had no effect on pigment granule position. Taken together, these results confirm that cAMP activates RPE pigment granule motility by the canonical cAMP-PKA pathway. Isolated RPE cells labeled with antibodies against PKA RIIα and against PKA-phosphorylated serine/threonine amino acids show diffuse, punctate labeling throughout the RPE cell body and apical projections. Immunoblotting of RPE lysates using the anti-PKA substrate antibody demonstrated seven prominent bands; two bands in particular at 27 and 64 kD showed increased levels of phosphorylation in the presence of cAMP, indicating their phosphorylation could contribute to the pigment granule aggregation mechanism.

Microtubule motor transport in the delivery of melanosomes to the actin-rich apical domain of the retinal pigment epithelium

Melanosomes are motile, light-absorbing organelles that are present in pigment cells of the skin and eye. It has been proposed that melanosome localization, in both skin melanocytes and the retinal pigment epithelium (RPE), involves melanosome capture from microtubule motors by an unconventional myosin, which dynamically tethers the melanosomes to actin filaments. Recent studies with melanocytes have questioned this cooperative capture model. Here, we test the model in RPE cells by imaging melanosomes associated with labeled actin filaments and microtubules, and by investigating the roles of different motor proteins. We found that a deficiency in cytoplasmic dynein phenocopies the lack of myosin-7a, in that melanosomes undergo fewer of the slow myosin-7a-dependent movements and are absent from the RPE apical domain. These results indicate that microtubule-based motility is required for the delivery of melanosomes to the actin-rich apical domain and support a capture mechanism that involves both microtubule and actin motors.

The cell biology of the retinal pigment epithelium

The retinal pigment epithelium (RPE), a monolayer of post-mitotic polarized epithelial cells, strategically situated between the photoreceptors and the choroid, is the primary caretaker of photoreceptor health and function. Dysfunction of the RPE underlies many inherited and acquired diseases that cause permanent blindness. Decades of research have yielded valuable insight into the cell biology of the RPE. In recent years, new technologies such as live-cell imaging have resulted in major advancement in our understanding of areas such as the daily phagocytosis and clearance of photoreceptor outer segment tips, autophagy, endolysosome function, and the metabolic interplay between the RPE and photoreceptors. In this review, we aim to integrate these studies with an emphasis on appropriate models and techniques to investigate RPE cell biology and metabolism, and discuss how RPE cell biology informs our understanding of retinal disease.

Orientation of actin filaments in teleost retinal pigment epithelial cells, and the effect of the lectin, Concanavalin A, on melanosome motility

Retinal pigment epithelial cells of teleosts contain numerous melanosomes (pigment granules) that exhibit light-dependent motility. In light, melanosomes disperse out of the retinal pigment epithelium (RPE) cell body (CB) into long apical projections that interdigitate with rod photoreceptors, thus shielding the photoreceptors from bleaching. In darkness, melanosomes aggregate through the apical projections back into the CB. Previous research has demonstrated that melanosome motility in the RPE CB requires microtubules, but in the RPE apical projections, actin filaments are necessary and sufficient for motility. We used myosin S1 labeling and platinum replica shadowing of dissociated RPE cells to determine actin filament polarity in apical projections. Actin filament bundles within RPE apical projections are uniformly oriented with barbed ends toward the distal tips. Treatment of RPE cells with the tetravalent lectin, Concanavalin A, which has been shown to suppress cortical actin flow by crosslinking of cell-surface proteins, inhibited melanosome aggregation and stimulated ectopic filopodia formation but did not block melanosome dispersion. The polarity orientation of F-actin in apical projections suggests that a barbed-end directed myosin motor could effect dispersion of melanosomes from the CB into apical projections. Inhibition of aggregation, but not dispersion, by ConA confirms that different actin-dependent mechanisms control these two processes and suggests that melanosome aggregation is sensitive to treatments previously shown to disrupt actin cortical flow.

Carbachol-mediated pigment granule dispersion in retinal pigment epithelium requires Ca2+ and calcineurin

Background

Inside bluegill (Lepomis macrochirus) retinal pigment epithelial cells, pigment granules move in response to extracellular signals. During the process of aggregation, pigment motility is directed toward the cell nucleus; in dispersion, pigment is directed away from the nucleus and into long apical processes. A number of different chemicals have been found to initiate dispersion, and carbachol (an acetylcholine analog) is one example. Previous research indicates that the carbachol-receptor interaction activates a G_q-mediated pathway which is commonly linked to Ca²⁺ mobilization. The purpose of the present study was to test for involvement of calcium and to probe calcium-dependent mediators to reveal their role in carbachol-mediated dispersion.

Results

Carbachol-induced pigment granule dispersion was blocked by the calcium chelator BAPTA. In contrast, the calcium channel antagonist verapamil, and incubation in Ca²⁺-free medium failed to block carbachol-induced dispersion. The calcineurin inhibitor cypermethrin blocked carbachol-induced dispersion; whereas, two protein kinase C inhibitors (staurosporine and bisindolylmaleimide II) failed to block carbachol-induced dispersion, and the protein kinase C activator phorbol 12-myristate 13-acetate failed to elicit dispersion.

Conclusion

A rise in intracellular calcium is necessary for carbachol-induced dispersion; however, the Ca²⁺ requirement is not dependent on extracellular sources, implying that intracellular stores are sufficient to enable pigment granule dispersion to occur. Calcineurin is a likely Ca²⁺-dependent mediator involved in the signal cascade. Although the pathway leads to the generation of diacylglycerol and calcium (both required for the activation of certain PKC isoforms), our evidence does not support a significant role for PKC.

Myosin II and Rho kinase activity are required for melanosome aggregation in fish retinal pigment epithelial cells

In the retinal pigment epithelium (RPE) of fish, melanosomes (pigment granules) migrate long distances through the cell body into apical projections in the light, and aggregate back into the cell body in the dark. RPE cells can be isolated from the eye, dissociated, and cultured as single cells in vitro. Treatment of isolated RPE cells with cAMP or the phosphatase inhibitor, okadaic acid (OA), stimulates melanosome aggregation, while cAMP or OA washout in the presence of dopamine triggers dispersion. Previous studies have shown that actin filaments are both necessary and sufficient for aggregation and dispersion of melanosomes within apical projections of isolated RPE. The role of myosin II in melanosome motility was investigated using the myosin II inhibitor, blebbistatin, and a specific rho kinase (ROCK) inhibitor, H-1152. Blebbistatin and H-1152 partially blocked melanosome aggregation triggered by cAMP in dissociated, isolated RPE cells and isolated sheets of RPE. In contrast, neither drug affected melanosome dispersion. In cells exposed to either blebbistatin or H-1152, then triggered to aggregate using OA, melanosome aggregation was completely inhibited. These results demonstrate that (1) melanosome aggregation and dispersion occur through different, actin-dependent mechanisms; (2) myosin II and ROCK activity are required for full melanosome aggregation, but not dispersion; (3) partial aggregation that occurred despite myosin II or ROCK inhibition suggests a second component of aggregation that is dependent on cAMP signaling, but independent of ROCK and myosin II.

Analysis of the linkage of MYRIP and MYO7A to melanosomes by RAB27A in retinal pigment epithelial cells

The apical region of the retinal pigment epithelium (RPE) typically contains melanosomes. Their apical distribution is dependent on RAB27A and the unconventional myosin, MYO7A. Evidence from studies using in vitro binding assays, melanocyte transfection, and immunolocalization have indicated that the exophilin, MYRIP, links RAB27A on melanosomes to MYO7A, analogous to the manner that melanophilin links RAB27A on melanocyte melanosomes to MYO5A. To test the functionality of this hypothesis in RPE cells, we have examined the relationship among MYRIP, RAB27A and MYO7A with studies of RPE cells in primary culture (including live-cell imaging), analyses of mutant mouse retinas, and RPE cell fractionation experiments. Our results indicate that the retinal distribution of MYRIP is limited to the RPE, mainly the apical region. In RPE cells, RAB27A, MYRIP, and MYO7A were all associated with melanosomes, undergoing both slow and rapid movements. Analyses of mutant mice provide genetic evidence that MYRIP is linked to melanosomes via RAB27A, but show that recruitment of MYRIP to apical RPE is independent of melanosomes and RAB27A. RAB27A and MYRIP also associated with motile small vesicles of unknown origin. The present results provide evidence from live RPE cells that the RAB27A-MYRIP-MYO7A complex functions in melanosome motility. They also demonstrate that RAB27A provides an essential link to the melanosome.

Shroom2 (APXL) regulates melanosome biogenesis and localization in the retinal pigment epithelium

Shroom family proteins have been implicated in the control of the actin cytoskeleton, but so far only a single family member has been studied in the context of developing embryos. Here, we show that the Shroom-family protein,Shroom2 (previously known as APXL) is both necessary and sufficient to govern the localization of pigment granules at the apical surface of epithelial cells. In Xenopus embryos that lack Shroom2 function, we observed defects in pigmentation of the eye that stem from failure of melanosomes to mature and to associate with the apical cell surface. Ectopic expression of Shroom2 in naïve epithelial cells facilitates apical pigment accumulation, and this activity specifically requires the Rab27a GTPase. Most interestingly, we find that Shroom2, like Shroom3 (previously called Shroom),is sufficient to induce a dramatic apical accumulation of the microtubule-nucleating protein γ-tubulin at the apical surfaces of naïve epithelial cells. Together, our data identify Shroom2 as a central regulator of RPE pigmentation, and suggest that, despite their diverse biological roles, Shroom family proteins share a common activity. Finally,because the locus encoding human SHROOM2 lies within the critical region for two distinct forms of ocular albinism, it is possible that SHROOM2mutations may be a contributing factor in these human visual system disorders.

Actin-dependent motility of melanosomes from fish retinal pigment epithelial (RPE) cells investigated using in vitro motility assays

Melanosomes (pigment granules) within retinal pigment epithelial (RPE) cells of fish and amphibians undergo massive migrations in response to light conditions to control light flux to the retina. Previous research has shown that melanosome motility within apical projections of dissociated fish RPE cells requires an intact actin cytoskeleton, but the mechanisms and motors involved in melanosome transport in RPE have not been identified. Two in vitro motility assays, the Nitella assay and the sliding filament assay, were used to characterize actin-dependent motor activity of RPE melanosomes. Melanosomes applied to dissected filets of the Characean alga, Nitella, moved along actin cables at a mean rate of 2 microm/min, similar to the rate of melanosome motility in dissociated, cultured RPE cells. Path lengths of motile melanosomes ranged from 9 to 37 microm. Melanosome motility in the sliding filament assay was much more variable, ranging from 0.4-33 microm/min; 70% of velocities ranged from 1-15 microm/min. Latex beads coated with skeletal muscle myosin II and added to Nitella filets moved in the same direction as RPE melanosomes, indicating that the motility is barbed-end directed. Immunoblotting using antibodies against myosin VIIa and rab27a revealed that both proteins are enriched on melanosome membranes, suggesting that they could play a role in melanosome transport within apical projections of fish RPE.

Role of myosin VIIa and Rab27a in the motility and localization of RPE melanosomes

Myosin VIIa functions in the outer retina, and loss of this function causes human blindness in Usher syndrome type 1B (USH1B). In mice with mutant Myo7a, melanosomes in the retinal pigmented epithelium (RPE) are distributed abnormally. In this investigation we detected many proteins in RPE cells that could potentially participate in melanosome transport, but of those tested, only myosin VIIa and Rab27a were found to be required for normal distribution. Two other expressed proteins, melanophilin and myosin Va, both of which are required for normal melanosome distribution in melanocytes, were not required in RPE, despite the association of myosin Va with the RPE melanosome fraction. Both myosin VIIa and myosin Va were immunodetected broadly in sections of the RPE, overlapping with a region of apical filamentous actin. Some 70-80% of the myosin VIIa in RPE cells was detected on melanosome membranes by both subcellular fractionation of RPE cells and quantitative immunoelectron microscopy, consistent with a role for myosin VIIa in melanosome motility. Time-lapse microscopy of melanosomes in primary cultures of mouse RPE cells demonstrated that the melanosomes move in a saltatory manner, interrupting slow movements with short bursts of rapid movement (>1 RR01183m/second). In RPE cells from Myo7a-null mice, both the slow and rapid movements still occurred, except that more melanosomes underwent rapid movements, and each movement extended approximately five times longer (and further). Hence, our studies demonstrate the presence of many potential effectors of melanosome motility and localization in the RPE, with a specific requirement for Rab27a and myosin VIIa, which function by transporting and constraining melanosomes within a region of filamentous actin. The presence of two distinct melanosome velocities in both control and Myo7a-null RPE cells suggests the involvement of at least two motors other than myosin VIIa in melanosome motility, most probably, a microtubule motor and myosin Va.

Actin-dependent, retrograde motility of surface-attached beads and aggregating pigment granules in dissociated teleost retinal pigment epithelial cells

Teleost retinal pigment epithelial (RPE) cells contain pigment granules within apical projections which undergo actin-dependent, bi-directional motility. Dissociated RPE cells in culture attach to the substrate and extend apical projections in a radial array from the central cell body. Pigment granules within projections can be triggered to aggregate or disperse by the presence or absence of 1 mM cAMP. Aminated, fluorescent latex beads attached to the dorsal surface of apical projections and moved in the retrograde direction, towards the cell body. Bead rates on RPE cells with aggregating or fully aggregated pigment granules were 2.2 +/- 0.5 and 2.6 +/- 0.2 microm/min (mean +/- SEM), respectively, similar to rates of aggregating (retrograde) pigment granule movement (2.0 +/- 0.4 microm/min). Bead rates were slightly slower on cells with fully dispersed or dispersing pigment granules (1.5 +/- 0.1 and 1.5 +/- 0.4 microm/min). Movements of surface-attached beads and aggregating pigment granules were closely correlated in the distal portions of apical projections, but were more independent of each other in proximal regions of the projections. The actin disrupting drug, cytochalasin D (CD), reversibly halted retrograde bead movements, suggesting that motility of surface-attached particles is actin-dependent. In contrast, the microtubule depolymerizing drug, nocodazole, had no effect on retrograde bead motility. The similar characteristics and actin-dependence of retrograde bead movements and aggregating pigment granules suggest a correlation between these two processes.

Bidirectional pigment granule migration in isolated retinal pigment epithelial cells requires actin but not microtubules

In the teleost retinal pigment epithelium (RPE), melanin pigment granules disperse into long apical projections in the light and reaggregate into the cell body in the dark. To investigate the cytoskeletal mechanisms responsible for these movements, we have examined the effects of cytoskeletal inhibitors on pigment granule transport in cultured, dissociated RPE cells using time-lapse video microscopy. The kinetics of pigment granule transport during normal aggregation and dispersion are quite distinct: during aggregation, all pigment granules undergo simultaneous, nonsaltatory centripetal movement (mean velocity 3.6 microm/min); during dispersion, individual granules undergo independent, bidirectional saltatations (mean velocities 3.7 microm/min centrifugal; 1.1 microm/min centripetal). Nocodazole disruption of microtubules within the RPE apical projections had little effect on the kinetics of pigment granule movement, and essentially no effect on extent of pigment granule aggregation or dispersion, or on maintenance of the fully aggregated or fully dispersed states. In contrast, cytochalasin D (CD) treatment blocked net aggregation and dispersion of pigment granules, and compromised maintenance of the fully aggregated and dispersed states. These observations suggest that the actin cytoskeleton plays an important role in both centripetal and centrifugal transport of pigment granules in teleost RPE cells.

Calcium-independent regulation of pigment granule aggregation and dispersion in teleost retinal pigment epithelial cells

In the eyes of teleosts and amphibians, melanin pigment granules of the retinal pigment epithelium (RPE) migrate in response to changes in light conditions. In the light, pigment granules disperse into the cells' long apical projections, thereby shielding the rod photoreceptor outer segments and reducing their extent of bleach. In darkness, pigment granules aggregate towards the base of the RPE cells. In vitro, RPE pigment granule aggregation can be induced by application of nonderivatized cAMP, and pigment granule dispersion can be induced by cAMP washout. In previous studies based on RPE-retina co-cultures, extracellular calcium was found to influence pigment granule migration. To examine the role of calcium in regulation of RPE pigment granule migration in the absence of retinal influences, we have used isolated RPE sheets and dissociated, cultured RPE cells. Under these conditions depletion of extracellular or intracellular calcium ([Ca2+]o, [Ca2+]i) had no effect on RPE pigment granule aggregation or dispersion. Using the intracellular calcium dye fura-2 and a new dye, fura-pe3, to monitor calcium dynamics in isolated RPE cells, we found that [Ca2+]i did not change from basal levels when pigment granule aggregation was triggered by cAMP, or dispersion was triggered by cAMP washout. Also, no change in [Ca2+]i was detected when dispersion was triggered by cAMP washout in the presence of 10 microM dopamine, a treatment previously shown to enhance dispersion. In addition, elevation of [Ca2+]i by addition of ionomycin neither triggered pigment movements, nor interfered with pigment granule motility elicited by cAMP addition or washout. Since other studies have indicated that actin plays a role in both pigment granule dispersion and aggregation in RPE, our findings suggest that RPE pigment granule migration depends on an actin-based motility system that is not directly regulated by calcium.

Changing the light intensity of the visual environment results in large differences in numbers of synapses and in photoreceptor size in the retina of the young adult rat

A quantitative light- and electron-microscopic study has been made of the retinae of rats which were exposed to different lighting conditions for between one and 15 weeks in young adulthood, having been reared in identical conditions during development. The width of the inner and outer segments of the photoreceptors and the width of the outer plexiform layer varied inversely with the light intensity under diurnal lighting conditions of 10 h light/14 h dark. Linear regression analysis showed that the widths were inversely related to the fourth root of the light intensity as measured in lux. Both central and peripheral areas of retina showed a similar change. No change was seen in the widths of the inner plexiform layer, or of the inner and outer nuclear cell layers. Nor was there a difference in the packing density or size of the nuclei in the nuclear cell layers. The number of ribbon synapses in the outer plexiform layer also varied inversely with the intensity of diurnal light. Linear regression analysis showed that the number of synapses was inversely correlated with the fourth root of the light intensity and was positively correlated with the width of the outer plexiform layer. The number of ribbon synapses was increased by up to two and a half times in constant darkness compared to diurnal light of 35 lux. The increase was present but not maximal after one week of exposure. The length of synaptic ribbons was unchanged. The nerve terminals forming such synapses were increased in size but not in number. After one week, there was little or no additional change in the retinal widths and number of synaptic ribbons with time. However, there was a progressive increase with time in nerve terminal size (two-fold in area) in constant darkness. There was some evidence of a slight decrease in nerve terminal number and increase in size of retinal nuclei with age. It is concluded that the adult retina responds to a different lighting environment by a relatively rapid change in the size of photoreceptor segments, by a progressive and large change in number of ribbon synapses and by a slower progressive and large change in the size of photoreceptor nerve terminals. The response is quantitatively determined by the strength of the stimulus but not in a linear fashion. These results are compared with the effects of environmental stimulation of other areas of the nervous system.

Transient sensitivity reduction and biphasic photoresponses observed when frog retinal rods are oxidized

1. Rod photoresponses and the effects of oxidation have been studied by recording either the transretinal voltage in aspartate-treated retinas or the outer segment current of single rods. 2. Oxidizing conditions transiently decreased, reducing conditions increased sensitivity. 3. Biphasic photoresponses were seen when the level of oxidation was rising and also in some other sensitivity-depressing conditions. 4. A model is proposed which explains the biphasic responses in terms of sensitivity differences between the tip and the base of the rod outer segment.

The bleaching and regeneration of rhodopsin in the cat

1. The processes of bleaching and regeneration were monitored by retinal densitometry in living cats.2. Neither bleaching nor regeneration of rhodopsin can be described by the simple kinetic equation (Alpern, 1971) found valid for man.3. After a strong 1 min bleach, the retina contains more unbleached rhodopsin than expected on the basis of the initial bleaching rate.4. During the first 9 min after a 1 min bleach, cats regenerate rhodopsin only slowly; density changes during this period are dominated by formation and decay of metarhodopsin III. Subsequently, rhodopsin regeneration accelerates to a rate of 50%/11 min.5. No such delay precedes recovery from a prolonged (20 min) bleach.

Studies on the eyes of New Zealand parrot-fishes (Labridae)

An investigation was made of the pigment epithelium and photoreceptor layer of the eyes of New Zealand parrot-fishes Pesudolabrus miles , P. celidotus and P. pittensis (Labridae). Eyes were studied by light and electron microscopy, and characteristics of the retinal pigments were investigated. The pigment epithelium contains numerous melanosomes in the basal region of the cell and red cylinders within the processes. Melanosomes and cylinders are dispersed slightly vitread during light- adaptation. The photoreceptors comprise long single cones, short single cones, double cones and rods. The cones possess a lateral sac connected to the outer segment, and well-developed calycal processes arising from the ellipsoid. Rods are without a lateral sac and have fewer and less regularly arranged calycal processes. The membranes of rod lamellae stacks are more widely spaced than those of cones and are deeply dissected by a few longitudinal fissures. Cones form a regular mosaic of squares throughout the depth of the retina scleral to the external limiting membrane. Rods are numerous and distributed throughout the cone mosaic with no particular patterns. Cones undergo only very limited radial movements, and long single cones are always buried in the pigment epithelium where they are surrounded by red pigment. Rods shorten in darkness and come to lie inside the pigment epithelium. The red material in the cylinders is apparently a new pigment; it absorbs strongly below 560 nm, and prevents all but red light from reaching the long single cones and the rods when extended. Several features and consequences of this organization are discussed.

Visual adaptation in the retina of the skate

The electroretinogram (ERG) and single-unit ganglion cell activity were recorded from the eyecup of the skate (Raja erinacea and R. oscellata), and the adaptation properties of both types of response compared with in situ rhodopsin measurements obtained by fundus reflectometry. Under all conditions tested, the b-wave of the ERG and the ganglion cell discharge showed identical adaptation properties. For example, after flash adaptation that bleached 80% of the rhodopsin, neither ganglion cell nor b-wave activity could be elicited for 10-15 min. Following this unresponsive period, thresholds fell rapidly; by 20 min after the flash, sensitivity was within 3 log units of the dark-adapted level. Further recovery of threshold was slow, requiring an additional 70-90 min to reach absolute threshold. Measurements of rhodopsin levels showed a close correlation with the slow recovery of threshold that occurred between 20 and 120 min of dark adaptation; there is a linear relation between rhodopsin concentration and log threshold. Other experiments dealt with the initial unresponsive period induced by light adaptation. The duration of this unresponsive period depended on the brightness of the adapting field; with bright backgrounds, suppression of retinal activity lasted 20-25 min, but sensitivity subsequently returned and thresholds fell to a steady-state value. At all background levels tested, increment thresholds were linearly related to background luminance.

Visual adaptation of the rhodopsin rods in the frogs retina

1. The threshold of the discharge from single ganglion cells in the excised and opened frog's eye has been measured with on/off stimuli and test parameters that make it possible to activate the rhodopsin rods only. The test stimuli have been restricted to the central part of the receptive field, where no nervous reorganization can be observed with changes in the state of adaptation.2. When such thresholds and the intensities of the background lights are expressed in terms of the number of quanta absorbed per unit time, it is found that three factors can be correlated with the thresholds measured in various states of light- and dark-adaptation: (i) the intensity of a steady background, (ii) the rate of regeneration of rhodopsin, and (iii) the amount of metarhodopsin II present in the rods.3. The threshold is found to be proportional both to the intensity of a background and to the rate of regeneration, whereas there is a linear relationship between the logarithm of the threshold and the amount of metarhodopsin II.4. The presence of metarhodopsin elevates all thresholds, the absolute threshold, increment thresholds and the thresholds elevated by regenerating rhodopsin in the same way.5. The saturation of the rods at high background intensities is found to be correlated with the accumulation of significant amounts of metarhodopsin in the rods, caused by the bleaching effect of the background.6. The effect of metarhodopsin on the threshold is independent of the amount of rhodopsin present in the rods.7. The combined effect of all three factors can be expressed in a general formula, given as eqn. (7) on p. 74.8. A background not only reduces the signals from the rods illuminated, but also those from neighbouring unilluminated rods. This effect is rapidly decreased with increasing distance from rods covered by the background. This kind of lateral spread in the retina probably occurs also when the rate of regeneration affects the threshold. The effect of metarhodopsin, on the other hand, appears restricted to those receptors that contain this substance.