Response transfer functions of Limulus ventral photoreceptors: interpretation in terms of transduction mechanisms

In recent years, our knowledge of the biochemical mechanisms underlying the transduction process in photoreceptors has expanded rapidly. However, a full picture of the temporal dynamics of these mechanisms remains elusive. To study the dynamics in the Limulus ventral photoreceptor, we measure its light-evoked transfer function under voltage clamp. Comparison of this transfer function to biochemically realistic theoretical models of transduction provides insights into the photoreceptor dynamics. This comparison supports the suggestion that the low-frequency behaviour of the Limulus photoreceptor, corresponding to light and dark adaptation, is that of a nonlinear negative feedback loop. The main reactions of this loop have time constants between about 1 and 40 s. Such a feedback loop does not account, however, for the high-frequency behaviour of the responses, which implies the existence of a further, fast-acting, mechanism.This work was supported by grants from the Binational Science Foundation (BSF) Jerusalem, Israel and the Israel Academy of Sciences and Humanities, by NIH grant EY 1428, and by NSF grant DMS 8505442     

Comparative photosynthesis of three gap phase successional tree species

Summary Photosynthesis was measured in situ on trees growing in an open, gap-like site and under a closed canopy. Photosynthetic responses also were monitored on trees grown in the laboratory under either a high or low light regime or on those trees transferred from a low to a high light regime. All three species studied, Liriodendron tulipifera, Acer rubrum and Cornus florida, were able to acclimate to a high light environment as evidenced by their higher photosynthetic rates. This acclimation was achieved by an increase in transfer conductance and was ultimately due to changes in leaf anatomy. Species avoided photo-bleaching primarily by changes in leaf orientation and canopy structure. Species-specific physiological responses to high light intensity may account for each species' known growth response following canopy opening.Study was partially funded by a grant in aid of research from Sigma Xi     

Photoelectric effects in Characean cells I. The influence of light intensity

Summary The kinetic features of the action of light on the membrane potential ofNitella mucronata were investigated by measuring the frequency responses at different light intensities ranging from 0.2 to 80 W/m². Frequencies from 1 cycle/3 h to 32 cycles/min were applied. This range exceeded that of earlier investigations and resulted in the demonstration of allpass elements at low frequencies. From the all-pass elements it was concluded that the system comprises parallel pathways.From the influence of the light intensity on the frequency responses it was seen that the kinetic data depend on the intensity. By means of the Laplace transformation the squarewave responses were calculated from the frequency responses, and it could be demonstrated that a single cell is able of exhibiting all those different types of curve shapes reported in literature, if only one parameter, the light intensity, is changed. With constant modulation depth the amplitude of the evoked changes in potential varied only little with the light intensity. This is in line with a logarithmic dose-effect function as known from many light effects.     

Rod vision is controlled by dopamine-dependent sensitization of rod bipolar cells by GABA

Dark and light adaptation of retinal neurons allow our vision to operate over an enormous light intensity range. Here we report a mechanism that controls the light sensitivity and operational range of rod-driven bipolar cells that mediate dim-light vision. Our data indicate that the light responses of these cells are enhanced by sustained chloride currents via GABA(C) receptor channels. This sensitizing GABAergic input is controlled by dopamine D1 receptors, with horizontal cells serving as a plausible source of GABA release. Our findings expand the role of dopamine in vision from its well-established function of suppressing rod-driven signals in bright light to enhancing the same signals under dim illumination. They further reveal a role for GABA in sensitizing the circuitry for dim-light vision, thereby complementing GABA's traditional role in providing dynamic feedforward and feedback inhibition in the retina.     

Comparison of the pattern reversal visual evoked potential mediated by separate cone systems

With the purpose of recording responses mediated by the 3 cone systems visual evoked potentials (VEPs) were elicited by the reversal of monochromatic checkerboards superimposed upon strong monochromatic backgrounds (yellow, purple and blue-green).The sensitivity to light of various wave lengths were measured as the reciprocal of the intensity necessary to elicit a VEP amplitude of 3 μV. The spectral sensitivity curves based on this VEP amplitude criterion in the presence of blue-green, purple and yellow adaptation showed peak sensitivities in the red, the green and the blue part of the spectrum, respectively. This indicates that the responses reflect separate modulation of the 3 different cone mechanisms.The potentials obtained with yellow adaptation differed from those obtained with purple and blue-green adaptation. The amplitude versus log intensity function was flatter and the latency of the major positive peak was increased by 20–25 msec.Repeated examinations of 4 subjects suggest that the method yields reliable latency measurements of responses mediated by separate cone mechanisms.     

Intracellular recordings from gecko photoreceptors during light and dark adaptation

Intracellular recordings were obtained from rods in the Gekko gekko retina and the adaptation characteristics of their responses studied during light and dark adaptation. Steady background illumination induced graded and sustained hyperpolarizing potentials and compressed the incremental voltage range of the receptor. Steady backgrounds also shifted the receptor's voltage-intensity curve along the intensity axis, and bright backgrounds lowered the saturation potential of the receptor. Increment thresholds of single receptors followed Weber's law over a range of about 3.5 log units and then saturated. Most of the receptor sensitivity change in light derived from the shift of the voltage-intensity curve, only little from the voltage compression. Treatment of the eyecup with sodium aspartate at concentrations sufficient to eliminate the beta-wave of the electroretinogram (ERG) abolished initial transients in the receptor response, possibly indicating the removal of horizontal cell feedback. Aspartate treatment, however, did not significantly alter the adaptation characteristics of receptor responses, indicating that they derive from processes intrinsic to the receptors. Dark adaptation after a strongly adapting stimulus was similarly associated with temporary elevation of membrane potential, initial lowering of the saturation potential, and shift of the voltage-intensity curve. Under all conditions of adaptation studied, small amplitude responses were linear with light intensity. Further, there was no unique relation between sensitivity and membrane potential suggesting that receptor sensitivity is controlled at least in part by a step of visual transduction preceding the generation of membrane voltage change.     

Frequency Characteristics of Retinal Neurons in the Carp

Frequency characteristics of various retinal neurons in the carp were studied using sinusoidally modulated light as an input. They were affected by both intensity and pattern of illumination. In the horizontal cells, in which the effect of light intensity was studied most extensively, an increase in the light intensity brought about a decrease of the gain, which was more marked at lower frequencies, resulting in a shift of cutoff frequency towards higher frequencies and in a slight low frequency attenuation. A decrease in the area illuminated had an effect similar to a decrease in the light intensity. In the receptor, the low frequency attenuation was not apparent even at high light intensities. The adaptation process in receptors was not sufficient to explain the low frequency attenuation in the horizontal cells, and a possible contribution of negative feedback from horizontal cells to receptors was suggested. In the bipolar cell, the lateral interaction played an important role. An increase in an area resulted in the suppression of the response at low frequencies where the phases of the center and the surround responses were opposed, but in the augmentation near 5 Hz where the two responses were in phase. In amacrine cells, a low frequency attenuation and a phase advance at low frequencies were very prominent, and were considered to be due mainly to a process designated here as the neural adaptation.     

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.     

Motion detection and adaptation in crayfish photoreceptors. A spatiotemporal analysis of linear movement sensitivity

Impulse and sine wave responses of crayfish photoreceptors were examined to establish the limits and the parameters of linear behavior. These receptors exhibit simple low pass behavior which is well described by the transfer function of a linear resistor-capacitor cascade of three to five stages, each with the same time constant (tau). Additionally, variations in mean light intensity modify tau twofold and the contrast sensitivity by fourfold. The angular sensitivity profile is Gaussian and the acceptance angle (phi) increases 3.2-fold with dark adaptation. The responses to moving stripes of positive and negative contrast were measured over a 100-fold velocity range. The amplitude, phase, and waveform of these responses were predicted from the convolution of the receptor's impulse response and angular sensitivity profile. A theoretical calculation based on the convolution of a linear impulse response and a Gaussian sensitivity profile indicates that the sensitivity to variations in stimulus velocity is determined by the ratio phi/tau. These two parameters are sufficient to predict the velocity of the half-maximal response over a wide range of ambient illumination levels. Because phi and tau vary in parallel during light adaptation, it is inferred that many arthropods can maintain approximately constant velocity sensitivity during large shifts in mean illumination and receptor time constant. The results are discussed relative to other arthropod and vertebrate receptors and the strategies that have evolved for movement detection in varying ambient illumination.     

Sensitivity facilitation in the insect eye

Summary ERG amplitude facilitation, observed in the eye ofAtta sexdens after light adaptation, was studied as a function of duration and intensity of adaptation, of dark interval between adapting and test stimuli, and of level of steady background illumination. Results show that sensitivity facilitation in this eye cannot be regarded as a minor effect since it covers a 2 log unit range, the same as that obtained for conditions that produce sensitivity reduction. Maximum facilitation occurs with short and intense light adaptation. The time span of the effect is close to 2 min, and its maximum amplitude may be attained up to 20 s after light adaptation. Increase in background illumination gradually erases facilitation. However, the facilitated response is less sensitive to background illumination than the dark adapted response. Long durations of light adaptation cause ERG decrease, or inhibition. A comparison of these two end results of light adaptation suggests that they arise from different processes, perhaps with distinct origins.Supported by a grant from Fundação de Amparo à Pesquisa do Estado de São Paulo, to the senior author (Contract n 71/1141)With a Fellowship from Fundação de Amparo à Pesquisa do Estado de São Paulo (N 74/388)We wish to express our appreciation to Henrique Fix for his editorial assistance, and to Celia Jablonka for laboratory help.     

Contrast gain, signal-to-noise ratio, and linearity in light-adapted blowfly photoreceptors

Response properties of short-type (R1-6) photoreceptors of the blowfly (Calliphora vicina) were investigated with intracellular recordings using repeated sequences of pseudorandomly modulated light contrast stimuli at adapting backgrounds covering 5 log intensity units. The resulting voltage responses were used to determine the effects of adaptational regulation on signal-to-noise ratios (SNR), signal induced noise, contrast gain, linearity and the dead time in phototransduction. In light adaptation the SNR of the photoreceptors improved more than 100-fold due to (a) increased photoreceptor voltage responses to a contrast stimulus and (b) reduction of voltage noise at high intensity backgrounds. In the frequency domain the SNR was attenuated in low frequencies with an increase in the middle and high frequency ranges. A pseudorandom contrast stimulus by itself did not produce any additional noise. The contrast gain of the photoreceptor frequency responses increased with mean illumination and the gain was best fitted with a model consisting of two second order and one double pole of first order. The coherence function (a normalized measure of linearity and SNR) of the frequency responses demonstrated that the photoreceptors responded linearly (from 1 to 150 Hz) to the contrast stimuli even under fairly dim conditions. The theoretically derived and the recorded phase functions were used to calculate phototransduction dead time, which decreased in light adaptation from approximately 5-2.5 ms. This analysis suggests that the ability of fly photoreceptors to maintain linear performance under dynamic stimulation conditions results from the high early gain followed by delayed compressive feed-back mechanisms.     

Temporal tuning and nonlinearity of intraretinal pathways in turtle: Effects of temperature,stimulus intensity,and size

Flash responses, amplitude and phase transfer functions, and nonlinearities were measured in turtle retina for pathways with photoreceptor inputs and outputs from horizontal (HC), hyperpolarizing bipolar (HBC), sustained amacrine (AC), and on-off ganglion (GC) cells. Flash responses slowed and attenuated in all cells as temperature decreased. Whitenoise transfer properties of sustained-type cells (HC, HBC, AC) were of low- or bandpass type; highfrequency cut-off (f _c) and phase crossover frequency decreased with temperature. f _c increased as spot diameter was increased. Nonlinearity of these sustained-response pathways (distortion product frequencies in response to a sum-of-sinusoids input probe) increases with intensity and may depend on amplitude saturation limiting. On/off GC synaptic and spike activity increased as spot diameter decreased and intensity increased. Amplitude transfer functions had a low-frequency peak (PSP activity) and monotonically decreasing amplitude vs. frequency shape (spikes and transient PSP activity). Nonlinearity increased with stimulus intensity; it was maximal with 1 mm spot size, less with smaller (500 m) and larger (5 mm) spots. It may depend on the functional equivalent of full-wave rectification (on-off response).This work was supported by NEI grant R01 EY03383     

Retinal responses inNautilus

Summary Electrical responses (ERG) were recorded from the retinal surface of the eye ofNautilus pompilius. Brief light flashes and step changes in intensity were used to explore linear and nonlinear response properties. The linear responses could be fitted well with cascaded exponential decay processes and a time delay. Nonlinear phenomena included a saturating amplitude of response with increased light intensity, and shape changes induced by adaptation and by background illumination.This study, of the R/V Alpha Helix Southeastern Asia Expedition, was supported by the National Science Foundation under Grants OFS74-01830 and OCE74-02888 to Scripps Institution of Oceanography. It was also supported by the National Science Foundation Grant BMS75-01149 to Iowa State University. — We would like to thank Walter Schneider for his technical assistance while on shipboard. This work would not have been possible without the help and hospitality of the wonderful people of Bindoy, Negros Oriental, The Republic of the Philippines.     

Biophysical evidence that light adaptation in Limulus photoreceptors is due to a negative feedback.

The steady-state stimulus-response curve of the Limulus ventral photoreceptor comprises a linear portion followed by a less-than-unity power law dependence, which is maintained over at least 4 decades of intensity. This progressive desensitization corresponds to light adaptation. For flash stimulation of dark-adapted cells, the stimulus-response curve again has an initial linear portion, but this is followed by a region of supralinearity before the curve saturates. In a previous article, we showed that the distribution of time integrals of the single-photon responses is consistent with a model of a single chain of first-order reactions. Starting with such a model, we have looked at relevant elementary nonlinear biochemical mechanisms to determine which of them can modulate the enzymatic amplifications of the chain in such a way as to lead to these behaviors. We assume that each of the two phenomena, adaptation and supralinearity, derives from a single mechanism that acts on a single enzymatic stage. We then conclude that the adaptation must be a cooperative negative feedback, in which an accessory material activated by a late stage of the transduction chain acts cooperatively to inhibit an earlier enzymatic amplification. In Limulus, the number of molecules that cooperate is between 3 and 5. We were not able to discard any of the mechanisms tested for the supralinearity, except to say that they must act at a stage of the chain later than that on which the adaptive material acts. If we assume the conclusions of a previous work which shows that the supralinearity mechanism is active during the steady state, we can also conclude that the supralinearity stage must precede the stage that is the source of the adaptive material.     

Studies on the electroretinogram. II. Model of an a-wave regenerator mechanism

A model is proposed to relate the regeneration of the ERGa-wave after partial light adaptation to the level of the light adaptation. The model assumes that thea-wave amplitude is a function of some reactive substance associated with ana-wave generator. The maximuma-wave amplitude occurs when the eye is fully dark adapted, and thea-wave generator initiator concentration is at a maximum. Thea-wave generator initiator concentration can be decreased by interacting with a product of the rhodopsin-light energy reaction, and increased by removal of this inhibitor. The removal of the inhibitor depends upon the isomerization of the all-trans-retinene to the 11-cis form. An excess of inhibitory material overa-wave generator initiator would cause a delay in the appearance of thea-wave until the excess inhibitory material is removed. This delay is a linear function of the logarithm of the adapting energy. The agreement of this model with the experimental ERG data is very good. Supported in whole by Public Health Service Research Grant No. NB-02522, from the National Institute of Neurological Diseases and Blindness, Bethesda, Maryland.     

Two temporal phases of light adaptation in retinal rods

Vertebrate rod photoreceptors adjust their sensitivity as they adapt during exposure to steady light. Light adaptation prevents the rod from saturating and significantly extends its dynamic range. We examined the time course of the onset of light adaptation in bullfrog rods and compared it with the projected onset of feedback reactions thought to underlie light adaptation on the molecular level. We found that adaptation developed in two distinct temporal phases: (1) a fast phase that operated within seconds after the onset of illumination, which is consistent with most previous reports of a 1-2-s time constant for the onset of adaptation; and (2) a slow phase that engaged over tens of seconds of continuous illumination. The fast phase desensitized the rods as much as 80-fold, and was observed at every light intensity tested. The slow phase was observed only at light intensities that suppressed more than half of the dark current. It provided an additional sensitivity loss of up to 40-fold before the rod saturated. Thus, rods achieved a total degree of adaptation of approximately 3,000-fold. Although the fast adaptation is likely to originate from the well characterized Ca(2+)-dependent feedback mechanisms regulating the activities of several phototransduction cascade components, the molecular mechanism underlying slow adaptation is unclear. We tested the hypothesis that the slow adaptation phase is mediated by cGMP dissociation from noncatalytic binding sites on the cGMP phosphodiesterase, which has been shown to reduce the lifetime of activated phosphodiesterase in vitro. Although cGMP dissociated from the noncatalytic binding sites in intact rods with kinetics approximating that for the slow adaptation phase, this hypothesis was ruled out because the intensity of light required for cGMP dissociation far exceeded that required to evoke the slow phase. Other possible mechanisms are discussed.     

Mathematical and computational analysis of adaptation via feedback inhibition in signal transduction pathways

We perform a systematic analysis of mechanisms of feedback regulation that underlie short-term adaptation in intracellular signaling systems. Upon receiving an external cue, these systems generate a transient response that quickly returns to basal levels even if the stimulus persists. Signaling pathways capable of short-term adaptation are found in systems as diverse as the high osmolarity response of yeast, gradient sensing in Dictyostelium, and the cytokine response in vertebrates. Using mathematical analysis and computational experiments, we compare different feedback architectures in terms of response amplitude and duration, ability to adapt, and response to variable stimulus levels. Our analysis reveals three important features of these systems: 1), multiple step signaling cascades improve sensitivity to low doses by an effect distinct from signal amplification; 2), some feedback architectures act as signal transducers converting stimulus strength into response duration; and 3), feedback deactivation acts as a dose-dependent switch between transient and sustained responses. Finally, we present characteristic features for each form of feedback regulation that can aid in their identification.     

Nonlinear mechanisms for gain adaptation in locust photoreceptors.

Intracellular membrane potential responses were recorded from locust photoreceptors under two stimulus conditions: pairs of flashes to dark-adapted receptors, and white-noise modulated light at a range of background intensities from 500 to 15,000 effective photons per second. Nonlinear analysis of the input-output relationships were performed by estimating the Volterra and Wiener kernels of the system. The Volterra kernels obtained from the double-flash experiments were similar to the Wiener kernels obtained from the white-noise experiments, except for a change of time scale. The structure of the second-order kernels obtained with either method gave evidence for a gain control mechanism acting at an early stage of the cascade. Both feedforward and feedback nonlinearities could account for the observed system behavior at any one background level. The differences in amplitude between the kernels obtained at different background levels could be accounted for by an adaptation process which further decreased the gain of the system, acting on a slower time scale, also at some early stage of the cascade.     

Light-induced changes of sensitivity in Limulus ventral photoreceptors

The responses of Limulus ventral photoreceptors to brief test flashes and to longer adapting lights were measured under voltage clamp conditions. When the cell was dark adapted, there was a range of energy of the test flashes over which the peak amplitude of the responses (light-induced currents) was directly proportional to the flash energy. This was also true when test flashes were superposed on adapting stimuli but the proportionality constant (termed peak currently/photon) was reduced. The peak current/photon was attenuated more by brighter adapting stimuli than by less bright adapting stimuli. The peak current/photon is a measure of the sensitivity of the conductance-increase mechanism underlying the light response of the photo-receptor. The response elicited by an adapting stimulus had a large initial transient which declined to a smaller plateau. The peak current/photon decreased sharply during the declining phase of the transient and was relatively stable during the plateau. This indicates that the onset of light adaptation is delayed with respect to the onset of the response to the adapting stimulus. If the adaptational state just before the onset of each of a series of adapting stimuli was constant, the amplitude of the transient was a nearly linear function of intensity. When the total intensity was rapidly doubled (or halved) during a plateau response, the total current approximately doubled (or halved). We argue that the transition from transient to plateau, light-elicited changes of threshold, and the nonlinear function relating the plateau response to stimulus intensity all reflect changes of the responsiveness of the conductance-increase mechanism.     

Correspondences in the Behavior of the Electroretinogram and of the Potentials Evoked at the Visual Cortex

Electrical potentials from the eye (ERG) and from the contralateral visual cortex were recorded in response to flashes of white and of colored light of various intensities and durations. The evoked potentials were found to parallel the behavior of the ERG in several significant respects. Selective changes in the ERG brought about by increasing the light intensity and by light adaptation led to parallel selective changes in the cortical responses. The dual waves (b₁, b₂) of the ERG were found to have counterparts in two cortical waves (c₁, c₂) which, in respect to changes in light intensity and to light adaptation, behaved analogously to the two retinal components. The responses evoked at high intensity showed only the diphasic c₁-potential. As stimulus intensity was lowered the c₁-wave decreased in magnitude and a delayed c₂-component appeared. The c₂-potential increased in amplitude as light intensity of the flash was further reduced. Eventually the c₂-wave, too, decreased as stimulus reduction continued. There was no wave length specificity in regard to either the duplex b-waves or duplex cortical waves. Both appeared at all wave lengths from 454 mµ to 630 mµ. The two cortical waves evoked by brief flashes of colored light showed all the behavior to changes in stimulus intensity and to light adaptation that occurred with white light.