Afterpotentials in dronefly retinula cells

Summary The wavelength dependence of the afterpotentials following a bright illumination was studied in single photoreceptor cells of the droneflyEristalis. Cells with only a spectral sensitivity peak in the blue were selected. As previously demonstrated, these cells contain a rhodopsin absorbing maximally at about 450–460 nm, which upon photoconversion transforms into a metarhodopsin absorbing maximally at about 550 nm (Tsukahara and Horridge, 1977).With the visual pigment initially all in the rhodopsin form, a high rate of visual pigment conversion results in an afterhyperpolarization (AHP) when the fraction of metarhodopsin remains negligible after illumination as occurs at longer wavelengths if the intensity is high. Intensive illumination at short wavelengths is followed by a prolonged depolarizing afterpotential (PDA). The magnitude of the PDA peaks at low intensities at about 450–460 nm, corresponding to the peak of the cell's spectral sensitivity (i.e. the rhodopsin peak). With increasing intensity of illumination, however, the peak shifts progressively towards 430 nm, which corresponds to the photoequilibrium with maximum metarhodopsin that can be established by monochromatic light. From this result, it is inferred that the PDA is related to the induced fall in the rhodopsin fraction. The PDA can be abolished, or knocked down, by a long-wavelength flash which reconverts remaining metarhodopsin into rhodopsin. Therefore the decline of the PDA is restrained by the existing amount of metarhodopsin. Possible theories of afterpotentials are discussed.     

Spatial properties of the prolonged depolarizing afterpotential in barnacle photoreceptors. I. The induction process

In invertebrate photoreceptors, when the light stimulus results in substantial net transfer of the visual pigment from the rhodopsin (R) to the metarhodopsin (M) state, the ordinary late receptor potential (LRP) is followed by a prolonged depolarizing afterpotential (PDA). The dependence of the amplitude of the PDA on the amount of pigment conversion is strongly supralinear, and the PDA duration also depends on this amount. These observations indicate an interaction among the elements of the PDA induction process and also make possible a test of the range of this interaction. The test consists of a comparison of the PDA after localized pigment conversion, obtained by strong spot illumination, to that after weaker diffuse illumination converting a comparable total amount of pigment. The experiment was performed on the barnacle lateral eye. The effective spot size was measured by the early receptor potential (ERP), in seawater saturated with CO2, which considerably reduced the electrical coupling between the photoreceptors. The ERP was also used to determine whether there is diffusion of R molecules into the illuminated spot. The spot illumination induced a PDA with small amplitude and long duration, while no detectable PDA was induced by the diffuse light. This indicates that the range of the PDA interaction is much smaller than the entire cell. In addition, the ERP results showed that there was no detectable diffusion of R molecules into the illuminated spot area over 30 min. This measurement, with a calculated correction for the microvillar geometry of the photoreceptor, enabled us to put an upper limit on the diffusion coefficient of the pigment molecules in the inact, unfixed barnacle photoreceptor of D less than 6 X 10(-9) cm2 s-1.     

Transduction in photoreceptors with bistable pigments: Intermediate processes

The prolonged depolarizing after potential (PDA) in the R1–6 receptors of the fly was used to isolate intermediate processes in phototransduction which are not manifested directly in the voltage response. It is first demonstrated that a pigment shift by light from metarhodopsin to rhodopsin in four species of the flies: Drosophila, Calliphora, Chrysomya and Musca induces an independent antagonistic process to the PDA, which is manifested in a strong inhibitory effect on PDA induction and is called the anti-PDA.By using mutants of Drosophila the existence of processes underlying the PDA were examined. The norpA ^H52and the trp mutant were used in which the voltage response of the photoreceptors could be reversibly abolished by elavated temperature and long intense light respectively. It is shown that the excitatory process underlying the PDA could be induced and depressed in conditions that block the voltage response of the photoreceptors, thus indicating the existance of intermediate processes which link the pigment activation by light to the PDA voltage response.Based on material presented at the European Neurosciences Meeting, Florence, September 1978     

Mutation that selectively affects rhodopsin concentration in the peripheral photoreceptors of Drosophila melanogaster

A Drosophila mutant (ninaAP228) that is low in rhodopsin concentration but identical to the wild-type fly in photoreceptor morphology has been isolated. R1-6 photoreceptors of the mutant differ from those of wild type in that (a) the prolonged depolarizing afterpotential (PDA) is absent, (b) concentrations of rhodopsin and opsin are substantially reduced, and (c) intramembrane particle density in the membranes of the rhabdomeres is low. Each of these traits is mimicked by depriving wild- type flies of vitamin A. The ninaAP228 mutation differs from vitamin A deprivation in that in the mutant (a) the rhabdomeric membrane particle density is reduced only in the R1-6 photoreceptors and not in R7 or R8, (b) the PDA can be elicited from the R7 photoreceptors, and (c) photoconversion of R1-6 rhodopsin to metarhodopsin by ultraviolet (UV) light is considerably more efficient than in vitamin A-deprived flies. The absorption properties of the mutant rhodopsin in the R1-6 photoreceptors appear to be identical to those of wild type as judged from rhodopsin difference spectra. The results suggest that the mutation affects the opsin, rather than the chromophore, component of rhodopsin molecules in the R1-6 photoreceptors. The interaction between the chromophore and R1-6 opsin, however, appears to be normal.     

The contribution of pigment transitions to sensitivity changes in the barnacle photoreceptor and the correlation with the prolonged depolarizing afterpotential

A conditioning light can cause a decrease (adaptation) or an increase (facilitation) in the sensitivity of barnacle photoreceptors, as measured by the amplitude of the late receptor potential (LRP). We show that a net transfer of visual pigment from the rhodopsin (R) to the metarhodopsin (M) state induces a large facilitation whereas the reverse transfer results in a much smaller facilitation or even an adaptation. These effects were not due to the response to the conditioning light but to the pigment reactions. When the conditioning light did not alter the pigment population (i.e., M M, R R) it was followed by an intermediate degree of facilitation. These conclusions are correct for cells which have relatively low sensitivity. In sensitive cells, all pigment transitions produce adaptation.LRP facilitation and the prolonged depolarizing afterpotential (PDA) show several common characteristics with respect to pigment transitions: 1.Their magnitude increases with the amount of pigment transferred from R to M. 2. Both are depressed by the M R transition. 3. Their production is impeded by the M R transition. 4. The PDA itself is facilitated by the R M transition and this facilitation decays with a time course comparable to that of LRP facilitation. These results suggest that there may be an underlying process common to LRP facilitation and PDA.     

Reversible phosphorylation of opsin induced by irradiation of blowfly retinae

Summary Light-induced phosphorylation and dephosphorylation of the visual pigment protein, opsin, was investigated in isolated retinae of the blowfly making use of the fact that photon capture by rhodopsin leads to the formation of a thermostable metarhodopsin. Retinae were exposed, in the presence of exogenous³²P-orthophosphate, to an intense blue light which initiated the phosphorylation of opsin (half-time about 5 min at 25 °C). Subsequent exposure of the retina to red light converted all the metarhodopsin present into rhodopsin and triggered a relatively rapid dephosphorylation of rhodopsin (half-time less than 20 s). It is proposed that the phosphorylated forms of rhodopsin and metarhodopsin represent inactive states of the pigment, i.e. phosphorylated metarhodopsin does not initiate reactions leading to the excitation of the photoreceptor cell and phosphorylated rhodopsin cannot be converted into physiologically active metarhodopsin without first being dephosphorylated.Abbreviations R1–6 peripheral retinula cells of the blowfly ommatidium - PDA prolonged depolarizing afterpotential - R rhodopsin - M metarhodopsin - R-P _n phosphorylated rhodopsin - M-P _n phosphorylated metarhodopsin - SDS-PAGE sodium dodecylsulphate polyacrylamide gel electrophoresis     

Photoconvertible pigment states and excitation in Calliphora; the induction and properties of the prolonged depolarising afterpotential

1. The proposed models of two independent groups, which relate the different states of the visual pigment to the excitation of the membrane in invertebrate photoreceptors (with particular reference to the prolonged depolarising afterpotential, the PDA) are compared and evaluated. 2. The validity of the late receptor potential (the "normal" receptor response) as an index of photoreceptor sensitivity, i.e., an index of the number of rhodopsin to metarhodopsin transitions, is verified by concurrent spectrophotometry. 3. Electrophysiological observations alone allow the calculation of 1.3 x 10(8) photopigment molecules in the rhabdom of an R1-6 photoreceptor of a vitamin A-bred Calliphora. 4. The PDA is shown to be quantifiable in terms of the number of rhodopsin to metarhodopsin conversions by the absorption of single light quanta. 5. The comparison of discrete membrane fluctuations (quantum bumps) during the PDA and during exposure to sustained light stimuli that mimic the PDA suggest that, the PDA, similar to the late receptor potential, may be due to the summation of quantum bumps.     

Photoreceptor responses of fruitflies with normal and reduced arrestin content studied by simultaneous measurements of visual pigment fluorescence and ERG

We have simultaneously measured the electroretinogram (ERG) and the metarhodopsin content via fluorescence in white-eyed, wild-type Drosophila and the arrestin2 hypomorphic mutant (w ⁻ ;arr2 ³ ) at a range of stimulus wavelengths and intensities. Photoreceptor response amplitude and termination (transition between full repolarization and prolonged depolarizing afterpotential, PDA) were related to visual pigment conversions and arrestin concentration. The data were implemented in a kinetic model of the rhodopsin–arrestin cycle, allowing us to estimate the active metarhodopsin concentration as a function of effective light intensity and arrestin concentration. Arrestin reduction in the mutant modestly increased the light sensitivity and decreased the photoreceptor dynamic range. Compared to the wild type, in the mutant the transition between full repolarization and PDA occurred at a lower metarhodopsin fraction and was more abrupt. We developed a steady-state stochastic model to interpret the dependence of the PDA on effective light intensity and arrestin content and to help deduce the arrestin to rhodopsin ratio from the sensitivity and PDA data. The feasibility of different experimental methods for the estimation of arrestin content from ERG and PDA is discussed.     

Spatial properties of the prolonged depolarizing afterpotential in barnacle photoreceptors. II. Antagonistic interactions

In the preceding article, we investigated the spatial properties of the induction of the prolonged depolarizing afterpotential (PDA) by shifting visual pigment from the rhodopsin (R) to the metarhodopsin (M) state in the barnacle photoreceptor. In this work, we have studied the ranges within the cell of the antagonistic effects on the PDA of M-to-R transfer. When this transfer occurs during a PDA, it depresses the PDA; when it precedes PDA induction, it impedes that induction ("anti-PDA"). These ranges were previously shown (by a statistical technique) to be at least a few tens of nanometers within a half-second (D greater than 10(-13) cm2 s-1). We now demonstrate, with local illumination techniques in which a PDA was induced in one side of the cell and PDA depression or anti-PDA was induced in the other side, that both ranges are much smaller than the cell diameter (approximately 100 microns) within 30 s (D less than 10(-6)). We further show, using a less direct but shorter-range technique involving colored polarized light, that the interaction of the PDA with the anti-PDA is restricted to less than approximately 6 microns (D less than 6 X 10(-9)). This figure is quite low and suggests that the interaction may be confined to the pigment molecules, possibly in a complex of the type suggested in the preceding article.     

Absorption of light by metarhodopsin modifies the effect of a conditioning light on the barnacle photoreceptor

We show that the effect of an adapting light on the sensitivity of barnacle photoreceptors depends on the direction of net pigment transfer [rhodopsin (R) to metarhodopsin (M) or reverse] occasioned by the adapting light. For stimuli giving no net pigment transfer the state of the pigment appears irrelevant, R R having the same effect as M M. With respect to these, R M gives enhanced facilitation and M R depressed facilitation. This suggests a correlation with the prolonged depolarising after-potential (PDA) and the anti-PDA, which follow R M and M R stimuli respectively. These effects appear mainly in less sensitive cells and for higher amounts of conditioning light — but still well within the physiological range and well below the threshold for PDA and anti-PDA induction. The special interest of these results is that they appear to be interpretable only by assuming that absorption of light by metarhodopsin exerts an effect on the stimulus coincident response (LRP), the first demonstration of such an effect.Based on material presented at the European Neurosciences Meeting, Florence, September 1978     

Sensitivity and photopigments of R1-6, a two-peaked photoreceptor,inDrosophila, Calliphora andMusca

Summary Low vitamin A rearing decreases sensitivity and eliminates the ultraviolet but not the blue sensitivity maximum in R1-6 inDrosophila, Calliphora andMusca (Figs. 2–4). Spectral adaptation functions for control and vitamin A deprived flies yielded derived stable metarhodopsin absorption spectra from spectral sensitivity. Metarhodopsin has a long wavelength maximum and also has an ultraviolet maximum especially in the normal vitamin A condition (Figs. 2–4). M-potentials (fast early-receptor-like potentials) were obtained (Fig. 1) from all three genera in normal vitamin A rearing and were used for spectral adaptation studies (Figs. 2–3); the latter data are approximate inverses of sensitivity based spectral adaptation data. Thus, sensitivity must reflect proportion of rhodopsin, with metarhodopsin being inert in receptor potential generation.Vitamin A effects on spectral functions were further investigated inDrosophila. Ultraviolet (370 nm) and visible (470 nm) sensitivities varied approximately linearly with dietary vitamin A dose (Fig. 5); 370 nm sensitivity decreased more than 470 nm sensitivity at lower doses. Increasing adaptation intensities of 370 and 470 nm caused parallel decreases in spectral sensitivity assayed at 370 and 470 nm in normal vitamin A flies (Fig. 6); the adapting intensities were sufficient to convert photopigment. These and previous results suggest that the two R1-6 spectral peaks are ultimately mediated by one rhodopsin. R1-6 rhabdomeres were structurally similar in high and low vitamin A flies but emitted a long wavelength fluorescence to ultraviolet excitation in high vitamin A flies only (Fig. 7). These results suggest some form of energy transfer; i.e., a carotenoid may capture ultraviolet quanta and transfer energy to rhodopsin via inductive resonance. Spectral adaptation data are consistent with a calculated high rhabdomeric optical density of ECL=0.26 (i.e., 45% of incident light is absorbed) derived from presently available data onDrosophila. Calculations show electro-retinographic sensitivity to be extremely high, perhaps measurable at less than one absorbed quantum per rhabdomere.Supported by NSF grants BMS-74-12817 and BNS-76-11921. We thank M. Chapin, K. Hu, D. Lakin, G. Pransky, D. Sawyer and W. Zitzmann for technical assistance. We are indebted to numerous colleagues especially W. Harris, for comments and suggestions.Chalky Calliphora were obtained from the laboratories of Dr. G. McCann at Caltech and Dr. L. Bishop at the University of Southern California.W-II Musca were from Dr. D. Wagoner at the U.S.D.A. in Fargo, North Dakota.     

In support of the “photopigment model” of vision in invertebrates

Summary Observations of the prolonged depolarising afterpotential (PDA) show that the rate of decay of a PDA is directly proportional of the extent of conversions of rhodopsin to metarhodopsin (regulated by controlled light stimuli). The experiments were designed to detect the effects of a hypothetical inhibitor (proposed by the Excitor-Inhibitor model of invertebrate vision); the results do not support the existence of an inhibitor, but further corroborate the already proposed Photopigment Model.Abbreviations E-I model Excitor-Inhibitor model - LRP late receptor potential - PDA prolonged depolarising afterpotential This work was part of the programme of the Sonderforschungsbereich 114, financed by the Deutsche Forschungsgemeinschaft     

Different ionic conductances are modulated during the late receptor potential and the prolonged depolarizing afterpotential in Hermissenda type A photoreceptors

Wavelength-dependent, bistable phenomena were found in the receptor potential of Hermissenda crassicornis type A photoreceptors. Short exposure to blue light induced a prolonged depolarizing afterpotential (PDA) following the cessation of the light stimulus. Stronger adaptation to blue light, as caused by prolonged exposure and/or high intensity stimulation, effected a reduction in the early depolarizing transient of the late receptor potential (LRP) as elicited by subsequent stimuli. Vast separation of LRP emergence and PDA emergence could be obtained in photoreceptors in which a strong cancellation of the LRP was accomplished but a PDA still emerged after cessation of the light stimulus. Short exposure to yellow light cancelled the PDA, and stronger adaptation restored the LRP (opposite effect to blue light). The initial depolarizing part of the LRP had earlier been demonstrated to be mediated by the lightdependent increase of an inward conductance. In contrast, in this study the PDA was found to be accompanied by the reduction of an outward conductance, most likely a K⁺ conductance. A bistable photopigment system is thought to control the bistable receptor potential phenomenology by regulating the different membrane conductances during the LRP and the PDA.Abbreviations LRP late receptor potential - PDA prolonged depolarizing afterpotential - PHA prolonged hyperpolarizing afterpotential     

Sensitivity and adaptation in R7, an ultraviolet photoreceptor,in theDrosophila retina

Summary Receptor deficient mutants and chromatic adaptation were used to isolate the contribution of R7 to the electroretinogram (ERG) ofDrosophila. R7 was found to be a single-peaked ultraviolet (UV) receptor (Fig. 1). Photoconversion of the UV absorbing rhodopsin (R) to its stable 470–495 nm metarhodopsin (M) was shown to elicit a long-lived negative (depolarizing) afterpotential (Fig. 3) while inactivating R7. Photoreconversion ofM toR reactivates R7 (Fig. 2) and repolarizes the ERG (Fig. 3). The intensities of light needed to elicit afterpotentials by photointerconverting R7 photopigment were found to be about 2 log units greater than for R1-6 photopigment (Fig. 4). Vitamin A deprivation decreases R7 (as well as R8) sensitivity by about 2 log units (through decreased photopigment levels) without changing spectral sensitivity shape (Fig. 5). Vitamin A deprivation further eliminates the light-induced inactivation of R7 allowing experiments designed to characterize the in vivo spectral absorption of R7M. R7M was found to have UV and 495 nm maxima (Fig. 6). No polarization sensitivity was detected in the R7 ERG component. The adaptational properties of R7 are similar to the properties previously established for R1-6 but different from the properties of R8.Supported by NSF grants BMS-74-12817 and BNS 76-11921. I thank M. Chapin, R. Greenberg, K. Hu, A. Ivanyshyn, D. Lakin, G. Pransky, D. Sawyer, J. Walker and W. Zitzmann for technical assistance.     

Photopigment and receptor properties in Drosophila compound eye and ocellar receptors

A review of the spectral sensitivity and the rhodopsin and metarhodopsin characteristics in three compound eye receptor types (R1–6, R7, and R8) and ocellar receptors is presented (Fig. 1). Photopigment properties were determined from measures of conversion efficiency. The photopigments of R1–6 were studied using in vivo microspectrophotometry in the deep pseudopupil of white-eyed flies. These studies yielded a refined estimate of the R1–6 metarhodopsin spectrum (Fig. 2). The quantum efficiency relative to the spectral sensitivity estimate of the rhodopsin spectrum was factored out. The quantum efficiency of rhodopsin is about 1.75 times that of metarhodopsin. The peak absorbance of metarhodopsin was estimated to be about 2.6 times that of rhodopsin. The mechanism of the two-peaked R1–6 spectral sensitivity and metarhodopsin spectrum is discussed in terms of evidence that there is only one rhodopsin in R1–6 and that vitamin A deprivation preferentially lowers ultraviolet sensitivity. The prolonged depolarizing afterpotential is reviewed from the standpoint of the internal transmitter hypothesis of visual excitation. A careful comparison of the intensity-responsivity for photopigment conversion and its adaptional consequences is made (Fig. 3).     

On the implications of bistability of visual pigment systems

The characteristics of different responses of invertebrate photoreceptors are reviewed. Invertebrate photopigment bistability has made possible the functional operational dissection of the pigment transition scheme. Outlasting the usual stimulus-coincident late receptor potential (LRP), additional antagonistic responses have been found: the prolonged depolarizing after-potential (PDA) arising from a net rhodopsin to metarhodopsin pigment shift, and a PDA-depression and an anti-PDA effect which arise from a reverse shift and cancel the PDA when induced during or closely before it. The characteristics of these aftereffects and of the LRP are reviewed, analyzed and compared. Both potentials require rhodopsin activation and they share the characteristics of a common ionic conductance-change mechanism. However, for the LRP response to weak stimuli, no antagonistic metarhodopsin-dependent effect has been found analogous to PDA-depression and the anti-PDA. However, this is just the response level where interactive effects would be weakest. For more intense stimuli, pigment-state effects on the shape of the LRP have been found, and net pigment shifts affect the strength of a facilitatory effect.Based on material presented at the European Neurosciences Meeting, Florence, September 1978     

Photopigment and receptor properties in Drosophila compound eye and ocellar receptors

A review of the spectral sensitivity and the rhodopsin and metarhodopsin characteristics in three compound eye receptor types (R1-6, R7, and R8) and ocellar receptors is presented (Fig. 1). Photopigment properties were determined from measures of conversion efficiency. The photopigments of R1-6 were studied using in vivo microspectrophotometry in the deep pseudopupil of white-eyed flies. These studies yielded a refined estimate of the R1-6 metarhodopsin spectrum (Fig. 2). The quantum efficiency relative to the spectral sensitivity estimate of the rhodopsin spectrum was factored out. The quantum efficiency of rhodopsin is about 1.75 times that of metarhodopsin. The peak absorbance of metarhodopsin was estimated to be about 2.6 times that of rhodopsin. The mechanism of the two-peaked R1-6 spectral sensitivity and metarhodopsin spectrum is discussed in terms of evidence that there is only one rhodopsin in R1-6 and that vitamin A deprivation preferentially lowers ultraviolet sensitivity. The prolonged depolarizing afterpotential is reviewed from the standpoint of the internal transmitter hypothesis of visual excitation. A careful comparison of the intensity-responsivity for photopigment conversion and its adaptional consequences is made (Fig. 3).     

Metarhodopsin control by arrestin,light-filtering screening pigments,and visual pigment turnover in invertebrate microvillar photoreceptors

The visual pigments of most invertebrate photoreceptors have two thermostable photo-interconvertible states, the ground state rhodopsin and photo-activated metarhodopsin, which triggers the phototransduction cascade until it binds arrestin. The ratio of the two states in photoequilibrium is determined by their absorbance spectra and the effective spectral distribution of illumination. Calculations indicate that metarhodopsin levels in fly photoreceptors are maintained below ~35% in normal diurnal environments, due to the combination of a blue-green rhodopsin, an orange-absorbing metarhodopsin and red transparent screening pigments. Slow metarhodopsin degradation and rhodopsin regeneration processes further subserve visual pigment maintenance. In most insect eyes, where the majority of photoreceptors have green-absorbing rhodopsins and blue-absorbing metarhodopsins, natural illuminants are predicted to create metarhodopsin levels greater than 60% at high intensities. However, fast metarhodopsin decay and rhodopsin regeneration also play an important role in controlling metarhodopsin in green receptors, resulting in a high rhodopsin content at low light intensities and a reduced overall visual pigment content in bright light. A simple model for the visual pigment–arrestin cycle is used to illustrate the dependence of the visual pigment population states on light intensity, arrestin levels and pigment turnover.     

The contribution of a sensitizing pigment to the photosensitivity spectra of fly rhodopsin and metarhodopsin.

Most of the photoreceptors of the fly compound eye have high sensitivity in the ultraviolet (UV) as well as in the visible spectral range. This UV sensitivity arises from a photostable pigment that acts as a sensitizer for rhodopsin. Because the sensitizing pigment cannot be bleached, the classical determination of the photosensitivity spectrum from measurements of the difference spectrum of the pigment cannot be applied. We therefore used a new method to determine the photosensitivity spectra of rhodopsin and metarhodopsin in the UV spectral range. The method is based on the fact that the invertebrate visual pigment is a bistable one, in which rhodopsin and metarhodopsin are photointerconvertible. The pigment changes were measured by a fast electrical potential, called the M potential, which arises from activation of metarhodopsin. We first established the use of the M potential as a reliable measure of the visual pigment changes in the fly. We then calculated the photosensitivity spectrum of rhodopsin and metarhodopsin by using two kinds of experimentally measured spectra: the relaxation and the photoequilibrium spectra. The relaxation spectrum represents the wavelength dependence of the rate of approach of the pigment molecules to photoequilibrium. This spectrum is the weighted sum of the photosensitivity spectra of rhodopsin and metarhodopsin. The photoequilibrium spectrum measures the fraction of metarhodopsin (or rhodopsin) in photoequilibrium which is reached in the steady state for application of various wavelengths of light. By using this method we found that, although the photosensitivity spectra of rhodopsin and metarhodopsin are very different in the visible, they show strict coincidence in the UV region. This observation indicates that the photostable pigment acts as a sensitizer for both rhodopsin as well as metarhodopsin.     

Drosophila mutant with a transducer defect

The trp is a conditional phototransduction mutant of Drosophila. Direct electrical measurements and shot noise analysis suggest that a prolonged intense light causes in the mutant a reduction in the quantum efficiency for quantum bump production that does not arise from bleaching of the visual pigment. This effect depends on the duration of the light and only weakly on its intensity. In the normal fly, an intense blue light that shifts the visual pigment from rhodopsin to metarhodopsin, induces an excitatory process manifested by a prolonged depolarizing after potential (PDA). In the mutant, the PDA has a small amplitude and bump noise is superimposed on the response. It can thus be shown that the excitatory process underlying the PDA is also present in those trp mutants where the PDA voltage response is small or absent. It is suggested that the absence of the PDA voltage response in the mutant is probably due to a defect in an intermediate process, which links the excitatory process to the membrane conductance change.Presented at the EMBO-Workshop on Transduction Mechanism of Photoreceptors, Jülich, Germany, October 4–8, 1976