Light adaptation in the rat retina: evidence for two receptor mechanisms

Rods progressively escape saturation to drive visual responses in daylight conditions

Rod and cone photoreceptors support vision across large light intensity ranges. Rods, active under dim illumination, are thought to saturate at higher (photopic) irradiances. The extent of rod saturation is not well defined; some studies report rod activity well into the photopic range. Using electrophysiological recordings from retina and dorsal lateral geniculate nucleus of cone-deficient and visually intact mice, we describe stimulus and physiological factors that influence photopic rod-driven responses. We find that rod contrast sensitivity is initially strongly reduced at high irradiances, but progressively recovers to allow responses to moderate contrast stimuli. Surprisingly, rods recover faster at higher light levels. A model of rod phototransduction suggests that phototransduction gain adjustments and bleaching adaptation underlie rod recovery. Consistently, exogenous chromophore reduces rod responses at bright background. Thus, bleaching adaptation renders mouse rods responsive to modest contrast at any irradiance. Paradoxically, raising irradiance across the photopic range increases the robustness of rod responses.

Rod photoreceptors are thought to be saturated under bright light. Here, the authors describe the physiological parameters that mediate response saturation of rod photoreceptors in mouse retina, and show that rods can drive visual responses in photopic conditions.

Light color importance for circadian entrainment in a diurnal (Octodon degus) and a nocturnal (Rattus norvegicus) rodent

The central circadian pacemaker (Suprachiasmatic Nuclei, SCN) maintains the phase relationship with the external world thanks to the light/dark cycle. Light intensity, spectra, and timing are important for SCN synchronisation. Exposure to blue-light at night leads to circadian misalignment that could be avoided by using less circadian-disruptive wavelengths. This study tests the capacity of a diurnal Octodon degus and nocturnal Rattus norvegicus to synchronise to different nocturnal lights. Animals were subjected to combined red-green-blue lights (RGB) during the day and to: darkness; red light (R); combined red-green LED (RG) lights; and combined red-green-violet LED (RGV) lights during the night. Activity rhythms free-ran in rats under a RGB:RG cycle and became arrhythmic under RGB:RGV. Degus remained synchronised, despite the fact that day and night-time lighting systems differed only in spectra, but not in intensity. For degus SCN c-Fos activation by light was stronger with RGB-light than with RGV. This could be relevant for developing lighting that reduces the disruptive effects of nocturnal light in humans, without compromising chromaticity.

Spectral Sensitivity Measured with Electroretinogram Using a Constant Response Method

A new method is presented to determine the retinal spectral sensitivity function S(λ) using the electroretinogram (ERG). S(λ)s were assessed in three different species of myomorph rodents, Gerbils (Meriones unguiculatus), Wistar rats (Ratus norvegicus), and mice (Mus musculus). The method, called AC Constant Method, is based on a computerized automatic feedback system that adjusts light intensity to maintain a constant-response amplitude to a flickering stimulus throughout the spectrum, as it is scanned from 300 to 700 nm, and back. The results are presented as the reciprocal of the intensity at each wavelength required to maintain a constant peak to peak response amplitude. The resulting S(λ) had two peaks in all three rodent species, corresponding to ultraviolet and M cones, respectively: 359 nm and 511 nm for mice, 362 nm and 493 nm for gerbils, and 362 nm and 502 nm for rats. Results for mouse and gerbil were similar to literature reports of S(λ) functions obtained with other methods, confirming that the ERG associated to the AC Constant-Response Method was effective to obtain reliable S(λ) functions. In addition, due to its fast data collection time, the AC Constant Response Method has the advantage of keeping the eye in a constant light adapted state.

Circadian and light-driven regulation of rod dark adaptation

Continuous visual perception and the dark adaptation of vertebrate photoreceptors after bright light exposure require recycling of their visual chromophore through a series of reactions in the retinal pigmented epithelium (RPE visual cycle). Light-driven chromophore consumption by photoreceptors is greater in daytime vs. nighttime, suggesting that correspondingly higher activity of the visual cycle may be required. However, as rod photoreceptors are saturated in bright light, the continuous turnover of their chromophore by the visual cycle throughout the day would not contribute to vision. Whether the recycling of chromophore that drives rod dark adaptation is regulated by the circadian clock and light exposure is unknown. Here, we demonstrate that mouse rod dark adaptation is slower during the day or after light pre-exposure. This surprising daytime suppression of the RPE visual cycle was accompanied by light-driven reduction in expression of Rpe65, a key enzyme of the RPE visual cycle. Notably, only rods in melatonin-proficient mice were affected by this daily visual cycle modulation. Our results demonstrate that the circadian clock and light exposure regulate the recycling of chromophore in the RPE visual cycle. This daily melatonin-driven modulation of rod dark adaptation could potentially protect the retina from light-induced damage during the day.

Regulation of mammalian cone phototransduction by recoverin and rhodopsin kinase

Cone photoreceptors function under daylight conditions and are essential for color perception and vision with high temporal and spatial resolution. A remarkable feature of cones is that, unlike rods, they remain responsive in bright light. In rods, light triggers a decline in intracellular calcium, which exerts a well studied negative feedback on phototransduction that includes calcium-dependent inhibition of rhodopsin kinase (GRK1) by recoverin. Rods and cones share the same isoforms of recoverin and GRK1, and photoactivation also triggers a calcium decline in cones. However, the molecular mechanisms by which calcium exerts negative feedback on cone phototransduction through recoverin and GRK1 are not well understood. Here, we examined this question using mice expressing various levels of GRK1 or lacking recoverin. We show that although GRK1 is required for the timely inactivation of mouse cone photoresponse, gradually increasing its expression progressively delays the cone response recovery. This surprising result is in contrast with the known effect of increasing GRK1 expression in rods. Notably, the kinetics of cone responses converge and become independent of GRK1 levels for flashes activating more than ∼1% of cone pigment. Thus, mouse cone response recovery in bright light is independent of pigment phosphorylation and likely reflects the spontaneous decay of photoactivated visual pigment. We also find that recoverin potentiates the sensitivity of cones in dim light conditions but does not contribute to their capacity to function in bright light.

Retino-hypothalamic regulation of light-induced murine sleep

The temporal organization of sleep is regulated by an interaction between the circadian clock and homeostatic processes. Light indirectly modulates sleep through its ability to phase shift and entrain the circadian clock. Light can also exert a direct, circadian-independent effect on sleep. For example, acute exposure to light promotes sleep in nocturnal animals and wake in diurnal animals. The mechanisms whereby light directly influences sleep and arousal are not well understood. In this review, we discuss the direct effect of light on sleep at the level of the retina and hypothalamus in rodents. We review murine data from recent publications showing the roles of rod-, cone- and melanopsin-based photoreception on the initiation and maintenance of light-induced sleep. We also present hypotheses about hypothalamic mechanisms that have been advanced to explain the acute control of sleep by light. Specifically, we review recent studies assessing the roles of the ventrolateral preoptic area (VLPO) and the suprachiasmatic nucleus (SCN). We also discuss how light might differentially promote sleep and arousal in nocturnal and diurnal animals respectively. Lastly, we suggest new avenues for research on this topic which is still in its early stages.

Rod and cone visual pigments and phototransduction through pharmacological, genetic, and physiological approaches

Activation of the visual pigment by light in rod and cone photoreceptors initiates our visual perception. As a result, the signaling properties of visual pigments, consisting of a protein, opsin, and a chromophore, 11-cis-retinal, play a key role in shaping the light responses of photoreceptors. The combination of pharmacological, physiological, and genetic tools has been a powerful approach advancing our understanding of the interactions between opsin and chromophore and how they affect the function of visual pigments. The signaling properties of the visual pigments modulate many aspects of the function of rods and cones, producing their unique physiological properties.

Evidence for learned skill during cocaine self-administration in rats

RATIONALE

It has been proposed that cocaine abuse results in skilled or "automatic" drug-taking behaviors. Brain regions important for skill learning are implicated in cocaine self-administration. However, the development of skill during self-administration has not been investigated.

OBJECTIVES

The present experiment investigated the development of skilled self-administration over extended drug use by employing a novel operant vertical head movement under discriminative stimulus (S(D)) control. In addition, the capacity of the head movement to serve as an operant was tested by manipulating drug levels above or below satiety drug levels via frequent noncontingent microinfusions (0.2 s) of cocaine.

RESULTS

Animals acquired the vertical head movement operant, which increased in number over days. Task learning was demonstrated by reduced reaction time in response to the S(D), increased propensity to self-administer upon S(D) presentation, and escalated drug consumption over days. Skill learning was demonstrated by (1) an increase over days in the velocity of operant movements, as a function of shorter duration but not altered distance, and (2) an increase over days in the probability of initiating the operant at the optimal starting position. Evidence that responding was specific to self-administration was revealed during periods of experimenter-manipulated drug level: maintaining drug levels above satiety decreased responding while maintaining drug levels below satiety increased responding.

CONCLUSIONS

Under the specific set of circumstances tested herein, cocaine self-administration became skilled over extended drug use. The vertical head movement can be used as an operant comparable to lever pressing with the additional benefit of quantifying skill learning.

Role of guanylyl cyclase modulation in mouse cone phototransduction

A negative phototransduction feedback in rods and cones is critical for the timely termination of their light responses and for extending their function to a wide range of light intensities. The calcium feedback mechanisms that modulate phototransduction in rods have been studied extensively. However, the corresponding modulation mechanisms that enable cones to terminate rapidly their light responses and to adapt in bright light, properties critical for our daytime vision, are still not understood. In cones, calcium feedback to guanylyl cyclase is potentially a key step in phototransduction modulation. The guanylyl cyclase activity is modulated by the calcium-binding guanylyl cyclase activating proteins (GCAP1 and GCAP2). Here, we used single-cell and transretinal recordings from mouse to determine how GCAPs modulate dark-adapted responses as well as light adaptation in mammalian cones. Deletion of GCAPs increased threefold the amplitude and dramatically prolonged the light responses in dark-adapted mouse cones. It also reduced the operating range of mouse cones in background illumination and severely impaired their light adaptation. Thus, GCAPs exert powerful modulation on the mammalian cone phototransduction cascade and play an important role in setting the functional properties of cones in darkness and during light adaptation. Surprisingly, despite their better adaptation capacity and wider calcium dynamic range, mammalian cones were modulated by GCAPs to a lesser extent than mammalian rods. We conclude that a disparity in the strength of GCAP modulation cannot explain the differences in the dark-adapted properties or in the operating ranges of mammalian rods and cones.

Contrast responses to bright slits of visual cells in the superior colliculus of the albino rat

Contrast is the most effective stimulus in the visual system. The response of single cells to changes in stimulus contrast has been studied in a large variety of animals and the contrast response function determined. In the rat, studies on responses to contrast have been focused primarily in the geniculocortical pathway and there are relatively few in subcortical structures. We report here for the first time the contrast response function of single units located in the superior colliculus (SC) of the albino rat to several stimulus contrast. Cells in the SC require a relatively high contrast to elicit a reliable response and the dynamic response range is restricted to a short contrast interval.

The reflection of retinal light response information onto the superior colliculus in the rat

BACKGROUND

The functional principles of mediation of retina-encoded visual information through the optic nerve to the superior colliculus (SC) of the contralateral brain hemisphere were investigated in non-drugged and unrestrained albino rats by considering the following issues: (1) the type of information transmitted, (2) the response components of the retina and SC involved in encoding the transmitted information, and (3) the timing of related processes.

METHODS

The field potential responses for different intensities of flashes, under different background illuminations, were simultaneously recorded from the sclera area of the eye and the optic layer of the contralateral SC.

RESULTS

It was found that the b-wave crest of the retinal electroretinogram (ERG) and the peak-1 or peak-2 of the SC correlate by their amplitude, while the a-wave trough of the retinal ERG and the peak-1 of the SC correlate by their latency. The values of these mutually correlating response components were invariably determined by the given light response bias of the retina (photoreceptors), the change in the photon flux of the light stimulus and, obviously, the change in the wavelength of the light stimulus. The a-wave trough, peak-1, b-wave crest and peak-2 were invariably induced in this time-order.

CONCLUSIONS

The data suggest that the information properties of (a) intensity, (b) presentation time and, obviously, (c) colour of the light stimulus, such as are shed on the retina, and information about the light response bias of the retina are mediated correlatively and quantitatively to the cell network system of the SC through the optic nerve. These processes must happen during the a-to-b-wave phases of the ERG. The data indicate that the random-type variations in the activity of the related cellular systems may actually be harnessed in mediating the defined information properties of the visual stimulus from the retina to the SC of the brain through the optic nerve. This study shows a method of measuring the function of the optic nerve.

Evidence that L-AP5 and D,L-AP4 can preferentially block cone signals in the rat retina

Several lines of evidence suggest that, as concentrations of two agonists of group III metabotropic glutamate receptors are increased, cone contributions to the b-wave are blocked before rod contributions. Application of L-AP5 (L-2-amino-5-phosphonobutyric acid) at concentrations of 50 microM and D,L-AP4 (D,L-2-amino-4-phosphonobutyric acid) at concentrations 2 microM had a greater effect in reducing the amplitude of the rat ERG b-wave at high light intensities than at low light intensities. The amplitude reduction occurs at flash intensities that saturate rod photoreceptor responses. When steady backgrounds are used to saturate rod photoreceptors, the b-wave responses show increased long-wavelength sensitivity. Responses on a rod saturating background are blocked by adding L-AP5 or AP4 at the above concentrations to the perfusate. Further evidence for metabotrophic receptors being involved comes from the observation that even when ionotropic glutamate receptors are pharmacologically blocked with MK801 and DNQX, AP4 selectively blocks cone contributions to the b-wave. Thus we suggest that the type III metabotrophic receptors on depolarizing cone bipolar cells or cone synaptic terminals are affected by concentrations of L-AP5 and D,L-AP4 that have minimal effects on rod bipolar cells or rod synaptic terminals.

Flicker assessment of rod and cone function in a model of retinal degeneration

Critical flicker frequency (CFF) is the lowest frequency for which a flickering light is indistinguishable from a non-flickering light of the same mean luminance. CFF is related to light intensity, with cone photoreceptors capable of achieving higher CFF than rods. A contemporaneous measure of rod and cone function can facilitate characterization of a retinal degeneration. We used sinusoidal flicker ERG to obtain CFF values, over a wide range of light intensities, in RCS dystrophic (RCS-p(+)) and wild type rats. Recordings were made at PN23, PN44, and PN64. The CFF curve in control animals increased in proportion to the log of stimulus intensity, with a gentle slope over the lowest 4 log-unit intensity range. The slope of the CFF curve dramatically increased for higher intensities, indicating a rod-cone break. In the RCS rats the rod driven CFF was significantly lower in amplitude compared to normal rats at the earliest age tested (PN23). By PN64 the rod driven CFF was immeasurable in the RCS rats. The amplitude of the cone driven CFF approached normal values at PN23, but was greatly reduced by PN44. By PN64 the entire CFF function was greatly depressed and there was no longer a discernable rod-cone break. These CFF/ERG data show that RCS rats exhibit significant early degeneration of the rods, followed soon after by degeneration of the cones. Using this approach, rod and cone function can be independently accessed using flicker ERG by testing at a few select intensities.

Behavioral visual responses of wild-type and hypopigmented zebrafish

Zebrafish possess three classes of chromatophores that include iridophores, melanophores, and xanthophores. Mutations that lack one or two classes of chromatophores have been isolated or genetically constructed. Using a behavioral assay based on visually mediated escape responses, we measured the visual response of fully and partially pigmented zebrafish. In zebrafish that lack iridophores (roy mutants), the behavioral visual responses were similar to those of wild-type animals except at low contrast stimulation. In the absence of melanophores (albino mutants) or both melanophores and iridophores (ruby mutants), the behavioral visual responses were normal under moderate illumination but reduced when tested under dim or bright conditions or under low contrast stimulation. Together, the data suggest that screening pigments in the retina play a role in the regulation of behavioral visual responses and are necessary for avoiding "scatter" under bright light conditions.

Background light adaptation of the retinal neuronal adaptive system. I. Effect of background light intensity

The behaviour of the neuronal adaptive retinal mechanisms to environmental light exposures was studied by measuring the oscillatory potentials (OPs) of the electroretinogram. Dark adapted rats were exposed to four levels of background light (BG), starting at a 'low scotopic' level of 1.43x 10(6) cd/m2, increased by steps of two log units, through 'high scotopic' -, 'low mesopic' - and finally the 'high mesopic' BG of 1.43x 10(0) cd/m2. The summed oscillatory response significantly increased as the BG intensity was raised, except at the 'high mesopic' level. The amplitudes of the a- and b-waves reduced as the BG light increased above the 'high scotopic' level. Each OP responded individually to the different BGs. O1 and O2, significantly enhanced at the 'low scotopic' BG. The amplitudes of the three later OPs increased significantly at the 'low mesopic' BG. The adaptational behaviour of the retinal oscillatory response to BG illumination was different to that of the a- and b- waves. The results indicate that the adaptational neuronal system, as reflected by the OPs, seems to be relatively robust and is separate from the slower photochemical adaptive process in the distal retina. The tentative corollary suggests the oscillatory system to play a vision-preserving role, possibly as an alert against undue depletion of the slowly regenerating visual pigment. The enhancement of the oscillatory response at the 'mesopic' illumination levels indicate both scotopic and photopic processes to contribute to neuronal adaptive activity of the retina.

Immunocytochemical identification of cone bipolar cells in the rat retina

We studied the morphology of bipolar cells in fixed vertical tissue sections of the rat retina by injecting the cells with Lucifer Yellow and neurobiotin. In addition to the rod bipolar cell, nine different putative cone bipolar cell types were distinguished according to the position of their somata in the inner nuclear layer and the branching pattern and stratification level of their axon terminals in the inner plexiform layer. Some of these bipolar cell populations were labeled immunocytochemically in vertical and horizontal sections using antibodies against the calcium-binding protein recoverin, the glutamate transporter GLT-1, the alpha isoform of the protein kinase C, and the Purkinje cell marker L7. These immunocytochemically labeled cell types were characterized in terms of cell density and distribution. We found that rod bipolar cells and GLT-1-positive cone bipolar cells occur at higher densities in a small region located in the upper central retina. This area probably corresponds to the central area, which is the region of highest ganglion cell density. A second peak of rod bipolar cell density in the lower temporal periphery matches the retinal area of binocular overlap. The population densities of the immunocytochemically characterized bipolar cells indicate that at least 50% of all bipolar cells are cone bipolar cells. The variety and total number of cone bipolar cells is surprising because the retina of the rat contains 99% rods. Our findings suggest that cone bipolar cells may play a more important role in the visual system of the rat than previously thought.

Behavioral estimates of absolute threshold in rat

Dark-adapted thresholds of albino and pigmented rats were estimated using behavioral methods. Albino and pigmented rats who had been water deprived learned to bar press for water reinforcement when a light stimulus was presented. Absolute threshold was defined to be the light intensity at which bar pressing behavior was significantly modified by the presence of the light stimulus. Albino rats had an average threshold of -5.23 log cd/m2 and the pigmented rats had a threshold of -5.0 log cd/m2. These values are close to -5.3 log cd/m2, the psychophysical threshold of human observers in the same apparatus. Consistent with our earlier electrophysiology, these behavioral experiments provide no evidence for an albino/pigmented sensitivity difference. Comparisons are made between behavioral and electrophysiological determinations of absolute threshold in albino and pigmented rats. Thresholds determined behaviorally agree remarkably well with those derived from visual evoked potentials.

Elevated dark-adapted thresholds in hypopigmented mice measured with a water maze screening apparatus

In previous electrophysiological experiments from hypopigmented animals (mice, rats, rabbits), single-unit recordings from both retinal ganglion axons and cells in the superior colliculus have demonstrated an increase in threshold in the dark-adapted state which is roughly proportional to the ocular melanin concentration. In the present study we compared an albino mouse strain which is relatively resistant to light damage and the beige mouse mutant to their wild-type controls in a situation that involved unanesthetized, unrestrained mice as a control to the electrophysiological single unit experiments. We used a six-chambered water maze. Animals were trained to swim to an illuminated ramp until their performances leveled off (about 10 days). The animals were then dark-adapted for 24 h and tested after reducing the luminance level of the water maze. We found that the albino mice failed to find the ramp when the luminance fell to 1.58 x 10(-3) cd/m2 (p < or = .0001), the beige mice failed at 2.00 x 10(-4) cd/m2 (p < or = .0001), and the normally pigmented controls performed to 5.00 x 10(-5) cd/m2 (p < or = .0001). These results support our previous findings that the sensitivity defect in hypopigmented animals is proportional to the degree of ocular hypopigmentation.

Two cone types of rat retina detected by anti-visual pigment antibodies

The presence of two distinct cone types was demonstrated in the retina of the rat using two cone-specific monoclonal anti-visual pigment antibodies. Cones labelled by antibody COS-1 constituted the large majority (about 93%) of cones, and are most probably responsible for the green photopic sensitivity of the rat. About 7% of the cones were recognized by antibody OS-2, and are thought to be blue-sensitive elements. While OS-2 positive cones were evenly distributed throughout the retina, there were slight differences in the distribution of COS-1 positive cones. The cones made up about 0.85% of all photoreceptor cells. Although the OS-2 positive cones occur in a very low number (0.05% of all photoreceptors) and probably do not appreciably contribute to the photopic system of the rat, their presence in the rat strengthens the presumption that most mammalian species exhibit a dual cone system with a shortwave and a middle-to-longwave sensitivity.

The postnatal development of the oscillatory potentials of the electroretinogram. IV. Mesopic characteristics

The postnatal development of the oscillatory potentials (OP) of the rat's electroretinogram (ERG) was studied during relatively mesopic adaptational conditions. The behaviour of the OPs induced at an interstimulus interval (ISI) of 30 sec entailed photopic qualities at an individual level, but when summated conformed to a scotopic course of postnatal maturation. An ISI of 1 min elicited OPs with individual scotopic characteristics. The total oscillatory activity culminated at 17 days of age at both ISIs, similar to more scotopically induced oscillations. These results show that relatively mesopic states of adaptation do not promote any earlier appearance of the OPs, and that the immature mesopically induced OPs seem to contain both scotopic and photopic qualities reflecting a postnatal change of balance between these components. Furthermore, photopic mechanisms seem to govern the oscillatory activity in the mature rat retina. We suggest that the differences in relation to adaptational conditions between the immature and mature OPs may reflect the postnatal status of the dopamine and its synthetizing enzyme, tyrosine-hydroxylase.

The postnatal development of the oscillatory potentials of the electroretinogram. III. Scotopic characteristics

The postnatal development of the oscillatory potentials (OPs) of the rat electroretinogram (ERG) was studied during more extreme scotopic conditions. Enhancement of scotopic conditions did not facilitate any earlier appearance of the OPs, including the later ones, compared to previously studied less scotopic conditions. The oscillatory activity appeared at Days 12 to 15, and increased rapidly up to Day 17, which coincided with the major period of development of the photoreceptors. After the physiological opening of the eyelids there was a decline of the OPs. We propose that the decline of the oscillatory activity induced during more extreme scotopic conditions is related to early cell death in the distal retina and/or to developmental neuronal plasticity in the proximal retina.

Impaired visual thresholds in hypopigmented animals

Ocular hypopigmentation is associated with neurological defects in structure and function. This paper investigates the absolute visual thresholds in dark-adapted hypopigmented animals compared to their normally pigmented controls. Here we asked (1) whether the threshold elevation found in hypopigmented animals is a general consequence of the reduction in melanin content; (2) if so, which melanin components in the eye are likely to influence visual thresholds; and (3) whether similar threshold defects can be detected in orders other than rodents. By single-unit recordings from the superior colliculus, we compared incremental thresholds of normal black mice of the C57BL/6J strain to hypopigmented mutants: beige (bg/bg), pale ear (ep/ep), and albino (c2J/c2J) mice, three mutants in which melanin pigment throughout the body is affected; and Steel (Sl/Sld) and dominant-spotting/W-mice (W/Wv), two mutants with normal pigmentation in the retinal pigment epithelium (RPE) but without any melanin in the choroid or the rest of the body. We found that all mutants had elevated thresholds that varied with the reduction in melanin. The albinos were 25 times less sensitive than black mice, pale ear mice 20 times, beige mice 11 times, and Steel and W-mice 5 times. The mean thresholds of dark-adapted black mice were 0.008 cd/m2. Recordings from rabbits showed a similar impairment of visual sensitivity; incremental thresholds were elevated 40 times in New Zealand-White albino rabbits (0.0008 cd/m2) compared to Dutch-Belted pigmented controls (0.00002 cd/m2).(ABSTRACT TRUNCATED AT 250 WORDS)

Light adaptation in retinal rods of the rabbit and two other nonprimate mammals

The responses of rabbit rods to light were studied by drawing a single rod outer segment projecting from a small piece of retina into a glass pipette to record membrane current. The bath solution around the cells was maintained at near 40 degrees C. Light flashes evoked transient outward currents that saturated at up to approximately 20 pA. One absorbed photon produced a response of approximately 0.8 pA at peak. At the rising phase of the flash response, the relation between response amplitude and flash intensity (IF) had the exponential form 1-e-kappa FIF (where kappa F is a constant denoting sensitivity) expected from the absence of light adaptation. At the response peak, however, the amplitude-intensity relation fell slightly below the exponential form. At times after the response peak, the deviation was progressively more substantial. Light steps evoked responses that rose to a transient peak and rapidly relaxed to a lower plateau level. The response-intensity relation again indicated that light adaptation was insignificant at the early rising phase of the response, but became progressively more prominent at the transient peak and the steady plateau of the response. Incremental flashes superposed on a steady light of increasing intensity evoked responses that had a progressively shorter time-to-peak and faster relaxation, another sign of light adaptation. The flash sensitivity changed according to the Weber-Fechner relation (i.e., inversely) with background light intensity. We conclude that rabbit rods adapt to light in a manner similar to rods in cold-blooded vertebrates. Similar observations were made on cattle and rat rods.

Structural features and adaptive properties of photoreceptors in the skate retina

In the duplex (rod/cone) retinae of most vertebrates, electrical responses associated with nocturnal vision can be recorded in relative isolation only over a limited intensity range before there is encroachment by signals from the cone system; e.g., at mesopic levels of illumination, the rods begin to saturate, and the cone mechanism is brought into play. On the other hand, the retinae of some sub-species of skate appear to contain only one class of photoreceptors, namely rods. Nevertheless, skate photoreceptors, as well as other retinal neurons, are able to respond to incremental stimuli presented on background fields so intense that 90% or more of the available rhodopsin is bleached during the exposure. These and other findings raise some doubts as to whether the skate has, in fact, an all-rod retina. In this paper, we present a body of evidence--based on the results of photochemical, anatomical, and electrophysiological studies--to support the view that only a single class of photoreceptor subserves vision in the skate retina. In addition, recent findings will be described that: a) demonstrate how the visual cells transform from sluggishly responding rods to brisk, cone-like elements, and b) may provide some insight into the functional significance of this unusual adaptive property.

Action spectra of the lateral eyes recorded from mammalian pineal glands

Single neuronal units from the pineal stalk and the pineal body of hamsters, guinea pigs and rats were recorded during photic stimulation of the lateral eyes in order to identify the retinal photoreceptor that mediates the environmental control of the mammalian pineal. Two cell types could be distinguished: one type was characterized by spontaneous spike discharges that were irresponsive to light stimulation of the eyes and the pineal body; the other, also spontaneously active, responded to flash stimulation of the lateral eye with On- and Off-discharges. With increasing light intensity, the spike frequency of the second response type followed a sigmoidal function up to a saturation level. Spectral sensitivity curves of all dark-adapted animals peaked at 500 nm. During light adaptation (18 microW/cm2) action spectra exhibited an additional maximum in the red (560 nm, rats and hamsters) and in the blue (450 nm, guinea pigs) light, respectively. Chromatic adaptation to orange light diminished the sensitivity at longer wavelengths, whereas adaptation to blue-green light enhanced the sensitivities at longer wavelengths. Thus, the spectral sensitivity recorded from pineal units of hamsters, guinea pigs and rats corresponds to those described for retinal ganglion cells, which indicates that both rods and cones contribute to the light-sensitivity of the mammalian pineal gland. Direct illumination of the pineal gland did not influence the activity of pineal units.

Retinal degenerations in the dog: IV. Early retinal degeneration (erd) in Norwegian elkhounds

A new early onset hereditary retinal degeneration is characterized in Norwegian elkhound dogs. This disease, termed early retinal degeneration (erd), was studied in 10 affected dogs, from 30 days- to 7 years old, clinically, by electroretinography, and by light- and electron-microscopic morphology. Control studies were performed on 49 non-affected dogs. Affected dogs are initially nightblind, and become totally blind between 12- and 18 months of age. The postnatal development of their rod and cone photoreceptors is abnormal both structurally and functionally. Morphologically, rod and cone inner- and outer-segment growth occurs but appears uncoordinated. Adjacent rods become very disparate in the size and proportions of their inner- and outer segments. Prominent villiform processes extend from the inner segments of rods and, to a lesser extent, cones. Synaptic terminals of rods and cones fail to develop properly. The b-wave of the electroretinogram fails to develop and the electroretinogram (ERG) remains a-wave-dominated. Subsequent to these abnormalities of development, the rods and cones degenerate, rapidly at first and later more gradually. In normal dogs, development of the ERG a- and b-waves is shown to follow, respectively, morphologic development of the photoreceptor outer segments and synaptic terminals. Similarly the abnormal development and subsequent degeneration of photoreceptor outer segments and synaptic terminals in affected dogs, correspond in time course to development and degeneration of the ERG a- and b-waves.

"Self-screening" of rhodopsin in rod outer segments

Microspectrophotometry (MSP) shows rhodopsin highly concentrated (about 3.0 mmol/l) in rod outer segments (ROS). Calculation of the in vivo absorption spectrum of human rhodopsin from such data reveals a striking failure to agree with the action spectrum of human rod vision. Agreement is good between the spectral distribution of absorption coefficients and the action spectrum, but the "concentration-broadening" (or "self-screening") introduced by the high end on absorbance at this concentration results in a misfit among the largest in the 93 years comparisons of this kind have been made! To deal with this anomaly, it has been suggested that "concentration-broadening" is inappropriate for rhodopsin in rod vision. This proposal was tested by comparing rod action spectra of 15-day-old and adult rats, since the lengths of ROS increase by a factor of about two in maturation. Three lines of evidence are inconsistent with it. Although the conundrum remains unexplained, it cannot be dismissed by supposing "self-screening" inappropriate for night vision.

Flicker detection in the albino rat following light-induced retinal damage

The effect of light-induced retinal damage on the behaving rat's critical flicker-fusion frequency (CFF) was studied by determining the CFF at scotopic and photopic luminances both before and after exposure to damaging light. The CFF was reduced but not abolished following damaging light exposure. The shapes of the functions relating CFF to luminance before and after exposure suggested that scotopic visual function may have survived the light damage better than did photopic function. Anatomical and biochemical measures of retinal damage indicated that 91-93% of the outer nuclear layer (ONL) and 99% of the rodopsin had been lost.

Rod saturation in b-wave of the rat electroretinogram under two different anesthetics

B-wave increment threshold experiments in the rat show that "rod saturation" occurs at different background levels with different anesthetics. Rod saturation builds up over the first 60 sec of light adaptation in pentobarbital anesthetized but not in urethane anesthetized animals. These and other findings suggest that "rod saturation" can occur when the rod photoreceptors themselves are not saturated.

Effects of constant bright illumination on reproductive processes in the female rat

Physiological and behavioral reproductive changes in the female rat which occur under constant bright illumination (LL) are examined. The development of LL-induced persistent estrus (PE) is discussed first in relation to other conditions in which PE is displayed. Next, mechanisms are reviewed which may account for the LL-induced changes. These include: (1) role of the retina, the retinohypothalamic tract and the suprachiasmatic nucleus; (2) influence of adrenal, pineal and Harderian glands; and (3) disruptions in either the 4-day endocrine rhythms or circadian neural component of the estrous cycle. Additional topics which are examined include the ontogeny of age-induced PE and the effects of LL on hormone receptor binding, puberty, sexual receptivity and mating.

Mechanisms of light adaptation in rat retina

Green, Dowling, Siegel and Ripps (1975) J. gen. Physiol. 65, 483-502 found that both receptors and post-receptoral elements regulate the process of light adaptation in skate retina. As a test of the generality of this conclusion, we repeated their experiments on the retina of the intact albino rat. Increment threshold and intensity-response functions of aspartate-isolated receptor potentials, ERG b-waves and single retinal ganglion cell axons were measured, and all were found to be similar to those described by Green et al. (1975). In particular, the adaptive properties of the b-wave and ganglion cells were similar to each other, but different from those of the receptor potential. We conclude that the retinal mechanisms of light adaptation are similar in rat and skate.

Light adaptation within the receptive field centre of rat retinal ganglion cells

1. Responses from axons of single retinal ganglion cells in the rat's optic tract were used to measure the pooling of adaptive signals within the cells' receptive field. Computer-aided analyses of response measurements were used to evaluate sensitivity at a number of field locations. 2. A small adapting spot caused a localized decrease in sensitivity within the receptive field centre of ON- and OFF-centre ganglion cells. 2. The functions describing response versus test luminance were similar in shape for all test and adaptation configurations. This assured that, using a fixed criterion response, sensitivity determinations could be made just as well in any receptive field location and under any of the experimental conditions. 4. A concentric surround, antagonistic to the receptive field centre, was readily apparent only under conditions of light adaptation. Experiments on the local effects of small adapting spots, conducted with selective surround adaptation, showed that the non-uniform spread of adaptation within the receptive field centre was not linked to surround intrusion. 5. The possibility that the photopic mechanism intruded to contaminate these results was considered and rejected. 6. When a suprathreshold spot was alternated between two equally sensitive positions, the ganglion cell gave an approximately balanced response. An upset of this balance was produced by placing a small adapting spot at either position, thus demonstrating, in another way, the non-uniform spread of adaptation within the receptive field centre. 7. It is concluded that significant pooling of adaptation effects occurs prior to the combination of influences which contribute to the centre response of a ganglion cell.

Rods and cones in the mouse retina. I. Structural analysis using light and electron microscopy

Rods and cones of the C57BL/6J mouse retina have been examined by light and electron microscopy to distinguish the structural features of the two photoreceptor types. By light microscopy, cone nuclei are conspicuously different from rod nuclei in 1-2 micrometer plastic sections. Cone nuclei have an irregularly shaped clump of heterochromatin that appears in single sections to be one to three clumps, whereas rod nuclei are more densely stained and have one large, central clump of heterochromatin. Cone nuclei make up approximately 3% of the photoreceptor nuclei in both the central and peripheral retina at all ages examined up to 267 days. Cone nuclei are confined to the outer half of the outer nuclear layer, and more than 50% of the cone nuclei lie adjacent to the outer limiting membrane. By electron microscopy, cones in the mouse retina meet virtually every morphological criterion of mammalian cones. The outer segments are conically shaped. Many, if not all of the outer segment discs are continuous with the outer plasma membrane, whereas almost all of the rod discs are not. Cone outer segments are only about half the length of the rod outer segments, and they are contacted by long, villous pigment epithelial cell processes. The cone inner segment diameter is greater than the outer segment diameter, and the accumulation of mitochondria present at the apical end of the inner segment forms a more conspicuous ellipsoid than in rods. The internal fiber or axon of the cone is larger in diameter than that of the rod, and it terminates in a large synaptic pedicle with multiple ribbon synapses, whereas the rod terminal is a smaller spherule with only a single ribbon synaptic complex.

Suprachiasmatic nucleus neurones: excitation and inhibition mediated by the direct retino-hypothalamic projection in female rats

The suprachiasmatic nucleus (SCN) of female rats was surveyed with microelectrodes under urethane anaesthesia. In rats with bilateral transection of the optic tracts, repetitive three pulses of 100 Hz applied to the contralateral optic nerve excited 8 and inhibited 11 other of the 86 SCN units examined. Transection of the optic tract did not significantly influence frequency of occurrence of the SCN units that were excited or inhibited by stimulation of the optic nerve. Certain SCN units responded to both of contralateral and ipsilateral stimulations of the optic nerve, indicating that bilateral visual inputs converge on the same single SCN neurones. Oscillatory responses with a period of 100--200 msec were occasionally produced by stimulation of the optic nerve. Flash stimuli with relatively weak intensity, even insufficient for producing wavelets in electroretinograms, produced an excitation and inhibition in SCN units. The mean firing rates were significantly altered by either electrical or flash stimuli repeated 500 times at 0.97 Hz in those units which showed no transitory response. Some of the SCN neurones receiving visual inputs were identified to be the tuberoinfundibular neurone and some other SCN neurones were found to receive converging inputs both from the optic nerve and from the axon collaterals of tuberoinfundibular neurones.

Dark-adaptation in abnormal (RCS) rats studied electroretinographically

1. Electroretinogram (e.r.g.) responses recorded from dark-reared rats with inherited retinal dystrophy (RCS) showed progressive decline in b-wave ampliture and prolongation of the time to the peak of the b-wave with age when compared with records obtained from dark-reared normal albino rats. 2. Dark-adaptation was followed in RCS and normal rats by recording the light intensity needed to evoke a criterion e.r.g. response at different time intervals after bleaching and 90% of the rhodopsin. 3. In normal rats, dark-adaptation was governed by two mechanisms. The first 25--35 min of recovery was determined by cones. The second branch, determined by the recovery of rods, lasted for about 3 hr and proceeded along an exponential time course with time constant of 41.4 +/- 2.4 min (S.E. of mean). 4. In RCS rats, the time course of the dark-adaptation after a 90% bleach depended on age. In 25--30 day old rats the recovery curve had at least three breaks separating three different mechanisms. Rats, 35--40 days old, exhibited double exponential recovery curves, while 45--70 day old rats recovered along a single exponential curve similar in time course to the cone branch of dark-adaptation found in normal rats. 5. Action spectra obtained from RCS rats at different time intervals of the recovery curve showed that in young rats, 25--30 days old, small e.r.g. responses recorded before bleaching and at the end of the recovery period were determined by rhodopsin while those recorded during the first part of the recovery from 90% bleach were determined by a combination of rods and cones. In RCS rats of advanced age (45--70 days old), rhodopsin was the major contributor to the e.r.g. responses recorded either before bleaching or at the end of the recovery period. 6. The gradual deterioration with age of the e.r.g. in RCS rats cannot be explained by either the decrease in quantum catch due to the decrease in rhodopsin content or by the linear relationship between log e.r.g. threshold and pigment concentration. 7. Using estimates of rhodopsin density within surviving rods obtained from retinal densitometry, it was shown that in RCS rats where more than 30% of normal levels of rhodopsin was located within the functioning rods, the log intensity needed for a criterion e.r.g. response measured at the end of the recovery period from a 90% bleach was linearly related to the fraction of 'functional' rhodopsin. 8. No simple relationship between log e.r.g. threshold and rhodopsin concentration could be found during the course of recovery in the dark from a strong bleaching exposure in RCS rats of all ages.

The damaging effects of light on the retina. Empirical findings, theoretical and practical implications

Light well below the intensity which causes thermal burns physiologically damages the retina. This damage is primarily localized in the receptors. The outer segments are most sensitive and slow recovery is possible if damage does not proceed to destruction of the inner segment. Many variables affect the extent and severity of light damage. Damage is correlated with continuity of source, light intensity, elevated body temperature, nocturnality, and albinism. Light damage has been considered only minimally in visual research with light preferences, reinforcement and discrimination, or in clinical settings. Based on the available evidence, it is suggested that retinal damage may be produced by such common light sources as room lighting, phototherapy techniques, ophthalmoscopes and fundus cameras. Further studies are recommended.

Rat model for hereditary retinal degeneration

The disorder in Wag/Rij rats is a spontaneous, bilateral retinal deneration. It is characterized by an early onset, slowly progressive degeneration of the photoreceptor cells leading to destruction of the retina. Degeneration affects both rod cells and cone cells, and to a lesser degree the cells in the inner nuclear layer. The remarkable alterations in the retinal pigment epithelium during the course of the disease suggest a profound change in metabolism and function of the pigment epithelium and implicate a possibility of interaction between the pigment epithelium and the photoreceptors. Since degenerated cells have also been observed in the inner nuclear layers, there is a possibility that Muller's cells are involved in the retinal degeneration. Controlled experiments have demonstrated that the disease is not induced by light damaging effects of the retina, and initial breeding experiments suggest that the disease is inheritable, probably as an autosomal dominant trait. The retinal degeneration in Wag/Rij rats is a new, unique system and it is a potentially very useful animal model of retinitis pigmentosa.

Cones survive rods in the light-damaged eye of the albino rat

Exposure to constant light causes extensive rod photoreceptor damage but spares the photopic system in albino rats. The rod branch of the dark-adaptation curve shows considerable elevation in threshold; the cone branch is hardly affected. Longer exposure and chromatic adaptation suggest that there are three cone mechanisms with peaks near wavelengths of 450,520, and 560 nanometers.

Properties of visual cells in the lateral eye of Limulus in situ

Excitatory properties of visual cells in the lateral eye of Limulus, investigated by optic nerve recordings in situ, differ significantly from the properties of cells in the classical, excised eye preparation. The differences suggest the possibility that two receptor mechanisms function in the eye in situ: one mechanism encodes low light intensities and the other responds to high intensities. The two mechanisms enable each ommatidium to respond over an intensity range of approximately 10 log units. This hypothesis was tested by measuring the increment threshold and the spectral sensitivity, by studying light and dark adaptation, and by analyzing the variability of the impulse discharge. Although the results do not conclusively identify two receptor mechanisms, they indicate that a process or a part of a process that functions in the eye in situ is abolished by excising the eye or cutting off its blood supply.

Action spectra and adaptation properties of carp photoreceptors

The mass photoreceptor response of the isolated carp retina was studied after immersing the tissue in aspartate-Ringer solution. Two electro-retinogram components were isolated by differential depth recording: a fast cornea-negative wave, arising in the receptor layer, and a slow, cornea-negative wave arising at some level proximal to the photoreceptors. Only the fast component was investigated further. In complete dark adaptation, its action spectrum peaked near 540 nm and indicated input from both porphyropsin-containing rods (lambda(max) approximately 525 nm) and cones with longer wavelength sensitivity. Under photopic conditions a broad action spectrum, lambda(max) approximately 580 nm was seen. In the presence of chromatic backgrounds, the photopic curve could be fractionated into three components whose action spectra agreed reasonably well with the spectral characteristics of blue, green, and red cone pigments of the goldfish. In parallel studies, the carp rod pigment was studied in situ by transmission densitometry. The reduction in optical density after a full bleach averaged 0.28 at its lambda(max) 525 nm. In the isolated retina no regeneration of rod pigment occurred within 2 h after bleaching. The bleaching power of background fields used in adaptation experiments was determined directly. Both rods and cones generated increment threshold functions with slopes of +1 on log-log coordinates over a 3-4 log range of background intensities. Background fields which bleached less than 0.5% rod pigment nevertheless diminished photoreceptor sensitivity. The degree and rate of recovery of receptor sensitivity after exposure to a background field was a function of the total flux (I x t) of the field. Rod saturation, i.e. the abolition of rod voltages, occurred after approximately 12% of rod pigment was bleached. In light-adapted retinas bathed in normal Ringer solution, a small test flash elicited a larger response in the presence of an annular background field than when it fell upon a dark retina. The enhancement was not observed in aspartate-treated retinas.

Adaptation in skate photoreceptors

Receptor potentials were recorded extracellularly from the all-rod retina of the skate after the application of sodium aspartate. This agent suppresses the responses of proximal elements, but leaves relatively unaffected the electrical activity of the photoreceptors (a-wave) and pigment epithelium (c-wave). Since the latter develops too slowly to interfere with the receptor response, it was possible to isolate receptor potentials and to compare their behavior in light and dark adaptation with earlier observations on the S-potential, b-wave, and ganglion cell discharge. The results show that the photoreceptors display the full complement of adaptational changes exhibited by cells proximal to the receptors. Thus, it appears that visual adaptation in the skate is governed primarily by the photoreceptors themselves. Of particular interest was the recovery of sensitivity in the presence of background fields that initially saturate the receptor potential. Analysis of this recovery phase indicates that a gain-control mechanism operates within the receptors, at a distal stage of the visual process.