Light-dependent channels from excised patches of Limulus ventral photoreceptors are opened by cGMP

Nitric oxide signaling pathways at neural level in invertebrates: functional implications in cnidarians

Nitric oxide (NO) is a small molecule with unconventional properties. It is found in organisms throughout the phylogenetic scale, from fungi to mammals, in which it acts as an intercellular messenger of main physiological events, or even as an intracellular messenger in invertebrates. In both vertebrates and invertebrates, NO is involved in many processes, regulated in part by cyclic guanosine monophosphate (cGMP), and reacts with different oxygen molecular species. The presence of NO in the early-diverging metazoan phylum of Cnidaria, of which Hydra represents the first known species having a nervous system, supports a role of this molecule as an ancestral neural messenger with physiological roles that remain to be largely elucidated. Therefore, our novel findings on the presence of NO in Hydra are here integrated in such a comparative frame.

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.

Photosensitive signal transduction induces membrane hyperpolarization in Paramecium bursaria

The protozoan ciliate Paramecium bursaria exhibits membrane hyperpolarization in response to photostimulation, accompanied with an increased swimming speed. The external addition of cyclic nucleotide phosphodiesterase (PDE) inhibitors, either theophylline (1,3-dimethylxanthine) or 3-isobutyl-1-methylxanthin (IBMX), increased in both amplitudes of the membrane hyperpolarization and the increase in swimming speed. Moreover, the addition of membrane permeable cyclic nucleotide analogs, either 8-bromo-adenosine 3',5'-cyclic monophosphate (Br-cAMP) or 8-Br-guanosine 3',5'-cyclic monophosphate (Br-cGMP), increased these amplitudes. On the other hand, the addition of l-cis-diltiazem, known to block the conductance of cyclic nucleotide-gated channels, partially decreased both amplitudes of the membrane hyperpolarization and the increase in swimming speed. An enzyme immunoassay of cellular cyclic nucleotide contents showed that photostimulation induced a rapid increase in adenosine 3',5'-cyclic monophosphate (cAMP), but little increase in guanosine 3',5'-cyclic monophosphate (cGMP), raising the possibility that a rapid increase in cAMP mediates the light-induced hyperpolarization in P. bursaria.

Whole-cell recording of light-evoked photoreceptor responses in a slice preparation of the cuttlefish retina

A new tissue slice preparation of the cuttlefish eye is described that permits patch-clamp recordings to be acquired from intact photoreceptors during stimulation of the retina with controlled light flashes. Whole-cell recordings using this preparation, from the retinas of very young Sepia officinalis demonstrated that the magnitude, latency, and kinetics of the flash-induced photocurrent are closely dependent on the magnitude of the flash intensity. Depolarizing steps to voltages more positive than -40 mV, from a membrane holding potential of -60 mV, induced a transient inward current followed by a larger, more sustained outward current in these early-stage photoreceptors. The latter current resembled the delayed rectifier (I(K)) already identified in many other nerve cells, including photoreceptors. This current was activated at -30 mV from a holding potential of -60 mV, had a sustained time course, and was blocked in a dose-dependent manner by tetraethylammonium chloride (TEA). The smaller, transient, inward current appeared at potentials more positive than -50 mV, reached peak amplitude at -30 mV and decreased with further depolarization. This current was characterized as the sodium current (I(Na)) on the basis that it was inactivated at holding potentials above -40 mV, was blocked by tetrodotoxin (TTX) and was insensitive to cobalt. Intracellular perfusion of the photoreceptors, via the patch pipette, demonstrated that U-73122 and heparin blocked the evoked photocurrent in a dose-dependent manner, suggesting the involvement of the phospholipase C (PLC) and inositol 1,4,5-triphosphate (InsP(3)), respectively, in the phototransduction cascade. Perfusion with cyclic GMP increased significantly the evoked photocurrent, while the inclusion of phorbol-12,13-dibutyrate reduced significantly the evoked photocurrent, supporting the involvement of cGMP and the diacylglycerol (DAG) pathways, respectively, in the cuttlefish transduction process.

Molecular and functional characterization of an I(h)-channel from lobster olfactory receptor neurons

We isolated a cDNA named PAIH encoding a member of the I(h)-channel family expressed in olfactory receptor neurons (ORNs) of the spiny lobster Panulirus argus. Functional expression of recombinant PAIH in HEK293 cells generated a slowly activating, noninactivating inward current under whole-cell voltage-clamp to hyperpolarizing voltage steps, the amplitude and activation rate of which increase with increasing hyperpolarization. The channel is weakly selective for K+. Intracellular cAMP or cGMP shifts activation of the current to less negative potentials in a concentration-dependent manner. Finally, the channel is blocked by the I(h)-channel blocker ZD7288. An I(h)-channel sharing the properties of the recombinant channel occurs in cultured lobster ORNs. PAIH immunoreactivity localizes the protein to the transduction compartment of the ORNs in situ, and selectively applying the blocker to the transduction compartment reduces spontaneous activity in the ORN. Collectively, these results implicate for the first time a functional role for an I(h)-channel in olfactory signal transduction.

Photosensitive neurons in mollusks

In addition to regular photoreceptors, some invertebrates possess simple extra ocular photoreceptors. For ex?ample, the central ganglia of mollusks contain photosensitive neurons. These neurons are located on the dorsal surface of the ganglia and based on their electrophysiological properties it has been postulated that they are internal photoreceptors. Besides the eye, transduction of light also occurs in these extra-ocular photoreceptors. In the present work, we analyze the reactivity of these nerve cells to light and describe the underlying mechanism mediating the light-induced response. <br><br><font color="red"><b> This article has been retracted. Link to the retraction <u><a href="http://dx.doi.org/10.2298/ABS150317025E">10.2298/ABS150317025E</a><u></b></font>

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.

Characterization of recombinant and native Ih-channels from Apis mellifera

Recently, a novel class of genes coding for Ih-channels has been identified in several vertebrates and invertebrates. We isolated a cDNA (AMIH) encoding a putative member of these ion channels from Apis mellifera heads by means of polymerase chain reaction and homology screening. High similarity (88% identical amino acids) to the putative Drosophila melanogaster Ih-channel suggests that the Apis cDNA codes for a hyperpolarization-activated and cyclic nucleotide-gated channel. Functional expression of recombinant AMIH in HEK293 cells gave unitary currents that were preferentially selective for potassium over sodium ions and were activated by hyperpolarizing voltage steps. Cyclic nucleotides shifted the voltage activation curve to more positive membrane potentials. The current kinetics, activation by hyperpolarizing voltage steps and modulatory influence of cyclic nucleotides properties closely resemble those of mammalian Ih-channels. RT-PCR analysis showed pronounced mRNA expression in the antennae, head and body of Apis mellifera. Investigation of hyperpolarization-activated currents in olfactory receptor neurons (ORNs) in a primary cell culture of Apis mellifera antennal cells revealed activation properties similar to the heterologously expressed Ih-channel. By in-situ hybridization and immunohistochemistry, expression of AMIH was seen in olfactory receptor neurons of the bee antennae. We conclude that AMIH is the ion channel responsible for the hyperpolarization-activated currents in olfactory receptor neurons of bee.

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.

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.

Light-dependent K(+) channels in the mollusc Onchidium simple photoreceptors are opened by cGMP

Light-dependent K(+) channels underlying a hyperpolarizing response of one extraocular (simple) photoreceptor, Ip-2 cell, in the marine mollusc Onchidium ganglion were examined using cell-attached and inside-out patch-clamp techniques. A previous report (Gotow, T., T. Nishi, and H. Kijima. 1994. Brain Res. 662:268-272) showed that a depolarizing response of the other simple photoreceptor, A-P-1 cell, results from closing of the light-dependent K(+) channels that are activated by cGMP. In the cell-attached patch recordings of Ip-2 cells, external artificial seawater (ASW) was replaced with a modified ASW containing 150 mM K(+) and 200 mM Mg(2+) to suppress any synaptic input and to maintain the membrane potential constant. When Ip-2 cells were equilibrated with this modified ASW, the internal K(+) concentration was estimated to be 260 mM. Light-dependent single-channels in the cell-attached patch on these cells were opened by light but scarcely by voltage. After confirming the light-dependent channel activity in the cell-attached patches, an application of cGMP to the excised inside-out patches newly activated a channel that disappeared on removal of cGMP. Open and closed time distributions of this cGMP-activated channel could be described by the sum of two exponents with time constants tau(o1), tau(o2) and tau(c1), tau(c2), respectively, similar to those of the light-dependent channel. In both the channels, tau(o1) and tau(o2) in ms ranges were similar to each other, although tau(c2) over tens of millisecond ranges was different. tau(o1), tau(o2), and the mean open time tau(o) were both independent of light intensity, cGMP concentration, and voltage. In both channels, the open probability increased as the membrane was depolarized, without changing any of tau(o2) or tau(o). In both, the reversal potentials using 200- and 450-mM K(+)-filled pipettes were close to the K(+) equilibrium potentials, suggesting that both the channels are primarily K(+) selective. Both the mean values of the channel conductance were estimated to be the same at 62 and 91 pS in 200- and 450-mM K(+) pipettes at nearly 0 mV, respectively. Combining these findings with those in the above former report, it is concluded that cGMP is a second messenger which opens the light-dependent K(+) channel of Ip-2 to cause hyperpolarization, and that the channel is the same as that of A-P-1 closed by light.

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels first identified in retinal photoreceptors and olfactory sensory neurons (OSNs). They are opened by the direct binding of cyclic nucleotides, cAMP and cGMP. Although their activity shows very little voltage dependence, CNG channels belong to the superfamily of voltage-gated ion channels. Like their cousins the voltage-gated K+ channels, CNG channels form heterotetrameric complexes consisting of two or three different types of subunits. Six different genes encoding CNG channels, four A subunits (A1 to A4) and two B subunits (B1 and B3), give rise to three different channels in rod and cone photoreceptors and in OSNs. Important functional features of these channels, i.e., ligand sensitivity and selectivity, ion permeation, and gating, are determined by the subunit composition of the respective channel complex. The function of CNG channels has been firmly established in retinal photoreceptors and in OSNs. Studies on their presence in other sensory and nonsensory cells have produced mixed results, and their purported roles in neuronal pathfinding or synaptic plasticity are not as well understood as their role in sensory neurons. Similarly, the function of invertebrate homologs found in Caenorhabditis elegans, Drosophila, and Limulus is largely unknown, except for two subunits of C. elegans that play a role in chemosensation. CNG channels are nonselective cation channels that do not discriminate well between alkali ions and even pass divalent cations, in particular Ca2+. Ca2+ entry through CNG channels is important for both excitation and adaptation of sensory cells. CNG channel activity is modulated by Ca2+/calmodulin and by phosphorylation. Other factors may also be involved in channel regulation. Mutations in CNG channel genes give rise to retinal degeneration and color blindness. In particular, mutations in the A and B subunits of the CNG channel expressed in human cones cause various forms of complete and incomplete achromatopsia.

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.

The ordered visual transduction complex of the squid photoreceptor membrane

The study of visual transduction has given invaluable insight into the mechanisms of signal transduction by heptahelical receptors that act via guanine nucleotide binding proteins (G-proteins). However, the cyclic-GMP second messenger system seen in vertebrate photoreceptor cells is not widely used in other cell types. In contrast, the retina of higher invertebrates, such as squid, offers an equally accessible transduction system, which uses the widespread second messenger chemistry of an increase in cytosolic calcium caused by the production of inositol-(1,4,5)-trisphosphate (InsP3) by the enzyme phospholipase C, and which may be a model for store-operated calcium influx. In this article, we highlight some key aspects of invertebrate visual transduction as elucidated from the combination of biochemical techniques applied to cephalopods, genetic techniques applied to flies, and electrophysiology applied to the horseshoe crab. We discuss the importance and applicability of ideas drawn from these model systems to the understanding of some general processes in signal transduction, such as the integration of the cytoskeleton into the signal transduction process and the possible modes of regulation of store-operated calcium influx.

Molecular cloning of a putative cyclic nucleotide-gated ion channel cDNA from Limulus polyphemus

Cyclic nucleotide-gated channels have been proposed to mediate the electrical response to light in the ventral photoreceptor cells of the horseshoe crab, Limulus polyphemus. However, a cyclic nucleotide-gated channel has not been identified from Limulus. We have cloned a putative full-length cyclic nucleotide-gated channel cDNA by screening cDNA libraries constructed from Limulus brain using a probe developed from Limulus ventral eye nerves. The putative full-length cDNA was derived from two overlapping partial cDNA clones. The open reading frame encodes 905 amino acids; the sequence shows 44% identity to that of the alpha subunit of the bovine rod cyclic GMP-gated channel over the region containing the transmembrane domains and the cyclic nucleotide binding domain. This Limulus channel has a novel C-terminal region of approximately 200 amino acids, containing three putative Src homology domain 3 binding motifs and a putative coiled-coil domain. The possibility that this cloned channel is the same as that detected previously in excised patches from the photoreceptive membrane of Limulus ventral photoreceptors is discussed in terms of its sequence and its expression in the ventral eye nerves.

Nitric oxide and cyclic GMP modulate photoreceptor cell responses in the visual system of the locust

Nitric oxide (NO) is a membrane-permeant messenger molecule which activates the cyclic GMP (cGMP)-synthesizing enzyme soluble guanylyl cyclase. Using cytochemical techniques, we recently reported NO-induced cGMP immunoreactivity in the photoreceptor cells of the compound eye of the locust Schistocerca gregaria and also detected NADPH diaphorase staining, a marker of NO synthase, in a subset of the monopolar cells of the lamina. By recording the corneal electroretinogram (ERG), we found that the application of neurochemicals that raise NO/cGMP levels in the optic lobe increased the ERG amplitude, whereas the experimental reduction of NO levels caused a decrease in the response to light. An increase in the light response was also found in intracellular recordings after application of a NO donor, suggesting that the NO-induced changes in the ERG are not caused by changes in the resistive isolation of the retina. Our cytochemical and electrophysiological data are both consistent with the hypothesis that NO synthesized in monopolar cells is a retrograde messenger to the presynaptic photoreceptor neurones.

Light-increased cGMP and K+ conductance in the hyperpolarizing receptor potential of Onchidium extra-ocular photoreceptors

The phototransduction mechanism of the extra-ocular photoreceptor cells Ip-2 and Ip-1 in the mollusc Onchidium ganglion was examined. Previous work showed that the depolarizing receptor potential of another extra-ocular photoreceptor cell, A-P-1 is produced by a decrease of the light-sensitive K+ conductance activated by a second messenger, cGMP and is inactivated by the hydrolysis of cGMP. Here, a hyperpolarizing receptor potential of Ip-2 or Ip-1 was associated with an increase in membrane conductance. When Ip-2 or Ip-1 was voltage-clamped near the resting membrane potential, light induced an outward photocurrent corresponding to the above hyperpolarization. The spectral sensitivity had a peak at 510 nm. The shift of reversal potentials of the photocurrent depended on the Nernst equation of K(+)-selective conductance. The photocurrent was blocked by 4-AP and L-DIL, which are effective blockers of the A-P-1 light-sensitive K+ conductance. These results suggested that the hyperpolarization is mediated by increasing a similar light-sensitive K+ conductance to that of A-P-1. The injection of cGMP or Ca2+ into a cell produced a K+ current that mimicked the photocurrent. 4-AP and L-DIL both abolished the cGMP-activated K+ current, while TEA suppressed only the Ca(2+)-activated K+ current. These results indicated that cGMP is also a second messenger that regulates the light-sensitive K+ conductance. The photocurrent was blocked by LY-83583, a guanylate cyclase (GC) inhibitor, but was unaltered by zaprinast, a phosphodiesterase (PDE) inhibitor. Together, the present results suggest that increasing the internal cGMP in Ip-2 or Ip-1 cells light-activates GC rather than inhibits PDE, thereby leading to an increase of the light-sensitive K+ conductance and the hyperpolarization.

Evidence for the involvement of inositol trisphosphate but not cyclic nucleotides in visual transduction in Hermissenda eye

Although several second messengers are known to be involved in invertebrate photoresponses, the mechanism underlying invertebrate phototransduction remains unclear. In the present study, brief injection of inositol trisphosphate into Hermissenda photoreceptors induced a transient Na+ current followed by burst activity, which accurately reproduced the native photoresponse. Injection of Ca2+ did not induce a significant change in the membrane potential but potentiated the native photoresponse. Injection of a Ca2+ chelator decreased the response amplitude and increased the response latency. Injection of cGMP induced a Ca2+-dependent, transient depolarization with a short latency. cAMP injection evoked Na+-dependent action potentials without a rise in membrane potential. Taken together, these results suggest that phototransduction in Hermissenda is mediated by Na+ channels that are directly activated by inositol trisphosphate without mobilization of cytosolic Ca2+.

Ca2+/calmodulin-binding peptides block phototransduction in Limulus ventral photoreceptors: evidence for direct inhibition of phospholipase C

Phototransduction in Limulus photoreceptors involves a G protein-mediated activation of phospholipase C (PLC) and subsequent steps involving InsP3-mediated release of intracellular Ca2+. While exploring the role of calmodulin in this cascade, we found that intracellular injection of Ca2+/calmodulin-binding peptides (CCBPs) strongly inhibited the light response. By chemically exciting the cascade at various stages, we found the primary target of this effect was not in late stages of the cascade but rather at the level of G protein and PLC. That PLCdelta1 contains a calmodulin-like structure raised the possibility that PLC might be directly affected by CCBPs. To test this possibility, in vitro experiments were conducted on purified PLC. The activity of this enzyme was strongly inhibited by CCBPs and also inhibited by calmodulin itself. Our results suggest that the calmodulin-like region of PLC has an important role in regulating this enzyme.

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.

Antagonists of the cGMP-gated conductance of vertebrate rods block the photocurrent in scallop ciliary photoreceptors

1. Hyperpolarizing scallop photoreceptors, like vertebrate rods, use cGMP as an internal messenger and their light-sensing structure is also of ciliary origin. To ascertain possible functional similarities between the light-sensitive conductances in the two classes of visual cells, we examined in scallop photoreceptors the effects of several antagonists of the photocurrent of rods. 2. Extracellular application of L-cis-diltiazem rapidly and reversibly suppressed the photocurrent. The effect was stereospecific and dose dependent, with a K1/2 of approximately 400 microM. Intracellular dialysis at lower doses (100-200 microM) also induced a substantial inhibition. 3. L-cis-Diltiazem reduced the light-activated conductance without shifting the intensity-response curve. Furthermore, the drug also blocked the current directly evoked by application of cGMP. These observations indicate that the inhibitory effects result from blockage of the conductance, rather than from impairment of the activating cascade. 4. The fractional blockage increased e-fold per approximately 55 mV depolarization, regardless of the side of drug application, as if the charged form of L-cis-diltiazem can only access the blocking site from the intracellular compartment. 5. The amiloride derivative 3',4'-dichlorobenzamil potently suppressed the photocurrent (K1/2 approximately 5 microM), without affecting its kinetics or operating range. Amiloride itself was also effective at higher concentrations. 6. The pharmacological resemblance of these light-dependent channels to those of rods and cones indicates that significant aspects of the transduction cascade are conserved across disparate sensory cells of ciliary origin.

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.

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.

The nitric oxide system in insects

It is well established that nitric oxide (NO) acts as a signalling molecule in the nervous system of both mammals and insects. In contrast to classical transmitters, the membrane-permeant NO can act on neighbouring targets limited by half-life and diffusion barriers. This type of diffuse signalling seems to be evolutionarily highly conserved and recent findings concerning the characterization and function of the NO system in insects are summarized in this review. Firstly, the properties and the localization of the NO forming enzyme, the NO synthase (NOS), are described. In the nervous system the brain contains by far the highest NOS activity. As an evolutionary peculiarity, a blood-feeding bug exhibits high NOS activity in the salivary glands. Secondly, the soluble guanylate cyclase (sGC), a major target of NO action, and cGMP-regulated enzymes like cGMP-dependent protein kinase and cyclic nucleotide gated channels are described. Anatomical organization of the NO/cGMP system in insects reveals evidence for a cellular separation of the release site and target site of NO, although in the antennal lobes of the locust an exception from this rule exists. Thirdly, the implication of the NO system in neuronal function in insects is described. In the honeybee, the NO/cGMP system in the antennal lobes is implicated in the processing of adaptive mechanisms during chemosensory processing, and recent findings support a specific role of the NO system in memory formation. Discussion of the results in insects with regard to properties and functions of the vertebrate NO system is attempted.

Cytochemical evidence for nitric oxide/cyclic GMP signal transmission in the visual system of the locust

Nitric oxide is a membrane-permeant messenger molecule which activates soluble guanylyl cyclase. Using NADPH diaphorase staining as a marker for the enzyme nitric oxide synthase and an antiserum against cyclic GMP (cGMP) we investigated the possible sites of nitric oxide and cGMP synthesis in the retina and lamina of Schistocerca gregaria. The photoreceptor cells did not express NADPH diaphorase staining but monopolar cells of the lamina were strongly stained. After inhibition of phosphodiesterase activity and incubation of tissue in a nitric oxide donor, the photoreceptor cells showed cGMP immunoreactivity. In contrast to the photoreceptors, the monopolar cells of the lamina were not stained. Since the presynaptic photoreceptors were cGMP-immunoreactive and the postsynaptic targets of the monopolar cells did not express immunoreactivity, it is conceivable that nitric oxide released by monopolar cells may play a role as a retrograde messenger in visual information processing.

Molecular and immunological characterization of a Gq protein from ventral and lateral eye of the horseshoe crab Limulus polyphemus

GTP binding proteins of the Gq family have been implicated in phototransduction in rhabdomeral photoreceptors. In this study we used molecular and immunochemical techniques to characterize a GTP-binding protein alpha subunit of the Gq family in ventral and lateral photoreceptors of the horseshoe crab, Limulus polyphemus. Both ventral photoreceptors and lateral eye retinular cells became strongly labeled with an antibody directed against the common carboxyl tail of two Gq family proteins, G alpha q and G alpha 11. This antibody also labeled a 42 kDa band on Western blots of proteins from ventral photoreceptor cell bodies, ventral photoreceptor axons, lateral eyes and lateral optic nerves. The reverse transcription-polymerase chain reaction (RT-PCR), along with degenerate oligonucleotide primers designed against conserved regions of known G alpha q and G alpha 11 proteins, was used to isolate a cDNA from ventral eye RNA which encodes a protein with high identity to known Gq proteins. Ribonuclease protection assays showed that the corresponding message was expressed in ventral eye, but these assays, as well as Northern blots, failed to detect expression in lateral eye. Therefore, while photoreceptors of both ventral and lateral eyes contain a Gq-like protein, the mRNA encoding the Gq protein in the ventral eye may differ in nucleotide sequence from its lateral eye counterpart.

Family of cyclic nucleotide gated ion channels

A variety of different cyclic nucleotide gated ion channels have recently been identified using molecular cloning and electrophysiological techniques. Current research is focussed on the specific molecular determinants that endow these channels with their distinctive character of gating, selectivity and modulation. In some cases, it has been possible to identify the specific physiological roles of different cyclic nucleotide gated channels. Their interactions with Ca2+ and calmodulin are particularly important, and determine the specific functions these channels subserve in distinct cells.

Molecular characterization of two Drosophila guanylate cyclases expressed in the nervous system

We have isolated, by interspecies hybridization, two classes of Drosophila cDNA each encoding a different guanylate cyclase (GC). One of them encodes an alpha subunit homolog of soluble GC, designated DGC alpha 1, and the other encodes a receptor-type GC, designated DrGC. The dgc alpha 1 cDNA encodes a protein of 676 amino acids and maps to 99B. In situ hybridization to adult tissue sections showed that dgc alpha 1 mRNA is found mainly in the cell bodies of the optic lobe, central brain, and thoracic ganglia. The DGC alpha 1 protein was also localized primarily to the nervous system by immunocytochemical staining, consistent with results of in situ hybridization. However, no detectable expression of this protein was found in the retina. The other class of cDNA, drgc, maps to 76C and encodes a 1525-amino acid protein displaying structural features similar to other known receptor-type guanylate cyclases. However, it has a C-terminal 430 amino acid region that has no homology to any known proteins. drgc RNA is expressed at low levels throughout development and in adult heads and bodies. In situ hybridizations to adult tissue sections showed that drgc mRNA is expressed in a wide range of tissues, including the optic lobe, central brain, thoracic ganglia, digestive tract, and the oocyte.

The dynamics of cytosolic calcium in photoreceptor cells

Analysis of the light-induced changes of cytosolic Ca2+ ([Ca2+]i) in photoreceptor cells has been taken a step further with two recently published studies. In one, changes in [Ca2+]i were measured in single detached rod outer segments from Gecko in response to various light intensities. The advances of the other are embodied in its employment of transgenic Drosophila, whose photoreceptors express a visual pigment that is insensitive to the wavelength of light used in the fluorescence imaging of [Ca2+]i. These studies provide a better basis for understanding the regulation of Ca(2+)-mediated events in photoreceptor cells.

Cyclic 3',5'-nucleotide phosphodiesterase: cytochemical localization in photoreceptor cells of the fly Calliphora erythrocephala

The distribution of cyclic 3',5'-nucleotide phosphodiesterase activity was determined in photoreceptor cells of the fly Calliphora erythrocephala. With cAMP as substrate, staining was most intense within the phototransducing region of these cells, the rhabdomeral microvilli and also in the extracellular space surrounding the microvilli and in the mitochondria. With cGMP as substrate, the intensity within the rhabdomeres was less marked, while their extracellular surroundings were stained heavily. Thus, compared to cGMP, cAMP is the better substrate for the phosphodiesterase in the rhabdomeres of the fly. For comparison, the same cytochemical method was used to localize the well-known phosphodiesterase activity in retinal tissue of the mouse. Under the same conditions as used for fly photoreceptors, a very intense reaction product was obtained in rod outer segments. With regard to the conflicting reports concerning the light-stimulated changes of cyclic nucleotides in invertebrate photoreceptor cells, the results presented here further argue for an important role of a cyclic nucleotide in the process of phototransduction of invertebrates.

Distinctive subtypes of bovine phospholipase C that have preferential expression in the retina and high homology to the norpA gene product of Drosophila

The Drosophila norpA gene encodes a phospholipase C involved in phototransduction. However, phospholipase C apparently is not directly involved in phototransduction in vertebrate photoreceptors, although light-activated phospholipase C activity has been reported in vertebrate rod outer segments. Conserved regions of norpA cDNA were used to isolate bovine cDNAs that would encode four alternative forms of phospholipase C of the beta class that are highly homologous to the norpA protein and expressed preferentially in the retina. Two of the variants are highly unusual in that they lack much of the N-terminal region present in all other known phospholipases C. The sequence conservation between these proteins and the norpA protein is higher than that between any other known phospholipases C. GTPase sequence motifs found in proteins of the GTPase superfamily are found conserved in all four variants of the bovine retinal protein as well as the norpA protein but not in other phospholipases C. Results suggest that these proteins together with the norpA protein constitute a distinctive subfamily of phospholipases C that are closely related in structure, function, and tissue distribution. Mutations in the norpA gene, in addition to blocking phototransduction, cause light-dependent degeneration of photoreceptors. In view of the strong similarity in structure and tissue distribution, a defect in these proteins may have similar consequences in the mammalian retina.

Intracellular injection of heparin and polyamines. Effects on phototransduction in limulus ventral photoreceptors

Heparin is thought to inhibit InsP3 binding to receptors involved in the intracellular release of Ca2+. Injection of heparin into Limulus ventral photoreceptors to high intracellular concentrations reduces the amplitude and slows the rate of rise of voltage-clamp currents induced by brief flashes, tends to make the responses to long flashes more "square," and tends to block the light-induced rise in [Ca2+]i detected by arsenazo III. In these ways, intracellular heparin mimics the effects of high concentrations of intracellular BAPTA or EGTA. In addition, the effects of heparin are attenuated by prior injection of BAPTA to high intracellular concentrations. Neomycin and spermine are thought to inhibit phospholipase C activity. Injections of spermine or neomycin to low intracellular concentrations largely mimic the effects of intracellular heparin. These findings suggest that the predominant effect of polyamines is to inhibit light-induced production of InsP3 by phospholipase C activity and thereby reduce the light-induced increase in [Ca2+]i. Our findings suggest that excitation can proceed in the absence of InsP3-induced increases in [Ca2+]i, but (a) the gain and speed of transduction are reduced and (b) adaptation is largely blocked.

Polarized distribution of Na,K-ATPase in honeybee photoreceptors is maintained by interaction with glial cells

Arthropod photoreceptors are polarized cells displaying distinct surface domains. The distribution of the Na,K-ATPase (sodium pump) over these domains was examined in the honeybee photoreceptor using a monoclonal antibody that specifically recognizes the sodium pump alpha-subunit (approximately 100 kDa). We find that the sodium pump is restricted to sites of the nonreceptive photoreceptor surface closely juxtaposed to glial cells; no sodium pumps were detected on the glia-free domains of the nonreceptive surface and on the light-sensitive microvillar membranes. In order to determine the role of photoreceptor-glia contact in maintaining this polarized pump distribution, we assayed the distribution of the Na,K-ATPase after experimentally influencing photoreceptor-glia contact. Sodium pumps were present on the entire nonreceptive photoreceptor surface when photoreceptor-glia contact was removed by isolating the photoreceptors. Remodeling photoreceptor-glia contact by incubation in hyperosmotic saline caused a redistribution of sodium pumps on the photoreceptor surface corresponding to the redistribution of glial cells. We show, further, that both photoreceptor-glia contact and Na,K-ATPase distribution are independent of extracellular Ca2+. No junctional structures were observed at the borders between Na,K-ATPase-positive and Na,K-ATPase-negative membrane domains. Together, these results suggest that adhesion of glial cells to the photoreceptors plays a crucial role in the maintenance of the polarized distribution of Na,K-ATPase in the honeybee photoreceptors. The Ca(2+)-independent adhesion of glial cells to the photoreceptor surface may trap the pump molecules at the sites of photoreceptor-glia contact.

Photosensory transduction in ciliates. IV. Modulation of the photomovement response of Blepharisma japonicum by cGMP

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.

Isolation of a Drosophila gene encoding a head-specific guanylyl cyclase

We have isolated and characterized a new guanylyl cyclase gene (dgc1) in Drosophila. The deduced amino acid sequence (683 amino acids) most closely resembled the mammalian soluble-type guanylyl cyclase alpha subunit. The cyclase catalytic domain was highly conserved between the mammalian and Drosophila guanylyl cyclases. The dgc1 mRNA was detected in wild-type heads but not in bodies, and its level was reduced in the mutant eyes absent (eya), indicating that dgc1 is preferentially expressed in the CNS and in the eye. The enriched distribution in the eye suggests that dgc1 may have a role in phototransduction.

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

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

Spontaneous activity of the light-dependent channel irreversibly induced in excised patches from Limulus ventral photoreceptors

We have studied the properties of membrane patches excised from the transducing lobe of Limulus ventral photoreceptors. If patches are excised into an "internal" solution that resembles the ionic composition of the cytoplasm, channel activity is typically absent, but can be turned on by cyclic GMP (cGMP). In contrast, if patches are excised directly into sea water and subsequently examined in internal solution, they exhibit a high channel activity in the absence of any second messenger (spontaneous channel activity). Because these patches contained only light-dependent channels when examined before excision and because these spontaneous channels have properties in common with the light/cGMP-dependent channel, we believe that the spontaneously active channels represent light/cGMP-dependent channels that have been damaged by exposure to sea water, perhaps due to proteolysis activated by the high Ca2+ levels of the sea water. One type of the spontaneously active channel resembles the light/cGMP-dependent channel in open time, reversal potential, conductance states and voltage dependence. Application of micromolar Ca2+ to this channel produces a reversible decrease in the opening rate, indicating a high affinity binding site for Ca2+ on this channel. Another type of spontaneously active channel has a conductance state and reversal potential similar to the light/cGMP-dependent channel, but has apparently lost its dependence and sensitivity to Ca2+ and voltage.

The latency of the response of Limulus photoreceptors to inositol trisphosphate lacks the calcium-sensitivity of that to light

The latent period before depolarization of Limulus ventral photoreceptors by light flashes was compared with that following brief, intracellular, pressure-injection of d-myo-inositol 1,4,5 trisphosphate. At temperatures between 18 degrees C and 22 degrees C and with an extracellular calcium concentration of 10 mM, the responses of 4 cells to light and to injections of 100 microM inositol trisphosphate displayed average latencies of 71 and 56 ms, respectively. The latencies of responses to InsP3 included an estimated 20 ms dead-time inherent in the injection method. Reducing the temperature lengthened the latency of the response to light (Q10 approximately 3.2 between 7 and 22 degrees C) more than that to inositol trisphosphate (Q10 approximately 2.3). Bathing the photoreceptors in seawater containing no added calcium and 1 mM of the calcium chelator EGTA greatly increased the latency of the light response at all temperatures, but did not increase the latency of the response to inositol trisphosphate. We conclude that the response to inositol trisphosphate lacks the calcium- and temperature-sensitive latent period which characterizes the response to light. If inositol trisphosphate acts, via the release of stored calcium, to stimulate an intermediate in the visual cascade, then that intermediate would appear to be downstream from the latency-generating mechanism.

Localization of phototransduction in Limulus ventral photoreceptors: a demonstration using cell-free rhabdomeric vesicles

Second messengers are involved in a number of cellular responses to a variety of stimuli. Diffusion of these second messengers likely will determine the speed and efficiency of such responses. Localization, particularly in large cells, would enhance the efficiency of such transduction systems by restricting the volume in which this diffusion takes place and thereby limiting the diffusion of soluble messengers. Phototransduction in Limulus ventral photoreceptors involves second-messenger systems; the volume of this cell is quite large, but the effect of a single photoexcited rhodopsin molecule is exerted over light-dependent channels localized within a very small area of the plasma membrane. In order to investigate localization of phototransduction in these photoreceptors, we have compared the light responses of small vesicles (photoballs) excised from these cells with those of the intact photoreceptors. We found that the basic kinetics of excitation and adaptation of the photoballs are essentially identical to those of the intact cell. This indicates that all of the necessary machinery for phototransduction is present and intact in the photoball and that any diffusion of second messengers that affect the normal light response of the cell must occur within a region at least as small as our photoballs (on the order of 1 micron3).

Photoreceptor deactivation and retinal degeneration mediated by a photoreceptor-specific protein kinase C

The protein kinase C (PKC) family of serine-threonine kinases has been implicated in the regulation of a variety of signaling cascades. One member of this family, eye-PKC, is expressed exclusively in the Drosophila visual system. The inaC (inactivation-no-afterpotential C) locus was shown to be the structural gene for eye-PKC. Analysis of the light response from inaC mutants showed that this kinase is required for the deactivation and rapid desensitization of the visual cascade. Light adaptation was also defective in inaC mutant flies. In flies carrying the retinal degeneration mutation rdgB, absence of eye-PKC suppressed photoreceptor cell degeneration. These results indicate that eye-PKC functions in the light-dependent regulation of the phototransduction cascade in Drosophila.