PHOTOSENSORY TRANSDUCTION IN CILIATES. I. AN ANALYSIS OF LIGHT-INDUCED ELECTRICAL AND MOTILE RESPONSES IN Stentor coeruleus

Abstract— Light-induced membrane potantial changes and motile responses have been studied in Stentor cells with intracellular microelectrodes and video microscopy, respectively. Intracellulae microelectrode showed that step-up increase in light 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 potetial is dependent on the intensity of light stimulus and the action potetials appears with a lag period (latency) after the onset of light stimuklus. The extent of the membrane established between the latency for te action poitential and the onset of ciliary reversal (stop responses). 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, iondicating 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 ehich the photoreceptors and the ciliary apparatus is mediated by the membrane potential canges.     

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.     

PHOTOSENSORY TRANSDUCTION IN CILIATES. IV. MODULATION OF THE PHOTOMOVEMENT RESPONSE OF Blepharisma japonicum BY cGMP

Abstract— The effect of various modulators of cytoplasmic guanosine 3',5'-cyclic monophosphate (cGMP) level on the step-up photophobic responses in Blepharisma japonicum has been investigated to clarify the possible role of cGMP in the mechanism of photosensory signal transduction. Membrane-permeable analogs of cGMP, 8-bromo-guanosine 3',5'-cyclic monophosphate or dibutyryl cGMP, caused a marked dose-dependent prolongation of the latency for the photophobic response, resulting in inhibition of the photophobic response in Blepharisma japonicum. A similar effect was observed when cells were treated with 3'-isobutylmethylxanthine (IBMX), a phosphodiesterase inhibitor, and pertussis toxin, a G-protein activity modulator. The G-protein activator, fluoroaluminate, and 6-anilino-5,8-quinolinedione (LY 83583), an agent which effectively lowers the cytoplasmic cGMP level, significantly enhanced the photoresponsiveness of these ciliates to visible light stimuli. These results suggest that cellular cGMP serves as a signal modulator in the photophobic response of Blepharisma japonicum.     

ACTION SPECTRUM AND ELECTROPHYSIOLOGICAL RESPONSES CORRELATED WITH THE PHOTOPHOBIC RESPONSE OF STENTOR COERULEUS

Abstract— The blue-green ciliate. Stentor coeruleus , is found predominantly in shady places. This concentration occurs because stentor responds when swimming from a shaded area to a lighted area by reversing the direction of its ciliary beat and reorienting its swimming direction until it once again is in the shaded area. A graded receptor potential is recorded from microelectrodes in vacuoles of stentor when the animal is photically stimulated. For all but very weak stimuli this receptor potential is sufficient to elicit a regenerative transmembrane response of variable amplitude in a swimming animal. Suprathreshold electrical stimuli also elicit this regenerative response. In turn the regenerative response is coupled to ciliary reversal. Thus ciliary reversal appears to be produced whenever the photic receptor potential crosses the threshold for elicitation of the regenerative response.
Using the threshold for production of ciliary reversal as a criterion response, an action spectrum was obtained. This action spectrum correlates well with the absorption spectrum of the major pigment of S. coeruleus , stentorin. Stentor bleached of pigment also have an elevated threshold for ciliary reversal. Thus stentorin seems to be the photosensitive pigment in stentor responsible for its photophobic behavior.     

Contribution of phosphoinositide-dependent signalling to photomotility of Blepharisma ciliate

The effect of experimental procedures designed to modify an intracellular phosphoinositide signalling pathway, which may be instrumental in the photophobic response of the protozoan ciliate Blepharisma japonicum, has been investigated. To assess this issue, the latency time of the photophobic response and the cell photoresponsiveness have been assayed employing newly developed computerized videorecording and standard macro-photographic methods. Cell incubation with neomycin, heparin and Li+, drugs known to greatly impede phosphoinositide turnover, causes evident dose-dependent changes in cell photomotile behaviour. The strongest effect on photoresponses is exerted by neomycin, a potent inhibitor of polyphosphoinositide hydrolysis. The presence of micromolar concentrations of neomycin in the cell medium causes both prolongation of response latency and decrease of cell photoresponsiveness. Neomycin at higher concentrations (> 10 microM) abolishes the cell response to light at the highest applied intensity. A slightly lower inhibition of cell responsiveness to light stimulation and prolongation of response latency are observed in cells incubated in the presence of heparin, an inositol trisphosphate receptor antagonist. Lithium ions, widely known to deplete the intracellular phosphoinositide pathway intermediate, inositol trisphosphate, added to the cell medium at millimolar level, also cause a slowly developing inhibitory effect on cell photoresponses. Mastoparan, a specific G-protein activator, efficiently mimics the effect of light stimulation. In dark-adapted ciliates, it elicits ciliary reversal with the response latency typical for ciliary reversal during the photophobic response. Sustained treatment of Blepharisma cells with mastoparan also suppresses the photoresponsiveness, as in the case of cell adaptation to light during prolonged illumination. The mastoparan-induced responses can be eliminated by pretreatment of the cells with neomycin. Moreover, using antibodies raised against bovine transducin, a cross-reacting protein with an apparent molecular mass of about 55 kDa in the Blepharisma cortex fraction is detected on immunoblots. The obtained results indirectly suggest that the changes in internal inositol trisphosphate level, possibly elicited by G-protein-coupled phospholipase C, might play a role in the photophobic response of Blepharisma. However, further experiments are necessary to clarify the possible coupling between the G-protein and the putative phospholipase C.     

PHOTOMOVEMENT RESPONSES OF THE HETEROTRICHOUS CILIATE Blepharisma Japonicum

Light-induced movement responses of the heterotrichous ciliate Blepharisma japonicum were studied by physiological experiments. Two photosensory responses could be identified. A step-up photophobic response is observed as a very rapid backward movement. Microbeam irradiations of individual cells showed that only the anterior part of the ciliate is able to perceive the light stimulus that mediates the phobic reaction. The action spectrum peaks at approximately 400 nm, which indicates that a blue light receptor is involved.
Positive photokinesis of Blepharisma could be shown as a forward movement that is accelerated by increasing the applied photon fluence rate. The steady state level of the velocity depends highly on wavelength and photon fluence rate of the actinic light. After specific inhibition of the phobic reaction bv 1 m/W NH₄⁺, photokinesis can be induced by microbeam irradiation at any part of the cell.
We isolated two main pigments by thin layer chromatography and characterized them as hypericin-like compounds: a red pigment that is obviously responsible for the red color of the ciliates (= blepharismin). and a yellow one with maximal absorption near 420 nm. The possible photoreceptor functions of these pigments are discussed.
We could not find in Blepharisma a distinct phototactic behavior which is so typical for the related ciliate Stentor.     

Analyses of structure of photoreceptor organelle and blepharismin-associated protein in unicellular eukaryote Blepharisma

In the ciliated protozoan Blepharisma, step-up photophobic response is believed to be mediated by a novel type of photosensory pigment known as "blepharismins" (BL) that are contained in the pigment granules located just beneath the plasma membrane. We examined the ultrastructure of the pigment granules by freeze-fracture and thin-section electron microscopy and proposed a schematic diagram showing the granules' three-dimensional inner membranous structure. Some of the BL are suggested to be associated with 200 kDa membrane protein. High-pressure liquid chromatography analysis of pigment species associated with 200 kDa protein obtained from blue forms of Blepharisma (oxyblepharisma) revealed that the 200 kDa protein was associated with five types of oxyblepharismin. The fluorescence intensity was increased when the pigments were dissociated from the 200 kDa protein. The result supports the hypothesis that the pigment-200 kDa complex is able to transduce light energy into signals mediating the photobehavior of Blepharisma.     

PHOTOTACTIC RESPONSE OF CHLAMYDOMONAS TO FLASHES OF LIGHT –II. RESPONSE OF INDIVIDUAL CELLS

Abstract— –Video-microscope studies provide further evidence that Chlamydomonas can become oriented in response to a single short flash of light. Following a flash, 50% of the cells in a negatively phototactic population undergo a transient deflection in swimming path ('turn response'), 10% show a 'stop response', and 40% continue to swim straight ahead. The direction of turning is related to the direction of the stimulus; a majority of cells turn away from the flash source. Repetitive flashing at 60 per s elicits oriented swimming, indistinguishable from that observed with continuous light. Responses at the onset of repetitive flashing resemble single-flash responses, reinforcing the idea that response to a single flash corresponds to the initial stages of orientation to continuous light. A stop response sometimes occurs at the onset of orientation to repetitive flashing, but it is apparently not an essential component of orientation. The fact that only 60% of the cells turn or stop in response to a flash is consistent with the hypothesis that light direction is perceived by comparing light absorbed in one photoreceptive region at two instants in time (before and during the flash). The only cells to turn or to stop would be those in which the photoreceptor organelle is appropriately oriented at the instant of the flash.     

ISOLATION OF BLEPHARISMIN-BINDING 200 kDa PROTEIN RESPONSIBLE FOR BEHAVIOR IN Blepharisma

Abstract— The ciliated protozoan, Blepharisma, shows an avoidance reaction (step-up photophobic response) in response to light stimulation. A profile of a gel-permeation of a crude detergent-solubilized sample of the cells resulted in several red-colored fractions. Among these blepharismin-containing fractions, the fractions III-V did not contain amino acids. The peak of fraction II monitored by 580 nm absorbance was much smaller. A prominent peak appeared in fraction I, which contained a large amount of amino acids. The absorption spectrum of fraction I was well fitted to the action spectrum of the step-up photophobic response, although free pigment (blepharismin) also fitted. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of this fraction resulted in a thicker band corresponding to molecular mass of 200 kDa. These results suggest that the 200 kDa chromoprotein (blepharismin-protein complex) is responsible for the step-up photophobic response in Blepharisma. The absorption spectrum of free chromophore dissociated from the chromophore-protein complex was identical to free red pigment termed blepharismin. The absorption spectrum of the other fractions agreed with that of thin-layer chromatography-purified red pigment, indicating that the pigments contained in these fractions are free pigment dissociated from the chromophore-protein complex.     

BEHAVIORAL RESPONSES IN Paramecium multimicronucleatum TO VISIBLE LIGHT

Specimens of colorless Paramecium multimicronucleatum were found to respond to visible light. They accumulated in the shaded region (photodispersal) of a half-shaded glass tube during 2 min exposure to visible light. The specimens showed avoiding reaction upon both spatial and temporal increase in light intensity (step-up photophobic response). Steady-state swimming velocity (orthokinesis) was higher, while steady-state frequency of spontaneous change in swimming direction (klinokinesis) was lower when the light intensity was kept higher. In a light with wavelength of 440 nm the velocity was highest, while the frequency was lowest. The specimens did not show phototaxis (light direction-oriented locomotion). Spectral sensitivity curves for both the photodispersal and the step-up photophobic response showed a major peak at 520 nm and a minor peak at 680 nm. The photodispersal seems to be caused mainly by the step-up photophobic response exhibited by the specimens at the dark-light border. The photokinetic responses enhance the degree of the photodispersal.     

TEMPERATURE DEPENDENCE OF DELAYED LIGHT EMISSION IN THE 6 to 340 MICROSECOND RANGE AFTER A SINGLE FLASH IN CHLOROPLASTS

Abstract. The delayed light emission decay rate (up to 120 μs) and the rise in chlorophyll a fluorescence yield (from 3 to 35 μs) in isolated chloroplasts from several species, following a saturating 10 ns flash, are temperature independent in the 0–35°C range. However, delayed light in the 120–340 μs range is temperature dependent. Arrhenius plots of the exponential decay constants are: (a) linear for lettuce and pea chloroplasts but discontinuous for bush bean (12–17°C) and spinach (12–20°C) chloroplasts; (b) unaffected by 3-(3,4 dichlorophenyl)-1,1-dimethylurea (inhibitor of electron flow), gramicidin D (which eliminates light-induced membrane potential) and glutaraldehyde fixation (which stops gross structural changes).
The discontinuities, noted above for bush bean and spinach chloroplasts, are correlated with abrupt changes in (a) the thylakoid membrane lipid fluidity (monitored by EPR spectra of 12 nixtroxide stearate, 12NS) and (b) the fluidity of extracted lipids (monitored by differential calorimetry and EPR spectra of 12 NS). However, no such discontinuity was observed in (a) chlorophyll a fluorescence intensity of thylakoids and (b) fluorescence of tryptophan residues of delipidated chloroplasts.
Microsecond delayed light is linearly dependent on light intensity at flash intensities as low as one quantum per 2 times 10⁴ chlorophyll molecules. We suggest that this delayed light could originate from a one quantum process in agreement with the hypothesis that recombination of primary charges leads to this light emission. A working hypothesis for the energy levels of Photosystem II components is proposed involving a charge stabilization step on the primary acceptor side, which is in a lipid environment.
Finally, the redox potential of P680 (the reaction center for chlorophyll of system II) is calculated to be close to 1.0–1.3 V.     

Stentor coeruleus SHOWS POSITIVE PHOTOKINESIS

Abstract— Stentor coeruleus responds to a sudden increase in light intensity with a step-up photophobic response (avoiding reaction), and to collimated light with negative phototaxis. The peaks of the action spectra for the photophobic response and for the phototaxis are in common, 610 nm.
5. coeruleus showed changes in its steady-state swimming velocity induced with varying intensities of light (photokinesis). The cells swam fast in light regions but slowly in dark ones (positive photokinesis); the mean velocity of swimming was about 0.6 mm/s at 100 lx but reached about 1.0 mm/s at 50000 lx. The peak of the action spectrum for this photokinesis was about 680 nm.
The organism is the first protozoan cell reported to show three types of photoresponse: photophobic response, phototaxis and photokinesis.     

Action Spectrum for the Photophobic Response of Ciona intestinalis (Ascidieacea, Urochordata) Larvae Implicates Retinal Protein

Ascidian tadpole larvae change swimming behavior during the course of development. The photic behavior of the larvae of Ciona intestinalis was monitored by a computerized cell-tracking system with a time resolution of 0.1 s. Newly hatched larvae swim at an average speed of 1.4 mm/s but show no response to light stimuli. The swimming speed of the larvae became slower (0.4 mm/s) 3 h after hatching and they were induced to swim more rapidly by a sudden decrease in light intensity 4 h after hatching. During the course of development, the maximal speed of swimming behavior increased with time until 8 h after hatching and then plateaued. The action spectrum for the step-down photophobic response of the larvae was determined at around 8 h after hatching and was fitted to Dartnall's nomogram with the absorbance maximum of the pigment located at 505 nm. These results suggest retinal proteins in the ocellus of the larvae are the photoreceptors for the photobehavior.     

PHOTOELECTRIC EFFECTS IN A BIMOLECULAR LIPID MEMBRANE IN THE PRESENCE OF LIGHT SENSITIVE INORGANIC IONS

Abstract— A new membrane photoeffect of high quantum efficiency is reported. The presence of a number of inorganic ions in the bathing solution of a bimolecular lipid film of oxidized cholesterol causes a change in the measured e.m.f. from -100 to -40 mV (photo e.m.f. effect) and increase in the conductivity by a factor of 100–200 (photoconductive effect) when the ions are excited by light. A model system, based upon the classical Becquerel Effect phenomenon of electronic charge injection into the membrane from electronically excited ions, is suggested and experiments verifying this model are reported. An extension of this model is used to suggest a new mechanism of the visual receptor process in biological photoreceptor structures.     

PHOTOSENSORY TRANSDUCTION IN CILIATES. II. POSSIBLE ROLE OF G-PROTEIN AND cGMP IN Stentor coeruleus

Abstract— The heterotrichous ciliate, Stentor coerulus , exhibits a welll defines photophobic response to a sudden increase in the intensity of visible light. the phobic reactions usually appear with a latency perios (i.e. a time delay between the onset of the stimulus and the stop response). This latency of phobic response was significatly increased when the cells werw incubated with 8-bromo-guanosine3',5'-cyclic monophospjhate. 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 lowerin cellular cGMP levels) showed opposite effects on hte cell photophobic response. These result indirectly suggesnt that the level of cytoplamic cGMP, possibly modulated by a G-protein-coupled CGMP phosphodiesterase, plays a phototreasducing 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 α-subunits 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.     

THE EFFECT OF POTASSIUM IODIDE ON PHOTOPHOBIC RESPONSES IN EUGLENA: EVIDENCE FOR TWO PHOTORECEPTOR PIGMENTS*

Abstract— Potassium iodide, a quencher of flavin fluorescence, inhibits the shock reaction which Euglena experiences upon a sudden decrease in light intensity (inverse photophobic response) completely at a concentration of 150 mM. The rate of swimming of the cells at the same concentration of KI is reduced to 30% of the control. The direct photophobic response, a shock reaction which appears identical but occurs upon an increase in light intensity, is unaffected by KI as is negative phototaxis of Euglena. It is concluded that a non-flavin pigment system mediates photoreception for the direct photophobic response and negative phototaxis.     

The "funny" current (I(f)) inhibition by ivabradine at membrane potentials encompassing spontaneous depolarization in pacemaker cells

Recent clinical trials have shown that ivabradine (IVA), a drug that inhibits the funny current (I(f)) in isolated sinoatrial nodal cells (SANC), decreases heart rate and reduces morbidity and mortality in patients with cardiovascular diseases. While IVA inhibits I(f), this effect has been reported at essentially unphysiological voltages, i.e., those more negative than the spontaneous diastolic depolarization (DD) between action potentials (APs). We tested the relative potency of IVA to block I(f) over a wide range of membrane potentials, including those that encompass DD governing to the SANC spontaneous firing rate. A clinically relevant IVA concentration of 3 μM to single, isolated rabbit SANC slowed the spontaneous AP firing rate by 15%. During voltage clamp the maximal I(f) was 18 ± 3 pA/pF (at -120 mV) and the maximal I(f) reduction by IVA was 60 ± 8% observed at -92 ± 4 mV. At the maximal diastolic depolarization (~-60 mV) I(f) amplitude was only -2.9 ± 0.4 pA/pF, and was reduced by only 41 ± 6% by IVA. Thus, I(f) amplitude and its inhibition by IVA at physiologically relevant membrane potentials are substantially less than that at unphysiological (hyperpolarized) membrane potentials. This novel finding more accurately describes how IVA affects SANC function and is of direct relevance to numerical modeling of SANC automaticity.     

INTERMEDIATE PROCESSES IN PHOTOTRANSDUCTION: A STUDY IN DROSOPHILA MUTANTS

Abstract— A variety of procedures were used to modify the light response of Drosophila photoreceptors in order to find out if these manipulations produce effects that mimic some aspects of light adaptation. The ultimate goal of our approach is to use these experimental manipulations to dissect the different stages of the phototransduction process.
The means which were used to modify the light response were as follows: (a) light adaptation of normal photoreceptors, (b) exposure of normal photoreceptors to various levels of CO₂ (c) light adaptation of the trp Drosophila mutant in which the receptor potential decays to baseline during illumination, (d) exposure of the temperature sensitive norpA ^H52 mutant of Drosophila to elevated temperatures. Intensity-response functions were obtained using intracellular and extracellular recordings. The maximal response amplitude ( V _max) was then plotted as a function of the light intensity () which evoked a response of half maximal amplitude. This plot was used to compare the effects of the procedures described above. Qualitatively all the manipulations had similar effects, they reduced V _max and shifted to higher levels of light intensity, however, quantitatively the various effects were different.
The finding that each experimental manipulation gave a different V _max versus plot suggests that they affect different stages in phototransduction. We suggest that the slope of the V _max versus a function can be interpreted in terms of an ordered cascade of events in phototransduction. This interpretation might give us a tool to distinguish between early and late stages in phototransduction.     

Action Spectra for UVB Impacts on Blepharisma japonicum Motility and Photobehavior

Abstract— The ciliate Blepharisma japonicum was exposed to artificial polychromatic and monochromatic UV radiation to evaluate the relative roles of UVB (280–315 nm UV radiation) and UVA (315–400 nm UV radiation) in altering its motility and photobehavior and to determine absolute weighting coefficients for these effects in the UVB range. Under polychromatic UV irradiation B. japonicum cells showed a severe reduction of cell speed and of the capability to respond to light stimuli. At low doses, however, UV caused a significant increase in the average velocity of a cell population. The UVB exclusion experiments indicated that UVA does not significantly alter motility and photoresponsiveness. The increase and the subsequent decrease in cell velocity was observed also under monochromatic irradiation at 281, 290 and 300 nm, whereas at 310 nm cells swim faster up to the highest photon flux density used. The cell capability of reacting to photic stimuli, conversely, steadily declined with increasing photon flux density at all the tested UVB wavelengths. The action spectra for the alteration of cell velocity and the impairment of photoresponsiveness show that the lower the irradiation wavelength, the more remarkable are the UVB effects and suggest different targets for the increase and the decrease in cell velocity.     

SINGLE PHOTONS ARE SUFFICIENT TO TRIGGER MOVEMENT RESPONSES IN Chlamydomonas reinhardtii

Abstract— The unicellular green alga Chlamy domonas reinhardtii shows two distinct movement responses upon changes in green light irradiance: These are direction changes and stop responses. The dependence of both responses on the intensity of applied flashes was measured and analysed with the help of Poisson statistics. In dark-adapted cells both responses were 'single-photon' events. Preillumination of cells or background irradiation decreased the sensitivity for the stop response, now requiring two or more photons to trigger it. In the blind, carotenoid-negative mutant FN 68 the light-sensitivity for a stop-response was reconstituted by addition of all trans retinal. This result indicated that not only the receptor for phototaxis but also that for the stop response is a retinal protein. In addition, stimulus-response curves from the literature were analysed by Poisson statistics.