Ionic mechanism of a quasi-stable depolarization in barnacle photoreceptor following red light

Drosophila locus with gene-dosage effects on rhodopsin

Mutations that decrease the amplitude of the prolonged depolarizing afterpotential (PDA) in Drosophila melanogaster have been shown to have reduced rhodopsin content in the rhabdomeres of photoreceptor cells. In the present study, a genetic analysis of a class of third chromosome PDA-defective mutants localized the ninaE locus to the salivary band region 92A-B. In flies with only one copy of this region instead of the normal two copies, and in ninaE heterozygotes, the rhodopsin content of the major class of photoreceptors is reduced. Three doses of this region increase the rhodopsin content of these photoreceptors. These characteristics of the ninaE locus are expected of the structural gene encoding the major species of opsin in the Drosophila compound eye.

Two light-dependent conductances in Lima rhabdomeric photoreceptors

Light-dependent membrane currents were recorded from solitary Lima photoreceptors with the whole-cell clamp technique. Light stimulation from a holding voltage near the cell's resting potential evokes a transient inward current graded with light intensity, accompanied by an increase in membrane conductance. While the photocurrent elicited by dim flashes decays smoothly, at higher stimulus intensities two kinetically distinct components become visible. Superfusion with TEA or intracellular perfusion with Cs do not eliminate this phenomenon, indicating that it is not due to the activation of the Ca-sensitive K channels that are present in these cells. The relative amplitude of the late component vs. the early peak of the light response is significantly more pronounced at -60 mV than at -40 mV. At low light intensities the reversal potential of the photocurrent is around 0 mV, but with brighter lights no single reversal potential is found; rather, a biphasic response with an inward and an outward component can be seen within a certain range of membrane voltages. Light adaptation through repetitive stimulation with bright flashes diminishes the amplitude of the early but not the late phase of the photocurrent. These observations can be accounted for by postulating two separate light-dependent conductances with different ionic selectivity, kinetics, and light sensitivity. The light response is also shown to interact with some of the voltage-sensitive conductances: activation of the Ca current by a brief conditioning prepulse is capable of attenuating the photocurrent evoked by a subsequent test flash. Thus, Ca channels in these cells may not only shape the photoresponse, but also participate in the process of light adaptation.

Intracellular calcium changes in Balanus photoreceptor. A study with calcium ion-selective electrodes and Arsenazo III

Intracellular Ca2+ concentration (Cai) in the dark and during light stimulation, was measured in Balanus photoreceptors with Ca2+ ion-selective electrodes (Ca-ISE) and Arsenazo III absorbance changes (AIII). The average basal Cai of 17 photoreceptors in darkness was 300 +/- 160 nM determined with liquid ion-exchanger (t-HDOPP) Ca-ISE. Ca-ISE measurements indicated that light increased Cai by 700 nM (average), whereas AIII indicated an average change of 450 nM. The time course of AIII absorbance changes matched the time course of changes in the receptor potential more closely than did the Ca-ISE. Changes in Cai were graded with light intensity but the change in Cai was much greater for a decade change in intensity at high light intensity than at low intensity. The peak light induced conductance change of voltage clamped cells had a relationship to light intensity similar to that of the change in Cai. The peak Cai level measured with Ca-ISE was in good agreement with the free Ca2+ concentration of injected buffer solutions. Control Cai levels were usually restored within 5 min following injection of Ca2+ buffers. Injection of Ca2+ buffers with free Ca2+ of 0.6 microM produced a membrane depolarization. Larger increases in Cai (greater than microM) produced by injection of CaCl2 or release of Ca2+ from injected buffers by acidifying the cell, produced a pronounced membrane hyperpolarization. Increasing Cai with all of these techniques reduced the amplitude of the receptor potential. The time course of the receptor potential recovery was usually similar to that of Cai recovery.

Light-induced oxygen consumption in Limulus ventral photoreceptors does not result from a rise in the intracellular sodium concentration

Illumination of Limulus ventral photoreceptors leads to an increase in the intracellular concentration of sodium, [Na+]i, and to an increase in the consumption of O2 (delta QO2). After a 1-s light flash, it takes approximately 480 s for [Na+]i to return to within 10% of its preillumination level, whereas delta QO2 takes approximately 90 s. Thus, the delta QO2 is complete long before [Na+]i has returned to its resting level. Pressure injection of Na+ into the cell in order to elevate [Na+]i to the same levels as attained by illumination causes a rise in [Na+]i that returns to baseline with the same time course as the light-induced rise in [Na+]i. However, the injection of Na+ does not lead to an increase of the consumption of O2. We conclude that activation of the Na pump by a rise in [Na+]i is not a factor involved in the light-induced activation of O2 consumption in these cells.

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.

Light-activated guanosinetriphosphatase in Musca eye membranes resembles the prolonged depolarizing afterpotential in photoreceptor cells

Measurement of light-dependent GTPase (EC 3.1.5.1) activity in a paradigm guided by electrophysiological experiments was used to examine the involvement of a guanine nucleotide binding protein in fly phototransduction. Cell-free membrane preparations of Musca eyes responded to blue light by a 10- to 20-fold increase in GTP-hydrolyzing activity. This light-dependent GTPase had a low Km for GTP (0.5 microM) and was effectively inhibited by guanosine (5'----O3)-1-thiotriphosphate and guanosine 5'-[beta-gamma-imino]triphosphate but not by adenosine 5'-[beta-gamma-imino]triphosphate and ATP. The action spectrum of GTPase activity measured with intense light resembled closely the photoequilibrium spectrum of metarhodopsin. After illumination with blue (less than 480 nm) light, which converted rhodopsin to metarhodopsin, the GTPase remained highly active for at least 60 min in the dark. Similarly, rhodopsin-to-metarhodopsin conversion in intact cells induced a prolonged excitation in the dark, known as the prolonged depolarizing afterpotential (PDA). The persistent GTPase activity (like the PDA) was suppressed to the low basal activity of the unilluminated membranes after conversion of metarhodopsin to rhodopsin with red light (greater than 570 nm), whereas during illumination with red light, some GTPase activity was maintained. The magnitude of the persistent GTPase activity in the dark, like the PDA, depended in a supralinear manner on the amount of pigment conversion. Thus, the dependence of GTPase activity of Musca membrane preparations on photopigment conversion resembles the induction and suppression of the PDA measured in intact photoreceptors of Musca. These findings indicate that a guanine nucleotide binding protein is part of the chain of events leading to both the generation of the receptor potential and the PDA.

Activation of electrogenic Na-Ca exchange by light in fly photoreceptors

Illumination of white-eyed Musca photoreceptors following hypoxia or the application of ruthenium red (RR, a known blocker of Ca2+ uptake into intracellular organelles) induced a transient after depolarization (TA). The TA was enhanced when external [Ca2+] was reduced; it was abolished when external [Na+] was reduced to a level that affected the receptor potential to a small degree. The TA was enhanced or depressed when the activity of Na/K pump, which controls the Na+ gradient, was enhanced or depressed respectively. This effect was observed even when the receptor potential was not affected. All of the above observations are consistent with the hypothesis that the TA is triggered by a light-induced increase in the concentration of intracellular free Ca2+ which appear to be very high, following treatments with hypoxia or RR. The high sensitivity of the TA to Na+ and Ca2+ gradients across the photoreceptors membrane strongly suggests that the TA is due to a transient activation of an electrogenic Na-Ca exchange mechanism which depolarizes the cell.

Colour dependence of the early receptor potential and late receptor potential in scallop distal photoreceptor

1. Intracellular voltage and current responses to short (blue) and long (red) wave-length lights were measured in the distal hyperpolarizing photoreceptor (;off receptor') of the isolated and perfused scallop (Pecten irradians) retina.2. The early receptor potential (e.r.p.) was isolated by holding membrane potential at the reversal potential for the late receptor potential (l.r.p.) or by working at temperatures (< 5.0 degrees C) that abolished the l.r.p.3. The e.r.p., measured using intense flashes of white light, consisted of a positive phase followed by a negative phase, but was converted to a monophasic, negative-going wave following pre-adaptation with red light and to a monophasic, positive-going wave following pre-adaptation with blue light.4. The spectral sensitivity curve for the negative e.r.p. was maximum at 500 nm, whereas the spectral sensitivity curve for the positive e.r.p. was maximum at 575 nm.5. The positive or negative e.r.p.s approached their maximum amplitude exponentially when tested with red or blue flashes of increasing intensity. The results suggest that the positive (or negative) e.r.p. is proportional to the number of photopigment molecules photo-isomerized.6. The photosensitivity maximum of rhodopsin calculated at 500 nm, using the exponential constant and the spectral sensitivity data, was estimated to be 2.1 x 10(-16) cm(2) photon(-1), whereas the photosensitivity maximum of metarhodopsin calculated at 575 nm was estimated to be 2.6 x 10(-16) cm(2) photon(-1).7. In cells pre-adapted with white light, stimulation with blue light caused a hyperpolarizing l.r.p. which was followed by a prolonged hyperpolarizing after-potential (p.h.a.). Stimulation with red light under similar conditions caused an initial hyperpolarization which was followed by a small depolarization during the stimulus, but no after-potential.8. The duration of the p.h.a. was increased by pre-adaptation with a red light, which caused the maximum net transfer of metarhodopsin to rhodopsin; however, its decay was always complete in 5 min or less.9. The photo-isomerization of metarhodopsin by red light suppressed the p.h.a. and caused an after-depolarizing response that decayed in less than 1 min.10. The spectral sensitivity curve for the induction of the p.h.a. was maximum at 500 nm and corresponded to the spectral sensitivity for the negative e.r.p. and for the l.r.p. studied in the dark-adapted retina, whereas the spectral sensitivity curve for the suppression of the p.h.a. and for the induction of the after-depolarization was maximum at 575 nm and corresponded to the spectral sensitivity for the positive e.r.p.11. In photoreceptors clamped to the resting potential in normal ASW, the photo-isomerization of rhodopsin, in the absence of light absorption by metarhodopsin, activated a persistent outward current that had the same time course of decay as the p.h.a. The photo-isomerization of metarhodopsin suppressed the persistent outward current and activated an inward current whose decay took longer than the decay of the after-depolarizing response.12. In the absence of external Ca(2+) and Na(+) ions, the persistent outward current produced by light absorption by rhodopsin, and the inward current produced by light absorption by metarhodopsin, both reversed at the K(+) equilibrium potential. The results show that the induction of the prolonged hyperpolarizing after-potential and the after-depolarizing response involve only the movement of K(+) ions through the same light-dependent K(+) channels that determine the hyperpolarizing l.r.p. of the distal cells.

The prolonged hyperpolarizing afterpotential in an invertebrate photoreceptor: wavelength and ionic dependence

A single electrode voltage clamp was used to examine the prolonged hyperpolarizing afterpotential (PHA) which accompanies photoconversion of a substantial fraction of rhodopsin (lambda max = 500 nM) to metarhodopsin (lambda max = 575 nM) in distal photoreceptor cells in the retina of the bay scallop, Pecten irradians. The PHA appears to result from a persistent light-activated outward K+ current passing through the same channels responsible for the normal receptor potential in these cells.

Properties of tetraethylammonium ion-resistant K+ channels in the photoreceptor membrane of the giant barnacle

After the offset of illumination, barnacle photoreceptors undergo a large hyperpolarization that lasts seconds or minutes. We studied the mechanisms that generate this afterpotential by recording afterpotentials intracellularly from the medial photoreceptors of the giant barnacle Balanus nubilus. The afterpotential has two components with different time-courses: (a) an earlier component due to an increase in conductance to K+ that is not blocked by extracellular tetraethylammonium ion (TEA+) or 3-aminopyridine (3-AP) and (b) a later component that is sensitive to cardiac glycosides and that requires extracellular K+, suggesting that it is due to an electrogenic Na+ pump. The K+ conductance component increases in amplitude with increasing CA++ concentration and is inhibited by extracellular Co++; the Co++ inhibition can be overcome by increasing the Ca++ concentration. Thus, the K+ conductance component is Ca++ dependent. An afterpotential similar to that evoked by a brief flash of light is generated by depolarization with current in the dark and by eliciting Ca++ action potentials in the presence of TEA+ in the soma, axon, or terminal regions of the photoreceptor. The action potential undershoot is generated by an increase in conductance to K+ that is resistant to TEA+ and 3-AP and inhibited by Co++. The similarity in time-course and pharmacology of the hyperpolarization afterpotentials elicited by (a) a brief flash of light, (b) depolarization with current, and (c) an action potential indicates that Ca++-dependent K+ channels throughout the photoreceptor membrane are responsible for all three hyperpolarizing events.

Sensitization in voltage clamped barnacle photoreceptors

1. The light response of Balanus lateral eye photoreceptor was studied using intracellular recording and voltage clamp techniques. 2. The effect upon the light response of a prior depolarization was studied. Two effects were observed: a transient sensitization which appeared not to involve a change in time course, and a long-lived desensitization which was accompanied by a reduction in response time course. 3., The sensitizing effect of depolarizing prepulses increased monotonically with prepulse duration; it was maximal for pulses to around +20 mV. 4. Sensitization was inhibited by perfusion with salines containing cobaltous ions or with calcium-free saline. 5. It is concluded that while desensitization is an effect upon the transduction mechanism, sensitization is a direct influence of a prior depolarization on the light-activated channels.

Light induced changes of internal pH in a barnacle photoreceptor and the effect of internal pH on the receptor potential

1. Intracellular pH (pH1) was measured in Balanus photoreceptors using pH-sensitive glass micro-electrodes. The average pH1 of twelve photoreceptors which had been dark adapted for at least 30 min was 7.3 +/- 0.07 (S.D.). 2. Illumination reduced the recorded pH1 by as much as 0.2 pH unit. The change in pH1 was graded with light intensity. 3. When the cells were exposed to CO2 in the dark, pH1 declined monophasically. Saline equilibrated with 2% CO2; 98% O2 produced a steady reduction in pH1 of about 0.25 unit in 2--3 min. The buffering capacity of the receptor cell cytoplasm calculated from such experiments is approximately 15 slykes. 4. In the presence of HCO3-1, CO2 saline produced smaller, biphasic changes in pH1. 5. The membrane depolarization produced by a bright flash (depolarizing receptor potential) was reversibly reduced in the presence of external CO2 or by injection of H+. Iontophoretic injection of HCO2- increased the amplitude of the receptor potential. 6. In individual cells there was a close correlation between the amplitude of the receptor potential and pH1. 7. Saline equilibrated with CO2 reduced the light induced current (recorded under voltage-clamp) by 40--50% without affecting its reversal potential. 8. Exposure of the receptor to 95% CO2 saline for several minutes (pH0 5.5) not only abolished the receptor potential but also reversibly decreased the K conductance of the membrane in the dark. These effects were not reproduced by pH0 5.5 buffered saline or by a 5 min exposure to saline equilibrated with N2. 9. It is suggested that changes in pH1 induced by light modulate the sensitivity of the receptor under physiological conditions.

Dual role for potassium in Balanus photoreceptor: antagonist of calcium and suppression of light-induced current

1. The mechanism of reduction and final abolition of the depolarizing receptor potential of Balanus eburneus photoreceptors in K+-free saline was examined with electro-physiological techniques including voltage-clamp and ion specific electrodes. 2. An extended exposure to K+-free saline reduces the transient peak and the steady phases of the depolarizing receptor potential by approximately equal amounts. The process can be reversed in normal saline although the wave form of the response is often more rectangular upon recovery. Restoration of K+ induces a transient hyperpolarization of the resting membrane for several minutes. 3. The depolarizing receptor potential can also be restored in K+-free solution by reducing the Ca2+ concentration. This saline depolarizes the resting membrane, and the wave form of the depolarizing receptor potential assumes a rectangular configuration. 4. Voltage-clamp experiments revealed that an extended exposure to K+-free saline produced an extreme reduction of the inward light-induced current (LIC), but no detectable change in the membrane potential at which the current reverses sign. Membrane conductance in darkness showed little change. Reduction of the Ca2+ concentration from 20 to 0-2 mM in K+-free restored the current and produced a negative 8-10 mV shift in the zero current potential. There was also a significant decrease in membrane conductance in darkness. 5. Current-voltage relations of the membrane in K+-free, low Ca2+, or K+-free low Ca2+ salines were somewhat dependent upon the order the salines were presented. 6. Low Ca2+ saline (0-2 mM) by itself produced a -5 mV shift in the zero-current potential. Removing K+ in low Ca2+ produced an additional shift (-5 mV) in the zero-current potential.

Spectral correlates of a quasi-stable depolarization in barnacle photoreceptor following red light

1. Illumination of B. eburneus photoreceptors with intense red light produces a membrane depolarization that persists in darkness. This quasistable depolarization (latch-up) can be terminated with green light. The phenomenon was investigated with electrophysiological, spectrochemical, and microspectrophotometric techniques. 2. Latch-up was associated with a stable inward current in cells with the membrane potential voltage-clamped at the resting potential in darkness. The stable current could only be elicited at wave-lengths greater than 580 nm. 3. Light-induced current (LIC) was measured at various wave-lengths in dark-adapted photoreceptors with the membrane voltage-clamped to the resting potential. The minimum number of photons required to elicit a fixed amount of LIC occurred at 540 nm, indicating that the photoreceptor is maximally sensitive to this wave-length of light. The photoreceptor was also sensitive to wave-lengths in the near-U.V. region of the spectrum (380-420 nm). 4. Steady red adapting light reduced the magnitude of the LIC uniformly at all wave-lengths except in the near-U.V. region of the spectrum; sensitivity was reduced less in this region. 5. The spectrum for termination of the stable inward current following or during red light was shifted to the blue (peak about 510 nm) compared to the peak for LIC (peak about 540 nm). 6. Absorbance of single cells prepared under bright, red light decreased maximally at 480 nm following exposure to wave-lengths of light longer than 540 nm. 7. A pigment extract of 1000 barnacle ocelli prepared under dim, red light had a maximum absorbance change at 480 nm when bleached with blue-gree light. 8. There was no evidence in the latter two experiments of photointerconversion of pigments with absorbance maxima at 480 and 540 nm. Rather, the maximum absorption of the bleaching products seemed to occur at wave-lengths shorter than 420 nm. 9. Since latch-up induction occurs at wave-lengths longer than 580 nm, it may depend on the 540 pigment or on an undetected red absorbing pigment. 10. A photolabile pigment at 480 nm correlated most closely with termination of the stable inward current associated with latch-up.