Light adaptation in Drosophila photoreceptors: I. Response dynamics and signaling efficiency at 25 degrees C

Besides the physical limits imposed on photon absorption, the coprocessing of visual information by the phototransduction cascade and photoreceptor membrane determines the fidelity of photoreceptor signaling. We investigated the response dynamics and signaling efficiency of Drosophila photoreceptors to natural-like fluctuating light contrast stimulation and intracellular current injection when the cells were adapted over a 4-log unit light intensity range at 25 degrees C. This dual stimulation allowed us to characterize how an increase in the mean light intensity causes the phototransduction cascade and photoreceptor membrane to produce larger, faster and increasingly accurate voltage responses to a given contrast. Using signal and noise analysis, this appears to be associated with an increased summation of smaller and faster elementary responses (i.e., bumps), whose latency distribution stays relatively unchanged at different mean light intensity levels. As the phototransduction cascade increases, the size and speed of the signals (light current) at higher adapting backgrounds and, in conjunction with the photoreceptor membrane, reduces the light-induced voltage noise, and the photoreceptor signal-to-noise ratio improves and extends to a higher bandwidth. Because the voltage responses to light contrasts are much slower than those evoked by current injection, the photoreceptor membrane does not limit the speed of the phototransduction cascade, but it does filter the associated high frequency noise. The photoreceptor information capacity increases with light adaptation and starts to saturate at approximately 200 bits/s as the speed of the chemical reactions inside a fixed number of transduction units, possibly microvilli, is approaching its maximum.     

Light adaptation in Drosophila photoreceptors: II. Rising temperature increases the bandwidth of reliable signaling

It is known that an increase in both the mean light intensity and temperature can speed up photoreceptor signals, but it is not known whether a simultaneous increase of these physical factors enhances information capacity or leads to coding errors. We studied the voltage responses of light-adapted Drosophila photoreceptors in vivo from 15 to 30 degrees C, and found that an increase in temperature accelerated both the phototransduction cascade and photoreceptor membrane dynamics, broadening the bandwidth of reliable signaling with an effective Q(10) for information capacity of 6.5. The increased fidelity and reliability of the voltage responses was a result of four factors: (1) an increased rate of elementary response, i.e., quantum bump production; (2) a temperature-dependent acceleration of the early phototransduction reactions causing a quicker and narrower dispersion of bump latencies; (3) a relatively temperature-insensitive light-adapted bump waveform; and (4) a decrease in the time constant of the light-adapted photoreceptor membrane, whose filtering matched the dynamic properties of the phototransduction noise. Because faster neural processing allows faster behavioral responses, this improved performance of Drosophila photoreceptors suggests that a suitably high body temperature offers significant advantages in visual performance.     

Changes in the dynamic properties of locust photoreceptors at three levels of light adaptation

The frequency response function of phototransduction was studied in the locust compound eye at three levels of dim light adaptation. The eyes were stimulated with light from a green light emitting diode (LED) and the resulting changes in membrane potential were measured with intracellular electrodes. Absolute sensitivities and light adaptation levels were established by counting the arrivals of single photons in dark adapted eyes. Frequency response functions for phototransduction could be well fitted by a model developed earlier for fly compound eyes, which includes underdamped second order poles and a pure time delay. However, the locust data requires longer time constants for the poles and a longer delay than were used to fit the fly data, reflecting the slower response characteristics of the locust. Changing the level of light adaptation at dim levels caused changes in both the sensitivity and the dynamic properties of the photoreceptors. Sensitivity was reduced and significant decreases were seen in the delay and several time constants. The behavior also became much more damped, with one of the second order poles decomposing into first order poles.     

Dispersion of latencies in photoreceptors of Limulus and the adapting- bump model

To light stimuli of very low intensity, Limulus photoreceptors give a voltage response with a fluctuating delay. This phenomenon has been called "latency dispersion." If the generator potential is the superposition of discrete voltage events ("bumps"), and if the effect of light upon bump size is negligible, then the latency dispersion and the bump shape completely characterize the frequency response to sinusoidal flicker. For very low light intensities, the latency dispersion of the bumps, the bump shape, and the frequency response are measured. It is found that for data obtained at 20 degrees C, the frequency response can be accounted for completely by the latency dispersion and by the bump shape derived from steady-state noise characteristics. At 10 degrees C, the time scale of the response of the photoreceptor is lengthened. The dispersion of latencies and the bump shape are found not to have the same temperature dependence. However, just as those measured at 20 degrees C, the bump shape and the dispersion of latencies measured at 10 degrees C can predict the frequency response measured under the same conditions. These results strongly suggest that the major mechanisms involved in the generator potential are the latency process and the bump process. At high light intensities, the time scale of the generator potential shortens. The decrease in time scale of the generator potential can be attributed to the decreases in time scales of the bumps and of the latency dispersion process.     

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.     

Voltage noise accompanying chemically-induced depolarization of insect photoreceptors

Intracellular potentials are recorded from photoreceptors in a superfused preparation of the retina of a locust compound eye. Chloral hydrate and alkyl alcohols induce a rapid, superfusing reversible depolarization of these photoreceptors when dissolved in the saline. Analysis of voltage noise accompanying depolarization by chloral hydrate suggests that depolarizing ionic pathways are opened briefly and randomly in time in the photoreceptor membranes. This conclusion is supported by measurements of the cell resistance and of voltage noise amplitude as a function of membrane potential. Replacement of superfusate sodium by choline reversibly reduces the effects of chloral hydrate, suggesting that the ionic pathways opened are permeable by sodium. The voltage noise induced by chloral hydrate is compared to that during depolarization by steady illumination of the same cell. As the illumination intensity is increased, the amplitude and the shape of the power spectrum of light-induced voltage noise approach those of drug-induced noise at the same depolarization level. The possibility that these phenomena represent alterations in the mechanism of phototransduction is discussed.     

Current issues in invertebrate phototransduction

Investigation of phototransduction in invertebrate photoreceptors has revealed many physiological and biochemical features of fundamental biological importance. Nonetheless, no complete picture of phototransduction has yet emerged. In most known cases, invertebrate phototransduction involves polyphosphoinositide and cyclic GMP (cGMP) intracellular biochemical signaling pathways leading to opening of plasma membrane ion channels. Excitation is Ca²⁺-dependent, as are adaptive feedback processes that regulate sensitivity to light. Transduction takes place in specialized subcellular regions, rich in microvilli and closely apposed to submicrovillar membrane systems. Thus, excitation is a highly localized process. This article focuses on the intracellular biochemical signaling pathways and the ion channels involved in invertebrate phototransduction. The coupling of signaling cascades with channel activation is not understood for any invertebrate species. Although photoreceptors have features that are common to most or all known invertebrate species, each species exhibits unique characteristics. Comparative electrophysiological, biochemical, morphological, and molecular biological approaches to studying phototransduction in these species lead to fundamental insights into cellular signaling. Several current controversies and proposed phototransduction models are evaluated.     

Membrane filtering properties of the bumblebee (Bombus terrestris) photoreceptors across three spectral classes

Filtering properties of the membrane form an integral part of the mechanisms producing the light-induced electrical signal in insect photoreceptors. Insect photoreceptors vary in response speed between different species, but recently it has also been shown that different spectral photoreceptor classes within a species possess diverse response characteristics. However, it has not been quantified what roles phototransduction and membrane properties play in such diversity. Here, we use electrophysiological methods in combination with system analysis to study whether the membrane properties could create the variation of the response speed found in the bumblebee (Bombus terrestris) photoreceptors. We recorded intracellular responses from each photoreceptor class to white noise-modulated current stimuli and defined their input resistance and linear filtering properties. We found that green sensitive cells exhibit smaller input resistance and membrane impedance than other cell classes. Since green sensitive cells are the fastest photoreceptor class in the bumblebee retina, our results suggest that the membrane filtering properties are correlated with the speed of light responses across the spectral classes. In general, our results provide a compelling example of filtering at the sensory cell level where the biophysical properties of the membrane are matched to the performance requirements set by visual ecology.     

The phosphoinositide signaling cascade is involved in photoreception in the leech Hirudo medicinalis.

The effects of BAPTA, heparin, and neomycin on electrical light responses were studied in the photoreceptors of Hirudo medicinalis. Light activation produces a fast increase in intracellular Ca2+ concentration (Cai) as detected with the fluorescent Ca2+ indicator calcium green-5N. Chelating intracellular calcium by injections of 10 mmol(-1) BAPTA suppresses spontaneous quantum bumps, reduces light sensitivity by more than 2 log(10) units, and substantially increases the latent period of light responses. BAPTA strongly inhibits the plateau phase of responses to long steps of light. Injections of 45-100 mg ml(-1) of heparin act in a similar manner to BAPTA, affecting the latency of the light responses even more. De-N-sulfated heparin, an inactive analog, is almost ineffective at the same concentration compared with heparin. Heparin diminishes the light-induced Cai elevation significantly, whereas de-N-sulfated heparin does not. Intracellular injections of 50-100 mmol l(-1) of the aminoglycoside neomycin, which inhibits phospholipase-C-mediated inositol 1,4,5-trisphosphate formation, acts similar to BAPTA and heparin. Pressure injections of the hydrolysis resistant analog of inositol 1,4,5-trisphosphate, inositol 2,4,5-trisphosphate, strongly depolarize leech photoreceptors and mimic an effect of light adaptation. These results suggest a close similarity between phototransduction mechanisms in leech photoreceptors and existing models for visual transduction in other invertebrate microvillar photoreceptors.     

Long-term light adaptation in photoreceptors of the housefly, Musca domestica

The photoreceptors of many animals adapt, when illuminated, by reducing their sensitivities to light and improving their response speeds. Light adaptation is usually considered to be rapid and complete within minutes. However, under bright light conditions, I show that functionally significant improvements in impulse response amplitude and speed continue over the course of an hour in photoreceptors of the fly, Musca domestica. After sustained illumination, the average information rate, a measure of signalling performance, improved by 28% in a sample of sixteen photoreceptors. This long-term light adaptation is a robust phenomenon across animals and is repeatable within the same cell when light-adapting sessions are separated by a period of darkness. White-noise analysis of voltage responses to light and current stimuli indicate that much of the long-term changes observed are attributable to an improvement in the reliability with which photoreceptors register the timing of photon absorptions. It is also found that the impedance amplitude of the photoreceptor increases during long-term adaptation, suggesting that the area of the photoreceptor's membrane is reduced.     

Light reduces the excitation efficiency in the nss mutant of the sheep blowfly Lucilia

The nss (no steady state) phototransduction mutant of the sheep blowfly Lucilia was studied electrophysiologically using intracellular recordings. The effects of the nss mutation on the receptor potential are manifested in the following features of the light response. (a) The responses to a flash or to dim lights are close to normal, but the receptor potential decays close to the baseline level during prolonged illumination after a critical level of light intensity is reached. (b) The decline of the response is accompanied by a large reduction in responsiveness to light that recovers within 20 s in the dark. (c) The full reduction in responsiveness to light is reached when approximately 13% of the photopigment molecules are converted from rhodopsin (R) to metarhodopsin (M). (d) A maximal net pigment conversion from R to M by blue light induces persistent inactivation in the dark, without an apparent voltage response. This inactivation could be abolished at any time by M-to-R conversion with orange light. The above features of the mutant indicate that the effect of the nss mutation on the light response of Lucilia is very similar to the effects of the transient receptor potential (trp) mutation on the photoreceptor potential of Drosophila. Noise analysis and voltage measurements indicate that the decay of the receptor potential is due to a severe reduction in the rate of occurrence of the elementary voltage responses (bumps). The bumps are only slightly modified in shape and amplitude during the decline of the response to light of medium intensity. There is also a large increase in response latency during intense background illumination. These results are consistent with the hypothesis that separate, independent mechanisms determine bump triggering and bump shape and amplitude. The nss mutation affects the triggering mechanism of the bump.     

Does Ca2+ reach millimolar concentrations after single photon absorption in Drosophila photoreceptor microvilli?

The quantum bump, the elementary event of fly phototransduction induced by the absorption of a single photon, is a small, transient current due to the opening of cation-channels permeable to Ca2+. These channels are located in small, tube-like protrusions of the cell membrane, the microvilli. Using a modeling approach, we calculate the changes of free Ca2+ concentration inside the microvilli, taking into account influx and diffusion of Ca2+. Independent of permeability ratios and Ca2+ buffering, we find that the free Ca2+ concentrations rise to millimolar values, as long as we assume that all activated channels are located in a single microvillus. When we assume that as much as 25 microvilli participate in a single bump, the free Ca2+ concentration still reaches values higher than 80 microM. These very high concentrations show that the microvilli of fly photoreceptors are unique structures in which the Ca2+ signaling is even more extreme than in calcium concentration microdomains very close to Ca2+ channels.     

The organization of INAD-signaling complexes by a multivalent PDZ domain protein in Drosophila photoreceptor cells ensures sensitivity and speed of signaling

Phototransduction in Drosophila has emerged as an attractive model system for studying the organization of signaling cascades in vivo. In photoreceptor neurons, the multivalent PDZ protein INAD serves as a scaffold to assemble different components of the phototransduction pathway, including the effector PLC, the light-activated ion channel TRP, and a protein kinase C involved in deactivation of the light response. INAD is required for organizing and maintaining signaling complexes in the rhabdomeres of photoreceptors. This macromolecular organization endows photoreceptors with many of their signaling properties, including high sensitivity, fast activation and deactivation kinetics, and exquisite feedback regulation by small localized changes in [Ca2+]i. Assembly of transduction components into signaling complexes is also an important cellular strategy for ensuring specificity of signaling while minimizing unwanted cross-talk. In this report, we review INAD's role as a signal transduction scaffold and its role in the assembly and localization of photoreceptor complexes.     

Long rise times of sound pulses in grasshopper songs improve the directionality cues received by the CNS from the auditory receptors

Auditory receptors of the locust (Locusta migratoria) were investigated with respect to the directionality cues which are present in their spiking responses, with special emphasis on how directional cues are influenced by the rise time of sound signals. Intensity differences between the ears influence two possible cues in the receptor responses, spike count and response latency. Variation in rise time of sound pulses had little effect on the overall spike count; however, it had a substantial effect on the temporal distribution of the receptor's spiking response, especially on the latencies of first spikes. In particular, with ramplike stimuli the slope of the latency vs. intensity curves was steeper as compared to stimuli with steep onsets (Fig. 3). Stimuli with flat ramplike onsets lead to an increase of the latency differences of discharges between left and right tympanic receptors. This type of ramplike stimulus could thus facilitate directional hearing. This hypothesis was corroborated by a Monte Carlo simulation in which the probability of incorrect directional decisions was determined on the basis of the receptor latencies and spike counts. Slowly rising ramps significantly improved the decisions based on response latency, as compared to stimuli with sudden onsets (Fig. 4). These results are compared to behavioural results obtained with the grasshopper Ch. biguttulus. The stridulation signals of the females of this species consist of ramplike pulses, which could be an adaptation to facilitate directional hearing of phonotactically approaching males.Abbreviations HFR high frequency receptor - ILD interaural level difference - LFR low frequency receptor - SPL sound pressure level - WN white noise     

Nucleo-cytoplasmic partitioning of the plant photoreceptors phytochromes

Phytochromes in harmony with blue light photoreceptors play a major role in controlling plant growth and development from germination to seed maturation. Light absorption by phytochromes triggers a signaling cascade, phototransduction, which culminates in regulated gene expression. A major regulatory step at the cellular level, which affects specificities of light-induced physiological responses, seems to be the light-quality and light-quantity dependent nuclear import of the phytochromes themselves. The correlations found between the nuclear import of phytochromes (phyA and phyB) and various physiological responses regulated by these photoreceptors provides strong support for this hypothesis.     

Two components of the receptor current are developed from distinct elementary signals in Limulus ventral nerve photoreceptor

Transient elementary currents, bumps, stimulated by short dim light flashes were measured in ventral nerve photoreceptors of Limulus. It is demonstrated that light activates two types of bumps, which form two distinct components of the receptor current at higher light intensities. The two bump types, which are both assumed to be activated by single absorbed photons, differ in current amplitude and kinetic parameters. The current amplitude of one bump type is smaller than 0.3 nA and that of the other type is in the usual current range of up to several nanoamperes. The average latency of small bumps measured from the short stimulus flash is shorter than that of the large bumps. The small bumps have slower activation kinetics than the large bumps. It is demonstrated that with increasing flash intensity the small bumps overlap first and form a macroscopic current, on top of which the large bumps are superimposed. Results indicate that a single absorbed photon selectively activates only one kind of the enzyme cascades evoking one bump type. We conclude that the active meta conformation of a rhodopsin molecule selectively binds a specific type of G-protein, which is involved in the stimulation of one of the transduction cascades. The two bump types, which are the elements of two macroscopic current components support the previous assumption that light activates different transduction mechanisms in Limulus photoreceptors.     

Circadian rhythms in Limulus photoreceptors. II. Quantum bumps

The light response of the lateral eye of the horseshoe crab, Limulus polyphemus, increases at night, while the frequency of spontaneous discrete fluctuations of its photoreceptor membrane potential (quantum bumps) decreases. These changes are controlled by a circadian clock in the brain, which transmits activity to the eye via efferent optic nerve fibers (Barlow, R. B., S. J. Bolanski, and M. L Brachman. 1977. Science. 197:86-89). Here we report the results of experiments in which we recorded from single Limulus photoreceptors in vivo for several days and studied in detail changes in their physiological and membrane properties. We found that: (a) The shape of (voltage) quantum bumps changes with the time of day. At night, spontaneous bumps and bumps evoked by dim light are prolonged. The return of the membrane potential to its resting level is delayed, but the rise time of the bump is unaffected. On average, the area under a bump is 2.4 times greater at night than during the day. (b) The rate of spontaneous bumps decreases at night by roughly a factor of 3, but their amplitude distribution remains unchanged. (c) The resting potential and resistance of the photoreceptor membrane do not change with the time of day. (d) the relationship between injected current and impulse rate of the second order neuron, the eccentric cell, also remains unchanged with the time of day. Thus the efferent input from the brain to the retina modulates some of the membrane properties of photoreceptor cells. Our findings suggest that the efferent input acts on ionic channels in the membrane to increase the sensitivity of the photoreceptor to light.     

The phosphoinositide signaling cascade is involved in photoreception in the leech Hirudo medicinalis

The effects of BAPTA, heparin, and neomycin on electrical light responses were studied in the photoreceptors of Hirudo medicinalis. Light activation produces a fast increase in intracellular Ca²⁺ concentration (Ca_i) as detected with the fluorescent Ca²⁺ indicator calcium green-5N. Chelating intracellular calcium by injections of 10 mmol l^-1 BAPTA suppresses spontaneous quantum bumps, reduces light sensitivity by more than 2 log₁₀ units, and substantially increases the latent period of light responses. BAPTA strongly inhibits the plateau phase of responses to long steps of light. Injections of 45-100 mg ml^-1 of heparin act in a similar manner to BAPTA, affecting the latency of the light responses even more. De-N-sulfated heparin, an inactive analog, is almost ineffective at the same concentration compared with heparin. Heparin diminishes the light-induced Ca_i elevation significantly, whereas de-N-sulfated heparin does not. Intracellular injections of 50-100 mmol l^-1 of the aminoglycoside neomycin, which inhibits phospholipase-C-mediated inositol 1,4,5-trisphosphate formation, acts similar to BAPTA and heparin. Pressure injections of the hydrolysis resistant analog of inositol 1,4,5-trisphosphate, inositol 2,4,5-trisphosphate, strongly depolarize leech photoreceptors and mimic an effect of light adaptation. These results suggest a close similarity between phototransduction mechanisms in leech photoreceptors and existing models for visual transduction in other invertebrate microvillar photoreceptors.     

Lanthanum mimicks the trp photoreceptor mutant of Drosophila in the blowfly Calliphora

The effect of lanthanum on the light response of blowfly (Calliphora erythrocephala) photoreceptors was studied. The electrophysiological behaviour of the photoreceptors in the presence of La can be summarized as follows: 1. Upon long stimulation the photoreceptors responded with a 'transient receptor potential', i.e. the cells depolarized at the onset of the stimulus and then repolarized to (or below) the resting potential. This effect was dependent on stimulus intensity and occurred only at high intensities. During illumination membrane noise was reduced. 2. The light-induced changes in membrane potential were paralleled by changes in membrane resistance. 3. The time course of the receptor response was slowed down. 4. Light adaptation led to an increase in response latency. 5. The recovery of the receptor response after light adaptation was slowed down. 6. The sensitivity of the receptor cells measured by the response to short light stimuli was reduced. In summary, the electrophysiological behaviour of Calliphora photoreceptors in the presence of La was very similar to that of the photoreceptors of the trp (transient receptor potential) mutant of Drosophila melanogaster. This result suggests that La and trp mutation affect the same cellular processes in the photoreceptors.