Light activated calcium release in Limulus ventral photoreceptors as revealed by laser confocal microscopy

An invertebrate‐like phototransduction cascade mediates light detection in the chicken retinal ganglion cells

Prebilaterian animals perceived ambient light through nonvisual rhabdomeric photoreceptors (RPs), which evolved as support of the chordate visual system. In vertebrates, the identity of nonvisual photoreceptors and the phototransduction cascade involved in nonimage forming tasks remain uncertain. We investigated whether chicken retinal ganglion cells (RGCs) could be nonvisual photoreceptors and the nature of the photocascade involved. We found that primary cultures of chicken embryonic RGCs express such RP markers as transcription factors Pax6 and Brn3, photopigment melanopsin, and G-protein q but not markers for ciliary photoreceptors (alpha-transducin and Crx). To investigate the photoreceptive capability of RGCs, we assessed the direct effect of light on 3H-melatonin synthesis in RGC cultures synchronized to 12:12 h light-dark cycles. In constant dark, RGCs displayed a daily variation in 3H-melatonin levels peaking at subjective day, which was significantly inhibited by light. This light effect was further increased by the chromophore all-trans-retinal and suppressed by specific inhibitors of the invertebrate photocascade involving phosphoinositide hydrolysis (100 microM neomycin; 5 microM U73122) and Ca2+ mobilization (10 mM BAPTA; 1 mM lanthanum). The results demonstrate that chicken RGCs are intrinsically photosensitive RPs operating via an invertebrate-like phototransduction cascade, which may be responsible for early detection of light before vision occurs.

Testing the role of calmodulin in the excitation of Limulus photoreceptors

The phototransduction cascade in Limulus ventral photoreceptors involves multiple second messengers, including Ca(2+) and cGMP. Light-induced Ca(2+) release from intracellular stores is an intermediate step, but the subsequent Ca(2+)-activated reaction remains to be determined. The possibility that Ca(2+)/calmodulin (Ca(2+)/CaM) might be involved is suggested by the high calmodulin content of the transducing lobe. To test whether CaM can excite the transduction cascade we injected a 25 microM Ca(2+)/CaM solution. This produced a rapid, brief depolarization similar to that produced by light, suggesting a role for CaM in the cascade. However, an important caveat is that Ca(2+) dissociating from the Ca(2+)/CaM complex might excite this process. Several control experiments argue against, but do not entirely eliminate this possibility. To test whether endogenous CaM has a function in excitation, trifluoperazine was pressure injected into the rhabdomeric region. The response to brief flashes was not affected, but the response to steady illumination was transiently attenuated by each injection. We conclude that calmodulin should be considered a candidate to couple intermediate and late stages of the transduction cascade.

The excitation cascade of Limulus ventral photoreceptors: guanylate cyclase as the link between InsP3-mediated Ca2+ release and the opening of cGMP-gated channels

Background

Early stages in the excitation cascade of Limulus photoreceptors are mediated by activation of G_q by rhodopsin, generation of inositol-1,4,5-trisphosphate by phospholipase-C and the release of Ca²⁺. At the end of the cascade, cGMP-gated channels open and generate the depolarizing receptor potential. A major unresolved issue is the intermediate process by which Ca²⁺ elevation leads to channel opening.

Results

To explore the role of guanylate cyclase (GC) as a potential intermediate, we used the GC inhibitor guanosine 5'-tetraphosphate (GtetP). Its specificity in vivo was supported by its ability to reduce the depolarization produced by the phosphodiesterase inhibitor IBMX. To determine if GC acts subsequent to InsP₃ production in the cascade, we examined the effect of intracellular injection of GtetP on the excitation caused by InsP₃ injection. This form of excitation and the response to light were both greatly reduced by GtetP, and they recovered in parallel. Similarly, GtetP reduced the excitation caused by intracellular injection of Ca²⁺. In contrast, this GC inhibitor did not affect the excitation produced by injection of a cGMP analog.

Conclusion

We conclude that GC is downstream of InsP3-induced Ca2+ release and is the final enzymatic step of the excitation cascade. This is the first invertebrate rhabdomeric photoreceptor for which transduction can be traced from rhodopsin photoisomerization to ion channel opening.

Simultaneous roles for Ca2+ in excitation and adaptation of Limulus ventral photoreceptors

The ventral photoreceptors of Limulus have been one of the main preparations for the study of invertebrate phototransduction. The study of ventral photoreceptors has revealed that they have remarkable performance characteristics, most notably the very large amplification of the transduction process. This amplification is critically dependent upon the coupling of photoactivated rhodopsin to the phosphoinositide cascade, resulting in the release of Ca2+ from intracellular stores. The consequent elevation of Ca2+ within the photoreceptor's cytosol is amongst the most rapid and dramatic known to be activated by the phosphoinositide cascade. This review summarizes the evidence that intracellular Ca2+ is a key regulator of transduction in Limulus photoreceptors. The mechanisms that regulate Ca2+ as well as the possible targets of the action of Ca2+ are reviewed. Ca2+ elevation is critical for triggering both excitation and adaptation processes in the photoreceptor. The question of how a single second messenger can produce these two opposing effects is of obvious interest and is a topic dealt with throughout this review.

Calcium homeostasis in fly photoreceptor cells

In fly photoreceptor cells, two processes dominate the Ca2+ homeostasis: light-induced Ca2+ influx through members of the TRP family of ion channels, and Ca2+ extrusion by Na+/Ca2+ exchange. Ca2+ release from intracellular stores is quantitatively insignificant. Both, the light-activated channels and the Ca2+-extruding exchangers are located in or close to the rhabdomeric microvilli, small protrusions of the plasma membrane. The microvilli also contain the molecular machinery necessary for generating quantum bumps, short electrical responses caused by the absorption of a single photon. Due to this anatomical arrangement, the light-induced Ca2+ influx results in two separate Ca2+ signals that have different functions: a global, homogeneous increase of the Ca2+ concentration in the cell body, and rapid but large amplitude Ca2+ transients in the microvilli. The global rise of the Ca2+ concentration mediates light adaptation, via regulatory actions on the phototransduction cascade, the voltage-gated K+ channels and small pigment granules controlling the light intensity. The local Ca2+ transients in the microvilli are responsible for shaping the quantum bumps into fast, all-or-nothing events. They achieve this by facilitating strongly the phototransduction cascade at early stages ofthe light response and subsequently inhibiting it. Many molecular targets of these feedback mechanisms have been identified and characterized due to the availability of numerous Drosophila mutant showing defects in the phototransduction.

Ca(2+)-dependent and Ca2+ release-dependent excitation in leech photoreceptors: evidence from a novel "inside-out" cell model

We have developed a novel, electrophysiologically intact and light-sensitive "inside-out" cell model (IOCM) of microvillar photoreceptors of the leech Hirudo medicinalis. Light responses recorded from the IOCM with sharp microelectrodes are depolarizations with amplitudes of up to 50-60 mV. In darkness, graded elevations of the free Ca(2+) concentration in the "intracellular medium" (ICM) reversibly increase the conductance of the microvillar membrane leading to Ca(2+)-induced graded voltage changes up to approximately 50 mV. The threshold for Ca(2+)-induced voltage changes is approximately 0.06 microM, EC(50) is approximately 1.2 microM, and saturation occurs at approximately 20 microM free Ca(2+). Small Ca(2+) elevations (<0.6 microM) produce discrete waves of depolarization resembling quantum bumps. Stimulating IOCMs with short (20-ms) and long (5-s) light stimuli produces transient light responses (repolarization within ca. 200 ms) in an ICM containing only 10nM free Ca(2+). At 0.44 microM free Ca(2+) in the ICM, the microvillar membrane depolarizes by 10-20 mV and responses to 5-s light steps have an initial transient component and a plateau component, similar to responses in intact cells. Generation of the plateau component in IOCMs is suppressed by heparin and cyclopiazonic acid (CPA), agents that block inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3))-induced Ca(2+) release from and Ca(2+) uptake into the endoplasmic reticulum (ER). These results indicate that there is a Ca(2+)-dependent conductance in the microvillar membrane and that the light-induced Ins(1,4,5)P(3)- and Ca(2+) release-mediated intracellular Ca(2+) elevation in leech photoreceptors contributes to the generation of the receptor potential, particularly the plateau component of responses to long steps of light.

Inositol 1,4,5-trisphosphate-induced calcium release is necessary for generating the entire light response of limulus ventral photoreceptors

The experiments reported here were designed to answer the question of whether inositol 1,4,5-trisphosphate (IP3)-induced calcium release is necessary for generating the entire light response of Limulus ventral photoreceptors. For this purpose the membrane-permeable IP3 receptor antagonist 2-aminoethoxydiphenyl borate (2APB) (Maruyama, T., T. Kanaji, S. Nakade, T. Kanno, and K. Mikoshiba. 1997. J. Biochem. (Tokyo). 122:498-505) was used. Previously, 2APB was found to inhibit the light activated current of Limulus ventral photoreceptors and reversibly inhibit both light and IP3 induced calcium release as well as the current activated by pressure injection of calcium into the light sensitive lobe of the photoreceptor (Wang, Y., M. Deshpande, and R. Payne. 2002. Cell Calcium. 32:209). In this study 2APB was found to inhibit the response to a flash of light at all light intensities and to inhibit the entire light response to a step of light, that is, both the initial transient and the steady-state components of the response to a step of light were inhibited. The light response in cells injected with the calcium buffer 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) was reversibly inhibited by 2APB, indicating that these light responses result from IP3-mediated calcium release giving rise to an increase in Cai. The light response obtained from cells after treatment with 100 microM cyclopiazonic acid (CPA), which acts to empty intracellular calcium stores, was reversibly inhibited by 2APB, indicating that the light response after CPA treatment results from IP3-mediated calcium release and a consequent rise in Cai. Together these findings imply that IP3-induced calcium release is necessary for generating the entire light response of Limulus ventral photoreceptors.

2-Aminoethoxydiphenyl borate inhibits phototransduction and blocks voltage-gated potassium channels in Limulus ventral photoreceptors

2-Aminoethoxydiphenyl borate (2-APB) is a membrane-permeable modulator that inhibits the activation of inositol (1,4,5) trisphosphate (InsP(3)) receptors, store operated channels (SOCs) and TRP channels in cells that utilize the phosphoinositide cascade for cellular signaling. In Limulus ventral photoreceptors, light-induced calcium release via the phosphoinositide cascade is thought to activate the photocurrent. Injection of either exogenous InsP(3) or calcium ions can therefore mimic excitation by light. One hundred micromolar 2-APB reversibly inhibited the photocurrent of ventral photoreceptors in a concentration-dependent manner, acting on at least two processes thought to mediate the visual cascade. 2-APB reversibly inhibited both light and InsP(3)-induced calcium release, consistent with its role as an inhibitor of the InsP(3) receptor. In addition, 2-APB reversibly inhibited the activation of depolarizing current flow through the plasma membrane caused by pulsed pressure injection of calcium ions into the light-sensitive lobe of the photoreceptor. We also found that 100 micro M 2-APB reversibly inhibited both transient and sustained voltage-activated potassium current during depolarizing steps. 2-APB has previously been shown to block phototransduction in Drosophila photoreceptors. The lack of specificity of the action of 2-APB in Limulus indicates that this blockade need not necessarily arise from inhibition of InsP(3)-induced calcium release.

The effect of intensity and duration on the light-induced sodium and potassium currents in the Hermissenda type B photoreceptor

Light duration and intensity influence classical conditioning in Hermissenda through their effects on the light-induced currents. Furthermore, the contribution of voltage-dependent potassium currents to the long-lasting depolarization in type B photoreceptors depends on light-induced currents active at resting potentials. Thus, the present study measures the effect of holding potential, duration, and intensity on the light-induced currents in discontinuous single-electrode voltage clamp mode. Three distinct current components are distinguished by their temporal and voltage characteristics and sensitivity to pharmacological agents. One current component is a transient sodium current, I(Nalgt); another is a plateau sodium current, I(plateau), which persists for the duration of the light stimulus. Substitution of trimethylammonium chloride for sodium reduces both currents equally, suggesting that I(plateau) represents partial inactivation of I(Nalgt). The third current component is a prolonged reduction in potassium currents, I(Klgt); it is accompanied by an increase in input resistance, and it appears at potentials close to rest. An increase in light duration or intensity causes an increase in the peak conductance of both I(Nalgt) and I(Klgt). Latency of I(Nalgt) is decreased by intensity, whereas rise time is increased by duration. An increase in light duration or intensity causes an increase in the time-to-peak and duration of I(Klgt). Characteristics of these currents suggest that I(Klgt) is responsible for the long-lasting depolarization seen after light termination, and thus plays a role in classical conditioning.

Endoplasmic reticulum of animal cells and its organization into structural and functional domains

The endoplasmic reticulum (ER) in animal cells is an extensive, morphologically continuous network of membrane tubules and flattened cisternae. The ER is a multifunctional organelle; the synthesis of membrane lipids, membrane and secretory proteins, and the regulation of intracellular calcium are prominent among its array of functions. Many of these functions are not homogeneously distributed throughout the ER but rather are confined to distinct ER subregions or domains. This review describes the structural and functional organization of the ER and highlights the dynamic properties of the ER network and the mechanisms that support the positioning of ER membranes within the cell. Furthermore, we outline processes involved in the establishment and maintenance of an anisotropic distribution of ER-resident proteins and, thus, in the organization of the ER into functionally and morphologically different subregions.

InsP(3)-induced Ca(2+) release in permeabilized invertebrate photoreceptors: a link between phototransduction and Ca(2+) stores

Using the low-affinity fluorescent Ca(2+) indicators, Mag-Fura-2 and Mag-Fura Red, we studied light- and InsP(3)-induced Ca(2+) release in permeabilized microvillar photoreceptors of the medicinal leech, Hirudo medicinalis. Two major components of the phosphoinositide signaling pathway, phospholipase-C and the InsP(3) receptor, were characterized immunologically and appropriately localized in photoreceptors. Whereas phospholipase-C was abudantly expressed in photoreceptive microvilli, InsP(3) receptors were found mostly in submicrovillar endoplasmic reticulum (SER). Permeabilization of the peripheral plasma membrane with saponin allowed direct measurements of luminal free Ca(2+) concentration (Ca(L)) changes. Confocal Ca(2+) imaging using Mag-Fura Red demonstrated that Ins(1,4,5)P(3) mobilizes Ca(2+) from SER. As detected with Mag-Fura-2, a brief 50ms light flash activated rapid Ca(2+) depletion of SER, followed by an effective refilling within 1min of dark adaptation after the light flash. Sensitivity to Ins(1,4,5)P(3) of the Ca(2+) release from SER in leech photoreceptors was accompanied by irreversible uncoupling of phototransduction from Ca(2+) release. Depletion of Ca(2+) stores was induced by Ins(1,4,5)P(3)(EC(50)= 4.75 microM) and the hyper-potent agonist adenophostin A (EC(50)/40nM) while the stereoisomer L-myo Ins(1,4,5)P(3) was totally inactive. Ins(1,4,5)P(3)- or adenophostin A-induced Ca(2+) release was inhibited by 0.1-1mg/ml heparin. The Ca(2+) pump inhibitors, cyclopiazonic acid and thapsigargin, in the presence of Ins(1,4,5)P(3), completely depleted Ca(2+) stores in leech photoreceptors.

Cyclic GMP in lateral eyes of the horseshoe crab Limulus

In Limulus photoreceptors, light induces a depolarization. The gating mechanism of the cation channels involved is not understood but evidence exists that cyclic nucleotides may act as ligands. Using an antiserum against a cGMP-protein conjugate, we found strong immunoreactivity localized to the light-sensitive rhabdoms of lateral eye photoreceptors. In homogenated tissue, the cGMP concentration was on average 0.2 pmol/mg retinal tissue protein both in light- and dark-adapted eyes. Phosphodiesterase inhibitors caused a 2-3-fold increase in the cGMP level. Despite our failure to detect a light dependence of the cGMP concentration, these results support the proposed role of cGMP in Limulus phototransduction.

Evidence for a distinct light-induced calcium-dependent potassium current in Hermissenda crassicornis

A model of phototransduction is developed as a first step toward a model for investigating the critical interaction of light and turbulence stimuli within the type B photoreceptor of Hermissenda crassicronis. The model includes equations describing phototransduction, release of calcium from intracellular stores, and other calcium regulatory mechanisms, as well as equations describing ligand-gating of a rhabdomeric sodium current. The model is used to determine the sources of calcium in the soma, whether calcium or IP3 is a plausible ligand of the light-induced sodium current, and whether the light-induced potassium current is equivalent to the calcium-dependent potassium current activated by light-induced calcium release. Simulations show that the early light-induced calcium elevation is due to influx through voltage-dependent channels, whereas the later calcium elevation is due to release from intracellular stores. Simulations suggest that the ligand of the fast, light-induced sodium current is IP3 but that there is a smaller, prolonged component of the light-induced sodium current that is activated by calcium. In the model, the calcium-dependent potassium current, located in the soma, is activated only slightly by light-induced calcium elevation, leading to the prediction that a calcium-dependent potassium current, active at resting potential, is located in the rhabdomere and is responsible for the light-induced potassium current.

Distribution of ryanodine receptor Ca(2+) channels in insect photoreceptor cells

Physiological studies have provided evidence for the existence of ryanodine receptor (RyR) Ca(2+) channels in compound eyes of insects. The present study identifies and localizes RyR in insect photoreceptors by use of an affinity-purified antibody against lobster muscle RyR. Western blotting and indirect immunofluorescence staining confirm cross-reactivity of the antibody with insect muscle RyR. In both honeybee and fly eyes, the antibody identifies a single protein that comigrates with muscle RyR on sodium dodecylsulfate (SDS) polyacrylamide gels demonstrating that RyR is present in this tissue. By confocal immunofluorescence microscopy on honeybee eyes, RyR is detected within the photoreceptors and shows a nonhomogeneous distribution over the endoplasmic reticulum (ER). Double labeling studies have demonstrated further that RyR is localized at distinct ER elements close to the light-sensitive microvilli and juxtaposed to adherens junctions. RyR has also been observed within the remaining soma of honeybee photoreceptors, being concentrated on ER cisternae close to mitochondria and the nonreceptive plasma membrane. For comparative purposes, the distribution of RyR has also been assayed in compound eyes of flies. In both Calliphora and Drosophila photoreceptors, the anti-RyR antibody provides punctate labeling throughout the cell body. The submicrovillar ER cisternae associated with the base of the microvilli, however, are only lightly labeled for RyR. These results suggest that RyR is involved with Ca(2+) regulation in the nonreceptive cell area of both fly and honeybee photoreceptors, but that it may contribute to Ca(2+) regulation close to the phototransduction compartment only in the latter cell.

Timing of Ca(2+) release from intracellular stores and the electrical response of Limulus ventral photoreceptors to dim flashes

Light-induced release of Ca(2+) from stores in Limulus ventral photoreceptors was studied using confocal fluorescence microscopy and the Ca(2+) indicator dyes, Oregon green-5N and fluo-4. Fluorescence was collected from a spot within 4 microm of the microvillar membrane. A dual-flash protocol was used to reconstruct transient elevations of intracellular free calcium ion concentration (Ca(i)) after flashes delivering between 10 and 5 x 10(5) effective photons. Peak Ca(i) increased with flash intensity to 138 +/- 76 microM after flashes delivering approximately 10(4) effective photons, while the latent period of the elevation of Ca(i) fell from approximately 140 to 21 ms. The onset of the light-induced elevation of Ca(i) was always highly correlated with that of the receptor potential. The time for Ca(i) to exceed 2 microM was approximately equal to that for the receptor potential to exceed 8 mV (mean difference; 2.2 +/- 6.4 ms). Ca(i) was also measured during steps of light delivering approximately 10(5) effective photons/s to photoreceptors that had been bleached with hydroxylamine so as to reduce their quantum efficiency. Elevations of Ca(i) were detected at the earliest times of the electrical response to the steps of light, when a significant receptor potential had yet to develop. Successive responses exhibited stochastic variation in their latency of up to 20 ms, but the elevation of Ca(i) and the receptor potential still rose at approximately the same time, indicating a shared process generating the latent period. Light-induced elevations of Ca(i) resulted from Ca(2+) release from intracellular stores, being abolished by cyclopiazonic acid (CPA), an inhibitor of endoplasmic reticulum Ca(2+) pumps, but not by removal of extracellular Ca(2+) ions. CPA also greatly diminished and slowed the receptor potential elicited by dim flashes. The results demonstrate a rapid release of Ca(2+) ions that appears necessary for a highly amplified electrical response to dim flashes.

Normal phototransduction in Drosophila photoreceptors lacking an InsP(3) receptor gene

The Drosophila light-sensitive channels TRP and TRPL are prototypical members of an ion channel family responsible for a variety of receptor-mediated Ca(2+) influx phenomena, including store-operated calcium influx. While phospholipase Cbeta is essential, downstream events leading to TRP and TRPL activation remain unclear. We investigated the role of the InsP(3) receptor (InsP(3)R) by generating mosaic eyes homozygous for a deficiency of the only known InsP(3)R gene in Drosophila. Absence of gene product was confirmed by RT-PCR, Western analysis, and immunocytochemistry. Mutant photoreceptors underwent late onset retinal degeneration; however, whole-cell recordings from young flies demonstrated that phototransduction was unaffected, quantum bumps, macroscopic responses in the presence and absence of external Ca(2+), light adaptation, and Ca(2+) release from internal stores all being normal. Using the specific TRP channel blocker La(3+) we demonstrated that both TRP and TRPL channel functions were unaffected. These results indicate that InsP(3)R-mediated store depletion does not underlie TRP and TRPL activation in Drosophila photoreceptors.

Calcium transients in the rhabdomeres of dark- and light-adapted fly photoreceptor cells

The light response of fly photoreceptor cells is modulated by changes in free Ca(2+) concentration. Fly phototransduction and most processes regulating it take place in or very close to the rhabdomere. We therefore measured the kinetics and the absolute values of the free Ca(2+) concentration in the rhabdomere of fly photoreceptor cells in vivo by making use of the natural optics of the fly's eye. We show that Ca(2+) flowing into the rhabdomere after light stimulation of dark-adapted cells causes fast Ca(2+) transients that reach peak values higher than 200 microM in <20 msec. Approximately 500 msec later, the free Ca(2+) concentration has declined again to approximately 20 microM. The duration of the Ca(2+) transients becomes still shorter, and their size reduced, when the photoreceptor cell is light-adapted. This reduction in duration and size of the Ca(2+) transients is graded with the intensity of the adapting light. The kinetics and absolute values of the free calcium concentration found to occur in the rhabdomere are suitable to mediate the fast feedback signals known to act on the fly phototransduction cascade.

Protein kinase C activators inhibit the visual cascade in Limulus ventral photoreceptors at an early stage

The phosphoinositide cascade mediates visual transduction in invertebrate photoreceptors. Phospholipase C (PLC) catalyzes the hydrolysis of phosphatidylinositol bisphosphate, producing inositol trisphosphate (InsP(3)) and diacylglycerol (DAG). Protein kinase C (PKC) is a major target of DAG in many cell types. We have used PKC activators to investigate the function of the kinase in the phototransduction cascade in Limulus polyphemus ventral photoreceptors. Extracellular application of (-)-indolactam V (0. 03-30 microM) or phorbol-12,13-dibutyrate (10 microM) reversibly reduced the sensitivity of the electrical response of the photoreceptors to light by up to 1000-fold. The inert stereoisomer (+)-indolactam V and 4alpha-phorbol had no effect. The effect of (-)-indolactam V was antagonized by the PKC inhibitors bisindolylmaleimide I and Gö 6976. Coapplication of bisindolylmaleimide V, used as a negative control compound for PKC inhibition, did not reduce the effectiveness of (-)-indolactam V. These findings are consistent with (-)-indolactam V activating PKC and desensitizing the light response. Furthermore, our pharmacological results indicate that PKC activation does not appear to play a role in light adaptation. We localized the position of the target of PKC in the visual cascade. We chemically excited the cascade at various stages to determine the kinase's target. PKC activation by (-)-indolactam V decreased the light-induced elevation of intracellular calcium but had no effect on the photoreceptor's excitatory response to intracellular injection of InsP(3). However, the PKC activator greatly reduced the excitation caused by GTP-gamma-S injection. We propose that PKC inhibits the visual transduction cascade at the G-protein and/or PLC stage.

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

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

Light dependence of calcium and membrane potential measured in blowfly photoreceptors in vivo

Light adaptation in insect photoreceptors is caused by an increase in the cytosolic Ca2+ concentration. To better understand this process, we measured the cytosolic Ca2+ concentration in vivo as a function of adapting light intensity in the white-eyed blowfly mutant chalky. We developed a technique to measure the cytosolic Ca2+ concentration under conditions as natural as possible. The calcium indicator dyes Oregon Green 1, 2, or 5N (Molecular Probes, Inc., Eugene, OR) were iontophoretically injected via an intracellular electrode into a photoreceptor cell in the intact eye; the same electrode was also used to measure the membrane potential. The blue-induced green fluorescence of these dyes could be monitored by making use of the optics of the facet lens and the rhabdomere waveguide. The use of the different Ca2+-sensitive dyes that possess different affinities for Ca2+ allowed the quantitative determination of the cytosolic Ca2+ concentration in the steady state. Determining the cytosolic Ca2+ concentration as a function of the adapting light intensity shows that the Ca2+ concentration is regulated in a graded fashion over the whole dynamic range where a photoreceptor cell can respond to light. When a photoreceptor is adapted to bright light, the cytosolic Ca2+ concentration reaches stable values higher than 10 microM. The data are consistent with the hypothesis that the logarithm of the increase in cytosolic Ca2+ concentration is linear with the logarithm of the light intensity. From the estimated values of the cytosolic Ca2+ concentration, we conclude that the Ca2+-buffering capacity is limited. The percentage of the Ca2+ influx that is buffered gradually decreases with increasing Ca2+ concentrations; at cytosolic Ca2+ concentration levels above 10 microM, buffering becomes minimal.

Intracellular Ca2+ and adaptation of voltage responses to light in Hermissenda photoreceptors

Fluorescent imaging of Ca2+ and intracellular recordings were used to assess Ca2+ increases and voltage responses during light presentations in Hermissenda B photoreceptors. Ca2+ levels increased and were sustained during a relatively long exposure to light. Repeated presentations of a brief light induced an elevation of intracellular Ca2+ that persisted throughout short interlight intervals, but which dissipated during long interlight intervals. In all instances, the magnitude of the intracellular Ca2+ signal was inversely related to the amplitude of the light-induced generator potential. Blocking of voltage-dependent Ca2+ channels did not significantly affect the magnitude of the Ca2+ signal, suggesting that the intracellular Ca2+ response arises primarily from release from intracellular stores. These results indicate that Ca2+ plays an important role in the modulation of the voltage responses to light, acting to suppress the response during repetitive or prolonged stimulation.

The Limulus ventral photoreceptor: light response and the role of calcium in a classic preparation

The ventral nerve photoreceptor of the horseshoe crab Limulus polyphemus has been used for many years to investigate basic mechanisms of invertebrate phototransduction. The activation of rhodopsin leads in visual cells of invertebrates to an enzyme cascade at the end of which ion channels in the plasma membrane are transiently opened. This allows an influx of cations resulting in a depolarization of the photoreceptor cell. The receptor current of the Limulus ventral photoreceptor consists of three components which differ in several aspects, such as the time course of activation, the time course of recovery from light adaptation, and the reversal potential. Each component is influenced in a different, characteristic way by various pharmacological manipulations. In addition, at least two types of single photon-evoked events (bumps) and three elementary channel conductances are observed in this photoreceptor cell. These findings suggest that the receptor current components are controlled by three different light-activated enzymatic pathways using three different ligands to increase membrane conductance. Probably one of these ligands is cyclic GMP, another one is activated via the IP3-cascade and calcium, the third one might be cyclic AMP. Calcium ions are very important for the excitation and adaptation of visual cells in invertebrates. The extracellular and intracellular calcium concentrations determine the functional state of the visual cell. A rise in the cytosolic calcium concentration appears to be an essential step in the excitatory transduction cascade. Cytosolic calcium is the major intracellular mediator of adaptation. If the cytosolic calcium level exceeds a certain threshold value after exposure to light it causes the desensitization of the visual cell. On the other hand, from a slight rise in cytosolic calcium facilitation results, i.e. increased sensitivity of the photoreceptor.

A role for hydrolysis of inositol 1,4,5-trisphosphate in terminating the response to inositol 1,4,5-trisphosphate and to a flash of light in Limulus ventral photoreceptors

Injection of inositol 1,4,5-trisphosphate (Ins 1,4,5-P3) into Limulus ventral photoreceptors produces excitation similar to that produced by light. One process which might contribute to rapid termination of the responses to Ins 1,4,5-P3 and to light is the hydrolysis of Ins 1,4,5-P3 by an InsP3-5-phosphatase to form inositol 1,4-bisphosphate. Inositol 2,4,5-trisphosphate (Ins 2,4,5-P3) is known to be less hydrolysable by the InsP3-5-phosphatase than is Ins 1,4,5-P3. Whereas ventral photoreceptors respond to an injection of Ins 1,4,5-P3 with a single wave of depolarization, the response to Ins 2,4,5-P3 is a burst of waves of depolarization. Our hypothesis is that it is the resistance to hydrolysis by the InsP3-5-phosphatase which accounts for the burst of waves produced by Ins 2,4,5-P3. To test this idea we injected ventral photoreceptors with Ins 1,4,5-P3 in the presence of the non-specific phosphatase inhibitors, vanadate and fluoride, which prolong the response to a flash of light in ventral photoreceptors (D.W. Corson, A. Fein, W.W. Walthall, J. Gen. Physiol. 82 (1983) 659-677). In the presence of fluoride or vanadate the response to Ins 1,4,5-P3 was composed of a burst of waves rather than a single wave of depolarization. We conclude that hydrolysis of Ins 1,4,5-P3 by the InsP3-5-phosphatase plays a role in terminating the ventral photoreceptors response to Ins 1,4,5-P3 and also to light.

Current issues in invertebrate phototransduction. Second messengers and ion conductances

Investigation of phototransduction in invertebrate photoreceptors has revealed many physiological and biochemical features of fundamental biological importance. Nonetheless, no complete picture of phototransduction has yet emerged. In most known cases, invertebrate phototransduction involves polyphosphoinositide and cyclic GMP (cGMP) intracellular biochemical signaling pathways leading to opening of plasma membrane ion channels. Excitation is Ca(2+)-dependent, as are adaptive feedback processes that regulate sensitivity to light. Transduction takes place in specialized subcellular regions, rich in microvilli and closely apposed to submicrovillar membrane systems. Thus, excitation is a highly localized process. This article focuses on the intracellular biochemical signaling pathways and the ion channels involved in invertebrate phototransduction. The coupling of signaling cascades with channel activation is not understood for any invertebrate species. Although photoreceptors have features that are common to most or all known invertebrate species, each species exhibits unique characteristics. Comparative electrophysiological, biochemical, morphological, and molecular biological approaches to studying phototransduction in these species lead to fundamental insights into cellular signaling. Several current controversies and proposed phototransduction models are evaluated.

Rapid coupling of calcium release to depolarization in Limulus polyphemus ventral photoreceptors as revealed by microphotolysis and confocal microscopy

Microphotolysis and confocal microscopy were used to investigate the timing of calcium release and of the electrical response in Limulus polyphemus ventral photoreceptors. The fluorescent dyes Fluo-3 and Calcium Green-5N were used to monitor local Ca2+ elevations. Photolysis of caged inositol trisphosphate (InsP3) close to the plasma membrane of the light-sensitive rhabdomeral (R-) lobe resulted in Ca2+ elevation within 10-20 msec, 20-45 msec before the physiological response to light normally would be detected. Inward ionic current flow and depolarization followed InsP3-induced calcium release within 2.5 +/- 3.3 msec. Voltage-clamping the cells and removal of extracellular Ca2+ did not affect the timing of the Ca2+ elevation that followed the photolysis of caged InsP3 or its relationship to the electrical response. In contrast to the physiological response to light, which only released calcium within the R-lobe, photolysis of InsP3 elevated Cai in both lobes, although with much greater effect in the R-lobe, as compared with the bulk of the A-lobe, suggesting the presence of InsP3-sensitive calcium stores in both lobes. Photolysis of caged calcium [o-nitrophenyl EGTA (NPE)] at the edge of the R-lobe activated an inward ionic current within 1.8 +/- 0.7 msec. This NPE-induced current reversed at a membrane potential of 10 +/- 6 mV in the range typical of that of the light-activated current under physiological conditions. Calcium release, therefore, activates an inward current rapidly enough to contribute to the electrical response to light.

Limulus ventral photoreceptors contain two functionally dissimilar inositol triphosphate-induced calcium release mechanisms

Injections of inositol 1,4,5 triphosphate (InsP3) into Limulus ventral photoreceptors give rise to a rapid depolarization and an elevation of intracellular calcium concentration (Cai). This response to InsP3 is followed by a period of desensitization that persists as long as Cai remains elevated (feedback inhibition). Limulus ventral photoreceptors have two types of lobe: a light-sensitive rhabdomeric lobe (R lobe), and a light-insensitive arhabdomeric lobe (A lobe). Evidence showing the presence of feedback inhibition has been so far demonstrated only in the R lobe. In this study, simultaneous measurements of Cai were made using aequorin and double-barreled calcium-sensitive electrodes in each type of lobe. We carefully checked the location of the R lobe and A lobe by scanning a microspot of light across the whole photoreceptor. We then inserted a double-barreled calcium-sensitive microelectrode with InsP3 in either type of lobe. In the R lobe, injections of InsP3 led to a large Cai increase, a rapid depolarization and feedback inhibition; a brief flash of light induced a rapid depolarization and a Cai increase measured by both aequorin and the calcium-sensitive electrode. In the A lobe, injection of InsP3 led to an increase in Cai measured by the calcium-sensitive electrode but to no depolarization or aequorin luminescence. Further there was no evidence of feedback inhibition in the A lobe; the elevation of Cai caused by the first injection did not desensitize the photoreceptor to a second injection of InsP3 3 s later. To verify that the aequorin and the cell membrane respond to an increase in Cai, we presented a brief flash of light. Following a uniform illumination, there is indeed a typical large luminescence increase and a receptor potential. The calcium-sensitive electrode measures a small and slow Cai increase because its tip is located in the A lobe and it is measuring Ca2+ diffusing from the R lobe. Our observation that the InsP3-induced Cai increase in the A lobe is not apparently accompanied by a subsequent desensitization to InsP3 may suggest that there are more than one type of InsP3 receptor in the same cell. Alternatively, the InsP3 receptor could be the same but some additional factor, which confers feedback inhibition, could be missing in the A lobe.

Latencies of calcium elevation and depolarization in Limulus ventral photoreceptors injected with GDP-beta S

We have used confocal fluorescence microscopy and fluorescent calcium indicators to investigate the relationship between light-induced elevation of intracellular calcium ion concentration (Cai) and depolarization in small volumes of cytosol close to the microvillar plasma membrane of the ventral photoreceptor of Limulus polyphemus. We prolonged the latency of the light response by treatment of cells with hydroxylamine and injection of the G-protein blocker, guanosine-5'-O-(2-thiodiphosphate (GDP-beta S). Such treatment increased the latency of the cell's response from approximately 20 to 50 ms. In both treated and untreated cells we observed a close correlation between the times at which we first detected the electrical response and the elevation of Cai. We obtained 18 out of 54 and 12 out of 22 recordings, in untreated and treated cells respectively, for which the elevation of Cai was detected simultaneous with, or 1-4 ms prior to, the electrical response to light. The prolonged latent period exhibited by treated cells may make possible future investigation of the effects on the initial response to light of local photolytic release of caged compounds at the microvillar membrane.

INDO-1 measurements of absolute resting and light-induced Ca2+ concentration in Drosophila photoreceptors

Absolute Ca2+ levels in dissociated Drosophila photoreceptors were measured using the ratiometric indicator dye INDO-1 loaded via patch pipettes, which simultaneously recorded whole-cell currents. In wild-type photoreceptors, the ultraviolet (UV) excitation light used to measure fluorescence elicited a massive Ca2+ influx that saturated the dye (>10 microM Ca2+), but lagged the electrical response by 2.8 msec. Resting Ca2+ levels in the dark, measured during the latent period before the response, averaged 160 nM in normal Ringer's (1.5 mM Ca2+). Ca2+ increases in response to weak illumination were estimated (1) by using a weak adapting stimulus before the UV excitation light and measuring Ca2+ during the latent period; and (2) by using ninaE mutants with greatly reduced rhodopsin levels. Ca2+ rose linearly as a function of the time integral of the light-sensitive current with a slope of 2.7 nM/pC. In the transient receptor potential (trp) mutant, which lacks a putative light-sensitive channel subunit, the slope was only 1.1 nM/pC, indicating a 2.5-fold reduction in the fractional Ca2+ current. From these data, it can also be estimated that >99% of the Ca2+ influx is effectively buffered by the cell. In Ca2+-free Ringer's, resting cytosolic Ca2+ was reduced (to 30-70 nM), but contrary to previous reports, significant light-induced increases (approximately 250 nM) could be elicited. This rise was reduced to <20 nM when extracellular Na+ was replaced with N-methyl-D-glucamine, suggesting that it could be attributed to Na+ influx altering the Na/Ca exchanger equilibrium. It is concluded that any light-induced release from internal stores amounts to <20 nM.

Distinguishing between roles for calcium in Limulus photoreceptor excitation

The role for Ca2+ in the excitation process by which light opens membrane channels in Limulus photoreceptors is discussed. Light initiates a phospholipase C/IP3 pathway that results in a rapid elevation of intracellular Ca2+, but whether this elevation is causal in triggering the light response or merely synergistic to some other second messenger pathway has been unclear. We have developed a procedure using progressive injection of Ca2+ buffers that distinguishes between mediation and synergy models [Shin J-H. Richard EA. Lisman JE. (1992) Ca2+ is an obligatory intermediate in the excitation cascade of Limulus photoreceptors. Neuron, 11, 845-855]. Our conclusion is that Ca2+ mediates all phases of the light-response. Models of this kind had previously been rejected because intracellular injection of Ca2+ buffer can lead to an increase of the late component (> 200 ms) of the response to bright, sustained light. We have used computer simulations of IP3 mediated Ca2+ release to show that the positive and negative regulation of this process by Ca2+ itself together with other feedback loops can explain counterintuitive effects of Ca2+ buffers.

Structure and cellular physiology of Ca2+ stores in invertebrate photoreceptors

Invertebrate microvillar photoreceptors contain an extensive, morphologically continuous endoplasmic reticulum (ER) that comprises several distinct subregions. Most prominent is the smooth submicrovillar ER, a sponge-like cisternal network underneath the photoreceptive microvillar membrane. The submicrovillar ER spatially separates the microvilli and a narrow space of submicrovillar cytoplasm from the remaining cell body, and, thus, defines a transduction compartment. In bee and locust photoreceptors, the shape and position of these submicrovillar ER cisternae is maintained by interaction with actin filaments. The structural layout of the ER is either rather static, or, in some invertebrate species, the ER undergoes dramatic rearrangements during illumination. The submicrovillar ER has a high Ca content in dark-adapted cells (47.5 mmol/kg dry weight in bee photoreceptors), and acts as a source and sink for Ca2+ mobilized by illumination. About 50% of the Ca content is released by a 3 s, non-saturating light stimulus, and an almost equimolar amount of Mg is taken up to maintain electroneutrality within the ER. Ca2+ release is initiated by Ins(1,4,5)P3. In addition, the submicrovillar ER contains a heparin-insensitive, caffeine- and ryanodine-sensitive Ca2+ release pathway in bee photoreceptors. Both the Ins(1,4,5)P3-dependent and the ryanodine-sensitive Ca2+ release mechanism are modulated by cytosolic Ca2+, but at different Ca2+ concentrations. The presence of two release pathways with different Ca2+ sensitivities may be a prerequisite for highly localized, exceptionally fast and large Ca2+ elevations during the illumination of invertebrate photoreceptors.