Step-up photophobic response in the ciliate, Stentor coeruleus

Electrical Signaling Beyond Neurons

Neural action potentials (APs) are difficult to interpret as signal encoders and/or computational primitives. Their relationships with stimuli and behaviors are obscured by the staggering complexity of nervous systems themselves. We can reduce this complexity by observing that "simpler" neuron-less organisms also transduce stimuli into transient electrical pulses that affect their behaviors. Without a complicated nervous system, APs are often easier to understand as signal/response mechanisms. We review examples of nonneural stimulus transductions in domains of life largely neglected by theoretical neuroscience: bacteria, protozoans, plants, fungi, and neuron-less animals. We report properties of those electrical signals-for example, amplitudes, durations, ionic bases, refractory periods, and particularly their ecological purposes. We compare those properties with those of neurons to infer the tasks and selection pressures that neurons satisfy. Throughout the tree of life, nonneural stimulus transductions time behavioral responses to environmental changes. Nonneural organisms represent the presence or absence of a stimulus with the presence or absence of an electrical signal. Their transductions usually exhibit high sensitivity and specificity to a stimulus, but are often slow compared to neurons. Neurons appear to be sacrificing the specificity of their stimulus transductions for sensitivity and speed. We interpret cellular stimulus transductions as a cell's assertion that it detected something important at that moment in time. In particular, we consider neural APs as fast but noisy detection assertions. We infer that a principal goal of nervous systems is to detect extremely weak signals from noisy sensory spikes under enormous time pressure. We discuss neural computation proposals that address this goal by casting neurons as devices that implement online, analog, probabilistic computations with their membrane potentials. Those proposals imply a measurable relationship between afferent neural spiking statistics and efferent neural membrane electrophysiology.

Switching of behavioral modes and their modulation by a geometrical cue in the ciliate Stentor coeruleus

Protists ubiquitously live in nature and play key roles in the food web chain. Their habitats consist of various geometrical structures, such as porous media and rigid surfaces, affecting their motilities. A kind of protist, Stentor coeruleus, exhibits free swimming and adhering for feeding. Under environmental and culture conditions, these organisms are often found in sediments with complex geometries. The determination of anchoring location is essential for their lives. However, the factors that induce the behavioral transition from swimming to adhering are still unknown. In this study, we quantitatively characterized the behavioral transitions in S. coeruleus and observed the behavior in a chamber with dead ends made by a simple structure mimicking the environmental structures. As a result, the cell adheres and feeds in narrow spaces between the structure and the chamber wall. It may be reasonable for the organism to hide itself from predators and capture prey in these spaces. The behavioral strategy for the exploration and exploitation of spaces with a wide variety of geometries in their habitats is discussed.

Photosensory transduction in unicellular eukaryotes: A comparison between related ciliates Blepharisma japonicum and Stentor coeruleus and photoreceptor cells of higher organisms

Blepharisma japonicum and Stentor coeruleus are related ciliates, conspicuous by their photosensitivity. They are capable of avoiding illuminated areas in the surrounding medium, gathering exclusively in most shaded places (photodispersal). Such behaviour results mainly from motile photophobic response occurring in ciliates. This light-avoiding response is observed during a relatively rapid increase in illumination intensity (light stimulus) and consists of cessation of cell movement, a period of backward movement (ciliary reversal), followed by a forward swimming, usually in a new direction. The photosensitivity of ciliates is ascribed to their photoreceptor system, composed of pigment granules, containing the endogenous photoreceptor -- blepharismin in Blepharisma japonicum, and stentorin in Stentor coeruleus. A light stimulus, applied to both ciliates activates specific stimulus transduction processes leading to the electrical changes at the plasma membrane, correlated with a ciliary reversal during photophobic response. These data indicate that both ciliates Blepharisma japonicum and Stentor coeruleus, the lower eukaryotes, are capable of transducing the perceived light stimuli in a manner taking place in some photoreceptor cells of higher eukaryotes. Similarities and differences concerning particular stages of light transduction in eukaryotes at different evolutional levels are discussed in this article.

Detection and localization of a putative cyclic-GMP-activated channel protein in the protozoan ciliate Stentor coeruleus

Immunoblotting and immunocytochemical assays were employed to identify and localize a channel protein activated by cyclic GMP (cGMP) in the protozoan ciliate Stentor coeruleus. Analysis of whole-cell homogenate with antibodies raised against the alpha-subunit of the cGMP-activated channel protein from bovine rod outer segments and against cGMP revealed four major protein bands with molecular masses of 40 kDa, 63 kDa, and over 120 kDa, which bound cGMP. However, only a cGMP-binding protein of 63 kDa, corresponding to the alpha-subunit of the cGMP-activated ion channel protein from bovine rod outer segments, was found in the ciliate cortex fraction. The functional cGMP-activated channel protein was also shown to be present in the cortex fraction of S. coeruleus by patch-clamp measurements of artificial liposomes. Incorporation of the cortex fraction into liposomes resulted in the appearance of ion channel activity related to cGMP. The reconstituted protein channels were strongly inhibited by l-cis-diltiazem, a known potent blocker of many types of cyclic-nucleotide-activated channels. The results presented here are the first demonstration of the existence and localization of a putative cGMP-activated channel protein in the ciliate S. coeruleus. Cyclic-nucleotide-activated channel proteins are nonspecific cation channels which mediate the receptor potentials in photoreceptor cells and in cells of the olfactory epithelium. On the basis of these data, we suggest that the 63 kDa protein identified in Stentor coeruleus is also a cGMP-activated ion channel and that it may be involved as an effector in the photosensory transduction pathway leading to the motile photophobic response in this ciliate protist.

A videomicroscopic study of the effect of l-cis-diltiazem on the photobehavior of Stentor coeruleus

The protozoan ciliate Stentor coeruleus displays a step-up photophobic response to an increase in light intensity in its environment. The motile response consists of a delayed stop of ciliary beating and transient ciliary reversal period. Such light-avoiding behavior was significantly influenced by an incubation of cells with l-cis-diltiazem, a common blocker of cyclic guanosine monophosphate (cGMP)-gated ion channel conductance. The introduction of l-cis-diltiazem to the medium induced ciliary reversal in control cells, mimicking the step-up photophobic response. In light-stimulated ciliates, the presence of this inhibitor caused a substantial decrease of the latency of ciliary stop response, prolongation of the ciliary reversal duration and also an increase of cell photoresponsiveness in a dose- and time-dependent manner. The obtained behavioral results support the suggestion that the photosensitive ciliate S. coeruleus possesses cGMP-gated channels, which may be involved in the process of light signal transduction for the motile photophobic response.

Additional evidence for the cyclic GMP signaling pathway resulting in the photophobic behavior of Stentor coeruleus

We report that exo- and endogenous guanosine 3',5'-cyclic monophosphate (cGMP) specifically influenced the photophobic response. In behavioral experiments the slowly hydrolyzable and membrane-permeable analogs of cGMP (8-bromo-cGMP [Br-cGMP] and N6,2'-o-dibutyryl-cGMP) dramatically prolonged the time for ciliary stop response and decreased the duration of ciliary reversal in a dose-dependent manner. When analogs of adenosine 3',5'-cyclic monophosphate (cAMP) (8-bromo-cAMP or N6,2'-o-dibutyryl-cAMP) were used, no essential effects were detected on the kinetics of the photophobic response. Both nonspecific cyclic nucleotide phosphodiesterase (PDE) activity inhibitors (3-isobutyl-1-methylxanthine [IBMX] and 1,3-dimethylxanthine [theophylline]) and the highly specific cGMP-PDE activity inhibitor 1,4-dihydro-5-[2-propoxyphenyl]-7H-1,2,3-triazolo[4,5-d]pyrimidine-7-one (zaprinast) mimicked the effects of cGMP analogs. Treatment of cells with an inhibitor of guanylate cyclase activity (6-anilino-5,8-quinolinedione [LY 83583]) exerted an effect opposite to that of cGMP analogs and PDE activity inhibitors. The positive physiological effect of LY 83583 was significantly diminished in ciliates that were treated simultaneously with Br-cGMP. In an assay of cell cyclic nucleotide content, the exposure of dark-adapted Stentor to light evoked a transient decrease in the basal level of intracellular cGMP. Alterations in internal cGMP levels were more distinct when the intensity of applied illumination was increased. In the presence of IBMX or theophylline the basal content of cGMP was markedly enhanced, and the photoinduced changes in cGMP level were less pronounced. In this paper the possible whole molecular mechanism by which the ciliary orientation in Stentor is controlled by light is presented.

Phototaxis in the ciliated protozoan Ophryoglena flava: dose-effect curves and action spectrum determination

The sensitivity of positive phototactic orientation of cells of the ciliated protozoan Ophryoglena flava has been measured for white light, broad-band blue and red light, and narrow-band monochromatic light, using a laboratory-developed computer aided system. The white-light fluence rate-response curve shows that there is no negative phototaxis in the fluence rate range investigated (0-15 W/m2) and no adaptation phenomena; it is very well fitted by a hyperbolic function; the fluence rate curves under broad band blue and red light (full width at half maximum, FWHM= 100 nm) can be fitted by the same model. The saturation level is, within experimental errors, the same for the three curves, indicating that there are no chromaticity effects and that if there is more than one photoreceptor pigment, they act independently of each other. The fluence rate-response curves determined under narrow band monochromatic light (FWHM = 10 nm) can also be fitted by the same model and show, within experimental errors, the same saturation level. An action spectrum for positive phototaxis at 10-nm intervals has been calculated from fluence rate-response curves: it shows three maxima, at 420, 540 and 590 nm. This action spectrum is significantly different from the ones for photomotile responses in Blepharisma japonicum, Stentor coeruleus and Chlamydodon mnemosyne, whereas it resembles the ones of Paramecium bursaria and Fabrea salina.

Do plant photoreceptors act at the membrane level?

All of the photoreceptors involved in the absorption and transduction of light energy in photosynthesis are integral (carotenoid, chlorophyll) or peripheral (phycobilin) membrane proteins. The informational photoreceptors (phytochrome) and the flavoprotein (carotenoprotein?) cryptochrome, could be integral (carotenoprotein, flavoprotein) or peripheral or soluble (phytochrome, flavoprotein) pigment-protein complexes. The primary activity of the informational photoreceptors is unlikely to involve energization of primary active transport: the solute fluxes produced in this way would not form a quantitatively significant link in the perception-transduction-response sequence. By contrast, regulation of mediated solute fluxes at the plasmalemma could effect a substantial amplification of the absorbed photon signal, i.e. a large change in moles of solute transported could result from the absorption of 1 mol of photons. Modulation of the passive influx (or active efflux) of protons or calcium ions at the plasmalemma are likely targets for regulation by photoreceptors. Calcium flux regulation is particularly attractive in view of the ubiquity of calmodulin activity in eukaryotes, although problems could arise in maintaining the uniqueness of phytochrome messages vis-à-vis cryptochrome messages. Temporal analysis of the relation between photoreceptor changes and electrical effects resulting from changes in ion fluxes cannot, in general, rule out the involvement of intermediates between the redox or conformational change in the photoreceptor and the observed change in ion flux. Although slow in terms of the potential rate of change on solute fluxes resulting from direct interaction of a photoreceptor and a solute porter, the observed rates of signal transduction are well in excess of any obvious ‘need’ on the part of the plant in terms of rates of response to environmental changes.

Photodynamic effects of hypericin on lipid peroxidation and antioxidant status in melanoma cells

Photodynamic-induced cytotoxicity by hypericin (HYP) was studied on three human melanoma cell lines: one pigmented cell line (G361) and two amelanotic cell lines (M18 and M6). No significant variation in the rate of uptake and in the maximum level of HYP incorporation for the different cells was observed. In the dark, no cytotoxicity was observed in the range 0-10-6 M HYP for the three cell lines. Amelanotic cells were found to be more sensitive than pigmented cells to irradiation of HYP with visible light (lambda > 590 nm). In addition, for the three cell lines HYP-induced photocytotoxicity was found to be drug-dose and light-dose dependent. Under the conditions used, thiobarbituric acid-reacting substances (TBARs) were significantly increased in amelanotic cells after irradiation (P < 0.0001). By contrast, the amount of TBARS remained unchanged in pigmented cells. Antioxidant defenses including enzymes and glutathione (GSH) were assayed before and after HYP photosensitization. Significantly increased total SOD activity was observed after photosensitizaton for amelanotic cells (P < 0.05), while glutathione peroxidase (GSHPx) and catalase (Cat) activities but also GSH levels were significantly decreased (P < 0.01). In pigmented cells a significantly increased Cat activity was found (P < 0.05), whereas GSHPx was unaffected after irradiation. It can be inferred that (a) HYP may be an effective PDT agent for melanoma and (b) there is a relationship between melanin content and sensitivity to HYP phototoxicity in human melanoma cells.

Initial spectroscopic characterization of the ciliate photoreceptor stentorin

Stentorin serves as the primary photosensor in the single cell ciliate, Stentor coeruleus, for its photophobic and phototactic response to light of visible wavelengths. We separated two subunits, stentorin-2A and -2B, from the previous stentorin complex ('stentorin-2') of greater than half a million molecular mass isolated from the photoreceptor organelle (pigment granule). Stentorin-2B bears the chromophore covalently linked to an approx. 50 kDa apoprotein, as determined by SDS-urea-PAGE. Partial amino acid sequences were obtained from this 50 kDa subunit. Its visible and CD spectra were found to be similar to those of stentorin-2. The steady-state and time-resolved fluorescence spectra of stentorin-2B, in H2O and D2O buffers, were also similar to those of stentorin-2. This suggests that the 50 kDa subunit retains the spectral integrity and primary photoreactivity of the stentorin-complex. The picosecond time-resolved fluorescence study revealed that the short picosecond emission component (tau F approximately equal to 8-10 ps) was the predominant emitting species in stentorin-2B and -2, followed by longer decaying species. No deuterium solvent effect was seen in this fast-decaying species. The possible mechanism for the primary photoreaction appears to involve electron transfer coupled with proton transfer.

Spectroscopic and photoacoustic studies of hypericin embedded in liposomes as a photoreceptor model

In photoresponsive ciliates, like Blepharisma japonicum and Stentor coeruleus, the photoreceptor pigments responsible for photomotile reactions are hypericin-type chromophores packed in highly osmiophilic subpellicular granules. Lipopsomes loaded with hypericin can constitute a simple model system, appropriate for understanding the primary light-induced molecular events triggering the sensory chain in these microorganisms. Optical absorption, steady-state and time-resolved fluorescence and pulsed photoacoustic calorimetry have been used to measure spectral distributions, fluorescence lifetimes, radiative and radiationless transition quantum yields of hypericin when assembled into egg L-alpha-phosphatidylcholine liposomes. With respect to hypericin ethanol solutions, both absorption and fluorescence maxima are 5 nm red shifted when the pigment is inserted into the lipidic microenvironment, regardless of the hypericin local concentration. Increasing by 100 times the hypericin local concentration decreases the relative fluorescence quantum yield by a factor of around 150 and the fraction of thermally released energy, conversely, increases from 0.6 to 0.9. From the analysis of fluorescence lifetimes and their relative amplitudes it appears that a subnanosecond living component is predominant at the highest hypericin local concentrations.

Photosensory transduction in ciliates. Role of intracellular pH and comparison between Stentor coeruleus and Blepharisma japonicum

To test the hypothesis that light signal transduction in the unicellular ciliates Stentor coeruleus and Blepharisma japonicum involves a change in intracellular pH as an initial signal following photoexcitation, we studied the dependence of the photophobic responses of the cells to changes in extracellular pH and to reagents that specifically affect intracellular pH. The extracellular pH can modify not only the intracellular pH, but can even reverse the sign of the pH gradient across the cell membrane. Thus, as predicted by the hypothesis, low extracellular pH reversibly inhibited the photophobic response of the ciliates. The intracellular pH-modulating reagents tested included ammonium chloride, a membrane-permeable weak acid that lowers the intracellular pH, and the protonophores carbonylcyanide m-chlorophenyl-hydrazone (CCCP) and carbonylcyanide p-(trifluoromethoxy)-phenyl-hydrazone (FCCP), which collapse the pH gradient across the cell membrane. The low pH and protonophore treatments caused a gradual inhibition of the photophobic responses in both ciliates. The observed reduction of the responsiveness of the cells to visible light can be attributed to the alteration of the intracellular pH, which is suggested to play a specific role in the photosensory transduction in both Stentor coeruleus and Blepharisma japonicum.

Photosensory transduction in ciliates. I. An analysis of light-induced electrical and motile responses in Stentor coeruleus

Light-induced membrane potential changes and motile responses have been studied in Stentor cells with intracellular microelectrodes and video microscopy, respectively. Intracellular microelectrode recordings showed that step-up increase in light intensity induced an electrical membrane response which consisted of an initial membrane depolarization (photoreceptor potential) followed by an action potential and maintaining phase of depolarization (afterdepolarization). The amplitude of the receptor potential is dependent on the intensity of light stimulus and the action potential appears with a lag period (latency) after the onset of light stimulus. The extent of the membrane afterdepolarization is dependent on the intensity and duration of stimulus used. A close time correlation has been established between the latency for the action potential and the onset of ciliary reversal (stop response). A time correlation was also observed between the duration of the membrane afterdepolarization and the duration of backward swimming. The action spectrum for the photoreceptor potential amplitude of Stentor resembled the action spectra for the latency of ciliary reversal and the photoresponsiveness, indicating that the photomovement response and membrane potential changes are coupled through the same photosensor system. A hypothesis on the photosensory transduction chain in Stentor is discussed according to which the photoreceptors and the ciliary apparatus is mediated by the membrane potential changes.

Photosensory transduction in ciliates. II. Possible role of G-protein and cGMP in Stentor coeruleus

The heterotrichous ciliate, Stentor coeruleus, exhibits a well-defined photophobic response to a sudden increase in the intensity of visible light. The phobic reactions usually appear with a latency period (i.e. a time delay between the onset of the stimulus and the stop response). This latency of phobic response was significantly increased when the cells were incubated with 8-bromo-guanosine 3',5'-cyclic monophosphate. In the presence of this nucleotide, a reduction of cell responsiveness (i.e. the number of photophobically responding cells) was also observed. Similar effects were observed when cells were treated with pertussis toxin, a G-protein activity modulator, and 3'-isobutyl-methylxanthine, an inhibitor of guanosine 3',5'-cyclic monophosphate (cGMP) phosphodiesterase. The G-protein activator fluoroaluminate and 6-anilino-5,8-quinolinedione (LY 83583) (an effective agent for lowering cellular cGMP levels) showed opposite effects on the cell photophobic response. These results indirectly suggest that the level of cytoplasmic cGMP, possibly modulated by a G-protein-coupled cGMP phosphodiesterase, plays a phototransducing role in Stentor. In addition, using an antiserum raised against bovine transducin, a cross-reacting protein with an apparent molecular mass of 39 kDa was detected on immunoblots. The alpha-subunit of a Stentor G-protein has also been partially cloned and sequenced. However, the possible coupling between the G-protein and the putative phosphodiesterase remains to be established.

Effects of green light on biological systems

Green light (510-565 nm) constitutes a significant portion of the visible spectrum impinging on biological systems. It plays many different roles in the biochemistry, physiology and structure of plants and animals. In only a relatively small number of responses to green light is the photoreceptor known with certainty or even provisionally and in even fewer systems has the chain of events leading from perception to response been examined experimentally. This review provides a detailed view of those biological systems shown to respond to green light, an evaluation of possible photoreceptors and a review of the known and postulated mechanisms leading to the responses.

Photoactivation of hypericin generates singlet oxygen in mitochondria and inhibits succinoxidase

Photosensitized inhibition of mitochondrial succinoxidase by hypericin was measured in vitro and found to be drug-dose, light-dose, and wavelength dependent. Singlet oxygen generation, monitored using the singlet oxygen trap tetramethylethylene, and oxygen consumption in isolated mitochondria sensitized by hypericin were also light-dose and wavelength dependent. Unequivocal evidence for the generation of singlet oxygen was obtained using kinetic isotope ratios of products from the reaction between singlet oxygen and geminally deuterated tetramethylethylene. An action spectrum for the inhibition of succinoxidase was measured at wavelengths between 400 and 700 nm and found to parallel the recorded visible absorption spectrum of hypericin in isolated mitochondria. The greatest singlet oxygen generation, oxygen consumption, and succinoxidase inhibition occurred with white light or 600 nm irradiation. These data are consistent with a type II singlet-oxygen-mediated mechanism for hypericin induced photosensitized inhibition of mitochondrial succinoxidase.

Structure and function of the photoreceptor stentorins in Stentor coeruleus. I. Partial characterization of the photoreceptor organelle and stentorins

The unicellular ciliary protozoan, Stentor coeruleus, exhibits photophobic and phototactic responses to visible light stimuli. The pigment granule contains the photoreceptor chromoproteins (stentorins). Stentorin localized in the pigment granules of the cell serves as the primary photoreceptor for the photophobic and phototactic responses in this organism. An initial characterization of the pigment granules has been described in terms of size, absorbance spectra and ATPase activity. Two forms of the stentorin pigments have been isolated from the pigment granules. Stentorin I has an apparent molecular weight of 68,600 and 52,000 by SDS-PAGE (at 10 and 13% gel, respectively) or 102,000 by steric exclusion HPLC, whereas stentorin II is a larger molecular assembly probably composed of several proteins (mol. wt. greater than 500,000). Stentorin I is composed of at least two heterologous subunits corresponding to apparent mol. wts. of 46,000 (fluorescent, Coomassie blue negative) and 52,000 (fluorescent, Coomassie blue positive) on SDS-PAGE (13% gel). However, these values were found to be strongly dependent on the degree of crosslinking in the acrylamide gel. Stentorin II appears to be the primary photoreceptor whose absorption and fluorescence properties are consistent with the action spectra for the photoresponses of the ciliate to visible light.

Photoreceptor pigments for photomovement of microorganisms: some spectroscopic and related studies

Optical spectroscopy of photoreceptor pigments can substantially contribute to our understanding of the molecular processes which are the basis of photoreception and sensory transduction in photomotile microorganisms. The main spectroscopic techniques are briefly illustrated, together with the most significant types of progress that can be achieved. A few "case examples" are discussed in some detail: Halobacterium, with particular attention to the contribution of flash photolysis studies to the identification and characterization of sensory rhodopsins; Euglena, and the role of in vivo microspectrofluorometry in confirming the flavin nature of its photoreceptor pigment; the first suggestions on the rhodopsin-like nature of the Chlamydomonas photosensing system; Stentor and Blepharisma and the contribution of static and time-resolved fluorescence studies to a molecular model of the primary events in their photoreceptor pigments (stentorin and blepharismin) and systems.

The ratio of extracellular Ca2+ to K+ ions affects the photoresponses in Stentor coeruleus

1. Stentor coeruleus exhibits negative phototaxis (due to phototactic orientation response) and step-up photophobic response (avoiding reaction) to visible light. 2. The effect of Ja-value ([K+]/[Ca2+]1/2) and calcium ion concentration of the surrounding medium on the photoresponses in Stentor were studied. 3. The both types of photoresponses in Stentor are greatly affected by the Ja-value. A higher Ja-value medium suppressed the step-up photophobic response of Stentor, whereas the organism showed a higher degree of phototactic orientation response in higher Ja-value solutions. 4. The effect of the Ja-value on the step-up photophobic response was opposite to that on the phototactic orientation response. 5. With increasing calcium concentration but at a constant Ja-value, the number of Stentor showing the step-up photophobic response increased, whereas the phototactic orientation response of Stentor was suppressed at higher Ca2+ concentrations. 6. The effect of the calcium concentration on the photophobic response was also opposite to that on the phototactic orientation response, as in the case of Ja-value effect.

Light-adaptation in the photophobic response by Stentor coeruleus

Effects of preillumination on photophobic response (light-adaptation) and recovery of the photophobic sensitivity in the dark (dark-adaptation) in Stentor coeruleus were examined. When the cells were preilluminated with white light of 7.80 W/m2 for 2 min, the fluence-rate response curve of photophobic response was shifted toward higher light intensities by half an order of magnitude compared to the one without preillumination. Preillumination with a higher light intensity resulted in a further shift of the fluence-rate response curve. An action spectrum for light-adaptation showed a primary peak at 610 nm and secondary peaks at 540 and 480 nm which are almost identical to the peaks observed in the photophobic action spectrum. The light-adapted cells showed a recovery of their photophobic sensing ability following dark treatment. Dark-adaptation resulted in total recovery of photophobic sensing ability in 8 minutes for the most cases examined.

Fast stages of photoelectric processes in biological membranes. II. Visual rhodopsin

The functioning of visual rhodopsin as a photoelectric generator has been demonstrated with a direct method. Photoreceptor discs were incorporated into a phospholipid-impregnated collodion film. Illumination of the resulting system with continuous light was found to induce formation of an electric potential (the disc-free side positive) that was measured with two electrodes separated by the film. A photopotential exceeding 40 mV was shown. It dissipated before the light source was switched off. A 15 ns 530-nm laser flash induced the formation of a photopotential of up to 35 mV whose appearance was preceded with a small oppositely directed electrogenic phase. This "negative" photoresponse took less than 200 ns. The "positive" photoresponse was composed of at least two phases (t 1/2 about 500 microseconds and several milliseconds). The latter was shown to correlate with formation of metarhodopsin II. A 347-nm laser flash added after a 530-nm flash resulted in a photoelectric effect similar to that initiated by 530-nm flash but of opposite direction. The 347-nm response was completely abolished by hydroxylamine preventing the accumulation of metarhodopsin II. The response at 530 nm proved to be hydroxylamine-resistant. Both the amplitude and the decay time of the flash-induced potential were maximal in the response to the first flash, each subsequent flash being less effective than the preceding one. Flashes were found to cause acceleration of the photopotential decay. The latter effect proved to be due to a increase of membrane conductance that developed faster than in 50 ms. Addition of 11-cis retinal after illumination improved the amplitude of the photoresponse but not the conductance. The light-induced increase in conductance was insensitive to hydroxylamine. It is suggested that a function of visual rhodopsin consists of generating a potential difference across the photoreceptor disc membrane which responds with a increase in membrane permeability to a rise of the membrane potential. A possible role of an electric break-down of the membrane, induced by the rhodopsin-generated local or partially delocalized electric field has been discussed.

The pH dependence of photosensory responses in Stentor coeruleus and model system

1. Live Stentor coeruleus exhibits a substantially red-shifted fluorescence maximum, corresponding to the anionic species of the photoreceptor chromophore. No change was observed in either the absorption or fluorescence excitation spectrum, indicating an efficient deprotonation of the photoreceptor pigment upon excitation by light. 2 Changes in external pH exhibit a dramatic effect on the pulmonary response of Stentor. Phototaxis is specifically inhibited at pH less than 6, with loss of photosensory perception which is restored when the pH is returned to pH greater than 6. 3. Fluorescence changes of 9-aminoacridine in suspensions of live Stentor indicate the generation of a pH gradient upon irradiation with light. Both pH gradient and phototaxis were inhibited by the addition of nigericin and p-tri-fluoromethoxy carbonyl cyanide phenylhydrazone (FCCP). 4. Incorporation of the Stentor photoreceptor protein in to artificial liposomes demonstrates the ability of the system to generate pH gradients across model membranes as monitored by the quenching of 9-aminoacridine fluorescence. The effect of external pH on net proton movement in the model system is strikingly similar to the pH dependent of the liver Stentor, thus lending support for transient proton flux being an important mode of light signal processing for photosensory transduction.