Photocurrents of single retinal rods from Rana pipiens tadpoles

Outer segment membrane current was recorded from single rod photoreceptors of Taylor-Kollros State III and Stage XXIII tadpoles. Response-intensity relations and kinetics of the photocurrent were similar to those of audit amphibian photoreceptors, suggesting that phototransduction is quantitatively similar in developing rods and adult rods. Maximum response amplitude was about 1/3 that of adult rods, probably because of the shorter length of the outer segment in tadpoles (20 vs 50 micrometers).

The connections of the retinal on and off pathways to the lateral geniculate nucleus of the monkey

This study examined the manner in which the On and Off systems originating in the retina make their connections with the lateral geniculate nucleus (LGN) of the rhesus monkey. 2-amino-4-phosphonobutyric acid (APB) was either infused or injected into the vitreous chamber of the eye to block the On channel while the response properties of LGN cells were assessed. APB in concentrations of 50-600 microns effectively blocked On-center cell responses in the LGN. The responses of Off-center cells were not affected significantly, although some cells showed a brief period of response increment following APB infusion while other cells showed a small response decrement. The sustained component of the response of On-center cells was blocked more effectively, more quickly and at lower concentrations than was the transient component of the response. Parvocellular cells were blocked at lower concentrations and for longer time periods than were magnocellular cells. Both the center and the surround responses of On-center cells were blocked by APB. The center and the surround responses of Off-center cells were not affected differentially by APB administration. Blue On-center responses were also blocked by APB. These results suggest that in the monkey the On and Off channels originating the retina remain functionally separated at the level of the LGN.

Absorptance and spectral sensitivity measurements of rod photoreceptors of the tiger salamander, Ambystoma tigrinum

The spectral sensitivity of the extracellularly-recorded photoresponse of isolated rods of the tiger salamander, Ambystoma tigrinum, was compared to the absorptance spectrum. Both measurements were made with the same optical system on the same portion of each cell to avoid errors that could occur when the two kinds of measurement were made under different conditions. The relative spectral sensitivity and absorptance spectrum were found to be in excellent agreement between 450 and 700 nm.

Single anion channels of frog rod plasma membrane

Anion channels have been found in the plasma membrane of the outer and inner segment of the isolated retinal rod by means of the patch voltage-clamp technique. The permeability of the channels for different anions follows a sequence: Cl- greater than F- greater than NO3- greater than propionate; the channel conductance in the fully open state is 200 +/- 30 pS measured in 108 mM NaCl. The non-linear character of the current-voltage relationship at membrane potentials from -40 to -20 mV suggests that these anion channels may be involved in receptor potential generation through an electrogenic mechanism.

Rod-cone interaction in flicker detection

There is considerable evidence in the literature that rod-cone interaction occurs when both rods and cones simultaneously detect a test target. More recent evidence, however, has shown a parafoveal rod-cone interaction during dark adaptation for a purely cone-detected flickering test stimulus; this influence on cone threshold appears to be mediated by surrounding rods. In this study, we demonstrate a similar rod-mediated influence on parafoveal cone-detected flicker threshold. More surprisingly, foveal cone-detected thresholds are also influenced by rods. This effect occurs over at least a 2 log unit intensity range of mesopic background level; cone-detected 25 Hz flicker sensitivity is enhanced by increasing the radiance of the background. The action spectrum of this effect fits the scotopic spectral sensitivity curve. At higher background levels, this rod-cone interaction disappears and surrounding cone activity then influences the cone flicker threshold. The results suggest that, as rods recover sensitivity, they reduce cone-detected flicker sensitivity, even at the fovea. The rod influence on cone flicker is most apparent for long wavelength test stimuli. Our results, in agreement with recent reports, suggest that the rod-cone interaction is laterally-mediated and may be specific for the long wavelength-sensitive cone type.

Spatial information capacity of eyes: roles of Na+ channels and photon noise in photoreceptor

The spatial information capacity of the human eye for photopic vision has been determined taking into account the intensity response function of the photoreceptor. It has been found that spatial information capacity increases with the mean luminance upto a certain value of mean luminance and after that it starts decreasing. The decrement occurs below the damage threshold. These results are in agreement with the reported experimental observations. It has been concluded that the limited number of Na+ channels in the photoreceptor outer segment and the photon noise are responsible for the fall in the information capacity below the damage threshold.

Responses of synaptic ribbons in pineal photoreceptors under normal and experimental lighting conditions

The size of synaptic ribbons (SR) in photoreceptor cells of the goldfish pineal organ was quantified over 24-h light:dark cycles of long (16:8) and short (10:14) photoperiods during summer and winter months, respectively. The amplitude of both rhythms was similar with peak values occurring toward the latter part of the photophase or early dark. When fish were entrained to the long photoperiod and exposed to continual light, SR size continued to increase during the expected dark time. The effect of extending the photoperiod into the expected dark time was diminished when fish were entrained to a short photoperiod and presented with 6 h of darkness at the end of the 24-h period. The size increase in response to environmental lighting is believed to reflect a greater demand for either vesicle attachment sites or neurotransmitter storage sites since vesicles (neurotransmitter) have been hypothesized to accumulate in the synaptic pedicles during inhibition by light. From a comparative standpoint it is noteworthy that synaptic ribbons (vesicle-crowned rods) in mammals react in a similar manner to both normal and experimental lighting conditions.

Intracellular dynamic response characteristics of pineal photoreceptors

The dynamic properties of single pineal photoreceptors of Lampetra japonica were measured intracellularly by sinusoidal modulated light stimuli. According to the Bode plots the response amplitude and phase characteristic of pineal photoreceptors could be simulated by a linear model with 4th-order transfer function. A similarity to the receptor response of the vertebrate lateral eye was observed.

Cyclic renewal of whole pineal photoreceptor outer segments

In the light-sensitive epiphysis cerebri of adult frogs (Rana esculenta) adapted in summer to an artificial long day (L:D = 17:7; L = 90 lx at 2,300 degrees K; controlled temperature of 8 degrees C) outer segments of cone-like pineal photoreceptor cells exhibit a cyclic and complete renewal. Shortly after 'lights off' inner segments protruding into the pineal lumen bear exclusively bulbous cilia of the sensory type. Considering these results, concepts dealing with different types of sensory pinealocytes have to be re-evaluated.

Spatial properties of red-green and yellow-blue perceptual opponent-color response

Opponent color responses for an equal illuminance spectrum were measured for field size from 10' to 2 degrees, by means of a hue cancellation procedure. Results showed that when the field diameter was increased, the red and yellow response relatively increased and the green and blue response relatively decreased. There existed a different spatial property between the red-green and yellow-blue opponent-color response function. The results were compared with the optical density hypothesis of the cone visual pigments and with the neurophysiologically obtained receptive field properties of opponent-color cells.

Amacrine cells in scotopic vision

Signals from rod bipolar cells of cat retina are processed by a variety of rod amacrine cells before finally arriving at ganglion cells. Three of these rod amacrine cells (AII, A13, and A17 ) have been studied at the physiological and anatomical levels; the results suggest that each carries out a unique visual function: AII cells appear to quicken the response time of the rod system in the mid-scotopic range, while A17 cells may increase the light-gathering area of rod bipolars near visual threshold. Stimulation of A13 cells may disinhibit ganglion cells, thus heightening their responsiveness at low levels of illumination.

Image sampling properties of photoreceptors: a reply to Miller and Bernard

Miller and Bernard argue that photoreceptor sampling occurs at the inner rather than outer segments. Foveal inner segments form a lattice-like array that should create visible Moiré patterns when frequencies above 60 c/deg are image by interferometry. Despite a checkered past, this prediction is confirmed by recent experiments. Extrafoveally, frequencies above the nominal Nyquist limits of the cones are routinely present in the retinal image. Spectral analysis shows that the inner segments there form optimally irregular sampling arrays that avoid Moiré distortion by scattering supra-Nyquist frequencies into broadband noise. Thus is appears that topological disorder in the receptor mosaic prevents aliasing outside the fovea--the only place it could occur in normal vision.

Activation of electrogenic Na-Ca exchange by light in fly photoreceptors

Illumination of white-eyed Musca photoreceptors following hypoxia or the application of ruthenium red (RR, a known blocker of Ca2+ uptake into intracellular organelles) induced a transient after depolarization (TA). The TA was enhanced when external [Ca2+] was reduced; it was abolished when external [Na+] was reduced to a level that affected the receptor potential to a small degree. The TA was enhanced or depressed when the activity of Na/K pump, which controls the Na+ gradient, was enhanced or depressed respectively. This effect was observed even when the receptor potential was not affected. All of the above observations are consistent with the hypothesis that the TA is triggered by a light-induced increase in the concentration of intracellular free Ca2+ which appear to be very high, following treatments with hypoxia or RR. The high sensitivity of the TA to Na+ and Ca2+ gradients across the photoreceptors membrane strongly suggests that the TA is due to a transient activation of an electrogenic Na-Ca exchange mechanism which depolarizes the cell.

Photophysiological functions of visual pigments

In order to gain a fundamental understanding of functions of a visual pigment, i.e., photoreception and phototransduction, it is essential to elucidate the molecular structure of visual pigment, its photochemical behavior and connection of the pigment to the molecular physiological amplification mechanism for excitation of a visual cell. A rhodopsin, a rod visual pigment, is composed of an 11-cis-retinal bound with an apo-protein, opsin, through a protonated Schiff-base. Competitive inhibition of beta-ionone on regeneration of rhodopsin from an 11-cis-retinal and cattle opsin demonstrated the existence of a hydrophobic linkage between the beta-ionone ring of the retinal and the hydrophobic region of opsin. Owing to these two linkages, the 11-cis-retinal is fixed in an opsin cleft. As a result, it is endowed with new physiological functions as a chromophore of rhodopsin; the change of 11-cis-retinal (lambda max = 369 nm, epsilon = 26,400) to rhodopsin (lambda max = about 500 nm, epsilon = 40,600) brings not only a spectral shift from near ultraviolet to visible regions and an intensification of the molecular extinction coefficient, but also an increase of a quantum yield of 11-cis-retinal to all-trans form. High quantum yield of rhodopsin suggests rapid formation of the first photoproduct. Study of the first photoproduct was accelerated by the finding of bathorhodopsin, which was formed by irradiation at liquid nitrogen temperatures. The change of rhodopsin to bathorhodopsin has been inferred to be due to a photoisomerization of the chromophore from 11-cis to a twisted all-trans form. This isomerization hypothesis has been verified by the following experimental results. Irradiation of 7-cis- and 9-cis-rhodopsins at liquid nitrogen temperature produced the same bathorhodopsin as that from 11-cis-rhodopsin, indicating that the chromophore of bathorhodopsin should be in all-trans or transoid form. 7-Membered-rhodopsin, in which the rotation of 11-12 double bond of the retinylidene chromophore is locked, did not form bathorhodopsin by excitation of picosecond laser photolysis. This fact indicates that bathorhodopsin is a product formed by photoisomerization of the chromophore. Rhodopsin showed a positive circular dichroism (CD) in the visible while bathorhodopsin showed a remarkable negative CD. The reversal of the sign of CD indicates that not only large conformational change of the retinylidene chromophore occurs during the conversion of rhodopsin to bathorhodopsin, but also the direction of twist of the chromophore reverses. It is inferred that the chromophore of bathorhodopsin is a twisted trans form.(ABSTRACT TRUNCATED AT 400 WORDS)

Depolarizing responses of L-type external horizontal cells in the goldfish retina under intense chromatic background

The external horizontal cells (EHCs) in the goldfish retina are known to respond with hyperpolarization to monochromatic flashes of all wavelengths. From their spectral responses they are referred to as "monophasic" horizontal cells. However, when the retina is exposed to an intense red background, EHCs showed a depolarizing response to flashes of intermediate and short wavelength of moderate intensity. The depolarizing component is presumably driven by the input from green-sensitive cones.

Electronic simulation of cones, horizontal cells and bipolar cells of generalized vertebrate cone retina

Electronic analogue of my theoretical model of generalized vertebrate cone retina [Siminoff: J. Theor. Biol. 86, 763 (1980)] is presented. Cone mosaic is simulated by 25 X 21 grid of phototransistors that have colored filters mounted in front of then to produce red-, green-, and blue-sensitive cones arranged in a "trichromatic" retina. Each retinal element is simulated by Summator-Integrator and unit gain voltage invertes are used to give correct polarities to output voltages. Dynamic properties of retinal elements are developed solely by temporal interplay of antagonistic input voltages with differing time courses, and spatial organization of receptive fields is developed by unit hexagons that precisely define cone input voltages to subsequent elements. Electronic model contains both color- and non-color-coded channels. Negative feedback from L-horizontal cells to cones, electrical coupling of like-cones, and electrical coupling of like-horizontal cells are simulated by feedforward circuits. Stray light is present due to light scattering properties of colored filters used to simulate color selectivety of cones. Stationary and moving spots of white and colored lights of varied sizes and intensities are used to study characteristics of electronic analogue. Results demonstrate practicality of electronic simulation to function analogues to real cone retinas to process visual stimuli and give information to higher centers as to size, shape, color and motion of objects in visual world.

Adaptive processes controlling sensitivity of short-wave cone pathways to different spatial frequencies

The sensitivity of the short-wave cone pathways was measured with violet interference gratings on monochromatic adapting fields. Spectral field sensitivity functions for test gratings of 2, 4 and 8 c/deg approximately matched Stiles' pi 3 function, which presumably reflects light-adaptation of the short-wave cones. Previous work had suggested that field sensitivity varies as a function of test spatial frequency, but this now seems an artifact of spatial adaptation. (The present study shows the pathway is highly susceptible to spatial adaptation induced by fine patterns.) For various spatial frequencies that affect the short-wave pathway, signals also pass through a cancellate adaptation site where signals from the short-wave and the middle- and long-wave cones act in opposition. Transient inputs in either opponent direction polarize or desensitize the opponent site. Additional transient inputs of the opposite sign then reduce the polarization and increase sensitivity. Such effects were demonstrated with test gratings as high as 12 c/deg. Transient effects at the opponent site are particularly evident at higher spatial frequencies.

Dark-adaptation of the human rod system

Following substantial bleaching, dark-adaptation thresholds of a complete rod monochromat and of a subject with normal colour vision were measured using a Wright colorimeter. When precautions were taken to ensure that the fixation point fell on the same retinal area during the threshold measurements as during the bleaching period, the dark-adaptation threshold curves of the rod monochromat followed exactly the same course as those of the normal subject subsequent to the cone-rod break of the long-term, normal dark-adaptation curve; irrespective of the intensity and the duration of the bleaching and the wavelength of the test stimulation. In contrast to the normal subject, however, the dark-adaptation curves of the rod monochromat showed no evidence of any cone function at photopic intensities. Furthermore, as opposed to previous measurements which show a simple linear relationship between fraction of bleached rhodopsin and log threshold, the present results show that there is a close linearity between log fraction of bleached rhodopsin and log threshold. This linear relationship is obtained despite varying extents of bleaching and subsequent dark-adaptation periods.

The time-course of rod-cone interaction

The time-course of rod-cone interaction (change of scotopic sensitivity caused by photopic background stimulation) was measured in the presence of briskly exchanged, scotopically matched, 490- and 630-nm background disks. In all conditions, interaction rose and fell quickly with changes of photopic stimulation. When the background was a small 0.6 degree-diameter disk, photopic stimulation produced relatively constant maintained interaction of about 0.6 log units. When the background was a large 7.8 degree-dia disk, photopic stimulation produced larger initial (0.6-1.0 log unit) than maintained (0.2 log unit) interaction. When a 0.6 degree by 7.8 degree annulus was used instead of a background, photopic stimulation produced substantial interaction only at offset, a transitory interaction. Thus, the spatial dependence of transitory interactions differs from that of maintained interaction: transitory interactions can be large even when maintained interaction is small or absent. The results are discussed in terms of a simple center-surround model of rod-cone interaction that unifies both maintained and transient interaction.

Electrically evoked calcium responses in rods of the frog retina

In the rod of the isolated frog retina perfused with normal Ringer solution, a transretinal current pulse flowing from the receptor side to the vitreous side evoked a response consisting of a transient depolarization and a subsequent damped oscillation. The transient depolarization was not an all-or-none response, but was a graded one which depended on the intensity and duration of the electrical stimulation. The amplitude of the initial depolarization varied with Ca2+ concentrations of the perfusate, being enhanced in the high Ca2+ and reduced in the low Ca2+ medium. Furthermore, it was enhanced by perfusion with a Ba2+-containing solution, and suppressed by exposure to Co2+. These observations suggest that the initial depolarization can be attributed to an increase of Ca2+ conductance. The late hyperpolarization which followed the initial depolarization was suppressed by membrane depolarization and was enhanced by membrane hyperpolarization. In addition, the evoked responses were not affected by application of either L-aspartate or L-glutamate, both of which blocked the synaptic transmission from the photoreceptors to the second order neurons. Thus it is unlikely that the late hyperpolarization is an IPSP-like response evoked by the negative feedback input from horizontal cells. Rather the rod membrane itself seems to have an oscillatory property.

On spectral sensitivity in the goldfish. Evidence for neural interactions between different "cone mechanisms"

Spectral sensitivity was measured in freely moving, light adapted goldfish using a behavioral training technique. The curve reveals 3 pronounced maxima at 470, 540 and 660 nm. Compared with the absorption spectra of the cone photopigments with lambda max at 450, 530 and 625 nm, the maxima were much narrower and shifted towards longer wavelengths. Results under chromatic adaptation indicate that spectral sensitivity measured under the white adaptation light is determined by inhibitory interactions between the spectrally different "cone mechanisms". For the short-wave maximum, a summation process probably has to be assumed in addition.